Information Engineering Department
Institute of Communication, Information and Perception Technologies
University of Pisa, Scuola Superiore Sant’Anna
Model-Based Development of an
Aircraft Environmental Control System
Master Degree in Embedded Computing Systems
Supervisors:
Prof. Marco Di Natale Prof. Giorgio C. Buttazzo
External Supervisor: Dr. Giusi Quartarone
Author: Salvatore Armenia
Acknowledgements
I am going to write this brief section of acknowledgements to thank the people who have supported me and helped me throughout this period. The first thanks goes to my supervisor Marco Di Natale, associate Professor at the Scuola Superiore Sant’Anna of Pisa. I would like to thank him for his availability when I needed to solve some issues and his precious advices.
I would like to thank Philip Harris for giving me the opportunity to embark on this grateful work experience, my supervisor Giusi Quartarone who has helped me during this period and Erica Zavaglio who taught me the fundamentals of thermodynamics. Last but not least touching thanks goes to my colleagues at the United Technologies Research Center with whom I shared my 6-months traineeships and supported me also just by talking and drinking a coffee in the canteen.
Thanks to everybody!
The aECS has received funding from the Clean Sky 2 Joint Undertaking under the European Union’s Horizon 2020 research and innovation programme under grant agreement No CS2-SYS-GAM-2014-2015-01.
Abstract
Model-Based Development (MBD) is a consolidated approach that is providing different strategies to effectively manage the development of increasingly complex systems. This work aims to exploit the Model-Based Development approach to analyse and document the key aspects of an aircraft Environmental Control System. The usage of models to drive the development process allows the definition of the system architecture and the formally specification of the components within the system taken into account. Specifically, the system architecture and the components models are defined by using the standardised modelling languages SysML. Moreover, this document aims to highlight how the models can be used in the next steps of the development process to generate runnable code for the creation of a Simulink library and to enable the generation of the documentation.
Table of contents
List of figures viii
List of tables xi
Glossary xii
1 Introduction 1
1.1 Model Based design . . . 2
1.2 System Modelling Language . . . 3
1.2.1 SysML constructs . . . 5
1.2.1.1 Stereotype . . . 5
1.2.1.2 Block Description Diagram . . . 5
1.2.1.3 Internal Block Diagram . . . 5
1.2.1.4 Block and Value type . . . 6
1.2.1.5 Generalization . . . 6 1.2.1.6 Association . . . 6 1.2.1.7 Port . . . 7 1.2.1.8 Operation . . . 7 1.3 Avionics field . . . 7 1.3.1 Clean Sky 2 . . . 8
1.3.2 Adaptive Environmental Control System . . . 10
1.3.3 Partners . . . 12 1.4 Intellectual Property . . . 12 2 Tools 13 2.1 Papyrus . . . 13 2.2 Acceleo . . . 13 2.3 Gendoc . . . 14
Table of contents v
2.4.1 Matlab . . . 14
2.4.2 Simulink . . . 14
3 Aircraft Environmental Control System 16 3.1 Environmental Control System . . . 16
3.2 Air-source System . . . 19
3.3 Air-conditioning Pack . . . 22
3.4 Ventilation and Distribution System . . . 25
3.5 Cabin Pressurization . . . 27 3.6 Temperature Control . . . 29 3.7 Recirculation System . . . 30 4 Model Definition 33 4.1 ECS simulator . . . 34 4.1.1 Simulator features . . . 34 4.1.2 Simulator analysis . . . 36 4.1.3 Results . . . 38 4.1.4 Test . . . 40 4.2 Papyrus project . . . 41 4.3 Standard methodology . . . 42 4.4 Stereotypes . . . 43 4.4.1 Bounds . . . 43 4.4.2 Units . . . 44 4.5 TYPEs package . . . 45 4.5.1 Fluid type . . . 45 4.5.1.1 Humid air . . . 46 4.5.1.2 Ideal air . . . 46 4.5.1.3 Water . . . 47 4.5.2 Rotation type . . . 47 4.5.3 Components type . . . 48 4.6 COMPONENTs package . . . 49
4.6.1 Bleed-air system component . . . 49
4.6.2 Heat-Exchanger component . . . 50
4.6.2.1 Pre-cooler . . . 51
4.6.2.2 Plate fin heat-exchanger . . . 51
4.6.2.3 Condensing heat-exchanger . . . 52
Table of contents vi 4.6.4 Splitter component . . . 53 4.6.5 Duct component . . . 54 4.6.6 Valve component . . . 54 4.6.7 Filter component . . . 55 4.6.8 Compressor component . . . 55 4.6.8.1 Compressor simple . . . 56
4.6.8.2 Compressor based on Universal Map . . . 56
4.6.9 Turbine component . . . 58
4.6.10 Shaft component . . . 59
4.6.11 Fan component . . . 59
4.6.11.1 Fan simple . . . 60
4.6.11.2 Fan based on Universal Map . . . 60
4.6.12 Cabin . . . 61
4.7 INTEGRATION package . . . 62
4.7.1 Example - Environmental Control System . . . 62
4.8 Results . . . 66
5 Generation of the Simulink library 68 5.1 Acceleo and Code-generation . . . 68
5.1.1 Acceleo templates . . . 69 5.1.1.1 generateMatlabCode . . . 69 5.1.1.2 parsingBlock . . . 70 5.1.1.3 createInOutport . . . 70 5.1.1.4 parsingPort . . . 70 5.1.1.5 createBusObject . . . 70 5.1.1.6 applyBoundsSstereotype . . . 71 5.1.1.7 createSimMatBlock . . . 71 5.1.1.8 concatFunctionAggr . . . 71 5.1.1.9 concatFunDataType . . . 72 5.1.1.10 manageFunOutputDataType . . . 72 5.1.1.11 createSimulinkBlockMask . . . 72 5.1.1.12 parsingReadOnlyAttribute . . . 72 5.1.1.13 manageAttributeOverRide . . . 73 5.1.1.14 manageAssociationReadOnly . . . 73 5.1.1.15 connectBlockInOut . . . 73 5.1.1.16 repositioningSubsystems . . . 73 5.2 Result . . . 74
Table of contents vii
5.3 Opportunities . . . 76
6 Generation of the documentation 77 6.1 Gendoc template . . . 77
7 Conclusion 80 7.1 Future work . . . 81
7.1.1 More generic SysML models . . . 81
7.1.2 Code generation from Internal Block Diagrams . . . 82
7.1.3 Requirements formalization . . . 83
7.1.4 Validation and Verification . . . 83
References 84 Appendix A TYPEs package 87 A.1 Fluid type . . . 87
Appendix B COMPONENTs package
SysML BDDs - IBDs - Blocks features
90 B.1 Engine component . . . 90 B.2 Heat-Exchanger component . . . 93 B.3 Mixer component . . . 97 B.4 Splitter component . . . 97 B.5 Filter component . . . 98 B.6 Duct component . . . 99 B.7 Compressor component . . . 100 B.8 Turbine component . . . 102 B.9 Shaft component . . . 103 B.10 Fan component . . . 104 B.11 Cabin . . . 107List of figures
1.1 Model-Based design development process - V-model [Matd] . . . 3
1.2 Relationship between SysML and UML2 - from [Gro15a] . . . 4
1.3 SysML diagram taxonomy [Gro15a] . . . 4
1.4 Clean Sky 2 - ITD Systems Work Packages. . . 9
3.1 Environmental Control system schema. . . 18
3.2 Pratt & Whitney 4000 engine [EHHT95] . . . 19
3.3 Bleed system architecture [EHHT95] . . . 20
3.4 (3.4a) Honeywell GTCP36 APU [APU]. (3.4b) External duct supplying conditioned air [Adm12, pg. 36] . . . 21
3.5 4-wheel thermodynamic process 3.5a. 4-wheel ACM architecture 3.5b. [CRdA13] . . . 23
3.6 Common cabin air distribution system [oS02] . . . 25
3.7 Variation of atmospheric pressure 3.7a and oxygen pressure 3.7b due to altitude variations [Adm12]. . . 27
3.8 Relative cabin pressure maintained due to atmospheric pressure variation [EHHT95] . . . 28
3.9 Variation of atmospheric temperature according to the altitude. Data from [Adm12]. The different variation of temperature around the 18000 feet is due to the changes of Troposphere composition that influence the speed in the temperature decreasing [Shr]. . . 29
3.10 Cabin air supply with recirculation: total air flow going into the cabin is a combination of fresh-air and filtered recirculated air [oS02] . . . 30
3.11 Filter efficiency [EHHT95, pg. 6] . . . 31
4.1 ECS simulator and the optimization schema. . . 34
4.2 Dependencies in the components’ models definition. . . 37
List of figures ix
4.4 Papyrus SysML project structure. . . 41
4.5 Papyrus bounds stereotype. . . 43
4.6 Papyrus units stereotype. . . 44
4.7 Papyrus Fluid SysML block. . . 45
4.8 Papyrus Humid air SysML block. . . 46
4.9 Papyrus Ideal air SysML block. . . 46
4.10 Papyrus Water SysML block. . . 47
4.11 Papyrus BDD for the Rotation block. . . 47
4.12 Engine SysML block. . . 50
4.13 Heat-exchanger SysML block. . . 50
4.14 Pre-cooler SysML block. . . 51
4.15 Plate fin heat-exchanger SysML block. . . 52
4.16 Condensing heat-exchanger SysML block. . . 52
4.17 Mixer SysML block. . . 53
4.18 Splitter SysML block. . . 53
4.19 Duct SysML block. . . 54
4.20 Valve SysML block. . . 54
4.21 Filter SysML block. . . 55
4.22 Compressor SysML block. . . 55
4.23 CP_simple SysML block. . . 56
4.24 Example of a universal turbo-compressor map [MAP]. . . 57
4.25 Compressor SysML block. It is based on the universal compressor map. . . 57
4.26 Turbine SysML block. It is based on the Universal Turbine Map (UTM). . . 58
4.27 Shaft SysML block. . . 59
4.28 Fan-simple SysML block. . . 60
4.29 Fan-simple SysML block. . . 60
4.30 Fan SysML block. It is based on the Universal turbo-machines Map. . . 61
4.31 Cabin SysML block. . . 61
4.32 3-wheel Air-cycle machine architecture [CRdA13]. . . 62
4.33 Papyrus BDD implementing the 3-Wheel_ECS environmental control system based on 3-wheel air-cycle machine [CRdA13]. . . 64
4.34 Papyrus IBD implementing the 3-Wheel_ECS environmental control system based on 3-wheel air-cycle machine [CRdA13]. . . 65
5.1 Acceleo script overall working. . . 69
5.2 Mixer Subsystem block mask. . . 74
List of figures x
5.4 Subset of the Simulink library generated. . . 75
5.5 Possible implementation of the Cabin Air-conditioning system. . . 76
6.1 Piece of dynamic text inside the Gendoc template . . . 78
6.2 Dynamic text inside the Gendoc template filled with the model content (Mixer component). . . 79
7.1 Generic Mixer SysML block. . . 81
7.2 Improvement of the Mixer implementation by handling a variable number of air flows as input. . . 82
A.1 Papyrus BDD of the Fluid type. . . 89
B.1 Papyrus BDD of the Bleed-air system component. . . 91
B.2 Papyrus IBD of the Bleed-air system component. . . 92
B.3 Papyrus BDD of the Heat-Exchanger component. . . 96
B.4 Papyrus BDD of the Filter component. . . 98
B.5 Papyrus BDD of the Compressor component. . . 101
List of tables
3.1 Compressor-stage used during typical flight phases [oS02] . . . 20
A.1 Fluid’s SysML block features . . . 87
A.2 Humid-air’s SysML block features . . . 88
A.3 Water’s SysML block features . . . 88
B.1 Bleed-air system’s SysML block features . . . 90
B.2 Heat-Exchanger’s SysML block features . . . 93
B.3 Pre-cooler’s SysML block features . . . 93
B.4 Plate-fin heat-exchanger’s SysML block features . . . 94
B.5 Condensing heat-exchanger’s SysML block features . . . 95
B.6 Mixer’s SysML block features . . . 97
B.7 Splitter’s SysML block features . . . 97
B.8 Duct’s SysML block features . . . 99
B.9 CP-simple’s SysML block features . . . 100
B.10 CP-umap’s SysML block features . . . 100
B.11 Turbine’s SysML block features. It is based on the Universal Turbine Map (UTM) . . . 102
B.12 Shaft’s SysML block features . . . 103
B.13 Fan-simple’s SysML block features . . . 104
B.14 Fan-umap’s SysML block features . . . 106
Glossary
Acronyms / Abbreviations ACM Air-cycle machine
aECS simulator It refers to the environmental control system developed within the aECS project
aECS adaptive Environmental Control System ANSI American National Standards Institute APU Auxiliary Power Unit
ASHRAE American Society of Heating, Refrigerating and Air-Conditioning Engineers BDD Block Definition Diagram
CS2 Clean Sky 2
ECS Environmental Control System HEPA High Efficiency Particulate Air
IADP Innovative Aircraft Demonstrator Platform IBD Internal Block Diagram
ITD Integrated Technology Demonstrator JTI Joint Technology Initiatives
ppm Parts per million
Psi Pounds per Square Inch TE Technology Evaluator TRL Technology Readiness Level
UTRC-I United Technologies Research Centre-Ireland WP Work Package
Chapter 1
Introduction
This work describes part of the activities performed during my intern-ship of six months at the United Technologies Research Centre-Ireland (UTRC-I). Specifically, I joined a team involved in an European avionics project: the adaptive Environmental Control System (aECS) under the Clean Sky 2 (CS2) programme, which is addressing the next generation of environ-mental control system for future airlines.
My contribute focuses on the modelling of an aircraft Environmental Control System. The team has received from another research centre, the United Technologies Research Centre-East Hartford, a tool simulating the behaviour of an environmental control system. This tool was designed for other purposes and has different drawbacks with respect to the aECS project needs (section 4.1). For this reason my first activities aim to analyse this simulator tool and use the model based approach to define its system architecture and specify the components characteristics and their interconnections. In particular, the system and components models are defined by using the graphical modelling language SysML (section 1.2) and Papyrus (section 2.1) as developing tool. The methodology and the models are described in more detail in the chapter 4.
The SysML models are descriptive models used to define the system architecture and specify the component features. Unfortunately, they can not be executed to simulate the behaviour of the environmental control system. Within this work, the Simulink tool is used for simulating and investigating the system and components behaviour. Since our approach aims to exploit the usage of models to drive all the development phases, the maintenance of the consistency between the SysML models and their implementation is crucial. For this purpose, techniques for the code-generation are used: the information specified in the SysML models are leveraged for generating programming code. Specifically, Acceleo (section 2.2) is
1.1 Model Based design 2
used for parsing and extracting the contents from the SysML models and generating a Matlab script that if executed creates a Simulink library containing the modelled components. In the chapter 5 the Acceleo templates used to link the SysML models to their implementation are described.
Furthermore, this work aims to present a technique to enable the auto-generation of the documentation. By using Gendoc (section 2.3) it is possible to retrieve the information from the SysML models and mix them in a Microsoft Word template. This process is described in the chapter 6.
1.1
Model Based design
The Model-Based design has been introduced and evolved in many engineering disciplines since it is providing different strategies to effectively manage the development of increasingly complex systems. Model-based design is a model-centric approach relying on the use of explicit models to describe the development process.
A model, considered as a limited representation of a real system or process, is created to deal with complexity since it allows the abstraction from reality by eliminating unnecessary components or aspects. The model is seen in term of language specifying its semantics (meaning) and notation (representation of meaning). This is critical to successful system de-sign since the model becomes an its rigorous and unambiguous description. Furthermore, the expressiveness of the model allows to capture system behaviour and structure by describing its complex information in ways that are easily understood.
The models can drive the whole development process starting from the system design to its verification, validation and deployment. The image 1.1 highlights a typical design flow chart. It describes the main phases characterizing the development process. Based on its shape, it is called V-shaped lifecycle. The V-cycle splits the product development process into a design and an integration phase. Within this work, the System Design, the Component Design and the Implementation phases are taken into account. Specifically, at the development phase of the system design, the environmental control system architecture layer in terms of number and typologies of component is specified. It is the most general and high level model allowing to visualize the entire system. Increasing the level of detail and following decomposition of the system into components or sub-systems, the models can be mapped into physical architectures. This procedure corresponds to the components
1.2 System Modelling Language 3
Fig. 1.1 Model-Based design development process - V-model [Matd]
design or sub-system definition. Finally, using the techniques of code-generation the models implementation can be obtained.
1.2
System Modelling Language
SysML® or System modelling language is a general-purpose graphical modelling language for specifying, analysing, designing and verifying complex systems that may include hard-ware, softhard-ware, information, personnel, procedures and facilities [Gro15a]. It is developed by the Object Management Group (OMG®) [Gro]. The OMG proposes to use standard language such as UML® (Unified Modelling Language) and SysML® to represent the system concepts in a model based on these language and implement the system design approach. Actually, SysML is a specialized UML2 profile targeted to system engineering: SysML representing a sub-set of UML2 since it is reusing some of UML2 elements, but at the same time SysML is extending it specifying the elements needed to support the systems engineering process. To visualize the relationship between the UML2 and SysML languages, consider the image 1.2.
As highlighted in the figure above, the SysML extensions indicate the new modelling constructs defined in SysML that have no representation in UML2 or which replace an UML2
1.2 System Modelling Language 4
Fig. 1.2 Relationship between SysML and UML2 - from [Gro15a]
construct; while there are parts of UML2 that are not required by SysML. The basic unit of structures in SysML that can be used to respresent hardware, software and any other system elements are highlighted in figure 1.3.
Fig. 1.3 SysML diagram taxonomy [Gro15a]
The Behavioural diagrams describe or define the functionality of the system or sub-system, its interaction between users and external or internal systems (or sub-systems). The Requirementsdiagram is used to represent the stakeholder requirements expressed in natural language. For the purpose of this work, only the Structural diagrams are used. Specifically, the Structural diagrams are used for statically defining the system by using block for system, composed blockfor sub-systems, ports, etc. [Gro15b]. Within the Structural diagrams the ones on our interest are the Block Description and the Internal Block diagram. They are described in the next section 1.2.1.
1.2 System Modelling Language 5
However, before highlighting the principle features of the SysML language used, it is good to make some other observations. SysML is not a methodology or a tool since it is methodology and tool independent, SysML is providing a semantics that is a "meaning" and a notation that is a "representation of meaning". From a practical point of view, there are different tools which allow creating system models based on SysML, both commercial and open-source. Among the commercial tool, one of the more famous is Rhapsody [IBM]. It is developed by the IBM and allows to create real-time and embedded system model based on UML/SysML. It also helps to generate documentation automatically and code in C, C++, Java and Ada. Among the open-source tools Topcased and Papyrus must be mentioned. Topcased [Polb] by the PolarSys [Pola] is a toolkit dedicated for the realization of critical embedded systems by using editors for specification, design and implementation of systems. Within this work, the tool adopted is Papyrus, that is described in more detail in the next chapter 2.1.
1.2.1
SysML constructs
In this section some of the SysML constructs used within this work are described. For the complete list, we recommend to the reader of referring to the SysML specification document [Gro15b].
1.2.1.1 Stereotype
The stereotypes represent the way in which it is possible to extend SysML by defining new customized SysML constructs and features based on the modelling needs.
1.2.1.2 Block Description Diagram
The Block Definition Diagram or BDD defines features of blocks and relationships between blocks such as associations, generalizations and dependencies. It can be used at the specifica-tion level to capture the definispecifica-tion of blocks representing the system environment. It provides a black box of the system. It is the SysML construct to redefine the Class diagram of UML, replacing classes with blocks.
1.2.1.3 Internal Block Diagram
The Internal Block Diagram or IBD captures the internal structure of a block in terms of properties and connectors between properties. Ports are a special class of property used to specify allowable types of interactions between blocks. Such a diagram can be used at
1.2 System Modelling Language 6
specification level to capture precisely the interfaces between the system and its environment. It provides a white box of the system. Composite blocks from the BDD are instantiated on the IBD as parts. These parts are connected through connectors, linking them directly by using the ports.
1.2.1.4 Block and Value type
The Block is shown as a stereotyped UML class. It is used to redefine the UML class. Value typeis one of the new SysML extensions and can be used as reusable types for properties and attributes in the model, for instance as type of Block attributes.
1.2.1.5 Generalization
A generalization is a kind of relationship that can be typically used on BDDs. This rela-tionship conveys inheritance between two elements: a more generalized element, called the supertype, and a more specialized element, the subtype. A generalizations can be used to define abstraction within the system design: in this work, for example, it is used to define a generic component Filter that is an abstraction of a more specialized class of filter such as the HEPA filter 4.6.7.
1.2.1.6 Association
It defines another kind of relationship. It could be a reference, composite or shared association. A reference association two blocks means that a connection can exist between instances of those blocks in an operational system. And those instances can access each other for some purpose across the connection. In this work only composite or shared associations are used.
Composite A composite association between two blocks conveys structural decomposition. An instance of the block at the composite end is made up of some number of instances of the block at the part end. For instance, it is used to model the relationship between a heat-exchanger of type plate-fin and the block Side (for instance, 4.6.2.2).
Shared It is a relation displayed on a BDD and aims to represent a structure that is internal to a block: a structure that the block is composed of. It also aims to highlight that the internal block or sub-system can also exist or be used outside the relationship ( for instance, 4.6.1).
1.3 Avionics field 7
1.2.1.7 Port
Ports are points at which external entities can connect to and interact with a block in different or more limited ways than connecting directly to the block itself. They are properties with a type that specifies features available to the external entities via connectors to the ports. SysML identifies two usage patterns for ports, one where ports act as proxies for their owning blocks or its internal parts (proxy ports), and another where ports specify separate elements of the system (full ports). Both are ways of defining the boundary of the owning block as features available through external connectors to ports. Proxy ports define the boundary by specifying which features of the owning block or internal parts are visible through external connectors, while full ports define the boundary with their own features.
1.2.1.8 Operation
An operation defines a Block’s behaviour. It represents a behaviour that a block performs when a client calls it. Stated formally, an operation is invoked by a call event. With respect to this work, a block’s operation is linked to the components’ physic behaviour: it expresses the function containing the component’s equations.
1.3
Avionics field
Nowadays, one of the most recurring theme is the Ecologically sustainable development: it regards many aspects of our life such as transportation, building, food, economy. It is especially true for almost all engineering branches that are starting to deal with the daily challenges in a green way: they are approaching the products and processes design by applying technologically feasible solutions in order to decrease the amount of pollution, minimize the risks of potential hazards and protect human health.
At the end of the day, aviation is one of the key service that changed our life-style: it contributes to quality of life allowing fast and secure people transportation, carrying the 0.5% of world trade shipments, that represents about 35% of the value of all world trade (it was achieved by consuming just the 2.2% of world energy) [Adm15]. Aviation must challenge with the continue increase of demands and the growing of population: in 2015, over 3 billion people used the world’s airlines, and air traffic is expected to triple by 2050. Today, the air transportation contributes about 3% to global greenhouse gas emission. Based on the prediction for the future increase of air traffic, many improvements must be taken into account. The so called Greening of Aviation is thinking to improve sustainability starting from the production phase into the fabrics (reducing the greenhouse gases and the usage of
1.3 Avionics field 8
resources such as water) up to improve the aircraft performances (reducing the amount of fuel, the particulate emissions and decreasing the noise produced) [UTC]. Since 2015, the Federal Aviation Administration[Adm15] is working with other stakeholders to transform the aviation system in order to address the challenges and opportunities of the future. Among the stakeholders it is included the International Civil Aviation Organization (ICAO) [ICA]: this highlights that it is not only an American initiative, but a world effort. The goals are to establish standards and recommended practices concerning transport safety (such as flight inspection, technologies’ requirements and maintenance) and aviation emissions. The aircraft is responsible of producing many of the compounds that have impacts on the climate change and the decreasing of the air quality such as Carbon Dioxide (CO2), Nitrogen Oxides (NOx),
oxides of sulphur (SOx), partially combusted or unburned hydrocarbons (HC) and particulate
matter (PM) [Adm15].
Meeting the new climate and energy objectives will require a drastic reduction of the avionics environmental impact by reducing its emissions. Maximising fuel efficiency, reduc-tion of air and noise pollureduc-tion are only the starting point. But the innovareduc-tion process in this sector is risky, complex and expensive, and needs long-term commitment: research study, proof-of-concept demonstrators and at the end prototypes that must be tested for many hours. In this challenging research scenario, the European Union is entrusting these activities under the Clean Sky 2 programme.
1.3.1
Clean Sky 2
The Clean Sky 2 [CS2a] is a Joint Technology Initiative (JTI) [JTI] and represents the largest European aeronautics research programme. Founded in the 2014 by the European Union’s Horizon 2020 programme [HOR], the CS2 gathers 24 countries and more than 600 participating entities among public research organizations, universities and companies (Europe’s aeronautics industrial leaders and small and medium-sized enterprises). The main goal of the programme is to identify, develop and validate the key technologies in order to reduce environmental pollution, noise but, at the same time, improve the air transportation service. How presented in the CS2 bi-annual work plan [CS2b], the environmental goal for the 2020 (whit respect to the 2000 levels) is to reduce the CO2emissions by 50%, Nitrous
oxides emissions by 80% and perceived external noise of 50%. In order to develop the new generations of greener aircraft, the CS2 is dealing with several topics. It is composed of four different parts:
1.3 Avionics field 9
• Three Innovative Aircraft Demonstrator Platforms (IADPs): Large Passenger Aircraft, Regional Aircraft and Fast Rotorcraft. The demonstrator shall operate at the vehicle level;
• Three Integrated Technology Demonstrators (ITDs): they include Airframe, Engines and Systems. The demonstrator shall operate at the system level;
• Two Transverse Activities: Eco-Design and Small Air Transport. They have the responsibility of integrating the different ITDs and IADPs for specific applications; • The Technology Evaluator (TE). It has the aim to evaluate the environmental and
societal value of the technologies developed in the IADPs and ITDs stage.
Due to the huge number of activities, the CS2 programme is organized into different main packages, each one of them is split into other sub-packages. For our purpose the third main package, ITD Systems, is presented. It is organized into seven different Work Packages (WP). The image 1.4 highlights the the main class of projects inside each WP.
Fig. 1.4 Clean Sky 2 - ITD Systems Work Packages.
Furthermore, as result of the Research & Development process in the different areas, the CS2 aims to regulate aeronautical security, quality and comfort standards (for example about the standardization of the cabin air filtering and quality) and increase the dissemination of knowledge.
1.3 Avionics field 10
1.3.2
Adaptive Environmental Control System
With reference to the image 1.4, the adaptive Environmental Control System [CS215] is a project belonging to the ITD Systems (SYS) programme area inside the sixth work package (Major Loads), intended to work under the Aircraft Loads Architecture (WP6.0). It is a recent project, started approximately on April 2016, lasting 96 months and in strictly collaboration with Airbus.
The Environmental Control System [oS02] is an essential system inside an aircraft since it enables survival, safety and comfort: it is designed to pressurize the aircraft cabin and flight deck, maintain their temperature within tolerable bounds and provide a healthy and comfortable environment during all the flight phases. It manages the amount of air coming from the outside and supplied into the cabin in order to meet the requirements of cabin pressure, temperature and air quality. The outside air (at the cruise altitude it could reach the temperature of −20◦ Celsius) flows through the compressor stages of the engine (in literature it is known as bleed-air) that raise the air temperature and pressure. After this air is properly filtered, cooled, mixed from other components inside the ECS and finally it is supplied to the cabin. A more detailed description about the ECS features and functionalities will be provided in the paragraph 3.1. Inside the current aircraft, the ECS is the largest energy consumer, about the 75% of non-propulsive energy [Mar12]: it takes around 200 KW for the air-conditioning and other ten kW for instrumentation, lighting, entertainment, kitchen, etc.. These values are self-explanatory. In order to meet the environmental goal established from the CS2 programme for the 2020 year, it is not only understandable invest time and resources to innovate this system, it is mandatory. This research topic is under the responsibility of the adaptive Environmental Control System.
The aECS is thought as a new greener and more efficient version of the traditional ECS. The key concept is minimize the amount of compressed air (bleed air) coming from the compressor stages of the engines and increasing the re-circulated cabin air. This shall lead to a reduction of the annual aircraft fuel burn up to the 2%. This result is going to be obtained in two steps:
• 1% of fuel saving. It will be reach by leveraging the current technologies; the existing technologies for air quality sensing and air treatment must be integrated into the Adaptive ECS. The target of 1% fuel saving shall not violate the requirements of air quality: the cabin air quality shall be maintained or improved. Summarizing, the first step of the project will be focused in developing a completely new control logic that, adopting the current technologies, operates accordingly with the requirements for air
1.3 Avionics field 11
quality and energy efficiency. A technology demonstration (TRL6) must be achieved by year 2019.
• 2% of fuel saving. New air quality sensing and air treatment technologies have to be developed. These technologies are expected to achieve better performances: technolo-gies from other industries could be taken into account, but they must be adapted for aerospace applications. Old and new technologies must be performed and tested in order to reach a demonstrator (TRL6) by year 2023.
The new control strategies introduced by the aECS are entailing a series of new challenges. The reduction of the fresh air coming from the outside, with a resulting increasing of the cabin air recirculation, will produce some drawbacks such as an increasing of the air compounds and CO2levels. These and more other aspects must be properly managed. For these reasons,
the aECS is split into four work-areas. Each work-area is mapped into a sub-package within the WP6.0:
6.0.1 - Architecture definition and Development of Adaptive ECS system & control logic. It regulates air treatment and optimizes the mix of outside and treated recirculated air. This work area will address both existing and advanced technologies.
6.0.2 - Development of reliable and high performance air quality sensors. It in-cludes integration of existing CO2, hydrocarbons and particulate sensors. Furthermore,
due to the objective of improving air quality standards, it will deal with the development of new advance sensing technologies.
6.0.3 - Development of air treatment technologies. It includes the integration and adaptation of existing technologies and development of new advanced air treatment components for hydrocarbons and odour removal, CO2removal and O2generation.
6.0.4 - System and aircraft level demonstration. This work area will address both existing technologies and advanced ones. Firstly, system demonstrators will be done on existing applicant’s facilities; as second step aircraft demonstration will be done on integration test bench AVANT (ZAL facility [ZAL]).
1.4 Intellectual Property 12
1.3.3
Partners
Not all the aECS activities are under the responsibility of the UTRC-I. For this purpose, a Consortiumwas created. Accordingly with the work-areas, the activities and responsibilities are split among the different partners.
United Technologies Research Centre-Ireland (UTRC-I) is the coordinator of the project. It will be responsible for the new control & logic implementation and for a sub-part of the system simulation and demonstration.
Nord-Micro (NM) is responsible for the air ventilation system, for the Temperature Control and for the Cabin Pressure Control. It will provide the instrumentation and the facility in order to run the first control logic algorithm’s test and technologies’ integration [nor].
AirSense Analystics is responsible for the development, test and integration of the high-performance quality sensoring technologies [Air].
Pall Corporation is dealing with the development, test and integration of the filtration, separation and purification technologies [Pal].
1.4
Intellectual Property
This document describes part of my work at the United Technologies Research Centre-Ireland. Since many information are proprietary, they can not be disclosed. For this reason some data were dropped or partial information were discussed. In particular, the SysML models presented have undergone a review process and some of their features have been removed. The same proceedings has been applied to the Environmental Control System architecture described as a case study. It does not represent the real architecture developed within my work. Whenever the information reported are down-selected, it will be highlighted.
Chapter 2
Tools
This chapter aims to briefly describe the tools used within this work. In particular, their main characteristics and their usage inside this work will be highlighted.
2.1
Papyrus
Papyrus [Ecld] is a graphical modelling tool for UML2 applications as defined by the Object Management Group (OMG) [Gro]. It is an open-source project, designed as an Eclipse platform’s component and based on the existing Eclipse Modelling Framework (EMF). Papyrus provides a complete support for SysML (1.2) modelling. Within this work it is used for the SysML models implementation of the system architecture and components since it provides specific graphical editors such as the Block Definition and Internal Block Diagrams. Furthermore, Papyrus enables the Model-Based System Engineering promoting support for cohesive architecture, verification and validation. Papyrus can either be used as a standalone tool or as an Eclipse plug-in as well as it can import other Eclipse plug-ins. For the purpose of this work, the Acceleo and Gendoc Eclipse plug-in are added to Papyrus.
2.2
Acceleo
Acceleo [Ecla] is an open-source code-generator from the Eclipse framework. Before be included into Eclipse, it was firstly developed by the French company Obeo [Obe]. It is a pragmatic implementation of the Object Management Group (OMG) MOF Model to Text Language (MTL) standard [Gro08]. Model to Text transformation stands for the process or technique of extracting code from a model. Since the transformation process could be defined in several ways depending on the source model and destination language, Acceleo
2.3 Gendoc 14
needs a model from which extract the information and a templates to define the target code language. With this approach, a template is a text containing dedicated part and Acceleo commands [Eclb], where the text will be computed from elements provided by the inputs models. With relation to this work, the input model is the Papyrus SysML project containing the specification of the system and components’ models; while the target language is Matlab: the Matlab commands will be properly filled with the information contained in the models in order to generate the Simulink implementation of the ECS SysML models provided as input.
2.3
Gendoc
Gendoc [Eclc] is the Eclipse project for document generation. It is an open source solution that extract the models information using document templates based on OpenOffice Writer or Microsoft Word. The template contains static and dynamic text: static indicates that the text in the template is traditional Word (or OpenOffice) text, while dynamic indicates that portion of text is dynamically filled with the model content. For the work’s purpose, each dynamic section contains Acceleo commands that aim to parse the SysML models. The result is an official Word paper documenting the SysML models features.
2.4
Matlab and Simulink
2.4.1
Matlab
MATLAB® or MATRix LABoratory is a multi-paradigm numerical computing environment developed by MathWorks [Matb]. It is one of the most used tool in the field of engineering or science. The matrix-based MATLAB language is optimized for analysing larger data sets and performing matrix manipulations. It can boast a large number of libraries and toolbox that increase its usage fields. It is written in proprietary programming language but it can be integrated with programming languages such as C, C++, Python, etc.. Within this work, it is used to write the components’ equations and perform tests.
2.4.2
Simulink
Simulink® is a block diagram environment enabling multi-domain simulation and the Model-Based Design [Sim]. It means that Simulink is a suitable tool both for modelling and simulation. It provides a graphical editor for building models as block diagrams enabling the system and components design. Models are hierarchical and allows top-down or bottom-up
2.4 Matlab and Simulink 15
approach: starting from high level system models to drill down increasing the level of model detail. It also includes several libraries to facilitate and speed up the design, but new blocks or library can be easily created.
The simulation is enabled since the models are executable. Simulink uses block diagrams to model algorithms and physical systems that can be simulated in continuous or discrete time simulation. This is an interesting features also for the purpose of this work. Since the SysML models are descriptive, they can not be simulated: a tool enabling the simulation and the model-based approach like Simulink is a perfect candidate. Furthermore, as reported in 6, techniques of code-generation to maintain the coherency between the SysML models and their Simulink implementation can be adopted.
Simulink is incorporated in MATLAB: it is possible to integrate MATLAB algorithm into models. This a another key feature used for the purpose of this work. In fact, the MATLAB function regarding the system’s components equations written and tested in Matlab are embedded in MATLAB-Function block in order to continue the development process by using the Simulink as simulation tool.
Chapter 3
Aircraft Environmental Control System
As reported in 1.3.2, this work fits within the aECS project, which is addressing the next generation of environmental control systems for future aircraft. The key point enabling the development of the new technologies is a complete understanding of the working of the current ECS.
In this chapter, the main characteristics of an Environmental Control System will be highlighted. It will be discussed about its purposes and the different architectures taking into account the technologies adopted, their functionalities as well as the advantages and the drawbacks of the different solutions.
3.1
Environmental Control System
The Environmental Control System (ECS) refers to the set of technologies adopted for maintaining safe and comfortable a specific environment taking into consideration a given payload such as people, goods, living matter. Generally, it controls the environmental air temperature, pressure and composition within an acceptable limits.
The features provided and the measurements considered in the control operations are strongly dependent on the specific application. For example, the ECS for buildings (like offices, houses, data centres) is known as HVAC, that stands for Heating, Ventilation and Air Conditioning. It is mainly used for keeping the environment at a constant temperature, but in more complex and advanced system it can also integrate fire detection and suppression
3.1 Environmental Control System 17
(adopting particular strategies such as limiting the circulating oxygen in order to not feed the fire). Instead an industrial plant, like a chemical laboratory, requires a more sophisticated ECS able to acquire and manage very specific measurement for example about air compo-sition (carbon dioxide, oxygen, humidity or other chemical substances dangerous for the process taking into account).
The ECS inside an aircraft, with respect to other vehicles’ ECS (such as car, trains, etc.), has higher safety-critical requirements because it is dealing with hostile external environment for the human life: it is operating in extreme conditions of temperature, ambient air quality and air pressure. During a flight, the passengers are totally dependent on the air provided by the environmental control system. As reported in [Mar12], an aircraft ECS is responsible not only of the Cabin conditioning but also to manage the power and electricity supplied to other aircraft’s facilities. It includes:
• Cabin air conditioning managing the cabin pressure, temperature, air ventilation, humidity and fire protection;
• Water and Sanitation: part of the ECS is responsible of heating up the water temperature to be used into the aircraft lavatory but also to prevent the water freezing (especially the water present in part of the aircraft that are closer to the external surfaces);
• Food and Solid waste;
• Fuel tank inertization, lighting, noise and entertainment.
In many aircraft this responsibility to protect the occupants from the extreme external conditions is under Environmental Protection System (EPS) [Mar12]. For example, it in-cludes the protection against low and high external temperature, high winds or turbulences, water and ice, radiations and electrical shock as well as biological attack.
This work focused in the part of the ECS regarding the Cabin Air Conditioning system. Furthermore, considering that large commercial passenger aircraft are typically using ECSs based on engine bleed air, the architectures and technologies taking into account are adopting this type of air-source.
Taking into consideration the image 3.1, the Cabin air-conditioning system can be split in three main functional sub-systems: the Air-source system, the Air-conditioning Pack and the Air-ventilation and distribution system. The first is responsible for providing the fresh1and
1The term fresh air is not referred to the air temperature, but it refers to uncontaminated compressed
3.1 Environmental Control System 18
Fig. 3.1 Environmental Control system schema.
compressed air to be supplied into the other ECS sub-systems. The air-conditioning pack is responsible to pressurize, heat, cool the air that will be supplied into the cabin. Finally, the air-ventilation and distribution system is responsible of supplying, distributing and circulating the fresh air into the different section of the aircraft.
In more details, the working of the ECS sub-system can be summarized in this way. Compressed and hot air called bleed air is taken from the compressors stages of the en-gine and supplied to one or more Air-conditioning pack. Inside the pack the air is cooled, re-compressed, expanded by using air-cycle machine producing air at the correct value of tem-perature and pressure that can be supplied into the cabin. The air exiting the air-conditioning pack flows into a mix chamber in which it will be properly mix with the air coming from the recirculation cycle and the trim air. Recirculation fans extract air from the cabin, that is filtered and forced again into the mix chamber. Trim air is hot bleed air that bypasses the air-conditioning pack: small amounts of this air are mixed with the air exiting the mix chamber in order to properly control the temperature flowing into the cabin. Compressed bleed air is used to maintain the cabin pressurized: the pressurization is managed by the cabin pressure control and one or more outflow valve that automatically regulate the pressure in the cabin.
In the following part of the chapter the main features of each sub-system inside the ECS are discussed in more detail. Specifically, it will be analysed the air-source system; the air-conditioning pack architectures and components; and the air-ventilation and distribution system, also taking into consideration the pressurization and temperature control and the recirculation cycle.
3.2 Air-source System 19
3.2
Air-source System
It is responsible to provide the air that will be supplied to the air-conditioning pack and after to the cabin. For this reason it is the heart of the environmental control system. Most ECSs take compressed air from the engines (bleed-air source) instead of no-bleed systems having dedicated compressors for producing compressed air (such as the Boeing 787 [Sin]). All the considerations in this document are referred to this category of bleed-air driven environmental control systems).
Fig. 3.2 Pratt & Whitney 4000 engine [EHHT95]
The air entering the engine passes through a fan and is split into two flows: the external flow does not receive compression and it will be used for propulsion; while the inner one it is used from the turbine that powers the engine, it will pass to multiple-stages compressor that rises its pressure and forced to the combustion chamber. The combustion chamber does not increase the pressure, but it is responsible of the rise of temperature. At the end of the process, this high pressure-temperature air passes through a turbine that expands the air (the expansion is producing energy which will be used to move the compressor) with an increasing of its speed that provides additional thrust for propulsion [oS02].
As showed in the figure 3.2, the air to supply the air-conditioning system is taken from the bleed ports. Most bleed-air system have two extraction ports, that are corresponding at different compressor-stages: since each compressor-stage increases the air pressure, the air coming from the two bleed ports has different value of pressure (the high stage has an highest
3.2 Air-source System 20
air pressure). The bleed system has this design because in this way it will be possible to take air at two pressures depending on the operational condition: for example, at low engine power (like during the descent or the cruise phase) the high stage it is the only source of air at sufficient pressure in order to meet the needs of the bleed system and the other ECS systems. The following table shows some typical operational conditions and the related compressor stage.
Operation Mode Temperature◦Celsius Pressure KPa (psi) Extraction stage
Take-off 350 1170 (170) Low pressure
Top of climb 310 690 (100) Low pressure
Cruise 250 340 (50) Low pressure
Initial Descent 185 200 (29) High pressure Table 3.1 Compressor-stage used during typical flight phases [oS02]
The bleed system in addition to the air-source includes some other component: a pre-cooler, valves and optionally an ozone converter. The image below 3.3 shows a common bleed air system.
Fig. 3.3 Bleed system architecture [EHHT95]
The amount of the air taken from the Intermediate or the High stage can be varied by properly command the valve. The high temperature-pressure air passes through a Pre-cooler: it is an heat-exchanger that uses the cool air coming from the outside called ram air in order to cool down the bleed air. This phase is needed because the temperature and the pressure of the air going into the ducts can not exceed some value: bleed pressure shall not exceed
3.2 Air-source System 21
300 KPa (possible damage to the system ducts), and generally it is regulated around 250 KPa because lower value may not be able to force the air through into some component of the air-conditioning pack (such as the other heat-exchanger); bleed temperature shall not exceed 250◦C(damage to composite structures and some plastic components), and typically it is set to 180◦Cbecause lower value may be insufficient for the anti-icing system (hot-air flows into dedicated ducts near the surfaces most directly in contact with the outside in order to prevent the formation of ice on the aircraft external surfaces) [Mar12]. If it is present the air will pass through an ozone converter because at high altitude, the air can contain excessive O3.
This component has the scope of reducing the ozone percentage of the bleed air in order to keep it within acceptable concentrations. At 39000 feet the air ozone concentration is about 0.8 ppm (while at the sea level is closed to 0.6 ppm): with this concentration of ozone, the occupants of the aircraft could experience symptoms like coughing,headache or eye irritation [EHHT95]. Finally, the bleed-air, properly cooled (and filtered), is supplied to the other part of the environmental control system.
(a) (b)
Fig. 3.4 (3.4a) Honeywell GTCP36 APU [APU]. (3.4b) External duct supplying conditioned air [Adm12, pg. 36]
As additional air-sources many aircraft have an Auxiliary Power Unit (APU) placed in the aft part of the aircraft. The APU is a small gas turbine including an electric generator for providing electric power to the aircraft. Generally, when the aircraft’s engines are off (ground operations) or they are not providing sufficient bleed-air or electric power (taxi phase), compressed and unfiltered air from the APU is used to feed the air-conditioning pack. The image 3.4a shows the Honeywell GTCP36 APU. Nowadays, during ground operations most
3.3 Air-conditioning Pack 22
aircraft are using high-efficiency ground refrigeration systems providing proper conditioned air ready to be supplied into the aircraft cabin. The image 3.4b shows the external duct used to supplied air to the aircraft ECS during ground operations.
3.3
Air-conditioning Pack
Bleed air from the engines is used to pressurize, heat, cool and ventilate the aircraft cabin. This actions are under the responsibility of the Air-conditioning pack (also called Air cycle machine (ACM)). An ACM is an air-cycle refrigeration system that uses as refrigerant the outside air (ram air). Inside an aircraft, there could be more than one air-conditioning pack based on its configuration and the number of air-source systems. Typically, considering an aircraft with two different air-source systems, there are two air-conditioning packs, one for each air-source system. Anyway, there are configurations with up to four air-conditioning packs [oS02].
The number and the typologies of the components inside an ACM depend on the ap-plications. The simpler versions of an air-conditioning pack are composed of: one turbine and one fan (on the common shaft driven by the turbine) for forcing the ram air to pass through the only heat-exchanger (Simple cycle ACM); one fan (it is not on the same shaft moved by the turbine) to control the amount of ram air, two heat-exchangers plus one tur-bine and one compressor on a common shaft (2-wheel Bootstrap ACM). This version are commonly used for smaller ECS and are not so much interesting for avionics application. Commonly, inside an aircraft are used 3-wheel Bootstrap ACM and 4-wheel ACM [whe]. They are respectively composed of one turbine, one compressor and one fan on the same shaft (3-wheel ACM) and two turbines, one compressor and one fan for the 4-wheel ACM. The results presented in [CRdA13] about the performance comparison of 3 and 4-wheel cycle machine are highlighting that the 4-wheel cycle machine provides, under the same operational conditions, higher air-cycle Coefficient of Performance2. Moreover, the 4-wheel ACM architecture allows obtaining the same temperature drop of the 3-wheel architecture but with smaller heat-exchangers. For this reason, the rest of the document refers to a 4-wheel cycle machine. The 4-wheel cycle machine architecture is shown in the figure 3.5b, while the thermodynamic process about the variations of temperature and enthalpy due to the
2The coefficient of performance (COP) of a heat pump, refrigerator or air conditioning system is a ratio of
3.3 Air-conditioning Pack 23
contribution of each component is shown at the sub-figure 3.5a.
(a) (b)
Fig. 3.5 4-wheel thermodynamic process 3.5a. 4-wheel ACM architecture 3.5b. [CRdA13]
Based on this configuration 3.5b and the thermodynamic phases 3.5a, the bleed air enter-ing the air-conditionenter-ing pack firstly passes through the Primary Heat-exchanger (operational functionality 4a - 4b). It works similarly to the radiator in a car: the hot-bleed air is cooled down by using the outside ram air forced into the system by means of the fan. The amount of ram air depends on the speed of the fan, that is driven by the two turbines because it is linked to the same shaft. The air exiting the primary heat-exchanger is called trim air. This cooled air is split in three different flows by using two valves: one part is flowing into the compressor; one part commanded by first valve bypasses the compressor in order to be mixed to the output of the first turbine (the purpose is to avoid that the cold air coming out from the turbine is freezing before entering the water separator [Adm12, pg. 39]); the last part commanded by the other valve bypasses all the ACM in order to be mixed with the air coming out the second turbine. The last mechanism has two purposes. The first is about the temperature control: by using the valve it is possible to mix the cold air exiting the second turbine with the desired amount of hotter trim air supplying air to the mix manifold at the proper temperature. The second purpose is about operational conditions when cooling demands are low or when the aircraft is at the cruise altitude. In both cases the ACM could be bypasses. For example, at the cruise altitude due to the very low temperature of the ram air, the primary heat-exchanger is enough to cool down the bleed-air at the proper temperature and there will be no need to pass through the other components of the ACM. For this reason this valve is called bypass valve [Adm12, pg. 39].
3.3 Air-conditioning Pack 24
Returning to the case in which all the ACM components are active. The air from the heat-exchanger is entering the Compressor. Here, its value of pressure and consequently of temperature will be raised (operational behaviour 4b - 5). Now the air is passing on the Secondary heat-exchanger: it has a similar behaviour of the primary heat-exchanger, using the ram air for cooling down the air coming from the compressor, but works ad different value of temperature and enthalpy (operational functionality 5 - 6). The air is going to a Water separator. It is used to remove the water from the saturated air before entering into the mix chamber and after into the cabin. Actually, the water separator is working mostly on or in proximity of the ground because during other flight phases the air has no enough humidity for producing water. Furthermore, it has no contribution in the temperature-enthalpy variation. Finally, the air-cycle, the air is further cooled by the first Turbine (operational working 6 - 7) and the second one (operational working 7 - 8). So at the end of the process, the air-conditioning pack provides essentially dry, sterile and dust free conditioned air into the mixing manifold (see next section 3.4) at the proper temperature, flow rate and pressure in order to meet the cabin pressurization and temperature requirements .
3.4 Ventilation and Distribution System 25
3.4
Ventilation and Distribution System
The ventilation system governs air supply. As shown in fig.3.6, it has various purposes varying from supplying fresh air to distributing and circulating air into the different sections of the aircraft such as the cabin, the flight deck and many bays. This system includes cabin air recirculation, avionics cooling, lavatory/galley ventilation, vacuum waste blowing, galley chilling, cargo heating and air particle separation. In addition, it is also responsible to provide air to the electrical and electronic components in the avionics compartment. It is also called Air distribution and Circulationsystem.
Fig. 3.6 Common cabin air distribution system [oS02]
With reference to the image 3.6, in large aircraft, air-distribution system can supply air to different cabin zones. Each zone has its own temperature panel that is able to maintain the temperature independent from the others: the crew or the pilots can manually select the temperature for each zone or just using the automatic mode. Generally, the air distribution system is managing three different zones of the aircraft independently: the Flight Deck3, the cabin’s Forward and the Aft [A32]. The air-ventilation and distribution, fig. 3.6, is mainly composed of a mixing manifold and a series of ducts, fans and valves. The mixing manifold mixes the fresh air from the air-conditioning packs with the recirculated-air coming from the
3The Flight Deck (sometimes called Cockpit [A32]) in some configurations receive the fresh air directly
from the air-conditioning pack. That air has a better quality than the one coming from the mixing manifold in which the air from the ACM is mixed with the air coming from the cabin.
3.4 Ventilation and Distribution System 26
different cabin zones. The recirculated-air is extracted by the recirculation fans and forced to pass through a filter before entering the mix manifold. Before entering the cabin, the air coming out of the mixing manifold is combine with a portion of hot trim-air. This procedure is provided in order to supply the air to the cabin to the proper temperature: the amount of hot trim-air is commanded by the valves.
A good air circulation is a key feature in order to achieve uniform temperature conditions, while a good distribution system is important for supplying fresh-air in the different cabin zones in order to meet the air quality requirements. Moving outside air into the cabin at one or a few locations will not provide adequate contaminant removal and acceptable thermal conditions: parts of the cabin would get very cold and other hot, parts of the cabin would have clean air and other parts stagnant and unpleasant air. Air quality is primarily measured by CO2concentration [ANS89] that shall be less than 5000 ppm: this value is linked to the
human mental ability, that above this value is generally lost. For the pilots, value just below this threshold are not feasible and less concentrations of CO2 must be guaranteed.
Of course the CO2 concentration is related to the amount of fresh air supplied to the
cabin but also to cabin passenger density and occupants breathing frequency. Early jet liners (in the 1940s) pressurized the cabin with 10 L/s per passenger of fresh air, but modern jets (since 1970) only supply and renew 5 L/s per passenger using fresh-air and forcing another 5 L/s per passenger from cabin air recirculation. According to the European Aviation Safety Agency (EASA), each passenger and crew compartment must be ventilated with enough fresh air (not less than 0.25 kg/min [oFR02]), with CO less than 50 ppm, CO2around 0.17%
(it was 3% up to 1997)[Mar12] corresponding to 1700 ppm , O3 less than 0.25 ppm, and
filter particles grater than 10 nm (for virus and other particles like dust and tobacco). Cabin air should also be renewed every 2 or 3 per minutes. This is achieved by discarding some air out of the cabin through outflow valve. With reference to the fig.3.6, this air is called exhausted air. It is generally extracted from the cabin, by using proper fans, at the floor level and at the side walls.
Moreover, due to the huge amount of air entering the cabin4, the distribution system shall be properly designed in order to not violate the requirements about the speed of air coming into the cabin close to people: it must be in the range of [0.05, 0.2] m/s [ASH97].
4Considering a minimum flow rate of fresh air of 0.25 kg/min per passenger [oFR02], in an A320 with 200
3.5 Cabin Pressurization 27
3.5
Cabin Pressurization
It is responsible of monitoring and controlling the pressure inside the cabin in order to maintain it at pre-fixed values. It represents the most critical part inside the environmental control system guaranteeing safety and comfort of the occupants and aircraft structural protection. Indeed the pressurization of the cabin is crucial due to the low level of pressure reached at the cruise altitude. The pressurization of the cabin is needed since the outside air pressure is varying with respect to the cruise altitude. As highlighted in the figure below, the atmospheric pressure (3.7a) and the oxygen pressure (3.7b) are not constant, but they are changing accordingly with the variation of altitude. For example, at a typical cruise altitude of 36000 feet, the atmospheric pressure is only about one-fifth that at the sea level [oS02].
(a) (b)
Fig. 3.7 Variation of atmospheric pressure 3.7a and oxygen pressure 3.7b due to altitude variations [Adm12].
These variation of altitude pressure have an impact to passengers and aircraft structure. As shown in the figure 3.7b, the oxygen pressure variation affects the safety of the aircraft passengers since it is varying the amount of oxygen that can be absorbed into the blood: greater pressure pushes the oxygen from the lung alveoli into the bloodstream; while as the pressure is reduced, less oxygen is forced into and absorbed by the blood. Below the 7000 ft. above the sea level, the relative oxygen pressure is still suitable to guarantee the blood saturation; but it will not be true for higher altitudes. So, even if the concentration of oxygen at 36000 ft. altitude is closed to the one at the sea level, the partial pressure of the oxygen (PO2) is only about 0.76 psi compared with the 3.08 at the sea level. That value is far from
what is necessary to sustain human life: . A reduction in the normal oxygen supply alters the human condition changing the body functions starting from sleepiness, headache, increase of respiration, a lesser degree of consciousness until the hypoxia [Adm12]. These symptoms
3.5 Cabin Pressurization 28
must be avoid for all the people, but of course the health of the pilots shall be far away from that risk. The cabin pressure is commonly expressed as a "pressure altitude" equivalent. It can be represented as the distance above sea level at which the atmosphere exerts the same pressure as the actual pressure in the aircraft cabin. The cabin pressure control system continuously monitors the air-planes altitude and consequently adjusts the cabin pressure. The image 3.8 highlights how the two pressures are correlated.
Fig. 3.8 Relative cabin pressure maintained due to atmospheric pressure variation [EHHT95]
The minimal cabin pressure is set by the Federal Aviation Regulation (FAR): under normal operating conditions, it shall not exceed the pressure at 8000 feet. At that altitude (circa 2.400 meters) the PO2is 2.32 psi, corresponding at the 74% of the oxygen pressure at
the sea level: this value represents the requirement set by FAR as the minimal PO2allowed
in the aircraft cabin.
This modality of varying the aircraft cabin pressurization has another critical purpose about the aircraft protection: the difference between internal and external pressure is not allowed to exceed about 55-62 KPa [oS02]; otherwise the cabin structural characteristic will be compromised. Moreover, the Cabin Pressure Control handles the cabin pressurization in order to prevent rapid pressure changes: they can influence both passengers’ comfort (producing changes in the volume occupied by gases in the body cavities with the result of passengers’ discomfort) and aircraft safety. Conditions of under-pressure and overpressure are controlled by valves: the outflow valve is constantly being positioned (by redundant motors) to maintain cabin pressure. Generally, pressure variations occurs during the ascent and descent phase. The [ASH99] limits the rate of change in cabin pressure to not more than 0.26 psi per minute during climb and 0.16 psi per minute descending.
3.6 Temperature Control 29
3.6
Temperature Control
The temperature control in the aircraft cabin is important both for safety and comfort. As highlighted in the figure 3.9, the outside air temperature has a huge changing related to the altitude. This makes the cabin temperature control a critical features due to the extreme conditions at typically cruise altitude: the outside temperature can easily reach −50◦C (fig.3.9).
Fig. 3.9 Variation of atmospheric temperature according to the altitude. Data from [Adm12]. The different variation of temperature around the 18000 feet is due to the changes of Troposphere composition that influence the speed in the temperature decreasing [Shr].
The cabin temperature, as well as comfortable condition, must be managed at all altitudes: the system shall be able to supply cool or warm air to the cabin as needed. Because of the high occupant density, the cooling of the cabin is required in most circumstances, particularly during the ground operations. However, it shall be designed in order to properly work in all the worst case condition such as cooling on ground at a hot and humid place, aircraft full and doors closes (worst cooling case) or heating on ground at a cold and humid place, aircraft empty and door closed [Mar12]. The reason way the ground is the worst scenario for both the control modality is because generally the air supplied into the cabin is taken from the engine, after being properly filtered, cooled and mixed: to heat up the cabin temperature the system takes air from the engine at higher temperature, but at ground the engines are off or in idle mode so they are not able to produce that compressed-hot air; instead for decreasing the cabin pressure the system increase the cooling factor of specific components (heat-exchangers and turbines) inside the air-pack, but for cooling down the air from the engines, as reported in 3.3, they are using the outside air that at ground is not enough.
Typically, the cabin temperature is maintained around 23◦C. Moreover, the [ASH97] establish as requirement a lower limit to the air temperature of the air supplied to the cabin:
3.7 Recirculation System 30
the air-conditioning system shall supply air without creating uncomfortable drafts (air speed less than 0.2 m/s close to people and less than 2 m/s anywhere) and at a minimum temperature of 10◦C[Mar12]. Consequently, the system must be designed with an air flow rate that is adequate to meet the largest heat load with this temperature of supply air constraint.
3.7
Recirculation System
It is responsible to manage the amount of air taken and supplied to the cabin. As shown in the figure 3.10 recirculation is achieved by extracting air form the cabin, mixing it with the conditioned outside flow and supplying it again into the cabin. The use of recirculation is a key feature in the design of building environmental control systems. In the building field the recirculation is designed in order to operate with up to the 90% of recirculated air. This kind of environmental control maintains flow rates of outside air about 0.5 kg/min per person, because the air quality requirements are more relaxed. Since in avionic application, there are more stringent requirements, the amount of fresh and recirculated air must be managed. Typically, the percentage of recirculated air is within the 30-55% of the total air supply [oS02, Table 2-1, pg.48]. Some models of aircraft can use different strategies to control the air recirculation: in some case the recirculated air is turn off, while in other case the amount of the recirculated air is programmed to change accordingly to the phase of the flight (climb, cruise or descent) [oS02, Table 2-1].
Fig. 3.10 Cabin air supply with recirculation: total air flow going into the cabin is a combina-tion of fresh-air and filtered recirculated air [oS02]
The air-recirculation provides two main benefits. Firstly, since the air flow rate entering the cabin must be fixed, an increasing of the recirculated air enables a reduction in the
