Master of Science in Space Engineering
Thermal Vacuum Test of a Pulsating Heat Pipe
Radiator for Space Application
Thesis Advisor:
Candidate:
Prof. S. Filippeschi
Giovanni Postorino
Research Supervisors:
Ing. M. Mameli
Ing. E. Mancini
Ing. L. Bianchi
1 Introduction 1
1.1 Spacecraft thermal control generalities . . . 1
1.2 Active and passive thermal control devices . . . 3
1.3 Pulsating Heat Pipe . . . 7
1.3.1 Characteristics and configurations . . . 7
1.3.2 Working principle . . . 9
1.3.3 Defining parameters . . . 14
2 PHPs for space application 22 2.1 State of development on PHP application for space environment 22 2.2 Radiators for PHP application . . . 28
2.3 Thermal vacuum test technology for space environment sim-ulation . . . 34
2.3.1 Thermal vacuum chamber . . . 37
2.3.2 Outgassing . . . 40
2.4 Open issues and aim of the thesis . . . 43
3 Test cell design and modifications 45 3.1 Space PHP test cell for 58° Parabolic Flight Campaign . . . . 45
3.3 Design of the radiator . . . 59
3.3.1 State of the art analysis . . . 60
3.3.2 Thermal FEM analysis . . . 73
3.4 Thermal Vacuum test facility setup . . . 83
3.4.1 Setup of vacuum plant and thermal supply . . . 84
3.4.2 Structural supports . . . 89
3.4.3 Sensors setup and wiring. . . 91
4 Experimental campaign and results 97 4.1 Test procedure . . . 97
4.2 Test campaign and experimental results . . . 107
4.3 Results analysis . . . 121
4.3.1 Comparison between FEM simulation and test results 121 4.3.2 Start-up and blockage analysis . . . 124
4.3.3 Performance analysis . . . 129
5 Conclusions and future developements 134
Appendices 138
A Datasheets 139
1.1 Generic scheme of PHP . . . 8
1.2 CEPHP (i) CLPHP (ii) . . . 9
1.3 Pressure-entalphy diagram . . . 10
1.4 Detailed zoom on internal forces acting on the working fluid . 11 1.5 Experimental Eö -√F rdiagram [White and Breadmore, 1962] [4]. . . 16
1.6 Example of hysteresis cycle of a PHP. . . 18
2.1 Blackbody radiator heat rejection [Gilmore] . . . 30
2.2 Example of body-mounted radiator (a) and deployable radia-tor on ISS (b) [Gilmore] . . . 31
2.3 Example of a Thermal Vacuum Chamber . . . 34
2.4 Generic trend for temperature and pressure during a thermal cycle [ACS Angelantoni website][16] . . . 36
2.5 Typical configuration of a vacuum plant [Paganucci][17] . . . . 39
2.6 Classification of vacuum pumps [Paganucci] . . . 40
2.7 Scheme of outgassing particles through diffusion. . . 41
3.1 Flux diagram of experiment concept . . . 46
3.2 Test-cell geometry and thermocouples location. . . 48
3.4 Air fan system used during Parabolic Flight Campaign. . . 50
3.5 Application of kapton tape along the milled housing of PHP for thermal isolation. Extended view (a), zoomed view (b). . . 53
3.6 Upper plate removed during disassembly. Thermal paste in white. . . 54
3.7 Upper plate with thermal paste on it (a) and after cleaning (b). 55 3.8 Cleaned PHP. Zoom on heaters. . . 56
3.9 Lateral zoomed view on TIM. . . 58
3.10 Application of the TIM upon the PHP envelope. . . 58
3.11 Phase-change variable surface area radiator concepts . . . 62
3.12 Concept of morfing radiator [Bertagne, 2017][20] . . . 63
3.13 Morphing radiator in series. . . 64
3.14 Conceptual scheme of a Liquid Droplet Radiator. . . 66
3.15 Pipeline integrated in Carbon-Carbon sheet. . . 67
3.16 Pipeline integrated in honeycomb panel. . . 68
3.17 Titanium water Heat Pipes with S-bonded radiators. [Ad-vanced Cooling Technlogies][21]. . . 69
3.18 Drawing of the radiator panel (dimensions in mm). . . 72
3.19 Radiator after painting, fixed to the test cell support. . . 73
3.20 Views of the assembly CAD model. . . 73
3.21 Example of a meshed part, a section of the PHP (a) and the whole assembly (b). . . 74
3.22 Simulation first iteration. . . 76
3.23 Simulation second iteration (a), Vadiraj experimental result (b). 77 3.24 20W radiator (a), and PHP (b). . . 79
3.25 30W radiator (a), and PHP (b). . . 79
3.27 50W radiator (a), and PHP (b). . . 80
3.28 60W radiator (a), and PHP (b). . . 81
3.29 70W radiator (a), and PHP (b). . . 81
3.30 80W radiator (a), and PHP (b). . . 82
3.31 Result of transient analysis, time to reach equilibrium tem-perature. . . 83
3.32 Thermal vessel, closed (a), and open (b). . . 86
3.33 Thermal cycle of the vessel. . . 86
3.34 Thermal vessel inside the chamber. . . 87
3.35 Scheme of the thermal-vacuum system. . . 89
3.36 Drawing of the structural support for the thermal vessel. . . 90
3.37 Drawing of the structural support for the test cell. . . 91
3.38 Wiring scheme of the system. . . 93
3.39 Thermocouples passing-through element. . . 94
3.40 Power inputs and DAS. . . 94
3.41 Pressure sensor passing-through element connection. . . 95
3.42 Hardware inside the thermal vessel after completed setup. . . 96
4.1 Results for ground tests of the 58PFC for horizontal and ver-tical configuration. . . 99
4.2 Temperature (a) and pressure (b) over time during outgassing 106 4.3 Evaporator, condenser and radiator Temperature vs Input Power vs Time (60UD). . . 109
4.4 Condenser Pressure vs Input Power vs Time (60UD). . . 110
4.5 Thermal resistance vs Input Power vs Time (60UD). . . 110
4.6 Evaporator, condenser and radiator Temperature vs Input Power vs Time (60D). . . 111
4.8 Thermal resistance vs Input Power vs Time (60D). . . 112
4.9 Evaporator, condenser and radiator Temperature vs Input Power vs Time (80UD). . . 113
4.10 Condenser Pressure vs Input Power vs Time (80UD). . . 114
4.11 Thermal resistance vs Input Power vs Time (80UD). . . 114
4.12 Evaporator, condenser and radiator Temperature vs Input Power vs Time (80D). . . 115
4.13 Condenser Pressure vs Input Power vs Time (80D). . . 116
4.14 Thermal resistance vs Input Power vs Time (80D). . . 116
4.15 Evaporator, condenser and radiator Temperature vs Input Power vs Time (100UD). . . 117
4.16 Condenser Pressure vs Input Power vs Time (100UD). . . 118
4.17 Thermal resistance vs Input Power vs Time (100UD). . . 118
4.18 Evaporator, condenser and radiator Temperature vs Input Power vs Time (100D). . . 119
4.19 Condenser Pressure vs Input Power vs Time (100D). . . 120
4.20 Thermal resistance vs Input Power vs Time (100D). . . 120
4.21 FC-72 Viscosity vs Temperature. . . 126
4.22 Zoom on the blockage of the test 60UD. . . 127
5.1 Scheme of the modified test cell. . . 137
A.1 Electrical Wire Heater material Datasheet. . . 139
A.2 FC-72 Datasheet. . . 140
1.1 Temperature limits for crucial spacecraft components [SMAD]
[1] . . . 2
1.2 Thermal Control Devices and Techniques . . . 3
2.1 Literature review from S.S. Ferreira (2017) on experimental works that analyzed the effect of gravity on the performance of PHPs. . . 25
2.2 Literature review for space environment related tests . . . 26
2.3 Goals of the experimental campaign. . . 44
3.1 TIM technical comparison. . . 57
3.2 Material comparison for flat plate radiator. . . 66
3.3 Comparison between radiator principal solutions. . . 70
3.4 Radiator specifics. . . 72
4.1 Flight test procedure for 58PFC. . . 98
4.2 Experimental procedure for the thermal vacuum test. Oper-ating points matrix. . . 102
4.3 Test starting procedure checklist. . . 103
4.4 Parameters control table. . . 104
4.6 Comparison matrix for 60D test. . . 122
4.7 Tests summary matrix. . . 124
4.8 Start-Up and Blockage condition summary. . . 128
Heat transfer requirements in space are constantly growing as the elec-tronics power consumption increases. Pulsating Heat Pipes (PHPs) are an efficient and totally passive way to transfer thermal power from the hottest components to the external radiator. Although PHPs have been extensively studied in the literature, the relative TRL is low for space applications. The low TRL is mainly due to a gap in the knowledge of standard testing proce-dures of radiative PHP in space environment. This thesis aims to testing a PHP radiator at boundary conditions typical of space thermal environment (low temperature and high vacuum). This work is the result of a collabo-ration between the Heat Transfer Laboratory at the University of Pisa and the Aerospazio Tecnologie srl company, whose core business is in the space environment simulation testing.
An already existing device that was tested under forced convection during parabolic flights, has been modified to adapt it to the boundary conditions typical of space thermal environment. For this reason, the heat sink of the test cell has been modified in a radiator spreader, previously designed and optimized with a commercial Finite Element Code. A vacuum test facility was made available by Aerospazio Tecnologie srl. The modified test cell has been adapted to the thermal vacuum facility and an experimental campaign was designed and set up. Measurements of temperature and pressure have been acquired at different input powers (from 40 W up to 80 W) and at different sky temperatures (-60°C/-80°C/-100°C).
agreement. From the experimental campaign it results that a minimum power must be supplied to activate the efficient thermal PHP performance and the lower is the sky temperature the higher is the minimum activation power (80W for -100°C). The PHP can be deactivated by decreasing the power supplied and a really viscous blockage of the oscillating fluid has been observed.
In conclusion, this work has been useful to define a standard experimen-tal procedure to test the operative performance of a radiative PHP for space application. An unknown viscous blockage of the fluid oscillation has been observed at low temperature. It could be crucial for space missions because the PHP can’t operate at low input powers. A theoretical hypothesis to explain it has been proposed. A larger experimental and theoretical investi-gation of this phenomenon must be done in future in order to setup predictive model tools.
Introduction
The aim of this chapter is to give the reader a first hint on the extensive topic that is the thermal control of spacecrafts, with particular focus on the physics behind the innovative technology of Pulsating Heat Pipe.
1.1 Spacecraft thermal control generalities
Spacecraft thermal control has always been one of the key factors in the concept and design of a space mission. Overheating or freezing of crucial components can be the cause of major mission failures and need to be avoided. This task is not trivial due to the extreme and counterintuitive environments that are both planetary and deep space. An accurate thermal design is mandatory ithough the design of the whole spacecraft and of each of its subsystems. Indeed, every spacecraft has an entirely dedicated subsystem aimed at Thermal Control. The task of TC is to maintain the spacecraft and the payload components within their required temperature limits during each phase of the mission.Two limits are generally defined:
• Operational limit: the component must remain within while operating • Survival limit: the component must always remain within, even when
not powered.
In the following table these limits are listed for some crucial components to give the idea of the width and magnitude of the temperature ranges in-volved.
Component/
Operating
Survival
System
Temperature [°C] Temperature [°C]
Digital elctronics 0 to 50 -20 to 70 Analog electronics 0 to 40 -20 to 70 Batteries 10 to 20 0 to 35 IR detectors -269 to -173 -269 to 35 Solid-state -35 to 0 -35 to 35 particle detector Momentum wheels 0 to 50 -20 ti 70 Solar panels -100 to 125 -100 to 125
1.2 Active and passive thermal control
de-vices
Various techniques can be adopted to maintain the temperature within the required limits, depending on the specific of the mission or the system. From a macroscopic point of view the devices utilized for thermal control are gen-erally distinguished in two main categories: active and passive. This charac-terization is made on whether the device requires some kind of power (usually electrical) to operate or not. Obviously, active thermal control devices re-quire a power input while passive does not. In the following some examples of both categories:
Active Passive
Heaters Shape and Orientation Thermo-electric coolers Optical properties, coatings
Shutters Insulation blankets Phase-change materials
Radiators Heat pipes
Table 1.2: Thermal Control Devices and Techniques • Active TC devices:
– Heaters: resistors that are used to heat up a component or a
specific spot due to Joule heating.
– Thermo-electric coolers: use current to cool down a component
– Shutters: surfaces controlled by an electrical motor that can cover
or expose a component or a certain area of the spacecraft to ex-ternal radiation.
• Passive TC devices and techniques:
– Shape and Orientation: the shape of the spacecraft is designed to
expose or not some critical areas to a certain direction (the Sun for example).
– Optical properties, coatings: exploiting absorptivity and
emissiv-ity of materials (such as paintings) to radiate or shield a deter-mined wavelength.
– Insulation blankets: covering of surfaces with sheets to screen
specific wavelengths (example: Multi-Layer Insulation MLI).
– Phase-change materials: hinges made of a material able to change
shape as consequence of a temperature change.
– Radiators: high emitting surfaces used to radiate to deep space
and dispose of excess heat.
– Heat pipes: tubular ducts filled with a working fluid (one or two
phases) that transport heat from a hot point to a cold area (usu-ally a radiator) due to pressure and temperature gradients. When compared to motors, passive thermal control devices (such as ra-diators or heat pipes) are usually more reliable, are less heavy and simpler. As a general rule, active thermal control is only used if a passive solution is not possible or not sufficient.
One of the limits of current thermal dissipation techniques is the heat dis-sipation capability of the system. Miniaturization of electronic components and increases of their power consumptions make high heat flux capability and low thermal resistance the driving factor of thermal design of both terrestrial and space technologies. To adapt to the continuously increasing trend, mod-ern heat transfer devices are designed to exploit the favorable properties of three fundamental physics phenomena: capillarity, gravity and phase change. • Capillarity and gravity: these forces substitute mechanical and/or ac-tion for the sustainment of the fluid behavior, making the device actu-ally passive. These phenomena can be simultaneously relevant in the dynamic of the device (as in thermosyphons) or act individually (as in heat pipes in space)
• Phase change: the increase in thermal capacity associated with the transition from liquid to vapor (and vice versa) make it possible to use considerably lower mass flow rates with respect to single phase devices A passive device is so completely thermally driven: the heating power activates the evaporation/condensation process and the consequent vapor expansion while capillarity and/or gravity forces provide the action needed to maintain the fluid behavior (circulation, oscillation or both).
The main benefits of a general two-phase heat transfer device with respect to single phases are:
• Greater heat transfer capability. Increase in heat loads (up to kilo-watts) and heat fluxes (up to 200W/cm2)
• Lower thermal resistance (down to hundreds of K/W )
• Lower mass flow rate (implying smaller size and weight, crucial for space application)
• No external work required • Smaller temperature gradients
• Wider range of acceleration fields compatibility
The main two-phase passive heat transfer devices currently available are: • Heat Pipes (HP)
• Capillary Pumped Loops (CPL) and Loop Heat Pipes (LHP) • Thermosyphons (TS)
• Pulsating Heat Pipes (PHP)
These are divided based on geometry and working principles. Heat pipes are surely the most used in space application thanks to its capability of car-rying high heat loads for long distances with negligible losses.
For the purpose of this thesis work, the following sections will focus on two of the technologies listed above: PHPs and radiators. Indeed, those are the principal components of the test cell used for the experiment associated
with this work and that will described in Chapter 3.
1.3 Pulsating Heat Pipe
1.3.1 Characteristics and configurations
Pulsating Heat Pipes, or shortly PHPs, have been firstly proposed by Smyrnov and Savchenkov in 1971 and then simplified and made versatile by a patent from Akachi [2] in 1990. This technology is often grouped as a subclass of the heat pipes family, but it is instead very different from a classical heat pipe for two main reasons: the absence of a capillarity wick and different dynamic behavior of the working fluid . Whereas a heat pipe requires a wick struc-ture to induce capillary action to sustain the stationary flux, in a PHP this phenomenon is given by the capillary diameter of the tube itself. Regarding the dynamic behavior of the working fluid, it is important to explain that no strict steady state is established, contrary to HPs functioning. Indeed, the process of evaporation and condensation inside the channel is alternating (the so-called “slug-plug flow”) and from here the denomination of pulsating. This concept is going to be explained more in detail in the following, but first a clarification of what a PHP is from a structural point of view is given.
Citing Khandekar [3] “a PHP consists of a plain meandering tube of
capillary dimensions with many U-turns”. The tube is firstly evacuated,
then filled by a determined volumetric fraction with the working fluid (in liquid phase) and then sealed. After sealing, the working fluid in the inside will fill the internal volume of the tube as a mixture of liquid and vapor (slugs and plugs) according to the external equilibrium temperature. The
Figure 1.1: Generic scheme of PHP main features of a generic PHP (Figure 1.1) are here listed:
• Absence of wick
• Absence of storage volumes
• A heat receiving section (evaporator) and a heat dissipating section (condenser)
• An “adiabatic” section between evaporator and condenser (optional) • Capillary slug-plug self-sustained and thermally driven oscillating flow
(no external mechanical power)
• Surface tension as predominant physical phenomenon (gravity may be relevant)
• Latent and sensible heat transport possible by oscillating working fluid.
Figure 1.2: CEPHP (i) CLPHP (ii)
Figure 1.2 shows the two most common configurations for a PHP: i Closed Ends PHP (CEPHP): the ends of the tube are not connected each
other but sealed on their own. This configuration allows flow oscillation but not circulation
ii Closed Loop PH (CLPHP): the tube is connected in a closed loop; no end is recognizable. This configuration allows both flow oscillation and circulation.
Beside the circulation of the fluid, both configurations work conceptually in the same way.
1.3.2 Working principle
Since the fluid is in saturated conditions, the heating source at the evapora-tor cause evaporation of the thin liquid film which surround each vapor plug.
The vapor expansion pushes the adjacent liquid toward the condenser where the adsorbed heat can be released to a cold sink.
Figure 1.3: Pressure-entalphy diagram
From a thermodynamic standpoint, the PHP is characterized by strong non-equilibrium phenomena so multiple local thermodynamic states are es-tablished during operation in different zones. A temperature gradient be-tween evaporator and condenser cause non-equilibrium pressure conditions. Referring to a generic p-h diagram of a working fluid control volume in non-equilibrium conditions (Figure 1.3 ), the heat transfer from the hot source to the evaporator causes the bubbles in that section to grow. The thermo-dynamic state tends to move from point A to point B, at higher pressure and temperature. This pushes the liquid column toward the condenser, at
lower temperature. Simultaneously, the vapor condensation at the cold end increase the pressure difference between the two ends and point A is forced to move towards point C, at lower pressure and temperature. A non-equilibrium is established throughout the thermal gradient that forces the whole system to equalize the internal pressure. The flow in a channel cause slugs and plugs in adjacent channels to move accordingly to the described motion. A self-sustained thermally driven oscillating flow is so obtained in the device. The inter-play between driving force (thermal gradient) and restoring force (pressure difference) makes the system oscillate in the axial direction (being the longitudinal direction of the tube the relevant axis). By means of this oscillation regime it is possible to transfer high heat fluxes (with respect to a comparable single-phase device) from a heat source to a heat sink.
Figure 1.4: Detailed zoom on internal forces acting on the working fluid Looking the system from a local perspective, as shown in Figure 1.4, it is
possible to analyze the mass transfer process and the force and heat balance between a adjacent liquid plug (in blue) and a vapor slug (in white) and with the outside. The major processes involved in the oscillating behavior of the working fluid can be recognized as exchange of forces (related to pressure difference) and exchange of heat (related to temperature gradient). These processes are listed in the following.
For what concerns force balance:
• Liquid and vapor plugs are subjected to pressure forces from the ad-joining plugs.
• Liquid slugs have menisci on their edges and are formed due to surface tension forces related to the capillary dimension of the PHP tube. The liquid film stability and thickness depends on the specific fluid-solid combination. If a liquid plug is moving or tends to move in a certain direction, then the leading contact angle (advancing) and the lagging contact angle (receding) may be different. This happens because the leading edge moves on a dry surface while the lagging edge moves on the surface that has just been wetted by the passage of the plug. The major contribution to the pressure difference in the flow comes from these two faces of the bubble, being the other part generally irrelevant. What might be relevant is the overall number of bubbles formed in the duct.
• Gravity force may have a significant role in the stabilization of the flow regime and so the performance of the device (thermal resistance) but does not modifies the slug flow regime behavior. The PHP can operate with or without gravity assistance. Up-header vertical configuration
(condenser at higher altitude than the evaporator) shows better overall performance with respect to horizontal configuration (condenser and evaporator at same altitude).
• Liquid and vapor plugs are subjected to internal viscous dissipation and wall shear stress as they move in the PHP tube.
For what concerns heat balance:
• Heat is adsorbed or released mostly radially (towards the tube wall) but also axially (towards adjacent plugs).
• At the evaporator:
– If the liquid slug enters in sub-cooled condition, sensible heating
plays the main role
– If the liquid slug is already saturated, heating is followed by either
evaporation mass transfer to the adjacent vapor plug or breaking up of the liquid slug itself with generation of new bubbles. In-creasing in saturation pressure and temperature.
• When a vapor bubble enters the evaporator zone evaporation mass transfer occurs. The mass trasfert increases the local saturation pres-sure and temperature, providing pump work to the system.
• Conditions described in the two previous points are valid at the con-denser but in opposite direction.
• In the adiabatic section strongly non-equilibrium phenomena occurs so no classical steady state can be identified in an operating PHP.
1.3.3 Defining parameters
1. Channel diameter
As already stated, the diametral dimension of the duct is fundamental to achieve the slug flow pattern. This pattern is the reason behind the self-sustained oscillatory behavior during operation. In hydrodynam-ics, the problem of the motion of a bubble inside a tube is influenced by three fundamentals physical properties: the liquid inertia, the liq-uid viscosity and the surface tension. These three parameters can be arranged in three non-dimensional parameters that fully describe the problem: F r = ρl· u 2 ∞ d · g · (ρl− ρv) = inertia buoyancy (1.1) P o = µ · u∞ d2· g · (ρ l− ρv) = viscous buoyancy (1.2) E ¨o = d 2 · g · (ρ l− ρv) σ = buoyancy surf ace (1.3)
being respectively the Froude number, the Poiseuille number and the Eötvos number. The terms present in the equations are: d for the typical dimension of the duct (diameter in case of circular duct), ρ for the density (with obvious notation on liquid and vapor phase), u for the velocity of the bubble, µ for the viscosity and σ for the surface tension.
The Bond number (Bo =√E ¨o) can be found in literature instead of Eö. This non-dimensional group explains how each of the aforementioned
physical properties affects the dynamic of a bubble in a duct. For instance, if surface tension and viscosity can be neglected, the system is dominated by the Froude number equation. As clarified before, the case of interest for the study of PHPs is when both viscous and inertia forces are negligible. The system is dominated by surface tension force, so by the Eötvos number equation.
Evidences from experimental data (Figure 1.5) show three regions of interest for the Eö number in relation with increasing bubble velocity (√F r):
• Eö>70: bubble velocity approaches a constant value. Fr≈0.345 dominates the behavior.
• 4<Eö<70: bubble velocity decreases for decreasing Eö.
• Eö≈4: bubble velocity becomes zero. Eötvos number (surface tension) is predominant over Fr and Po.
From this last condition (Eö≈4) it is possible to evaluate the maxim value of the diameter (dcr) needed to obtain a stable slug flow regime
(surface tension dominated). E ¨o = d 2 cr · g · (ρl− ρv) σ ≈ 4 → dcr = s σ g · (ρl− ρv) (1.4) If d< dcrsurface tension forces tend to dominate and stable liquid slugs
are formed. 2. Heat input
The level of heat flux used as input to the device is a defining factor of the flow pattern that is established during operation. Three
prin-Figure 1.5: Exp erimen tal Eö -√ F r diagram [White and Breadmore, 1962] [4].
cipal level can be characterized during experiments: start-up, nominal operation, dry-out.
• Start-up: when the incoming heat is below a certain threshold the oscillation of bubbles have high frequency and low ampli-tude. Hence, no significant dynamic effect is generated and no macroscopic oscillating regime can be observed. This regime is characterized by high thermal resistance (poor performance). • Nominal operation: as heat input increase, bubbles start to
oscil-late with increasing amplitude (comparable to the length of the single duct) and the flow tends to take a fixed direction. Thermal resistance reaches its minimum value (best performance).
• Dry-out: as the working fluid gets heated over a certain threshold, the liquid bubbles tend to evaporate and diminish in volume. Va-por volumetric fraction increases until the slug flow regime is not sustainable anymore and the PHP does not operate as a pulsating device.
If the heat flux given to the device starts from a level inside the nominal operation regime and decreases in time it is showed from experiments that the level of heat input at which the device stops to operate is not analogous to the start-up level but is actually lower (Figure 1.6). This is due to a certain inertia of the oscillating flow that permits the oscillation to persist in conditions in which the working fluid at rest could not start working. Activation energy is clearly involved in this hysteresis process.
Figure 1.6: Example of hysteresis cycle of a PHP .
3. Filling ratio
Filling ratio (α or FR) is defined as the volumetric portion of working fluid over the total internal volume of the pipeline (at room tempera-ture). This parameter can influence the performance of the device in a similar way it is affected by the dry-out phenomenon described in the section above. A PHP with a FR=0 transfers heat like an empty metal tube, with thermal resistance significantly higher than a PHP in nomi-nal operation. This is obvious since the absence of working fluid makes impossible for bubbles to be generated and so the oscillating slug flow regime cannot take place. If the same PHP has a FR=1 the whole inter-nal volume is filled with liquid, with no room for the formation of vapor bubbles. This implies a single-phase thermosyphon alike behavior. For FR close to these two practical limits, similar characteristics can be found. In theory an optimum value for the filling ratio is possible and it can be evaluated experimentally considering a fixed set of defined parameters for a certain PHP geometry. Generally, 10%<FR<90% is defined as a true working range. A higher FR traduces in more liquid mass for sensible heat transportation but lower degree of freedom so less pumping power, lower FR on the contrary gives the system more perturbation pumping effect but less heat transfer capability.
4. Working fluid properties
The most important properties of the fluid are listed in the following. • Surface tension: the maximum allowable diameter increases with
number). Larger diameters improve the performance but require higher heat input (higher pressure difference between evaporator and condenser).
• Latent heat: the evaporation rate increases with decreasing latent heat. Liquid slug oscillation velocity increases with corresponding improvement in PHP performances. Dry-out phenomenon occurs at decreasing heat input for decreasing latent heat (due to quicker evaporation).
• Specific heat: the transferable sensible heat increases with higher specific heat of the fluid. Fluids with high specific heat are ad-vantageous.
• Viscosity: since µ ∝ τ (shear stress at the wall), lower viscosity of the liquid implies lower stress along the wall with consequent lower pressure drop in the channel and lower heat input required to sustain the oscillating flow. For increasing values of dP
dT|saturation
(pressure change rate with respect to temperature, at saturated condition) the pressure difference between evaporator and con-denser increases, improving the pumping effect.
5. Number of turns
The effect of increasing the number of turns in this kind of device, keeping fixed the other parameters, is to increase the point of local instability and so the number of perturbations, improving the local pumping effect. The main consequence is that the effect of gravity becomes negligible in the overall behavior.
6. Inclination angle
Three general configurations may be distinguished:
• Bottom Heat Mode (BHM): vertical operation with evaporator at lower height than condenser
• Horizontal: no height difference between the two ends
• Top Heat Mode (THM): vertical operation with condenser at lower altitude than evaporator.
Experimental results [5] have shown that performances (thermal re-sistance) are in general better in BHM configuration and that PHPs with low number of turns (significantly affected by gravity) might not operate in horizontal configuration.
PHPs for space application
In this second chapter a literature overview of the most important articles regarding PHPs in space environment is given in order to spot gaps in the knowledge of the topic. How this thesis work wants to contribute in the scientific and technical panorama is the outcome of the chapter.
2.1 State of development on PHP application
for space environment
Since the first patent in 1990 by Akachi [2], the vast majority of studies on PHPs have been carried out under standard conditions in order to under-stand the behavior of this particular device and to characterize the main parameters involved. In this regard, one of the most accurate reviews that can be found is given by Yuwen Zhang & Amir Faghri (2008) in their pub-lication: Advances and Unsolved Issues in Pulsating Heat Pipes [6]. The most interesting points from a space environment simulation test that can be highlighted from this literature review are:
• Effect of gravity is not negligible for low number of turns.
• Orientation is significant in terms of performance. Bottom heat mode perform better than top heat mode.
• FC-72, water, ethanol are valid fluids for operation.
• Optimum filling ratio is generally between 50% and 70%, depending on fluid and geometry.
• A minimum heat input is necessary to initiate pulsating flow.
These results are surely valid for terrestrial application under standard conditions (same conditions under wich relatives experiments has been con-ducted), but yet to be proven for other environments such as loss of gravity or purely radiative heat flux exchange.
A more updated review regarding tests on gravity effects on PHPs has been carried out by S.S. Ferreira and C.B. Tibirica (2017) in their paper:
Study of the effect of gravity on the performance of Pulsating Heat Pipes
[7]. Table 2.1 reports the reviewd paper analyzed by Ferreira and Tibirica. The outcome of this report is that PHPs performance is ’closely related to
gravity, since different values for thermal resistance, different flow patterns are obtained from the variation of gravity and / or inclination angle of the tube’.
Given the works described up to now it seems clear that gravity is a well studied parameter for PHPs. This is due to the necessity of perfor-mance prediction under various working conditions. From the perspective of a space-simulating test can it be stated that a certain level of predictability
can be applied with the current level of knowledge, even if very few tests have been conducted in real space gravity conditions. The the Parabolic Flight Campaigns tests were able to simulate microgravity for a time of or-der of hundreads of seconds as well as those tests that have flown for REXUS sounding rocket program (PHOS and U-PHOS). The test campaings in the FOX (Flat-plate Oscillating heat pipe eXperiment) project by JAXA [13][14] were able to test a PHP (called OHP) in a LEO small satellite from 2011 to 2016. Besides gravity effects, other aspect of space environment still need to be investigated since very few literature can be found.
A more focused research has been made to find out the publications on PHPs working in environment relevant for space application. The search criteria is to first identify the features needed to define as ”space relevant” a particular environment, then to identify the publishing and compare them into a matrix (Table 2.2). The most relevant features has been identifies as: • How the heat flux is removed from the condenser region (Cooling
method).
• Presence of a vacuum control volume • Presence of radiator for heat exchange • Presence of gravity field
In addition to this criteria, a comparison between the temperatures of the working fluid is reported. Classically in literature it is easier to find a heat power input value instead of a straightforeward temperature range dataset, but since one of the purpose of this work is to investigate on fluid viscosity (temperature dependant), a temperature range may be more appropriate.
A uthor (et al) ID/OD Tub e W orking FR Inclination Po w er year [mm] material fluid Gra vit y input [W] Khandekar 2/4,2 glass w ater, 50% 0°,45°,90° 5 to 15 2002 ethanol Charo ensa w an 1/2 Cu w ater, 50% 0°-90° 0 to 1,2 2003 ethanol,R123 W annapakhe 2 Cu Nano silv er NA 0°-90° 0 to 12 2009 fluid Ay el 1,2/2,5 Cu acetone, ethanol NA vertical, 0 to 4500 2010 pen tane, w ater horizon tal Verma 1,45/2,54 Cu w ater, 40%, vertical, NA 2013 methanol 50% horizon tal, 45° Burban 2,5/3,2 Cu acetone, methanol 50% vertical, NA 2013 w ater,n-p en tane horizon tal, 45° Jahan 2/3 Cu w ater, 70% 0°,15°,30°,45° NA 2013 ethanol 60°,75°,90° Ay el NA Cu w ater 50% 0g ,1,8g 50,75,100, 2013 200,300 Mameli 1/1,2 Cu FC-72 50% ,70%, 0°,15°,30°,45° 10 to 100 2014 90% 60°,75°,90° Xue and Qu 2/6 Quartz ammonia 50% 0°,30°, 40 to 280 2014 glass 60°,90° Mangini 3/5 Al FC-72 50% vertical,horizon tal 10 to 160 2015 0g,1g,1,8g Gosha yeshi 1,25/3 Cu Fe2O3 50% 0°,15°,30°,45° 15 to 90 2015 60°,75°,90° Table 2.1: Literature review from S.S. Ferreira (2017) on exp erimen tal w orks that analyzed the effect of gra vit y on the performance of PHPs.
Moreover, from a temperature comparison it is clear that two main cate-gories of tests have been carried out: close-to-ambient temperature tests and cryogenic tests.
Author (et al.) Working Fluid temp. Cooling Vacuum Radiator Gravity
year fluid range [K] method
A. Vadiraj [8] water, 300-380 air environment NO YES YES
2011 ethanol
Hy-PHP FC-72 300-380 fans NO NO NO
2013
L. D. Fonseca [9] N 70-80 cryocooler YES NO YES
2015
PHOS/UPHOS [10] FC-72 metal foam YES NO NO
2015-2017 & paraffin
D. Mangini [11] FC-72 290-370 fans NO NO NO
2016
Qing Liang [?] Ne 30-40 cryocooler NO NO YES
2017
T. Daimaru [13] HFC-134a 0-25 irradiation YES YES NO
2017
M. Ando [14] HFC-134a 0-25 irradiation YES YES NO
2017
Monan Li [15] He 10-100 cryocooler YES NO YES
2018
Table 2.2 shows that gravity is present in all of the mentioned test with the exception of Hy-PHP, PHOS/U-PHOS[10] and FOX[13][14]. The former has been conducted under microgravity condition during a parabolic flight while PHOS/U-PHOS have flown in a sounding rocket able to reach an al-titude of approximately 90km. FOX programme was tested in a LEO small satellite.
PHOS/U-PHOS [10] and FOX[13][14] are also the only reported tests in which vacuum is not artificially induced. In the case of PHOS/U-PHOS, vacuum is present for a short period of time (hundreads of seconds). In the case of FOX experiments, a timespan of 5 years was tested for 5 days per week.
Vacuum is instead induced in a containment vessel if cryogenic tempera-ture of the test cell is required. Vacuum is used to ensure that heat flux is taken away only by the cryocooler system adopted, preventing natural con-vection and conduction. In those tests [9], radiation is reduced to a negligible quantity by means of multy-layer insulation of the vessel.
The only test conducted to investigate on the radiative response of a PHP attached by a radiating surface is the one by Vadiraj [8]. This test is interesting for the usage of this technology on spacecraft. The experiment demonstrates an improvement in the heat transfer capability and a more uniform temperature distribution for a PHP embedded in a sandwich struc-ture. This characteristic is fundamental in the design of a spacraft heat sink connected to a PHP. Indeed, in a spacecraft the only possible way to dispose of heat is radiating it away to deep space.
The tests in the FOX project are reported as using a radiator, but the system was styl tested as a payload and not as part of the thermal control subsystem of the spacecraft. Indeed, the radiator used was not integrated in subsystems other than the payload. Moreover, the temperature range achieved is considered not wide enough to be characteristic of a space radi-ator (0-25°C) and no informations are reported on the radiradi-ator temperature distribution.
In summary, the conclusions that can be made on the experimental ac-tivities conducted up to now for relevant space-simulating environment are:
• Performance and gravity correlation is relevant and qualitative predic-tion is possible for gravity and microgravity environments.
• Cryogenic temperature of the working fluid is a topic of current in-terest but only for particular working fluids (noble gases and nitrogen expecially), generally not used in space applications.
• Tests have been performed under vacuum condition but no real heat sink for space application has been proved for pulsating devices. • Knowledge on radiator effects on temperature distribution for pulsating
devices is not sufficient for accurate prediction.
2.2 Radiators for PHP application
Since every spacecraft needs to use electrical power to operate, radiators are an essential part of any space mission. It was mentioned in the first chapter that an increasing in power consumption coupled with a continuosly decrising
size of electronics will require higher and higher heat flux capability for fu-ture generation of spacecrafts. The main function of other elements of the thermal control subsystem is generally to transport heat from the hot source to another point of the spacecraft. The role of the radiator is to dispose the excessive heat generated by components, joule heating and external irradia-tion, to outer space.
It is well known that the emitting power of a surface is proportional to its area (A), to the Stefan–Boltzmann constant (σ=5.67037*10−8 W
m2K4), to the surface emissivity (ε) and varies with the forth power of the surface temperature:
W = σεAT4
Given the Stefan-Boltzmann equation, since weight is the most concerning parameter in spacecraft design, it is clear that the driving elements for the sizing of the radiator must be surface reduction and temperature distribu-tion optimizadistribu-tion. In particular, as shown by figure 2.1, temperature can be extremely effective in decreasing the radiating power of the surface given its strong dependance. This is why a strong temperature gradient on the radi-ating surface can lead to heat transfer inefficiency, especially at low absolute values.
Usual radiators for spacecrafts can be grouped in the following categories: • Passive structure: an existing aluminum honeycomb-panel wall of the spacecraft serves both as part of the structure and as a radiator. In the mass budget of the spacecraft the weight of this kind of radiator is normally counted in the structural mass.
Figure 2.1: Blackbody radiator heat rejection [Gilmore]
dissipation, heat pippes can be mounted on a structural element to spread the heat across the radiating surface.
• Body-mounted: if it is required that the radiator must not be a part of the vehicle structure (radiator may need to run at a temperature different from that of the rest of the spacecraft). Heat is transported from the heat-dissipating components to the radiator using heat pipes, loop heat pipes, or capillary pumped loops, and additional heat pipes may be used to spread the heat out in the radiator panel itself (Figure 2.2 (a)).
• Deployable: expecially for crewed missions, the satellite bus may lacks enough area to reject the internally generated waste heat. In such a situation, radiator surfaces that extend beyond structural panels are required to increase available radiating area (with relative weight
pe-nalization) (Figure 2.2 (b)).
(a)
(b)
Figure 2.2: Example of body-mounted radiator (a) and deployable radiator on ISS (b) [Gilmore]
As a general rule, a solution that exploits structural elements already present in the design of the spacecraft is preferable due to weight saving. Body-mounted and deployable panels are associated with mass addiction and pressure losses as well as severe temperature gradients (in the case of long pipelines). For example, ISS deployable radiators system requires a pumped ammonia heat transport that adds even more complexity to the system due to moving parts reliability (pumps and deployment mechanism).
As already showed in Table 2.2, only few studies on a radiator applied to a PHP can be found, and no studies have been conducted in a relevant testing environment. This lack of knowledge makes it necessary to figure out the possible employment of this technology starting from no real case of study. Therefore, the next step should be to prove the advantages of using
pulsating passive device turns out as an advantage in terms of performance improvement and/or weight saving.
It is already clear from experimental studies that PHPs have an overall lower thermal resistance than metal sheets (and honeycomb panels presum-ably). This fact suggests the possibility of using PHPs as heat spreader to obtain a more uniform temperature distribution on the radiator surface. Heat spreaders are mainly used in structural panels connected to a heat surce with a high heat dissipation demand, or in body-mounted radiators with a surface large enough to induce a strong temperature non-uniformity.
As suggested by the study of Vadiraj [8], a PHP embedded in an alu-minum milled plate has an overeall thermal conductivity 4 times higher than if a plate with the same thickess (but not milled) is adherent on the same PHP. From this result is clear that the best way to improve the heat exchange capability of a metal panel using a PHP is to incorporate it in a sandwich structure.
Usual heat spreading techniques imply the oversizing of the panel thick-ness to increase its ”fin efficiency” 1 with corresponding weight penalization.
To use heat pipes in a body-mounted radiator an honeycomb structure is nec-essary to give the panel structural integrity. Being extremely light, pulsating heat pipes may solve these weight penalization and provide comparable to better thermal performances. This topic is one of the investigation point of
1Fin efficiency is defined as the ratio of actual heat flow through the fin to the one
which would be obtained with a fin of constant temperature uniformly equal to the base surface temperature, that is, a fin with infinite thermal conductivity.
this master thesis work.
After a first overview on the state of the art for radiators on PHPs, it can be stated that:
• Heat pipes are a well known technology in thermal control for space. • Heat spreading is a necessity for high heat dissipation applications. • Pulsating Heat Pipes have a low TRL (Technology Readiness Level)
and have not been used in actual spacecrafts yet.
• The most promising application for PHPs devices is for heat spreading on radiator surface to obtain more uniform temperature distribution and relative better thermal performances.
2.3 Thermal vacuum test technology for space
environment simulation
Vacuum conditions simulate the unique context in which a thermal exchange is possible in space: by irradiation and conduction. As mentioned while talking about the thermal control subsystem, it is essential to ensure that all failure more are tested before the spacecraft actually encounter them dur-ing operation. Incorrect estimation of the thermal environment to which the satellite will be subjected frequently end up in freezing or overheating of sensitive components. Furthermore, it is stated by ECSS (European Co-operation for Space Standardization) that the minimum level of pressure for appropriate space environment testing is 10−6mbar.
Figure 2.3: Example of a Thermal Vacuum Chamber
A Thermal Vacuum Chamber (TVC) is an equipment used to simulate en-vironmental space conditions in terms of temperature and pressure. The most common applications of a Thermal Vacuum Chamber are related to the tests
of satellite performance, control of the thermal cycle and testing of compo-nents, subsystems, and complete satellites in a fully controlled environment. The tests are able to accurately reproduce space conditions through the si-multaneous control of the two principal environmental parameters: pressure and temperature. The thermal analysis conducted on the designed system using thermal models, are verified and tested in the TVC. In Figure 2.3 a large TVC for satellite testing is showed. Dimensions of vacuum chamber vary from fraction of meters to tens of meters in diameter. The Space Power Facility at NASA Glenn Research Center’s Plum Brook Station in Sandusky, Ohio, houses the world’s largest vacuum chamber. It measures 100 feet (al-most 33m) in diameter and is 122 feet tall.
Thermal cycling is used to subject the device under test to the alternation of high and low temperatures typical of space application. The temperature range is typically - 100 °C to + 100 °C, while the pressure is maintained below 10−6mbar (high vacuum). Figure 2.4 shows an example of a thermal
cycle.
Thermal balance tests are performed for the validation of the thermal-mathematical model of the satelitte. Tests are performed by creating an environment with a temperature range similar to that which the satellite will encounter in orbit (below -180°C). Some parts of the satellite are also subjected to heating from hot sources (lamps or IR emitters) to simulate the effect of incoming radiations that can cause temperatures to locally raise up to over +150°C.
Figure 2.4: Generic trend for temperature and pressure during a thermal cycle [ACS Angelantoni website][16]
2.3.1 Thermal vacuum chamber
The main caracterizing elemets of a thermal-vacuum system are:
• The chamber: the external body of the testing volume (vessel) is pro-duced from high-quality stainless steel and its design is supported by the FEM (Finite Element Method) analysis in order to optimise the steel thickness withstanding the pressure differences between the in-ternal and exin-ternal environments. Welding and surface are treated to minimise leak rates and outgassing, making it possible to reach deep vacuum.
• The thermal field: the test volume of the TVC is a thermoregulated stainless steel cylinder, known as the shroud, which transfers the heat to the device under test by its inner surface irradiation. Two disc-shaped heat shields close the two ends of the cylinder, in order to achieve a uniform temperature field around the tested device. The shroud consists of two laminated sheets with a gap of a few millimetres between them. This cavity is for the passage of the thermal fluid from the thermal power generation system. A special black paint is applied on this surface producing a layer with high emissivity (ε> 0.9) and with low recovered mass loss, which maximises the thermal exchanges in high vacuum conditions. Sometimes there is a “thermal plate” on which some specimens are placed to perform conductive thermal cycles. • The vacuum generation plant: consists of a set of high quality vacuum pumps. The first vacuum stage (primary or rough pumping) can be attained with dry pumps, eliminating the risk of oil back streaming and characterized by very low maintenance. It allows the transition from ambient pressure to values of around 10−2 mbar. The second
stage, consisting of more sophisticated pumps (e.g. turbomolecolar and/or cryogenic pumps), allows the achievement of an higher vacuum. Typical levels of final pressure inside the chamber are around 10−6 to
10−8 mbar.
• Control and management system: the control and management system of the TVC consists of a fully automated combination of hardware and software components.
In Figure 2.5 a typical vacuum plant is showed, featuring low to ultra high-vacuum pumps. Figure 2.6 illustrate the type of pumps available in the market and their attainable vacuum level.
1. Vacuum chamber 2. Low vacuum valve
3. Lowvacuum pump (e. g. rotary pump) 4. High vacuum valve (gate valve)
5. High vacuum pump (turbomolecular or diffusive) 6. High vacuum pump (cryogenic)
7. Venting valve 8. Low vacuum gauge 9. High vacuum gauge
There are serveral kind thermal shroud configurations, based on test re-quirements, economy and flexibility of the system. The most common are:
Figure 2.5: Typical configuration of a vacuum plant [Paganucci][17] • Liquid nitrogen flooding (boiling mode)
• Liquid nitrogen flooding (boiling mode) with heating elements (lamps or IR emitters)
• Liquid nitrogen partial flooding (boiling mode) with heating elements (lamps or IR emitters)
• Liquid nitrogen pressurized circuit with heating elements (lamps or IR emitters)
• Pressurized gaseous nitrogen circuit
• Combined liquid and gaseous nitrogen modes • Mechanical cooling with intermediate fluid
Figure 2.6: Classification of vacuum pumps [Paganucci]
2.3.2 Outgassing
For a material to be declared suitable for space application the assessment of its outgassing properties is required. Outgassing is defined as the mass loss of a sample in vacuum conditions at high temperature (e.g. >100°C). This specific test is ESA certified and it has to be performed in accordance to ECSS-Q-ST-70-02C: “Thermal vacuum outgassing test for the screening of space materials”.
NASA and ESA maintain a list of low-outgassing materials suitable for spacecrafts. The outgassing is an hazardous phenomena for spacecraft com-ponents. Indeed, the outgassing products can condense onto optical elements, thermal radiators or solar cells and obscure them. This problem is a peculiar-ity of vacuum application, very common in space engineering. Materials not
normally considered absorbent can release enough light-weight molecules and interfere with industrial or scientific vacuum processes. Moisture, sealants, lubricants, and adhesives are the most common sources of light-weight out-gassed molecules, but even vitrous materials and alloys can release gases from cracks or impurities. The rate of outgassing increases at higher temperatures because the vapor pressure and rate of chemical reaction increases. For most solid materials, the manufactoring and the preparation can significantly re-duce the level of outgassing. Also, cleaning of surfaces or heating either the individual components or the entire assembly (a process called ”bake-out”) can drive off volatiles.
Figure 2.7: Scheme of outgassing particles through diffusion.
What happens from a phisycal point of view is that a layer of vapour is in equilibrium with the solid over its surface. The partial pressure of the gas (pv) equals the saturation pressure (ps). On the outside of the layer,the
vapour partial pressure decreases due to diffusion processes. When the ex-ternal pressure is high, ps is normally negligible with respect to the overall
pressure and therefore no outgassing occurs. At low pressure ps becomes not
negligible with respect to the overall pressure and outgassing due to subli-mation must be considered.
Below the triple point a phase transition between solid and gas is possible (sublimation/ deposition). According to Clausius Clapeyron law, transition pressure (saturation pressure) is a function of temperature.
2.4 Open issues and aim of the thesis
Pulsating Heat Pipes are considered a relatively new technology that is far from being completely understood. Although the first patent dates back to 1990 and a large amount of papers can be found in literature on the topic. What is not immediate at first glance is their applications. Up to this point, the work done in this thesis was to understand the state of the art of PHPs and their future possible applications in the space industry.
Some of the open issues that are yet to be clarified are listed in the following:
• Hysteresis behavior
• Effect of extreme termperature of the working fluid • Effect of viscosity in start-up and blockage
In the same way, some important technological questions have to be an-swered in order to be able to correctly utilize the device.
• Which is the temperature distribution across the surface of the radiat-ing surface?
• What is the effect of external vacuum on an embedded radiating sys-tem?
• How are performaces influenced by viscosity?
• Is it possible to have a comparison criteria between a PHP dissipating heat exclusively by irradiation and one in standard environment?
The aim of the present thesis work is to develope an experiment capable of testing a known PHP device (whose characteristic and performance have been tested in a standard ambient conditions) in a space-simulating environment at a temperature at which the viscosity of the working fluid is not negligible in the dynamic of the motion of the fluid itself.
Goal Priority
To obtain a working fluid temperature of -10°C during operation Primary (TF C72< -10°C)
To characterize the radiator surface temperature distribution Secondary To verify the data obtained by numerical simulation Secondary
Test cell design and
modifications
This third chapter will discuss the experiment design. The hardware of the test cell and of the test facilities are presented, with a focus on the modifica-tion adopted to run a thermal-vacuum test. At the end of the chapter, the final test configuration is showed.
3.1 Space PHP test cell for 58° Parabolic Flight
Campaign
To investigate the possibility of comparison criterias between similar devices or similar environment conditions it is necessary that the experiment hard-ware and/or the experimental procedure are comparable to other studies. In this regard, the most straightforeward and safe approach is to use the exact same device that has been used or will be used for test in standard environment and test it under the desired conditions. Instead of building
a completely new device it has been considered simpler to take an already existing device and apply the needed modifications to make it suitable for the thermal-vacuum test. The generic concept of the experiment is described in Figure 3.1.
Figure 3.1: Flux diagram of experiment concept
At a high level of abstraction, the hardware has to be composed of three main elements:
• Heat Source: in order to investigate the behavior of the fluid inside the pipeline it is necessary to excite the thermodynamic state of the system. This has to be done providing heat power to the evaporator region of the PHP. This element idealizes the component of the spacecraft that produces the excessive heat that has to be disposed (an electronic box for example).
component that has to be characterized.
• Heat Sink: Altering the stationary state of the system with input power the PHP will react trasfering heat from the evaporator to the condenser, creating a temperature gradient. Without an element capable of releas-ing heat from the system, the temperature of the workreleas-ing fluid would increase over the point of complete dry-out of the device. A sink is needed to exchange output power to the outside of the system under analysis, reaching a pseudo-stationary behavior. This element simulate the thermal control component of the spacecraft (generally a radiator). From the literature review presented in Chapter 2, the ideal candidates for the working fluid are water, ethanol and FC-72. The filling ratio (FR) of the device to be tested should correspond to the optimal one (around 50-70% depending on the fluid) in order to obtain technically relevant results that can be used for future design.
The device chosen for the test is the PHP used by Mameli and his team during the experiments for the publication Thermal response of a closed loop
pulsating heat pipe under a varying gravity force [18]. This particular device
is an interesting one, especially for a test in space simulated environment, because it has been tested both in standard ambient conditions and in micro-gravity conditions during the 58th Parabolic Flight Campaign (58PFC). The results exposed by Mameli highlight how the PHP in horizontal configuration exhibits a behavior similar with respect to microgravity conditions. Quoting the paper [18] ”For a perfect 2D geometry, where in horizontal position the
flow motion is never assisted by the gravity head, the horizontal operation on ground is the most similar to the microgravity operation”. This result is
an horizontal orientation of the device during the test will give more accurate results in the context of space application.
This harware matches all the working features required to be operative for a test: it is easy to modify since there are no fixed parts, it is favorable for the budget of the test, it was available in a short period. For these reasons the presented PHP has been chosen for the test.
Figure 3.2: Test-cell geometry and thermocouples location.
The pulsating heat pipe used for both tests (58PFC and thermal vacuum test) is now described as it was built for the 58PFC tests. The applied mod-ifications are described in the following. The PHP is made of a copper tube (I.D./O.D. 1.1 mm/2.0 mm) bended into a planar serpentine (32 parallel channels) where all curvature radii are 3 mm. Two “T” junctions allow to
close the serpentine in a loop and to derive two ports at each side: one is devoted to the vacuum and filling procedure while the second one hosts a pressure transducer (Kulite, ETL/T 312, 1.2bar A). The PHP is equipped with 14 “T” type thermocouples with wire diameter 0.127 mm, with an ac-curacy of 0.1 C after calibration; nine are tin soldered on the external tube surface in the evaporator and four in the condenser in order to maximize the thermal contact, a last one measures the ambient temperature. The test cell geometry as well as the thermocouple locations is showed in Figure 3.2.
Figure 3.3: Electric wire heating system.
The chosen PHP is equipped with a wire electrical heater (Thermocoax Single core 1Nc Ac, 0.5 mm external diameter) wrapped 20 times around each evaporator “U-turn” in order to cover an evaporator length of 6 mm, as shown in Figure 3.3. This design allows to minimaze the thermal inertia in the evaporator section, in order to avoid heat conduction between adjacent channels and to provide the same heating power to each “U-turn” in the evaporator. Circular cross section channels are milled on the surface of the aluminum heat sink so as to host the copper tubes. The PHP condenser is embedded into the heat sink and fixed with an aluminum back plate. The PHP equipped with such a light heating system is able to reach the ther-mal pseudo-steady state in about 3 minutes for laboratory operation. A
different pseudo-equilibrium time will be evaluated to take into account the modifications. Electric power (up to 100W, corresponding to a radial heat flux up to 12 W
cm2 ) can be provided by an external power supply through the heaters. Thermal contact between the PHP and the aluminun plates was obtained by heat sink compound. Four air fans were located on the heat sink fins in suction mode to ventilate hot air out of the assembly (Figure 3.4).
Figure 3.4: Air fan system used during Parabolic Flight Campaign.
3.2 Modification applied for the vacuum test
In the described hardware it is possible to recognize four major type of com-ponents:• Heat source: electric wire heater • Heat transfer medium: the PHP • Heat sink: fans assembly
• Sensors package: thermocouples, pressure sensor, RTD
In this way it has been identified the role of the 58PFC experiment com-ponents with respect to the conceptual diagram presented by Figure 3.1. Having to design a vacuum test, it is clear that the fans are useless as heat sink for the purpose of our test. The first and principal modification identi-fied is so the necessity to rethink how the heat is going to be disposed out of the new test cell system. This topic is explained in detail in Section 3.2 but, for clarity, it has to be said that a flat aluminum plate (60x70 mm) has been chosen as radiator.
From an accurate review of the sensors datasheets (Appendix A: Datasheets) no sensor has resulted incompatible with vacuum environment or with the expected temperature and pressure conditions. The sensor package installed in the 58PFC test cell can be totally reused for the thermal-vacuum test. In particular, each thermocouple is directly connected to the external surface of the PHP and will remain in the same position during the thermal-vacuum test. The thermocouple labeled as TC-13 (Figure 3.2) will be put in contact with a strategic point of the radiator to register mesurements of its temper-ature.
The position of the Kulite pressure transducer and RTD is not optimal in the 58PFC test cell configuration. Temperature measurments from the T junction does not represent accurately the temperature of the inner pipes of the PHP. Pressure measurements could be affected by positioning too, with a less accurate measuring of the magnitude of pressure value. However, besides obtaining a sufficiently accurate measure of the absolute value, the purpose of the pressure transducer is to show the characteristic oscillation behavior
of the working fluid. This task is still obtained with the pressure sensor po-sitioned in the 58PFC configuration. As a compromise between complexity of modification and accuracy of measurement it has been decided to acquire pressure data from the pressure sensor but to not use the RTD for internal temperatures. Temperature of the working fluid will be estimated during the data post-processing as function of the thermocouples data (external surface of the pipeline temperature) and pressure measured with the pressure trans-ducer.
In addition to some minor modifications (e.g. changing of old screws and cables), the principal modifications to the 58PFC (58° Parabolic Flight Campaign) hardware are listed in the following.
• Disassemble of fans and fans support.
Air fans were positioned in suction of hot air from the finned plate. This way of ventilating the heat sink has no purpose in a vacuum en-vironment due to the absence of air. As a consequence, the four fans and their structural support have been removed. Under these condi-tions the fins on the outer surface of the envelope of the PHP lose their functionality, remaining as additional mass. The aluminum sheet with fins is internally milled to house the PHP. This makes this com-ponent difficult to replace with a non finned one due to the particular milling process required to reproduce it. In order to reduce the usless aluminium mass, which contributes negatively to the achievement of equilibrium conditions, this sheet is thermally isolate from the PHP with Kapton tape (Figure 3.5). This tape keeps the PHP attached to the panel connected to the radiator, enhanching the physical contact
and so the heat conduction between the outer surface of the PHP and the heat sink. This is needed due to the impossibility of using heat sink compounds in vacuum.
(a) (b)
Figure 3.5: Application of kapton tape along the milled housing of PHP for thermal isolation. Extended view (a), zoomed view (b).
• Disassemble of 58PFC support structure.
The structure used to fix the test cell in the 58PFC was designed to be realiable, to be hyperstatic for the rendundancy of fixing elements, and have some security elements against vibrations (locknuts, rubber vibration dampers). Therefore the PHP mechanical assembly is more complicated than required for the vacuum test and it is not easily adaptable to the vacuum facility interface. It was decided to remove the old structure and build a more suited one once the test facility was chosen. The final support structure is showed in the following of the chapter (Section 3.4.2, page 86).
• Remove heat sink compund and installation of TIM.
Commercial thermal paste has been used for ensuring thermal con-tact between the two aluminum plates. For what mentioned in Section 2.3.2, a colloidal material such as thermal compound is very prone to the outgassing process due to its saturation pressure with respect to the ambient pressure of the vacuum chamber. Impurity in the atmo-sphere inside the chamber can compromise crucial components of the test facility such as pressure gauges. Moreover, a polluted chamber requires a very expensive cleaning to ensure that sensitive future test devices will not be compromised.
Figure 3.6: Upper plate removed during disassembly. Thermal paste in white. Since outgassing of the thermal paste is an issue for the cleanliness of the vacuum chamber,it has been decided to complitely remove it and rethink how to ensure thermal contact between parts. To remove the paste the envelope has been completely disassembled and each part has been cleaned separately (Figure 3.6). A cloth soaked in isopropyl
alcohol has been used to roughly clean all the surfaces. Afterwards a vi-brating bath has been used to remove the paste solified on the surfaces of both plates of the envelope. Unfortunately this was unsufficient to remove micrometric dust trapped in the milled plate so a cotton swab soaked in isopropyl alcohol was needed to refine the cleaning. Figure 3.7 shows the effectiveness of the cleaning process on the surfaces of the envelope that were ready for vacuum.
(a) (b)
Figure 3.7: Upper plate with thermal paste on it (a) and after cleaning (b). As showed in Figure 3.6, the outer surface of the PHP required some cleaning too. The structural stifness of the copper pipeline was not sufficient to keep it standing still on its own. The PHP itself was fixed with cable ties to be able to clean it and the heating wires wrapped around it with cotton swab and isopropyl alcohol.
Figure 3.8 shows the cleaned PHP and highlights that it was not possi-ble to remove all the thermal past without compromising the integrity of the experiment. This little quantity of thermal paste is expected to
