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Chapter 6
6.
TURBOPUMP CONFIGURATIONS
As introduced in chapter 1, the aim of the present thesis is to study the rotordynamic forces and their effects on the performance, of different turbopump configurations which involves two pumps: an axial inducer called DAPROT3 and a centrifugal pump called VAMPIRE.
The experimental campaign has been developed on a total of three configurations: • DAPROT3,
• VAMPIRE,
• DAPROT3 and VAMPIRE.
Despite this, the configurations considered for the analysis in the present thesis are the DAPROT3 in stand-alone condition and the combination of the two pumps where the inducer is placed upstream with respect to the centrifugal pump. In the latter case, the configuration has been termed VAMPDAP. Nevertheless, the software developed (see Appendix A) is capable to consider the three pump configurations as well as other pumps (axial, radial and mixed flow pumps), by changing the input parameters.
This chapter will present the main features along with the geometrical dimensions and performance of the two pumps analyzed.
6.1.
DAPROT3
The first pump considered is the inducer called DAPROT3.
These kind of impellers are used to improve the performance of the pumps that are placed downstream by increasing their inlet pressure and thus the Net Positive Suction head (NPSH). This allows the pumps to operate at safe conditions, without cavitation, whereas inducers operate under controlled cavitation. Hence the geometry of the latter is important to control the cavitation inception, affecting the pressure field and the residence time in a region with a pressure higher than vapor pressure. This objective is reached with a design which provides a gradually raise in pressure by means of small incidence angles (few degrees) and thin blades to minimize the perturbation imparted to the flow. The inducer develops so small power to the fluid that there is virtually no noise, vibration, or mechanical problems.
A nonzero incidence angle is used to avoid the uncertainty of suction or pressure cavitating surfaces (or eventual oscillation between them), forcing the cavitation to occur on suction surface.
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The DAPROT3 (Figure 6.1) is a three-bladed inducer made of 7075-T6 aluminum alloy with tapered hub and variable pitch. This impeller has been designed by means of the reduced order model recently developed to this purpose at Alta S.p.A.. Its main geometrical and operational parameters are reported in Table 6.1. The solidity and diffusion factor of the inducer have been chosen with the aim of improving its suction performance by reducing the leading-edge cavity. At the same time the value of the tip incidence-to-blade angle ratio (α/βb < 0.5) has been selected with the purpose of providing
sufficient margin against the onset of surge instabilities under cavitating conditions.
Figure 6.1 DAPROT3 impeller, frontal view.
Design flow coefficient ΦD 0.065 [--]
Number of blades N 3 [--]
Tip radius rT 81.00 mm
Inlet tip blade angle γTle 82.10 deg Inlet hub radius (fully developed blade) rHle 44.50 mm
Outlet hub radius rHte 58.50 mm
Mean blade height hm 25.95 mm
Axial length (fully-developed blade) ca 63.50 mm
Inlet hub radius rH1 35.00 mm
Axial length L 90.00 mm
Diffusion factor D 0.47 [--] Tip incidence-to-blade angle ratio α/βb 0.33 [--]
Tip solidity σT 1.68 [--]
Incidence tip angle at design α 2.58 deg Outlet tip blade angle γTte 70.56 deg
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As can be seen in Figure 6.2 and in Figure 5.29 the DAPROT3 has been mounted with a radial diffuser (orange component in Figure 5.29) in order to reduce the suspended mass and the interference with the forces acting on the impeller, measured by the dynamometer.
Figure 6.2 DAPROT3 with diffuser mounted on CPRTF facility.
6.1.1.
DAPROT3 NONCAVITATING PERFORMANCE
From a series of tests, the noncavitating pumping performance and hydraulic efficiency at 20 °C of the DAPROT3 inducer in the eccentricity free configuration has been assessed. In this condition a blade tip clearance of 2 mm is present, corresponding to 7.7% of the mean blade height.
The tests have been performed in the same operating conditions with a main impeller speed of 1750 rpm at 13 different values of flow rate where an acquisition time of 5 seconds has been exploited for each test. The instruments and transducers used in the facility are as follows:
• The volumetric flow rate has been measured by means of an electromagnetic flowmeter (see Chapter 5.10) mounted on the discharge line,
• The static pressure rise across the pump (Δp) by means of a differential pressure transducer (see Chapter 5.20) where:
The low pressure tap has been placed on the suction line at approximately 6 inducer diameters upstream of the blade leading edges. The position is intended to eliminate the effect of the inlet flow prerotation (see Chapter 2.4).
The high pressure tap has been positioned on the discharge line at about 2.5 duct diameters downstream the test chamber exit. The reason of its position is that there is a certain amount of uncertainties due to the exit flow swirl at the discharge of the inducer.
As a consequence of the pressure taps positions, the measure of the head includes:
• the viscous losses developed between the inducer inlet and the upstream pressure tap, • the diffusion losses from the inducer exit into the test section,
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The losses due to the first component are negligible. Conversely, two main assumptions have been taken into account due to the other components, for the evaluation of the correct pressure rise across the pump. Hence, according to the model presented in chapter 5.21:
• the static pressure at discharge line can be considered equal to the static pressure at the exit of the inducer,
• the azimuthal velocity at the inducer exit can be obtained from Carter’s rule.
The torque has been measured by means of the rotating dynamometer and the hydraulic efficiency has been obtained by means of the equation 2.19, whereas for the evaluation of the head coefficient, equation 2.8 has been exploited.
Figure 6.3 shows the experimental curves obtained for the head coefficient and the total-to-total hydraulic efficiency as functions of the flow coefficient in the noncavitating condition. The results have been evaluated from the measurements obtained from the tests and the Matlab source code used for their calculation is reported in Appendix B.1. Even if in the present context the noncavitating performance in the presence of rotordynamic forces and moments, thus in whirling condition, is not taken into account since no particular behaviors have been observed, the Matlab source code exploited in this second case is reported in Appendix B.2.
Figure 6.3 Noncavitating pumping performance and hydraulic efficiency of the DAPROT3 inducer.
From previous figure, a maximum hydraulic efficiency of approximately 82% has been found in correspondence to a flow rate close to the design flow coefficient of Ф = 0.065.
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6.1.2.
DAPROT3 CAVITATING PERFORMANCE
As shown in chapter 3.4 the cavitating pump performance are important to understand the response of a turbomachine in the presence of a moderate or heavy cavitation. This information can be obtained from the family of curves (σ,Ψ) represented in Figure 3.10 where the inception, critical, and breakdown cavitation numbers are indicated.
The experimental curves have been obtained from a single continuous test in which the inlet pressure is linearly decreased at fixed flow rate. The experimental procedure for the centered (zero eccentricity, 2 mm clearance) cavitating performance consists of a first offset test in which the engine is at rest and of a test where the bladder is linearly depressurized in 4 minutes from a value sufficiently high of cavitation number to the lowest pressure that can be obtained from the pressurization system. Moreover, the data obtained have been divided in intervals of 1 second to allow the assumption of constant pressure and the intervals are overlapped by 75%. Hence each group of data contains 75% of the previous interval. This allows to obtain discrete points sufficiently close to each other, improving the precision of the experimental curve. For further details on the experimental procedure the reader is referred to A. Bonaguidi3.
The experimental curves obtained in terms of head coefficient and hydraulic efficiency are respectively presented in Figure 6.4 and Figure 6.5.
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Figure 6.5 Hydraulic efficiency of DAPROT3 inducer (at zero eccentricity and 2 mm clearance) as a function of the cavitation number.
The experimental procedure has been exploited also in order to evaluate the cavitating performance at nonzero eccentricity condition with a fixed whirl speed and flow rate. The operating condition at which these tests have been performed are indicated in Table 6.2. Hence, the results for cavitating performance are shown in Figure 6.6. From the same tests the rotordynamic forces as a function of cavitation number have been obtained (see Figure 6.7, 6.8, and 6.9). For this purpose, additional offset test, eccentricity test, residual eccentricity test, and air test are necessary. The procedure for these tests will be explained in the next chapter.
Ω [rpm] Φ T [°C] Samples per second Δt [s] ω/Ω
1750 0.065 19.5 5000 240 -0.5 19.7 -0.1 19.2 0.5 19.2 0.7
Table 6.2 Operating conditions for cavitating performance tests on DAPROT3 inducer.
From the following diagrams, the response of the fluid-induced rotordynamic force at highly cavitating condition is shown. It can be observed that at cavitating condition close to the breakdown, the rotordynamic force tends to diverge. For what concern the normal force it can be observed that for
ω/Ω = -0.1 it becomes highly stabilizing whereas in the other operating conditions it becomes highly
destabilizing. On the other hand the tangential force becomes highly destabilizing for any whirl speed. It is important to note that for ω/Ω = -0.5 the tangential force is stabilizing at noncavitating condition and even if the experimental curve does not reach negative values it is clear that the trend is to
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destabilize the force. Hence the presence of high cavitation may destabilize a force that is stabilizing at noncavitating condition.
Figure 6.6 Cavitating performance at nonzero eccentricity (ε = 1.063 mm) of DAPROT3 inducer at T = 20 °C and Φ = ΦD = 0.065.
Figure 6.7 Nondimensional normal forces as a function of the cavitation number for different whirl speeds at T = 20 °C and Φ = ΦD =
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Figure 6.8 Nondimensional tangential forces as a function of the cavitation number for different whirl speeds at T = 20 °C and Φ = ΦD = 0.065.
Figure 6.9 Nondimensional rotordynamic force intensity as a function of the cavitation number for different whirl speeds at T = 20 °C and Φ = ΦD = 0.065.
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The MATLAB code developed for the evaluation of the cavitating performance in the zero/nonzero eccentricity conditions is reported in Appendix C (see also Appendix A.3).
6.2.
VAMPDAP
The second configuration considered is the VAMPDAP, composed by the combination of the DAPROT3 inducer and the VAMPIRE radial pump.
The latter is the combination of a vaneless diffuser, a single-spiral volute (shown in Figure 6.12), and a centrifugal impeller (shown in Figure 6.10) designed by means of a reduced order model entirely developed at Alta S.p.A., Pisa (Italy), as a result of a natural extension of the model exploited for the inducers under more general assumptions suitable for centrifugal pumps. This model, proposed by d’Agostino et al. in 2011, allows the definition of the geometry and the prediction of noncavitating performance at the same time.
Figure 6.10 Impeller of VAMPIRE.
The centrifugal impeller is a six bladed unshrouded impeller made of 7075-T6 (same material used for DAPROT3) which is an aluminum alloy with zinc as primary alloying element and magnesium and copper in smaller quantities. This material presents a high strength and good fatigue strength with an average machinability. The T6 temper consists in a homogenizing at 450 °C for many hours, with a subsequent ageing at 120 °C for one day. This kind of temper increases the peak of strength of the 7075 alloy, reducing the effect of the collapsing bubble and therefore reducing the erosion.
As previously stated, the VAMPDAP is the configuration in which the inducer is positioned upstream with respect the centrifugal pump and therefore the assembly obtained is the one in Figure 6.11. Between the two impellers a flow conveyor is present (pink elements in Figure 6.13) made by an external and an internal element with the aim of both obtaining the correct clearance for the VAMPIRE pump, and optimizing its inlet flow. In particular, the external flow conveyor element can be regulated by rotating it, where a screw system adjusts its axial position by 1 mm for each complete rotation. This allows the impeller to rotate without rotor/stator contacts when an eccentricity is imposed during rotordynamic tests. Hence the nominal clearance has been set at 2 mm when tests with eccentricity at 2 mm has been performed, controlling the absence of contact stator/rotor during assembly.
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Figure 6.11 VAMPDAP pump assembly without volute.
The main geometrical dimensions of the VAMPIRE are shown in Table 6.3.
Design flow coefficient ΦD 0.092 [--]
Number of blades N 6 [--]
Outlet radius r2 105 mm
Inlet tip radius rT1 57.20 mm
Inlet hub radius rH1 31.80 mm
Axial length (fully-developed blade) zH2 46.40 mm
Inlet tip blade angle γT1 56.60 deg Inlet backsweep angle χ1 0 deg
Diffuser outlet radius r3 126 mm
Tip solidity σT 2.26 [--]
Incidence tip angle at design α 17.40 deg Outlet tip blade mean angle γT2 67.78 deg
Outlet tip backsweep angle χ2 66 deg Exit blade height B2 10.50 mm
Exit cross-section volute radius R4 38.20 mm
Volute maximum radial dimension rV4 201.50 mm
Predicted design head coefficient ΨD 0.31 [--]
Predicted specific velocity ΩS 0.74 [--]
Predicted efficiency η 83 %
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Once the VAMPIRE is mounted in the CPRTF facility as shown in Figure 6.12 (where the flow conveyor is not present for a better understanding of the elements), the DAPROT3 is placed by means of a tie rod and it is blocked in its position with a locking nut. A nose will complete the VAMPDAP pump and the result will be as in Figure 6.13 and Figure 6.14.
Figure 6.12 Volute design with discharge structural section (left) and picture of volute and centrifugal impeller during VAMPDAP assembly (right).
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Figure 6.14 Cut-off rendering with VAMPDAP mounted.
6.2.1.
VAMPDAP AND VAMPIRE NONCAVITATING
PERFORMANCE
From the same procedure exploited for the DAPROT3 inducer, a series of tests has been performed for the evaluation of the VAMPDAP pump noncavitating performance at 20 °C. From the experimental curves shown in Figure 6.15 it is possible to observe that the maximum of the pump is near the flow coefficient at design. Since the experimental campaign performed in order to obtain these result provides discrete points, the maximum is reached for Φ = 0.083 which corresponds to an hydraulic efficiency of about 62%. It has to be taken into account the fact that the tests have been performed with a clearance both of 2 mm for the inducer and the radial impeller. Hence the actual pumping performance will be higher in optimal operating conditions.
For what concerns the VAMPIRE pump, the noncavitating performance at 20 °C with a clearance of 0.16 mm (conditions at which the flow conveyor does not come into contact with the rotor) is shown in Figure 6.16. From the experimental curve, an hydraulic efficiency of 77% is reached at design flow coefficient, ΦD = 0.092.
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Figure 6.15 Noncavitating pumping performance and hydraulic efficiency of the VAMPDAP pump with a clearance of 2 mm.
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6.2.2.
VAMPDAP CAVITATING PERFORMANCE
The cavitating performance of the VAMPDAP pump has been obtained exploiting the same experimental procedure used for DAPROT3 and explained in detail in chapter 6.1.2. Hence a test at centered position has been performed to evaluate the cavitating performance of the pump at zero eccentricity and 2 mm of clearance. Additional tests have been performed to evaluate the cavitating performance at different whirl speeds and flow rates. The operating conditions of the tests are indicated in Table 6.4. Ω [rpm] Φ T [°C] Samples per second Δt [s] ω/Ω 1750 0.092 19.7 5000 240 0 19.9 -0.3 19.9 0.3 19.8 0.4 0.074 19.8 0.25
Table 6.4 Operating conditions for cavitating performance tests on VAMPDAP pump.
The head coefficient and hydraulic efficiency as a function of the cavitation number in the zero eccentricity condition are respectively shown in Figure 6.17 and 6.18. The results for nonzero eccentricity are reported in Figure 6.19-6.22.
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Figure 6.18 Hydraulic efficiency of VAMPDAP pump (at zero eccentricity and 2 mm clearance) as a function of the cavitation number.
Figure 6.19 Cavitating performance at nonzero eccentricity (ε = 1.160 mm) of VAMPDAP pump at Φ = ΦD = 0.092 and Φ = 0.074 (T
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Figure 6.20 Nondimensional normal forces as a function of the cavitation number for different whirl speeds at T = 20 °C, and Φ = ΦD
= 0.092 or Φ = 0.074.
Figure 6.21 Nondimensional tangential forces as a function of the cavitation number for different whirl speeds at T = 20 °C, and Φ = ΦD = 0.092 or Φ = 0.074.
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Figure 6.22 Nondimensional rotordynamic force intensity as a function of the cavitation number for different whirl speeds at T = 20 °C, and Φ = ΦD = 0.092 or Φ = 0.074.
6.3.
REFERENCES
[1] D. Valentini, A. Pasini, G. Pace, L. Torre, L. d’Agostino, Experimental validation of a reduced order for radial turbopump design, 49th AIAA/ASME/SAE/ASEE Joint
PropulsionConference. San Jose, CA, July 14-17 2013.
[2] L. Pecorari, Studio delle prestazioni cavitanti e delle forze rotodinamiche su induttori per uso spaziale, Tesi di Laurea in Ingegneria Aerospaziale, Università degli studi di Pisa, 2009.
[3] A. Bonaguidi, Caratterizzazione delle prestazioni e delle forze rotodinamiche di turbopompe per uso spaziale, Tesi di Laurea in Ingegneria Aerospaziale, Università degli
