Cryogenic propellant recirculation for orbital propulsion systems

Journal Pre-proofs Cryogenic Propellant Recirculation for Orbital Propulsion Systems Kiyoshi Kinefuchi, Hideto Kawashima, Daizo Sugimori, Koichi Okita, Hiroaki Kobayashi PII: DOI: Reference:

S0011-2275(19)30068-2 https://doi.org/10.1016/j.cryogenics.2019.102996 JCRY 102996

To appear in:

Cryogenics

Received Date: Revised Date: Accepted Date:

27 February 2019 24 June 2019 6 November 2019

Please cite this article as: Kinefuchi, K., Kawashima, H., Sugimori, D., Okita, K., Kobayashi, H., Cryogenic Propellant Recirculation for Orbital Propulsion Systems, Cryogenics (2019), doi: https://doi.org/10.1016/ j.cryogenics.2019.102996

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© 2019 Published by Elsevier Ltd.

Cryogenic Propellant Recirculation for Orbital Propulsion Systems

Kiyoshi Kinefuchi1, Hideto Kawashima2, Daizo Sugimori3 and Koichi Okita4 Japan Aerospace Exploration Agency, Tsukuba, Ibaraki, 305-8505, Japan Hiroaki Kobayashi5 Japan Aerospace Exploration Agency, Sagamihara, Kanagawa, 252-5210, Japan

Abstract

In the next generation orbital propulsion systems, cryogenic propellant recirculation will be applied to reduce engine chill-down consumption by recirculation chill-down and to achieve efficient propellant utilization by active propellant cooling. To understand of tank-to-tank recirculation, a recirculation test campaign was conducted using liquid nitrogen as a working fluid. An electrically-driven recirculation pump developed for this experiment has bleed holes on the impeller to bleed vapor rather than liquid so that it works under two-phase flow operation. In addition to this pump, the test facility includes a 600 mm-diameter cryogenic tank, void fraction meters, an ultrasonic flow meter, and temperature and pressure sensors. The tank has a double layered window on the top to visually observe the return flow inside. The void fraction meters and ultrasonic flow meter were used to evaluate the two-phase mass flow rate. The tests were carried out in two different return port positions in the tank, the side and bottom return port cases, and the results indicated that the pressure and temperature behaviors in the tank were affected by the return flow rate, phase of the return flow and the return port position in the tank. In the side return Associate Senior Research Engineer, Research and Development Directorate. Associate Senior Research Engineer, Research and Development Directorate. 3 Research Engineer, Space Technology Directorate I. 4 Director, Research and Development Directorate. 5 Assistant Professor, Institute of Space and Astronautical Science. 1 2

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port case, the flow returned to tank ullage and less disturbance of liquid in the tank was observed at a low flow rate while strong mixing of the liquid was observed at a high flow rate. The tank pressure was affected by the phase of return flow rather than the ullage temperature. In the bottom return port case, less disturbance was observed in vapor return flow, while in two-phase or liquid return flow, strong disturbance was observed even at lower flow rate. The tank pressure in this case was similar to that in the side return port case: the vapor return increased the pressure while the two-phase or liquid return decreased the pressure. The bleed holes on the impeller helped the inlet flow to recover to subcooled liquid condition and to keep the impeller working under saturation condition at the inlet although the system suffered decreased pump head, loss of liquid and unsteady oscillation of pump head and flow rate.

1. Introduction Liquid rocket engines using cryogenic propellants provide high specific impulse; therefore, liquid oxygen and liquid hydrogen have been widely used for space launch vehicles, especially in their upper stages, and liquid methane is now under study for practical application. For future orbital transfer vehicles to the Moon and other planet [1], investigations have been conducted to apply cryogenic propellants not only to conventional chemical propulsion systems but also to advanced electric propulsion systems [2-4]. because these propellants have an additional advantage: in-situ production on the surface on the Moon and Mars [5-7]. Cryogenic propellant transfer between vehicles in orbit, so-called a propellant depot, has been investigated to expand the capability of space exploration beyond the Earth orbit [8,9]. To realize more flexible operation and higher performance of cryogenic orbital vehicles, multiple-ignition capability of cryogenic engines and efficient use of the propellants (e.g., reduction of engine chill-down consumption [10], effective propellant settling under reduced gravity [11-13], and vaporization reduction) are strongly required. In cryogenic propulsion systems, the chill-down procedure is carried out before each ignition to condition the temperature of feedlines and turbopumps. In general, the chill-down requirement is achieved by the forced convection boiling of cryogenic propellants, and used propellants are vented outside of the vehicle; therefore, excessive amount of chill-down consumption lowers the vehicle’s launch performance. A recirculation chill-down system is a candidate to reduce this consumption for long-term deep space missions that require multiple ignitions of

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cryogenic engines [14]. In a recirculation chill-down system, a cryogenic pump draws propellant from the tank into the engine through feedlines. The enthalpy of the propellant increases to chill down the turbopump, then the warm propellant recirculates back to the tank again without venting outside. As for vaporization reduction, novel thermal insulations for propellant tanks [15,16] rather than conventional foam insulations are necessary for long term cryogenic storage. In addition, an active propellant cooling system with a cryocooler and/or thermodynamic vent system (TVS) is also crucial to reduce boil-off of cryogenic propellant [17-19]. Flachbarta experimentally explored a tank-to-tank loop by using a recirculation pump for liquid hydrogen and liquid methane [20,21]: the circulated liquid propellant is cooled through a cryocooler and TVS and then injected into the tank. The flow returns to the tank in the form of a spray or an axial jet which contributes to control of the tank pressure and the destratification of liquid temperature layer [22]. Recently, a broad area cooling or tank-on-tube concept has been proposed to achieve zero boil-off (ZBO) of cryogenic propellant [23]. A test campaign called the ZBOT (Zero Boil-Off Tank) experiment conducted in the International Space Station has successfully characterized an axial jet under reduced gravity [24]. Mer analytically optimized another type of active cooling system with a recirculation pump, TVS and subcooled jet [25]. As reviewed above, future cryogenic orbital propulsion systems require recirculation systems of cryogenic propellant for recirculation chill-down, active cooling and tank pressure and temperature controls for efficient propellant utilization. These recirculation systems basically consist of a recirculation pump and a tank-to-tank loop; therefore, in this study, recirculation experiment was conducted using liquid nitrogen as a working fluid. A centrifugal pump driven by a brushless motor was developed for cryogenic recirculation. One of critical issues using electrically driven cryogenic pump is that heat is transferred from the electric motor to the cryogenic flow. To curb the temperature rise in the flow, a long shaft was installed between the impeller and the motor, instead of using a submerged motor. Assuming the actual application in space, the pump draws saturated flow or liquid-vapor two phase flow in some cases; thus, the centrifugal impeller has vapor bleed holes to work even under saturation condition. The bleed holes realize recirculation under two-phase flow condition at the cost of a certain amount of propellant loss. For example, the bleed operation can support warm start of recirculation. Even in a ZBO system, the operation can also help the system to recover from malfunctions, e.g. cryocooler failure. A 600 mm-diameter liquid nitrogen tank having 25 thermocouples and a level sensor was made for the experiment. The tank has two fluid return ports: the side port to return the fluid into ullage and the bottom port to return it into liquid. A double layered

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window was installed on the top of the tank to visually observe the return flow inside. To observe the two-phase flow characteristic in detail, void fraction meters were installed in the recirculation line. Turbine flow meters are not suitable for this experiment because liquid-vapor two-phase flow may cause over rotation of the turbine. Hence, an ultrasonic flow meter was set to the recirculation line to do the experiment efficiently and to measure two-phase mass flow rate. In this paper, first, the basic performance of the recirculation pump will be shown using water and liquid nitrogen as a working fluid. Then, the results of liquid nitrogen recirculation experiment using the recirculation pump will be explored. In addition, two-phase flow suction characteristics of the pump will be discussed.

2. Experimental Apparatus 2.1. Cryogenic Recirculation Pump The recirculation pump used in the experiment is shown in Fig. 1. The pump has been designed to be low cost, to have cryogenic compatibility, and to be usable under two-phase flow condition at the inlet. A simple open radial vane Barske type impeller has been applied to the pump. The impeller, 80 mm in outer diameter, has 8 vanes. Between the vanes, there are 8 bleed holes near the hub as shown in Fig. 1 to bleed some fluid. Bled fluid through the holes is vented outside through the bearing room by the pressure difference between inside and outside of the pump. The bleed holes, located near the hub, can bleed as much vapor as possible under liquid-vapor two-phase flow operation because the centrifugal force causes the liquid to diffuse to the outer side (casing side), while the vapor remain at the center side (hub side). The impeller casing is made of transparent polycarbonate resin to visually observed the inside even under cryogenic operation. The pump is driven by the blushless motor and coupled with the shaft by the magnetic coupling, which eliminates the need of a complex and costly shaft seal system. The shaft is supported by two ceramic ball bearings which have cryogenic compatibility. This long shaft can reduce the heat input from the motor and the magnetic coupling to the impeller.

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Magnetic Coupling

Bleed Hole Centrifugal Impeller Transparent (OD = 80 mm, 8 vanes) (Ø4.0 mm × 8) Impeller Casing Bleed Line Shaft

Brushless Motor

Flight direction

Inlet

Ceramic Ball Bearings Flange

Outlet

736 mm

Fig. 1 Cryogenic recirculation pump.

2.2. Cryogenic Tank The drawing of the cryogenic tank is depicted in Fig. 2. The tank is 600 mm in inner diameter and 482 mm in inner height. The tank skin 4 mm in thickness is made of stainless steel and its outside is covered with polyurethane foam for thermal insulation. Thanks to this thermal insulation, the temperature rise rate of the liquid nitrogen in the tank is 2 mK/s. The tank has four ports: one each on the top for pressurization and ventilation, on the side wall and on the bottom tank head for return flow of recirculation, and on the bottom center for liquid feeding. These two return ports have no device in the tank such as diffuser, spray bar or jet mixer at the outlet to simply understand the physical process of the flow return to the tank. A baffle plate is installed on the feed port to prevent ullage gas from entering into the feedline at a low liquid level [26]. On the top plate of the tank, a double layered window made of polycarbonate is placed to visually observe the inside. The space between the layers is evacuated for thermal insulation. A capacitance type liquid level sensor and a total of 25 type-T thermocouples are installed inside the tank. The location of each thermocouple is given in Table 1 based on the coordinate system defined in Fig. 2. All the thermocouples, calibrated before the experiment, have the accuracy of ±0.1 K.

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Window

Thermocouples Level Sensor

Vacuum Port

Vent and Pressurization Port

Ø217

Window θ = 90 deg Tank Mount

θ = 0 deg 200 482

Ø600

Side Return Port

150

Baffle z Blind Port (Not in Use)

150

r

O Feed Port

Bottom Return Port Fig. 2 Drawing of cryogenic tank for recirculation experiment. Table 1 Temperature sensor location (the coordinate system is shown in Fig. 2) Sensor Tag T11 T12 T13 T14 T15 T21 T22 T23 T24 T25 T31 T32 T33 T34 T35 T41 T42 T43 T44 T45 T51 T52 T53 T54 T55

r, mm 240 240 240 240 240 120 120 120 120 120 0 0 0 0 0 120 120 120 120 120 240 240 240 240 240

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θ, deg 60 60 60 60 60 60 60 60 60 60 0 0 0 0 0 240 240 240 240 240 240 240 240 240 240

z, mm 328.7 262.4 196.1 129.8 63.5 319.2 243.4 167.6 91.8 16 320 245 170 95 20 319.2 243.4 167.6 91.8 16 328.7 262.4 196.1 129.8 63.5

2.3. Cryogenic Recirculation Facility Figure 3 shows the flow schematic of the cryogenic recirculation test facility in the side return configuration. The bottom return port, used for the bottom return test, is sealed in this configuration. The facility mainly consists of the recirculation pump and the tank. As shown in the figure, 5 pressure sensors (±0.3 % accuracy) and 5 temperature sensors (±0.1 K accuracy) are installed on the line, and each has a tag name, e.g. P01 and T01. The temperature sensors measure the flow temperature. The ultrasonic flow meter [27] (FM1), instead of a typical turbine flow meter, is used to measure the recirculation flow rate because such a turbine flow meter is difficult to use in this configuration where liquid-vapor two-phase flow may cause over rotation of the turbine. Two cryogenic void fraction meters, V01 and V02, are installed on the recirculation line [28]. The ultrasonic sensor provides the volume flow rate Q, and the void meter provides the void fraction ; thus, one can estimate two-phase flow mass flow rate W assuming dispersed and homogeneous flow using the following expression: 𝑊 = 𝑄{𝜌𝐺𝛼 + 𝜌𝐿(1 ― 𝛼)} Where 𝜌𝐺 is gaseous phase density and 𝜌𝐿 is liquid phase density. The accuracy of the mass flow rate is ±5 % in the case of liquid flow, while ±10 % in the case of two-phase flow. The drain valves, DV1 and DV2, are used to condition the temperature of the recirculation pump and the recirculation line. The main valve, MV, is on the recirculation line and used to start the recirculation. The bleed line from the pump is operated by the bleed valve, BV, and connected to the heat exchanger which vaporizes the liquid contained in the bleed flow. The vapor flows into the turbine flow meter with ±2 % accuracy and then is vented to the ambient. The tank is pressurized by helium gas through a regulator. The vent and relieve lines on the top of the tank are used for depressurization of the tank after the experiment.

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PV

P01 T01

He Bottle

HX

T

Temperature Sensor

V Void Fraction Meter Turbine Flow Meter

Tank

HX Heat Exchanger

MV P03 T03

Circulation Line

Pressure Sensor

Ultrasonic Flow Meter

Side Return Port V01

P

Hand Regulator

Bottom Return Port DV1

Valve

Liquid Nitrogen

DV2

P02 T02 Pump V02

Bleed Line

Dome Regulator

FM1

Relieve Check

P05 T05 P07 T07

BV

Insulated Line (3/8”)

FM2

HX

Fig. 3 Cryogenic recirculation test facility.

3. Results and Discussion 3.1. Cryogenic Recirculation Pump Performance The head performance of the recirculation pump has been evaluated using water and liquid nitrogen as a working fluid. Figure 4 indicates the head rise against the rotational speed at a volume flow rate of 100 cc/s. The head coefficient vs. the flow coefficient is depicted in Fig. 5. The head coefficient is calculated by H2/N, where H is head rise and N is rotational speed. The flow coefficient is calculated Q/N, where Q is volume flow rate. As seen in both graphs, almost the same head performance has been obtained in both experiments using water and liquid nitrogen.

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4 Flow Rate = 100 cc/s 3

Head Rise, m

2 1 0 Liquid Nitrogen -1

Water

-2 0

500

1000 1500 Rotational Speed, rpm

2000

2500

Fig. 4 Pump head rise against rotational speed using water and liquid nitrogen.

Head Coefficient (H/N2), m/rpm2 106

2 Water

1

Liquid Nitrogen 0 -1 -2 -3 -4 0

0.05 0.1 0.15 0.2 Flow Coefficient (Q/N), (cc/s)/rpm

0.25

Fig. 5 Pump head coefficient against flow coefficient using water and liquid nitrogen.

To evaluate the characteristic of the bleed holes on the impeller, gaseous nitrogen of 0.6 cc/s was injected into water at the upstream of the pump inlet in the water experiment at a rotational speed of 2000 rpm and a water flow rate of 100 cc/s [29]. The bleed line was kept opened to the ambient during the test. A photograph during the two-

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phase flow operation is shown in Fig. 6. The two-phase flow on the impeller was observed through the polycarbonate-made casing. The liquid phase and gaseous phase were segregated and the gaseous phase was stagnated inside of the impeller [30] as seen in the figure. The flow rate of the bled water from the bleed holes was 6 cc/s, that is, 6 % of the total water flow rate. The head rise was 2.2 m while it was 3.0 m without gaseous nitrogen injection as shown in Fig. 4. The head rise decreased to 0.9 m when the bleed line was closed. The advantage of the bleed holes was demonstrated under the two-phase flow inlet condition even though the head performance deteriorated and a small amount of water was lost to the ambient.

Front View

Liquid Close up

Gas

Side View Fig. 6 Pump operation at 2000 rpm and 100 cc/s with gaseous nitrogen injection.

3.2. Recirculation Test with Side Return The recirculation test was conducted in the cryogenic recirculation test facility with the recirculation pump and the cryogenic tank in two return configurations: the side return test using the side return port and the bottom return test using the bottom return port shown in Fig. 2. Figure 7 describes the time sequence and results of the recirculation test in the side return configuration. The names of the sensors in the graphs correspond with those in Fig. 3. Tsat (P03) is the saturation temperature with respect to the pressure at P03. Only Tsat (P03) is shown in the figure because P01 and P05 indicated almost the same pressure as P03. After filling the liquid nitrogen into the tank to a liquid level of 22 cm (z = 22 cm), gaseous

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helium was supplied to pressurize the tank to 0.3 MPaG. The side return port is located at 28.2 cm above the bottom, that means the return flow goes into the tank ullage, or gaseous phase. Then, DV1 valve (see Fig. 3) was opened to chill down the pump and the recirculation line. Through the chill-down operation, the two void fraction sensors indicated around 0 % (100 % liquid) and the pump inlet temperature (T05) of 81 K was achieved. The tank pressure decreased after the chill-down, thus, the tank was pressurized again to 0.3 MPaG at around 300 s. The recirculation characteristics after chill-down are discussed below in 6 phases from (I) to (VI) as shown in Fig. 7. Phase (I): With MV open and DV1 close, recirculation was started at a rotational speed of 500 rpm. The temperature just below the liquid surface (T23) increased just after the start of the recirculation, while the temperature at the tank bottom (T35) still indicated 2 mK/s rise rate; that is due to the external heat load to the tank as explained in subsection 2.2. The temperature behavior in the tank indicated that the return liquid affected the tank liquid only just near the surface due to low recirculation flow rate. The temperature around pump gradually increased because the flow rate gradually decreased. Phase (II): The pump inlet finally reached the saturation condition at 400 s, and the mass flow rate rapidly decreased. The void fractions indicated two-phase flow at both inlet and outlet of the pump. The external heat load along the recirculation line from the tank to the pump outlet is roughly estimated to be 1 kW from enthalpy rise of the recirculation flow. The degree of subcooling at the tank bottom was 10 K and the heat capacity of liquid nitrogen is 2 kJ/kg/K, hence, a mass flow rate of less than 50 g/s was saturated in the pump. Actually, the mass flow rate was around 50 g/s at 500 rpm under 100 % liquid condition. That was considered to be just the boundary between saturation and subcooling. Therefore, the vapor generation in the pump prevented the suction, then the flow rate suddenly decreased and the temperature increased. Phase (III): The bleed line valve (BV) was opened and the bleed flow rate started to increase with the pump total mass flow rate rising. The high pump flow rate cooled down the pump and feedline again, and the void fractions recovered to 100 % liquid condition. Phase (IV): Even after BV valve was closed and the rotational speed increased to 1000 rpm, there were no significant change in the recirculation flow and the tank temperature behavior. Phase (V): After the rotational speed was increased to 2000 rpm, the liquid surface temperature (T23) decreased while the rise rate of tank bottom temperature (T35) slightly increased. That indicated that the return flow at a higher flow rate slightly enhanced mixing of the tank contents.

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Phase (VI): The tank bottom temperature (T35) rapidly rose just after the rotational speed was increased to 3000 rpm. Then, T35 and the liquid surface temperature (T23) showed the same temperature at 740 s. Whole liquid in the tank mixed up due to the strong disturbance with the high return flow rate. The recirculation line temperatures increased because of the high supply temperature or tank bottom temperature (T35). Regarding the entire time evolution of the tank pressure (P01), it hardly depends on ullage temperature (see Appendix). Instead, it seems to have a relationship with the return flow temperature, T03. When T03 was greater than Tsat (P03), or superheated gas condition, the tank pressure kept rising. On the other hand, when T03 was less than Tsat (P03), or subcooled liquid condition, the tank pressure decreased. That indicated that the subcooled liquid injection to the tank ullage condensed ullage gas and decreased the tank pressure.

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0 95

200

300

Time,s 400

500

600

700

800

420 400 Pressurization Chill-down (VI) T02 (Pump 380 Tsat(P3) Outlet) 360 340 320 300 280 260 T05 (Pump 40 240 Inlet) P02 (Discharge 220 Pressure) 200 35 P01 (Tank 180 Pressure) 160 T54 (near Liquid T35 140 30 Surface in Tank) (Tank Bottom) 120 Rotational 100 V01 Speed 25 80 (Downstream Void Fraction) Liquid Level 60 Bleed Mass 40 Flow Rate V02 20 (Upstream Void Fraction) 0 -20 0 100 200 300 400 500 600 700 800 15 Time,Total s Pump Mass Flow 10 Rate

90

M V Open BV Open

Temperature, K

0.4 85

0.35

80 0.3

0.25 75 0.2

70 0.15 0.1 0.05

5

Temperature, K

0 95 0

100 200 T02 (Pump Outlet)

400 500 600 T03 (Recirculation Outlet) Time, s

300

400 Time, s

90

85

T05 (Pump Inlet)

80

75

70

V02 (Upstream Void Fraction) 0

100

200

0 800 420 400 Tsat(P03) 380 360 340 320 300 280 260 240 220 200 T23 (near Liquid Surface in Tank) 180 160 140 T35 (Tank Bottom) 120 100 80 V01 60 (Downstream Void Fraction) 40 20 0 -20 500 600 700 800

300

700

Fig. 7 Time sequence and results of recirculation test in side return test.

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Void Fraction, %

DV1 Open

Rotational Speed, rpm/100 Void Fraction, % cm Liquid Level, Bleed Mass Flow Rate, g/s

Line(IV) Outlet) (V) (I) T03 (II) (Circulation (III)

Phase

Pressure, MPaG Pump Total Mass Flow Rate, kg/s

100

The photographs inside the tank during the side return test are shown in Fig. 8. A liquid nitrogen column was observed flowing from the side return port. The liquid column landed on the liquid surface and disturbed the surface. Expectedly, the distance between the side port and the landing point on the surface became longer as the return flow velocity was rising. The magnitude of surface disturbance appeared to become greater as the rotational speed becomes higher, especially at 3000 rpm. This behavior corresponds to the tank temperature history in Fig. 7.

Liquid surface

Side return port

Return liquid

Bottom return port

Return liquid landing on surface (b) 345 s, 500 rpm

(a) Before recirculation

Return liquid

Return liquid

(c) 613 s, 1000 rpm

Return liquid landing on surface

(d) 730 s, 3000 rpm

Return liquid landing on surface

Fig. 8 Photographs in the tank in side return test.

3.3. Recirculation Test with Bottom Return Figure 9 depicts the time sequence and result of recirculation test in the bottom return configuration. The time sequence and initial conditioning before the recirculation were the same as those in the side return test. The result is discussed below based on Fig. 9 in the same manner as the side return test: Phase (I): The pump inlet flow unsteadily fluctuated between subcooled and two-phase conditions. During the bleed valve (BV) was open, the mass flow rate and the pump head increased, and the pump inlet flow remained in subcooled condition. Both the tank bottom temperature (T35) and the liquid temperature near the surface (T43) increased after the start of recirculation. The rise rate of T43 increases greatly, especially at high flow rate. Figure 10

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shows the photographs in the tank. As seen in Fig. 10 (b), less disturbance was observed in the vapor return case (downstream void fraction V01 = 100 %). However, in the two-phase return case (V01 < 100%), the return flow caused strong surface disturbance as shown in Fig. 10 (c). The tank pressure was affected by the downstream void fraction history rather than the ullage temperature (see Appendix) as in the side return case. In the vapor return case, the tank pressure increased even though the mass flow rate was small, while the tank pressure decreased in the twophase flow return case. In the vapor return case, the vapor bubbles rose from the bottom port through the liquid into the tank ullage, increasing the tank pressure. The time histories of the mass flow rate, the tank pressure (P01) rise rate and the liquid surface temperature (T43) rise rate are described in Fig. 11. There are apparent relationships among them: higher mass flow rate leads to mixing and temperature rise of tank liquid and the decrease in the tank pressure, while lower mass flow rate (vapor injection) causes the increase in the tank pressure but the temperature of tank liquid remains low due to less forced convection. Phase (II): At 1000 rpm, no head nor flow rate was obtained when BV valve was closed. After BV valve was opened, both inlet and outlet achieved a subcooled condition. Then, even when BV valve was closed, there is no change observed in the flow condition. The temperature rise rate of T43 was similar to that at high flow rates in the phase (I). The liquid surface disturbance by the return flow corresponded to this behavior: The similar disturbance was also observed in the two-phase return and liquid return cases as seen in Fig. 10 (c) and (d). The tank bottom temperature (T35) hardly rose in the side return configuration until the rotational speed reached 3000 rpm, while the temperature increased even at 500 rpm in the bottom return configuration.

15

0 95

200

Time,s 400

300

500

600

700

800

420 400 Pressurization Chill-down (II) T02 (Pump 380 Tsat(P3) Outlet) 360 340 320 300 280 260 T05 (Pump 35 240 Inlet) P02 (Discharge 220 Pressure) 200 30 180 160 T54 (near Liquid T35 140 25 Surface in Tank) P01 (Tank (Tank Bottom) 120 Pressure) 100 V01 20 80 (Downstream Void Fraction) Liquid Level 60 40 Bleed Mass V02 15 20 Flow Rate (Upstream Void Fraction) Rotational 0 Pump Total Speed -20 Mass 0 100 200 300 Flow Rate 400 500 600 700 80010 Time, s

90

M V Open BV Open

Temperature, K

0.35 85 0.3

80 0.25 0.2 75 0.15

70 0.1 0.05

5

0

Pump Pressure Rise

-0.05 100 0

T02 (Pump Outlet)

95

Temperature, K

100

200 300 T03 (Recirculation Outlet)

400 Time, s

500

600

700

Tsat(P03)

90 T05 (Pump Inlet)

85

80 V01 (Downstream Void Fraction)

75

T35 (Tank Bottom)

T43 (near Liquid Surface in Tank)

V02 (Upstream Void Fraction)

70

65 0

100

200

300

400 Time, s

500

600

700

Fig. 9 Time sequence and result of recirculation test in bottom return test.

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0 -5 800 420 400 380 360 340 320 300 280 260 240 220 200 180 160 140 120 100 80 60 40 20 0 -20 800

Void Fraction, %

DV1 Open

Rotational Speed, rpm/100 Fraction, Void Rise, kPaD% Pump Pressure Liquid Level, cm Bleed Mass Flow Rate, g/s

T03 (I) (Circulation Line Outlet)

Phase

Pressure, MPaG Pump Total Mass Flow Rate, kg/s

100

Babble from return port Bottom return port

(a) Before recirculation

(b) 360 s, vapor return, less disturbance

(c) 440 s, two-phase return, strong disturbance

(d) 700 s, liquid return , strong disturbance

Fig. 10 Photographs inside the tank in bottom return test.

0.2

0.001 Tank Pressure (P01) Rise Rate

0.18

Mass Flow Rate, g/s Temperature Rise Rate, K/s

0.14

0

0.12 0.1

Tank Temperature (T43) Rise Rate

0.08

-0.0005 Mass Flow Rate -0.001

0.06 0.04

Pressure Rise Rate, MPa/s

0.0005

0.16

-0.0015

0.02 0

-0.002 250

350

450

550

650

750

Time, s

Fig. 11 History of mass flow rate, tank temperature (T43) rise rate and tank pressure (P01) rise rate during bottom return test.

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3.4. Two-phase Flow Suction Test To evaluate the pump suction performance in a saturation or two-phase flow condition at the pump inlet, a twophase flow suction test was conducted using the same configuration as the bottom return test. In this test, however, the pump and the recirculation line were intentionally chill downed less than in the side and bottom return tests in order to achieve the two-phase flow suction of the pump. The result is shown in Fig. 12 and discussed in the same manner as the other tests. Phase (I): Due to less chill-down, the temperature was higher than that in other tests, and the upstream void fraction showed two-phase flow and the downstream void fraction indicated superheated gas. Phase (II): First, with the bleed valve (BV) close, no pump head nor flow rate was obtained even at a rotational speed of 3000 rpm. Phase (III): Opening BV valve increased the flow rate, then the recirculation starts with the two-phase flow at pump inlet. After 120 sec, the pump outlet temperature (T02) reached a saturation condition, but the recirculation outlet temperature (T03) still showed gaseous phase because of less chill-down. The pump pressure rise and the flow rate unsteadily oscillated during this phase. Figure 13 indicates the side view inside the impeller during this oscillation. At the low flow rate in the oscillation, low pressure rise and high void fraction at inlet were observed, and the impeller had vapor as seen in Fig. 13 (a). On the other hand, as indicated in Fig. 13 (b), no vapor was observed at high flow rate, high pump pressure rise and low void fraction. The flow around the impeller repeated the states between Fig. 13 (a) and (b) during the oscillation. The separation of vapor and liquid in the impeller as seen in the water test (Fig. 6) was not able to be confirmed from these photographs due to the limitation of observation method. However, the impeller provided pressure head even with vapor mixture at the inlet. The time average of the total mass flow rate of the pump during this oscillation period is approximately 50 g/s while the bleed flow rate is 5 g/s; that is, 10 % of the total flow was bled through the bleed line. Phase (IV): As BV valve was opened more, the bleed flow rate increased, and the inlet flow reached a subcooled condition. However, the downstream void fraction still showed superheated gas due to the less chill-down. Because of this, the mass flow rate was limited at 60 to 70 g/s, which is lower than that at 3000 rpm (150 g/s) in other tests with enough chill-down. To keep working, the pump required higher bleed mass flow rate of 10 to 20 g/s due to less degree of subcooling (4 K) at the tank bottom. Phase (V): When BV valve was closed, no pump head nor flow rate was obtained due to the less chill-down.

18

200

Time,s 400

300

500

600

700

800

95 (I) T02(II) (Pump

Phase

T03 (Circulation (IV) Line Outlet)

(III)

(V)

Tsat(P3)

Outlet)

DV1 Open

420 400 380 360 340 320 300 280 260 35 240 220 200 30 180 160 140 25 120 100 80 60 20 40 20 0 15 -20

90

M V Open BV Open

T05 (Pump Inlet)

Temperature, K

0.35 85

Rotational Speed

Pressure Rise, MPaD Pump Total Mass Flow Rate, kg/s

0.3

T54 (near Liquid Surface in Tank)

80

0.25

T35 (Tank Bottom)

Bleed Mass V01 Flow Rate (Downstream Void Fraction)

75 0.2 V02 Fraction) (Upstream VoidPump Total Mass Flow Rate

0.15 70

0

100

200

300

400 Time, s

500

600

700

0.1

10

0.05

0 100 0

5

Pump Pressure Rise 50

95

Temperature, K

800

100

150 200 T03 (Recirculation T02 (Pump Outlet) Time, s Outlet) Tsat(P02)

250

300

Tsat(P03) Tsat(P05)

90 T35 (Tank Bottom)

85 T23 (near Liquid Surface in Tank)

80

T05 (Pump Inlet)

V01 (Downstream Void Fraction) 75 V02 (Upstream Void Fraction) 70 0

50

100

150

200

250

300

Time, s

Fig. 12 Time sequence and result of two-phase flow suction test.

19

Rotational Speed, rpm/100 oid Fraction, Rate, g/s % Flow Bleed Mass V

100

0 350 420 400 380 360 340 320 300 280 260 240 220 200 180 160 140 120 100 80 60 40 20 0 -20 350

Void Fraction, %

0

Filled with liquid

Liquid contains vapor Vapor (a) 105 s

(b) 109 s

Flow

Fig. 13 Side view of impeller in two-phase flow suction test.

4. Conclusion To understand of tank-to-tank recirculation in cryogenic orbital propulsion systems, a cryogenic recirculation test has been conducted using an electrical driven recirculation pump. The pump head performance was similar in both water and liquid nitrogen operations. The recirculation pump has bleed holes on the impeller designed to bleed as much vapor as possible under the two-phase flow operation. The gas-liquid separation has been visually observed using gaseous nitrogen and liquid water. Two kinds of recirculation test, the side return and bottom return tests, were carried out using liquid nitrogen as a working fluid. These tests were aimed to measure not only the pressure and temperature of the flow, but also the two-phase mass flow rate using void fraction meters and an ultrasonic sensor. In addition, to visually observe the return flow in the tank, a double layered window was installed on the top of the tank. In the side return test, the return flow went back to the tank ullage, disturbing the liquid in the tank and mixing the liquid only near the surface at a lower flow rate (less than 100 g/s) or a lower pump rotational speed (less than 1000 rpm). However, at a flow rate of 150 g/s or a pump rotational speed of 3000 rpm, strong disturbance of the liquid in the tank was observed, showing that almost all the liquid was mixed and became the same temperature. The tank pressure depended on the phases of return flow: it increased with superheated vapor but decreased with subcooled liquid due to condensation of ullage gas. In the bottom return test where the return flow went back to the liquid in the tank, the liquid remained undisturbed in vapor return case. On the other hand, the liquid was easily mixed even at a lower flow rate of 50 g/s or a rotational speed of 500 rpm in two-phase or liquid return case. The tank pressure behavior was also changed by return flow characteristics: in vapor return case, the tank pressure increased due to vapor rising to the ullage despite

20

of lower return mass flow rate while the tank pressure decreased in two-phase or liquid return cases. In conclusion, the pressure and temperature behaviors in the tank were strongly affected by return mass flow rate, phase of the flow, and return positions in the tank, that means one can control temperature distribution and tank pressure by optimizing recirculation flow to achieve required net positive suction head of the engine turbopumps. It has been demonstrated that the bleed holes on the impeller have the advantage of managing two-phase flow inlet condition: the bleed line simply increased the mass flow rate to the pump by venting much more vapor than liquid and helped the pump inlet to recover to subcooled condition. Under the two-phase flow condition at the inlet, the bleed holes vented vapor rather than liquid and kept the impeller working although the system suffered decreased pump head, loss of liquid and unsteady oscillation of pump head and flow rate. The bleed hole can provide a wide range of choices of propellant utilization in actual applications.

21

Appendix. All temperature history inside tank Side Return Test

210 T11

T21

T31

T41

T11

T21

T31

T41

T51

210

190

190

Temperature, K

170

Temperature, K

Bottom Return Test

230 T51

150 130 110

170 150 130 110

90

90

70 160 0 150

100 T12

200 300 400 500 Time,s T42 T22 T32

600 700 T52

70 190 0

800

100 T12

200 300 400 500 T22 T32 Time,s T42

600 700 T52

800

100 T13

200 300 400 500 Time,s T43 T23 T33

600 700 T53

800

100

200

400 500 Time,s T44 T34

600

700

800

400 500 Time,s T45 T35

600

700

800

700

800

170

130

Temperature, K

Temperature, K

140

120 110 100

150 130 110

90

90

80 70 110 0

70 100 T13

200 300 400 500 Time,s T43 T23 T33

600 700 T53

95 0

800

105 90

Temperature, K

Temperature, K

100 95 90 85

85

80

80 75

75

95 0

100

T14

200

300

T24

400 500 Time,s T44 T34

600

700

95 0

800

T14

T54

Temperature, K

Temperature, K

T54

90

90

85

85

80

80

75

75 95 0

100 T15

200

300

T25

400 500 Time,s T45 T35

600

700

95 0

800

100 T15

T55

200

300

T25

T55

90

Temperature, K

90

Temperature, K

300

T24

85

85

80

80

75

75 0

100

200

300

400 Time,s

500

600

700

800

0

100

200

300

400 Time,s

Fig. A1 All temperature history inside tank.

22

500

600

Acknowledgments The authors are grateful to Mitsuo Watanabe, Hiroki Kannan and Yohei Ogawa of JAXA for their assistance on the experiment. The authors also acknowledge the technical support of Dynax Co. Ltd.

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Highlights  Tank-to-tank loop recirculation experiment with liquid nitrogen  Void fraction and ultrasonic flow meters are applied to know two-phase flow characteristics  Recirculation pump has vapor bleed holes on impeller to work under two-phase flow condition  Flow rate, phase of return flow and return position to tank affect tank pressure and temperature  Visual observation inside tank reveals surface disturbance corresponds to liquid temperature

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