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Viewing 1 to 30 of 36
2004-07-19
Technical Paper
2004-01-2504
Jentung Ku, Laura Ottenstein, Charles D. Butler, Theodore Swanson, Diane Thies
The Terra spacecraft is the flagship of NASA’s Earth Science Enterprise. It provides global data on the atmosphere, land, and oceans, as well as their interactions with solar radiation and one another. Three Terra instruments utilize Capillary Pumped Heat Transport Systems (CPHTS) for temperature control. Each CPHTS, consisting of two capillary pumped loops (CPLs) and several heat pipes and electrical heaters, is designed for instrument heat loads ranging from 25W to 264W. The working fluid is ammonia. Since the launch of the Terra spacecraft in December 1999, each CPHTS has been providing a stable interface temperature specified by the instrument under all modes of spacecraft and instrument operations. The ability to change the CPHTS operating temperature upon demand while in service has also extended the useful life of one instrument. This paper describes the design and on-orbit performance of the CPHTS thermal systems.
2005-07-11
Technical Paper
2005-01-2881
Jentung Ku
This paper describes the heat load sharing function among multiple parallel evaporators in a capillary pumped loop (CPL). In the normal mode of operation, the evaporators cool the instruments by absorbing the waste heat. When an instrument is turned off, the attached evaporator can keep it warm by receiving heat from other evaporators serving the operating instruments. This is referred to as heat load sharing. A theoretical basis of heat load sharing is given first. The fact that the wicks in the powered evaporators will develop capillary pressure to force the generated vapor to flow to cold locations where the pressure is lower leads to the conclusion that heat load sharing is an inherent function of a CPL with multiple evaporators. Heat load sharing has been verified with many CPLs in ground tests. Experimental results of the Capillary Pumped Loop 3 (CAPL 3) Flight Experiment are presented in this paper. Factors that affect the amount of heat being shared are discussed.
2007-07-09
Technical Paper
2007-01-3194
Triem T. Hoang, Jentung Ku
Passive cooling transport in the cryogenic temperature regime still remains a challenging task since problems regarding parasitic heat gains from the surrounding have not been resolved satisfactorily. A recently-introduced concept of Advanced Loop Heat Pipe (or ALHP) had demonstrated an ability to manage “excessive” vapor generation in the compensation chamber. Nitrogen and Neon were successfully utilized as the working fluids to provide cryocooling transports in the temperature range of 80-120K and 30-40K, respectively. A Hydrogen ALHP in 2004 became the first capillary-pumped system to operate in the 20-30K range. This paper will present the ALHP technology in general and the detailed description of the research program/test results in particular.
2007-07-09
Technical Paper
2007-01-3235
Triem T. Hoang, Jentung Ku
Loop Heat Pipes have proven as reliable heat transports for spacecraft thermal control systems. NASA Goddard Space Flight Center in collaboration with NASA Jet Propulsion Laboratory recently proposed a miniature dual pump/condenser LHP system for use in future Mars missions. Results of a ground test program indicated that the dual pump/condenser LHP performed very well, but in a complicated manner. No analytical model was available to facilitate the design/analysis of this emerging technology. A generalized LHP theory will be presented in this paper along with the derived governing equations and solution scheme. Model predictions were made and compared with test data for validation.
2007-07-09
Technical Paper
2007-01-3234
Jentung Ku, Hosei Nagano
This paper describes an experimental study on effects of gravity on the start-up and heat load sharing of a miniature loop heat pipe (MLHP) with two evaporators and two condensers. Each evaporator has an outer diameter of 9 mm and has its own integral compensation chamber (CC). For this experimental study, the MLHP was placed under five different configurations where the relative elevation and tilt among loop components were varied. The four well-known initial conditions between the evaporator and CC prior to the LHP start-up were created through combinations of: 1) the test configuration; 2) the method of pre-conditioning the loop prior to start-up, and 3) the heat load distribution between the evaporators. Effects of gravity on start-up transients and heat load sharing were clearly seen under otherwise the same heat load distribution and sink temperatures.
1989-07-01
Technical Paper
891566
Jentung ku, Edward J. Krollczek, Michael E. McCabe
The High-Power Spacecraft Thermal Management (HPSTM) system was modified and upgraded to facilitate improved performance testing. Modifications to the system included augmenting the heat dissipation capacity of the condenser sink for steady-state high power operation, adding more pressure transducers to monitor pressure drops in various components of the system, installing pressure contact thermocouples on the evaporators to measure the heating surface temperature, providing a coolant loop to one of the evaporator plates for heat load sharing operation, installing a load cell on the reservoir to monitor transient fluid flows, and re-orienting the reservoir to reduce the effects of compressed vapor during transient operations. The system demonstrated a steady, continuous operation at a power input of 20 kW for 10 hours in the capillary mode. Test results also showed about 33% less variation of the reservoir set point temperature during power transients.
1995-07-01
Technical Paper
951567
Jentung Ku, Tu Nguyen, Seokgeun Tun
The start-up of a capillary pumped loop under a fully flooded condition is often difficult. The problem is due to the presence of a high superheat at the onset of nucleate boiling, and the associated large pressure spike that results in possible vapor penetration through the wick. To overcome the start-up difficulty, a capillary starter pump has been developed. The starter pump is similar to the traditional evaporator pump except for a bayonet tube that is connected directly from the reservoir to the inside of the pump. Such an evaporator design is selfpriming since all displaced liquid must pass through the pump. The starter pump can replace the traditional capillary pump to provide capillary pumping for heat transport. It can further be integrated into a cold plate to receive heat from distributed sources such as spacecraft instruments. Isothermalization of the cold plate can be accomplished by incorporating heat pipes into the cold plate.
1995-07-01
Technical Paper
951566
Dan Butler, Laura Ottenstein, Jentung Ku
The Capillary Pumped Loop Flight Experiment (CAPL) employs a passive two-phase thermal control system that uses the latent heat of vaporization of ammonia to transfer heat over long distances. CAPL was designed as a prototype of the Earth Observing System (EOS) instrument thermal control systems. The purpose of the mission was to provide validation of the system performance in micro-gravity, prior to implementation on EOS. CAPL was flown on STS-60 in February, 1994, with some unexpected results related to gravitational effects on two-phase systems. Flight test results and post flight investigations will be addressed, along with a brief description of the experiment design.
1995-07-01
Technical Paper
951554
Tu M. Nguyen, Patrick J. Brennan, Jentung Ku, Matthew T. Buchko
An IBM Personal Computer (PC) version of the Groove Analysis Program (GAP) was developed to predict the steady state heat transport capability of axially grooved heat pipes for a specified groove geometry and working fluid. In the model, the heat transport capability of an axially grooved heat pipe, usually governed by the capillary limit, is determined by the numerical solution of the governing equation for momentum conservation with the appropriate boundary conditions. This paper discusses the theory behind the development of the GAP model. It also presents many useful capabilities of the model. Furthermore, correlations of flight test performance data using GAP are presented and discussed.
1994-06-01
Technical Paper
941599
Laura Ottenstein, Jentung Ku, Dan Butler
The Capillary Pumped Loop Flight Experiment (CAPL) is a prototype of the Earth Observing System (EOS) instrument thermal control systems, which are based on two-phase heat transfer technology. The CAPL experiment has been functionally tested in a thermal vacuum chamber at NASA's Goddard Space Flight Center (GSFC). The tests performed included start-up tests, simulated EOS instrument power profiles, low and high power profiles, a variety of uneven coldplate heating tests, subcooling requirement tests, an induced deprime test, reprimes, saturation temperature changes, and a hybrid (mechanical pump-assist) test. There were a few unexpected evaporator deprimes, but overall the testing was successful. The results of all of the tests are discussed, with emphasis on the deprimes and suspected causes.
2003-07-07
Technical Paper
2003-01-2386
Jentung Ku, Jose I. Rodriguez
This paper presents a theory that explains the low frequency, high amplitude temperature oscillations in loop heat pipe (LHP) operation. Temperature oscillations with amplitudes on the order of tens of Kelvin and periods on the order of hours have been observed in some LHPs during ambient testing. There is presently no satisfactory explanation for such a phenomenon in the literature. It is well-known that the operating temperature of an LHP with a single evaporator is governed by the compensation chamber (CC) temperature, which in turn is a function of the evaporator power, condenser sink temperature, and ambient temperature. As the operating condition changes, the CC temperature will change during the transient but eventually reach a new steady state. Under certain conditions, however, the CC temperature never reaches a true steady state, but instead displays an oscillatory behavior.
2003-07-07
Technical Paper
2003-01-2387
Jentung Ku
The operating temperature of a loop heat pipe (LHP) with a single evaporator is governed by the compensation chamber (CC) temperature, which in turn is a function of the evaporator power, condenser sink temperature, and ambient temperature. As the operating condition changes, the CC temperature will also change during the transient but eventually reach a new steady temperature. Under certain conditions, however, the LHP never really reaches a true steady state, but instead displays an oscillatory behavior. This paper presents a study on the oscillation of the loop operating temperature with amplitudes on the order of 1 Kelvin and periods on the order seconds to minutes. The source of the high frequency temperature oscillation is the fast movement of the vapor front in the condenser section, which usually occurs when the vapor front is near the condenser inlet or the condenser outlet.
2001-07-09
Technical Paper
2001-01-2189
Jentung Ku, Gajanana C. Birur
This paper presents a comprehensive experimental study of the loop operating temperature in a loop heat pipe (LHP) which has two parallel evaporators and two parallel condensers. In a single evaporator LHP, it is well known that the loop operating temperature is a function of the heat load, the sink temperature and the ambient temperature. The present study focuses on the stability of the loop operating temperature and parameters that affects the loop operation. Tests results show that the loop operating temperature is a function of the total system heat load, sink temperature, ambient temperature, and heat load distribution between the two evaporators. Under most conditions, only one compensation chamber (CC) contains two-phase fluid and controls the loop operating temperature, and the other CC is completely filled with liquid. Moreover, as the test condition changes, control of the loop operating temperature often shifted from one CC to another.
2001-07-09
Technical Paper
2001-01-2188
Jentung Ku, Gajanana C. Birur
This paper describes a test program on active control of the operating temperature in a loop heat pipe (LHP) with two evaporators and two condensers. Test results shoe that when the CCs were not actively controlled, the loop operating temperature was a function of the total heat load, heat load distribution among evaporators, condenser temperature and ambient temperature. Because of the many variables involved, the operating temperature also showed more hystereses than an LHP with a single evaporator. Tight operating temperature control can be achieved by controlling the compensation chambers (CCs) at a desired set point temperature. Temperature control was achieved by maintaining one or both CCs at the desired set point through cold biasing and external heating. Tests performed included start-up, power cycle, sink temperature cycle, CC temperature cycle, and capillary limit.
2001-07-09
Technical Paper
2001-01-2192
Jentung Ku, Laura Ottenstein, Paul Rogers, Kwok Cheung
This paper presents test results of an experimental study of low power operation in a loop heat pipe. The main objective was to demonstrate how changes in the vapor void fraction inside the evaporator core would affect the loop behavior. The fluid inventory and the relative tilt between the evaporator and the compensation chamber were varied so as to create different vapor void fractions in the evaporator core. The effect on the loop start-up, operating temperature, and capillary limit was investigated. Test results indicate that the vapor void fraction inside the evaporator core is the single most important factor in determining the loop operation at low powers.
2002-07-15
Technical Paper
2002-01-2502
Jentung Ku, Laura Ottenstein, Paul Rogers, Kwok Cheung
This paper describes a study on the capillary limit of a loop heat pipe (LHP) at low powers. The slow thermal response of the loop at low powers makes it possible to observe interactions among various components after the capillary limit is exceeded. The capillary limit at low powers is achieved by imposing an additional pressure drop on the vapor line through the use of a metering valve. A differential pressure transducer is also used to measure the pressure drop across the evaporator and the compensation chamber (CC). Test results show that when the capillary limit is exceeded, vapor will penetrate the primary wick, resulting in an increase of the CC temperature. Because the evaporator can tolerate vapor bubbles, the LHP will continue to function and may reach a new steady state at a higher operating temperature. Thus, the LHP will exhibit a graceful degradation in performance rather than a complete failure.
2002-07-15
Technical Paper
2002-01-2503
Jentung Ku, Gajanana Birur
This paper describes a study on the capillary limit of a loop heat pipe (LHP) with two evaporators and two condensers. Both theoretical analysis and experimental investigation are performed. Experimental tests conducted include heat load to one evaporator only, even heat loads to both evaporators, and uneven heat loads to both evaporators. Test results show that after the capillary limit is exceeded, vapor will penetrate through the wick of the weaker evaporator, and the compensation chamber (CC) of that evaporator will control the loop operating temperature regardless of which CC has been in control prior to the event. Because the evaporator can tolerate vapor bubbles, the loop can continue to work after vapor penetration. As the loop operating temperature increases, the system pressure drop actually decreases due to a decrease in liquid and vapor viscosities. Thus, the loop may reach a new steady state at a higher operating temperature after vapor penetration.
1997-07-01
Technical Paper
972328
Jentung Ku, Triem Hoang
The CAPL 2 Flight Experiment, flown on Space Shuttle STS-69 in 1995, was a flight demonstration of a full-scale prototype of a thermal control system planned for the Earth Observing System (EOS-AM) instruments Flight tests successfully demonstrated various CPL operations with simulated EOS-AM power profiles, including baseline and backup start-up procedures. In general, there were no significant differences in CPL performance between one-G and zero-G. However, some unusual behaviors were observed in several start-ups during the flight test. This paper describes CAPL 2 start-ups in detail, and offers explanations for the notably different zero-G behaviors.
1997-07-01
Technical Paper
972329
Jentung Ku, Triem Hoang
A capillary pumped loop (CPL) with a single starter pump in its evaporator section has been demonstrated to have very reliable start-ups and robust operation. In order to service payloads with large thermal footprints or to service multiple payloads, a CPL with multiple starter pumps seems a logical approach. However, questions were raised concerning its reliability for successful start-ups. In order to verify the feasibility of such a concept, a test program was conducted at NASA Goddard Space Flight Center, using four starter pumps plumbed in parallel. The main purpose of this experimental investigation was to verify the system's ability to provide a successful start-up and to retain performance characteristics demonstrated by a CPL with multiple evaporators of the traditional two-port pump design. Tests were conducted progressively by installing one, two and four pumps in the test loop.
1988-07-01
Technical Paper
881083
Jentung Ku, Edward J. Kroliczek
This paper describes the operational principles of a hybrid capillary pumped loop in general, and results on testing of a high power hybrid system in particular. A hybrid capillary pumped loop is a thermal control system which consists of a capillary pumped loop and a mechanical pump which is placed in series with the capillary evaporators in the liquid return line. The hybrid loop can be operated in either a passive capillary mode, or in a pump-assisted mode, whereby the mechanical pump augments the heat transport capability of the capillary evaporators. The high power hybrid system was built to demonstrate the feasibility of such a hybrid loop concept. Test results verified that a hybrid loop could be operated in either mode, and that transition between these two modes of operation required opening or closing a single valve on the liquid line.
1996-07-01
Technical Paper
961435
Triem Hoang, Jentung Ku
The the past, the design of a Capillary Pumped Loop involved mainly on the thermodynamics and heat transfer aspects of the system. The fluid flow dynamics of the working fluid were deemed benign to the system performance. Recently theoretical and experimental studies have revealed several mechanisms that led to the deprime of the capillary pumps. These mechanisms were all related to the dynamics of the fluid movement inside the loop.
1996-07-01
Technical Paper
961432
Jentung Ku, Laura Ottenstein, Dan Butler
This paper describes flight test results of the CAPL 2 Flight Experiment, which is a full scale prototype of a capillary pumped loop (CPL) heat transport system to be used for thermal control of the Earth Observing System (EOS-AM) instruments. One unique feature of CAPL 2 is its capillary starter pump cold plate design, which consists of a single capillary starter pump and two heat pipes. The starter pump enhances start-up success due to its self-priming capability, and provides the necessary capillary pumping force for the entire loop. The heat pipes provide the required isothermalization of the cold plate. Flight tests included those pertinent to specific EOS applications and those intended for verifying generic CPL operating characteristics and performance limits. Experimental results confirmed that the starter pump was indeed self-priming and the loop could be successfully started every time.
1996-07-01
Technical Paper
961431
Dan Butler, Laura Ottenstein, Jentung Ku
The Capillary Pumped Loop Flight Experiment (CAPL 2) employs a passive two-phase thermal control system that uses the latent heat of vaporization of ammonia to transfer heat over long distances. CAPL was designed as a prototype of the Earth Observing System (EOS) instrument thermal control systems. The purpose of the mission was to provide validation of the system performance in microgravity, prior to implementation on EOS. CAPL 1 was flown on STS-60 in February, 1994, with some unexpected results related to gravitational effects on two-phase systems. Start-up difficulties on CAPL 1 led to a redesign of the experiment (CAPL 2) and a reflight on STS-69 in September of 1995. The CAPL 2 flight was extremely successful and the new “starter pump” design is now baselined for the EOS application. This paper emphasizes the design history, the CAPL 2 design, and lessons learned from the CAPL program.
2004-07-19
Technical Paper
2004-01-2505
Jentung Ku, Seong-II Jeong, Dan Butler
This paper describes the design and testing of a miniature loop heat pipe (LHP) having a 7 mm outer diameter (O.D.) evaporator with an integral compensation chamber (CC). The vapor line and liquid line are made of 1.59mm O.D. stainless steel tubing. A thermoelectric (TEC) is installed on the CC and the hot side of the TEC is connected to the evaporator through a copper strap. By changing the direction of the electric current provided by a bi-polar power supply, the TEC can heat or cool the CC. Tests performed in the laboratory included start-up, power cycle, sink temperature cycle, and CC temperature control with the test article being placed in horizontal and vertical positions. The LHP demonstrated very robust operation in all tests where the heat load varied between 0.5W and 140W, and the sink temperature varied between 243K and 293K. The heat leak from the evaporator to the CC was extremely small.
2004-07-19
Technical Paper
2004-01-2506
Paul Rogers, Jeffrey Perez, Jentung Ku, Mark Kobel, Jay Ochterbeck, David Nelson
A Loop Heat Pipe (LHP) is a passive two-phase heat transfer device developed and successfully employed to cool spacecraft (satellite) electronics. The intrinsic benefits of this technology (lightweight, small volume, high thermal conductance) make it an attractive potential solution to many problems in ground vehicle thermal management. As most published LHP research has focused on cooling orbiting spacecraft components, there is little knowledge of how LHPs perform under the temperature extremes (−40°C to 40°C) and diurnal/seasonal fluctuations anticipated with terrestrial applications. Ambient temperature extremes mandate consideration of transport line heat exchange with the surroundings (parasitic losses/gains). This paper presents results from an experimental investigation of liquid line return temperature impact on system performance for sink temperatures from −30°C to 40°C and evaporator loads up to 700 Watts.
2008-06-29
Technical Paper
2008-01-1998
Jentung Ku
The operating temperature of a loop heat pipe (LHP) is governed by the saturation temperature of its compensation chamber (CC); the latter is in turn determined by the balance among the heat leak from the evaporator to the CC, the amount of subcooling carried by the liquid returning to the CC, and the heat exchanged between the CC and ambient. Thus, the operating temperature of an LHP is a function of the evaporator heat input and the condenser sink temperature. The LHP operating temperature can be controlled at a desired set point by actively controlling the CC temperature. Several methods have been developed to control the CC temperature, including direct heating of the CC, coupling block, heat exchanger and separate subcooler, variable conductance heat pipe, vapor by-pass valve, secondary evaporator, and thermoelectric converter. The paper discusses the operating principles, advantages and disadvantages of each method.
1993-07-01
Technical Paper
932235
Laura Ottenstein, Jentung Ku, Dan Butler
The Capillary Pumped Loop Flight Experiment (CAPL) is a prototype of the Earth Observing System (EOS) instrument thermal control systems. Four CAPL flight hardware components were tested in the Instrument Thermal Test Bed at NASA's Goddard Space Flight Center. The components tested were the capillary cold plates, capillary starter pump, heat pipe heat exchangers (HPHXs), and reservoir. The testing verified that all components meet or exceed their individual performance specifications. Consequently, the components have been integrated into the CAPL experiment which will be flown on the Space Shuttle in late 1993.
1993-07-01
Technical Paper
932236
Jentung Ku, Theodore D. Swanson, Keith Herold, Kim Kolos
A Capillary Pumped Loop (CPL) is an advanced two-phase heat transport device which utilizes capillary forces developed within porous wicks to move a working fluid. The advantage this system has over conventional thermal management systems is its ability to transfer large heat loads over long distances at a controlled temperature. Extensive ground testing and two flight experiments have been performed over the past decade which have demonstrated the potential of the CPL as a reliable and versatile thermal control system for space applications. While the performance of CPL's as “black boxes” is now well understood, the internal thermo-fluid dynamics in a CPL are poorly known due to the difficulty of taking internal measurements. In order to visualize transient thermohydraulic processes occurring inside an evaporator, a see-through capillary evaporator was built and tested at NASA's Goddard Space Flight Center.
1999-08-02
Technical Paper
1999-01-2627
Jentung Ku, Mark Kobel, David Bugby, Edward Kroliczek, Jane Baumann, Brent Cullimore
This paper describes the flight test results of the fifth generation cryogenic capillary pumped loop (CCPL-5) which flew on the Space Shuttle STS-95 in October of 1998 as part of the CRYOTSU Flight Experiment. This flight was the first in-space demonstration of the CCPL, a lightweight heat transport and thermal switching device for future integrated cryogenic bus systems. The CCPL-5 utilized nitrogen as the working fluid and operated between 75K and 110K. Flight results indicated excellent performance of the CCPL-5 in a micro-gravity environment. The CCPL could start from a supercritical condition in all tests, and the reservoir set point temperature controlled the loop operating temperature regardless of changes in the heat load and/or the sink temperature. In addition, the loop demonstrated successful operation with heat loads ranging from 0.5W to 3W, as well as with parasitic heat loads alone.
2001-07-09
Technical Paper
2001-01-2190
Jentung Ku, Gajanana C. Birur
Most existing loop heat pipes (LHPs) consist of one single evaporator and one single condenser. LHPs with multiple evaporators are very desirable for cooling multiple heat sources or a heat source with large thermal footprints. Extending the current LHP technology to include multiple evaporators and multiple condensers faces some challenges, including operating temperature stability, adaptability of loop operation to rapid power and sink temperature transients, and sizing of the compensation chambers (CCs). This paper describes an overview of an extensive testing program for an LHP with two evaporators and two condensers. Tests performed include start-up, power cycle, sink temperature cycle, CC temperature cycle, and capillary limit. Test results showed that the loop could be started successfully in most cases, and the operating temperature was a function of the total heat load, heat load distribution between the two evaporators, condenser sink temperature and ambient temperature.
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