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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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

Loop Heat Pipes (LHPs) are being considered for cooling of military combat vehicles and spinning spacecraft. In these applications, it is important to understand the effect of an accelerating force on the performance of LHPs. In order to investigate such an effect, a miniature LHP was installed on a spin table and subjected to variable accelerating forces by spinning the table at different angular speeds. Several patterns of accelerating forces were applied, i.e. continuous spin at different speeds and periodic spin at different speeds and frequencies. The resulting centrifugal accelerations ranged from 1.2 g's to 4.8 g's. This paper presents the second part of the experimental study, i.e. the effect of an accelerating force on the LHP operating temperature. It has been known that the LHP operating temperature under a stationary condition is a function of the evaporator power and the condenser sink temperature when the compensation temperature is not actively controlled.

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.

The loop heat pipe (LHP) was invented in Russia in the early 1980’s. It is a two-phase heat transfer device that utilizes the evaporation and condensation of a working fluid to transfer heat, and the capillary forces developed in fine porous wicks to circulate the fluid. The LHP is known for its high pumping capability and robust operation because it uses fine-pored metal wicks and the integral evaporator/hydro-accumulator design. It has gained rapid acceptance in recent years as a thermal control device in space applications. This paper presents an overview of the LHP operation. The physical processes and the thermal-hydraulic behaviors of the LHP are first described. Operating characteristics as functions of various parameters including the heat load, sink temperature, ambient temperature, and elevation are presented. Peculiar behaviors in LHP operation such as temperature hysteresis and temperature overshoot during start-up are explained.

The success of CAPL 2 flight experiment has stirred many interests in using capillary pumped loop (CPL) devices for spacecraft thermal control. With only one evaporator in the loop, CAPL 2 was considered a point design for the Earth Observing System (EOS-AM). To realize the full benefits of CPLs, a reliable system with multiple evaporators must be developed and successfully demonstrated in space. The Capillary Pumped Loop (CAPL 3) Flight Experiment was designed to flight demonstrate a multiple evaporator CPL in a space environment. New hardware and concepts were developed for CAPL 3 to enable reliable start-up, constant conductance operation, and heat load sharing. A rigorous ground test program was developed and extensive characterization tests were conducted. All performance requirements were met, and the loop demonstrated very reliable operation.

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.

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.

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.

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.

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.

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.