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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.

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.

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.

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 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.

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.

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.

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 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 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.

A parametric study of performance characteristics of a Loop Heat Pipe (LHP) is presented. A mathematical model, based on the steady-state energy conservation equations, is used. The calculations are performed by varying the operation conditions (heat load, sink and ambient temperatures, and elevation) and the LHP design parameters (working fluid, transport length size, external thermal conductance of the condenser and wick properties). The results are illustrated on LHP performance curves (saturation temperature as a function of applied power). All the results are compared with a baseline configuration to analyze the effects of different parameters. Operating limits due to various constraints such as heat transport limit, capillary pressure limit and the vapor pressure limit are discussed.