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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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.