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Abstract.Loop heat pipes (LHPs) are versatile two-phase heat transfer devices that have gained increasing acceptance for space and terrestrial applications. The operating temperature of an LHP is a function of its operating conditions. The LHP usually reaches a steady operating temperature for a given heat load and sink temperature. The operating temperature will change when the heat load and/or the sink temperature changes, but eventually reaches another steady state in most cases. Under certain conditions, however, the loop operating temperature never really reaches a true steady state, but instead becomes oscillatory. This paper discusses the temperature oscillation phenomenon using test data from a miniature LHP. INTRODUC_ONLoop Heat Pipes (LHPs) are two-phase heat transfer devices which utilize the evaporation and condensation of the working fluid to transfer heat, and the surface tension forces developed in the fine porous wicks to circulate the working fluid. Figure I shows the schematic of a typical LHP, which consists of an evaporator, a compensation chamber, a condenser, and vapor and liquid transport lines. The entire loop is made of smooth wall tubing except the evaporator and the compensation chamber. The evaporator contains fine pore wicks which develop the necessary pumping forces to circulate the fluid around the loop. The compensation chamber is usually made as an integral part of the evaporator, and is connected to the evaporator by a secondary wick with coarser pores. The secondary wick draws liquid from the compensation chamber to the evaporator, thus ensuring the evaporator will not be deprived of liquid and become vapor locked. Such a feature makes it possible to start the LHP by applying power directly to the evaporator without pre-conditioning.It also makes LHP operation very robust because the evaporator can accommodate vapor inside the core. On the other hand, the integral construction of the evaporator and compensation chamber also yields some unique characteristics in the LHP operation. The physical proximity and the secondary wick link between the https://ntrs.nasa.gov/search.jsp?R=20000114844 2018-05-12T23:42:13+00:00Z

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.

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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.