Loop Heat Pipe Testing and Analytical Model Verification at the U.S. Naval Research Laboratory
Heat transfer characteristics of a Loop Heat Pipe (LHP) is difficult to predict due to the complex nature of thermal interaction between the LHP itself and the environment it operates in. The overall thermal conductance varies not only with the power input, sink temperature and ambient temperature as expected, but also with the system initial condition and/or previous history of its operation. Hence, the analytical modeling of LHPs often yielded inaccurate results when compared with the actual data. The U.S. Naval Research Laboratory (NRL) recently completed the development of a transient LHP model. Accordingly, NRL carried out an extensive LHP test program, in part, to provide test data for the model verification. In this paper, description of the test program and results of the model verification will be presented.
Mathematical Modeling of Loop Heat Pipes with Multiple Evaporators and Multiple Condensers
†Loop Heat Pipes (LH Ps) have proven themselves as reliable and robust heat transport devices for spacecraft thermal control systems. So far, the LHPs in earth -orbit satellites perform very well as expected. Conventional LHPs usually consist of a single capillary pump for he at acquisition and a single condenser for heat rejection. Multiple pump/multiple condenser LHPs have shown to function very well in ground testing. Nevertheless, the test results of a dual pump/condenser LHP also revealed that the dual LHP behaved in a complicated manner due to the interaction between the pumps and condensers. Thus it is redundant to say that more research is needed before they are ready for 0 -g deployment. One research area that perhaps compels immediate attention is the analytical mod eling of LHPs, particularly the transient phenomena. Modeling a single pump/single condenser LHP is difficult enough. Only a handful of computer codes are available for both steady state and transient simulations of conventional LHPs. No previous effort was made to develop an analytical model (or even a complete theory) to predict the operational behavior of the multiple pump/multiple condenser LHP systems. The current research project offered a basic theory of the multiple pump/multiple condenser LHP o peration. From it, a computer code was developed to predict the LHP saturation temperature in accordance with the system operating and environmental conditions.
Effects of reservoir volume on performance of pulse tube cooler.
A pulse tube cooler has the advantages of long-life and low-vibration over conventional cryocoolers such as G-M and Stirling coolers because of the absence of moving parts at low temperature. On the other hand, the combination of a reservoir and orifice is indispensable to optimize the performance of pulse tube coolers. In order to make the pulse tube cooler compact for practical applications, the volume of reservoir should be minimized. This paper analyzes the effects of the reservoir volume on the thermodynamic performance of various components in a simple orifice and a double-inlet pulse tube cooler by combining a linearized model with a thermodynamic analysis. Expressions of entropy production for those components are presented. The results show that the reservoir volume has a significant influence on the entropy production in the various components when the reservoir to pulse tube volume ratio is smaller than about 5. The ratio is important to determine the minimum reservoir volume for a pulse tube cooler. Optimum settings for a double-inlet pulse tube cooler are also discussed.