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.
Technology Overview of a Multi-Evaporator Miniature Loop Heat Pipe for Spacecraft Applications
Aminiature loop heat pipe with multiple evaporators andmultiple condensers was developed for thermal control of small spacecraft and instruments requiring low mass, low power, and compactness. Multiple evaporators afford flexible placement of instruments inside the spacecraft and facilitate heat-load sharing among instruments. Multiple condensers allow the radiators to be placed at various locations on the spacecraft surface and exposed to different thermal environments. Thermoelectric converters are used to provide heating and cooling to the reservoir for loop operating temperature control. A breadboard and a protoflight unit of the miniature loop heat pipe with two evaporators and two condensers were built and tested in a thermal vacuum chamber to demonstrate the thermal performance. In addition, an analytical model was developed to simulate the steady-state and transient behaviors of the miniature loop heat pipe during thermal performance tests.
Mathematical Modeling of Loop Heat Pipes
The primary focus of this study is to model steady-state performance of a Loop Heat Pipe (LHP). The mathematical model is based on the steady-state energy balance equations at each component of the LHP. The heat exchange between each LHP component and the surrounding is taken into account. Both convection and radiation environments are modeled. The loop operating temperature is calculated as a function of the applied power at a given loop condition. Experimental validation of the model is attempted by using two different LHP designs. The mathematical model is tested at different sink temperatures and at different elevations of the loop. Tbc comparison of the calculations and experimental results showed very good agreement (within 3%). This method proved to be a useful tool in studying steady-state LHP performance characteristics.