Modeling and experimental validation of a loop heat pipe for terrestrial thermal management applications.

Abstract

The Loop Heat Pipe (LHP) is a passive, two-phase heat transfer device used, most commonly, for thermal management of aerospace and aeronautical electronic equipment. A unique feature is a porous wick which generates the necessary capillary action required to maintain circulation between the heat source and the heat exchanger. What differentiates LHP devices from traditional heat pipes, which also work through the use of a wick structure, is the constrained locality of the wick, placed solely in the evaporator, which leaves the remainder of the piping throughout the device as hollow. This provides the LHP with a number of advantages, such as the ability to transport heat over long distances, operate in adverse gravitational positions and to tolerate numerous bends in the transport lines. It is also self-priming due to the use of a compensation chamber which passively provides the wick with constant liquid access. These advantages make LHPs popular in aerospace and aeronautical applications, but there is growing interest in their deployment for terrestrial thermal management systems. This research had two aims. Firstly, to create and validate a robust mathematical model of the steady-state operation of an LHP for terrestrial high heat flux electronics. Secondly, to construct an experimental LHP, including a sintered porous wick, which could be used to validate the model and demonstrate the aforementioned heat exchange and gravity resistant characteristics. The porous wick was sintered with properties of 60% porosity, 6.77x10-13 m2 permeability and an average pore radius of 1μm. Ammonia was the chosen working fluid and the LHP functioned as expected during horizontal testing, albeit at higher temperatures than anticipated. For safety reasons the experimental LHP could not be operated past 18 bar, which translated into a maximum saturated vapour temperature of 45°C. The heat load range extended to 60 W, 50 W and 110 W for horizontal, gravity-adverse and gravity-assisted operation respectively. Because of certain simplifying assumptions in the model, the experimental results deviated somewhat from predicted values at low heat loads. Model accuracy improved as the heat load increased. The experimental LHP behaved as expected for 5° and 10° gravity-assisted and gravity-adverse conditions, as well as for transport line variation, in which performance was assessed while the total tubing length was increased from 2.5 m to 4 m. Overall, the construction of the LHP, particularly of the porous wick, its operation and the modeling of the constant conductance mode of operation proved to be successful. The variable conductance mode of operation was not accurately modeled, nor was expected behaviour in the elevation testing encountered, although the reasons for these results are suggested.