Analysis of overloaded micro heat pipes: Effects of solid thermal conductivity

Starting from the principles of mass, momentum, and energy conservations as well as the Young–Laplace capillary equation, a mathematical model of triangular MHPs has been developed, primarily to investigate the effects of the thermal conductivity of the solid wall on their performance under overloaded conditions. For this purpose, two solids of significantly different thermal conductivities, copper and nickel, have been selected for the wall material. Using the model, a map encompassing the various possible operation zones of an MHP was constructed, to provide insight into the modes of operation under a given operating temperature but with varying heat loads and charge levels of the working fluid. The model predicts that in the overloaded zones, the dryout and flooded lengths increase with the applied heat load, resulting in a decrease in the effective length of the MHP. Moreover, it is also observed that the existence of dryout is accompanied by a large temperature rise over the dry region and, thus, a large total axial temperature drop, which increases rapidly with the applied heat load. Finally, comparison between copper and nickel MHPs on how the key performance indicators, such as the dryout and flooded lengths, the effective length, and the total axial temperature drop, response to the applied heat load shows that an MHP of solid wall of higher thermal conductivity out-performs one with lower solid thermal conductivity.