EFFECT OF LOW WICK PERMEABILITY ON TRANSIENT AND STEADY-STATE PERFORMANCE OF HEAT PIPES

Novel wicks made of micro/nanopillar array or carbon nanotube array have the advantages of superhigh capillary pressure and compact dimensions, which is attractive for applications like smartphones. However, much lower permeability becomes the dominant shortcoming compared to conventional wicking structures. This work presents an investigation of the effects of low-permeability wick on various aspects of the heat pipe performance, by assigning the permeability as the parametric control variable. The transient process as well as steady state have been numerically solved by an efficient heat pipe simulation tool LVHPM. Heat pipe performances with wicks covering a wide range of permeability have been studied in terms of the mass flow, temperature, and pressure influences. Calculation results showed that with decrease of the permeability, the growth of flow resistance in the wick is faster than the capillary pressure, resulting in lower capillary limit. Further, the effect of permeability on the coupled fluid flow and heat transfer inside the heat pipe was investigated, assuming that the heat load is within moderate range. The velocity was shown to be more evenly distributed for lower permeability, but the total mass flux was insensitive to it. Steady-state temperature distribution was weakly related to wick permeability, whereas the transient temperature presented stronger dependence with permeability, showing faster response at smaller permeability. During the transition of external heat loads, pressure drop inside the wick was found being established in a short time.