Experimental study of evaporative heat transfer in sintered powder structures at low superheat levels

Abstract

Evaporation at the evaporator of a heat pipe plays an important role in its overall heat transfer performance, especially at low superheat levels. Used sintered powder structures as wicks, this study investigated the correlations between superheat levels and heat fluxes. The parameters included powder sizes of 45μm, 75μm, 150μm, and powder shapes of spherical, dendritic. A two-part measurement in this study consisted of effective thermal conductivity and evaporative heat transfer. For the experiment of evaporative heat transfer, an apparatus consisting of a thermal guard test chamber, a direct sintering design, a pressure control loop, and a data acquisition system was used to measure heat fluxes and corresponding superheat levels. The effective thermal conductivity measurement showed that smaller powder sizes achieved higher effective thermal conductivities for both powder shapes. Spherical powder structures achieved twice the effective thermal conductivity of dendritic powder ones for each powder size. Furthermore, the evaporative heat transfer measurement showed that the heat fluxes increased proportionally with the superheat between 2 and 6K. At the same superheat level, structures of smaller powder size and dendritic powder shape achieved higher heat fluxes. In conclusion, the effect of thin-film evaporation may be the primary factor affecting evaporative heat transfer among these structural parameters.