Effect of Liquid Properties on Phase Change Heat Transfer in Porous Wick Structures

Abstract

In a heat pipe, operating fluid saturates wick structures system and establishes a capillarydriven circulation loop for heat transfer. Thus, the thermophysical properties of the operating fluid inevitably impact the transitions of phasechange mode and the capability of heat transfer, which determine both the design and development of the associated heat pipe systems. This article investigates the effect of liquid properties on phasechange heat transfer. Two different copper wick structures, cubic and cylindrical in cross section, 340 خ¼m in height and 150 خ¼m in diameter or width, are fabricated using an electroplating technique. The phasechange phenomena inside these wick structures are observed at various heat fluxes. The corresponding heat transfer characteristics are measured for three different working liquids: water, ethanol, and Novec 7200. Three distinct modes of the phasechange process are identified: (1) evaporation on liquid–vapor interface, (2) nucleate boiling with interfacial evaporation, and (3) boiling enhanced interface evaporation. Transitions between the three modes depend on heat flux and liquid properties. In addition to the mode transition, liquid properties also dictate the maximum heat flux and the heat transfer coefficient. A quantitative characterization shows that the maximum heat flux scales with Merit number, a dimensionless number connecting liquid density, surface tension, latent heat of vaporization, and viscosity. The heat transfer coefficient, on the other hand, is dictated by the thermal conductivity of the liquid. A complex interaction between the mode transition and liquid properties is reflected in Novec 7200. In spite of having the lowest thermal conductivity among the three liquids, an early transition to the mode of the boiling enhanced interface evaporation leads to a higher heat transfer coefficient at low heat flux.