Effect of Heat Pipe Figure of Merit on an Evaporating Thin Film

Abstract

The performance of a two-phase heat transport device such as the loop heat pipe is influenced by the evaporative heat transfer coefficient in the evaporator. From previous experiments with loop heat pipes, it has been observed that fluids with a high heat pipe figure of merit have a high heat transfer coefficient. Considering an evaporating extended thin film, this paper theoretically corroborates this experimental observation by deriving a direct link between the evaporative heat flux at the interface and the fluid figures of merit (namely interline heat flow parameter and heat pipe figure of merit) in the thin film. Numerical experiments with different working fluids clearly show that a fluid with high figure of merit also has a high cumulative heat transfer in the microregion encompassing the evaporating thin film. Thus, a loop heat pipe or heat pipe that uses a working fluid with a high interline heat flow parameter and heat pipe figure of merit will lead to a high evaporative heat transfer coefficient.

Thermophysical phenomena of working fluids of thermocapillary convection in evaporating thin liquid films

“…Ammonia has the largest film thickness, followed by acetone, heptane and water, in a descending order. For the sake of comparison, it has also been shown that ammonia manifests the largest film thickness among those of acetone, pentane and octane, in a previous work employing nonpolar liquids without considering thermocapillary effect [12]. A pronounced deviation is induced by the thermocapillary effect on the film thickness such that the film thickness of Model 1 is larger than that of Model 2.…”

“…In addition to the latent heat, other thermo-physical properties are also important in characterizing the evaporation process. Judging from this, the selection of appropriate working fluid for a designated range of operating temperature is an important issue in delineating the thermal performance of a two-phase cooling device [9][10][11][12][13]. By considering different types of common working fluid, it has been pointed out that the heat transport capacity of such device is dominated by the circulation rate, which is the ratio of the surface tension and dynamic viscosity of the working fluid [10,13].…”