Capillary performance and heat transfer charateristics of the evaporating thin liquid film in a triangular micro groove

Abstract

For the capillary flow in a groove channel, the analysis of axial variation of the intrinsic meniscus as well as that of evaporating thin film region was carried out. Various approximate analytical solutions were developed in order to get some insight in the phenomena of the axial capillary performance. The perturbation method was used for the case of a very small tilt angle. For a general case, approximating the governing equation to Bernoulli equation yielded the axial profile of the radius of curvature which is very similar to the numerical result. Approximating the governing equation to an algebraic equation yielded simple and explicit expression of dryout point location. This equation has no any experimentally correlated constant, and showed the significant effect of Bond number and the capillary number on the capillary performance. The prediction from algebraic equation was recommended among the developed models. A theoretical model has also been developed for the heat transfer characteristics of the evaporating thin film on the groove wall. The combined heat transfer mechanisms of both liquid conduction and interfacial vaporization were used to describe the local interfacial mass flux in the interline region. Based on this approach, a local heat transfer coefficient was defined, and the average heat transfer coefficient was described by dimensionless film thickness and characteristic thermal resistance ratio. This model can be used to better understand the variation of the heat transfer coefficient and the effective evaporating length for the specified groove geometry, working fluid, heat flux, and temperatures of vapor and groove wall. It was found that if wall superheat is constant, one of main factors affecting the length of active interline region is the heat flux supplied from the bottom plate. When a high heat flux is given, the highest heat transfer coefficient not necessarily exists at the region of the axial dryout point. For a micro heat pipe analysis, careful consideration on Cotter's model (1984) was carried out, and Cotter's model was modified for a triangular micro heat pipe by introducing of new definition of a dimensionless shape factor. The result showed more reliable value of prediction for the heat transport capacity.