Geometrical effects of wick structures on the maximum phase change capability

Liquid-to-vapor phase change on porous wick structures has been proven efficient and capable of high density heat removal. This superior thermal performance is attributed to thin film evaporation on liquid–vapor interfaces, capillary enhanced liquid return mechanism, as well as rapid vapor ventilation. In this complicated physical process, wick geometrical parameters, such as wick thickness, particle diameter/gap size and heating area dimension, of the wick structures play critical roles in determining the maximum phase change capability. This article presents an analytical model to extract the wick geometrical effects on heat and mass transport limits, based on mono porous wick structures composed of cylindrical pillars. The model considers two extreme cases: (a) thin wick structure with large heating area; and, (b) thick wick structure with small heating area. Two dimensionless geometrical numbers are derived based on liquid and vapor phase flow resistances. For a thin wick structure with large heating area, wherein the liquid phase is dominant in flow resistance, the maximum heat flux is proportional to liquid phase geometrical number, defined as a product of the pillar diameter and the wick thickness divided by square of the heating area dimension. In contrast, phase change capability of a thick wick structure with small heating area is attributed to the vapor phase geometrical number, written as the ratio of the pillar diameter over the wick thickness. The analytical model is validated through experimental results by characterizing phase change performance of the silicon mono wick structures. A ratio between the heating area width and the wick thickness is presented to justify the broad applicability of the analytical model. The two analytic cases of thin wick structure with large heating area and thick wick structure with small heating area correspond to the ratios being >1.0 and <0.1, respectively.