Performance evaluation of evaporation from micropillar arrays with different pillar topologies

This paper reports the results of our studies on the performance of thin film evaporation, mostly used in the evaporator of an ultra-thin vapor chamber using micropillar arrays. The dry-out heat flux and evaporator surface temperature have been used as the performance metrics. A numerical model has been formulated to solve for the fluid flow and phase change heat transfer considering the capillary pumping action (wicking action) using a cell-by-cell forward approach and validated with results reported in the literature. The solutions of the model equation are then utilized to study the effects of geometry and arrangement of the micropillars within the vapor chamber and to gain further insights into the interplay of the various forces. It is seen that these variations strongly influence the pumping action as well as the shape of the liquid vapor interface thereby impacting the pressure field, flow rate and the dry-out heat flux. A conical shape of the micropillar (having the same volume as that of other shapes) has a significant impact on the dry-out heat flux e.g., reducing the top diameter from 10 to 8 μm results in an 18% reduction. For a rectangular arrangement of the micropillars, the performance is quite sensitive to the transverse and axial spacing. In the range of 30–40 μm of axial and transverse spacing, the dry out heat flux can be altered by as much as 20%. Higher transverse pitch with lower axial pitch is found to be a better combination for a given overall geometry and number of pillars.