Saturated flow boiling of isolated seed bubble across a heated square cylinder in two-dimensional microchannel

摘要

Two-phase flow boiling within pin-fin microchannel heat sinks has gain increasing attentions over the last decades owing to the demands for high efficiency cooling applications, while the detailed interactions between a vapor bubble and heated pillar structures are still not fully understood. In this paper, the saturated-interface-volume phase change model is coupled with VOF method to investigate the saturated flow boiling of an isolated vapor bubble traveling across a heated square cylinder in a microchannel (with Reynolds number Re ranging from 60 to 360 and dimensionless bubble diameter d_b* from 0.2 to 0.6). The numerical model is first validated against literatures, and a mesh-independent study is conducted to ensure the accuracy of the present simulations. Then the bubble dynamics and its influences on flow structures are presented, where the symmetrical vortices attached behind the cylinder at small Re (60 and 120) are found replaced with the vortex-shedding flow and the deformed bubble (d_b* = 0.6) squeezes the cylinder and forms evaporating thin film not only on the adjacent side but also the windward and leeward sides. The local wall superheat and Nusselt number are presented along all the four sides of the heated cylinder. Two competing mechanisms governing heat transfer enhancement, i.e. phase change heat transfer improved by the thin film evaporation and convective heat transfer suppressed by the wrap of the bubble, are revealed. Both the dynamic evolution and time-averaged heat transfer performance are obtained for each side of the cylinder and their sums. The global heat transfer enhancement is found decreases monotonically with increasing Re and decreasing d_b*.