Pressure drop characteristics of large length-to-diameter two-phase micro-channel heat sinks

Abstract

Unlike prior published two-phase micro-channel studies that concern mostly miniature heat sinks, the present study addresses transport characteristics of a heat sink containing large length-to-diameter micro-channels. The copper heat sink has a 609.6-mm long by 203.2-mm wide base area, and contains 100 parallel micro-channels having a 1×1mm2 cross-section, and a length-to-diameter ratio of 609.6 to 1. The study addresses pressure drop characteristics of R134a for subcooled inlet conditions with inlet pressures of 689.4–731.3kPa, mass velocities of 75.92–208.79kg/m2s, and base heat fluxes of 4036–28,255W/m2. The data are compared to predictions of the Homogeneous Equilibrium Model (HEM) in conjunction with six different two-phase mixture viscosity relations, correlations based on the Separated Flow Model (SFM) and intended for macro-channels, and correlations based on SFM and intended for micro-channels. Overall, fairly good predictions are achieved with HEM, compared to poor predictions with the macro-channel SFM correlations. The micro-channel SFM correlations fared better, yielding mean absolute error values as low as 6.66%. Flow visualization results show appreciable periodic fluctuations in two-phase flow patterns, which include four primary patterns: bubbly/slug, slug, transition, and annular, along with two other patterns associated specifically with downstream dryout: transition and annular. These observations point to the need for more research to address the role of flow instabilities and fluctuations in flow pattern development.