Liquid Velocity Field Measurements in Two-phase Microchannel Convection

Two-phase microchannel heat sinks are promising for future thermal management of integrated circuit (IC) chips. The modeling and design of these heat sinks, however, require a comprehensive understanding of the interplay between vapor bubble nucleation and growth and the associated liquid velocity field. Most of the flow visualization studies for two-phase convection in microchannels have provided qualitative observations of bubble sizes, densities, and trajectories. Jiang et al. observed local nucleate boiling in V-grooves with DH = 26 μm – 53 μm with an input power of 12 W [1]. Zhang et al. observed bubble formation and growth from cavities of 4 μm 8 μm in microchannels with DH < 150 μm [2]. Steinke et al. collected high speed video of flow boiling in DH = 207 μm and observed different flow regimes with bubbles ranging from 15 μm to 193 μm in diameter [3]. Qu et al. visually detected boiling incipience in microchannels when the first bubble was sighted growing primarily from the bottom channel wall [4]. Few studies have reported liquid velocity measurements in two-phase systems. Lindken et al. reported velocity measurements in bubbly two-phase flow where pseudo-turbulence could be characterized [5]. Grunefeld et al. showed the feasibility of measuring instantaneous velocity fields in both gaseous and liquid phases in an unsteady laminar flow system [6]. Molho reported velocity measurements around laser-induced cavitation bubbles [7]. Qiu et al. reported liquid velocity field measurements with PIV for sliding bubbles on inclined surfaces [8]. Heinzel et al. presented preliminary flow pattern measurements in a 200 μm x 200 μm heat exchanger and described particle fouling as a large problem in obtaining further velocity measurements [9] . This paper reports quantitative flow field measurements of the liquid-phase during bubble growth in a heated microchannel using micron-resolution particle image velocimetry (μPIV). These measurements are used to study the transient dynamics of bubble growth and the corresponding unsteady velocity fields in channels with D H < 150 μm. These velocity fields allow for the quantification of forces on the liquid/vapor interface and estimation of the pressure difference across the bubble interface. These forces will be helpful for the study of bubble departure. This preliminary study reports results for ReD << 1, where surface tension forces are dominant and the bubbles do not depart from the walls. However, the approach presented here establishes a methodology for future analysis of forces which can be extended to higher flowrate conditions typical in microchannel forced convection.