Two-Phase Flow Regimes and Heat Transfer Coefficients With R245fa in Manifolded-Microgap Channels

Manifold microchannels have shown high performance in embedded two-phase cooling of high heat flux electronics, demonstrating several examples of heat dissipation of 1 kW from 1-cm
²
chips, with low pumping power requirements. The heat transfer coefficients (HTCs) in previous studies of chip-scale manifold-microchannel coolers exhibited a peak at relatively low quality (about 0.2–0.4) and steeply declined thereafter with increasing quality. However, manifold microchannels—arrays of manifolded-microgap channels—are a more complex geometry than traditional, straight, and circular channels and therefore generate more complex two-phase flow phenomena, which influences the resulting heat transfer performance. To better characterize the flow and heat transfer in such channels, in this study, the fluid flow and wall heat transfer in a visualization test section was imaged with a high-speed optical camera and thermography. R245fa served as the working fluid due to its low-pressure operation (approximately 1.8 atm at 30 °C) and excellent thermal properties, including high latent heat, high specific heat, and high thermal conductivity. The results are reported in terms of the channel outlet flow regime and HTCs under varying heat fluxes and mass fluxes with two manifold designs. The flow regimes and HTCs are compared with well-regarded predictive relations developed for more traditional channel geometries. The emergence of intermittent, and then persistent, dryout was associated with intermittent rivulet flow at the channel outlet and was observed to cause declining average wall HTCs, even while high HTCs occurred at the consistently wetted inlet region.