Significant Heat Transfer Enhancement in Microchannels With a Segmented Flow of Two Immiscible Liquids

With the advancement in microfabrication technologies the size of electronic devises keeps decreasing. At the same time high density of circuits and the directly related heat generation requires simultaneous increase in the performance of cooling systems to keep the electronics operating at optimum temperatures. Liquid cooling incorporating microchannel networks as the heat sinks is known to be a promising approach to this heat transfer problem because of its plausible manufacturability at the chip level. However, even the upper range of heat fluxes achieved in such microchannel heat sinks will not meet the increasing demands of the electronics industry. We present herein a novel experimental study of convective heat transfer in serpentine microchannels with segmented liquid-liquid emulsions. It is demonstrated that this concept yields significant heat transfer enhancement in micro channel heat sinks compared to traditional single phase liquid cooling. The flow of water and mineral oil droplets is examined experimentally in microchannels with cross section 100 by 100 μm and flow rates up to 130 μl/min. The study is focused on the investigation of the local temperature of the liquid and on the influence of the oil droplets on the temperature distribution and heat transfer. Laser Induced Fluorescence (LIF) is employed to measure the temperature of the flow fields with and without droplets, and PIV measurements are conducted to determine the velocity field. The range of flow conditions which led to a stable segmented flow with enhancement of heat transport was identified. The increase of the oil fraction was accompanied by an increase of the recirculation zone in the transverse direction to the flow and by higher velocity gradients inside the zones thus resulting in enhanced mixing of water across the channel. The LIF measurements indicate that for the same total flow rate, the local temperature of the water region separating the droplets will be higher than in the case of single phase liquid flow. For segmented flow, up to a four-fold increase of the Nusselt number compared to pure water flow was observed.