Proposed models, ongoing experiments, and latest numerical simulations of microchannel two-phase flow boiling

A survey of the most recent work aimed at physically characterizing local heat transfer in flow boiling in microchannels is presented. This includes recent experimental work, new flow boiling prediction methods, and numerical simulations of microchannel slug flows with evaporation. Some significant developments in the measurement techniques provide simultaneous flow visualizations and measurements of 2D temperature fields of multi-microchannel evaporators. In particular, information on inlet micro-orifices has been gained as well as better ways to reduce such heat transfer and pressure drop data for very high resolution data (10,000 pixels at rate of 60 Hz). First of all, flow patterns are seen to have a significant influence on the heat transfer trends in microchannels (just like in macrochannels), and thus need to be accounted by visualization during experiments and during modeling. A clear distinction between steady, unsteady, well- and maldistributed flows needs to be made to avoid any confusion when presenting and comparing the heat transfer coefficient trends. In reducing the raw data to local heat transfer coefficients, the calculated values of several terms involved in the heat transfer coefficient determination are influenced by the data reduction procedure, especially the way to deduce the local saturation pressures/temperatures, and may lead to conflicting trends and errors approaching 100% in local heat transfer coefficients if done inappropriately. In addition to experiments, two-phase CFD simulations are emerging as a tenable tool to investigate the local heat transfer mechanisms, especially those details not accessible experimentally. In particular, a new prediction method based on numerical simulation results captures the heat transfer in the recirculating liquid flow between elongated bubbles. Thus, it is shown here that targeted computations can provide valuable insights on the local flow structures and heat transfer mechanisms, and thus be used to improve the mechanistic boiling heat transfer prediction methods.