A Critical Overview on the Recent Literature Concerning Flow Boiling and Two-Phase Flows Inside Micro-Scale Channels

Abstract

Studies on two-phase flow and flow boiling inside micro-scale channels published approximately until the middle of last decade were characterized by drastic discrepancies among authors. Frequently, differences of about one order of magnitude were observed for experimental results obtained at almost similar conditions. Parcels of these differences were related to the use of inaccurate experimental techniques and also for not taking into account all the relevant experimental parameters, such as surface roughness, presence of thermal instabilities, and flash gas effects. Therefore, it was not surprising that the proposed models and correlations worked reasonably well for restricted databases, mainly those considered during their development. Transitions between micro- and macro-scale were proposed based only on channel dimensions and manufacturing processes without taking into account heat transfer and two-phase flow mechanistic aspects. During this early period, most authors were just theorizing about the relevant physical mechanisms, and nucleate boiling was frequently proposed as the main heat transfer mechanism even at high vapor qualities under annular flow conditions. Most recently, as result of the large number of studies performed on this theme and the efforts of leading laboratories over the world, agreements between independent databases are observed and recent correlations and models are providing accurate predictions. The experimental techniques used by each laboratory were improved, resulting in more accurate results that have contributed to the actual status quo. Flow pattern predictive methods based on objective recognition techniques have been proposed. Researchers are suggesting that the transition between micro- and macro-behaviors may be due to the results of bubble confinement within the channel as well as the balance among gravitational, inertial, wetting, and surface tension forces. In addition, recent physical models have been proposed and their hypothesis investigated through numerical simulations and experimental studies focusing on determining the two-phase topology and temperature and velocity fields. Additionally, the knowledge of the mechanisms acting on the flow thermal instabilities has immerged as a critical aspect in order to predict the hydro-thermal performance of flow boiling in micro-scale channels. In this context, the present study concerns an overview on the recent literature (from 2005 to 2012) dealing with flow boiling and two-phase flow in micro-scale channels and was written having the main goal of identifying points of agreements among authors and aspects that remain unclear.