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

Mirco Magnini, Sylwia Szczukiewicz, John Richard Thome

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.

Elsevier2014

Critical Heat Flux of R134a and R245fa Inside Small-Diameter Tubes

Cristiano Bigonha Tibiriça, Sylwia Szczukiewicz, John Richard Thome

This article presents new experimental critical heat flux results under saturated flow boiling conditions for a macro/microscale tube. The data were obtained in a horizontal 2.20-mm inside diameter stainless-steel tube with heating lengths of 361 and 154 mm, R134a and R245fa as working fluids, mass velocities ranging from 100 to 1500 kg/m(2)-s, critical heat flux from 25 to 300 kW/m(2), exit saturation temperatures of 25, 31, and 35 degrees C, and critical vapor qualities ranging from 0.55 to 1. The experimental results show that critical heat flux (CHF) increases with increasing mass velocity and inlet subcooling but decreases with increasing saturation temperature and heated length. The data also indicated a higher CHF for R245fa when compared with R134a at similar conditions. The experimental data were compared against four CHF predictive methods and the results of the comparisons are reported.

Taylor & Francis2013

Fine-resolution two-phase flow heat transfer coefficient measurements of refrigerants in multi-microchannel evaporators

Navid Borhani, Sylwia Szczukiewicz, John Richard Thome

The time-averaged two-phase flow heat transfer coefficients were determined with a very fine resolution based on infra-red (IR) temperature measurements conducted over a 1 cm(2) heated area of a multi-micro-, channel evaporator. This experimental investigation included two low-pressure refrigerants, R245fa and R236fa, as well as one medium-pressure new refrigerant R1234ze(E) flowing in four different test sections. The number of microchannels and their cross-sectional areas were the same for all the tested micro-evaporators, respectively, 67 and 100 x 100 mu m(2). Channel entrances with and without inlet micro-orifices were tested for establishing stable flow. Using 90 x 90 pixel grid (and then averaged width-wise), 90 local heat transfer coefficients were measured from inlet to outlet, yielding a U-shape of the heat transfer coefficient trend, where the descending branch of the curve corresponds to the coalescing elongated bubble flow regime (CB), while ascending one represents the increasing heat transfer coefficient in the annular flow regime (AF). The effects of channel mass flux, wall heat flux, orifice expansion ratio, and fluid properties on the wall heat transfer coefficient were identified for a selected number of data sets. In terms of predicted heat transfer coefficient values, in both the CB and AF flow regimes, the experimental results were found to be in a good agreement with respective existing flow pattern-based heat transfer prediction methods. Whereas, in order to better reflect the obtained experimental U-shaped trend of the heat transfer coefficient close to the CB-AF flow transition (which is a churn flow), the joined model of these two was modified by proposing a new vapor quality buffer for the width of the transition. (C) 2013 Elsevier Ltd. All rights reserved.

Elsevier2013