Visualization of Two-Phase Refrigerant and Refrigerant-Oil Flow in a Microchannel

Abstract

Visualizations of adiabatic two-phase refrigerant flow in a glass channel of diameter 0.5 mm have been made for three refrigerants: R134a, Propane (R290), and Ammonia (R717), representing a wide span of fluid properties, which covers most of the refrigerants commercially in use. In these visualizations four flow regimes were observed: bubble-slug, slug, slug-annular, and annular. These flow regimes were compared to various flow maps, including some developed for small channels. Flow visualizations were also made with mixtures of R134a and 68-weight POE oil at oil circulation rates of approximately 0.5, 1.5 and 3 percent. This is of interest when considering refrigeration systems, which have a small percentage of oil in circulation that travels through system and through the heat exchangers. When the refrigerant is in a liquid state, this presents little variation in fluid properties, because the concentration is so small. However, when the refrigerant is partly vapor, the oil concentration in the remaining liquid can have significant effect on the fluid properties. In addition, the saturation temperature and pressure of the oil-refrigerant mixture changes with concentration, where a single-phase vapor is never observed in flows of oil-refrigerant mixtures, even at temperatures exceeding the saturation temperature of the pure mixture. This effect is known as “apparent superheat”.

A comprehensive flow regime map for microchannel flow boiling with quantitative transition criteria

“…In recent years, a few studies [7][8][9][10][11] have developed flow regime maps for boiling in microchannels using similar axes as conventional maps with flow regime definitions pertinent to microscale boiling, and have shown that flow regime maps developed for larger tubes are inapplicable for predicting flow regime transitions in microchannels. Flow regime maps for adiabatic two-phase flow in microchannels have also been proposed through high-speed visualizations [12][13][14];…”

Effects of channel dimension, heat flux, and mass flux on flow boiling regimes in microchannels

“…In order to readily illustrate the effect of channel size on the transitions between different flow regimes, the lines representing transition from bubbly or slug flows to intermittent churn/wispy-annular or churn/annular flows are plotted for the six microchannel widths in Figure 15 not suitable for predicting the boiling in microchannels as also discussed in the literature (Hetsroni et al 2003, Huo et al 2004, Revellin et al 2006, Field and Hrnjak 2007. In general, five flow regimes -bubbly, slug, churn, wispy-annular, and annular flow -are identified.…”

“…Field and Hrnjak (2007) showed that the flow maps developed for large channels were not suitable for prediction of the flow regimes in microchannels; also, flow maps were dependent on the specific fluid for which they were developed. Despite the inability of macroscale boiling maps or adiabatic two-phase flow regime maps to predict the flow patterns for boiling in microchannels, a review of the literature shows a dearth of investigations into flow regime maps specifically targeted at microchannels undergoing flow boiling.…”

“…Adiabatic two-phase flow patterns were investigated through high-speed visualizations and flow regime maps developed by Chung and Kawaji (2004), Hassan, Vaillancourt, and Pehlivan (2005), and Field and Hrnjak (2007). Field and Hrnjak (2007) showed that the flow maps developed for large channels were not suitable for prediction of the flow regimes in microchannels; also, flow maps were dependent on the specific fluid for which they were developed.…”

Flow Boiling in Silicon Microchannel Heat Sinks

“…Flow visualization has been employed to record the flow patterns under different flow conditions and to understand differences from the boiling regimes in conventional-sized channels [7,8,9,10,11,12,13,14,15]. However, systematic research on the effect of channel size and mass flux on the flow patterns during boiling in microchannels has not been performed.…”