Two-phase flow regimes of condensing R-134a at low mass flux in rectangular microchannels

At small hydraulic diameters, the two-phase flow morphology deviates from that predicted for larger tubes at equivalent operating conditions. The importance of gravity-dominated flow regimes, including stratified and wavy flow, decreases, while the prevalence of intermittent, slug-and-plug type flow increases. Qualitative two-phase flow regime data were obtained from high-speed visualization of condensing flows of R-134a at mass fluxes from 75 to 150 kg m−2 s−1 and quality from 0.1 to 0.8 in square microchannels (DH = 0.84 mm) cooled from a single side. Superheated R-134a was distributed into multiple parallel microchannels and then partially condensed, using a counterflow water loop, to the desired quality prior to the inlet of a visualization section. This experimental arrangement mitigates the potential for flow maldistribution. Annular or annular/wavy type flow were observed for all conditions, with no distinct intermittent flow. The data were compared with flow macro and mini/microchannel maps, which were shown to overpredict the occurrence of intermittent or wavy flow. R-134a was chosen as a working fluid due to its prevalence in the HVAC&R industry and the availability of prior work using this fluid. R-134a is in the process of being phased down due to its high global warming potential, but its thermophysical properties are comparable to new low GWP replacements such as R-1234yf and R-513A. The low mass flux data obtained in this study were compared to five different flow maps, four of which were specifically developed for condensing refrigerants. The classic Taitel and Dukler (1976) flow map for horizontal and inclined round tubes is still commonly used as the basis for many contemporary flow maps. Cavallini et al. (2002) developed a flow map based on a database of flow visualization and heat transfer data of condensing refrigerants, which concluded that there exists a dimensionless vapor velocity (jv*) above which all flow will be annular and below which transition to either stratified, intermittent, or stratified annular will be determined by a constant turbulent-turbulent Martinelli parameter (Xtt). The El Hajal et al. (2003) map predicts transitions between stratified, wavy, intermittent, annular, mist, and dispersed flow. The proposed flow map was also shown in a subsequent study (Thome et al., 2003), which made use of the map to predict condensation heat transfer. The results of this study can be extended to new refrigerants, such as R-1234yf and R-513A, due to their comparable thermophysical properties to R-134a.