Condensation of R134a in a Rectangular Microchannel to Visualize Flow Regimes at Low Mass Fluxes

The research paper by Maria Sattar and Nouman Ali focuses on visualizing flow regimes at low mass fluxes. The study aims to analyze the different flow regimes and vapor quality of R134a inside horizontal rectangular microchannels. The energy efficiency concerns of the refrigeration and air conditioning industry are significant, with the major portion of energy consumed in heating and cooling. The performance and efficiency of such systems depend on two building blocks: refrigerants and footprint of the system. Therefore, energy used by heating and cooling can be saved through refrigerants and footprints, mostly by 50% of its total energy production. The research paper presents the condensation of R134a inside horizontal rectangular microchannels at saturation temperatures of 40°C and 55°C, and saturation pressures of 147psi and 217psi, over lower mass fluxes of 75-150 kg/m²-s. The experiments were performed to visualize the different flow regimes and compare experimental data points with the Barnea and Breber flow maps. The literature review highlights the understanding of flow patterns inside the horizontal microchannel, which can be developed by analyzing the flow regime maps. Flow regime maps are generated by considering the fluid flows and channel parameters with thermo-physical properties of condensation fluid. Researchers have discussed the interrelation among mass fluxes, channel size, channel geometry, fluid properties, and directing forces, such as gravitational forces, viscous drag forces, inertial forces, buoyancy forces, and surface tension forces, during the flow inside the microchannel. The study by Suo and [5] developed the first understanding of flow regime map inside the capillary-sized flow channel during adiabatic heat transfer. Their findings stated that usually in small diameter tubes, flow transition will occur in two stages: first, slug flow to bubbly flow, and secondly, bubbly slug flow to annular flow at higher flow rates in zero gravity field. In this study, they considered the surface tension forces, viscous drag forces, and inertial forces as main governing forces of flow regime transition. Taitel and Dukler [6] presented a theoretical flow regime map based on dimensionless flow parameters. They formulated five flow regime transition groups, including stratified-annular, stratified-intermittent, intermittent-dispersed bubble, stratified smooth-stratified wavy, and annular dispersed liquid-intermittent-dispersed bubble. Their parametric study considered five different dimensionless parameters. They stated that at low flow rates, intermittent flow can only occur when the ratio between liquid level and channel size is 0.5. They suggested that intermittent flow can only be established in the horizontal channel if the liquid level inside the channel goes higher than the center of flow sphere. Primarily, the wave stability controlled the transition of flow regime. For stabilized waves, the intermittent flow will occur, while higher flow rates cause instability that will lead to annular flow regimes. The research paper aims to contribute to the understanding of flow regimes inside the microchannel and to provide a comprehensive analysis of the condensation of R134a. The study will help to improve the design and performance of microchannel heat exchangers, which is a promising technology to increase the system's operational efficiency of HVAC&R equipment and to reduce the carbon footprint of refrigeration and air conditioning units on the global environment.