Flow Visualization of Two-Phase R-245fa at Low Mass Flux in a Plate Heat Exchanger near the Micro-Macroscale Transition
plate heat exchanger, R-245fa, two-phase flow, microchannel, macrochannel, flow visualization
Two-phase R-245fa flow in a plate heat exchanger is experimentally investigated to understand the unique flow regimes found during operation at low refrigerant mass flux. A transparent plate heat exchanger replica with 3.4 mm hydraulic diameter is 3D-printed for flow visualization using high-speed videography. Observed flow regimes support that the thermofluidic characteristics peculiar to plate heat exchanger (PHE) operation are due to the macro-microscale transitional two-phase flow from the coexistence of fluid inertial force and surface tension effects, corresponding to the operation conditions. Maximum stable bubble diameter is bigger at low mass flux than at high mass flux, and the bubbles can become big enough to be fully confined in the millimeter-scale PHE channel to be deformed or elongated. This represents the main thermo-physical characteristics of two-phase flow in mini- and microchannels, which is different from turbulent mixing flow easily found at high-mass-flux operation or in channels of conventional macroscale. Flow morphology involving complex bubble coalescence and breakup dynamics is captured and analyzed in relation to the fluid properties and geometric obstructions provided by the plate heat exchanger channel. While there exist previous studies, and even heat transfer coefficient correlations, suggesting the potential microscale flow regimes in PHEs, this is the first time presenting evidences via flow visualization.