Flow visualization of two-phase R-245fa at low mass flux in a plate heat exchanger near the micro-macroscale transition

Two-phase R-245fa flow in a plate heat exchanger (PHE) is experimentally investigated to understand the unique flow regimes found during adiabatic operation at low mass flux. A transparent PHE 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 PHE operation are due to the macro-microscale transitional two-phase flow from the coexistence of fluid inertial forces and surface tension effects, corresponding to the operating 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 found at high-mass-flux operation or in channels of conventional macroscale. Flow morphology involving bubble coalescence and breakup dynamics is captured and analyzed in relation to the fluid properties and geometric obstructions provided by the heat exchanger channel. Current observations validate some previously published findings that suggest potential microscale boiling mechanisms in PHEs.