Convective boiling of refrigerants near the micro-macroscale transition inside plate heat exchangers

This work focuses on the refrigerant flow morphology and its impact on evaporative heat transfer and pressure drop during plate heat exchanger (PHE) operation at low mass flux conditions. While there is interest in this operating regime in the heating, ventilating, air conditioning, and refrigeration industries, especially for part-load conditions, many aspects of the two-phase flow are not well understood for a PHE with complex millimeter-scale passages formed by chevron-patterned plates. Some industrial practitioners and academic researchers suggest PHE channel designs are intended to induce turbulence for mixing and enhanced convection, and PHE thermal performance correlations have typically been based on semi-empirical approaches assuming turbulent flows. However, the prevailing two-phase flow regimes at low-mass-flux operation, with bubble confinement effects, are at low Reynolds numbers and might not be turbulent. In this research, liquid-vapor two-phase flow regimes and semi-local heat transfer and pressure drop during refrigerant evaporation are studied over varying mass flux, saturation pressure, heat flux, and vapor quality for a commercially available PHE with a 3.4 mm hydraulic diameter. For flow visualization studies, transparent replicas of the PHE are 3D-printed with clear resin using a stereolithography apparatus. High-speed videography is used to investigate flow morphologies, including bubble growth, elongation, coalescence, and breakup. Flow morphologies involving bubble confinement effects are captured and analyzed as they relate to fluid properties (such as surface tension and viscosity) and the PHE geometry. The maximum stable bubble diameter is larger at low mass flux than at high mass flux, with the increased significance of surface tension due to decreased inertial effects at lower flow velocity. The bubbles can grow large without breaking up, and they can be fully confined in the PHE channel to be deformed or elongated. These large non-spherical bubbles with surrounding thin liquid films characterize the two-phase flow morphology in microchannels. In this flow regime, effective heat transfer can be achieved by evaporating the thin liquid film surrounding the bubbles, rather than fully relying on mixing and convection. The two-phase heat transfer coefficient in the PHE increases with heat flux, but this behavior is not due to nucleate boiling as is often argued for macroscale channels. Convective heat transfer through unsteady film flow on the corrugated wall is an important mechanism. Heat transfer also improves with higher saturation pressure and the associated lower latent heat during convective boiling; heat transfer is improved by the decreased liquid film thickness surrounding confined bubbles inside the narrow PHE channels, a characteristic of microscale boiling. Experimental results are compared to several semi-empirical PHE correlations, and their ability to capture important thermal-fluid characteristics is assessed. Through flow visualization and thermal-hydraulic analyses, results from this research support the existence of microscale flow regimes in PHE flows. Instead of being solely in one regime or the other, microscale and macroscale flows can coexist in PHEs, due to the non-uniform, complex flows in the heat exchangers. With a clearer understanding of the flow morphology and operating principles, improved heat exchanger designs can be developed to serve a wide range of applications.U of I OnlyAuthor requested U of Illinois access only (OA after 2yrs) in Vireo ETD syste