Unveiling flow boiling hysteresis mechanisms and characteristics of R134a within minichannels

Abstract

Boiling hysteresis is the difference in boiling behavior observed between increasing and decreasing heat flux phases. Although hysteresis phenomenon in flow boiling has been reported, the underlying mechanisms and key influencing factors remain unclear. This study comprehensively analyzes the effects of flow pattern and surface morphology to reveal the hysteresis mechanism of flow boiling. Experiments were conducted at the refrigerant mass flow rates ($\dot{m}$) of 0.005 kg/s to 0.009 kg/s (corresponding to mass fluxes of 60.5 kg/m²·s to 187.4 kg/m²·s), and effective heat fluxes (q_eff) of 2.9 kW/m² to 151 kW/m², by supplying 7 °C subcooled R134a refrigerant to minichannels at a saturation pressure (P_sat) of 7.27 bar. The influence of heat flux, vapor quality, flow behavior, and nucleation site density on wall superheat (ΔT_sat), average heat transfer coefficient (h_ave), and pressure drop (ΔP) in the hysteresis loop are investigated and compared across different specimens. It has been demonstrated that open minichannels with high nucleation site density exhibit the most significant hysteresis, with a maximum h_ave enhancement of 120 % over a wide heat flux range. This is attributed to a sequential activation process of nucleation sites as heat flux increases. In contrast, hysteresis phenomenon is significantly diminished by the limited number of nucleation sites, or due to the early occurrence of vapor backflow in closed minichannels. The effects of inlet fluid subcooling, maximum heat flux in thermal history, and refrigerant mass flow rate on hysteresis phenomenon are also characterized with the aim of establishing a comprehensive guideline to maximize hysteresis-induced thermal enhancement in microstructured minichannels for practical cooling applications.