Enhanced pool boiling and hysteresis of refrigerant R-134a and its potential alternatives R-1336mzz(E), and R-1336mzz(Z)

• Two electrodeposited surfaces and one hierarchical surface developed to enhance pool boiling heat transfer of refrigerants. • Boiling heat transfer performance characterized using R-134a, R-1336mzz(E) and R-1336mzz(Z). • Hierarchical Cu surface enhances heat transfer by increasing surface area and nucleation site density. • Hierarchical Cu surface mitigates boiling hysteresis by enhancing bubble entrapment in R-1336mzz(E). • A model for boiling hysteresis and bubble accumulation is proposed based on optical visualization. The enhancement of refrigerant pool boiling has garnered significant attention for the design of compact shell-and-tube heat exchangers for large capacity residential, commercial, and industrial chillers. The increased need for compactness and effectiveness stems from the increasing demand for chillers to enable building air conditioning in an ever-warming world. This study develops three modified surfaces utilizing electroplating and cathodic deposition techniques to enhance refrigerant pool boiling. Among these, the low-current electroplated copper tube demonstrated suppressed boiling of R-134a and low Global Warming Potential (GWP) refrigerants R-1336mzz(E), and R-1336mzz(Z). In contrast, the high-current electrodeposited tube enhanced boiling in R-1336mzz(E) and R-1336mzz(Z) by increasing the surface area and cavity size, achieving an enhancement ratio of 1.5 at a specific heat flux. Notably, strong boiling hysteresis was observed in the high-current electrodeposited tube, with inactivated potential nucleation sites reducing the heat flux. The enhancement ratio also increased with the rise in the maximum activation heat flux. However, the addition of CuO surface roughness on top of the high-current electrodeposited copper tube (designated as Hierarchical Cu), effectively mitigated boiling hysteresis in R-1336mzz(E). The average overshoot temperature decreased from 3 °C to 1 °C. The varying effective cavity size range of the Hierarchical Cu tube results in different structures being effective for different refrigerants. In R-134a and R-1336mzz(Z), the effective structures are CuO with a structure size R structure = 1 µm and high-current electrodeposited Cu with R structure = 350 µm respectively. In R-1336mzz(E), both structures contribute to the boiling enhancement. This study discusses the mechanisms by which modified surfaces enhance pool boiling of refrigerants, providing insights into the design of surface structures to promote boiling for potential low-GWP refrigerants in future chiller applications.