Abstract:
The study of refrigerants and their blends has always been important due to their wide use and environmental impact. In this study, the thermophysical properties and nanoscale phase change characteristics of R152a/R1234ze(E) blends with varying molar fractions have been investigated using non-equilibrium molecular dynamics simulation (NEMD) under non-equilibrium heating conditions. In total, five different compositions of the refrigerant blend have been considered with the blends containing 100%, 67%, 50%, 33% and 0% molar fraction of R152a. Thermophysical properties such as density, liquid thermal conductivity and isobaric specific heat have been computed for the refrigerant blends considered. To study phase characteristics a three-phase domain is considered where liquid and vapor refrigerant molecules are placed over the solid platinum (Pt) surface. Two different heating conditions for the wall have been applied to induce various phase change modes. The first condition involved a linear increase in wall temperature, where temperature rise rates of 80, 160, and 240 K/ns are employed. The second condition maintained constant wall temperatures of 300 K and 320 K respectively.
The phase change characteristics of different refrigerant blends under various heating conditions have been reported utilizing various parameters such as net evaporation number, time averaged heat flux, bubble growth, mean square displacement, and energy contours. Three distinct boiling modes are observed, depending on the heating rate and the mixture composition of the refrigerant blend. Moreover, under isothermal heating conditions, diffusive evaporation of the refrigerant blends took place. The results indicate that the increase in percentage of R152a in the refrigerant blend enhances the heat transfer performance. However, considering both environmental impact and thermal performance, refrigerant blend with %R152a of 67% exhibit better overall performance. Furthermore, if environmental considerations are stricter refrigerant blend up to 50% R152a percentage can be considered without significant decline in phase change performance.
In addition, in this study the bubble nucleation process and interfacial characteristics of the refrigerant blends have also been examined to elucidate the mechanisms behind the phase change process. The interfacial behavior is analyzed in terms of interfacial thermal resistance, potential energy contours, and spatial density distribution. It is found that R1234ze(E) molecules exhibit a stronger attraction to the solid surface, resulting in lower interfacial thermal resistance.