Study on heat transfer and bubble behavior inside horizontal annuli: experimental comparison of R-134a, R-407C, and R-410A subcooled flow boiling

Abstract Subcooled flow boiling (SFB) is a complicated phenomenon regarding the significant heat transfer and vapor bubble formation, which is extremely critical either in daily life or industrial application. The present work pertains to the experimental study on heat transfer characteristics and bubble behaviors in narrow horizontal annuli with emphasis on diverse refrigerants of R-134a, R-407C and R-410A. From experimental results, the significant temperature undershoot during ONB is observed and the level of temperature drop is R-134a > R-407C > R-410A. Enhancing the input heat flux increases boiling heat transfer coefficient (hr) for all three refrigerants. The hr of R-410A always remains higher than R-134a and R-407C stemming from the physical property of lower surface tension. The visualization of bubble behavior addresses that enhancing the input heat flux improves buoyancy force, resulting in the increases of bubble departure size and departure frequency. To achieve a quantitative analysis of bubble characteristics, three indexes of bubble detachment diameter (dp), bubble departure frequency (fb) and the active nucleation site density (Nac) are introduced to explore bubble dynamic behaviors in SFB. The results show that the refrigerant of R-410A exhibits the smallest dp but the highest fb and Nac among these three refrigerants.