Bubble dynamics of R-123 and R-134a on pore/sub-tunnel surfaces

Structured enhanced surfaces are widely used to promote nucleate boiling heat transfer in refrigeration and process industries. Despite the frequent usage of the enhanced surfaces, there appears a significant lack of understanding on the boiling mechanism or predictive models. To enhance the understanding, the bubble dynamic data, which are the building blocks of the predictive models, are of necessity. This study continues that by Mehdi et al. [15], who reported R-134a bubble dynamic data on a pored surface. In this study, the tests were extended to R-123. The nine samples included those having 0.1–0.3 mm pore diameters and those having 0.75–3.0 mm pore pitches. The results showed that, R-123 yielded larger bubble departure diameters and more nucleation site densities than R-134a. The reason was attributed to the larger surface tension and liquid thermal conductivity of R-123 over R-134a. For both refrigerants, the trends of bubble dynamics according to the pore geometry were almost identical. The bubble departure diameter and the bubble generation frequency increased as the pore diameter increased. The nucleation site density increased as the pore pitch decreased. An improved model was developed extending that of Chien and Webb [13]. The model reasonably predicted the heat transfer coefficients as well as the bubble dynamic data of the present surfaces (95% of the heat transfer coefficients within ± 40%).