Bubble dynamics of R-134a/POE and R-123/MO mixture on enhanced surfaces having pores on sub-tunnels

Abstract

Structured enhanced surfaces are widely used to promote the nucleate boiling heat transfer in a refrigeration system, where a compressor oil circulates. However, there is a significant lack of understanding of the mechanism or predictive models, especially on the boiling of refrigerant/oil mixtures on enhanced surfaces. In this study, as a building block, the bubble dynamic data as well as the heat transfer coefficients were obtained for the refrigerant/oil mixtures on pored surfaces. The test mixtures included R-134a/polylester (POE) and R-123/mineral oil (MO). The results showed that the oil degraded the size of the departing bubbles. The degradation was more severe for R-123/MO than R-134a/POE, and it increased as the saturation temperature increased. The sequence of degradation among samples generally followed the reverse order to the magnitude of the heat transfer coefficient of the pure refrigerant. Similarly, the oil degraded the frequency. The degradation was, in general, in reverse order to the degradation of the bubble departure diameter. The oil also degraded the nucleation site density. As for the heat transfer coefficient, the degradation was more significant at a higher saturation temperature, and for R-134a/POE than for R-123/MO. The changes of bubble dynamic parameters as well as the thermo-physical properties of the refrigerants were shown to be responsible. A heat transfer prediction model was developed based on the bubble dynamic parameters.