Nucleate boiling of refrigerant–oil mixtures : bubble equilibrium and oil enrichment at the interface of a growing vapour bubble

Abstract

Equilibrium conditions of a vapour bubble, embedded in an infinite refrigerant–oil mixture, and oil enrichment at the interface of a bubble, growing in such a mixture are the main subjects of this paper. On the assumption of an ideal mixture behaviour and a non-volatile oil, an expression is derived that connects the temperature, the oil concnetration and the size of the bubble in equilibrium with the mixture. Using this expression, it is shown that oil, added to a refrigerant, can facilitate the bubble formation and improve the heat transfer. For this to occur, the oil must contain some surface active components. It is further demonstrated that the quantity of oil, corresponding to oil enrichment in the liquid due to bubble growth—the oil excess—suffices to envelop the bubble as a closed film already at a relatively low oil concentration. The film thickness increases as the bubble grows amounting to several monomolecular layers, if the oil excess is assumed to remain at the bubble surface. For the same bubble size, the number of such layers, ‘the coverage number’, is shown to be larger at a higher boiling temperature. When oil diffusion in the liquid occurs, a minimum driving temperature difference is needed for the film to establish. At a temperature difference above this minimum value, the oil film enveloping the bubble influences its growth kinetics. The film acts as ‘an interphase’ that must be passed through by refrigerant molecules to evaporate.