Flow Boiling Heat Transfer of CO_2 At Low Temperatures- Challenges and Opportunities

摘要

There is a growing use of CO_2 refrigeration to achieve low temperatures, particularly in the food industry, however, very limited information is available in the open literature on its boiling heat transfer characteristics below (-)30℃. This paper presents an overview of the flow boiling heat transfer characteristics of pure CO_2 at low temperatures, its' special thermal properties and their effect on the boiling heat transfer. The paper also presents an analysis of the experimental data collected from (-) 24.3℃ to (-) 40℃ in a novel experimental rig, specifically designed to achieve low temperatures down to (-) 50℃, using 4.5m long horizontal stainless steel tube of 4.57mm inner diameter. The paper highlights the limitations of existing empirical correlations by comparing their predictions with the experimental boiling heat transfer coefficients. It is expected that the data presented in this study would be beneficial to industry and designers of compact heat exchangers for CO_2 at low temperatures.