Effect of low heat and mass fluxes on the boiling heat transfer coefficient of R-245fa. (December 2021)

• Record Type: Journal Article
• Authors: Van den Bergh, W.J.; Moran, H.R.; Dirker, J.; Markides, C.N.; Meyer, J.P.
• Is Part Of: International journal of heat and mass transfer. Volume 180(2021)
• Publication Date: 2021-12
• Subjects: Flow boiling -- Heat flux -- Heat transfer -- Mass flux; Heat -- Transmission -- Periodicals; Mass transfer -- Periodicals; Chaleur -- Transmission -- Périodiques; Transfert de masse -- Périodiques; Electronic journals; 621.4022
• DOI: 10.1016/j.ijheatmasstransfer.2021.121743
• Languages: English
• ISSNs: 0017-9310

Abstract

Abstract: In-tube flow boiling at low mass and heat fluxes is of increasing interest particularly for low-concentration solar power systems, refrigerators, heat pumps, and other thermal management components and systems. In this study, the flow boiling of R-245fa was investigated experimentally for vapour qualities ranging from 0.05 to 0.90, mass fluxes of 40, 60 and 80 kg/m 2 s, and heat fluxes of 2.5, 5.0 and 7.5 kW/m 2 . Tests were done at quasi-steady-state conditions in a horizontal smooth tube with an inner diameter of 8.31 mm and a heated length of 0.8 m, at a saturation temperature of 35°C. It was found that the heat transfer coefficient was influenced by both the mass and heat fluxes. At any given vapour quality and heat flux combination, an increase in the mass flux resulted in an increase in the heat transfer coefficient. However, the magnitude of the increase and sensitivity to the mass flux was not the same in all of the test cases. Higher vapour quality cases were more sensitive to the mass flux than lower vapour quality cases, except at low heat flux conditions. In the most peculiar case, at the lowest considered heat flux of 2.5 kW/m 2, severe sensitivity to mass flux was observed at vapour qualities between 0.2 and 0.3. For all other heat fluxes, the heat transfer coefficient was found to be independent of the vapour quality except when the mass flux was high, where increased vapour quality resulted in improved heat transfer coefficients. Various correlations were investigated, but none of them captured the trends for the lowest heat flux. Highlights: Local heat transfer measured at mass fluxes up to 80 kg/m 2 s and heat fluxes up to 7.5 kW/m 2 Between 5 and 7.5 kW/m 2, the heat transfer coefficient is independent of vapour quality The heat transfer coefficient peaks near the upstream end (inlet) of the tube at a 2.5 kW/m 2 At 80 kg/m 2 s and 2.5 kW/m 2 the heat transfer coefficient is 70% higher at the inlet than the outlet Established correlations cannot predict the heat transfer coefficient at low heat fluxes