Upstream heating history effects on heat transfer of supercritical R134a in transcritical organic Rankine cycle

Abstract

A comprehensive understanding on the heat transfer of supercritical working fluid is critical for the design of vapor generator in transcritical Organic Rankine Cycles. The thermal entrance region under supercritical pressure conditions is relatively complex and remains unresolved. To this end, this study investigates heat transfer to supercritical fluids in heated horizontal flow, focusing on the impact of upstream heating history which has been rarely discussed. The Abe-Kondoh-Nagano (AKN) low Re k–ε turbulence model was employed, covering a wide range of upstream conditions: upstream heating length ranging from 10d (d is tube diameter) to 200d, and upstream heat flux up to 4.5 times that of the downstream test section. The results revealed that heating history didn’t matter when upstream Grq/Grth <5. For upstream mixed convection of Grq/Grth =10–220, heating history had a significant effect on the downstream flow, causing a maximum 50% to 60% increase in heat transfer coefficients within an affected length up to 100d. When the downstream flow was forced convection, thermophysical property variations mainly contributed to the above enhancement. Meanwhile for downstream mixed convection, upstream heating history notably enhanced turbulent intensity, further improving heat transfer. The upstream heating length was identified as a crucial factor influencing both forced and mixed convections downstream. Negligible differences were observed when the heating length was less than 20d. On the other hand, the radial distribution of flow and thermal fields remained constant beyond a critical heating length, indicating the achievement of a pseudo-fully-developed state with identical heat transfer behavior. Results of this work confirm that the effect of upstream heating history is closely tied to thermal developing length and holds significant importance in the thermal design of vapor generators.