Falling film evaporation heat transfer characteristics of R1336mzz (Z) for various tube types under high-temperature conditions

초록

Falling film evaporators designed for heat pumps can enhance energy efficiency compared to traditional flooded evaporators. However, experimental studies on the falling film heat transfer performance of low-global warming potential (GWP) refrigerants in enhanced tubes under high-temperature conditions remain limited. This study investigates the falling film evaporation heat transfer characteristics of R1336mzz(Z) in smooth, corrugated, and endcross tubes under high-temperature conditions. For all tube types, the evaporation heat transfer coefficient (HTC) of R1336mzz(Z) gradually increased with the film Reynolds number (Re) in the partial dry-out regime but stabilized within 10% beyond a threshold Re in the fully wetted regime. Moreover, in the fully wetted regime, the corrugated and endcross tubes exhibited 80% and 43% improvements in HTCs with an increase in heat flux from 9 to 23 kW m-2 , respectively. Additionally, when the saturation temperature increased from 60 degrees C to 70 degrees C, the HTCs of the corrugated and endcross tubes rose by 26% and 13%, respectively, due to improved wettability. Among the tested tubes, the endcross tube exhibited the highest HTC due to its superior wettability. At film Re values of 530-800, the endcross tube showed an improvement in HTC by a factor of 1.4-1.7, compared to the smooth tube. Finally, Nusselt number (Nu) correlations were developed separately for the partial dry-out and fully wetted regimes as functions of dimensionless operating parameters. This research provides the experimental dataset of R1336mzz(Z) in enhanced tubes under high-temperature conditions and reveals new insights into the distinct heat transfer mechanisms governing the partial dry-out and fully wetted regimes. Moreover, these results can be utilized to design falling film evaporators with enhanced tubes using low-GWP refrigerants in high-temperature conditions.

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