Frictional pressure drop during flow boiling in micro-fin tubes: A new general correlation

Due to the wide commercial application of micro-fin tube and eco-friendly refrigerants, a more general frictional pressure drop correlation is required for a better prediction, and this study is aimed at compared existing correlations and provide guides for the furthermore improvement. Experimental data points for frictional pressure gradients during flow boiling of different working fluids in horizontal helical micro-fin tubes were extracted from reported experimental investigations in the literature to evaluate numerous existing frictional pressure drop correlations and specify the applicability of each correlation to meet the urgent demand of extensive application of eco-friendly refrigerants. The database consists of 1014 data points covering eleven refrigerants (R1233zd(Z), R410A, R1234ze(E), R410A, R22, R32, R1234ze(Z), R22, R134a, R245fa and R1234yf included), and the covered operating conditions are as follows: mass velocity 94-888 kg m(-2) s(-1), vapor quality 0.04-0.99, heat flux 3.9-85.2 kW m(-2), and equivalent diameter 2.12-11.84 mm. Eight general frictional pressure drop models, including Cavallini et al., Kuo and Wang, Wongsangam et al., modified Friedel, Diani et al., Choi et al., Goto et al., and Rollman and Spindler correlation, were compared against the collected data points. The results indicated that none of these correlations could provide a satisfactory prediction for a general operating condition, though the Diani et al. correlation exhibits the highest predictive accuracy with 73.0% of the data points in an error band of +/- 30%. A new empirical correlation was developed by introducing the Churchill model for the effect of fins and optimizing the relationship between vapor quality and frictional pressure gradients. (C) 2020 Elsevier Ltd. All rights reserved.