Saturated flow boiling of pure refrigerants, zeotropic and azeotropic refrigerant blends in a small-diameter round tube: Experimental results and assessment of correlations

An experimental investigation of flow boiling of single-component refrigerants R-32, R-1234yf, R-134A and R-1234ze(E), zeotropic blends R-450A, R-454B, R-454C, R-455A and azeotropic blend R-513A in a untreated aluminum round tube with hydraulic diameter 4.572 mm and effective heated length of 2 m was conducted in the range of mass flux from 100 to 500 kg/(m 2 ˑs). A database consisting of 1058 measurements for pure refrigerants and 1769 measurements for refrigerant blends corresponding to saturated boiling was obtained and used for analysis. It was shown that flow asymmetry significantly affects experimental values of local heat transfer coefficient. The predictive capabilities of multiple heat transfer correlations and pressure drop models available in the literature were assessed, demonstrating that the Friedel friction pressure drop model provides excellent agreement with experimental pressure drop data. The flow boiling heat transfer correlations assessed provided mean absolute error (MAE) ranging from 36 to 55% for single-component (pure) refrigerants and were recalibrated using data not affected by flow asymmetry that yielded MAE values as low as 23.3% for pure refrigerants. Correction methods to account for mass transfer resistance in refrigerant blends were also assessed and it was found that none of the methods can provide acceptable MAE for practical applications. Hence, a correction method was developed which can achieve a MAE of 22% for refrigerant blends.