Experimental study on flow boiling heat transfer and pressure drop of R245fa/R141b mixture in a horizontal microfin tube

Abstract

• The HTC and frictional pressure drop of R245fa/R141b in a micro fin tube are tested. • Effects of mass fraction of R245fa on HTC and frictional pressure drop were investigated. • Experimental data of HTC and frictional pressure drop are compared against predicted values. The heat transfer and pressure drop characteristics of the mixture are the design basis of evaporators and optimization of Organic Rankine Cycle (ORC). As a promising mixture for ORC systems, R245fa/R141b with different mass fractions (1/0, 0.75/0.25, 0.5/0.5, 0.25/0.75 and 0/1) were selected as the working fluid. The heat transfer coefficient (HTC) and frictional pressure drop in a horizontal microfin tube with 8.6 mm equivalent diameter were measured. During the experimental tests, the saturation pressure is vary from 0.15 to 0.25 MPa (saturation temperature from 25.3 to 60.5 ℃), mass fluxes from 203.19 to 306.64 kg•m -2 s -1 , and heat fluxes from 7 to 15 kW m -2 . Additionally, based on the empirical correlations in the literature, we developed a correlation with higher prediction accuracy. The results indicate that the HTC and pressure drop of R245fa/R141b increase with mass flux, while decrease with saturation pressure. The increase of the mass fraction of R245fa results in an increase of the HTC and friction pressure drop for R245fa/R141b. The correlation of Cavallini–Kattan are consistent with the experimental data of heat transfer with mean absolute deviations of 16.68%. When mass flux is less than 200 kg m -2 s -1 and vapor quality is less than 0.2, modified Dittus–Boelter correlation can predict 87.3% data points in the error range of ±30% with a MAD of 6.75%. As to frictional pressure drop, the deviations of ±10.34% experimental data are obtained by the modified Filho correlation.