Experimental study on boiling flow patterns of the zeotropic mixtures of R290/R600a in a horizontal smooth tube

The mixture of R290/R600a is a potential refrigerant alternative to R134a and the study of its flow pattern is the basis for designing heat exchangers in refrigeration systems. In the present study, the flow pattern and transition of the zeotropic R290/R600a two-phase mixtures are investigated experimentally using a 6 mm inner diameter horizontal smooth tube. Aiming at analyzing the influence of affecting parameters on the results, the variation range of the mass flux, heat flux, and saturation pressure were set to 63–190 kg m⁻² s⁻¹, 10.6–73.0 kW m⁻², and 0.215–0.415 MPa, respectively. Three types of flow patterns were observed and recorded. The transitions between different flow patterns were compared with several flow pattern maps and none of them had enough accuracy. The reason can be attributed to the lack of consideration of the saturation pressure and the non-equilibrium characteristics in the boiling process of the zeotropic mixtures in the existing correlations. Consequently, a new model was proposed with a new dimensionless number F and X_tt. In the proposed dimensionless number F, the effects of inertia, gravity, shear tension, surface tension, heat flux, reduced pressure, and concentration were considered. The new flow pattern map showed satisfactory prediction accuracy and 94.9% of the experimental data could be accurately predicted.