Evaporation of pure fluids and mixtures in a microchannel tube

Two-phase flow in microchannel tube is taking huge interest by both the academic and engineering worlds. It is necessary to understand the flow when the diameter is at the scale of one millimeter. The decreasing in diameter enhances the heat transfer, but also increases pressure drop. The two-phase flow pattern in microchannel tube is simpler due to the importance of surface tension. However, existing flow pattern maps, heat transfer coefficient and pressure gradient correlations are not accurate enough in prediction. This study focuses on the measurements and modeling of two-phase flow in microchannel tube. Six pure fluids (R134a, R32, R1234ze(E), R1234yf, R1233zd(E), and R1336mzz(Z)) are tested, and they have significant difference in properties. The experiments are conducted in a 24-port microchannel tube with an averaged hydraulic diameter of 0.643 mm. The tested microchannel has hydraulic behavior as a round tube (fRe=64 in laminar flow). Heat transfer coefficient, pressure gradient, and flow patterns are measured or recorded simultaneously. Deep discussion is made for illustrating the two-phase flow in microchannel tube by connecting flow patterns to measurements. Measurements are done at varied mass fluxes, heat fluxes, saturation temperatures to understand the effect of the operating conditions. The results are compared with each other to discuss the effects of properties. Comparisons to existing predictive models are made and discussed. The flow patterns are also reported and compared to existing maps. Measurements in the video show the homogeneity in plug/slug flow when velocity is low. Novel video processing method is introduced for measuring vapor plug velocity in videos. A liquid droplet forming mechanism is reported. The liquid slug collides with liquid ring and breaks into several droplets in the vapor core. One commercial mixture (R448A) and three lab made mixtures (R32+R1234yf at 15/85, 50/50, and 85/15 mass fractions) experiments are also conducted on the same facility. Results are compared to the component fluids. The effect of the temperature glide on zeotropic mixtures is reported and discussed. A new flow pattern map based on force balance and kinetic energy analysis is introduced. The flow pattern map shows good agreement with the measurements. A mechanistic model based on liquid film thickness in annular flow is proposed. The model calculates void fraction, pressure gradient, and heat transfer coefficient at the same time and it has good agreement with the measurements.U of I OnlyAuthor requested U of Illinois access only (OA after 2yrs) in Vireo ETD syste