Flow Boiling Heat Transfer and Pressure Drop Characteristics of a 45/55% by Weight Mixture of R321R125

This study investigates the heat transfer and pressure drop characteristics of a 45/55% by weight mixture of R321R125. For mass fluxes above 150 x 103 Ibm/ft2_h (200 kglm2-s) and heat fluxes below 6400 Btu/ft2_h (20 kW/m2), the experimental Nusselt number is dependent on both heat flux and qUality. An increase in either of these qualities results in anincrease in the Nusselt number. For mass fluxes below 150 x 103 lbm/ft2-h (200 kglm2-s), the Nusselt number is independent of qUality. Pressure drop is dependent on heat flux and quality for the entire range of mass fluxes tested. The accuracy of several two-phase correlations in predicting the experimental data is examined. The Shah [1976] correlation underpredicts the experimental Nusselt number and this is attributed to an underestimation of the convective boiling contribution. The Jung and Radermacher [1989] correlation overpredicts the data. This is due, in part, to the absence of accurate surface tension data for the mixture. The Wattelet [1994] correlation predicts the experimental Nusselt number to within ??20%. The Sousa [1993] correlation predicts the experimental pressure drop to within ?? 10%. iiiAir Conditioning and Refrigeration Center Project 3

Condensation of a 50/50 Blend of R-32/R-125 in Horizontal Tubes with and Without Oil

This study examined local condensation heat transfer and pressure drop for a pure 50/50% mixture of R- 32/125 and for R-32/125 mixed with approximately 1%, 3%, and 5% concentrations of an ester oil. An apparatus was built to simulate conditions found in the condenser sections of domestic refrigerators/freezers. Experiments were performed to measure the internal heat transfer coefficients and pressure drops inside a 0.277 in. (7.0 mm) o.d. smooth, horizontal copper tube. It was observed from the oil samples that the oil concentration of the oil-refrigerant mixtures flowing through the test section changed for different mass fluxes and qualities. The data for the heat transfer coefficients were compared with existing correlations and the Dobson correlation demonstrated the best accuracy for the pure R-32/125 and predicted the performance of oil-refrigerant mixtures with less than 20% error when used with the Schlager enhancement factor. The experiments showed that oil addition degraded the heat transfer coefficient at vapor qualities greater than 50% and increased the pressure drop by as much as 25% at high mass fluxes

Condensation of a Zeotropic Refrigerant R-321R-125/R-134a (23o/al25o/al52%) in a Horizontal Tube

This experimental study examined the local heat transfer coefficients and frictional pressure gradients of a ternary zeotropic refrigerant mixture, R-32/R-125/R-134a (23%/25%/52%) condensing in a smooth, horizontal tube. The test section consisted of a 3 ft long, copper tube with a diameter of 0.277 in (7.04 mm) surrounded by a counter-flow, water-cooled annulus. Test conditions varied the average quality from 10% to 90% and the mass flux from 55 to 474 klb/ft2_hr (75 to 650 kg/m2-s) to insure that data points were taken all across the wavy and annular flow regimes. The temperature difference was held approximately constant at 5.4 OF (3.0 ??C). At mass fluxes higher than 364 klb/ft2_hr (500 kg/m2-s), the Dobson correlation consistently overpredicted the heat transfer coefficients requiring a least squares-determined factor of 0.836 to correct. As the flow regime approached wavy flow, the Dobson correlation increasingly overpredicted the heat transfer coefficient due to the dominance of zeotropic degradation. Frictional pressure gradient data showed similar trends to the Souza pressure drop correlation but were significantly lower.Air Conditioning and Refrigeration Center Project 3