Experimental determination of the forced convective boiling heat transfer coefficients of R407C in fluted-tubes
De Vos, Wouter Philip
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Due to the phase-out of all refrigerants with ozone depletion potential, a large void is left in the refrigeration market. This void was caused due to a lack of new, ozone friendly, pure refrigerants with similar thermodynamic properties to those of the banned refrigerants. As a result mixtures of refrigerants are used to create replacement refrigerants. These new mixtures have to be experimentally evaluated to derive correlations for the prediction of the heat transfer coefficients. One of these mixtures is R407C. With this new refrigerant and the need for smaller, more compact heat exchangers, a search was initiated for a correlation describing the heat transfer coefficients of R4O7C in fluted-tube compact heat exchangers. The need for such a correlation is to accurately design compact fluted-tube heat exchangers for use in heat pumps and refrigeration systems. Fluted-tube heat exchangers are in general, much smaller and more compact than standard smooth tube-in-tube heat exchangers. The purpose of this study was to experimentally determine the forced convective boiling heat transfer coefficients of R407C in fluted-tubes; furthermore, to test these experimental values against existing heat transfer correlations. The product of this study is experimental boiling data for R22 and R407C in fluted-tubes, together with a correlation to predict the boiling heat transfer coefficient of R4O7C in fluted-tubes. The test bench used, was a 15 kW heat pump with a split evaporator design. The split evaporator is made up of a bypass evaporator and a test evaporator, with the test evaporator consisting of three separate heat exchangers: the pre-evaporator, test section and the super-heater. Of the three heat exchangers only the test section was a fluted tube-in-tube heat exchanger with the other two smooth tube-in-tube heat exchangers. The test section was operated in a counter flow configuration with water flowing inside the fluted-tube and the refrigerant flowing in the annulus. The accuracy of the test bench was validated using R22 in a smooth tube-in-tube heat exchanger, resulting in a maximum deviation between the water and the refrigerant heat transfer of 2.5%. After the validation the smooth tube test section was replaced with the fluted-tube section and tested with R22 and R407C. The refrigerant mass flow rates ranged from 0.01 kg/s - 0.03 kg/s. Along with the mass flow rates the heat fluxes were varied from 0.89 kW/m2 - 20.34 kW/m2 and with evaporating pressures set at 4.0, 4.5, 5.0 and 5.5 bar respectively. The maximum deviation between the water heat transfer and the refrigerant heat transfer for all the tests was 2.77% with an overall average deviation of 0.83%. The experimental results for R22 and R407C were evaluated against seven correlations found in the literature consulted: Gungor and Winterton (1986:351), Gungor and Winterton (1 987: l48), Liu and Winterton (1 991 :2759), Pierre (ASHRAE Fundamentals, 1997:4.7), Chen (1963: I), Rousseau et al. (2003:232) and Kattan et al. (1 998c: 156). All the correlations were used as found in the literature, with the exception on the Rousseau et al. (2003:232) correlation. The enhancement factor used in this correlation was adapted to fit the experimental data better. Comparing results of all the correlations, it was found that Rousseau et al. (2003:232), with the adapted enhancement factor, gave the best results for R22 as well as R407C, with the respective average deviations of 12.67% and 3.83% and respective mean deviations of 35.16% and 22.01%. A new correlation was proposed combining the geometric properties of the Rousseau et al. (2003:232) correlation with the boiling heat transfer of the Gungor and Winterton (1 987: 148) correlation.
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