Conference Year

2012

Keywords

numerical simulation, finite volume, R-134a, minichannel evaporator, refrigerant mixtures

Abstract

The need for more compact and more efficient heat exchangers in the aerospace, automotive, and HVAC&R industries has led to the development of heat exchangers that utilize minichannel or microchannel tubes coupled with louvered fins. In this study, a finite volume, steady-state evaporator model that includes rectangular minichannel and microchannel tubes with louvered fins and headers was developed and validated in Matlab. The model provides the user with the option to select from multiple published correlations for calculating the air-side and refrigerant-side heat transfer and pressure drop within each control volume. Model validation was performed using the experimental data presented in Wu and Webb (2002), Yun et al. (2007), Qi et al. (2009) and Shi et al. (2011). The average error between the predicted and actual cooling capacity for these four studies was 8.54%, 12.62%, 4.94% and 7.93%, respectively, with an average deviation of 8.5% (n = 29). It should also be noted that the range of examined cooling capacities in this validation was fairly large (i.e. 325 W to 40,850 W), and the simulation underpredicted the cooling capacity with approximately the same frequency as it over-predicted it. The model was then used to explore the thermal-hydraulic performance of two ternary refrigerant mixtures— namely, R-125/R-32/R-161 (34%/15%/51%) versus R-22 and R-125/R-143a/R-161 (45%/40%/15%) versus R- 404A. The physical properties of these refrigerant mixtures were estimated using REFPROP 9.0 and (where possible) verified by actual experimental property data. Constant mass flux conditions of 60, 80 and 100 kg/m2s were used for these simulations. It was found that the heat transfer rate per surface area of the simulated evaporator containing R-125/R-32/R-161 (34%/15%/51%) was more than 58% higher than that of R-22 for the same mass flux for 1.7 mm < Dh < 3.7 mm. The refrigerant-side pressure drop of this mixture was also simulated over this range of Dh and found to be comparable to the pressure drop for R-22 at 60 kg/m2s and only slightly higher than R-22 at 80 and 100 kg/m2s.

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