Influence of the microfin tube structure on the thermal-hydraulic performance of mixed supercritical CO2/R32

Abstract

This research establishes 5 mm three-dimensional (3-d) flow and heat transfer microfin tube theoretical models with three different geometric structures. Using these models, the thermal-hydraulic performances of supercritical CO2/R32 in microfin tubes with different structures at various working conditions were investigated. The influences of each of three factors (pressure, mass flow, and microfin tube structures) on the thermal-hydraulic performance of CO2/R32 were evaluated respectively. Furthermore, orthogonal tests were undertaken to obtain the optimized combination of overall thermal-hydraulic performance. Results indicate that: the more the temperature of working media approximates to the critical temperature, the bigger the local convective heat transfer coefficient. Compared to non-critical temperatures, the convective heat transfer coefficient at critical temperature shows an eight-fold increase. The closer the pressure of the mixed working media is to the critical pressure, the greater the maximum convective heat transfer coefficient (CHTC) and the lower the temperature corresponding to the peak point, among which, the maximum CHTC under 7.5 MPa is three times as large as that at 8.5 MPa; the CHTC increases with increasing mass velocity, generally showing a linear relationship; through calculating the most optimal combination of thermal-hydraulic performance evaluation using orthogonal tests, the maximum CHTC is determined to be 96 kW/(m2?K).