Experimental and numerical comparison of the heat transfer behaviors and buoyancy effects of supercritical CO2 in various heating tubes

Abstract

For the different flow orientations and physical models, the influence of buoyancy effect on flow characteristics exist essential difference, the heat transfer behaviors change accordingly. To suppress heat transfer deterioration and improve heat transfer performance, it is quite necessary to discuss the heat transfer behaviors of supercritical CO2 in various heating tubes. The heat transfer and flow characteristics of supercritical CO2 heated in the vertically straight tube, horizontal tube, and vertical helical-coiled tube, with inner diameter of 4 mm, are comparatively studied by experiments and numerical simulations. The tests are conducted at operating pressures from 7.5 MPa to 9 MPa, the mass flow rate is in the range of 80–600 kg/(m2•s). The heat flux covers a range from 10 kW/m2 to 70 kW/m2. The coupling relationship of buoyancy effect and flow characteristics in different physical models are revealed, and the various buoyancy criteria are validated with experimental data. The experimental results confirm that the employment of buoyancy parameter needs to take full account of the interaction between buoyancy force and flow orientations. And experimental data indicate that the heat transfer deterioration in the vertical tube is more serious than it in the horizontal tube. Generally, the helical-coiled tube has a noteworthy advantage for the average heat transfer performance, especially in the case of strong buoyancy. The new empirical correlations for the horizontal tube and helical-coiled tube are proposed with experimental data.