NUMERICAL INVESTIGATION ON HEAT TRANSFER OF SUPERCRITICAL PRESSURE CO2 IN A BENT PIPE

The flow characteristics and heat transfer mechanism of supercritical pressure CO2 in a bent pipe were studied by the method of numerical simulation. The velocity, temperature, secondary flow, and turbulent kinetic energy distribution of the CO2 under different heating conditions are analyzed. The effects of pipe bend and sharp changes in the thermal properties of CO2 on the flow field and temperature field are discussed and compared. The results show that a narrow high-temperature region appears on the inside wall, where the turbulent kinetic energy is low. Here, the local heat transfer coefficient decreases by 60%, compared with the other areas of the bend. Under the conditions of high heat flux and low mass flux, this narrow region transfers from the inside wall to the outside wall, and a reduction in heat transfer occurred. More specifically, the heat flux increases from 20 to 50 kW·m-2, and the heat transfer coefficient decreases by 35%, while the mass flux decreases from 600 to 400 kg·m-2·s-1, and the heat transfer coefficient decreases by 50%. When the pressure changes from 7.58 MPa to 9.00 MPa, the heat transfer coefficient is maximum of 5% reduction. At higher pressures, the heat transfer coefficient is slightly affected by the pressure. Meanwhile, the secondary flow significantly enhances the convective heat transfer, and the range and intensity increase with the flow development.