Experimental investigation of flow and convection heat transfer of CO_{2} at super-critical pressures in small vertical tube with helical wire insertXu Ruina 摘要The flow and convection heat transfer of CO2 at super-critical pressures in a vertical small tube with helical wire inserts was investigated experimentally. The inner diameter of the tube is 0.986mm and a helical wire with an outer diameter of 0.2mm was inserted. This study investigated the effects of pressure, heat flux, mass flow rate, flow direction, properties variation, buoyancy and flow acceleration on convection heat transfer in small tube. The pressures were 7.6MPa, 8.5MPa and 9.5MPa; the heat flux ranged from 12kW/m^{2} to 63 kW/m^{2}; the mass flow rate were 0.65kg/h ,1.0kg/h,1.2kg/h,1.38kg/h and 1.75kg/h; the flow direction included both upward and downward. In downward flow, the heat transfer coefficient reached about five times the value obtained from the experiment with a plain tube of the same size near the pseudocritical temperature; in upward flow, helical wire insert could inhibit the heat transfer deterioration and the heat transfer coefficient reached almost sixteen times compared to the plain tube and the heat transfer of upward flow is stronger than downward flow. When the inlet pressure was higher than the critical pressure, the heat transfer coefficient with the increase of inlet pressure were reduced both upward flow and downward flow. The increase of mass flow rate nearly had no effect on heat transfer enhancement when mass flow rate reached 1.0kg/h. |
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