Numerical study on the enhancement of local and average heat transfer in supercritical CO2 horizontal flow by twisted tape inserts

Twisted structures have potential to reduce circumferential thermal inhomogeneity in horizontal flow, thereby enhancing the stability and operational safety of supercritical Rankine cycle systems. Here, the effects of key operating parameters on thermal performance of supercritical CO2 in horizontal tubes with twisted tape (TT) inserts are numerically investigated by quantitatively evaluating the performance evaluation criterion (PEC). Results indicated that the thermal field inside the tube with TT is more uniform, which can effectively reduce circumferential thermal non-uniformity up to 95.7 %. The beneficial effect of TT on the circumferential thermal uniformity is more pronounced with the increase of mass flux, heat flux, inlet temperature, and operating pressure. In particular, mass flux has a much larger effect on the overall heat transfer enhancement than other parameters. For the average performance, a maximum PEC of 1.81 is observed at mass flux of 1200 kg/(m(2)<middle dot>s). In general, PEC is larger than 1 when the ratio of heat flux to mass flux is between 0.09 kJ/kg and 0.2 kJ/kg. Further studied on the interaction between buoyancy and TT reveals that the above overall enhancement (PEC > 1) is due to the as the effective suppression of TT on the buoyancy effect within this q/G range.