Flow structure with mixed turbulent flow of supercritical CO2 heated in helically coiled tube

The mixed turbulent convection heat-transfer characteristics of supercritical CO2 (S-CO2) heated in both vertical and horizontal oriented helically coiled tube (HC-tube) is investigated via experimental and numerical methods. Firstly, the coupled equations of flow direction, buoyancy force and centrifugal force in HC-tube are established. Then, the influence of the three factors on axial and radial heat transfer coefficient (HTC) performance are discussed. In vertical oriented HC-tube, the HTC first increases progressively and then drop swiftly, presenting a peak value at the pseudo-critical point. But, in horizontal oriented HC-tube, due to buoyancy force effects, the HTC oscillates dramatically which indicates the poor heat-transfer stability. Meanwhile, the numerical simulation, corresponding to the experiment condition, is carried out. To capture the flow structures, 6 characteristic lines on each cross-section are selected and the corresponding velocity and the turbulent kinetic energy distributions are comparative analyzed. The results showed that the coupling effects of centrifugal force and buoyancy force mainly affect heat-transfer on the inside of a cross-section. In addition, different from the M-type velocity in straight tube, the salient point of M-type velocity in HC-tube always locate at the outer side of the buffer-layer and the heat-transfer is slightly affected. The local heat-transfer deterioration mainly occurs at inside of a cross section where the velocity is obviously suppressed.