Effect of intermittent heating on the heat transfer performance of supercritical R134a flowing in a pipe

Heat transfer performance of supercritical fluid is unique due to the significant variance of the thermo-physical properties of fluids around the pseudo-critical temperature. Aiming at exploring the effect of intermittent heating on heat transfer to supercritical fluid, experimental and numerical study on supercritical R134a flowing upward in an inner diameter 7 mm and 2.3 m long intermittent heated pipe have been carried out. The pipe is heated by Joel effect and 0.3 m-long part in the middle of pipe can be short-circuited. The test is carried out at 4.3 MPa and the mass flux is 600 kg/(m2 s). The heat flux ranges from 20 to 40 kW/m2. The Re number varies between 4.1 × 104 and 2.1 × 105. It's found that the variance of HTC (heat transfer coefficient) due to the intermittent heating effect and the length of the pipe where heat transfer affected by the intermittent heating effect at low, medium and high heat fluxes is different. Heat transfer enhancement is observed at the downstream of no-heating section for all three kinds of cases. The enhancement due to the intermittent heating effect at the heat transfer deterioration case (heat transfer deterioration happens if the pipe is uniformly heated under the same heat flux, inlet pressure and temperature) is much stronger than that at the normal heat transfer case (heat transfer deterioration does not happen if the pipe is uniformly heated under the same heat flux, inlet pressure and temperature). The maximum HTC ratio for the heat transfer deterioration case is as high as 90%. Moreover, the location where heat transfer deterioration happens is shifted by the intermittent effect. The length affected by the development of boundary layer due to intermittent heating effect is about 40 diameters for the normal heat transfer case and it is increased to 90 diameters for the heat transfer deterioration case since the heat transfer deterioration region is shifted further downstream. With the assistance of numerical simulation results using k-ω SST turbulence model, the reason for heat transfer enhancement and the shift of heat transfer deterioration is discussed.