Heat transfer characteristics of supercritical pressure water in vertical upward annuli

• Effects of annular gap size on heat transfer are investigated.
• Wire-wrapped spacer has local and global effects on heat transfer.
• Influence of buoyancy on heat transfer is evaluated.
• Heat transfer correlations were assessed against annuli test data.

Heat transfer characteristics of supercritical pressure water in vertical-upward annuli with annular gaps of 4 mm and 6 mm were investigated experimentally. The inner heated rod has an outer diameter of 8 mm with an effective heated length of 1400 mm. Experimental parameters covered the pressures of 23–28 MPa, mass fluxes of 350–1000 kg/m2s, heat fluxes of 200–1000 kW/m2 and inlet bulk temperature up to 400 °C. According to the experimental data, the effects of heat flux and mass flux on heat transfer of supercritical water were analyzed. Experimental results showed that heat transfer of various heat fluxes and mass fluxes in annuli are similar with those in tubes. Compare the heat transfer differences in the two annular gaps, it was found that heat transfer in 6 mm gap channel is better than that in 4 mm gap channel, especially in the pseudo-critical enthalpy region. Experimental results also showed that the spiral spacer, which was arranged on the outer surface of the heated rod, has a positive effect on enhancing local heat transfer. However, this enhanced phenomenon seems stronger in 4 mm gap compared to that in 6 mm gap. The criterion of Jackson–Hall was selected to distinguish the effect of buoyancy in annular channels. Predicted results demonstrated that this criterion achieves good agreements against the experimental data at various mass fluxes and pressures. The present paper compared the experimental data with eight heat transfer correlations for supercritical pressure water. It was found that the correlations of Jackson, Bishop and Cheng are most close to the test data for normal heat transfer region, whereas only the correlation of Cheng seems acceptable when heat transfer deterioration occurs.