Experimental Investigation of Heat Transfer to Supercritical Pressure Fluid in Tubes With Rough Inner Surface

The highly corrosive environment inside a supercritical water-cooled reactor (SCWR) places stringent demands on the fuel rod cladding material, particularly requiring it to have strong corrosion resistance. However, within a refueling cycle, an oxide layer is growing on the surface of the fuel rods. In any case, the heat transfer to water under supercritical pressure conditions (⁠⁠p$_{cr}$ = 220.64 bar, T$_{cr}$ = 373.95 °C) is an overly complex phenomenon, since the thermophysical properties of the fluid show drastic variations with respect to the temperature around the pseudo-critical temperature. An increase in the surface roughness height has an impact on the heat transfer. To provide insight into the effect of surface roughness on heat transfer an experimental database, using the surrogate fluid R134a (p$_{cr}$ = 40.59 bar, T$_{cr}$ = 101.06 °C⁠⁠), covering a range of flow conditions is established. The database consists of reference data, obtained in a conventional hydraulic tube and of data obtained in a tube with an artificially roughened inner surface. In this work, the impact of the surface roughness on heat transfer is evaluated, by comparing the results obtained in the smooth tube, to the results obtained in the tube with rough inner surface. ... mehrHeat transfer is enhanced when the Reynolds number is large enough and heat transfer deterioration can be suppressed or shifted to larger bulk enthalpy, due to the roughness. Furthermore, existing empirical correlations are assessed against the newly generated experimental database. It is concluded that none of the available correlations satisfactory predicts the experimental data over the entire range of Reynolds numbers, surface roughness, and wall-to-bulk temperature ratios.