Numerical analysis of buoyancy effect and heat transfer enhancement in flow of supercritical water through internally ribbed tubes

Heat transfer to supercritical water in vertical Internally Ribbed Tubes (IRT) with various geometries was numerically investigated in a range of p = 25 MPa, T-b = 590 similar to 700 K, G = 100 similar to 1000 kg.m(-2).s(-1), q = 200 similar to 800 kW/m(2) and q/G = 0.33 similar to 3 kJ/kg. The Shear-Stress Transport k-omega (SST) model was employed to solve the turbulent flow and conjugate heat transfer. In IRT with different rib geometries, buoyancy-induced deterioration of heat transfer did not occur and buoyancy forces continuously enhanced heat transfer at Bo < 3 x 10(-3). Causes for the superior performance of IRT over smooth tubes were carefully identified by comparing heat transfer characteristics of IRT with different helix angles (0 degrees similar to 90 degrees). Comparison shows that turbulence intensity in the transition region of buffer layer to log-law region (y(+) approximate to 30 similar to 50) is critical to supercritical heat transfer. Heat transfer can be improved once flow turbulence in this region is intensified, which can be achieved by either the rotational flow in helically rib roughened tubes or the vortex between adjacent ribs in transversely rib-roughened tubes. Rotational flow improves heat transfer by intensifying turbulent diffusion in the aforementioned region, rather than by rewetting the hotter wall with cooler bulk fluid. Both sharp property variations and rotational flow contribute to heat transfer enhancement in IRT, so in the case of supercritical water, less rib-induced additional disturbance is needed when compared with constant property fluid. The Webb-Narayanamurthy correlation can accurately estimate the frictional resistance of IRT. (C) 2016 Elsevier Ltd. All rights reserved.