One-side heating test and modeling of tubular receivers equipped with turbulence promoters for solar tower applications

Tubular receivers in central tower systems suffer the high mechanical stresses caused by the temperature gradient typically established along the tube and across its circumference due to the one-side heating. In the present work, the thermal behavior of three different absorber tubes is investigated both experimentally and numerically. The tubes, manufactured in Cr alloy 718 (Inconel®), were smooth or with repeated rib-roughness (annular or helical ribs), and were tested at the solar furnace SF60 of the Plataforma Solar de Almería (PSA) in 2017 within the international access program of SFERA II project, financed by the EU. The specific focus of the tests was the assessment of the role of turbulence promoters in reducing the peak wall temperature when a strong one-side heating is present, contributing to the reduction of the thermal gradients between the irradiated and the non-irradiated (back) side of the tube. The experimental results show that the use of turbulence promoters reduce the wall temperature with respect to the case of a smooth tube, as expected, although the comparison between the samples is not trivial in view of the change in the optical properties induced by the progressive oxidation of the irradiated surface. Computational Fluid Dynamic (CFD) 3D models have been developed for the three samples and they have proven the capability to very-well reproduce the experimental results. A fair comparison between the different simulated tubes in the same controlled conditions of one-side heating has been performed numerically, assessing quantitatively the temperature reduction induced by the turbulence promoters, and the best performance of the Inconel® tube equipped with helices.