Numerical Study of the Thermal and Flow Performance of a Novel S‐Shaped Absorber for Direct Steam Generation in Parabolic Troughs

ABSTRACT

Direct steam generation (DSG) represents a viable alternative to synthetic oil heat‐transfer fluids in parabolic trough solar power plants. In the absorber tube, evaporation induces two‐phase flow and a range of flow regimes that strongly influence the heat‐transfer coefficient, pressure drop, and circumferential wall temperature distribution. This work extends a previous parabolic trough DSG study by examining an S‐shaped absorber tube as an alternative to the conventional straight configuration. Numerical simulations were performed in OpenFOAM employing the Eulerian multiphase framework coupled with the Rensselaer Polytechnic Institute wall‐boiling model under reference operating conditions. To ensure a consistent comparison, the inlet conditions of each collector in the S‐shaped configuration were mapped from the corresponding straight‐absorber simulations using the MapFields utility. The nonuniform circumferential heat flux distribution applied to the absorber wall was determined through Monte Carlo Ray Tracing simulations performed in Tonatiuh. Key findings show that the S‐shaped absorber increases the overall pressure drop by 21.48% relative to the straight‐absorber configuration. The flow regime transitions from predominantly stratified flow in the straight absorber to stratified‐wavy, slug, and intermittent regimes in the S‐shaped geometry, enhancing phase mixing and heat transfer. However, the downward sections of the S‐shaped absorber promote liquid redistribution toward the upper wall, thereby forming an insulating vapor layer adjacent to the heated surface. This effect causes a significant increase in wall temperature, particularly in the downstream collectors subjected to the highest incident heat flux. The study provides new insights into the trade‐offs associated with S‐shaped absorber geometries for DSG systems and highlights the combined impact of three‐dimensional flow development, phase distribution, and nonuniform heating on thermal performance.