Effect of bend orientation on the heat transfer performance of U-bend vapor generator in transcritical ORC

•Bend orientation effect was proved closely to the buoyancy effect.•U-bend had the best performance within the bend when horizontally downward arranged.•Differences in the entire bend-affecting region are, however, minimal.•Horizontally downward orientation is more sensitive to the bend curvature change.•Nusselt correlations were recommended for all three bend orientations. Adequate recognition of supercritical heat transfer characteristics in U-bends is important to guide the optimization of the design and practical application of heat exchanges containing U-bends. Past studies have focused on the heat transfer performance under different operating conditions and geometric parameters, however, the influence of bend orientation on the local heat transfer and overall performance of U-bends is still unclear and rarely reported. This work fills this gap with numerical simulation to quantitatively analyze and explain the heat transfer performance of U-bend vapor generators with three different bend orientations (horizontal, horizontally downward, and horizontally upward). The results show that as the ratio of heat flux to mass flux increases, bend orientation effects start to appear only when the buoyancy parameter Grq/Grth at the bend inlet is above 80 ∼ 95. In all three orientations, horizontally downward flow U-bend has the best average heat transfer performance within the bend, followed by horizontal and horizontal upwardly. However, there is no significant difference in the average performance of the three orientations over the entire bend-affecting region, with a maximum difference of only 5%. Therefore, when the downstream length is much greater than the bend, there is no need to consider the influence of flow orientation. Increasing the bend curvature considerably improves the heat transfer within the bend, and the downstream bend-affecting distance becomes larger as well. However, larger curvature also causes greater fluctuation in the heat transfer coefficient. Finally, heat transfer correlations were recommended and developed for all three orientations. Correlations predict over 93% of the entire dataset with a maximum error of 20%.