Abstract

This study provides an in-depth analysis of the flow and heat transfer behavior of low-water-content jet fuel (α ≤ 2 %) in U-bend tubes, utilizing Large Eddy Simulations (LES) and Discrete Phase Models (DPM). The research focus on the influence of radii of curvature ratio (r/D), flow rates (Qtp) and α on flow dynamics and heat transfer mechanisms. Flow fields for r/D = 1 and r/D ≥ 2.5, where distinct secondary flow structures, play a critical role in shaping internal flow behavior. These vortices significantly enhance flow mixing and heat transfer, especially on the inner wall, through complex multi-level vortex interactions. The local wall Nusselt number shows a close correlation with vorticity trends, initially increasing and then decreasing, with notable deviations near the inlet and outlet regions. This non-uniform heat transfer is primarily driven by the interaction of secondary flow structures and adverse pressure gradients near the inner wall, while higher Qtp and smaller r/D can improve heat transfer uniformity under certain flow conditions. The presence of droplets increases the total wall Nusselt number by 4 % to 60 % compared to single-phase jet fuel flow, due to two-phase interaction and boundary layer disruption. Finally, the study proposes modified correlations for pressure drop gradients and total wall Nusselt numbers, accounting for effects of r/D and multiphase heat transfer processes. These correlations, validated with RRMSEs of 3.54 % and 6.80 % respectively, provide valuable insights for improving heat transfer efficiency in fuel systems and form a theoretical foundation for real-time monitoring technologies in aircraft fuel lines.

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