Abstract
Leveraging existing subsea natural gas pipelines for hydrogen blending offers a practical route for offshore low-carbon energy transport. However, traditional T-pipes require long distances to achieve uniform mixing conditions, raising the risks of stratification and hydrogen embrittlement. In this study, the conventional pipe fittings—elbows are innovatively adopted as passive mixing elements for hydrogen-blended natural gas pipes, expecting to use curvature-induced secondary flow and vortex reorganization to accelerate homogenization without extra flow mixers. Numerical simulations were performed to investigate the effects of the distance L1 between the hydrogen branch and the elbow inlet and the elbow curvature radius Rc. Hydrogen distributions, coefficient of variation, homogeneous mixing path, and flow vorticities were analyzed to make comparisons between the elbow configurations and the conventional T-pipe. The results show that a smaller L1 shortens the homogeneous mixing path Sh, while a smaller elbow curvature radius Rc generates stronger secondary flows but also intensifies the asymmetric hydrogen enrichment induced by centrifugal force. The mixing effect of the elbow configuration is more significant when HBR ≥ 20%, which gradually weakens as the HBR decreases. In this paper, the shortest mixing path Sh = 11.73 m is obtained in the optimal structure with L1 = 0 and Rc/D = 4, achieving a reduction of 67.3% relative to the conventional T-pipe. It proves that elbow structures can significantly enhance the mixing efficiency of hydrogen-blended natural gas pipes, providing new solutions for hydrogen blending in offshore pipelines.