Flame evolution and heat flux correlation of hydrogen-blended jet fires from moving trains in tunnels

With large-scale railway tunnel construction and low-carbon rail transit transition, hydrogen-blended fuel trains pose new thermal safety challenges to tunnel infrastructure, while existing fire safety frameworks lack targeted support. This study combines scaled experiments and validated computational fluid dynamics (CFD) simulations to investigate hydrogen-blended ethylene jet fires from moving vehicles in tunnels, covering five blending ratios and six speeds. Results show the piston effect transforms flames from buoyancy-dominated upright jets to momentum-dominated roof-attached flames with extended attachment length and altered thermal distribution. Hydrogen blending suppresses soot formation and elevates convective heat transfer contribution; under high speed and high blending ratio, heat transfer becomes nearly convection-dominated with negligible radiation, shifting tunnel thermal hazard from radiation-controlled global heating to convection-dominated localized roof exposure. Finally, a segmented dimensionless correlation integrating blending ratio and Froude number is established for roof heat flux prediction. The correlation is valid within the present tested tunnel geometry, blocking ratio, model-scale vehicle speed, heat release rate and hydrogen blending ratio. Its extrapolation to full-scale and extra-long tunnel conditions requires further verification. Results help support tunnel lining thermal safety design, hydrogen fire risk assessment and regulation development.