Confinement effects on a supersonic hydrogen jet flame in a duct

This study investigates high-speed, confined hydrogen-air jet flames, representative of leak scenarios from a high pressure hydrogen tank in practical applications. Using high-fidelity Large Eddy Simulation (LES) with detailed combustion chemistry and heat transfer modeling, the research examines a round supersonic jet flame confined within a duct, subjected to an air crossflow and impinging on the duct walls. The confined hydrogen jet flame interacts strongly with its surroundings (Viskanta, 1993), leading to significant thermal stresses and accelerated wall degradation. Two configurations are compared: Case W, where the flame has sufficient space to ignite and develop before interacting with the opposite wall (Bradley et al., 2019), and Case N, where the nozzle-to-plate spacing is smaller than the flame lift-off height, resulting in partial flame quenching due to confinement. Substantial differences are observed between cases W and N. For Case W, the flame is attached to the jet, exhibits a central premixed core followed by a stabilized diffusion zone like in free jet flames. For Case N, this jet region is quenched. However, the overall flame does not extinguish and combustion proceeds in a large diffusion flame, stabilized away from the jet. While higher quantities of unburnt hydrogen exit the duct compared to Case W, Case N exhibits similar heat loads onto the tunnel walls. This behavior is attributed to the changed flame stabilization as well as confinement effects on the crossflow jet interaction, that impact heat transfer characteristics. Last, an analysis of the probability density functions of temporally averaged local wall heat fluxes indicates a spatially more evenly distributed heat transfer for Case N than for Case W.

Novelty and significance statement

Supersonic hydrogen jet flames (issuing from a high-pressure tank through a nozzle) are usually studied in free space. Recent safety concerns for hydrogen leaks require to study how hydrogen jets will behave in very confined setups, where the nozzle exit to plate spacing is smaller than the nominal lift-off height of the jet flame. The investigation of these hydrogen jets into small ducts is crucial for safety in future hydrogen applications. This work presents the first detailed simulation of cases where the supersonic hydrogen jet is strongly confined in a duct. It shows that, for very small nozzle to plate spacings, hydrogen does not burn in the shear layer of the jet but diffuses and burns far away from the jet core, creating a new flame topology and different wall heat loads compared to standard free flames and classical impinging jets.