The structure and flame propagation regimes in turbulent hydrogen jets

Experiments on flame propagation regimes in a turbulent hydrogen jet with velocity and hydrogen concentration gradients have been performed. Horizontal stationary hydrogen jets released at normal and cryogenic temperatures of 290, 80 and 35 K with different nozzle diameters and mass flow rates have been investigated. Sampling probe method and laser PIV techniques have been used to evaluate the distribution of hydrogen concentration and flow velocity. High-speed photography combined with a Background Oriented Schlieren (BOS) system was used for the visual observation of the turbulent flame propagation. In order to investigate different flame propagation regimes the ignition position was changed along the jet axis. It was found that the flame propagates in both directions, up- and downstream of the jet flow if hydrogen concentration is >11%, whereas in case [H2] < 11%, the flame propagates only downstream. This means that at normal temperature the flame is able to accelerate effectively only if the expansion ratio σ of the H2-air mixture is higher than a critical value σ* = 3.75 defined for a closed geometry.