The knowledge of lower flammability limit (the lowest concentration below which the fuel-air mixture can no longer be ignited) of hydrogen-air mixtures is very important in fire safety and prevention where hydrogen is used as a fuel. The reported values of lower flammability limit substantially varied from 4% to 9.5% by volume of hydrogen in air, depending on the experimental configuration employed.

A simple yet effective experiment has been developed to determine the lower flammability limits of hydrogen mixtures. Effect of various test factors, such as the shape and size of the combustion vessel, the direction of flame propagation, ignition energy, ignition gap distance, heat losses, and mixing level are considered. A unifying semi-empirical model based on the burning velocity of hydrogen air mixtures has been developed that can explain all the experimental data including the present one. This approach will allow the predictions of flammability limits in any practical configurations such as the fire risks of accidental hydrogen leaks from high-pressure storage cylinders in transportation vehicles.  Dependence on the size of the combustion vessel, direction of flame propagation and heat losses due to conduction and convection are included in the model. The predicted results are compared with values reported in literature for different configurations and directions of flame propagation. The model is able to capture the trend and the results agreed well. Utilizing this model, the minimum recommended tube dimensions that would give lower flammability limit with negligible heat losses are predicted for different directions of flame propagation.

Because hydrogen has high diffusion coefficient in air and very low density when compared with that of air, a diffusion-buoyancy computational model has been developed to account for the diffusion of hydrogen affecting flammability limits using the Fluent software. This is to predict the variation of hydrogen concentration at different time intervals in a vertical cylinder. These simulations are to be extended to other geometries in order to explore safe practices for hydrogen delivery to fuel cell and for ventilation of hydrogen accidental leakage in closed environments such as a garage.

This research will be important for understanding safety issues that need to be fully addressed by developing proper codes and standards that are critical for the design and operation of hydrogen-powered transportation vehicles.