Ignition and Transition to Detonation - Interplay between Gas Dynamics and Chemical Kinetics

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Hydrogen is an attractive automotive fuel because it burns without releasing CO2 or other greenhouse gases, and because it is the fuel of choice for fuel cells. Hydrogen is also used in oil sand processing. Hydrogen production from hydro power or wind energy is carbon-neutral. However, because hydrogen has a markedly different behaviour when compared with hydrocarbon fuels, there are significant safety issues that need to be resolved before widespread hydrogen storage and distribution facilities such as refuelling stations are allowed in close contact with populated areas, which will be needed if hydrogen is used as an automotive fuel. Likewise, there are safety issues associated with hydrogen storage in vehicles in tunnels and public garages. Because hydrogen is very light and buoyant, outdoors installations can normally be made safe by ensuring that there is enough distance around where the public is not allowed. However, in enclosed environments such as tunnels and garages, in which hydrogen can accumulate, a serious safety issue is the risk of detonation, which is very difficult to deal with. This is particularly important because hydrogen is much more prone to than other fuels. Detonation is the most violent form of combustion, in which combustion rides on a shock wave which it also supports. Thus detonation waves travel at high supersonic speeds as long as they encounter combustible mixture, and they entail a pressure increase usually in the tens of atmospheres, which is quite destructive. Thus a detonation in a tunnel could result in very bad accidents more serious than for instant the Mont-Blanc tunnel fire, a few years ago, not only because of high pressure, but because it might propagate over considerable distances. Unfortunately, major uncertainties remain on the mechanisms leading to detonation, even for hydrogen, the chemistry of which is better known than other more complex fuels, at least at relatively low pressures. The first goal of the current work is to clarify the relationship between specific features of hydrogen chemistry and both the structure and the appearance of a detonation. To that effect, numerical simulation and mathematical analysis will be performed for a hierarchy of scenarios involving chemical kinetic models of increasing complexity, focusing upon two specific issues. The first is flame acceleration in a tube, which entails flame oscillations, potentially leading to appearance of a detonation wave. The second has to do with detonation cells. Detonation waves are observed to draw cell-like patterns on tube walls. The process whereby this occurs is well-understood, but the mechanism that determines the maximum size of these cells is not known. In addition, there is reliable experimental evidence showing that a sudden release of high pressure hydrogen into the atmosphere may ignite spontaneously, under a mechanism that is not well understood, called jet ignition or spontaneous ignition. From a safety perspective, this is a double-edged sword in that in some scenarios, jet ignition may favor safety, while in others, it will trigger a fire. The goal of the current work is to clarify the relationship between specific features of realistic hydrogen chemistry and spontaneous jet ignition. To that effect, numerical simulation and mathematical analysis will be performed for a hierarchy of increasingly more complex and realistic chemical kinetic models. Our previous work has identified one key property of hydrogen, in which it differs from hydrocarbon fuels, which plays a crucial role: with its small molecule, hydrogen diffuses more rapidly than air, hence bringing fuel where ignitino will likely take place. Proposed studies will confirm the role of this mechanism using realistic chemical kinetics.