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

氢是一种有吸引力的汽车燃料，因为它燃烧时不会释放二氧化碳或其他温室气体，而且它是燃料电池的首选燃料。氢也用于油砂加工。水力发电或风能产生的氢气是碳中性的。然而，由于氢气与碳氢化合物燃料相比具有明显不同的行为，因此，如果氢气被用作汽车燃料，那么在广泛的氢气储存和分配设施（如加氢站）被允许与人口稠密地区密切接触之前，需要解决重大的安全问题。同样，在隧道和公共车库的车辆中储存氢气也存在安全问题。由于氢气非常轻且容易浮起，因此只要确保在公众不允许进入的地方周围有足够的距离，就可以确保户外装置的安全。然而，在隧道、车库等氢气积聚的封闭环境中，爆炸的危险是一个严重的安全问题，很难处理。这一点尤其重要，因为氢比其他燃料更容易发生爆炸。爆轰是最猛烈的燃烧形式，在这种燃烧中，燃烧依靠它所支持的冲击波。因此，只要遇到可燃混合物，爆震波就会以高超音速传播，并且通常会使压力增加几十个大气压，这是相当具有破坏性的。因此，隧道中的爆炸可能导致非常严重的事故，比几年前勃朗峰隧道的瞬间火灾更为严重，这不仅是因为高压，还因为它可能传播相当远的距离。不幸的是，导致爆炸的机制仍然存在很大的不确定性，即使是氢，它的化学性质比其他更复杂的燃料更为人所知，至少在相对较低的压力下。当前工作的第一个目标是澄清氢化学的特定特征与爆炸的结构和外观之间的关系。为此，将对涉及日益复杂的化学动力学模型的一系列情景进行数值模拟和数学分析，重点关注两个具体问题。首先是管内的火焰加速，这需要火焰振荡，可能导致爆震波的出现。第二个与引爆装置有关。观察到爆震波在管壁上画出细胞样图案。发生这种情况的过程很容易理解，但决定这些细胞最大尺寸的机制尚不清楚。此外，有可靠的实验证据表明，高压氢气突然释放到大气中可能会自燃，其机制尚不清楚，称为喷射点火或自燃。从安全的角度来看，这是一把双刃剑，在某些情况下，喷气点火可能有利于安全，而在其他情况下，它会引发火灾。当前工作的目的是澄清现实氢化学的特定特征与自燃射流点火之间的关系。为此，数值模拟和数学分析将对越来越复杂和现实的化学动力学模型进行层次分析。我们之前的工作已经确定了氢的一个关键特性，这与碳氢化合物燃料不同，它起着至关重要的作用：氢分子小，扩散速度比空气快，因此将燃料带到可能发生引燃的地方。拟议的研究将利用现实的化学动力学证实这一机制的作用。

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, majoruncertainties 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.