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

氢气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。氢气也用于油砂加工。从水力发电或风能生产氢气是碳中性的。然而，由于氢与碳氢化合物燃料相比具有明显不同的特性，因此在允许广泛的氢储存和分配设施（如加油站）与人口稠密地区密切接触之前，需要解决重大的安全问题，如果氢用作汽车燃料，这将是必要的。同样，在隧道和公共车库中的车辆中储存氢也存在安全问题。由于氢气非常轻且具有浮力，因此通常可以通过确保周围有足够的距离来确保户外安装的安全性，而这些距离是不允许公众使用的。然而，在隧道和车库等封闭环境中，氢气可能会积聚，严重的安全问题是爆炸的风险，这是非常难以处理的。这一点特别重要，因为氢比其他燃料更容易发生。爆炸是燃烧的最猛烈的形式，其中燃烧依靠它也支持的冲击波。因此，爆震波只要遇到可燃混合物就以高超音速传播，并且它们通常需要数十个大气压的压力增加，这是相当具有破坏性的。因此，在隧道中爆炸可能导致非常严重的事故，比几年前的Mont-Blanc隧道火灾更严重，不仅因为高压，而且因为它可能传播相当长的距离。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，其化学性质比其他更复杂的燃料更好地了解，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具体特征与爆轰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设想方案进行数值模拟和数学分析，重点是两个具体问题。第一个是火焰加速管，这需要火焰振荡，潜在地导致爆震波的外观。第二个与爆炸细胞有关。爆轰波被观察到绘制细胞状图案管壁。这种情况发生的过程是众所周知的，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，其机理尚不清楚，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。目前工作的目标是澄清现实的氢化学和自发射流点火的具体功能之间的关系。为此，将对一系列越来越复杂和现实的化学动力学模型进行数值模拟和数学分析。我们以前的工作已经确定了氢的一个关键特性，它不同于碳氢化合物燃料，碳氢化合物燃料起着至关重要的作用：由于其小分子，氢比空气扩散得更快，因此将燃料带到可能发生点火的地方。拟议的研究将使用现实的化学动力学来证实这种机制的作用。

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, 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.