Thermodynamic and gas dynamic aspects of a BLEVE

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This first chapter will give an introduction to what is a BLEVE, Boiling Liquid Expanding Vapor Explosion, and its hazards particularly in relation to tunnel safety. It will be shown that several definitions of a BLEVE can be given, depending on the aspect put in focus. In particular distinction can be made between two groups of references, respectively giving an engineering definition and a physical definition. As a result of our literature survey, our own definition of BLEVE will be presented trying to bridge the gap between these two groups of definitions. In the following chapters, we stick to that definition unless specified otherwise. First of all, we would like to consider some basic concepts, i.e. superheated liquid, explosive boiling and bubble nucleations, by an easy example. It is well known that when we heat the water in a tea kettle up to the temperature of 100 ±C at the atmospheric pressure, the water will start to boil or vaporize. In this case, invisible active nuclei formed on the inner surface of the kettle or at any impurities in the water will grow to be a visible bubble which will detach from the wall and move upwards due to buoyancy. Such a type of bubble nucleation is called heterogeneous nucleation since the bubble nucleation only occurs at the locations where there is a boundary between two different phases. However if we put the water into a very smooth glass and heat it in the microoven, the boiling will not occur even the temperature already exceeds the normal boiling point at the atmospheric pressure. At this moment, the water is said to be superheated and if we keep heating the water, a rapid ’explosive-like’ boiling, termed explosive boiling, will suddenly occur and may cause serious damage to the microoven. In this explosive boiling, the active nuclei are evenly formed throughout the liquid, therefore it is called homogeneous nucleation. Superheating sometimes is referred to as boiling retardation, or boiling delay. It refers to the phenomenon in which a liquid is heated to a temperature higher than its boiling point, without actually boiling. A superheated state can also be reached in another way than by heating, namely by depressurization to a pressure lower than the saturation pressure at the prevailing temperature.