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Reactive ?uids are present in many situations of great importance, such as in combustion chambers or around spacecraft re-entering the atmosphere. - alyzing the ?ow properties of such ?uids represents one of the most di?cult challenges to current technology. Indeed, all of the most di?cult aspects of ?uid mechanics appear to be grouped together in this research ?eld! Such ?uids are complex mixtures with compositions that vary rapidly in time and space. They are not usually at thermodynamic equilibrium, since the reaction times of the chemical reactions involved may not be negligible in comparison with the transit time of the ?uid. However, the author of this book limits its scope to typical phenomena that are not very far from local equilibrium but can nevertheless exhibit the most important types of irreversible processes. The production of entropy is highly dependent on the chemical reactionpa- way, which is di?cult to simplify. Also, most of the classical problems that characterize?uid mechanics—such as turbulence, the presence of thin bou- ary layers or shear layers, and the propagation of acoustic waves and shock waves—are also present, and are much more di?cult to analyze and describe than they are for homogeneous?uids, because reactive mixtures interact with these phenomena. For example, density is highly dependent on the chemical pathway since it is determined by the local and instantaneous production of chemical species, and so its value a?ects many other quantities through the equation of state and the balances of mass, momentum, and energy.