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Chapter 1 Introduction Detonation is one of the most distinguished phenomena in gas dynamics�� and it is characterized by nonlinearity�� strong discontinuity and the tight coupling of chemical reaction and shock wave. A detonation wave is a supersonic combustion wave across which the pressure and temperature of combustion products increase sharply. Since the phenomenon of detonation was first observed scientifically over one hundred years ago�� there have been numerous studies on detonations�� from fundamental physics to application technologies�� such as severe explosion prevention�� supernovas in astrophysics�� and for military purposes. Detonation has been ��applied�� for military and mining and observed in nature but was not well understood. In the last two decades�� detonation applications in aerospace propulsion and high-enthalpy shock tunnels have attracted worldwide attention. In this chapter�� the origin and current understanding of gaseous detonation are briefly reviewed at first�� and then�� a description of the critical issues in detonation research is given�� which will be further discussed in the following chapters of this book. 1.1 Origin and Cognition of Gaseous Detonation Detonation is an intense supersonic combustion phenomenon with extreme speed that occurs in nature and influences the development of human civilization. Early observation and research on detonation are derived from accidental explosions in coal mines�� which were also observed in chemical plants. The terrible destruction cannot be explained by fire or the usual explosion�� thus attracting attention from scientific researchers. In coal mines�� methane leakage is unavoidable and some dust is produced. Their mixing with air creates a circumstance of detonation initiation�� which could be triggered by casual ignition�� such as a spark or mechanical impact. The initiated detonation propagates with a high speed led by a strong shock�� which could compress and ignite the combustible gases. For hydrogen-fueled detonation��the speed could reach as high as 2000 m/s. Early researchers paid attention to how to prevent the hazardous effects of detonation�� such as suppression of detonation initiation and weakening its destructive power�� which are important to coal mine production and chemical plant design. Detonation is essentially a chemical explosion that has a long research history. Abel [1] measured the detonation wave speed in a pioneering study in 1869. Berth-elot and Vieille from France [2] in 1881 performed a systematic study of the detonation wave speed for various reactants�� including H2�� C2H2�� and C2H4 fuels�� and the evidence of detonation existence has been proven substantially. Another critical experiment was performed by Mallard and Le Chatelier in 1883 [3]�� which confirmed that both deflagration and detonation could be ignited in one combustible mixture and recorded the deflagration-to-detonation transition (DDT) process. As further shown�� the leading shock of the detonation wave in front of the heat release zone compresses the gas and induces combustion�� whose mechanism is essentially different from that of deflagration. These early works provide a primary concept of detonation and its features. In the 1990s�� detonation phenomena were studied worldwide and became an interdisciplinary subject. There are a number of monographs on detonation from various viewpoints [4��9]�� reviewing the theoretical�� numerical and experimental progress at different periods. From the viewpoint of combustion�� the leading shock provides a circumstance of high pressure and improves a fast and efficient combustion mode�� in turn improving the thermodynamic cycle efficiency. Moreover�� under this pressure-gain combustion�� more energy is converted to mechanical energy than with constant-pressure combustion given the same initial conditions. Therefore�� advanced engines based on detonation have great potential to be superior to current engines based on deflagration�� leading to the concept of pulse detonation engines�� rotating detonation engines and oblique detonation engines. In addition to the engines�� detonation could also be used in shock tunnels�� especially high-enthalpy shock tunnels. Two driving patterns were proposed: backward-driven and forward-driven patterns�� which increase the total pressure of the shock tunnel to different extents [10-12]. We believe that more detonation applications will be proposed in the future and hope that gaseous detonation research will continuously promote engineering development. 1.2 Explosion�� Deflagration and Detonation Waves Although detonation has a clear definition and unique features�� some researchers in related communities may confuse detonation with two other waves: explosion waves and deflagration waves. Therefore�� we first distinguish these different concepts in this subsection. The explosion results in a strong shock�� which compresses the surrounding medium and changes the gas dynamic p