G D. Roy et al. Progress in Energy and Combustion Science 30(2004)545-672 551 Voitsekhovsky [58 and Denisov and Troshin [59] have ven s a discovered the multihead detonation and analyzed the flow shock wave reflection at contact discontinuities patterns at the triple wave configurations with transverse walls were visualized shock waves and reaction fronts arising at the detonation Flame acceleration, DDT, and detonation propagation in front and changes in the flow patterns upon collisions of these ough-walled tubes were first visualized by babkin and figurations. Instability of realistic detonation waves and Kozatchenko [80, 81]. It has been shown that the structures their 3D structure raised serious questions concerning the of detonations in rough and smooth tubes can differ validity of the Arrhenius kinetics with an average tempera- considerably. In a tube with rough walls, mixture ignition ture in ID ZND modeling of detonation initiation and is facilitated by roughness elements due to high local propagation. Direct photographs and soot imprints [60-62] temperatures behind reflected shock waves. One-dimen- uivocally the fish-scales like cellular structure sional model predicts that due to this fact, detonations in not only of cj detonations but of the initial detonation kernel rough tubes should exhibit higher stability and wider which meant that the mixture was actually ignited behind the concentration limits [55, 56]. However, experimental obser shock front in hot spots where temperature is significantly vations [82,83] show somewhat narrower concentration higher than the average temperature limits of low-velocity regimes as compared to detonation in Based on this understanding numerous models of single smooth tubes and quite large wave velocity fluctuations and head (spinning) and multihead detonations have been recovery of a detonation wave upon its entry from a rough suggested since 1950s(see review articles [63, 641). tube into a smooth tube occurs within still narrower limits With the growing availability of diagnostics with This is evidently attributable to an essentially multidimen- mproved temporal and spatial resolutions and powerful sional nature of the reactive waves in rough tubes omputing resources, the progress in the detonation science One of the questions of practical importance is, how after the 1960s has been overwhelming First of all, it became detonation wave originated in a narrow tube behaves when it possible to visualize the ignition process behind a reflected enters a tube of a larger volume or unconfined mixture? The shock wave and discover two different modes of shock- answer to this question should provide information about induced ignition of a reactive gas, namely, 'strong'an al ways of detonation initiation in large volumes, mild ignition[65, 66]. violent volumetric ignition of shock because a mixture in a narrow duct can be initiated much compressed gas in which no local fluctuations of the ignition easier than in wide ones. Transition of detonation waves delays were resolved by the photographic technique was from narrow to wide ducts has been systematically studied termed strong ignition in contrast to mild ignition of the by a number of investigators, starting as early as in 1956 shock-compressed gas in clearly visible exothermic centers 1541. Visualization of detonation transmission from a (hot spots) giving rise to an accelerating flame fronts that run channel into an unconfined volume was probably first up to detonation in some cases. It has been unambiguously lade by mitrofanov and Soloukhin [84 in 1964. demonstrated that it is strong ignition mode that is relevant to Extensive experimental data on detonability of various detonation. However, the ignition process still remains fuels has been provided by research groups from all over the pendent on flow fluctuations even in this case. A world [64, 85-88. Based on well-documented experimental xperimental evidence shows [67 the ignition front behind data on detonation initiation, propagation and transition. the lead shock is quite irregular. This is supported by the well- several important empirical criteria have been extracted. The known nonuniform pattern of soot prints of multihead characteristic size in the fish-scales like structure of realistic detonations. An anal Ref [68] shows that the driving detonation waves, referred to as the detonation cell size, was mechanism of ignition delay fluctuations are gasdynamic found to be a representative parameter to qualitatively grade pulsations of the flow parameters due to collisions of weak detonability of the mixture: the larger the cell size the less coustic and quasi-acoustic waves traveling behind the shock sensitive is the mixture. The cell size was found to be a wave front and affecting it(because of the subsonic nature of function of the initial pressure, temperature, mixture the flow behind the shock wave). Interestingly, these composition and tube diameter. The cell size was proved fluctuations show up even in overdriven waves in which to be directly relevant to detonation transition from a channel the heat release is relatively very low(the temperature rise to an unconfined volume [64 to the limiting tube diameter due to the reaction not exceeding 400 K[691 1891, and to the critical energy of detonation initiation [90 Numerous theoretical works on ID and two-dimensional Detonations in heterogeneous media containing gaseous (2D) analysis of detonation wave instability predict the oxidizer and liquid fuel spray or film, or solid fuel virtually all waves with realistic reaction kinetics are unstable uspension is a topic of growing interest since the 1950s and develop a spinning or multihead structure [70-76] in view of industrial safety and military applications. In the series of elaborate photographic studies Detonations in such media were extensively studied bot Oppenheim et al. [62,77-79 revealed various scenarios experimentally [91] and theoretically [92]. It has been found of detonation onset during DDT in tubes with smooth walls. that detonability of heterogeneous mixtures depends Fast ejection of fame tongues and detonation kernel significantly on the fuel vapor concentration, in particular, formation near the accelerating flame brush, as a result of for heavy hydrocarbon fuelsVoitsekhovsky [58] and Denisov and Troshin [59] have discovered the multihead detonation and analyzed the flow patterns at the triple wave configurations with transverse shock waves and reaction fronts arising at the detonation front and changes in the flow patterns upon collisions of these configurations. Instability of realistic detonation waves and their 3D structure raised serious questions concerning the validity of the Arrhenius kinetics with an average tempera￾ture in 1D ZND modeling of detonation initiation and propagation. Direct photographs and soot imprints [60–62] showed unequivocally the fish-scales like cellular structure not only of CJ detonations but of the initial detonation kernel, which meant that the mixture was actually ignited behind the shock front in hot spots where temperature is significantly higher than the average temperature. Based on this understanding numerous models of single￾head (spinning) and multihead detonations have been suggested since 1950s (see review articles [63,64]). With the growing availability of diagnostics with improved temporal and spatial resolutions and powerful computing resources, the progress in the detonation science after the 1960s has been overwhelming. First of all, it became possible to visualize the ignition process behind a reflected shock wave and discover two different modes of shock￾induced ignition of a reactive gas, namely, ‘strong’ and ‘mild’ ignition [65,66]. Violent volumetric ignition of shock￾compressed gas in which no local fluctuations of the ignition delays were resolved by the photographic technique was termed strong ignition in contrast to mild ignition of the shock-compressed gas in clearly visible exothermic centers (hot spots) giving rise to an accelerating flame fronts that run up to detonation in some cases. It has been unambiguously demonstrated that it is strong ignition mode that is relevant to detonation. However, the ignition process still remains dependent on flow fluctuations even in this case. As experimental evidence shows [67] the ignition front behind the lead shock is quite irregular. This is supported by the well￾known nonuniform pattern of soot prints of multihead detonations. An analysis in Ref. [68] shows that the driving mechanism of ignition delay fluctuations are gasdynamic pulsations of the flow parameters due to collisions of weak acoustic and quasi-acoustic waves traveling behind the shock wave front and affecting it (because of the subsonic nature of the flow behind the shock wave). Interestingly, these fluctuations show up even in overdriven waves in which the heat release is relatively very low (the temperature rise due to the reaction not exceeding 400 K [69]. Numerous theoretical works on 1D and two-dimensional (2D) analysis of detonation wave instability predict that virtually all waves with realistic reaction kinetics are unstable and develop a spinning or multihead structure [70–76]. In the series of elaborate photographic studies, Oppenheim et al. [62,77–79] revealed various scenarios of detonation onset during DDT in tubes with smooth walls. Fast ejection of flame tongues and detonation kernel formation near the accelerating flame brush, as a result of collision of flame-driven shock waves, and as a result of shock wave reflection at contact discontinuities and tube walls were visualized. Flame acceleration, DDT, and detonation propagation in rough-walled tubes were first visualized by Babkin and Kozatchenko [80,81]. It has been shown that the structures of detonations in rough and smooth tubes can differ considerably. In a tube with rough walls, mixture ignition is facilitated by roughness elements due to high local temperatures behind reflected shock waves. One-dimen￾sional model predicts that due to this fact, detonations in rough tubes should exhibit higher stability and wider concentration limits [55,56]. However, experimental obser￾vations [82,83] show somewhat narrower concentration limits of low-velocity regimes as compared to detonation in smooth tubes and quite large wave velocity fluctuations and recovery of a detonation wave upon its entry from a rough tube into a smooth tube occurs within still narrower limits. This is evidently attributable to an essentially multidimen￾sional nature of the reactive waves in rough tubes. One of the questions of practical importance is, how a detonation wave originated in a narrow tube behaves when it enters a tube of a larger volume or unconfined mixture? The answer to this question should provide information about optimal ways of detonation initiation in large volumes, because a mixture in a narrow duct can be initiated much easier than in wide ones. Transition of detonation waves from narrow to wide ducts has been systematically studied by a number of investigators, starting as early as in 1956 [54]. Visualization of detonation transmission from a channel into an unconfined volume was probably first made by Mitrofanov and Soloukhin [84] in 1964. Extensive experimental data on detonability of various fuels has been provided by research groups from all over the world [64,85–88]. Based on well-documented experimental data on detonation initiation, propagation and transition, several important empirical criteria have been extracted. The characteristic size in the fish-scales like structure of realistic detonation waves, referred to as the detonation cell size, was found to be a representative parameter to qualitatively grade detonability of the mixture: the larger the cell size the less sensitive is the mixture. The cell size was found to be a function of the initial pressure, temperature, mixture composition and tube diameter. The cell size was proved to be directly relevant to detonation transition from a channel to an unconfined volume [64], to the limiting tube diameter [89], and to the critical energy of detonation initiation [90]. Detonations in heterogeneous media containing gaseous oxidizer and liquid fuel spray or film, or solid fuel suspension is a topic of growing interest since the 1950s in view of industrial safety and military applications. Detonations in such media were extensively studied both experimentally [91] and theoretically [92]. It has been found that detonability of heterogeneous mixtures depends significantly on the fuel vapor concentration, in particular, for heavy hydrocarbon fuels. G.D. Roy et al. / Progress in Energy and Combustion Science 30 (2004) 545–672 551