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The paper is concerned with identification of the key mechanisms controlling deflagration-to-detonation transition in unconfined gaseous systems. The issue of thermal runaway triggered by positive feedback between the advancing flame and the flame-driven precompression wave is discussed in the context of expanding spherical flames. Depending on parameters of the system the transition may occur either within a subsonic or supersonic range of flame propagation. The paper is an extension of the previous study concerned with planar flames. [ABSTRACT FROM AUTHOR]

The problem of deflagration-to-detonation transition in an unconfined environment is revisited. With a freely expanding self-accelerating fractal-like hydrogen-oxygen flame as an example, it is shown that deflagration-to-detonation transition is indeed possible provided the flame is large enough. The transition occurs prior to merging of the flame with the flame-supported precursor shock. The pre-transition flame does not reach the threshold of CJ-deflagration. Numerical simulations employed are based on the recently developed pseudo-spectral method with time and space adaptation. [ABSTRACT FROM AUTHOR]

Within the Boussinesq approximation an elementary model for the deflagration-to-detonation transition triggered by self-acceleration of an expanding flame in a central gravitational field is formulated and explored numerically. The self-acceleration is sustained by the intrinsic Rayleigh-Taylor instability until the Deshaies-Joulin deflagrability threshold is reached, followed by an abrupt transition to detonation. Emergence of the threshold is caused by positive feedback between the accelerating flame and the flame-driven pressure shock that results in the thermal runaway when the flame speed reaches a critical level. The model offers a simple mechanism that may be responsible for the transition to detonation in thermonuclear supernovae. The present study is an extension of the preceding models dealing with constant gravitational fields. [ABSTRACT FROM AUTHOR]

The paper is concerned with identification of the key interactions controlling deflagration-to-detonation transition in confined and unconfined gaseous systems. The issue of thermal runaway triggered by positive feedback between the advancing flame and the flame-driven precursor shock is revisited. Predictions of the asymptotic analysis based on small Mach number approximation are validated by numerical simulations of the pertinent dynamical model. The linkage between the thermal runaway and deflagration-to-detonation transition is substantiated. A possible way to model transition to detonation of an expanding wrinkled flame is discussed. [ABSTRACT FROM AUTHOR]

With a freely expanding unwrinkled spherical flame as an example, it is shown that deflagration-to-detonation transition in unconfined gaseous systems is possible, provided the flame is close to its propagability threshold and the deficient reactant Lewis number is not too small. The transition is triggered by the thermal runaway induced by positive feedback between the accelerating flame and the flame-driven pressure build-up. A possible way to account for the impact of wrinkling is discussed. [ABSTRACT FROM AUTHOR]

This paper is concerned with the identification of the key interactions controlling the deflagration-to-detonation transition in narrow smooth-walled channels. Two agencies contributing to the transition are discussed: hydraulic resistance and flame folding. Depending on the kinetics and parameters of the system, nucleation of detonation may occur near the channel wall and/or in the channel interior. A possibly unexpected outcome of the resistance-folding interplay is the non-monotonicity of the dependence between the predetonation run up time/distance and the channel width. [ABSTRACT FROM PUBLISHER]

A weakly nonlinear model for a self-accelerating outward propagating corrugated flame is formulated and explored. The self-acceleration is sustained by the intrinsic Darrieus-Landau and Rayleigh-Taylor instabilities until the Deshaies-Joulin deflagrability threshold is reached, followed by an abrupt transition to detonation. Emergence of the threshold is caused by positive feedback between the accelerating flame and the flame-driven pressure shock that results in the thermal runaway when the flame speed reaches a critical level. The model offers a simple mechanism that may be responsible for the transition to detonation in thermonuclear supernovae. [ABSTRACT FROM AUTHOR]

To elucidate the key mechanisms responsible for the transition from deflagrative to detonative combustion in smooth-walled channels, a reactive flow with anisotropic thermal and molecular diffusivities is considered. Setting the transverse diffusivities large compared to longitudinal diffusivities, the initially formed deflagration (despite no-slip boundary conditions) appears to be nearly planar and not accelerating. This, however, does not prevent its eventual abrupt transition to Chapman–Jouguet detonation. [ABSTRACT FROM PUBLISHER]

Abstract: The piston–flow interaction induced by a piston pushing hydraulically resisted gas through a long tube is discussed. It is shown that the hydraulic resistance causes a significant precompression and preheating of the gas adjacent to the piston’s edge. In the case of an explosive premixture this development may lead to a localized autoignition triggering detonation. It is suggested that the problem may serve as a guide for understanding the deflagration-to-detonation transition in tubes, with the piston modeling the impact of the advancing flame. [ABSTRACT FROM AUTHOR]

Abstract: The experimentally known phenomenon of spontaneous transition from slow conductive to fast penetrative (convective) burning in a confined gas-permeable explosive is discussed. A reduced quasi-linear model, involving only the most essential physical ingredients, is formulated. A good qualitative agreement between theoretical and experimental dependencies is obtained. Similar to the previously studied case of an inert porous matrix filled with an explosive gas, the transition is triggered by a localized autoignition in the extended resistance-induced preheat zone gradually formed ahead of the advancing deflagration. The conventional concept of penetrative burning is re-examined. [Copyright &y& Elsevier]

Whereas deflagration-to-detonation transition in confined systems is a matter of common knowledge, feasibility of the transition in unconfined space is still a matter of controversy. With a freely expanding self-accelerating spherical flame as an example, it is shown that deflagration-to-detonation transition in unconfined gaseous systems is indeed possible provided the flame is large enough. The transition is caused by positive feedback between the accelerating flame and the flame-driven pressure buildup, which results in the thermal runaway when the flame speed reaches a critical level. [ABSTRACT FROM AUTHOR]