Origins of the deflagration-to-detonation transition in gas-phase combustion

The mechans of transition from deflagration to detonation in vapor cloud explosions

Strong interest in the use of coal-derived syngas in gas turbines has led to recent experimental studies that highlight the important features of H2/CO combustion at high pressures and relatively low temperatures. In the present study these investigations are reviewed, evaluated, and chemical kinetic updates based on these new results are discussed. Disparities observed between experimental measurements and kinetic model predictions of high pressure ignition delay and burning velocity are noted and the effect that surfaces, trace impurities, and contaminants may have on the H2/CO kinetic system are elucidated. In particular, the impurity coupling with NOx is discussed in relation to energy conversion processes involving hydrogen as a fuel component. An example of its importance to pre-ignition in reciprocating engine applications is also demonstrated.

Syngas Combustion Kinetics and Applications

Auto-ignition of hydrocarbons behind reflected shock waves

The paper elucidates the distinction between two regimes of ignition, one strong and the other weak, that characterize the onset of combustion in homogeneous gaseous mixtures whose reaction mechanism is controlled by a chain-branching step, such as that associated with the H+O2 collision in the hydrogen-oxygen system. On the basis of experimental observations, made primarily by the use of a stroboscopic laser-schlieren system yielding a sequence of photographic records of the entire flow field behind a reflected shock near the closed end of the tube at a repetition rate of 2 microseconds between frames, it has been revealed that strong ignition is manifested by a practically instantaneous appearance of a relatively plane pressure front associated with the generation of a flow field across the whole cross section of the tube, while mild ignition starts in the form of distinct flame kernels whose growth is comparatively slow and essentially devoid of any gasdynamic effects. The demarcation line between the two regimes of ignition is then shown to be associated with a critical value of the gradient of the induction time with respect to temperature at constant initial pressure. Specifically, for a stoichiometric hydrogen-oxygen mixture, this turns out to be −2 microseconds/°K. Although basically controlled by the same elementary processes as those that establish the second explosion limit, this so-called strong ignition limit differs from the locus of states obtained by direct extrapolation of the second limit. Moreover, it does not intersect with any of the explosion limits, encompassing a region that is fully contained within the classical explosion regime.

On the shock-induced ignition of explosive gases

A conventional shock tube modified so as to prevent local early detonation from the diaphragm burst or from wall crevasses was used to study the initiation of detonation behind the reflected shock wave. Three types of initiation behavior were observed, two of which appear to be nicely one dimensional. In all cases stoichiometric hydrogen‐oxygen mixtures diluted with argon were used in the tube. In the first case a simple acceleration of the reflected shock occurred when the pressure wave generated by the adiabatic explosion reached the reflected shock. In the second case a typical accelerating wave pattern headed by a shock wave was formed behind the reflected shock while in the third case (which does not appear to be strictly one dimensional) the adiabatic explosion quickly produced a Chapman‐Jouguet detonation wave behind the reflected shock wave. A simple qualitative theory for the occurrence of the wave patterns observed is presented and the adiabatic explosion delays are compared to Schott and Kinsey's recent results using a different shock‐tube technique.

Initiation of Detonation

Strip film and multiple spark schlieren pictures of reflected shocks were taken in the gases He, A, N2, O2, H2, NO, CO2, N2O, CH4, and C3H8. Interaction of the reflected shock with the boundary layer behind the incident shock was observed in all gases except He and A. The bifurcation and acceleration associated with the interaction limits the use of the reflected shock region for high‐temperature kinetic and equilibrium studies. It was possible to minimize the effects of this interaction by using gases with high γ and confining measurements to a region near the back wall. Reflected shock observations agreed with Mark's theory of the development of bifurcation but disagreed with his predicted deceleration of the reflected shock.From comparison of incident and reflected shock velocities, measured at the backwall, with ideal one‐dimensional theory, it was concluded that calculations of the temperature behind the reflected shock from incident shock velocities would yield high results.Decelerations of the refl...

Limitations of the Reflected Shock Technique for Studying Fast Chemical Reactions and Its Application to the Observation of Relaxation in Nitrogen and Oxygen

Arthur Cohen

Papers

Initiation of Detonation

Limitations of the Reflected Shock Technique for Studying Fast Chemical Reactions and Its Application to the Observation of Relaxation in Nitrogen and Oxygen

Shock‐Initiated Detonations