ABSTRACT
The paper features the key role of the turbulent burning velocity in deflagration to detonation transitions, DDT. When a flame propagates along a duct closed at one end, open at the other, and containing obstacles that generate sufficient turbulence, it is accelerated by a feed back mechanism. This is due to the associated increasing gas velocity ahead of the flame generating more turbulence and further increasing the turbulent burning velocity, ut, until it attains the maximum possible value, at which any further increase in turbulence would reduce the burning velocity due to flame quenching. The increased gas velocity ahead of the flame generates a shock wave further ahead. As the strength of the shock increases, the increase in temperature and pressure of the reactants between the shock and the flame can become sufficient for them to autoignite. This would generate further shocks that might couple with the chemically reacting autoignition front to generate a detonation. This mechanism demonstrates how a possible transition to detonation is dependent upon attaining a sufficiently high turbulent burning velocity.
Recent experimental studies suggest general expressions that yield the maximum attainable turbulent burning velocity. These are introduced into a previously developed simplified one dimensional analysis, which enables the temperature and pressure of the shocked reactants to be found. If the autoignition delay time of the mixture between flame and shock under these conditions is less than its lifetime, autoignition will occur. The procedure outlined rests upon a number of assumptions, yet it provides first order estimates of the onset of autoignition and focuses on salient phenomena. The factors governing the possible transition from autoignition to detonation are discussed.
Keywords: DDT, Turbulent burning velocity, Shock wave, Autoignition.
