Turbulent flame speeds at high pressures: Effects of flame-front instability

In recent studies valuable experimental results have been reported at very high turbulent Reynolds numbers and atmospheric conditions. Recognizing that while the wrinkling of flames, and thus, the propagation of turbulent flames, can be characterized by turbulent Reynolds numbers, high pressure induces a separate mechanism of wrinkling in form of hydrodynamic flame-front instabilities, and the high-pressure environments within internal combustion engines, we present herein an experimental study on the propagation of isobaric expanding turbulent flames at pressures up to 20 atm and moderately high turbulent Reynold numbers up to 8,000. We will show that under such conditions and for diffusionally neutral (unity Lewis number) mixtures, laminar flames display Darrieus-Landau (DL) instability, and as such self-accelerate while expanding. We will, then, present the corresponding propagation speeds of these turbulent flames to delineate the effects of DL instability on turbulent flame propagation. The validity of a previously proposed scaling, i.e. normalized flame speed being proportional to the square root of a flame-based Reynolds number, at such engine relevant conditions, will also be assessed.