Experimental analysis of laser-induced spark ignition of lean turbulent premixed flames

Abstract

Spark ignition in lean and highly turbulent premixed flows is experimentally investigated by measuring the Minimum Ignition Energy (MIE), for laser-induced spark ignition in methane/air mixtures, as a function of the velocity fluctuation <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msup> <mml:mrow> <mml:mi>u</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>′</mml:mo> </mml:mrow> </mml:msup> </mml:math> , the equivalence ratio and the focal length of the focusing lens. The burner provides a 2D stationary flow with a homogeneous and isotropic turbulence. A clear transition on the MIE values is observed, when <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msup> <mml:mrow> <mml:mi>u</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>′</mml:mo> </mml:mrow> </mml:msup> </mml:math> increases. Before the transition, the MIE increases gradually with <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msup> <mml:mrow> <mml:mi>u</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>′</mml:mo> </mml:mrow> </mml:msup> </mml:math> , whereas across the transition (for <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msup> <mml:mrow> <mml:mi>u</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>′</mml:mo> </mml:mrow> </mml:msup> <mml:mo>∼</mml:mo> <mml:mn>1</mml:mn> <mml:mtext> </mml:mtext> <mml:mtext>m</mml:mtext> <mml:mspace width="0.2em"/> <mml:msup> <mml:mrow> <mml:mtext>s</mml:mtext> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msup> </mml:math> ), the MIE increases strongly. As the MIE is proportional to the flame thickness to the power of three, this slope breakdown could indicate the existence of two distinct modes of flame structure. From a certain threshold of turbulence intensity, the flame kernel no longer develops in a flamelet regime, rather in a regime where its structure is strongly modified by the turbulence: the flame front would undergo an intense mixing by all the eddies of the flow ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mi mathvariant="italic">Da</mml:mi> </mml:mrow> <mml:mo>&lt;</mml:mo> <mml:mn>1</mml:mn> </mml:math> ) and a thickening by the small eddies ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mi mathvariant="italic">Ka</mml:mi> </mml:mrow> <mml:mo>∼</mml:mo> <mml:mn>10</mml:mn> </mml:math> ), leading to an intense stretch and possibly local extinctions. This ignition transition phenomenon in turbulent mixtures has already been reported by Shy et al. (2010) [1] and Huang et al. (2007) [2], but for different experimental conditions. The present study also reveals that there is a common criterion for the ignition transition phenomenon: whatever the focal length of the focusing lens and the equivalence ratio of the mixture, an ignition transition occurs when the turbulent flow reaches a Karlovitz number of the order of 10.