Derivation of burning velocities of premixed hydrogen/air flames at engine-relevant conditions using a single-cylinder compression machine with optical access

With respect to hydrogen internal combustion engines beside turbulence also flame front instabilities of high-pressure combustion provoke an acceleration of the flame. To account for this effect within engine simulations, it is suggested to include the impact of flame front instabilities directly into a “quasi-laminar” burning velocity that is an input for turbulent combustion models. Premixed hydrogen/air flames are investigated in a single-cylinder compression machine using OH-chemiluminescence and in-cylinder pressure analysis. Values of burning velocities are calculated from flame front velocities considering thermal expansion effects. A flame speed correlation is derived which covers temperatures and pressures of the unburned mixture, relevant for internal combustion engines, ranging from 350 K to 700 K and 5 bar to 45 bar. Values of air/fuel equivalence ratio cover lean and rich regimes between 0.4 ≤ λ ≤ 2.8. For an evaluation of stretch and instability effects a comparison to fundamental laminar burning velocities of a one-dimensional flame computed with a detailed chemical kinetic-mechanism is given. At high-pressure conditions flame speed measurements demonstrate that flame front instabilities have an accelerating effect on the value of laminar burning velocities, which cannot be reproduced by computations with a chemical model. A linear stability analysis is applied in order to estimate the magnitude of instabilities. The proposed “quasi-laminar” burning velocity does not account for interaction between turbulence and instability effects. Consequently, at increasing turbulence levels partially counter-balancing of instabilities by turbulence is not followed which may allegorize a possible limitation of the suggested approach.