Modeling of combustion in stratified hydrogen-air mixtures

Rebecca A Owston, Purdue University

Abstract

This work provides a detailed and fundamental study of the propagation behavior of flames from an ignition source in hydrogen-air stratified layers, and explores approaches to their modeling. The understanding gained can be useful for practical applications like direct-injection spark-ignited (DISI) hydrogen engines. Beyond hydrogen applications, this research is also relevant to flame propagation in stratified fuel-air mixtures in general. Initial comparisons with published results show that qualitative flame structures and quantitative flame speed agree with the prior results. Following the relatively short-lived transient phase during ignition, the flame kernel is observed to propagate as a triple flame with the triple point located along the stoichiometric isocontour. The triple flame speed is about the same as the laminar premixed flame speed in a stoichiometric mixture, but the propagation speed with respect to a fixed frame of reference is greater than the flame speed due to hydrodynamic effects. This work shows that while the local structure of the triple flame is similar to that of a premixed flame at the local equivalence ratio, differences arise because of the trailing diffusion flame and diffusion of species and heat normal to the propagation. Changing mixing layer thickness does not change the flame speed or structure, but changes the propagation speed. Pressure, temperature, and inert concentration effects on triple flame structure and speed are the same as for premixed flames. The similarity between the triple flame and the premixed flame suggests that the triple flame can be modeled as a premixed flame, provided the diffusion normal to the propagation direction is accounted for. Evaluation of combustion sub-models based on 1-D premixed and/or diffusion flamelets and 2-D triple flame data confirm the potential viability of using the premixed library and direct tabulation options for modeling. In this work, the normal diffusion is directly computed, but in RANS simulations and LES, models for scalar dissipation rates which account for such diffusion will have to be incorporated. Reduced reaction mechanisms have also shown promise for use in modeling stratified flames, provided the mechanisms can reproduce premixed flame speeds and structure over a wide range of equivalence ratios.

Degree

Ph.D.

Advisors

Abraham, Purdue University.

Subject Area

Automotive engineering|Mechanical engineering

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