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Licentiate seminar

Fundamental studies of non-premixed combustion in turbulent wall jets using direct numerical simulation


Defendant Main Advisor Extra Advisor Date
Zeinab Pouransari Arne Johansson 2011-09-28

Opponent
Christer  Fureby, FOI

Evaluation committee

Abstract

The present thesis deals with the fundamental aspects of turbulent mixing and non-premixed combustion in wall-jet flows. Direct numerical simulations (DNS) of compressible turbulent flows are performed in a wall-jet configura- tion, which has a close resemblance to many industrial combustion applica- tions. The triple ”turbulence-chemistry-wall” interactions are also present in this flow set-up. These interactions have been addressed by first focusing on turbulent flow effects on the isothermal reaction, including the near-wall issues. Then, by adding heat-release to the simulations, it has been concentrated on heat-release effects on various phenomena that occur in the reacting turbulent wall-jet flow. In the computational domain, fuel and oxidizer enter separately in a non-premixed manner and the flow is fully turbulent and subsonic in all simulations. In the first phase of this study, the case of a turbulent wall-jet including an isothermal reaction without heat release is addressed in order to isolate the near-wall effects and the mixing characteristics of the flow and the key statistics for combustion are studied in the absence of thermal effects. A deeper insight into three-dimensional mixing and reaction characteristics in a turbulent wall-jet has been gained through investigation of the probability den- sity functions, higher order moments of velocities and reacting scalars and the scalar dissipation rates of different species. In the second phase, DNS of turbu- lent reacting wall-jets including heat release is performed, where a single-step global exothermic reaction with an Arrhenius-type reaction rate is considered. The main target was to identify the heat-release effects on different mixing scales of turbulent wall-jet flow. The scalar dissipation rates, time scale ratios, two-point correlations, one and two-dimensional premultiplied spectra are used to illustrate the heat release induced modifications. It is observed that heat release effects delay the transition process in the chemically reacting cases and enlarge the fluctuation intensities of density and pressure, but have a damping effect on all velocity fluctuation intensities. Finer small mixing scales were ob- served in the isothermal simulations and larger vortical structures formed after adding significant amounts of heat-release. Simulations with different Damko ?h- ler numbers, but comparable temperature-rise are performed and the expected behavior, a thinner flame with increasing Damko ?hler number, is observed. Fi- nally, some heat transfer related quantities are examined. The wall heat flux and the corresponding Nusselt numbers are addressed. The near-wall reaction effects on the skin friction coefficient are studied and further the reaction char- acteristics are investigated throughout the domain.
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