Date of Award

8-26-2022

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Mechanical and Aerospace Engineering

Advisor(s)

Benjamin Akih Kumgeh

Subject Categories

Engineering | Mechanical Engineering

Abstract

This thesis investigates the effect of a boundary condition on the dynamics of a bluff-body stabilized f lame operating near blow-off condition. Special emphasis is given to the effect of inlet turbulence intensity. This work is motivated by the understanding that more stringent regulations on fossil fuels generated emissions necessitate the design of combustion systems that operate at very fuel-lean conditions. Combustion at very lean conditions, however, induces flame instability that can ultimately lead to flame fluttering and eventual extinction. The dynamics of the flame at lean conditions can therefore be very sensitive to its boundary conditions. To better understand this, a numerical investigation was needed as experimental research used for our model validation ceased to provide this information. The first stage of the numerical research is based on the experiment conducted in the Volvo Flygmotor AB program. The numerical models are validated by comparing the results with the available experimental data. The near blow-off equivalence ratio is then determined using the validated set of models. The effect of ITI on the flame dynamics is subsequently investigated for a lean flame that is near blow-off condition. For the computational analysis Large Eddy Simulation (LES) method was selected for its accuracy and efficiency. Combustion is accounted for through the transport of chemical species and the turbulence-combustion interaction through laminar finite-rate model. The sensitivity to inlet turbulence is assessed by carrying out simulations at near blow-off condition. The inlet turbulence intensity is varied in increments of 5%. It is observed that while the inlet intensity of 5% causes blow-off, further increase to 10% preserves a healthy flame on account of more heat release arising from greater entrainment of combustible mixtures into the f lame zone just behind the bluff-body. This balance is again lost as the inlet turbulence intensity is further increased to 15%. These conclusions are first obtained using 2D LES and selected cases are verified through 3D LES. Further, the importance of chemical kinetics is addressed by comparative analysis using global and detailed chemical kinetics models. The results jointly highlight strategies that can be used to reduce the required computational costs without loss of critical flow features of near blow-off bluff body turbulent flame.

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Open Access

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