Semester

Spring

Date of Graduation

2020

Document Type

Dissertation

Degree Type

PhD

College

Statler College of Engineering and Mineral Resources

Department

Mechanical and Aerospace Engineering

Committee Chair

Vyacheslav Akkerman

Committee Member

Hailin Li

Committee Member

Patrick Browning

Committee Member

Songang Qiu

Committee Member

Hayri Sezer

Abstract

Premixed combustion remains of fundamental interest in energy generation and propulsion systems as well as in implementation of safety measures for residential and industrial accidental fire explosions. While the fast pace and complex nature of the combustion process has previously necessitated the analytical and computational studies to employ the simplifying assumption of equidiffusivity, when the Lewis number defined as the thermal-to-mass diffusivities ratio is unity , the ongoing advancements in technology and the requirements for efficiently operating combustors over a wide range of conditions make the combustion process more non-equdiffusive ( ) than ever. The impact of non-equidiffusivity on the dynamics and morphology of a flame, and thereby on the combustion efficiency, becomes aggravated by the interactions with combustor geometric parameters as well as thermochemical properties of the fuel mixture.

Therefore, by representing combustors as channels with various extreme conditions (open channels, when both ends are open, or semi-open, when one end is closed, while the other one remains open), boundary conditions (non-slip or free slip, adiabatic or isothermal walls) and internal structures (obstructed or unobstructed), the current work addresses the effects of non-equidiffusivity and its interplays with other parameters on flame propagation in channels. Specifically, propagation of non-equidiffusive flames in channels is investigated by means of the computational simulations of the reacting flow equations with fully-compressible hydrodynamics and Arrhenius chemical kinetics. A detailed parametric study is performed for the Lewis numbers in the range ; the channel half-width , where is the thermal flame thickness; the blockage ratios, , being from to ; and the spacing between the obstacles being .

The diffusional-thermal combustion instability, associated with , and the flame thickening at are found to play a major role in determining the flame dynamics in a channel. Regarding finger flame acceleration in semi-open channels with adiabatic slip walls, it is shown that the flames accelerate slower than equidiffusive ones. In contrast, the flames acquire stronger distortion, associated with the diffusional-thermal combustion instability, and thereby accelerate much faster than at . Increased surface area of the flame front and thus, a higher burning rate and stronger acceleration is also obtained in wider channels. Presence of equally spaced obstacles in such channel produced higher acceleration, with the increase being more significant at and high blockage ratio.

When both ends of the channels are open, the flames show oscillations, acceleration or a sequence of both, depending on other parameters. For a channel with adiabatic non-slip walls, the oscillation amplitude and frequency decreases with , and the low- flames exhibiting different morphologies. A drastic change in flame dynamics is however seen for channel with isothermal wall. In narrow channels with small blockage ratios, the oscillations amplitude and frequency changes with , with the frequency decreasing and the amplitude increasing as grows from 0.3 to 2. In other conditions, a transition from flame oscillations to its sudden acceleration or propagation at constant velocity, is singularly influenced by the Lewis number, or by coupling to the geometric parameters. The delay time before the onset of flame acceleration, especially at , also varies as channel width and the blockage ratio changes. In all cases, the Lewis number shows both quantitative and qualitative effects on flame propagation in obstructed channel.

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