Semester

Spring

Date of Graduation

2019

Document Type

Dissertation

Degree Type

PhD

College

Statler College of Engineering and Mineral Resources

Department

Mechanical and Aerospace Engineering

Committee Chair

V’yacheslav Akkerman

Committee Co-Chair

Arvind Thiruvengadam Padmavathy

Committee Member

Arvind Thiruvengadam Padmavathy

Committee Member

Cosmin Dumitrescu

Committee Member

Terence Musho

Committee Member

Damir Valiev

Abstract

The understanding of the morphology and propagation of premixed flames in channels is vital for the design and development of efficient propulsion systems requiring high heat release, such as pulse detonation engines, and for resolving accidental fire in industrial conduits. In this work, extensive computational simulation of premixed flames in channels (with rectangular and cylindrical cross sections) with closely packed obstacles are carried out with the goal of providing insights into flame propagation under various conditions which would be invaluable to the development and safety of energy and propulsion systems as well as for the mitigation of accidental fires in industrial processes.

Factors that may intensify or attenuates premixed flame propagation includes the geometry, thermochemical properties based on fuel mixture Lewis numbers (the ratio of the thermal to mass diffusivities) and operating conditions. In semi-open channels, the flame accelerates while propagating from the closed end to the open end. Much stronger flame acceleration occurs when a channel is packed with a tooth-brush-like array of obstacles. This is the so-called Bychkov mechanism, according to which, flame acceleration is devoted to delayed burning in the pockets formed by adjacent obstacles. The starting point of this study is to investigate the effect of wall shear stress and the Lewis number on flame acceleration in semi-open channels. It is found that the effects of wall shear stress play a significant role only when the obstacle spacing exceeds the radius (half-width) of the channel. However, the impact of the Lewis number, Le, is significant. Specifically, flame acceleration weakens for Le > 1, inherent to fuel-rich hydrogen or fuel-lean propane burning. In contrast, the Le < 1 flames, corresponding to the fuel-lean hydrogen or fuel-rich propane mixtures, are prone to extra strong folding of the flame front and thereby accelerate faster. The later effect can be devoted to the onset of the diffusional-thermal combustion instability. A strong interplay is observed to exist between the Lewis number and the blockage ratio (the ratio of the obstacle height to the half-width of the channel).

While semi-open channels replicate the geometry of most propulsion systems, premixed flames in channels with both ends open may represent the scenario of an accidental fire in a coal mine tunnel or other numerous industrial and laboratory conduits. In such channels with closely-placed obstacles, flame oscillations, or acceleration, or their sequences have been observed. While the oscillations are generally inherent to relatively narrow channels, in wider ones, the accelerative trend eventually dominates over the oscillations. Both the accelerating and the oscillatory trends depend strongly on the thermochemical properties of the mixtures and geometrical factors. All these trends differ conceptually from an ultrafast acceleration in semi-open channels, where the entire flame-generated flow is pushed towards a single exit, while with two extremes open, the flow is distributed in both the upstream and downstream directions, thereby moderating flame propagation.

The operating condition of premixed combustion is likely to influence the flame propagation trend. The next part of this work is focused on supercritical combustion. Supercritical fluids are known to have unique thermophysical properties that can be harnessed in developing highly efficient energy systems with near-zero emission levels. To develop such advanced combustors, it is imperative to scrutinize the combustion of supercritical oxy-fuels with CO2-dilutions. For this purpose, the key characteristics of supercritical CO2 diluted, oxy-methane premixed flames such as the flame velocity, flame thickness as well as the internal flame structure at 300 bar and 800 K are investigated. It is observed that an increase in the CO2-dilution rate makes the flame thicker and reduces the unstretched laminar flame speed as compared to the non-diluted cases. Starting with a planar flame, the analysis is subsequently extended to the corrugated ones, with the dynamics and morphology of such a corrugated flame front identified.

Furthermore, the Markstein length and Markstein number are calculated for each dilution rate to characterize the influence of the local heat release on the flame morphology and its propagation. The Markstein length is largest (and positive) for the case of no dilution, and it decreases with the dilution rates, attaining a negative value at highly-diluted conditions. While a corrugated flame generally spreads faster than a planar one having the same thermal-chemical characteristics, the corrugated flame velocity reduces with the dilution rate. Unlike exponential acceleration observed for hydrocarbon flames at atmospheric pressure, here initial exponential acceleration occurs which terminates quite soon and supplanted with a quasi-linear one.

Lastly, the supercritical premixed oxy-methane flames with CO2-dilutions in a semi-open channel with a tooth-brush array of obstacles is studied to understand how highly-diluted flames could accelerate. The supercritical oxy-methane flame in this state propagates quite fast, with the appearance of a leading shock, thereby attaining a saturated velocity independent of the channel width (diameter). The observed vorticity and the shock wave are confined to the burnt matter for a high blockage ratio of 2/3. With low CO2-dilution of the oxy-methane flame, a similar trend exists, but as the flame tip velocity starts approaching the sound speed in the premixture, the flame front breaks-up and the hot gas propagates into the cold fuel thereby preheating the fuel ahead of the front.

Generally, dilution stabilizes the flame front, reduces small-scale wrinkling and moderates flame acceleration. In highly diluted cases, a flame front widens. As a result, the flame oscillates near a quasi-steady speed due to hydraulic resistance imposed by the narrow tube and the obstacles. These results show that the flame accelerates exponentially for all dilution rates and the presence of obstacles causes significant acceleration of the highly diluted flames in a relatively wide channel. Overall, the Bychkov theory predicts well the propagation velocity of a supercritical flame as long as the dilution is moderate.

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