Title

The development of a correlation to predict the lean blowout of bluff body stabilized flames with a focus on relevant timescales and fuel characteristics

Date of Award

2013

Degree Name

M.S. in Mechanical Engineering

Department

Department of Mechanical and Aerospace Engineering

Advisor/Chair

Advisor: Scott D. Stouffer

Abstract

In many high-speed reacting flows, a bluff body is used to locally slow the velocity and stabilize the flame. Gas turbine engines, both in ground-based industrial settings, as well as in aviation settings, utilize bluff body stabilized flames, often running at lean fuel-air ratios to extend the equipment's lifetime or to meet emissions regulations. However, running the equipment at a lean condition also puts the system at risk for lean blowout, which can result in facility inefficiencies, hardware damage, and a catastrophic reduction in aircraft performance. This thesis uses experimental data taken at the Air Force Research Laboratory, as well as data collected from a review of past literature, to develop a correlation to predict lean blowout using a least squares curve fit method. The laminar flame speed and ignition delay time were calculated for subsets of the data using the chemical kinetics software Cantera, and the results were incorporated into the correlations. The purpose of this effort was to provide an accurate, practical method of predicting lean blowout for designers and modelers, as well as to provide insight into the critical parameters and timescales that govern the blowout process by examining the significance of each parameter included in the correlation and the physical and chemical processes it may affect. The correlations presented in this thesis indicate that the lean blowout of bluff body stabilized flames is dependent on both the Damkohler number and the Lewis number. U/D is the inverse of the fluid mechanic timescale, likely that of the mixing time in the shear layer between the recirculation zone and the fresh reactants. Pressure, temperature, and the hydrogen to carbon ratio of the fuel all affect the reactivity of the mixture, contributing to the chemical timescale in the Damkohler number. The molecular weight of the fuel influences the mass diffusion, and thereby the Lewis number, of the fuel. As the Lewis number increases, various reaction rates, including the turbulent flame speed, decrease, also affecting the chemical timescale in the Damkohler number. The exact chemical timescale could not be determined from the laminar flame speed and ignition delay time data. A major contribution of this work is establishing the role that fuel characteristics play in the lean blowout process. Very little work has been done in the literature on fuel effects in the lean blowout of bluff body stabilized flames, but the correlation developed clearly shows that both the molecular weight and hydrogen to carbon ratio of the fuel influence the process.

Keywords

Gas-turbines Combustion Testing, Space vehicles Propulsion systems Testing, Mechanical engineering; bluff body flame; lean blowout; fuel effects; Damkohler number; premixed combustion; lean blowoff

Rights Statement

Copyright 2013, author

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