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Research Papers: Gas Turbines: Combustion, Fuels, and Emissions

# Fuel Flexibility Influences on Premixed Combustor Blowout, Flashback, Autoignition, and Stability

[+] Author and Article Information
Tim Lieuwen

School of Aerospace Engineering,  Georgia Institute of Technology, Atlanta, GA 30318

Vince McDonell

UCI Combustion Laboratory,  University of California, Irvine, CA 92697-3550

Eric Petersen

Mechanical, Materials & Aerospace Engineering,  University of Central Florida, Orlando, FL 32816

Domenic Santavicca

Department of Mechanical and Nuclear Engineering,  The Pennsylvania State University, University Park, PA 16802

The Lewis number, Le, is the ratio of the thermal to mass diffusivity.

In the $H2∕O2$ system, there are two pathways through which the $H+O2$ reaction proceeds: $H+O2=OH+O$, a chain-branching pathway, and $H+O2+M=HO2+M$, a chain-terminating pathway. The classical second explosion limit refers to the region of temperature and pressure where the chain termination reaction becomes important, hence slowing down the ignition process. This second limit generally occurs at higher pressures and lower temperatures. The first limit favors the chain-branching reaction and occurs at higher temperatures and lower pressures.

We can obtain insight into the effect of fuel composition on this phenomenon by considering the mechanism responsible for vortex breakdown. While there are undoubtedly a variety of mechanisms for the instabilities leading to breakdown, one likely candidate is an axisymmetric, inviscid mechanism discussed by Brown and Lopez (52). For this mechanism, the breakdown point will favor regions of adverse pressure gradient, or equivalently, flow divergence. Flame fronts that are inclined to the flow induce a radial component in the approach flow velocity because of the gas expansion process and deflection of streamlines across the flame. The magnitude of this induced flowfield is a function of the density ratio across the flame and the flame’s inclination angle with respect to the flow.

J. Eng. Gas Turbines Power 130(1), 011506 (Jan 11, 2008) (10 pages) doi:10.1115/1.2771243 History: Received June 13, 2006; Revised July 04, 2006; Published January 11, 2008

## Abstract

This paper addresses the impact of fuel composition on the operability of lean premixed gas turbine combustors. This is an issue of current importance due to variability in the composition of natural gas fuel supplies and interest in the use of syngas fuels. This paper reviews available results and current understanding of the effects of fuel composition on combustor blowout, flashback, dynamic stability, and autoignition. It summarizes the underlying processes that must be considered when evaluating how a given combustor’s operability will be affected as fuel composition is varied.

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## Figures

Figure 1

Sketch of basic approach for lean premixed combustion

Figure 2

Predicted dependence of laminar flame speed upon CH4 concentration (by volume) for CH4∕H2 and CH4∕CO2 mixtures. Adiabatic flame temperature fixed at indicated value. Pressure=1atm, reactant temperature=300K.

Figure 3

Dependence of laminar flame speed (cm∕s) upon fuel composition at fixed 1500K (left) and 1900K (right) adiabatic flame temperatures at 4.4atm with 460K reactants temperature

Figure 4

Dependence of turbulent flame speed upon turbulence intensity for several fuel blends with the same laminar flame speed (reproduced from Kido (25))

Figure 5

Predicted dependence of ST∕SL for CH4∕H2 mixtures upon CH4 concentration at fixed adiabatic flame temperatures of 1500K and 2000K, integral length scale=5.1cm. Results obtained from application of model developed by Kido (25).

Figure 7

Effect of mixture composition of CH4∕H2 (left) and CO∕H2 (right) blends on ignition delay times at 15atm and ϕ=0.4

Figure 11

Dependence of chemical time (ms) on fuel composition at fixed adiabatic flame temperature, 1500K (left) and 1900K (right), at 4.4atm with 460K reactant temperature

Figure 14

Normalized rms pressure fluctuation in the combustor versus the vortex convection time (τV=τconv) divided by the acoustic period, T

Figure 13

Two-dimensional chemiluminescence flame images showing the effect of equivalence ratio on the distance to the flame “center of mass” for a fixed inlet velocity of 72m∕s and inlet temperature of 200°C

Figure 12

Dependence of LBO equivalence ratio upon H2 mole fraction at constant premixer exit velocity of 60m∕s and 4.4atm combustor pressures, 460K inlet temperature

Figure 10

Ratios between residence time and chemical time at constant premixer exit velocity of 60m∕s, combustor pressure of 1.7atm, and 300K reactant temperature

Figure 9

Representation of combustion phenomena relative to stability loop

Figure 8

Effect of pressure on ignition delay times for 60∕40 blends of CH4 or CO with H2 at ϕ=0.4 (left, CH4∕H2 blend; right, CO∕H2 blend)

Figure 6

Dependence of characteristic kinetic time scales upon fuel composition at mixture temperatures of 1000K (left, in ms) and 1400K (right, in μs). Compositions are based on those that will produce an adiabatic flame temperature of 1500K at 4.4atm with a 460K reactant temperature.

Figure 15

Comparison of flow reactor data with kinetic modeling using different detailed mechanisms (35,40,56-59)

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