Bryan D. Quay

Spatially resolved measurements of soot volume fraction using laser-induced incandescence

Laser-induced incandescence is used to obtain spatially resolved measurements of soot volume fraction in a laminar diffusion flame, in which comparisons with laser scattering/extinction data yield excellent agreement. In addition, the laser-induced incandescence signal is observed to involve a rapid rise in intensity followed by a relatively long (ca. 600 ns) decay period subsequent to the laser pulse, while the effect of laser fluence is manifest in nonlinear and near-saturated response of the laser-induced incandescence signal with the transition occurring at a laser fluence of approximately 1.2 × 108 W/cm2. Spectral response of the laser-induced incandescence involves a continuous spectrum in the visible wavelength range due to the blackbody nature of the emission. Simultaneous measurements of laser-induced incandescence and light scattering yield encouraging results concerning the mean soot particle diameter and number concentration. Thus, laser-induced incandescence can be used as an instantaneous, spatially resolved diagnostic of soot volume fraction without the need for the conventional line-of-sight laser extinction method, while potential applications in two-dimensional imaging and simultaneous measurements of laser-induced incandescence and light-scattering to generate a complete soot property characterization are significant.

Effect of Flame Structure on the Flame Transfer Function in a Premixed Gas Turbine Combustor

The flame transfer function in a premixed gas turbine combustor is experimentally determined. The fuel (natural gas) is premixed with air upstream of a choked inlet to the combustor. Therefore, the input to the flame transfer function is the imposed velocity fluctuations of the fuel/air mixture without equivalence ratio fluctuations. The inlet-velocity fluctuations are achieved by a variable-speed siren over the forcing frequency of 75-280 Hz and measured using a hot-wire anemometer at the inlet to the combustor. The output function (heat release) is determined using chemiluminescence measurement from the whole flame. Flame images are recorded to understand how the flame structure plays a role in the global heat release response of flame to the inlet-velocity perturbation. The results show that the gain and phase of the flame transfer function depend on flame structure as well as the frequency and magnitude of inlet-velocity modulation and can be generalized in terms of the relative length scale of flame to convection length scale of inlet-velocity perturbation, which is represented by a Strouhal number. Nonlinear flame response is characterized by a periodic vortex shedding from shear layer, and the nonlinearity occurs at lower magnitude of inlet-velocity fluctuation as the modulation frequency increases. However, for a given modulation frequency, the flame structure does not affect the magnitude of inlet-velocity fluctuation at which the nonlinear flame response starts to appear.

Characterization of Forced Flame Response of Swirl-Stabilized Turbulent Lean-Premixed Flames in a Gas Turbine Combustor

Flame transfer function measurements of turbulent premixed flames are made in a model lean-premixed, swirl-stabilized, gas turbine combustor. OH∗, CH∗, and CO2∗ chemiluminescence emissions are measured to determine heat release oscillation from a whole flame, and the two-microphone technique is used to measure inlet velocity fluctuation. 2D CH∗ chemiluminescence imaging is used to characterize the flame shape: the flame length (LCH∗ max) and flame angle (α). Using H2-natural gas composite fuels, XH2=0.00–0.60, a very short flame is obtained and hydrogen enrichment of natural gas is found to have a significant impact on the flame structure and flame attachment points. For a pure natural gas flame, the flames exhibit a “V” structure, whereas H2-enriched natural gas flames have an “M” structure. Results show that the gain of M flames is much smaller than that of V flames. Similar to results of analytic and experimental investigations on the flame transfer function of laminar premixed flames, it shows that the dynamics of a turbulent premixed flame is governed by three relevant parameters: the Strouhal number (St=LCH∗ max/Lconv), the flame length (LCH∗ max), and the flame angle (α). Two flames with the same flame shape exhibit very similar forced responses, regardless of their inlet flow conditions. This is significant because the forced flame response of a highly turbulent, practical gas turbine combustor can be quantitatively generalized using the nondimensional parameters, which collapse all relevant input conditions into the flame shape and the Strouhal number.

Flame Response Mechanisms Due to Velocity Perturbations in a Lean Premixed Gas Turbine Combustor

The response of turbulent premixed flames to inlet velocity fluctuations is studied experimentally in a lean premixed, swirl-stabilized, gas turbine combustor. Overall chemiluminescence intensity is used as a measure of the fluctuations in the flame’s global heat release rate and hot wire anemometry is used to measure the inlet velocity fluctuations. Tests are conducted over a range of mean inlet velocities, equivalence ratios and velocity fluctuation frequencies, while the normalized inlet velocity fluctuation (V′ /Vmean ) is fixed at 5% to ensure linear flame response over the employed modulation frequency range. The measurements are used to calculate a flame transfer function relating the velocity fluctuation to the heat release fluctuation as a function of the velocity fluctuation frequency. At low frequency, the gain of the flame transfer function increases with increasing frequency to a peak value greater than one. As the frequency is further increased, the gain decreases to a minimum value, followed by a second smaller peak. The frequencies at which the gain is minimum and achieves its 2nd peak are found to depend on the convection time scale and the flame’s characteristic length scale. Phase-synchronized CH* chemiluminescence imaging is used to characterize the flame’s response to inlet velocity fluctuations. The observed flame response can be explained in terms of the interaction of two flame perturbation mechanisms, acoustic velocity fluctuations and vorticity fluctuations. Analysis of the phase-synchronized flame images show that when both perturbations arrive at the flame at the same time (or phase) they constructively interfere, producing the 2nd peak observed in the gain curves. And when the perturbations arrive at the flame 180 degrees out-of-phase, they destructively interfere, producing the observed minimum in the gain curve.Copyright

THE EFFECTS OF FUEL COMPOSITION ON FLAME STRUCTURE AND COMBUSTION DYNAMICS IN A LEAN PREMIXED COMBUSTOR

The stability characteristics of a laboratory-scale lean premixed combustor operating on natural gas - hydrogen fuel mixtures have been studied in a variable length combustor facility. The fuel and air were mixed upstream of the choked inlet to the combustor to eliminate equivalence ratio fluctuations and thereby ensure that the dominant instability driving mechanism was flame-vortex interaction. The inlet velocity, inlet temperature, equivalence ratio and percent hydrogen in the fuel were systematically varied, and at each operating condition the combustor pressure fluctuations were measured as a function of the combustor length. The results are presented in the form of two-dimensional stability maps, which are plots of the normalized rms pressure fluctuation versus the equivalence ratio and the combustor length, for a given inlet temperature, inlet velocity, and fuel mixture. In order to understand the effects of operating conditions and fuel composition on the observed stability characteristics, two-dimensional chemiluminescence images of the flame structure were recorded at all operating conditions and for all fuel mixtures under stable conditions. Changes in the stable flame structure, as characterized by the location of the flame’s “center of heat release”, were found to be consistent with the observed instability characteristics. The location of the flame’s “center of heat release” was found to lie along a single path for all operating conditions and fuel mixtures. It was also observed that there were regions of stable and unstable combustion as one moved along this path. Furthermore it was found that flames having the same “center of heat release” location, but different operating conditions and fuel composition, have very nearly the same flame shape. These results will be useful for developing phenomenological models for predicting unstable combustion.Copyright

Experimental Investigation of the Nonlinear Response of Swirl Stabilized Flames to Equivalence Ratio Oscillations

The nonlinear response of a swirl-stabilized flame to equivalence ratio oscillations was experimentally investigated in an atmospheric-pressure, high-temperature, lean-premixed model gas turbine combustor. To generate high-amplitude equivalence ratio oscillations, fuel was modulated using a siren type modulating device. The mixture ratio oscillations at the inlet of the combustion chamber were measured by the infrared absorption technique and the flame’s response, i.e., the fluctuation in the flame’s rate of heat release, was estimated by CH* chemiluminescence emission intensity. Phase-resolved CH* chemiluminescence images were taken to characterize the dynamic response of the flame. Results show that the amplitude and frequency dependence of the flame’s response to equivalence ratio oscillations is qualitatively consistent with the flame’s response to inlet velocity oscillations. The underlying physics of the nonlinear response of the flame to equivalence ratio oscillations, however, is associated with the intrinsically nonlinear dependence of the heat of reaction and burning velocity on the equivalence ratio. It was found that combustion cannot be sustained under conditions of high amplitude equivalence ratio oscillations. Lean blowoff occurs when the normalized amplitude of the equivalence ratio oscillation exceeds a threshold value. The threshold value is dependent on the mean equivalence ratio and modulation frequency.Copyright

Flame Transfer Function Measurement and Instability Frequency Prediction Using a Thermoacoustic Model

The dynamic response of a turbulent premixed flame to an acoustic velocity perturbation was experimentally determined in a lean-premixed, swirl-stabilized, lab-scale gas turbine combustor. Fuel was injected far upstream of a choked inlet to eliminate equivalence ratio oscillations. A siren-type modulation device was used to provide acoustic perturbations at the forcing frequency of 100 ∼ 400 Hz. To measure global heat release rate, OH*, CH*, and CO2 * chemiluminescence emissions were used. The two-microphone method was utilized to estimate inlet velocity fluctuations, and it was calibrated by direct measurements using a hot wire anemometer under cold-flow conditions. Gain of the flame transfer function (FTF) shows a low pass filter behavior, and it is well-fitted by a second-order model. Phase difference increases quasi-linearly with the forcing frequency. Using the n-τ formulation, gain and phase of FTF were incorporated into an analytic thermoacoustic model in order to predict instability frequencies and corresponding modal structures. Self-excited flame response measurements were also performed to verify eigenfrequencies predicted by the thermoacoustic model. Instability frequency predicted by the thermoacoustic model is supported by experimental results. Two instability frequency bands were measured in the investigated gas turbine combustor at all operating conditions: f ∼ 220 Hz and f ∼ 350 Hz. Results show that the self-excited instability frequency of f ∼ 220 Hz results from the fact that the flames amplify flow perturbations with f = 150 ∼ 250 Hz. This frequency range was observed in the flame transfer function measurements. The other instability frequency of f ∼ 350 Hz occurs because the whole combustion system has an eigenfrequency corresponding to the 1/4-wave eigenmode of the mixing section. This was analytically and experimentally demonstrated. Results also show that the flame length, LCH*max , plays a critical role in determining self-induced instability frequency.Copyright

An Experimental Study on the Coupling of Combustion Instability Mechanisms in a Lean Premixed Gas Turbine Combustor

An experimental study was conducted to characterize the combined effects of flame-vortex interactions and equivalence ratio fluctuations on self-excited combustion instabilities in a swirl-stabilized lean premixed gas turbine combustor. The combustor was designed so that the fuel injector location and the combustion chamber length could be independently varied. In addition, the fuel and air could be mixed upstream of the choked inlet to the combustor, thereby eliminating the possibility of equivalence ratio fluctuations. Experiments were performed over a broad range of operating conditions and at each condition both the combustor length and the fuel injection location were varied. Dynamic pressure in the combustor, acoustic pressure and velocity in the mixing section, and the overall rate of heat release were simultaneously measured at all operating conditions. Two distinct instability regimes were observed; one near 220 Hz and the other near 345 Hz. It was also found that the strength of the instability changed significantly as the fuel injection location was varied, while the phase of the acoustic pressure and velocity fluctuations in the mixing section did not change. A time series of pressure and CH* chemiluminescence signals confirmed constructive or destructive coupling of the two instability mechanisms; the flame-vortex interaction and the equivalence ratio fluctuation interact each other and determine the instability characteristics in partially premixed conditions.Copyright

The Effect of Confinement on the Structure and Dynamic Response of Lean-Premixed, Swirl-Stabilized Flames

The effect of flame-wall interaction on the forced response of a lean-premixed, swirl-stabilized flame is experimentally investigated by examining flames in a series of three combustors, each with a different diameter and therefore a different degree of lateral confinement. The confinement ratios tested are 0.5, 0.37 and 0.29 when calculated using the diameter of the nozzle relative to the combustor diameter. Using both flame images and measured flame transfer functions, the effect of confinement is investigated and generalized across a broad range of operating conditions. The major effect of confinement is shown to be a change in flame structure in both the forced and unforced cases. This effect is captured using the parameter Lf,CoHR/Dcomb, which describes the changing degree of flame-wall interaction in each combustor size. The measured flame transfer function data, as a function of confinement, is then generalized by Strouhal number. Data from the two larger combustors is collapsed by multiplying the Strouhal number by the confinement ratio to account for the flow expansion ratio and change in convective velocity within the combustor. Trends at the transfer function extrema are also assessed by examining them in the context of confinement and by using flame images. A change in the fluctuating structure of the flame is also seen to result from an increase in confinement.Copyright

Experimental Investigation of the Nonlinear Response of Swirl-Stabilized Flames to Equivalence Ratio Oscillations

The nonlinear response of a swirl-stabilized flame to equivalence ratio oscillations was experimentally investigated in an atmospheric-pressure, high-temperature, lean-premixed model gas turbine combustor. To generate high-amplitude equivalence ratio oscillations, fuel was modulated using a siren-type modulating device. The mixture ratio oscillations at the inlet of the combustion chamber were measured by the infrared absorption technique, and the flame’s response, i.e., the fluctuation in the flame’s rate of heat release, was estimated by CH chemiluminescence emission intensity. Phase-resolved CH chemiluminescence images were taken to characterize the dynamic response of the flame. Results show that the amplitude and frequency dependence of the flame’s response to equivalence ratio oscillations is qualitatively consistent with the flame’s response to inlet velocity oscillations. The underlying physics of the nonlinear response of the flame to equivalence ratio oscillations, however, is associated with the intrinsically nonlinear dependence of the heat of reaction and burning velocity on the equivalence ratio. It was found that combustion cannot be sustained under conditions of high-amplitude equivalence ratio oscillations. Lean blowoff occurs when the normalized amplitude of the equivalence ratio oscillation exceeds a threshold value. The threshold value is dependent on the mean equivalence ratio and modulation frequency. DOI: 10.1115/1.4001999