Domenic A. Santavicca

Experimental Diagnostics for the Study of Combustion Instabilities in Lean Premixed Combustors

An improved understanding of the mechanisms of unstable combustion in lean premixed combustors is essential to the development of stable gas turbine combustion systems. To obtain such understanding, detailed experimental studies of the phenomenology of unstable combustion are required. A number of experimental diagnostic techniques for characterizing unstable combustion and the underlying instability mechanisms are discussed. This includes techniques based on pressure, chemiluminescence emission, infrared absorption, and laser-induced fluorescence measurements. The techniques themselves are discussed briefly; however, the primary objective is to present and discuss results illustrating how these techniques can be used to characterize the mechanisms of unstable combustion, to gain an improved understanding of unstable combustion, and to develop strategies for suppressing unstable combustion in lean premixed gas turbine combustors.

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

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. Of particular concern is the effect of fuel composition on combustor blowout, flashback, dynamic stability, and autoignition. This paper reviews available results and current understanding of the effects of fuel composition on the operability of lean premixed combustors. 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.Copyright

Mechanism of Combustion Instability in a Lean Premixed Dump Combustor

Results from an experimental study of the mechanism of unstable combustion in a coaxial, optically accessible, bluff-body-stabilizeddumpcombustorwithnaturalgasasthefuelarereported.Aparametricstudywasperformed to investigate the effects of equivalence ratio, inletvelocity, inlet fuel distribution, inlet swirl, and centerbody recess oncombustionstability. It wasfoundthatall of theseparametershadan effectonthestability characteristicsofthis combustor.Atselectedunstableoperatingconditions,phase-resolvedCHchemiluminescenceimageswerecaptured to study the heat-release structure during one period of pressure oscillation. The e ame‐ e owe eld interaction that is depicted in these images indicates that e ame‐ vortex interactions, and the resultant e ame area changes, play a signie cant role in the instabilities that occur when there is no swirl. A simple analysis of these images, however, showedthate uctuatinge ameareaandequivalenceratioe uctuationsbothcontributetotheheatreleasee uctuations that drive the instability. Unstable combustion with swirl appears to be fundamentally different from unstable combustion without swirl in that instabilities with swirl occur near lean blowout and appear to be associated with repeated detaching and reattaching of the e ame from the centerbody.

Planar laser Rayleigh scattering for quantitative vapor-fuel imaging in a diesel jet

Abstract Quantitative images of vapor-phase fuel concentrations were obtained in an evaporating and combusting diesel jet using planar laser Rayleigh scattering. The diagnostic has been calibrated, evaluated, and successfully applied to an optically accessible direct-injection diesel engine for fired and nonfired operating conditions. The measurements were obtained in the leading portion of the diesel jet (the zone beyond 27 mm from the injector nozzle), where the fuel is entirely evaporated, and which corresponds to the main combustion zone in this engine. The technique was shown to be effective for quantitative imaging of the fuel-vapor concentration before ignition, with high spatial and temporal resolution. Additionally, images of the fuel-vapor concentration were further reduced to imagers of the equivalence ratio using an adiabatic mixing assumption to model the local temperature of the evaporating diesel jet. This procedure also yielded temperature distribution images. The results show that, at 4.5° crank angle (0.63 ms) after the start of injection, which corresponds to the time just before the first indicated heat release, the fuel and air are relatively well mixed in the leading portion of the diesel jet. At this crank angle, the equivalence ratio in the majority of the jet ranges from 2 to 4. The edges of the jet are well defined, with the signal level rising sharply from the background level up to levels corresponding to equivalence ratios in the jet. The temperature of the richest mixture regions in the jet is as low as 700 K, with the ambient air temperature at 1000 K. Finally, comparisons of Rayleigh images of the reacting and nonreacting jet show that the initial breakdown of the fuel, indicated by a significant decrease in the Rayleigh signal intensity, occurs throughout the cross section of the leading portion of the diesel jet.

Measurement of equivalence ratio fluctuation and its effect on heat release during unstable combustion

This paper presents the first quantitative measurements of equivalence ratio fluctuations during unstable combustion in a lean premixed model dump combustor operating on natural gas with a nominal heat release rate of 75 kW. Equivalence ratio was measured using an infrared absorption probe based on the absorption by methane of the 3.39 μm output of a He−Ne laser. In order to make quantitative equivalence ratio measurements it was necessary to independently measure the absorption coefficient over a wide range of temperatures (293–683 K) and pressures (up to 600 kPa). Using simultaneous equivalence ratio, pressure, and chemiluminescence-based heat release measurements, it was demonstrated that new insights could be obtained regarding the role of equivalence ratio fluctuations during unstable combustion. For example, the measurements indicate that the observed heat release can be primarily attributed to the measured equivalence ratio fluctuations, and they show that the equivalence ratio fluctuations exhibit significantly higher harmonics, whereas the heat release and pressure fluctuations occur predominately at the fundamental frequency of the instability.

Stability and emissions characteristics of a lean premixed gas turbine combustor

The effect of incomplete fuel-air mixing on NO x emission and combustion stability has been studied in an atmospheric pressure, optically accessible, laboratory-scale, premixed dump combustor. The degree of fuel-dimensional acetone fluorescence. Combustion stability was characterized both in terms of the level of combustion-generated noise during combustion oscillations and the lean blowouts limit. The structure of the flame zone was characterized using two-dimensional OH fluorescence measurements that were phase locked with the combustion-generated pressure oscillations. As has been observed previously, it was shown that incompletes fuel-air mixing results in increased NO x emissions at overall fuel-lean conditions. The results, however, indicate that “reaction zone” NO mechanisms, enhanced by superequilibrinm O-atom concentrations, are in part responsible for more than 50% of the increased NO production. It was also shown that incomplete fuel-air mixing in a premixed combustor can significantly reduce the stable operating range of the combustor both in terms of increasing the tendency for combustion oscillation and raising the lean blowont limit. Lastly, it was observed that during unstable combustion, the flame moves in a well-defined periodic manner: however, the details of the flame behavior and instability mechanism can vary significantly depending on the overall equivalence ratio and the degree of fuel-air mixing.

Burner Development and Operability Issues Associated with Steady Flowing Syngas Fired Combustors

This article addresses the impact of syngas fuel composition on combustor blowout, flashback, dynamic stability, and autoignition in premixed, steady flowing combustion systems. These are critical issues to be considered and balanced against emissions considerations in the development and operation of premixed combustors. Starting with blowout, the percentage of hydrogen in the fuel is suggested to be the most significant fuel parameter, which is more fundamentally related to the hydrogen flames resistance to stretch induced extinction. Turning to flashback next, it is shown that multiple flashback mechanisms are present in swirling flows, and the key thermophysical properties of a syngas mixture that influence its flashback proclivity depend upon which flashback mechanism is considered. Flashback due to turbulent flame propagation in the core flow and the interaction of heat release with pulsations are less critical, whereas flame propagation in boundary layers and flashback due to the interaction of the heat release with vortex breakdown dynamics are most significant. Then, combustion instability is considered. The key flame parameter impacting the conditions under which instabilities occur is the spatial distribution of the flame. As such, fuel composition influences dynamics through impacts upon flame speed and the flame stabilization point. Furthermore, certain syngas fuel compositions are not more inherently stable than others – rather, each mixture has particular islands in the parameter space of, e.g., velocity and fuel/air ratio, at which instabilities occur. Changes in fuel composition move these islands around but do not necessarily eliminate or introduce instabilities. Relative to autoignition, measurements indicate that the ignition delay time exceeds typical premixer residence times, though by a substantially less margin than suggested by the calculations. Recent experiment work suggest that current detailed kinetic mechanisms developed for hydrogen/carbon monoxide ignition overestimate the ignition delay time, indicating the need for additional kinetic work in the low temperature, high pressure regime.

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.

Effect of injection location on the effectiveness of an active control system using secondary fuel injection

The effect of secondary fuel injection location on the effectiveness of active combustion control was studied in an atmospheric pressure, laboratory-scale dump combustor operating on natural gas with a nominal heat release rate of 75 kW. The combustor provided for secondary fuel injection from three different locations. The effectiveness of both open- and closed-loop control was found to be strongly dependent on the secondary fuel injection location. To understand this behavior, CO2 chemiluminescence imaging measurements were made of the spatial and temporal heat release distribution in the combustor, both to characterize the evolution of the heat release distribution during unstable combustion and to characterize the heat release produced by the injection of secondary fuel. The results indicate that the most effective approach to preventing unstable combustion is not necessarily to add heat in a manner which is out of phase with the instability but that effective control can also be achieved by disrupting the mechanism that is driving the instability. These results serve to illustrate that gaining an understanding of the phenomenology of the instability is a critical step in designing and optimizing an active combustion control system.