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The introduction of the pilot flame can stabilize the lean premixed flame and promote its industrial application. However, the interaction mechanism between the pilot and main flames is complicated. To reveal the effect of the pilot flame on the main flame, a laminar lean premixed flame adjacent to a rich premixed pilot flame on one side and a similar lean premixed flame on the other side was considered. A two-dimensional numerical model was adopted with detailed chemistry and species transport, also with no artificial flame anchoring boundary conditions. The results show that the pilot flame could promote the main flame stabilized in different locations with various shapes, by adjusting the stretch, heat transfer, and preferential diffusion in a complicated manner. The pilot flame improves the local equivalence ratio and transfer more heat to the main flame. The growth of the pilot flame equivalence ratio and inlet velocity enhances the combustion on the rich side of the main flame and helps it anchor closer to the flame wall. Both the curvature and strain rate show a significant effect on the flame root, which contributes to the main flame bending towards the pilot flame.

This work presents an experimental study on the influence of the pilot flame characteristics on the flame morphology and exhaust emissions of a turbulent swirling flame. A reduced-scale burner, inspired by that fitted in the AE-T100 micro gas turbine, was employed as the experimental platform to evaluate methane (CH4) and an ammonia-methane fuel blend with an ammonia (NH3) volume fraction of 0.7. The power ratio (PR) between the pilot flame and the main flame and the fuel composition of the pilot flame was investigated. The pilot power ratio was varied from 0 to 20% for both fuel compositions tested. The NH3 volume fraction in the pilot flame ranged from pure CH4 to pure NH3 through various NH3–CH4 blends. Flame images and exhaust emissions, namely CO2, CO, NO, and N2O were recorded. It was found that increasing the pilot power ratio produces more stable flames and influences most of the exhaust emissions measured. The CO2 concentration in the exhaust gases was roughly constant for CH4-air or NH3–CH4–air flames. In addition, a CO2 concentration reduction of about 45% was achieved for XNH3 = 0.70 compared with pure CH4, while still producing stable flames as long as PR ≥ 5%. The pilot power ratio was found to have a higher relative impact on NO emissions for CH4 than for NH3–CH4, with measured exhaust NO percentage increments of about 276% and 11%, respectively. The N2O concentration was constant for all pilot power ratios for CH4 but it decreased when the pilot power ratio increased for NH3–CH4. The pilot fuel composition highly affected the NO and N2O emissions. Pure CH4 pilot flames and higher power ratios produced higher NO emissions. Conversely, the NO concentration was roughly constant for pure NH3 pilot flames, regardless of the pilot power ratio. Qualitative OH-PLIF images were recorded to further investigate these trends. Results showed that the pilot power ratio and the pilot fuel composition modified the flame morphology and the OH concentration, which both influence NO emissions.

Computational fluid dynamics (CFD) modelers require high-quality experimental data sets for validation of their numerical tools. Preferred features for numerical simulations of a sooting, turbulent test case flame are simplicity (no pilot flame), well-defined boundary conditions, and sufficient soot production. This paper proposes a non-premixed C2H 4/air turbulent jet flame to fill this role and presents an extensive database for soot model validation. The sooting turbulent jet flame has a total visible flame length of approximately 400 mm and a fuel-jet Reynolds number of 10,000. The flame has a measured lift-off height of 26 mm which acts as a sensitive marker for CFD model validation, while this novel compiled experimental database of soot properties, temperature and velocity maps are useful for the validation of kinetic soot models and numerical flame simulations. Due to the relatively simple burner design which produces a flame with sufficient soot concentration while meeting modelers' needs with respect to boundary conditions and flame specifications as well as the present lack of a sooting \"standard flame\", this flame is suggested as a new reference turbulent sooting flame. The flame characterization presented here involved a variety of optical diagnostics including quantitative 2D laser-induced incandescence (2D-LII), shifted-vibrational coherent anti-Stokes Raman spectroscopy (SV-CARS), and particle image velocimetry (PIV). Producing an accurate and comprehensive characterization of a transient sooting flame was challenging and required optimization of these diagnostics. In this respect, we present the first simultaneous, instantaneous PIV, and LII measurements in a heavily sooting flame environment. Simultaneous soot and flow field measurements can provide new insights into the interaction between a turbulent vortex and flame chemistry, especially since soot structures in turbulent flames are known to be small and often treated in a statistical manner. © 2011 Springer-Verlag.

Substituting hydrocarbon fuels such as methane (CH4) with ammonia (NH3) reduces CO2 emissions in gas turbines, but ammonia’s low reactivity challenges flame stability. Dual-swirl staged combustors using a low thermal power (Ppilot) pilot flame can stabilise the main flame. This work compares the morphologies and stabilities of NH3/air premixed swirl flames using ammonia and methane pilot flames (APF and MPF). Flame imaging and simultaneous OH-NH planar laser-induced fluorescence (PLIF) are employed to analyse flame morphology. Main flame stability is assessed by measuring the lean blow-off equivalence ratio (ϕb,main). The results show that MPF significantly outperforms APF in main flame stabilisation. At Ppilot = 1.2–1.8 kW (14.2–21.3% of Pmain), the dual-swirl flames exhibit a stratified structure, with OH concentrated in the pilot stage. Flames with MPF exhibit considerably lower ϕb,main than those with APF. For example, at Ppilot = 1.6 kW, ϕb,main is reduced to 0.42 with MPF, compared to 0.56 with APF, demonstrating MPF’s superior stabilisation capability. MPF can reduce CO2 emissions by 82.4–87.6% compared to a CH4 flame of equivalent thermal power. Two stabilisation modes are identified, namely primary recirculation zone-dominated and pilot-dominated modes. These findings demonstrate that a low-power MPF provides an effective strategy for enhancing ammonia flame stability and reducing CO2 emissions in gas turbines.

The pilot flame serves as the primary anchor for global flame stabilization in a centrally staged combustor. In engineering practice, it typically operates in the diffusion mode. The fuel non-uniformity and diffusion kinetics of the pilot flame may have a significant impact on the flow and flames within the combustor. The flame structure and flame–flow interaction in a centrally staged burner featuring a diffusion pilot flame are investigated in the present paper, using high-frequency CH2O planar laser-induced fluorescence (CH2O-PLIF), CH* chemiluminescence, and particle image velocimetry (PIV) measurements. The stratified flame (S-flame) and the lifted flame (L-flame) are identified under two-stage conditions. The S-flame and L-flame correspond to the separated flow and the merged flow of the two stages, respectively. Significant radial oscillation of the pilot stage airflow is also found. Extensive tests demonstrate that the pilot equivalence ratio (Φp) plays an important role in flame mode switching. Silicone droplets with extremely fine sizes are introduced into the pilot fuel to trace its transportation. When the oscillating pilot stage airflow rushes towards the lip in an instant, it can entrain the pilot fuel to reach the inner side of the main stage outlet. With a low pilot fuel supply and relatively low injection velocity, the pilot fuel and the hot radicals are more likely to be entrained and accumulate in larger amounts at the inner side of the main stage outlet. Consequently, the main stage premixed mixture can be ignited at the main stage outlet, forming the S-flame. The flame mode switches from S- to L-flame when the equivalence ratio increases to the point where the corresponding velocity ratio of pilot fuel to air (Vfp/Vap) approaches 1.0, with a reduced entrainment of the pilot fuel and radicals. Simultaneous CH2O-PLIF and flow field results show that when the main stage is ignited downstream, hot products cannot recirculate to the pilot stage outlet, causing the extinction of the pilot flame root. This paper reveals that the fuel diffusion characteristics of the pilot stage can dramatically change the flame structure. To achieve the ideal designed flame shape, the interaction between the pilot fuel and pilot air requires very careful treatment in practical centrally staged combustors.

The interaction mechanism of internally-staged-swirling stratified flame is complex, and the pilot flame has a manifest influence on flame stability. To study this, a series of experimental investigations for the pilot flame has been carried out in a model swirl combustor by only supplying the pilot fuel. The CH * chemiluminescence images of the pilot flame are acquired by a high-speed camera with a CH * bandpass filter, whose dynamic characteristics are identified by image statistical analysis and proper orthogonal decomposition (POD) analysis. And the fast algorithm based on matrix theory proposed in this paper increases the operation efficiency and operability of POD. With the pilot equivalence ratio Φ increase, the pilot flame gradually shows an unstable state, whose POD energy distribution is significantly different. In the unstable state, the flame dynamics include three modes—spiral motion mode, flame shedding mode, and axial oscillation mode, whose formation reasons have also been further analyzed in combination with the experimental characteristics. And the fast Fourier transform (FFT) analysis of the time coefficients for the first four POD modes indicates all the dominant frequency is 280 Hz, which means the model combustor is in resonance. In addition, a sensitivity analysis based on the different image resolutions further reveals the robustness of the POD method and its optimization direction. These results emphasize the important influence of the pilot fuel flow rate on the stability of the pilot flame.

Combustion instabilities are a problem facing the gas turbine industry in the operation of lean, pre-mixed combustors. Secondary flames known as “pilot flames” are a common passive control strategy for eliminating combustion instabilities in industrial gas turbines, but the underlying mechanisms responsible for the pilot flame’s stabilizing effect are not well understood. This dissertation presents an experimental study of a pilot flame in a single-nozzle, swirl-stabilized, variable length atmospheric combustion test facility and the effect of the pilot on combustion instabilities. A variable length combustor tuned the acoustics of the system to excite instabilities over a range of operating conditions without a pilot flame. The inlet velocity was varied from 25 – 50 m/s and the equivalence ratio was varied from 0.525 – 0.65. This range of operating conditions was determined by the operating range of the combustion test facility. Stability at each operating condition and combustor length was characterized by measurements of pressure oscillations in the combustor. The effect of the pilot flame on the magnitude and frequency of combustor stability was then investigated. The mechanisms responsible for the pilot flame effect were studied using chemiluminescence flame images of both stable and unstable flames. Stable flame structure was investigated using stable flame images of CH* chemiluminescence emission. The effect of the pilot on stable flame metrics such as flame length, flame angle, and flame width was investigated. In addition, a new flame metric, flame base distance, was defined to characterize the effect of the pilot flame on stable flame anchoring of the flame base to the centerbody. The effect of the pilot flame on flame base anchoring was investigated because the improved stability with a pilot flame is usually attributed to improved flame anchoring through the recirculation of hot products from the pilot to the main flame base. Chemiluminescence images of unstable flames were used to identify several instability mechanisms and infer how these mechanisms are affected by the pilot flame. Flame images of cases in which the pilot flame did not eliminate the instability were investigated to understand why the pilot flame is not effective in certain cases. The phase of unstable pilot flame oscillations was investigated to determine how the phase of pilot flame oscillations may affect its ability to interfere with instability mechanisms in the main flame. A forced flame response study was conducted to determine the effect of inlet velocity oscillation amplitude on the pilot flame. The flame response was characterized by measurements of velocity oscillations in the injector and chemiluminescence intensity oscillations determined from flame images. As the forcing amplitude increases, the pilot flame’s effect on the flame transfer function magnitude becomes weaker. Flame images show that as the forcing amplitude increases, the pilot flame oscillations increase, leading to an ineffective pilot. The results of the flame response portion of this study highlight the effect of instability amplitude on the ability of a pilot flame to eliminate a combustion instability.

Abstract In a stratified charge engine, a glow plug pilot flame ignition system has been compared with a spark-ignition system for a model stratified charge Wankel combustion chamber. A motored two-stroke diesel engine was operated as a rapid compression and expansion machine with the cylinder head replaced by a model Wankel combustion chamber designed to simulate the temporal changes of air flow and pressure fields inside the chamber of an actual engine. It was found that the pilot flame ignition system had better ignitability and improved combustion characteristics, especially in the lean mixture range, relative to the spark-ignition system.

The LTC1541, which combines a micropower amplifier, comparator and 1.2V reference in an 8-pin package, is described.