Lean Blowout Limit Research Articles

Effect of mainstream-forced-entrainment control strategy on the combustion performance of a cavity-based combustor

The trapped vortex combustor (TVC) displays potential for use in advanced aircraft engines because of its excellent combustion stability with a compact geometry. However, the existence of forced-entrainment phenomenon in the mainstream poses obstacles to the practical application of TVC. This study introduces passive control strategies using modified radial struts to address the issue of mainstream-forced-entrainment in a TVC. Additionally, experimental and numerical studies were conducted to investigate how the mainstream-forced-entrainment control strategy on the combustion performance of a TVC. The findings suggest that the inclined strut (case-2) and longer-vertical strut (case-3) are more effective than the short-vertical strut (case-1) in reducing mainstream entrainment into the cavity. This results in a larger vortex structure and a more concentrated fuel distribution within the cavity of case-2 and case-3, as compared to case-1. Consequently, case-2 and case-3 achieved much better ignition and lean blowout limit and combustion efficiency performances compared to case 1.

Characterizing lean blowout dynamics of DME/air premixed swirl flames in a gas turbine model combustor with and without confinement

This work investigates lean blowout (LBO) dynamics of dimethyl ether (DME)/air premixed swirl flames in a gas turbine model combustor with and without confinement. CH* chemiluminescence imaging and 2 kHz high-speed OH* chemiluminescence imaging are performed to capture the swirl flame macrostructures. Particle image velocimetry (PIV) and planar laser-induced fluorescence (PLIF) are used to visualize the flow fields and reaction zones, respectively. Compared to the unconfined flame, the addition of confinement results in the appearance of an outer recirculation zone (ORZ), variation in flame topology, enhanced overall heat release (HR) rate, increased turbulent kinetic energy (TKE), and extended LBO limits. Far from LBO, all flames are stabilized in the shear layer. Approaching LBO, the confinement enhances the flow vortex structures along the shear layers, which is associated with extinction at the flame base and shear layer. However, the unconfined flame is still stabilized in the shear layer due to the weaker flow vortex structures. Different extinction mechanisms during the LBO process are also discussed between the unconfined and confined flames. The unconfined flame suddenly leaves the inner shear layer (ISL) and recedes to the inner recirculation zone (IRZ) within about 100 ms before LBO. Abundant CH2O is transported into IRZ mainly by recirculation, which reduces the HR region area and give rise to LBO. For the confined flame, more and more cold reactants gather near the base plate and then are gradually brought into IRZ by ISL vortices, which makes the flame lift from the base plate and decrease the HR region area until LBO happens.

Effects of wall confinement on flame topologies and lean blowout characteristics of partially premixed DME/air flames in a gas turbine model combustor

Dimethyl ether (DME) is a promising alternative fuel for gas turbines, considering its renewable nature and potential for large-scale synthesis from CO2 feedstock. However, there are limited studies on the swirl combustion characteristics of DME, particularly regarding the stability performance. This work investigates the effects of wall confinement on flow, mixing, and lean blowout (LBO) characteristics of partially premixed DME/air swirl flames in a gas turbine model combustor. Flow and flame macrostructures are captured using particle image velocimetry and planar laser-induced fluorescence (PLIF) measurements, respectively. The results show remarkable differences in flame topologies between unconfined and confined flames, especially the formation of inner recirculation zone and outer recirculation zone. The LBO limits are measured over various Reynolds numbers and flame dynamics approaching LBO are captured using 10 kHz OH* chemiluminescence imaging. Unconfined flames have unexpectively lower LBO limits (∼0.4) than the LBO limits of confined flames (over 0.5). Approaching LBO, unconfined flames display a quiet blowout mode featuring swirling spiral blow-off behaviors. In contrast, confined flames exhibit a violent blowout mode featuring large-scale quasi-instability behaviors, including periodic events of local extinction, flame lift-off, and re-ignition. Based on the numerical simulations and simultaneous OH- and CH2O-PLIF measurements, the large discrepancy in the measured LBO limits between the two configurations is found to result from different re-ignition processes related to the low-temperature ignition before the final blowout. These findings highlight the crucial roles of wall confinement in determining the stable flame topologies, LBO characteristics, and mechanisms driving the LBO physics for partially premixed DME/air swirl flames.

The Effect of Swirl Number on Lean Blow Out Limits of Lean Direct Injection Combustors

Abstract This is the first study where a single variable sweep of SN is conducted to assess its impact on lean blowout limits (LBO) in a liquid-fueled lean direct injection (LDI) combustor. This study uses a scaled NASA SV-LDI (Swirl Venturi—Lean Direct Injection) hardware and is concerned with the impact of swirl number on the lean blow-out limit of a single-element LDI system at atmospheric pressure. The swirl numbers (SN) were varied from 0.31 to 0.66 using continuously variable active swirl number control system that was developed in-house. It is shown that the minimum operating equivalence ratio is a linearly increasing function of swirl number. While previous literature agrees with the positive slope for this correlation, past work has suggested that the LBO limit is proportional to the swirler vane angle which is shown to be untrue for LDI systems. By actively varying the swirl number, it is proven that LBO is proportional to SN, and it is well known that SN is not proportional to swirler vane angle. Increased SN reduces LBO margin because the better-mixed, high swirl dilutes locally rich pockets of fuel–air mixture in a globally lean flow. In addition to a baseline venturi, which was scaled from NASA's geometry, two other venturis were tested. A low-pressure loss venturi with a large throat diameter showed poor blow-out performance whereas a parabolically profiled venturi improved LBO over the baseline for the same swirl number.

Experimental and numerical study on flow/flame interactions and pollutant emissions of premixed methane-air flames with enhanced lean blowout hydrogen injection

This study investigates experimentally and numerically the stability, combustion, and emissions characteristics of premixed swirl-stabilized methane/air flames with enhanced lean blowout (ELBO) injection of hydrogen in the non-premixed mode for higher turndown ratio gas turbines. The study analyzed the effects of hydrogen fraction (HF: volumetric percentage of H2 in the total fuel) ranging from 0 to 25% and equivalence ratio (φ) ranging from 0.5 to 0.893 on flow/flame interactions and flame stability, structure, and emissions. The introduction of non-premixed H2 yields two conflicting outcomes, with enhancements in reaction kinetics and flame speed yet counteracted by increased hydrogen jet momentum dispersing the flame. The combustor lean blowout (LBO) limit remains relatively unaffected (φoverall≈ 0.5) by H2 injection. Flame temperature is primarily influenced by φ and minorly by HF. Increasing HF resulted in localized temperature rises around H2 jets without an overall increase in flame temperature, offering a promising strategy for reduction of NOx emissions. Nevertheless, using H2 as a pilot fuel leads to an increase in intermediate species like OH, H, and O, contributing to flame stabilization. However, this practice also results in higher CO and unburned hydrocarbons emissions due to the inherent characteristics and heightened reactivity of H2.

Experimental and numerical study on a cavity-swirler-based combustion strategy for advanced gas turbine engine

A cavity-swirler-based combustor has been found to possess the capability of achieving robust stability, high combustion efficiency, low emissions, and a short flame length. However, the effect of cavity air-injection mode on the combustion and emission performances of the cavity-swirler-based combustor was not well understood to date. In this work, numerical and experimental investigations were conducted to analyze the flowfield, combustion efficiency, combustion stability, pollutant emissions, temperature distribution, and flame characteristics of the combustor under two different cavity air-injection modes, namely case-1 and case-2. The air-injection hole of case 1 was designed on the cavity inner-wall, while the air-injection hole of case 2 was designed on the cavity outer-wall. The results indicate that the combustion performance of the combustor is significantly influenced by the air-injection mode, leading to different flow fields within the combustor. Case 1 demonstrates notable advantages in terms of lean blowout limit compared to case 2. However, case 2 achieves a much more uniform distribution of outlet temperature. Higher combustion efficiency was achieved by case-1 under low fuel equivalence conditions, whereas case-2 exhibited greater combustion efficiency under high fuel equivalence conditions. Both case-1 and case-2 displayed distinct combustion zones that were separate from each other.

Enhanced ammonia combustion by partial pre-cracking strategy in a gas turbine model combustor: Flame macrostructures, lean blowout characteristics and exhaust emissions

Cofiring with hydrogen presents a reasonable approach to achieve enhanced ammonia (NH3) combustion without introducing an extra carbon footprint. A promising strategy for NH3/H2 cofiring in gas turbines involves on-site partial pre-cracking of NH3 and burns of NH3/H2/N2 mixtures, eliminating additional hydrogen transportation and storage. This work investigates the effects of the pre-cracking ratio (γ) on flame macrostructures, lean blowout characteristics and exhaust emissions of the partially pre-cracked NH3 flames in a single-swirl gas turbine model combustor. Flow and flame macrostructures were captured using particle image velocimetry (PIV) and OH planar laser-induced fluorescence (OH-PLIF) measurements. Lean blowout limits (ϕLBO) were assessed under varying γ, and emissions at the burner outlet were measured using a Fourier transform infrared spectroscopic (FTIR) gas analyzer. Results show that as γ increases, the flame exhibits a shortened height, strengthened OH fluorescence, amplified core jet velocities and significantly reduced ϕLBO, indicating an effective enhancement of NH3 combustion by partial pre-cracking strategy. Nevertheless, NO and NO2 emissions exhibit a substantial increase with larger γ. Opposite trends of NO and NH3 emissions versus equivalence ratio (ϕ) suggest a trade-off between NO and NH3 emissions, with relatively low NO/NH3 window appearing under slightly-rich (ϕ = 1.0–1.1) conditions. Low NO emissions are also noted under ultra-lean conditions (ϕ = 0.4–0.5) with the penalty of high NH3 and N2O emissions, making it an unacceptable trade-off. Furthermore, the effect of N2 separation from the partially pre-cracked NH3 mixtures was evaluated at γ = 0.4. The results show deteriorating effects on NOx emissions, resulting in 13 % and 21 % increases in peak NO and NO2 emissions, respectively, which implies more feasibility to burn the partially pre-cracked NH3 in a direct manner rather than N2 separation.

Co-firing ammonia and hydrogen with butane under methane-equivalent calorific value and Wobbe index: Insights into transition in flame propagation and swirl flame characteristics

Fuel flexibility stands as a critical requirement for the development of combustors in future carbon-free energy systems. Carbon-free fuels including ammonia (NH3) and hydrogen (H2) are substantial supports for achieving this target, raising strong needs to counterbalance their distinct fuel thermochemical properties from methane (CH4), such as volumetric calorific value (CV) and Wobbe index (WI). In this work, butane (C4H10) which is an emerging renewable synthetic fuel is proposed to co-fire NH3 and H2 under methane-equivalent calorific value (MECV) and Wobbe index (MEWI). Laminar flame propagation of NH3/H2/C4H10 mixtures under MECV and MEWI are investigated in a cylindrical constant-volume combustion vessel. C4H10 co-firing can simultaneously tune the fuel thermochemical properties to CH4 levels and regulate the laminar flame propagation of NH3 and H2. Kinetic simulation and modeling analysis are performed to provide insights into the dominant kinetics and transition in laminar flame propagation. CV-compensated transition variables can generally well constrain the transition profiles of laminar burning velocity (LBV), while WI-compensated transition variables can linearize the transition profile from NH3 to C4H10 but meet challenges in constraining the transition profiles of H2-blending mixtures. Chemical effects are found to play the dominant role in the transition of laminar flame propagation, while thermal effects also have positive contributions. Test experiments in a gas turbine model combustor are also conducted to assess the swirl flame characteristics of NH3/H2/C4H10 mixtures under MECV and MEWI. The measured swirl flame morphologies and lean blowout limits indicate enhanced fuel reactivity and flame stability with the transition in fuel content, which is in accordance with the observed trends in LBV. Furthermore, comparison with CH4 provides a comprehensive evaluation of NH3/H2/C4H10 mixtures from the aspects of combustion characteristics and thermochemistry.

Experimental Investigation of the Effect of Superheated Liquid Fuel Injection on the Combustion Characteristics of Lean Premixed Flames

Abstract The effect of superheated liquid fuel injection on the performance and emissions of a single nozzle combustor was investigated. Combustion of the lean premixed flames was achieved using a combination of jet and swirl as a stabilization method. In a nonreactive setup, the optimum transition temperature of Jet A-1 fuel from liquid to superheated vaporized state was analyzed. In a subsequent reactive setup, a series of tests were conducted with the liquid fuel at low and elevated temperatures. The experiments were conducted at ambient pressure and various air and fuel preheat temperatures, axial swirlers, thermal powers, adiabatic flame temperatures, and flame tube diameters. Concentrations of nitric oxide (NOx) and carbon monoxide (CO) in the flue gas were measured. The operating conditions were systematically selected according to the design of experiments (DOE) method. The results showed that the adiabatic flame temperature caused the most significant change in combustion emissions and the position and shape of the reaction zone, while the superheated fuel injection had only a minor effect because the liquid fuel droplets were largely vaporized before entering the reaction zone through the integration of a swirler and a prefilmer. The use of the axial swirler and prefilmer allowed the combustor to operate in both spray and fully vaporized fuel conditions. As a result, very low emission concentrations of NOx (≈5 ppm) and CO (≈6 ppm) were achieved. The median flame length and height above the burner of the characterized flames showed competitive values of 32 mm and 50 mm, respectively. Lean blowout limits of less than 1500 K were achieved. Two different flame modes were observed during the experiments. By increasing the bulk velocity of the combustor, its hysteresis was resolved, resulting in stable and reliable flames with a wide low-NOx operating range.

Determination of Cetane Numbers Via Chemical Kinetic Mechanism

Abstract Minimizing global warming is a major task of todays' society. For air transport, sustainable aviation fuels (SAF) produced from renewable sources are a promising key solution. While electric flight is intriguing for short distances, SAF are required for mid- and long-distance flights and in addition, enable fuel design strategies to minimize environmental effects. The qualification and approval for SAF are standardized in the ASTM D4054, which include fuel properties as an essential part. Among others, lean blow-out (LBO) limits are a key performance parameter. The experimental determination of LBO is very time-consuming and cost-effective. The LBO of a specified engine is highly dependent on the fuel properties affecting evaporation, mixing, and ignitability. Therefore, prediction tools are desired to identify early promising SAF for decreasing the certification cost. Due to the correlation between LBO and derived cetane numbers (DCN), a tool for the prediction of the DCN is presented in this study. The DCN model uses chemical kinetic ignition delay time (IDT), simulated in a constant volume combustion chamber based on the ASTM D6890 standard, and seven representative physical properties of a fuel. A high agreement of the predicted DCN to the literature DCN with root-mean-square errors of 4.7 and correlation coefficients of 0.95 was found.

Enhancement of lean blowout limits of swirl stabilized NH3-CH4-Air flames using nanosecond repetitively pulsed discharges at elevated pressures

This study experimentally examines the impact of nanosecond repetitively pulsed (NRP) discharges on lean premixed methane-ammonia-air swirl flames at pressures up to 4 bar. The results reveal the extension of flame blowout limit by 10–12 % under atmospheric conditions and 5–6 % at 4 bar, when NRP discharges are applied. This decrease in the plasma effect could be attributed to the detrimental impact of strong shock waves at high pressure, destabilizing the flame. NH3-CH4-air flames shows higher lean blowout limits in comparison to CH4-air flames. NRP discharges exhibit consistent impact on the lean blowout limit enhancement across ammonia fractions (20–60 %), irrespective of pressure conditions. These results are obtained for a ratio of NRP discharge power to flame thermal power of 0.63 % and constant pulse repetition frequency of 30 kHz.

Synergistic effect of non-thermal plasma and CH4 addition on turbulent NH3/air premixed flames in a swirl combustor

The synergistic effect of non-thermal plasma (NTP) induced by a dielectric barrier discharge (DBD) and CH4 addition on turbulent swirl-stabilized NH3/air premixed flames in a laboratory-scale gas turbine combustor is experimentally investigated by varying the mixture equivalence ratio, φ, the mixt velocity, U0, and the mole fraction of CH4 in the fuel, Xf,CH4. It is found that the streamer intensity is significantly increased by adding CH4 to NH3/air flames compared with that by adding H2. This is because positive ions generated by CH4 addition play a critical role in generating streamers. Such streamers intensified by CH4 addition enhance the ammonia combustion more together with CH4, and hence, the lean blowout (LBO) limits of NH3/CH4/air flames are significantly extended compared with those without applying NTP. The maximum streamer intensity is found to be linearly proportional to φ⋅Xf,CH4⋅U0 in wide ranges of φ, Xf,CH4, and U0. NTP is also found to significantly reduce the amount of NOx and CO emissions simultaneously. All of the results suggest that NTP can be used more effectively with CH4 addition to stabilize turbulent premixed NH3/air flames and reduce NOx/CO emissions, which is attributed to their synergistic effect on the ammonia combustion.

Influence of Fuel Atomization on Combustor Lean Blowout: An Empirical Approach

Empirical approach has been adopted to study the effect of operating conditions on atomizing parameters and influence of such parameters on lean blowout limit in a gas turbine combustor. Poor atomization represented by higher spray drop size is found to affect adversely the lean blowout limit. Low air pressure and low fuel temperature results in a higher fuel drop size and require more energy for evaporation and combustion initiation which has to come from more fuel or higher FAR thus raising the blowout limit. Empirical relations can predict reasonably when validated and updated with more and more experimental data.

Rotating gliding arc discharge induced flame oscillation near the lean blowout limit

This work investigates the rotating gliding arc (RGA) assisted swirling methane combustion characteristics near the lean blowout limit by utilizing the high-speed videography. As the flow rate and the global equivalence ratio were varied, four flame modes, namely self-sustained flame, RGA-sustained stable flame, RGA-induced oscillating flame, and RGA anchored flame modes were detected. In particular, the oscillating flame mode with an oscillating frequency of 2∼4 Hz occurs with the air flow rates above 13 L/min and the equivalence ratio between 0.52 and 0.56. It is an RGA-induced low-frequency flame oscillation phenomenon. The RGA discharge characteristics can impact the oscillating behaviors that with the increase of the transformer input voltage, the flame oscillation frequency is enhanced and the averaged flame area becomes larger to result in a higher combustion efficiency. Besides, increasing the wall temperature of the combustion chamber promotes the combustion during the oscillating stage. Based upon the experimental and numerical results this RGA-induced low-frequency oscillating flame are further interpreted by a three-staged mechanism. It implies that the first stage, during which the fuel/oxidant mixture is refreshed in the chamber and the temperature increases through recirculating flow gradually, determines the oscillating frequency. The fast accumulation of released energy from plasma discharge and partial fuel burning by recirculation can accelerate the temperature growth in the recirculation zone to increase the oscillation frequency. In all, this unique oscillating phenomenon and related explanations indeed broaden our knowledge of oscillating combustion behaviors and GA assisted combustion.

Experimental study on the effect of liquid loading on n-heptane spray jet flame stability

Flame stability significantly influences the performance of a practical combustor. The mechanisms governing the stability limits of spray jet flames are examined here using an annular co-flow spray burner. This study focuses on investigating the impact of liquid loading on spray jet flame stability, represented by flame lift-off height (LOH) and lean blow-out (LBO) limit. This is achieved by utilizing a parametric experimental study in which co-flow temperatures (Tco-flow = 298 K, 350 K, 400 K, and 450 K), spray nozzle sizes (dorifice = 0.209 mm, 0.219 mm, 0.232 mm, and 0.237 mm), fuel/air flow rates, and velocities are independently varied. A single liquid fuel, n-heptane, is used so that the vapor/liquid fractions (droplet lifetime) can be easily predicted via simple vapor–liquid equilibrium calculations to assist in the explanation of the stability mechanisms involved. The results show that the spray flame LOH mechanisms are affected by both chemical flame speed and, to a lesser extent, vaporization. On the other hand, the LBO limit is found to be controlled by the amount of liquid fuel in the presence/entering the flame. When the co-flow temperature is increased, while keeping constant co-flow velocity, the flame stabilizes near the burner lip. However, an opposite trend is noticed for flame blow-out limits, with the flame being more difficult to blow out when lower co-flow temperatures are applied. The variation of nozzle sizes leads to similar conclusions, i.e., when larger nozzle sizes are used, leading to the formation of larger droplets, the flame stabilizes further downstream and becomes more difficult to reach blow-out. The interpretation of the results indicates that delaying the vaporization and generating local enrichment regions near the flame edge enhances the flame LBO performance. Such an approach may be used as an efficient passive control mean to enhance flame stabilization; however, secondary impacts on emissions must be considered.

Experimental and numerical prediction of LBO performance in a centrally staged combustor

A new type of centrally staged combustor was proposed for an aeroengine combustor with a fuel–air ratio of 0.037. The combustor dome comprised a three-stage swirler, which included a pilot stage and main stage. The pilot stage was composed of Stage 1 and Stage 2 swirlers, and all swirlers were radial. The effects of different inlet Reynolds numbers and swirler structural parameters, including the installation angle of the first radial swirler, installation angle of the second radial swirler, installation angle of the main stage, and co– and counter-rotation of Stage 1 and Stage 2 swirlers on the lean blow-out (LBO) performance of the centrally staged combustor were examined at an atmospheric pressure and inlet temperature of 473 K. Experiments with single-dome rectangular test pieces and numerical methods were used. The results showed that the fuel–air ratio decreases with an increase in the inlet Reynolds number, indicating that the increase in inlet Reynolds number plays an important role with respect to the LBO. When the rotation directions of Stage 1 and Stage 2 swirlers were opposite, the fuel–air ratio of the LBO was smaller than that of Stage 1 and Stage 2 swirlers in the same rotation direction, indicating that the rotation directions of Stage 1 and Stage 2 swirlers significantly affect the LBO performance. Stage 1 and Stage 2 swirlers exhibited the best LBO performance when they rotated in opposite directions, and the rotation direction of the main stage slightly affected the LBO performance. The best combination of rotation directions is that the rotation directions of Stage 1 and Stage 2 swirlers should be opposite and rotation direction of Stage 2 swirler should be the same as that of the main stage. With the increase in the installation angle of Stage 1 and Stage 2 swirlers, the LBO fuel–air ratio decreased by nearly 4% and 9%, respectively, and the LBO performance improved. The prediction of the LBO limit of 0.00553, using the fuel steady-state successive approximation method, was less than 0.0066, which was measured in the test, indicating that the method should be improved. The flow field characteristics of the LBO process, which could not be obtained in the test process, were obtained via numerical simulation.

Blowout dynamics and plasma-assisted stabilization of premixed swirl flames under fuel pulsations

The present work investigates blowout dynamics of premixed swirl flames under low-frequency high-amplitude flow pulsations from the fuel feedline, and applies microsecond repetitively pulsed discharges with extremely-low power consumption to precisely control the stability of pulsating flames. First, both positive and negative fuel pulsations deteriorate the flame stability, causing much-narrowed lean blowout regimes. The fuel pulsation causes the flame to remain at the fuel-lean condition for a long period of time, which is considered to be the main cause of flame blowout. The flame response can be further explained by the convection process of the equivalence ratio oscillation. Secondly, the microsecond pulsed discharge with the same repetition rate as the fuel pulsation is used to alter the lean blowout via thermal, kinetic, and hydrodynamics effects. More importantly, the time delay between the discharge pulse and the fuel pulsation determines whether the plasma has an enhancement effect or not. It is found that the discharge prior to the convection of the equivalence ratio pulsation reaches the optimal effectiveness for flame stabilization, extending the blowout limit by approximately 10%. Finally, the lean blowout limit can be continuously extended with the increasing discharge repetition rate until saturation.

An FV-EE model to predict lean blowout limits for gas turbine combustors with different structures and sprays

The occurrence of Lean Blowout (LBO) is a disadvantage that endangers a stable operation of gas turbines. A determination of LBO limits is essential in the design of gas turbine combustors. A semiempirical model is one of the most widely used methods to predict LBO limits. Among the existing semiempirical models for predicting LBO limits, Lefebvre’s LBO model and the Flame Volume (FV) model are particularly suitable for gas turbine combustors. On the basis of Lefebvre’s and FV models, the concept of effective evaporation efficiency is introduced in this paper, and a Flame Volume-Evaporation Efficiency (FV-EE) model is derived and validated. LBO experiments are carried out in a model combustor with 23 different structures and 10 different sprays. The prediction uncertainty of the FV-EE model is less than ± 13% for all of these 33 structures and sprays, compared with ± 50% for the FV model and ± 60% for Lefebvre’s model. Furthermore, the prediction uncertainty of the FV-EE model is also less than ± 13% for other combustors from available literature.

Control of Lean Blowout in a Swirl-Stabilized Dump Combustor at Different Levels of Premixing Based on Flame Colour

ABSTRACT Presently, the use of lean combustion has been growing in industrial furnaces, aero-engine, and rocket engine applications due to their low N O X emission and minimum maintenance requirements. On the other hand, due to the excessively low combustion reaction rate, the operation at a low fuel–air ratio leads to the sudden shutdown of the engines, which is termed lean blowout (LBO). Therefore, to operate land-based and aircraft engines at an extremely lean fuel–air mixture, control of LBO has become indispensable. In the present work, we develop several feedback in-loop control strategies using a pixel-averaged intensity ratio between the red and blue components of the flame emission. During a lean operation, based on the quantification of the proximity of the combustion dynamics to the LBO limits using this color ratio, the control strategies make the solenoid valves active to inject a secondary pilot fuel and energize the flame base. We notice that a proper choice of the color ratio as a threshold enhances the stability of both premixed and partially premixed combustion using a low pilot fuel. Besides, we test N O X emission for unpiloted and piloted operations and find that the emission is only marginally affected during a pilot injection.

Experimental investigation of the stability and emission characteristics of premixed formic acid-methane-air flames in a swirl combustor

Formic acid (FA) is a potential hydrogen energy carrier and low-carbon fuel by reversing the decomposition products, CO2 and H2, back to restore FA without additional carbon release. However, FA-air mixtures feature high ignition energy and low flame speed; hence stabilizing FA-air flames in combustion devices is challenging. This study experimentally investigates the flame stability and emission of swirl flames fueled with pre-vaporized formic acid-methane blends over a wide range of formic acid fuel fractions. Results show that by using a swirl combustor, the premixed formic acid-methane-air flames could be stabilized over a wide range of FA fuel fractions, Reynolds numbers, and swirl numbers. The addition of formic acid increases the equivalence ratios at which the flashback and lean blowout occur. When Reynolds number increases, the equivalence ratio at the flashback limit increases, but that decreases at the lean blowout limit. Increasing the swirl number has a non-monotonic effect on stability limits variation because increasing the swirl number changes the axial velocity on the centerline of the burner throat non-monotonically. In addition, emission characteristics were investigated using a gas analyzer. The CO and NO concentrations were below 20 ppm for all tested conditions, which is comparable to that seen with traditional hydrocarbon fuels, which is in favor of future practical applications with formic acid.