Flashback Limits Research Articles

Combustion and emission characteristics for various swirler geometries and fuel heating values in a premixed low swirl combustor system

This study investigated combustion and emission characteristics with various swirler parameters (i.e., swirl angle and perforated screen blockage) and fuel heating values (4,000 kcal/Nm3∼9,560 kcal/Nm3, 16.3 MJ/kg∼55.5 MJ/kg in SI unit) using the premixed low swirl combustor system. Flashback limits were evaluated as a flame stability and flame structures were analyzed by acquiring chemiluminescence (OH* and CH*) and OH-Planar Laser Induced Fluorescence (PLIF) signals. NOx and CO emissions were evaluated by calculating emission indices (EINOx and EICO). As a result, the swirl angle did not affect flame stability, whereas screen blockage increased flashback limits. The highest screen blockage (β = 0.8) showed the flashback limit of 5.6 kW, around 1.5 times higher than the lowest screen blockage (β = 0.6). Also, the blockage affected the radical distributed position due to the ratio between swirling and non-swirling flow. The lower heating value fuel showed reduced flashback limits, decreased radical intensities, and lower emissions (EINOx and EICO). The lowest heating value gas (HV 4000) had an EINOx of less than 0.5 g/kg and an EICO of less than 5 g/kg, respectively. In conclusion, these findings would be helpful databases for optimizing heating values to solve environmental pollution issues.

Experimental and Numerical Study on the Combustion Characteristics of Premixed Methane/Air in Porous Media Burner

ABSTRACT The porous media burner is considered as the promising alternative for residential water heaters. In this work, a two-layer porous media burner was designed and the maximum NOx emission was 48.7 mg/m3 under the firing rate of 666 kW/m2. A two-dimensional model was established; the effect of the pore parameters on the flame stability limit and NOx emission was investigated. The flashback, blow-off and flame instability were observed in the experimental and numerical investigations. Through the numerical optimization, the preferred pore parameters were 0.8 mm & 0.4 (preheating zone) and 1.6 mm & 0.7 (combustion zone); the predicted NOx emission was 32.9 mg/m3. The preferred porous media were produced and studied experimentally. Under the same condition, NOx emission was 28.7 mg/m3, which was less than 35 mg/m3 (the standard requirement, GB 18,111-2021) and reduced by 41% compared with that of the initial burner (48.7 mg/m3). The flash back limit was increased and the flame was stable at φ = 0.71.

Characterization of a 5-nozzle array using premix/micromix injection for hydrogen

Hydrogen is one of the most promising fuels to decarbonize energy systems since it has a high specific energy, a carbon-free combustion process, and may be produced sustainably through electrolysis. Direct fuel drop-in replacement cannot be done in traditional lean, premixed combustion burners because of the high reactivity of the hydrogen-air mixture and its inherently unstable nature that might lead to flashback and hardware failure. Consequently, new injection strategies and burner geometries are investigated to mitigate these risks. Here we present hydrogen capabilities of a premix/micromix injector that relies on a two-staged fuel injection strategy to offer wide fuel flexible capability (methane to hydrogen) within a single design. Five injectors are placed in a cross-shaped array to simulate a sector found in multi-element combustion systems. Stability and combustion dynamics maps are obtained for the array and OH planar laser-induced fluorescence (PLIF) provides additional insight into the combustion process of these novel burners when placed in an array. The use of micromixing is shown to drastically improve the acoustics of this geometry and expand the flashback limits compared to the fully premixed configuration. Significant variability in flashback limits are observed for different additively-manufactured injector configurations. Phase-averaged OH-PLIF measurements, obtained by registering the acquisition with the acoustics module, and the 3D reconstruction of a partially-premixed flame highlight the complexity of the stabilization mechanisms for these highly three-dimensional, non-axisymmetric flames that may be subjected to large thermoacoustics. This first investigation into premix/micromix clustered injectors demonstrates the importance of better understanding the impact of flame–flame interaction in multi-element combustion systems with micromixed, or partially-premixed, combustion.

Quantification of Mixing Quality Effects on Flashback Limits for H2-Rich Fuel Gases in Lean Premixed Combustion Using Laser-Induced Fluorescence of Acetone

Abstract One of the main challenges arising from hydrogen-rich fuel mixtures is to prevent flame flashback. In typical gas turbines, the fuel is commonly injected through holes in the axial swirler vanes to achieve a high mixing quality. However, this injection method is not perfect and can cause nonhomogeneous mixing regions, so that locally rich fuel clusters can significantly increase flashback propensity. This work aims to establish a link between the local mixing quality of fuel and air and flashback limits obtained experimentally under elevated pressure conditions. The nonreacting experiments have been conducted at an atmospheric mockup test rig and acetone-planar laser-induced fluorescence (PLIF) has quantified the local fuel concentration. Zones of high equivalence ratio are evident close to the center body wall. The near-wall equivalence ratio fields reveal that the critical probability of the local equivalence ratio being greater than the one for perfect premixing is between 20% and 35% for all hydrogen concentrations at the flashback limits observed. A probability of 35% is selected as a critical threshold to derive a correlation between the local and the global equivalence ratio in technical premixing. Even though the correlation is specific to the investigated burner configuration, the presented methodology offers valuable insights into the impact of the local mixing quality on flashback propensity, which can improve flashback prediction models formulated for perfect premixing conditions.

Study of the Impact of LPG Composition on the Blowoff and Flashback Limits of a Premixed Flame in a Swirl Burner

Liquefied petroleum gas (LPG) is considered one of the gases widely used in many industrial and residential sectors. Still, due to its different compositions, mainly propane (C3H8) and butane (C4H10), it can have other combustion characteristics. This paper aims to conduct an experimental analysis to study the impact of LPG composition on the stability map (limits of blowoff and flashback) of the premixed flame in a tangential swirl burner. Four LPG mixtures were used with different proportions of ethane (C2H6), propane (C3H8), butane (C4H10), and pentane (C5H12). Three burner nozzles at diameters of 20, 25, and 30 mm have been used, which gave three swirl numbers of 0.918, 1.148, and 1.377, respectively. The results indicate that increasing the swirl number (S) from 0.918 to 1.377 for all LPG mixtures accelerated the flashback propensity (getting worse) while the blowoff resistance improved; thus, a rising S gave a better stability map. As for the effect of the LPG composition, it was found that the maximum flame temperature was for the LPG mixture containing high percentages of butane (C4H10), while the lowest was for the mixture containing fewer percentages of butane. Changing the LPG composition had an apparent effect on the flashback limits and a slight effect on the blowoff limits; it was found that mixtures containing high percentages of butane increased flame speeds and increased the flashback propensity. Compared to LPG mixtures, the flame stability map was widest for LPG mixtures containing lower percentages of butane. Therefore, LPG with propane (C3H8) proportions higher than butane (C4H10) reduces flame temperature, flame speeds, and flashback propensity, thus improving the stability map.

Prediction of boundary layer flashback limits of hydrogen flame using an LES/non-adiabatic FGM approach

Prediction of boundary layer flashback limits of hydrogen flame using an LES/non-adiabatic FGM approach

Performance of biogas blended with hydrogen in a commercial self-aspirating burner

A natural gas and carbon dioxide fuel mixture was enriched by hydrogen to experimentally measure the performance of a hydrogen and surrogate biogas fuel. The performance of various biogas qualities and hydrogen blend ratios was investigated for use in a commercial self-aspirating burner by quantifying the impact on primary air entrainment, blow-off and flashback limits, NOx emissions, radiant heat transfer and flame appearance. The results showed operating limits of a biogas or hydrogen flame alone could be extended by blending one with the other and by increasing fuel injector size (lowering primary air entrainment), with the latter being more effective. The collateral impact of carbon dioxide and hydrogen addition on radiant heat transfer could be mitigated by increasing injector size, with the carbon dioxide content in biogas maintaining NOx levels at or below the level of natural gas. Overall, there appears to be merit for hydrogen and biogas to operate as a mutually beneficial fuel in a commercial self-aspirating burner design.

Predicting flashback limits in [formula omitted] enriched [formula omitted]/air and [formula omitted]/air laminar flames

The influence of hydrogen blending on the flashback limit of dihedral premixed laminar flames stabilized on a slit burner is investigated via two-dimensional direct numerical simulations with conjugate heat transfer and detailed chemistry. Flashback limits are determined for CH4/H2/Air and C3H8/H2/Air mixtures with hydrogen contents ranging from 0% to 100% and varying fuel-to-air ratios adjusted according to the iso–Tad and the iso–δT hybridization strategies. The analysis reveals the existence of two different flashback regimes and a critical effective Lewis number for the fuel mixture, Lec=0.5, controlling the switch from one regime to the other. Simulations are also used to explore the dynamics of flames during flashback in these two regimes. They show that for mixtures above the critical effective Lewis number, flashback is symmetric whereas for mixtures below the critical value, an asymmetric flame propagation is observed through the burner slit. These results highlight the impact of preferential diffusion on the stabilization mechanism of hydrogen fuel blends.

Impacts of Flow Swirl on Stability and Flow/Flame Interactions of Premixed Oxy-Methane Swirl Flames

Abstract Effects of flow swirl on stability and flow/flame interactions of premixed oxy-methane flames (CH4/O2/CO2) are investigated experimentally and numerically in a premixed model gas turbine combustor. Two swirlers of 55-deg and 45-deg swirl angles were considered to perform this study over a range of combustor operating equivalence ratio (Φ = 0.1–1.0) and oxygen fraction (OF = 21%–70%) at a constant inlet flow velocity of 5.2 m/s. Combustor stability maps (representing flashback and blowout bounds) were identified experimentally in the Φ-OF space for the two swirlers, and the results were plotted over the calculated contours of adiabatic flame temperature (AFT). Specific flames were photographed using a camera to investigate the impact of flow swirl on flame macrostructure. Also, the shapes of the selected flames were calculated numerically using the contours of OH radicals, and the results showed good agreement with the photographed flame shapes. Contours of temperature and flow streamlines were plotted based on numerical calculations to figure out the influence of flow swirl on flame/flow interactions. The results showed that CH4/O2/CO2 swirl flames blow out at fixed AFT of ∼1600 K with no effect of swirl on flame stability near the blowout. Flow/flame interactions significantly affect flame stability near the flashback limit. Flame speed (FS) and AFT correlate with one another as log(FS) ∝ 1/AFT. The 45-deg swirler resulted in a wider stable combustion zone than that of the 55-deg swirler.

Effect of CO[formula omitted] dilution on structures of premixed syngas/air flames in a gas turbine model combustor

The impact of CO2 dilution on combustion of syngas (a mixture of H2, CO, and CH4) was investigated in a lab-scale gas turbine model combustor at atmospheric pressure conditions. Two mild dilution levels of CO2, corresponding to 15% and 34% of CO2 mole fraction in the syngas/CO2 mixtures, were experimentally investigated to evaluate the effects of CO2 dilution on the flame structures and the emissions of CO and NOx. All experiments were performed at a constant Reynolds number (Re = 10000). High-speed flame luminescence, simultaneous planar laser-induced fluorescence (PLIF) measurements of the OH radicals and particle image velocimetry (PIV) were employed for qualitative and quantitative assessment of the resulting flame and flow structures. The main findings are: (a) the operability range of the syngas flames is significantly affected by the CO2 dilution, with both the lean blowoff (LBO) limit and the flashback limit shifting towards fuel-richer conditions as the CO2 dilution increases; (b) syngas flames exhibit flame-pocket structures with chemical reactions taking place in isolated pockets surrounded by non-reacting fuel/air mixture; (c) the inner recirculation zone tends to move closer to the burner axis at high CO2 dilution, and (d) the NOx emission becomes significantly lower with increasing CO2 dilution while the CO emission exhibits the opposite trend. The flame-pocket structure is more significant with increased CO2 dilution level. The low NOx emissions and high CO emissions are the results of the flame-pocket structures.

Modeling the boundary-layer flashback of premixed hydrogen-enriched swirling flames at high pressures

We model the boundary-layer flashback (BLF) of CH4/H2/air swirling flames via large-eddy simulations with the flame-surface-density method (LES-FSD), in particular, at high pressures. A local displacement speed model tabulating the stretched flame speed is employed to account for the thermo-diffusive effects, flame surface curvature, and heat loss in LES-FSD. The LES-FSD well captures the propagation characteristics during the BLF of swirling flames. In the LES-FSD for lean CH4/H2/air flames at 2.5 bar, the critical equivalence ratio for flashback decreases with the increasing hydrogen volume fraction, consistent with the experiments. This is due to the improved modeling of effects of the flame stretch and heat loss on the local displacement speed. We also develop a simple model to predict the BLF limits of swirling flames. The model estimates the critical bulk velocity for given reactants and swirl number, via the balance between the flame-induced pressure rise and adverse pressure for boundary-layer separation. We validate the model against 14 datasets of CH4/H2/air swirling flame experiments, with the hydrogen volume fractions in fuel from 50% to 100%. The present model well estimates the flashback limits in various operating conditions.

Numerical investigation of hydrogen addition effects to a methane-fueled high-pressure combustion chamber

To get a general understanding of hydrogen fuel usage in a real-condition high-pressure industrial gas turbine burner operating at 15 atm pressure, the effects of hydrogen addition to methane fuel are studied numerically. Methane-hydrogen premixed fuel-lean flame is investigated while hydrogen is substituted for methane at a fixed fuel and air mass flow rate at the burner inlet. Steady-state reacting and swirling flow equations (RANS) are solved using the standard k−ε model as the turbulence model. A detailed methane combustion mechanism GRI 2.11 is used to simulate methane-hydrogen combustion with Finite Rate formulation more accurately. Temperature and axial velocity variation and important pollutant species emissions such as CO, CO2, and NOx were investigated at the combustion chamber exit. Flame shape, flow pattern, and flame location variation with hydrogen addition were studied due to hydrogen flashback limits. The hydrogen addition leads to higher temperatures inside the burner and the hot zone inside the combustion chamber extends and moves toward the fuel inlet. Flow structure including recirculation zones change with hydrogen addition. CO emission increases by hydrogen addition to the fuel mixture near the burner exit while CO2 mass fraction is reduced. A significant increase in NO mass fraction with hydrogen addition was observed near the burner exit due to higher temperature.

Prediction of flashback limits for laminar premixed hydrogen-air flames using flamelet generated manifolds

Development of models that can help predict flashback limits of premixed flames at an affordable computational cost is essential for the safe and efficient design of combustion chambers. For flames with strong preferential diffusion effects, usually the focus has been on the development of at least a three dimensional flamelet database that can predict the enthalpy and mixture fraction mapped on to the reaction progress variable. However, in this study, we show that a 3D FGM table is sufficient to predict flashback limits for lean laminar methane-air flames but is not sufficient to predict the same for lean hydrogen flames and an over-prediction of 100% could occur in the calculation of the flashback limits. We trace the root cause of this over-prediction to be related to the thickness of the reaction zone in the progress variable for hydrogen flames. This results in the development of a novel correction factor for the progress variable source term using 1D flame simulations where the flame experiences strong enthalpy gradients. In the end, we successfully show for the first time that the flashback limits for hydrogen flames can be predicted accurately using flamelet generated manifolds with a source term corrector function.

Confined Boundary-Layer Flashback Flame Dynamics in a Turbulent Swirling Flow

Understanding boundary-layer flashback is critical to the design of safe and efficient gas turbines, especially as the addition of highly reactive hydrogen to these devices becomes a prevailing trend for reduction of carbon dioxide emissions. In this work, the boundary-layer flashback of lean hydrogen–air mixtures is studied using large-eddy simulations based on a generic turbulent swirl burner previously investigated experimentally. Simulations at increasing equivalence ratios are used to identify the flashback limits, showing reasonable qualitative agreement with experimental measurements. Flame dynamics during flashback are compared to previous experimental studies, indicating that important physics are captured in the simulations even if the exact flashback limits are not. In particular, a change in the swirl vane angle is shown to dramatically change the flame dynamics, with flame propagation occurring in the direction of swirl at high angles and against the direction of swirl at low angles, consistent with experimental observations. The near-wall nonreacting mean velocity field is shown to be controlled by a temporally stationary flow instability, which is directly responsible for the counterswirl flame flashback exhibited at low swirl angles. Finally, the interaction of local axial flow velocity and flame propagation speed of the leading flame point is investigated with varying swirl angles both before and after the onset of flashback, elucidating the differences in flame dynamics in all four of these cases. In particular, the importance of local flame extinction on flashback limits is emphasized.

A compilation of operability and emissions performance of residential water heaters operated on blends of natural gas and hydrogen including consideration for reporting bases

The impact of hydrogen added to natural gas on the performance of commercial domestic water heating devices has been discussed in several recent papers in the literature. Much of the work focuses on performance at specific hydrogen levels (by volume) up to 20–30% as a near term blend target. In the current work, new data on several commercial devices have been obtained to help quantify upper limits based on flashback limits. In addition, results from 39 individual devices are compiled to help generalize observations regarding performance. The emphasis of this work is on emissions performance and especially NOx emissions. It is important to consider the reporting bases of the emissions numbers to avoid any unitended bias. For water heaters, the trends associated with both mass per fuel energy input and concentration-based representation are similar For carbon free fuels, bases such as 12% CO2 should be avoided. In general, the compiled data shows that NOx, NO, UHC, and CO levels decrease with increasing hydrogen percentage. The % decrease in NOx and NO is greater for low NOx devices (meaning certified to NOx <10 ng/J using premixing with excess air) compared to conventional devices (“pancake burners”, partial premixing). Further, low NOx devices appear to be able to accept greater amounts of hydrogen, above 70% hydrogen in some cases, without modification, while conventional water heaters appear limited to 40–50% hydrogen. Reporting emissions on a mass basis per unit fuel energy input is preferred to the typical dry concentration basis as the greater amount of water produced by hydrogen results in a perceived increase in NOx when hydrogen is used. While this effort summarizes emissions performance with added hydrogen, additional work is needed on transient operation, higher levels of hydrogen, system durability/reliability, and heating efficiency.

Numerical Study of the Influence of the Thermal Gas Expansion on the Boundary Layer Flame Flashback in Channels with Different Wall Thermal Conditions

In recent years, boundary layer flame flashback (BLF) has re-emerged as a technological and operational issue due to the more widespread use of alternative fuels as a part of a global effort to promote carbon neutrality. While much understanding has been achieved in experiments and simulations of BLF in the past decades, the theoretical modeling of BLF still largely relies on the progress made as early as the 1940s, when the critical gradient model (CGM) for the laminar flame flashback was proposed by Lewis and von Elbe. The CGM does not account for the modification of the upstream flow by the flame, which has been recently shown to play a role in BLF. The aim of the present work is to gain additional insight into the effects of thermal gas expansion and confinement on the flame-flow interaction in laminar BLF. Two-dimensional simulations of the confined laminar BLF in a channel are performed in this work. The parametric study focuses on the channel width, the thermal gas expansion coefficient, and the heat losses to the wall. This study evaluates the influence of these factors on the critical condition for the flame flashback. By varying the channel width, it is demonstrated that at the critical condition, the incoming flow in narrow channels is modified globally by the thermal gas expansion, while in wider channels, the flow modification by the flame tends to be more local. In narrow channels, a non-monotonic dependence of the critical-condition centerline velocity on the channel width has been identified. The variation of the heat loss to the wall confirms that the wall’s thermal conditions can significantly alter the flashback limit, with the flashback propensity being larger when the thermal resistance of the wall is high. To assess the general applicability of the CGM, the flame consumption speed and the flow velocity near the wall are quantified. The results confirm that the assumption of flame having no influence on the upstream flow, employed in the CGM, is not fulfilled under confinement for a realistic thermal gas expansion. This results in a general disagreement between the simulations and the CGM, which implies that the thermal expansion effects should be accounted for when considering the confined boundary layer flashback limits. It is shown that the critical velocity gradient increases with the gas expansion coefficient for the given channel width and wall thermal condition.

Humidification Towards Flashback Prevention in a Classical Micro Gas Turbine: Thermodynamic Performance Assessment

Combustion air humidification has proven to be effective to stabilize hydrogen combustion and to avoid flashback apparition in a typical micro Gas Turbine (mGT). However, both the fuel alteration and combustion air dilution will impact the cycle performance. A complete characterization of this thermodynamic impact is essential to ensure that the mGTs become cleaner, and fully flexible to fit with the expectation of future small-scale decentralized power production. Therefore, the objective of this work is twofold: the determination of the necessary dilution for combustion stabilization, depending on the type of fuel, as well as the impact assessment on the cycle performance. In this framework, a hybrid model of the Turbec T100 mGT combustor, combining a 0D Chemical Reactor Network and 1D Laminar flame calculations, is used to first assess the flashback limits. The laminar flame speed is evaluated to predetermine the necessary minimal water dilution of the combustion air to avoid flashback for several CH4/H2blends. Second, a thermodynamic analysis is performed to assess the impact of the flame stabilization measures on the cycle performance of the mGT using Aspen Plus. The 0D/1D simulation results show that the combustor of the Turbec T100 can operate with fuels containing up to 100% hydrogen. However, the thermodynamic analysis shows that the water dilution leads to a decreased electrical performance. Future work consists in the iterative coupling of both 0D/1D and the Aspen model to correctly predict the flashback limits, considering the altering operating conditions. To conclude, with this work, we provide a framework for future mGT operations with alternative fuels.

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.

Investigation of flame evolution and stability characteristics of H2-enriched natural gas fuel in an industrial gas turbine combustor

Enhancing the H2 blending ratio in industrial gas turbines is essential to reduce fossil energy consumption and achieve carbon neutrality. However, several uncertainties still exist in their safe operation with H2 addition. In this work, a numerical investigation is conducted to evaluate the behavior and combustion stability of an H2-enriched natural gas fuel premixed swirl flame in an industrial gas turbine. The results indicate that the flame exhibits three types of behaviors upon increasing the H2 blending ratio, i.e., stable combustion, central flashback, and boundary layer flashback. The flame structure is highly sensitive to the inlet air temperature. The combustion reaction zone moves to the swirler as the inlet air temperature decreases, which in turn aggravates the central flashback. Injecting recirculated flue gas into the inlet air is proposed to restrain the central flashback. It is found that the flashback and blow-out limits exhibit quadratic polynomial and linear relationships with the recirculated flue gas ratio, respectively. The laminar flame speed of H2-enriched natural gas premixed flames with recirculated flue gas dilution at the flashback limit is 35.8 ± 0.5 cm/s. It is recommended to design or control H2-blended gas turbine burners with a laminar flame speed to ensure operational safety.

Accurate Prediction of Confined Turbulent Boundary Layer Flashback Through a Critically Strained Flame Model

Abstract A novel boundary layer flashback model is developed based on previous measurements that showed flashback limits may be related to strained premixed flame extinction. According to the model, flashback occurs at the equivalence ratio where the strained extinction limit flame speed matches the mean axial flow velocity one thermal distance from the wall. The model is validated by comparison with experimental measurements of flashback of confined nonswirling turbulent hydrogen-air flames. This comparison shows that the proposed model is capable of predicting confined turbulent boundary layer flashback across a large range of wall velocity gradients and preheat temperatures. The model is extended to methane-hydrogen-air flames in a swirling configuration using information about a single flashback event and shows good agreement with experimental measurements as a function of both hydrogen mole fraction in the fuel and pressure. In addition, inclusion of a mean nonreacting velocity field computed via large Eddy simulation allows for a significant increase in the accuracy of the model when applied to swirling flows. Ultimately, this model provides a new pathway for the design of flashback resistant gas turbines, even with the addition of fuels like hydrogen.