Boundary Layer Flashback Research Articles

Evaluation of hydrogen/ammonia substitute fuel mixtures for methane: Effect of differential diffusion

One strategy for utilizing ammonia as an energy vector is to replace conventional hydrocarbons with hydrogen-enriched ammonia in existing or retrofitted combustors. However, the strong differential diffusion effect of hydrogen can significantly alter the combustion properties and cause challenges in combustor operability. Therefore, this numerical study performs a systematic investigation on the effect of differential diffusion on fundamental combustion properties of ammonia/hydrogen fuel blends. The investigated combustion properties span from the initiation of combustion, i.e., ignition, to stretched flame propagation and ultimately to flame stabilization mechanisms. The objective is to evaluate the feasibility of hydrogen/ammonia/air mixtures as substitutes for methane/air particularly considering the implications of differential diffusion effects. To this end, four fuel blends with similar unstretched burning properties are selected: fuel lean ammonia/hydrogen (AH-L), fuel rich ammonia/hydrogen (AH-R), fuel lean methane (M-L) and fuel lean methane/hydrogen (MH-L). First, the forced ignition and stretched flame propagation are evaluated. It is found that the AH-L (AH-R) mixture has a large negative (positive) Markstein length, and thereby among the selected fuel blends, AH-L (AH-R) has the lowest (highest) minimum ignition energy. Then flame stabilization mechanisms are investigated. For a stable flame, AH-L (AH-R) has an intensified (weakened) flame base and a weak (strong) flame tip, which shows a higher flashback (blow-off) propensity. Based on the critical gradient theory, a novel stability regime diagram is proposed. With the regime diagram, the flame stabilization limits (central flashback, boundary layer flashback, stable, and blow-off) and flow conditions can be determined for burners of different sizes.

Thermal runaway effect of the centerbody on boundary layer flashback in a swirl-stabilized gas turbine burner operated with hydrogen

The effect of centerbody heating by the flame on flashback propensity is experimentally investigated. The burner is technically premixed with 100% hydrogen as fuel under atmospheric pressure. The centerbody temperature was monitored by thermocouples revealing a thermal runaway of the centerbody temperature at the onset of flashback. Post-processed OH*-chemiluminescence imaging data of the flame show an increase in OH*-chemiluminescence intensity during thermal runaway indicating either less quenching or auto-ignition events of hydrogen/air mixtures being in contact with the heated centerbody surface. The authors recorded the formation of an auto-ignited flame kernel upstream of the flame front. An analytical model involving an auto-ignition Damköhler number DaAI, the Reynolds number Re and the normalized burner pressure drop Δpbur/Δpburref is introduced predicting whether thermal runaway takes place under certain operating conditions. Experimental data from two different burners are applied to the model’s safety map and good agreement is found.

Combined Impact of the Lewis Number and Thermal Expansion on Laminar Flame Flashback in Tubes

The understanding of the boundary layer flame flashback (BLF) has considerably improved in recent decades, driven by the increasing focus on clean energy and the need to address the operational issues associated with flashback. This study investigates the influence of the Lewis number (Le) on symmetric flame shapes under the critical conditions for a laminar boundary layer flashback in cylindrical tubes. It has been found that the transformation of the flame shape from a mushroom to a tulip happens in a tube of a given radius, as the thermal expansion coefficient and Le are modified. A smaller Lewis number results in a local increase in the burning rate at the flame tip, with the flame being able to propagate closer to the wall, which significantly increases the flashback propensity, in line with previous findings. In cases with a Lewis number smaller than unity, a higher thermal expansion results in a flame propagation happening closer to the wall, thus facing a weaker oncoming flow and, consequently, becoming more prone to flashback. For Le > 1, the effect of the increase in the thermal expansion coefficient on the flashback tendency is much less pronounced.

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

Experimental investigation and modeling of boundary layer flashback for non-swirling premixed prevaporized n-propanol/air and /isopropanol/air flames

The idea of utilizing lean, premixed and prevaporized (LPP) combustion could be a step towards sustainable aviation in the future, with the absolute absence of soot particles and very low nitrogen oxide emissions. However, premixing with compressed hot air presents a challenge for both conventional jet fuel and synthetically produced sustainable aviation fuel (SAF) due to self-ignition and flashbacks. Our group is thus seeking LPP-compatible advanced sustainable aviation fuels, which are evaluated based on their flashback propensity. Preliminary studies indicate that short-chain alcohols, with their higher ignition delay times and lower flame velocities, may be appropriate for LPP combustion concepts. In this study, the isomers of propanol, n-propanol, and isopropanol, are the first representatives of the short-chain alcohols to be investigated experimentally and numerically with respect to their flame stability limits. The experimental determination of stability limits for n- and isopropanol/air premixed flames was conducted on a generic, non-swirled atmospheric burner for an equivalence ratio range of 0.82 to 1.11. A fully automated measurement procedure was used to generate 119 measurement points with Reynolds numbers in the range from 1430 to 2750. The influence of the positional isomerism and of the preheat temperature (422 K, 446 K and 466 K) of the unburned mixture are investigated. A clear influence of the isomeric fuel structure is found. A theoretical analysis shows that it can be related to the different laminar flame speed and laminar flame thickness of the isomers. Moreover, a detailed analysis shows that the critical velocity gradient concept to describe boundary layer flashback is well usable with a Péclet number model.

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.

Flame self-interaction during turbulent boundary layer flashback of hydrogen-rich premixed combustion

This work presents the statistics of flame-self interaction during boundary layer flashback of a hydrogen-rich premixed flame. It is shown that the flame topology is significantly affected by the turbulent boundary layer and the heat loss to the wall. Furthermore, it is shown that the choice of the species used to define the progress variable has a significant influence on the distribution of flame-self interaction topologies.

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.

The Mathematical Model and Research Progress of the Boundary Layer Flashback in Premixed Combustion

Flashback is a key problem influencing the normal operation of power equipment such as gas turbines. As one of the main mechanisms that cause flashback, the boundary layer flashback has an important effect on the design and operation of gas turbine combustors and other combustion devices. Since the critical gradient model for the boundary layer flashback was put forward by Lewis et al. in 1945, the theoretical models for the boundary layer flashback, such as the Peclet number model, the Damköhler number model and the flame angle theory, were developed one after another. However, these theoretical models still need improvements. Until now, the theoretical models for the boundary layer flashback are still in continuous development and modification. The history of the boundary layer flashback was reviewed, and the background, pertinence and shortcomings of the theoretical models were elucidated in the order of the model establishment time. In addition, the development status and research progress of the theoretical models for the boundary layer flashback in recent years were summarized, especially the progress made with new methods such as numerical simulation and statistical analysis. Further, the theoretical research direction and breakthrough points of the combustion boundary layer flashback at present and in the future were put forward.

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.

Evaluation of a Turbulent Jet Flame Flashback Correlation Applied to Annular Flow Configurations With and Without Swirl

Abstract The adaptation of high hydrogen content fuels for low emissions gas turbines represents a potential opportunity to reduce the carbon footprint of these devices. The high flame speed of hydrogen air mixtures combined with the small quenching distances poses a challenge for using these fuels in situations where a significant premixing is desired. In particular, flashback in either the core flow or along the walls (i.e., boundary layer flashback) can be exacerbated with high hydrogen content fuels. In this work, the ability of a flashback correlation previously developed for round jet flames is evaluated for its ability to predict flashback in an annular flow. As a first step, an annular flow is generated with a centerbody located at the centerline of the original round jet flame. Next, various levels of axial swirl is added to that annular flow. Additional flashback data are obtained for various mixtures of hydrogen and methane and hydrogen and carbon monoxide for all the annular flow configurations. Pressures from 3 to 8 bar are tested with mixture temperatures up to 750 K. Flashback is induced by slowly increasing the equivalence ratio. The results obtained show that the same form of the correlation developed for round jet flames can be used to correlate flashback behavior for the annular flow case with and without swirl despite the presence of the centerbody. Adjustments to some of the constants in the original model were made to obtain the best fit, but in general, the correlation is quite similar to that developed for the round jet flame. A significant difference with the annular flow configurations is the determination of the appropriate gradient at the wall, which in the present case is determined using a cold flow computational fluid dynamics simulation.

Investigation of boundary layer flashback for non-swirling premixed hydrogen/ammonia/nitrogen/oxygen/air flames

Hydrogen and ammonia are attracting more attention as a carbon free energy carrier for gas turbines and other combustion systems. Hydrogen/ammonia fuel blends allow the flexible adjustment of the flame properties over a broad range, as the reactivity can be controlled with the hydrogen/ammonia ratio content. In this study, the boundary layer flashback (BLF) is investigated for various H2/NH3/N2/O2/air mixtures in a non-swirling premixed burner at normal pressure (101 kPa) and partly increased temperature (293 K to 557 K). The effect of nitrogen addition is studied by varying H2 to N2 ratios from 1:9 to 1:1 in H2/N2/NH3 mixtures. A H2 to N2 ratio of 3:1 is used to simulate dissociated ammonia. The results show that for an increasing nitrogen content and increasing dissociated ammonia content the flashback propensity decreases and increases, respectively. It was further found that the content of ammonia in the fuel mixture reduces much stronger the flashback propensity than the nitrogen content. The impact of oxygen enrichment and increased unburned mixture temperature are also studied for two selected H2/NH3/air mixtures. By increasing the oxygen content from 21 vol% to 31 vol% or the unburned mixture temperature from 293 K to 556 K, the mean flow velocity at flashback onset is found to double approximately. Furthermore, the boundary layer flashback of potential methane substitute mixtures are investigated as these mixture might allow to substitute methane in gas turbines. The substitute mixtures with a methane like laminar burning velocity show a higher flashback propensity compared to methane due to flame instabilities. By increasing the ammonia content and therefore reducing the laminar burning velocity, new substitute mixtures with a methane like flashback propensity were found. In order to describe the flashback propensity in terms of laminar burning velocity and other relevant parameters, a previously proposed model for the critical velocity gradient is extended for the new experimental data. The extended model is able to predict the measured data with good accuracy.

Computational Modeling of Boundary Layer Flashback in a Swirling Stratified Flame Using a LES-Based Non-Adiabatic Tabulated Chemistry Approach.

When operating under lean fuel–air conditions, flame flashback is an operational safety issue in stationary gas turbines. In particular, with the increased use of hydrogen, the propagation of the flame through the boundary layers into the mixing section becomes feasible. Typically, these mixing regions are not designed to hold a high-temperature flame and can lead to catastrophic failure of the gas turbine. Flame flashback along the boundary layers is a competition between chemical reactions in a turbulent flow, where fuel and air are incompletely mixed, and heat loss to the wall that promotes flame quenching. The focus of this work is to develop a comprehensive simulation approach to model boundary layer flashback, accounting for fuel–air stratification and wall heat loss. A large eddy simulation (LES) based framework is used, along with a tabulation-based combustion model. Different approaches to tabulation and the effect of wall heat loss are studied. An experimental flashback configuration is used to understand the predictive accuracy of the models. It is shown that diffusion-flame-based tabulation methods are better suited due to the flashback occurring in relatively low-strain and lean fuel–air mixtures. Further, the flashback is promoted by the formation of features such as flame tongues, which induce negative velocity separated boundary layer flow that promotes upstream flame motion. The wall heat loss alters the strength of these separated flows, which in turn affects the flashback propensity. Comparisons with experimental data for both non-reacting cases that quantify fuel–air mixing and reacting flashback cases are used to demonstrate predictive accuracy.

Boundary Layer Flashback Limits of Hydrogen-Methane-Air Flames in a Generic Swirl Burner at Gas Turbine-Relevant Conditions

AbstractOperating stationary gas turbines on hydrogen-rich fuels offers a pathway to significantly reduce greenhouse gas emissions in the power generation sector. A key challenge in the design of lean-premixed burners, which are flexible in terms of the amount of hydrogen in the fuel across a wide range and still adhere to the required emission levels, is to prevent flame flashback. However, systematic investigations on flashback at gas turbine relevant conditions to support combustor development are sparse. The current work addresses the need for an improved understanding with an experimental study on boundary layer flashback in a generic swirl burner up to 7.5 bar and 300 °C preheat temperature. Methane-hydrogen-air flames with 50 to 85% hydrogen by volume were investigated. High-speed imaging was applied to reveal the flame propagation pathway during flashback events. Flashback limits are reported in terms of the equivalence ratio for a given pressure, preheat temperature, bulk flow velocity, and hydrogen content. The wall temperature of the center body along which the flame propagated during flashback events has been controlled by an oil heating/cooling system. This way, the effect any of the control parameters, e.g., pressure, had on the flashback limit was decoupled from the otherwise inherently associated change in heat load on the wall and thus change in wall temperature. The results show that the preheat temperature has a weaker effect on the flashback propensity than expected. Increasing the pressure from atmospheric conditions to 2.5 bar strongly increases the flashback risk, but hardly affects the flashback limit beyond 2.5 bar.

Boundary Layer Flashback Model for Hydrogen Flames in Confined Geometries Including the Effect of Adverse Pressure Gradient

Abstract The combustion properties of hydrogen make premixed hydrogen-air flames very prone to boundary layer flashback. This paper describes the improvement and extension of a boundary layer flashback model from Hoferichter et al. (2017, “Prediction of Confined Flame Flashback Limits Using Boundary Layer Separation Theory,” ASME J. Eng. Gas Turbines Power, 139(2), p. 021505) for flames confined in burner ducts. The original model did not perform well at higher preheat temperatures and overpredicted the backpressure of the flame at flashback by 4–5×. By simplifying the Lewis number-dependent flame speed computation and by applying a generalized version of Stratford's flow separation criterion (Stratford, 1959, “The Prediction of Separation of the Turbulent Boundary Layer,” J. Fluid Mech., 5(1), p. 1), the prediction accuracy is improved significantly. The effect of adverse pressure gradient flow on the flashback limits in 2 deg and 4 deg diffusers is also captured adequately by coupling the model to flow simulations and taking into account the increased flow separation tendency in diffuser flow. Future research will focus on further experimental validation and direct numerical simulations to gain better insight into the role of the quenching distance and turbulence statistics.

Neural Network Prediction of Boundary Layer Flashback

Abstract Flashback is a major concern for engine operation and safety, particularly with progress toward renewably producible and cleaner-burning fuels, such as hydrogen fuel blends. This work extends prior progress in developing models for predicting the onset of boundary layer flashback. While prior attempts have developed models based on analytical theory or through phenomenological considerations, problem complexity has inhibited flashback understanding and, hence, model performance. The goal of this work is to address current model performance limitations by leveraging the representational flexibility offered by neural networks (NNs) in predicting boundary layer flashback. This is demonstrated through two applications. The first demonstrates the utility of training an NN on only a subproblem, thereby preserving model intuition. The second presents a predictive boundary layer flashback model using only a NN. Focus is placed on developing NN models which are practical; the input and output variables are easily measurable and controllable prior to experimentation.

Numerical study of the influence of wall roughness on laminar boundary layer flashback

Boundary layer flame flashback is a phenomenon that may constitute a key challenge for efficient combustion of novel fuels at gas turbine conditions. In the present work, the effect of wall roughness on the laminar boundary layer flashback is studied systematically using numerical simulation. The results indicate that the wall roughness can attenuate flashback speed due to enhanced heat loss in case of low thermal resistance of the wall. The critical velocity gradient of the oncoming flow is shown to decrease with wall roughness level and increase with gas thermal expansion ratio.

Investigation of turbulent premixed methane/air and hydrogen-enriched methane/air flames in a laboratory-scale gas turbine model combustor

Methane and hydrogen-enriched (25 vol% and 50 vol% H2-enriched CH4) methane/air premixed flames were investigated in a gas turbine model combustor under atmospheric conditions. The flame operability ranges were mapped at different Reynold numbers (Re), showing the dependence on Re and H2 concentrations. The effects of equivalence ratio (Φ), Re, and H2 enrichment on flame structure were examined employing OH-PLIF measurement. For CH4/air cases, the flame was stabilized with an M shape; while for H2-enriched cases, the flame transitions to a П shape above a specific Φ. This transition was observed to influence significantly the flashback limits. The flame shape transition is most likely a result of H2 enrichment, occurring due to the increase in flame speed, higher resistance of the flame to the strain rate, and change in the inner recirculation zone. Flow fields of CH4/air flames were compared between low and high Re cases employing high-speed PIV. The flashback events, led by two mechanisms (combustion-induced vortex breakdown, CIVB, and boundary-layer flashback, BLF), were observed and recorded using high-speed OH chemiluminescence imaging. It was found that the CIVB flashback occurred only for CH4 flames with M shape, whereas the BLF occurs for all H2-enriched flames with П shape.

Influence of gas expansion on the propagation of a premixed flame in a spatially periodic shear flow

It has been previously demonstrated that thermal gas expansion might have a role in boundary layer flashback of premixed turbulent flames [Gruber et al., J Fluid Mech 2012], inducing local flow-reversal in the boundary layer's low-velocity streaks on the reactants’ side of the flame and facilitating its upstream propagation. We perform a two-dimensional numerical investigation of the interaction between a periodic shear flow and a laminar premixed flame. The periodic shear is a simplified model for the oncoming prolonged streamwise velocity streaks with alternating regions of high and low velocities found in turbulent boundary layers in the vicinity of the walls. The parametric study focuses on the amplitude and wavelength of the periodic shear flow and on the gas expansion ratio (unburnt-to-burnt density ratio). With the increase of the amplitudes of the periodic shear flow and of the gas expansion, the curved flame velocity increases monotonically. The flame velocity dependence on the periodic shear wavelength is non-monotonic, which is consistent with previous theoretical studies of curved premixed flame velocity. The flame shape that is initially formed by the oncoming periodic shear appears to be metastable. At a later stage of the flame propagation, the flame shape transforms into the stationary one dominated by the Darrieus-Landau instability.