Boundary Layer Flashback Research Articles

Experimental investigation and modeling of boundary layer flashback for non-swirling premixed hydrogen/ammonia/air flames

Carbon free fuels such as hydrogen/ammonia blends show a promising potential to become sustainable and renewable fuels for gas turbines and other combustion systems. One interesting aspect about these blends is the possibility to adjust different combustion properties like the laminar burning velocity or ignition delay time by changing the ratio between H2 and NH3. Such fuel blends can be produced via partial catalytic decomposition of NH3. However, such mixtures can lead to flame instabilities such as flashback, especially if the hydrogen content is high. In the present study, the boundary layer flashback of premixed hydrogen/ammonia/air mixtures is investigated experimentally for non-swirling flows at normal temperature (293 K) and normal pressure (101 kPa). A new experimental setup for boundary layer flashback investigation with a fully automated measurement procedure is introduced. With preliminary studies, the influence of various measurement procedures on the flashback limits is firstly investigated. For a broad flashback study, the data of 351 flashback experiments are collected. The ammonia content in H2/NH3 fuel mixtures is varied from 0 vol% to 50 vol% in 10 vol% steps. The fuel–air equivalence ratio is ranging from 0.38 to 1.17. As the ammonia content is increasing, the mean flow velocities at flashback are exponentially decreasing. Additionally, theoretical modeling is performed. A model is derived based on the concept of the critical velocity gradient which is able to predict the measured data with high accuracy. For two exemplary cases with H2/air and 80% H2/20% NH3/air mixtures, the process of boundary layer flashback is investigated in detail with low and high speed direct imaging and image post-processing. During the flashback onset of H2/NH3/air flames a separate reaction of H2 followed by the reaction of NH3 can be observed. Also, a flame-oscillation between fused silica tube and burner head with approximately 10 Hz was observed. Furthermore, indications for an adverse pressure gradient based on the flame propagation speed is seen. Details about the flame structure during the flashback process of H2/NH3/air flames are shown. During the upstream flame propagation of H2/NH3/air flames, high frequency oscillations with about 830 Hz of the leading flame tip are observed, which are assumed to be related to thermoacoustic instabilities.

Flashback and combustion stability in swirl burners: review paper

Flame flashback phenomena and combustion stability in a swirl burners represent the major of instability problems that lead to causing a considerable damage to combustion systems as well as the pollution troubles. Recently, researchers suggested many techniques in order to enhancing flame flashback resistance especially against a combustion induced vortex breakdown mechanism in addition to boundary layer flashback. Therefore, the present paper represent a general review about flashback and combustion stability in swirl burners to be as a scientific reference for researchers in this field.

Swirl flame boundary layer flashback at elevated pressure: Modes of propagation and effect of hydrogen addition

Operating lean-premixed gas turbine burners with fuels containing large amounts of hydrogen severely increases the risk of flame flashback. Enabling fuel-flexible burners, i.e. mitigating that risk for a wide range of hydrogen admixed to natural gas, for instance, is particularly challenging. An improved understanding of the responsible mechanisms and their impact on flashback limits is required to aid the development of such fuel-flexible and yet flashback-resistant burners. Only few detailed and systematic investigations on flame flashback have been conducted at elevated pressure and preheat temperature. In the current experimental study, boundary layer flashback of methane-hydrogen air flames has been investigated in an optically accessible swirl burner installed in a high-pressure test rig. High-speed imaging and laser diagnostics have been applied to characterize the flame propagation pathway and flame shapes. The current findings show that the flame propagation pathway, which determines the relevant flow-flame interaction for flashback, differs compared to previous swirl flame studies. The characteristics of flashback in the current configuration are found to be comparable to flashback in non-swirling boundary layers. Flashback limits have been measured at 2.5 bar and 200∘C preheat temperature to quantify the increase in flashback propensity as the amount of hydrogen is increased from 50% to 100% by volume. Counterflow premixed flame simulations have been performed to investigate the increase in flashback propensity with hydrogen addition. The results suggest that the investigated flames are close to being critically stretched when flashback occurs in the turbulent boundary layer, which allows deriving a Karlovitz number based criterion for an improved prediction of flashback limits.

Statistics and Dynamics of Intermittent Boundary Layer Flashback in Swirl Flames

Flame behavior at conditions approaching boundary layer flashback was studied in fuel-lean premixed swirl flames with a central bluff-body flame holder. High-speed chemiluminescence images were collected at over 450 different combinations of fuel composition (hydrogen/methane blends), equivalence ratio, flow rate, and reactant temperature. A selected group of conditions was further examined using simultaneous high-speed stereoscopic particle image velocimetry and OH planar laser induced fluorescence. Over a range of conditions between stable burning in the combustor and total flashback, the flame would intermittently propagate through the bluff-body boundary layer into the reactant feed tube for a period of time before retreating back to the combustion chamber. Statistical characteristics of the flame dynamics, such as the depth and duration of the flashback events, showed consistent relationships across all combinations of operating parameters. Hence, tracking these statistics provides a potential means of anticipating an upcoming flashback event. The number of transient flashback events per second showed particular promise as an early warning sign due its rather gradual change with flame propagation depth. Laser diagnostics revealed local reductions in axial velocity ahead of the tip of the flame protrusions. The strength of these reductions increased as the flame moved farther upstream; however, no total flow reversal was observed.

DNS analysis of boundary layer flashback in turbulent flow with wall-normal pressure gradient

The presence of swirl in combustion systems produces a marked change in their boundary layer flashback behaviour. Two aspects of swirling flow are investigated in this study: the effect of the swirl-generated wall-normal pressure gradient, and the effect of misalignment between the mean flow direction and the direction of flame propagation. The analysis employs Direct Numerical Simulation (DNS) of fuel-lean premixed hydrogen-air flames in turbulent planar channel flow with friction Reynolds number of 180. The effect of swirl on the flashback process is investigated by imposing a wall-normal pressure gradient profile. Analysis of the DNS data shows how the resulting differences in flow field and flame topology contribute to the differences in the overall flashback speed. Misalignment of the flow and propagation directions leads to asymmetry in the flame shape statistics as streaks of high velocity fluid in the boundary layer cleave into the flame front at an angle, yielding an increase in flame surface density away from the wall. Swirl has a stabilising effect on the turbulent flame front during flashback along the centre-body of a swirling annular flow due to the density stratification across the flame front, and produces a reduction in turbulent consumption speed. However the swirl also sets up a hydrostatic pressure difference that drives the flame forward, and the net effect is that the flashback speed is increased. The dominance of hydrostatic effects motivates development of relatively simple modelling for the effect of swirl on flashback speed. A model accounting for the inviscid momentum balance and for confinement effects is presented which adequately describes the effect of swirl on flashback speed observed in previous experimental studies.

Surface density function evolution and the influence of strain rates during turbulent boundary layer flashback of hydrogen-rich premixed combustion

The statistical behavior of the magnitude of the reaction progress variable gradient [alternatively known as the surface density function (SDF)] and the strain rates, which govern the evolution of the SDF, have been analyzed for boundary layer flashback of a premixed hydrogen-air flame with an equivalence ratio of 1.5 in a fully developed turbulent channel flow. The non-reacting part of the channel flow is representative of the friction velocity based Reynolds number Reτ = 120. A skeletal chemical mechanism with nine chemical species and twenty reactions is employed to represent hydrogen-air combustion. Three definitions of the reaction progress variable (RPV) based on the mass fractions of H2, O2, and H2O have been considered to analyze the SDF statistics. It is found that the mean variations of the SDF and the displacement speed Sd depend on the choice of the RPV and the distance away from the wall. The preferential alignment of the RPV gradient with the most extensive principal strain rate strengthens with an increase in distance from the cold wall, which leads to changes in the behaviors of normal and tangential strain rates from the vicinity of the wall toward the middle of the channel. The differences in displacement speed statistics for different choices of the RPV and the wall distance affect the behaviors of the normal strain rate due to flame propagation and curvature stretch. The relative thickening/thinning of the reaction layers of the major species has been explained in terms of the statistics of the effective normal strain rate experienced by the progress variable isosurfaces for different wall distances and choices of RPVs.

Statistical behavior of turbulent kinetic energy transport in boundary layer flashback of hydrogen-rich premixed combustion

Statistical behavior of turbulent kinetic energy (TKE) transport in boundary layer flashback of hydrogen-rich premixed combustion is analyzed for the first time. Flame alters the structure of the turbulent boundary layer, which is evident from the changes in the budget of the TKE transport equation.

Numerical Investigation of Pressure Influence on the Confined Turbulent Boundary Layer Flashback Process

Boundary layer flashback from the combustion chamber into the premixing section is a threat associated with the premixed combustion of hydrogen-containing fuels in gas turbines. In this study, the effect of pressure on the confined flashback behaviour of hydrogen-air flames was investigated numerically. This was done by means of large eddy simulations with finite rate chemistry as well as detailed chemical kinetics and diffusion models at pressures between 0 . 5 and 3 . It was found that the flashback propensity increases with increasing pressure. The separation zone size and the turbulent flame speed at flashback conditions decrease with increasing pressure, which decreases flashback propensity. At the same time the quenching distance decreases with increasing pressure, which increases flashback propensity. It is not possible to predict the occurrence of boundary layer flashback based on the turbulent flame speed or the ratio of separation zone size to quenching distance alone. Instead the interaction of all effects has to be accounted for when modelling boundary layer flashback. It was further found that the pressure rise ahead of the flame cannot be approximated by one-dimensional analyses and that the assumptions of the boundary layer theory are not satisfied during confined boundary layer flashback.

Effects of different nozzle configurations on swirl flow topology in tangential swirl burners

Flame flashback is one of the central combustion instabilities, especially when it appears in the form of boundary layer flashback (BLF) or combustion induced vortex breakdown (CIVB) flashback, some of the most common instabilities in swirl combustors. This paper focuses on mitigating the phenomenon of CIVB and BLF flashback mechanisms using different nozzle configurations while using central air injection. Studies were conducted on a 150-kW tangential swirl burner manufactured and previously characterised at Cardiff University. The effects of different nozzle heights (hn) with and without microstructure on the swirl flow characteristics were investigated experimentally by utilising an LDA system. Different strip heights (hm) of a wire woven mesh have been employed as a liner on the smooth nozzle to change its surface roughness. It was found that longer smooth nozzles (hn/Ro=2.3) led to promotion of stability in the swirl burner by minimising the axial velocity defect while decreasing turbulence downstream the dump plane. Moreover, the average measurements show that the burner nozzles with microstructured surfaces enable improvements in controlling the BLF flashback and hence reduces outflow drag. It was found that the microstructured mesh alters the flow structure near the wall by increasing the velocity adjacent to this region delivering further resistance to BLF. On the other hand, using both central air injection and the nozzle with and without the microstructured surface can affect the operability of the gas turbine combustors.

Enhancing flame flashback resistance against Combustion Induced Vortex Breakdown and Boundary Layer Flashback in swirl burners

Swirl combustors have proven to be effective flame stabilisers over a wide range of operation conditions thanks to the formation of well-known swirl coherent structures. However, their employment for lean premixed combustion modes while introducing alternative fuels such as high hydrogenated blends results in many combustion instabilities. Under these conditions, flame flashback is considered one of the major instability problems that have the potential of causing considerable damage to combustion systems hardware in addition to the significant increase in pollutant levels. Combustion Induced Vortex Breakdown is considered a very particular mode of flashback instability in swirling flows as this type of flashback occurs even when the fresh mixture velocity is higher than the flame speed, a consequence of the interaction between swirl structures and swirl burner geometries. Improvements in burner geometries and manipulation of swirling flows can increase resistance against this type of flashback. However, increasing resistance against Combustion Induced Vortex Breakdown can lead to augmentation in the propensity of another flashback mechanism, Boundary Layer Flashback. Thus, this paper presents an experimental approach of a combination of techniques that increase Combustion Induced Vortex Breakdown resistance, i.e. by repositioning a central injector and using central air injection, while simultaneously avoiding Boundary Layer Flashback, i.e. by changing the wall boundary layer characteristics using microsurfaces on the nozzle wall. Results show that using these techniques together has promising potentials regarding wider stable operation for swirl combustors, enabling them to burn a broader variety of fuel blends safely, while informing developers of the improvements obtained with the combined techniques.

Large Eddy simulation of confined turbulent boundary layer flashback of premixed hydrogen-air flames

Flame flashback from the combustion chamber into the upstream burner mixing section is an inherent threat associated with premixed combustion. Especially for hydrogen-containing fuels, boundary layer flashback (BLF) can occur and damage the combustor. It is thus important to develop methods for the accurate prediction of the onset of BLF. In this study, the ability of large eddy simulations (LES) to reproduce experimentally determined flashback limits of flames confined in a channel is investigated. First, inert LES of turbulent channel flow are conducted. Subgrid turbulence is modelled via the Smagorinsky turbulence model. The unsteady velocity profiles obtained from the inert simulations are then used as inlet boundary conditions for the reactive LES. The combustion process is modelled via finite rate chemistry and detailed chemical kinetics. By directly calculating the species reaction rates from filtered species and temperature distributions, turbulence-chemistry interaction of the subgrid scales is neglected. Differential diffusion and Soret diffusion are included in the species transport equations. It is shown that LES with the Smagorinsky turbulence model is capable of reproducing the turbulence characteristics of the channel flow. With the correct unsteady inlet velocity profile the chosen combustion model is capable of accurately predicting the onset of confined BLF. It can thus be concluded that the chosen combustion model reproduces all physical effects relevant to BLF. Furthermore, it is shown that BLF is initiated when the size of local flow separation regions clearly exceeds the quenching distance of the flame.

Coupling between premixed flame propagation and swirl flow during boundary layer flashback

Flashback of premixed methane–air flames in the turbulent boundary layer of swirling flows is investigated experimentally. The premix section of the atmospheric model swirl combustor features an axial swirler with an attached center-body. Our previous work with this same configuration investigated the flame propagation during flashback using particle image velocimetry (PIV) with liquid droplets as seed particles that precluded making measurements in the burnt gases. The present study investigates the transient velocity field in the unburnt and burnt gas region by means of solid-particle seeding and high-speed stereoscopic PIV. The global axial and circumferential lab-frame flame propagation speed is obtained simultaneously based on high-speed chemiluminescence movies. By combining the PIV data with the global flame propagation speed, the quasi-instantaneous swirling motion of the velocity field is constructed on annular shells, which provides a more intuitive view on the complex three-dimensional flow–flame interaction. Previous works showed that flashback is led by flame tongues. We find that the important flow–flame interaction occurs on the far side of these flame tongues relative to the approach flow, which we henceforth refer to as the leading side. The leading side is found to propagate as a classical premixed flame front relative to the strongly modified approach flow field. The blockage imposed by flame tongues is not limited to the immediate vicinity of the flame base, but occurs along the entire leading side.

Comparison of Two Methods to Predict Boundary Layer Flashback Limits of Turbulent Hydrogen-Air Jet Flames

Flame flashback into the premixer is a serious issue in gas turbine combustion, especially for high hydrogen content fuels. Of particular interest is the risk of upstream flame propagation inside the wall boundary layer. Consequently, methods to predict the minimum flow velocities to prevent boundary layer flashback are sought by designers. In the first part of this paper two methods to predict boundary layer flashback limits are summarized and compared. The first method is a Damkohler correlation based on non-dimensional parameters developed at University of California Irvine (UCI). The correlation was developed based on the gathered experimental data at elevated pressures and temperatures (i.e. p = 3–7 bar, T u = 300–500 K, ϕ = 0.3–0.6) and successfully applied to a commercial gas turbine combustor. Due to its simplicity the Damkohler correlation is attractive for the design of gas turbine burners. But its applicability is limited to the turbulent combustion regime for which it was originally designed. The second method is called the “flame angle theory”. It was developed at Technische Universitat Munchen (TUM) and is based on a description of the physical process of boundary layer flashback. This method has been validated with experimental data at atmospheric pressure and a wide range of preheating temperatures and equivalence ratios (T u = 293–673 K, ϕ = 0.35–1.0). Since it describes the physical process of boundary layer flashback based on a set of sub-models it should be generally applicable to all operating conditions if the sub-models are appropriate. To verify this, the flame angle theory is applied to high pressure conditions in the second part of this paper. A comparison with results from the Damkohler correlation shows that the predicted flashback limits are in a reasonable range. However, the degree of agreement between Damkohler correlation and flame angle theory strongly depends on equivalence ratio because the Damkohler correlation does not account for the changing susceptibility of different hydrogen-air mixtures to flame stretch. For that reason, a modified Damkohler correlation has been derived at TUM from the flame angle theory and is presented in the third part of this paper. This correlation combines the advantages of the other two methods as it features high usability and is generally applicable to all operating conditions.

Experimental study to enhance swirl burner against boundary layer flashback

In gas turbine systems, operation stability represents the major challenge to any successful device deployment. Climate change combined with fossil fuel pollution has led to the need of considering high hydrogen content fuels, thus putting more pressure to stabilise gas turbines at operation conditions. Flashback is one of the main operation stability problems that represent a real challenge for gas turbine designers when using fast reacting fuels with high hydrogen content. One mechanism that has shown to contribute to flashback considerably is the propagation of the flame through its boundary layer. Although the latter has been studied, there are still several unknowns in its evolution through the system. Thus, boundary layer flashback of a swirling turbulent flame was investigated in a 150 kW tangential swirl burner previously characterised. To produce controlled changes to the boundary layer, the internal side of the burner was covered by woven wire steel mesh to mimic biological skin techniques in flow drag improvement. Two different wire meshes were used to study the effect of the regular roughness size on the boundary flashback. Moreover, the effects of using the wire mesh in such swirling flow with and without central air injection for reduction of other flashback phenomena were studied. The results show good enhancement of the system to boundary layer flashback, and a new map of the combustion stability of the rig has been produced.

Enhancement flame flashback resistance against CIVB and BLF in swirl burners

Swirl combustors have proven as effective flame stabilisers over a wide range of operation conditions thanks to the formation of well-known swirl coherent structures. However, employment of swirl combustors to work on lean premixed combustion modes while introducing alternative fuels such as high hydrogen blends result in many combustion instabilities. Under these conditions, flame flashback has been considered as one of the major instability problems that have the potential of causing considerable damages of the combustion systems hardware in addition to the significant increase in pollutant levels. Combustion Induced Vortex Breakdown (CIVB) is considered a very particular mode of flashback mechanism in swirling flows as this type of flashback occurs even when the fresh mixture’s velocity is higher than the flame speed, consequence of the interaction between swirl structures and swirl burner geometries. Improvements of burner geometries and manipulation of swirl flows can produce good resistance against this type of flashback. However, increase flame flashback resistance against CIVB can lead to an increase in the propensity of another flashback mechanism, Boundary Layer Flashback (BLF). Thus this paper presents an experimental and numerical approach that allows the increase in CIVB resistance by using diffusive air injection and simultaneously avoid BLF by changing the wall boundary layer characteristics using microsurface grids as a liner for the nozzle wall. Results show that using those two techniques together has promising potentials regarding wider stable operation for swirl combustors, enabling them to burn a great variety of fuel blends safely.

Boundary Layer Flashback in Premixed Hydrogen–Air Flames With Acoustic Excitation

Lean premixed combustion is prevailing in gas turbines to minimize nitrogen oxide emissions. However, this technology bears the risk of flame flashback and thermoacoustic instabilities. Thermoacoustic instabilities induce velocity oscillations at the burner exit which, in turn, can trigger flame flashback. This article presents an experimental study at ambient conditions on the effect of longitudinal acoustic excitation on flashback in the boundary layer of a channel burner. The acoustic excitation simulates the effect of thermoacoustic instabilities. Flashback limits are determined for different excitation frequencies characterizing intermediate frequency dynamics in typical gas turbine combustors (100–350 Hz). The excitation amplitude is varied from 0% to 36% of the burner bulk flow velocity. For increasing excitation amplitude, the risk of flame flashback increases. This effect is strongest at low frequencies. For increasing excitation frequency, the influence of the velocity oscillations decreases as the flame has less time to follow the changes in bulk flow velocity. Two different flashback regimes can be distinguished based on excitation amplitude. For low excitation amplitudes, flashback conditions are reached if the minimum flow velocity in the excitation cycle falls below the flashback limit of unexcited unconfined flames. For higher excitation amplitudes, where the flame starts to periodically enter the burner duct, flashback is initiated if the maximum flow velocity in the excitation cycle is lower than the flashback limit of confined flames. Consequently, flashback limits of confined flames should also be considered in the design of gas turbine burners as a worst case scenario.

Preliminary study on lean premixed combustion of ammonia-hydrogen for swirling gas turbine combustors

Hydrogen has been considered one of the most promising materials for energy storage during the last decade with considerable research having been undertaken to demonstrate the use of the molecule in power production systems. However, hydrogen presents drawbacks in terms of global commercialisation and deployment since its distribution is only feasible with significant dedicated infrastructure investment including liquefaction or if it is combined with other gases such as methane. The latter will still produce carbon emissions, whilst the former is not economically viable with current technologies. Therefore, an alternative is to use ammonia as a hydrogen storage vector. Ammonia, a molecule that has been used for more than a century, is a well-known material distributed across the world. Moreover, its properties allow its liquefaction at a relatively low pressure under atmospheric temperature compared to hydrogen, serving as a compound that can be used from fertilising to industrial processes. For power generation, ammonia has demonstrated to have a very slow reaction hence flame speeds, thus one option is to dope the fuel with a more reactive molecule such as hydrogen, which conveniently can be obtained from cracking ammonia. Hence, this paper presents the results of a numerical and experimental campaign where a 50:50 (vol%) ammonia-hydrogen blend was used for lean premixed combustion in a generic swirl combustor used in gas turbine studies. The results show that whilst the mixture can produce a good flame velocity similar to methane with the mixture having near equivalent laminar flame speed characteristics, the high diffusivity of hydrogen under these conditions leads to a narrow operational envelope with the potential for boundary layer flashback. High NOx emissions are produced due to the excess production of OH and O radicals. Recommendations for further studies and future developments are also discussed.

Application of a Turbulent Jet Flame Flashback Propensity Model to a Commercial Gas Turbine Combustor

Because flashback is a key operability issue associated with low emission combustion of high hydrogen content fuels, design tools to predict flashback propensity are of interest. Such a design tool has been developed by the authors to predict boundary layer flashback using nondimensional parameters. The tool accounts for the thermal coupling between the flame and burner rim and was derived using detailed studies carried out in a test rig at elevated temperature and pressure. The present work evaluates the applicability of the model to a commercial 65 kW microturbine generator (MTG). Two sets of data are evaluated. One set is obtained using the combustor, removed from the engine, which has been configured to operate like it does in the engine but at atmospheric pressure and various preheat temperatures. The second set of data is from a combustor operated as it normally would in the commercial engine. In both configurations, studies are carried out with various amounts of hydrogen added to either natural gas or carbon monoxide. The previously developed model is able to capture the measured flashback tendencies in both configurations. In addition, the model is used to interpret flashback phenomena at high pressures and temperatures in the context of the engine conditions. An increase in pressure for a given preheat temperature and velocity reduces the equivalence ratio at which flashback occurs and increases the tip temperature due to lower quenching distance. The dependency of the flashback propensity on the injector tip temperature is enhanced with an increase in pressure. The variation of critical velocity gradient with equivalence ratio for a constant preheat temperature is more pronounced at higher pressures. In summary, the model developed using the high-pressure test rig is able to predict flashback tendencies for a commercial gas turbine engine and can thus serve as an effective design tool for identifying when flashback is likely to occur for a given geometry and condition.

Analytic prediction of unconfined boundary layer flashback limits in premixed hydrogen–air flames

Flame flashback is a major challenge in premixed combustion. Hence, the prediction of the minimum flow velocity to prevent boundary layer flashback is of high technical interest. This paper presents an analytic approach to predicting boundary layer flashback limits for channel and tube burners. The model reflects the experimentally observed flashback mechanism and consists of a local and global analysis. Based on the local analysis, the flow velocity at flashback initiation is obtained depending on flame angle and local turbulent burning velocity. The local turbulent burning velocity is calculated in accordance with a predictive model for boundary layer flashback limits of duct-confined flames presented by the authors in an earlier publication. This ensures consistency of both models. The flame angle of the stable flame near flashback conditions can be obtained by various methods. In this study, an approach based on global mass conservation is applied and is validated using Mie-scattering images from a channel burner test rig at ambient conditions. The predicted flashback limits are compared to experimental results and to literature data from preheated tube burner experiments. Finally, a method for including the effect of burner exit temperature is demonstrated and used to explain the discrepancies in flashback limits obtained from different burner configurations reported in the literature.

Prediction of Confined Flame Flashback Limits Using Boundary Layer Separation Theory

Premixed combustion is a common technology applied in modern gas turbine combustors to minimize nitrogen oxide emissions. However, early mixing of fuel and oxidizer opens up the possibility of flame flashback into the premixing section upstream of the combustion chamber. Especially, for highly reactive fuels, boundary layer flashback (BLF) is a serious challenge. For high preheating and burner surface temperatures, boundary layer flashback limits for burner stabilized flames converge to those of so-called confined flames, where the flame is stabilized inside the burner duct. Hence, the prediction of confined flashback limits is a highly technically relevant task. In this study, a predictive model for flashback limits of confined flames is developed for premixed hydrogen–air mixtures. As shown in earlier studies, confined flashback is initiated by boundary layer separation upstream of the flame tip. Hence, the flashback limit can be predicted identifying the minimum pressure rise upstream of a confined flame causing boundary layer separation. For this purpose, the criterion of Stratford is chosen which was originally developed for boundary layer separation in mere aerodynamic phenomena. It is shown in this paper that it can also be applied to near-wall combustion processes if the pressure rise upstream of the flame tip is modeled correctly. In order to determine the pressure rise, an expression for the turbulent burning velocity is derived including the effects of flame stretch and turbulence. A comparison of the predicted flashback limits and experimental data shows high prediction accuracy and wide applicability of the developed model.