A Semi-Analytical Model for Prediction of Wall Quenching Distances of Premixed Flames

In order to predict the variation of the wall quenching distance of a premixed flame under different equivalence ratios and incoming flow velocities, a semi-analytical model applied to both lean single-layer flame and rich double-layer flame has been derived based on the conservation of energy in the quenching zone. In this model the flame surface radiation plays an important role. Factors influencing the radiation have been analyzed, respectively. The model indicates that the factors affecting the quenching distance in premixed flame are more complicated than that in single-wall flames or flames in tube. To fit the empirical coefficient in this model, a methane-air premixed flame quenching distance experiment under both lean and rich conditions has been performed. The comparison between the theoretical prediction and the experiment result shows that this semi-analytical model gives a suitable description of the quenching distance. The relative error of the quenching distance under different equivalence ratios and incoming flow velocities is less than ±15%.

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

The combustion properties of hydrogen make premixed hydrogen-air ?ames very prone to boundary layer ?ashback. This paper describes the improvement and extension of a bound- ary layer ?ashback model from Hoferichter [1] for ?ames con- ?ned in burner ducts. The original model did not perform well at higher preheat temperatures and overpredicted the backpres- sure of the ?ame at ?ashback by 4-5x. By simplifying the Lewis number dependent ?ame speed computation and by applying a generalized version of Stratford's ?ow separation criterion [2], the prediction accuracy is improved signi?cantly. The effect of adverse pressure gradient ?ow on the ?ashback limits in 2? and 4? diffusers is also captured adequately by coupling the model to ?ow simulations and taking into account the increased ?ow sep- aration tendency in diffuser ?ow. 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.

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

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 [1] 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 4–5x. By simplifying the Lewis number dependent flame speed computation and by applying a generalized version of Stratford’s flow separation criterion [2], the prediction accuracy is improved significantly. The effect of adverse pressure gradient flow on the flashback limits in 2° and 4° 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.

Investigating Boundary Layer Flashback of a High Turbulence Intensity Jet Flame at Gas Turbine Conditions

Hydrogen derived from non-fossil sources is an attractive candidate to replace carbon based fuels in gas turbines, as it is inherently carbon free. Yet the unusual combustion properties of hydrogen requires some care to successfully use it in gas turbines. To attain the lowest NOx emissions, uniformly low reaction temperatures must be assured thus the reactants must be well mixed. This is accomplished in low emission gas turbines by mixing the reactants within a pre-mixer section prior to entry into the combustor. With the addition of hydrogen into the fuel, certain issues arise such as higher flame speeds compared to carbon based fuels. Flashback is a phenomena that occurs when the flame no longer propagates beyond the exit of the premixer/injector but instead retracts and propagates upstream towards, and ultimately into the pre-mixer, causing significant damage due to such high temperatures. Flashback occurs when the flame speed exceeds either the local or bulk flow velocity. In practice, the question arises regarding the impact of turbulence levels. While an increase in turbulence intensity may help improve mixing, it also known to increase turbulent burning velocity. In the present work, the influence of bulk turbulence intensity of the flow on boundary layer flashback is investigated. Data are acquired for a different turbulence intensities at pressures from 3 to 8 bar with preheated reactants up to 750 deg. K. Various mixtures of hydrogen and methane are evaluated. The results show that even with significantly different bulk flow turbulence intensities (based on the ratio of flow centerline turbulence to centerline axial velocity) boundary layer flashback is not strongly affected. This is attributed to the role of the quenching distance in connection with damping within the boundary layer. It is noted that core flow flashback or other flashback mechanisms may be affected differently.