Flame-wall Interaction Research Articles

Direct measurement of laminar flame quenching distance in a closed vessel

The study of flame–wall interactions is of crucial importance for the understanding of near wall combustion. One of the parameters directly responsible for heat exchange near the wall is the quenching distance. The results of direct measurements of the quenching distance during side-wall quenching and head-on quenching of premixed methane/air flame are presented. A laminar flame is produced at the centre of a rectangular combustion chamber. The experiments are performed using different wall materials: polished steel, steel covered with soot and ceramics. Flame interacts with a rectangular obstacle placed at the bottom of the combustion chamber. The quenching distance near the top of the obstacle and in the vicinity of its side-wall is measured using direct photography of the flame position near the quenching surface. The quenching phenomenon depends on the wall material. These circumstances must be taken into account during heat flux measurements. For ceramics, the side-wall quenching distance is less than for polished steel by a factor of 1.1–1.8 in the pressure range of 0.05–0.15 MPa. The coating of polished steel by a 15–20 μm thick soot layer decreases the quenching distance by a factor of 1.1–1.5. The head-on quenching distance for polished steel is 1.4–2.5 times less than side-wall quenching in the pressure range of 0.08–0.19 MPa. An evaluated value of the maximum heat flux to the wall during quenching is obtained from the value of quenching distance using a simple relationship. For atmospheric pressure and cold wall conditions, the peak heat flux to the polished steel surface is found to be 0.46 and 0.26 MW/m 2 for the head-on and side-wall quenching, respectively. Near the ceramic surface these values are 0.8 and 0.39 MW/m 2 for head-on and side-wall quenching, respectively.

Flame/wall interaction and maximum wall heat fluxes in diffusion burners

Determining maximum wall heat fluxes in diffusion burners (rocket engines, diesel engines, or gas turbines) is a critical question for many combustion chambers. These fluxes are often due to flame/wall interaction (FWI) phenomena. This interaction may be due to either premixed or diffusion flamelets impinging on walls. Possible FWI configurations in diffusion burners are analyzed, and a proper laminar configuration for FWI studies (a diffusion flame impinging on a wall in a stagnation point flow) is identified. This case is studied for a variety of strain rates in terms of minimum distances between flame and wall and maximum wall heat fluxes. Results show that diffusion flames interacting with walls can generate heat fluxes which are larger than a premixed stoichiometric flame and that these mechanisms must be accounted for to predict wall heat fluxes in real burners.

A numerical study of side wall quenching with propane/air flames

The head-on (i.e., stagnation) configuration has generally been used to numerically and experimentally characterize the flame-wall interaction with complex chemistry and multicomponent transport. Other studies have treated the transient case of a flame propagating toward a wall, and combustion in a boundary layer has also been dealt with. In this paper, a two-dimensional stationary model has been used to study the sidewall quenching of laminar propane/air flames in a boundary-layer flow. This geometry may be described as a flame parallel to the wall that is swept away with a laminar boundary-layer flow while propagating toward and interacting with the wall. The main purpose has been to examine the extent to which the flame can propagate toward the cooled wall for lean flames compared to stoichiometric flames. A detailed kinetic model is used to examine the oxidation of both the fuel and the intermediate hydrocarbons (IHCs). For stoichiometric and near stoichiometric mixtures the thermal coupling between the flame and the wall is small but significant. However, for very lean flames, the thermal coupling between the flame and the wall is found to be very significant. The intermediate hydrocarbons are the dominant emissions for stoichiometric and near-stoichiometric flames in contrast to the leaner flames in which the fuel becomes more significant. This implies that the IHCs are very important for the overall hydrocarbon emissions from flame quenching: as a result detailed kinetics of complex fuels should be used when determining the unburned hydrocarbon emissions.

Numerical studies of wall effects with laminar methane flames

Wall effects in the combustion of lean methane mixtures have been studied numerically using the CHEMKIN software. To gain a deeper understanding of the flame-wall interaction in lean burn combustion, and in particular the kinetic and thermal effects, we have simulated lean and steady methane/air flames in a boundary layer flow. The gas-phase chemistry is modeled with the GRI mechanism version 1.2. Boundary conditions include an inert wall, a recombination wall and catalytic combustion of methane. Different pressures, wall temperatures and fuel-air ratios are used to address questions such as which part of the wall effects is most important at a given set of conditions. As the results are analyzed it can be seen that the thermal wall effects are more significant at the lower wall temperature (600 K) and the wall can essentially be modeled as chemical inert for the lean mixtures used. At the higher wall temperature (1,200 K), the chemical wall effects become more significant and at the higher pressure (10 atm) the catalytic surface retards homogeneous combustion of methane more than the recombination wall because of product inhibition. This may explain the increased emissions of unburned fuel observed in engine studies, when using catalytic coatings on the cylinder walls. The overall wall effects were more pronounced for the leaner combustion case (φ = 0.2). When the position of the reaction zone obtained from the boundary layer calculations is compared with the results from a one-dimensional premixed flame model, there is a small but significant difference except at the richer combustion case (φ = 0.4) at atmospheric pressure, where the boundary layer model may not predict the flame position for the given initial conditions.

Blowoff Characteristics and Flame Structure of Edge Flame in the Stagnation Flow.

The relation between blowoff characteristics of the edge flame in a methane-air diffusion flame and its flame structure has been investigated by using our original burner. The burner can form an edge flame without premixed flame, as a hole, in the stagnation region of an axisymmetric impinging jet. Varying the hole diameter, blowoff limits and maximum flame temperature were measured for an edge flame's blowoff character, and also, for an edge flame's structure, temperature profile and flame location were measured and a thermal boundary layer around the edge flame was visualized with laser tomographic technique. It is found that all the edge flames in the stagnation flow have a critical stagnation velocity gradient, beyond which the flame can never be existed. The critical stagnation velocity gradient that represents the overall reaction rate in the edge flame zone decreases as the hole diameter is increased. The increase in a hole diameter leads to change of the edge flame's structure and to addition of two heat loss factors to the edge flame. One heat loss factor is the edge flame-wall interaction, which occurs due to decrease in the edge flame's location. Another is penetration of cold flow into a hole in the flame, which occurs due to decrease in the overlapping range of thermal boundary layer of the edge flame in the hole. These additional heat losses occur at lower stagnation velocity gradient and have stronger influence on the edge flame as a hole diameter becomes larger. Consequently the overall reaction rate in the edge flame zone is reduced by the increase in a hole diameter. Finally it is clarified that the edge flame shows qualitatively same extinction as a pure diffusion flame, when the flame zone of the edge flame lies spatially as the boundary that divides between oxidizer side and fuel side in spite of existence of a partially premixture of fuel and oxidizer ahead of the reaction zone in the edge flame region.

A Study of the Interaction Between a Jet Flame and a Lateral Wall

The dynamic process of the interaction between a jet flame and a lateral wall is experimentally studied. The evolution of the outer buoyant vortices, which are involved in the jet flame bulge and flame tip-cutting phenomena, is found to play the central role in the flame-wall interaction process for low speed jet flames. The flame response as the lateral wall approaches from infinity can be categorized into five characteristic stages based on the ratio of the flame bulge size on both sides. As the wall approaches, the flame is observed to first increase in the size of the flame bulge on the wall side. Then, flame is wiggling with off-set flame bulges on both sides. The flame is then seen to incline and attach to the wall, and finally flame on the wall side is complete quenched as the separation distance is further reduced. These variations in flame structure are closely related to the retarded evolution of the buoyant vortices due to the wall and are scaled and explained based on similarity as well as the induced strain rate due to the variation of the outer vortices.

Turbulence, scalar transport, and reaction rates in flame-wall interaction

Turbulent premixed flames are studied in a Couette channel flow using direct numerical simulation (DNS). The combustion is simulated by a single-step reaction with heat release, and the flow is fully developed turbulence at a Reynolds number of 504 based on channel half-width The flow configuration consists of a V-shaped flame held in place by a flame holder. One flame is far from the walls and remains adiabatic, while the other flame interacts with a wall and is nonadiabatic. The flame alters the turbulent velocity profile. Near-wall viscous sublayer scaling is found provided both the pressure gradient and the local, average wall shear stress are used. However, in the log layer, the standard scaling does not collapse the velocity profiles, indicating the importance of additional transport mechanisms caused by the flame. The turbulent length scale is reduced in the flame brush region. In the adiabatic flame, the reduction is to approximately the laminar flame thickness. In the nonadiabatic wall flame, the reduction is not as dramatic due to wall heat loss causing weaker reaction rates. The probability density function (PDF) of the scalar progress variable has a classical bi-modal shape in the adiabatic flame. Thus, in the adiabatic flame, the DNS results compare well with the standard Bray-Moss-Libby (BML) expressions for the turbulent scalar flux and flame surface density. However, in the nonadiabatic flame, which interacts with the wall, the scalar PDF is less bi-modal. In this case, the DNS results and the BML expression for the flame surface density does not correctly capture the heat loss effects of the wall. Modifications to the flame surface area are suggested and compare well with the DNS results.

A flame-wall interaction model for combustion and heat transfer in S.I. engines

A new premixed charge turbulent combustion model has been developed to take flame-wall interaction into account. The flame-wall interaction model is based on the combined turbulent-dissipation/laminar-mixing flamelets based on fractal theory. The model features a variable fractal dimension formulated as a function of the turbulence Reynolds number. The computed heat release rate and pressure are shown to be in good agreement with measurements. The results exhibit realistic outward-curved flame shapes. The fractal dimension is shown to vary from 2.7 in the core regions to around 1.9 in the near wall regions. A fractal dimension less than 2 is consistent with the framework of the present model. Computed local wall heat fluxes are also presented and indicate reasonable time histories. It is shown that the constant fractal-dimension model gives incorrect flame shapes, being inward-curved, and unreasonable local heat flux curves. © 1998 Scripta Technica, Heat Trans Jpn Res, 27(3): 205–215, 1998

Premixed flame–wall interaction in a turbulent channel flow: budget for the flame surface density evolution equation and modelling

Turbulent premixed flame propagation in the vicinity of a wall is studied using a three-dimensional constant-density simulation of flames propagating in a channel. The influence of the walls is investigated in terms of the flamelet approach, where flamelet speed and flame surface density transport are used to describe the flame. The walls have constant temperature and lead to flamelet quenching for sufficiently small wall–flame distances. Starting from the exact evolution equation for the surface density of propagating interfaces (Trouvé & Poinsot 1994; Candel & Poinsot 1990; Pope 1988), a budget for the flame surface density equation is presented before, during, and after the interaction with the wall. Before the flame interacts with the wall, flame propagation is controlled by a balance between surface production and annihilation. During the interaction, high flame surface density gradients near the wall are responsible for the predominance of the transport terms. Closures of all terms of the flame surface density equation are proposed. These models are based on flamelet ideas and take into account wall effects. Enthalpy loss through the wall affects flamelet speed, flamelet annihilation and flame propagation. Decrease of turbulent scales near the wall affects turbulent diffusion and flame strain. This model is compared to DNS results using two types of tests: (i) a priori tests, where individual terms of the modelled flame surface density equation are compared to the terms of the exact interface density propagation equation, calculated with the DNS; (ii) a posteriori tests, where the final model is used to obtain total reaction rate, mean fuel mass fraction, heat flux at the wall and fuel mass fraction at the wall in the configuration used in the DNS. For both types of tests the model compares well with the DNS results.

Modeling of flame-wall interaction for combustion and heat transfer in S.I. engines

A numerical combustion model was developed for an engine CFD code in spark ignition engine combustion chambers. The combustion model features the following new concepts: (1) The introduction of preheated reactants which are produced in the turbulent mixing process and consumed in the reaction process; (2) the modification of the probability of burned and unburned gas blobs meeting each other in consideration of a change in turbulence length scale, which becomes smaller in the near wall region. With the first concept, the model deals with turbulence-reaction controlled combustion in the near wall region as well as turbulence-controlled combustion in the core region. The second concept avoids the unrealistic flame shape due to the near wall acceleration of the turbulent flame propagation, which is inherently seen in combustion models based simply on the turbulent mixing time such as the Magnussen model. It is also shown that the present model gives more realistic local heat fluxes.

Flame-wall interaction simulation in a turbulent channel flow

The interaction between turbulent premixed flames and channel walls is studied. Combustion is represented by a simple irreversible reaction with a large activation temperature. A low heat release assumption is used, but feedback to the flowfield can be allowed through viscosity changes. The effect of wall distance on local and global flame structure is investigated. Quenching distances and maximum wall heat fluxed computed in laminar cases are compared to DNS results. It is found that quenching distances decrease and maximum heat fluxes increase relative to laminar flame values, scaling with the turbulent strain rate. It is shown that these effects are due to large coherent structures which push flame elements towards the wall. The effect of wall strain in flame-wall interaction is studied in a stagnation line flow; this is used to explain the DNS results. The effects of the flame on the flow through viscosity changes is studied. It is also shown that “remarkable” flame events are produced by flame interaction with a horseshoe vortex: burned gases are pushed towards the wall at high speed and induce quenching and high wall heat flux while fresh gases are expelled from the wall region and form finger-like structures. Effects of the wall on flame surface density are investigated.

Heterogeneous/homogeneous reaction and transport coupling during flame-wall interaction

The effect of surface chemistry and transport modeling on the head-on quenching process of laminar propane/air and methane/air flames is investigated numerically. The fully compressible, one-dimensional Navier-Stokes equations with detailed mechanisms for diffusion and chemical kinetcs (gas-phase and surface) are solved using a sixth-order finite-difference scheme in space and a third-order Runge-Kutta scheme in time. Important applications are in the modeling of flame-wall interaction processes in piston engines, their related experimental measurements, and general surface chemistry modeling. The latter is due to the interaction being unsteady in contrast to typical boundary layer or stagnation point flow studies over reactive surfaces. Also the surface response takes place in the high coverage zone. It is found that in order to simulate quenching at higher wall temperatures accurately, the process of diffusion, adsorbtion, and surface recombination especially for the free radical H and OH have to be described. This results in an increasing wall heat flux with wall temperature in excellent agreement with existing experimental data If the participation of the surface in the reaction channels of the near-wall gas-phase species is excluded (either because of the physical surface properties or because of the boundary treatment in the simulations), the quenching structure is completely changed with strongly exothermic radical recombination reactions dominating the interaction. The developed detailed surface mechanism contains 10 reactions involving four surface species, and seven gas-phase species. It takes into account the breakdown of the Langmuir adsorption concept for high surface coverages and includes a precursor-state model for the adsorption of H 2 , H, and OH.

Applications of direct numerical simulation to premixed turbulent combustion

This paper describes recent progress in the field of Direct Numerical Simulations (DNS) for premixed turbulent combustion. A classification of Computational Fluid Dynamics techniques used in reacting flows is first proposed. Numerical methods and problems specific to DNS of combustion like grid resolution or boundary conditions treatment are discussed. The presentation of DNS results is done in the framework of flamelet modeling. The domain of validity of the flamelet assumptions is studied with simulations of flame vortex interactions. The local structure of flamelets in premixed turbulent combustion is investigated using two and three-dimensional DNS with simple chemistry models. Effects of complex chemistry are considered through recent two-dimensional DNS performed with realistic chemical schemes. Flame surface densities, flame strain and curvature statistics obtained from various three dimensional simulations are given and implications for modeling are presented. DNS is also exploited to deal with some specific problems like turbulent flame ignition or flame-wall interactions. This review is concluded with a discussion of current limitations and a list of future challenges.

Direct simulation and modeling of flame-wall interaction for premixed turbulent combustion

The interaction between turbulent premixed flames and walls is studied using a two-dimensional full Navier-Stokes solver with simple chemistry. The effects of wall distance on the local and global flame structure are investigated. Quenching distances and maximum wall heat fluxes during quenching are computed in laminar cases and are found to be comparable to experimental and analytical results. For turbulent cases, it is shown that quenching distances and maximum heat fluxes remain of the same order as for laminar flames. Based on simulation results, a “law-of-the-wall” model is derived to describe the interaction between a turbulent premixed flame and a wall. This model is constructed to provide reasonable behavior of flame surface density near a wall under the assumption that flame-wall interaction takes place at scales smaller than the computational mesh. It can be implemented in conjunction with any of several recent flamelet models based on a modeled surface density equation, with no additional constraints on mesh size or time step. Preliminary tests of this model are presented for the case of a spark-ignited piston engine.

Studies on Turbulent flames: A. Flame Propagation Across velocity gradients B. turbulence Measurement in flames

This work focuses on sidewall quenching (SWQ) for premixed methane/air flames that are forced by a periodic oscillatory inflow with excitation frequency f = 100 Hz. The effects of steady-state and transient flame stretch on the near-wall flame dynamics are evaluated using two-dimensional direct numerical simulation (2D-DNS) and the GRI 3.0 reaction mechanism. The velocity fluctuations lead to significant changes in flame speed and flame stretch, as well as the associated Markstein numbers. The phenomenon of SWQ is analyzed using flame quenching distance, wall heat flux and heat release rate. For steady-state conditions, there is a strong correlation between the maximum wall heat flux WHFmax and the flame quenching Peclet Number (Peq), as well as between the flame speed and the flame stretch; for transient conditions, the flame quenching distance (dq) increases continuously from phase angles of 1/4f−1 to 3/4f−1 in one cycle as time progresses, and the fluctuation of the quenching distance (Δdq) decreases gradually with increasing equivalence ratio (∅). The flame stretch changes from negative to positive in the process from 1/4f−1 to 3/4f−1, while the heat release rate and fuel reaction rate near the wall gradually decrease. Furthermore, the FWI region is dominated by negative flame stretch while positive flame stretch is present at the base of the flame. Moreover, the methane/air flame has a nearly twofold increase in the consumption speed during the oscillation from phase angle 3/4f−1 to the next cycle at 1/4f−1 at ∅ = 0.5 and ∅ = 1.0. These results show that flow field perturbations are not negligible in elucidating the effects of flame-wall interactions.