Flame-wall Interaction Research Articles

Simultaneous Measurements of Temperature and CO Concentration in Stagnation Stabilized Flames

An impinging jet burner was developed to investigate flame-wall interactions (FWI) using laser based diagnostics. CO concentrations were measured with two-photon laser-induced fluorescence (LIF) in combination with coherent anti-Stokes Raman spectroscopy (CARS) gas phase temperature measurements. Besides being the principal factor in chemical kinetics, temperature data is required to correct the CO LIF data for various factors like density variation, quenching and variation in the Boltzmann population. Phosphor thermometry was used to determine surface temperatures of the wall and to estimate the heat flux. In an parameter study Reynolds numbers and fuel equivalence ratio were varied.

Micro and mesoscale combustion

A review of research and development on micro and mesoscale combustion is presented, with an emphasis on fundamental understandings achieved in the field during the last decade. Due to its small scale nature, increasing effects of flame–wall interaction and molecular diffusion are the characteristic features of micro and mesoscale combustion. After brief review of device developments, overview of fundamentals in micro and mesoscale combustion as well as possible future directions is presented.

Impact of flame-wall interaction on premixed flame dynamics and transfer function characteristics

In this paper, we numerically investigate the response of a perforated-plate stabilized laminar methane–air premixed flame to imposed inlet velocity perturbations. A flame model using detailed chemical kinetics mechanism is applied and heat exchange between the burner plate and the gas mixture is incorporated. Linear transfer functions, for low mean inlet velocity oscillations, are analyzed for different equivalence ratio, mean inlet velocity, plate thermal conductivity and distance between adjacent holes. The oscillations of the heat exchange rate at the top of the burner surface plays a critical role in driving the growth of the perturbations over a wide range of conditions, including resonance. The flame response to the perturbations at its base takes the form of consumption speed oscillations in this region. Flame stand-off distance increases/decreases when the flame-wall interaction strengthens/weakens, impacting the overall dynamics of the heat release. The convective lag between the perturbations and the flame base response govern the phase of heat release rate oscillations. There is an additional convective lag between the perturbations at the flame base and the flame tip which has a weaker impact on the heat release rate oscillations. At higher frequencies, the flame-wall interaction is weaker and the heat release oscillations are driven by the flame area oscillations. The response of the flame to higher amplitude oscillations are used to gain further insight into the mechanisms.

Turbulent flame–wall interaction: a direct numerical simulation study

A turbulent flame–wall interaction (FWI) configuration is studied using three-dimensional direct numerical simulation (DNS) and detailed chemical kinetics. The simulations are used to investigate the effects of the wall turbulent boundary layer (i) on the structure of a hydrogen–air premixed flame, (ii) on its near-wall propagation characteristics and (iii) on the spatial and temporal patterns of the convective wall heat flux. Results show that the local flame thickness and propagation speed vary between the core flow and the boundary layer, resulting in a regime change from flamelet near the channel centreline to a thickened flame at the wall. This finding has strong implications for the modelling of turbulent combustion using Reynolds-averaged Navier–Stokes or large-eddy simulation techniques. Moreover, the DNS results suggest that the near-wall coherent turbulent structures play an important role on the convective wall heat transfer by pushing the hot reactive zone towards the cold solid surface. At the wall, exothermic radical recombination reactions become important, and are responsible for approximately 70% of the overall heat release rate at the wall. Spectral analysis of the convective wall heat flux provides an unambiguous picture of its spatial and temporal patterns, previously unobserved, that is directly related to the spatial and temporal characteristic scalings of the coherent near-wall turbulent structures.

Temperature and species measurement in a quenching boundary layer on a flat-flame burner

A detailed understanding of transport phenomena and reactions in near-wall boundary layers of combustion chambers is essential for further reducing pollutant emissions and improving thermal efficiencies of internal combustion engines. In a model experiment, the potential of laser-induced fluorescence (LIF) was investigated for measurements inside the boundary layer connected to flame-wall interaction at atmospheric pressure. Temperature and species distributions were measured in the quenching boundary layer formed close to a cooled metal surface located parallel to the flow of a premixed methane/air flat flame. Multi-line NO-LIF thermometry provided gas-phase temperature distributions. In addition, flame species OH, CH2O and CO were monitored by single-photon (OH, CH2O) and two-photon (CO) excitation LIF, respectively. The temperature dependence of the OH-LIF signal intensities was corrected for using the measured gas-phase temperature distributions. The spatial line-pair resolution of the imaging system was 22 μm determined by imaging microscopic line pairs printed on a resolution target. The experimental results show the expected flame quenching behavior in the boundary layer and they reveal the potential and limitations of the applied diagnostics techniques. Limitations in spatial resolution are attributed to refraction of fluorescence radiation propagating through steep temperature gradients in the boundary layer. For the present experimental arrangements, the applied diagnostics techniques are applicable as close to the wall as 200 μm with measurement precision then exceeding the 15–25% limit for species detection, with estimates of double this value for the case of H2CO due to the unknown effect of the Boltzmann fraction corrections not included in the data evaluation process. Temperature measurements are believed to be accurate within 50 K in the near-wall zone, which amounts to roughly 10% at the lower temperatures encountered in this region of the flames.

Heat flux measurements in stagnation point methane/air flames with thermographic phosphors

Light-induced phosphorescence from thermographic phosphors was used to study the wall temperatures and heat fluxes from nearly one-dimensional flat premixed flames. The investigated flames were stoichiometric, lean and rich laminar methane/air flames with equivalence ratios of φ = 1, φ = 0.75 and φ = 1.25 at ambient pressure. The flames were burning in a stagnation point arrangement against a water-cooled plate. The central part of this plate was an alumina ceramic plate coated from both sides with chromium-doped alumina (ruby) and excited with a Nd:YAG laser or a green light-emitting diode (LED) array to measure the wall temperature from both sides and thus the heat flux rate from the flame. The outlet velocity of the gases was varied from 0.1 to 1.2 m/s. The burner to plate distance (H) ranged from 0.5 to 2 times the burner exit diameter (d = 30 mm). The measured heat flux rates indicate the change of the flame stabilization mechanism from a burner stabilized to a stagnation plate stabilized flame. The results were compared to modeling results of a one-dimensional stagnation point flow, with a detailed reaction mechanism. In order to prove the model, gas phase temperatures were measured by OH-LIF for a stoichiometric stagnation point flame. It turns out that the flame stabilization mechanism and with it the heat fluxes change from low to high mass fluxes. This geometry may be well suited for further studies of the elementary flame wall interaction.

Some Features of Propane-Air Flames Under Quenching Conditions in Narrow Channels

Knowledge of flame–wall interaction helps in the understanding of flame phenomena near a wall and of flame extinction. In the present work different flame characteristics were measured in square, narrow quenching channels. A propane/air mixture was employed and direct visualization was used to observe flame behavior under quenching conditions. Flame propagation velocity was determined during downward propagation. As expected, close to quenching condition it was found to be lower than that under adiabatic conditions and close to the values predicted by the quenching theory. The dead space appeared to be larger for rich mixtures than that found with lean ones. Flame curvature reached its maximum value for stoichiometric mixtures decreased for both leaner and richer mixtures. Numerical simulation revealed the structure of limit flames during their propagation in quenching channels. It was confirmed that flame quenching depends on the relation between heat release rate and heat loss rate.

Oscillations of Premixed Flames in Tubes Near the Flashback Conditions

We present a theoretical and experimental study of the oscillatory propagation of lean premixed methane-air flames in tubes of low thermal conductivity. The effect is observed experimentally near flashback conditions when the flame velocity relative to the wall is much smaller than the planar burning velocity. The theoretical and numerical analysis is based on a simplified constant-density model. It is shown that the principal mechanism of oscillation results from a thermal flame-wall interaction, and that there can be multiple auto-oscillatory states for the same set of parameters.

Pollutant formation modelling in natural gas SI engines using a level set based flamelet model

The level set flamelet model for turbulent premixed combustion was recently extended to improve the modelling of the spark ignition process, to incorporate the influence of unsteady flame kernel development on turbulent flame propagation, to embrace flame—wall interaction, and to comprise pollutant prediction models. The formation of nitrogen oxides (NO x) is predicted using the extended Zeldovich mechanism. The concentrations of the involved intermediate species are determined using a chemical equilibrium assumption. The formation of engine-out unburnt hydrocarbons (UHCs) and carbon monoxides (COs) is directly linked to incomplete combustion due to flame quenching at the time the flame hits the wall. In this paper, the new model is used for the analysis of combustion in a homogeneous charged natural gas SI engine operating on lean conditions. The investigated engine operating (PAM) points differ in the applied equivalence ratio, ignition timing, and rate of exhaust gas recirculation. The influence of the load on the peak pressures, peak pressure locations, and pollutant formations is discussed in detail. A good agreement with experimental data was obtained among all engine operating points.

Numerical study of turbulent flame velocity

A premixed flame propagating through a combination of vortices in a tube/channel is studied using direct numerical simulations of the complete set of combustion equations including thermal conduction, diffusion, viscosity, and chemical kinetics. Two cases are considered, a single-mode vortex array and a multimode combination of vortices obeying the Kolmogorov spectrum. It is shown that the velocity of flame propagation depends strongly on the vortex intensity and size. The dependence on the vortex intensity is almost linear in agreement with the general belief. The dependence on the vortex size may be imitated by a power law ∝ D 2 / 3 . This result is different from theoretical predictions, which creates a challenge for the theory. In the case of the Kolmogorov spectrum of vortices, the velocity of flame propagation is noticeably smaller than for a single-mode vortex array. The flame velocity depends weakly on the thermal expansion of burning matter within the domain of realistically large expansion factors. Comparison to the experimental data indicates that small-scale turbulence is not the only effect that influences the flame velocity in the experimental flows. Large-scale processes, such as the Darrieus–Landau instability and flame–wall interaction, contribute considerably to the velocity of flame propagation. Still, on small scales, the Darrieus–Landau instability becomes important only for a sufficiently low vortex intensity.

A thermal formulation for single-wall quenching of transient laminar flames

Improving our knowledge of flame–wall interaction is of relevance to performing near-wall combustion calculations. Quenching distance is to be determined accordingly, as a major parameter of flame quenching. For this purpose, an equation describing the behavior of single-wall flame quenching has been derived from a simplified model of laminar flame–wall interaction. It allows evaluating quenching distance from wall heat flux and mixture properties; a significant advantage of this formula is the absence of any empirical coefficient. To assess its reliability, the results computed with this equation have been compared to experimental data concerning laminar flame–wall interaction. For this purpose, single-wall quenching parameters have been recorded in both head-on and sidewall configurations. Quenching distance and wall heat flux have been measured simultaneously, during the combustion of quiescent methane–air mixtures in a constant-volume vessel. Quenching distance is determined through direct visualization, whereas wall heat flux is processed from the time evolution of wall surface temperature. The equation has been verified over the pressure range 0.05–0.35 MPa in stoichiometric and lean mixtures. It shows good agreement with experimental data at first order, with less than 20% variation.

Flame-wall interaction for a non-premixed flame propelled by a vortex ring

The characteristics of non-premixed flame–wall interaction have been studied in a reacting vortex ring configuration. A finite duration axisymmetric methane jet is injected into a quiescent oxidizer ambient to generate a laminar vortex ring. Fuel and oxidizer temperatures are adjusted such that auto-ignition takes place and results in the formation of a non-premixed flame surrounding the vortex ring. The methane combustion is modelled using detailed kinetic mechanism GRI3.0 with the reactions and the species containing nitrogen removed, except N2. The flame is propelled by the ring induced velocity and interacts with an isothermal inert wall which is perpendicular to the direction of the flame propagation. Interactions have been studied for two different wall conditions – one with and one without a thermal boundary layer. Heat release rate, vorticity, and temperature fields have been examined during the interaction. Various flame–wall interaction parameters like strain rates, flame power, Peclet number and non-dimensional wall heat flux are studied. For all runs, the values of quenching Peclet number are in the range 0.26–0.45. The non-premixed flame quenching distance is found to be approximately of the same order as that of the premixed flame quenching distance obtained in experiments. The quenching values of non-dimensional wall heat flux for non-premixed flames in this study are of the same order of magnitude as those obtained for premixed flames. Flame structure and near wall reaction mechanisms are analysed. In the absence of a thermal boundary layer, various radical mole fractions are increased in the near wall region. In the presence of a thermal boundary layer, HO2 accumulates and contributes primarily to the heat release rate at the wall. Products and unburned hydrocarbon mole fractions are found to be higher for the runs without a thermal boundary layer compared to the runs with a thermal boundary layer.

Unsteady heat transfer during the turbulent combustion of a lean premixed methane–air flame: Effect of pressure and gas dynamics

The objective of this work is to investigate the experimental behaviour of wall heat losses according to pressure and gas dynamics. Another goal is to improve the knowledge of unsteady flame–wall interaction. In a constant volume chamber, turbulent combustion occurs in a tumbling charge of lean methane–air mixture at equivalence ratio 0.7. Heat flux is calculated from wall surface temperature, and velocity is obtained by high-speed Particle Image Velocimetry. At first order, the low-frequency time evolution of heat flux is that of pressure. High-frequency time variations of heat flux are attributed to the cyclic fluctuations of large-scale velocity. As for large-scale velocity, its magnitude influences heat losses as well as its direction relatively to the wall. Compared to large-scale velocity, turbulence seems to have only second order effects on heat losses, in the case of a structured flow motion. Finally, the observed tendencies are in good agreement with previous results from thermal correlations and with laminar flame quenching measurements.

마이크로 연소기에서 발생하는 열 소염과 화학 소염 현상 (II)- SiOx(x≤2) 플레이트의 물리, 화학적 성질이 소염에 미치는 영향 -

In order to realize a stably propagating flame in a narrow channel, flame instabilities resulting from flame-wall interaction should be avoided. In particular flame quenching is a significant issue in micro combustion devices; quenching is caused either by excessive heat loss or by active radical adsorptions at the wall. In this paper, the relative significance of thermal and chemical effects on flame quenching is examined by means of quenching distance measurement. Emphasis is placed on the effects of surface defect density on flame quenching. To investigate chemical quenching phenomenon, thermally grown silicon oxide plates with well-defined defect distribution were prepared. ion implantation technique was used to control defect density, i.e. the number of oxygen vacancies. It has been found that when the surface temperature is under , the quenching distance is decreased on account of reduced heat loss; as the surface temperature is increased over , however, quenching distance is increased despite reduced heat loss effect. Such abberant behavior is caused by heterogeneous surface reactions between active radicals and surface defects. The higher defect density, the larger quenching distance. This result means that chemical quenching is governed by radical adsorption that can be parameterized by oxygen vacancy density on the surface.

Numerical study of thermodiffusive flame structures interacting with adiabatic walls using an adaptive multiresolution scheme

We report on numerical simulations of planar, circular, and spherical thermodiffusive flames in one, two and three space dimensions, respectively. Attention is focused on the interaction of spherical flames with adiabatic walls or equivalently on their interaction with their mirror image. The numerical scheme is based on an adaptive multiresolution discretization which allows self-adaptive grid refinement in regions of the thin reaction zone. We show that the Lewis number determines the behaviour of the flame–wall interaction. When the flame is approaching the wall, we observe for Lewis numbers smaller than unity that the reaction rate is decreased, for unitary Lewis number the reaction rate neither increases nor decreases, and for Lewis numbers larger than unity the reaction rate increases. Due to tangential diffusion the flame front curvature is also modified, i.e. for small Lewis numbers the spherical flame contracts, for large Lewis numbers it spreads out, while for a unitary Lewis number the flame front remains perpendicular to the wall. The observed phenomena present similarities with capillarity effects in fluid mechanics when a droplet hits a wall.

Direct numerical simulation of nonpremixed flame–wall interactions

The objective of the present study is to use detailed numerical modeling to obtain basic information on the interaction of nonpremixed flames with cold wall surfaces. The questions of turbulent fuel–air–temperature mixing, flame extinction, and wall-surface heat transfer are studied using direct numerical simulation (DNS). The DNS configuration corresponds to an ethylene–air diffusion flame stabilized in the near-wall region of a chemically inert solid surface. Simulations are performed with adiabatic or isothermal wall boundary conditions and with different turbulence intensities. The simulations feature flame extinction events resulting from excessive wall cooling and convective heat transfer rates up to 90 kW/m 2. The structure of the simulated wall flames is studied in terms of a classical mass-mixing variable, the fuel–air based mixture fraction, and a less familiar heat loss variable, the excess enthalpy variable, introduced to provide a measure of nonadiabatic behavior due to wall cooling. In addition to the flame structure, extinction events are also studied in detail and a modified flame extinction criterion that combines the concepts of mixture fraction and excess enthalpy is proposed and then tested against the DNS data.

HEAD-ON QUENCHING OF TRANSIENT LAMINAR FLAME: HEAT FLUX AND QUENCHING DISTANCE MEASUREMENTS

ABSTRACT This work presents results of quenching distance and heat flux measurements during the head-on quenching of transient laminar stoichiometric methane/air flame in the pressure range 0.05–1.7 MPa. The results of direct visualization have been used to measure the quenching distance and to prove the relationship relating the quenching distance and heat flux to the wall over the global combustion reaction rate. It is shown that quenching distance decreases from 0.43 to 0.016 mm with pressure rise from 0.05 to 1.7 MPa. The maximum wall heat flux increases nonlinearly from 0.35 to 2.3 MW/m2 with pressure rise from 0.05 to 1.7 MPa. The dimensionless value of the heat flux depends only slightly on the pressure and decreases from 0.3 to 0.2 with rise of pressure in this pressure range. Flame–wall interaction time is about constant and equal to 0.15–0.155 ms in all ranges of pressure variation.

Direct numerical simulation of non-premixed flame-wall interactions

The overall objective of this paper is to illustrate how detailed numerical modelling may be used to bring basic information on fundamental problems in combustion science. We consider in the following the interaction of non-premixed flames with cold solid wall surfaces. Flame-wall interactions are an important feature in many combustion systems, that result in significant changes in the flame and wall dynamics: the flame strength is reduced near cold wall surfaces, leading possibly to (partial or total) quenching, while the gassolid heat flux takes peak values at flame contact. The questions of turbulent fuel-air-temperature mixing, flame extinction and wall surface heat transfer are here studied using direct numerical simulation (DNS). The DNS configuration corresponds to an ethylene-air diffusion flame stabilized in the near-wall region of a chemically-inert solid surface. Simulations are performed with adiabatic or isothermal wall boundary conditions, and with different turbulence intensities. The simulations feature flame extinction events resulting from excessive wall cooling, and convective heat transfer up to 90 kW/m2. The structure of the simulated wall flames is studied in terms of a classical mass mixing variable, i.e. the fuel-airbased mixture fraction, and a less familiar heat loss variable, i.e. the excess enthalpy variable, introduced to provide a measure of non-adiabatic behavior due to wall cooling.

Interaction of a premixed flame with a liquid fuel film on a wall

In piston engines and in gas turbines, the injection of liquid fuel often leads to the formation of a liquid film on the combustor wall. If a flame reaches this zone, undesired phenomena such as coking may occur and diminish the lifetime of the engine. Moreover, the effect of such an interaction on maximum wall heat fluxes, flame quenching, and pollutant formation is largely unknown. This paper presents a numerical study of the interaction of a premixed flame with a cold wall covered with a film of liquid fuel. Simulations show that the presence of the film leads to a very rich zone at the wall in which the flame cannot propagate. As a result, the flame wall distance remains larger with liquid fuel than it is for a dry wall, and maximum heat fluxes are smaller. The nature of the interaction of flame wall interaction with a liquid fuel is also different from the classical flame/dry wall interaction: it is controlled mainly by chemical mechanisms and not by the thermal quenching effect observed for flames interacting with dry walls: the existence of a very rich zone created above the liquid film is the main mechanism controlling quenching.

Flame wall interaction: effect of stretch

An experimental study of flame-wall interaction is presented. The flame is a laminar, flat methane–air one at three different equivalent ratio: 1, 0.8, 0.6. Temporal PIV system was used to acquire images sequences of the interaction. Evolution of velocity field and flame contour shows that stagnation points appear between the reactive zone and the wall. Even if the maximum velocity value is relatively low, fresh gases motion amplifies wrinkle perturbation of the flame front. To explore more precisely the curvature evolution and its repercussion on the stretch rate, the stretch rate was calculated by determining local strain rate and curvature stretch term. Evolution of the stretch rate versus the time shows that the quenching stretch rate can be reached and locally, the extinction of the flame can appear for the equivalent ratio near the stoichiometry. We show that the interaction between a flame and a wall is controlled and disturbed by the modification of the velocity field of the fresh gases trapped between the reactive zone and the wall.