Flame Wall Research Articles

On the flame height definition for upward flame spread

Flame height is defined by the experimentalists as the average position of the luminous flame and, consequently is not directly linked with a quantitative value of a physical parameter. To determine flame heights from both numerical and theoretical results, a more quantifiable criterion is needed to define flame heights and must be in agreement with the experiments to allow comparisons. For wall flames, steady wall flame experiments revealed that flame height may be defined by a threshold value on the wall heat flux. From steady wall flame measurements, three definitions of flame height from wall heat flux are retained: the first is based on the continuous flame while the two others are based on threshold values of 4 kW/m 2 and 10 kW/m 2. These definitions are applied to determine flame heights from a two-dimensional time-dependent CFD model used to describe flame spread on a vertical slab of PMMA. Results show that the predicted flame heights are consistent with the available data of the literature. Defining flame height by threshold values on the wall heat flux of 4 and 10 kW/m 2 allows the correlation of the wall heat flux in terms of ( x− x p)/( x fl− x p), which is the dimensionless characteristic length scale for upward flame spread. It is also found that the continuous flame is not a characteristic length for the heat transfer to the unburnt fuel and is not really appropriate to define flame height in upward flame spread.

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.

A numerical study of thermal and chemical effects in interactions of n-heptane flames with a single surface

The thermal and chemical effects of a one-dimensional, premixed flame quenching against a single surface are studied numerically. Fuels considered include n-heptane and molar-based mixtures of 95/5 and 70/30 percent n-heptane and hydrogen, respectively. A reduced gas-phase kinetic mechanism for n-heptane is employed. Wall boundary conditions investigated include both an adiabatic and an isothermal wall with temperatures ranging from 298 to 1200 K. The effects of equivalence ratio variations between 0.7 and 3 are investigated. The computations with n-heptane and n-heptane/hydrogen mixtures show that for wall temperatures greater than 400 K heat release rates have a higher value for the wall-interacting flame than for the freely propagating flame. It is also seen that the peak wall heat flux increases with increasing wall temperatures up to 1000 K. Chemical pathway analysis reveals the importance of radical recombination reactions at the surface to the heat release profiles of this study. The effect of H, O, and OH radical recombination near the inert wall is observed to lower the heat release spike on a 750 K isothermal boundary. The concentrations of intermediate hydrocarbons in the near-wall region are studied and related to unburned hydrocarbon formation in an engine cylinder. It is shown that a simple one-step global reaction rate expression for n-heptane fuel conversion cannot reproduce the flame–wall trends observed with the reduced n-heptane mechanism.

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.

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.

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.

Theoretical and experimental studies on mesoscale flame propagation and extinction

Mesoscale flame propagation and extinction of premixed flames in channels are investigated theoretically and experimentally. Emphasis is placed on the effect of wall heat loss and the wall–flame interaction via heat recirculation. At first, an analytical solution of flame speed in mesoscale channels is obtained. The results showed that channel width, flow velocity, and wall thermal properties have dramatic effects on the flame propagation and lead to multiple flame regimes and extinction limits. With the decrease in channel width, there exist two distinct flame regimes, a fast burning regime and a slow burning regime. The existence of the new flame regime and its extended flammability limit render the classical quenching diameter inapplicable. Furthermore, the results showed that at optimum conditions of flow velocity and wall thermal properties, mesoscale flames can propagate faster than the adiabatic flame. Second, numerical simulation with detailed chemistry demonstrated the existence of multiple flame regimes. The results also showed that there is a non-linear dependence of the flame speed on equivalence ratio. Moreover, it is shown that the Nusselt number has a significant impact on this non-linear dependence. Finally, the non-linear dependence of flame speed on equivalence ratio for both flame regimes is measured using a C 3H 8–air mixture. The results are in good agreement with the theory and numerical simulation.

BURNING VELOCITIES AND A HIGH-TEMPERATURE SKELETAL KINETIC MODEL FOR n-DECANE

ABSTRACT Laminar flame speeds of n-decane/air mixtures were determined experimentally over an extensive range of equivalence ratios at 500 K and at atmospheric pressure. The effect of N2 dilution on the laminar flame speed was also studied at these same conditions. The experiments employed the stagnation jet–wall flame configuration with the flow velocity field determined by particle image velocimetry. Reference laminar flame speeds were obtained using linear extrapolation from low to zero stretch rate. The determined flame speeds are significantly different from those predicted using existing published kinetic models, including a model validated previously against high-temperature data from flow reactor, jet-stirred reactor, shock tube ignition delay, and burner-stabilized flame experiments. A significant update of this model is described that continues to predict the earlier validation experiments as well as the newly acquired laminar flame speed data and other recently published shock-tube ignition delay measurements.

Modeling fire growth in a combustible corner

A fire growth model was developed to predict the flame spread and total heat release rate of a fire in a corner configuration with a combustible lining. Input data for the combustible lining were developed using small-scale test data from the ASTM E1354 cone calorimeter and ASTM E1321 LIFT. The fire growth model includes a flame spread model linked with a two zone compartment fire model, CFAST Version 3.1.2. At a user selected time interval, the flame spread model uses the gas temperature from CFAST to predict the heat release rate of the fire at that time interval, and then provides CFAST with a new heat release rate to predict conditions during the next time step. The flame spread model is an improved version of the flat wall flame spread model previously developed for the US Navy. The model is capable of predicting flame spread in a variety of configurations including a flat wall, a corner with a ceiling, flat wall with a ceiling, unconfined ceiling, and parallel walls. The model has been validated against ISO 9705 test data and was used in this study to simulate conditions that develop in three open corner tests each with a different lining material. The model was able to predict the heat release rate of the fire and provide a reasonable estimate of the flame fronts and flame lengths during the growing fire.

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: 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.

Corner Fire Growth in a Room With a Combustible Lining

The flammability of interior finish materials for many applications is regulated using the ISO 9705 large-scale room corner fire test. To aid designers in developing new materials, a fire growth model has been developed to predict material performance in the ISO 9705 test using small-scale test data from ASTM E1354 cone calorimeter. The fire growth model includes a flame spread model linked with a two zone compartment fire model, CFAST Version 3.1.2. At a user selected time interval, the flame spread model uses the gas temperature from CFAST to predict the heat release rate of the fire at that time interval, and then provides CFAST with a new heat release rate to predict conditions during the next time step. The flame spread model is an improved version of the flat wall flame spread model previously developed for the U.S. Navy. The model is capable of predicting flame spread in a variety of configurations including a flat wall, a corner with a ceiling, flat wall with a ceiling, unconfined ceiling, and parallel walls. The fire growth model was used to predict the heat release rate and smoke production rate measured in eight ISO 9705 room corner fire tests on composite materials used in marine applications.

Prediction of knock onset and the autoignition site in spark-ignition engines

A three-dimensional model of knock is developed to predict knock occurrence and the autoignition site in spark-ignition engines. A premixed turbulent combustion model, which falls within the flame area evolution category, and a flame wall quenching model are applied to simulate flame propagation. A reduced kinetic model is used to simulate autoignition in the unburned gas region. Engine experiments are performed to calibrate the reduced kinetic model and to verify the results of this modelling. A spark plug integrated pressure transducer and 11 ion probes are installed in the cylinder head to detect knock occurrences, flame arrival angles and autoignition sites. The model predictions show good agreement with the cylinder pressures, flame arrival angles, knock occurrences and autoignition site of the engine experiments with the variations in spark timing.

Prediction of Autoignition Site in Spark Ignition Engines

A three-dimensional knock model is developed to predict the knock occurrence and autoignition site in spark ignition engines. Weller's 1-equation combustion model and a flame wall quenching model a re applied to simulate flame propagation. A Reduced kinetic model developed by Keck and Hu is used to simulate autoignition in the unburned gas region. Engine experiments are performed to calibrate the reduced kinetic model and to verify the knock model. A spark plug integrated pressure transducer and 11 ion probes are installed in the cylinder head to detect knock occurrences, flame arrival angles, and autoignition sites. Autoignition site can be determined from the knock onset angle determined by cylinder pressure and flame arrival time determined by ion probes. Before the reduced kinetic model is combined with the combustion model, it was calibrated with measured cylinder pressure in a spark ignition engine. The cylinder pressure and flame arrival angles of numerical results are compared with the experiments. The numerical results are in good agreement with the experiments. The knock model developed in this study can predict knock occurrence and autoignition sites with reasonable accuracy.

Ignitor and Thickness Effects on Upward Flame Spread

The effects of a material's ignitor characteristics and burning duration on upward wall flame spread are investigated. The ignitor is represented as an energy line source. Its energy release rate and its duration after ignition are considered. The material is represented as a finite, thick noncharring material with properties representative of polymethylmethracrylate (PMMA). A Volterra integral equation is solved for upward flame speed by numerical methods, and a transient, noncharring burning rate model is included. Results show the influence on propagation of ignitor effects and material thickness. A propagation map is computed showing the domains of flames that spread and flames that stop. Criteria for propagation and how propagation affects fire growth are considered in a standard room-corner test.

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.

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.