Local Mass Burning Rate Research Articles

Local Burning Behavior of Wind-Driven Flames under the Influence of Mixed-Convective Turbulent Flow Conditions

ABSTRACT Experiments were conducted to investigate the burning behavior of wind-driven turbulent flames stabilized over a condensed fuel surface. A controlled turbulent crossflow environment was established to examine the impact of external flow velocity and turbulence intensity on the flame profile parameters and mass burning rates. To address the complexities of practical fire scenarios, a mixed-convection parameter ξ x in the form of G r x 1 / ψ x 2 n was defined, encapsulating the combined effects of momentum, buoyancy, and freestream turbulence. Furthermore, the property of fuel (in terms of mass transfer number B) was integrated into the mixed-convection parameter ( ξ x ) to establish a fuel-dependency factor within the formulation . The interdependence of flame characteristics and the mass burning process was observed using ξ x , that yielded several meaningful correlations. In this regard, a power-law trend was obtained between flame standoff distance and the dimensionless parameter ξ x . Finally, the local mass burning rates were estimated and plotted against mixed-convective parameter ( ξ x ), resulting in a unified local mass burning rate correlation. This correlation incorporates the fuel-dependency aspect in its formulation and is applicable in both laminar and turbulent crossflow environments.

Heat transfer in wind-driven fires stabilized under a mixed-convective turbulent flow environment

This study experimentally investigates the heat flux distribution over a horizontally placed condensed fuel surface under mixed-convective turbulent flow conditions. The study explores different freestream conditions, including variations in crossflow velocity (ranging from 1 m/s to 1.8 m/s) and turbulent intensity (ranging from 3% to 9%) on local heat fluxes at the condensed fuel surface. Further, to decipher the collective influence of momentum, buoyancy, and turbulence on local heat fluxes, a mixed-convection variable ξx was defined in the form of ξx=Grx1/ψx2n. The defined variable also encapsulates the property of fuel as a function of mass transfer number B, thereby establishing a fuel-dependency factor in the formulation of the mixed-convective parameter (ξx). A methodology was adopted to quantify heat flux components (convective and radiative) with the knowledge of local mass burning rates and local temperature gradients at the condensed fuel surface in the pyrolysis zone. An inverse relationship was observed for incident heat flux with flame standoff distance and ξx. The present study considered the experimental data for two fuels, n-heptane, and ethanol, which showed almost similar trends. However, the radiative heat flux was dominant for the n-heptane case due to the higher sooting propensity of heptane.

Burning behavior of mixed-convection wind-driven flames under varying freestream conditions

Wind-driven fires were experimentally-investigated to understand the influence of varying flow conditions on the local burning behavior of a condensed fuel surface. These flames imitate the fire dynamics of hazardous fires occurring in a forced-flow environment sharing the likes of wildland fires or ventilated compartment fires. The flame structure was probed using a digital camera. Small-scale wind-driven flames usually operate in a mixed-convection regime. Therefore, to decipher the competitive effect of buoyancy, momentum, and turbulence intensity, a non-dimensional parameter ξx=Grx1/ψx2n was evaluated at multiple streamwise locations within the pyrolysis zone. To quantify the dependence of each flame parameter, first, a comparison between the flame standoff and ξx was conducted, where a power-law trend was observed. Also, the convective heat transfer towards the fuel surface varied inversely with the flame standoff distance and ξx. Finally, the local mass burning rates were evaluated and plotted against the convective heat flux and ξx, where a positive linear scaling was observed for the former, and a negative power-law trend was shown by the latter. Based on the experimental measurements, a unified local mass burning rate correlation is proposed in the present work that performs exceptionally well for both laminar and turbulent boundary layer diffusion flames.

Assessment of spark-energy allocation and ignition environment on lean combustion in a twin-plug Wankel engine

Due to the elongated shape of the combustion chamber and strong one-way flow characteristic, changing the location of enhanced combustion is an effective means to ameliorate the indicated efficiency of the Wankel engine. For this purpose, strengthening combustion can happen by allocating spark-energy in the desired location along with a favorable ignition environment. In this study, three-dimensional CFD simulations were implemented and the numerical results were compared with existing measured data, in which ignition environment variations were achieved by hydrogen enrichment within the intake manifold of a spark-ignition Wankel engine. Based on CONVERGE code with the SAGE detailed chemistry solver, the twin-plug engine model was then used to assess the location of strengthened combustion employing a spark-energy allocation from the perspective of species evolution, combustion characteristics, and emissions formation. The results indicate that enhancing spark-energy at the leading side of the rotor chamber has an overall higher burned volume rate and faster species evolution due to the later connection of two burning sources prior to quenching. Increasing the hydrogen content in the ignition environment causes richer active radicals and higher turbulence, ultimately promoting flame propagation and combustion intensity. A larger T-plug spark-energy decreases the indicated mean effective pressure, heat release rate, and thermal efficiency because of the intensified wall heat flux and the lower mass burning rate at the leading side. There is no doubt that the introduction of hydrogen enrichment presents a positive effect on both thermal efficiency and specific fuel consumption at the price of a slighter increase of nitrogen oxide formation due to higher mass fraction of high-temperature regions. It is recommended that enhancing local mass burning rate in the leading part of the rotor chamber under the hydrogen-enriched ignition environment can provide a great potential in the quest towards high-efficiency Wankel rotary engine.

Numerical simulation of small pool fires incorporating liquid fuel motion

For small-scale pool fires, Vali et al. [1] showed a pair of vortices in the liquid pool. The first vortex appeared just close to the sidewall of the container, and the second one emerged slightly away from the first vortex. Large-eddy simulations of small methanol pool fires coupled with liquid fuel convective flow were conducted using an in-house version of FireFOAM to investigate the above phenomenon. In this study, a three-dimensional liquid phase model is newly developed. The model incorporates the effects of thermocapillary Marangoni convection, buoyancy, shear stress, and evaporation. For the gas phase, the combustion model is the extended eddy dissipation concept model coupled with the laminar combustion model. This combustion model uses the viscous diffusion rate to consider laminar-turbulent transition. The predictions were in reasonably good agreement with the measured local mass burning rate, flame height and distributions of liquid temperature. The error of the mass burning rate was within 4%. The present predictions captured a pair of vortices in line with Vali et al.'s experiment [1]. Their sizes increased with increasing the liquid temperature. The Reynolds analogy could explain the sensible reason behind this trend. Shear stress and thermocapillary force caused convection in the liquid pool, and this convection formed a pair of vortices. Thermocapillary force was due to the different distributions of convective and radiative heat transfer. Sensitivity test for sub-models for the liquid phase demonstrated that their effects on the mass burning rate were all less than 5.1%. Conversely, the simulation assuming zero gravity only in the liquid phase resulted in almost 64% reduction in the mass burning rate.

Steady and transient pyrolysis of a non-charring solid fuel under forced flow

In previous work, the Reynolds analogy was used to develop a theoretical expression that allowed for the estimation of local mass burning rates in steady laminar boundary layer diffusion flames established over liquid and solid fuels. This technique was used to elucidate the mechanisms responsible for pyrolysis of both solid and liquid fuels in forced and free convective environments. These previous studies, however, focused on steady results that occur early in the combustion process, before regression of the fuel surface begins to influence results. In this work, a thorough experimental investigation of steady and transient pyrolysis of clear cast Poly Methyl Methacrylate (PMMA) is presented using both local pyrolysis rates and heat feedback to the condensed fuel surface measured at different streamwise locations in a bench-scale wind tunnel. A functional form of the Nusselt number is derived that can be readily used to identify these steady and transient regimes of PMMA burning in the form of local convective heat transfer coefficients. At early times (<150 s), a steady burning regime is identified where heat feedback properties are constant and the gas phase can be assumed to be in a steady state. At later times, a transient burning regime dominated by solid-phase effects occurs. Heat feedback from the flame and hence local mass loss rates measured at later times are transient in nature and do not correspond well with the steady state theoretical solution. Investigation under different forced-flow wind conditions reveals this transient phenomena most likely occurs due to both deformation of the surface of PMMA and solid-phase conduction into the fuel, which eventually influences the gas phase. The results presented will be useful for future modeling of transient solid-phase combustion, especially as it is applied to studies of flame spread.

Experimental Methodology for Estimation of Local Heat Fluxes and Burning Rates in Steady Laminar Boundary Layer Diffusion Flames.

Modeling the realistic burning behavior of condensed-phase fuels has remained out of reach, in part because of an inability to resolve the complex interactions occurring at the interface between gas-phase flames and condensed-phase fuels. The current research provides a technique to explore the dynamic relationship between a combustible condensed fuel surface and gas-phase flames in laminar boundary layers. Experiments have previously been conducted in both forced and free convective environments over both solid and liquid fuels. A unique methodology, based on the Reynolds Analogy, was used to estimate local mass burning rates and flame heat fluxes for these laminar boundary layer diffusion flames utilizing local temperature gradients at the fuel surface. Local mass burning rates and convective and radiative heat feedback from the flames were measured in both the pyrolysis and plume regions by using temperature gradients mapped near the wall by a two-axis traverse system. These experiments are time-consuming and can be challenging to design as the condensed fuel surface burns steadily for only a limited period of time following ignition. The temperature profiles near the fuel surface need to be mapped during steady burning of a condensed fuel surface at a very high spatial resolution in order to capture reasonable estimates of local temperature gradients. Careful corrections for radiative heat losses from the thermocouples are also essential for accurate measurements. For these reasons, the whole experimental setup needs to be automated with a computer-controlled traverse mechanism, eliminating most errors due to positioning of a micro-thermocouple. An outline of steps to reproducibly capture near-wall temperature gradients and use them to assess local burning rates and heat fluxes is provided.

Experimental Methodology for Estimation of Local Heat Fluxes and Burning Rates in Steady Laminar Boundary Layer Diffusion Flames

Modeling the realistic burning behavior of condensed-phase fuels has remained out of reach, in part because of an inability to resolve the complex interactions occurring at the interface between gas-phase flames and condensed-phase fuels. The current research provides a technique to explore the dynamic relationship between a combustible condensed fuel surface and gas-phase flames in laminar boundary layers. Experiments have previously been conducted in both forced and free convective environments over both solid and liquid fuels. A unique methodology, based on the Reynolds Analogy, was used to estimate local mass burning rates and flame heat fluxes for these laminar boundary layer diffusion flames utilizing local temperature gradients at the fuel surface. Local mass burning rates and convective and radiative heat feedback from the flames were measured in both the pyrolysis and plume regions by using temperature gradients mapped near the wall by a two-axis traverse system. These experiments are time-consuming and can be challenging to design as the condensed fuel surface burns steadily for only a limited period of time following ignition. The temperature profiles near the fuel surface need to be mapped during steady burning of a condensed fuel surface at a very high spatial resolution in order to capture reasonable estimates of local temperature gradients. Careful corrections for radiative heat losses from the thermocouples are also essential for accurate measurements. For these reasons, the whole experimental setup needs to be automated with a computer-controlled traverse mechanism, eliminating most errors due to positioning of a micro-thermocouple. An outline of steps to reproducibly capture near-wall temperature gradients and use them to assess local burning rates and heat fluxes is provided.

Local Burning Rates and Heat Flux for Forced Flow Boundary-Layer Diffusion Flames

A methodology based on the Reynolds analogy was developed earlier that allowed for the estimation of local mass burning rates and heat fluxes in free-convection laminar boundary-layer diffusion flames. In this study, the relationship was examined in a forced-convective environment using methanol as a liquid fuel. The gas-phase temperature profiles across the laminar boundary layer with a methanol diffusion flame established over it were measured with the freestream air flowing parallel to the condensed fuel surface. Local and averaged mass burning rates were measured along with shear stresses at the fuel surface. The fuel consumption rate and flame lengths were observed to increase monotonically with an increase in the freestream velocity. Although the initial study was taken in the laminar regime, further extensions of the technique could be applicable to turbulent boundary-layer combustion in propulsion-oriented research.

A methodology for estimation of local heat fluxes in steady laminar boundary layer diffusion flames

A unique methodology has been used for the estimation of local mass burning rates and flame heat fluxes over a laminar boundary layer diffusion flame with a high accuracy by utilizing micro thermocouple measurements in the gas phase close to the condensed phase surface. Both liquid and solid fuels were investigated. Convective and radiative heat feedback from the flames were measured both in the pyrolysis and plume regions by using temperature gradients near the wall. As expected, for small laminar flames, convective heating was found to be the dominant mode of heat transfer to the condensed fuel surface and accounted for nearly 85–90% of the total flame heat flux in both liquid and solid fuels. The total average incident flux to the condensed fuel surface was estimated to be approximately 22, 20 and 27kW/m2 for methanol, ethanol and Poly Methyl Methacrylate (PMMA) wall-bounded flames, respectively. The average convective heat flux from the flame to the wall in the pyrolysis zone was estimated to be 18.9, 17 and 22.9kW/m2 for methanol, ethanol and PMMA, respectively. The radiative component in these small flames was observed to be small, never accounting for more than 20% of the total wall heat flux. Temperature gradients normal to the wall, proportional to the dominant convective heat flux, were found to decrease from the leading edge towards the trailing edge in the pyrolysis zone, however they were found to remain relatively constant in the combusting plume region (∼450K/mm) until the tip of the flame was reached. Thereafter, they were found to decrease significantly downstream of the combusting plume. The work presented here also discusses the selection of transport properties at appropriate temperatures to calculate convective fluxes by using a crude approximation as opposed to detailed temperature measurements in such flames.

Estimation of local mass burning rates for steady laminar boundary layer diffusion flames

A thorough numerical and experimental investigation of laminar boundary-layer diffusion flames established over the surface of a condensed fuel is presented. By extension of the Reynold’s Analogy, it is hypothesized that the non-dimensional temperature gradient at the surface of a condensed fuel is related to the local mass-burning rate through some constant of proportionality. First, this proportionality is tested by using a validated numerical model for a steady flame established over a condensed fuel surface, under free and forced convective conditions. Second, the relationship is tested by conducting experiments in a free-convective environment (vertical wall) using methanol and ethanol as liquid fuels and PMMA as a solid fuel, where a detailed temperature profile is mapped during steady burning using fine-wire thermocouples mounted to a precision two-axis traverse mechanism. The results from the present study suggests that there is indeed a unique correlation between the mass burning rates of liquid/solid fuels and the temperature gradients at the fuel surface. The correlating factor depends upon the Spalding mass transfer number and gas-phase thermo-physical properties and works in the prediction of both integrated as well as local variations of the mass burning rate as a function of non-dimensional temperature gradient. Additional results from precise measurements of the thermal field are also presented.

Steady laminar flame characteristics over methanol surface with air co-flow

A numerical investigation of the flame characteristics and mass burning rates of steady laminar diffusion flames established over methanol surface under co-flow configuration is presented. A numerical model that solves the transient, two-dimensional gas-phase governing conservation equations using global single-step reaction kinetics for methanol-air oxidation is employed for the present investigations. The effect of liquid-phase transport is neglected for the present quasi-steady burning process by assuming that the fuel pool is thin and its level is maintained constant by supplying fuel at a rate at which it is being consumed. Since the gas-phase combustion occurs following evaporation from the liquid fuel surface, appropriate interfacial boundary conditions are specified. The model is validated using experimental and numerical temperature profile data across a methanol flame for a particular co-flow configuration reported in literature. Thereafter, parametric investigations are carried out by varying the co-flow air velocity in the range of 0.05–0.75 m/s, keeping the fuel pool length as 20 mm. A detailed discussion of the effect of co-flow air velocity on the thermal and flow fields, as well as on the local and average mass burning rates, is presented. It is observed that as the co-flow air velocity is increased, the mass burning rate attains a minima at an intermediate velocity of around 0.4 m/s for the present configuration. The thermal and flow fields, variation of fuel vapor mass fraction normal to the interface and the diffusion of oxidizer into the flame zone are investigated at various co-flow velocities to explore and explain the flame characteristics.

Application of Flamelet-Generated Manifolds and Flamelet Analysis of Turbulent Combustion

Three-dimensional direct numerical simulations are performed of turbulent combustion ofinitially spherical flame kernels. The chemistry is described by a progress variable which isattached to a flamelet library. The merits of this reduction process is studied in detail toassess the validity of the method. In the turbulent cases the influence of flame stretch andcurvature on the local mass burning rate is studied and compared to an analytical model.It is found that there is a good agreement between the simulations and the model. Thenapproximations to the model are evaluated.

Direct Numerical Simulations of Premixed Turbulent Flames with Reduced Chemistry: Validation and Flamelet Analysis

Direct numerical simulation is a very powerful tool to evaluate the validity of new models and theories for turbulent combustion. In this paper, direct numerical simulations of spherically expanding premixed turbulent flames in the thin reaction zone regime and in the broken reaction zone regime are performed. The flamelet-generated manifold method is used in order to deal with detailed reaction kinetics. The computational results are analyzed by using an extended flame stretch theory. It is investigated whether this theory is able to describe the influence of flame stretch and curvature on the local burning velocity of the flame. It is found that if the full profiles of flame stretch and curvature through the flame front are included in the theory, the local mass burning rate is well predicted. The influence of several approximations, which are used in other existing theories, is studied. When flame stretch is assumed constant through the flame front or when curvature of the flame front is neglected, the theory fails to predict the local mass burning rate. The influence of using a reduced chemistry model is investigated by comparing flamelet simulations with reduced and detailed chemistry.

A flamelet analysis of the burning velocity of premixed turbulent expanding flames

Direct numerical simulation is a very powerful tool to evaluate the validity of new models and theories for turbulent combustion. In this paper, direct numerical simulations of spherically expanding premixed turbulent flames in the corrugated flamelet regime are performed. The flamelet-generated manifold method is used to deal with detailed reaction kinetics. The numerical method is validated for both laminar and turbulent expanding flames. The computational results are analyzed by using an extended flame stretch theory. It is investigated whether this theory is able to describe the influence of flame stretch and curvature on the local burning velocity of the flame. If the full profiles of flame stretch and curvature through the flame front are included in the theory, the local mass burning rate is predicted accurately. The influence of several approximations, which are used in other existing theories, is studied. When flame stretch is assumed to be constant through the flame front or when curvature of the flame front is neglected, the theory fails to predict the local mass burning rate.

A numerical study of confined triple flames using a flamelet-generated manifold

Laminar triple flames are investigated numerically using detailed and reduced chemical reaction mechanisms. Triple flames are believed to play an essential role in flame propagation in partially premixed systems. In order to reduce the computational cost of the simulations, the flamelet-generated manifold (FGM) method is used to simplify the chemistry model. The FGM method is based on a library of premixed laminar flamelets. Mixture fraction variations in partially premixed flames are taken into account by using the mixture fraction as an extra degree of freedom. A comparison of the results computed with FGM and with detailed chemistry shows that FGM predicts the structure of triple flames very accurately. The gradient of the mixture fraction in the unburnt mixture is varied, and its influence on the structure and propagation of triple flames is studied. For decreasing gradients, the curvature of the premixed flame branch decreases and the propagation velocity increases. Due to the diffusive nature of the flow and the use of symmetry boundary conditions in the lateral direction, high mixture fraction gradients could not be realized at the position of the flame. As a result, the trailing diffusion flame branch is only weakly present. The heat release in the premixed flame branches is two orders of magnitude higher than in the diffusion flame branch. The premixed and diffusion flame branches are studied using a flamelet analysis. Flame stretch and curvature appear to be significant in the premixed flame branch of a triple flame. These effects cause a decrease in the local mass burning rate of almost 15%. The structure of the diffusion flame branch appears to be similar to the structure of a counterflow diffusion flame with the same scalar dissipation rate.

A flamelet description of premixed laminar flames and the relation with flame stretch

A laminar flamelet description is derived for premixed laminar flames. The full set of 3D instationary combustion equations is decomposed in three parts: (1) a flow and mixing system without chemical reactions, described by the momentum, enthalpy, and element conservation equations, (2) the G-equation for the flame motion, and (3) a flamelet system describing the inner flame structure and the local mass burning rate. Local fields for the flame curvature and the flame stretch couple the flamelet system with the flow and flame motion. To derive an efficient model, the flamelet equations are analyzed in depth, using the Integral Analysis, first introduced by Chung and Law [1]. It appears that the flamelet response is governed by algebraic equations describing the influence of flame stretch on the local mass burning rate, the enthalpy variation, and element composition. Known expressions for the mass burning rate, found by Joulin, Clavin, and Williams are recovered in some special cases. Furthermore, the validity of the expressions has been shown for weak and strong stretch by comparing the results with numerical results of lean stretched premixed methane/air flames, computed with skeletal chemistry. Finally, the theory is illustrated for the tip of a 2D stationary Bunsen flame.

Experimental and numerical investigation of the structure of flames under engine conditions

The turbulent combustion of premixed gases is investigated under engine conditions. The chemiluminescence of the CH radical generated by a two-dimensional flame is registered quantitatively with high spatial resolution at different times. A telecentric optical integration evaluates volume elements of identical state and directly provides adequately averaged quantities. The spatial distribution of the local mass burning rate and of the local flame surface density is obtained from the emission measurements. This analysis is based on the flamelet assumption and uses the global mass burning rate calculated from the pressure record as a calibration basis. It was found that the maximum of the flame surface density hardly depends on the turbulence intensity and that the increase of turbulent flame speed with increasing turbulence is essentially achieved by a growth of the thickness of the flame brush. The turbulent flame speed does not reach a quasi-steady state during the phase of free propagation of the flame under our experimental conditions. The measured flame surface density is compared to the results of a numerical simulation using the transport equation for the flame surface density of the coherent flame model (CFM). This model was not able to reproduce the qualitative trends of the measurements. In particular, the calculated maximum of the flame surface density significantly increased with higher turbulence intensities. whereas the flame thickness remained constant. Modifications of the CFM are suggested, leading to a qualitative agreement with the experimental observation.

Mixed Convective Droplet Combustion with Internal Circulation

Abstract The combustion of a fuel droplet in a mixed convective environment once the initial transient heating of the droplet has been completed is analyzed considering the internal motion of the liquid. The coupled gas and liquid phase analyses are solved simultaneously by a series expansion approach in terms of the polar angle. A previously developed mixed convection formulation is used to provide solutions ranging from the forced to the free convective limit. Detailed results are presented for the forced convective combustion of a hexadecane droplet in a standard atmosphere. The pure evaporation of the hexadecane droplet is presented as a particular case of the analysis. The effect on the local mass burning and droplet regression rates of the liquid motion and mixed flow intensities is presented in terms of the parameters that represent the respective processes. It is shown that the internal circulation of the liquid has only a moderate influence in enhancing the evaporation rate of the droplet. A sing...

Theory of the mixed convective combustion of a spherical fuel particle

An analysis is developed for the mixed, forced and free, convective combustion of a spherical fuel particle. A mixed convection parameter (( 3Re 2 ) 4 + Gr 2) 1 8 is introduced in the nondimensionalization of the governing boundary layer equations which provides solutions as a function of a Froude number-related parameter φ = Gr ( 3Re 2 ) 2 that are uniformly valid from one convective limit to the other. Explicit expressions for the local mass burning rate and overall regression rate are obtained in terms of these two parameters and the thermophysical properties of the fuel and oxidizer. The predicted dependences on the Reynolds, Grashof, and mass transfer numbers agree well qualitatively with existing experimental data. It appears that a single explicit expression for the particle regression rate will suffice over the whole range of mixed flow intensity.