Heat Release Rate Distribution Research Articles

Numerical study of influence of surface reaction and heat-loss on flame intensity of methane-air flames

This paper reports a numerical study of influence of radical quenching and heat loss on bulk flame characteristics in narrow parallel channels. Flame-wall interaction is an important phenomenon on combustors. Especially, the wall effects on the flame characteristics in a small scale combustor become larger than those on normal scale one. The wall effects are caused by heat loss and surface reaction. The surface reaction on many common non-catalytic materials may weaken or quench the flames, although those for a catalytic wall can strengthen the flames. Authors have investigated the influence of the surface reaction and the heat loss on a noncatalytic wall using numerical simulation. In this study, a two-dimensional slit burner between two parallel plates with or without surface reaction is modelled. The wall temperature is 500 and 1200 K. The flame behavior and heat release rate distributions are examined when the distance between two plates is changed.

A new pattern of instability observed in an annular combustor: The slanted mode

In annular combustion chambers of aero-engines and gas turbines, acoustic coupling may arise from azimuthal modes which are less well damped than axial modes. Also, since the circumference is the largest length in the combustor, the azimuthal modes have the lowest resonance frequencies and are most prone to instability. Such a coupling raises many scientific issues which are considered in a small number of fundamental experiments. The present investigation focuses on this problem and provides experimental data on a special type of combustion instability in which the thermo-acoustic resonant coupling involves a combination of modes. This produces an unusual pattern of flame responses in which the distribution of heat release rate is slanted. Data are provided in the form of free radical light intensity patterns (interpreted as heat release rate distributions) and microphone signals detected in the plenum and chamber. It is shown that the slanted pattern is the signature of a combination of two modes with coinciding frequencies, the first being a standing azimuthal mode while the second is an axial mode. Measurements of the flame describing function on a single matrix burner at the fundamental frequency are used to explain the observed phase shift and amplitude in the flame responses of the different injectors in the annular combustor.

Analysis of Fire Spread and Fire Extinguishing Agent Distribution in Nacelle of Helicopter under No-ventilation Condition

Nacelle fire is seen as one of the most difficult to put out. To clarify the characteristics of fire spread in nacelle and determine the minimum usage of fire extinguishing agent, a numerical study is conducted to investigate fire activity in helicopter nacelle by analyzing the distribution of heat release rate, velocity vector field, temperature field and the distribution of fire extinguishing agent concentration with FDS (Fire Dynamics Simulator). Full scale experiment is implemented and three fire sources are set in the nacelle away from front fire wall of 1/4 position, 1/2 position and 3/4 position, respectively. Fire extinguishing system is designed by the results of fire spread. The distribution of fire extinguishing agent concentration is investigated from 6 s to 10 s. Ten scenarios are studied, which are established by changing mass flow rate or injection time. The results indicate that the fire spread in nacelle is affected by the position of inlet and fire source significantly under no ventilation condition. More attention should be paid on 1/4 position which locates at the opposite face of inlet. In order to improve the dependability of helicopter and reduce the waste of fire extinguishing agent, minimum usage of agent is obtained. Decreasing agent mass flow rate is propitious to improve reliability of fire extinguishing system when meets fire suppression requirement.

Determination of the Heat Release Distribution in Turbulent Flames by a Model Based Correction of OH* Chemiluminescence

Imaging of OH* or CH* chemiluminescence with intensified cameras is often employed for the determination of heat release in premixed flames. Proportionality is commonly assumed, but in the turbulent case this assumption is not justified. Substantial deviations from proportionality are observed, which are due to turbulence-chemistry interactions. In this study a model based correction method is presented to obtain a better approximation of the spatially resolved heat release rate of lean turbulent flames from OH* measurements. The correction method uses a statistical strain rate model to account for the turbulence influence. The strain rate model is evaluated with time-resolved velocity measurements of the turbulent flow. Additionally, one-dimensional simulations of strained counterflow flames are performed to consider the nonlinear effect of turbulence on chemiluminescence intensities. A detailed reaction mechanism, which includes all relevant chemiluminescence reactions and deactivation processes, is used. The result of the simulations is a lookup table of the ratio between heat release rate and OH* intensity with strain rate as parameter. This lookup table is linked with the statistical strain rate model to obtain a correction factor which accounts for the nonlinear relationships between OH* intensity, heat release rate, and strain rate. The factor is then used to correct measured OH* intensities to obtain the local heat release rate. The corrected intensities are compared to heat release distributions which are measured with an alternative method. For all investigated flames in the lean, partially premixed regime the corrected OH* intensities are in very good agreement with the heat release rate distributions of the flames.

Heat release rate correlation and combustion noise in premixed flames

The sound emission from open turbulent flames is dictated by the two-point spatial correlation of the rate of change of the fluctuating heat release rate. This correlation in premixed flames can be represented well using Gaussian-type functions and unstrained laminar flame thermal thickness can be used to scale the correlation length scale, which is about a quarter of the planar laminar flame thermal thickness. This correlation and its length scale are observed to be less influenced by the fuel type or stoichiometry or turbulence Reynolds and Damkohler numbers. The time scale for fluctuating heat release rate is deduced to be about τc/34 on an average, where τc is the planar laminar flame time scale, using direct numerical simulation (DNS) data. These results and the spatial distribution of mean reaction rate obtained from Reynolds-averaged Navier–Stokes (RANS) calculations of open turbulent premixed flames employing the standard model and an algebraic reaction rate closure, involving a recently developed scalar dissipation rate model, are used to obtain the far-field sound pressure level from open flames. The calculated values agree well with measured values for flames of different stoichiometry and fuel types, having a range of turbulence intensities and heat output. Detailed analyses of RANS results clearly suggest that the noise level from turbulent premixed flames having an extensive and uniform spatial distribution of heat release rate is low.

G601 渦糸ソリトンによる高速火炎伝播メカニズムの検証(GS6 反応を伴う流れ,一般セッション)

In order to verity the high-speed flame propagation mechanism by a vortex filament soliton, 3-D numerical simulation was carried out. In the 3-D visualization of vortex line and heat release rate distributions, the helical vortex line was seen near the flame top. Furthermore, in the 1-D analysis along the vortex line, the peak of curvature (the solitary wave like the vortex filament soliton) and the peak of heat release rate (the flame zone) overlapped and propagated together along the vortex line. This result also proved the validity of the original idea of high-speed flame propagation mechanism proposed by Shinoda et al. (1993).

05/01712 Utilizing Philippine Calatrava coal-diesel oilmixture (CDOM) as alternative fuel for industrial steam generator

The two-dimensional flow field in the vicinity of the interface between laminae of a solid oxidizer and solid fuel under steady-state conditions is solved for empirical inputs, taking into account the nonplanarity of the burning surfaces of the two laminae. The experimentally observed nonplanar burning surface under steady-state conditions is used to prescribe the upstream boundary conditions to the gas phase domain. The regression rate of the burning surface along the laminar interface, determined experimentally, is also used as an input. This decouples the condensed phase energy balance from the gas phase fluid dynamics, and assigns the distribution of surface temperature along the burning surface. The equation for two-dimensional energy conservation in the condensed phase is also solved, with the heat flux balance giving a boundary condition across the burning surface. A distribution of heat release rate is assumed in the gas phase, and the gas phase energy equation is solved iteratively with a convergence criterion set by the interfacial heat flux balance across the burning surface. This obviates the need for a knowledge of the chemical kinetic details of the gas phase flame. The goal is to determine a realistic distribution of heat release rate which satisfies the burning surface profile and its regression rate, measured for a laminar system of ammonium perchlorate and polymethyl methacrylate at pressures below the self-deflagration limit of the former. The results indicate lateral heat conduction in the condensed phase from the oxidizer lamina to the fuel lamina. The computed distribution of heat release rate in the gas phase indicates the need to include heat release near the surface, as well as farther downstream at typical stand-off distances for the flame.

Combustion Oscillation Analysis of Premixed Flames at Elevated Pressures

A new analytical time lag flame model based on Bloxidge’s flame model was introduced, which calculates the combustion oscillation of premixed flame to take into account the distribution of heat release rate and flame speed that was calculated by analytical formulas dependent on pressure, temperature, fuel-to-air ratio and velocity. The transfer matrix technique using the new flame model was applied to the calculation of acoustic resonance characteristics. To verify the model, combustion oscillation experiments were performed for methane-air premixed flames stabilized by a swirl burner at elevated pressures in a range of 0.6-0.9MPa. The fluctuating pressure had a maximum peak at a specific value of fτf, where f is the resonance frequency and τf is the passing time of premixed gas through the flame zone. The analytical model could simulate the dependency of the fluctuating pressure local peak on the fuel-to-air ratio and the static pressure.

Combustion at the interface of a laminate system of solid oxidizer and solid fuel

The two-dimensional flow field in the vicinity of the interface between laminae of a solid oxidizer and solid fuel under steady-state conditions is solved for empirical inputs, taking into account the nonplanarity of the burning surfaces of the two laminae. The experimentally observed nonplanar burning surface under steady-state conditions is used to prescribe the upstream boundary conditions to the gas phase domain. The regression rate of the burning surface along the laminar interface, determined experimentally, is also used as an input. This decouples the condensed phase energy balance from the gas phase fluid dynamics, and assigns the distribution of surface temperature along the burning surface. The equation for two-dimensional energy conservation in the condensed phase is also solved, with the heat flux balance giving a boundary condition across the burning surface. A distribution of heat release rate is assumed in the gas phase, and the gas phase energy equation is solved iteratively with a convergence criterion set by the interfacial heat flux balance across the burning surface. This obviates the need for a knowledge of the chemical kinetic details of the gas phase flame. The goal is to determine a realistic distribution of heat release rate which satisfies the burning surface profile and its regression rate, measured for a laminar system of ammonium perchlorate and polymethyl methacrylate at pressures below the self-deflagration limit of the former. The results indicate lateral heat conduction in the condensed phase from the oxidizer lamina to the fuel lamina. The computed distribution of heat release rate in the gas phase indicates the need to include heat release near the surface, as well as farther downstream at typical stand-off distances for the flame.

97/04038 Effects of pressure on fuel-rich combustion of methane-air under high pressure

The techniques developed for detailed flame structure studies have been used to analyze the stabilizing region of a fuel-rich methane-air flame, with and without vitiation of the secondary combustion air, with a view to resolving various uncertainties as to the stabilizing mechanism of burner-stabilized flames. Detailed temperature, velocity, and composition measurements are given, covering a two-dimensional stabilizing region, also given are heat release rate distributions and species fluxes derived from these measurements. The flame structure analysis shows that the maximum heat release rate in the stabilizing region of these fuel-rich flames is much less than for the stoichiometric methane/air flame considered in a previous paper. This is not in accord with the classical view that fuel-rich flames are, in effect, stabilized by a stoichiometric mixture formed by mixing with the ambient atmosphere. Further, the results indicate that the greater stability of a fuel-rich flame, compared with a stoichiometric flame, is due to a heat interaction between the primary and secondary combustion zones in the stabilizing region of a fuel-rich flame. The decrease in the magnitude of this interaction, observed in the present work when the secondary combustion air was vitiated, is put forward as the reason for the large reductions in stability of fuel-rich flames that can be brought about by secondary vitiation.

Intrinsic combustion instability of solid energetic materials

In the first part, a review is offered of the intrinsic stability of pressure- or radiation-driven burning of solid propellants with chemically inert condensed phase. The two main approaches today available [flame modeling and the Zeldovich-Novozhilov (ZN) method] are both considered. Within the current quasi-steady homogeneous one-dimensional framework, the intrinsic burning stability boundaries and frequency of the self-sustained oscillatory solution predicted by several techniques are the same; the classical boundaries predicted by the ZN are somewhat extended. The intrinsic stability boundaries for pressure-driven or radiation-driven burning are identical in the limit of fully opaque condensed phase (surface absorption). However, the burning configurations are affected by the impinging radiant flux. Some peculiar effects of radiation assisted burning are illustrated. In the second part of this article, an extension is presented to chemical reactions volumetrically distributed in the condensed phase with a uniform heat release rate distribution. Frequency response functions and unbounded response limits are obtained; both recover the particular configuration of chemically inert condensed phase. Nomenclature A = nondimensional function, Eq. (31); Denison and Baum nondimensional parameter, Eq. (53); Krier-T'ien-Sirignano-Summerfield transient flame mode nondimensional parameter, Eq. (54) As = nondimensional function, Eq. (6) a = nondimensional function defined by Eq. (13) ax = average volumetric absorption coefficient, cm1

Fuel Mixing Effects on Propagation of Premixed Flames. I. Hydrogen + Carbon Monoxide Flames

Abstract The combustion mechanisms in premixed flames having a mixture of hydrogen and carbon monoxide as the fuel and the stoichiometric amount of air as the oxidizer were investigated by numerical simulation to elucidate the fuel-mixing effects on the burning velocity. Hydrogen and carbon monoxide have similar physical properties and their stoichiometric flames with air are known to propagate through the same type of mechanism, governed by active species diffusion. The flames of the mixed fuel hydrogen+carbon monoxide were found here to have their heat-release rate distributions changing with conformity in shape when the fuel composition is changed. The burning velocity was found to vary almost linearly with the hydrogen content in the fuel. Furthermore, the flux of chemical energy which is carried into low-temperature regions by hydrogen atoms diffusing from the flame front was also found to be roughly proportional to the burning velocity.

On modeling of pulse combustors

Experimental investigations of reignition and combustion in a pulse combustor were performed to establish a realistic physical model useful in numerical simulation of these processes. The model takes into consideration all phenomena observed during a working cycle, including weak reignition by recirculation of combustion products, low temperature burning at the boundary layer of entering air, initiation of explosive combustion and fast flame spreading through the combustor chamber. Results of these experimental investigations were applied to determine some simplifying assumptions for simulation of the combustion process in a mathematical model. One-dimensional flow through the pulse combustor was assumed and two methods of simulation of the combustion process were examined. The first method, based on a plug flow reactor with a space-time distribution of heat release rate, was unable to take into consideration mixing processes affecting temperature variations in the combustor. The second method, applying a set of two well-stirred reactors, was the one that effectively imitated the operation of actual pulse combustors.

A flame structure study of the stabilizing region of a fuel-rich flame and the effects of vitiation

The techniques developed for detailed flame structure studies have been used to analyze the stabilizing region of a fuel-rich methane-air flame, with and without vitiation of the secondary combustion air, with a view to resolving various uncertainties as to the stabilizing mechanism of burner-stabilized flames. Detailed temperature, velocity, and composition measurements are given, covering a two-dimensional stabilizing region, also given are heat release rate distributions and species fluxes derived from these measurements. The flame structure analysis shows that the maximum heat release rate in the stabilizing region of these fuel-rich flames is much less than for the stoichiometric methane/air flame considered in a previous paper. This is not in accord with the classical view that fuel-rich flames are, in effect, stabilized by a stoichiometric mixture formed by mixing with the ambient atmosphere. Further, the results indicate that the greater stability of a fuel-rich flame, compared with a stoichiometric flame, is due to a heat interaction between the primary and secondary combustion zones in the stabilizing region of a fuel-rich flame. The decrease in the magnitude of this interaction, observed in the present work when the secondary combustion air was vitiated, is put forward as the reason for the large reductions in stability of fuel-rich flames that can be brought about by secondary vitiation.

A flame structure study of the stabilizing region of a near-stoichiometric laminar burner flame

The techniques that have been developed for detailed flame-structure studies have been used to analyse the stabilising region of a near stoichiometric methane-air flame with a view to resolving various uncertainties as to the stabilising mechanism of burner-stabilised flames. Detailed temperature, velocity, and composition measurements are given, covering a two-dimensional stabilising region, as are heat-release rate distributions and species fluxes derived from these measurements. The flame-structure analysis shows that exothermic reactions continue at an appreciable rate in the so-called dead space underlying the base of the flame. Also, it was quite clear from the results that mixture from the primary stream flows out through the dead space or dark space and there is no intermixing of ambient air through the dark space. This is contrary to the recent suggestions of Edmondson and Heap and to the original concept which formed the basis of Lewis and von Elbe's blow-off theory. The lack of mixing with ambient air is strong evidence in support of the flame-stretch theory of blow off.