Heat Release Zone Research Articles

Numerical simulation of turbulent propane–air combustion with nonhomogeneous reactants

High-resolution two-dimensional numerical simulations have been performed for premixed turbulent propane–air flames propagating into regions of nonhomogeneous reactant stoichiometry. Simulations include complex chemical kinetics, realistic molecular transport, and fully resolved hydrodynamics (no turbulence model). Aerothermochemical conditions (pressure, temperature, stoichiometry, and turbulence velocity scale) approach those in an automotive gasoline direct-injection (GDI) engine at a low-speed, part-load operating condition. Salient findings are as follows: (1) There is no leakage of the primary fuel (propane) behind an initial thin premixed heat-release zone. This “primary premixed flame” can be described using a monotonic progress variable and laminar premixed flamelet concepts. (2) For the conditions simulated, differences in global heat release and flame area (length) between homogeneous and nonhomogeneous reactants having the same overall stoichiometry are small. (3) Beyond three-to-four flame thicknesses behind the primary flame, practically all hydrocarbon fuel has broken down into CO and H 2. (4) The rate of heat release in the “secondary reaction zone” behind the primary premixed flame is governed by turbulent mixing and the kinetics of CO 2 production. Mixture-fraction-conditioned secondary heat release, CO, and CO 2 production rates are qualitatively similar to results from a first-order conditional moment closure (CMC) model; CMC gives poor results for H 2, H 2O, and radical species. Description of the secondary heat release using steady laminar diffusion flamelet concepts is problematic. (5) Of the chemical species considered, HCO mass fraction or the product of CH 2O and OH mass fractions correlates best with local heat-release rate [1]. (6) Computational considerations demand modifications to chemical mechanisms involving C 3H 7 and CH 3CO. Specific changes are proposed to strike a satisfactory balance between accuracy and computational efficiency over a broad range of reactant stoichiometry.

Experimental and modeling studies of two-dimensional ammonium perchlorate diffusion flames

We investigated two-dimensional axisymmetric diffusion flames in which the surface decomposition products of ammonium perchlorate (AP) combustion provided the coflow for a methane fuel stream. The two-dimensional problem was solved using the fully elliptic formulation of the governing equations. By utilizing recent developments in hydrocarbon, chlorine, NO x , and AP kinetics, we formulated a detailed transport, finite rate chemistry system for the temperature, velocities, vorticity, and species mass fractions of the combined flame system. We compared the results of this model with a series of experimental measurements in which the temperature was measured with OH rotational population distribution, and the transient species OH, CN, and NH were measured with planar laser-induced fluorescence and emission spectroscopy. The kinetic mechanism, previously validated in one-dimensional experiments, was found to give good results in the two-dimensional model when compared with the experiments. The two-dimensional model can be used to understand the effects of solid propellant heterogeneity on combustion zone microstructure and the location and structure of heat release zones that control the propellant ballistic properties. It is a first step in developing computer codes that can a priori predict propellant ballistic behavior via modeling, thereby partially supplanting expensive physical formulation and testing.

An experimental estimation of mean reaction rate and flame structure during combustion instability in a lean premixed gas turbine combustor

The interactions between the local flame structure produced by a premixed swirl-stabilized injector with combustion instabilities were experimentally studied for a model gas turbine combustor operating at high pressure and temperature. The model gas turbine combustor studied utilizes a sudden-expansion dump combustor with a single swirler and bluff body for enhancing mixing rate and flame stabilization, respectively. Laser-based measurements were made for both stable and unstable operating conditions. The local flame front structure was visualized using planar laser-induced fluorescence (PLIF) from the OH⊙, and the global heat release zone was interpreted from flame emission measurements. For stable combustion conditions, the mean reaction rate estimated independently from both OH-PLIF and OH * chemiluminescence measurements showed good agreement, thereby indicating confidence in the use of OH-PLIF measurements for extracting the local mean reaction rate. For unstable combustion conditions, the flamefront characteristics, including flame surface density and mean reaction rate, were evaluated together with the information from the OH * chemiluminescence measurements to identify the boundary of the heat release region at discrete phases of the unstable flame. Analysis of the flame structures during combustion instability indicated significant variations during different phases of the instability. The heat release flow field, particularly in the recirculation regions appearing at the corner and inner face of the dump plane, varied substantially. Rayleigh index information indicated that the recirculation zones play an important part in driving the instability. In contrast, the high shear layer formed along the interface between reactants and hot products produced a region where the instability was damped due to a lowering of the heat release.

99/00604 Reburning using biomass for NO x control

This study examines the use of wood reburning for NOx reduction in a cyclone-fired boiler. Reburning is an in-furnace NOx control technology that uses fuel injected above the main firing system to reduce NO formed in the main heat release zone. Bench- and pilot-scale work done over the past several years suggests that NOx reductions up to sixty percent are possible using reburning under the right conditions. The practical implementation of reburning technology depends upon the ability to contact the reburning fuel with NO formed in the cyclone barrel at the optimum stoichiometry for he maximum time. Thus, reburning effectiveness in practical systems is determined by the performance of he mixing system. This paper describe the results of a computer modeling study of the TVA Allen Station Unit No. 2 to evaluate various reburning fuel injection and overfire air injection designs and estimate he NOx reduction potential using wood as a reburning fuel. Results of this study showed the best mixing was obtained using opposed fired reburning injectors located as close to the cyclone barrels as possible with opposed-wall overfire air injectors at as high a velocity as pressure drop constraints allow. Under these conditions, NOx reductions of ∼ 45% are expected. When the wood carrier air is replaced by fuel gas, the expected NOx reduction increases to ∼ 55%. Further NOx reduction was possible if the stoichiometric constraints in the reburning zone could be relaxed to allow deeper staging.

Reburning using biomass for NO x control

This study examines the use of wood reburning for NOx reduction in a cyclone-fired boiler. Reburning is an in-furnace NOx control technology that uses fuel injected above the main firing system to reduce NO formed in the main heat release zone. Bench- and pilot-scale work done over the past several years suggests that NOx reductions up to sixty percent are possible using reburning under the right conditions. The practical implementation of reburning technology depends upon the ability to contact the reburning fuel with NO formed in the cyclone barrel at the optimum stoichiometry for he maximum time. Thus, reburning effectiveness in practical systems is determined by the performance of he mixing system. This paper describe the results of a computer modeling study of the TVA Allen Station Unit No. 2 to evaluate various reburning fuel injection and overfire air injection designs and estimate he NOx reduction potential using wood as a reburning fuel. Results of this study showed the best mixing was obtained using opposed fired reburning injectors located as close to the cyclone barrels as possible with opposed-wall overfire air injectors at as high a velocity as pressure drop constraints allow. Under these conditions, NOx reductions of ∼45% are expected. When the wood carrier air is replaced by fuel gas, the expected NOx reduction increases to ∼55%. Further NOx reduction was possible if the stoichiometric constraints in the reburning zone could be relaxed to allow deeper staging.

Towards the development of a universal fire emulator-detector evaluator

Past measurements are examined of CO, CO2, H2O, H2, O2, smoke and temperature produced in standard fires of the UL and EN type. Additional measurements just above the heat release zone are suggested to obtain a more complete footprint of each standard fire, and the concept of a universal fire emulator-detector evaluator (FE-DE) is introduced. The objective of the emulator is to produce more well-controlled environments that eliminate the unavoidable run-to-run variations associated with full-scale tests. Numerical fluid dynamic computations are recommended to insert the fire source into the space being protected as a guide for detector placement and to predict system per formance under realistic conditions.

Structural response of counterflow diffusion flames to strain rate variations

The structural response of counterflowing methane/oxygen/nitrogen diffusion flames to aerodynamic straining was experimentally and computationally investigated. The temperature and major species concentration profiles were experimentally determined as functions of the applied strain rate by using spontaneous Raman scattering. The experimental situations were further computationally simulated with detailed reaction mechanisms and transport properties. The computed results were found to be in close quantitative agreement with the experimental data. Results demonstrate that, in contrast to counterflow premixed flames, a strained, counterflow diffusion flame has less flexibility to freely adjust its location in response to strain rate variations such that its structure in the direction normal to the flame surface is quite sensitive to variations in the strain rate. Specifically, the counterflow diffusion flame becomes thinner with increasing strain rate a, with its thickness varying inversely with √ a. This leads to increased amount of reactant leakage, progressive reduction in the flame temperature, and eventually extinction of the flame. Computational results further show that while the heat release rate of premixed flames is characterized by a single sharp maximum, for diffusion flames a secondary maximum described by a distinctively different reaction submechanism exists on the oxidizer side of the primary heat release zone. The nature and extent of the negative heat release rate on the fuel side of the flame, as well as its relation to the presence of a dent around the same location on the temperature profile as observed in previous experiments, were also discussed.

Thermoacoustic Analysis of Combustion-Driven Oscillation.

Thermoacoustic analysis of heat-driven oscillation with steady flow is performed. Comparing the results with Katto's experiment on the Rijke tube and our experiment on combustion-driven oscillation with multispud gas burner which is previously reported, the following results are obtained : (1) In acoustic-type combustion-driven oscillation, one of the generating sources of oscillation is the fluid vibration on temperature boundary layer which promotes the heat transmission through unsteady conduction perpendicular to the temperature gradient. (2) The effects of steady flow velocity on heat driven-oscillation may be summarized by the volume flow rate in the normal state. (3) There are many steady flow regions exciting the oscillation when the path width of an air jet at the heat release zone is large. (4) The effects of heat source position on the oscillation are not so great. (5) The narrower the path width of steady flow and the higher the heating temperature, the smaller the oscillating flow region.

Direct numerical simulation of H2/O2/N2 flames with complex chemistry in two-dimensional turbulent flows

Premixed H2/O2/N2 flames propagating in two-dimensional turbulence have been studied using direct numerical simulations (DNS: simulations in which all fluid and thermochemical scales are fully resolved). Simulations include realistic chemical kinetics and molecular transport over a range of equivalence ratios Φ (Φ = 0.35, 0.5, 0.7, 1.0, 1.3). The validity of the flamelet assumption for premixed turbulent flames is checked by comparing DNS data and results obtained for steady strained premixed flames with the same chemistry (flamelet ‘library’). This comparison shows that flamelet libraries overestimate the influence of stretch on flame structure. Results are also compared with earlier zero-chemistry (flame sheet) and one-step chemistry simulations. Consistent with the simpler models, the turbulent flame with realistic chemistry aligns preferentially with extensive strain rates in the tangent plane and flame curvature probability density functions are close to symmetric with near-zero means. For very lean flames it is also found that the local flame structure correlates with curvature as predicted by DNS based on simple chemistry. However, for richer flames, by contrast to simple-chemistry results with non-unity Lewis numbers (ratio of thermal to species diffusivity), local flame structure does not correlate with curvature but rather with tangential strain rate. Turbulent straining results in substantial thinning of the flame relative to the steady unstrained laminar case. Heat-release and H2O2 contours remain thin and connected (‘flamelet-like’) while species including H-atom and OH are more diffuse. Peak OH concentration occurs well behind the peak heat-release zone when the flame temperature is high (of the order of 2800 K). For cooler and leaner flames (about 1600 K and for an equivalence ratio below 0.5) the OH radical is concentrated near the reaction zone and the maximum OH level provides an estimate of the local flamelet speed as assumed by Becker et al. (1990).

Combustion-Driven Oscillation in a Furnace with Multispud-Type Gas Burners. 4th Report. The Effects of Position of Secondary Air Guide Sleeve and Openness of Secondary Air Guide Vane on Combustion Oscillation Condition.

In the preceeding papers, "the resonance factor", which predicts the degree of combustion oscillation, was proposed, and applied to both the furnace with a single burner and that with multiple burners. It was found that the combustion oscillation occurred in the region of low primary and secondary air flow rates in both furnaces and that the region of combustion oscillation was affected by the openness of secondary air vane and the burner arrangement in a furnace under a fixed flame holder condition. If the operational conditions of the practical burners are not within this region, the furnace will not oscillate at all. This study's objective is to clarify the effects of the position of the secondary air guide sleeve and the openness of the secondary air guide vane on the combustion oscillation region in a multiburner furnace, since the combustion oscillation condition varies with position and strength of the heat release zone is a furnace and thus is governed by the configurations of flame holder which is composed of an impeller, air guide sleeves and air guide vanes. Futhermore, a procedure of burner operation while avoiding combustion oscillation is demonstrated.

Structure of a nonequilibrium high-frequency discharge

The problem for a one-temperature plasma is considered with allowance for the finite ionization rate. It is shown that in the developed nonequilibrium case the leading effect of ambipolar electron diffusion and ionization lag lead to a qualitatively different picture as compared with the equilibrium case. In particular, the heat release zone covers only a relatively small leading part of the ionization zone with a relatively low (ne ≪ nef) electron concentration.

Density fluctuations of flames in grid-induced turbulence

Density fluctuations have been measured using Raleigh scattering in the heat release zone of a rod-stabilized C 2H 4/air flame propagating into a grid-induced turbulent flow field. Probability density functions show that intermediate states between burned products and cold reactants are significant in determining density statistics. A comparison of the statistics for fluctuation intensity with results predicted by the model of Bray, Moss, and Libby, in which intermediate states are neglected, further confirms this point. However, the skewness is accurately predicted by the Bray-Moss-Libby model. Since intermediate states are approximately symmetrically distributed, they do not contribute significantly to the skewness.

Erosive Burning Study of Composite Solid Propellants by Turbulent Boundary-Layer Approach

Erosive burning of composite solid propellants is investigated by analyzing a steady, two-dimensional, chemically reacting, turbulent boundary layer over a propellant surface. Predicted erosive burning rates agree closely with experimental data. The erosive burning rate augmentation is found to be caused by the increase in heat feedback introduced by the increase in transport coefficients, and the turbulence enhanced mixing and reaction of the oxidizer and fuel gases. The increase in freestream gas velocity brings the location of the peak turbulence intensity and the heat release zone closer to the propellant surface, thereby increasing the burning rate of a propellant.

Kinetic mechanism, structure and properties of premixed flames in hydrogen—oxygen—nitrogen mixtures

Kinetic mechanism, structure and properties of premixed flames in hydrogen—oxygen—nitrogen mixtures

Langmuir probe measurements of ion density in an atmospheric-pressure air-propane flame

A measurement of the ion density in the flame front of an atmospheric-pressure, premixed air-propane flame is presented. It is based on the sheath-convection model of current collection by a Langmuir probe. In contrast to the results of previous authors, ion densities as high as 4 × 10 17 ions/m 3 have been measured in a stoichiometric flame. The half width of the density profile appears to be the same width as the heat-release zone reported by previous authors. This result implies that the probe is capable of spatially resolving the density profile. Observations of the change in the ion-density profile of the flame with respect to the air-fuel mixture ratio and the effect of seeding the flame with KOH are also presented.

The prediction of nitric oxide formation in a direct injection diesel engine

The development of a modular model capable of predicting heat release and nitric oxide formation in a direct injection compression ignition engine is described. The model consists of separate modules which describe cylinder thermodynamics, heat transfer, fuel spray dispersion, ignition delay, heat release and pollutant production. The model includes several empirical constants, and the sensitivity to the calculated exhaust NO concentration to the value of these constants is discussed. Model predictions have been compared with experimental data generated in a single cylinder diesel engine for several operational and engine parameters (load, injection timing, engine speed, air swirl). Based upon comparison with single cylinder test data, it is concluded that the overall predictive capability of the model with respect to pressure history and exhaust NO concentration is good. Experience gained during the development of the model indicates that calculated pollutant levels are most sensitive to fuel/air mixing parameters and nonpremixed heat release zones cannot be modeled adequately by single droplet diffusion flames.

Strong explosion in a combustible gas mixture

Assume that a planar, cylindrical, or spherical point explosion takes place in a combustible mixture of gases. As a result of the explosion a strong shock wave develops and triggers chemical reactions with the release of heat. The solution of the problem for the case in which the thickness of the heat release zone is neglected (the infinitely thin detonation wave model) was obtained in [1–3].

Surface temperature of burning nitroglycerine powder

1. It has been established that the temperature at the end of the first heat release zone for N powder burning in air is practically independent of the initial temperature, is equal to about 275°C, and is evidently the boiling point of a mixture of nitroglycerine, and dinitrotoluene in the presence of nitrocellulose and semi-decomposition products. 2. Calculations based on the assumption that the temperature at the surface of the powder is equat to the boiling point satisfactorily describe the relation between the burning rate of N powder and initial temperature at T0<+20°–40°C. 3. The results obtained point to the important role of the evaporation of volatile components in the combustion of nitroglycerine powders. 4. An explanation of the sharp change in the course of the “burning rate-initial temperature” curve for nitroglycerine powder is proposed.

Structure of flame quenched by cold surfaces

The region of interaction of a cooled metallic heat sink with a premixed lean flat ethylene/air flame is investigated. The detailed distribution of burning velocity is measured, by means of a refinement of the particle track method. Two dimensional distributions of temperature and composition in this region are obtained from thermocouples and microprobe sampling, followed by gas-chromatographic analysis. The burning velocity-heat loss relation is found to depend on the orientation of the flame front to the heat sink. Heat release rates deduced by substituting temperature and flow data into conservation equations reveal some unexpec­ted features close to the heat sink; in particular the zone of heat release becomes narrower and the maximum rate of heat release does not decrease appreciably towards the heat sink, in spite of a considerable reduction in temperature. These features are interpreted in terms of the composition pattern, which suggests that the narrowing is due to quenching of the second stage of reaction, while the reaction rate effect is caused by diffusion of hydrogen atoms to the surface, which acts as a sink for them. It is shown that a reaction such as H + O 2 → Surface HO 2 would account for all the features observed; conclusions are drawn regarding the status of quenching theory and the role of flame-surface interactions in flame structure studies.

The structure of flat, counter-flow diffusion flames

It is shown that the establishment of a large flat diffusion flame in the counter-flow régime of opposed jets of two gaseous reactants could very considerably extend the range of applicability of flame-kinetics studies by structure analysis. Suitable flames are stabilized and their characteristics and behaviour described. The flow patterns, spectrum , refractive index fields, temperature distributions and gas composition at a few selected points are studied for ethylene flames by methods including the use of thermocouples, sodium line reversal, illuminated particle tracks, interferometry and gas chromatography. The aerodynamic and thermal structures are analyzed to yield the distribution of the rate of heat release per unit volume. The following are among the conclusions: two stagnation points and two planes which particles cannot cross occur when the centres of the reaction and aerodynamic systems are made to coincide—this can be prevented by displacing the plane of stoichiometry from that of aerodynamic symmetry; the radial component of flow velocity is proportional to radial distance in the central parts. Isotherms are parallel to each other and to the flame, away from the edges, but maximum temperatures occur at the periphery, at least for some flames. The heat release profile shows regions where small amounts of heat are absorbed; C 2 H 6 and H 2 appear on the oxidant side of the flame. The most significant observation is that the zone of heat release is about ten times wider than would be expected of the equivalent pre-mixed flame and this makes the method applicable to the study of faster flame reactions.