Simple Jet Flame Research Articles

Relationships between Soot and the Local Instantaneous Strain Rate in Turbulent Non-Premixed Bluff-Body Flames

In this paper, the simultaneous measurements of the local velocity using polarized particle image velocimetry (P-PIV) and soot volume fraction, SVF, using planar laser-induced incandescence (LII) are reported for turbulent non-premixed ethylene–nitrogen bluff-body flames. The measured data were used to assess the correlation between soot volume fraction and the shear-strain rate (S), both mean and instantaneous, in different regions of the bluff-body flames. In the recirculation zone, an exponential function was found to reasonably describe the relationship between total SVRZ and S. It was also observed that the strain rate associated with the maximum SVF in the recirculation zone and the jet region are ≲1000 s–1, consistent with the soot inception region of simple jet flames. The joint PDFs show that the SVF is not well correlated with the local instantaneous strain rate in either the recirculation zone or the jet region of these flames. Qualitatively, the correlation of coefficient, R2, between the local instantaneous SVF and inverse of the strain rate (1/S) reveals a low to weak correlation, 0.3 < R2 < 0.6, in the recirculation zone and the jet region, which is consistent with the joint PDFs. This suggests that the timescales for SVF are significantly greater than those driving local fluctuations. In addition, a high-fidelity experimental dataset has been provided for model development and validations.

Computational Fluid Dynamics Study of a Nonpremixed Turbulent Flame Using openfoam: Effect of Chemical Mechanisms and Turbulence Models

Abstract This article presents a study of the influence of chemical mechanisms and turbulence models on Reynolds-averaged Navier–Stokes (RANS) simulations of the CH4/H2/N2-air turbulent diffusion flame, i.e., the so-called DLR-A flame. The first part of this study is focused on the assessment of the influence of four chemical models on predicted profiles of the DLR-A flame. The chemical mechanisms considered are as follows: (i) a C2 compact skeletal mechanism, which is derived from the GRI3.0 mechanism using an improved multistage reduction method, (ii) a C1 skeletal mechanism containing 41 elementary reactions amongst 16 species, (iii) the global mechanism by Jones and Lindstedt, (iv) and a global scheme consisting of the overall reactions of methane and dihydrogen. RANS numerical results (e.g., velocities, temperature, species, or the heat production rate profiles) obtained running the reactingFOAM solver with the four chemical mechanisms as well as the standard k − ɛ model, the partially stirred reactor (PaSR) combustion model, and the P − 1 radiation model indicate that the C2 skeletal mechanism yields the best agreement with measurements. In the second part of this study, four turbulence models, namely, the standard k − ɛ model, the renormalization group (RNG) k − ɛ model, realizable k − ɛ model, and the k − ω shear stress transport (SST) model, are considered to evaluate their effects on the DLR-A flame simulation results obtained with the C2 skeletal mechanism. Results reveal that the predictions obtained with the standard k − ɛ and the RNG k − ɛ models are in very good agreement with the experimental data. Hence, for simple jet flame with moderately high Reynolds number such as the DLR-A flame, the standard k-epsilon can model the turbulence with a very good accuracy.

Hierarchical model form uncertainty quantification for turbulent combustion modeling

All models invoke assumptions that result in model errors. The objective of model form uncertainty quantification is to translate these assumptions into mathematical statements of uncertainty. If each assumption in a model can be isolated, then the uncertainties associated with each assumption can be independently assessed. In situations where a series of assumptions leads to a hierarchy of models with nested assumptions, physical principles from a higher-fidelity model can be used to directly estimate a physics-based uncertainty in a lower-fidelity model. Turbulent nonpremixed combustion models fall into a natural hierarchy from the full governing equations to Conditional Moment Closure to flamelet-like models to thermodynamic equilibrium, and, at each stage of the hierarchy, a single assumption can be isolated. In this work, estimates are developed for the uncertainties associated with each assumption in the hierarchy. The general method identifies a trigger parameter that can be obtained with information only from the lower-fidelity model that is used to estimate the error in the lower-fidelity model using only physical principles from the higher-fidelity model. Starting from the lowest fidelity model, the trigger parameters identified in this work are the product of a chemical time scale and the scalar dissipation rate for the equilibrium chemistry model, which characterizes the errors associated with neglecting finite-rate chemistry and transport; the reciprocal of the product of a generalized Lagrangian flow time and the scalar dissipation rate for the steady flamelet model, which characterizes the errors associated with neglecting flow history effects; and the relative magnitude of the conditional fluctuations for Conditional Moment Closure. The approach is applied in LES to a turbulent nonpremixed simple jet flame to quantify the errors associated with the equilibrium chemistry model and the steady flamelet model. The results indicate that errors associated with neglecting finite-rate chemistry and transport in the equilibrium chemistry model are dominant upstream while errors associated with neglecting flow history in the steady flamelet model become more important downstream.

Effects of hydrogen and nitrogen on soot volume fraction, primary particle diameter and temperature in laminar ethylene/air diffusion flames

This study reports the effects of hydrogen (H2) and nitrogen (N2) blended into the fuel on soot evolution and flame temperature in axisymmetric ethylene/air diffusion flames at atmospheric pressure. Two series of laminar ethylene (C2H4) diffusion flames blended with H2 and N2 were experimentally investigated, in which soot volume fraction (fv), primary particle diameter (dp) and flame temperature (T) were measured using planar laser-based techniques. These twelve flames are grouped into two sets. In the first one, H2 or N2 are added into a constant volumetric flow of C2H4 to separate their complementary effects. In the second set, the total volumetric flow rate of the mixture of C2H4/H2/N2 was kept constant for the same ratios of dilution to isolate the influence of exit velocity. For both sets, a reference flame corresponding to a mixture of C2H4/H2/N2 =40%/40%/20%, by volume, was included to match the fuel composition of a turbulent ‘target’ sooting flame (termed the “Adelaide simple jet flame” in the International Sooting Flames Workshop). Planar laser-induced incandescence (LII), time-resolved LII (TiRe-LII) and two-line atomic fluorescence (TLAF) thermometry (using atomic indium as tracer) were applied to measure fv, dp and T. Radial profiles of the flame temperature were also measured using a thermocouple positioned 3mm above the burner lip, which is just upstream from the sooting region. It is found that dilution either with H2 or N2 causes a significant reduction in fv and dp, while the influence on the peak flame temperature is weaker. Flame structure (length and spatial profiles) is also influenced by the blending with H2 and N2. This database can serve as benchmark for model development due to its systematic nature and combination of measured scalars.

Analysis of flame shapes in turbulent hydrogen jet flames with coaxial air

This paper addresses the characteristics of flame shapes and flame length in three types of coaxial air flames realizable by varying coaxial air and/or fuel velocity. Forcing coaxial air into turbulent jet flames induces substantial changes in flame shapes and NOx emissions through the complex flow interferences that exist within the mixing region. Mixing enhancement driven by coaxial air results in flame volume decrease, and such a diminished flame volume finally reduces NOx emissions significantly by decreasing NOx formation zone where a fuel/air mixture burns. It is found that mixing in the vicinity of high temperature zone mainly results from the increase of diffusive flux than the convective flux, and that the increase of mass diffusion is amplified as coaxial air is increased. Besides, it is reaffirmed that non-equilibrium chemistry including HO2/H2O2 should be taken into account for NOx prediction and scaling analysis by comparing turbulent combustion models. In addition, it is found that coaxial air can break down the self-similarity law of flames by changing mixing mechanism, and that EINOx scaling parameters based on the self-similarity law of simple jet flames may not be eligible in coaxial air flames.

Flame Blowout Limits of Landfill Gas Mixed Fuels in a Swirling Nonpremixed Combustor

Recently, landfill gas (LFG) has attracted considerable interest as a source of regenerative energy for the production of heat and power. In the present study, the characteristics of flame blowout limits for landfill gas−liquified propane gas (LFG−LPG) mixed fuels in a swirling nonpremixed combustor were investigated. Validations of existing blowout models were conducted, and global parameters were suggested to explain the characteristics of the blowout limit for LFG−LPG mixed fuels in swirling nonpremixed flames. It was found that the flame stability of LFG−LPG mixed fuels was lower than that of CH4 under weak swirl conditions. Under strong swirl conditions, however, LFG−LPG mixed fuels had a similar or larger stable flame region with an increase of the air flow rate compared to those of CH4. Blowout velocities estimated by the Kalghatgi model as well as the Dahm−Mayman model, in simple jet flames and nonswirling flames with a coaxial air flow, respectively, were in good agreement with measurements from ...

Structure and stabilization mechanism of a microjet methane diffusion flame near extinction

The flame structure and stabilization mechanism of a microjet methane diffusion flame near extinction are numerically investigated using multi-component transport model coupled with GRI-Mech 3.0 chemical kinetic mechanisms. The small flame size and intensive heat loss to the burner wall suggest that the microjet flame may always operate in a severe condition near extinction with distinct stabilization feature. Of particular interest is the flame structure in the standoff region, as it would directly relate to the mixing, diffusion, and chemical kinetic processes as well as flame stabilization. Computed results show that near extinction the fuel burns in a diffusion flame, in contradiction to a simple jet flame prediction. Neither a double flame nor a triple flame is observed in the computed structures, suggesting that the standoff flame is stabilized by the hot zone that connects to the reaction kernel, through the formation of HO 2 layer and subsequent key radical reactions.

A Study of the Influence of Oxygen Index on Soot, Radiation, and Emission Characteristics of Turbulent Jet Flames

Many combustion applications benefit from the use of oxygen-enriched air or pure oxygen as an oxidizer. To provide guidance in the use of oxygen-enriched or oxy-fuel combustion, research was conducted to understand how key parameters affect the radiation and emissions characteristics of jet flames for a range of oxygen indices from 21% (air) to 100% (pure O 2 ). Experiments were conducted on simple jet flames created by fuel issuing from a 3-mm i.d. straight tube into a low-velocity oxidizer stream for a variety of conditions. In addition to O 2 content of the oxidizer, fuel jet velocity and fuel type were varied. Laser light extinction was used to measure mean soot volume fractions as functions of axial distance. Other measurements include axial profiles of total radiant heat flux and NO x and CO emission indices. Results of these experiments were compared with predictions from a modified version of the two-stage Lagrangian (TSL) model of Broadwell and Lutz. The TSL code was modified by the incorporation of a detailed soot model from Frenklach et al. and the addition of a radiation submodel that accounts for both gas-band and soot blackbody radiation. In general, the TSL model captures the trends observed in the experiments for various parameters as functions of oxygen index, fuel type, and fuel-jet velocity, although absolute values are not predicted with engineering accuracy. The model was also exercised to predict the soot contribution to the total radiant heat transfer with soot volume fractions adjusted to be more representative of experimentally measured values.

Characteristics of microscale diffusion flames

This paper describes an investigation of microscale diffusion flames, which are potential sources of energy for future micro power generators. Of particular interest was the extinction process, as it would have direct bearing on the minimum size and power of the devices in which such flames would be used. Propane diffusion flames stabilized on top of vertical stainless steel hypodermic tubes were studied. Since intrusive probes could not be used at small scales, flame visualization and chemiluminescence measurements were used to investigate the extinction process. The causes of extinction and blowoff were elucidated by correlating measured microflame behavior with the predictions of a simple jet flame model. The flame lengths, predicted by the model were in good agreement with observations. Moreover, provided with one experimental parameter, the flame standoff distance, the model properly predicted the dependence of the flame shape upon the fuel flow rate. The model also predicted that at fuel flow rates near extinction, the fuel jet completely premixes with air within the standoff distance to form a combustible mixture just upstream of the flame, suggesting that near extinction the fuel burns in a flat premixed flame. The model accurately determined the fuel flow rate at quenching by equating the predicted flame length to the measured standoff distance. Finally, experiments conducted under this study showed that heating the surrounding air significantly reduces the fuel flow rate at extinction.

The Influence of Changes to Mixing on the Sooting and NOx Emission Characteristics of Unconfined Turbulent Jet Diffusion Flames

AbstractResults are presented from new measurements of vertically fired unconfined diffusion flames issuing from precessing jet nozzles and new calculations of existing data from simple jet flames. These results provide quantitative information about large‐scale mixing, NOx emissions and sooting tendencies of precessing jet flames. It is shown that a characteristic large eddy strain rate at the tip of a precessing jet flame is reduced by an order of magnitude when compared to that in simple jet flames. The reduction in large eddy strain rate correlates with an increased sooting tendency that is consistent with findings for simple jet flames presented elsewhere. The reduction in strain rate correlates with decreased NOx emissions and is found to be contrary to flamelet model predictions for simple jet flames.

Large–Scale Dynamics of an Unconfined Precessing Jet Flame

Visual observations, high speed movie sequences and image processing techniques have been used to examine unconfined vertical lifted turbulent diffusion flames issuing from precessing jeta (PJ) nozzles. These techniques provide qualitative information about the dynamic motions in the flame and quantitative data on the size and number of flame structures, the celerity of those structures, flame dimensions, residence times and characteristic strain rates. The information is used to provide new insight into the flame stabilisation mechanism of, and combustion processes occurring in, a PJ flame and to enable a comparison with similar studies in free turbulent jet diffusion flames and pool fires. Large-scale structures are seen to form near to the base of the PJ flame and move downstream with a slow, nearly constant speed in a manner reminiscent of the puffing motions in a pool fire. The entrainment and mixing of these structures is such that the flame tip oscillates as the residual unburned mixture in each structure, becomes combustible and burns out rapidly as a single entity. It is proposed that the puffing motions in these unconfined flames are the result of a buoyant instability, which is analogous to that in a pool fire. However the buoyant puff structures are the result, not the cause, of the stabilisation process. The high stability of the PJ flame, previously reported, is deduced to be related to the rapid decay in jet velocity and to the increase in entrainment which occurs in and upstream from the stabilisation region. The frequency of puffing is found to be uncorrelatetagging.bd with the frequency of precession. Global residence times and characteristic rates of shear are also measured and found to be reduced relative to those in a simple jet flame. The implications of the reduced shear on models of NOx generation in jet flames is discussed.

Oxides of nitrogen emissions from turbulent jet flames: Part I—Fuel effects and flame radiation

Measurements of oxides of nitrogen emission indices, flame radiant fractions, and visible flame dimensions were made for turbulent jet diffusion flames covering a wide range of flow conditions. Objectives of the study were to explain the observed scaling of NO x emissions with flow variables and to understand the interrelationships among NO x , flow conditions, and flame radiation. The flames were vertical and stabilized with hydrogen pilot flames on straight tube burners. Flow conditions were varied by changing the initial jet velocity and/or the burner tube diameter. Four burner sizes were used, with diameters ranging from 2.18 to 6.17 mm; and four fuel types, having a wide range of sooting tendencies, were employed: methane, ethylene, propane, and a 57% CO 43% H 2 (by volume) mixture. The ranges of Reynolds numbers and Froude numbers explored were 3,130–88,500 and 218 to 2.8 × 10 6, respectively. The effects of flow parameters and fuel type on radiant losses are shown to be important in determining the NO x emissions from simple jet flames. For high-temperature flames (T > 2050 K), overall NO x production rates for all four fuels were found to scale with characteristic flame temperatures deduced from the measured radiant fractions in a manner consistent with Zeldovich kinetics. This successful scaling of NO x production rates with global flame temperatures and residence times is consistent with, but does not prove, the view that much of the NO x emitted by jet flames is formed in large-scale eddies at the flame tip. NO x production rates higher than expected from the thermal mechanism alone are observed for the hydrocarbon fuels at lower flame temperatures (<2050 K), with the NO x production rates ranking in the same order as sooting tendencies. This suggests that gas-molecular radiation is more relevant than broadband radiation from soot for determining temperatures in NO formation zones. Prompt NO and/or other soot-NO interactions may also be important for the hydrocarbon fuels in this temperature regime. Previously reported Reynolds and Froude number dependencies for NO x production rates are examined and found to be consistent with flame heat loss characteristics.

Nitric oxide levels of jet diffusion flames: Effects of coaxial air and other mixing parameters

This paper quantifies the effects of certain fluid mechanical mixing perameters, such as coaxial air velocity, Reynolds number and Damkohler number, on the total nitric oxide emission index of non-premixed jet flames. It is found that forcing coaxial air into the flame can result in a significant (up to sixfold) reduction of the NOx emission index (in g/kg fuel) as compared to simple jet flames. The large NOx reduction is explained by the fact that coaxial air reduces the flame length and therefore forces the jet flame to exist closer to the nozzle where the velocities are larger, which reduces the local residence time as well as the reaction zone volume. The thermal NOx scaling proposed by Peters for jet flames with no coaxial air appears to explain the present results successfully for the case of a hydrogen-air flame but with only partial success for the case of a methane-air flame. The different scaling of the methane-air flame may be due to its greater sensitivity to the strain induced by coaxial air, or may be due to prompt NO. Coaxial air also significantly changes the fraction of NO2 within the NOx, but the NO2 scales in a manner that is different from the overall NOx. Parameters that were systematically varied include the Reynolds number, the coaxial air velocity, fuelto-air diameter ratio and fuel type. The measured trends can be used to assess recent NOx/mixing models.

The role of the recirculation vortex in improving fuel-air mixing within swirling flames

An important example that shows how a flame interacts with a vortex is the case of a swirlstabilized flame. Such a flame essentially is a toroidal vortex into which a jet of fuel is injected along the torus centerline. Because of the central toroidal vortex, the overall fuel-air mixing rate within a swirl-stabilized flame is found to be a factor of five times greater than that of a simple jet flame, as evidenced by a fivefold shortening of flame length. Swirl flames are unique in that the fuel-air mixing rate can be varied by controlling the amount of swirl. Flow visualization shows how internal recirculation helps to enhance the mixing. The englufment mechanisms is similar to that of a simple jet flame but is enhanced because of two factors that increase the fuel-air contact area. First, the recirculation zone acts like a large eddy with a characteristic velocity and length scale that are much larger than those associated with eddies in a simple jet. The active role of the recirculation zone contradicts a previouslyheld concept that the recirculation zone is a passive obstacle and that its internal velocity is not important. Air is entrained into the toroidal vortex primarily in the downstream region of the vortex. A second reason why mixing is more intense within swirling flames than within jet flames is that there is impingement of opposed jets at the forward stagnation point. Thus pressure gradients, which are not present in simple jets, enhance the mixing rates. Flame length, which is a measure of overall fuel-air mixing, was found to have a different scaling in swirl flames than for simple jet flames. The physical reasons are explained by a scaling which was developed and which explains four trends in the flame length data. It is proposed to use a general, nondimensional circulation parameter that allows for general comparison of mixing efficiencies of swirl, bluff-body, and dump-combustor flames.