Modelling of non-premixed swirl burner flows using a Reynolds-stress turbulence closure

In this computational study, the performance of a differential Reynolds-stress turbulence model has been assessed in predicting a turbulent, non-premixed combusting swirling flow of the type frequently found in practical combustion systems. Calculations are also performed using the widely employed eddy-viscosity based k-ε turbulence model in order to examine the relative performances of these two closure models. The predictions are compared against the experimental data of mean axial and tangential velocities, turbulence quantities, gas temperatures and oxygen concentration collected in a 400 kW semi-industrial scale combustor fired with coke-oven gas using an industry-type swirl burner at the International Flame Research Foundation [17]. Computations of a corresponding non-combusting flow are also carried out and the predictions are compared with limited data available. The overall agreement between the measurements and the predictions obtained with both the k-ε and Reynolds-stress turbulence models are reasonably good, in particular, the flame properties. However, some features of the isothermal and combusting flow fields, and the flame are better predicted by the Reynolds-stress model. The subcritical nature of the isothermal flow and the effects of combustion on the size and shape of the swirl-induced internal recirculation zone in the corresponding combusting flow are well simulated by this model. The k-ε model fails to reproduce the subcritical nature of the isothermal flow. The predictions of this model erroneously show a general trend of the mean tangential velocity distribution to assume a forced-vortex profile. The levels of gas temperature and oxygen concentration in the internal recirculation zone and the enveloping shear region are on the whole better predicted by the Reynolds-stress model.     

电磁场强度对层流火焰和NO生成特性的影响

在液化气与空气燃烧的层流火焰两侧施加放电磁场,测定磁场强度,采用双铂铑热电偶和综合烟气分析仪检测层流自由射流火焰温度和NO浓度,分析了不同磁场强度下层流自由射流火焰特性和NO生成特性. 结果表明,在电磁场作用下火焰长度变短,火焰下部直径增大;随磁场强度增大,火焰面下部温度提高. 电磁场可减少火焰中N, HCN, CN等离子和离子团与氧的碰撞几率,导致NO浓度降低,最大下降值为4.26 mg/m3,最大降幅为78.60%.     

Regimes of Unstable Expansion and Diffusion Combustion of a Hydrocarbon Fuel Jet

Results of an experimental study of hydrodynamics and diffusion combustion of hydrocarbon jets are presented. Various regimes of instability development both in the jet flame proper and inside the source of the fuel jet are considered. The experiments are performed for the case of subsonic gas jet expansion into the air from a long tube 3.2 mm in diameter in the range of Reynolds numbers from 200 to 13 500. The fuel is the propane–butane mixture in experiments with a cold jet (without combustion) and pure propane or propane mixed with an inert dilutant (CO₂ or He) for the jet flame. The mean velocity and velocity fluctuations in the near field of the jet without combustion are measured. Among four possible regimes of cold jet expansion (dissipative, laminar, transitional, and turbulent), three last regimes are investigated. The Hilbert visualization of the reacting flow is performed. The temperature profiles in the near field of the jet are measured by a Pt/Pt–Rh thermocouple. An attached laminar flame is observed in the transitional regime of propane jet expansion from the tube. In the case of combustion of C₃H₈ mixtures with CO₂ or with He in the range of Reynolds numbers from 1900 to 3500, the transitional regime is detected in the lifted flame. Turbulent spots formed in the tube in the transitional regime exert a significant effect on the flame front position: they can either initiate a transition to a turbulent flame or lead to its laminarization.     

Presumed joint probability density function model for turbulent combustion

A presumed joint probability density function (pdf) model of turbulent combustion is proposed in this paper. The turbulent fluctuations of reactant concentrations and temperature are described using a presumed joint pdf of three-dimensional Gaussian distribution based on first and second-order moments of reactant concentration and temperature. Mean reaction rates in both premixed and diffusion combustion are obtained by mean of integration under the presumed joint pdf. This model is applied to predict turbulent premixed combustion of sudden-expansion flow and turbulent jet diffusion methane/air flame. For turbulent premixed combustion, the predicted results of temperature distribution and maximum temperature using the proposed model agree better with the experiment than that using the conventional eddy-breakup (EBU)-Arrhenius model. For the turbulent jet diffusion methane/air flame, the predicted results of velocity, temperature and species concentrations using the proposed model, the Arrhenius, EBU-Arrhenius, and laminar flamelet models are compared with experiment data. Results obtained with the presumed pdf model and that obtained by the laminar flamelet model both agree well with experiments, while results using the other models have a significant difference. The presumed joint pdf model is used to predict the NO formation process, which also agrees well with the experiment data. A unified turbulent combustion model, in which both effects of turbulent diffusion and chemical dynamics are considered, is established for both premixed and diffusion combustion, especially for the process of NO formation.     

Nitrogen dilution effect on flame stability in a lifted non-premixed turbulent hydrogen jet with coaxial air

The nitrogen dilution effect on flame stability was experimentally investigated in a lifted non-premixed turbulent hydrogen jet with coaxial air. Hydrogen gas was used as the fuel and coaxial air was injected to initiate flame liftoff. Hydrogen was injected into an axisymmetric inner nozzle (d_F = 3.65 mm) and coaxial air jetted from an axisymmetric outer nozzle (d_A = 14.1 mm). The fuel jet and coaxial air velocities were fixed at u_F = 200 m/s and u_A = 16 m/s, while the mole fraction of the nitrogen diluent gas varied from 0.0 to 0.2 with a 0.1 step. For the analysis of the flame structure and the flame stabilization mechanism, the simultaneous measurement of PIV/OH PLIF was performed. The stabilization point was in the region of the flame base with the most upstream region and was defined as the point where the turbulent flame propagation velocity was found to be balanced with the axial component of the local flow velocity. The turbulent flame propagation velocity increased as the nitrogen mixture fraction decreased. The nitrogen dilution makes the flame structure more premixed. That is, the stabilization mechanism shifts from edge flame propagation based mechanism toward premixed flame propagation based mechanism. We concluded that the turbulent flame propagation velocity was expressed as a function of the turbulent intensity and the axial strain rate, even though the mole fraction of the nitrogen diluent varied.     

Predictions of CO and NOx emissions from steam cracking furnaces using GRI2.11 detailed reaction mechanism – A CFD investigation

This investigation develops a three-dimensional Computational Fluid Dynamics (CFD) model to simulate the turbulent diffusion flame on the fire-side of the radiation section of a thermal cracking test furnace coupled with a non-premixed low NO_x floor burner. When this type of burners which uses the internal Flue Gas Recirculation (FGR) technique is coupled with large scale furnaces, both the turbulent mixing and chemical reaction rates are comparable and hence this should be considered in the model. Different combustion models are used to simulate the turbulence–chemistry interactions for this flame. The CFD model, based on the Eddy Dissipation Concept (EDC) combustion model coupled with the detailed GRI2.11 reaction mechanism, gives the most reasonable predictions compared with the available experimental data or empirical correlations for the diffusion flame in the thermal cracking test furnace, especially for the flame length and the CO and NO_x emissions.     

Real-fluid flamelet modeling for gaseous hydrogen/cryogenic liquid oxygen jet flames at supercritical pressure

The primary goal of this study is to numerically model the transcritical mixing and reacting flow processes encountered in liquid propellant rocket engines. In order to realistically represent turbulence-chemistry interactions, detailed chemical kinetics, and non-ideal thermodynamic behaviors related to the liquid rocket combustion at supercritical pressures, the flamelet approach is coupled with real-fluid modeling based on the Soave-Redlich-Kwong (SRK) equation of state. To validate the real-fluid flamelet model, a gaseous hydrogen/cryogenic liquid oxygen coaxial jet flame at supercritical pressure has been chosen as a benchmark case. Numerical results are compared with experimental data obtained for the OH radical and the temperature distribution. It was found that weak flow recirculation is induced by the sudden expansion of cold core cryogenic oxygen associated with the pseudo-boiling process. This weak recirculation zone substantially influences the fundamental characteristics of liquid propellant reacting flows at supercritical pressures in terms of the spreading and the flame length. For the flame conditions employed in this study, the predicted contours of the OH radical are in good agreement with the experimental Abel transformed emission image in terms of the flame spreading angle and the flame location. Numerical results suggest that the real-fluid based flamelet model is capable of realistically predicting the overall characteristics of a turbulent non-premixed GH₂/LOx flame at supercritical pressures.     

Numerical simulation of the formation of nitric oxides in the combustion of coal-dust fuel

A mathematical model and a method of calculating two-phase flows and heat and mass transfer in industrial combustion chambers operating on coal-dust fuels are given. The numerical model of coal-dust combustion in a high-temperature turbulent flow is based on three-dimensional steady-state equations of mechanics of heterogeneous media and describes the processes of yield and combustion of fuel volatiles, afterburning of the coke residue, radiative heat transfer in the combustion chamber, and the effect of the disperse phase on the turbulent structure of the carrier medium. To predict the concentration of nitric oxides inside a combustion chamber, an effective numerical model of NO formation during jet burning of coal dust, which is based on the Mitchell-Tarbell kinetic scheme, is proposed. The results obtained in the use of the proposed complex numerical model are in good agreement with experimental data for industrial combustion chambers. The degree of detail ensured by the calculations allows one to make effective decisions to organize coal-dust burning with a decreased yield of hazardous nitric oxides.     

Liftoff height behavior in a non-premixed turbulent hydrogen jet with coaxial air

To understand hydrogen lifted flames, the experimental approximation of liftoff height in non-premixed turbulent conditions was studied. The objectives were to analyze liftoff height behavior and to derive the normalized expression for lifted jet with the effective diameter (d_F,eff). Hydrogen flow velocity varied from 100 m/s to 300 m/s. Coaxial air velocity was regulated from 12 m/s to 20 m/s. For the simultaneous measurement of velocity field and reaction zone, particle image velocimetry using hydroxyl radicals (PIV/OH) planar laser-induced fluorescence (PLIF) techniques with neodymium-droped yttrium–aluminum-gamet (Nd:YAG) lasers and charge-coupled device/intensified charge-coupled device (CCD/ICCD) cameras were used. Liftoff height decreased with increased fuel velocity. The flame stabilized in a lower velocity region next to the faster fuel jet due to the mixing effects of the coaxial air flow. The stabilization point was defined as the point where local flow velocity is balanced with turbulent flame propagation velocity. On the basis of the far field concept, we could derive the experimental approximation of the liftoff height divided by the effective diameter.     

Chemistry and radiation in oxy-fuel combustion: A computational fluid dynamics modeling study

In order to investigate the role of combustion chemistry and radiation heat transfer in oxy-fuel combustion modeling, a computational fluid dynamics (CFD) modeling study has been performed for two different oxy-fuel furnaces. One is a lab-scale 0.8 MW oxy-natural gas flame furnace whose detailed in-flame measurement data are available; the other is a conventional 609 MW utility boiler which is assumed to be operating under oxy-fuel combustion condition with dry flue gas recycle. A new model for gaseous radiative properties is developed, validated, and then implemented in the CFD simulations. The CFD results are compared to those based on the widely used model in literature, as well as the in-flame measurement data. The importance and advantage of the new model for gaseous radiative properties have been well demonstrated. Different combustion mechanisms are also implemented and compared in the CFD simulations, from which significant difference in the predicted flame temperature and species is observed. This difference is consistent with those expected from the equilibrium calculation results. As a conclusion, the appropriate combustion mechanisms applicable to oxy-fuel combustion modeling are identified. Among the key issues in combustion modeling, e.g., mixing, radiation and chemistry, this paper derives useful guidelines on radiation and chemistry implementation for reliable CFD analyses of oxy-fuel combustion, particularly for industrial applications.     

Spatial Structure of a Reacting Turbulent Swirling Jet Flow with Combustion of a Propane–Air Mixture

Results of an experimental study of the spatial structure of a reacting flow during combustion of a propane–air mixture in a turbulent swirling jet escaping into atmospheric air are presented. The fuel-to-air equivalence ratio is φ = 0.7, and the Reynolds number of the jet is Re = 5 · 10³. The time-averaged spatial distributions of velocity, local density, and concentrations of the main species of the gas mixture are measured in low-swirl and high-swirl flows. In both cases, the flame front is stabilized in the internal mixing layer formed by the axial region of jet retardation, where hot combustion products are concentrated. In a high-swirl flow, the temperature distributions in the cross section y/d = 0.5 show that the region with the maximum temperature of the gas is located at the periphery of the central recirculation zone. Moreover, in the case of a high-swirl flow, there exists a recirculation zone at the axis, and the CO₂ concentration is twice higher than in a low-swirl jet. The opposite situation is observed for O₂.     

Formation and reduction of nitric oxide in fixed-bed combustion of straw

Mechanisms of nitric oxide (NO) formation and reduction in fixed-bed combustion of straw have been modeled mathematically and verified experimentally. The model for the straw combustion and nitrogen chemistry consists of sub-models for evaporation, pyrolysis, tar and char combustion, nitrogen conversion, and energy and mass conservation. Twenty chemical reactions are included, of which 12 belong to the fuel nitrogen reaction network. Volatile nitrogen is assumed to be NO, NH₃, HCN and HNCO, and char nitrogen is converted to NO during char oxidation. The model predictions are in qualitative agreement with the measurements during the ignition phase, i.e. when the combustion front passes through the un-burnt fuel. The yield of NO can be reduced considerably by using a low primary air flow due to the longer gas residence in the fixed-bed, while the NO exhaust concentration is insensitive to the bed temperature. The NO exhaust concentration initially reaches a maximum and then decreases towards a stable value after the straw bed is ignited. Variations of NO, NH₃, HCN, and HNCO concentrations in the ignition flame front indicate that a large quantity of NO can be reduced in the thin flame front zone. The developed model is further validated by separate experiments in which NO or NH₃ was added at the middle through tubes or at the bottom of the bed with the primary air flow. Both the simulations and measurements showed that the variation of the NO exhaust concentration is small as compared with the injected NO or NH₃ concentration. According to the simulations and experiments, it is proposed that flue gas recirculation may be a very effective method of reducing NO emissions from flue gas in the fixed-bed combustion of straw. Calculations indicated that about 20% of the flue gas may be recirculated without significantly affecting the combustion behavior.     

方型多火嘴石油化工管式炉中辐射室三维流动及传热的数学模拟

提出方型石油化工管式炉中辐射室的三维流动和传热的数学模型.对某焦化炉计算的结果与实测数据进行了初步比较并作出讨论.     

A double-k-ε and multi-δ pdf model for turbulent gas-liquid reacting flows

The gas-liquid reacting two-phase flow of a gaseous SF₆ jet submerged in liquid metal Li is numerically simulated by a two-fluid model. To describe the changes of the flow pattern in the two-phase flow, a double-k-ε two-phase turbulence model is developed and adopted. A multi-δ-pdf model is proposed and applied to simulate the turbulent combustion of gaseous SF₆ with liquid metal Li. The gas and liquid velocities, void fraction, shape of the penetrating region (the region containing the gas phase) and the mixture fraction related to gas temperature and species concentration are predicted. Comparison with the results using a single-fluid model shows that current predictions are reasonable in reflecting the influence of velocity slip between the two phases and capable of predicting Li/SF₆ submerged combustion.     

Flame and radiation characteristics of gas-fired O2/CO2 combustion

This paper presents an experimental study on the flame properties of O₂/CO₂ combustion (oxy-fuel combustion) with focus on the radiation characteristics and the burn-out behaviour. The experiments were carried out in a 100 kWth test unit which facilitates O₂/CO₂ combustion with real flue gas recycle. The tests comprise a reference test in air and two O₂/CO₂ test cases with different recycled feed gas mixture concentrations of O₂ (OF 21 @ 21 vol.% O₂, 79 vol.% CO₂ and OF 27 @ 27 vol.% O₂, 73 vol.% CO₂). In-furnace gas concentration, temperature and total radiation (uni-directional) profiles are presented and discussed. The results show that the fuel burn-out is delayed for the OF 21 case compared to air-fired conditions as a consequence of reduced temperature levels. Instead, the OF 27 case results in more similar combustion behaviour compared to the reference conditions in terms of in-flame temperature and gas concentration levels, but with significantly increased flame radiation intensity. The information obtained from the radiation and temperature profiles show that the flame emissivity for the OF 21 and OF 27 cases both differ from air-fired conditions. The total emissivity and the gas emissivity of the OF 27 and the air-fired environment are discussed by means of an available model. The gas emissivity model shows that the increase in radiation intensity (up to 30%) of the OF 27 flame compared to the air flame can partly, but not solely, be explained by an increased gas emissivity. Hence, the results show that the OF 27 flame yields a higher radiative contribution from in-flame soot compared to the air-fired flame in addition to the known contribution from the elevated CO₂ partial pressure.     

Simulation of turbulent combustion and NO formation in a swirl combustor

A presumed probability density function (PDF) model for temperature fluctuation is proposed and formulated in this paper. It incorporates a two-step reaction mechanism for propane combustion and the thermal and prompt NO formation mechanisms. The present model, together with a new algebraic Reynolds stress model (ASM), is employed to simulate the turbulent combustion and NO formation in a swirl combustor. The calculated propane, carbon monoxide, and carbon dioxide concentrations agree with the measurement. The calculated gas temperature and oxygen and NO concentrations are in general agreement with the measured data. The simulated results show that NO forms mainly in the upstream region of the combustor. The flue gas recirculation effectively abates the nitrogen oxides (NO_x) emission in the combustor.     

梭式窑预混富氧燃烧预热阶段换热特性的模拟研究

通过计算流体力学(CFD)软件—FLUENT研究了富氧浓度对预热阶段梭式窑内换热特性的影响。结果表明火焰最高温度随富氧浓度的增加非线性增大。梭式窑内的富氧燃烧可以减少高温高速烟气射流直接对窑墙的冲刷。由于烟气不能充分冲刷烧嘴附近区域和烟气射流顶部回流的影响,窑炉断面出现温差。为使窑内温度尽量均匀,预热阶段也可通过控制燃料量,点燃全部烧嘴。富氧助燃可以使窑内换热增强,减小窑内温差。     

Flame structure of LPG-air Inverse Diffusion Flame in a backstep burner

The present experimental study characterizes the turbulent LPG Inverse Diffusion Flame (IDF) stabilized in a backstep burner in terms of visible flame length, dual flame structure, centerline temperature distribution, and oxygen concentration. The visible flame length for a fixed fuel jet velocity is found to reduce with increase in air jet velocity. Besides this, the effect of air and fuel jet velocities on visible flame length is interpreted using a new parameter, Global Momentum Ratio (GMR). Interestingly, GMR seems to be correlating well with the visible flame length for the air and fuel velocity ranges considered in the present study. Moreover, the dual flame structure of IDF is identified with the help of CH-chemiluminescence signature. The existence of dual flame structure of IDF is confirmed further with the centerline temperature and oxygen concentration measurements.     

Refining the criterial description of flame stabilization during gas jet combustion

Analytical dependence for the diffusion flame lift-off length with fuel jet discharge into the atmosphere is obtained in criterial form. The dependence can be used to determine the limiting parameters at which blow-off occurs and describe the lift-off length variation under pre-blow-off conditions, and it correlates well with earlier generalizations of experimental data. The experiments on the combustion of propane-butane and natural gas confirm the correctness of the proposed form of the analytical generalization, show the marked influence of the external shape of the gas nozzle on the lift-off parameters, and indicate that the blow-off of the turbulent diffusion flame is slower than the characteristic combustion time.