Large eddy simulation of swirling jet in a bluff-body burner

The large eddy simulation (LES) is applied to an unconfined swirling flow of an air surrounding a bluff-body having a central jet of air, and the complicated flowfield that involves the recirculation and vortex breakdown is investigated. The Smagorinsky model is used as the sub-grid scale model. The results of the present numerical simulation are compared with the experimental data of the mean and stochastic root mean square (RMS) variations of two velocity components. Although the inflow conditions are specified in a simple manner, the obtained numerical results are in reasonable agreement with the experiments, except for a part of RMS variation values near downstream of the bluff body. The present numerical calculations can successfully reproduce the two characteristics of the flow, i.e., an upstream recirculation zone established just downstream of the burner plane and the additional recirculation zone established at the more downstream location.     

The hysteresis phenomenon in flame lift-off on a bluff-body burner under airflow dominant conditions

We investigated the behavior of the lifted flame on a bluff-body burner under the airflow dominant condition by the higher annular airflow velocity and the lower central fuel jet one and found the appearance of the hysteresis phenomenon in lift-off height of the flame that depends on the history of the fuel jet velocity. The hysteresis behavior is entirely different from the case of the fuel flow dominant condition by the higher central fuel jet velocity and lower annular airflow one. The observation by shadowgraph revealed that the occurrence of the phenomenon has a relation to the interaction between the fuel jet and the recirculation airflow region on the burner.     

Experimental study on turbulent structure of humid air flame in a bluff-body burner

The main objective of the present experimental study is to analyze the turbulent structure in humid air non-premixed flame, and determine the effect of humidity on the flow field and the flame stability limit in turbulent non-premixed flame. Particle Image Velocimetry (PIV) is used to capture the instantaneous appearance of vortex structures and obtain the quantitative velocity field. The distributions of Reynolds shear stress, mean and root-mean squared fluctuating (rms) velocities are examined to get insight into the effect of fuel-to-air velocity ratio on velocity flow field. The results show that with steam addition, the air-driven vortex in the bluff-body wake is thinner; the biggest peaks of rms velocity and Reynolds shear stress are lower; the distance between the peaks of rms velocity on the sides of centerline reduces. Besides these, the flame stability is affected. Both central fuel penetration limit and partially quenching limit reduce with steam addition.     

Mixing and Recirculation Characteristics of A double COncentric Burner with Bluff—Body

The concentric bluff-body jet burner is widely used in industrial combustion systems. This kind of burner often generates a considerably complex recirculation zone behind the bluff body. As a result, the fuel often remains in the recirculation zone, achieving stability of flame. This study investigates, by means of experiments, the variations of the aerodynamics as the fluid is injected into a combustion chamber through a double concentric burner with a bluff-body. The observation and measurement of the aerodynamics in our experiment are conducted under a cold flow. The controlled parameters in our experiment are: variations in the blockage ratio of the center bluff body, the cone angle of the bluff body, and the velocity ratio (U _s/U_p) of the secondary jet and primary jet; the injection of helium bubbles into the primary and secondary jets to observe the recirculation zone behind the bluff body; using Tufts for observing the characteristics of corner recirculation zone in a combustion chamber, measuring the average velocity of each point within the aerodynamics by the 5-hole pitot tube; measuring the distribution of static pressure of the combustion chamber walls with a static pressure tap.     

Shape effect of cavity flameholder on mixing zone of hydrogen jet at supersonic flow

Cavity flameholder is known as an efficient technique for providing the ignition zone. In this research, computational fluid dynamic is applied to study the influence of the various shapes of cavity as flameholder on the mixing efficiency inside the scramjet. To evaluate different shapes of cavity flame holder, the Reynolds-averaged Navier–Stokes equations with (SST) turbulence model are solved to reveal the effect of significant parameters. The influence of trapezoidal, circle and rectangular cavity on fuel distribution is expansively analyzed. Moreover, the influence of various Mach numbers (M = 1.2, 2 and 3) on mixing rate and flow feature inside the cavity is examined. The comprehensive parametric studies are also done. Our findings show that the trapezoidal cavity is more efficient than other shapes in the preservation of the ignition zone within the cavity. In addition, the increase of free stream Mach number intensifies the main circulations within cavity and this induces a stable ignition zone within cavity.     

Large-eddy simulation of a piloted premixed jet burner

A three-stream flamelet/progress variable model is applied to the Sydney piloted premixed jet burner (PPJB). Using experimental data, a prior model evaluation is performed to assess critical modeling assumptions regarding the applicability of this formulation to partially-premixed combustion, the statistical representation of the scalar mixing, and the joint PDF-closure. A Dirichlet distribution, as generalization of the beta distribution, is introduced to represent the interaction between the two mixture fractions that are associated with the fuel, pilot, and coflow streams. Comparisons with experimental data are performed to demonstrate the accuracy of this closure-model. Following this prior model evaluation, the three-stream combustion model is applied to large-eddy simulation, and calculations of all four burner configurations, designated as PM1-{50, 100, 150, 200}, are performed. Through comparisons with experimental data and equilibrium computations it was found that the flow-field is sensitive to the scalar inflow composition, and scalar boundary conditions consistent with experimental measurements were used for all simulations. The effect of wall heat-losses on the temperature and species profiles is assessed in an approximate way, suggesting that species profiles are unaffected by the heat-transfer between pilot and coflow streams. Comparisons of statistical results and thermo-chemical correlations show that the model is capable of predicting flow-field, temperature, and major species profiles. The simulations over-predict the fuel-consumption for PM1-150 and PM1-200, which has also been observed in previous investigations. Aspects regarding model extensions to account for heat-losses and transient extinction/re-ignition processes are discussed.     

The effect of chemical reaction on the mixing flow between aqueous solutions of acetic acid and ammonia

The basic characteristics of the reacting mixing flow of two streams were investigated. The reaction between aqueous solutions of ammonia and acetic acid, which produces ammonium acetate, was investigated in terms of the effect on the fluid–fluid interface of the mixing flow relative to fluids that did not react. The reaction between these solutions was negligibly exothermic, and there were minimal differences in density. The velocity field in the reacting mixing flow was quantitatively measured using high-speed time-resolved particle image velocimetry (PIV) and the behavior of the mixing flow was qualitatively investigated using laser-induced fluorescence (LIF). The jet width, the velocity field, the kinetic energy and the turbulent intensities are qualitatively estimated and discussed. It was found that the chemical reaction resulted in the suppression of the mixing flow.     

A stochastic model of turbulent mixing with chemical reaction: Nitric oxide formation in a plug-flow burner

A stochastic model of turbulent mixing has been developed for a reactor in which mixing is represented by n-body fluid particle interactions (n = 2, 3, ... 6). The model has been used to justify the assumption made in previous investigations of the role of turbulent mixing on burner generated thermal nitric oxide and carbon monoxide emissions that, for a simple plug flow reactor, composition nonuniformities can be described by a Gaussian distribution function in the local fuel: air equivalence ratio. Rate-limited upper and lower bounds of the nitric oxide produced by thermal fixation of molecular nitrogen and oxidation of organically bound fuel nitrogen are estimated on the basis of the stochastic model for a plug flow burner; these are compared with experimental measurements obtained using a laboratory burner operated over a wide range of test conditions; good agreement is obtained.     

LES analysis of flow and turbulent mixing in an unsteady jet

The flow and mixing process of unsteady jets are fundamentally analyzed by large eddy simulations. The effects of nozzle velocity and turbulence intensity on the turbulent eddy structure and mixing process between the nozzle fluid and ambient fluid were investigated. The results show that a toroidal‐shaped vortex, which emerges around the jet tip, primarily accelerates the entraining flow. Also, increasing the turbulence intensity in the nozzle encourages mixing in the jet without changing the jet‐contour. Furthermore, when the rise‐up time of the initial nozzle velocity is elongated, turbulent mixing is suppressed. © 2007 Wiley Periodicals, Inc. Heat Trans Asian Res, 36(5): 303–313, 2007; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20158     

Diameter of alumina balls effect on stabilization operation region for a reciprocal flow burner

A reciprocal gas flow porous burner is computationally simulated for obtaining its stable operation region as function of equivalence ratio, filtration velocity and particle diameter, for two energy losses configurations schemes. Equivalence ratio and filtration velocity were considered over the range [0.1, 1.0] and [0.25, 1.0] (m/s). Lateral heat losses were studied at two scenarios: reactor coupled with and without heat exchangers. Results demonstrated increased stabilization in temperature profile, thus increased stabilization region for the reactor coupled with heat exchangers. Increase of particle diameter results in increased stabilization region for both energy losses schemes.     

一次风弯头对旋流燃烧器出口气固相流动的影响

利用数值模拟方法对一种新型的双通道旋流燃烧器的流场和一次风出口处的气固分布进行冷态模拟,并通过搭建1∶2的冷模试验台来对冷态和气固两相的流场进行测量,然后与模拟结果进行对比。数值模拟与试验表明,弯头对燃烧器出口的气固两相流速度和浓度分布有影响,其中速度偏差达32%,浓度偏差达17%,设计和运行过程中应重视弯头的布置。     

Operational characteristics of a parallel jet MILD combustion burner system

This study describes the performance and stability characteristics of a parallel jet MILD (Moderate or Intense Low-oxygen Dilution) combustion burner system in a laboratory-scale furnace, in which the reactants and exhaust ports are all mounted on the same wall. Thermal field measurements are presented for cases with and without combustion air preheat, in addition to global temperature and emission measurements for a range of equivalence ratio, heat extraction, air preheat and fuel dilution levels. The present furnace/burner configuration proved to operate without the need for external air preheating, and achieved a high degree of temperature uniformity. Based on an analysis of the temperature distribution and emissions, PSR model predictions, and equilibrium calculations, the CO formation was found to be related to the mixing patterns and furnace temperature rather than reaction quenching by the heat exchanger. The critical equivalence ratio, or excess air level, which maintains low CO emissions is reported for different heat exchanger positions, and an optimum operating condition is identified. Results of CO and NO_x emissions, together with visual observations and a simplified two-dimensional analysis of the furnace aerodynamics, demonstrate that fuel jet momentum controls the stability of this multiple jet system. A stability diagram showing the threshold for stable operation is reported, which is not explained by previous stability criteria.     

The effect of confinement on the flow and turbulent heat transfer in a mist impinging jet

Numerical study of the effect of confinement on a flow structure and heat transfer in an impinging mist jets with low mass fraction of droplets (M_L1 ? 1%) were presented. The turbulent mist jet is issued from a pipe and strikes into the center of the flat heated plate. Mathematical model is based on the steady-state RANS equations for the two-phase flow in Euler/Euler approach. Predictions were performed for the distances between the nozzle and the target plate x/(2R) = 0.5–10 and the initial droplets size (d₁ = 5–100 μm) at the varied Reynolds number based on the nozzle diameter, Re = (1.3–8) × 10⁴. Addition of droplets causes significant increase of heat transfer intensity in the vicinity of the jet stagnation point compared with the one-phase air impinging jet. The presence of the confinement upper surface decreases the wall friction and heat transfer rate, but the change of friction and heat transfer coefficients in the stagnation point is insignificant. The effect of confinement on the heat transfer is observed only in very small nozzle-to-plate distances (H/(2R) < 0.5) both in single-phase and mist impinging jets.     

Liftoff behavior and combustion characteristic of jet flame in a coflow burner

Stabilization and autoignition mechanisms of lifted flames have been widely investigated to improve combustion efficiency and safety of combustion equipment. This paper focuses on liftoff behavior and combustion characteristic of methane and propane flames under various coflow conditions in a coflow burner. Unlike the case of free jet flame in ambient air, the different tendencies of liftoff height changes with jet velocity for both methane and propane flames in vitiated coflow illustrate a transition from conventional combustion to Moderate & Intense Low Oxygen Dilution (MILD) combustion. Flame temperature difference with radial position measured by primary spectrum pyrometry proves the transition regime.     

A filtered-laminar-flame PDF sub-grid-scale closure for LES of premixed turbulent flames: II. Application to a stratified bluff-body burner

Large eddy simulation of the two stratified nonswirling configurations of the Cambridge burner studied by Sweeney et al. (2012) is presented. The sub-grid-scale combustion closure relies on a physical space filtering operation with a filter size determined locally depending on the resolved and sub-grid-scale flame properties, which is discussed in a companion paper. Similarly to the premixed configuration of the same burner, the modeling reproduces the differential diffusion effects leading to accumulation of carbon and an enhancement of mixture fraction in the recirculation zone, an effect that is less pronounced than in the fully lean premixed case, because of the modification of the topology of the reaction zone that is induced by the mixture stratification. The study of the LES combustion regimes shows that the reaction zones develop under a quite large range of flame topologies, from wrinkled flamelets up to thin reaction zones. Instantaneous and time-averaged LES data were analyzed to extract information concerning the degree of stratification and the orientation of flame and mixing vectors. A decomposition of the flame response into premixed, diffusion, and partially premixed flamelets is performed, to conclude that the premixed mode dominates close to the burner, with a partially premixed burning regime further downstream. Overall, the length scales associated with stratification were found to be much larger than that of the reaction zone and flame, resulting in a quasi-homogeneous propagation, predominantly in a back supported stratified combustion regime. Overall good agreement between simulation and measurements was obtained for either configurations.     

Cold flow characteristics of a novel bluff body hydrogen burner

The cold flow characteristics of a novel partial premixed bluff body (PPBB) low NO_x burner, capable of operating with hydrogen as well as methane-hydrogen blends, were investigated numerically. The PPBB burner features a frustum shaped conical bluff body generating a flame stabilizing recirculation zone. Fuel is partially premixed via jets in an accelerating cross-flow. Steady-state and transient non-reacting simulations using five different turbulence models, i.e. standard k-ε, realizable k-ε, shear stress transport (SST) k-ω, stress-blended eddy simulation (SBES) and large eddy simulation (LES), were conducted. The simulations were validated against particle image velocimetry (PIV) measurements of an unconfined non-reacting flow. All turbulent models were able to predict the recirculation zone length in good agreement with the experimental data. However, only scale resolving simulations could reproduce velocity magnitudes with sufficient accuracy. Time averaged and instantaneous results from the scale resolving simulation were analysed in order to investigate flow characteristics that are special about the PPBB burner design and of relevance for the combustion process. Two different burner configurations were studied and their effects on the flow field were examined. The recirculation zone volume as well as the entrainment into the wall jet around the bluff body were found to correlate with the elevation of the bluff body relative to the burner throat. Both of these parameters are expected to have a strong impact on the overall NO_x emission, since the near burner region is typically one of the main contributors to the NO_x formation.     

结构参数对平焰燃烧器流场的影响

针对现有的天然气平焰燃烧器,采用RNGk-ε湍流模型和EDM燃烧模型,运用计算流体力学软件和建模工具对天然气平焰燃烧器流场进行了数值模拟,并应用五孔探针等工具对天然气平焰燃烧器冷态流场进行了测试,模拟结果分析表明:燃料径向喷出、空气进口偏心距l/D为0.476、烧嘴砖曲率半径R/D为1.14~1.71、旋流数S为0.61~1.68、混合距离L/D为0.6~0.9时能获得较佳的燃烧效果(其中,D为燃烧器喷口直径)。通过分析比较模拟结果和试验结果,发现两者的分布规律基本一致,数据基本吻合,回流区和主流区基本相同,并且与理论分析一致,符合实际情况,说明了天然气平焰燃烧器流场数值模拟结果是真实可信的,可为天然气平焰燃烧器的工业应用提供重要参考依据,也为天然气平焰燃烧器的设计、研究提供了有效的研究方法和途径。     

The influence of swirl burner structure on the gas/particle flow characteristics

Improvements were made to a low-NO_x axial swirl burner (LNASB), aimed at mitigating slagging in a 600-MWe boiler burning bituminous coal. The new design is referred to as improved low-NO_x axial swirl burner (ILNASB). This paper describes investigations of the influence of swirl burner structure on the gas/particle flow characteristics using a three-dimensional particle-dynamics anemometer. In comparing results from both ILNASB and LNASB, a central recirculation zone is seen to form in the region x/d = 0.1–0.3 within the ILNASB. This zone had shifted from the region between primary and secondary air in LNASB to a region between inner and outer secondary air. In the vicinity of the burner outlet, particle volume flux is reduced significantly in the central recirculation zone. In contrast, this flux is high near the central axis in ILNASB, thus concentrating a great fraction of pulverized coal near the central axis. Form the study, the gas/particle flow characteristics of the ILNASB show that the improved burner has the ability to ease slagging and reduce NO_x emissions.     

The effect of jet array arrangement on the flow characteristics of the outlet hole in short confined channels

This experimental study is part of the research program related to the aerodynamic characteristics of impingement in a confined channel. Experimental research on better understanding flow structure in confined channel with impingement cooling is rarely found in open literature. The main purposes of this project are (1) gaining greater insight into the structure of the impingement jet flow and flow in the outlet hole; (2) understanding the effect of impingement hole arrangement build up on the flow structure and aerodynamic parameters within holes and channels with different aspect ratios. The test models are two confined channels with three staggered circular jet holes, and a single large size outlet hole placed downstream of the jet plate. The structure parameters of these orifices are different. In this paper, detailed flow field in the outlet hole was measured with a straight five‐hole probe, and the discharge coefficient of outlet holes with different aspect ratios was also studied in each channel. Experimental data shows that the jet arrangement has little influence on the flow behavior of the outlet hole when the aspect ratio of the passage was 1, but it played an important role on the discharge coefficient. Distinctively different flow patterns were found in two models with the aspect ratios of 3 and 5, while the variation of the discharge coefficient showed a slight influence on impingement hole arrangement. © 2007 Wiley Periodicals, Inc. Heat Trans Asian Res, 37(1): 20–28, 2008; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20188