Study of the turbulent inverse diffusion flame in recessed backstep and coaxial burners

A comparative study of the turbulent Inverse Diffusion Flame (IDF) in recessed coaxial and backstep burners is carried out, based on visible flame appearance, flame length, flame stability, centerline temperature distribution, centerline oxygen concentration, and NO _x emissions. The backstep burner is observed to produce a compact flame shape with less luminosity at a higher air-fuel velocity ratio, as compared to the coaxial burner. Moreover, slightly better thermal characteristics and marginal reduction in NO _x emissions are provided by the backstep IDF, as compared to the recessed coaxial IDF. Besides this, the centerline oxygen concentration is marginally increased in the backstep IDF due to higher entrainment of ambient air. Interestingly, a lower flame stability limit is seen in the backstep burner than in the coaxial IDF, which can be attributed to its enhanced fuel-air mixing.     

Computational modeling of a utility boiler tangentially-fired furnace retrofitted with swirl burners

The paper presents 3D numerical investigation of OP-380 boiler tangentially-fired furnace utilizing bituminous coal. The boiler was retrofitted by replacing traditional jet burners with RI-JET2 (Rapid Ignition — JET) swirl burners. This kind of solution is unique in power generation systems. The purpose of this work is to show how the flow, combustion performance and heat exchange in the furnace are affected by introducing rapid ignition phenomena in RI-JET2 burners instead of delayed ignition associated with the traditional jet burners. Results were compared to simulations of similarly designed boiler equipped with traditional jet burners. Furnace simulation was preceded with a single RI-JET2 burner simulation at the inlet to a virtual combustion chamber. The results have shown that pulverized coal (PC) concentrator separates the PC into two streams: concentric with fine particles and axial with coarse particles. Stable flame operation was noticed even without secondary and tertiary air swirl. 3D simulations of combustion chamber have shown that in a burner zone a visibly isolated, concentrated flame exists in the furnace axis. This kind of flame shape reduces corrosion risk and furnace walls slagging as a result of RI-JET2 burner's long range.     

An experimental and modeling study of heat radiation characteristics of inclined ceiling jet in an airplane cargo compartment

A series of pool fires were carried out in an airplane cargo compartment to investigate the effect of the pressure on the heat radiation flux (HRF) of the inclined ceiling jet fire. During the tests, different static chamber pressures ranging from 50 to 101 kPa were controlled by the air flow in and out; both free fire and inclined ceiling jet fire were conducted with five different heat release rates (HRRs), which were produced by a 17‐cm square porous gas burner using propane as fuel. Vertical flame temperature, thermal plume temperature beneath the inclined ceiling, and HRF to the horizontal floor were measured and analyzed; at the same time, the flame shape was recorded by a video. It was found that the HRF was increased with the HRR, and there was a sudden rise for these fire impinging on the ceiling. The flame radiation fraction had a weak correlation with the environment pressure, while the flame emissivity was increased with the increasing ambient pressure. Besides, on the basis of the assumption that the flame emissivity is equaled in both free flame and the inclined ceiling jet fire, HRF calculated model was established and compared with the experimental results.     

Air Entrainment During Flame Aerosol Synthesis of Nanoparticles

Enclosed flames typically produce substantially larger particles than open flames under identical reactant flows and composition. The enclosure hinders air entrainment to the flame and reduces heat losses by radiation and convection, facilitating particle coagulation and coalescence. Here the effect of natural air entrainment on flame aerosol synthesis is investigated by lifting off the enclosing tube from the burner surface and utilizing tracer gas (Ne) analysis after calibration with forced air entrainment. That way the effect of air entrainment on product primary particle diameter and mobility size distribution dynamics is investigated by microscopy, scanning mobility particle sizing, and N₂ adsorption, while temperature is measured by Fourier-transform infrared spectroscopy. So air entrainment during flame spray pyrolysis is examined here for its versatility in scalable manufacture of an array of material compositions, while copper oxide (CuO) is used for its electro-chemical applications (e.g., battery electrodes). It is shown that natural air entrainment facilitates rapid gas-to-particle conversion and high process yields by minimizing vortex recirculation and particle deposition on the enclosing tube walls and burner surface. For example, the average primary particle diameter of CuO can be controlled from 42 to 10 nm and the yield from 40 to 90% by gradually lifting off the enclosing tube, resulting in up to 250 L/min natural air entrainment at the present CuO synthesis conditions.

Copyright 2014 American Association for Aerosol Research     

Prediction of Droplet Velocities and Rain Out in Horizontal Isothermal Free Jet Flows of Air and Viscous Liquid in Stagnant Ambient Air

Two‐dimensional phase Doppler anemometer measurements of droplet size and velocity conducted under several nozzle conditions and a systematic variation of the air mass flow quality and liquid phase viscosity show that the air entrainment process is enhanced when keeping all test conditions constant except for increasing the Newtonian liquid viscosity above of that of water. A two‐zone entrainment model based on a variable two‐phase entrainment coefficient is proposed with the normalized axial distance allowing for a change in the jet angle. Thus, the jet perimeter is lower and the breakup length is longer in the case of air/relatively higher viscosity liquid phase. It provides the most accurate reproduction of the experimental droplet velocity in comparison with that of other models in the literature and, hence, is recommended for the prediction of the droplet velocity in the case of two‐phase air/liquid phase free jet flow in stagnant ambient air. A model for predicting the droplet rain out, considering the droplet trajectories in the free jet flow, allows also for an adequate reproduction of the experimental data.     

Experimental study of the burner for FAA fire test: NexGen burner

The NexGen (Sonic) burner is the new burner developed by the Federal Aviation Administration, FAA, to replace old oil burners used for the required fire certification tests on power plant‐related materials, as it provides the capability to control both air and fuel flow rates. During a fire test, the burner is supposed to simulate a certain fire condition, so the flame properties should be robust and repeatable. The NexGen burner can achieve this due to the precise fuel and air controls. However, the current calibration criterion (ISO2685:1998 and AC20‐135) may not be good enough to ensure consistent flame properties. In the presented work, the sensitivity of the burner performance to air and fuel flow rate, as measured by the temperature and heat flux for calibration purposes, was studied. Additionally, the influence of the turbulator and the thermocouple size used for flame calibration was also studied. The impact of varying fuel/air ratio and thermocouple sizes was studied by conducting fire tests on aluminum samples, to show the inadequacies in the current calibration standards.     

Turbulent diffusion in the flame of a rotary kiln

Conclusions Experimental data on the distribution of velocities in the stream in the flow of air from models of a burner with a normal annular Laval nozzle and a burner with an increase in the angle of opening of the stream under supersonic conditions was obtained. The results of the work may be used in the design of burners for rotary kilns. According to the experimental data the coefficient of turbulent diffusion reaches a value of 0.0071 m²/sec for a pure flow and is reduced by 30% with an increase in the dust content from 0 to 1 kg/kg. It is desirable to use the data obtained in calculations of the flame processes and selection of means of intensification of the process of mixing of the fuel with the oxidizer in the presence of dust.Translated from Ogneupory, No. 1, pp. 46–49, January, 1986.     

Adiabatic premixed combustion in a gaseous fuel porous inert media under high pressure and temperature: Novel flame stabilization technique

This work presents an experimental investigation to study the characteristics of combustion using a premixed methane-air mixture within a non-homogeneous porous inert medium (PIM) under high pressure and temperature. In order to obtain a stable flame under these operating conditions within PIM, a novel flame stabilization technique in porous inert media (PIM) combustion under high pressure and temperature has been developed and evaluated. The proposed technique avoids the draw backs of the hitherto developed techniques by properly matching the flow and flame speeds and, consequently, ensuring a stable combustion, for a wide range of operating pressure and temperature. The success of this technique permits the extension of PIM combustion to new applications such as gas turbines. The validity of this new technique has been assessed experimentally in detail by analyzing combustion inside a prototype burner. The effects of various operating conditions, such as initial preheating temperature and elevated pressure, have been examined for an output power range between 5 and 40 kW. The experiments covered a broad spectrum of operating conditions ranging from a mixture inlet temperature of 20 °C and pressure ratio of 1 up to a temperature of 400 °C and a pressure ratio of 9. Evaluation of the results revealed excellent flame stability with respect to both flashback and blow-out limits throughout all the operating conditions studied, including relative air ratios far beyond the normal lean limit. While the blow-out stability showed no significant dependence on pressure, it was strongly determined by the preheating mixture inlet temperature. A remarkable broadening of the stability range from 0.6 to 1.0 on preheating to 400 °C was observed. This reveals the potential of pre-heat temperature to improve the dynamic modularity of the burner.     

Flame stability and emission characteristics of turbulent LPG IDF in a backstep burner

The stability characteristics and emissions from turbulent LPG inverse diffusion flame (IDF) in a backstep burner are reported in this paper. The blow-off velocity of turbulent LPG IDF is observed to increase monotonically with fuel jet velocity. In contrast to normal diffusion flames (NDF), the flame in the present IDF burner gets blown out without getting lifted-off from the burner surface. The soot free length fraction, SFLF, defined as the ratio of visible premixing length, H_p, to visible flame length, H_f, is used for qualitative estimation of soot reduction in this IDF burner. The SFLF is found to increase with central air jet velocity indicating the occurrence of extended premixing zone in the vicinity of flame base. Interestingly, the soot free length fraction (SFLF) is found to be correlated well with the newly devised parameter, global momentum ratio. The peak value of EINO_X happens to occur closer to stoichiometric overall equivalence ratio.     

新型低氮燃气分级燃烧器燃烧特性和NOx排放的CFD研究

应用CFD软件Fluent数值模拟了某二甲苯塔再沸炉在役油气联合燃烧器燃烧和NOx排放特性,分析了其NOx排放浓度较高的原因,提出了新型低NOx燃气分级燃烧器的改造方案,并数值模拟了新型燃烧器空气预热温度Tair、过剩空气系数α和主辅喷枪燃气质量分率Rp对辐射室壁面热通量、出口温度、火焰高度和炉膛出口NOx排放浓度的影响。针对在役燃烧器的模拟结果与现场运行数据吻合良好,说明所选模型能够正确模拟炉膛内部的流动、辐射、燃烧和NOx生成过程。新型燃烧器模拟结果表明,增加Tair会增加辐射壁面热通量,同时也会增加NO的排放;辐射壁面热通量随α增加而降低,NOx排放浓度随α增加而增加;Rp对炉内传热和NOx排放的影响并不明显。当Tair = 220℃、α = 1.05及Rp= 0.24时,新型燃烧器在模拟范围内达到最佳运行工况,辐射壁热通量为37.45kW/m2,NOx排放浓度为12.1μL/L。     

Study of catalysts for catalytic burners for fuel cell power plant reformers

Catalytic burners for fuel cell power plant reformers are alternatives to conventional flame burners. Their application is expected to provide uniform temperatures in the reformer, efficient use of low-calorific gaseous by products and reduction of pollutant emissions. For testing in the burners, a series of spherical Pd/CeO₂/Al₂O₃ catalysts were prepared. An optimum concentration of ceria providing the highest thermal stability of catalysts was determined. An effect of catalyst activation in the reaction mixture-1% methane in air was observed. A series of Mn containing oxide catalysts on spherical γ-Al₂O₃ or (γ+Χ)-Al₂O₃, both pure and doped with La, Ce and Mg oxides were prepared. The catalysts were characterized by chemical analysis, X-ray phase analysis, BET surface area and activity measurements in methane oxidation. A batch of Mn-Mg-La-Al-O catalyst was prepared for further long-term testing in a model reformer with a catalytic burner. A model reformer with a catalytic burner was designed and fabricated for testing in the composition of the bench-scale Fuel Cell Power Plant. Preliminary testing of this catalyst showed that it provided complete methane combustion at the specified operational temperatures over 900 °C.     

Numerical simulations of the slagging characteristics in a down-fired, pulverized-coal boiler furnace

Numerical studies of the slagging characteristics under different operational conditions in a 300 MW down-fired boiler were carried out using slagging models coupled with gas-solid two phase flow and combustion models. Combined with the real operating conditions; comparative and detailed analysis on the slagging position, extent, and causes is presented. The results show that the serious slagging is mainly on the side walls of the lower furnace. Because of the more rapid expansion of the flue gas under the higher temperature, the flue gas in the furnace center makes the flue gas on both sides deflect and flow to the side walls; and the pulverized-coal flame impinges on the side walls. This results in the slagging on the side walls. Under off-design operating conditions, such as stopping some burners, the local flow field is asymmetric and impinges on the local arch burner, front and rear wall regions where the stopped burners are located. It leads to slight slagging on the arch burner regions and the front and rear wall regions of the lower furnace. Based on the investigation, it has been found that the serious slagging on the side walls can be effectively alleviated by cutting off the burners close to the side walls, reducing boiler load and burning low slagging-tendency coals.     

逆向射流燃烧技术研究进展

逆向射流燃烧技术是可同时适用于燃气和燃煤领域的高效低污染燃烧技术,逆喷结构和射流流速比决定了其流场特性。笔者综述了逆向射流燃烧技术在燃气和燃煤领域的发展历史、研究现状和发展趋势。在燃气领域,逆向射流主要起稳定火焰作用,具有良好的燃料-空气混合条件,形成一个近似均匀的热流场,避免燃烧过程中出现局部热点,但目前仅为一种为燃气轮机和飞机发动机提供的探索性技术,工程应用还需克服燃料和空气在一个狭小空间里的流场合理控制以及从简化装置到工程放大等问题。在燃煤领域,对于煤粉燃烧器,逆向射流可形成一个可控组分、大小、形状和位置的回流区,且将煤粉直接送进回流区,还可控制煤粉在回流区内的停留时间,该技术与传统火焰稳定方式相比,火焰稳定能力更强、停留时间更长、污染更低,更适用于低阶煤的高效燃烧,目前,逆向射流燃烧技术耦合其他稳燃、低氮技术为煤粉高效清洁利用发展提供了新方向,且已有实际工程应用,但对于其机理研究不够深入,限制了其进一步发展与推广。对于电站锅炉,部分一次风或燃尽风逆向偏转射入炉内,可缓解四角切圆燃烧方式下炉膛出口烟气的烟速和烟温偏差,目前主要是燃尽风反切的工业应用,但如何合理控制燃尽风反切角度、反切动量以及反切层数等关键问题还需进一步研究。     

Development of an oxycoal swirl burner operating at low O2 concentrations

This work is to clarify the underlying mechanisms of burning pulverised coal in a mixture of CO₂/O₂. The performance of two different burner designs, single central orifice-type (SCO) and single annular orifice-type (SAO), under oxycoal conditions was examined in a down-fired test facility. Based on detailed in-flame measurements, combined with numerical simulations, the main parameters influencing the stability of a CO₂/O₂ pulverised coal swirl flame were investigated. The oxycoal flame was stabilised at the burner quarl by: increasing the O₂ concentration above 34 vol% without changes to the air-firing burner design and by modifications of the burner geometry thus changing its aerodynamics. The modification of the burner allowed a decrease of the O₂ concentrations to 23 vol% for SCO burner and to less than 21 vol% for SAO burner. Comprehensive measurement data for axial and tangential velocity, flue gas temperature and oxygen concentration for stable oxy-firing at 21 vol% O₂ is presented. The results reported can be used as a guideline for a development of an industrial swirl burner capable of stable operation in both regimes, namely: air and oxycoal.     

Factors influencing the ignition of flames from air-fired swirl pf burners retrofitted to oxy-fuel

Combustion tests were undertaken in a vertical pilot-scale furnace (1.2 MWt) at the IHI test facility in Aioi, Japan, to compare the performance of an air fired swirl burner retrofitted to oxy fired pf coal combustion with the oxy fired feed conditions established to match the furnace heat transfer for the air fired case. A turn down test at a reduced load was also conducted to study the impact on flame stability and furnace performance.Experimental results include gas temperature measurements using pyrometry to infer the ignition location of the flames, flue gas composition analysis, and residence time and carbon burnout. Theoretical computational fluid dynamics (CFD) modelling studies using the Fluent 6.2 code were made to infer mechanisms for flame ignition changes.Previous research has identified that differences in the gas compositions of air and oxy systems increase particle ignition times and reduce flame propagation velocity in laminar systems. The current study also suggests changes in jet aerodynamics, due to burner primary and secondary velocity differences (and hence the momentum flux ratio of the flows) also influence flame shape and type.For the oxy fuel retrofit considered, the higher momentum flux of the primary stream of the oxy-fuel burner causes the predicted ignition to be delayed and occur further distant from the burner nozzle, with the difference being accentuated at low load. However, the study was limited to experimental flames being all Type-0 (low swirl with no internal recirculation), and therefore future work consider higher swirl flames (with internal recirculation) more common in industry.     

Stability boundaries for spinning of fibres based on poly-p-phenylene terephthalamide through an air gap

The restriction of the zone of stationary jet flow of LC sulfuric-acid solutions of PPTA and the conditions of stable spinning of fibres with small air gaps using spinnerets with small-diameter channels is explained with the well-known theory of resonance in drawing for viscoelastic, non-Newtonian media in continuation of the effect of shear flow in the channels on longitudinal flow in the jets. In spinning fibres from 19.5% LC sulfuric acid solutions of PPTA and copolyamide, drawing of the jet in the spinning bath supplements drawing of the jet in the air gap. The former is slight and is a function of the stress created in deformation of the LC solution in the air gap, area of the jet cross-section on the surface of the spinning bath, and type of polymer. The conditions of extreme spinning regimes with breaks in PPTA and polyamide monofilaments in the spinning bath medium in the absence of resonance in drawing of the jet in an air gap are presented. Terlon Co., Mytishchi. Translated fromKhimicheskie Volokna, No. 1, pp. 12–16, January–February, 1999.     

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。     

Pulverized coal flame structures at elevated pressures. Part 1. Detailed operating conditions

The tests and simulations in this study characterize the chemical structure of pressurized pulverized coal flames, particularly (1) how the O₂ in simulated near-burner flame zones is apportioned among the various fuel components; and (2) the burner operating conditions and mechanisms that most strongly affect flame structure. CFD simulations resolved the structures of flames of a subbituminous and two hv bituminous coals for stoichiometric ratios (SR) from 0 to 1.8 for pressures from 1.0 to 3.0 MPa. The structures of all flames were largely determined by the accumulation of particles in the turbulent boundary layer on the flow tube wall. Gaseous fuel compounds always ignited first on the wall at the burner inlet, and this flame propagated toward the flow axis to form a 2D parabolic flame surface. Within the core, residual gaseous fuels, soot, and char may have eventually reached their ignition threshold and burned in a premixed mode. Residual CO, H₂, and char burned in the near-wall region after the volatiles flame had propagated deeper into the core.Whether or not the flame closed on the centerline was mainly determined by pressure and SR. Inlet conditions that formed closed flames at a lower test pressure eventually sustained open flames at progressively higher pressures. The impact of decreasing SR was qualitatively similar, due to the lower heat release rates for progressively lower SR. As the pressure is increased, flame ignition and, by association, flame stability will become more problematic due to the greater thermal capacitance of air streams at progressively higher pressures.     

Effects of recessed air jet on turbulent compressed natural gas inverse diffusion flame shape and luminosity

Effects of the recession of the central air jet on the visible flame height, necking zone, and luminosity of a turbulent compressed natural gas-air inverse diffusion flame in a coaxial burner are investigated in this experimental study. The inner circular tube of the coaxial burner is recessed by 0.25d _a , 0.5d _a , and 1.0d _a , where d _a is the central tube inner diameter. From the visual observation, the flame height and the necking zone height are observed to decrease exponentially with the air jet Reynolds number with no recession of the central air jet. However, only a marginal reduction in the visible flame height is observed with an increase in the recession height of the air jet as compared to the necking zone height. Interestingly, the necking zone at the flame base disappears beyond the critical recession height of the central jet. Moreover, the recession is found to be effective in eradicating the fuel rich zone and soot ring at the flame base of turbulent compressed natural gas inverse diffusion flame at lower air jet Reynolds numbers.     

Experimental investigation of the effect of fuel nozzle geometry on the stability of a swirling non-premixed methane flame

An experimental investigation on the stability of a swirling non-premixed methane flame is reported in this paper. Methane gas is supplied through a central nozzle, and combustion (co-flow) air is supplied through an annulus surrounding the nozzle. Two main parameters were varied independently, which are the nozzle geometry and swirl strength; however the exit velocity of the central (fuel nozzle) jet and co-airflow were also varied to provide a wide range of test conditions. Two nozzles were tested: a contracted circular (referred to hereafter as CCN) and a rectangular (referred to hereafter as RN), which have similar equivalent diameter, D_e (defined as the diameter of a round slot having the same exit area as the nozzle geometry). The contracted circular nozzle has a diameter of 4.82 mm, and the rectangular nozzle has a diameter of 4.71 with an aspect ratio of 2:1. The swirl strength of the co-flow was varied by changing the vanes’ angle. The main results obtained from this study show that the rectangular nozzle exhibits higher entrainment and jet spreading rates compared with its CCN counterpart. In addition, the results revealed that increasing the swirl strength creates a flow recirculation zone which is larger with the RN compared with that of the corresponding CCN. These flow features associated with the RN lead to an enhanced mixing which consequently promotes better flame stability compared with its CCN counterpart.