变磨煤机组合下的墙式对冲燃煤锅炉炉内燃烧特性数值模拟

为了研究亚临界墙式对冲燃煤锅炉变燃烧器组合工况下的炉内燃烧特性,建立了锅炉炉膛的研究模型,利用数值模拟方法对某电厂600 MW亚临界对冲燃煤锅炉开展了数值模拟研究,并分析了锅炉变燃烧器工况下对炉膛温度场和燃尽率的影响。结果表明:停用前墙上层燃烧器比停用后墙上层燃烧器的炉膛出口温度稍高;停用上层燃烧器比停用下层燃烧器更有利于煤粉燃尽和降低排放量;以上模拟分析为以后的锅炉运行的经济性和环保性提供指导依据。     

超低负荷电站锅炉燃烧特性CFD模拟

为了研究超低负荷下电站锅炉不同燃烧器布置方式对炉内煤粉燃烧等特性的影响，采用CFD方法对一台600 MW前后墙对冲电站锅炉进行了数值模拟研究。结果表明，超低负荷下，与投运1层燃烧器相比，投运2层燃烧器的炉内温度分布更均匀，火焰充满度更好，燃烧更加稳定，炉内气氛还原性更强。将燃烧器布置于下层，NO排放量减少，燃尽率提升，炉膛出口烟温降低。综合考虑炉内温度、气氛分布和炉膛尾部参数，在25%BMCR超低负荷下，投运2层燃烧器并布置于下层为最佳工况。     

两层多孔介质燃烧器的数值模拟

通过一维数值模拟研究了预混气体在两层多孔介质燃烧器内的燃烧特性,着重研究两层多孔介质燃烧器中的超绝热燃烧和火焰的稳定区域。结果表明,预混气体在两层多孔介质内可以发生一定程度的超绝热燃烧,贫燃极限可以扩展到0.45。两层多孔介质能够在较宽的流速范围内将火焰稳定在它的交界面上。数值预测的最小和最大火焰传播速度与实验取得了相同的趋势,其火焰传播速度至少是自由空间中的3倍。     

660MW旋流对冲锅炉低NO_x燃烧优化数值模拟

对一台660 MW旋流对冲锅炉在不同层燃烧器组合运行工况下进行了炉内流动、燃烧和NOx排放特性的数值模拟,模拟结果与试验值相对误差小于10%。模拟结果表明:额定负荷时,5层燃烧器运行,停前墙和停后墙同层燃烧器时NOx排放和飞灰含碳量基本相同;停上层燃烧器相比于停中层燃烧器,空气分级效果强化,煤粉颗停留时间增加,NOx和飞灰含碳量分别降低9.5%和9.8%,在锅炉实际运行过程中,停上层燃烧器更有利于降低NOx排放和提高燃烧效率。     

利用火焰推举距离判定燃煤锅炉低负荷稳燃性能的研究

为研究低负荷工况下锅炉的稳燃能力,定义了火焰推举距离,即以900℃的温度作为火焰边界,以火焰边界与燃烧器出口之间的距离作为火焰推举距离。采用ANSYS Fluent模拟软件对某电厂350 MW前后墙对冲锅炉内的燃烧状况进行模拟,并根据试验确定了该锅炉的最低发电负荷为24. 8%时,火焰推举距离的极限值为3. 15 m。进一步分析了煤种、煤粉粒径、一次风量、二次风量、二次风温度、再循环烟气量和燃烧器燃烧方式等因素对火焰推举距离的影响,研究表明:减小煤粉粒径、减少一次风量、关闭再循环烟气量和燃烧器对冲燃烧方式,这四种因素能显著地减小火焰推举距离。最后综合这四个有利因素,将煤粉粒径减小至5. 6×10~(-5)m,通过磨煤机和燃烧器的阀门调节把一次风量降低至60%的风量,关闭再循环烟气挡板,采用上层燃烧器冲燃烧方式,能够将该锅炉的发电负荷降低至7. 39%的最低负荷,并保持安全稳定运行。     

燃烧模型在NexGen燃烧器出口流场模拟中的应用

通过求解三维定常雷诺平均的N-S方程,对NexGen燃烧器的出口流场进行数值模拟。首先,利用冷态流场的实验数据验证燃烧器几何模型和数值计算方法的有效性。然后,在计算中分别选取火焰面模型、混合分数PDF模型和涡耗散模型3种燃烧模型,比较燃烧模型对燃烧器出口流场模拟结果的影响。研究结果表明:燃烧模型对Nex Gen燃烧器出口的速度场、火焰形状和热流密度分布基本没有影响,但是对火焰长度、火焰最高温度、最高热流密度及校准面上温度分布和温度值有较大影响。相比火焰面模型和涡耗散模型,混合分数PDF模型的计算结果与实验结果吻合较好,可以为防火试验方案设计提供参考。     

H2/CO合成气层流扩散燃烧产物NOx生成机理的数值分析

采用Chemkin软件中的OPPDIF(对冲扩散火焰)模型对H_2/CO合成气燃烧产生的NO_x排放特性进行了数值研究,分析了合成气扩散火焰中H_2体积分数对NO_x形成的影响。使用OPT(光学薄辐射)模型考察了辐射散热对模拟火焰的影响,采用GRI-Mech 3.0详细化学反应机理研究了NO_x的生成机制。数值模拟结果表明:非绝热条件下,随着合成气中H_2体积分数的增加,层流对冲扩散火焰的峰值温度单调增加;在绝热条件下,合成气火焰的峰值温度会随H_2体积分数的增加而略微下降;同时,随着H_2体积分数的增加,合成气燃烧产生的NO含量明显增加,其中热力型NO的生成量随H_2体积分数的增加变化明显,主导着NO生成量的变化趋势。     

壁面厚度和材料对微通道内H_2/空气燃烧特性影响的数值模拟

通过数值计算对二维平板微燃烧器内H_2/空气的预混燃烧特性进行研究,讨论了壁面厚度和材料对火焰位置、外壁面温度、燃烧效率、热循环比和散热损失比的影响.结果表明:平板的热导率越高、平板厚度越大,火焰的稳定性就越好,吹熄极限和偏斜极限就越大;平板厚度越大、热导率越高,火焰位置越靠近燃烧器的上游,且平板外壁面温度分布更加均匀,燃烧效率和热循环比更高;燃烧器的散热损失比受到火焰温度的影响,并随着火焰温度的升高而降低,但是当进气速度达到偏斜极限时,燃烧效率和火焰温度开始下降,从而导致燃烧的散热损失比开始急剧下降.     

130 t/h四角切圆煤粉炉低氮燃烧改造的试验研究及数值模拟分析

运用试验研究及数值模拟方法,分析了130 t/h四角切圆煤粉炉进行低氮燃烧改造后,空气分级对炉内火焰特性、NO_x排放和灰渣结焦的影响。研究表明:分离燃尽风(SOFA)比率为24. 8%时,燃烧器区域的平均燃烧温度下降120℃,SOFA风上部的烟气温度上升80℃;改造后锅炉NO_x排放量下降了36. 4%; CFD数值模拟与NO_x实测排放的相对误差在±5%以内,证明了数值模拟的可靠性;随着SOFA比率的增大,燃烧器上部火焰切圆直径增大,较高温度下灰中结焦液体含量增多,反映了炉内SOFA风上部炉壁更易结焦,这与实际情况相符。     

300 MW墙式切圆锅炉水冷壁超温的数值模拟与燃烧优化

某300 MW四墙切圆燃烧锅炉在实际运行过程中一直存在着燃烧器区域的火焰刷墙与高温腐蚀现象,导致燃烧器区域水冷壁易超温,严重影响机组的安全运行.通过现场试验与数值模拟,分析了产生以上问题的原因,并针对性地提出了相应的燃烧器改造与优化方案.结果表明:造成水冷壁超温与高温腐蚀的主要原因是燃烧时火焰产生的切圆过大、火焰气流距...     

On scaling and mathematical modelling of large scale industrial flames

Gaseous flames and pulverized coal flames are considered to examine relationships between lab-scale flames, semi-industrial scale and industrial-scale flames. The experimental data spans the thermal input range from the lowest scale of 30 kW to the largest of 12 MW with several intermediate scales. The primary questions are whether effects observed in lab-scale flames are scalable to industrial applications and whether mathematical models developed on the basis of lab-scale data are directly applicable to industrial flames.It has been observed that disparity between the in-flame temperatures measured in lab-scale and in large-scale flames can be as large as 100–200 K due to different measurement techniques used. In lab-scale experiments one observes a strong interaction between turbulence and chemistry and the measured data is sensitive to small alterations to burner inputs and/or boundary conditions. The sensitivity almost disappears at large-scales since the convective mixings is the dominant (the slowest) mechanism. In other words, different effects are seen at small- and large-scales and different mechanisms are controlling. Although the paper is concerned with single flames, in our opinion, the conclusions are also relevant to gas turbines. Until genuine efforts are taken to develop a good understanding of combustion system scaling, the worlds of combustion science and combustion engineering will remain parted.     

Effect of elliptic burner geometry and air equivalence ratio on the nitric oxide emissions from turbulent hydrogen flames

An experimental study on turbulent hydrogen flames from circular and elliptic burners with varying degrees of premixedness (diffusion, fuel-rich, stoichiometric, and fuel-lean) is presented. Flame stability, visible flame height, flame radiation, global nitric oxide (NO) concentration, and inflame temperature and NO concentration profiles were measured. We found that the elliptic burner flames had lower liftoff velocity, were shorter, and radiated less heat to the surrounding as compared to circular burner flames. Global NO concentration decreased with an increase in air equivalence ratio for both circular and elliptic burner flames. Peak in-flame NO concentration along the flame centerline increased with a decrease in air equivalence ratio. Elliptic burner flames produced higher peak in-flame temperatures. Overall, the elliptic burner flames produced less peak NO as compared to circular burner flames at all air equivalence ratios except zero (diffusion flames) in accordance with the global emission measurements.     

How “flat” is the rich premixed flame produced by your McKenna burner?

McKenna burners are widely used in the combustion community for producing “flat” premixed flames. These flames are considered as standards for the development and calibration of optical techniques. Rich premixed flames produced by McKenna burners are frequently investigated in order to understand soot formation processes both by optical and by sampling techniques. Measurements are normally performed along the axis of the flames, with a uniform distribution of temperature and species concentration assumed in the radial direction. In this work it is shown that the soot radial profiles of rich premixed ethylene–air flames produced by a McKenna burner with a stainless steel porous plug may be far from being “flat.” Soot is mainly distributed in an annular region and nonsoot fluorescing species are present in the core of the flames. This surprising result was verified under several working conditions. Furthermore, flames cannot be considered axial-symmetric but present a skewed soot distribution. Another McKenna burner with a bronze porous disk was used to produce flames of the same equivalence ratio and flows. These flames show a completely different soot radial profile, closer to the claimed flat distribution. These results cast doubts about the conclusions drawn in several studies on soot formation performed with a stainless steel McKenna burner.     

A new burner for studying auto-ignition in turbulent dilute sprays

This paper introduces a new burner to study the auto-ignition characteristics of dilute turbulent spray flames. The spray is formed upstream of the exit plane and carried with air or nitrogen into a hot co-flowing stream of vitiated combustion products. The fluid dynamics is kept intentionally simple and the boundary conditions are well-defined such that the burner lends itself easily to computations. The stability characteristics are presented here for a range of liquid fuels as a function of the controlling parameters. For each fuel, three types of flames are identified based on their visual appearance. These flames are all lifted but differ in the shape of the leading edge and heat release zones. These characteristics are similar for all fuels and three flames, one from each type, are selected for further study. Measurements of mean temperature, velocity and droplet fields as well as high-speed LIF images of OH, Mie scattering from droplets and chemiluminescence of CH^∗ are made in the selected flames of ethanol. It is found that the leading edge of the flame, which is a hardly visible light-blue zone close to the jet exit plane, is marked by the presence of OH (but not CH^∗), and slight increases in peak temperature above the levels of the co-flow. Further downstream, there exists an intense blue region that is characterized by higher OH levels, significant emission of CH^∗, and temperatures that are much higher than those of the co-flow. This is the region where the bulk of the heat release is taking place. Pockets of CH^∗ are seen to appear in this region and grow as they are advected further downstream.     

旋流对天然气高温空气燃烧特性影响的数值模拟

以工业炉的高温空气燃烧技术应用为背景,对一个新型轴向旋流式单烧嘴燃烧室内天然气的高温空气燃烧特性进行了数值研究。采用数值模拟的方法研究了同心式轴向旋流燃烧器（HCASbumer）中螺旋肋片的旋转角度对燃烧特性的影响,其中湍流采用Reynolds应力模型,气相燃烧模拟采用β函数形式的PDF燃烧模型,采用离散坐标法模拟辐射换热过程,NOx模型为热力型与快速型。计算结果表明,对预热空气采用旋转射流时,能明显降低NOx生成量。对于HCAS型燃烧器,随着空气射流旋转角度的增大,燃烧室内的回流区域增大增强,降低了局部的氧体积分数分布,燃烧室中平均温度和最高温度都有所增加,且燃烬程度大幅度提高,而局部高温区缩小,只在靠近入口处出现。总的NOx排放量随着空气射流旋转角度的增大先减小,后增大。因此,适当调整肋片的旋转角度可以降低NOx生成量。     

Pilot impact on turbulent premixed methane/air and hydrogen-enriched methane/air flames in a laboratory-scale gas turbine model combustor

The impact of pilot flame operation on the combustion of pure methane and hydrogen-enriched methane (H₂/CH₄: 50/50 in vol%) fuels was investigated in a gas turbine model combustor under atmospheric conditions. The burner assembly was designed to mimic the geometry of an industrial burner, the Siemens DLE Burner, in which a concentric annular ring equipped with pilot flame burners is implemented in the dome of the combustor. Two pilot burner configurations have been investigated: a non-premixed and a partially premixed pilot arrangement. The performance of the pilot burners was evaluated for varying Reynolds number (Re) and H₂ enrichment. High-speed OH1 chemiluminescence imaging, as well as simultaneous planar laser-induced fluorescence measurements of the OH radicals and formaldehyde (CH₂O) were used for evaluating the dynamics and structures of the flames for different conditions. Furthermore, emission measurements were carried out to determine the influence of hydrogen dilution on the NO_x and CO emission levels. The main findings are (a) the effect of the pilot flame is sensitive to the Reynolds number of the main flame and the type of the pilot flame, (b) the stability range becomes narrower with increasing hydrogen ratio, due to the tendency to flashback, (c) non-premixed pilot flames lower the NO_x and increase the CO emissions, albeit rather small differences in the emissions have been detected, and (d) the NO_x and CO emissions become significantly lower with increasing hydrogen ratio.     

Evaluation of the influence of OEC on soot formation and thermal radiation in confined acetylene diffusion flames

The effect of OEC on soot formation and thermal radiation was studied in confined acetylene diffusion flames. Confined flames are widely used in industrial settings. The flames were produced in a combustion chamber with a burner operating with a parallel annular coaxial flow of the oxidizer. The soot concentration was calculated by the laser-induced light extinction method. The thermal radiation was measured with a radiometer in the narrow band of influence of soot radiation. The oxygen content in the combustion air was less than 30% – 21 to 25% – which does not require significant retrofitting of existing equipment when combustion conditions are varied. The results suggest that the use of OEC enables soot formation and thermal radiation in confined acetylene flames to be managed and controlled.     

Tubular premixed and diffusion flames: Effect of stretch and curvature

Tubular flames are ideal for the study of stretch and curvature effects on flame structure, extinction, and instabilities. Tubular flames have uniform stretch and curvature and each parameter can be varied independently. Curvature strengthens or weakens preferential diffusion effects on the tubular flame and the strengthening or weakening is proportional to the ratio of the flame thickness to the flame radius. Premixed flames can be studied in the standard tubular burner where a single premixed gas stream flows radially inward to the cylindrical flame surface and products exit as opposed jets. Premixed, diffusion and partially premixed flames can be studied in the opposed tubular flame where opposed radial flows meet at a cylindrical stagnation surface and products exit as opposed jets. The tubular flame flow configurations can be mathematically reduced to a two-point boundary value solution along the single radial coordinate. Non-intrusive measurements of temperature and major species concentrations have been made with laser-induced Raman scattering in an optically accessible tubular burner for both premixed and diffusion flames. The laser measurements of the flame structure are in good agreement with numerical simulations of the tubular flame. Due to the strong enhancement of preferential diffusion effects in tubular flames, the theory-data comparison can be very sensitive to the molecular transport model and the chemical kinetic mechanism. The strengthening or weakening of the tubular flame with curvature can increase or decrease the extinction strain rate of tubular flames. For lean H₂-air mixtures, the tubular flame can have an extinction strain rate many times higher than the corresponding opposed jet flame. More complex cellular tubular flames with highly curved flame cells surrounded by local extinction can be formed under both premixed and non-premixed conditions. In the hydrogen fueled premixed tubular flames, thermal-diffusive flame instabilities result in the formation of a uniform symmetric petal flames far from extinction. In opposed-flow tubular diffusion flames, thermal-diffusive flame instabilities result in cellular flames very close to extinction. Both of these flames are candidates for further study of flame curvature and extinction.     

燃气燃烧器燃烧特性的数值模拟

运用Fluent软件对40 t/h锅炉用燃气燃烧器进行三维数值模拟,研究进风量对燃烧器性能及污染物NOx排放特性的影响。结果表明:合适的进风量是保证燃烧器稳定燃烧的重要条件,也是减少NOx排放的关键因素。数值模拟的计算结果为燃烧器优化设计、改造提供了参考依据。     

天然气在渐变型多孔介质中的预混燃烧

设计了孔径沿程变化的渐变型多孔介质(GVPM)燃烧器，为了解天然气在其中的预混燃烧特性，对燃烧室气体、固体温度分布和CO、NO；污染物排放进行了测量．试验研究了渐变型多孔介质中燃烧的温度场分布、火焰移动、污染物排放、稳定性及多孔介质孔径结构对燃烧特性的影响规律．将研究结果与几种均匀型多孔介质(HPM)中的燃烧进行比较，发现渐变型多孔介质中的燃烧可以有更多的优点，包括均匀温度场分布、极低污染物排放、高火焰速度、高稳定性、宽燃烧极限和有很大的负荷调节范围等．