燃烧参数影响高温空气燃烧特性的数值分析

高温空气燃烧技术具有高效节能和低NOx排放等多重优越性，是一种新型燃烧技术。为了深入研究高温空气燃烧机理和低氮氧化物排放特性，将湍流N—S方程与扩散燃烧模型和热力型NO生成模型相结合，研究了低氧浓度条件下，燃烧参数，如燃气供应量，过量空气系数，进口空气预热温度以及进口空气氧含量对燃烧的影响，为发展高温空气燃烧技术提供了理论依据。     

平焰半预混燃烧NOx排放特征的试验研究

实验针对采用了半预混、强旋流平焰燃烧的方法,以液化石油气为燃料,组织通过改变空气过剩系数和旋流数工况来进行燃烧实验,用热电偶测量温度,烟气分析仪测量NOX浓度,研究分析了炉膛温度、及过剩空气系数和旋流数对平焰燃烧NOX排放的影响。结果表明,分别取三组不同的空气过剩系数1.06,1.23和1.37后,它们在1.76旋流数下所达到的最高温度是1 159,1 057,993℃;而NOX浓度分别为60×10-6,60×10-6,30×10-6。     

低NOx高温空气燃烧技术

低NOx高温空气燃烧技术将传统的低NOx燃烧技术与高温热式燃烧系统有地结合起来,具有热效率高、炉内温度分布均匀、NOx排放量低等特点。本文介绍了高温空气燃烧技术,重点分析了高温空气燃烧技术中的低NOx排放的原理,并对两种采用烟气再循环和分级燃烧技术的NOx高温空气燃烧器进行阐述。     

高温低氧燃烧条件下氮氧化物的生成特性

高温低氧燃烧原理是高温空气燃烧技术赖以发展的基础，使得高温燃烧条件下的氮氧化物的生成与排放受到大大抑制。为了掌握这种非常规燃烧现象及污染物生成的基本规律，采用扩散燃烧模型、热力NO生成模拟与湍流N－S方程，数值研究了燃烧空间中空气氧浓度对燃烧特性和氮氧化物排放浓度的影响，再现了高温与低氧两种条件相结合，形成的稳定的低氮氧化物排放的燃烧特性。计算结果与实验数据吻合，为发展高温空气燃烧技术提供了理论基础。     

高温空气燃烧技术

高温空气燃烧技术是一项燃烧领域的高新技术。具有不同于传统燃烧技术的新特点。燃料在1200℃高温空气和氧浓度约5％的炉内环境中燃烧，达到高效低污染物排放的要求。本文简要介绍了该技术的基本原理和其中的关键技术，以及最新的低NOx高温空气燃烧器。     

高温空气燃烧技术的最新进展

高温空气燃烧技术是90年代得到迅速发展的一项燃烧领域的高新技术。具有不同于传统燃烧技术的新特点,燃料在1200℃高温空气和氧浓度约5％的炉内环境燃烧,达到高效低污染物排放的要求。本文简要介绍该技术的基本原理和最新进展。     

高温空气燃烧技术的开发应用、技术优势及其展望

高温空气燃烧技术和高温空气气化技术是当前世界节能与环保领域中的两大新技术,二者均采用高于燃料着火点温度的高温空气作氧化剂或气化剂。介绍了利用蓄热式高温烟气余热回收装置和专门的高温空气发生器产生高温空气的方法,前者主要用于高温空气燃烧技术,后者主要用于高温空气气化技术。概括了高温空气燃烧技术和高温空气气化技术的应用状况,总结了其技术优势,并指出高温空气燃烧技术和高温空气气化技术符合中国国情,具有巨大的开发潜力和广阔的市场前景。     

涡流室式柴油机准维相关火焰微元燃烧模型的研究

把涡流室式柴油机不同区域与不同时期的燃烧过程分开处理，将涡流室的燃烧过程分为５个时期，即：低温着火化学动力学反应期，向高温预混燃烧化学动力学反应过渡期，高温预混燃烧化学动力学反应期，向空气和燃料混合控制的扩散燃烧过渡期和火焰微元的扩散燃烧期。而主燃烧室的燃烧只有火焰微元的扩散燃烧期。用Ｓｈｅｌ着火模型、Ａｒｈｅｎｉｕｓ方程和相关火焰微元模型来分别模拟其中的低温着火、高温预混燃烧和扩散燃烧过程以建立准维燃烧模型。模型预测的示功图和燃烧放热率与实验值吻合良好。本文还研究了模型中拉伸因子和耗散因子对示功图的影响。     

高温空气燃烧技术(HTAC)及其应用效果

高温空气燃烧技术是近十年来高速发展的一种新型燃烧技术，因同时具有高效、节能和低污染等特性，目前正得到越来越广泛的应用。本文介绍了高温空气燃烧技术的由来、工作原理、特点及应用效果，并分析了这种燃烧技术在我国的应用前景。     

预混燃烧数值模拟与结构改进

对煤气-空气预混燃烧进行了数值模拟,通过模拟研究了燃烧器的结构对煤气-空气预混效果的影响,优化了燃烧器的结构,使煤气-空气预混效果达到最佳。模拟结果与实际燃烧过程情况相符。     

进气参数对LPP燃烧室振荡燃烧特性和火焰结构的影响

为研究贫预混预蒸发(LPP)燃烧室振荡燃烧规律和LPP火焰结构,利用动态压力传感器测量了LPP燃烧室内不同进气参数下时域及频域上的压力脉动;利用激光诱导荧光(PLIF)测量系统研究了不同进气参数下的LPP火焰结构变化规律。结果表明:随着燃烧室入口流速的增加,激励出的振荡燃烧的当量比区域会减小;在一定的入口流速下,所激励的振荡燃烧主频会随着当量比的增加而增加;随着燃烧室入口空气温度的提高,激励出振荡燃烧的区域会减小,激励出的振荡燃烧的强度会下降,但振荡燃烧的主频均会增加;稳定燃烧时,LPP火焰为V型火焰;振荡燃烧则会将LPP火焰转化为平整型火焰。     

Numerical studies on flame inclination in porous media combustors

Inclinational instability developing during propagation of a filtration combustion wave in an inert porous medium is studied using two-dimensional numerical model. Stable and unstable combustion waves are generated by varying combustion parameters such as pressure, equivalence ratio, filtration velocity, effective conductivity of porous media, pellet diameter and combustor scale. The wave propagation velocity of inclinational flame is studied and compared with flat flame. The growth and reduction of inclinational instability are analyzed at different conditions. The numerical results show that a development of inclinational instability causes essential flow non-uniformity and can result in a separation of the flame front in the multiple flame zones. The limited conductive and radiant heat transfer in the solid phase, small pellet diameter of packed bed, high inlet velocity, large combustor scale and low equivalence ratio promote the instability growth. The inclinational instability is suppressed in a reciprocal combustor.     

阻火器型式对防爆柴油机性能影响的对比分析

通过CFD-Fluent对排气平板/波纹阻火器内部流场、压力场进行流体数值模拟。结果表明,对于降低排气背压,波纹阻火器效果优于平板阻火器,且能保证防爆效果。对防爆柴油机排气防爆系统进行台架试验,分析了TY4100QFB型防爆柴油机加装相等废气流通面积、不同规格平板/波纹阻火器时,外特性下不同转速排气背压对柴油机动力性、经济性和排放的影响规律。试验结果表明,波纹阻火器因其较大的空隙率对于降低排气阻力,优化燃烧,降低CO、NO_x、PM排放效果显著,CO、NO_x、PM平均降幅分别为8.5%、4.6%、11.45%,转矩、燃油消耗无显著变化。同时波纹阻火器有利于减小设计尺寸,降低质量。     

n-Octane combustion in a flat flame

In the present paper new results on the combustion of n-octane obtained by molecular beam mass spectrometry in a flat flame are presented. Corresponding experiments on i-octane have been reported [1] and comparisons to that compound are made to explore the different ignition and combustion behaviour of n- and i-octane.     

The effect of chemical energy release on heat transfer from flames in small channels

Stable combustion in a heated tube, with a radius on the order of the flame thickness, is investigated experimentally and numerically. The downstream portion of the tube is heated by an external heat source resulting in a steady, axially varying temperature gradient along the tube wall. Strongly burning, axisymmetric methane/air flames are stabilized inside this wall temperature profile which are observed to be “flat” for sufficiently small tube dimensions. The position of these flames is dictated by a competition between the energy required to preheat the reactants, that released by combustion, and the heat lost to the wall. To model such flames, an extension to the standard 1-D, volumetric flame formulation is proposed to solve for wall/gas heat transfer by employing a thermal boundary layer. The boundary layer utilizes a non-linear, radially-varying heat source to account for combustion and captures the effect of enhanced interfacial heat transfer inside the reaction zone. The proposed numerical model gives improved quantitative predictions for flame stabilization position than approaches which neglect the effect of heat release by modeling heat transfer with Newton’s law of cooling and a local Nusselt number.     

Radiation effects on flame stabilization on flat flame burners

This study investigates the impact of radiative heat transfer on the behavior of flat flame burners within the framework of a simplified one-dimensional model. Flat flame burners stabilize planar premixed flames downstream of a porous plug. Within this study, the porous plug is modeled as a thermally conducting, optically thick medium, allowing for both conductive and radiative heat transfer. Based on the simplified model, the impact of radiative heat exchange between the porous plug exit and the downstream environment is investigated. In “surface” combustion, flame stabilization occurs due to heat transfer between gas phase and porous solid. Results demonstrate that radiative heat transfer from a hot downstream environment to the porous plug significantly increases maximum attainable mass fluxes. For a cold downstream environment, plug properties do not affect the maximum supportable mass flux, although plug porosity and heat transfer between gas and solid have a significant impact on the “stand-off” distance between flame and plug exit. In addition, the model provides insight to a second “submerged” combustion mode, where the flame is stabilized within the porous plug of the burner. Here, increased flame temperatures lead to a dramatic increase of the maximum supportable mass flux. Overall, results show that radiative heat losses play a critical role in both combustion modes: in surface combustion, they are an important mode of heat dissipation, where they can prevent “flash-back” conditions with the flame moving into the porous matrix; in submerged combustion, they prevent flame stabilization close to inlet and exit faces and enable a “slow” solution branch that does not exist without radiative losses.     

Combustion characteristics of boron nanoparticles

An experimental investigation of the combustion characteristics of boron nanoparticles in the post flame region of a flat flame burner has been conducted. Boron is attractive as a fuel or a fuel supplement in propellants and explosives due to its high heats of combustion on both a gravimetric and volumetric basis. A relatively large database exists for combustion characteristics of large (greater than 1 μm) boron particles, but very little exists for nano-sized boron. Ignition and combustion characteristics have been studied in the post flame region of a fuel lean CH₄/Air/O₂ flame, with burner temperatures ranging from about 1600 K to 1900 K, and oxygen mole fractions ranging between 0.1 and 0.3. As in earlier investigations on boron combustion, a two-stage combustion phenomenon was observed. Ensemble-averaged burning times of boron nanoparticles were obtained, while the ignition time measurements for boron nanoparticles were extended into a lower temperature range previously unavailable in the literature. The measured burning times were between 1.5 ms and 3.0 ms depending on both the temperature and oxygen mole fraction. The ignition times were relatively insensitive to oxygen concentration in the range studied, and were affected only by temperature. The measured ignition times were inversely related to the temperature, ranging from 1.5 ms at 1810 K to 6.0 ms at 1580 K. The burning time results were compared to both diffusion and kinetic limited theories of particle combustion. It was found that the size dependence on particle burning times did not follow either theory.     

Characteristics of premixed hydrogen/air squish flame in a confined vessel

The appearance of the squish flame is of great significance to accelerate burning progress and improve the combustion efficiency. In this paper, we experimentally studied the characteristics of the squish flame in a cylindrical constant volume vessel under different initial pressures and equivalence ratios by using high-speed schlieren photometry. Due to the compression of the main flame front, “squish flow” was induced in the analogous triangular vertebrae region besieged by the convex flame front, the concave wall and the flat optical windows, which provided the perturbation of large wavelength to promote the appearance of the squish flame. When the squish flames occur, as the initial pressure increases, the main flame propagation distance becomes shorter, the main flame propagation velocity increases first and then gradually saturates to a certain value; as the equivalence ratio increases, the main flame propagation distance becomes longer, the main flame propagation velocity rises first and then declines, and the maximum is obtained in the vicinity of Φ = 1.0. There exists a critical initial pressure at each equivalence ratio below which no squish flame appears, and it takes on a U-shaped trend with the increase of equivalence ratio. The hydrodynamic instability plays a key role in the formation of the squish flame. The squish flame tends to appear at higher hydrodynamic instability. The formation mechanism and the critical feature of the squish flame obtained in this paper can provide a theoretical guide to achieve fast controllable combustion.     

Assessment of the release of atomic Na from a burning black liquor droplet using quantitative PLIF

The quantitative measurement of atomic sodium (Na) release, at high concentration, from a burning black liquor droplet has been demonstrated using a planar laser-induced fluorescence (PLIF) technique, corrected for fluorescence trapping. The local temperature of the particle was measured to be approximately 1700 °C, at a height of 10 mm above a flat flame burner. The PLIF technique was used to assess the temporal release of atomic Na from the combustion of black liquor and compare it with the Na concentration in the remaining smelt. A first-order model was made to provide insight using a simple Plug Flow Reactor model based on the independently measured concentration of residual Na in the smelt as a function of time. This model also required the dilution ratio of the combustion products in the flat flame entrained into the plume gas from the black liquor particle to be estimated. The key findings of these studies are: (i) the peak concentration of atomic Na from the combustion of the black liquor droplets is around 1.4 ppm; (ii) very little atomic Na is present during the drying, devolatilisation or char combustion stages; and (iii) the presence of atomic Na during smelt phase dominates over that from the other combustion stages.     

Bulk and wall flame quenching in nonuniform concentration fields

Schlieren photographic experiments on laminar flame quenching both in bulk gases and on a flat wall have been conducted in nonuniform concentration fields that simulate local gaseous conditions in an engine. The combustion takes place at atmospheric pressure and room temperature through a combustible layer formed in an interface between layers of fuel and air. Bulk flame quenching is observed in methane/air mixtures under conditions such as increased concentration gradient, reduced ignition energy, and addition of nitrogen in methane. This raises the possibility that any causes, even turbulence, generating spatial nonuniformity in concentration in an engine may result in a decrease in combustion efficiency. That is, bulk flame quenching of the present type may contribute appreciably to unburned hydrocarbon formation. Both head-on wall quenching and side-on quenching were observed for methane/air and hydrogen/air mixtures, respectively. The results show that the present combustor may be capable of providing proper data reduction of the wall flame quench layer thickness in practical mixtures.