PIV measurement and turbulence scale in turbulent combustion

We have investigated turbulent combustion by PIV (Particle Image Velocimetry) technique. Comparing with LDV data, the validity of PIV measurements has been confirmed. Particularly, the conditions of sampling number and spatial resolution have been shown to yield reliable data using PIV. Based on the velocity fields in cold flow and combustion, the interaction between flame and flow has been discussed. It was observed that the flow field is changed by combustion and the turbulence is reduced. In order to determine statistical quantities such as mean velocity and RMS of velocity fluctuation, a sampling number of 1000 is needed. Moreover, the velocity correlation coefficient was evaluated to obtain the integral length scale of the flow. For both cold flow and combustion, the PIV estimated scale is very close to that of LDV based on the assumption of Taylor's hypothesis. As a result, the spatial resolution in this study is about 6 times smaller than the integral length scale. © 2006 Wiley Periodicals, Inc. Heat Trans Asian Res, 35(7): 501–512, 2006; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20129     

室内可燃气体泄漏过程中的着火与火焰蔓延

采用小尺度箱体及液化石油气模拟室内燃气泄漏的条件,通过电火花点火,实验研究了在燃气泄漏过程中的着火与火焰蔓延的现象与规律.实验结果表明,火焰首先在燃料燃烧的浓限和稀限之间的可燃区域传播,并且在化学恰当比附近区域的传播速度最大,因此燃气的动态浓度场决定了火焰蔓延的形状.并表明可燃区域是一个逐渐发展的区域,最终其高度与窗沿高度相当,在此区域内点火最容易成功,因此是气体燃料泄漏后发生火灾的最危险的区域.实验结果还揭示了在火焰蔓延过程中,存在着由火焰面推进传播向空间整体燃烧过渡的现象.     

花瓣稳燃器回流区的特性分析

通过数值模拟的方法,详细研究了花瓣稳燃器的回流区特性,给出了花瓣稳燃器回流区的三维立体形状图和回流区长度、宽度、回流量、最大回流速度等特性参数,并对衡量煤粉颗粒与高温回流烟气间掺混速度和掺混强度的掺混系数进行了分析;与普通扩流锥稳燃器的回流区进行了比较.结果表明：花瓣稳燃器的特殊设计能够形成多种回流区,对于旋流燃烧器燃烧低挥发分煤和低负荷运行具有良好的稳燃性能.图6表1参4     

Effect of Turbulence on Flame Propagation in Comstarch Dust-Air Mixtures

Following the quantitative determination of dust cloud parameters, this study investigated the flame propagation through cornstarch dust clouds in a vertical duct of 780 mm height and 160×160 mm square cross section, and gave particular attention to the effect of turbulence on flame characteristics. The turbulence induced by dust dispersion process was measured using a particle image velocimetry (PIV) system. Upward propagating dust flames were visualized with direct light and shadow photography. The results show that a critical value of the turbulence intensity can be specified below which laminar flame propagation would be established. This transition condition is about 10 cm/s. The measured propagation speed of laminar flames appears to be in the range of 0.45-0.56 m/s, consistent with the measurements reported in the literature. For the present experimental conditions, the flame speed is little sensitive to the variations in dust concentration. Some information on the flame structure was revealed from the shadow records, showing the typical heterogeneous feature of dust combustion process.     

GT13E2燃气轮机爆炸事故分析及改进措施

就1台GT13E2型燃气轮机发生的一起燃烧室及排气烟道爆炸事故进行了详细的分析,认为在控制油压力消失和燃烧室熄火的情况下,燃气轮机仍然不能发出跳机指令,进而在燃气轮机转速下降过程中发电机低频保护动作断开主变高压侧断路器,造成厂用电丢失,导致燃气轮机在燃油系统的事故水冲洗过程中发生爆炸。指出了爆炸的原因是由于制造厂燃气轮机控制系统逻辑及控制油管道设计错误,爆炸是设计错误发展的必然结果,以及事故处理和预防措施。     

体积力场对预混火焰面形状的影响

介绍了一种在燃烧场中产生大于1g体积力的实验系统,分析了影响体积力场的因素．利用实时摄像的方法,研究了在燃烧过程中体积力对预混火焰面形状的影响．结果表明,高温烟气在体积力场中的浮力效应,使火焰面向与体积力相反的方向弯曲、偏转．体积力增加将导致火焰面变形程度增加,并导致燃烧过程不稳定,以致发生熄火．气流的射流角不同,体积力对火焰面的影响效果不同,火焰面会发生弯曲、拉伸或压缩．当射流角为负值时,火焰容易发生吹熄．     

Soot concentration and temperature measurements in co-annular, nonpremixed CH4/air laminar flames at pressures up to 4 MPa

Laminar nonpremixed methane-air flames were studied over the pressure range of 0.5 to 4 MPa using a new high-pressure combustion chamber. Flame characterization showed very good flame stability over the range of pressures, with a flame tip rms flicker of less than 1% in flame height. At all pressures, soot was completely oxidized within the visible flame. Spectral soot emission (SSE) and line-of-sight attenuation (LOSA) measurements provided radially resolved measurements of soot volume fraction and soot temperature at pressures from 0.5 to 4.0 MPa. Such measurements provide an improved understanding of the influence of pressure on soot formation and have not been reported previously in laminar nonpremixed flames for pressures above 0.4 MPa. SSE and LOSA soot concentration values typically agree to within 30% and both methods exhibit similar trends in the spatial distribution of soot concentration. Maximum soot concentration depended on pressure according to a power law, where the exponent on pressure is about 2 for the range of pressures between 0.5 and 2.0 MPa, and about 1.2 for 2.0 to 4.0 MPa. Peak carbon conversion to soot also followed a power-law dependence on pressure, where the pressure exponent is unity for pressures between 0.5 and 2.0 MPa and 0.1 for 2.0 to 4.0 MPa. The pressure dependence of sooting propensity diminished at pressures above 2.0 MPa. Soot concentrations measured in this work, when transformed to line-integrated values, are consistent with the measurements of Flower and Bowman for pressures up to 1.0 MPa [Proc. Combust Inst. 21 (1986) 1115-1124] and Lee and Na for pressures up to 0.4 MPa [JSME Int. J. Ser. B 43 (2000) 550-555]. Soot temperature measurements indicate that the overall temperatures decrease with increasing pressure; however, the differences diminish with increasing height in the flame. Low down in the flame, temperatures are about 150 K lower at pressures of 4.0 MPa than those at 0.5 MPa. In the upper half of the flame the differences reduce to 50 K.     

The effects of heat loss on the burning velocity of cellular premixed flames generated by hydrodynamic and diffusive-thermal instabilities

The effects of heat loss on the burning velocity of cellular premixed flames are investigated by two-dimensional unsteady calculations of reactive flows based on the compressible Navier-Stokes equation and on the diffusive-thermal model equation. Hydrodynamic and diffusive-thermal instabilities are taken into account as contributing to the intrinsic instability of premixed flames. A sufficiently small disturbance is superimposed on a planar flame to obtain the relation between the growth rate and the wavenumber, i.e., the dispersion relation. As the heat loss becomes larger, the growth rate decreases and the unstable range narrows. This is because hydrodynamic instability caused by thermal expansion weakens for nonadiabatic flames. To investigate the characteristics of cellular flames, the disturbance with the linearly most unstable wavenumber, i.e., the critical wavenumber, is superimposed. As the superimposed disturbance evolves, the cellular-flame front forms due to the intrinsic instability. The lateral movement of cellular flames is observed at low Lewis numbers, and the behavior of cellular-flame fronts becomes more unstable for nonadiabatic flames. As the heat-loss parameter increases, the burning velocity of a cellular flame normalized by that of a planar flame increases at Lewis numbers lower than unity. By contrast, when the Lewis number is not less than unity, the burning-velocity increment decreases by increasing the heat loss. Diffusive-thermal instability thus has a pronounced influence on the unstable behavior and burning velocity of nonadiabatic cellular flames.     

Assessment of closure schemes in second-order conditional moment closure against DNS with extinction and ignition

The performance of second-order conditional moment closure (CMC) depends on models to evaluate conditional variances and covariances of temperature and species mass fractions. In this paper the closure schemes based on the steady laminar flamelet model (SLFM) are validated against direct numerical simulation (DNS) involving extinction and ignition. Scaling is performed to reproduce proper absolute magnitudes, irrespective of the origin of mismatch between local flamelet structures and scalar dissipation rates. DNS based on the pseudospectral method is carried out to study hydrogen-air combustion with a detailed kinetic mechanism, in homogeneous, isotropic, and decaying turbulent media. Lewis numbers are set equal to unity to avoid complication of differential diffusion. The SLFM-based closures for correlations among fluctuations of reaction rate, scalar dissipation rate, and species mass fractions show good comparison with DNS. The variance parameter in lognormal PDF and the constants in the dissipation term have been estimated from DNS results. Comparison is made for the resulting conditional profiles from DNS, first-order CMC, and second-order CMC with correction to the most critical reaction step according to sensitivity analysis. Overall good agreement ensures validity of the SLFM-based closures for modeling conditional variances and covariances in second-order CMC.     

Soot formation in laminar diffusion flames

Laminar, sooting, coflow diffusion flames at atmospheric pressure have been studied experimentally and theoretically as a function of fuel dilution by inert nitrogen. The flames have been investigated with laser diagnostics. Laser extinction has been used to calibrate the experimental soot volume fractions and an improved gating method has been implemented in the laser-induced incandescence (LII) measurements resulting in differences to the soot distributions reported previously. Numerical simulations have been based on a fully coupled solution of the flow conservation equations, gas-phase species conservation equations with complex chemistry, and the dynamical equations for soot spheroid growth. The model also includes the effects of radiation reabsorption through an iterative procedure. An investigation of the computed rates of particle inception, surface growth, and oxidation, along with a residence time analysis, helps to explain the shift in the peak soot volume fraction from the centerline to the wings of the flame as the fuel fraction increases. The shift arises from changes in the relative importance of inception and surface growth combined with a significant increase in the residence time within the annular soot formation field leading to higher soot volume fractions, as the fuel fraction increases.