燃气轮机燃烧室液雾自燃延迟时间预测方法

为减少一体化加力燃烧室内支板火焰稳定器高度与进口试验参数较高所导致的昂贵基础试验成本,采用经试验数据验证的数值计算方法,对不同高度的一体化模型加力燃烧室燃烧性能进行数值模拟,分析模型加力燃烧室高度变化和侧壁边界层效应对一体化加力燃烧室回流区、总压恢复系数以及燃烧效率的影响。在保持空间油雾场分布均匀与阻塞比一致的前提下,简化扇形加力燃烧室模型为矩形加力燃烧室模型,其中模型加力燃烧室高度H分别为200,150和100 mm,总长L=1 480 mm,宽B=125 mm。结果表明：模型加力燃烧室高度的降低对燃烧性能影响较小,其中回流率最大降幅为0.16%,总压恢复系数最大降幅为0.15%,燃烧效率的最大降幅为1.9%;模型加力燃烧室侧壁面边界的引入对燃烧性能影响较小,回流率、总压恢复系数最大降幅均小于1%,燃烧效率的最大降幅仅为0.7%;可以采用单支板火焰稳定装置降低高度的方法简化试验件设计。     

半封闭狭窄通道内甲烷/空气预混火焰传播不稳定性实验研究

针对贫油预混预蒸发燃烧室主燃级中横喷液雾现象进行研究,综合考虑RP-3航空煤油横喷液雾的雾化、蒸发和自燃过程构建自燃预测模型,基于CH基团随时间的变化规律对自燃延迟时间进行预测。结合试验测试结果对模型进行校验,并进一步分析温度、压力、流速、射流动量比等变量对自燃延迟时间的影响规律。结果表明：对于直射式喷嘴形成的横喷液雾,其下游的油气分布主要受射流动量比和流动速度的影响,射流动量比决定了液雾的总体油气比,流动速度则主要影响液滴的粒径及其蒸发时间;随着压力、射流动量比及气流速度的增加,自燃延迟时间均会缩短,相比于预混燃料液雾的自燃延迟时间受负温度效应的影响较弱。     

Direct measurement of entrainment during nanoparticle synthesis in spray flames

Using phase Doppler anemometry the scale of turbulence and the spray dilution by air entrainment are investigated during nanoparticle synthesis by flame spray pyrolysis (FSP). Ethanol and a solution of 0.5 M zirconium propoxide in ethanol are dispersed and combusted using an external-mixing, gas-assisted atomizer resulting in product ZrO₂ particles of about 11 nm in diameter as determined by nitrogen adsorption at a production rate of 100 g/h. Solution droplet size distributions of the solid cone spray are measured and related to standard correlations of spray atomization. Air entrainment and the radial spread of the expanding jet are determined from the gas velocities in horizontal planes across the spray cone at different heights above the nozzle. The isotropy of the turbulence is investigated using measured axial and radial velocity fluctuations. The turbulent flow is characterized by the integral time and Kolmogorov scales as well as the average shear rates acting on droplets and particles. The flow structure of these spray flames is of major importance as it determines product particle size, polydispersity, morphology, and crystallinity.     

"Bump燃烧室"内新概念稀扩散燃烧混合气形成机理的研究

基于自行研制的实验装置,用片状激光诱导荧光法(PLIF)对普通商用柴油喷雾的撞壁混合过程进行了实验研究,并用CFD数值分析软件对其进行了模拟计算,二者结果基本吻合．平板和实际燃烧室的实验及计算结果均表明,撞壁射流在遇到限流沿(Bump)后会剥离壁面,形成二次空间射流,扩大撞壁射流与空气的空间混合体积及混合速率,出现与周围空气迅速混合的“闪混”现象,减少壁面燃油堆积量．计算结果还表明,Bump的存在改变了缸内气流运动的流场结构,Bump附近旋向相反的“双涡结构”极大地增强了二次空间射流对周围空气的卷吸,促进了燃油与空气的混合,是Bump燃烧室内稀混合气形成及稀扩散燃烧的关键所在．     

Primary air entrainment characteristics for a self-aspirating burner: Model and experiments

Experimental and theoretical investigations of primary air entrainment characteristics of a self-aspirating burner are presented. Emphasis was made on experiments, which were performed using both hot and cold tests; and a correlation between them is proposed. The level of primary air entrainment is measured using an oxygen sensor and a particle image velocimetry system. Experimental results are used to validate the predicted ones, which are obtained by constructing a theoretical model basing on simple momentum and energy conservation principles. It is found that the model predictions agree with the experimental data for a similar system. Primary air entrainment is a function of fuel gas flow rate, fuel gas type, injector geometry, mixing tube geometry, and burner port geometry. The level of primary air entrainment increases with increasing momentum rate of the fuel gas. The hot test gives about a 22 percentage point (37% relative) lower PA value than that of the cold test because of the preheating effect caused by combustion. A first correlation between the hot test and the cold one for primary air entrainment is proposed. It is recommended that the preheating effect caused by combustion in a self-aspirating burner not be neglected when designing the burner.     

Laser doppler velocimetry investigation of the turbulence structure of axisymmetric diffusion flames

Measurements of mean velocity components, turbulent intensities, velocity probability density functions, power spectra and autocorrelation functions of axial velocity fluctuation, and spatial turbulence macroscale, are reported in a turbulent round jet flow, issuing vertically into stagnant air, in non-combusting and combusting situations. The fuel density (a mixture of methane and argon) is chosen to be equal to the cold flow gas density (a mixture of air and helium) in order to minimize cold fuel/cold gas mixture density difference effects on measured turbulence properties. The objectives are to study the influence of the combustion process on the turbulence structure of the combustible jet flows considered, and to provide data against which results of numerical prediction methods for such flows embodying various turbulence and combustion models can be compared, with a view to improving our understanding of relevant transport processes and on guiding modelling and prediction efforts of such flows. A one-dimensional laser velocimeter operating in forward scatter differential Doppler mode was used to obtain the measurements. Gas temperatures were measured by thermocouples. A visual study by schlieren photography has also been conducted. It is found that the existence of the flame suppresses turbulence in the upstream region of the jet flow and enhances it in the downstream region, where turbulence intensities are substantially higher than in the corresponding cold jet flow. However, the relative intensities, i.e. the ratio of the local turbulent intensity to the local mean velocity, are smaller in the jet diffusion flame and become comparable to relative turbulent intensities found in the cold jet flow in the downstream region of the flow. Turbulence in the jet diffusion flame is appreciably more anisotropic than in the corresponding cold jet in all regions of the flow, suggesting the eventual desirability of multi-stress models of turbulence for the prediction of such flames. The combustion process has been found to have also a marked influence on the turbulence macroscale. It is significantly smaller than in the cold jet flow in the upstream region and increases appreciably at downstream distances, the rate of this increase closely following the rate of temperature increase. The experimental results obtained will guide the development of an improved prediction method for such combusting systems.     

喷雾撞壁油膜流动的研究

对燃油喷雾撞壁形成的油膜运动进行了三维数值模拟计算 ,在应用边界层理论建立的油膜流动模型中将油膜视为极薄湍流边界层 ,考虑了雾滴与壁面油膜的相互作用、油膜蒸发、卷吸、与气体间的传热传质等物理过程 ,通过对已有试验数据的比较 ,分析了平均油膜厚度、速度、喷雾粘附比、卷吸量等变化及其影响因素 ,预测结果与试验结果相符 ,计算结果表明喷射撞壁后液滴的质量、动量和能量分布是决定油膜动力学特性的主要因素     

Influence of evaporation on spray flamelet structures

The structure of laminar spray flames considerably differs from their gaseous counterpart. However, most often flamelet models employed in the simulation of turbulent spray combustion are based on laminar gas flame structures neglecting the influence of spray evaporation in the laminar spray flamelets. In this work, a combined theoretical and numerical study of the impact of spray evaporation on the structure of laminar spray flames is presented. Spray flamelet equations are derived, which explicitly take into account evaporation effects – the classical gas flamelet equations are recovered for non-evaporating conditions. Two new terms accounting for evaporation and for combined mixing and evaporation, respectively, are identified, and their relative importance is evaluated by means of numerical simulations of an axisymmetric laminar mono-disperse ethanol/air counterflow spray flame. The results show that the distribution of the spray evaporation rate plays a key role in the characterization of the spray flame structure. The new source terms overweigh the dissipation term of the gas phase in most situations even for non-evaporating species. Therefore, spray evaporation should always be considered. The relevance of the present formulation for turbulent spray modeling is evaluated and discussed, and a novel spray flamelet formulation is suggested.     

多流体碱雾发生器内蒸发喷雾射流的数值模拟

通过对多流体碱雾发生器中伴随气固两相流的蒸发喷雾射流的数值模拟,得到了碱雾发生器内气液固三相的速度矢量场.计算结果表明,气体轴向速度呈中间高两头低的对称分布;对于不同粒径的液滴,呈现出不同的空间分布规律,大液滴由于惯性大,可以穿越周围的气流区,比小液滴有更大的扩展角;在喷嘴出口2倍管道直径区域,由于雾化液滴与固体颗粒存在较大的速度差,有利于固体颗粒的碰撞增湿.     

Verification on spray simulation of a pintle injector for liquid rocket engine

The pintle injector used for a liquid rocket engine is a newly re-attracted injection system famous for its wide throttle ability with high efficiency. The pintle injector has many variations with complex inner structures due to its moving parts. In order to study the rotating flow near the injector tip, which was observed from the cold flow experiment using water and air, a numerical simulation was adopted and a verification of the numerical model was later conducted. For the verification process, three types of experimental data including velocity distributions of gas flows, spray angles and liquid distribution were all compared using simulated results. The numerical simulation was performed using a commercial simulation program with the Eulerian multiphase model and axisymmetric two dimensional grids. The maximum and minimum velocities of gas were within the acceptable range of agreement, however, the spray angles experienced up to 25% error when the momentum ratios were increased. The spray density distributions were quantitatively measured and had good agreement. As a result of this study, it was concluded that the simulation method was properly constructed to study specific flow characteristics of the pintle injector despite having the limitations of two dimensional and coarse grids.     

Simulation of small-scale releases from liquid hydrogen storage systems

Knowledge of the concentration field and flammability envelope from small-scale leaks is important for the safe use of hydrogen. These small-scale leaks may occur from leaky fittings or o-ring seals on liquid hydrogen-based systems. The present study focuses on steady-state leaks with large amounts of pressure drop along the leak path such that hydrogen enters the atmosphere at near atmospheric pressure (i.e. Very low Mach number). A three-stage buoyant turbulent entrainment model is developed to predict the properties (trajectory, hydrogen concentration and temperature) of a jet emanating from the leak. Atmospheric hydrogen properties (temperature and quality) at the leak plane depend on the storage pressure and whether the leak occurs from the saturated vapor space or saturated liquid space. In the first stage of the entrainment model ambient temperature air (295 K) mixes with the leaking hydrogen (20-30 K) over a short distance creating an ideal gas mixture at low temperature (∼65 K). During this process states of hydrogen and air are determined from equilibrium thermodynamics using models developed by NIST. In the second stage of the model (also relatively short in distance) the radial distribution of hydrogen concentration and velocity in the jet develops into a Gaussian profile characteristic of free jets. The third and by far the longest stage is the part of the jet trajectory where flow is fully developed. Results show that flammability envelopes for cold hydrogen jets are generally larger than those of ambient temperature jets. While trajectories for ambient temperature jets depend solely on the leak densimetric Froude number, results from the present study show that cold jet trajectories depend on the Froude number and the initial jet density ratio. Furthermore, the flammability envelope is influenced by the hydrogen concentration in the jet at the beginning of fully developed flow.     

Effect of turbulence on vaporization, mixing, and combustion of liquid-fuel sprays

The effect of turbulence properties on spray flame characteristics has been investigated experimentally in detail. A fine scale fluctuation was imposed on a spray by setting a grid in front of the spray nozzle. This simple way of changing the turbulence characteristics was proved to be a very effective way of increasing evaporation rate of the spray. It was found that the faster evaporation does not necessarily lead to faster combustion. As the turbulence characteristics change, evaporated fuel does not burn instantly but the flame whose characteristics are similar to those of a gaseous diffusion flame rather than to those of a heterogeneous spray flame can be observed. The results indicate that with the increase of evaporation rate, mixing of gaseous fuel and air becomes a controlling process of combustion. In the case of a jet mixing with the ambient air, the mixing between heterogeneous phases is more efficient than that between two homogeneous species. This fact is well known from the study of particle-laden jets. In this study its effects in reacting heterogeneous flows are shown.     

脉冲液体射流泵性能的研究

为获得脉冲液体射流泵性能方程中各参数对其性能的影响规律,用数值计算的方法对脉冲液体射流泵的基本性能方程进行了系统的研究,得出了脉冲射流主要是提高射流泵的卷吸率的结论。定量分析了喉管流速系数和喉管进口段的流速系数等参数的变化规律及其对性能的影响程度,进一步探明了其内部流动规律,对设计性能优良的射流泵具有理论指导意义。     

Effects of inlet conditions on film evaporation along an inclined plate

The evaporation of falling water liquid film in air flow is used in different solar energy applications as drying, distillation and desalination, and desiccant systems. The good understanding of the hydrodynamics and heat exchange in falling liquid film and gas flow, with interfacial heat and mass transfer, can be applied in improving solar systems performance. The solar system performance is dependent on the operating conditions, system conception and related to several physical parameters, where the effects of some of these parameters are not completely clarified. In the present numerical study, we examine the effects of inlet conditions on the evaporation processes along the gas–liquid interface. The liquid film streams over an inclined plate subjected to different thermal conditions. Liquid and gas flows are approached by two coupled laminar boundary-layers. The numerical solution is obtained by utilizing an implicit finite-difference box method. In this analysis an air–water system is considered and the coupled effects of inclination, inlet liquid mass flow rate and gas velocity are examined. The results show that, for imposed heat flux or uniform wall temperature, the effect of inclination is highly dependent on the liquid mass flow rate and gas velocity. An increase in the liquid mass flow rate causes an enhancement of the effect of inclination on the heat and mass transfer. The inclination affects the heat and mass transfer, especially at lower gas velocities. In the range of inclination angles of 0–10°, an increase in the inclination improves the evaporation by increasing the vapor mass flow rate. The maximum effect of inclination is nearly achieved at an inclination angle of 10°.     

循环流化床锅炉低氮燃烧的CPFD数值模拟

针对某75 t/h循环流化床锅炉炉膛出口NOx排放超标问题进行分析探讨,以合理的低氮燃烧控制技术为主,辅以SNCR烟气脱硝技术,争取达到NO x超净排放要求。采用CPFD计算方法对循环流化床锅炉炉膛内的气固流动和燃烧特性进行数值模拟,运用低过量空气燃烧法和空气分级技术对锅炉进行低氮燃烧控制,研究一、二次风配比、二次风射流、过量空气系数、循环倍率和颗粒粒径等因素对炉内燃烧及NO x排放的影响。结果表明:通过低氮燃烧控制后,炉内速度场和温度场分布均匀,炉膛出口处烟气流速增加,炉膛平均烟温和出口氧浓度降低,还原性气体CO浓度和优化前基本相同,炉膛出口NOx浓度降低,减排效果显著,为以后的锅炉运行提供实际指导经验。     

Effects of turbulence and carrier fluid on simple, turbulent spray jet flames

This paper presents simultaneous LIF images of OH and the two-phase acetone fuel concentration as well as detailed single-point phase-Doppler measurements of velocity and droplet flux in three turbulent spray flames of acetone. This work forms part of a larger program to study spray jets and flames in a simple, well-defined geometry, aimed at providing a platform for developing and validating predictive tools for such flows. Spray flames that use nitrogen or air as droplet carrier are investigated and issues of flow field, droplet dispersion, size distribution, and evaporation are addressed. The joint OH/acetone concentration images reveal a substantial similarity to premixed flame behavior when the carrier stream is air. When the carrier is nitrogen, the reaction zone has a diffusion flame structure. There is no indication of individual droplet burning. The results show that evaporation occurs close to the jet centerline rather than in the outer shear layer. Turbulence does not have a significant impact on the evaporation rates. A small fraction of the droplets escapes the reaction zone unburned along the centerline and persists far downstream of the flame tip. The proportion of this droplet residue increases with shorter residence times as observed for the higher velocity flame.     

Non-Boussinesq turbulent buoyant jet resulting from hydrogen leakage in air

This paper is devoted to introduce a numerical investigation of a vertical axisymmetric non-Boussinesq buoyant jet resulting from hydrogen leakage in air as an example of injecting a low-density gas jet into high-density ambient. As the domain temperature is assumed to be constant and therefore the density of the mixture is a function of the concentration only, the binary gas mixture is assumed to be of a linear mixing type. Also, it is assumed that the rate of entrainment to be a function of the plume centerline velocity and the ratio of the mean plume and ambient densities. On the other hand, the local rate of entrainment may be considered to be consisted from two components; one is the component of entrainment due to jet momentum while the other is the component of entrainment due to buoyancy. Firstly, the integral models of the mass, momentum and concentration fluxes are obtained and transformed to a set of ordinary differential equations using some non-dimensional transformations known as similarity transformations. The given ordinary differential system is integrated numerically and the mean centerline mass fraction, jet width and mean centerline velocity are obtained. In the second step, the mean axial velocity, mean concentration and mean density of the jet are obtained. Finally in the third step of this article, several quantities of interest, including the cross-stream velocity, Reynolds stress, velocity-concentration correlation (radial flux), turbulent eddy viscosity and turbulent eddy diffusivity, are obtained. In addition, the turbulent Schmidt number is estimated and the normalized jet-feed material density and the normalized momentum flux density are correlated.     

Correlations for the mass transfer rate of droplets in vertical upward annular flow

New correlations for the deposition rate and entrainment rate of droplets in vertical upward annular flow were developed from simple models and available experimental data. In the correlation for the deposition rate, the superficial gas velocity was used as the parameter of primary importance at low droplet concentration while the droplet concentration itself at high droplet concentration. In correlating the rate of droplet entrainment, the ratio of interfacial shear force to the surface tension force acting on the surface of liquid film was the appropriate scaling parameter to correlate the experimental data measured in varied conditions. The experimental data for air–water annular flow were used in the development of the present correlations since extensive databases were available. It was however confirmed that the present model provides satisfactory agreements with the experimental data for high-pressure steam–water annular flow.     

Numerical simulation on the behavior of a liquid jet into an air flow

A numerical simulation has been performed to clarify the effects of turbulence in a liquid on the deformation of the liquid jet surface into an air flow. The turbulences in the liquid jet were simulated by the Rankin vortices, and the liquid jet surface was tracked numerically by the volume of fluid method. By numerical simulations, the onset of the protrusions on the liquid jet surface is caused by the vortices in the liquid, and the surrounding air flow plays an important role in the amplification of the protrusions. The amplification rate of the trough displacement is proportional to the air‐to‐liquid velocity ratio. At large imposed vortex intensities, the trough displacement increases with the vortex intensity. On the other hand, at small imposed vortex intensities, the amplification of the trough displacement is also affected by factors other than vortex intensity. © 2001 Scripta Technica, Heat Trans Asian Res, 30(6): 473–484, 2001