Influences of spray and operating parameters on penetration of vaporizing fuel droplets in a gas turbine combustor

The effects of inlet spray and operating parameters on penetration and vaporization histories of fuel droplets of a liquid fuel spray injected into a turbulent swirling flow of air through a typical can type gas turbine combustor, have been evaluated from numerical solutions of the conservation equations in gas and droplet phases. The computational scheme is based on the typical stochastic separated flow model of the gas-droplet flow within the combustor. A κ–ε model with wall function treatment for near wall region has been adopted for the solution of conservative equations in gas phase. The initial spray parameters are specified by a suitable PDF size distribution and a given spray cone angle. It has been recognized that the penetration of vaporizing droplets is reduced with an increase in inlet air swirl and spray cone angle. An increase in inlet air pressure or a decrease in inlet air temperature also results in a reduction in droplet penetration. The inlet air pressure and spray cone angle are found to be the most influencing parameters in this regard.     

Experimental investigations of spray generated by a pressure swirl atomizer

This paper has focused on the droplets behavior of kerosene RP-3 spray produced by a pressure swirl atomizers in terms of spray pattern, droplet size spatial distribution, mean droplet size, and distribution index with variations of pressure differential. The analyses have been carried out experimentally with the aid of optical diagnostic methods. The spray pattern, such as spray cone angle and fuel spatial distribution, has been measured by the technique of planar laser induced fluorescence of kerosene. A method for correction of fuel distribution measurement error induced by laser attenuation in spray is proposed and validated. The droplet size spatial distribution in central axis plane of the spray has been measured by a planar droplet sizing method which combining laser induced fluorescence and Mie scattering. The spray pattern in axial center plane and cross-sectional plane perpendicular to axis of the atomizer indicate that the droplets in spray concentrate around the outer periphery and in a narrow annular zone at the near-field of fuel injector exit, and then disperse to produce a solid spray at downstream of the spray. The analyses of droplet size spatial distribution, Sauter mean diameter, and distribution index with pressure differential clearly show the presence of droplets collision and its adverse effects on droplet size uniformity. The spray outline, droplet mass spatial distribution, and droplet size spatial distribution, droplets dispersion and collision in the process of atomization provide a great insight into the processes of atomization and spray development, which are key information for fuel injector design and quality control. The visualizations of spray pattern and droplet size spatial distribution with variations of pressure differential for pressure swirl atomizer are key issues in swirl cup or internally staged airblast fuel injectors because pressure swirl atomizer provides primary atomization or pilot spray which affects the quality of air/fuel mixing in lean-burn combustion. Moreover, a well-defined and complete database regarding the isothermal hollow cone spray is provided for validation of spray model.     

Investigation of heat and mass transfer between the two phases of an evaporating droplet stream using laser-induced fluorescence techniques: Comparison with modeling

Heat and mass exchanges between the two phases of a spray is a key point for the understanding of physical phenomena occurring during spray evaporation in a combustion chamber. Development and validation of physical models and computational tools dealing with spray evaporation requires experimental databases on both liquid and gas phases. This paper reports an experimental study of evaporating acetone droplets streaming linearly at moderate ambient temperatures up to 75 °C. Two-color laser-induced fluorescence is used to characterize the temporal evolution of droplet mean temperature. Simultaneously, fuel vapor distribution in the gas phase surrounding the droplet stream is investigated using acetone planar laser-induced fluorescence.Temperature measurements are compared to simplified heat and mass transfer model taking into account variable physical properties, droplet-to-droplet interactions and internal fluid circulation within the droplets. The droplet surface temperature, calculated with the model, is used to initiate the numerical simulation of fuel vapor diffusion and transport in the gas phase, assuming thermodynamic equilibrium at the droplet surface. Influence of droplet diameter and droplet spacing on the fuel vapor concentration field is investigated and numerical results are compared with experiments.     

The role of droplet fragmentation in high-pressure evaporating diesel sprays

The relative importance of the physical processes taking place during the development of Diesel sprays is evaluated through use of a dense-particle Eulerian–Lagrangian model. The physical processes considered include the influence of the injection conditions, as determined by a nozzle cavitating flow model, liquid-core atomisation, droplet break-up, turbulent dispersion, droplet-to-droplet interactions and vaporisation. For the latter, different physical mechanisms are included, considering high pressure and temperature as well as multi-component effects. Droplet aerodynamically-induced break-up is the dominant mechanism determining the contact area between the droplets and the surrounding air during their fragmentation period. Furthermore, a new model is considered for the droplet deformation induced during the fragmentation processes of the moving droplets. That is found to increase substantially the interface area available for heat transfer and vaporisation and to reproduce the observed trend of liquid penetration being independent of injection pressure. Model predictions are successfully compared against a wide range of experimental data for the liquid and vapour penetration, spray CCD (Charge Coupled Device) images and PDA (Phase Doppler Anemometry) measurements for various injector nozzle geometries. The results are found to predict trends as well as actual values of the penetrating fuel plumes, as function of nozzle geometry, injection pressure and air thermodynamic conditions covering the range of conditions of modern supercharged DI Diesel engines.     

柴油机燃油喷射雾化的PIV测量试验研究

介绍了先进的粒子图像测速技术(PIV)应用于燃油喷雾过程的试验研究结果，燃油通过多种孔径的单孔和多孔喷油嘴喷入一个定容压力室内，采用高分辨率的CCD摄像机记录整个喷射过程，可以观察到喷雾场内部结构．结果分析显示，高速射流表面存在不稳定扰动波，在气动力作用下，液滴发生分裂破碎，呈现枝状分离表面．瞬态速度矢量图显示，气液界面处存在大尺度的卷吸涡流运动，有助于油气扩散混合。     

Effects of ambient pressure,gas temperature and combustion reaction on droplet evaporation

The effects of ambient pressure, initial gas temperature and combustion reaction on the evaporation of a single fuel droplet and multiple fuel droplets are investigated by means of three-dimensional numerical simulation. The ambient pressure, initial gas temperature and droplets’ mass loading ratio, ML, are varied in the ranges of 0.1–2.0 MPa, 1000–2000 K and 0.027–0.36, respectively, under the condition with or without combustion reaction. The results show that both for the conditions with and without combustion reaction, droplet lifetime increases with increasing the ambient pressure at low initial gas temperature of 1000 K, but decreases at high initial gas temperatures of 1500 K and 2000 K, although the droplet lifetime becomes shorter due to combustion reaction. The increase of ML and the inhomogeneity of droplet distribution due to turbulence generally make the droplet lifetime longer, since the high droplets’ mass loading ratio at local locations causes the decrease of gas temperature and the increase of the evaporated fuel mass fraction towards the vapor surface mass fraction.     

烟气除湿喷淋塔出口液滴逃逸率研究

为了降低火电厂烟气含湿量,以新型的上喷淋上进气喷淋塔为研究对象,采用FLUENT模型模拟喷淋塔内部两相流体的运动,并与实验验证,采用单变量分析方法研究烟气流速、喷淋液滴直径、喷淋速度、喷淋流量四种因素对出口液滴逃逸量的影响。研究结果表明：各操作条件对出口液滴逃逸量的影响程度：液滴直径>喷淋流量>进气速度>喷淋速度;出口液滴逃逸量随着喷淋流量和进气速度的增长而线性增长;在进气速度3.5 m/s下,当液滴直径小于1 mm时,逃逸量随液滴直径减小而迅速上升;当直径大于1 mm时,液滴逃逸量几乎为0,不受喷淋流量影响。     

Large eddy simulations for radiation-spray coupling for a lean direct injector combustor

Large Eddy Simulations (LESs) for a lean-direct injection (LDI) combustor are performed and compared with experimental data. The LDI emissions characteristics, and radiation-spray coupling effect on the predictions are analyzed. The flamelet progress variable approach is employed for chemistry tabulation coupled with a stochastic secondary breakup model. Good comparisons are shown with the experimental data mean and root mean square for both the gas phase and spray droplets profiles. The effect of combustion is found to change the shape and structure of the central recirculation zone to be more compact in length but larger in diameter in the transverse direction. In-addition the results show that the gas phase radiation alters the spray dynamics by changing the local gas-phase temperature distribution. This impacts the spray evaporation rate, the local mixture fraction, and consequently the combustion heat released rate and the predicted emissions. The simulation with no radiation modeling shows over prediction in the temperature distribution, pollutants emissions, higher fuel evaporation rate, and narrower range of droplet size distribution with lower number density for the smaller size particles. The current study suggests that, even for low pressure systems, radiation modeling can be important for accurate emissions prediction.     

Percolation theory for flame propagation in non- or less-volatile fuel spray: A conceptual analysis to group combustion excitation mechanism

A percolation theory for flame propagation in non- or less-volatile fuel spray is developed based on a cubic lattice model representing a local spray state. The interdroplet flame propagation characteristics found from microgravity experiments on flame spread along a linear droplet array are applicable to describing interdroplet flame propagation between neighboring droplets in any distribution of droplets because the effect of heat conduction from the flame front is shielded by the nearest unburned droplet, which acts as a heat sink. Thus, once the method by which the unburned droplet nearest to the flame front is ignited is identified and formulated into a simple algorithm rule, we can examine by computer simulation the statistical flame propagation behavior in a non- or less-volatile fuel spray in the framework of the percolation theory. In non- or less-volatile fuel, an unburned droplet swallowed by an envelope diffusion flame of other droplets is heated and becomes a new supplier of fuel vapor to the flame front, allowing the flame front to advance. For randomly distributed droplets, the flame front selects the path that minimizes its propagation time. These two phenomena occur when the grid spacing of the cubic lattice model is equal to the maximum flame radius of an isolated droplet immersed in the same air conditions as the local spray state. Furthermore, physical considerations reveal that the lattice size that leads to statistically meaningful information can be rather small, i.e., 20×20×20 vertices. Therefore, the proposed percolation theory is tractable and useful in finding the probability that a flame front propagates across a spray element and for exploring the mechanism of the excitation of group combustion for non- or less-volatile fuel sprays.     

二甲醚液滴高压蒸发的数值模拟

在分析燃油液滴高压蒸发规律的基础上,考虑液滴内部的热传导过程、内部环流和非理想气体效应,建立了高压蒸发模型,并利用该模型对二甲醚(DME)单液滴的蒸发过程进行了数值模拟分析。采用状态方程法计算了DME-N2体系的气液相平衡。结果表明:高压有利于燃料液滴蒸发;即使环境压力超过燃油的临界压力,其平衡蒸发温度也未必能达到临界温度。     

NUMERICAL MODELING FOR INTERACTION OF A WATER SPRAY WITH SMOKE LAYER

Interaction of a water spray with a smoke layer was studied numerically. A one-dimensional model was developed by taking smoke and air as two quasi-steady layers. W ater droplets of the spray were divided into several typical classes based on the droplet distribution function. Spray characteristics such as droplet diameter and velocity, shape of the spray envelope, gas temperature, and flow rate inside the spray envelope were calculated with heat and mass transfer to the induced air flow. The simulated results agreed with the experiments on full cone water spray in a normal atmospheric environment, and indicated some physical principles for using water spray to minimize smoke damage.     

Transient vaporization and burning in dense droplet arrays

Three-dimensional droplet-array combustion with an unsteady liquid-phase and a quasi-steady gas-phase is studied computationally by a generalized approach using a mass-flux potential function. Symmetric and asymmetric droplet arrays with non-uniform droplet size and non-uniform spacing are considered. Burning rates are computed and correlated with the number of droplets, an average droplet size, and an average spacing for the array through one similarity parameter for arrays as large as 1000 droplets. Total array vaporization rates are found to be maximized at a specific droplet number density that depends on liquid volume within the array. An unsteady liquid-phase model with either a uniform or a radially varying temperature distribution is coupled with the quasi-steady gas-phase solution for decane, heptane, and methanol fuels. Droplet interactions and liquid-phase heating have been shown to almost double the lifetime when compared to an isolated droplet. Depending on fuel type, initial temperature, and array geometry, droplets may initially burn with individual flames, transition to a single group flame, and transition back to individual flames as vaporization progresses. In most cases, group combustion occurs upon ignition and is the dominant mode of combustion.     

柴油机高压喷雾碰壁前后粒子速度的LDA试验研究

利用3光束二维氩离子激光多普勒测速仪对柴油机高压喷雾碰壁前和碰壁后的粒子速度进行了试验研究。结果表明：喷雾中液体粒子速度分布范围很宽,在距离喷嘴70mm处,粒子速度仍可达100m/s左右,且沿径向分布平坦,与气体射流速度的分布不同。在喷雾可见边界之外测量到有高速粒子存在。喷雾碰壁后,反弹粒子速度的统计值大大衰减,且主要为沿壁面爬行。滴群的运动形态类似于碰壁后的气体射流而不同于碰壁反弹的固体颗粒。     

Evolution of liquid and gas phases in multi‐plume spray injection

In this study, the unsteady development of multi‐plume sprays has been investigated by large eddy simulations with Eulerian–Lagrangian multiphase approach for both global spray characteristics and local flow features. Multi‐plume sprays are injected at the injection pressures of 10 MPa and 15 MPa, and the temperature of T_s = 297.65 K into the ambient air at the atmospheric pressure and temperature of T_a = 293.15 K. Experimentally obtained global multi‐plume spray characteristics in terms of spray shape and penetration are used to validate the present numerical simulations. The present numerical predictions for Sauter mean diameter and its temporal variation agree well with the empirical correlations. The predicted droplet size distribution evolves temporally and spatially, and exhibits bimodal distribution, until eventually the mode for small droplet sizes dominates. The spray plumes are found to have limited interaction due to the relatively large orientation angles between the plumes. Because of the momentum transfer from the liquid to gas phase, spray‐induced air jets appear in the multi‐plume sprays. Using vorticity, pressure, and λ₂ − criterion fields, it is shown that the spray‐induced air jets form similar vortical structures as single phase jets. Similarities between the spray‐induced air jets and single phase jets in terms of the shear layer vortical structures such as hairpin‐like vortices improves our understanding of the entrainment and mixing processes in multi‐plume sprays. Copyright © 2016 John Wiley & Sons, Ltd.     

Numerical simulation of spray performance based on the Euler-Lagrange approach

Numerical simulations have been carried out to investigate the liquid atomization and spray process using the Discrete Phase Model of the commercial CFD code combined with the Wall-Film boundary conditions. The effects of spray parameters on droplets Sauter mean diameter (SMD), droplet collision speed, the thickness of liquid-film, the surface temperature and its uniformity were analyzed in the present study. The simulation results and the experimental data obtained in the available literature agree within 13.8%. The computational results show that the spray pressure is the main factor to realize the atomization. Increasing the mass flux and the spray pressure, the droplet collision speed increases while the corresponding maximum film thickness on the heated surface declines. The surface temperature changes indistinctively with the increase of the spray distance, but the temperature distribution tends to be uniform.     

Dispersion and vaporization of biofuels and conventional fuels in a crossflow pre-mixer

In lean premixed pre-vaporized (LPP) combustion, controlled atomization, dispersion and vaporization of different types of liquid fuel in the premixer are the key factors required to stabilize the combustion process and improve the efficiency. The dispersion and vaporization process for biofuels and conventional fuels sprayed into a crossflow pre-mixer have been simulated and analyzed with respect to vaporization rate, degree of mixedness and homogeneity. Two major biofuels under investigation are Ethanol and Rapeseed Methyl Esters (RME), while conventional fuels are gasoline and jet-A. First, the numerical code is validated by comparing with the experimental data of single n-heptane and decane droplet evaporating under both moderate and high temperature convective air flow. Next, the spray simulations were conducted with monodispersed droplets with an initial diameter of 80 μm injected into a turbulent crossflow of air with a typical velocity of 10 m/s and temperature of around 800 K. Vaporization time scales of different fuels are found to be very different. The droplet diameter reduction and surface temperature rise were found to be strongly dependent on the fuel properties. Gasoline droplet exhibited a much faster vaporization due a combination of higher vapor pressure and smaller latent heat of vaporization compared to other fuels. Mono-dispersed spray was adopted with the expectation of achieving more homogeneous fuel droplet size than poly-dispersed spray. However, the diameter histogram in the zone near the pre-mixer exit shows a large range of droplet diameter distributions for all the fuels. In order to improve the vaporization performance, fuels were pre-heated before injection. Results show that the Sauter mean diameter of ethanol improved from 52.8% of the initial injection size to 48.2%, while jet-A improved from 48.4% to 18.6% and RME improved from 63.5% to 31.3%. The diameter histogram showed improved vaporization performance of jet-A.     

AN ADVANCED SPRAY MODEL FOR APPLICATION TO THE PREDICTION OF GAS TURBINE COMBUSTOR FLOW FIELDS

It is well known that fuel preparation, its method of injection into a combustor, and its atomization characteristics have a significant impact on emissions. A simple dilute spray model, which assumes that droplet heating and vaporization occur in sequence, has been implemented in the past within computational fluid dynamics (CFD) codes at General Electric (GE) and has been used extensively for combustion applications. This spray model coupled with an appropriate combustion model makes reasonable predictions of the combustor pattern factor and emissions. To improve upon this predictive ability, a more advanced quasi-steady droplet vaporization model has been considered. This article describes the evaluation of this advanced model. In this new approach, droplet heating and vaporization take place simultaneously (which is more realistic). In addition, the transport properties of both the liquid and vapor phases are allowed to vary as a function of pressure, gas phase temperature, and droplet temperature. These transport properties, which are most up to date, have been compiled from various sources and appropriately curve-fit in the form of polynomials. Validation of this new approach for a single droplet was initially performed. Subsequently calculations of the flow and temperature field were conducted and emissions (NOx, CO, and UHC) were predicted for a modern single annular turbofan engine combustor using both the standard spray model and the advanced spray model. The effect of the number of droplet size ranges as well as the effect of stochastic treatment of the droplets were both investigated.     

小型直喷式柴油机喷雾特性的试验研究

为了掌握喷雾的结构物特性,利用高速摄像装置和高压容器拍摄了喷雾的逆光图像,并研究了喷雾周围介质压力对喷雾贯穿距离、喷雾锥角以及蒸发等喷雾特性的影响。在液滴数密度较大的条件下,燃料液滴的蒸发速率传热过程和雾化质量的影响;喷射妆期喷雾的贯穿距离和贯穿速度几乎不受介质物影响;喷雾的贯穿跨离受喷油压力的影响不大,但随着喷油压力的提高,喷雾锥角增大,喷雾质量得到改善。     

直喷式柴油机高压喷射特性的研究

结合国产６１１０柴油机高压喷射技术的实用性开发，研究了供油系统中喷油泵，高压油管和喷油嘴对喷油压力的影响，并以激光纹影高速摄影和激光粒子尺寸分析仪，测量了喷油压力４０－１００ＭＰａ变化时的喷雾特性。结果表明：实现高压喷射的有效措施是采用多孔数，小孔径喷嘴，小孔径喷嘴在高喷射压力下，喷注的贯穿度和喷雾的破碎期减少，雾化改善。     

A parametric study of burning of multicomponent droplets in a dilute spray

The model for sphericosymmetric thin‐flame combustion of a multicomponent fuel droplet in a dilute spray using a unit cell approach, developed in the companion paper, has been used for studying the interaction effect between droplets. The effects of droplet spacing, ambient oxidizer concentration, ambient temperature and pressure have been considered. Droplet life increases with decrease in droplet spacing, ambient temperature and ambient oxidizer concentration. However, droplet life has a weak dependence on ambient pressure. Copyright © 2001 John Wiley & Sons, Ltd.