Effects of Spray and Turbulence Modelling on the Mixing and Combustion Characteristics of an n-heptane Spray Flame Simulated with Dynamic Adaptive Chemistry

Accurate modelling of spray combustion process is essential for efficiency improvement and emissions reduction in practical combustion engines. In this work, both unsteady Reynolds-averaged Navier-Stokes (URANS) simulations and large eddy simulations (LES) are performed to investigate the effects of spray and turbulence modelling on the mixing and combustion characteristics of an n-heptane spray flame in a constant volume chamber at realistic conditions. The non-reacting spray process is first simulated with URANS to investigate the effects of entrainment gas-jet model on the penetration characteristics and fuel vapor distributions. It is found that the droplet motion near the nozzle has significant influence on the fuel vapor distribution, while the liquid penetration length is controlled by the evaporation process and insensitive to gas-jet model. For the case considered, both URANS with the gas-jet model and large eddy simulations can properly predict the vapor penetration. For the combustion characteristics, it is found that LES yields better predictions in the global combustion characteristics. The URANS with gas jet model yields a comparable flame length and lift-off-length (LOL) to LES, but results in a larger ignition delay time compared to the experimental data. Another focus of this work is to qualify the convergence characteristics of the dynamic adaptive chemistry (DAC) method in these transient combustion simulations, where DAC is applied to reduce the mechanism locally and on-the-fly to accelerate chemistry calculations. The instantaneous flame structures and global combustion characteristics such as ignition delay time, flame lift-off length and emissions are compared between simulations with and without DAC. For URANS, good agreements are observed both on instantaneous flame structures and global characteristics. For LES, it is shown that the errors incurred by DAC are small for scatter distributions in composition space and global combustion characteristics, while they may significantly affect instantaneous flame structures in physical space. The study reveals that for DAC application in transient simulations, global or statistic information should be used to assess the accuracy, such as manifolds in composition space, conditional quantities and global combustion characteristics. For the cases investigated, a speed-up factor of more than two is achieved by DAC with a 92-species skeletal mechanism with less than 0.2 % and 3.0 % discrepancy in ignition delay and LOL, respectively.     

Autoignition of n-decane Droplets in the Low-, Intermediate-, and High-temperature Regimes from a Mixture Fraction Viewpoint

Detailed numerical simulations of isolated n-decane droplets autoignition are presented for different values of the ambient pressure and temperature. The ignition modes considered included single-stage ignition, two-stage ignition and cool-flame ignition. The analysis was conducted from a mixture fraction perspective. Two characteristic chemical time scales were identified for two-stage ignition: one for cool-flame ignition, and another for hot-flame ignition. The appearance and subsequent spatial propagation of a cool flame at lean compositions was found to play an important role in the ignition process, since it created the conditions for activating the high-temperature reactions pathway in regions with locally rich composition. Single-stage ignition was characterized by a single chemical time scale, corresponding to hot-flame ignition. Low-temperature reactions were negligible for this case, and spatial diffusion of heat and chemical species mainly affected the duration of the ignition transient, but not the location in mixture fraction space at which ignition first occurs. Finally, ignition of several cool flames of decreasing strength was observed in the cool-flame ignition case, which eventually lead to a plateau in the maximum gas-phase temperature. The first cool flame ignited in a region where the fuel / air mixture was locally lean, whereas ignition of the remaining cool flames occurred at rich mixture compositions.     

A Comprehensive Study of Effects of Mixing and Chemical Kinetic Models on Predictions of n-heptane Jet Ignitions with the PDF Method

The composition Probability Density Function (PDF) model is coupled with a Reynolds-averaged k???ε turbulence model and three computationally efficient, yet widely used chemical mechanisms to simulate transient n-heptane spray injection and ignition in a high temperature and high density ambient fluid. Molecular diffusion is modelled by three mixing models, namely the interaction by exchange with the mean (IEM), modified Curl (MC) and Euclidean minimum spanning trees (EMST) models. The liquid phase is modelled by a discrete phase model (DPM). This represents among the first applications of the PDF method in practical diesel engine conditions. A non-reacting case is first considered, with the focus on the ability of the model to capture the spray structure, e.g., vapour penetration and liquid length, fuel mixture fraction and its variance. Reacting cases are then investigated to compare and evaluate the three different chemical mechanisms and the three mixing models. It is concluded that the EMST mixing model in conjunction with a reduced chemical kinetic model (Lu et al., Combust Flame 156(8):1542–1551, 2009) performs the best among the options considered. The sensitivity of the results to the choice of the mixing constant is also studied to understand its effect on the flame ignition and stabilisation. Finally, the PDF model is compared to a well-mixed model that assumes turbulent fluctuations are negligible, which has been widely used in the diesel spray combustion community. Significant structural differences in the modelled flame are revealed comparing the PDF method with the well-mixed model. Quantitatively, the PDF model exhibits excellent agreement with the measurements and shows much better results than the well-mixed model in all ambient O₂ and temperature conditions.     

Experimental and numerical analysis of spray-atomization characteristics of biodiesel fuel in various fuel and ambient temperatures conditions

The purpose of this work is to reveal the effects of fuel temperatures and ambient gas conditions on the spray-atomization behavior of soybean oil methyl ester (SME) fuel. The spray-atomization behavior was analyzed through spray parameters such as the axial distance from the nozzle tip, local and overall Sauter mean diameter (SMD). These parameters were obtained from a spray visualization system and a droplet measuring system. In addition, the experimental results were compared with the numerical results calculated by the KIVA-3V code. It was revealed that the increase of the fuel temperature (from 300 K to 360 K) little affects the spray liquid tip penetration. The increase of the ambient gas temperature (from 300 K to 450 K) caused a increase in the spray liquid tip penetration. Also, biodiesel fuel evaporation actively occurred due to the increase in the fuel temperature and the ambient gas temperature. Of special significance was that the highest vapor fuel mass concentration was observed at the center region of the spray axis. In the results of the microscopic characteristics, the detected local droplet size at the axial direction and overall droplet size at the axial and radial direction in a control volume increased when the fuel temperature increased. This is believed to be due to an increase in the number of small droplets that quickly evaporated. In addition, the increased fuel temperature caused the decrease of the number of droplets and the increase of the vapor fuel mass. The mean axial velocity of droplets decreased with increasing fuel temperature.     

Large Eddy Simulation of Spark Ignition in a Gas Turbine Combustor

Ignition in an aircraft gas turbine combustion chamber is simulated using Large Eddy Simulation (LES) in conjunction with the filtered probability density function (pdf) equation approach, which is solved using the Eulerian stochastic field method. The LES-pdf methodology is used for both dispersed (liquid) and gas phases. The liquid phase is described using a Lagrangian formulation whilst an Eulerian approach is employed for the gas phase. The spark energy deposition was mimicked by a distributed energy source term added to the enthalpy equation. Unsuccessful and successful ignition sequences have been simulated and the results suggest that spark ‘size’ is an important parameter in the ignition of kerosene fuelled combustion chambers.     

Flow Structure of Swirling Turbulent Propane Flames

Flow structure of premixed propane–air swirling jet flames at various combustion regimes was studied experimentally by stereo PIV, CH* chemiluminescence imaging, and pressure probe. For the non-swirling conditions, a nonlinear feedback mechanism of the flame front interaction with ring-like vortices, developing in the jet shear layer, was found to play important role in the stabilisation of the premixed lifted flame. For the studied swirl rates (S = 0.41, 0.7, and 1.0) the determined domain of stable combustion can be divided into three main groups of flame types: attached flames, quasi-tubular flames, and lifted flames. These regimes were studied in details for the case of S = 1.0, and the difference in the flow structure of the vortex breakdown is described. For the quasi-tubular flames an increase of flow precessing above the recirculation zone was observed when increased the stoichiometric coefficient from 0.7 to 1.4. This precessing motion was supposed to be responsible for the observed increase of acoustic noise generation and could drive the transition from the quasi-tubular to the lifted flame regime.     

Finite differences study of ignition in a methane–air mixture flow

A detailed simulation of the ignition process and premixed flames propagation, taking into account molecular transport, chemical reaction, thermodynamics and convection, is built by making use of the implicit finite difference scheme with the help of the Tridiagonal Matrix Algorithm. The velocity of chemical reaction is expressed by means of Arrhenius form of first order in both fuel and oxygen. The main objective of this work is to define numerically in two cases, u=0.1 m/s and u=0.4 m/s, the ignition temperature of the methane–air mixture with different air excess coefficients in the mixture. In addition, the effect of the thickness of the region ignition and of ignition location on the transient behavior of the flame was studied.     

A RANS simulation toward the effect of turbulence and cavitation on spray propagation and combustion characteristics

A multidimensional computational fluid dynamic code was developed and integrated with probability density function combustion model to give the detailed account of multiphase fluid flow. The vapor phase within injector domain is treated with Reynolds-averaged Navier–Stokes technique. A new parameter is proposed which is an index of plane-cut spray propagation and takes into account two parameters of spray penetration length and cone angle at the same time. It was found that spray propagation factor (SPI) tends to increase at lower r/d ratios, although the spray penetration tends to decrease. The results of SPI obtained by empirical correlation of Hay and Jones were compared with the simulation computation as a function of respective r/d ratio. Based on the results of this study, the spray distribution on plane area has proportional correlation with heat release amount, NO _x emission mass fraction, and soot concentration reduction. Higher cavitation is attributed to the sharp edge of nozzle entrance, yielding better liquid jet disintegration and smaller spray droplet that reduces soot mass fraction of late combustion process. In order to have better insight of cavitation phenomenon, turbulence magnitude in nozzle and combustion chamber was acquired and depicted along with spray velocity.     

Experimental Study on Lifted Flames Operated with Liquid Kerosene at Elevated Pressure and Stabilized by Outer Recirculation

This study deals with the impact of the operating conditions, e.g. pressure, preheating temperature, pressure drop across the nozzle, nozzle size and stoichiometry, on the reaction zone location and spray evaporation progress in case of a lifted flame. Lifted flames are highly valued for their NO_x reduction potential and for their low susceptibility to flash-back and thermo-acoustic instabilities. These advantageous features arise from the improved homogeneity of the fuel-air mixture provided to the reaction zone. One distinctive feature of the lifted flames is the presence of the so called lift-off zone located between nozzle outlet and main reaction zone. Within the lift-off zone fuel and oxidizer remain a certain time in contact and mix together prior to the onset of the combustion reaction. This leads to a more uniform heat release distribution and lowers the nitrogen oxides emissions at lean conditions by reducing the temperature spikes. In contrast to many other studies the subject of investigation was not a plain jet flame, but a modified version of the airblast nozzle, widely used in industrial applications. The nozzle was operated with liquid kerosene. As liquid fuels are easier to handle than gaseous or solid, it is expected that many efforts in the future will focus on the development of liquid fuels surrogates. Our previous investigations have shown, that the nozzle is well suited to be operated with gaseous fuels as well (Fokaides et al, J Eng Gas Turbine Power 130, 011508 2008). The position of the reaction zone was determined by means of chemiluminescence of the OH^? radicals and from its location the lift-off height was derived. In addition the fuel evaporation progress was measured by means of light scattering, revealing that fuel droplets and main reaction zone are well separated. It was found that the operating conditions have a versatile impact on the length of the lift-off zone and spray cone and thus on the degree of pre-evaporation and premixing. Thus, it may be concluded, that through a proper choice of operating conditions and combustor size a desired lift-off height can be adjusted in accordance with criteria, like available space, required emission levels etc.     

Propagation of Flames in the Limit of Zero Ignition Temperature

This paper is devoted to the study of planar deflagration flames. The combustion takes place in a gaseous mixture in which N chemical species B ₁,…,B _N are subject to a complex chemical network of R chemical reactions: In the limit of zero ignition temperature, existence of traveling waves for any speed in a range [c ₀,+∞) is proved for exothermic networks. Non-existence is proved for c∈(0,c ^*). A new index argument is introduced which is of independent interest. In the case of Lewis number less than 1 and of non-competitive networks, the set of speeds is exactly the interval [c ₀,+∞) (c ₀?=?c ^*).     

基于光学诊断的某新型含铝推进剂燃烧特性分析

为探究某新型含铝固体推进剂燃烧特性和规律,在模拟固体发动机的高压条件下,采用可调功率激光器结合高速摄影、发射光谱等光学诊断技术对该新型含铝固体推进剂开展了系统的点火及燃烧过程研究。通过对该推进剂的点火延迟、退移速率、燃烧温度以及团聚物颗粒尺寸的定量测量和分析,明确了该推进剂的点火延迟量级;证实此推进剂的退移速率严格遵循Summerfield燃速公式;判断出其最高燃烧温度高于3 300 K,且随压力增大而升高;通过对燃烧过程中发光凝聚相产物面积的量化分析得出推进剂产物中团聚物粒径尺寸受环境参数的影响规律。     

Experimental Characterization of Gelled Jet A1 Spray Flames

Gelled propellants provide energetic performance similar to conventional liquid propellants and safety during storage and handling like a solid propellant. Experiments on unconfined gelled Jet A1 spray flames and the comparison with ungelled spray flames are reported for the first time in this paper in terms of the global features, burning regimes, stability limits, visible flame height, emission spectra, natural luminosity, and CH ^? chemiluminescence. Propellants were atomized by an internally impinging two-fluid atomizer, developed specifically for efficient atomization of non-Newtonian gels. Swirling and non-swirling spray flames were successfully stabilized on a burner incorporating bluff body and annular jet of combustion air over a wide range of operating parameters. Structural features of the atomizer impart high momentum to the (central) spray jet, such that the recirculation zone could be penetrated under all conditions. Long-exposure smoke and high-speed visualizations were employed to study cold flow structures and droplet-vortex interactions. Short-exposure direct and backlit imaging were used to observe global features of spray flames. Stability limits and visible flame heights were mapped for different thermal inputs, swirl numbers, and flow rates of atomizing and combustion air jets. Non-swirling stable anchored, partially blown off, and neck-blown off flames were observed. Lifted, and a transition regime, in which the flame could burn in stable and lifted mode repetitively, were observed for the swirling flames. Interactions between central and annular jets are important in these regimes, determining flame shape, symmetry, and flame height. Jet-like propagation zone determines the flame height through its dependence on momentum of spray jet. The length of this zone is affected by variations in thermal input, gas-liquid ratio, and air-fuel ratio. The gelled Jet A1 flames are remarkably shorter despite having a larger average droplet size than ungelled Jet A1. This experimental observation directly supports theoretical predictions reported in literature. These flames are more luminous than ungelled Jet A1, especially at the base and the neck regions. While, majority of the heat is released in the jet-like propagation zone for both the flames, significant heat is released in the neck zone of ungelled Jet A1 spray flame in comparison to ungelled Jet A1 spray flame due to intense turbulence and smaller droplet size.     

Propagation of Reaction Between Bubbles with a Gas Burning in a Fluidised Bed

A bubbling fluidised bed reactor has been used for investigating how combustion propagates between bubbles of premixed fuel + air, rising from the distributor towards the surface. Earlier work has shown that when the temperature of the sand gradually rises, the gases first burn in flames above the bed, regime A, then combustion moves under its surface, to occur in bubbles travelling up the bed, under regime B. Above a certain temperature, characteristic of the gas mixture composition, combustion descends towards the bottom of the bed. Ignition then occurs in small bubbles near the distributor, under the stable regime C. The kinetic model, used to calculate the delay for thermal ignition inside gas bubbles rising through the bed, gives correct predictions for regimes A and C but not for B. Under regime B, bubbles of the mixture begin to ignite under the bed’s surface while their residence time in the bed is remains shorter than the delay times for thermal ignition derived from the kinetic model. As the temperature rises, the ignition delay rapidly decreases, and regime C is reached, in accordance with the model. In this work attention was focused on regime B. A laboratory reactor of quartz glass was used, with a bed of quartz sand. Fast video recording was employed to capture ignition phenomena as the bed’s temperature was raised or lowered. Records of freeboard concentrations of O₂, CO and of total hydrocarbons, VOCs, were obtained, confirming the specific aerodynamic and chemical character of regime B. It has been shown that combustion spreads from the surface to bubbles near the bed’s surface and then to other bubbles close by. Such transfer of the reaction stabilizes combustion inside the bed, at temperatures appreciably lower than that for the thermal ignition of the mixture, given by the kinetic model. This is consistent with earlier findings, which have shown that the combustion of gaseous mixtures in bubbling fluidised beds is controlled by gas phase processes, as in flames.     

Large-Eddy Simulations of Spray a Flames Using Explicit Coupling of the Energy Equation with the FGM Database

This paper provides a numerical study on n-dodecane flames using Large-Eddy Simulations (LES) along with the Flamelet Generated Manifold (FGM) method for combustion modeling. The computational setup follows the Engine Combustion Network Spray A operating condition, which consists of a single-hole spray injection into a constant volume vessel. Herein we propose a novel approach for the coupling of the energy equation with the FGM database for spray combustion simulations. Namely, the energy equation is solved in terms of the sensible enthalpy, while the heat of combustion is calculated from the FGM database. This approach decreases the computational cost of the simulation because it does not require a precise computation of the entire composition of the mixture. The flamelet database is generated by simulating a series of counterflow diffusion flames with two popular chemical kinetics mechanisms for n-dodecane. Further, the secondary breakup of the droplet is taken into account by a recently developed modified version of the Taylor Analogy Breakup model. The numerical results show that the proposed methodology captures accurately the main characteristics of the reacting spray, such as mixture formation, ignition delay time, and flame lift-off. Additionally, it captures the “cool flame" between the flame lift-off and the injection nozzle. Overall, the simulations show differences between the two kinetics mechanisms regarding the ignition characteristics, while similar flame structures are observed once the flame is stabilised at the lift-off distance.

     

Numerical Simulation of Non-premixed Turbulent Combustion Using the Eddy Dissipation Concept and Comparing with the Steady Laminar Flamelet Model

Numerical simulations of the Sandia flame CHNa and the Sydney bluff-body stabilized flame HM1E are reported and the results are compared to available experimental data. The numerical method is based on compressible URANS formulations which were implemented recently in the OpenFOAM toolbox. In this study, the calculations are carried out using the conventional compressible URANS approach and a standard k- ?? turbulence model. The Eddy Dissipation Concept with a detailed chemistry approach is used for the turbulence-chemistry interaction. The syngas (CO/H₂) chemistry diluted by 30 % nitrogen in the Sandia flame CHNa and CH₄/H₂ combustion in the Sydney flame HM1E are described by the full GRI-3.0 mechanism. A robust implicit Runge-Kutta method (RADAU5) is used for integrating stiff ordinary differential equations to calculate the reaction rates. The radiation is treated by the P1-approximation model. Both target flames are predicted with the Steady Laminar Flamelet model using the commercial code ANSYS FLUENT as well. In general, there is good agreement between present simulations and measurements for both flames, which indicates that the proposed numerical method is suitable for this type of combustion, provides acceptable accuracy and is ready for further combustion application development.     

Studies on Lifted Jet Flames in Coflow: The Stabilization Mechanism in the Near- and Far-Fields

This paper presents the results of a parametric study concerning the phenomenon of liftoff of a nonpremixed jet flame. The dependence of liftoff height on jet exit velocity and coflow velocity is described. It is shown that lifted flames become less sensitive to jet exit velocity as the stabilization point recedes from the burner exit. The results reveal that in cases of extreme liftoff height, increases in jet exit velocity with a constant coflow cause some ethylene flames to stabilize closer to the burner. The success of current theories on lifted flame stabilization in comparison to the experimental results of this study are assessed. The existence of multiple regimes for flame stabilization, incorporating aspects of both premixed and nonpremixed combustion, is proposed.     

Temperature and related statistics in turbulent jet flames

This paper describes the use of digitally-compensated thermocouples to characterise the time-resolved behaviour of the temperature field in turbulent non-premixed jet flames for Re?2×10⁴, including the transport of heat and the related temperature dissipation. The experimental procedures used are analysed, including the thermocouple time constant determination and the numerical derivative compensation method, and assessments of accuracy are provided for the cross-correlation techniques used in order to estimate high-order temperature statistics in the flames studied. The results quantify the influence of the spatial resolution of the measuring systems on the accuracy of correlation values, regarding the characteristics scales of the flames studied, and demonstrate the applicability of fine-wire thermocouples to characterise the turbulent transport and the dissipation of temperature in non-premixed jet flames.     

温度和浓度对甲醇喷雾爆炸特性参数的影响

采用20 L球形喷雾爆炸实验系统,探究甲醇在不同环境温度、物料温度及喷雾浓度下的爆炸特性规律。结果表明:20 L爆炸球内甲醇喷雾液滴爆炸极限范围为118.8~594.0 g/cm^3,与纯气相爆炸极限范围(78.6~628.6 g/cm^3)相比,甲醇喷雾液滴爆炸极限范围较窄,喷雾液滴的爆炸敏感性比纯气相甲醇蒸汽低。随着爆炸球内环境温度的升高,甲醇喷雾爆炸极限范围变宽,受限空间内甲醇气液喷雾点火成功概率增大。当甲醇物料自身温度或爆炸容器内环境温度保持不变时,相应爆炸特性参数在Φ=1.8拐点处均呈现先增大后减小的趋势。当Φ=1.8时,甲醇喷雾爆炸存在最大超压峰值。环境温度、物料温度的升高可以提高雾化质量,促进扩散燃烧。但是环境温度的变化较之物料温度的改变对于甲醇液滴喷雾爆炸特性参数的影响更为显著。环境温度和化学当量比二元变量共同影响着甲醇喷雾爆炸强度值,当Φ=1.8,环境温度为303.15 K时,甲醇的喷雾爆炸强度大于甲烷气体爆炸的爆炸强度。     

Investigation of Two-Fluid Methods for Large Eddy Simulation of Spray Combustion in Gas Turbines

An extension of the large eddy simulation (LES) technique to two-phase reacting flows, required to capture and predict the behavior of industrial burners, is presented. While most efforts reported in the literature to construct LES solvers for two-phase flow focus on Euler–Lagrange formulation, the present work explores a different solution (‘two-fluid’ approach) where an Eulerian formulation is used for the liquid phase and coupled with the LES solver of the gas phase. The equations used for each phase and the coupling terms are presented before describing validation in two simple cases which gather some of the specificities of real combustion chamber: (1) a one-dimensional laminar JP10/air flame and (2) a non-reacting swirled flow where solid particles disperse (Sommerfeld and Qiu, Int. J. Multiphase Flow 19(6):1093–1127, 1993). After these validations, the LES tool is applied to a realistic aircraft combustion chamber to study both a steady flame regime and an ignition sequence by a spark. Results bring new insights into the physics of these complex flames and demonstrate the capabilities of two-fluid LES.     

点火过程和初始条件对燃烧轻气炮内弹道性能的影响

采用计算流体力学方法对燃烧轻气炮膛内燃烧过程进行数值模拟,分析不同的点火点数目和点火能量以及初始温度和压力对燃烧轻气炮内弹道特性的影响。结果表明,采用合理的点火点数目、初始温度和压力条件可以有效控制氢气的燃烧过程,减弱燃烧室内的压力波动。模拟结果对燃烧轻气炮膛内燃烧过程控制具有重要参考价值。