Combustion of partially premixed spray jets

Gas turbines, liquid rocket motors, and oil-fired furnaces utilize the spray combustion of continuously injected liquid fuels. In most cases, the liquid spray is mixed with an oxidizer prior to combustion, and further oxidizer is supplied from the outside of the spray to complete diffusion combustion. This rich premixed spray is called “partially premixed spray.” Partially premixed sprays have not been studied systematically although they are of practical importance. In the present study, the burning behavior of partially premixed sprays was experimentally studied with a newly developed spray burner. A fuel spray and an oxidizer, diluted with nitrogen, was injected into the air. The overall equivalence ratio of the spray jet was set larger than unity to establish partially premixed spray combustion. In the present burner, the mean droplet diameter of the atomized liquid fuel could be varied without varying the overall equivalence ratio of the spray jet. Two combustion modes with and without an internal flame were observed. As the mean droplet diameter was increased or the overall equivalence ratio of the spray jet was decreased, the transition from spray combustion only with an external group flame to that with the internal premixed flame occurred. The results suggest that the internal flame was supported by flammable mixture through the vaporization of fine droplets, and the passage of droplet clusters deformed the internal flame and caused internal flame oscillation. The existence of the internal premixed flame enhanced the vaporization of droplets in the post-premixed-flame zone within the external diffusion flame.     

Large Eddy Simulation of a swirl stabilized spray flame

Large Eddy Simulations (LES) of kerosene spray combustion in an axial-swirl combustor have been carried out focusing on the effect of the evaporating droplets on the flame temperature and species concentrations. 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 predictive capability of LES with sub-grid scale models for spray dispersion and evaporation is assessed placing emphasis on the effect of the unresolved velocity and temperature fields on the droplet evaporation rate. The results of the fully coupled LES formulation exhibit good agreement between the measured and simulated mean velocity fields. The global behaviour of the spray combustion, such as droplet dispersion and evaporation, are captured reasonably well in the simulations. It was found that the large velocity fluctuations observed in the shear layer strongly affect the evaporation rate and thus the temperature distributions. The present work also demonstrated the feasibility of LES to study complex flow features which are typical of gas-turbine combustion chambers.     

Dynamics of spray and swirling flame under acoustic oscillations : A joint experimental and LES investigation

The dynamics of spray swirling flames is investigated by combining experiments on a single sector generic combustor and large eddy simulations of the same configuration. Measurements and calculations correspond to a self-sustained limit cycle operation where combustion coupled by an axial quarter wave acoustic mode induces large amplitude oscillations of pressure in the system. A detailed analysis of the mechanisms controlling the process is carried out first by comparing the measured and calculated spray and flame dynamics. Considering in a second stage that the spray and flame are compact with respect to the acoustic wavelength the analysis can be simplified by defining state variables that are obtained by taking averages over the combustor cross section and representing the behavior of these average quantities as a function of the axial coordinate and time. This reveals a first region in which essentially convective processes prevail. The convective heat release rate then couples further downstream with the pressure field giving rise to positive Rayleigh source terms which feed energy in the axial acoustic mode. In the convective region, the swirl number features oscillations around its mean value with an impact on the flow aerodynamics and flame radial displacement. Fluctuations in the fuel flow rate are initiated at the injector exhaust and likewise convected downstream. The total mass flow rate that exhibits strong convective disturbances is dominated further downstream by the acoustic motion. This information provides new insights on the convective-acoustic coupling that controls the heat release rate disturbances and reveals the time delays governing the combustion oscillation process.     

A spray flamelet/progress variable approach combined with a transported joint PDF model for turbulent spray flames

A spray flamelet/progress variable approach is developed for use in spray combustion with partly pre-vaporised liquid fuel, where a laminar spray flamelet library accounts for evaporation within the laminar flame structures. For this purpose, the standard spray flamelet formulation for pure evaporating liquid fuel and oxidiser is extended by a chemical reaction progress variable in both the turbulent spray flame model and the laminar spray flame structures, in order to account for the effect of pre-vaporised liquid fuel for instance through use of a pilot flame. This new approach is combined with a transported joint probability density function (PDF) method for the simulation of a turbulent piloted ethanol/air spray flame, and the extension requires the formulation of a joint three-variate PDF depending on the gas phase mixture fraction, the chemical reaction progress variable, and gas enthalpy. The molecular mixing is modelled with the extended interaction-by-exchange-with-the-mean (IEM) model, where source terms account for spray evaporation and heat exchange due to evaporation as well as the chemical reaction rate for the chemical reaction progress variable. This is the first formulation using a spray flamelet model considering both evaporation and partly pre-vaporised liquid fuel within the laminar spray flamelets. Results with this new formulation show good agreement with the experimental data provided by A.R. Masri, Sydney, Australia. The analysis of the Lagrangian statistics of the gas temperature and the OH mass fraction indicates that partially premixed combustion prevails near the nozzle exit of the spray, whereas further downstream, the non-premixed flame is promoted towards the inner rich-side of the spray jet since the pilot flame heats up the premixed inner spray zone. In summary, the simulation with the new formulation considering the reaction progress variable shows good performance, greatly improving the standard formulation, and it provides new insight into the local structure of this complex spray flame.     

Direct numerical simulations and analysis of three-dimensional n-heptane spray flames in a model swirl combustor

Three-dimensional n-heptane spray flames in a swirl combustor are investigated by means of direct numerical simulation (DNS) to provide insight into realistic spray evaporation and combustion as well as relevant modeling issues. The variable-density, low-Mach number Navier–Stokes equations are solved using a fully conservative and kinetic energy conserving finite difference scheme in cylindrical coordinates. Dispersed droplets are tracked in a Lagrangian framework. Droplet evaporation is described by an equilibrium model. Gas combustion is represented using an adaptive one-step irreversible reaction. Two different cases are studied: a lean case that resembles a lean direct injection combustion, and a rich case that represents the primary combustion region of a rich-burn/quick-quench/lean-burn combustor. The results suggest that premixed combustion contribute more than 70% to the total heat release rate, although diffusion flame have volumetrically a higher contribution. The conditional mean scalar dissipation rate is shown to be strongly influenced, especially in the rich case. The conditional mean evaporation rate increases almost linearly with mixture fraction in the lean case, but shows a more complex behavior in the rich case. The probability density functions (PDF) of mixture fraction in spray combustion are shown to be quite complex. To model this behavior, the formulation of the PDF in a transformed mixture fraction space is proposed and demonstrated to predict the DNS data reasonably well.     

Optical diagnostics of diesel spray injections and combustion in a high-pressure high-temperature cell

We report on spatially and temporally resolved optical diagnostic measurements of propagation and combustion of diesel sprays introduced through a single-hole fuel injector into a constant volume, high-temperature, high-pressure cell. From shadowgraphy images in non-reacting environments of pure nitrogen, penetration lengths and dispersion angles were determined for non-vaporizing and vaporizing conditions, and found to be in reasonable agreement with standard models for liquid jet propagation and break-up.Quasi-simultaneous two-dimensional images were obtained of laser elastic light scattering, shadowgraphs and spectrally integrated flame emission in a reacting environment (cell temperature 850 K). In addition laser-induced incandescence was employed for the identification of soot-loaded regions. The simultaneously recorded spray images exhibit remarkable structural similarity and provide complementary information about the spray propagation and combustion process. The measurements also reveal the fuel vapor cloud extending well beyond the liquid core and close to the nozzle tip. Ignition takes place close to the tip of the spray within the mixing layer of fuel vapor and surrounding air. Soot is formed in the vapor core region at the tip of the liquid fuel jet. Our results support recently developed phenomenological model on diesel spray combustion.     

LES-CMC of high-altitude relight in an RQL aeronautical combustor

Large-Eddy Simulations with the Conditional Moment Closure sub-grid combustion model and detailed chemistry for kerosene were performed for the ignition process in an Rich-Quench-Lean aviation gas turbine combustor at high-altitude conditions. The simulations used realistic boundary conditions for the flow inlet and spray droplet size distributions and velocity. Due to the large droplets, the Central Recirculation Zone (CRZ) is filled with fuel, mostly in liquid form. The first phase of the ignition process is critical and the results show that the spark kernel must provide enough energy to evaporate the spray and pyrolyse the fuel for the flame to grow and establish in the corner of the combustor. The second phase is characterised by the flame burning the mixture in the scorner and propagating around the Inner Shear Layer. This phase is also critical, as the flame needs the prevaporised fuel and smaller droplets in the corner to sufficiently increase the temperature and be able to propagate inside the CRZ, filled with liquid fuel and cold air. If this propagation inside the CRZ is achieved, phase three is accomplished and the burner is fully ignited. The simulations demonstrate the particular importance of detailed chemistry and proper boundary conditions for flame ignition simulations in high-altitude relight conditions.     

Effects of flash boiling injection on in-cylinder spray,mixing and combustion of a spark-ignition direct-injection engine

Higher engine efficiency and ever stringent pollutant emission regulations are considered as the most important challenges for today's automotive industry. Fast evaporation and combustion technique has caused unprecedented attention due to its potential to solve both of the above challenges. Flash boiling, which features a two-phase flow that constantly generates vapor bubbles inside the liquid spray is ideal to achieve fast evaporation and combustion inside direct-injection (DI) gasoline engines. In this study, three spray conditions, including liquid, transitional flash boiling and flare flash boiling spray were studied for comparison under cold start condition in a spark-ignition direct-injection (SIDI) optical gasoline engine. Optical access into the combustion chamber includes a quartz linear and a quartz insert on the piston. In separate experiments, we recorded the crank angle resolved spray morphology using laser scattering technique, and distribution of fuel before ignition employing laser induced fluorescence technology, as well as time-resolved color images of flame with high-speed camera. The spray morphology during the intake stroke shows stronger plume-plume and plume-air interaction under flash boiling condition, as well as smaller penetration. Then around the end of compression (before ignition), the fuel distribution is also shown to be more homogeneous with less cyclic variation under flash boiling. Finally, from the color images of the flame, it was found that with the increase of superheat degree, the diffusion rate of blue flame (generated by excited molecules) is higher, which is considered to be related with the larger fractal dimension of the flame front. Also, the combustion is more complete with less yellow flame under flash boiling.     

“Slow” active control of combustion instabilities by modification of liquid fuel spray properties

This paper describes an experimental investigation of the feasibility of using “slow” active control approaches, which “instantaneously” change liquid fuel spray properties, to suppress combustion instabilities. The objective of this control approach was to break up the feedback between the combustion process heat release and combustor pressure oscillations that drive the instability by changing the characteristics of the combustion process (e.g., the characteristic combustion time). To demonstrate the feasibility of such control, this study used a proprietary fuel injector (Nanomiser^TM), which can vary its fuel spray properties, to investigate the dependence of acoustics–combustion process coupling, i.e., the driving of combustion instabilities, upon the fuel spray properties. This study showed that by changing the spray characteristics it is possible to significantly damp combustion instabilities. Furthermore, using combustion zone chemiluminescence distributions, which were obtained by Abel’s deconvolution synchronized with measured acoustic data, it has been shown that the instabilities were mostly driven midway between the combustor centerline and wall, a short distance downstream from the flame holder, where the mean axial flow velocity is approximately zero in the vortex near the flame holder. The results of this study strongly suggest that a “slow” active control system that employs controllable fuel injectors could be effectively used to prevent the onset of detrimental combustion instabilities.     

Ethanol turbulent spray flame response to gas velocity modulation

A numerical investigation of the interaction between a spray flame and an acoustic forcing of the velocity field is presented in this paper. In combustion systems, a thermoacoustic instability is the result of a process of coupling between oscillations in heat released and acoustic waves. When liquid fuels are used, the atomisation and the evaporation process also undergo the effects of such instabilities, and the computational fluid dynamics of these complex phenomena becomes a challenging task. In this paper, an acoustic perturbation is applied to the mass flow of the gas phase at the inlet and its effect on the evaporating fuel spray and on the flame front is investigated with unsteady Reynolds averaged Navier-Stokes numerical simulations. Two flames are simulated: a partially premixed ethanol/air spray flame and a premixed pre-vaporised ethanol/air flame, with and without acoustic forcing. The frequencies used to perturb the flames are 200 and 2500 Hz, which are representative for two different regimes. Those regimes are classified based on the Strouhal number St = (D/U)f_f: at 200 Hz, St = 0.07, and at 2500 Hz, St = 0.8. The exposure of the flame to a 200 Hz signal results in a stretching of the flame which causes gas field fluctuations, a delay of the evaporation and an increase of the reaction rate. The coupling between the flame and the flow excitation is such that the flame breaks up periodically. At 2500 Hz, the evaporation rate increases but the response of the gas field is weak and the flame is more stable. The presence of droplets does not play a crucial role at 2500 Hz, as shown by a comparison of the discrete flame function in the case of spray and pre-vaporised flame. At low Strouhal number, the forced response of the pre-vaporised flame is much higher compared to that of the spray flame.     

Prediction of soot and thermal radiation in a model gas turbine combustor burning kerosene fuel spray at different swirl levels

Combustion of kerosene fuel spray has been numerically simulated in a laboratory scale combustor geometry to predict soot and the effects of thermal radiation at different swirl levels of primary air flow. The two-phase motion in the combustor is simulated using an Eulerian–Lagragian formulation considering the stochastic separated flow model. The Favre-averaged governing equations are solved for the gas phase with the turbulent quantities simulated by realisable k–? model. The injection of the fuel is considered through a pressure swirl atomiser and the combustion is simulated by a laminar flamelet model with detailed kinetics of kerosene combustion. Soot formation in the flame is predicted using an empirical model with the model parameters adjusted for kerosene fuel. Contributions of gas phase and soot towards thermal radiation have been considered to predict the incident heat flux on the combustor wall and fuel injector. Swirl in the primary flow significantly influences the flow and flame structures in the combustor. The stronger recirculation at high swirl draws more air into the flame region, reduces the flame length and peak flame temperature and also brings the soot laden zone closer to the inlet plane. As a result, the radiative heat flux on the peripheral wall decreases at high swirl and also shifts closer to the inlet plane. However, increased swirl increases the combustor wall temperature due to radial spreading of the flame. The high incident radiative heat flux and the high surface temperature make the fuel injector a critical item in the combustor. The injector peak temperature increases with the increase in swirl flow mainly because the flame is located closer to the inlet plane. On the other hand, a more uniform temperature distribution in the exhaust gas can be attained at the combustor exit at high swirl condition.     

Simulation of spray combustion in a lean-direct injection combustor

Large-eddy simulation (LES) of a liquid-fueled lean-direct injection (LDI) combustor is carried out by resolving the entire inlet flow path through the swirl vanes and the combustor. A localized dynamic subgrid closure is combined with a subgrid mixing and combustion model so that no adjustable parameters are required for both non-reacting and reacting LES. Time-averaged velocity predictions compare well with the measured data. The unsteady flow features that play a major role in spray dispersion, fuel–air mixing and flame stabilization are identified from the simulation data. It is shown that the vortex breakdown bubble (VBB) is smaller with more intense reverse flow when there is heat release. The swirling shear layer plays a major role in spray dispersion and the VBB provides an efficient flameholding mechanism to stabilize the flame.     

液体火箭有机凝胶喷雾液滴蒸发模型及仿真研究

凝胶推进剂虽然兼具有液体推进剂流量可控和固体推进剂长期可储存等优点, 但凝胶喷雾液滴蒸发燃烧问题却一直困扰着凝胶推进剂研制及燃烧室设计工作, 阻碍了凝胶推进剂实际工程应用.设计实现了凝胶单液滴蒸发燃烧实验系统, 通过某型有机凝胶偏二甲肼(UDMH)单液滴在四氧化二氮蒸气中的蒸发燃烧实验现象, 进一步深入分析了凝胶液滴蒸发燃烧机理.根据实验中凝胶单液滴在不同阶段的蒸发特性, 建立了有机凝胶喷雾液滴在胶凝剂膜形成、膨胀、破裂三个不同蒸发阶段的多组分蒸发模型, 采用初步选定的模型参数及物性参数对凝胶单液滴在高温气体环境中的蒸发全过程进行了仿真计算, 并与常规液体液滴的仿真结果进行了对比分析.结果表明,凝胶喷雾液滴表面胶凝剂含量在蒸发初期增加比较缓慢, 但在某临界时刻后的极短时间内迅速升高至形成胶凝剂膜的质量分数95%, 导致表面质量流率迅速下降至0,表面温度则快速上升至UDMH推进剂沸点.胶凝剂膜形成后, 液滴半径及表面UDMH蒸气质量分数出现了实验现象中凝胶液滴反复膨胀-破裂的震荡现象, 液滴表面温度维持在略高于沸点的某温度范围内,凝胶液滴内部的沸腾蒸发明显强于液体液滴表面稳态蒸发流率, 使得凝胶喷雾液滴生存时间小于常规液体液滴.     

Onset of detonation in polydispersed fuel–air mixtures

Investigations of detonation onset in pulverized fuel–air mixtures were carried out. Combustion and detonation processes in sprays differ greatly from that in homogeneous mixtures, because not only chemical reactions, but physical processes of combustible mixture formation take place within the combustion zone (droplets atomization and evaporation). The polydispersed character of mixture and non-uniformity of droplet spatial distribution strongly affects spray combustion and detonation onset. The present paper contains the results of theoretical and experimental investigations of detonation onset peculiarities in polydispersed non-uniform hydrocarbon–air mixtures.     

On the behaviour of organic gel multi-size spray diffusion flames

The recently reported, experimentally observed, unusual behaviour of organic gellant-based fuel droplets which, under appropriate ambient thermal conditions, evaporate and burn in an oscillatory fashion is incorporated in a phenomenological manner in a model of a two-dimensional arbitrary multi-size spray diffusion flame. Non-unity Lewis numbers are permitted for the fuel vapour and oxidant. A combined analytical/numerical solution of the governing equations is presented and used to investigate how a spray's initial polydispersity and the frequency of oscillatory evaporation influence the combustion field. It is demonstrated that the initial droplet size distribution and the frequency of evaporation of the burning gel droplets can have an acute impact both on the homogeneous diffusion flame shape, height and width and on the thermal field downstream of the flame front. Hot spots of individual (or clusters of) burning droplets can be created and under certain operating conditions can lead to hotter temperatures than experienced in the main homogeneous flame. The intensity of these hotspots, their number and location are sensitive to spray related parameters. In realistic combustion chambers there is a danger inherent in the existence of hotspots in undesirable regions as they can damage the structural integrity. Other computed results demonstrate that, in relation to the spray diffusion flames obtained using an equivalent purely liquid fuel spray, the use of a gel fuel spray can lead, under certain operating conditions, to a reduction in flame height and temperature. The latter effect is critical when considering flame extinction.     

A study of ignition and combustion of liquid hydrocarbon droplets in premixed fuel/air mixtures in a rapid compression machine

The combustion of two fuels with disparate reactivity such as natural gas and diesel in internal combustion engines has been demonstrated as a means to increase efficiency, reduce fuel costs and reduce pollutant formation in comparison to traditional diesel or spark-ignited engines. However, dual fuel engines are constrained by the onset of uncontrolled fast combustion (i.e., engine knock) as well as incomplete combustion, which can result in high unburned hydrocarbon emissions. To study the fundamental combustion processes of ignition and flame propagation in dual fuel engines, a new method has been developed to inject single isolated liquid hydrocarbon droplets into premixed methane/air mixtures at elevated temperatures and pressures. An opposed-piston rapid compression machine was used in combination with a newly developed piezoelectric droplet injection system that is capable of injecting single liquid hydrocarbon droplets along the stagnation plane of the combustion chamber. A high-speed Schlieren optical system was used for imaging the combustion process in the chamber. Experiments were conducted by injecting diesel droplet of various diameters (50 µm < d_o < 400 µm), into methane/air mixtures with varying equivalence ratios (0 < ϕ < 1.2) over a range of compressed temperatures (700 K < T_c < 940 K). Multiple autoignition modes was observed in the vicinity of the liquid droplets, which were followed by transition to propagating premixed flames. A computational model was developed with CONVERGE™, which uses a 141 species dual-fuel chemical kinetic mechanism for the gas phase along with a transient, analytical droplet evaporation model to define the boundary conditions at the droplet surface. The simulations capture each of the different ignition modes in the vicinity of the injected spherical diesel droplet, along with bifurcation of the ignition event into a propagating, premixed methane/air flame and a stationary diesel/air diffusion flame.     

对流速度对燃料液滴火焰形态及燃烧的影响

考虑气相非稳态及液滴内部环流,建立运动液滴非稳态蒸发燃烧模型.模型采用动网格方法精确追踪液滴表面位置,采用守恒方程组更新液滴表面边界条件.根据单步全局化学反应机理,仿真研究正庚烷燃料液滴在不同对流速度下的火焰形态及燃烧.结果表明:运动液滴内部环流使液滴内部低温区向环流中心移动.当液滴运动速度大于某临界值后,火焰形态由包覆火焰转变为尾迹火焰.包覆火焰的富燃区范围、高温区范围及燃烧速率明显较尾迹火焰大;包覆火焰的液滴表面温度及表面蒸发流率分布也明显不同于尾迹火焰.     

Effect of non-zero relative velocity on the flame speed of two-phase laminar flames

A numerical study of one-dimensional n-heptane/air spray flames is presented. The objective is to evaluate the flame propagation speed in the case where droplets evaporate inside the reaction zone with possibly non-zero relative velocity. A Direct Numerical Simulation approach for the gaseous phase is coupled to a discrete particle Lagrangian formalism for the dispersed phase. A global two-step n-heptane/air chemical mechanism is used. The effects of initial droplet diameter, overall equivalence ratio, liquid loading and relative velocity between gaseous and liquid phases on the laminar spray flame speed and structure are studied. For lean premixed cases, it is found that the laminar flame speed decreases with increasing initial droplet diameter and relative velocity. On the contrary, rich premixed cases show a range of diameters for which the flame speed is enhanced compared to the corresponding purely gaseous flame. Finally, spray flames controlled by evaporation always have lower flame speeds. To highlight the controlling parameters of spray flame speed, approximate analytical expressions are proposed, which give the correct trends of the spray flame propagation speed behavior for both lean and rich mixtures.     

High-pressure fuel spray ignition behavior with hot surface interaction

Fuel-flexible aircraft propulsion systems using compression ignition engines will require novel strategies for reducing the ignition delay of low-reactivity fuels to feasible timescales. Hot surface ignition of fuel sprays has been implemented in some practical situations, but the complex nature of flame formation within the spray structure poses significant challenges. In order to design next-generation ignition devices, the capacity of hot surface heating elements to promote fuel spray ignition must be investigated. In this study, a rapid compression machine (RCM) was used to examine the ignition process of a single kerosene-based F-24 jet fuel spray with a cylindrical heating element inserted into the spray periphery. The experiments, performed with moderately high injection pressures of 40 MPa, have demonstrated two modes of ignition governed by surface temperature and insertion depth of the heating element. There exists an optimal position where the heating element tip is located in the fuel vapor cone around the liquid spray. For this configuration, a critical surface temperature was identified (～1250 K), above which short ignition delays associated with a “spray ignition” mode are consistently achieved. In this case, a local ignition flame kernel propagates downstream to the flame lift-off length before full ignition of the spray. In comparison, below the critical temperature a slower “volumetric” mode results. The extended ignition delays associated with this mode may be impractical for compression ignition engines operating at high speeds and increased altitude.     

Simultaneous rainbow schlieren deflectometry and OH* chemiluminescence imaging of a diesel spray flame in constant pressure flow rig

The physical and chemical phenomena that take place during fuel injection, entrainment and fuel-air mixing, cool-flame and ignition reaction, and combustion in diesel sprays still require extensive study. Global parameters such as liquid and vapor jet penetration lengths and spreading rates render useful yet still limited information. Understanding of the temporal evolution of the spray as it progresses through various steps is needed to develop advanced clean combustion modes and high-fidelity predictive models with sufficient accuracy. In this study, high-speed rainbow schlieren deflectometry (RSD) and OH* chemiluminescence are used to simultaneously image fuel-air mixing, cool-flame reactions, ignition, flame propagation and stabilization, and combustion in a transient diesel-like flame. A constant pressure flow rig (CPFR) is used to conduct multiple injections in quick succession to obtain a statistically relevant dataset. n-heptane was injected at nominal supply pressure of 1000 bar from a single-hole diesel injector into ambient at pressure of 30 bar and temperature of 800 K. About 500 injections were performed and analyzed to reveal structural features of non-reacting and reacting regions of the spray, quantify jet penetration and spreading rates, and study cool-flame behavior, ignition, flame propagation and stabilization at lift-off length, and combustion at upstream and downstream locations.