Structure of evaporating single- and multicomponent fuel sprays for 2nd generation gasoline direct injection

In the present paper the effect of fuel properties on spray formation and evaporation was investigated for a hollow-cone spray of a piezoelectric injector for Direct Injection Spark Ignition (DISI) engines. Late injection timing in a high-pressure atmosphere (1.5 MPa, 200 °C) was simulated in an injection chamber. Liquid and vapor phase structure of the hollow-cone spray were studied with 2D-Mie scattering, laser-induced fluorescence (LIF) as well as phase-Doppler anemometry (PDA). The spray structure was investigated for several alkanes with high and low volatility (n-hexane, n-heptane, iso-octane, n-decane) and a three-component mixture of the n-alkanes with similar fuel properties like a multicomponent gasoline fuel. It is found that the rapid evaporation of high volatility fuels can lead to spray destabilization, whereas low volatility single-component fuels overestimate radial spray propagation and vortex formation. For iso-octane the droplet size distribution is shifted to smaller droplets and the spray appeared to be less dense compared to n-heptane despite almost identical boiling behavior. However, the much higher viscosity of iso-octane determines the internal nozzle flow which results in a reduced injected fuel mass and changed atomization. A well defined three-component fuel models the global spray characteristics as well as the droplet size, droplet momentum distribution and evaporation behavior of the used multicomponent gasoline fuel very precisely. Small amounts of low volatility fractions delay the droplet evaporation and support the overall spray stability also for multicomponent mixtures. This leads to an increased spray width as well as larger droplet sizes and momenta. The evaporation characteristic of multicomponent fuels at increased ambient pressure is complex. At the studied injection conditions it is situated between the two limiting cases of distillation-like behavior and coevaporation of the components. Moreover, the results in comparison with theoretical estimations indicate a demixing of light and heavy boiling fractions in the three-component and multicomponent fuel under conditions which are typical for DISI strategies with late injection.     

Droplet size distribution and evaporation characteristics of fuel spray by a swirl type atomizer

Spray atomization and evaporation play extremely important roles in mixture formation and combustion processes of direct injection (DI) gasoline engines. In this study, the fundamental characteristics of a swirl spray injected into a constant volume vessel are investigated by means of several laser diagnostic techniques including the laser diffraction-based method for droplet size distribution, the laser induced fluorescence-particle image velocimetry for velocity distributions of droplets and spray-induced ambient air flow, and the two-wavelength laser absorption-scattering technique for concentration distributions of liquid and vapor phases in the spray. The results show that the droplets at outer zone of the spray exhibit larger diameter than those at inner zone under both ambient pressures 0.1 and 0.4 MPa. While this can be partially attributed to the effect of spray-induced ambient air flow, the strength of ambient air flow become small when increasing the ambient pressure from 0.1 to 0.4 MPa, indicating the strong influence of spray dynamics on the droplet size distribution. In the evaporating spray, there are higher vapor concentrations near the spray axis than at peripheral zones. At 4.0 ms after start of injection, spray droplets almost completely evaporate under ambient temperature 500 K and pressure 1.0 MPa, but there are significantly amount of fuels with equivalence ratio below 0.5 in the spray. Reduction in ambient pressure promotes the air entrainment and droplet evaporation, but lowered ambient pressure results in more fuel vapor of equivalence ratio above 1.3 along the spray axis.     

A model for fuel spray penetration

A new model for fuel spray penetration (location of spray tip) is suggested and validated against available experimental data. Simple analytical expressions for fuel spray penetration are derived in two limiting cases: the initial stage and the two-phase flow regime. At the initial stage, the effects of droplet drag and entrainment of air are accounted for. In the case of the two-phase flow, it is assumed that the spray droplets have the same velocities as the entrained air. The characteristic time of droplet break-up in the spray is estimated for both bag and stripping break-up mechanisms. For realistic diesel spray parameters, the droplet break-up takes place almost immediately after the droplets leave the nozzle. This leads to a considerable shortening of the initial stage and a rapid conversion of the flow to the two-phase stage. This allows the analysis to be restricted to the two-phase flow approximation for this type of spray. The expressions for spray penetration derived in this paper give more accurate predictions compared with those suggested earlier.     

Airborne Dust Capture and Induced Airflow of Various Spray Nozzle Designs

Water spray characteristics, including droplet size and velocity, airborne dust capture potential, and induced airflow quantity for various spray nozzle designs were evaluated to provide basic information for improving spray applications. Water droplet size and velocity characteristics were initially measured by a Phase Doppler Particle Analyzer (PDPA) for hollow cone, full cone, flat fan, and air atomized spray nozzles at similar operating parameters. Airflow inducement and dust capture experiments were also conducted under the same operating parameters to examine any salient features of the spray nozzle type, droplet characteristics, induced airflow, and airborne dust capture.

Test results indicate that there are trade offs between airflow inducement and dust capture efficiency. A spray nozzle with a wider discharge angle was observed to induce more airflow, but at reduced dust capture efficiencies. Increasing spray nozzle fluid pressure(s) generally reduced water droplet sizes with concurrent increases in droplet velocity, airflow inducement, and airborne dust capture. Placing a three-sided barrier around the spray nozzles normally reduced spray air induction and increased dust capture efficiency. A direct relationship between airborne dust capture efficiency and spray input power normalized per unit of airflow induced was observed. This information can be utilized to improve the performance of water sprays for reducing airborne dust levels.     

Validation of the Lagrangian Approach for Predicting Turbulent Dispersion and Evaporation of Droplets within a Spray

The accuracy of the Lagrangian approach for predicting droplet trajectories and evaporation rates within a simple spray has been addressed. The turbulent dispersion and overall evaporation rates of droplets are modeled reasonably well, although the downstream velocity decay of the larger droplets is underpredicted, which is attributed to a poor estimate of the radial fluctuating velocity of these droplets at the inlet boundary. Qualitative agreement is found between the predicted and experimental evolution of the droplet size distribution downstream of the nozzle. These results show that smaller droplets evaporate preferentially to larger droplets, because they disperse more quickly toward the edge of the jet, where the entrainment of fresh air from the surroundings produces a significant evaporative driving force. Droplet dispersion and evaporation rates are highly influenced by the rate of turbulence generation within the shear layer. This work demonstrates the potential of the Lagrangian approach for analyzing particle trajectories and drying within the more complex spray dryer system.     

Validation of the Lagrangian Approach for Predicting Turbulent Dispersion and Evaporation of Droplets within a Spray

The accuracy of the Lagrangian approach for predicting droplet trajectories and evaporation rates within a simple spray has been addressed. The turbulent dispersion and overall evaporation rates of droplets are modeled reasonably well, although the downstream velocity decay of the larger droplets is underpredicted, which is attributed to a poor estimate of the radial fluctuating velocity of these droplets at the inlet boundary. Qualitative agreement is found between the predicted and experimental evolution of the droplet size distribution downstream of the nozzle. These results show that smaller droplets evaporate preferentially to larger droplets, because they disperse more quickly toward the edge of the jet, where the entrainment of fresh air from the surroundings produces a significant evaporative driving force. Droplet dispersion and evaporation rates are highly influenced by the rate of turbulence generation within the shear layer. This work demonstrates the potential of the Lagrangian approach for analyzing particle trajectories and drying within the more complex spray dryer system.     

APPLICATION OF LASER DIAGNOSTIC TECHNIQUES FOR THE EXAMINATION OF LIQUID FUEL SPRAY STRUCTURE

Results are presented on the spatial distribution of droplet mean size and number density obtained from a hollow-cone kerosene spray, introduced into nonswirling and swirling flow fields. An ensemble light scattering technique, based on measurement of the polarization ratio, has been employed to determine local droplet characteristics in both dense and dilute regions of the spray. The measurements are complemented with Lorenz-Mie calculations of the scattering characteristics for a polydispersion of droplets; the calculations were carried out for different mean sizes and refractive indices. The results reveal that the degree of swirl imparted to the surrounding air flow has a strong influence on spray structure. For all conditions examined the droplet mean size is found lo be larger on the spray boundary than towards the centerline. Droplet mean size is also found to increase with axial distance at all radial positions of the spray; this trend is attributed to the vaporization of smaller droplets and/or possible coalescence between the droplets.

In addition to the ensemble technique, measurements have also been obtained with the phase/Doppler interferometry and light intensity deconvolution techniques under identical experimental conditions. The droplet mean sizes obtained with the ensemble approach are in general smaller than those measured with the phase/Doppler technique; however, general features of the radial profiles obtained with both techniques are similar. The deconvolution technique also indicates the presence of smaller size droplets and supports the results obtained with the ensemble technique. The selective sensitivity of these sizing techniques to different ranges of droplet size and number density is discussed.     

Influence of ambient flow conditions on the droplet atomization characteristics of dimethyl ether (DME)

This paper describes the effects of ambient flow conditions on the droplet atomization characteristics of dimethyl ether (DME) both experimentally and numerically.In this investigation, the droplet atomization of DME fuel affected by ambient flow conditions was studied in terms of droplet mean size and detected droplet percentage under elevated ambient pressures and temperatures. In order to predict the DME spray atomization, the hybrid breakup model combined with KH-RT (Kelvin-Helmholtz and Rayleigh-Taylor) and KH-DDB (Kelvin-Helmholtz and Drop Deformation Breakup) models was applied in this study.It was revealed that the spray arrival time of DME fuel under a high ambient pressure increased in accordance with the increase in ambient pressure in the spray chamber. It can be seen that more small droplets are distributed at high ambient flow pressure conditions than at atmospheric conditions. This is a consequence of enhanced atomization of DME fuel. On the other hand, when the ambient pressure increases to 2 MPa, the Sauter mean diameter (SMD) increases only slightly compared with that at 1 MPa of pressure. The SMD value of droplets is increased as ambient temperature is increased. Under the high temperature condition in the chamber, the small droplets of DME fuel evaporate quickly and mix with the ambient air. As a result, it promotes the air-fuel mixing in a combustion chamber.     

Evaporation of pure liquid droplets: Comparison of droplet evaporation in an acoustic field versus glass-filament

The rate of heat and mass transfer to droplets in sprays is a critical issue in the design of many industrial spray systems. Applications like fuel injection in internal combustion engines or spray drying stimulate interest in studying the processes related to the evaporation of droplets. In this study an acoustic levitator and the glass filament method are used to observe single droplets during evaporation. The introduction of a droplet into the acoustic field leads to the formation of two steady toroidal vortices close to the droplet surface, known as outer acoustic streaming. The results of this study illustrate how this “outer acoustic streaming” affects the heat and mass transfer. The elimination of these vortices has been achieved through applying a ventilating air flow. Particle Image Velocimetry has been used to characterize the flow field and the interfacial transfer. Furthermore, the resulting evaporation rates have been verified by comparing them with existing model predictions. A comparison of these results to those obtained with the glass-filament method revealed good agreement when the air flow was increased to a limit where the inner acoustic streaming is eliminated, i.e. when forced convection was the primary mechanism in determining the evaporation rate of the liquid droplet. For other air-flow regimes no direct comparisons between results obtained with the two techniques were permissable, as confirmed by the differing Sherwood numbers obtained in the experiments.     

Analysis of droplet impingement of different atomizers used in spray coating processes

This paper presents numerical studies on spray painting processes using three different atomizers, i.e., a high-speed rotary bell atomizer, an airless gun and a pneumatic air spray gun. A commercial CFD code was applied using an Euler-Lagrangian approach for two phase flow calculations. Special attention was paid to the droplet impingement process on the substrate. Orthogonal impact velocities and size distributions of the droplets depositing on substrates were analyzed in detail. The obtained simulation results show substantial differences of the impacting droplet characteristics of these three atomizers, which will have significant influences on the properties of the forming paint films.     

An Experimental Investigation of Agglomeration with One and Two Nozzle Atomisation

Water-droplet size and velocity measurements were taken throughout two different sprays produced by a single nozzle and two nozzles pointed towards each other. The aim of this investigation was to understand the manner in which the motion of the droplets in a spray leads to agglomeration of these droplets. It appears that the inertia of the droplets plays an important role in the redistribution of droplets throughout a spray. Larger droplets tend to concentrate at the outer portions of the spray, because they are able to maintain their radial momentum farther downstream of a nozzle, while the smaller droplets follow the airflow more closely and thus collect in the core of the spray. Agglomeration can result from both turbulent collisions and collisions due to the relative velocities of the droplets. The difference between the agglomeration rates in the sprays from a single nozzle and two-nozzles pointed towards each other was difficult to resolve in these experiments, although the results suggest that th     

单双喷嘴雾化凝聚的实验研究

Water-droplet size and velocity measurements were taken throughout two different sprays produced by a single nozzle and two nozzles pointed towards each other. The aim of this investigation was to understand the manner in which the motion of the droplets in a spray leads to agglomeration of these droplets. It appears that the inertia of the droplets plays an important role in the redistribution of droplets throughout a spray. Larger droplets tend to concentrate at the outer portions of the spray, because they are able to maintain their radial momentum farther downstream of a nozzle, while the smaller droplets follow the airflow more closely and thus collect in the core of the spray. Agglomeration can result from both turbulent collisions and collisions due to the relative velocities of the droplets. The difference between the agglomeration rates in the sprays from a single nozzle and two-nozzles pointed towards each other was difficult to resolve in these experiments, although the results suggest that the outer portions of both sprays should be investigated more closely for evidence of agglomeration.     

Transport and Removal of Expiratory Droplets in Hospital Ward Environment

The transport and removal characteristics of expiratory droplets at different supply airflow rates and “coughing” orientations were investigated both numerically and experimentally in a three-bed hospital ward setting. A Lagrangian-based particle-tracking model with near-wall correction functions for turbulence was employed to simulate the fate of the expiratory droplets. The model was tested against experimental droplet dispersion data obtained in an experimental hospital ward using Interferometric Mie Imaging and a light-scattering aerosol spectrometer. The change in airflow supply rate had insignificant effect on the transport and deposition of very large droplets (initial sizes ≥ 87.5 μm) due to the dominance of gravitational settling on these behaviors. Smaller droplets (initial sizes ≤ 45 μm) exhibited certain airborne behaviors. The effect of thermal plumes from heat sources was observed only when the supply airflow was low and when the droplet size was small, as observed in the vertical mixing patterns of the droplets of various sizes. Larger droplets tended to settle lower and lateral dispersion of the droplets became weak at the low supply airflow rate. The deposition characteristics for different surfaces in the room are described. The heat plumes seemed to obstruct small droplets from being deposited onto heated surfaces. More deposition was predicted in the lateral injection case compared with the vertical injection case. Adopting near-wall correction for turbulence in the model reduced the predicted deposition removal fraction by 25% for 1.5 μm droplets. This reduction became less significant for larger droplets due to the smaller dependence on turbulent diffusion in their deposition.     

Dispersion of Expiratory Droplets in a General Hospital Ward with Ceiling Mixing Type Mechanical Ventilation System

This study investigated the dispersion characteristics of polydispersed droplets in a general hospital ward equipped with ceiling-mixing type ventilation system. Injections of water test droplets containing non-volatile content were produced. The injections simulate human coughs with a similar droplet size distribution (peak size at 12 μ m) and airflow rate (0.4 L/s). The dispersion of test droplets was measured in-situ by interferometric Mie imaging (IMI) method combined with an aerosol spectrometer. A multiphase numerical model was employed to simulate the droplet dispersion tracks to provide additional transient position tracking data. Results show that the small size group of droplets or droplet nuclei (initial size ≤45 μ m) behaved airborne transmittable as some nuclei stayed airborne for more than 360 s. The dispersions were strongly affected by the ventilation airflow pattern. The expiratory droplets exhibited a two-stage lateral dispersion behavior, in which rapid dispersion was found in the early “initial dispersion” stage and then the dispersion became much slower in the subsequent “stable” stage. The exhaust air vents significantly enhanced lateral dispersions towards their direction. Droplets in the large size group (initial size = 87.5 μ m and 137.5 μ m) were subjected to heavy gravitational effect and stayed airborne for less than 30 s. Results indicate that the location of exhaust air vents has significant impact on the dispersion pattern of expiratory droplets. It should be carefully considered in designing ventilation systems for health-care settings.     

Experimental study on fusion and break-up motion after droplet collision

The interactions between droplets have an important influence on the atomization of liquid fuel, the combustion efficiency, and the reduction of particulate matter emissions for an engine. For this reason, this paper presents results from an experimental study on the coalescence and break-up of droplets after collision. According to the shape and parameters of the droplets at different times after the collision of the droplets was captured by a high speed camera, analysis was done for the following effects of droplet collisions: the collision-coalescence motion for the collision between the droplets, the change history of the dimensionless length-to-width ratio of the oscillation motion, the critical size ratio of the breakup motion, and the liquid physical properties of the particles. The results show that the droplets collide and exhibit two forms of coalescence oscillation and break-up: for oscillating motion, at higher droplet collision velocities and dimensionless size ratios, there will be a larger dimensionless length-to-width ratio for the droplet oscillation; for the break-up motion, at higher collision velocities, there will be lower dimensionless size ratios, and lower liquid surface tension, shorter times over which the droplet breaks, and facilitated droplet break-up. The research results presented here can be used for atomization in engine cylinder, increasing the gas/liquid contact area and enhancing the combustion efficiency of gas/liquid heat transfer to improve the combustion efficiency of the engine.     

Atomization and combustion of canola methyl ester biofuel spray

The spray atomization and combustion characteristics of canola methyl ester (CME) biofuel are compared to those of petroleum based No. 2 diesel fuel in this paper. The spray flame was contained in an optically accessible combustor which was operated at atmospheric pressure with a co-flow of heated air. Fuel was delivered through a swirl-type air-blast atomizer with an injector orifice diameter of 300 μm. A two-component phase Doppler particle analyzer was used to measure the spray droplet size, axial velocity, and radial velocity distributions. Radial and axial distributions of NO, CO, CO₂ and O₂ concentrations were also obtained. Axial and radial distributions of flame temperature were recorded with a Pt–Pt/13%Rh (type R) thermocouple. The volumetric flow rates of fuel, atomization air and co-flow air were kept constant for both fuels. The droplet Sauter mean diameter (SMD) at the nozzle exit for CME biofuel spray was smaller than that of the No. 2 diesel fuel spray, implying faster vaporization rates for the former. The flame temperature decreased more rapidly for the CME biofuel spray flame than for the No. 2 diesel fuel spray flame in both axial and radial directions. CME biofuel spray flames produced lower in-flame NO and CO peak concentrations than No. 2 diesel fuel spray flames.     

Predictive models for characterizing the atomization process in pyrolysis of methyl ricinoleate

Pyrolysis of methyl ricinoleate(MR) can produce undecylenic acid methyl ester and heptanal which are important chemicals. Atomization feeding favors the heat exchange in the pyrolysis process and hence increases the product yield. Herein, predictive models to characterize the atomization process were developed. The effect of spray distance on Sauter mean diameter(SMD) of atomized MR droplets was examined, with the optimal spray distance to be 40–50 mm. Temperature mainly affected the physical properties of feedstock, with smaller droplet size obtained at increasing temperature. In addition, pressure had significant influence on SMD and higher pressure resulted in smaller atomized droplets. Then, a model for SMD prediction, combining temperature, pressure, spray distance, and structural parameters of nozzle, was developed through dimensionless analysis. The results showed that SMD was a power function of Reynolds number(Re), Ohnesorge number(Oh), and the ratio of spray distance to diameter of swirl chamber in the nozzle(H/dsc), with the exponents of -1.6618,-1.3205 and 0.1038, respectively. The experimental measured SMD was in good agreement with the calculated values, with the error within ±15%. Moreover, the droplet size distribution was studied by establishing the relationship between the standard deviation of droplet size and SMD. This study could provide reference to the regulation and optimization of the atomization process in MR pyrolysis.     

单喷雾射流在受限空间内的错流混合(英文)

In order to achieve uniform mixing between spray droplets and crossflow, cold-model experiment of a hollow-cone water spray in an air crossflow is investigated via a numerical simulation. The simulation cases are designed by using the orthogonal design method. The Eulerian-Lagrangian formulation is employed for modeling the droplets-crossflow two-phase flow while the realizable k-ε turbulence model is used to describe the turbulence. A new index, mixedness quality, is proposed to assess the overall mixing of the droplets in the crossflow. The simulation results demonstrate that the counter-rotating vortex pair (CVP) imposes a more significant impact on the spatial distribution than on the size distribution of the droplets. Pairs of CVP with smaller scales are preferable for achieving a better mixing. The influencing factors are listed in the following order in terms of the degree of their impact from the greatest to the least: the Sauter diameter of the initial droplets, the mixing tube diameter, the spray angle, the velocity of the inlet crossflow, and the vertical velocity of the initial droplets. A moderate droplet diameter, a smaller tube diameter, a moderate spray angle, a greater crossflow velocity and a moderate vertical velocity of the droplet are favorable for achieving a higher mixedness quality of the jet spray in a confined crossflow.     

Convective vaporization of a fuel droplet with thermal radiation absorption

The ‘extended’ and ‘effective-conductivity’ droplet vaporization models developed by Abramzon and Sirignano [Abramzon B, Sirignano WA. Droplet vaporization model for spray combustion calculations. Int J Heat Mass Transfer 1989;32(9):1605-18] are generalized to take into account the contribution of thermal radiation and the temperature dependence of liquid fuel properties. The thermal radiation effect is simulated using the simplified model for thermal radiation absorption suggested by Dombrovsky and Sazhin [Dombrovsky LA, Sazhin SS. Absorption of thermal radiation in a semi-transparent spherical droplet: a simplified model. Int J Heat Fluid Flow 2003;24: 919-27]. Physical properties of liquid fuel, including density, are evaluated at the average liquid temperature and updated at each time-step. These generalized models are applied to the analysis of the vaporization process of n-decane and diesel fuel droplets injected into hot air. It is pointed out that the radiation absorption in diesel fuel is generally stronger than in n-decane, and it needs to be taken into account in modelling the combustion processes in diesel engines. Calculations of the droplet vaporization rate performed using the simplified ‘effective-conductivity’ model with the internal radiation heat source uniformly distributed show exceptionally good agreement with results obtained based on the more accurate ‘extended’ vaporization model with the non-uniform distribution of radiation absorption. This allows us to recommend using the ‘effective-conductivity’ model with uniform radiation absorption for spray combustion calculations, including applications in internal combustion engines.     

喷淋塔液滴粒径分布及比表面积的实验研究

以水和空气为实验介质,通过拍照法获得喷淋塔内液滴粒径分布,考察了不同喷淋量及空塔气速对塔内不同高度处液滴Sauter平均直径(SMD)的影响,并对液滴粒径分布进行了理论分析.结果表明,喷淋塔顶部液滴分布密集,底部稀疏,液滴群在下落过程中,平均粒径减小且趋于均匀化;塔顶处液滴SMD随喷淋量的增加而增大,处在塔中下部的液滴SMD则随喷淋量增大而减小,提高空塔气速,可减小平均粒径;理论分析认为,液滴粒径减小主要是由于发生了碰撞破碎的缘故,而塔内液滴大小不一是碰撞的主要原因;通过量纲1化拟合得到喷淋塔内液滴SMD经验关联式,其计算结果与实验值吻合较好;考虑液滴破碎的喷淋塔比表面积比不考虑破碎的比表面积大70%左右.