Evaporation of bi-component droplets in a heated, highly turbulent flow

This work aims to understand the phenomena that occur in a combustion chamber where multi-component fuel droplets are injected. Many evaporation models exist but the influence of turbulence on spray vaporization is not yet well understood. This study gives a useful database to improve these models. The objective of the work is to measure the dispersion and the evaporation of bi-component (octane/3-pentanone) droplets and the resulting vapor mixing in a well-known, heated, highly turbulent channel flow. The carrier flow shows high turbulence levels, flat profiles for the mean velocity and the velocity fluctuations. The injected droplets have a large variety of behaviors due to the large polydispersion and to the turbulence. The evolution of 3-pentanone liquid concentration, mass flux, and droplet clusters are described. Mean concentration, fluctuations of concentration, and mixing of the vapor phase are characterized.     

Modeling droplet dispersion and interphase turbulent kinetic energy transfer using a new dual-timescale Langevin model

Dispersion of spray droplets and the modulation of turbulence in the ambient gas by the dispersing droplets are two coupled phenomena that are closely linked to the evolution of global spray characteristics, such as the spreading rate of the spray and the spray cone angle. Direct numerical simulations (DNS) of turbulent gas flows laden with sub-Kolmogorov size particles, in the absence of gravity, report that dispersion statistics and turbulent kinetic energy (TKE) evolve on different timescales. Furthermore, each timescale behaves differently with Stokes number, a non-dimensional flow parameter (defined in this context as the ratio of the particle response time to the Kolmogorov timescale of turbulence) that characterizes how quickly a particle responds to turbulent fluctuations in the carrier or gas phase. A new dual-timescale Langevin model (DLM) composed of two coupled Langevin equations for the fluctuating velocities, one for each phase, is proposed. This model possesses a unique feature that the implied TKE and velocity autocorrelation in each phase evolve on different timescales. Consequently, this model has the capability of simultaneously predicting the disparate Stokes number trends in the evolution of dispersion statistics, such as velocity autocorrelations, and TKE in each phase. Predictions of dispersion statistics and TKE from the new model show good agreement with published DNS of non-evaporating and evaporating droplet-laden turbulent flow.     

Modeling and Simulation of Effects of Turbulence on Vaporization, Mixing and Combustion of Liquid-Fuel Sprays

The objective of this work is twofold. Firstly, the effects of turbulence intensity variations on the turbulent droplet dispersion, vaporization and mixing for non-reacting sprays (with and without swirl) are pointed out. Secondly, the effects of the coupling of the turbulence modulation with external parameters, such as swirl intensity, on turbulent spray combustion are analyzed in configurations of engineering importance. This is achieved by using advanced models for turbulence, evaporation and turbulence modulation implemented into FASTEST-LAG3D-codes: (1) To highlight the influence of turbulence modulation on some spray properties, a thermodynamically consistent modulation model has been considered besides the standard assumption and the well known Crowe's model. For turbulent droplet dispersion, we rely on the Markov-sequence formulation. (2) In order to characterize phase transition processes ongoing on droplets surfaces, a non-equilibrium evaporation model shows better agreement with experiments in comparison with the quasi-equilibrium-based evaporation models often used. (3) The results of turbulence intensity variations reveal the existence of a limited range out of which the increase or decrease of the turbulence intensity affects no more the efficiency of the heat and mass transfer. A derived characteristic number, a vaporization Damkhöler number, possesses a critical value which separates two different behavior regimes with respect to the turbulence/droplet vaporization interactions. (4) Under reacting conditions, it is shown how the evaporation characteristics, mixing rate and combustion process are strongly influenced by swirl intensity and turbulence modulation. In particular, the turbulence modulation modifies the evaporation rate, which in turn influences the mixing and the species concentration distribution. In the case under investigation, it is demonstrated that this effect cannot be neglected for low swirl intensities (Sw.Nu. ≤ 1) in the region far from the nozzle, and close to the nozzle for high swirl number intensities. In providing these particular characteristics, a reliable control of the mixing of gaseous fuel and air in evaporating and reacting sprays, and a possible optimization of the mixing process can tentatively be achieved.     

Drag coefficients of spherical liquid droplets Part 2: Turbulent gaseous fields

The drag of non-evaporating, spherical, liquid droplets was measured in turbulent flow fields at parametric ranges relevant to spray combustion, characterized by the droplet Reynolds number, and the intensity and spatial scales of turbulence. The experimental apparatus comprised a wind-tunnel and a piezo-electric droplet generator. The procedure was to inject water droplets of uniform size co-currently and continuously with vertical turbulent air flows while droplet velocity was measured at different elevations using laser-Doppler velocimetry. Turbulence was characterized using hot-wire anemometry prior to droplet injection. Drag coefficients were calculated using these main measurements and the law of conservation of mechanical energy. Reynolds numbers were investigated in the range 10–100, in terms of the equivalent spherical diameter of a droplet, and the mean relative speed between the ambient gaseous field and the droplets. Weber numbers were much less than unity so droplets were effectively spherical. Relative intensities of turbulence were investigated in the range 20–65 percent, in terms of the mean relative speed. Spatial scales of turbulence were large in comparison to the droplets; the ratio between the spatial integral scale and the droplet diameter was in the range 11–38, and the Kolmogorov scale was comparable in size or smaller than the droplet diameter. Experimental data showed that the drag in turbulent fields under these conditions is not significantly different than that of solid spheres in a quiescent field at the same Reynolds number.The financial support of the Natural Sciences and Engineering Research Council of Canada and the Manufacturing Research Corporation of Ontario is gratefully acknowledged.     

A two-equation turbulence model for jet flows laden with vaporizing droplets

A two-equation turbulence model for steady incompressible two-phase flows including phase change has been recently developed by Mostafa & Elghobashi (1984). This model is tested for the flow of a turbulent axisymmetric gaseous jet laden with evaporating liquid droplets. To avoid the problem of density fluctuations of the carrier phase at this stage, only isothermal flow is considered and vaporization is assumed to be due to the vapor concentration gradient. The continuous size distribution of the droplets is approximated by finite size groups. Each group is considered as a continuous phase interpenetrating and interacting with the carrier phase. Two test cases have been predicted by the model. The first is for a Freon-11 spray issuing from a round nozzle, where experimental data are available at distances equal to or greater than 170 nozzle diameters. Good agreement between the data and the predictions was achieved. The second is for a methanol spray where no experiments are available yet and the predictions consider the flow region close to the nozzle ($\text{z/D < 40}$). The results of the methanol spray include distributions of the mean velocity, volume fractions of the different phases, concentration of the evaporated material in the carrier phase, turbulence intensity and shear stress of the carrier phase, droplet diameter distribution, and the jet spreading rate. In this case the results are analyzed based on a qualitative comparison with the corresponding single phase jet flow.     

Acetone Droplet Behavior in Reacting and Non Reacting Turbulent Flow

Acetone droplet characteristics in reacting and non-reacting turbulent flow are predicted and compared to experimental data. Investigations are conducted to study the effects of surrounding environment properties on the velocities, dispersion, and evaporation of a relatively volatile spray fuel that featured a wide range of Stokes numbers. The simulations are performed in the framework of Reynolds Averaged Navier Stokes equations along with the Eulerian-Lagrangian approach in which 12 different classes of the dispersed phase. The phase transition is modeled by the Langmuir-Knudsen law that accounts for non equilibrium effects based on a consistent determination of the molar mass fraction on the droplet surfaces. For the droplet dispersion, the Markov sequence model is improved by adding a correction drift term to the fluid fluctuation velocity at the parcel position along the droplet trajectory. This correction term aimed at accounting for the non-homogeneity effects in the turbulent flow. The combustion is captured using the Bray-Moss-Libby model that is extended to account for the partially premixed spray combustion. The chemistry is described with the flamelet model using a recent detailed reaction mechanism that involves 84 species and 409 reactions for which the Lewis number is not set to the unity. Mean droplet velocities for reacting and non-reacting test cases are compared with experimental data. Good agreement is observed. The spray is interacting with the nozzle edge developing new classes and relatively dense region. Hence the RMS-velocities close to the nozzle exit plan demonstrate discrepancies. The droplets group combustion effect is found to be important in the modeling of the burning velocity which influences the flame propagation. Reasonable agreements between the numerical and the experimental results are also observed in the spray flux and temperature profiles.     

Influence of Gravitational Settling on Turbulent Droplet Clustering and Radar Reflectivity Factor

This study investigates the influence of gravitational settling of droplets on turbulent clustering and the radar reflectivity factor. A three-dimensional direct numerical simulation (DNS) of particle-laden isotropic turbulence is performed to obtain turbulent droplet clustering data. The turbulent clustering data are then used to calculate the power spectrum of droplet number density fluctuations. The results show that the gravitational settling modulates the power spectrum more significantly as the settling becomes larger. The gravitational settling weakens the intensity of clustering at large wavenumbers for St≤1, whereas it significantly enlarges the intensity for St>1. The dependence on the Taylor-microscale-based Reynolds number is also investigated to discuss the contribution of large-scale eddies to the settling influence. The results show that large-scale eddies modulate the small scale clustering structure of large St droplets. The increment of radar reflectivity factor due to turbulent clustering is estimated from the power spectrum for the case of St=1.0. The result shows that the influence of gravitational settling on the radar reflectivity factor can be significant for the case of large settling velocity droplets.     

Large Eddy Simulation of a Polydisperse Ethanol Spray Flame

Ethanol is identified as an interesting alternative fuel. In this regards, the predictive capability of combustion Large Eddy Simulation approach coupled to Lagrangian droplet dynamic model to retrieve the turbulent droplet dispersion, droplet size distribution, spray evolution and combustion properties is investigated in this paper for an ethanol spray flame. Following the Eulerian-Lagrangian approach with a fully two way coupling, the Favre-filtered low Mach number Navier-Stokes equations are solved on structured grids with dynamic sub-grid scale models to describe the turbulent carrier gas phase. Droplets are injected in polydisperse manner and generated in time dependent boundary conditions. They evaporate to form an air-fuel mixture that yields spray flame. Part of the ethanol droplets evaporates within the prevaporization area before reaching the combustion zone, making the flame to burn in a partially premixed regime. The chemistry is described by a tabulated detailed chemistry based on the flamelet generated manifold approach. The fuel, ethanol, is modeled by a detailed reaction mechanism consisting of 56 species and 351 reversible reactions. The simulation results including excess gas temperature, droplet velocities and corresponding fluctuations, droplet mean diameters and spray volume flux at different distances from the exit plane show good agreement with experimental data. Analysis of combustion spray features allows gaining a deep insight into the two-phase flow process ongoing.     

Modeling of drag reduction in turbulent channel flow with hydrophobic walls by FVM method and weakly-compressible flow equations

In this paper the effects of hydrophobic wall on skin-friction drag in the channel flow are investigated through large eddy simulation on the basis of weaklycompressible flow equations with the MacCormack's scheme on collocated mesh in the FVM framework. The slip length model is adopted to describe the behavior of the slip velocities in the streamwise and spanwise directions at the interface between the hydrophobic wall and turbulent channel flow. Simulation results are presented by analyzing flow behaviors over hydrophobic wall with the Smagorinky subgrid-scale model and a dynamic model on computational meshes of different resolutions. Comparison and analysis are made on the distributions of timeaveraged velocity, velocity fluctuations, Reynolds stress as well as the skin-friction drag. Excellent agreement between the present study and previous results demonstrates the accuracy of the simple classical second-order scheme in representing turbulent vertox near hydrophobic wall. In addition, the relation of drag reduction efficiency versus time-averaged slip velocity is established. It is also foundthat the decrease of velocity gradient in the close wall region is responsible for the drag reduction. Considering its advantages of high calculation precision and efficiency, the present method has good prospect in its application to practical projects.     

A moment method for splashing and evaporation processes of polydisperse sprays

A new moment method for the modelling of polydisperse sprays is proposed that simultaneously takes into account the dispersion in droplet size and droplet velocity. For the derivation of this Eulerian method the kinetic spray equation is used which constitutes a partial differential equation for the probability density function of droplets. To reduce the complex kinetic spray equation to a form that can be managed with the available numerical procedures, moment transforms with respect to the droplet velocity and the droplet size are conducted. The resulting moment equations are closed by choosing an approximate probability density function which applies to polydisperse sprays. The method is successfully tested for configurations in which a polydisperse spray is either splashed, evaporated or effected by a Stokes drag force. The tests are organised in such a way that crossing of two spray distributions is always included. The new method is able to capture the polydisperse nature of sprays as well as the bi-(or multi-) modal character of the droplet velocity distribution function, for example, when droplets cross each other.     

两种颗粒湍流扩散模型数值模拟气液两相流泄漏扩散的比较

采用相间耦合的欧拉-拉格朗日方法，模拟了装有液化气（丙烷）的容器出现小孔或裂缝时，发生泄漏后的气液两相扩散过程。分别应用随机轨道模型及颗粒群模型来考察湍流对液滴扩散的影响，并与确定性轨道进行了比较。给出了数学物理模型，计算结果与实验数据进行了对比。结果表明:采用随机轨道模型能较好地描述液滴的湍流扩散，适用于有液相蒸发的两相流扩散问题。     

Accurate analysis of multicomponent fuel spray evaporation in turbulent flow

The aim of this paper is to perform an accurate analysis of the evaporation of single component and binary mixture fuels sprays in a hot weakly turbulent pipe flow by means of experimental measurement and numerical simulation. This gives a deeper insight into the relationship between fuel composition and spray evaporation. The turbulence intensity in the test section is equal to 10%, and the integral length scale is three orders of magnitude larger than the droplet size while the turbulence microscale (Kolmogorov scales) is of same order as the droplet diameter. The spray produced by means of a calibrated droplet generator was injected in a gas flow electrically preheated. N-nonane, isopropanol, and their mixtures were used in the tests. The generalized scattering imaging technique was applied to simultaneously determine size, velocity, and spatial location of the droplets carried by the turbulent flow in the quartz tube. The spray evaporation was computed using a Lagrangian particle solver coupled to a gas-phase solver. Computations of spray mean diameter and droplet size distributions at different locations along the pipe compare very favorably with the measurement results. This combined research tool enabled further investigation concerning the influencing parameters upon the evaporation process such as the turbulence, droplet internal mixing, and liquid-phase thermophysical properties.     

Flow characteristics of a large-size pressure-atomized spray using DTV

The purpose of this study is to characterize the atomization of a jet of water sprayed into the air at high velocity through a commercial nozzle widely used for sprinkler irrigation. The typical diameter of the droplets present in the spray is in the range of several tens of micrometers to several millimeters. They are visualized by ombroscopy. A specific Droplet Tracking Velocimetry (DTV) technique is developed to estimate the size and velocity of these highly polydispersed droplets that are distinctly non spherical. This analysis is performed from the rupture of the liquid core region (about a distance of 550 nozzle diameters) to the dispersed zone (about a distance of 900 nozzle diameters). With this technique, we obtain joint size-velocity measurements that are rarely produced. Especially two velocity components and also a large diameter range are characterized at the same time; while with other techniques, such as Particle Doppler Anemometry (PDA), the diameter range is quite reduced and requires specific settings. Additional measurements of the liquid volume fraction are performed using a single mode fiber-optic probe. In the light of our experimental data, it appears that the turbulent droplet motion in the spray is strongly anisotropic. This anisotropy is quite unexpected because other studies on sprays (generally concerned with engine applications) show a relatively low anisotropy. We attribute this increase of anisotropy to the fact that, for this type of spray, the droplet relaxation time is long in comparison to the characteristic time of the turbulence and that biggest droplets are still submitted to atomization process. This strong anisotropy is responsible for the poor radial dispersion of the spray.     

Numerical Investigation of the Time Scales of Single Droplet Burning

The effect of gas phase velocity fluctuations on single droplet burning is investigated numerically. The main objective of this study is to understand the effect of gas phase turbulence on nitric oxide formation in single droplet flames. Since the interaction of gas phase velocity fluctuations with droplet burning is of sequential character, a separate investigation of droplet momentum coupling and droplet burning is performed. Momentum coupling controls droplet relaxation against changes of the gas phase velocity along the droplet trajectory and, thereby, determines to what extend gas phase velocity fluctuations translate into droplet slip velocity fluctuations. This coupling effect acts as a high pass filter with a cutoff frequency determined by the droplet Reynolds number and diameter. In the simulation of single droplet burning detailed models for chemical reaction, diffusive species transport and evaporation are used. A significant effect of slip velocity fluctuations on the mean values of NO formation rate is observed. The effect of slip velocity fluctuations on the mean NO formation rate is frequency dependent. The frequency response of the droplet flame is similar to that of a low pass filter. The droplet flame time scale characterizing the response to slip velocity fluctuations is found to correlate with chemical time scales. This time scale is not affected by droplet diameter.     

Experiments in Turbulent Pipe Flow with Polymer Additives at Maximum Drag Reduction

In this paper we report on (two-component) LDV experiments in a fully developed turbulent pipe flow with a drag-reducing polymer (partially hydrolyzed polyacrylamide) dissolved in water. The Reynolds number based on the mean velocity, the pipe diameter and the local viscosity at the wall is approximately 10000. We have used polymer solutions with three different concentrations which have been chosen such that maximum drag reduction occurs. The amount of drag reduction found is 60–70%. Our experimental results are compared with results obtained with water and with a very dilute solution which exhibits only a small amount of drag reduction. We have focused on the observation of turbulence statistics (mean velocities and turbulence intensities) and on the various contributions to the total shear stress. The latter consists of a turbulent, a solvent (viscous) and a polymeric part. The polymers are found to contribute significantly to the total stress. With respect to the mean velocity profile we find a thickening of the buffer layer and an increase in the slope of the logarithmic profile. With respect to the turbulence statistics we find for the streamwise velocity fluctuations an increase of the root mean square at low polymer concentration but a return to values comparable to those for water at higher concentrations. The root mean square of the normal velocity fluctuations shows a strong decrease. Also the Reynolds (turbulent) shear stress and the correlation coefficient between the stream wise and the normal components are drastically reduced over the entire pipe diameter. In all cases the Reynolds stress stays definitely non-zero at maximum drag reduction. The consequence of the drop of the Reynolds stress is a large polymer stress, which can be 60% of the total stress. The kinetic-energy balance of the mean flow shows a large transfer of energy directly to the polymers instead of the route by turbulence. The kinetic energy of the turbulence suggests a possibly negative polymeric dissipation of turbulent energy. This revised version was published online in July 2006 with corrections to the Cover Date.     

壁面展向周期振动的槽道湍流减阻机理的研究

利用直接数值模拟研究了带有壁面展向周期振动的槽道湍流.壁面在展向的周期运动使湍流受到抑制,并使壁面摩擦阻力减小.通过对雷诺应力输运方程的分析研究了壁面展向周期振动的减阻机理,进一步揭示了压力变形项在湍流抑制中的关键作用.     

Evaporation of polydispersed droplets in a highly turbulent channel flow

A model experiment for the study of evaporating turbulent two-phase flows is presented here. The study focuses on a situation where pre-atomized and dispersed droplets vaporize and mix in a heated turbulent flow. The test bench consists in a channel flow with characteristics of homogeneous and isotropic turbulence where fluctuations levels reach very high values (25% in the established zone). An ultrasonic atomizer allows the injection of a mist of small droplets of acetone in the carrier flow. The large range diameters ensure that every kind of droplet behavior with regards to turbulence is possible. Instantaneous concentration fields of the vaporized phase are extracted from fluorescent images (PLIF) of the two phase flow. The evolution of the mixing of the acetone vapor is analyzed for two different liquid mass loadings. Despite the high turbulence levels, concentration fluctuations remain significant, indicating that air and acetone vapor are not fully mixed far from the injector.     

Events of High Polymer Activity in Drag Reducing Flows

The mechanism of drag reduction in turbulent flows due to polymers has been investigated with help of a direct numerical simulation. In particular, we consider the interaction between turbulent velocity fluctuations and polymers in terms of elastic energy that can be stored in the polymer. To this end all the terms of the elastic energy budget have been computed. The most interesting term is the production of elastic energy due to turbulent fluctuations, because it describes the interaction between polymers and turbulence. Although this term appears to be small in the average, it turns out that it can reach very large values instantaneously and intermittently, and the energy transfer from polymer to turbulence is located in very well defined areas inside the channel. This implies that locally there is a strong interaction between the polymer and the turbulent flow structure, and this strong interaction is mostly seen in areas of high velocity fluctuations.