Large Eddy Simulation of the spray formation in confinements

The particle and powder properties produced within spray drying processes are influenced by various unsteady transport phenomena in the dispersed multiphase spray flow in a confined spray chamber. In this context differently scaled spray structures in a confined spray environment have been analyzed in experiments and numerical simulations. The experimental investigations have been carried out with Particle-Image-Velocimetry to determine the velocity of the gas and the discrete phase. Large-Eddy-Simulations have been set up to predict the transient behaviour of the spray process and have given more insight into the sensitivity of the spray flow structures in dependency from the spray chamber design.     

Atomization of liquid droplets on surfaces exposed to moving shock waves

Many engineering applications involve the stripping of liquid droplets from surfaces, one example being the entrainment of surface fuel from the inlet valves, ports, cylinder walls and piston crowns of internal combustion engines during the induction process. This configuration is likely to exhibit differences from the more commonly studied case of suspended droplets. In order to study the atomization of liquids from surfaces, shock waves at low Mach numbers (M = 1.05 and 1.12) have been used in the present work to initiate the flow over water droplets with visualization obtained from shadowgraph photographs, high-intensity flash photography and a CCD camera. Visualization paths both normal and angled at ±45° to the flow were used in order to obtain improved examination of the atomization details. Surface wave formation and a specific pattern of droplet distortion followed by stripping, was observed. There are similarities in the processes to those of suspended droplets that are modified by the boundary layer effects. At the Weber numbers considered, a cave-like formation occurs near the wall due to surface flow around the droplet with a major liquid flow directed tangentially across the air flow towards the cave peak where bag or chaotic type break-up and stripping takes place.     

Experimental and analytical investigation of liquid sheet breakup characteristics

The main objective of this research is to study analytically and experimentally the liquid sheet breakup of a flat fan jet nozzle resulting from pressure-swirling. In this study the effects of nozzle shape and spray pressure on the liquid sheet characteristics were investigated for four nozzles with different exit widths (1.0, 1.5, 2.0 and 2.5 mm). The length of liquid sheet breakup, liquid sheet velocity and the size of formed droplets were measured by a digital high speed camera. The breakup characteristics of plane liquid sheets in atmosphere are analytically investigated by means of linear and nonlinear hydrodynamic instability analyses. The liquid sheet breakup process was studied for initial sinuous and also varicose modes of disturbance. The results presented the effect of the nozzle width and the spray pressure on the breakup length and also on the size of the formed droplets. Comparing the experimental results with the theoretical ones for all the four types of nozzles, gives a good agreement with difference ranges from 4% to 12%. Also, the comparison between the obtained results and the results due to others shows a good agreement with difference ranged from 5% to 16%. Empirical correlations have been deduced describing the relation between the liquid sheet breakup characteristics and affecting parameters; liquid sheet Reynolds number, Weber number and the nozzle width.     

Atomization of viscous and non-newtonian liquids by a coaxial,high-speed gas jet. Experiments and droplet size modeling

This paper describes a collaborative theoretical and experimental research effort to investigate both the atomization dynamics of non-Newtonian liquids as well as the performance of coaxial atomizers utilized in pharmaceutical tablet coating. In pharmaceutically relevant applications, the coating solutions being atomized are typically complex, non-Newtonian fluids which may contain polymers, surfactants and large concentrations of insoluble solids in suspension. The goal of this investigation was to improve the understanding of the physical mechanism that leads to atomization of viscous and non-Newtonian fluids and to produce a validated theoretical model capable of making quantitative predictions of atomizer performance in pharmaceutical tablet coaters. The Rayleigh–Taylor model developed by Varga et al. has been extended to viscous and non-Newtonian fluids starting with the general dispersion relation obtained by Joseph et al. The theoretical model is validated using droplet diameter data collected with a Phase Doppler Particle Analyzer for six fluids of increasing rheological complexity. The primary output from the model is the Sauter Mean Diameter of the atomized droplet distribution, which is shown to compare favorably with experimental data. Critical model parameters and plans for additional research are also identified.     

Modeling the evolution of droplet size distribution in two-phase flows

A theoretical model is developed in the present study to simulate droplet motion and the evolution of droplet size distribution (DSD) in two-phase air/dispersed water spray flows. The model takes into account several processes which influence DSD and droplet trajectory: droplet collision and coalescence, evaporation and cooling, gravitational settling, and turbulent dispersion of dispersed phase. The DSDs determined by the model at different locations in a two-phase flow are evaluated by comparing them to experimental observations obtained in an icing wind tunnel. The satisfactory coincidence between simulation and experimental results proves that the model is reliable when modeling two-phase flows under icing conditions. The model is applied for two particular examples in which the modification of DSD is calculated in two-phase flows under conditions describing in-cloud icing and freezing drizzle.     

砂岩球体法向恢复系数实验研究

法向恢复系数是岩崩块石运动分析的关键参数,其取值直接决定了块石的运动轨迹。本文中采用自行设计的碰撞实验装置和声频采样技术,测定了砂岩球体碰撞的法向恢复系数,研究了粒径、碰撞速度、含水状态和板的弹性特性4个因素对恢复系数的影响。结果表明：砂岩球体法向恢复系数存在复杂的尺寸效应,恢复系数随粒径的增大先增大后减小;碰撞过程中存在的黏弹性耗能机理和弹塑性损伤耗能机理共同作用产生了复杂的尺寸效应;受砂岩非均质特性的作用,粒径较小时,恢复系数的速度效应较明显(随速度增大而增大),粒径较大时速度对恢复系数的影响消失;砂岩饱和使黏弹性耗能和弹塑性损伤耗能增加,使恢复系数比风干时低;等效弹性模量对恢复系数的影响较大,等效弹性模量越大,法向恢复系数越小。     

Experimental study of pre-swirl flow effect on the heat transfer process in the entry region of a convergent pipe

Experiments have been performed to study the heat transfer process of swirling flow issued into a heated convergent pipe with a convergent angle of 5° with respect to the pipe axis. A flat vane swirler situated at the entrance of the pipe is used to generate the swirling flow. During the experiments, the Reynolds number ranges from 7970 to 47,820, and the swirl number from 0 to 1.2. It is found that the convergence of the pipe can accelerate the flow which has an effect to suppress the turbulence generated in the flow and reduce the heat transfer. However, in the region of weak swirl (S = 0-0.65), the Nusselt numbers increase with increasing swirl numbers until S = 0.65, where turbulence intensity is expected to be large enough and not suppressible. In the region of strong swirl (S > 0.65), where recirculation flow is expected to be generated in the core of the swirling flow, the heat transfer characteristic can be altered significantly. At very high swirl (S ? 1.0), the accelerated flow in the circumferential direction is expected to be dominant, which leads to suppress the turbulence and reduce the heat transfer. The Nusselt number is found proportional to the swirl number. Correlations of the Nusselt numbers in terms of the swirl number, the Reynolds number and the dimensionless distance are attempted and are very successful in both the weak and the strong swirl regions.     

Efficient approaches of determining the motion of a spherical particle in a swirling fluid flow using weighted residual methods

The motion of a spherical particle released in a swirling fluid flow is studied employing the least-squares method and method of moments. The governing equations are obtained and solved employing the two methods. The accuracy of the results is examined against the results of a fourth-order Runge–Kutta numerical method. The effects of various parameters, namely the initial radius, initial radial velocity, initial angular velocity, and drag-to-inertia ratio, on the non-dimensional velocity profiles and particle position distribution are considered. The results show that the radial velocity increases over time while the angular velocity decreases, and that an increase in the initial radial velocity increases the particle radial distance and angular velocity but decreases the radial velocity profile.     

Effects of the gas phase molecular weight and bubble size on effervescent atomization

The effervescent atomization from an industrial Coker feed nozzle is compared for two different gas densities (air and mixed gas of 81.4 vol.% helium/18.6 vol.% nitrogen) at equivalent operating temperatures. The application is to observe the similarity of lab tests using air at 20 °C to the industrial process using steam at 300-400 °C. The effects of operating conditions, such as gas to liquid mass ratio, mixing pressure and void fraction on the flow regime, bubble size, and droplet size distribution were also examined in this study. The experiments were performed using mixtures of water with air or mixed gas, which resulted in gas to liquid mass ratios ranging from 1% to 4%.Stroboscopic back scattered imagery (SBSI) indicates that the average bubble size inside the nozzle conduit is similar when air and water are used as the process fluids, when compared to the case when mixed gas and water are used as the process fluids. Under similar conditions, the Phase Doppler Particle Anemometer (PDPA) data indicate that the droplet size in the spray is similar when using either mixed gas or air as the atomization gas.Experimental results obtained by high-speed video shadowgraphy (HSVS) indicate that the flow pattern inside the nozzle feeding conduit was slug flow with a tendency to attain annular flow with increased air to liquid mass ratios. Thus, from the experimental results it is evident that the smaller molecular weight of the mixed gas versus air (8.4 versus 29) does not significantly reduce the bubble (<±10% difference) and droplet size (<±1.5% difference), indicating a weak dependence of the gas phase density on two-phase atomization. This confirms that laboratory experiments on effervescent nozzles using air have reliable similarity to systems that use high temperature steam for the gas phase.     

Large-eddy structures of turbulent swirling flows and methane-air swirling diffusion combustion

Turbulent swirling flows and methane-air swirling diffusion combustion are studied by large-eddy simulation (LES) using a Smagorinsky-Lilly subgrid scale turbulence model and a second-order moment (SOM) SGS combustion model, and also by RANS modeling using the Reynolds Stress equation model with the IPCM+wall and IPCM pressure-strain models and SOM combustion model. The LES statistical results for swirling flows give good agreement with the experimental results, indicating that the adopted subgrid-scale turbulence model is suitable for swirling flows. The LES instantaneous results show the complex vortex shedding pattern in swirling flows. The initially formed large vortex structures soon break up in swirling flows. The LES statistical results of combustion modeling are near the experimental results and are as good as the RANS-SOM modeling results. The LES results show that the size and range of large vortex structures in swirling combustion are different from those of isothermal swirling flows, and the chemical reaction is intensified by the large-eddy vortex structures. The project supported by the Special Funds for Major State Basic Research (G-1999-0222-07). The English text was polished by Keren Wang.     

On the nonlinear stability of a swirling liquid jet

The nonlinear deformation and atomization of a rotating column is considered using an axisymmetric boundary element formulation. Swirl has been considered by superposing a potential vortex to the bulk flow of the jet. The resulting model has been shown to reproduce the classical linear result due to Ponstein and parametric studies are conducted in the nonlinear regime to determine wave shapes and droplet sizes. As with prior nonlinear column breakup studies, results indicate that satellite drops are formed from the main wave under virtually all conditions. The ratio of the main drop to satellite drop diameter is shown to be remarkably constant over a variety of wave numbers/column lengths thereby providing a potential approach to produce tightly controlled bimodal sprays.     

Modelling and simulation of sprays in laminar flames

In the present paper we model and numerically simulate steady, laminar, premixed spray flames. The gasphase is described in Eulerian form by the equations governing the conservation of overall mass, momentum, energy and species mass. The liquid phase is described in Lagrangian form by the overall continuity equation, which reduces to an equation for the droplet size, the equations of motion, the energy equation and a droplet density function transport equation. The latter is the so-called spray equation, which, at any position in the chemically reacting flowfield, describes the joint distribution of droplet size, droplet velocity and droplet temperature. Herein the spray equation is solved using a Monte Carlo method. Detailed models of the exchange of mass, momentum and energy between the gaseous and the liquid phase are taken into account. The results presented in this paper are for an octane-air flame, where small amounts of liquid octane in form of a liquid spray are added to a fresh, unburnt gaseous octane-air mixture.Presented at Euromech Colloquium 324: The Combustion of Drops, Sprays and Aerosols, 25th–27th July 1994, Marseilles, France.     

Rheology and processing of salad dressing emulsions

In order to study the influence that the processing parameters exert on the rheology and stability of salad dressing emulsions, both steady-state shear and oscillatory measurements, as well as droplet size distribution tests were performed. Emulsions containing a mixture of egg yolk and sucrose stearate as emulsifier were prepared using two different emulsification machines, a rotor-stator turbine and a colloidal mill. An increase both in energy input and in the temperature of processing yields higher values of the steady-state viscosity, an increase in emulsion stability and, generally, lower droplet size and lower polydispersity. Furthermore, a plateau region in the loss modulus versus frequency plots appears as the energy input and processing temperature increase. This effect has been analyzed by calculating the relaxation spectra of these emulsions. The results have been discussed taking into account the relationship between several structural parameters, such as interparticle interactions and droplet size distribution, and the rheological response of these emulsions.     

Development of a hybrid k-ε turbulence model for swirling recirculating flows under moderate to strong swirl intensities

A hybrid k-ε turbulence model, based on the concept that the modification of anisotropic effects should not be made in the flow regions inherent to small streamline curvatures, has been developed and examined with the swirling recirculating flows, with the swirl levels ranging from 0·6 to 1·23 in abrupt pipe expansion. A fairly satisfactory agreement of model predictions with the experimental data shows that this hybrid k-ε model can perform better simulation of swirling recirculating flows as compared to the standard k-ε model and the modified k-ε model proposed by Abujelala and Lilley.     

Ligament formation in sheared liquid–gas layers

We perform numerical simulations of two-phase liquid–gas sheared layers, with the objective of studying atomization. The Navier–Stokes equations for two-dimensional incompressible flow are solved in a periodic domain. A volume-of-fluid method is used to track the interface. The density ratio is kept around 10. The calculations show good agreement with a fully viscous Orr–Sommerfeld linear theory over several orders of magnitude of interface growth. The nonlinear development shows the growth of finger-like structures, or ligaments, and the detachment of droplets. The effect of the Weber and Reynolds numbers, the boundary layer width and the initial perturbation amplitude are discussed through a number of typical cases. Inversion of the liquid boundary layer is shown to yield more readily ligaments bending upwards and is thus more likely to produce droplets.     

Effect of droplet size on the rheological properties of highly-concentrated w/o emulsions

Rheological properties of highly concentrated emulsions of the water-in-oil type were studied. Water phase (concentration approximately 91%) consists of a supersaturated aqueous solution of nitrate salts; water comprises less than 20% by mass. The average size of droplets, D, in the emulsions was varied. It was found that the emulsions are non-Newtonian liquids and flow curves measured in a sweep regime of shearing have clear low-shear-rate Newtonian domain. The complete flow curves are fitted by the Cross equation. The elastic modulus is practically constant in a very wide frequency range. Hence the viscoelastic relaxation processes might be expected at times >>100 s and in the short-term side of the curve at approximately 0.01 s. The elastic modulus (measured in oscillating testing and in elastic recovery as well) is proportional to D^-2 while the Newtonian viscosity is proportional to D^–1.The time effects were observed: it was found that the emulsions behave as rheopectic materials because prolonged shearing results in an increase of viscosity in the low shear rate domain of several orders of magnitude.Presented in part at the First Annual European Rheological Conference, Guimarães, Portugal, 11–13 September 2003     

Rheology and microstructure of gluten and soya-based o/w emulsions

Highly concentrated oil-in-water (o/w) emulsion stabilised by means of gluten and soya protein isolate (SPI) at low pH have been characterized by means of linear dynamic viscoelasticity and droplet size distribution analysis (DSD). The microstructure of these emulsions has been characterized at a colloidal level by using confocal laser scanning microscopy (CLSM) and light microscopy (LM). These emulsions always exhibited a behaviour characteristic of highly flocculated emulsions with a mechanical spectrum showing a well-developed plateau region. DSD results generally showed log normal bimodal profiles. Microstructure images revealed occurrence of a close packing of droplets with a broad distribution of sizes participating in the formation of a three dimensional flocculated network. The Mason model of elasticity of compressed emulsions has been used to correlate viscoelastic and microstructural parameters giving adequate fitting but underestimating the elastic properties obtained for the highest concentration of gluten. These deviations may be explained in terms of an enhancement of the elastic network formed in the aqueous phase in which the glutenin fraction must play an important role. Paper presented at the Annual European Rheology Conference (AERC) 2005, April 21-23, Grenoble, France.