Electric Charging of Soot Particles in Aircraft Engine Exhaust Plumes

A physico-mathematical model is developed and the variation of the charge on soot particles interacting with positive and negative ions in the exhaust of modern subsonic aircraft is simulated numerically. The calculations are based the gas dynamic system of equations for an axisymmetric turbulent isobaric jet. The system is supplemented with the thermodynamic relations, kinetic equations, and equations for the turbulent viscosity. Ion and neutral and charged soot particle concentration distributions over the exhaust jet on the ground and under cruising flight conditions are obtained.     

Impacts of biomass-burning on aerosol properties of a severe haze event over Shanghai

Anthropogenic aerosols have significant impacts on the environment and human health in the Yangtze River Delta region, one of the most densely populated regions in the world. A biomass-burning plume swept across this area (Shanghai) in May 2009, leading to changes in the physical and optical properties of aerosols, which were investigated using ground-based remote sensing and in situ measurements via comparisons with dust pollution and background conditions. Experiments show that the biomass-burning plume led to an increase in the average aerosol optical depth (AOD) at 500 nm from 0.73 to 1.00 (37% higher), an absorption Angstrom exponent (AAE) of 1.48, and an increase in the Angstrom exponent (α) up to 1.53. Furthermore, local dust aerosols derived from road dust and/or construction dust also led to higher values of AOD (2.68) and AAE (2.16), and a daily average value of α of 1.05. For the biomass-burning plume, the aerosol particles exhibited significant variations in short-wavelength spectra. The single scattering albedo at 670 nm decreased remarkably under the influence of the biomass-burning plume, indicating the significant absorptive ability of the biomass-burning pollution and higher ratio of absorption aerosols within the plume. Under the effects of the biomass-burning, the volume concentration of fine-mode aerosols increased significantly and the PM-fine/PM-coarse volume concentration ratio reached 12.33. This relatively large change in fine-mode particles indicates that biomass-burning has a greater impact on fine-mode aerosols than on coarse-mode aerosols.     

NUMERICAL STUDY ON THE GROWTH PROCESS OF SECONDARY AEROSOL IN THE FOG

雾环境气溶胶之间的碰撞和凝并是气溶胶迁移、生长的动力学基础. 基于颗粒群平衡方程和多重蒙特卡洛方法,针对雾环境中二次液态气溶胶生长过程开展数值研究,着重分析了湍流和布朗作用机制对单分散和指数分布的二次气溶胶碰撞凝并的影响. 结果表明:雾环境气溶胶之间的碰撞凝并使得颗粒的总数目减少,颗粒平均体积逐渐增大. 对于初始尺度为自由分子区、单分散的气溶胶,布朗运动时间为600 s时,二次气溶胶平均尺度增大至初始的202.7倍左右,数目降至初始的0.006. 对于湍流作用下有布朗运动的二次气溶胶,在较短的时间内(如100 s)气溶胶尺度增至初始的163倍,颗粒数目降至初始的0.025倍, 说明雾环境流场的湍流运动引起二次气溶胶较强的输运和聚集,导致颗粒碰撞凝并概率增加,颗粒尺度增大.     

A REVIEW OF CURRENT KNOWLEDGE CONCERNING SIZE-DEPENDENT AEROSOL REMOVAL

The status of current knowledge on size-dependent aerosol removal by dry and wet processes, including dry deposition and impaction and nucleation scavenging, is reviewed. The largest discrepancies between theoretical estimations and measurement data on dry deposition and below-cloud scavenging are for submicron particles, Early dry deposition models, which developed based on chamber and wind tunnel measurements, tended to underestimate dry deposition velocity （Vσ） for submicron particles by around one order of magnitude compared to recent field measurements. Recently developed models are able to predict reasonable Vσ values for submicron particles but shift unrealistically the predicted minimum Vσ to larger particle sizes. Theoretical studies of impaction scavenging of aerosol particles by falling liquid drops also substantially underestimate the scavenging coefficients for submicron particles. Empirical formulas based on field measurements can serve as an alternative to the theoretical scavenging models. Future development of size-resolved impaction scavenging models needs to include more precipitation properties （e,g., droplet surface area） and to be evaluated by detailed cloud microphysical models and available measurements. Several recently developed nucleation scavenging parameterizations for in-cloud removal of interstitial aerosol give comparable results when evaluated against parcel models; however, they need to be verified once suitable field measurements are available. More theoretical and field studies are also needed in order to better understand the role of organic aerosols in the nucleation scavenging process.     

Convective transport of combustion products in the atmosphere above large fires

A nonstationary axisymmetric model of the development of a turbulent convective combustion product column above a fire in a stratified atmosphere is proposed. The model takes into account the compressibility of the gas and the diffusion of the aerosol particles and makes it possible to predict the dynamics of ascent of the convective column, the height of the cloud and the distribution of the aerosol in the atmosphere. The numerical and experimental data are compared.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 4, pp. 47–52, July–August, 1990.     

Large-eddy simulation of a turbulent forced plume

This paper reports on an application of large-eddy simulation (LES) to a spatially-developing round turbulent buoyant jet. The numerical method used is based on a low-Mach-number version of the governing equations for compressible flow which can account for density variations. The second-order centre-difference scheme is used for spatial discretization and an Adams–Bashforth scheme for temporal discretization. Comparisons are made between LES results, experimental measurements and plume theory for the forced plume under moderate Reynolds number and good agreement has been achieved. It is found that the plume spreading and the centerline maximum mean velocity strongly depend on the forcing conditions imposed on the inflow plane. The helical mode of instability leads to a larger spreading rate as compared to an axisymmetric mode. The enhanced entrainment is directly related to the strong turbulent momentum and energy transports between the plume and surrounding fluid induced by vortex dynamics. The entrainment ratio is about 0.09 and falls into the range of experimentally determined values. Budgets of the mean momentum and energy equations are analyzed. It is found that the radial turbulent transport nearly balances the streamwise convection and the buoyancy force in the axial momentum equation. Also, the radial turbulent stress is balanced by the streamwise convection in the energy equation. The energy-spectrum for the axial velocity fluctuations shows a −5/3 power law of the Kolmogorov decay, while the power spectrum for the temperature fluctuations shows both −5/3 and −3 power laws in the inertial-convective and inertial-diffusive ranges, respectively.     

Eulerian-Lagranigan simulation of aerosol evolution in turbulent mixing layer

The formation and evolution of aerosol in turbulent flows are ubiquitous in both industrial processes and nature. The intricate interaction of turbulent mixing and aerosol evolution in a canonical turbulent mixing layer was investigated by a direct numerical simulation (DNS) in a recent study (Zhou, K., Attili, A., Alshaarawi, A., and Bisetti, F. Simulation of aerosol nucleation and growth in a turbulent mixing layer. Physics of Fluids, 26, 065106 (2014)). In this work, Monte Carlo (MC) simulation of aerosol evolution is carried out along Lagrangian trajectories obtained in the previous simulation, in order to quantify the error of the moment method used in the previous simulation. Moreover, the particle size distribution (PSD), not available in the previous works, is also investigated. Along a fluid parcel moving through the turbulent flow, temperature and vapor concentration exhibit complex fluctuations, triggering complicate aerosol processes and rendering complex PSD. However, the mean PSD is found to be bi-modal in most of the mixing layer except that a tri-modal distribution is found in the turbulent transition region. The simulated PSDs agree with the experiment observations available in the literature. A different explanation on the formation of such PSDs is provided.     

NUMERICAL SIMULATION WITH A COMPREHENSIVE CHEMICAL TRANSPORT MODEL OF NITRATE, SULFATE, AND AMMONIUM AEROSOL DISTRIBUTIONS OVER EAST ASIA

The transport and chemical production processes of nitrate, sulfate, and ammonium aerosols over East Asia were investigated by use of the Models-3 Community Multi-scale Air Quality （CMAQ） modeling system coupled with the Regional Atmospheric Modeling System （RAMS）. For the evaluation of the model＇s ability in depicting their 3-dimensional concentration distributions and temporal variations, modeled concentrations of nitrate, sulfate, and ammonium aerosols are compared with the observations obtained at a ground station in Japan in March 2001 and onboard of an aircraft DC-8 on 18 and 21 March 2001 during the Transport and Chemical Evolution over the Pacific （TRACE-P） field campaign. Comparison shows that simulated values of nitrate, sulfate, and ammonium aerosols are generally in good agreement with their observed data, and the model captures most important observed features, and reproduces temporal and spatial variations of nitrate, sulfate, and ammonium aerosol concentrations reasonably well, e.g., the timing and locations of the concentration spikes of nitrate, sulfate, and ammonium aerosols are well reproduced, but large discrepancies between observed and simulated values are also clearly seen at some points and some times due to the coarse grid resolution and uncertainties of the emissions used in this study. This comparison results indicate that CMAQ is able to simulate the distributions of nitrate, sulfate, and ammonium aerosols and their related species in the troposphere over East Asia reasonably well.     

Simulations of dispersed turbulent multiphase flow

Direct numerical simulations of homogeneous isotropic turbulence are used to investigate the effects of turbulence on the transport of particles in gas flows or bubbles in liquid flows. The inertia associated with the bubbles or the particles leads to locally strong concentrations of these in regions of instantaneously strong vorticity for bubbles or strain-rate for particles. This alters the average settling rates and other processes. If the mass-loading of the dispersed phase is significant a random “turbulent” flow is generated by the particle settling. A simple demonstration of this is given, showing the statistically axisymmetric character of this flow and how it can modify an ambient turbulent flow.     

Calculation of the flow of a two-phase mixture in a Laval nozzle,allowing for the turbulent diffusion of the particles

The flow of a mixture of gas and condensed particles in an axisymmetric Laval nozzle is considered. The motion of the particles is calculated in a specified field of gas flow, with due allowance for their turbulent diffusion. The results of calculations indicating the necessity of allowing for this phenomenon when considering the motion of particles toward the wall of a profiled nozzle are presented.Translated from Izvestiya Akademii Nauk SSSR, No. 2, pp. 161–165, March–April, 1973.     

Dynamics of Sulfate Aerosol Formation in Engine Jets

The effect of SO₂, SO₃, HSO₃, and H₂SO₄ formation in the jet duct on the dynamics of sulfate H₂O/H₂SO₄ aerosol formation in the wake of a subsonic aircraft is analyzed numerically. It is shown that the presence, in addition to SO₂, of SO₃, HSO₃, and H₂SO₄ at the nozzle edge leads to an increase in the nucleation rate and intensification of the coagulation process. A significant quantity of large particles (of radius greater than 9 nm) is formed in the jet. This quantity depends on the sulfur content of the fuel.     

Direct numerical simulation of a liquid sheet in a compressible gas stream in axisymmetric and planar configurations

A thin liquid sheet present in the shear layer of a compressible gas jet is investigated using an Eulerian approach with mixed-fluid treatment for the governing equations describing the gas–liquid two-phase flow system, where the gas is treated as fully compressible and the liquid as incompressible. The effects of different topological configurations, surface tension, gas pressure and liquid sheet thickness on the flow development of the gas–liquid two-phase flow system have been examined by direct solution of the compressible Navier–Stokes equations using highly accurate numerical schemes. The interface dynamics are captured using volume of fluid and continuum surface force models. The simulations show that the dispersion of the liquid sheet is dominated by vortical structures formed at the jet shear layer due to the Kelvin–Helmholtz instability. The axisymmetric case is less vortical than its planar counterpart that exhibits formation of larger vortical structures and larger liquid dispersion. It has been identified that the vorticity development and the liquid dispersion in a planar configuration are increased at the absence of surface tension, which when present, tends to oppose the development of the Kelvin–Helmholtz instability. An opposite trend was observed for an axisymmetric configuration where surface tension tends to promote the development of vorticity. An increase in vorticity development and liquid dispersion was observed for increased liquid sheet thickness, while a decreasing trend was observed for higher gas pressure. Therefore surface tension, liquid sheet thickness and gas pressure factors all affect the flow vorticity which consequently affects the dispersion of the liquid.      

Numerical modeling of a two-dimensional vertical turbulent two-phase jet

The results of numerically modeling two-dimensional two-phase flow of the “gas-solid particles” type in a vertical turbulent jet are presented for three cases of its configuration, namely, descending, ascending, and without account of gravity. Both flow phases are modeled on the basis of the Navier-Stokes equations averaged within the framework of the Reynolds approximation and closed by an extended k-? turbulence model. The averaged two-phase flow parameters (particle and gas velocities, particle concentration, turbulent kinetic energy, and its dissipation) are described using the model of mutually-penetrating continua. The model developed allows for both the direct effect of turbulence on the motion of disperse-phase particles and the inverse effect of the particles on turbulence leading to either an increase or a decrease in the turbulent kinetic energy of the gas. The model takes account for gravity, viscous drag, and the Saffman lift. The system of equations is solved using a difference method. The calculated results are in good agreement with the corresponding experimental data which confirms the effect of solid particles on the mean and turbulent characteristics of gas jets.     

Effect of Interphase Heat Transfer on the Stability of a Turbulent Multicomponent Flow in a Vortex

The parameters of an axisymmetric turbulent two-phase swirling flow of a viscous heat-conducting gas containing a liquid dispersed phase in the presence of water vapor condensation on the particles are calculated. For the dispersed phase, a model taking into account the variation of the vapor concentration and the particle size due to condensation or evaporation is proposed. The distributions of the parameters of the basic unperturbed flow obtained numerically are used in the numerical solution of the linear problem of hydrodynamic stability within the time-dependent formulation. The parameters of small-amplitude harmonic perturbations propagating along the vortex axis are investigated in the linear formulation. A significant effect of heat release in the gas due to water vapor condensation on the parameters of the neutral perturbations and the neutral-stability curves is detected.     

Wavy flow of a liquid film in the presence of a cocurrent turbulent gas flow

Wavy downflow of viscous liquid films in the presence of a cocurrent turbulent gas flow is analyzed theoretically. The parameters of two-dimensional steady-state traveling waves are calculated for wide ranges of liquid Reynolds number and gas flow velocity. The hydrodynamic characteristics of the liquid flow are computed using the full Navier-Stokes equations. The wavy interface is regarded as a small perturbation, and the equations for the gas are linearized in the vicinity of the main turbulent flow. Various optimal film flow regimes are obtained for the calculated nonlinear waves branching from the plane-parallel flow. It is shown that for high velocities of the cocurrent gas flow, the calculated wave characteristics correspond to those of ripple waves observed in experiments.     

Homogeneous condensation in turbulent submerged isobaric jets

Turbulent isobaric vapor-air jet flows with homogeneous condensation are examined. A general system of equations, including the gas dynamic and kinetic equations, the thermodynamic relations and the equations for the turbulence model, is formulated. The moment kinetic equations valid for the free-molecular regime of drop growth in the surrounding medium are extended to other drop mass transfer regimes. The structure of the condensation shock, which includes the nucleation zone and the zone of drop growth on pre-existing nuclei, is investigated on the basis of a general asymptotic approach. Additional conditions at the nucleation and condensation shocks, the need for which follows from the requirement that the shocks be evolutionary, are obtained. Certain problems of averaging of the source terms in the moment equations are discussed, and with reference to the simple example of averaging of the frozen nucleation rate it is shown that the latter is nonzero for a mean supersaturation less than unity and that the condensation zone is displaced upstream. Condensation in a turbulent jet into which condensation-intensifying charged particles (corona discharge ions) are introduced is studied. A numerical method of analyzing homogeneous condensation in turbulent jets, which makes it possible to obtain the gas dynamic and disperse flow characteristics for various temperature conditions with allowance for the averaging of the source terms, is developed.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 2, pp. 43–52, March–April, 1988.The authors wish to thank V. R. Kuznetsov for discussing various aspects of their work.     

Numerical study of aerosol particle deposition in simple and converging-diverging micro-channels with a slip boundary condition at the wall

The design of micro-devices involving aerosol transport requires the study of the deposition of aerosols in micro-channels. In this study, the slip and no-slip boundary conditions for the gas flow regime were applied to the Navier-Stokes equations to obtain the particle deposition in simple and converging-diverging micro-channels. The equation of particle motion included inertial, viscous, Brownian, and gravity terms. It was found that the ratio of gravity to inertial effects controls the deposition of particles with diameters of 0.1-1 μm, and the ratio of diffusion to inertial effects controls the deposition of particles with diameters of 0.01-0.001 i~m. Comparison between the no-slip and slip flow regimes showed that the deposition of 0.1- to 1-μm-diameter particles was less and the deposition of 0.01- to 0.001-1μm-diameter particles was greater for the slip flow regime. There was no significant difference between slip and no-slip flow regimes for the deposition of 0.01- to 0.1-μm-diameter particles. Finally, it was shown that the stagnated gas in the corners of the converging-diverging micro-channel produced similar gas velocity profiles under the slip and no-slip flow regimes.     

Problem of Deceleration of a Supersonic Conducting Channel Flow by a Magnetic Field

Problems of the deceleration of a supersonic conducting flow by a magnetic field are investigated. A conducting gas flow in a circular tube is considered in the presence of an axisymmetric magnetic field induced by a unit current loop or solenoid of finite length. The analysis is carried out on the basis of both the Euler equations (inviscid gas) and the complete system of Navier-Stokes equations for laminar viscous gas flow and turbulent flow using a one-parameter turbulence model. The numerical simulation is based on an implicit relaxation finite-difference scheme which is a modification of the Godunov method. The total pressure losses are determined for various values of the magnetohydrodynamic (MHD) interaction, the initial Mach number, and different magnetic field geometries and it is shown that the irreversible losses are significant in MHD supersonic flow deceleration.     

Investigation of Different Condensation Regimes in Isobaric Turbulent Air-Steam Jets

Condensation in axisymmetric turbulent air-steam jets is studied theoretically and experimentally under bench experiment conditions in which a hot mist jet is injected from a nozzle into air. On the basis of the physico-mathematical model developed, four problems are considered: homogeneous condensation in the jet at a fairly low ambient air temperature, heterogeneous condensation on particles introduced into the jet at the nozzle outlet, heterogeneous condensation on particles ejected into the jet from the surrounding space, and condensation on ions entering the jet from a corona point on the flow axis. The local characteristics of the dispersed phase (mean particle size, standard deviation of the particle size, particle number and volume concentrations) and its integral characteristics (coefficient of vapor conversion into condensed phase and the optical thickness of the jet in different sections) are determined. The calculation results are compared with experimental data. As an application of the model developed, the characteristics of heterogeneous condensation in the jets of certain modern aircraft engines (IL-96-300, Tu-204, MiG-29, Boeing-707) are found on the assumption that the condensation occurs on particles entering the jet at the nozzle outlet and the particle growth rate in all stages (including the initial stage of particle irrigation) coincides with the growth rate of liquid drops.     

Turbulent flow structure and bubble distribution in an axisymmetric nonisothermal impinging gas-liquid jet

The flow structure of a bubbly impinging jet in the presence of heat transfer between the two-phase flow and the surface is numerically investigated on the basis of the Eulerian approach. The model uses the system of Reynolds-averaged Navier–Stokes equations in the axisymmetric approximation written with account for the inverse effect of the bubbles on the average and fluctuating flow parameters. The influence of the gas volumetric flow rate ratio and the dimensions of the bubbles on the flow structure in a gas-liquid impinging jet is studied, In the presence of gas bubbles the liquid velocity is higher than the corresponding value in the single-phase flow. A considerable, more than twofold, anisotropy between the axial and radial turbulent fluctuations in the gas-liquid impinging jet is shown to exist. An addition of air bubbles leads to a considerable growth in the liquid velocity fluctuations in the two-phase flow (up to 50% compared with the single-fluid liquid impinging jet). An increase in the disperse phase dimensions leads to intensification of turbulence of the liquid.