Clayton T. Crowe

On Models for Turbulence Modulation in Fluid-Particle Flows

Abstract A model is introduced for the carrier phase turbulence in a fluid–particle flow based on the volume-averaged equations for the kinetic energy of the carrier phase. The model shows the trends observed by experiment, that is, small particles attenuate turbulence while large particles augment turbulence. The change in turbulence intensity is correlated with the particle loading and the ratio of the particle diameter to the turbulence length scale.

Particle mixing in free shear flows

Abstract The mixing of particles or droplets in free shear layers is encountered in a variety of combustion systems and industrial applications. Free shear layers are characterized by large scale vortical structures which evolve and interact with time. These vortex structures can play a major role in particle or droplet dispersion. It has recently been postulated that the organized rotating motion of the large-scale structures can enhance the dispersion of intermediate size particles. This paper first reviews the currently-accepted mechanisms and models for particle dispersion in homogenoue, isotropic turbulence and addresses the difference between such flows and free shear layers. The essential features of free shear flows are then described and experiments on particle dispersion in free jets and mixing layers are reviewed. Numerical models which have been developed for particle dispersion in free shear layers, such as plane mixing layers, jets and wakes, are outlined and the results are interpreted in light of the postulated physical model. Both experimental results and numerical simulations strongly imply that particle dispersion in free shear layers is controlled by the motion of large scale vortex structures.

PARTICLE DISPERSION BY COHERENT STRUCTURES IN FREE SHEAR FLOWS

ABSTRACT The dispersion of particles in turbulent flows is poorly understood. Previous approaches to this problem have been found to be inadequate for nonisotropic turbulent flows. An approach involving a new physical concept is presented. This approach assumes that coherent vortex structures control the particle dispersion process in free shear flows. A simple computational model employing Stuarts vortices is used to simulate particle motion in a two-dimensional free shear layer. The results of this simulation are in reasonable agreement with previous experiments. For the first time, experimental observations indicating particle dispersion rates greater than fluid dispersion rates in free shear flows can be plausibly explained.

Particle Dispersion by Vortex Structures in Plane Mixing Layers

The dispersion of particles in a plane mixing layer between two air streams is investigated using experimental and numerical techniques. The results show that large-scale spanwise vortices strongly influence the particle dispersion process. Particles with aerodynamic response times on the order of the large scale vortex time scales are found to concentrate near the outer edges of the vortex structures. Time average velocity measurements also demonstrate that these particles tend to move away from the center of the mixing layer

Modulation of Turbulence by a Dispersed Phase

The results of an earlier investigation that demonstrated the significance of the particle diameter/fluid length scale ratio in determining whether or not the addition of a dispersed phase would cause an increase or decrease in the carrier phase turbulent intensity are extended to radial locations other than the centerline.

Experiments on particle dispersion in a plane wake

Abstract Detailed experimental results are presented concerning the effects of vortex structures on the solid particle dispersion process in a plane wake. Previous numerical results have indicated that vortex structures in plane wakes can disperse intermediate Stokes number particles into highly organized patterns. The cross stream spatial dispersion values associated with these particles were computed to be several times greater than that associated with fluid elements. The major objective of this study was to obtain direct experimental results concerning the time dependent particle dispersion process in a plane wake. The experimental approach used in this work primarily involves laser sheet pulsed imaging of glass bead particles in a wake downstream of a blunt trailing edge. Two sizes of glass beads with nominal diameters of 10 and 30 μm were used as particles in an air flow. The associated Stokes numbers of the particles were 0.15 and 1.4. Digital image analysis techniques were employed to identify and determine particle locations and velocities. The results demonstrate that particle dispersion in plane wakes can produce highly organized patterns of particle concentrations. The particles at intermediate Stokes number are focused into sheet-like regions near the boundaries of the large scale vortex structures. In addition the spatial dispersion of the intermediate Stokes number particles was much larger than the smaller Stokes number particles. These experimental results strongly support previous numerical simulation findings.

Particle Interactions with Vortices

The interaction of particles with vortices is important in many technological and environmental applications. The primary effect of vortical flows on suspended particles and droplets is to generate a dispersion pattern in which the particulate phase tends to migrate from the vortex core and concentrate on the edges of the vortices. Note that this is the exact opposite to the behaviour of bubbles interacting with vortices (Chapter XVIII), because the particulate phase is more dense than the fluid phase. The resulting dispersion of the particles is important in the design of combustion systems in which the degree of mixing controls combustion efficiency and the generation of soot. Also, the effectiveness of wire and plate electrostatic precipitators depends on the interaction of the particles with the vortices generated by the supporting structure. The spread of fires by firebrands entrained by large scale vortices in atmospheric flows is another example of the importance of particle interaction with vortices. Several other important industrial and environmental applications are discussed in a recent review article by Hunt (1991).

Effects of turbulent mixing and chemical kineticson nitric oxide production in a jet-stirred reactor

A conical jet-stirred reactor was utilized to study the formation of nitric oxide under nearstoichiometric conditions, and at pressures varying from 0.25 to 1 atm. Measurements of temperature, mass-flow rate, equivalence ratio, nitric oxide and oxygen concentrations were made and compared with perfectly stirred reactor (PSR) predictions, based on kinetic rate data and mechanisms chosen to maximize predicted NO formation and to correctly predict reactor blowout conditions. Observed blowout flow rates were greater by a factor of 2 than that predicted using PSR assumptions and rate data and mechanisms due to Marteney and due to Seery and Bowman. Measured NO levels were higher by a factor of 3 to 10 than predicted by the PSR computer model. A one-parameter “two-environment” mixing model was unsuccessfully applied to the analytical model, in an attempt to correct for imperfect stirring. Since factors controlling formation of NO by the Zeldovich mechanism were suppressed (short residence time and low combustion temperature), the high levels of NO observed are postulated to be due to fast NO-producing reactions occurring during pyrolysis steps of the reaction mechanism.

Experimental study of the flow properties of a homogenous slurry near transitional reynolds numbers

Abstract The results of an experimental study on the pressure drop and velocity distribution of a homogeneous slurry flowing in a duct are reported. The slurry consisted of chloroform and silica gel with matched index of refraction to enable laser-Doppler measurements through the mixture. Slurries with two particle sizes at solids concentrations up to 30% were used. Velocity and pressure drop measurements were made in fully-developed flows with Reynolds numbers from 1200 to 30,000. The results show that the velocity distribution fits the logarithmic law of the wall with a modified Karman constant and turbulent boundary factor. The pressure drop measurements for the slurry with small particles (96 μm) indicate a transitional Reynolds number near that for a single-phase fluid. The corresponding measurements with large particles (210 μm) show no transitional effects over the Reynolds number range tested.

Gas—Particle Flow

The parameters relating to the interaction of particles and gas in a dispersed-phase flow are presented in this chapter. These parameters are essential to the development of numerical and analytic submodels of pulverized-coal combustion.