Powder Flow
Summary:
Over the past several years, IFPRI-supported work at Duke has focused on understanding jaming and flow properties in a quasi-2D hopper by using photoelastic particles. Two final goals of the proposed work were
- to extend studies using cohesionless photoelastic particles to similar particles, but with cohesive interactions, and
- to studies of three dimensional systems. During the final year of this project, we have addressed the goal of using cohesive particles by carrying out i) studies of particles with cohesion and ii) studies of the stress response of systems of agglomerate particles, which have the property that they can break. We have
- carried out additional studies of hopper flow for cohesionless photoelastic particles, where we have used synchronized high speed video imaging for both particle tracking and photoelastic imaging. The goals of 3) are to understand the coupling between force chains (as a measure of stresses), flow velocity, and density. A key rationale of this last set of studies is to understand the relation between the various granular states that occur for hoppers: flowing and jammed, and to understand whether shear jammed states (recently discovered at Duke) are in fact the same states that occur for jamming of a hopper. We have also developed a new approach for studying fully 3D granular systems at the particle scale, which includes the measurement of inter-particle forces and particle motion. This report provides information on the last year’s work, as well as highlights of previous results.
IFPRI support has led to the Ph.D. of Junyao Tang, now employed in industry. Two additional students, Audrey Melville, and Yiqiu Zhao, have worked on this project in the past year.
Executive summary
In this work our objective has been to develop quantitative methods for the prediction of all technically important features of the flow of gas and solid particles through ducts. Flows of this sort are of great importance in pneumatic transport of particulate material and in the circulation of particulate materials within certain chemical process plants; for example, catalytic crackers used in oil refining, and circulating fluidized beds, which are important in coal processing and a number of other chemical processes. Design of these systems is made difficult by the tendency of the particles to distribute themselves over the cross section of the duct in a highly non-uniform way. Consequently it has not been possible to predict properties such as the holdup of particles, the distribution of residence times of the particles in a given section of duct, or the gas pressure drop. Even scaling these quantities between ducts of different sizes, operated under different conditions, has been uncertain, at best.
In an earlier period of IFPRI funding (see Final Report FRR 09- 10) we developed equations of motion for gas-particle mixtures and applied them to streamline flow through ducts. The results predicted a rich variety of behavior, which conformed well with observations in systems of this type, but the quantitative predictions were unrealistically sensitive to the values of, ‘certain parameters representing physical properties of the particles. We speculated that this shortcoming was a consequence of limiting our attention to steady, streamline flows since, in most situations of technical interest, the flow is markedly unsteady, like the fast, turbulent flow of a simple liquid or gas through a pipe. Accordingly, we have extended our treatment to cover this type of flow, and we now find that the unrealistic sensitivity is suppressed, while the qualitative features of interest are conserved. There is a shortage of experimental measurements against which the quantitative predictions can be tested, but comparisons with the data presently available in the open literature are encouraging.
Executive Summary
It is now well established that meso-scale structures, whose characteristic size is on the order of a few millimeters, arise in rapid gas-solid flows. These structures significantly affect the overall flow behavior and should therefore be accounted for in CFD simulations.
Unfortunately, the meso-scale structures cannot be resolved adequately in CFD simulations of risers of practically relevant dimensions. As coarse-grid simulations of such gas-solid flows do not resolve the meso-scale structures, quantities such as the effective inter-phase drag and the net rate of dissipation of pseudo-thermal energy are not properly accounted for in the computations. Therefore, the coarse-grid simulations, which have been published in the literature, should be viewed with some skepticism, as there is little basis to argue that these results are indeed true solutions of the differential equation models one is trying to solve.
In the last four years, we have made significant progress in understanding the origin of the meso-scale structures. It is established that they arise as a result of the inertial instability associated with the relative motion between the gas and particle phases and/or inelastic collisions, both of which are local events occurring on a length-scale comparable to the size of the meso-scale structures. This allowed us to assemble a tentative sub-grid model to account for the effects of the (unresolved) meso-scale structures in coarse-grid simulations. The proposed sub-grid model interrogates the stability of uniform motion on a length scale smaller than the grid size of a coarse-grid simulation and incorporates corrections to quantities such as effective drag, etc. accordingly. In that sense, it is indeed based on the differential equation model, which one is trying to solve. The sub-grid model is internally consistent, in the sense that, as the grid size goes to zero, the sub-grid corrections become smaller and smaller. Thus, the intent of the sub-grid model is not to change the original system of differential equations one is trying to solve, but to help us simulate the macro-scale structures correctly without having to resolve the meso-scale structures. Some of the elements of the sub-grid model are speculative and remain to be tested.
We then embarked on a program of research aimed at gathering statistics on fluctuations associated with the meso-scale structures in a model two-phase flow problem, so that we can verify the validity of the speculative elements of the sub-grid model. This work is in progress and some of the initial results on fluctuation statistics are described in this report. Results obtained at different levels of solids loading manifest qualitatively similar fluctuation statistics, giving us hope that a validated sub-grid model is indeed within reach.
EXECUTIVE SUMMARY
Vertical risers constitute an important class of reactors for contacting solid particles and gases in the chemical engineering, petroleum refining and power generation industries. In an effort to inform recent models of their fluid dynamics, we employed a unique facility that recycles fluidization gases of adjustable properties.
In that facility, we investigated the effects of gas density, scale and operating conditions while achieving hydrodynamic similarity with generic high-temperature risers operating at pressures of 1 and 8 atm. We interpreted our results in the upper riser using steady, fully-developed momentum balances for the gas and solid phases. The analysis showed that the “atmospheric” and “pressurized” experiments conform to distinct viscous and inertial regimes. It also provided quantitative predictions for the suspension density in the upper riser.
By recording radial profiles of volume fraction and axial gradients of gas pressure, we inferred the shear stress at the wall and found conditions where the solid recirculation produces shear stresses directed along the flow, rather than against Tt.
From a study of solid clusters, we produced a robust correlation for their descending velocity at the wall and concluded that their dynamics in that region is mainly governed by particle interactions.
We also compared our measurements of pressure losses and efficiency of a cyclone operating at high pressure and solid loading with available models. Finally, we developed new process instrumentation to record local values of the solid volume fraction in high-temperature industrial vessels and we produced an exhaustive state-of-the-art review of experimental techniques for dense gas-solid flows.
EXECUTIVE SUMMARY
With the conclusion of this project, my students and I wish to thank the IFPRI member companies for their generous support and invaluable discussions during the past several years. The work summarized below, triggered by the invitation by IFPRI to submit a proposal for work in the nanorheology area, has been perhaps the most productive period during my research career.
Goals of this project
The objective of this project was systematic understanding of particle-particle nanorheology based on the single particle-particle contact of two atomically-smooth solid surfaces in molecularly-thin proximity. The main relevance was to understand the origins of suspension rheology, especially the origins of rheological anomalies that arise when interfacial films between two solid bodies are so thin that the intuition of what to expect based on bulk rheology no longer applies. Based on this understanding, we sought to develop new methods to control and manipulate the properties of their interfacial films. The premise for this work was the conviction that progress in understanding fine powder applications is impeded by difficulties in separating the overall rheology of a macroscopic-sized sample into various mechanistic subprocesses. Much is known about interparticle forces (van der Waals, electrostatic, hydrogen bonding, capillary, steric, etc.) and the information obtained in rheology experiments has often been interpreted in these terms. This project took the different approach of seeking to understand rate-dependent influences of nanorheological response. We were concerned with the rate-dependence of nanorheological responses not just in shear but also in adhesive mode.
This report is the annual statement of progress on the Loughborough powder flow project. The project concerns the aerated flow of powders, that regime of flow where the voidage of the bed increases and the resistance of the powder to flow is greatly reduced. This investigation is a broad topic and may be considered to have three broad perspectives:
- Measurement of aerated flow of properties
- Manipulation of material properties and behaviour
- Behaviour of materials in engineering situations
Introduction
The knowledge of strain distributions of granular materials is required in various kinds of particle handling processes in chemical, ceramic, food processing and other related industries. However, it is very difficult to obtain such a knowledge as compared with the case of fluids. If we want to obtain a velocity distribution of a gas or liquid flow, we can utilize the Navier-Stokes equation. We have had, however, no basic equations for the corresponding calculation of granular materials. The aim of this paper, is therefore, to derive a new equation for granular materials like the Navier-Stokes equation for fluids.
I will derive a new simple equation for obtaining the strain-distribution based on the classical hydrodynamics. Then, the equation will be applied to simple one-dimensional problem, and compared with the experimental results.
SUMMARY
The literature on the subject of the flow of aerated powders has been reviewed and is presented in this report under 5 headings.
- THE FLOW OF COARSE MATERIALS AND THE EFFECT OF INTERSTITIAL PRESSURE GRADIENTS.
It is concluded that the flow of unaerated coarse materials is understood and that correlations and theories exist from which it is possible to predict the discharge rate with precision. The effect of deliberately imposed pressure gradients on the flow of a coarse material is also understood. Some progress has been made in the understanding of the effects of self-generated pressure gradients on the flow of materials of mean size less than 500 um.
CONSOLIDATION
The expulsion of pore water from clays has been well studied in the soil-mechanics literature. The case of deaeration of a loosely fitted material has been analysed by Jenike and by Murfitt using similar techniques. Methods are discussed.
5. FLOW OF AERATED POWDER DOWN CHUTES
Only one paper specifically on this topic has been found and this is discussed. T. Rathbone Under the Supervision of Dr. R.M. Nedderman Cambridge November 1983 The case of deaeration.
2. FLOODING
The empirical work of Carr seems to provide a criterion for identifying which materials are liable to flood, though the mechanism of flooding is not understood. Jenike and co-workers maintain that flooding is more likely to occur in core flow hoppers and that materials liable to flood should therefore be stored in mass flow hoppers.
3. THE FLUIDISATION OF FINE POWDERS
It is concluded that the Geldart classification of powders is the most reliable available for predicting the type of fluidisation that will occur with a given material.
4. CONSOLIDATION
The expulsion of pore water from clays has been well studied in the soil-mechanics literature. The case of deaeration of a loosely fitted material has been analysed by Jenike and by Murfitt using similar techniques. The approximations in their methods are discussed.
5. FLOW OF AERATED POWDER DOWN CHUTES
Only one paper specifically on this topic has been found and this is discussed. T. Rathbone Under the Supervision of Dr. R.M. Nedderman Cambridge November 1983 The case of deaeration.
SUMMARY The objectives of our project are to establish an industrial estimation method for any powder bed yield loci and to develop a design procedure for an industrial powder process by use of the yield loci. In this report, research works on the phenomena of powder, in which fluidization, packing structure, adhesion and agglomeration, mechanical yield etc. are contained, are systematically reviewed. Examples of the application of the method for estimating powder bed yield loci to actual particulate processes are discussed. Finally, the transmission of particle bulk density in powder is described qualitatively and experimentally.