Tong Deng

The influence of particle rotation on the solid particle erosion rate of metals

Abstract It has long been recognised that particle spin may have a significant effect on the impact erosion rate, particularly of ductile metals. However, no work has previously been carried out to quantify this effect, partly due to the practical difficulty of measuring the magnitude of the rotational speed. Particle spin is a feature of the centrifugal accelerator erosion tester. In this tester it has proved possible to examine the effect on erosion of particle spin direction by varying the target orientation. The results indicated a strong effect of the spin direction on erosion rate at low impact angles when the targets were impacted by angular particles. A quantitative model was developed to explain the effect of particle spin direction on the observed differences. The model is a modification of the Finnie–Bitter model [Wear 3 (1960) 87; Wear 6 (1963) 5; Wear 6 (1963) 160], and is the first to explicitly incorporate the effect of rotating particles on the subsequent erosion rate when the particles impact a metal target. The model supposes that the effective impact velocity, the contact velocity between the particle and the target, is altered due to spin of the particles. The predictions of the model were validated through actual measurement of particle rotational speed by high-speed photographic techniques; the first such measurements. Experimental erosion results conformed to the predictions of the model. An effect of particle spin on the peak erosion rate is also predicted by the model and confirmed by the experimental results.

A novel sensing technique for measurement of magnitude and polarity of electrostatic charge distribution across individual particles

Electrostatic charge is generated during powder handling due to particle-particle and particle-wall collisions, rubbing, sliding, and rolling. In case of bipolar charge generation, the electrostatic forces may significantly change the inner forces and increase powder adhesion and cause a serious problem in material handling process. Therefore, the knowledge of distribution of charge across the individual particles is helpful to identify the role of triboelectrification and the effects of various relevant variables especially change in the contact materials, environmental conditions during processing, etc. A novel approach based on inductive sensor has been developed to detect the either polarity of charged particle and to characterise the bipolar charge distribution in the population of particulate material. To achieve this, an amplification unit configured as a pure integrator and signal processing techniques has been used to de-noise and correct the baseline of signal and MATLAB algorithm developed for peak detection. The polarity of charged particles obtained by this method is calibrated with Faraday pail method and the results are promising. Experimental study has been carried out by using two distinct populations of oppositely charged particles (glass beads-PVC, olivine sand, and silica sand). The obtained results indicate that the method is able to detect the distribution of polarities of charged particles.

An investigation of particle dynamics within a centrifugal accelerator type erosion tester

Abstract Particle impact erosion is usually tested experimentally using one of two major types of erosion testing device; the gas-blast tester and the centrifugal accelerator type tester. The influence of the choice of the tester on the erosion results obtained has been recognised, together with the need for a better understanding of particle dynamics within the testers to allow correct interpretation of the erosion test results. To date, relatively little work has been carried out on understanding particle dynamics in the centrifugal tester, and this paper attempts to redress this. The paper considers the dynamics of particles travelling down the acceleration tube in the tester. A comprehensive physical model is described taking account of airflow in the acceleration tubes, particle rotation and friction effects. This led to the development of a computational model to predict the particle velocity vector (particle velocity and exit angle). The predictive model indicates the important influence of particle shape on particle dynamics and suggests that particle size has little effect. This implies a sensitivity of particle velocity and exit angle to the coefficient of friction of the particle in the acceleration tubes. Experimental measurements were carried out and the results verified the predictions of the model.

Anomalies in the results obtained from rotating disc accelerator erosion testers: a discussion of possible causes

During recent erosion research at the University of Greenwich using a rotating disc accelerator erosion tester no peak has been observed in the curve of erosion damage vs. angle of impingement. This lack of peak has been observed for tests on a range of steels at a variety of particle impact velocities and fluxes and is contrary to previously reported results. Results of erosion tests on targets at various orientations are given in this paper. The targets used in this work were all made from 0.8% carbon steel (SAE1074). It is shown that changing the orientation of the target in such a tester can lead to different mechanisms of impact erosion occurring owing to changes in the particle dynamics at impact. Several reasons for this behaviour are suggested by the authors including: (a) the variation of particle flux across the target surface as the angle of orientation of the target to the flow of particles is changed, (b) the variation in the occurrence of inter-particulate collisions with the angle of target orientation, and (c) the effects of particle sliding on the walls of the acceleration mechanism inducing particle spin such that the mechanism of cutting is increased at low angles of particle impingement.

Influence of particle dynamics on erosion test conditions within the centrifugal accelerator type erosion tester

Abstract It is well known that particle dynamics behaviour in an erosion tester has a significant influence on test parameters and conditions and, consequently, the erosion test result. This paper examines aspects of particle dynamics in the centrifugal accelerator type erosion tester and their influence on the erosion test conditions, particularly the particle concentration in the particle jet stream and the mass flux on the surface of the eroded target. The effect of particle and particle jet characteristics on particle jet dispersion, particle positional distribution within the jet and particle velocity distribution, and how these in turn effect the mass flux on the target, are examined both theoretically and experimentally. Experimental results showed that a narrow velocity distribution was generally obtained for this type of erosion tester. The velocity distribution was, however, seen to be sensitive to particle characteristics. Experimental measurement of jet dispersion was necessary to allow the mass flux to be estimated. The jet dispersion was determined using a novel technique in which an optical photo-densitometer was utilised to define and measure the diameter of the wear scar on a target. The particle positional distribution was also indicated by this technique. Mass flux was found to vary with position on the target surface and additionally at any given position varied with impact angle. The effect of inter-particulate collisions on mass flux was also considered.

A model to predict the life of pneumatic conveyor bends

Abstract A new approach to the prediction of bend lifetime in pneumatic conveyors, subject to erosive wear is described. Mathematical modelling is exploited. Commercial Computational Fluid Dynamics (CFD) software is used for the prediction of air flow and particle tracks, and custom code for the modelling of bend erosion and lifetime prediction. The custom code uses a toroidal geometry, and employs a range of empirical data rather than trying to fit classical erosion models to a particular circumstance. The data used was obtained relatively quickly and easily from a gas-blast erosion tester. A full-scale pneumatic conveying rig was used to validate a sample of the bend lifetime predictions, and the results suggest accuracy of within ±65%, using calibration methods. Finally, the work is distilled into user-friendly interactive software that will make erosion lifetime predictions for a wide range of bends under varying conveying conditions. This could be a valuable tool for the pneumatic conveyor design or maintenance engineer.

Evaluation of Particle Degradation Due to High- Speed Impacts in a Pneumatic Handling System

Particle degradation can be a significant issue in particulate solids handling and processing, particularly in pneumatic conveying systems, in which high-speed impact is usually the main contributory factor leading to changes in particle size distribution (comparing the material to its virgin state). However, other factors may strongly influence particles breakage as well, such as particle concentrations, bend geometry, and hardness of pipe material. Because of such complex influences, it is often very difficult to predict particle degradation accurately and rapidly for industrial processes. In this article, a general method for evaluating particle degradation due to high-speed impacts is described, in which the breakage properties of particles are quantified using what are known as “breakage matrices.” Rather than a pilot-size test facility, a bench-scale degradation tester has been used. Some advantages of using the bench-scale tester are briefly explored. Experimental determination of adipic acid has been carried out for a range of impact velocities in four particle size categories. Subsequently, particle breakage matrices of adipic acid have been established for these impact velocities. The experimental results show that the “breakage matrices” of particles is an effective and easy method for evaluation of particle degradation due to high-speed impacts. The possibility of the “breakage matrices” approach being applied to a pneumatic conveying system is also explored by a simulation example.

Analysis on flowability of cement based on powder flow tester

Flow functions and time consolidation effect of CEM Ⅱ/A-LL 32.5R Portland limestone were analyzed based on Jenike theory and powder flow tester (PFT). Wall friction tests were carried out using five kinds of materials of 304/2B stainless steel, Tivar plastic, mild steel, rubber and PTFE tape. The results show that the flowability of materials becomes weak by time consolidation effect. The cement wall friction angle is the smallest taking 304/2B stainless steel as wall materials. According to diagrams of flow factor and overall flow hopper half angle function, the maximum half hopper angles of conical hopper and wedge hopper made by mild steel wall materials are respectively 5 and 15°，and the minimum open size that is outlet diameter and exit slit width are 0.112 and 0.057 m respectively.