Monitoring particle impact energy using acoustic emission technique

Abstract

The estimation of energy dissipated during multiple particle impact is a key aspect in evaluating the abrasive potential of particle-laden streams. A systematic investigation of particle impact energy using acoustic emission (AE) measurements is presented in this thesis with experiments carried out over a range of particle sizes, particle densities and configurations. A model of the AE impact time series is developed and validated on sparse streams where there are few particle overlaps and good control over particle kinetic energies. The approach is shown to be robust and extensible to cases where the individual particle energies cannot be distinguished. For airborne particles, a series of impact tests was carried out over a wide range of particle sizes (from 125 microns to 1500 microns) and incident velocities (from 0.9 ms-1 to 16 ms-1). Two parameters, particle diameter and particle impact speed, both of which affect the energy dissipated into the material, were investigated and correlated with AE energy. The results show that AE increases with the third power of particle diameter, i.e. the mass, and with the second power of the velocity, as would be expected. The diameter exponent was only valid up to particle sizes of around 1.5mm, an observation which was attributed to different energy dissipation mechanisms with the higher associated momentum. The velocity exponent, and the general level of the energy were lower for multiple impacts than for single impacts, and this was attributed to particle interactions in the guide tube and/or near the surface leading to an underestimate of the actual impact velocity in magnitude and direction. In order to develop a model of the stream as the cumulation of individual particle arrival events, the probability distribution of particle impact energy was obtained for a range of particle sizes and impact velocities. Two methods of time series processing were investigated to isolate the individual particles arrivals from the background noise and from particle noise associated with contact of the particles with the target after their first arrival. For the conditions where it was possible to resolve individual impacts, the probability distribution of particle arrival AE energy was determined by the best-fit lognormal probability distribution function. The mean and variance of this function was then calibrated against the known nominal mass and impact speed. A pulse shape function was devised for the target plate by inspection of the records, backed up by pencil lead tests and this, coupled with the energy distribution functions allowed the iv records to be simulated knowing the arrival rate and the nominal mass and velocity of the particles. A comparison of the AE energy between the recorded and simulated records showed that the principle of accumulating individual particle impact signatures could be applied to records even when the individual impacts could not be resolved. For particle-laden liquid, a second series of experiments was carried out to investigate the influence of particle size, free stream velocity, particle impact angle, and nominal particle concentration on the amount of energy dissipated in the target using both a slurry impingement erosion test rig and a flow loop test rig. As with airborne particles, the measured AE energy was found overall to be proportional to the incident kinetic energy of the particles. The high arrival rate involved in a slurry jet or real industrial flows poses challenges in resolving individual particle impact signatures in the AE record, hence, and so the model has been further developed and modified (extended) to account for different particle carrier-fluids and to situations where arrivals cannot necessarily be resolved. In combining the fluid mechanics of particles suspended in liquid and the model, this model of AE energy can be used as a semi-quantitative diagnostic indicator for particle impingement in industrial equipments such as pipe bends.