Measurement of Electrostatic Dipoles and Net Charge on Air Dispersed Particles
Thesis DisciplineChemical Engineering
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
Dipoles are expected to often dramatically enhance the mutual collision rate of diffusing particles (above the effects of Brownian or turbulent motion). However, this spreading awareness of the possible influence of discrete dipoles on particles is still based largely on theory, and some qualitative experience of particle behaviour from microgravity experiments. Individual particle dipoles have not yet been definitely isolated in experiments, nor measured in practical situations. In this project, it was intended to measure, for the first time, distributions of dipole strength (as well as net charge and particle size) on particulates dispersed into air by typical industrial and pharmaceutical processing methods.
The instruments designed to do this were built around a sampling head which allowed examination of a flow of dust dispersed into an air stream. During dispersal, the particles suffered tribocharging by mutual separation and collision on walls. Examination of the particles involved recording the path of particles as they moved through a non-uniform electric field around a central electrode, which was supplied with high voltage. Particles were attracted towards the central electrode (of 0.5 mm diameter in this study) if they contained dipoles, independent of the polarity of the field or their net charge. Particles to be examined were illuminated by a laser sheet as they moved past, and a high speed video captured their trajectories (over a field of view of around 5 mm).
The equation of motion of a particle which involved the forces of both particle net charge and dipole strength was applied to the particle path to evaluate both these parameters. The particle trajectories were modelled, and checked against the observed experimental trajectories. The voltage applied to the probe varied from 4 kV to 18 kV but for most of the runs 6 kV voltage was used. The electric field around the probe tip was assumed to be same as that for a spherical electrode of the same size as the probe. The flow field axially towards and around the probe tip was calculated using the Stokes creeping flow equations around a sphere. The calculated electric and flow fields were checked against COMSOL Multiphysics models applied to actual geometries and flow regimes. The rotation dynamics of the particles was also considered important in the technique, requiring possibly extra knowledge of the initial direction of the dipole. The flow was led through a lateral field between two plates in order to orient the direction of any dipoles in the direction of the lateral field. The expected orientation of dipoles coming out of the plates was used as an initial guess of their orientation for modelling the rotation of the dipoles when they entered the probe field. Misalignment after leaving the plate field and before entering the probe field was also considered, and was found to be important due to vortices characterised by smoke and particle studies. However, the trajectory modelling revealed that the particles studied quickly rotated into alignment with the probe field, providing maximum attractive force to the probe, and so the values of net charge and dipole strength obtained did not depend on the initial orientation.
Estimated errors of particle position and diameter used in all the calculation steps were judged to be well within a basic image error limit of ±1 pixel. Some particle trajectories showed unexplainable shapes which was traced to the influence of large mixing eddies around the gas/particle jet. A check for corona discharge at the probe tip was made both at the beginning and at the end of the sampling experiments. No corona was detected initially (up to 18 kV), but a discharge could be observed at voltages close to 7 kV in the later checks.
Particles of acrylic, glass bubbles, whole milk and fertiliser powder were sampled and net charges and dipole charges were estimated. The sampled particles overall had net charge and dipole charge in the range of 10-15 C to 10-12 C on individual particles with diameters 20 μm -130 μm. Dipoles were more evident (more easily measured) for glass bubbles but the presence of dipoles on other particle samples was found and could not be completely ruled out for many of them. The analysis procedure is presently time consuming but can be automated so it is recommended in the future that it should be automated. The work can be extended into industrial situations by sampling moving dust suspensions, e.g. fluid bed overflows and pneumatically conveyed outflows, useful in the dairy and fertilizer industries.