Flow structures in solid-liquid suspensions in mixing and confined jets

Licentiate thesis, 2012

This thesis investigates the influence of solid particles on different liquid flow systems. Large-scale periodic fluctuations such as macro instability (MI) phenomena in a mixing vessel and jet flows in a confined jet were experimentally analyzed by means of LDV. The influences of different sized (0.5mm-2mm) glass particles (0%vol - 6.2%vol) on the velocity and turbulence distribution in a jet were investigated, as well as the effects of varying the amounts of solid particles (up to 11.8%vol) on the frequency and amplitude of macro instabilities. Two component LDV measurements were conducted at different locations in the jet and in the mixing vessel. The values were evaluated using the Lomb algorithm to obtain frequency spectra of the liquid flow. The average axial and lateral velocities, as well as RMS distributions, were determined together with the integral length scale in the confined jet to analyse particle effect. The particle Stoke number was evaluated for the different sized particles and the different flow situations. The results showed that the MI frequency is not influenced by the addition of solids. However, the MI amplitude was reduced by the addition of the solid phase although still detectable up to the highest concentration measured (11.8 %vol). The particles (1.5mm) with a Stokes number of 1.1, related to the time scale of MI phenomena, caused an increase in MI strength at intermediate solids loading. This phenomenon could not be observed for smaller particles. No change in the rate of decrease of the centreline velocity or the jet expansion ratio could be observed in the confined jet configuration. The RMS values in the jet increased at high particle loadings; the increase was particularly pronounced in the shear layer close to the nozzle. With increasing particle size a greater effect on the RMS values could be observed. The frequency spectra of the flow and the strength of the large-scale instabilities were only affected by the suspension with the 2mm particles. The 2mm particles had a stabilizing effect on the jet and moved the instability further downstream while the frequency was unaffected.