Flow structures in solid-liquid suspensions in mixing and confined jets
Doctoral thesis, 2014
This thesis is an investigation of the influence of solid particles on different liquid flow systems. Large-scale periodic fluctuations such as macro instabilities (MI) and particle cloud dynamics in a mixing vessel and confined jets were studied experimentally and compared to numerical results from CFD simulations. The influence of particle size (0.5mm-2mm), volumetric concentration (0%vol-20%vol) and impeller speed (20Hz-37.5Hz) on turbulent structures and large-scale periodic phenomena were examined. Two-component Laser Doppler Velocimetry (LDV) measurements were conducted at different locations in a confined jet and in a mixing vessel. Video processing was used to observe the dynamic behaviours of a particle cloud and to track the cloud height. Spectral analyses were conducted using the Lomb algorithm to collect information about dominant periodicities in a continuous flow and particle cloud behaviour.
The flow conditions were numerically modelled with a Euler-Lagrange approach, treating the particles as a discrete phase and solving flow structures with an LES model. Numerical findings concerning MI phenomena, particle cloud spectra and local particle concentrations were compared to experimentally obtained data.
The results showed that all identified dominant frequencies were linear with impeller speed, resulting in a constant Strouhal number. The frequency of continuous-phase MI phenomena and of particle cloud dynamics was not influenced by the addition of solids. However, the amplitudes of the dominant frequencies in fluid MI phenomena, as well as in variations of particle cloud height, were reduced by increasing the concentration of solids. Spectra obtained with numerical simulations showed the same dominant frequency peaks as identified in the experiments. It was possible to track the local particle concentration inside a particle cloud and to detect the instantaneous cloud surface.
The Root Mean Square (RMS) values in the confined 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. Large-scale instabilities in the jet could be identified in the spectra of the flow. The particles had a slightly stabilizing effect on the jet, and moved the instability downstream, while the frequency was unaffected.