eTheses Repository

# CFD studies of complex fluid flows in pipes

The flow of rheologically complex fluids in industrial equipment poses a number of challenges, not least from a modelling point of view. Research is needed to further understand and be able to predict the flow behaviour of such materials and to investigate ways of improving their processing. This work investigates the numerical modelling of complex fluids in three areas: flow and heat transfer under an externally imposed mechanical vibration, and steady-state solid-liquid flows as a first step in extending the vibration studies to these multiphase systems. Validated CFD simulations were used to study the effects of rotational and transversal mechanical vibrations on the pipe flow of viscous non-Newtonian fluids of the power-law, Bingham plastic, and Herschel-Bulkley types. Vibration frequencies in the sonic range of 0-300 Hz and linear amplitudes of 0-4 mm were used. The results showed that rotational and transversal vibrations give rise to substantial enhancements in flow for shear thinning and viscoplastic fluids, while shear thickening fluids experienced flow retardation. The flow enhancement was found to depend on vibration frequency and amplitude, fluid rheological properties, and pressure gradient. These vibrations can be effective at enhancing the flow of low to moderately viscous fluids in industries such as the confectionery industry. For extremely viscous fluids (consistency index ~10 kPa s$$^n$$ and yield stress ~200 kPa), ultrasonic frequencies (> 16 kHz) were found to produce orders of magnitude enhancements in flow. These results suggest that vibration can increase the fluidity of highly viscous fluids in industrial applications such as polymer extrusion. Results are also reported for the effects of transversal vibration on heat transfer and temperature uniformity in Newtonian and non-Newtonian shear thinning fluids. Vibration was found to generate sufficient chaotic fluid motion that led to considerable radial mixing which translated into a large enhancement in wall heat transfer as well as a near-uniform radial temperature field. Vibration also caused the temperature profile to develop very rapidly in the axial direction, thus, reducing the thermal entrance length by a large factor, so that much shorter pipes can be used to achieve a desired exit temperature. These effects increased with both vibration frequency and amplitude but were more sensitive to the amplitude. Higher fluid viscosities required larger amplitudes and/or frequencies to achieve substantial temperature uniformity. These results have significant implications for processes where a wide temperature distribution over the pipe cross-section is undesirable as it leads to an uneven distribution of fluid heat treatment, such as in the thermal sterilisation of food products. A numerical study was also conducted of the laminar pipe transport of coarse spherical particles (d = 2-9 mm) in non-Newtonian carrier fluids of the power law type using an Eulerian-Eulerian CFD model. The predicted flow fields were validated by PEPT experimental measurements of particle velocity profiles and passage times, whilst solid-liquid pressure drop was validated using relevant correlations gleaned from the literature. The study was concerned with nearly-neutrally buoyant particles (density ~1020 kg m$$^{-3}$$) flowing in a horizontal or vertical pipe at concentrations up to 40% v/v. The effects of various parameters on the flow properties of such mixtures were investigated over a wide range of conditions. Whilst the effects of varying the power law parameters and the mixture flow rate for shear thinning fluids were relatively small over the range of values considered, particle size and concentration had a significant bearing on the flow regime, the uniformity of the normalised particle radial distribution, the normalised velocity profiles of both phases, and the magnitude of the solid-liquid pressure drop. The maximum particle velocity was always significantly less than twice the mean flow velocity for shear thinning fluids, but it can exceed this value in shear thickening fluids. In vertical down-flow, particles were uniformly distributed over the pipe cross-section, and particle diameter and concentration had little effect on the normalised velocity and concentration profiles. Pressure drop, however, was greatly influenced by particle concentration. These results can help in understanding and predicting the flow behaviour of such solidliquid mixtures in industrial applications, such as the conveying of particulate food suspensions.