Wall Slip in Pipe Rheometry of Multiphase Fluids

Abstract

Multiphase fluids are widely available in our everyday life. Many of the materials we use and eat every day are classed as multiphase and characterisation of their properties is required to improve quality and manufacture. Being typically non-Newtonian fluids, rheological characterisation of multiphase fluid systems is indeed a complex procedure. Apparent wall slip, or more precisely wall depletion effect near the wall, is an important phenomenon which often occurs in the flow of multiphase fluids in pipes. Wall slip has its own advantages and disadvantages in the processing and pipe flow of multiphase fluids. One of the main problems it causes is the underestimation of the viscosity and the true flow curve of the fluids reported during experimental measurements. Standard correction methods often account for this effect, but there have been many instances reported where the data does not comply with the technique. This study aimed to present a wall slip analysis of a selection of non-Newtonian multiphase fluids during flow in pipes. A pipe rheometry rig was specially designed and built for the purpose of the research which includes four interchangeable pipes with different diameters; three sets of pressure and temperature transducers located at three different points along the test section; an electronic mass balance; and a PC data logger for control, monitoring and data collection purposes. Three distinct non-compliant multiphase fluids were chosen for study i.e. ice cream, citrus dietary fibre (CDF) suspensions and magnesium silicate slurries. The experiments were carried out in pressure driven shear flow. The flow data were analysed using the classical Mooney method along with a Tikhonov regularisation-Mooney method. For ice cream flow data, the analysis indicated that significant apparent wall slip occurred in all flows and there was a small but significant increase in the temperature near the wall which indicated the occurrence of viscous heating phenomenon. Energy balances indicated that the apparent wall slip effect was not due to the existence of a thin slip layer of rarefied low viscosity fluid next to the wall. It was found that the results were better understood as being the result of a moderately thick layer of slightly heated ice cream next to the wall. Mooney and Tikhonov regularisation-Mooney methods were confirmed to be incompatible with the wall slip behaviour of CDF suspensions and magnesium silicate slurries. The incompatibility of the method to analyse wall slip is attributed to the inconsistent ratio of Vslip/τw and delta/μslip at constant wall shear stress. It is concluded that this was due to the microstructure changes and shear-induced re-orientation of the particles in CDF suspensions and magnesium silicate slurries during flows which resulted in the inconsistency of the slip layer thickness and consequently affected the wall slip characterisation process. The principal contribution of this research work is to present a comprehensive wall slip analysis in pipe rheometry of multiphase non-Newtonian fluids, which are of particular interest in engineering process design. 

Date of Award	31 Dec 2013
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Peter Martin (Supervisor)