Abstract:
Multiphase systems are ubiquitous in industrial applications aimed at the generation of products either by chemical/biological reaction or physical separation based on density, electrical charge or surface properties such as hydrophobicity. The physical processing of these multiphase systems is carried out at all scales of operation and within an endless variety of vessel shapes and ancillary devices. Underpinning each process is a complex interaction between phases involving hydrodynamic, heat and mass transport. These phenomena are in turn governed largely by the nature of the flow, and in particular whether laminar or turbulent conditions prevail. In large-scale industrial processes the flows are almost always turbulent, whilst for microscale operations the flow will be laminar. Each condition provides its own challenge in being able top redict (and optimise) performance in terms of operational stability and efficiency of energy utilisation.Turbulent systems are particularly difficult to optimise, where the efficiency of utilisation of turbulent energy to enhance the transport phenomena is poor, usually less than five percent. The major loss of efficiency can be attributed to the non-optimal distribution of turbulent energy dissipation rate and the length scale of energetic flow structures. The length scale of turbulence and spatial distribution of energy dissipation rate, especially in the region of the interface between the phases, must be tailored to the desired application in order to achieve higher efficiency. This might sound easy but tailoring energy dissipation to the operational requirements is very difficult to achieve in practice. Not all systems operate under turbulent conditions. Microdevices are increasingly being used for mixing and reacting operations since they are able to offer uniformity of flow, enhanced heat and mass transfer, and simplicity and low cost of construction. However, microdevices have their own limitations in that the flow is almost certainly to be in thelaminarregime whereby any velocity fluctuations, and resultant enhancement of dispersion behaviour, are likely to be attenuated. Irrespective of whether the system is operating under either laminar or turbulent conditions it is important to be able to control energy dissipation rate at a length scale that optimises the transport process. Multiphase systems operate under different flow regimes. For example, fluidised beds and bubble columns typically operate in either the homogeneous (bubbly) or heterogeneous (churn -turbulent) regimes depending on the volumetric flow rates, and dispersed phase (bubbles, droplets or particles) characteristic size and volume fraction. The rate of heat, mass and momentum exchange will depend on the operatingregime, and for this reason it is important to be able to predict when the transition will take place for a given (conventionalor micro -) scale of operation and energy input rate In this study, the velocity field inside a typical multiphase system (fluidised bed and bubble column) is quantified using particle image velocimetry (PIV). The information is then processed to obta in both overall energy dissipation rate and spatial and temporal energy spectrums, which allows us to compute/comment upon the energy utilisation within the system. Measurements are performed within both conventional and microscale fluidised beds and compared with expectations of pressure drop and dispersed phase volume
fractions obtained from Richardson–Zaki/Ergun and discrete element modelling. Finally, both drift-flux and linear stability analysis are applied in order to predict regime transition as a function of system operating parameters, including inlet flow conditions of the continuous phase.