Fluid mechanics, multiphase flows, combustion and pyrolysis
Rebei Bel-Fdhila, department of public technology

The numerical and physical modeling of industrial multiphase flows is an increasingly important issue with implications on safety, design and performance optimization, as well as fundamental understanding of complex processes. Multiphase flow processes are indeed found in a wide range of industrial applications: boiling systems, such as nuclear power plants or distillation plants, solid rocket motor exhaust flows, liquid combustion processes, chemical process unit operations and food processing, lubrication systems, gas and oil extraction and transport, steam turbines, fire mitigation, etc. In the present we propose three possible PhD subjects which are of intense scientific interest world wide and also which are tightly connected to various advanced industrial processes. The two first projects are focusing particularly on gas/liquid dispersed flows where we study the properties and mechanisms of the various transfer between the gas bubbles and the surrounding turbulent liquid. These bubbly flows are encountered in a number of technical and industrial processes, such as sewage water purification, flotation devices, bubble columns, stirred tanks and gas liquid contactors in chemical and biomedical processes. They are also very important within the petroleum industry as a whole, extraction, transportation, separation, storage and even in refining and a number of downstream operations in the oil and gas industry. The hydrodynamics in the bubble columns and a lot of other mixing vessels is determined by the bubble rise, the bubble size distributions, the gas hold-up, the bubble/bubble interactions and also the bubble /liquid and bubble/wall interactions. Moreover, bubble rise introduced turbulence, shear produced in the vicinity of the bubbles, bubble oscillations and wakes influence dramatically the local mixing and the mass and heat transfer from or to the bubbles. There is a lack of detailed physical understanding and prediction tools to design and optimize vessels and reactors where bubble mixing is occurring. We propose the two following projects: 1) MODELLING OF BUBBLY FLOWS IN MIXING VESSELS USING INTERFACIAL AREA DENSITY MODEL Model the bubbly flow in bubble columns and possibly in other mixing devices using the two-fluid model approach coupled with the interfacial area density model that accounts for mass transfer as well as for bubble break-up and coalescence. The model includes an additional transport equation that governs the interfacial area density. The main task will be physical and mathematical modeling within the frame of Computational Fluid Dynamics and will be based on one of the most leading commercial CFD codes in the word. Part of the work could also be experimental measurements. 2) MODELLING OF BUBBLY FLOWS IN MIXING VESSELS USING POPULATION MODEL In this project we would like to use a population balance transport model coupled with the two-fluid model. In fact, to overcome the restrictions of the basic formulation of the two-fluid model, we to extend it by introducing population classes where each class covers a range of bubble radii and is treated as an independent fluid. This approach can also allow the bubble fragmentation and coalescence to be properly taken into account. The third PhD project proposal is within the energy area and is concerning the fluidized bed combustion using both coal and biomass. 3) MODELLING OF GAS/SOLID FLOWS WITH HEAT AND MASS TRANSFER IN FLUIDISED BEDS USING THE GRANULAR FLOW APPROACH Model the two-phase gas/solid flow in fluidized bed including heat and mass transfer using the approach of the kinetic theory for granular flow coupled with the appropriate models representing pyrolysis or combustion. Special focus will be given to the interaction between particles and between the particles and the gas and particularly in the case multi-sized particulate systems with heat and mass transfer. This is a continuation of a PhD which will finish in late September.