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Cyclonic Flows for Reactor Applications with High Engulfment Levels

This thesis aims at designing, developing and optimizing novel cyclonic vortex reactor configurations in the context of innovative technologies development for increasing the efficiency and reducing the environmental impact of energy systems. Furthermore, not depending on the specific engineering application, the thesis aims at improving our knowledge on cyclonic vortex flows, clarifying several aspects of their aerodynamic. In particular, the interaction mechanism between a jet and the vortex structure generated by jet itself and/or by cross interference process of multiple jets is investigated. The mixing process between inlet and recirculated fluid, due to the presence of a vortex flow is analyzed with the aim to identify reactor configurations for which the mixing process results maximized, strong, fast and easy to control. Mixing process rate analysis has been carried out by means of engulfment process quantification, a process that governs the mass exchange between a jet and fluid coming from the environment. Furthermore, the identification of the key parameters and the assesment of their effects on the cyclonic vortex structure in terms of vortex characterization and stabilization have been performed. 
MILD Combustion and Solar Thermal Gasification are the technologies investigated in this thesis. Cyclonic flows can be naturally coupled with such a processes due to peculiar characteristics.
The first study is related to the development of a novel Vortex burner configuration for a MILD post-combustion process for the purification of CO2-rich exhaust streams from non-condensable reactive species in the context of Carbon-Capture and Storage Technologies (CCS).
The process requires strong and fast mixing of fuel and oxidant to be stabilized and it is characterized by kinetic times longer than a conventional combustion process. The proposed configuration matches with all these constrains. Furthermore, with the configuration proposed, the cyclonic vortex is characterized by a quasi 2D fluid-dynamic structure, so that the scale up/down of the vortex burner can be easily performed.
Vortex structure characterization and the mixing process analysis have been performed by means of Particle Image Velocimetry and a CFD analysis with RANS approach. The effect of the inlet jet Reynolds number, number of inlet jets, jet type, level of confinement and total inlet flow rate on the engulfment process has been assessed. Results highlighted that the engulfment process is enhanced by increasing the inlet jet Reynolds number and the number of inlet jets, by adopting low level of confinement and an off-set wall jet.
The second study investigates the use of a cyclonic flow to couple Lean Premixed (LP) and MILD Combustion concepts, for gas turbine applications. This type of process need to take place in combustion chamber where the internal recirculation is significantly enhanced to yield a high level of mixing and subsequent heating of the fresh air-fuel by means of recirculated hot products. Strong and fast mixing of inlet fluid with the hot products is necessary for stabilizing lean and ultra-lean mixture in MILD conditions and represents the critical point for the successful application of this concept to LP gas turbines. The novel vortex reactor configuration proposed allows to analyze some key aspects of mixing process in a vortex combustor. In particular, the effects of inlet jet Reynolds number, number and position of inlet jets are investigated by means of Large Eddy Simulation (LES) approach with the aim to analyze the mixing process between inlet and recirculated fluid. The reactor configuration proposed ensures intense, fast and stable engulfment process. 
Results highlighted that the engulfment process is enhanced for a jet that interacts with a vortex structure with respect to a free jet, due to an increase of the local vorticity. Furthermore, it has been found that a strong/stable mixing is achieved for a vortex reactor configuration with two or four inlet jets meanwhile a not stable mixing and a not stable vortex structure is obtained for one single inlet reactor configuration. 
Also the presence of fluid-dynamic flow instabilities such as Precessing Vortex Core (PVC) phenomena, generally connected with a strong 3D cyclonic vortex structure has been investigated. Results showed that with the proposed reactor configuration, characterized by a H/dreactor <1 and a sudden contraction as outlet section, a stable quasi 2D vortex structure is obtained, so that PVC is avoided.
Finally, the third study concerns the development of a novel Solar Vortex Gasifier (SVG)configuration. In a directly irradiated solar gasifier, the reactor window is a critical part being used to control the atmosphere in the reactor and prevent particles egress from it. The window can be subjected to overheating due to particles deposition, leading to a reduction in the solar power absorption (decreased transparency) and potential failure. The second issue that needs to be addressed is the short residence time of the particles within the SVG. The effectiveness of this reactor concept can be increased by increasing the residence time of the gas. This prolonged resident time of the particles in the reactor can be achieve by either reducing the inlet volumetric flow rate or generating stronger vortices that keep the particles in the reactor for a longer time. In a conventional SVG configuration, residence time distribution (RTD) of the particles are principally related to the main flow rate, and it is not easy to control. For a successful application of this process is mandatory that RTD results a function of other parameters, principally the particle size and that it can be easily controlled. 
In this view, a novel design of a solar vortex gasifier is proposed in order to develop an efficiently and flexible reactor in which at the same time the window appears clear, long particles residence time can be obtained and the particle RTD results a function of the particle size. To address this scope, a CFD Analysis has been performed with RANS approach coupled with a Lagrangian particle tracking in order to evaluate the effects of changing key parameters, namely geometrical factors, total flow rate and particle size, on window state, mean residence time and the residence time distribution of solid particles.