Flow separation characteristics within rectangular and conical diffusers

The concept of a diffuser relies on transforming the dynamics or velocity pressure into static pressure. This simple concept found numerous application including in the jet engine, sewage, optics, automotive, ventilation systems. During the first half of the 20th century, a multitude of diffuser designs were investigated to provide valuable information and guideline in optimising their efficiency. The performance of diffuser is extremely sensitive to its geometry and inlet condition, which can lead to the flow separation of the turbulent boundary layer and the development of reversal flow. This reversal flow is the main contributor to the pressure losses leading to a drop in the diffuser's efficiency. The three-dimensional nature and the unsteadiness of the reversal flow makes it experimentally and numerically challenging to study and quantify its characteristics and underlying mechanisms. This research operates and analyses high-resolution Large Eddy Simulation (LES) to further the understanding of the flow mechanisms and interactions with the coherent structures responsible for the emergence of instantaneous reversal flow pockets.The flow separation geometrical sensitivity is assessed through the investigation of two different asymmetric rectangular diffusers. Moreover, the flow separation temporal behaviour and the arrangement of the energy-containing motions are evaluated within a conical diffuser when subjected to a fully developed pipe inlet and a swirling boundary condition. Prior to these parametric studies, the hydrodynamics of each diffuser were calibrated and validated extensively with experimental data. Two publications derived from the work undertaken in this thesis have been submitted: 1. A. Hajaali, T. Stoesser. Flow separation behaviour within three-dimensional asymmetric diffusers . Submitted to Journal of Flow, Turbulence and Combustion. 2. A. Hajaali, T. Stoesser. Impact of swirling in flow on conical diffusers' efficiency of a conical diffuser and their coherent structure . Submitted to Journal of Fluids Engineering, ASME