Fluidic Control of Shear Layer in Dump Combustors Using Microjets
thesisposted on 01.11.2015 by Harish P. Kanchi
In order to distinguish essays and pre-prints from academic theses, we have a separate category. These are often much longer text based documents than a paper.
There is a worldwide interest to investigate efficient approaches to energy conversion. This thesis explores the use of microjets as an effective means to improve the performance of dump combustors using computational fluid dynamics (CFD) tools. The introduction of microjets from the dump plane at an angle of 45 degrees to the dump plane in dump combustors increases the complexity of the problem. Reynolds averaged Navier-Stokes (RANS) simulations are well suited for performing various parametric studies to arrive at a finalized design for the implementation of microjets in dump combustors. The RANS simulations in this thesis show that adding microjets in dump combustors significantly increases the heat release rate in a round (axisymmetric) and planar (asymmetric) dump combustors. For a more accurate understanding of the fluid dynamics, large-eddy simulation (LES) is used to study the effect of single microjet in flow over backward-facing step (BFS) geometry. A Legendre spectral element code is utilized for the LES studies. The microjet is placed on the step face and enters the BFS geometry at an angle of 45 degrees to the step face. Two microjet velocities, 20 m/s and 40 m/s, are simulated to study the effect of microjet in a backward-facing step flow. The microjet cases are simulated with a passive scalar to study the effect of microjet on scalar field and quantify scalar mixing. The introduction of a microjet changes the flow structure in the shear layer and creates a three-dimensional flow pattern as compared to the case without microjet, where the shear layer is primarily two-dimensional. The 40 m/s microjet case penetrates deeper into the flow field, has higher impact on the shear layer, and gives rise to higher turbulent kinetic energy in the shear layer as compared to the 20 m/s microjet case. Microjets entering at an angle of 45 degrees to the step face impact the primary shear layer and give rise to a counter-rotating vortex pair. This counter-rotating vortex pair entrains the surrounding fluid and enhances the mixing of the passive scalar due to the microjet. Various indices are used in the analysis of mixing efficiency. The indices show that mixing is higher in 20 m/s microjet case as compared to the 40 m/s microjet case.