Numerical study of the effect of expansion angle on turbulent swirling flow

Abstract

This thesis presents the numerical simulation of swirling turbulent flow through different expansion angles, which have many practical applications in industrial furnaces, combustors, etc. Such a complex flow possesses several distinctly different flow regimes, either one, two or even three recirculation regions and extremely high level of turbulence. Due to the expansion geometry, substantially higher mixing rates are produced. The elevated mixing rates are due to very high level of turbulence kinetic energy generated by shearing as the core flow issues into the larger pipe. Adding swirl to the flow field having expansion causes further increase in turbulence kinetic energy and consequently speeds up the mixing process in the combustion chamber. The type of rotational motion, solid body or free-vortex type also have different effects on the stresses and turbulence energy. Hence detail knowledge of the properties are required before manufacturing the equipment encountering these types of flows. Experimental investigations are quite expensive. This thesis, therefore, suggests to numerically investigate the effect of different flow parameters including expansion angle and to generate information which will be helpful for production purpose in a cost effective way. The governing differential equations using k-E turbulence model closure are solved by a control-volume based iterative finite difference technique. A non uniform staggered grid is used. The discretized equations with boundary condition modifications are solved utilizing the SIMPLE algorithm with TDMA. Computations are done for the cases having solid body rotation type and constant swirl vane angle type swirl generation at inlet. Different swirl numbers up to 1.5 are considered. The effect of expansion angle is studied by repeating the computations for different expansion angles between 300 and 900. Predicted distribution for the mean axial and tangential velocities, streamline plots, turbulence kinetic energy and turbulence dissipation rate are presented. With the increase of swirl strength, secondary on-axis or off-axis recirculation is observed in addition to the primary corner recirculation. Swirl produces larger turbulence kinetic energy and enhances mixing rate, thus require shorter combustor length. Swirl generation by a constant vane angle swirl generator at inlet is found to have higher turbulence kinetic energy generation and consequently better mixing compared to that with solid body rotation swirl generation. It is also observed that, the value of the transition swirl strength, beyond which the secondary recirculation grows continuously with swirl strength, is lower for smaller expansion angle. As a result, for any particular swirl strength beyond the transition value, at smaller expansion angle higher turbulence kinetic energy generation and consequently better mixing is found. The capability of the computational code for predicting recirculating flows is also tested by comparing the results with the available experimental data and found to have reasonable matching.