IMECE2008-69186 SIMULATION OF VORTEX SHEDDING BEHIND SQUARE AND CIRCULAR CYLINDERS Hossein Moin IMECE2008-69186

ABSTRACT Many studies have been conducted on downstream flows behind two-dimensional cylindrical sections for low Reynolds numbers. The vortex shedding phenomenon occurs in fluid flows over buildings, trailer trucks, bridge piers, heat exchangers, pipelines in the sea, etc. This phenomenon, which is due to the flow instability in the wake region results in a periodic oscillation of drag and lift forces. In experimental studies, visualization techniques such as hot wire and Laser Doppler Velocimetry (LDV) are usually employed. By performing extensive measurements and using the concept of curve fitting, correlations have been obtained for Strouhal number variation with Reynolds number. In addition to experimental works, some analytical studies on complex wake structures of forced and freely oscillating cylinders have been undertaken. Recently, numerical models have been introduced in order to simulate this phenomenon. Khalak and Williamson made several Direct Numerical Simulation (DNS) studies on freely oscillating cylinders for Re up to 350. Low Reynolds flows (up to 800) over square and circular cylinders are simulated based on a numerical method where transient 2D Navier-Stokes equations are solved. In simulations, the fluid was assumed water with properties at 25°C. The model predictions for pressure fluctuations and the variation of Strouhal number (St) with Reynolds (Re) were compared with those obtained from experiments and correlations. In this numerical model we also compared drag force (CD) against Reynolds (Re). Under sharp rising distribution and horizontal asymptotic regime which are two major parts of St-Re variations, the model results agree well with measurements. Both simulations and experiments reveal that the St-Re variations do not depend on the shape of the cylinder. The model results agreed well with measurements

SIMULATION OF LIQUID FUEL ATOMIZATION IN AN INDUSTERIAL SPRAY NOZZLE OF A POWERPLANT BOILER

ABSTRACT In this study, the liquid fuel atomization in the injector nozzle of the combustion chamber of a powerplant boiler is numerically simulated. The atomization of a liquid fuel injector is characterized by drop size distribution of the nozzle. This phenomenon plays an important role in the performance of the combustion chamber such as the combustion efficiency, and the amount of soot and NOx formation inside the boiler. The injector nozzle, considered in this study, belongs to a powerplant boiler where the liquid fuel is atomized using a high pressure steam. First, the geometric characteristics of the injector are carefully analyzed using a wire-cut process and a CAD model of the nozzle is created. Next, one of the nozzle orifices and the atomization zone where the high pressure steam meets the liquid fuel is recognized. The computational domain is extended long enough to cover the whole atomization zone up to the end of the orifice. The flow governing equations are the continuity and Navier-Stokes equations. For tracking the liquid/gas interface, the Volume-ofFluid (VOF) method along with Youngs' algorithm for geometric reconstruction of the free surface is used. The simulation results show the details of the liquid and steam flow inside the nozzle including velocity distribution and shape of the liquid/gas interface. It is found that the liquid breakup to ligaments and the atomization of liquid to droplets do not occur inside the nozzle orifice. A liquid jet with certain cross sectional shape leaves the orifice surrounded by a high speed steam. The numerical model provides the shape of the liquid jet, and the steam and fuel velocity distributions at the exit of the nozzle orifice. These parameters are then correlated to the final drop size distribution using analytical/experimental correlations available in literature