An effective numerical method for calculating unseparated flows in building aerodynamics

The problem of the mixing of heated gases is of interest in connection with the development of pollution-free technologies for natural-fuel combustion in modern heat and electricity generating plants. The operating principle of the tall structures designed for this purpose, which combine a smokestack and a cooling tower, is as follows. At the base of the stack flue gas, from which the sulfur has been removed, is fed into a flow of air heated in a heat exchanger. As it moves through the stack, the gas mixes with the hot air and is carried into the atmosphere by the natural draft. The design must satisfy certain requirements. The temperature of the flue gas must not fall below a certain limit at which condensation that leads to corrosion develops. The gas outlet velocity must be higher than 4 m/s to prevent downdraft. The concentrations of pollutants released into the atmosphere must be within the permissible limits. One promising means of enhancing the efficiency of such structures is to swirl the flue gas ahead of the stack inlet. Flow swirling considerably intensifies the heat and mass transfer processes, improves the mixing of the hot gases, reduces the pollutant concentrations at the stack outlet, and prevents flow separation on the walls. The general formulation of the problem of the mixing of two nonisothermal turbulent flows is based on the complete Reynolds equations. This system closed by a turbulence model (algebraic or differential) is fairly complicated and its solution is laborious. A simplified model is based on the parabolized Navier-Stokes equations, which restricts the area of applicability to unseparated flows. However, in view of the mechanical nature of the problem considered, unseparated flows are of most interest. In this paper, an effective method of solving boundary-layer equations is discussed, in which the initial system reduces to ordinary differential equations written on the streamlines. (paper)