COMPUTATIONAL MODELING STUDY OF THE HYDRODYNAMICS IN A SUDDEN EXPANSION - TAPERED CONTRACTION REACTOR GEOMETRY

The hydrodynamics and recirculating flow characteristics of a reactor geometry consisting of a sudden expansion followed by a tapered contra ction have been studied using the computational fluid dynamics package CFX-F3D. The results provide important information for the design and optimization of such reactor configurations. The pressure difference across the reactor can be well predicted based on a simple theory deri ved from first principles. The value of Delta P-R is strongly dependen t on both the Reynolds number and inlet expansion ratio, and reaches a maximum at a chamber exit angle of beta(E) = 60 degrees. The correspo nding values of the energy dissipation rate indicate that this paramet er is between 1.5 and 2 times higher for the reactor inlet compared to the exit. Specific energy dissipation rates of up to 1000-kW kg(-1) a re typical in the entrance region. The length of the recirculation zon e (L-RE/H similar to 7), maximum backflow velocity (U-B(MAX)/U-N simil ar to 0.12), position of the recirculation centre [L(P-B(MAX))/L-RE si milar to 0.533 and position of the wall pressure minimum [L(P-W(MIN))/ L-RE similar to 0.35] all remain approximately constant over a wide ra nge of reactor geometries and operating conditions. For chamber length s less than the reattachment length in an infinite chamber, the size o f the recirculation zone adjusts to fill the entire chamber volume ava ilable. As the inlet expansion ratio is increased, the reattachment le ngth (L-RE/H) passes through a slight maximum at a ratio value of D-N/ D-C = 0.4, corresponding to the point at which the potential core exte nds the entire length of the recirculation zone. An increase in the ma ximum back flow velocity, a decrease in the minimum wall pressure and an upstream movement of the recirculation centre with increasing expan sion ratio are also observed. The centreline velocity profiles and jet radii results in the recirculation zone at the reactor entrance show that the confined jet can be modelled as a free jet over an initial se ction of the recirculation where there appears to be little direct int eraction between the downstream wall and the shear layer. (C) 1997 Els evier Science Ltd.