The hydrodynamics and recirculating flo~ characteristics of a reactor geometry consisting of a sudden expansion followed by a tapered contraction have been studied using the computational fluid dynamics package CFX-F3I). 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 derived from first principles. The value of APR is strongly dependent on both the Reynolds number and inlet expansion ratio, and reaches a maximum at a chamber exit angle of[Ij: = 60 . The corresponding values of the energy dissipation rate indicate that this parameter is between 1.5 and 2 limes higher for the reactor inlet compared to the exit. Specific energy dissipation rates of up to I(X)0 kW kg ~ are typical in the entrance region. The length of the recirculation zone (LRfH -71. maximum backflow velocity (U~AX"u~. ,~ 0.12), position of the recirculalion centre [L(P~XX)/LR~ ~0.53] and ~L'PMI~) L 0.35] all remain approximately con-position of the wall pressure minimum t ~ w . R~: stant over a wide range of reactor geometries and operating conditions. For chamber lengths less than the reattachment length in an infinite chamber, the size of the recirculation zone adjusts to fill the entire chamber w)lume available. As the inlet expansion ratio is increased, the reattachment length (LRv:;tt) passes through a slight maximum at a ratio value of DN 'De = 0.4, corresponding to the point at which the potential core extends the entire length of the recirculation zone. An increase in the maximum back flow velocity, a decrease in the minimum wall pressure and an upstream movement of the recirculation centre with increasing expansion 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 section of the recirculation where there appears to be little direct interaction between the downstream wall and the shear layer. , 1997 Elsevier Science l.td