Sigma theory is commonly used to predict the conditions that result in half of the entering particles being retained within a continuous-flow centrifuge. In this research, a 2D computational fluid dynamics (CFD) model of flow through a tubular bowl centrifuge (Sharples, 5 cm radius) was developed with Fluent software. A volume-of-fluid (VOF) approach was used for most of the simulations to track the motion of the liquid and gas phases, because the location of the gas/liquid interface depends on the centrifuge operating conditions. The CFD results indicate that the assumptions of plug flow and rigid body motion, which are fundamental assumptions of Sigma theory, were not valid for the conditions tested (700 < Re, < 3500 and 1000 < speed (rpm) < 3000). The average interfacial axial velocities (V-i theta) were 6-18 times greater than the plug-flow velocity (V-pf), and the average interfacial swirl velocities (V-i theta) were 0.12-0.41 of that corresponding to rigid body motion (V-rbm). Despite these differences, the CFD results agreed with that predicated by Sigma theory in that the retention of particles, comparable in size to animal cells (5-50 mu m), increased with increasing speed (RPM) and particle density and with decreasing flow rate. Furthermore, Sigma theory and CFD estimated similar particle sizes attaining 50% retention under similar operating conditions. In contrast to Sigma theory, the CFD model predicts an increase in particle retention with increasing fluid viscosity resulting from higher Vie values that give rise to higher centrifugal forces on the particles. This viscosity effect, which would not be expected to be as significant in centrifuges that use accelerators to promote liquid swirl, is a geometric feature not accounted for in Sigma theory. Finally, the CFD model predicted that the maximum shear rates (7 000-25 000 s(-1)) generally occurred in the inlet region near the deflector plate. These shear rates increased in a predictable way with RPM and were less affected by flow rate.