accepted after revision 15 March 1995The air core diameter in a hydrocyclone is analysed using the physics of uniform density, inviscid dow at each outlet, modified by an empirical factor to account for viscous effects. It is expressed in terms of flow variables at the underflow and overflow, using the principle that the diameter of the air core adjusts so that the flow is a maximum for a given total pressure head. The resulting expression for the air core diameter can be applied iteratively during a hydrocyclone flow calculation, and the computational grid adjusted accordingly at each step. The new air core relationship is incorporated in the computational hydrocyclone flow model of Davidson. Predictions of air core diameter are compared with corresponding measured values for an extensive set of vortex chamber experiments, and for a limited number of hydrocyclone experiments which include flows both with and without solids and cover a range of viscosities. It is shown that accurate predictions for the size of the air core in both hydrocyclones and vortex chambers can be achieved using the new air core sub-model.