A hydrocyclone is cyclone that uses water as a bulk fluid. The advantages like low cost, no moving parts, little maintenance cost and high capacity make it attractive for industrial use. During the operation, due to the low pressure at cyclone axis, a back-flow of gas can occur which then forms a gas-core. This gas-core affects the separation efficiency. This gas-core was eliminated by inserting a solid rod. Experiments were made in a newly constructed hydrocyclone for different inlet flow rate in presence as well as in absence of gas-core to qualify the pressure drop characteristics of the hydrocyclone. Effect of gas-core on the pressure between different specific locations was studied. FLUENT as a CFD-modelling tool was used for hydrodynamic study. (C) 2008 Elsevier B.V. All rights reserved.