It is an engineering requirement that gases entrained in the coolant flow of an engine must be removed to retain cooling performance, while retaining a volume of gas in the header tank for thermal expansion and pressure control. The main gases present are air from filling the system, exhaust emissions from leakage across the head gasket, and also coolant vapour. These gases reduce the performance of the coolant pump and lower the heat transfer coefficient of the fluid. This is due to the reduction in the mass fraction of liquid coolant and the change in fluid turbulence. The aim of the research work contained within this paper was to analyse an existing phase separator using CFD and physical testing to assist in the design of an efficient phase separator.This study analysed a commercially available phase separator of the ‘swirl pot’ type using CFD to provide a benchmark performance of the gas extraction efficiency and to analyse the flow characteristics using both Eulerian-Lagrangian and Eulerian-Eulerian models. This data was compared to physical test data taken on a newly commissioned engine cooling system test rig and conclusions were drawn about the suitability of the CFD models. This initial report is the first in a series to produce the data required to provide a suitable design tool to design higher efficiency centrifugal phase separators. These are now required to increase the efficiency of cooling systems as the cooling demands increase with new high energy-density engines.