Side chambers of centrifugal turbomachinery resemble rotor–stator cavities. The flow in these cavities develops complex patterns which substantially influence the axial thrust on the shaft and the frictional torque on the rotor. Axial thrust caused by the flow pattern in side chambers accumulates in multistage single shaft radial compressors where it is often balanced by a single axial bearing. Miscalculation of axial thrust may lead to axial loads significantly higher than predicted or even undefined load situations which may cause early bearing failure. Likewise, a wrong prediction of friction losses may lead to lower efficiency than originally intended. Current models for axial thrust and friction torque are limited to circumferential Reynolds numbers of Re ≤ 107. New models are needed for modern high-pressure centrifugal compressors which reach circumferential Reynolds numbers up to Re = 109.
The rotor–stator cavity flow model by Kurokawa and Sakuma  for merged boundary layers is analysed. It is based on the assumptions of axisymmetric and time invariant flow. Functional forms of the mean tangential and radial velocity and the surface stress vectors on the rotor and stator are assumed. Reynolds averaging is applied to consider turbulence effects in the model. The modelling assumptions are compared with detailed RANS CFD analyses at Reynolds numbers of 4 · 106 ≤ Re ≤ 2 · 108 to investigate their accuracy. Based on these CFD results, a way towards a high Reynolds number model is presented, providing prediction of disc torque, radial pressure distribution and axial thrust in rotor–stator cavities.