The flow split between the products of a hydrocyclone is one of the le
ast understood aspects of hydrocyclone operation. This split is greatl
y influenced by the air core diameter, so an understanding of the air
core behaviour would greatly assist in predicting flow splits. A progr
am of combined theoretical and experimental investigation of the facto
rs that control the air core in a hydrocyclone is being undertaken. Ri
etema type simplifications of the Navier Stokes equations have been ex
tended to include the prediction of air core diameter. The equations w
hich are indirect in their original form have been converted to give a
direct solution for the dimensionless pressure and air core diameter
ratio. An experimental program using simplified test units has been un
dertaken. This has obtained pressure drop, air core diameter and veloc
ity profile data over a considerable range of operating conditions. Th
is data is being used to identify and correct deficiencies in the theo
retical analysis, and to investigate the range of validity of the resu
lting relations. The aim is to produce simple relations that can be us
ed to improve the accuracy of typical simulations of hydrocyclone perf
ormance, and also to provide some guidance in the design of hydrocyclo
nes.