Modeling of a homogenous gas-water two phase flow through a Venturi and vertical pipe (A prediction of pressure drop sign change in two phase flow)

In two phase flow, differential pressures technique can be used to measure the volume fraction of the gas phase. In the
case where no restriction is available in the pipeline, the differential pressure technique can be used only in vertical or
inclined pipelines. Two phase air-water pressure drop across a Venturi meter may change its sign from positive to negative
due to change in the compressibility of the gas phase. In other words, the inlet of the venturi (upstream section) is not
always positive as in a single phase flow. A new model to predict the sign change of the two phase pressure drop across a
Venturi was developed and checked against data recently obtained from an air-water flow rig at the University of
Huddersfield. The predication of a two phase pressure drop through a vertical pipe was also investigated and compared with
experimental data. Four sets of data were investigated. In each set the water volumetric flow rate was fixed while the gas
volumetric flow rate was varied (see table-1). It was inferred from the model proposed in this paper and the experimental
data that the sign of the differential pressure drop across Venturi meter and parallel pipe for homogenous air-water flow
depends mainly on the parameters C1, C2, Uh and K. Therefore, if C1 > C2 then, the differential pressure drop across Venturi
tends to be negative and if Uh
2 > K then the differential pressure drop across a 1m long Perspex pipe will be negative.