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