Interpretation of Pitot Pressure in Compressible Two-Phase Flow

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Abstract
An analysis is presented of the dynamics of small droplets near the mouth of a purged or unpurged pitot tube in two-phase flow. Under certain conditions, that part of the total pressure which is attributable to loss of droplet momentum is shown to be only weakly dependent upon droplet size and density, vapour viscosity, tube size and purging rate. A new method is formulated for calculating the velocity from measured static and total pressures in compressible flow, which does not include the assumption that the fluid behaves as a homogeneous equilibrium mixture. When compared with results based upon the mixture assumption, for condensing flows of steam at low pressures, the new method yields velocities which are greater by one or two per cent and which approach the dry vapour values as the wetness fraction tends to zero. The absolute accuracy of the method has yet to be determined through experiment.

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