Gamma tomography with application to two-phase flow imaging in hydrodynamic machines

We discuss the application of gamma tomography to the fully three-dimensional measurement of gas and fluid distributions inside rotating hydrodynamic machines. For such investigations we use a gamma tomography setup that currently employs a Cs-137 source of 180 GBq activity and approximately 5 mm focal spot together with a detector arc made of 64 scintillation crystals (BGO) with photomultiplier read-out. The detector is operated in pulse counting mode with energy discrimination. In the presentation we will introduce the principle of phase-correlated projection acquisition. Thus, projection data of a rotating object is acquired by a fast detector read-out and subsequent integration of the data for a set of equal angular intervals which are recorded by an angle sensor at the rotating object. As a first example we demonstrate the measurement of gas fractions in an axial pump that is in two-phase flow operation. In this study we were able to visualise the gas profiles at 10% gas fraction within the impeller of the pump at different axial measurement planes. The results directly provided an understanding of the interrelation between the hydraulic conveyance characteristics of the pump in two-phase flow operation and the flow structure in the impeller. In a second example we introduce the measurement of fluid distributions in a hydrodynamic coupling. In this study we successfully applied gamma tomography to visualise flow patterns inside the coupling for varying filling levels and slips. These results now find application in the optimisation of coupling designs and in the validation of computational fluid dynamics simulations for such devices.