Void Fraction and Flow Regime Determination

by Low-Energy Multi-Beam Gamma-Ray Densitometry

E. Åbro1 and G.A. Johansen2

1 Christian Michelsen Research AS, P.O. Box 6031, N-5020 Bergen, Norway

2 University of Bergen, Department of Physics, Allègaten 55, N-5007 Bergen, Norway.

Abstract ? Gamma-ray densitometry is a frequently used method for non-intrusive measurement of the void fraction in two- and multi-phase gas liquid pipe flows. Here it is demonstrated how a multi-beam configuration using a low-energy gamma-ray source and several detectors, enables the void fraction to be determined almost independent of the flow regime.

An experimentally verified EGS4 Monte Carlo simulation has been developed of the multi-beam gamma-ray densitometer. This is an efficient tool in developing the densitometer as detector responses to different void fractions and flow regimes easily are generated. The simulations cover the full range of void fractions with homogeneous, annular and stratified flows. The model has also been applied to generate training data for a neural network which then was tested with experimental data. This has been done for a variety of detector positions in order to optimise the geometry of the multi-beam densitometer. Polypropylene phantoms (density = 0.91 g/cm3) were used to represent oil in these experiments in order to have reliable and accurate references.

By using experimental data as input to the neural networks, the void fraction was determined with an error of 3% regardless of the flow regime. The flow regimes were successfully recognised in all cases studied here, meaning that the system also provides tomographic information.