Experimental investigation of two-phase bubbly flow in a fuel assembly with spacers

Full text of publication follows: Accurate local measurements of time-averaged two-phase flow characteristics in a rod bundle are of primary importance for the understanding of phase distribution mechanisms between the subchannels and for the proper modelling validation. The present understanding of phase distribution is based on experimental and theoretical investigations of two-phase flow in tubes. The current two-fluid models are actually capable of accurately predicting the radial distribution of void fraction in tubes. However, these models are based on a number of empirical coefficients derived from air-water bubbly flow experiments in tubes only. For the proper numerical modelling validation for realistic geometries, encountered in fuel assemblies, reliable data have to be presented not only for local void fractions but also for interfacial area concentration, local gas and liquid velocity components. In recent years, very detailed measurements of the void fraction distribution in a rod bundle have been performed using X-ray or?-ray computer tomography [1,2 and 3]. However, no corresponding measurements of local phasic velocities and turbulence have been made. Lack of such complete set of data is a major obstacle in a proper validation of mechanistic Computational Fluid Dynamics models of phase distribution in fuel assemblies. The present work is designed to provide a set of local experimental data for the square subchannels of a 24-rod bundle using an optical probe. The probe is mounted on a traverse system, which enables the acquisition of experimental data in various points of a subchannel as well as in axial direction. An additional advantage of the probe is the possibility of 'one-time' acquisition of void fraction, interfacial area concentration and bubble velocity. The full paper will contain a detailed description of the measuring technique and will present the measured data obtained in a 24-rod bundle. 1. Windecker G., and Anglart H., 2001, Phase Distribution in a BWR Fuel Assembly and Evaluation of a Multidimensional Multi-field Model, Nuclear Technology, Vol. 134, pp. 49-61; 2. Inoue A., Futakuchi M., Yagi M., Mitsutake T. and Morooka S., 1995, Void Fraction Distribution in a Boiling Water Reactor Fuel Assembly and the Evaluation of Subchannel Analysis Codes, Nuclear Technology, Vol. 112, pp. 388-400; 3. Kok H., van der Hagen T. and Mudde R., 1997, Measurements of the Void-Fraction Distribution in a Simulated Fuel Assembly and the Role of the Void-Fraction on the Dynamics of a Natural Circulation Loop, Annals of Nuclear Energy, Vol. 24, pp. 1333-1347. (authors)