A NEW CRITERION FOR THE INSTABILITY THRESHOLD OF A SQUARE TUBE BUN-DLE SUBJECT TO AN AIR-WATER CROSS-FLOW
Résumé
In this paper, we investigate the fluid-elastic instability of a square tube bundle subject to a two-phase cross-flow. A dimensional analysis is carried out, leading to a new criterion of stability. This criterion establishes a direct link with the stability thresholds in single-phase flows and as such constitutes an extension to the Connors equation for two-phase flows. In parallel to the dimen-sional analysis, an experimental work is conducted to test the validity of the criterion in air-water cross-flow. The bundle consists on 5 rows of 3 tubes (plus two end-rows of half-tubes), with reduced pitch P/D = 1.5, tubes diameter and length D = 30 mm, L=300 mm. The central tube is mounted on two flexible blades allowing a vibra-tion in the transverse direction, whereas all the other tubes are rigid. Three sets of blades with different rigidi-ties are tested to investigate the stability of the central tube, considering, for an homogeneous void fraction in the range 0% to 100%. We show that the criterion of stability derived from the dimensional analysis is in very good agreement with the air-water experiments. This new criterion is of theoretical interest for the understanding of complex two-phase flow excitations, as well as of practical significance for the predictive analysis of indus-trial components.