Damping in Tube Arrays Subjected to Two Phase Cross Flow

Abstract

An experimental study was conducted to investigate the mechanism of damping in tube arrays subjected to two-phase cross-flow, mainly focusing on the influence of void fraction and flow regime.  The model tube bundle had a parallel-triangular configuration, with a pitch ratio of 1.49.  The two-phase flow loop used in theis research utilized Refrigerant 11 as the working fluid, which better models steam-water than air-water mixtures in terms of vapour-liquid mass ratio as well as permitting phase changes due to pressure fluctuations.  The void fraction was measured using a gamma densitometer, introducing an improvement over the homogeneous equilibrium model (HEM).  Three different damping measurement methodologies were implemented and compared in order to obtain a more reliable damping estimate: the traditionally used half-power bandwidth, the logarithmic decrement and an exponential fitting to the tube decay response.  The experiments showed that the half-power bandwidth produces higher damping values that the other two methods, due to the tube frequency shifting triggered by fluctuations in the added mass and coupling between the tubes, which depend on void fraction and flow regime.  The exponential fitting proved to be the more consistent and reliable approach to estimating damping.  A dimensional analysis was carried out to investigate the relationship between damping and two-phase flow related parameters.  As a result, the inclusion of surface tension in the form of the capillary number appears to be useful when combined with the two-phase component of the damping ratio (interfacial damping).  A strong dependence of damping on flow regime was observed when plotting the interfacial damping versus the void fraction, introducing and improvement over the previous results obtained by normalizing the two-phase damping, which does not exhibit this behavior.