On the 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 twophase crossflow, mainly focusing on the influence of void fraction and flow regime. The model tube bundle had a paralleltriangular configuration, with a pitch ratio of 1.49. The twophase flow loop used in this research utilized Refrigerant 11 as the working fluid, which better models steamwater than airwater mixtures in terms of vapourliquid 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 halfpower bandwidth, the logarithmic decrement and an exponential fitting to the tube decay response. The experiments showed that the halfpower bandwidth produces higher damping values than 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 twophase 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 twophase 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 an improvement over the previous results obtained by normalizing the twophase damping, which does not exhibit this behavior.