Mechanical Behaviour of Variable Nozzle Guide Vane Systems Based on Full-Scale Testing

A full-scale test bench for the analysis of frictional and modal behavior of Nozzle Guide Vanes (NGV) is an advanced tool enabling high design accuracy for these components equipped in an increasing number of turboexpanders. This sub-assembly is a key feature enabling operational flexibility for the expander. Great attention must be paid during the NGV's design to the natural frequencies of its kinematic chain, which are influenced by internal clearances and by friction. In this paper the results of a testing campaign performed on an NGV assembly employing actual blade geometry are presented together with details concerning the test bench commissioning and set-up procedure. The testing campaign involves three different combinations of blade orientation and pre-load corresponding to typical design conditions of the expander. In addition, two different bushing clearances values corresponding to worn out conditions were investigated. The measurements of the global friction coefficient based on actuator force detection are summarized. After that, the reconstruction of mode shapes based on experimental modal analysis is explained. The results highlight the importance of loading conditions on the actual value of friction force and their influence on blade natural frequencies. The testing campaign was used to properly validate Finite Element Models to be used for further investigations.