Aerodynamic and Aerothermal Investigation of the Flow Around an HPT Rotor Shroud: PIV Measurements

Abstract

An experimental study has been conducted to investigate the aerothermal performance of a shrouded high pressure turbine blade in a large scale rotating rig. The rotor blade and the associated shroud and casing geometry have been modelled in a large scale low speed turbine rig that was designed to investigate a novel passive shroud cooling methodology. The objective of the present paper is to provide a detailed description of the flow field around the rotor blade shroud. The improved physical understanding of the shroud flow gained from this study will be used to analyse the aerothermal performance of the shroud cooling strategy as reported in a companion paper, Lehmann et al. [1]. Experiments have been carried out using endoscopic PIV to identify and understand salient flow features that exist upstream and downstream of the shroud as well as within the shroud cavities. The measurements are complemented by steady and unsteady numerical computations of the turbine stage. The study identifies the existence of important vortical structures within the shroud cavities that not only interact with the main passage flow but also modify the amount and distribution of the shroud leakage flow in a manner that has major implications for shroud cooling and heat transfer. A detailed shroud flow model is derived and used to elucidate the causes and consequences of the flow pattern observed. The model emphasises the circumferentially asymmetric nature of the cavity flow structures caused by the presence of the inter shroud gap that in turn influences the production, interaction and dissipation of such vortical structures.

Impact Of Different Shroud Configurations On Leakage Flow Of a LP Rotor

“…The generated jet was supposed to have a blockage effect, which is even stronger than the blockage effect caused by the vortex structures inside the cavity (see e.g. Lehmann et al (2011)). In order to get a deeper understanding of the occurring flow phenomena, the measurements were supported by a steady state CFD.…”

“…However, the passive tip injection through the bore leads to a suppression of the vortex B (see left part of Figure 8) and significantly reduces vortex C. Generally, the vortices look a little bit different than expected, see scheme in Figure 2. As described by Lehmann et al (2011), these vortices are the main reason for the reduction of tip leakage mass flow due to their blockage effect. The question is, whether the blockage caused by the passive tip injection is strong enough to really represent an improvement compared to the conventional shroud geometry.…”

“…Several authors carried out very basic studies in order to get detailed information about the flow field in the tip near regions of turbine and compressor rotors. Lehmann et al (2011) for example investigated a shrouded turbine rotor, which was equipped with three sealing fins in order to reduce the leakage flow. The outcome of their work was a very detailed description of the flow features inside the shroud cavities and at the inlet and exit of the shroud.…”

“…The first thing to be observed is that the Mach number in the second and in the third shroud cavity is significantly lower for the shroud with bore than for the shroud without bore. Lehmann et al (2011), who also focused on the flow through a rotor shroud, described the flow structure in each of the cavities. In the inlet cavity, they found a vortical structure which can also be seen in the shroud with bore (structure A), but not in the shroud without bore.…”