No abstract

Computational thermo-fluid dynamics in the field of turbo machinery research tend to account for transient interaction mechanisms to predict the convective heat transfer within the hot gas path. In this context, the rotor hub side endwall region of the high pressure turbine depicts an object of interest as the near wall flow field may be dominated by rotating flow structures emerging from the disc space cavities. The validation of the applied numerical tools rely on experimental heat transfer setups reproducing such transient boundary conditions. This paper describes an experimental approach to quantify the heat transfer coefficient and the adiabatic wall temperature on the rotating endwall of a large scale test turbine. The wall heat flux distribution in a thin film isolator coated to a well conducting support structure is quantified for a series of quasi-isothermal boundary conditions. A high-resolution infrared camera is used to capture triggered thermograms of the rotating surface. Distributed thermocouples in the base body serve as reference points for camera calibration and to deduce the temperature distribution at the interface to the isolator. The calibration is in-situ and includes the pixel-wise quantification of uncertainties in the surface temperatures. An advanced linear fit approach is applied to derive the unknown adiabatic quantities and their uncertainties. For the examined operating point with a rim seal purge flow rate of 1% the random part of the relative measurement uncertainty is clearly below 10% for the heat transfer coefficient and below 5% for the adiabatic wall temperature. As the evaluation algorithm is designed to consider covariances between the thermocouple and infrared readings, the surface wall heat flux can be evaluated for every single infrared image.

Unsteady large scale flow structures rotating in rim seal cavities are identified to possibly influence the flow field near the hub side endwalls in axial turbines. Recently, a number of investigations were published on their identification, quantification and interpretation. As the formation of the cavity structures can be dominated by the unsteady interaction between stator and rotor, complex test setups are necessary requiring rotating facilities. Concerning numerical investigations, transient full circle CFD-setups or at least multiple sectors are necessary to allow the formation of the rotating cavity structures. This conference contribution presents a combined experimental and numerical study of rotating cavity structures identified in the hub side region of the Large Scale Turbine Rig at Technische Universität Darmstadt, Germany. Unsteady aerodynamic probe data close to the endwall but in the main annulus are combined with unsteady wall pressure information located inside the cavity. The data is used to validate an unsteady 60°-sector CFD. Using infrared based instantaneous wall temperatures on the rotor hub side endwall the purge flow distribution is evaluated and again accompanied by CFD-results. For the investigated purge flow rate, the instantaneous rim seal distribution is shown to be dominated by the identified cavitymodes leading to strong inhomogeneities between the single rotor passages.