Session: 15-05 Rotating Heat Transfer 2

Paper Number: 81291

81291 - Experimental Investigation of Local Heat Transfer in a Rotating Two-Pass Cooling Channel Using the Transient Thermochromic Liquid Crystal (TLC) Technique 

In an effort to increase the overall efficiency of aero engines, the turbine blade cooling air consumption of modern engines is being constantly reduced. However, with decreasing cooling air mass flow rates, the influence of rotation on the flow and thus the heat transfer distribution inside rotating cooling channels increases. Future cooling system designs will be more and more dependent on the understanding of these rotational effects.

A test rig for the investigation of rotating turbine blade internal cooling channel configurations is presented. Local heat transfer distributions are evaluated using the transient thermochromic liquid crystal (TLC) technique. The rig can be operated at rotational speeds of up to 900 rpm, mass flow rates between 3 g/s and 30 g/s, fluid temperatures between -100°C and +80°C and fluid pressures of up to 10 bar. This allows for a broad range of possible test conditions. The investigated cooling channel model is a two-pass leading edge configuration manufactured out of Perspex. It consists of a first pass with a trapezoidal cross-section and radial outward flow, a 180-degree bend, and a second pass with a rectangular cross-section and radial inward flow. The suction side and pressure side surfaces are equipped with 60° ribs.

Air cooled with liquid nitrogen is used as test fluid while the Perspex model is initially kept isothermal at ambient temperature. This way the heat flux is in the correct direction, i.e. from a warm channel wall to a colder cooling fluid, as required to reproduce the correct sense of the buoyancy forces. Precooling of the air supply pipes inside the rotor prevents the test air to heat up excessively before reaching the test model. This is essential especially for low mass flow rates. A radio telemetry system collects temperature and pressure data from inside the rotating model housing and allows real time monitoring.

The evaluation method is based on the measurement of TLC indication times. A sudden fluid temperature change is applied to cool down the TLC-coated heat transfer surfaces. This induces a TLC colorplay which is captured using cameras that are co-rotating with the test model. Assuming 1D heat conduction inside a semi-infinite wall, heat transfer coefficients are then calculated from the TLC indication times and the fluid reference temperature measurements. The validity of semi-infinite wall assumption is verified in preliminary tests using thermocouples positioned in different depth inside the Perspex wall.

The data evaluation procedure and the results of a typical experiment are presented. Heat transfer data are shown in contour plots of the Nusselt number distribution. Furthermore, a method to directly compare a rotating with a corresponding non-rotating experiment using normalized Nusselt number ratios is shown. Finally, data reduction methods yielding line-averaged values as well as histograms of the normalized Nusselt number ratios are described.

Presenting Author: Christian Waidmann Institute of Aerospace Thermodynamics (ITLR)

Presenting Author Biography: Research assistant at the Institute of Aerospace Thermodynamics, University of Stuttgart

Authors:

Christian Waidmann Institute of Aerospace Thermodynamics (ITLR)
Rico Poser Institute of Aerospace Thermodynamics (ITLR)
David Gutiérrez de Arcos Institute of Aerospace Thermodynamics (ITLR)
Michael Göhring Institute of Aerospace Thermodynamics (ITLR)
Jens von Wolfersdorf Institute of Aerospace Thermodynamics (ITLR)
Klaus Semmler MTU Aero Engines AG
Bernhard Jäppelt MTU Aero Engines AG