ExecSummSR064

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AIRFOIL TRAILING EDGE COOLING

Final Technical Report

February 1, 1998 to July 31, 2000

Principal Investigators:

Satish Ramadhyani, Michael W. Plesniak, and Patrick B. Lawless

Graduate Research Assistants:

Aaron Brundage and Neal Venters

Purdue UniversitySchool of Mechanical Engineering

Maurice J. Zucrow Laboratories

West Lafayette, IN 47907-1288

Contract No. 98-01-SR064

Clemson University Research FoundationSouth Carolina Institute for Energy Studies (SCIES)

Advanced Gas Turbine Systems Research (AGTSR)

Clemson, South Carolina 29634-5181

January 2001


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Executive Summary

The objective of this project was to develop an understanding of the fundamental

physical processes that determine heat transfer rates at the trailing edges of gas-turbine

vanes, and, through this knowledge, to provide insight into the optimal balance betweenheat transfer and aerodynamic performance. Through a combination of experiments and

analyses, we considered various parameters including the trailing edge geometry, and the

Reynolds and Mach numbers of the freestream. The focus of the study was on the region

within 20% axial chord upstream of the trailing edge.

An existing wind tunnel was modified to facilitate the investigation of the trailing

edge heat transfer in a 5X-scale model of an advanced-design vane geometry provided by

Rolls-Royce. A new test section was fabricated, along with several instrumented models

over which we recreated the actual pressure gradient and Mach number distribution, as

well as the appropriate boundary layer parameters. Design, fabrication, and performance

testing of this facility involved significant effort. Instrumentation of the models to obtain

quantitative heat flux information in this region also proved quite challenging.

Heat flux measurements were made with Vatell Corporation model HFM-7 E/L

heat flux microsensors over the final 87-93% axial chord of the test article. These gages

have the advantage that they directly measure the wall heat flux and are not subject to

conjugate heat transfer effects. However, they were too large for applications past the

93% chord location. Miniature heat flux gages (TSI Inc. model 1471), driven by an IFA

300 constant temperature anemometer, were used to measure from 93-99% axial chord.

Although the test articles were fabricated from low-conductivity ceramic (Rescor 310-

M), the conjugate heat transfer (conduction from the sensors to the model) was large

compared to heat transferred by convection. Surface static pressures were acquired over

the entire test article, along with surface temperature, to confirm their streamwise

development along the model. Wake profiles of total pressure were obtained with a pitot

probe rake.

Companion computations were performed using the Fluent software suite on an

IBM RS 6000 workstation. Inviscid (Euler code) calculations were done to design the

wall shape for the desired pressure gradient distribution, as well fully viscous

computations to compute the surface heat transfer. Resolution of the extremely thin


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boundary layers was a major limitation for the viscous computations. Compressible

boundary layer computations were also performed, using an in-house code 2DEBLC.

Extensive benchmarking was done for all of the computational tools.

Information on aerodynamic loading for both pressure and suction surfaces was

obtained from measurements of static pressure along the surface of the test article. These

results were compared with a one-dimensional, isentropic analysis of the test section

flow, as well as a two-dimensional, viscous flow solution obtained using Fluent for

computational fluid dynamics. The pressure ratio measured experimentally and that

predicted by the viscous flow CFD solution differed by less than 5% in the 80-100%

axial chord region. Although there was a slight disparity in Reynolds number between

the two was less than 15%. Total pressure losses were quantified using a rake of total

pressure probes placed in the wake of the trailing edge. The total pressure losses were

confined to a flow region within the thickness of the test article.

As a result of the heat flux measurements, fundamental data were obtained to

quantify typical values of convective heat transfer coefficient in the trailing edge region

of a first stage turbine vane flow. In the 85-93% axial chord region of the internally

heated airfoil test article, time histories of the surface temperature and heat flux were

measured using the embedded heat flux microsensors. From the surface temperature and

heat flux data, heat transfer coefficients were computed. The heat transfer coefficient

steadily decreased along the suction surface. On the other hand, the magnitude of heat

transfer coefficient on the pressure surface remained roughly the same at all three chord

locations.

In the 93-100% axial chord region, platinum thin film sensors were mounted in a

different airfoil test article containing a low thermal conductivity trailing edge material.

The thin film sensors were operated at a specified temperature using a thermal

anemometer circuit, and the power needed to maintain the sensor at each operating

temperature was used to determine heat flux. In addition, a thermal resistance network

analysis was used to provide an estimate of correction needed to account for conjugate

conduction effects. Experiments revealed that the power supplied to the thin film sensors

increased dramatically toward the trailing edge. The increase in power input to sensors in


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this region was reflected in the acute increase in heat transfer coefficient there. However,

conjugate heat transfer was quite dominant, even at these downstream sensor locations.

In summary, the primary results of the research program included the

development and benchmarking of a versatile facility for the testing of vane elements at

operating Mach numbers, with the correct pressure gradient and boundary layer

characteristics. In addition, companion computational capability was developed. The

project contributed to the technical training of two graduate students at the M.S. and

Ph.D. level, as well as that of undergraduate assistants.

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