Test Data AC6-06
Gas Turbine nozzle cascade
Application Challenge 6-06 © copyright ERCOFTAC 2004
Overview of Tests
In order to have experimental results useful to validate turbulence and transition models implemented in CFD codes for a highly loaded turbine guide vane, different gas to wall temperature ratio, Mach and Reynolds numbers should be simulated. Different inlet total temperature, total pressure and mass flow should be provided.
In this regard the facility is made up of three parts: a 5 metres long and 1 metre diameter cylinder, the test section and a 15 m3 downstream dump tank. The cylinder is isolated from the test section by a slide valve. As the piston is pushed forward, the gas located in front of it is nearly isentropically compressed until it reaches the requested pressure and temperature levels. The fast opening of the valve allows the pressurised and heated gas to flow through the test section providing constant freestream conditions (total temperature, total pressure and mass flow) until the piston completes the stroke. The free-stream gas conditions can be varied between 300 and 600 K and 0.5 and 7 bar respectively.
The downstream dump tank allows a static pressure adjustment in the range of 0.15 to 3 bar allowing independent selection of Mach and Reynolds number. The typical test duration is 400 ms.
The test section is 250x100 mm2 wide. The blade has been designed by means of an inverse method for a nominal isentropic 0.9 mach number and is mounted on a linear cascade made of 5 profiles and 4 vanes. The central blade has been instrumented for both pressure and heat transfer measurements.
Free-stream total pressure and temperature, static pressure and turbulence intensity were measured 55 mm upstream the leading edge plane. The total pressure is measured by means of a Pitot probe connected to a variable reluctance Valydine differential pressure transducer, the total temperature by a K type thermocouple probe. Upstream wall static pressure is measured by means of tappings connected to National Semi Conductor differential pressure transducers.
Wall static tappings were also installed downstream of the cascade, in a plane parallel to the TE and located 16 mm downstream the LE itself. They covered a distance of 130 mm (more than two pitches) to verify the downstream periodicity of the flow field and to determine the exit Mach number to the cascade. Blade velocity distribution were obtained from 27 static pressure measurements performed along the central blade profile and referred to the upstream total pressure.
The downstream loss coefficient evolution and the exit flow angle were obtained by a traversing mechanism, transporting a Pitot probe over two pitches.
Local wall convective heat fluxes were obtained from the corresponding time dependent surface temperature evolutions, provided by platinum thin film gauges painted onto the central blade, made of machinable glass ceramic. The wall temperature/wall heat flux conversion was obtained from an electrical analogy, simulating a one dimensional semi-infinite body configuration. The convective heat transfer is computed as the ratio of the measured wall heat flux and the difference between the total freestream and the local temperatures.
One of the most important parameters in this investigation is represented by free-stream turbulence which can be completely defined only by means of two parameters: intensity and spectrum. In the CT-2 facility these measurements represented a challenge due to the nature of the flow which provoked difficulties in the calibration procedure of the hot wire probe. The free-stream turbulence is generated by a grid of a spanwise oriented parallel bars (d=3mm; s/d =4). The intensity is varied by a displacement of the grid upstrem of the blade model. In this way a maximum level of 6% can be obtained starting from a natural turbulence of the facility of 1.0 %. The turbulence intensity is measured using a constant temperature hot wire probe while the freestrem turbulence spectrum is obtained processing by means of a Fast Fourier transform analysis the raw signal coming from the hot wire probe.
The uncertainty on the various measured quantities is ±0.5% on pressure measurements, ±1.5K on temperature measurements, ±5% on heat transfer coefficient, ±0.2 points on the integrated loss coefficient and ±0.5 deg on the exit flow angle.
A comprehensive set of experimental data is available for comparing blade pressure profiles, heat transfer distributions, downstream loss coefficient and angle distributions. The experimental data available are listed in Table 1 , 2, and 3.
|Re2||M2is||Total Pressure (bar)||Data Available|
Table 1- Blade Velocity Distribution Data
|M2is||Tu%||Total Temp (K)||Total Pres (bar)||Wall Temp (K)||detailed data|
Table 2- Blade Heat Transfer Results
Table 3-Downstream Loss Coefficient and Angle Distribution
|Loss & Angle||LOSS_ANGLE.dat|
Table 3- Summary of Measured Parameters & Available Data-Files
Blade Velocity Distribution
Description of Experiment
For this particular set of experiments isentropic Mach number distributions were obtained for different loading from local static pressure measurements. Mach number is referred to the upstream total pressure. The central blade of the cascade was equipped with 27 static pressure tappings whose location is shown in Fig 2a-b, each of them is connected to a National Semi Conductor differential pressure transducer. The low pressure ports were connected to a vacuum pump to allow a regular verification of the calibration characteristics. Different experiments have been performed for different values of Mach number and total inlet pressure while the Reynolds number has been kept constant. The data are reported in file VELOCITY_DISTR.dat, where for each test case the measured static pressure in bars is reported for each tap on suction and pressure side as a function of the curvilinear abscissa on the blade profile (in mm). The total inlet pressure, Re and Mach numbers characteristic of the simulation are reported under the test reference number while taps locations are listed in the file PRESSURE_MEASURE_TAPS.dat.
The uncertainty on the various measured quantities, the frequency response of the measurement chain and the sampling rate are the same described in 2.1. The repeatability of the results remained within 0.5%. The influence of turbulence intensity and Reynolds number are not considered at this stage. The useful testing time was of the order of 450 ms.
Blade Velocity Pressure measurements: tappings measurements of the static pressure for 27 location on the blade profile: 20 on the suction side 1 on the TE and 6 on the pressure side (Fig 2). The results are reported in the file VELOCITY_DISTR.dat.
Blade Heat Transfer Results
Description of Experiment
For this set of experiments heat transfer distribution were obtained for different Mach and Reynolds numbers and freestream turbulence intensities by means of 45 platinum thin films whose location is shown in Figure 2 and listed in file PRESSURE_MEASURE_FILMS.dat. The latter were painted on a machinable glass ceramic blade replacing the central profile of the cascade.
The frequency response of the measurement chain associated whit the thin films (gauges, analogs, amplifier) is above 1kHz. The sampling rate was selected to be 1Khz and signals were filtered at 800 Hz. The useful testing time was of the order of 300 ms. The repeatability of the results was remained within 1%. All tests were performed for un upstream total temperature of about 415-420K.
Heat Transfer Distribution: The final heat transfer values for the 45 thin films used, 27 of which located on the blade suction side (Fig 2), are reported in the file HEAT_TRANSFER.dat. The heat transfer coefficients have been computed from measured heat flux and wall temperature reported in file HEAT_FLUX.dat.
Downstream Loss Coefficient and Angle Distribution
Description of Experiment
For this set of experiments the downstream integrated loss coefficient distribution (area averaged) was obtained as a function of the isentropic exit Mach number. The measurements were performed for three different reynolds number.
The uncertainty on the loss coefficient was estimated to be 0.2. The general level of the losses, measured for 1% freestream turbulence, is quite low in the subsonic region. This is explained by the late transiction observed for all configurations with mach below 1.0.
Loss and Angle Distribution:
Loss and angle are reported in file LOSS_ANGLE.dat.
© copyright ERCOFTAC 2004
Contributors: Elisabetta Belardini, Francesco Martelli - Universita di Firenze