ISTP-16, 2005, PRAGUE 16TH INTERNATIONAL SYMPOSIUM ON TRANSPORT PHENOMENA

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Abstract The paper discusses the possibility of flow field analysis in high-speed measurements by combination of different measurement techniques. The information about velocity field is obtained by means of Particle Image Velocimetry, which is supported by the schlieren method. The measurements are performed on a transonic turbine blade cascade.

1 Introduction Particle Image Velocimetry (PIV) is a very

popular technique, which is being used with success in low-speed measurements. The quality of the measured data depends on the ability of seeding particles to follow the flowing fluid. However, this ability decreases with increasing gradients of velocity. If the PIV method is to be used in compressible flow with shock-waves (with abrupt changes of velocity) the question how the movement of seeding particles corresponds to movement of fluid arises. Therefore, it is needed to verify such data from PIV by some different technique. One way how to verify PIV data is a comparison with schlieren photographs.

2 Experiments

2.1 Experimental Equipment All tests have been performed in a small high-speed wind tunnel of the laboratory of the institute of Fluid Dynamics and Power Engineering at CTU in Prague. The tunnel consists of test section with model and vacuum

tank, the volume of which is 10m3. Due to the limited volume of vacuum tank, time of measurement has been less then 10 seconds.

Fig. 1. Test section of wind tunnel

As the tested model a turbine blade cascade has been used. Flow conditions in this model are apparently distinguished by large areas with significant dissipative processes.

THE QUALITATIVE ANALYSIS OF FLUID FLOW IN TRANSONIC BLADE CASCADE

Jan Slanec*, Milan Matějka*, Kateřina Vališová*

* Czech Technical University in Prague, Faculty of Mechanical Engineering, Corresponding author: Jan.Slanec@fs.cvut.cz

Keywords: blade cascade, high-speed, shock-wave, PIV, schlieren method

Jan Slanec, Milan Matejka, Katerina Valisova

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Fig. 2. Tested blade cascade in test section

The model of blade cascade has following parameters: • chord 50 mm • span 30 mm

2.2 PIV system The measurement has been carried out by PIV system produced by Dantec which consists of: • two Nd –YAG lasers - New Wave Gemini

15 Hz , 120 mJ. • CCD camera Dantec HiSence PIV,

1280 x 1024 pixel • PIV processor Dantec FlowMap1500, 1 GB

buffer

2.3 Schlieren device Schlieren device made by Zeiss has been employed. Mercury discharge lamp of 500 W power has served as a source of light. Visible area is a circular one of 80mm in diameter.

3 Experiments

3.1 PIV measurement As the first step for obtaining information about flow field the visualization by Particle Image Velocimetry was used. Type of seeding particles was important for obtaining good results by this method. To avoid problems of fouling of vacuum tank, solid particles couldn’t be used. Water droplets have been used instead. The choice of these seeding particles wasn’t ideal because of high density of water, but only water

droplets have sufficient size for investigated area.

Fig. 3. Water droplets as seeding particles

Because the operating time of the high-speed wind tunnel in supersonic regime was very short (less then 15 seconds), the PIV data had to be obtained during several operating cycles. Repeatability of measurements was guaranteed by keeping the same starting conditions of the wind tunnel.

Fig. 4. PIV visualization – supersonic regime

On the Fig.4, the PIV visualization of velocity field in supersonic regime is shown. This map of velocity is only qualitative information about flow field. It can’t give relevant information

THE QUALITATIVE ANALYSIS OF FLUID FLOW IN TRANSONIC BLADE CASCADE

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about values of velocity, because it represents statistics from only few PIV frames.

Fig. 5. PIV visualization – subsonic regime

In comparison with subsonic regime (Fig.5) the PIV visualization on Fig.4 shows different structure of flow field. On Fig.4 there is marked area with decrease of velocity, which is missing on Fig.5. It is supposable that this decrease of velocity is caused by occurrence of shock-wave.

3.2 Schlieren visualization For obtaining information about shock-waves the schlieren method has been used.

Fig. 6. Schlieren photograph

Fig.6 is one of the schlieren photograph. This photo is one of the sequence of photographs,

which have been taken by CCD camera with frequency of 8fps.

Fig. 7. Sequence of schlieren photos (0.5 second)

This sequence of pictures (Fig.7) enables to observe an oscillatory movement of shock waves. However, the recording frequency isn’t sufficient to evaluate the frequency of movement described.

Fig. 8 Power spectral density – supersonic regime

Jan Slanec, Milan Matejka, Katerina Valisova

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Fig. 9 Power spectral density – subsonic regime

In order to determine the frequency of shock-wave oscillations, the spectral analysis of sound emissions has been done. On Fig.8 and Fig.9 there are shown power spectral densities of sound, which has been recorded during the measurement. This analysis didn’t detect unique main frequency and didn’t give satisfactory information about frequency of shock-wave movement.

3.3 Comparison of PIV and schlieren data On Fig.4 and Fig.5 are shown different velocity distributions in flow field for subsonic and supersonic regimes of the wind tunnel. It is supposable that this difference is caused by occurrence of shock-wave.

Fig. 10. Comparison of PIV velocity map and schlieren photograph

Comparison of PIV visualization of velocity field in supersonic regime and schlieren photograph is on Fig.10. On this picture is demonstrated, that area with decrease of velocity really corresponds with location of oscillated shock-wave.

4 Conclusions Although only the simple configuration of PIV tests has been used, the PIV visualization has been able to give basic information about velocity distribution. From the comparison with schlieren photographs it is visible, that it’s possible to find information about shock-waves from these data, too. These experiments have been done to obtain basic quantitative information about flow field that have been able to use to preparation of more detailed and accurate (and heavy on time and money) measurements. For this purpose the simple performance of experiments has been fully sufficient and the comparison with schlieren photographs has been very useful for correct interpretation of PIV data.

Acknowledgements The research was supported by the FRVŠ under the project No. G1/1969.

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[3] Knob M, Slanec J, Šafařík P, Nožička J, Sobolík P. The Coanda Effect in a Compressible Fluid Flow. Colloquium Fluid Dynamics 2003, Prague: Ústav termomechaniky AV ČR, pp 59-62, 2003