AEROSPACE
Iowa State University in the USA to develop technology that can test and improve cooling strategies for gas turbine engines. Te technique uses lasers to carry out particle image velocimetry (PIV) to measure the turbulent flow as it leaves the cooling holes on turbine blades. PIV is becoming a standard technique for
studying turbulent flows, but applying it to turbine blades poses its own challenges. At the university, Dr Blake Johnson and Dr Hui Hu have set up a mock film-cooling test rig inside a wind tunnel where the PIV system is applied, consisting of a Quantel EverGreen dual-pulsed laser and a frame-straddling digital camera. GE prescribes test scenarios for the team who then report their results back to GE. A typical experiment consists of a test plate
mounted in a wind tunnel filled with smoke, with air flowing through at a rate of 30 metres per second. Two pulses of laser light illuminate the particles in the smoke, at two intervals in time, which are then captured by a camera. Te two high-speed images, taken with a specified time gap between them, are then analysed to determine how groups of particles have travelled between the first and second image. ‘It is possible to observe how the cold air
moves through the hole and mixes with the hot stream of gas above it,’ explained Dr Blake Johnson, post-doctoral research associate at Iowa State University. ‘Te concentration of coolant gas against the mainstream gas can lead to a measurement of film cooling effectiveness.’ Te experiment is repeated using different parameters, such as
Example of temperature image from thermographic particle image velocimetry, showing the temperature and velocity field from a heated jet
hole spacing ratio, to determine the best way the holes can be distributed in order to achieve the greatest cooling performance. Te dual-cavity pulsed lasers that are used in this technique have a flash rate of about 15Hz. Using lasers that have a repetition rate in the kilohertz range would yield high speed measurements. However, these lasers tend to have a lower energy per pulse, which limits the field of view. ‘It would be nice to see a high-energy
high-pulse rate laser for PIV – that could be very helpful,’ said Johnson.
Stretching the imagination A sheet of laser light is generated to illuminate the wind tunnel. Tis involves focusing the laser into a very small concentrated beam, and then stretching the beam horizontally by using cylindrical lenses. Te optical components need to be secured tightly and in the correct position so that no misalignment is caused by vibrations in the system. Tis can be a lengthy task, according to Johnson: ‘Sometimes it can take one to two weeks to get the system lined up correctly and bolted down securely so you’re sure it will be reliable from one day to the next.’ To reduce the set-up time, they are using a LaVision laser light arm, which consists of an enclosed series of tubes attached to the outlet aperture of the laser. Te rotatable joints on the end of these tubes contain high-energy mirrors, which make it possible to deliver the laser beam from any direction, so it then can be spread out using a pre-positioned cylindrical lens to give a sheet of laser light. ‘Te purpose for us is to provide idealised
Experimental set-up for thermographic particle image velocimetry, developed by Dr Frank Beyrau at Imperial College London
16 LASER SYSTEMS EUROPE ISSUE 21 • WINTER 2013
film-cooling measurements. GE will ideally try to develop some simulation codes that can replicate our results,’ explained Johnson. ‘Once they confirm that their codes are sufficiently robust to do that, they will apply their codes to more complicated technologies related to film cooling. ‘GE’s goal ultimately is to optimise the geometry and distribution of film-cooling
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Dr Frank Beyrau, Imperial College London
Dr Frank Beyrau, Imperial College London
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