AEROSPACE
holes on the gas turbine blade, and determine whether they can achieve any improvement in the efficiency of the film cooling system,’ said Johnson. ‘As the technology develops, perhaps they will be able to use fewer cooling holes on a turbine blade, which could possibly even lead to some amount of acceleration in the manufacturing process.’ As an extension of conventional PIV, a new
technique adds an ultraviolet laser to measure the gas phase temperature and velocity simultaneously. Dr Frank Beyrau and researchers at Imperial College London developed this high-speed thermographic particle image velocimetry technique, which was introduced at Optical Instrumentation for Energy and Environmental Applications conference in November.
Exciting techniques Previously, simultaneous application of optical techniques for thermometry, such as Raman or Rayleigh spectroscopy, has proven to be challenging due to the presence of the PIV particles. Te method that Dr Beyrau developed uses tracer particles made from
thermographic phosphor material. In addition to the dual cavity pulse laser as used in standard PIV, an ultraviolet laser excites these optically active particles and causes them to emit temperature dependent phosphorescence. Te two lasers emit different colours, allowing the two light sheets to be overlapped. Te cameras in the system then capture the spectrally resolved particle phosphorescence, and the pictures can be converted into a temperature image to provide the gas phase temperature. ‘Tis will, for example, give an insight into how cool you can keep the air close to turbine blades, which has been difficult to study in the past. Tis is what we are going to investigate,’ said Beyrau. Te team are using diode-pumped solid
PIV will provide aviation companies with a more efficient way of testing
Thermographic
speeds,’ Beyrau remarked. Tis technique is now being developed so that it can be used commercially. Standard PIV is already widely available and is a shop standard technique used to measure flow fields in reactive and non-reactive flows. If this new technique becomes the standard, it will provide aviation companies with more extensive data relating to the cooling effectiveness. ‘It will, in the near future, be possible to buy such a system commercially,’ said Beyrau. By providing high-speed
simultaneous thermometry and velocity measurements, thermographic PIV will provide
state lasers at 355nm and 532nm, which makes it possible to measure the temperature and the velocity in a planar field, at a multi-kilohertz repetition rate. ‘It is the first time that such a technique works at high
Lasermet_LSE_NovAd_213x130mm.qx8_(v) 14/11/2013 17:30 Page 1
Modular, Active and Passive Laser Safety Enclosures CERTIFIED TO EN 60825-4 AND ANSI Z 136.1 FOR ALL LASER POWERS AND WAVELENGTHS
aviation companies with a more efficient way of testing the performance and efficiency of cooling holes. In addition, development in laser design and capability will decrease the need for post-process testing and improve the ability to create idealised shapes. Both will ultimately lead to advancements in the design and manufacture of more fuel-efficient jet engines.
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ISSUE 21 • WINTER 2013 LASER SYSTEMS EUROPE
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