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A cooler way to fly

How can you take the temperature of a jet engine? At the Optical Instrumentation for Energy and Environmental Applications conference in the USA in November, Dr Frank Beyrau from Imperial College London outlined one technique: high-speed thermographic particle imaging velocimetry, which uses lasers to measure gas-phase temperatures, potentially for investigating cooling performance in jet turbine engines. His work highlights the increasingly important role of lasers – in the manufacture of more efficient jet engines, and also in research to monitor and improve that efficiency.

The demand for more fuel-efficient jet engines is growing. In September, the European aeroplane manufacturer Airbus forecast that more than a third of aircraft to be produced over the next decade would be fuel-efficient replacements for older, inefficient planes. Growth in air travel, rising fuel prices, and environmental pressures have all combined to increase the requirement for more fuel-efficient engines.

Monitoring temperatures, as Dr Beyrau’s technique is intended to do, is vital because new engines are being designed to run at higher temperatures – in order to improve thermal efficiency and power output, and to reduce pollutants such as nitrogen oxide gases. However, combustion temperatures can often exceed the melting point of the metal alloys used in the turbine engine. To overcome this, lasers are used to produce hundreds of cooling holes in the turbine blades, shaped specifically for heat removal. This allows cold air from the compressor of the engine to keep these parts at a temperature where they remain stable and can still operate.

Advanced cooling allows for higher operating temperatures, which helps drive efficiency and extend part life and performance. Matt Benvie of GE Aviation remarked: ‘Our advanced designs and current new product introduction (NPI) engines are relying heavily on shaped cooling holes that are generated by laser processing. The laser allows for tailoring of shapes for each region or component without high consumable cost, as well as generating those shaped features in short cycle times.’

Pulsing for precision

According to GE Aviation, the drive towards more shaped holes will require post-process inspection of the shapes to ensure the required performance of the hole. Higher-quality laser processing would allow for reduced inspection of the laser generated shapes.

‘The growth of diode pumped lasers, both fibre and disk laser designs, will improve both the performance and stability of the beam quality, and the ability to have better control of pulse widths and frequencies, which should improve the capability to create shapes that are closer to idealised designs,’ explained Benvie. ‘These aspects should help to reduce post-process inspection and also reduce or eliminate cleaning of laser generated by-products. Pulsed disk lasers have some very interesting promise in this respect, but are probably still several years away from introduction into the value stream.’

Richard Baxter, global capital systems and marketing manager for Winbro Group Technologies, said: ‘It’s all about using new technology to produce these turbine engine blades more rapidly and in a more efficient way.’

In addition to the holes, further thermal protection is provided by a thermal barrier coating (TBC), but this ceramic layer makes the holes more difficult to drill. A high-peak pulsed laser can be used to drill through the ceramic coating and create the hole on the turbine blade. However, if the peak power is too high, the thermal shock of the beam hitting the TBC can cause cracking or even removal of the coating. This is particularly apparent in the case of percussion drilling, where the laser stays in the same position to deliver a number of pulses to the material until the hole is finished. For manufacturing companies that produce parts for turbine blades, this type of drilling could result in failure to meet the strict standards laid down for these engine components.

Reducing thermal schock

Laser ablation removes the TBC at the location of the hole in a controlled manner, without damaging the surrounding coating. This process employs a fibre laser, which has a much lower energy and much shorter pulses than a traditional high peak pulse laser used for drilling. The TBC is broken down a thin layer at a time so there is reduced thermal shock to the material. Laser ablation can also be applied to the re-opening of cooling holes that become blocked during re-coating of the engine parts after repair.

According to Winbro’s Baxter: ‘The materials of the engine parts are changing in that they are coated with TBC. So, we have had to develop ways of machining those materials, which laser ablation does very effectively.’

As it happens, laser ablation is also a good way of creating both two- and three-dimensional shaped cooling holes. A three-dimensional model of the shape is first designed, and then the shape is recreated on the surface of the engine part, by taking away the material layer by layer. ‘Laser ablation is a more efficient way of producing shaped cooling holes on turbine blades. Shaped holes allow cool air to be spread over the surface of the part more effectively,’ said Baxter.

Manufacturers such as Winbro produce machines that can combine laser ablation and drilling processes into a single machine. So, on a turbine blade, a fibre laser can be used for removal of the TBC and creation of 3D shapes, and a high-power pulse laser can then drill the holes. ‘This makes the whole operation more efficient as it eliminates alignment issues of switching between two processes,’ said Baxter.

Measured approach

Laser technology is also useful in researching how the geometry and distribution of cooling holes can improve cooling performance. GE Global Research is funding researchers from the aerospace engineering department at Iowa State University in the USA to develop technology that can test and improve cooling strategies for gas turbine engines. The 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. The 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. ‘The concentration of coolant gas against the mainstream gas can lead to a measurement of film cooling effectiveness.’ The experiment is repeated using different parameters, such as hole spacing ratio, to determine the best way the holes can be distributed in order to achieve the greatest cooling performance. The 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. This involves focusing the laser into a very small concentrated beam, and then stretching the beam horizontally by using cylindrical lenses. The optical components need to be secured tightly and in the correct position so that no misalignment is caused by vibrations in the system. This 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. The 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.

‘The purpose for us is to provide idealised 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 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. The 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. The two lasers emit different colours, allowing the two light sheets to be overlapped. The 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.

‘This 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. This is what we are going to investigate,’ said Beyrau.

The team are using diode-pumped solid 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 speeds,’ Beyrau remarked. This 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 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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