Danger: blast area

Novel optical sensors that can withstand the extreme environments within an explosion or a jet engine simulation rig are now being developed in the UK.

Observing explosions close-up is not advisable for humans or conventional sensors. Yet, without a better understanding of what happens at the heart of a blast, research into making everything from buildings to jet turbines safer is likely to stall. What are needed are sensors that can survive in the ultimate danger zone.

Novel optical sensors that can withstand the extreme environments within an explosion or a jet engine simulation rig are being developed by a team at Heriot-Watt University, Edinburgh, with EPSRC funding. The emerging technology could help researchers understand in more detail what happens in confined spaces when there is an explosion – whether controlled, accidental or deliberate – as well as provide engineers with important insights into the effects of turbine design on jet engine performance and wear.

EPSRC Advanced Research Fellow Dr. Bill MacPherson and Dr. Jim Barton are working on two aspects of fibre optic sensors that could be used in two very different but equally extreme environments. The sensors could provide information that is otherwise almost impossible to access.

According to Dr. Barton, air-borne blast waves from high explosives are difficult to measure experimentally. There is little reliable experimental data on how explosions occur in confined spaces and how corners and gaps in everyday structures affect the blast.

More detailed measurements could lead to much better models. “These would improve our understanding of blast damage,” explains Dr. Barton, “and aid in the design of structures that minimise the damage, such as in buildings, military vehicles, industrial plant, and body armour.”

Dr. Barton, working in the Fibre Optics Group at Heriot-Watt University and with colleagues at the University of Sheffield, is devising miniature fast response pressure sensors to get inside explosions.

“Blast measurement introduces challenges because the sensor must be very robust!” he says. “By the nature of explosive air-blasts there is a likelihood of airborne debris travelling at very high speed.” In collaboration with Dr. Andy Tyas, Department of Civil Engineering, University of Sheffield, and Dr. Richard Allen of the Defence, Science and Technology Laboratory at Fort Halstead, the team hopes with EPSRC and MoD funding to overcome the various problems associated with such sensors. “For explosives research it’s beneficial if the sensors can be low cost – due to the potential damage they might incur,” comments Dr. Barton.

“Conventional sensors concentrate their cost at the measurement point, so sensor damage can be costly, however fibre sensors have the expensive interrogation system remotely located, therefore loss of the sensor is, in principle, less costly.” He points out that economies of scale will cut costs further when it becomes possible to use mass production techniques such as micromachining.

To this end the team is also working with the Central MicrostructureFacility at the Rutherford Appleton Laboratory and the Scottish MicroelectronicsCentre, in Edinburgh, on the microfabrication of the sensor bodies.

The new sensors being developed have very low mass diaphragms as the pressure-sensitive element. This diaphragm vibrates as the pressure wave hits it and the movement is detected by a variation in the light signal reflected back along the optical fibre to which the sensor is attached. Conventional electrical pressure sensors in explosives research are larger, resulting in sensitivity to vibration which can appear as a spurious signal in blast tests.

“An additional bonus of fibre sensors is the absence of electrical leads and long wire connections which need careful screening against electromagnetic pick-up, a particular issue when setting up electrically-detonated explosive tests,” adds Dr. Barton.

Systematic trials will allow the researchers to record the variation in pressure at the surfaces of model structures in an enclosed space as an explosion progresses. Because of the small size of the fibre optic pressure sensors a range of scale models will be tested to allow the scientists to investigate the effects of a blast and improve computational models for military, civil, and industrial structures.

The second harsh environment in which these sensors could be used is in gas turbines, such as those found in aircraft engines. Aerodynamic flows in gas turbines can exceed the speed of sound as well as becoming hotter than the melting point of the engine components, requiring active cooling. “Clearly engine design is critical for efficiency and low pollution emission,” says Dr. Barton, “however, although computational techniques are excellent at predicting complex flows, there is still a requirement to validate these predictions experimentally.”

The problem is, of course, any conventional sensor placed in such a harsh environment will quickly be destroyed. Many of the necessary validation experiments on new turbine blades can be done in wind tunnel test facilities under slightly less harsh conditions that simulate real engine conditions. The flow velocities are similar but the temperatures used are hundreds of degrees lower, which gets around the damage problem.

Optical fibres provide the solution to getting the sensors into a vantage point, says Dr. Barton. “The advantage of optical fibres is that they can be routed to the point at which the measurement is to be made,” he explains. The researchers also point out that the sensors must be tiny and so capable of being embedded in small jet engine components. “We’ve placed sensors on the trailing edge of a blade less than 1mm thick,” adds Dr. Barton, “we’ve also now installed miniaturised probes that perturb the flow less than conventional-sized probes might.”

The turbine engine sensor research is being carried out with Professor Terry Jones of the Department of Engineering Science at the University of Oxford, and Kam Chana of Experimental Programmes at QinetiQ Pyestock. “The Holy Grail of turbine engine instrumentation is a sensor that could survive combusting flow, at temperatures approaching 2000 degrees Celsius,” says Dr. Barton. Direct measurements in test engines rather than simulation rigs could then be done allowing efficiency losses to be pinpointed accurately for better engine design.

There are two further types of sensor being developed. The first uses multicore fibre: Unlike conventional fibre which carries light in only one central core, this fibre has typically two to four separate cores, which opens the door to multi-parameter/multisensor probes by allowing different sensors to be integrated on the end of a single fibre. For example, a diaphragm pressure sensor can be combined with a thin film temperature-sensitive coating on a single fibre end.

Multicore sensors are only just being investigated but offer plenty of potential in both the explosive and turbine applications.

Dr. MacPherson is investigating a second branch of the optical sensing technology that could offer its own advantages over conventional optical fibres. He hopes to learn how photonic crystal fibre (PCF) technology might be exploited in optical sensor systems. He is building on existing links with Bath University, which is at the forefront of PCF fabrication in the UK and Dr. MacPherson is undertaking a systematic investigation into the use of PCFs for optical sensing for environmental, structural, and multi-parameter measurement. PCFs, he points out, could offer far greater control over the fibre properties than is available with conventional doped fused silica fibres, greatly extending the type and performance of possible sensors. In particular, these materials could lead to miniaturised sensors (for engineering research and structural monitoring) and high power beam delivery for full field measurements and spectroscopic chemical detection schemes.

This article has been reproduced from the Summer issue of ‘Newsline’ by kind permission of the EPSRC.

On the web