Turbine blade implants measure at 32,000g.

By combining modern micro-machined silicon technology
with 19th Century type vitreous enamel, an Oxford University
research group has come up with a means of mounting
sensors on turbine blades which does not interfere with
gas flow.

True turbulent flow conditions inside a turbine operating in a realistic
manner can now be accurately monitored for the first time, unhindered
by possible interactions between sensors, their leads and the gases
rushing past.

 Mounting a turbine rotor in a test rig.

The shaft mounted electronics associated with testing. This will lead to the ability to model fluid flow under conditions of extreme turbulence much more accurately and confidently than has been possible up to now. And it also demonstrates a neater, more effective way of building in pressure, temperature and other types of sensors into test rigs, than is normally the case. Fluid flow under extreme conditions of turbulence,-those to be expected in turbines, hydraulic valves and most other situations of real engineering interest- have so far proved impossible to model fully on computers. The problems are caused not so much by lack of computing power, as a mathematics which does not lend itself to solution with conventional machines. One Oxford physicist is even now engaged in developing a totally new kind of computer to try to solve the problem but until he succeeds, proper mathematical modelling and design of systems involving highly turbulent flow still has to be based on painstaking experimental measurement. Micro-machined silicon pressure sensors bonded into a turbine blade. Nowhere is this more true than in the case of gas turbines, and for some time now, Roger Ainsworth and his team from the university's Department of Engineering Science have been making full scale measurements of turbine gas flow behaviour in what used to be the old Oxford Power Station. They have a test bed, which allows 1 .6m3 of air at 115 lb/in2 (8 bar) to be exhausted through a single 2ft diameter rotor and stator in 0.2s (representing a transient power output of 2MW). It would all be in vain were it not for the effort the team has put into finding a way of building sensors and circuitry into the turbine blades without distorting gas flow. Both pressure and temperature can be measured, by making use of the properties of vitreous enamels of the same types as used on domestic cookers. In the case of temperature sensors, the blades are enamelled before platinum resistance gauges and gold leads are painted on, using four coats of 'Hanovia Liquid Bright' from Englehard. The blades are then fired at 640°C, a process which results in measurement sensors and leads being endowed with an overall resistance of about 50ohms. But even more remarkable are the measurements with micro-machined silicon pressure chips from Kulite. First, a coating of vitreous enamel is laid down and fired to provide an insulating layer, followed by wires, in a machined groove. Then follows a second coating and firing followed by milling to expose the wires. Fifteen Kulite micro-machined silicon pressure sensors are then stuck in the groove with epoxy and connected up to provide a ring of sensors capable of withstanding 32,000g. Personally developed by the president of Kulite Semiconductor products, Anthony Kurtz, the chips use piezo-resistive strain gauge elements diffused into a flexing diaphragm bonded onto glass. Most remarkable from Roger Ainsworth's point of view has been Kulite's ability to turn round new chip designs in no more than a week. The project has been referred to by Geoff Bancroft, Kulite's British managing director as the "Jewel in their Crown", both because of the importance they are giving it and the jewel-like look of the sensors mounted on the blade. 'Explosive' acceleration The turbine test rig is also of more than passing interest. Invented in its original form in the early 70s by Professor Jones to test stationary cascades of turbine blades, the team have modified it to incorporate a fully rotating turbine stage, supported by Rolls-Royce and SERC. The driving reservoir of air is compressed by a piston in a cylindrical chamber, 5.5m long and two feet in diameter, pressed on its outer side by compressed air from cylinders. The air in the reservoir is then held in place by a rotary gate valve of 30 segments, across the entire cross-section of the cylinder. The turbine is initially run up to 6,500rev/min by an air motor, whereupon a perspex cylinder is shattered by a detonator to release the gate valve, which until then has been held open against compressed air at 65 lb/in2 (4.5 bar). The valve opens fully in 40ms and in the subsequent 200ms, the turbine accelerates by a further 3,000 rev/mm. Data is grabbed from 8 of the 32 sensors as the turbine passes 8,400 rev/mm via slip rings, gathering signals which have already been pre-amplified by electronic circuitry built into the turbine shaft. The bearing have to be preloaded during the run up, since they were designed to run only under load, and the outward side of the turbine is initially evacuated to 0.1 lb/in2 (7 mbar), a pressure which rises to 0 lb/mm2 (0.7 bar) by the end of the run. The same embedding and painting techniques using vitreous enamel could also be applied to the painting or implanting of strain gauge and other sensors and the whole method is of general application to the mounting of sensors on test rigs, whether to maintain unhindered gas flow or merely to protect them. The ideas can be further extended to the production of vitreous enamel on steel-printed circuit boards. Such items have been manufactured experimentally but are not in production because of micro-cracking problems which appear in long-term use - particularly in the domestic cooker environment for which they were originally intended. _________________________________________________________ From an article by Tom Shelley, published in the magazine 'Eureka' April 2001

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