SOUE News Issue 6

Jet Engines, and Plastics

David Witt

The Jenkin Lecture was preceded by two half-hour "research talks".

In the first, David Gillespie told us about work on improving the efficiency of jet engines by reducing the leakage through the radial gap between turbine discs and the surrounding case. Any seal has to be capable of withstanding the high temperatures involved, and changes in the radial gap due to differential expansion, ovality and gyroscopic distortions. But a mere 1% reduction in specific fuel consumption would permit 15 extra passengers on a long-haul flight, and much larger reductions than this were conceivable.

A century ago, Charles Parsons devised the "labyrinth seal" for the shafts of his steam turbines, which works by creating a tortuous path that leaking gas has to find its way past. Research in Oxford had been started 15 years ago by Terry Jones, and had progressed first to the "bristle seal" (a sort of brush with alloy steel bristles) and then to the "leaf seal". This has a large number of thin metal leaves projecting from the circumference of the turbine wheel and rubbing against a stationary ring. The leaves are straight in the axial direction, but bent into involute form in the plane of the turbine wheel. The latest version has proved very successful, less fatigue-prone than its predecessors. Samples were passed around for the audience to inspect.

In the second lecture, Paul Buckley talked about "The place of plastics in sustainable engineering". His aim was to defend plastics, or more specifically polymers, against the rather bad press they seem to have had recently. He pointed out that polymers need much less energy to create and form them than metals do, and although metals have some advantage over polymers where tensile strength and stiffness are concerned, the situation is reversed when one is more interested in their bending properties.

About 30% of polymer use is for packaging, and there has been a lot of progress here in "doing more with less". For instance, plastic bags used to be 50 µm thick, but are now made down to 10 µm, an advance which has required a lot of research. And we were invited to compare the weight of a PET bottle, e.g. for lemonade, of about 100 µm wall thickness, with that of its glass alternative. Some of the improvements in this area come from molecular alignment, achieved by forming components rapidly and then quenching them to stop re-alignment.

Better material properties can also be got by using composites. Some fibre-reinforced materials are familiar, but there are others less well-known. For instance, polypropylene, reinforced with glass fibre, can withstand 100% strain. Fibres have a high surface-to-volume ratio, which gives them their strength, but then so do very small particles. Paul instanced Montmorillonite clay, which consists of platelets 10 Å (1 nm) thick, adhering to each other electrostatically. By adding 5% of this, the Young's Modulus of nylon-6 can be increased by 70%, which is better than can be got with glass fibres.

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