FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1961
1961 - 0327.PDF
FLIGHT, 16 March 1961 335 BETTER SMALL TURBINES SOME THOUGHTS BY THE TECHNICAL EDITOR DURING the past 20 years there has been some justificationfor the feeling that all turboshaft engines tend to fall intoone of two clearly defined categories. Those for aircraft usehave been designed ever more boldly, by adopting the highest possible pressure ratio and turbine entry temperature. Thus—without any modification to the basic thermodynamic cycle—engines such as the Tyne can demonstrate specific fuel consumption betterthan that of nearly all piston engines, not excepting even diesels. In contrast, non-aeronautical turbines appear to have been designedto ridiculously unambitious pressures and temperatures, so that their performance has been fundamentally uninspiring, even whenthe thermodynamic cycle has been improved by all sorts of inter- mediate heating and cooling. It is surely for this reason primarilythat the gas turbine has so far failed to win a place as a prime mover outside the field of aviation. The accompanying pictures illustrate some ideas which may go along way towards remedying the competitive performance of other- wise unambitious gas turbines; and they look particularly promisingfor small engines. No engineer working in this field needs to be reminded of the manifold shortcomings of the heat exchangers sofar developed in an attempt to bring down the s.f.c. of small turbo- shaft engines intended for automotive applications, where efficiencyunder part-load conditions is of the first importance. Engineers in France and Sweden have evolved improved hardware for a workingcycle which appears likely to live up to the encouraging claims being made for it. There are really three ideas involved; the Pouit by-passturbine, the Holmquist heat-exchanger and the SODIM turbine wheel. Pouit's By-pass Future gas turbines intended for extensive part-load running may be expected to have an "inner heat pump" evolved by the Frenchman Robert Pouit. Under small and varyingconditions of output torque, any desired proportion of the combustion air is automatically by-passed through a second heatexchanger, so that the compressor turbine can work in an approxi- mately constant gas temperature. The claim is made that, between40 per cent and 100 per cent power, the compressor-turbine entry temperature can be held at 800°C and the s.f.c. maintained at165gm/hr/CV, equivalent to O.3691b/hr/s.h.p. The need for a by-pass line of this type is essential in surface applications involving extensiverunning at low loads, but may not be worth while in light aircraft, which can readily be designed to cruise continuously at over 70per cent power. Holmquist's Heat Exchanger Ernst Holmquist of Gothenburgis a specialist in heat-transfer. His heat exchanger overcomes most of the crippling handicaps which have made such devices ex-cellent things not to have in an aircraft gas turbine. To the writer's knowledge, the Bristol Theseus was the only aircraft gas turbine toincorporate a heat exchanger; and it was dispensed with before the engine flew. Holmquist's idea may well bring such devices backinto favour, particularly in relatively small engines in which high cycle-efficiency is very difficult to achieve. The basic idea is wonderfully simple. The medium for heattransfer is a stack of 100 or more belts or sleeves made of very thin stainless-steel sheet. As the perspective sketch shows, the belts areof steadily increasing diameter, the increment being one or two millimetres. The concentric stack of sleeves is wrapped round acentral drum, with a diameter slightly less than that of the smallest Suggested circuit diagram for a small engine incorporat- ing Pouit's by-pass and Holmquist's heat-exchanger: A, compressor (I5°C in, 115 Cout); B, first heat-exchanger (500 C exhaust); C, by-pass valve (480 C); D, second heat-exchanger; £, by-pass duct ;F, combustion chamber; C, compressor turbine (800 C); H, power turbine; J, exhaust (135 C) sleeve, and held tightly at top and bottom by outer rollers. Thisdeforms the sleeves to an oblate form, with maximum separation between adjacent sheets on either side. One side is placed in theintake and the other in the exhaust, and the complete stack of belts is driven at any desired speed (by a toothed wheel, it is suggested,engaging in teeth cut along the edges of the bands). Advantages of this arrangement are its low weight and bulk, thefact that spacing between the bands can be extremely fine without incurring excessive pressure-drop in the gas flow and, above all, theelimination of gas leakage. The latter is prevented by the fact that the thin sheets enter and leave the gas ducts compressed to a solidpack, passing through sliding fits in graphite-lined slots lubricated by oil mist. Development problems do not appear to be insuperable;obvious queries concern component life, and in particular the behaviour of the thin belts under conditions of corrosion, or withingested particles (which could be hard or abrasive) in the gas stream. The concept has at least attracted the attention of suchrenowned companies as Rover in Britain, Hispano-Suiza in France and Chrysler in the USA; and Hispano in Paris are actually aboutto run a lOOhr endurance test with a Holmquist heat exchanger, with Inconel belts, for a 13,000 h.p. gas-turbine locomotive on order forthe SNCF (French State Railways). SODIM'S turbine It is appropriate to bring into this dissertationa brief reference to the turbine rotor developed by SODIM, a French company for the practical evaluation of new inventions. The typicalexample illustrated in the photograph consists of two discs bolted together to clamp 48 stumpy blades around their periphery. Coolingair is fed through the drive shaft and escapes between the discs and out through holes in the blades. Each blade is sintered in 87 percent aluminium and 13 per cent aluminium oxide, weighs about 0.18oz and costs Is 6d. The turbine shown, from a 120 s.h.p. auto-motive engine, runs at only 12,500 r.p.m. and blade temperature does not rise above 450°C in a gas temperature of 800CC. Thisparticular wheel has run 1,000hr on a fuel containing 1 per cent sulphur and 0.07 per cent vanadium, both of which are viciouslycorrosive on hot steel. Last year SNECMA began investigation into light and cheap wheels of this nature. Left, schematic diagram of a pair of Holmquist heat-ex- changers in series: A, intake duct; B, exhaust duct; C, stock of moving belts (only a few are shown, for clarity); D, central roller; E, pressure rollers; F, drive pinion The SODIM turbine wheel after running I.OOOhr in a corrosive gas at 8C0°C
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
