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Aviation History
1942
1942 - 2165.PDF
OCTOBER 15TK, 1942 FLIGHT 4*9 the turbine-compressor combination for use with jet propul sion, many projects of various countries having already been discussed in this series of articles. So far as these major components, the rotary compressor and the gas turbine, are concerned, when used for propul sion they are, of course, much larger and more complicated. But with an already complicated multi-cylinder petrol engine, the necessary supercharging arrangements, either i;ear or exhaust driven, add further complication. It' is readily apparent, therefore, comparing a turbine-compres sor unit with a complete normal type four-stroke engine, that cost, complication, man-hours in production and par ticularly weight can, or should, be very materially reduced in the case of turbine plant. Considered from the viewpoint oi power generation, these claims are important. But schemes employing gas turbines for motive power in com bination with normal type airscrews are the main burden of these notes, and examples are illustrated and referred to later. Research on units for aircraft propulsion is divided be tween steam and gas turbines. A review of steam turbine projects appeared in these columns on April 30th and July 30th last. For example, the Great Lakes Aircraft Corpora tion of America has produced a design of a propelling unit developing 2,300 h.p., in which the turbine is said to rotate at 20,000 r.p.m., and is actuated by steam at 70 kg./sq. cm. and 535 deg. C. The unit weighs 1.6 kg./h.p. More recent developments should enable that creditable power/ weight figure to be improved upon. At the moment the tooling condenser remains the heaviest part of a steam- propelled unit, and attention is now being given to the pos sibility of utilising the surface of the wings for cooling, and at the same time provide a simple solution of the problem of de-icing. Constant Volume or Constant Pressure Research on the combustion gas turbine has followed two schools of thought. One advocates combustion of fuel on the " constant volume " cycle and the other on the "constant pressure" cycle. In the first, for instance, the Holzwarth turbine, the combustion of fuel and air occurs in closed chambers, each of a "constant volume," and the resultant gases are released through valves and nozzles to impinge on the blades of the turbine rotor. Energy is thus supplied intermittently and at varying pressure and the approach to continuity is by multiplying the number of combustion chambers In the second system, typified in Brown-Boveri designs, the compressor delivers air con tinuously to a combustion chamber into which fuel is con tinuously sprayed arid the resultant gases reach the turbine nozzle ring at " constant pressure." There is a compromise system which, at least at the present stage of development, possesses certain attractive Fig. 4. Experimental Brown-Boveri axial compressor with adjustable pitch blading. possibilities. A reciprocating diesel engine is very highly supercharged by a compressor driven directly from the engine crankshaft. The engine is proportioned so that all the mechanical power delivered is utilised in driving the supercharging compressor. Effluent gases from the engine cylinders drive the turbine and provide the sole source of useful energy of the complete unit. This, incidentally, is tantamount to an admission that in a reciprocating engine more energy escapes by way of the exhaust pipe than is delivered at the engine shaft. The Swiss firm of Sulzer Bros, has made great progress with this system for indus trial power installations and has supercharged diesel engines to operate at the remarkably high mean effective pressure of 225 lb./sq. in. It is not essential to employ a conven tional engine and compressor and, in fact, some advantage may be gained by diesel operated, free-piston compressors of the Junkers or Pcscara types. These reciprocating units have no connecting rods or crankshafts. The two opposed engine pistons each carry a compressor piston and the motion is cushioned by air in coaxial compressor cylinders. Only a light linkage is necessary to synchronise the piston motion. Recent Developments In recent years, knowledge acquired by aerodynamic research on gas flow and airfoils has enabled more efficient rotary components to be built. The blades of modern compressors and turbines are of a carefully designed airfoil section, as will be seen in Fig. 2, illustrating the turbine rotor of the 2,000 h.p. Brown-Boveri set for a main line locomotive. Concurrently, better heat resisting steels have been developed and turbines can now be operated continuously with a gas admission temperature of 1,000-1,100 deg. F. Thus the respective efficiencies of the complementary com ponents have been raised and the overall efficiency improved. There are reasonably good prospects of further substantial progress in both components and some advan tage can be gained by compounding and employing auxiliary heat exchange equipment. Of course, an axial blower with fixed blading has only a limited range of air delivery volumes. With adjustable rotor blades, however, it could be regulated to deliver any volume from zero to maximum, and pressure from zero to maximum at constant speed. Such an innovation would be as important as the variable pitch airscrew has proved to be in normal aircraft propulsion. Fig. 4 shows the rotor of an experimental three-stage, high-speed blower of this type, built by Brown-Boveri for test purposes. The blades can be adjusted between o deg. and 45 deg., whilst run ning, and normal maximum air delivery is 6,395 cu. ft. per minute at a pressure of 2.1 atm. (30.9 lb./sq. in. absolute) at 7,800 r.p.m. .Gas Turbines in other Fields As an independent power unit, the gas turbine may be regarded as in its infancy. Already, however, it can com pete with diesel or steam plants in certain industrial applications where low first cost, small space occupied, and ability to be brought into immediate operation are im portant requirements. This applies with particular emphasis where liquid or gaseous fuels are available and water is scarce or unobtainable. They are used to generate electric power and for driving blowers for blast furnaces and chemical plants. Mention has been made of a 2,000 h.p. gas turbine locomotive built for the Swiss Federal Railway (see Fig. 3), and projects are at present under consideration for a 4,000 h.p. marine propulsion set driving through a two-stage reduction gear. In America, it was reported at the end of 1940, the Allis Chalmers Company built sets of Brown-Boveri type for generating electric power up to 6,000 kW. (approximately 8,000 h.p.) in the oilfields. Several such sets " have been operating almost constantly at full load since 1936." To-day, Brown- Boveri are able to build gas turbine electric generating plant for outputs up to 10,000 kW. (13,400 h.p.). The overall thermal efficiency of these industrial sets is from 20 to 22 per cent for the simplest types. By
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