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Aviation History
1946
1946 - 0361.PDF
FEBRUARY 2IST, 1946 IRCRAFT . . . rubbing surfaces and only two or three shaft bearings, lubrication is greatly simplified. It is possible to start a turbine from cold and take off in less than a minute at full throttle and the engine kept at maximum rating. The only critical factor is the temperature of the gas flow at the turbine, indicated by a thermometer on the instru- ment board. The overall thermal efficiency, that is the heat value of the power delivered as thrust, expressed as a per- centage of the heat value of the fuel consumed, is often adversely criticised by those who quote figures on a pinjts-per-hour basis. If considered in relation to dis- ftce traversed and weight carried in a given time, such a ton-miles comparison is favourable to gas turbines. At speeds of 500 m.p.h the simple turbine/jet is as economical as a TURBINE ROTOR piston engine ; with multi-stage com- pressors an increase of compression EXHAUST CONE GAS UNIT IN OPERATION.TURBINE/JET A DOUBLE-SIDED centrifugal compressor delivers air to the combustion chambers, into which kerosene is injected and burnt continuously. The heated air and combustion products force their way through the blades of the turbine rotor, so creating mechanical power necessary to drive the compressor. ratio is possible with enhanced economy. Heat ex- changers also offer great possibilities in this connection. At high altitudes in regions of low temperature, gas turbines operate more efficiently. Component efficien- cies, too, are improved by the ram effect of air entering at high forward speed. Again comparing the two engine types, the constantly varying processes of induction, compression, expansion and exhaust take place in one organ, the cylinder, whereas a turbine does not have this handicap as the processes are carried out in separate specialised components which can be individually studied. DERWENT FEATURES Having grasped the main differences in the cycle of operations of a four-stroke and a turbine engine, the reader should now study the sectioned diagram of the Rolls-Royce Derwent I turbine/jet unit which has been coloured to illustrate the functions. This design is based upon the designs of Air Comdre. Frank Whittle, R.A.F., and Power Jets, Ltd., with the important change from reverse flow to straight through ' combustion chambers evolved by the Rover Co. at their Barnoldswick factory and developed in association with M.A.P. and Joseph Lucas, Ltd.* Rolls- Royce took over turbine development work from the Rover Co. in April, 1943, to supplement their own active research instituted in 1938. The turbine/jet unit illustrated incorporates at the front a centrifugal compressor having an impeller about 21 in. diameter with twenty-nine radial blades on each side. At the other end of the shaft is a single-stage axial flow tur- bine. The main shaft is carried in three bearings. Two of the ten combustion chambers grouped around the shaft are shown in section. Air is induced on both sides of the rapidly spinning impeller and fed past the diffuser necks to the combustion chambers into which kerosene is pumped through burners as a finely atomised spray. The supply of fuel is governed by a throttle valve controlled by the pilot. An automatic barometric control is also fitted to reduce the fuel supply to the burners at altitude. When starting up, the fuel is ignited by two ignition plugs, thereafter the burning mixture maintains con- tinuous combustion. Combustion Chamber Design Each combustion chamber consists of an outer casing in which is arranged a co-axial flame tube furnished with two series of holes to admit primary combustion air and dilution air respectively from the annular space between the two elements to the interior of the flame tube. The air casing is capped by a connect- ing pipe flanged to both the casing and the diffuser neck of the compressor. At its oppo- site end, to permit thermal expansion, the casing makes a sliding joint with the discharge nozzle leading to the turbine guide vanes. The flame tube is located from the casing by three radial supports in the region of the primary combustion zone, and at its rear end is guided with freedom to expand in the end of the casing. Two of the three radial supports are in the form of tubular stubs to receive the short pipes by which the casings of all ten chambers are interconnected. Socketed on the forward end of the flame tube is a perforated domed cap known as the "colander." Probably coined in the workshop, this designation has now passed into official use. In the centre of the colander is a sleeve to receive the fuel burner and around the sleeve a ring of vanes to promote a helical swirling of air around the fuel spray. A perforated inner cone extends from the swirl vanes to the junc- tion of the flame tube. The function of the swirl vanes is to ensure a continuous flow of air with an organised turbulencB to the atom- ised fuel whilst the colander, lowering the velo- city and damping out any pulsation, provides locally stabilised ambient conditions and avoids any possibility of the flame being " blown out." The work of compression raises the air tempera- ture from, say 15 deg. C. to 205 deg. C, and through the first series of holes in the flame tube the air feeds into the interior to maintain an air-fuel ratio of approximately 18 to 1. The second series of holes admits the so-called dilution air which increases the fuel-air ratio to about 60 to 1 and reduces the temperature from 1,800 deg. C. in the combus- tion zone to 850 deg. C. for delivery to the turbine. Heat is expended in the passage through the turbine and the jet temperature is approximately 690 deg. C. Connecting the combustion chambers is a series of balance pipes to equalise the pressure and allow the flames * See Flight, January 3rd or loth. 1946.
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