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Aviation History
1942
1942 - 2204.PDF
440 FLIGHT OCTOBER 22ND, 1942 AIRCRAFT PROPULSION SYSTEMS (PART II) Fig. 9. Two types of Brown-Boveri generators for the continuous pro duction of combustion gases. at about one-third the turbine rotor speed. The torque delivered by the turbine is necessarily transmitted wholly to the compressor impellor and, as no stationary reaction members intervene, the same torque will be passed to the output rotor. In proportion to the energy required to compress the air the speed of the output rotor is reduced. After passing the output rotor the air enters a frusto- conical combustion chamber G into which fuel is injected by nozzle H. The chamber is built up of overlapping shrouds, as shown in the detail section (J in Fig. 8) thereby furnishing longitudinal, tangentially directed air inlets. Excess air flows between the chamber and the casing and enters the chamber by way of the tangential slots to mix with the combustion gases which reach the turbine inlet at a lowered temperature. The gases enter the turbine through a stationary ring of guide blades K, which are made hollow for cool ing purposes. At the centre of the turbine rotor the gases, entering radially, are guided into opposite axial directions by a suitably shaped flange member L of a heat-resisting steel. Separate Unit Combustion Chambers It will have been noted that the diagram matic sketch of the 2,000 h.p. locomotive set, Fig. 3, showed a combustion chamber which is • virtually a separate unit interposed between compressor and turbine. A component of this type might well be used as a central gas genera tor for two wing-mounted turbine units driving airscrews. Two such generators of combustion \J3 gases, both of Brown-Boveri design, are sketched in section in Fig. 9. The one having the inner lining formed of a series of nesting conical sections is of the type actually em ployed on the locomotive. They are con- Fig. 10 structed of sheet metal, the requirements being an inner casing capable of withstanding without deformation a temperature of about 1,800 deg. F., * and an outer shell supporting the pressure of the com pressed air and combustion gases. At the present stage of development this pressure will probably be not more than 45-50 lb. /sq. in. They can be mounted in any position. Air from the compressor is delivered to the annular space around the axially disposed fuel injection nozzle A and the necessary air for combustion enters the inner casing through a ring of guide vanes B which impart a helical swirling motion to ensure intimate mixing with the atomised fuel. Usually only 20-25 per cent, of the air is required for combustion and the remainder is utilised to lower the temperature of the combustion gases to 900-1,000 deg. F. prior to admission to the turbine. At the same time it serves to conserve heat by forming a high velocity layer between inner casing C and outer shell D and avoiding losses by radiation. Air flow is indicated in the sketch and at each peri pheral slot formed by the nesting sections some of this excess air flows into the inner casing to form a boundary layer, protecting the casing from heat and * eventually mixing with the gases. The main mixing • to produce the prescribed driving gas is effected in the terminal space E. Corrugations F in the outer shell and inlet and outlet ducts serve to accommodate ex pansion and avoid the stressing of the structure. In the second sketch, Fig. 9, is a plain cylindrical combustion unit embodying another feature further to reduce radiation loss. To avoid the transmis sion of heat from inner casing G to outer shell H a cylindrical screen J is inserted to divide the cooling space. The contact surface giving up heat to the cooling air is thus increased by the area of both inner and outer surfaces of screen J. Providing the cross- sectional area of the cooling space is maintained several such screens may be employed, suitably spaced concen trically to one another. Alternatively, a single screen may be longitudinally corrugated, as in the detail section K in order to increase the area of the contact surface. As indicating the efficiency of this device, the following approximate particulars are cited for a combustion chamber 39m. in diameter supplied with air at a pressure of 44 lb. sq. in. and a temperature of 300 deg. at a velocity of 66ft. per second and burning mazut, a highly radiating, heavy residual oil. Without a screen the temperature of the inner Baynes-Muntz layout of a multi-screw turbine large, high-altitude transport aircraft. plant
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