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Aviation History
1957
1957 - 1873.PDF
20 December 1957 961 GAS TURBINE DEVELOPMENT Further Abstracts from Hayne Constant's Sir Henry Royce Memorial Lecture IN our first series of abstracts from Mr. Hayne Constant's Sir HenryRoyce Memorial Lecture, published last week, an account was given of some of the development work carried out at the National GasTurbine Establishment, Pyestock, on compressors and turbines. The next section of the paper (which was delivered before the Derby Branchof the Royal Aeronautical Society) was concerned with the improve- ment of turbine cooling to make possible the use of higher gas tempera-tures. After describing the initial approach to this problem, in 1946, the lecturer referred to the construction of an experimental cooledturbine and a cascade tunnel with cooled blades. After unrewarding work with free convective cooling and liquid cooling, it was decided thatan air cooling technique was the most promising. - THE first tests on an air-cooled turbine [continued thelecturer] were made in 1952 and the first runs with a meangas temperature of over 1,100 deg C in 1953. Since then much further work has been done on turbines operating under a variety of conditions (Fig. 6) and at higher gas temperatures. In parallel with the experimental work on the cascade rig and the air-cooled turbine, analytical work was needed to build up a supporting theory. At the same time subsidiary tests were done on a water-cooled turbine and on one whose blades were cooled by a water spray; and novel methods of constructing cooled blades, e.g., the use of lost-wax castings, were developed. All this was real pioneering work carried out some years in advance of anyone else, work which I think has resulted in our gaining a reasonably exact understanding of the problems of turbine cooling. (Refs. 9, 10, 11.) WhUe this work on the turbine was going on we spent some effort on the related problem of cooling the walls of ducts, such as the flame-tubes of combustion chambers. A criterion of cool- ing effectiveness was developed and this was used to compare the relative efficiency of a number of different cooling systems. As a result of this work we suggested in 1950 that the best practical method of cooling a duct was by a series of films of air, bleeding into the hot gas stream, fed from a chamber so jacketed as to give good convection cooling. We have since successfully used this combination of external convection cooling and internal boundary-layer or film cooling on a number of occasions. Combustion. The lecturer next turned to work on combustion carried out at Pyestock during the past 20 years, referring to analysis methods employed to isolate the various physical and chemical processes that occur in a combustion chamber. As examples he cited the study of the burning of individual droplets of fuel and the determination of ignition delays. The filling in of basic data on elementary combustion processes such as these [continued Mr. Constant] is gradually giving per- spective and balance to the view of the combustion engineer and enabling him to make his judgments on a quantitative basis. A good example is the success that has attended our efforts to define precisely the effect of linear scale in combustion chambers (Ref. 12). Our early work gave the first effective analysis of the significance of linear scale in high-speed combustion processes and showed how model tests could be used to simulate full-scale Fig. 6. Test rig for air-cooled turbine. performance. Later work by ourselves and others—notably Rolls-Royce—has extended our understanding by explaining the effects of fuel injection and heat transfer. For some years we have been trying with some success to make more rapid progress in increasing the rate of heat release from a chamber. The most recent and most notable advance here has been a chamber having a heat release of 6 x 106 CHU/cu ft/ atm/hr—if I may be permitted to use a rather unfashionable criterion—about three times that of current chambers, without a penalty being paid in other aspects of its performance. This great compactness has been achieved by building into a single chamber four novel features (Fig. 7). The first of these is the elimination of the compressor diffuser, thus shortening the length of the chamber. The secondary air is led straight through the chamber at high velocity to a sandwich mixer, in the cool-air passages of which lie the turbine nozzles. Mixing of secondary and primary air takes place in the nozzles, which are at the same time effec- tively cooled. Uniform peripheral distribution of fuel into the annular primary zone is effected by dispensing with the normal type of injectors and passing the fuel off the lip of a spinning disc. The turbulence in this primary zone and the strength of the stabilising vortex are increased by paddle wheels attached to this disc. This may prove to be a revolutionary step forward in com- bustion chamber design. Reheat. Work started in 1943 with an attempt to check by experiment the theoretical prediction that increased thrust should result from burning fuel after the turbine and before the pro- pelling nozzle. The first experiments were done in an air supply simulating, as we thought, the conditions in the exhaust pipe of a jet engine. These experiments led us to the conclusion that it would be possible to burn fuel in the exhaust in spite of the very high gas velocity. A Power Jets W.1A engine was then obtained and the first reheat combustion system was built into its exhaust pipe. The main feature of the system was the perforated pilot chamber formed as an extension of the exhaust bullet. Within this chamber were mounted a sparking plug for ignition and an upstream pilot jet. Downstream of the pilot chamber was a single main jet which directed a spray of fuel into the pilot chamber. Results were sufficiently promising to justify going ahead. Reheat was forced into the air before its time by the flying- bomb attacks on London in 1944. Although these attacks were repulsed before reheat could be demonstrated in action a signifi- cant improvement in aircraft performance was obtained. The Fig. 7 (left). Details of compact combustion system. Fig. 8 (below). Reheat installation in a Meteor, 1945.
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