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Aviation History
1946
1946 - 0029.PDF
JANUARY 3RD, 1946 FLIGHT y COMBUSTION RESEARCH chilled and the reactions brought to a stand- still. Subsequent analysis then shows, as nearly as possible, the composition existing at the sampling point at the time of with- drawal. Main gas analyses are carried out on the Haldane apparatus, which permits an accuracy of ±0.02 per cent. Complications are introduced when samples are taken from the primary combustion zone as it is possible to have unburnt fuel and "cracked '•' gaseous products as well as combustion gases. A special analytical technique and the neces- sary apparatus has been devised to meet these conditions. Air distribution through the chamber is the subject of detailed study. It is intimately dependent upon flow conditions from the compressor, and particular care is taken in the design of the delivery duct. Pulsation, surging or shock from the compressor delivery' would, of course, radically affect combustion conditions. A special traversing apparatus developed in the laboratory for investigating the airflow at the compressor outlet. This enables yawmeter, pitot- static tube and temperature traverses to be carried out at any position in the passage. The first measurement is of yaw in two perpendicular planes. When this is ascer- tained a pitot tube having the appropriate droop is set up with the correct yaw. "Pin-cushion" Models Velocity diagrams obtained by this system of airflow measurement are visually demonstrated by so-called "pin-cushion" models. In a base block of shape and area corresponding to the section of the delivery passage, wires of a scale length to show velocity are set at the appropriate angle to indicate flow direction. In the case of a reverse flow in turbulent conditions this is repre- sented by a loop. Optical methods of investigating airflow are also being developed. It is intended to observe flow lines in a cascade box fitted with Perspex sides by means of a Cascade and exhaust traverses on de Havilland Ghost chamber. has been Rolls-Royce Nene chamber on test. Schlieren set-up and to record them with an ultra-high- speed camera. Similar traversing equipment .is employed to deter- mine pressure and temperature at the opposite end of the chamber, where the gases are delivered to the nozzle guide vanes "of the turbine. The total pressure loss through the chamber is thus readily ascertainable. Combustion efficiency may be defined as Actual Temperature Rise Theoretical Temperature Rise It will be appreciated that extremely careful measure- ments of air supply, fuel supply and assessment of calorific value are necessary to obtain an accurate esti- mate of the theoretical temperature rise. Traversing instruments carrying thermo-couples are used to deter- mine the actual mean temperature at both the inlet and outlet of the chamber. All the apparatus was produced in the Lucas workshops. At the discharge end the gases have a mean temperature of about 800 deg. C. and a velocity of the order of 5OO-6ooft./sec. It is, of course, essential that combustion be complete before reaching the turbine nozzle ring. Flame, unburnt fuel, or a stratified flow at different temperatures would have most detrimental effect on the turbine blading. Mechanical design and methods of fabricat- ing chambers and equipment are developed complementary to the combustion research. A wide range of materials has been tested for chamber components. Flame tubes are of Inconel or Nimonic 80 nickel-chrome alloys, whilst the outer casing is usually of mild steel. In view of the thin sections, the character of the material and the severe operating conditions, special welding tech- niques had to be developed.' Apart from obtaining satisfactory combustion perform- ance, it is obviously most important to extend the reliable working life of the components. Already the life of flame tubes in fully de- veloped engines is in excess of two hundred hours. This was well in advance of the German units in service at the end of the war. (Concluded on page 15.)
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