|Fig. 4-9 Annular combustion chamber.|
23. A combustion chamber must be capable of allowing fuel to burn efficiently over a wide range of operating conditions without incurring a large pressure loss. In addition, if flame extinction occurs, then it must be possible to relight. In performing these functions, the flame tube and spray nozzleatomizer components must be mechanically reliable.
24. The gas turbine engine operates on a constant pressure cycle, therefore any loss of pressure during the process of combustion must be kept to a minimum. In providing adequate turbulence and mixing, a total pressure loss varying from about 3 to 8 per cent of the air pressure at entry to the chamber is incurred.
|Fig. 4-10 Combustion efficiency and air/fuel|
25. The heat released by a combustion chamber or any other heat generating unit is dependent on the volume of the combustion area. Thus, to obtain the required high power output, a comparatively small and compact gas turbine combustion chamber must release heat at exceptionally high rates.
26. For example, at take-off conditions a Rolls- Royce RB211-524 engine will consume 20,635 lb. of fuel per hour. The fuel has a calorific value of approximately 18,550 British thermal units per lb., therefore the combustion chamber releases nearly 106,300 British thermal units per second. Expressed in another way this is an expenditure of potential heat at a rate equivalent to approximately 150,000 horsepower.
27. The combustion efficiency of most gas turbine engines at sea-level take-off conditions is almost 100 per cent, reducing to 98 per cent at altitude cruise conditions, as shown in fig. 4-10.