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1 Introduction One of the principal requirements of military aircraft of the 1990's, which will operate with high performance, energy efficient engines, is to have superior engine performance over a wide range of flight conditions.
In any aircraft gas turbine engine, it is the flight conditions that have a predominant effect on its performance. It is well-known that the pressure and temperature vary considerably with altitude and ambient condition. This affects the temperature and pressure of the air entering the compressor of the gas turbine engine.
Thus, it would not be possible to design any engine for optimum performance at all flight conditions, since a conventional engine with a fixed design point will give optimum performance at that point only and not for the entire flight spectrum.
2. Design Considerations Hitherto, the practice has been to design the engine for optimum performance at a particular operating condition viz., sea level static at international standard atmosphere (iSA), and to select the engine parameters based on this operating condition.
Thus the ISA sea level static (SLS) condition was the design point which corresponded to a compressor inlet total temperature TI of 288°K and inlet total pressure Pa of 1013 millibar (14.7 psia). Consider now the increase of TI and PI due to varying flight ambient conditions as mentioned above.
An increase in P, is generally advantageous to engine performance, whereas an increase in T, causes a deterioration in performance. The correspondence of the design value of T, of 288°K to different conditions of the flight spectrum is shown in Fig. I.
This design value of inlet temperature is fairly well matched to the transonic Mach, numbers at high altitudes of flight and to the low subsonic Mach numbers at low altitudes of flight, and with the value of TI around 28g°K, the compressor operating point is maintained about the same as that at ISA SLS condition (Fig. 2). For compressor inlet temperatures.
TI lower than 288"K, the compressor performance would be better than that at ISA SLS. But when compressor inlet temperature TI is higher than 288"K, the compressor operating point would move away on the operating line from the design point. As a result, the air mass flow parameter W~%/P, and the pressure ratio P,/P1 would be inferior to the corresponding design point values.
The engine performance would thus be inferior to the design intent. It has been recognized that this fact has to be counteracted for high supersonic cruise at high altitude and for high subsonic operation at low altitude (both cases corresponding to high values of TI).
As a matter of fact, the contemporary high pressure ratio engines lose thrust, often rapidly. Under these conditions, this loss of thrust with increase in TI becomes significant in the case of bypass engines, with thrust drop increasing with increase in bypass ratio.
3. Concept
Now, if the engine cycle could be varied so that the engine operates in most of the flight spectrum nearer the design point, then significant improvement in engine performance could be expected. A variable cycle engine would be an ideal solution for this problem.
The thermodynamic cycle of the engine can be changed either by providing variable bypass ratios andlor by energy inputs in the different flow passages of the engine. The thermodynamic cycle of the engine is primarily dependent on the maximum cycle temperature and compressor pressure ratio as well as on component efficiencies, mechanical losses, etc.
In conventional engines that have to operate at off-design conditions, it has been the practice to maintain the maximum cycle temperature constant i.e. turbine entry temperature is constant at the design value. Consider now the case of high inlet temperature TI discussed above.
At constant maximum turbine entry conditions (under high TI) the corrected engine speed N/~K falls with corresponding drop in W ~/T/P,. It would, therefore, appear that there is a scope for restoring the cycle to operate at the design point, thereby recovering the thrust drop.
A comparatively simple and direct approach to the problem could be achieved by increasing maximum cycle temperature at the design (high TI)
condition
1. This increase in maximum cycle temperature under high TI condition is accompanied by increase in mechanical RPM to maintain the corrected RPM thereby keeping air mass flow parameter and pressure ratio same as that at ISA SLS condition.
In other words, the compressor is made to operate such that the operating point under high T, conditions is the same as that at ISA SLS condition; the operating point is aerothermodynamically retained
2. It is interesting to note that this concept of variable cycle achieved by varying the maximum cycle temperature has been recognised in recent days for the design of engines for combat aircraft with particular reference to supersonic cruise at altitude
3. The ratio of the maximum cycle temperature to that at ISA SLS has been referred to as 'throttle ratio' in recent literature. By employing the concept of high throttle ratio design, one can compensate the significant
thrust drop associated with high pressure ratio engines under high T, conditions. This results in a 'flat rated engine'.
The above concept can be applied both for high Ta pressure ratio straight jet engines and to a limited extent to bypass engines. In a conventional engine, with increase in TI under maximum engine operating conditions, the maximum cycle temperature will be essentially constant.
Hence as TI increases, for fixed maximum cycle temperature, the heat energy added will be less, whereas in the case of an engine employing high throttle ratio design, the maximum cycle temperature increases with increase in TI thus tending to maintain the input of energy.
4. GTX Engine Design
The GTX engine was conceived from the basic observation that if the overall pressure ratio can be retained and simultaneously the turbine entry temperature (maximum cycle temperature) is increased above the design point value, then with increase of TI, the available dry thrust can be significantly increased within the allowable aerothermodynamic limits of the components of the gas turbine engine.
Thus GTX 37-14U is a flat rated engine design based on an early and simplified approach to the variable cycle engine high throttle ratio concept with particular reference to the Indian operating requirements of good dry combat performance at low level, high speed t and high ambient condition.
The engine is a twin spool turbojet with a high compressor pressure ratio having a throttle ratio of 1.13. The performance of this engine and a conventional engine of throttle ratio of unity are compared in Figs 3 to 6. These figures show that both the corrected air flow and pressure ratio with forward speed are well below the design values and there is scope to make use of this underused capacity.
The simplest way would be to open the throttle and thereby increase the thrust. The throttle can be opened till the corrected air flow and compressor pressure ratio arerestored to the design values. This then is the background of the GTX concept.
The effect of throttle ratio higher than unity as in the case of the GTX engine is clearly seen from Fig. 7. Selection of throttle ratio is limited by the maximum cycle temperature which the turbine technology can permit. In the case of the GTX engine, the maximum cycle temperature is limited at present to 1450°K so as not to exceed the material limits of the turbine blades.