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Aviation History
1952
1952 - 0092.PDF
36 FLIGHT BOOSTING GAS TURBINES Why it is Desirable and How it is Done: Water-Methanol and other Injectants By W. T. GUNSTON WHY is water-injection a desirable feature of gas-turbine design ? Quite apart from the obvious advantage of having an increased power output available for emergencies, it is fair to say that most turbines are cruised nearer to their maximum output than are most aircraft reciprocating engines. One evident result is that, in a tur bine, there is a smaller reserve of power available when it is most needed. Some people will be inclined to disagree strongly on this point, but the fact remains that with a turbojet or turboprop there is less "in the bag"; it is being used already. Consequently, it is doubly * important to provide some means of increasing the power rating for emergency use, even if it is only available for short periods. Afterburning, or jet-pipe reheat, is one solution that is becoming popular for fighters. It is undoubtedly of value for turbojets, particularly in very fast aircraft. It carries, however, a heavy weight and space penalty, and has a further drawback in that the whole operation of a turbojet so equipped is a compromise in which the cruising condition y usually suffers. A very interesting approach to the problem is seen in the Double Mamba, which, in its application to long-range naval aircraft, may be said virtually to embody a complete spare engine for take-off, combat and emergencies. It cannot be denied, however, that a power-boosting system capable of being (almost) clapped on to an installed power plant has a great appeal, and in this field water injection has an apparent monopoly. How does water injection work? Considering the basic Newtonian P=Mf equation behind every turbojet (and turboprop), it is clear that increased output can be obtained only by increasing either the total mass-flow or the mean gas acceleration. Continuing with the formula?, many readers will, at some time or another, have had drummed into them something like: Power required=^^ RWTi (ft/Pi) "~-1 They may even recognize it as referring to air compressors. The important part here is the factor 7*i which shows that the power required to drive an air compressor is usually dependent directly upon the ambient intake-air temperature. This fact is the key to the whole business. One of the simplest ways of cooling the intake air is to inject into it a spray of volatile liquid. Cooling is then achieved by reason of the fact that the latent heat of vaporization required to convert the spray into vapour <-> 36°I"S. I 1—I—1—I oc A O \ $ 280 Vt; UJ 4-4' 4-2 40 3-8. z 0 <«o r$« hi 0 0 tfoolp p2 £ooj<;5 Fig. 1. Effect upon compressor perfor mance and tail-pipe temperature of a varying injection rate -.General Elec tric 1-40 unit, with injection of pure water. INJECTION RATE (qal/min)OF WATER MOST of us are familiar with the use of water and other liquids as anti-detonants in piston engines. We know, also, that there is no detonation problem in the gas turbine ; and some of us are puzzled by the fact that "wet" ratings are becoming as common on this new prime mover as they ever were on its reciprocating predecessors. Why this is so, and how water can make such a difference to the output of a gas turbine, is explained here. is extracted from the air. From this a number of effects are likely to result. If the power taken by the compressor is to remain con stant, the better pressure ratio P2/P1 across the compressor will result in increased mass-flow and jet velocity; thus we have increased both M and /, since in any given engine / is proportional to final velocity. For an unchanged fuel consumption, this will give a "weaker mixture" and will reduce operating temperatures throughout. Alternatively, increasing the intake-air cooling will eventually result in a decrease in compressor power requirements and a corre sponding rise in jet-pipe temperature. Again, it may be necessary to burn more fuel in order to vaporize all the cooling liquid and heat the steam to combustion temperature; this extra fuel is partly wasted. In actual practice, of course, all these factors are inter dependent and with varying intake-cooling the overall result is interesting. Some experimental results from the U.S.A. are shown in Fig. 1. Cooling liquid injected into the intake- air will begin to evaporate at once, at a rate principally determined by the spray-droplet size and the air turbulence. As the spray passes through the compressor, more and more of it will evaporate, until there is no liquid left; some liquids will vaporize completely before the compression stage is over. This will result not only in "continuous intercooling," but also in a progressive increase in volume-flow and hence in both flow-velocity and acceleration. There will also be a reduced temperature-rise in the compression stage and, therefore, a better pressure ratio. If all the cooling liquid can be made to evaporate before entering the compressor, the resulting increase in pressure ratio will be a maximum, but this ideal is possibly unattainable. For any given engine condition, the proportion of the spray evaporating before the compression stage will be dependent upon the boiling point of the liquid injected; clearly, this should not be greatly above the intake-air temperature at the ram pressure pre vailing. The ideal cooling liquid will have a high specific heat and (more important) a high latent heat of evaporation and low boiling-point, as well as a high calorific value. It will be non-toxic, non-corrosive, will leave no deposit when burnt, and will be cheaply obtainable in a pure state. There is, unfortunately, no such liquid. A number, however, approxi mate to this ideal, but all are deficient in one or more properties. Nevertheless, these possible coolants are worth considering individually; their chief properties in this con nection are shown qualitatively in Fig. 2. Water.—The most readily available coolant, water also has by far the greatest latent heat and specific heat. Unfor tunately, its boiling-point at usual ram pressures is consider ably above ambient temperature, so that it is difficult to evaporate large amounts ahead of the compressor. Further more, as its calorific value is zero, it merely tries to put the fire out. One interesting effect, more useful on "heavy engineering" gas turbines, is a tendency towards the water- gas equation, 'C+HiO=CO+Hi, reducing carbon forma tion. Methanol.—Methyl alcohol, CHiOH, is very attractive. Its latent heat is less than half that of water, but is still high. It also has a lower boiling-point and a high calorific value. In actual practice, when neat methanol is injected only about half actually burns, but it still gives one of the
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