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Aviation History
1952
1952 - 0093.PDF
11 January 1952 37 greatest boosts for a given percentage injection. Its partial combustion does result in increased temperature at the turbine and tail-pipe, and its use must be carefully regulated. In combination with water, it forms an extremely attractive combination, in which the best of both worlds (boiling-point and latent heat) are obtained, quite apart from the anti freeze provided by the methanol. Ammonia.—Pure ammonia, NH3, is a gas at the pressure and temperature found in the usual air-intake, and thus there is no boiling-point problem at all. Further, among available liquids, it has a latent heat second only to that of water and it dissociates in the combustion chamber to yield hydrogen, which partly burns to give a fair overall calorific value. At first sight it might appear ideal for the job, and certainly can give startling results, but it, too, has its draw backs. Ammonia vapour with air can form a violently explosive mixture; it also has dangerously offensive proper ties, both to human beings and to copper-base alloys (it may meet these alloys in turbine bearings). Finally, its use is rather limited by atmospheric conditions in that ammonia gas, when combined with water, liberates heat; thus, on a rainy day, the heating may cancel out the cooling. Paraffin.—The great advantage here is that there is a ready supply available. Unfortunately, paraffin has so high a boiling-point that, taken in conjunction with a mediocre latent heat, its cooling effect is insignificant. Its famous scouring action is, however, very beneficial to the whole power plant, of which fact more later. Sulphur Dioxide, Nitric Acid, Nitrous Oxide, Benzene, and Turpentine.—All these are "possibles" with low boiling- points and fair to good calorific values, and they are readily available in a pure state. All, however, suffer from the drawback of very poor latent heat, quite apart from their generally undesirable qualities—some, in fact, would clearly ruin an engine in a few minutes, though all have at some period been considered as possible coolants. Cost and bulk-availability will always be the deciding ~K° AMMONIA SO* METHANOL BENZENE WATER TURPENTINE PARAFFIN -K>0 WATER N,0 o 100 200 BOILING POINT AT 76 cm Ho DEGREES CENT. 300 AMMONI* S02 METHANOL BENZINE TURPENTINE PARAFFIN o 4,000 aooo 12,000 16,000 | 20,000 2,000 6000 10,000 14,000 18,000 GROSS CALORIFIC VALUE B.Th.U./LB N.O TURPS BENZENE so4 PARAFFIN METHANOL AMMONIA WATER Z3 100 200 300 LATENT HEAT OF EVAPORATION CALORIES/gm. AT I5°C 400 500 factors in choosing the normal injectant. It must be remembered that, al though the cooling liquid may not comprise more than one-twentieth of the total flow by weight, and a far smaller proportion by volume, the total through put of a big gas turbine is so enormous that the bulk of coolant required for any useful period assumes formidable proportions. It has, in fact, been found that at least half of any operational coolant must be water. Before we go on to study results obtained it must be appreciated that security must compel the 24 16 8 30 o 10 20 INJECTION RATE (cjol/min) OF WATER Fig. 3. Variation of percentage boost with injection rate: General Electric 1-40, with pure water injection. Fig. 2. Important physical properties of possible injectants. drawing of a veil over all present experimental work. We may, however, note that most of the liquids specified as "possible" have, in fact, been tried in actual installations, and generally found wanting. First of all, research upon the injection of plain water has taken place in almost every country. Shortly after the end of the war, experimental results began to be made available. As example of these early results, the following figures from Power Jets (Research and Development), Ltd., may be quoted : using an elderly Whittle W2/700 double-sided centrifugal pure-jet engine, a water injection rate of 102 lb/min (i.e., approximately 4 per cent of the total mass-flow) gave a thrust increase of 18 per cent. In the U.S.A. a full report on trial injections of plain water was published in 1947 (Ref. 1). The trial concerned a General Electric 1-40 (U.S.A.F. designation J-33) engine installed in an early Lockheed F-80A; being of Whittle descent, this unit had a double-sided centrifugal compressor. The straight-line relation obtained between injection rate and thrust increase is shown in Fig. 3. It was found that the maximum injection rate of 26 gal/min lowered the intake temperature by 20 to 30 deg F, indicating that between one-ninth and one-sixth of the water was evaporating. These results are no doubt to be considered poor by present standards. A great deal of time has been given to experiments with ammonia, and some very great thrust-increases recorded. Though they refer to work done several years ago, the follow ing results, again from Power Jets, may be quoted as examples. On a Whittle W.i, Mk III, a neat-ammonia injection equal to 6.15 per cent of the total flow by weight resulted in a net thrust increase of 28 per cent. The same report (Ref. 2) shows that on a W2B engine (early Rolls- Royce Welland) a net thrust-increase of 22.4 per cent was obtained for an ammonia injection of only 4.4 per cent. A different source (Ref. 3) quotes the interesting result of a "negligible" thrust-augmentation when a 5 per cent ammonia injection was used in conditions of high atmospheric humid ity; this, of course, was due to the previously mentioned ammonium-hydroxide formation, with resulting liberation of heat. The greatest published thrust increase with coolant injection is a gain of 41 per cent (against a theoretical 57-58 per cent from a 10 per cent ammonia injection; in the author's opinion, however, this figure is of academic interest only; certainly a 10 per cent ammonia injection is most un likely ever to become standard operating procedure). Turning finally to methanol and methanol-water mixtures, there is little published information upon the result of past experiment. This may be because they are clearly the most useful liquids for this purpose and are of great operational importance, while the previously quoted results may largely concern work which has since been abandoned. Suffice to say that research upon their use is being carried out by both private firms and Government establishments in a great many countries, and that water-methanol injection is already a standard operating procedure on all kinds of turbine- powered aeroplanes, including such diverse types as heavy bombers, carrier fighters and turboprop airliners.
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