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Aviation History
1963
1963 - 1880.PDF
FLIGHT International, 24 October 1963 687 REGENERATIVE TURBOPROP Preliminary Details of the Allison T78 THIS journal has from time to time drawn attention to the contrasting philosophies which have governed the design of gas turbines f<*r aeronautical use and for surface appli cations. Broadly, the modern aircraft engine is designed to an ambitious pressure ratio and turbine entry temperature (say, 16:1 and 1,400°K), and thereby achieves excellent economy while having minimum bulk and weight. The automotive, industrial or marine engine appears to suffer from a mistaken belief that long parts-life and time between overhauls can be attained only by choosing a low pressure ratio and turbine entry temperature (say, 5 :1 and 950°K), which not only makes the unit exceedingly large and heavy and expensive for a given power output but also means that a heat exchanger must be incorporated if the specific fuel consumption is not to be excessive. Through all heat exchangers pass two flows, one hotter and the other colder, and as they do so the hotter flow gives up part of its heat to the colder flow. A simple example of a heat exchanger is a car radiator. The hot flow is water, which gives up part of its heat to the flow of cold air; the air emerges hotter than before and the water emerges cooler. Heat exchangers are invariably key items in the design of nuclear powerplants. In a gas turbine the heat exchanger is used to extract heat from the hot gas downstream of the turbine and pass it to the cooler air delivered from the compressor. This increases the tofal energy of the flow entering the combustion chamber, so that less fuel need be burned to reach a given turbine entry temperature; and it also extracts heat from the efflux which would otherwise be discharged into the atmosphere and wasted. The regenerative function of a heat exchanger in an elementary gas turbine is shown in Fig 1 on this page. Obviously the addition of a heat exchanger into a gas turbine improves the efficiency of the overall thermodynamic cycle. Clearly for a heat exchanger to be effective it must have a very large surface area, which must be heated by the exhaust gas and give up its heat to the compressor air. Accordingly heat exchangers are of necessity bulky and heavy, they cause a direct pressure-drop in the airflow due to their internal drag, they increase an engine's cost and com plexity and provide additional sources of potential trouble. In the past the penalties of using a heat exchanger have out weighed the advantages, as far as aircraft gas turbines are concerned Table-top model of the 778 is discussed by Roswell £. Cutler (left), Allison chief project engineer for regenerative engines, and Gordon E. Holbrook, chief engineer for product design and development —at least no such engine has ever been put into production. Nevertheless there have been numerous interesting attempts, one of the best known having concerned the Bristol Theseus turboprop of 1947-50. This engine was projected in one form with a matrix- type heat exchanger occupying the entire rear of the engine enve lope. And it could also be argued that reverse-flow engines like the Python and Proteus have, or had, a degree of heat exchange because the delivery from the combustion chambers passes through the air intake around the engine immediately ahead of the turbine. Be that as it may, nobody has thought it worth while developing a really advanced regenerative aircraft engine until now. In any case, it would be unthinkable in a turbojet or turbofan, because the installational losses, added to the other penalties, would make the engine worse than before. With a turboprop, however, the addition of a heat exchanger has a more beneficial effect on the operating cycle, since it does not reduce flow energy at the turbine but in the jetpipe. Moreover, a study of modern turboprop installations will show that there is usually plenty of room available even for a really large and effective heat exchanger (Fig 2). An American company—the only one to have produced a suc cessful modern turboprop (even Pratt & Whitney could not call the T34 modern)—has now made the following announcement:— "Allison Division of General Motors has been awarded a con tract by the US Navy for design and development of a regener ative gas-turbine engine that will radically improve the long-range search and patrol capabilities of Navy aircraft. "Harold H. Dice, vice-chairman of General Motors and Allison general manager, said in the announcement: 'Award of this con tract is a major milestone in the regenerative research programme that has been underway at Allison for some five years. During this period, we have successfully operated a series of gas-turbine engines fitted with regenerators. These experiments have demon strated exceptionally improved performance and appreciably •CE> HE JP cc T Fig I Simple flow diagrams showing a basic gas turbine (top) and one with heat exchanger added; C, compressor; CC, combustion chamber; HE, heat exchanger (or regenerator); T, turbine; JP, jetpipe Fig 2 A "Flight International" sketch clarifying the method of oper ation of the Allison S45-R2 (T78), and illustrating the ease with which it can be accommodated in a nacelle of normal dimensions. I, air intake ; 2, compressor; 3, rotary-drum heat exchanger; 4, seal; 5, turbine; 6, combustion chamber. The compressor delivery pipes blow inwards through the heat-exchanger matrix, while the hot gas from the turbine passes outwards through the intermediate spaces around the drum B
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