FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1943
1943 - 1172.PDF
474 FLIGHT MAY 6TH, 1943 NEW CAS TURBINE PROJECTS A system and is indicated diagrammatically in Fig. 3. The working medium is compressed in an axial" flow compressor A and is passed through a tube in the furnace C and thence to the inlet side of turbine B. The heater may be oil fired, although pulverised fuel, or indeed any type of fuel may be used. The gas leaving the turbine on the return half of the circuit still retains a considerable amount of heat and is therefore passed through a regenera- tor D in order to preheat the cool gas on its way from the compressor to the furnace. If, after leaving the re- generator, the return gas is still at a higher temperature than desirable, it may be passed through a cooler, not -hewn in the diagram, before entering the compressor for recirculation. The products of combustion do not, of course, pass into the compressor and turbine system. The outstanding feature of the English Electric system is the use of a monatomic gas as the working medium. A monatomic gas may be briefly defined as one in which the molecule, the smallest division retaining all the characteristics of the gas, contains only one atom. The molecules of diatomic* and triatomic gases, carbon monoxide and carbon dioxide, for instance, have respectively two and three atoms. For f» monatomic gas the ratio of the specific heat at constant pressure to the specific heat at constant volume is approxi- mately 1.66 as compared with 1.4 for a diatomic gas and 1.3 for a triatomic gas. The higher the value of this ratio the lower will be the pressure ratio required for a compressor aud turbine designed to give maximum efficiency at chosen values of maximum and minimum limits oi tempera- ture in the system. Furthermore, within given temperature limits and with a given pressure ratio, the greater the density of the gas the smaller will be the adiabatic heat drop in the turbine. As a consequence, providing that rotor diameters, speeds of rotation and ratios of blade speed to gas speed remain the same, the number of stages required in the turbine and in the compressoi will be fewer. Additionally, the greater the density of the gas the smaller will be the dimen- sions of the flow passages. A monatomic gas is not liable to dissociation at high temperatures, and if a relatively inert gas is used, the corrosive action on the turbine and compressor blades will be less than that of air. By using a monatomic gas, such as argon, all these advan- tages can be realised. This inert gas has a density approxi- Fig. 3. Diagram of English Electricclosed-cycle turbine plant employ- ing a monatomic gas as the work-ing medium. mately 1.38 times that of air at the same temperatur.e and pressure. The rare gases krypton and xenon could also be used to further advantage, as their densities are respec- tively 2.87 times and 4.53 times that of air. Some pro- portion of neon, another monatomic gas, could be included in the working medium, although its low density, only 0.696 that of air, is a disadvantage. The question of relative density is of considerable im- portance. As an alternative to the comparatively rare gases mentioned, the English Electric Co. propose the use of carbon dioxide for the working medium, as it has a density of 1.52 times that of air at the same temperature and pressure. This despite the fact that it is a triatomic gas and consequently less suitable for the purpose than a monatomic gas. From the U.S.A. comes a proposal by B. B. Holmes to employ a liquid medium in a closed thermal cycle to operate a turbine specifically for driv- ing ar airscrew. In the past various fluids such as alcohol, aniline, benzol, carbon-tetrachloride, ether, sulphur dioxide, toluol, .water and xylol have been suggested as a working fluid for converting heat into motive power, but all had disadvantages which rendered them impracticable or dangerous in operation and therefore not suitable for commercial use In the main, the criti- cal temperatures and pressures of such fluids are excessively high and thereby necessitate extremely heavy equipment, or they .arf highly toxic, corrosive or inflammable and consequently liable to be dangerous to life and property. The liquids proposed are stable chemical combinations of carbon, chlorine and flourine, and known as "freons." Four of these compounds, trichloro-monofluoro-methane (F-n), dichloro-difluoro- methane (F.12), dichloro-monofluoro-methane (F.21), and trichloro-trifluoro-ethane (F-113), possess physical and thermodynamic properties which render them suitable for use in a turbine motive unit for aircraft. All are non-toxic, non-combustible and non-corrosive to metal. Each has a low melting point, low critical temperature and low critical pressure. F-ll' Trichioro-monofluoro methane ...F-12 Dichloro - difluoro- methaneF-21 Dicbloro-monoBuoro methaneF-113 Trichloro - trifluoro- ethane SYMBOL CC1,F CC!ZF2 CHClaF CC1.F, MELTING POINT dee. C. — 111-1 ~ 157-7 -135-0 - 35-0 CRITICAL PRESSURE U>. sq. in. abs. C;f5 582 750 400 CRITICAL TEMPERA- TURE dee. C. 107-7 111-7 3 78-7 21:i-9 Fig- 4- The Holmes thermodynamic plant shown diagrarnmaticallyinstalled in an aircraft fuselage. The choice of the liquid for a specific aircraft application will depend upon the prevailing con- densing temperature, that is, whether the aircraft travels in sub-zero or higher temperature con- ditions, and upon the thermal efficiency obtainable from the liquid under the conditions obtaining. Thus an aircraft may be operated in winter or at high altitudes with one liquid, whilst in summer or at low altitudes another might be employed to better advantage. The liquid is raised to a pressure of between 400 and 1,000 Ib./sq. in. ; heat is applied ex- ternally to raise the temperature to between 93 deg. C. and 215 deg. C. to vaporise the liquid; the vapour is superheated to a temperature sufficiently high to ensure dryness throughout the desired ex-
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
