TURBINE. A turbine is a water wheel fixed upon a vertical revol ving axis, receiving and discharging water in various directions round its surface. This description of prime mover consists of a drum, bearing a number of suitably formed vanes, curved in such a manner as to allow the water leaving them, after it has glanced off from them, to escape with as little velocity or power as possible. Turbines possess the advantage of occupying, comparatively with the power exercised, a very small bulk ; and of being equally efficient with large or small falls.
There may be considered to be three classes of turbines : 1st. Those in which the water is supplied and discharged in a direction parallel to the axis; they are technically known as the parallel-flow turbines, according to the classification proposed by Professor Rankine, in his ' Treatise on Prime Movers.' 2nd. Those in which the water is sup plied and discharged in currents radiating from the axis : or the out ward flow turbines. And 3rd. Those in which the water is supplied and discharged in currents converging radially towards the axis, or the inward fiow turbines. The leading principles affecting the working of these various machines are the same, though the details are neces sarily dissimilar; they may be briefly described as follows : Whatever description of turbine be used, it is desirable that little or no change of velocity of flow should take place in the current, during the passage of the water through the wheel; and that the water should enter without shock, and leave without a whirling motion. The latter condition is attained when the ratio of the entering velocity is to the outgoing velocity in the same ratio as the radius of the receiving to that of the discharging sides; or calling this ratio n, it =1, in parallel flow turbines; outward flow turbines; and in inward flow turbines. The angles of obliquity of
the blades to the line of flow, differ according to the velocity required in the wheels, and to their peculiar form : thus, in the parallel flow turbines, the angle may vary between 20° and in the outward flow turbines, between 141t° and 26.r ; and in the inward flow ones, between 36° and 54°, the best velocity at the middle of the rings of the vanes appears to be ascertained by the formula (for all kinds of turbines)—calling v the velocity, h the head in the supply chamber, and g the accelerating force of gravity : then V2gh.
From General Morin's experiments it would appear that the efficiencies of the respective descriptions of turbines, with reference to the powers exerted upon them, are comprised within the limits of from 75 to 30 per cent. of the power exerted in the parallel flow turbines; in inward flow turbines the limits approach closely the average of 73 per cent. ; and in outward flow ones, they are on the average 68 per cent.; on the average then, the efficiency of all classes may be taken to be about 70 per cent. of the total power. The reaction wheel may be considered to constitute a modification of the turbine ; and it will be discussed more fully under WATER POWER.
The best descriptions of this class of engines are to be found either in Rankine's work before quoted ; in 3Iorin's Lecons de 316canique Pratique ; ' in Armengaud's 'Trait6 Pratique des Moteurs Hydrau liques ; ' and in several of the practical papers ou machinery by Mr. Fairbairn. M. Fourneyron has published a description of the outward flow turbines he himself invented, with full instructions for ascer taining the proportions of the several details.