Turbine (Lat. Turdo A Whirling, Or That Which Whirls), a water wheel through which the water passes, guided by channels in the wheel itself, and usually by other passages exterior to the wheel which cause it to impinge on the wheel buckets at the proper angle to secure efficiency. The guide curves (as the walls of the last named channels are called) and the buckets of the wheels are usually both curved in such manner that the water shall enter the wheel as nearly as possible without shock, and shall leave it with the least possible velocity. Turbines are generally, but not always, set in the horizontal plane, their axes being vertical; their size diminishes as the height of fall increases, and for falls of ordinary height they are very much smaller than the ordinary forms of so-called " vertical " water wheels, an advantage which increases with the height of fall. Their smaller size gives necessarily a high velocity of rotation, which constitutes their most important advantage over the older forms of wheel; it permits the adoption of less heavy and expensive machinery for transmitting the power, dispenses with gearing, and gives greater regularity of speed and nearly equal efficiency under all heights of fall.
The turbine was introduced into general use by Fourneyron in France in 1827, and soon after by Fairbairn in England and by Boyden in the United States. Turbines are classed.as outward-flow, inward-flow, and parallel-flow wheels, according to the direction taken by the water in passing through them; but the principle already enunciated applies to all. Could the water be entered upon the wheel absolutely without shock, and discharged absolutely without velocity, the efficiency of the wheel would be perfect, and the energy of the fall would be all transformed into work. The efficiency of good turbines, under favorable circumstances, approaches 80 per cent., and has been known to exceed that figure; the usual value is about 75 per cent. The efficiency is determined as follows: The amount of water flowing through the wheel is ascertained by gauging; its weight, measured by the height of fall, indicates the maximum power of the stream, or the power available. The actual amount of power utilized by the wheel is determined by measurement with the dynamometer.
If R = the resistance and v = the velocity with which the wheel overcomes that resistance, R x v = the work done in the unit of time, and Rv = Whc, in which expression W is the weight of water flowing per second, h the height of fall, and 0 the coefficient of efficiency, or that fraction of the total available fall which is actually utilized by the wheel; the value of 0 is the "modulus" of the wheel. This value is capable of being estimated with approximate accuracy by the designer of the wheel, and the performance thus predicted, by the use of formulas involving quantities dependent in magnitude upon the forms of the guiding channels. Turbines give the highest efficiency when their speed is between 0.5 and 0.7 of that due to the height of fall. The velocity of direct flow, or that with which the water passes through the wheel, is to be preserved as nearly uniform as possible, and the passages are to be given such form and magnitude of cross section as will insure that uniformity. The velocity of whirl is made as nearly as possible equal to the rotary velocity of the wheel, and the water is thus passed upon the wheel without shock.
It should glide over the buckets without sudden change of velocity, and should finally pass out with a speed opposite in direction and equal in magnitude to that of the wheel, thus dropping out of the wheel with the least possible velocity of flow, and with its original vis viva transformed into mechanical energy. Fig. 1 is a vertical section exhibiting the construction of the Boyden outward-flow turbine, made by the Holyoke machine company. A is a quarter-turn leading the water smoothly upon the wheel; B is the lower curb; C the disk carrying the guides; D the wheel with its guide channels, shown with the guide curves more perfectly in the plan, fig. 2; E is a disk connecting the wheel to its vertical shaft; F, G, G' are supporting beams; S the shaft; I the support for the bearings; J the driving gear; and R the apparatus for moving the gate. In the Boyden wheel is illustrated the outwardflow turbine. In the inward-flow wheel, the water enters between guide curves, or is carried in a spiral channel, and the form of the buckets of the wheel is modified in accordance with the principles already stated, and gives the form seen in plan in fig. 3, in which A is the wheel disk and B is the shaft.
In the parallel-flow turbine the water enters upon the wheel as in fig. 4, which represents the Bodine turbine. It is cased in so that it may be set at any point in the fall, utilizing the so-called suction of that part below it, as well as the pressure due to the column above it, a method of arrangement first introduced by Henschel and Jonval. The wheel was invented by Fontaine. The upper set of guides are fixed; the lower set are the wheel buckets. In the Burnham turbine, fig. 5, the inward and downward flow forms are combined to make a wheel of very high efficiency, while yet cheap in construction and durable. In the Leffel wheel, fig. 6, the stream is divided to obtain the combined inward and downward flow and to secure greater effectiveness, and also to obtain more perfect regulation. This form has been extensively introduced in the United States, as has also the preceding. The same letters denote the same parts in each figure. The arrows indicate the direction of flow. In Schiele's inward-flow turbine the water divides on entering the wheel, a part passing out above, the remainder emerging below, the wheel disk.
The Fourneyron outward-flow and the Jonval parallel-flow turbines are most used in Europe. Regulation is effected by a vertically sliding gate (R, fig. 1), by a set of valves at the entrances to the wheel (figs. 4, 5, 6), or by varying the positions of the guide blades themselves. The most perfect method would be by varying the velocity ratio of the wheel and the driven mechanism. The loss of efficiency in reducing the power of the wheel by regulation is often serious. Rankine states this loss as follows:
Fig. 1. - Section of Boyden Turbine.
Fig. 2. - Plan of Boyden Turbine.
Fig. 3. - Inward-flow Turbine, Plan.
Fig. 4. - Bodine Turbine, Parallel Flow.
Fig. 5. - Burnbam Turbine. Inward and Downward Flow.
Ratio of efficiency
Whitelaw's turbine is a simple form of wheel without guide blades. Barker's mill was a very rude apparatus consisting of a vertical spout surrounding the shaft and conducting the water to hollow horizontal arms, from the extremities of which it emerged tangentially to the orbit of the orifices. These wheels are usually known as reaction wheels; their efficiency is comparatively small. - See Francis, "Lowell Hydraulic Experiments" (Boston, 1855); Rankine, "Steam Engine and Prime Movers" (London, 1859); and Mahan, "Hydraulic Motors" (New York, 1873).
Fig. 6. - Leffel Turbine. Divided Flow.