All About Engines

Steam Turbines I5I Fig. 88.—Diagram to explain principle of impulse turbine on leaving the nozzle, passes to another moving object, viz., the vane, and the speed at which it slides over the surface will be less than its speed at the instant of discharge—just as a man running to catch a moving train may be travelling at a greai speed along the platform, and yet have only a slow rate of motion relative to the train. Now, while the water is moving over the vane its speed over the surface will be practically the same at B as at a, and, in this re- spect, the action is the same for a moving as for a fixed vane. But the water is not merely moving over the surface —it is also following up the vane as it retreats. Its speed will be less than that of the original jet, and its direction will have been changed. It will not leave the vane exactly as it appears to do in the diagram, but will be thrown forward by the motion of the vane—that is, to the left. It is clear, therefore, that the result- ing effect of the jet depends very much upon the inclination of the face at A and b, and it is here, more particularly, that scientific design is required. From the foregoing account it follows that (i.) from the time the water or steam leaves the nozzle it suffers no change of pressure, and (ii.) the work is done by the water or steam upon the moving vane by virtue of the change, both in magnitude and