Mechanics of Engineering and of Machinery - Softcover

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1878 Excerpt: ...backwater. Sometimes an apparatus is used for raising and lowering the wheel or the race. Such an arrangement is shown in Fig. 429. Such contrivances for accommodating the wheel to variable level of water are best replaced by turbines, the more so as the delivery of these latter wheels is greater. § 213.--L,o« of Water in Straight Rare.--If c is the velocity of the water, and v that of the wheel, then, for the theoretical delivery of an undershot-wheel in straight race, we have and hence the force of rotation, (see Vol. I., Art. 501). Here Q, denotes the amount of water which actually strikes the floats, and we have, therefore, to find the ratio which this bears to the whole amount of water. The loss of water for a wheel in a straight race is a double one. First, the water escapes through the clearance between wheel and race, and, second, certain deeper elements of the water do not strike at all upon some of the foremost floats. Let us consider first the loss through the clearance between the race and lowest float end. The height of this clearance is variable. Thus, if the float A B, Fig. 432, is at its lowest position, the clearance AF=a is the least possible. If, however, the two floats A, B, and A, Bt are equally distant from F, the height E F oi the clearance is greatest. If the radius CA =r and the number of floats is «, we have Therefore, the greatest height of clearance lit"r E F--o 11 I 2 The mean value between these extremes is, then, This multiplied by width of race i gives for the crosssection through which water escapes, 'n I 4 and we have only to determine the velocity w with which the water passes through this cross-section. If the level of the issuing water is the same as that of the approaching water, the velocity through...