By R. A. Wallis
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50) Pressure The growth of a turbulent layer in a pressure gradient can be approximately calculated from eq. 37) provided there is no 42 BOUNDARY LAYER A N D SKIN FRICTION RELATIONS flow separation. The science of predicting flow separation in adverse pressure gradients is, at the present, not an exact one, but a method such as t h a t outlined by Spence*8) can be very useful in assessing the deterioration of the layer as it enters a region of rising pressure. e. δ*/0, along the surface. The parameter H increases as flow separation is approached and hence an upper limit is usually set above which flow separation is considered to be imminent.
In axial compressor design, however, design methods are being evolved which may lead to a special aerofoil type for each machine, with the aim of increasing the working efficiency to the limit and providing a deterrent to compressor surging. 3. Lift An aerofoil at zero incidence constitutes an obstruction in a stream of air and hence local speeding up of the air takes place. With increasing incidence the velocity at any point on the top surface is progressively increased, while, in general, the reverse is true on the lower surface.
48) and, by substituting in eq. 49) In developing eqs. 3) and hence 0* is zero. When Rx is large, the difference between xe and x (see Fig. 6) will usually be small. Before applying the foregoing equations, it should be established whether the relations are suitable for the task in hand. (ii) Fully developed flow in a pipe. The mean velocity in the pipe and the pipe diameter are usually employed in defining skin friction. From eqs. 44), eq. 6. 50) Pressure The growth of a turbulent layer in a pressure gradient can be approximately calculated from eq.