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Aviation History
1942
1942 - 0193.PDF
JANUARY 2"2ND, 1942 FLIGHT 71 S U P E R C H A R C E R S Fig. 21—and the required tip speed u2 of the impeller, obta.ned from the equation gKai=qa4 M% (2) A succession of further empirical factors are required to fix the dimensions of the impeller—in particular, a suit able diameter ratio D2/D, (good re suits have been obtained with a ratio of D3/D1 = i.4-2.o, see Fig. 14) ; and the ratio of the meridian flow velocity Cm to the tip speed u, for which a suit able limit is vm fu = 0.35 — 0.60. The blade-width at the impeller discharge is obtained from the condition of con tinuity, a suitable basis for design, allowing for the thermody namic state- change in the flow through the blower, be ing the assump tion of a con -*C"stant rate of increase of the meridian com ponent of" the absolute velo city between in take and outlet; suitable effi ciency can be ensured by keeping the de sign values at the mean of the limits given. Limitation o f the running .v" M #L // 8000 £ 50 100 150 2O0 2SO 300 35 0 400 450 ROTOR TIP SPEED u2. (m/sec) Fig. 21. DVL measurements for the head H„d at the point of optimum efficiency, in relation to the rotor tip speed u2 for aircraft engine blowers with radial blades. Averages of 313 measurements. speed owing to peculiarities in the driving mechanism, e.g., a gear-drive of known efficiency, or an exhaust turbo- drive, can easily be adjusted within the stipulated limits of the above ratios. The setting angles of the entry impeller blajies and diffuser vanes and the dimensions of the volute casing can be determined from the ratio of flow velocity to tip speed. Blowers without diffuser vanes can have smaller casing diameters and are thus more convenient to instal. They should be used principally for higher flow velocities, i.e., higher specific running speeds. Their qnd values are always lower than when guide-vane diffusers are used. The char acteristic curves of blowers with vaneless diffusers can be made very flat. Fig. 22 shows the characteristics of a - 0 02 04- 06 06 10 INDUCTION VOLUME V, (cu.m/sec.) 12 14 Figs. 24 (top) and 25 (bottom). Characteristic curves ot a DVL experimental blower with adjustable straight-exit vanes. a = inclination of guide-vane median lines to the direction of the periphery. DVL blower with an exit volute, an impeller diameter ratio about 1.65, and a short-radius annular clearance space, at average flow velocities. Fig. 23 shows similar curves for a DVL blower with a large rotor-diameter ratio, rela tively-large radial clearance, and high flow velocity. These curves have a particularly flat gradient and the blower 04 OS 12 16 20 nrys 24 INDUCTION VOLUME V, (cu m/SeC.) ' 04 06 08 10 12 14- 16 m^ INDUCTION VOLUME V(CU.m/S6C.) 20 Characteristic diagrams of DVL blowers. u, = rotor tip speed ; ti,_ad — internal blower efficiency. Fig. 22 (left) : Blower with singlg-exit spiral volute without exit vanes and of small radial extension. (See also Fig. 12 in last week's issue.) Fig. 23 (right) • Blower with single-exit spiral volute without exit vanes and large annular diffuser between rotor and volute casing
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