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Handbook of Turbomachinery Second Edition Revised - Ventech!

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Referring to Fig. 15(e), we can see the variables in play. With a‘‘small’’ inlet<br />

area, the fluid inlet meridonal velocity is relatively high, and the impeller<br />

blade speed is relatively low, due to its placement on a small radius. With a<br />

‘‘large’’ inlet area, although the fluid inlet meridonal velocity is reduced, the<br />

impeller blade speed has been increased due to the placement <strong>of</strong> impeller<br />

blade leading edges on a large radius. Somewhere between these two cases<br />

the velocity <strong>of</strong> the fluid relative to the blade leading edges at the inlet will be<br />

minimized. Minimizing this velocity energy (specifically W 2 1t =2g) acts to<br />

minimize the diffusion that must be performed near the impeller inlet, and<br />

thus minimizes the energy lost in this process. The process <strong>of</strong> inlet sizing for<br />

minimum impeller diffusion losses fulfills the criteria <strong>of</strong> sizing for maximum<br />

efficiency.<br />

We have seen in previous discussions that propellant pump thermodynamic<br />

efficiency is <strong>of</strong>ten not a significant problem (within bounds <strong>of</strong><br />

reason). A much more pressing issue in most cases is the ability <strong>of</strong> a pump to<br />

operate with a very low inlet pressure in relation to the fluid’s vapor<br />

pressure. This ‘‘surplus’’ <strong>of</strong> pressure (or head) above the fluid vapor pressure<br />

(or head) is referred to as NPSH. The urgency <strong>of</strong> this issue relates to the<br />

weight savings that can be afforded a launch vehicle by a pump’s ability to<br />

operate at inlet pressures that are consistent with very lightweight propellant<br />

tanks (it should be noted here that the larger the launch vehicle, the larger<br />

are the available weight savings from a given reduction in pump inlet<br />

pressure). For this reason, a more common optimization procedure for inlet<br />

sizing <strong>of</strong> rocket propellant pumps is to minimize the required inlet NPSH. A<br />

pump inlet that is sized according to this procedure will invariably be larger<br />

than that which would be derived for maximum efficiency. However, it<br />

Figure 15e Typical characteristic <strong>of</strong> pump efficiency versus specific speed.<br />

Copyright © 2003 Marcel Dekker, Inc.

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