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Handbook of Turbomachinery Second Edition Revised - Ventech!

Handbook of Turbomachinery Second Edition Revised - Ventech!

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<strong>of</strong>f by a selector valve. This design has only two modes <strong>of</strong> operation, exhaust<br />

gas flow with one or both passages open, and therefore only two A=R ratios<br />

are available. Disadvantages <strong>of</strong> this approach include reduced turbine<br />

efficiency caused by wake loss and additional surface friction, plus hightemperature<br />

stresses on the thin dividing wall. An alternate version <strong>of</strong> this<br />

concept uses a horizontally sliding gate that closes <strong>of</strong>f part <strong>of</strong> the turbine<br />

housing volute at low speeds.<br />

One more flexible variable-nozzle boost control concept uses a single<br />

movable vane placed at the turbine inlet (Fig. 22). This device changes the<br />

A=R ratio by changing the effective throat area over a relatively large range.<br />

The design is potentially quite reliable because <strong>of</strong> the limited number <strong>of</strong><br />

moving parts and <strong>of</strong>fers a relatively large operating range. There is a<br />

reduction in efficiency because <strong>of</strong> gas leakage through the side gap between<br />

the vane and housing, wake loss, and a nonuniform velocity distribution.<br />

A more complex variable-geometry design uses multiple nozzle vanes<br />

located on the perimeter <strong>of</strong> the turbine housing. (The VNT design<br />

previously mentioned is one example.) The individual vanes are linked<br />

together so they mechanically move in unison. Changing the passage area<br />

between the vanes is used to control the turbine speed. While more complex,<br />

more costly to manufacture, and somewhat less reliable, the design provides<br />

excellent efficiency and a very wide operating range. A larger turbine<br />

Figure 22a Single vane used to control exhaust gas velocity supplied to turbine<br />

wheel is a simple means <strong>of</strong> controlling boost pressure. (Courtesy Mitsubishi Heavy<br />

Industries America, Inc.)<br />

Copyright © 2003 Marcel Dekker, Inc.

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