28.02.2013 Views

Handbook of Turbomachinery Second Edition Revised - Ventech!

Handbook of Turbomachinery Second Edition Revised - Ventech!

Handbook of Turbomachinery Second Edition Revised - Ventech!

SHOW MORE
SHOW LESS

You also want an ePaper? Increase the reach of your titles

YUMPU automatically turns print PDFs into web optimized ePapers that Google loves.

RADIAL-INFLOW TURBINE SIZING<br />

Differences Between Radial-Inflow and Axial-Flow Turbines<br />

Radial-inflow turbines enjoy widespread use in automotive turbochargers<br />

and in small gas turbine engines (auxiliary power units, turboprops, and<br />

expendable turbine engines). One advantage is their low cost relative to<br />

machined axial turbines, as most <strong>of</strong> these applications use integrally bladed<br />

cast radial-inflow turbine wheels.<br />

The obvious difference between radial-inflow and axial-flow turbines is<br />

easily seen in Fig. 1; aradial-inflow turbine has asignificant change in the<br />

mean radius between rotor inlet and rotor outlet, whereas an axial-flow<br />

turbine has only a minimal change in mean radius, if any. Because <strong>of</strong> this<br />

geometric difference, there are considerable differences in the performance<br />

characteristics <strong>of</strong> these two types <strong>of</strong> turbines. Referring to the ‘‘typical’’<br />

radial-inflow vector diagram <strong>of</strong> Fig. 9(c), the radius change causes a<br />

considerable decrease in wheel speed U between rotor inlet and outlet. For<br />

zero exit swirl, this results in a reduced relative exit velocity compared to an<br />

axial turbine with the same inlet vector diagram (since U2&U1 for an axial<br />

rotor). Since frictional losses are proportional to the square <strong>of</strong> velocity, this<br />

results in higher rotor efficiency for the radial-inflow turbine. However, the<br />

effect <strong>of</strong> reduced velocity level is somewhat <strong>of</strong>fset by the long, low-aspectratio<br />

blade passages <strong>of</strong> a radial-inflow rotor.<br />

Compared to the axial-flow diagram <strong>of</strong> Fig. 9(a), there is a much larger<br />

difference between the rotor inlet relative and absolute velocities for the<br />

radial-inflow diagram. Referring to Eq. (17), this results in a lower relative<br />

inlet total temperature at design point for the radial-inflow turbine. In<br />

addition, due to the decrease in rotor speed with radius, the relative total<br />

temperature decreases toward the root <strong>of</strong> radial-inflow turbine blades (see<br />

Mathis [23]). This is a major advantage for high inlet temperature<br />

applications, since material properties are strongly temperature-dependent.<br />

The combination <strong>of</strong> radial blades at rotor inlet (eliminating bending stresses<br />

due to wheel rotation) and the decreased temperature in the high-stress<br />

blade root areas allows the radial-inflow turbine to operate at significantly<br />

higher wheel speeds than an axial-flow turbine, providing an appreciable<br />

increase in turbine efficiency for high-pressure-ratio, high-work applications.<br />

For applications with moderate inlet temperatures (less than 500 8F)<br />

and pressure ratios (less than 4:1), the blade speed <strong>of</strong> an axial wheel is not<br />

constrained by stress considerations and the radial-inflow turbine is at a size<br />

disadvantage. Due to bending stress considerations in the rotor blades,<br />

radial blades are used at the inlet to eliminate bending loads. This limits the<br />

Vy1=U1 ratio to 1 or less, meaning that the tip speed for an equal work<br />

Copyright © 2003 Marcel Dekker, Inc.

Hooray! Your file is uploaded and ready to be published.

Saved successfully!

Ooh no, something went wrong!