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Handbook of Turbomachinery Second Edition Revised - Ventech!

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e provided in the simulation environment. It is a known that turbulent flow<br />

imposes a different momentum balance because <strong>of</strong> the existence <strong>of</strong> turbulent<br />

shear forces in addition to the shear forces due to molecular viscosity. Fluid<br />

elements also experience additional forces due to inherent periodic<br />

unsteadiness in the turbine. Thus the convective heat transfer to the surface<br />

is increased by the additional heat flux due to turbulence as well as inherent<br />

unsteadiness.<br />

To simulate the external convective heat transfer, the local Reynolds<br />

number at the inlet <strong>of</strong> the blade (or nozzle) is calculated under engine<br />

conditions. The same Reynolds number needs to be reproduced at the inlet<br />

<strong>of</strong> the hot cascade. The total pressure at the inlet section <strong>of</strong> the cascade can<br />

then be calculated in such a way that both the inlet Reynolds number and<br />

the Mach number values are matched for the prescribed cascade operating<br />

temperature. The geometric similarity condition or the characteristic length<br />

in the expression for the Nusselt and Reynolds numbers can be satisfied<br />

easily by using a full-scale model in the cascade test section. For the same<br />

Reynolds number in the turbine and hot cascade, by using the equation <strong>of</strong><br />

state and the definition <strong>of</strong> the Mach number, the free-stream mass flux rate<br />

can be defined as<br />

rhcUhc ¼ mhc rturWtur ¼<br />

mtur Phc pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi<br />

Ma ghcRhcThc RhcThc<br />

If the component simulated is a rotor blade, then Wtur corresponds to the<br />

relative velocity with respect to the rotor. The static pressure, Phc, and<br />

temperature Thc, values for the hot cascade can be replaced with the total<br />

quantities by using the isentropic relations. The expression for the total<br />

pressure <strong>of</strong> the hot cascade corresponding to an operator-selected cascade<br />

total temperature can then be obtained from<br />

Pohc ¼ mhcrturWtur qffiffiffiffiffi 1 þ ðghc 1Þ<br />

2<br />

Ma2<br />

m turMa<br />

g hc<br />

Rhc<br />

ghc ghc 1<br />

sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi<br />

Tohc<br />

ð1 þ ðghc 1Þ<br />

2 Ma2Þ At this stage, turbine conditions including the local Mach and Reynolds<br />

numbers are known. The total temperature can easily be controlled by<br />

changing the combustor operating conditions. Calculation <strong>of</strong> the hot<br />

cascade total pressure requires an iterative procedure because the specific<br />

heat ratio and the gas constant in the hot cascade are not known initially.<br />

Although the static temperature in the cascade is not known yet also, the<br />

total temperature is defined. The specific heat ratio has some nonnegligible<br />

static temperature dependence. Since the hot cascade static temperature is<br />

not known initially, the fluid properties can be approximately evaluated at<br />

Copyright © 2003 Marcel Dekker, Inc.

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