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Basics of Fluid Mechanics, 2014a

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7.1. THE FIRST LAW OF THERMODYNAMICS 199<br />

The energy equation (7.2) for system is<br />

∫<br />

∫<br />

dT<br />

k T<br />

A sys<br />

dn dA+ (S S n + τ ) dV<br />

A sys<br />

+Ẇshaft = D ∫<br />

Dt<br />

V sys<br />

ρ<br />

(E U + m U 2<br />

2 + gz )<br />

dV<br />

(7.8)<br />

Equation (7.8) does not apply any restrictions on the system. The system can<br />

contain solid parts as well several different kinds <strong>of</strong> fluids. Now Reynolds Transport<br />

Theorem can be used to transformed the left hand side <strong>of</strong> equation (7.8) and thus<br />

yields<br />

∫<br />

A cv<br />

k T<br />

dT<br />

dn dA+<br />

Energy Equation<br />

∫<br />

(S S n + τ ) dA + Ẇshaft =<br />

A cv ∫<br />

d<br />

ρ<br />

(E u + m U 2 )<br />

dt V cv<br />

2 + gz dV<br />

+<br />

(E u + m<br />

∫A U 2 )<br />

cv<br />

2 + gz ρU rn dA<br />

(7.9)<br />

From now on the notation <strong>of</strong> the control volume and system will be dropped since all<br />

equations deals with the control volume. In the last term in equation (7.9) the velocity<br />

appears twice. Note that U is the velocity in the frame <strong>of</strong> reference while U rn is the<br />

velocity relative to the boundary. As it was discussed in the previous chapter the normal<br />

stress component is replaced by the pressure (see equation (6.8) for more details). The<br />

work rate (excluding the shaft work) is<br />

flow<br />

work<br />

∫{ }} { ∫<br />

Ẇ ∼ = P ˆn · UdA−<br />

S<br />

S<br />

τ · U ˆndA (7.10)<br />

The first term on the right hand side is referred to in the literature as the flow<br />

work and is<br />

∫<br />

S<br />

∫<br />

P ˆn · UdA =<br />

Equation (7.11) can be further manipulated to become<br />

∫<br />

S<br />

S<br />

P<br />

S<br />

U { }}<br />

rn<br />

{ ∫<br />

(U − U b )ˆndA+ PU bn dA (7.11)<br />

S<br />

work due to<br />

the flow<br />

{ ∫ }} {<br />

P<br />

P ˆn · UdA =<br />

ρ ρU rn dA +<br />

work due to<br />

boundaries<br />

movement<br />

∫{ }} {<br />

PU bn dA (7.12)<br />

S

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