05.04.2016 Views

Modern Engineering Thermodynamics

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

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

Problems 309<br />

Multiplying this equation through by dt and integrating over time from system state 1 to state 2 gives the<br />

open system modified entropy balance (MSB) equation as<br />

Z 2<br />

1Q 2<br />

+ _m ðs in − s out Þdt + 1 S P Þ<br />

T 2 = 0 (9.11)<br />

b 1<br />

2. For flow in nozzles, diffusers, or throttles, we have the entropy production rate of an incompressible fluid in<br />

an adiabatic nozzle, diffuser, or throttle:<br />

<br />

_S P <br />

adiabatic<br />

incompressible<br />

fluid<br />

= _mc ln T out<br />

T in<br />

> 0 (9.14)<br />

and the entropy production rate of an incompressible fluid in a nozzle, diffuser, or throttle with heat<br />

transfer<br />

<br />

_S P incompressible<br />

fluid<br />

= _mc ln T out<br />

T in<br />

3. For flow in heat exchangers, we have the heat exchanger entropy production rate:<br />

− _ Q<br />

T b<br />

> 0 (9.15)<br />

_S P = _m H ðs out − s in Þ H<br />

+ _m C ðs out − s in Þ C<br />

(9.24)<br />

See Eqs. (9.25) and (9.26) when the hot and cold fluids are known to be incompressible liquids or ideal<br />

gases.<br />

4. When two flow streams, 1 and 2, are combined to form a mixed outlet flow stream, 3, the entropy<br />

production rate is<br />

_S P<br />

mixing = _m 3½ys ð 2 − s 1 Þ+ ðs 3 − s 2 ÞŠ> 0 (9.29)<br />

See Eq. (9.32) for the entropy production rate in mixing identical incompressible liquids or identical ideal<br />

gases.<br />

5. The entropy production rate for shaft work machines is<br />

<br />

_S P shaft work<br />

machine<br />

<br />

= _m <br />

ðh 1 − T b s 1 Þ− ðh 2 − T b s 2 Þ+ V2 1 − V2 2<br />

+ gZ <br />

ð 1 − Z 2 Þ<br />

−<br />

T b<br />

2g c g c<br />

_ W actual<br />

T b<br />

(9.36)<br />

6. The entropy production in adiabatically filling a rigid tank with an incompressible liquid is<br />

1ðS P<br />

<br />

Þ 2 incomp:<br />

liquid<br />

ðadiabaticÞ<br />

= m 2 c ln T <br />

2<br />

= m 2 c ln 1 + ðpv<br />

T in<br />

Þ in<br />

cT in<br />

<br />

(9.41)<br />

7. The entropy production in adiabatically filling a rigid tank with an ideal gas is<br />

1ðS P<br />

<br />

Þ 2 ideal<br />

qas<br />

ðadiabaticÞ<br />

= m 2 c p ln T 2<br />

T in<br />

− m 2 R ln p 2<br />

p in<br />

= m 2 c p ln k (9.42)<br />

8. The entropy production in isothermally filling a rigid tank with an incompressible liquid is<br />

1ðS P<br />

<br />

Þ 2 incomp:<br />

liquid<br />

ðisothermalÞ<br />

9. The entropy production in isothermally filling a rigid tank with an ideal gas is<br />

1ðS P<br />

<br />

Þ 2 ideal<br />

gas<br />

ðisothermalÞ<br />

= m 2 ðpvÞ in /T (9.45)<br />

= m 2 c v ðk − 1Þ = m 2 R = p 2 V/T (9.46)

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

Saved successfully!

Ooh no, something went wrong!