06.09.2021 Views

Basics of Fluid Mechanics, 2014a

Basics of Fluid Mechanics, 2014a

Basics of Fluid Mechanics, 2014a

SHOW MORE
SHOW LESS

Create successful ePaper yourself

Turn your PDF publications into a flip-book with our unique Google optimized e-Paper software.

468 CHAPTER 11. COMPRESSIBLE FLOW ONE DIMENSIONAL<br />

11.7.11 More Examples <strong>of</strong> Fanno Flow<br />

Example 11.22:<br />

To demonstrate the utility in Figure (??) consider the following example. Find the<br />

mass flow rate for f =0.05, L =4[m], D =0.02[m] and pressure ratio P 2 /P 1 =<br />

0.1, 0.3, 0.5, 0.8. The stagnation conditions at the entrance are 300K and 3[bar] air.<br />

Solution<br />

First calculate the dimensionless resistance, 4 fL<br />

D .<br />

4 fL<br />

D = 4 × 0.05 × 4 =40<br />

0.02<br />

From Figure ?? for P 2 /P 1 =0.1 M 1 ≈ 0.13 etc.<br />

or accurately by utilizing the program as in the following table.<br />

M 1 M 2<br />

4fL<br />

D<br />

4fL∣ D 1<br />

4fL∣ P 2<br />

D 2 P 1<br />

0.12728 1.0000 40.0000 40.0000 0.0 0.11637<br />

0.12420 0.40790 40.0000 42.1697 2.1697 0.30000<br />

0.11392 0.22697 40.0000 50.7569 10.7569 0.50000<br />

0.07975 0.09965 40.0000 107.42 67.4206 0.80000<br />

Only for the pressure ratio <strong>of</strong> 0.1 the flow is choked.<br />

M<br />

T<br />

T 0<br />

ρ<br />

ρ 0<br />

A<br />

A ⋆<br />

P<br />

P 0<br />

A×P<br />

A ∗ ×P 0<br />

0.12728 0.99677 0.99195 4.5910 0.98874 4.5393<br />

0.12420 0.99692 0.99233 4.7027 0.98928 4.6523<br />

0.11392 0.99741 0.99354 5.1196 0.99097 5.0733<br />

0.07975 0.99873 0.99683 7.2842 0.99556 7.2519<br />

Therefore, T ≈ T 0 and is the same for the pressure. Hence, the mass rate is a<br />

function <strong>of</strong> the Mach number. The Mach number is indeed a function <strong>of</strong> the pressure<br />

ratio but mass flow rate is a function <strong>of</strong> pressure ratio only through Mach number.<br />

The mass flow rate is<br />

ṁ = PAM<br />

√<br />

k<br />

RT<br />

= 300000 ×<br />

π × 0.022<br />

4<br />

× 0.127 ×<br />

√ ( )<br />

1.4<br />

kg<br />

287300 ≈ 0.48 sec

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

Saved successfully!

Ooh no, something went wrong!