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Basics of Fluid Mechanics, 2014a

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

11.7.7.2 Short 4 fL/ D<br />

Figure 11.29 shows different pressure pr<strong>of</strong>iles for different back pressures. Before the<br />

flow reaches critical point a (in the Figure 11.29) the flow is subsonic. Up to this<br />

stage the nozzle feeding the tube increases the mass flow rate (with decreasing back<br />

pressure). Pressure between point a and point b the shock is in the nozzle. In this<br />

range and further reduction <strong>of</strong> the pressure the mass flow rate is constant no matter<br />

how low the back pressure is reduced. Once the back pressure is less than point b the<br />

supersonic reaches to the tube. Note however that exit Mach number, M 2 < 1 and is<br />

not 1. A back pressure that is at the critical point c results in a shock wave that is at<br />

the exit. When the back pressure is below point c, the tube is “clean” <strong>of</strong> any shock 32 .<br />

The back pressure below point c has some adjustment as it occurs with exceptions <strong>of</strong><br />

point d.<br />

2.0<br />

1.8<br />

1.6<br />

M<br />

1.4<br />

1.2<br />

1.0<br />

0.8<br />

shock location at:<br />

75%<br />

50%<br />

5%<br />

0.6<br />

0.4<br />

0.0 0.05 0.1 0.15 0.2 0.25 0.3<br />

4fL<br />

D<br />

Fig. -11.30. The effects <strong>of</strong> pressure variations on Mach number pr<strong>of</strong>ile as a function <strong>of</strong> 4 fL<br />

D<br />

when the total resistance 4 fL<br />

D<br />

=0.3 for Fanno Flow.<br />

11.7.7.3 Long 4 fL<br />

D<br />

In the case <strong>of</strong> 4 fL<br />

D<br />

> 4 fL<br />

D<br />

∣ reduction <strong>of</strong> the back pressure results in the same<br />

max<br />

process as explained in the short 4 fL<br />

D<br />

up to point c. However, point c in this case is<br />

different from point c at the case <strong>of</strong> short tube 4 fL<br />

D<br />

< 4 fL<br />

D<br />

∣ . In this point the<br />

max<br />

exit Mach number is equal to 1 and the flow is double shock. Further reduction <strong>of</strong> the<br />

32 It is common misconception that the back pressure has to be at point d.

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