Compressible Flow Free Textbook

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The Pressure Ratio, P2/P1, effects

In this section the studied parameter is the variation of the back pressure and thus, the pressure ratio $P_2 \over P_1$ variations. For very low pressure ratio the flow can be assumed as incompressible with exit Mach number smaller than

. As the pressure ratio increases (smaller back pressure,

), the exit and entrance Mach numbers increase. According to Fanno model the value of $\frac{4fL}{D}$ is constant (friction factor,

, is independent of the parameters such as, Mach number, Reynolds number et cetera) thus the flow remains on the same Fanno line. For cases where the supply come from a reservoir with a constant pressure, the entrance pressure decreases as well because of the increase in the entrance Mach number (velocity).

Again a differentiation of the feeding is important to point out. If the feeding nozzle is converging than the flow will be only subsonic. If the nozzle is ``converging-diverging'' than in some part supersonic flow is possible. At first the converging nozzle is presented and later the converging-diverging nozzle is explained.

**Figure:** The pressure distribution as a function of 4fL/D for a short 4fL/D
$\begin{figure}\centerline{\includegraphics {cont/fanno/tubePressureLoss}}\end{figure}$

Subsections

Next: Choking explanation for pressure Up: Working Conditions Previous: Fanno Flow Supersonic Branch Index

Created by:Genick Bar-Meir, Ph.D.
On: 2007-11-21