Page 197 - 48Fundamentals of Compressible Fluid Mechanics

9.7. WORKING CONDITIONS 159

plained.

a shock in
the nozzle
fully subsoinic
flow

critical Point a
criticalPoint b critical Point c

critical Point d

Fig. 9.11: The pressure distribution as a function of for a short

Converging nozzle

9.7.2.1 Choking explanation for pressure variation/reduction

decreasing the pressure ratio or in actuality the back pressure, results in increase
of the entrance and the exit velocity until a maximum is reached for the exit ve-
locity. The maximum velocity is when exit Mach number equals one. The Much
number, as it was shown in the chapter 4, can increase only if the area increase.
In our model the tube area is postulated as a constant therefore the velocity can-
not increase any further. However, the flow to be continuous the pressure must
decrease and for that the velocity must increase. Something must break since the
conflicting demands and it result in a “jump” in the flow. This jump and it is referred
to as a choked flow. Any additional reduction in the back pressure will not change
the situation in the tube. The only change will be at tube surroundings which are
irrelevant to this discussion.