Figure 5 shows the variation of the mass flow

Figure 5 shows the variation of the mass
flow rate against back pressure through the orifice. The mass flow rate remains
constant until the point of 320 back pressure where it then begins to drop. The
drop is quite sharp and as it reaches the back pressure, the mass flow rate
eventually decreases to zero. The theoretical maximum flow rate, max, is 0.008 kg s-1 which is when the probe is fully inserted
into the nozzle. The maximum value based on the results is 0.0056 kg s-1.
The reason for this difference is due to the theoretical equation not accounting
for the frictional losses occurring in the pipe of turbulence created from the
orifice.

The analysis of figure 5 shows that the
mass flow rate for was exactly the same for the values of back pressure from 0
to 150 kPa with a negligible drop until 280 kPa where the flow of the fluid is
initially subsonic, sonic at the throat, supersonic at the exit of the throat
until it experiences a shock and settles down to subsonic speeds. The mass flow
rate then sharply declines from back pressure 320 to 370 kPa. Here the flow of
the liquid is as before except that when it leaves the throat, it exits at
subsonic speeds without a shock. Finally, at the 390 kPa back pressure, the
mass flow rate is quite low hence the fluid is flowing at subsonic speeds
throughout. Pipe

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Figures 6 and 7 show the variation of streamwise
pressure within the nozzle for various back pressures at supply pressures of
250 kPa and 350 kPa respectively. For the back pressure values of 200 kPa in
figure 6, the system flows at subsonic speeds throughout. This is because the trend
line maintains a relatively steady gradient and never reaches a sharp turning
point. For the back pressure values of 150 kPa in figure 6 and 250 kPa in
figure 7, the adverse pressure gradient changes at the sonic throat which means
they reach Mach 1 and then slow down to subsonic speeds. This checks out as
both have a turning point from a region of the sonic throat at approximately
100 to 120 mm. Note that a lower supply pressure shows a lower pressure
gradient (p/p0).

For all other back pressure
values, they initially pass through the regime of subsonic speed followed by sonic
speed at the throat. After passing the throat, they briefly go through
supersonic speed before experiencing shock and returning to subsonic speeds.
The shock wave accompanied by flow deceleration is when the outlet pressure is
above the design value. This can be seen on the graph as the remaining trendlines
start with a fairly constant gradient before slowing decreasing in pressure and
as the value of the back pressure gets closer to the supply pressure, there is a
sharp decrease on gradient to zero and subsequently levelling off.

Conclusion

From these
experiments, it is concluded that the method of accelerating a gas from rest to
supersonic speeds is by accelerating the fluid subsonically through a
convergent-divergent nozzle is successful. It confirms the theory that the mass
flow rate remains constant when pressure at the nozzle exit is reduced. Furthermore,
the speed of the flow cannot exceed Mach 1 at the throat as the mass flow rate
is negligible and fluid cannot flow at supersonic speeds at the throat. The
Mach number is the ratio between the fluid velocity and the velocity of sound.