Following this experiment, it
is evident that the analysis of a simple device like the converging diverging
nozzle is more involved than was originally anticipated. For the results
obtained for the maximum mass flow rates against the back pressure in Figure 6, it is felt that the values
are counter intuitive and prove to be correct. These results can be used could
be used safely for non-critical approximations if desired. The second part of
the lab, measuring the pressure distribution along the axis of the nozzle as a
function of the pressure ratio gave a good indication of the flow patterns that
occur in the nozzle and how they can be used to achieve desired flows if
It has to be mentioned that there could
have been possible errors made by students when reading the manometer (although
these are unnecessary mistakes they still need to be mentioned)
There possibly may have been errors
that went unnoticed in the lab with pressure gauges which could account for a
small level of error.
One of these errors may be the positive
displacement of the compressor in the apparatus. There were minor fluctuations
that occurred due to its method of operation in load and unload cycles. Even
with the pressure regulator in the system, it provided the system to smoothen
out these fluctuations but there still may be some margin of error present. A
very high level of maintenance and calibration would be necessary to reduce
these to a certain extent.
It was felt after the conclusion of the
experiment that a number of factors could have caused discrepancies between the results of the experiment.
During the experiment, these values
were recorded in the table given to us in the brief and plotted in Figures 6,7 and 8. To achieve a flow
pattern which can be attributed to the design condition, where the supersonic flow
at the nozzle exit contains no shock waves and is not chocked at the throat. The
plotted line must decrease smoothly to a value above the unexpanded flow
condition. On examining Figure 7 it
is seen that we achieve no value consistent with the design condition. This can
be attributed to the pressure drop, step size of approximately 50 kPa.
The objective of the Compressible Flow
lab experiment was to measure the pressure distribution within the nozzle over
a range of pressure ratios. In the nozzle, the isentropic expansion of the
fluid to a supersonic flow is dependent on the back-pressure ratios applied to
the system. By measuring the linear variation of pressure at different lengths
through the nozzle it can be determined from Figures 7 and 8 with what
type of flow is occurring within the system.