Chapter 5
: Applications of Equations of Motion and Mechanical Energy
Lecture 15 :
Flow Nozzle
The flow nozzle as shown in Fig.15.4
is essentially a venturi meter with the divergent part omitted.
Therefore the basic equations for calculation of flow rate
are the same as those for a venturimeter.
The dissipation of energy downstream
of the throat due to flow separation is greater than that
for a venturimeter. But this disadvantage is often offset
by the lower cost of the nozzle.
The downstream connection of the manometer
may not necessarily be at the throat of the nozzle or at
a point sufficiently far from the nozzle.
The deviations are taken care of in the
values of Cd, The coefficient Cd
depends on the shape of the nozzle, the ratio of pipe to
nozzle diameter and the Reynolds number of flow.
Fig 15.4
A Flow nozzle
A comparative picture of the typical values of Cd, accuracy, and the cost of
three flow meters (venturimeter, orificemeter and flow nozzle) is given below:
Type of Flowmeter
Accuracy
Cost
Loss of
Total Head
Typical Values
of Cd
Venturimeter
High
High
Low
0.95 to 0.98
Orificemeter
Low
Low
High
0.60 to 0.65
Flow Nozzle
Intermediate between a venturimeter and an orificemeter
0.70 to 0.80
End
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