CONTINUITY EQUATION
Fluid Flow
As shown in Figure 1 the pressure at different levels in the tank varies and this causes the fluid
to leave the tank at varying velocities. Pressure was defined to be force per unit area. In the
case of this tank, the force is due to the weight of the water above the point where the pressure
is being determined.
Example:
Pressure =Force
Area
=Weight
Area
P =
m g
A gc
=
r V g
A gc
where:
m = mass in lbm
g = acceleration due to earth’s gravity 32.17
ft
sec2
gc = 32.17
lbm ft
lbf sec2
A = area in ft2
V = volume in ft3
r = density of fluid in
lbm
ft3
The volume is equal to the cross-sectional area times the height (h) of liquid. Substituting
this in to the above equation yields:
P =
r A h g
A gc
P =
r h g
gc
HT-03
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