110
4 Mine Ventilation Networks
0
10000
20000
30000
40000
50000
60000
0
20
40
60
80 100 120 140 160 180 200
Pressure (Pa)
Flow rate (m 3 ·s -1 )
A =1 m 2
A =2 m 2
A =3 m 2
Fig. 4.7 Flow rates generated by different pressure differences for three equivalent orifice areas.
Note the conceptual similarity with Fig. 4.1
tendency for the system to lose pressure due to friction. After the orifice, we find
the vena contracta. In it, the fluid accelerates due to the smaller cross section, which
results in an increase in speed at the expense of a reduction of the static pressure.
Static
pressure
Permanent loss
Recovery
Distance
P 1
P 2
P 3
Vena
contracta
Fig. 4.8 Recovery and permanent pressure losses for a stream crossing an orifice
After the vena contracta, the air stream expands so that the speed decreases and
the static pressure increases again. If the static pressure measurement is carried out
at a sufficient distance from the wall, the speed will have recovered again to the
pre-orifice values. This measurement represents the permanent losses of the system.