E1C10 09/14/2010
13:4:48 Page 465
10.28 A thermal mass flow meter is used to meter 30
C air flow through a 2-cm-i.d. pipe. If 25 W of power
are required to maintain a 1
C temperature rise across the meter, estimate the mass flow rate through
the meter. Clearly state any assumptions. c p ¼ 1:006 kJ/kg-K.
10.29 Research available thermal mass flow meters. Discuss the assumptions necessary for their use.
Describe situations where they would be appropriate and not appropriate.
10.30 When used with air (or any perfect gas), a sonic nozzle can be used to regulate volume flow rate.
However, uncertainty arises due to variations in density brought on by local atmospheric pressure
and temperature changes. For a range of 101:3 Æ 7 kPa abs and 10
Æ 5
C, estimate the error in
regulating a 1.4-m
3 /min flow rate at critical pressure ratio due to these variations alone.
10.31 Estimate an uncertainty in the determined flow rate in Example 10.5, assuming that dimensions are
known to within 0.025 mm, the systematic uncertainty in pressure is 0.25 cm H 2 O, and the pressure
drop shows a standard deviation of 0.5 cm H 2 O in 31 readings. Upstream pressure is constant. State
your reasonable assumptions.
10.32 A thermal mass flow meter is used to meter air in a 1-cm-i.d. tube. The meter adds 10 W of energy to
the air passing through the meter, from which the meter senses a 3
C temperature gain. What is the
mass flow rate? c p ¼ 1:006 kJ/kg-K.
10.33 A vortex meter is to use a shedder having a profile of a forward facing equilateral triangle
(Table 10.1) with a characteristic length of 10 mm. Estimate the shedding frequency developed for
20
C air at 30 m/s in a 10-cm-i.d. pipe. Estimate the meter constant and measured flow rate.
10.34 The flow of air is measured to be 30 m
3 /min at 50 mm Hg and 15
C. What is the flow rate in SCMM?
10.35 A 6 in. Â 4 in. i.d. cast venturi is used to measure the flow rate of water. Estimate the flow rate and its
uncertainty at 95%. The following values are known from a large sample:
x
Value
b
x
s
x
C
0.984
0.00375
0
d o
3.995 in.
0.0005
0
d 1
6.011 in.
0.001
0
r
62.369 lb m /ft
3
0.002
0.002
H
100 in H 2 O @ 68
F
0.15
0.4
10.36 A 60-mm-i.d. pipe will be used to transport alcohol r ¼ 790 kg/m
3
; m ¼ 1:2 Â 10
À3 N-s/m
2
À
Á
from
storage to its point of use in a manufacturing process. A flow nozzle with flange taps is selected to
meter the flow. If a pressure drop of 4kPa is desired at 0.003 m
3 /s, specify the nozzle diameter, d o .
Problems 465
13:4:48 Page 465
10.28 A thermal mass flow meter is used to meter 30
C air flow through a 2-cm-i.d. pipe. If 25 W of power
are required to maintain a 1
C temperature rise across the meter, estimate the mass flow rate through
the meter. Clearly state any assumptions. c p ¼ 1:006 kJ/kg-K.
10.29 Research available thermal mass flow meters. Discuss the assumptions necessary for their use.
Describe situations where they would be appropriate and not appropriate.
10.30 When used with air (or any perfect gas), a sonic nozzle can be used to regulate volume flow rate.
However, uncertainty arises due to variations in density brought on by local atmospheric pressure
and temperature changes. For a range of 101:3 Æ 7 kPa abs and 10
Æ 5
C, estimate the error in
regulating a 1.4-m
3 /min flow rate at critical pressure ratio due to these variations alone.
10.31 Estimate an uncertainty in the determined flow rate in Example 10.5, assuming that dimensions are
known to within 0.025 mm, the systematic uncertainty in pressure is 0.25 cm H 2 O, and the pressure
drop shows a standard deviation of 0.5 cm H 2 O in 31 readings. Upstream pressure is constant. State
your reasonable assumptions.
10.32 A thermal mass flow meter is used to meter air in a 1-cm-i.d. tube. The meter adds 10 W of energy to
the air passing through the meter, from which the meter senses a 3
C temperature gain. What is the
mass flow rate? c p ¼ 1:006 kJ/kg-K.
10.33 A vortex meter is to use a shedder having a profile of a forward facing equilateral triangle
(Table 10.1) with a characteristic length of 10 mm. Estimate the shedding frequency developed for
20
C air at 30 m/s in a 10-cm-i.d. pipe. Estimate the meter constant and measured flow rate.
10.34 The flow of air is measured to be 30 m
3 /min at 50 mm Hg and 15
C. What is the flow rate in SCMM?
10.35 A 6 in. Â 4 in. i.d. cast venturi is used to measure the flow rate of water. Estimate the flow rate and its
uncertainty at 95%. The following values are known from a large sample:
x
Value
b
x
s
x
C
0.984
0.00375
0
d o
3.995 in.
0.0005
0
d 1
6.011 in.
0.001
0
r
62.369 lb m /ft
3
0.002
0.002
H
100 in H 2 O @ 68
F
0.15
0.4
10.36 A 60-mm-i.d. pipe will be used to transport alcohol r ¼ 790 kg/m
3
; m ¼ 1:2 Â 10
À3 N-s/m
2
À
Á
from
storage to its point of use in a manufacturing process. A flow nozzle with flange taps is selected to
meter the flow. If a pressure drop of 4kPa is desired at 0.003 m
3 /s, specify the nozzle diameter, d o .
Problems 465