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7/23/2019 2134-2discussion
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Comment on the relative advantages and disadvantages of
Venturimeter, orifice plate meter and rotameter as flow measuring devices.
From table 3, it is observed that loss factor Kv is the smallest.
This suggests that the venturimeter may give more accurate measurement as
compared to that of orifice plate.
its experimental flow rates that are quite close to the theoretical value (d
= 1.0178 1).
This implies that it is also able to measure a greater range of values of flow rate
as the will be less energy lost at high flow velocities.
The meter has a smooth internal surface that helps to reduce friction as the water
flows through it.
!oreover, the gradual reduction and expansion in the diameter of the tube
reduces this loss.
"owever, to cater for the gradual reduction and expansion, a longer tube is
required and the #enturimeter occupies quite a lot of space($%& ratio ofapproximate ') and is an expensive device as it is more difficult to construct
compared to the other device.
*t also cannot be altered for measuring pressure beyond a maximum velocity.
The orifice plate meter is fairly accurate in its measurement
*t is also the easiest to construct.
+rifice meter occupies less space thus it could be useful to measure fluid flows within space
constraints
The orifice plate meter has a significant head loss.
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*ts level of accuracy falls as the flow rate rises. The sudden contraction of the
diameter causes high loss factor due to turbulent flow.
tube with diameter larger then the orifice meter plate is required to reduce this
energy loss.
Furthermore, as the diameter of the tube is reduced, the angle between the surface of t
he tube with the hori-ontal should not be too large, so as to reduce energy loss and improve
accuracy, but that would demand even more hori-ontal space.
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Comment on the head losses associated with all the flow meters
studied in this experiment, emphasising the relationship between the
mechanism of loss generation and its magnitude
For the venturimeter, its long section enables gradual constriction and expansion
of diameter.
The head loss is thus caused by the dissipation of energy via the friction betweenthe fluid and the inner surface of the pipe.
*t has been observed that the head loss in the venturimeter is relatively
small relative to the orifice plate.
*n the #enturi meter, the gradual reduction and expansion of the diameter
reduces the separation of flow as well as reduces the separation in the
deceleration portion of the meter.
"ence, the energy loss is mainly due to friction with the wall of the tube rather to
inefficient mixing and separate flow
The orifice plate has a significant head loss when compared to the other parts of
the apparatus. #+is observed to be very high too.
s fluid velocity across the plate is relatively high, the sudden contraction
and expansion of the diameter before and after the orifice place may result in a
sharp and significant change in momentum about the orifice, causing
turbulence to form on both ends.
The turbulence increases as the flow increases.
onsequently, this turbulence dissipates energy, thus resulting in the head loss
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"ence the energy loss may be due to the high energy flow across the small
orifice plate, resulting in a significant loss in energy.
ith the orifice plate experiencing as much as ' times more head loss
it can be implied that turbulence contributes to significantly higher energy loss than friction
with the internal walls.
Explain with the aid of simple sketches what is the
vena contractor of an
orifice meter.
#ena contracta is the point in a fluid stream where the
diameter of the stream isthe least. The contraction ta/es place at a section slightl
y downstream from the
orifice, where the fluid flow is hori-ontal. 0treaml
ines will converge 1ust
downstream of the diameter change, and a region of sep
arated flow occurs from
the sharp corner of the diameter change and extends past
the vena contracta.
#ena contracta is the narrowest central flow region
of a 1et that occurs 1ust downstream to the
orifice plate. *t is characteri-ed by high velocity
, laminar flow. $aminar flow, sometimes
/nown as streamline flow, occurs when a fluid flows
in parallel layers, with no disruption
between the layers.
The vena contracta refers to the point in the fluid stre
am where the diameter of
the streamlines is the smallest, and it occurs 1ust slightly d
ownstream of the
orifice, where the flow is nearly hori-ontal and is
concentric with the orifice and
flow channel. fter the vena contracta, the streamlines
diverge and a region of
separated flow occurs and extends past it.
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cross section of a circular orifice of diameter &o
is shown. The thic/ness of the wall is
assumed small compared to the diameter of the orifice.
&ue to the convergence of the
streamlines approaching the orifice, the cross section o
f the 1et decreases slightly until the
pressure is equali-ed over the cross2section, and the ve
locity profile is nearly rectangular.
This point of minimum area is called the
vena contracta
. eyond the vena contracta, friction
with the fluid outside the 1et (air) slows it down, an
d the cross section increases perforce.
This divergence is usually quite small, and the 1et is ne
arly cylindrical with a constant
45
velocity. The 1et is held together by surface tension, o
f course, which has a stronger effect the
smaller the diameter of the 1et
. omment on the limitations and ma1or sources of err
or in this experiment
$imitations of the experiment6
2 The experiment enables only a small range of flow measured. The
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apparatus does not allow measurement over larger range.
2 This experiment is conducted using water. pparatus w
ill not wor/ for fluid
with higher viscosity as very large energy will be requi
red.2 The weighing tan/ is of limited si-e, not large enou
gh to measure
rotameter reading of value lesser than 7. The water i
n tan/ will overflow.
!a1or sources of errors6
8
The reading of manometer. The readings may not be acc
urate as it tends
to 1ump, even after allowing some time before the reading is ta/en. Thus
reading ta/en might not be the actual result.
8
The manometer is not totally transparent, due to prese
nce of water vapour
and bubbles in the tube, thus readings may not be accurat
e.
8
!easuring of a few meters using the same apparatus mayresult in more
error in the measurements and calculation.
8
9sage of stopwatch and weight to measure the mass flow
rate may not be
accurate due to human errors.
8
:arallax error reading from the apparatus may occur du
e to the colour ofthe fluid used.
onclusion
From the experiment, we are able to recogni-e the wor
/ing mechanism of the
venturimeter, orifice meter and rotameter used to measu
re flow.
e have also compared the advantages and disadvantages of
the three meters,
as well as calculated the head loss from each type of meter. From these, we are
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able to better understand the principles behind the w
or/ing mechanism of each
meter, the efficiency of each meter and thus determine
wisely which meter is
better suited for various applications.
*n this experiement, the 3 devices 2 the venture me
ter, orifice meter and the rotameter has been close
ly
examined.
lso, the coefficient of discharge , d
, for the
#
enturi meter and orifice meter is determined; where
d
for
the
#
enturi meter is .4, while ?d
for the orifice meter is .3
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measurement over a larger range of flow rates.
2 This experiment cannot be conducted with fluids of gre
ater viscosity as a
lot of energy will be required to pump the fluid aro
und the experiment setup. lso corrosive fluids or fluids at higher temperatur
es cannot be
experimented with as some of the connections and tubing
s are less
resistant to corrosion and% or high temperatures.
2 The weighing tan/ is of limited si-e, and is not suitab
le to be used to time
greater mass flow rates of fluids and inaccuracies will ari
se if it fills up toofast. lso, the water in tan/ will overflow easily if the
flow is too high.
2 lso, the experiment may not be suitable for dar/ colo
ured or opaque
fluids as the rotameter float has to be visible to ta/e
that reading.
!a1or sources of errors6
8
The pressure of the pump is not constant and fluctuates quite a bit.
This will result in estimation errors while trying to r
ead the measurement
values off the instruments. The readings will fluctuate,
even after allowing
the set up some time to settle. This problem may be solved
by using a
stabiliser for the pump pressure or a & pump.
8 ondensation and dirt build up occurs inside the manome
ter tubes,
ma/ing them not totally transparent. This may cause inaccura
cies in the
readings, even if efforts are ta/en to reduce parallax e
rrors. onducting
the experiment in a dryer environment may reduce the r
is/ of
condensation.8
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9sing a hand stopwatch and manually loading weights to
measure the
mass flow rate may not be accurate due to human errors an
d bouncing of
the lever armature. *t may be better to use an electronic timer attached to
the contact point between the arm and the frame and u
sing mechanically
loaded weights or a calibrated spring to counter the
weight of the water.
*mplementing this will ma/e the experiment more accura
te but will also
increase the overall cost.
8 :arallax error reading from the apparatus may occur as
the manometer
diameter is small. lso, at the thic/ness, surface tension
effects may
become significant, resulting in inaccurate readings.
8
The tubings and connections may not be totally airtight
or watertight and
any lea/age of either fluid will result in inaccuracies in the reading ta/en.
onclusion
From this experiment, we have become more familiar wit
h the wor/ing
mechanisms of the various types of flow measuring devices such
as the #enturi
meter, orifice meter and rotameter. e have also dete
rmined the coefficient of
discharge of the #enturi meter and calibrated the rotameter.
e have also compared the advantages and disadvantages of
the three meters,
and calculated the head loss from each measuring apparat
us. From these, we
are able to better understand the principles behind t
he wor/ings of each meter,
and determine which meter is better suited for various a
pplications. Thus, * feelthat this experiment has been a satisfactory one.
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)
"uman reaction time and parallax is one possible error.
b)
ater is used in this experiment. ny other substances th
at have higher viscosity may not
be used. There still exist some air bubbles inside the tub
e that could have led to
inaccurate readings and affect subsequent calculations.
c)
The flow rate in the experiment is control by the pum
p which may not pump water at a
constant rate
.
d)
Friction at the 1oints of the lever system may cause
inaccuracies when weighing the
water.
e)
The readings on the manometer are always fluctuating and
consistent results cannot be
obtained.
+E$90*+E 6The usage of the venture meter, orifice meter and the r
otameter has been examined. From the
three, the venture meter gives us the most accurate re
ading and the rotameter involves the
most heat loss. e can also see that the main reaso
n for any energy loss is due to turbulent
flow, friction and drag loss. *n order to obtain an acc
urate reading, the pressure and velocitydifference had to be significant across the meters.
oefficient of discharge for venturimeter and orifice
!eter,
d
A 4.4=>
A .'
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