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Information Report

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Experiment

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Abstract
This experiment was conducted to comprehend the devices used for flow measurement and todetermine and compare the coefficient of discharge, C
d
for a series of flow measurement devices. Since the coefficient of discharge is the factor that influences the actual discharge by the flow meter, the C
d
for both venturi meter and orifice plate meter [Ref.2 were being compared based on the experimental results. The outlet pipe was ad!usted to four different height for each of the flow measurement devices, of which the corresponding reading is recorded, tabulated and analysed. The experimental result depicted that coefficient of discharge of devices varies with the type of devices. The venturi meter gave the higher coefficient of discharge than the orifice plate meter. ence, the actual discharge by the venturimeter was higher to the orifice plate meter. The experiment was concluded that the venturi meter gives more accurate reading in flow measurement compared to orifice plate meter.
1.0 Introduction
#n many systems, the measurement of the rate of flow of a fluid is of primary importance.The water flow measuring apparatus is designed as free$standing apparatus for the use on thehydraulics bench. %mong the types of flow measuring devices, the most commonly used onesare the flow obstruction devices. These flow meters are based on the principle that the changein the static pressure or pressure drop or head can be related to the change in the velocitycaused by a change in the cross$sectional area of the flow. The relationship between flow rateand pressure difference is illustrated by the &ernoulli e'uation, assuming that changes inelevation, wor( and heat transfer to be negligible. Some typical head meters are described briefly in the following.%n orifice plate is a restriction with an opening smaller than the pipe diameter which isinserted in the pipe) the typical orifice plate has a concentric, sharp edged opening *figure +.
-igure + /rifice plateThe small aperture between orifice plate gives a smaller area and accelerates the flow of fluid, causing a corresponding decrease in pressure. &y using &ernoulli0s e'uation, the flowrate can then be calculated from the measured pressure drop across the orifice plate, 1
+
$1
.The orifice plate is commonly used, however, the plate will bring about turbulence an largeamount of non$recoverable pressure drop, contributing to inefficiency in measuring flow rate.% venturi meter *figure 2 wor(s similar to an orifice meter, but it is designed to closelyeliminate boundary layer separation, and thus form drag. #t is manufactured from transparentacrylic materials and follows the classic 2 $ + convergent$divergent design. The change in
1 0
cross$sectional area in the venturi tube allows the flow rate to be determined from the pressure change between the convergent section and the throat. Though venture meter is moreexpensive than an orifice plate) it gives substantially lower non$recoverable pressure drop.-igure 2 3enturi 4eter
2.0 Theory
132
3enturi 4eter Referring to the venturi tube diagram *figure 2, the &ernoulli e'uation can be applied to points + and 2. -ollowing the analysis, the e'uations for flow rate can be derived.3olumetric flow rate
2++
+,*2
−−==
A Ahh g AV AQ
th
*+5here)
Q
th
6 theorietical volumetric flow rate *m
7 s%
+
6 cross sectional area at + *m
2
%
6 cross sectional area at *m
2
h+6 height of manometer column + in meters *mh26 height of manometer column in meters *mThe discharge coefficient is defined as the ratio of actual volume rate to theoretical volumeflow rateCoefficient of discharge,
ltheoreticaactual
QQC
d
=
*2The discharged coefficient is less than unity due to the losses caused by the wall shear stress,the losses in contraction and the losses during expansion. *
This e'uation can be written as8og 9
act
6 8og n : a8og ;h *<in order to find n and hence C
d
experimentally, using a graph 8og 9
act
versus 8og ;h/rifice 1late 4eter Referring to the orifice plate diagram *figure +, the &ernoulli e'uation can be applied to points + and . -ollowing the analysis, the e'uations for flow rate can be derived.3olumetric flow rate
( )
2
+.2
mh g aQ
th
−∆=
*=5here
Q
th
6 theorietical volumetric flow rate *m
7 sa 6 cross$sectional area of plate *m
2
m6 ratio of cross$sectional area of plate to pipe *a7%;h6 difference in height of manometer column *mThe discharge coefficient is defined as the ratio of actual volume flow rate to theoreticalvolume flow rate Coefficient of discharge,
ltheoriticaactual
QQC
d
=
(6)
(7)

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