Question: What kinds of data will be gathered from the experiment? Explain what we're looking for, what formulas we're using, and what we're learning.

Lab given:

Flow through Orifice and Venturi meter

For Orifice

Objective of lab

1. To determine the pressure drop due to the flow of the fluid through an orifice plate.

2. Determination of the flow using an orifice meter.

3. Determination of the corrective coefficient C of an orifice plate.

Theory Governing experiment

Orifice plates serve as a type of indirect flow meter. They are used to measure volumetric flow based on pressure and velocity variations of fluid flow in a pipe. Orifice plates consist of mostly three shapes: sharp edges, quarter of a circle, and conical inlet.

(Edibon, practical exercises manual)

The orifice plate causes an abrupt change in flow within a pipe. The decrease in cross-sectional area will cause an increase in velocity and decrease in pressure. The design of the orifice plate in the pipe can be seen as.

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Pressure taps can be attached, as seen above, to obtain the change in pressure. Velocity can be calculated from a known diameter and flow rate within the pipe. A closer analysis of the pressure drop can be seen by,

(Edibon, practical exercises manual)

The figure above displays the pressure drop in an orifice plate. Since there is no pump or turbine, Bernoullis equation can be used to analyze the flow across the orifice plate. Subscripts 1, 2, and 3 will indicate the location of interest.

11+1+1212=22+2+1222

(1)

One limitation in Bernoullis equation is that flow must be incompressible. That means that density does not change. The flow must also be in steady state conditions. Mass and volume flow rate must not change.

111=222

(2)

P: Pressure (Pa)

: density (kg/m^3)

: acceleration due to gravity (m/s^2)

: velocity (m/s)

: elevation (m)

: cross-sectional area (m^2)

Q: volumetric flow rate (m^3/s)

Bernoullis equation can be applied to any location within the orifice. It is not only applicable to location 1 and 2. Solving for 1

in equation 2 and plugging to equation 1 at location 3 then solve for 3

,

3=213212

(3)

Volumetric flow rate does not change, it can be set to,

=33

(4)

Plugging equation 3 into equation 4,

=1312322

(5)

The change in pressure in the manometer can be described as

=

(6)

is the change in height in the manometer connected to the pressure taps. Plugging equation 6 into equation 5,

=1312322

(7)

Equation 7 gives an ideal flow across the orifice plate. A corrective coefficient can be introduced to obtain an actual flow. Also, 3

is unknown, 2

is used,

=1212222

(8)

It is important to note the C is not the same as discharge coefficient,

. Corrective coefficient, C, is approximately 0.6. The value of discharge coefficient is very small in the orifice plate. That is because discharge coefficient is affected by geometry of the orifice edge. In comparison with the venturi tube, there is an abrupt change in diameter in the orifice plate. The corrective coefficient in the venturi tube is very small, and therefore uses the discharge coefficient to determine the actual flow.

Experimental set up

The overall set would look like,

(Edibon, practical exercises manual)

• The orifice plate is number 3.

• Venturi meter is number 7

• Connection to hydraulic bench is number 8

• Variable area meter is number 5

• Flow control valve is number 2

• The manometer is number 6

The following are also needed,

• Hydraulic bench

• Graduated test tube

• FME 18 (tube)

• Chronometer

Experimental procedure

Connect the hydraulic bench and switch on the bench. Fill the manometric tubes. Vary the flow of 5 1/min. Take the measurements of the manometric tubes no. 6 and 7. Then, the hydraulic bench is switched off.