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Name:____________________ Lab Section : ________________ Date:____________

ME4751, Energy Systems Laboratory

Fan Performance Experiment

Objective

The objective of this experiment is to obtain a performance and an efficiency curve of a

centrifugal fan.

Experimental Set-Up

The experimental apparatus is shown in Fig. 1 and schematics of flow passage are shown in

Figure 2. Air is drawn from the atmosphere through a centrifugal fan and passes through

rectangular duct with varying cross section and exits to the atmosphere through a thin plate orifice

at the end of the flow passage. Pressure of the air is measured at two locations: one near the fan

and the other at the orifice. Temperature of the air in the duct is also measured.

Figure 1. Experimental Apparatus

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Theory

One dimensional energy equation between any two points in the flow passage is

��� � ����� � � �� ��� � ����� � � � �� � �� (1)

Let the flow conditions of the atmosphere are denoted by subscript ‘1’, and the conditions after

passing through the fan by subscript ‘2’. Neglecting elevation change and all losses, application of

Eq. (1) to the air flow passing through the fan results in

�� ������ � ����� (2)

Multiplied by the specific weight of the air (� ���, Eq. (2) becomes

���� ∆���� � � ���� (3)

where the velocity is the average air speed at area A1 (Figure 2) pressure rise is measured by a

manometer.

Eq. (3) shows that the energy added to a unit volume of air by the fan is the summation of static

pressure rise through the fan and the kinetic energy of air leaving the fan. The total rate of energy

added to the air by the fan is then

���� ����� (4)

where Q is the volume flow rate of air. The volume flow rate is measured by a thin orifice

method.

The efficiency of the fan is defined by

!"#$!%&'( (5)

where the ‘Power’ is the electrical energy supplied to the motor and is directly measured by a

watt meter.

The static pressure rise through the fan is measured directly by a manometer. The velocity is

however, measured indirectly using the volume flow rate obtained by a thin orifice attached at the

end of the duct.

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Applying Eq. (1) on the air stream passing through the orifice, we obtain the air flow speed at the

orifice (see Figure 2 for symbols.)

�%()�)*' + �,�-./"/0123�34-./"/014� ��� (6)

where A2 is the duct area near the orifice and the pressure drop across the orifice is measured by

a manometer. Eq. (6) gives an ideal flow speed since all losses in the flow are neglected. The

orifice correction coefficient is obtained by experimental correlation in terms of Reynolds number

and diameter ratio,

56 0.5959 � 0.0312>�.-0.184>? � @.AB�.CD'E.FC (7)

The diameter ratio is defined by

> GHI =%()�)*' G)�K'L'(M�G(�N()* G)�K'L'( %� LM' GN*L (8)

Reynolds number is based on the flow conditions in the duct area A2,

OP 2�4�Q RM (9)

Where flow speed in the larger duct is calculated by using mass conservation

��� �%()�)*' S�-./"/01�� T (10)

For the most of turbulent flow, C0 is close to 0.6.

The total air flow rate is then calculated by

� 56�%()�)*'U%()�)*' (11)

Flow speed in the duct near the fan, needed in Eq. (3), is then obtained by ��� �/U (12)

Laboratory Procedure

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1. Select a fan speed to the desired constant rpm using a tachometer.

2. Mount an orifice securely. Start with smaller size first.

3. Measure ���� and �%()�)*'. Make sure the manometer is leveled and the manometer

fluid level indicates zero reading before taking the measurement.

4. Measure the air temperature. Also measure the power going to the fan motor.

5. Repeat steps 2, 3 and 4 for each orifice diameter. Record the raw data in Table 1.

6. Perform a sample calculation for a selected orifice case to obtain �, ����, and . Include

the results in your report. Assume C0=0.6.

7. Use the spreadsheet program and calculate the remaining cases.

8. Plot Q vs. �_��� and Q vs. .

9. Attach the spreadsheet and the plot to your report.

Figure 2. Schematics of Flow Passage: A1 (0.312 ft2) and A2 (1.1 ft2) are area of rectangular

cross sections.

fan

orifice

thermometer

∆Pfan

∆Porifice

honey comb

A0rifice

A1

A2

power input

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Table 1. Fan Performance Raw Data

Rpm__________

Temperature (F) Orifice diameter,

d (inch)

����

Inch H2O

ΔP[\]^]_` Inch H2O

Power

Watts

.