2. FAN TEST - EQUIPMENT AND PROCEDURES
2.1 Setting up
the rotor, the fans, and the equipment
Description of the rotor model
The VAWT model, as shown in Photo 1, is a simple small fixed pitch type
straight bladed Darrieus rotor. The size of the model is 0.5m x 0.5m.
The rotor has 3 NACA0015 airfoil blades with solidity of 0.42. The solid
airfoils were shaped from ‘Jelutong’ wood strip. Each of the
airfoil is supported at the centre by a pair of flat plates. The rotor
is mounted to a shaft, which rotates on a pair of deep groove bearings.
A pipe and frame structure supports the rotor. A small pulley with reflective
tape is attached to the lower end of the shaft. More components and fabrication
details can be found in the Detail and Assembly Drawing section.
Photo 1: The rotor clamped to stools and positioned in front of 3 large
fans
Positioning the fans and the rotor
The rotor was placed on top of 4 stools. The base of frame was clamped
to the stools. Pieces of tire rubber were inserted in between the frame
and stools to help tighten the clamps and to damp the vibration. Weights
were placed on top of the frame to further steady the rotor as shown in
Photo 1.
3 large fans were lined side by side at about 1.65m in the front of the
rotor as shown in Photo 1. Height of fans were adjusted until its’
hub were leveled with rotor’s hub. The centre fan was pointed straight
to the rotor centre while the two side fans were aimed slightly more toward
the rotor instead of pointing straight forward so that the wind can reach
the gap area between the fans. A thin stick with thread tied at one end
can be used to roughly check the direction and strength of the wind at
any point in front of rotor.
Measuring the wind speed
Wind speed was measured using a handheld propeller type anemometer. Instead
of holding the anemometer directly, the meter was tied to one end of a
thin stick to reduce obstruction by the reader’s body. Wind speeds
from 9 points in front of the rotor (swept area divided equally to 9 squares,
speed taken at centre of each squares) were collected and averaged. Prior
to the measurement, the rotor blades were held stationary in an angular
position where it did not obstruct the wind flow. A horizontal wood bar
and a vertical stick with markings, as shown in Photo 2, were used to
roughly locate these points. Wind speeds at distance of 1.5 times radius
from both sides of the rotor were also collected.
Photo 2: The wood bar and the vertical stick with markings for locating
points for wind speed measurement using the blue anemometer
2.2 Band brake
testing method
Setting up the band brake and the stroboscope
All the three airfoils were marked with the sign ‘-‘, ‘x’,
and ‘o’ respectively at the centre to facilitate stroboscope
measurement. The light of stroboscope was aimed toward the centre of rotor
as shown in Photo 3. If blade images with a single sign were captured
by the stroboscope then the corresponding frequency is the rotor rpm.
If the images’ sign change at same pattern every time, in this case
‘-xo’, then the corresponding frequency is 3 times the rpm.
Photo 3: The band brake loads the rotor while the stroboscope captures
the markings on the blades
The band brake, as shown in Figure 1 and Photo
4, consists of a cotton rope with one end pulled by a spring scale while
the other end was loaded with small weights. The cotton rope was tightened
around the small pulley below the rotor. Rope type, rope contact angle,
and pulley size can be adjusted to obtain the best reading from the brake.
Figure 1: Band brake system
Photo 4: The band brake system consisting of a spring scale, weights,
and a cotton rope
Measuring rotor torque from band brake
First, the rotor was accelerated to the maximum rpm in unloaded condition.
When the rpm stabilized, the maximum rpm was obtained from the stroboscope.
Next, small weights were added to the brake. The corresponding rpm and
spring scale measurement were collected. The small weights were added
until the rotor stalled. The time required to read the fluctuating rpm
was long so the test was performed for 3 fans at ‘High’ speed
condition only.
2.3 Inertia acceleration testing
method
Setting up the tachometer and the reflective
tape
The tachometer was positioned to emit the beam straight to the reflective
tape so that the deflected beam will fall back on the tachometer as shown
in Photo 5. Cloth clipper was used to continuously press down the ON button.
The timer was placed near to the meter.
Photo 5: The tachometer aimed to the reflective tape on the rotor with
the timer nearby
Recording the rpm changes from the tachometer
The rpm of the unloaded rotor was recorded from startup to maximum rpm
for every 5 seconds interval. The test was performed under 3 fans with
3 different speed setting (high, medium, and low). Most of the time, the
rotor was rotated to the highest torque position (one of the airfoils
exactly parallel to the wind at counter wind half while the other two
get pushed by wind at the wind assisted half) first before releasing it
so that testing time can be reduced. Also, during the ‘low’
fan speed setting, the rotor remained at slow rotation refusing to accelerate
up so the rotor had to be assisted by rotating the pulley using hand until
the acceleration pick up. The rpm was recorded only when the rotor acceleration
start to pick up. The test was repeated for 2 fans and a single fan as
well but using the maximum fan speed only.
2.4 Additional testing and calibration of tachometer and stroboscope
Calibrating rpm readings from tachometer to stroboscope
The 3x rotor rpm was obtained from the stroboscope and at the same time
the tachometer reading was jotted down. Readings were taken conveniently
between the maximum rotor rpm of 450 to 520.
Estimating the refresh rate of the tachometer
reading display
The time needed for the tachometer to display 10 readings under constant
rotor rpm was measured. A few measurements were taken at 120, 200, and
500rpm.
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