3. FAN TEST - RESULTS AND DISCUSSIONS
-> continued pg.4
3.7
Observation, calculation, and errors in band brake testing
When weight were added into the band
brake, the tension difference increased generally while the rotor decelerated
to a lower rpm or simply stalled. The rotor rpm was constantly fluctuating
with a big amplitude so it was hard to determine the stalling point or
the maximum possible torque. The situation is further worsened when this
high inertia rotor decelerated slowly. During testing, readings from one
weight increment needed at least 5 minutes to finalize. Due to limited
time, only 3 sets were taken. By increasing number of sets and making
smaller weight increment, this problem might be solved but it is too time
consuming to be practical in current situation.
Rotor torque was calculated from the equation below.
Graph 10 and 11 show the torque and power increased when rpm decreased.
The highest data points are questionable because of the fluctuation and
slow respond. Left portion of the torque curve is not obtainable because
of the shape of the curve itself. The rotor will either stall or accelerate
to right portion of curve when operating in this left portion.
Graph 10: Torque vs. rpm for 3 fans at highest speed using band brake
method
Graph 11: Power vs. vs. rpm for 3 fans at highest speed using band brake
method
The major causes of errors in the results are:
- High fluctuation might cause earlier stalling at lower torque while
slow respond might lead to the recording of higher torque than stalling
point. Both problems also produced inaccurate rpm reading.
- Band brake might not applying uniform friction. The small but fast vibration
on the spring scale might indicate that the friction is not applied continuously.
- Wind from fans was not uniform, not continuous but in intermittent gust
form, and fluctuate randomly.
3.8
Band brake result and comparison with inertia result
Ct and Cp curves for band brake method using
3 fans at highest fan speed are shown in Graph 12 and 13. The wind with
speed of 4.25m/s produced maximum Cp of 7% at tsr of 2.75 when measured
using band brake. The maximum Cp might not be the peak Cp as the curve
did not show the peak shape. The maximum tsr is 3.3.
Graph 12: Ct curves for both brake and inertia method
Graph 13: Cp curves for both brake and inertia method
The curves from inertia acceleration method with
similar number fans and fan speed are also shown in the same graphs. Although
the fans’ setting was similar, the average wind speed was different
because the fans were unfortunately rearranged between the two testings.
Additional curve from medium fan speed were added as comparison. The average
wind speeds for inertia acceleration method under ‘high’ and
‘medium’ fan speed are 4.46 and 4.03m/s. The graphs show that
the result from both methods did not agree well. The data points from
brake method were slightly higher (ignore the 1st set last and highest
point because very probable erroneous) even when its’ wind speed
was lower. More data points from brake method will be needed to study
the agreement between the two methods.
From the observation in these testings,
the inertia acceleration method is clearly better than brake method. Both
methods did not give similar result but the inertia acceleration method
is less subjected to critical errors so the results are deemed as more
accurate. Furthermore, the inertia testing yielded more complete curve
with acceptable data scatter, shorter testing time, easier procedure,
and less equipment. Therefore, this method will be used for future testings
in wind tunnel.
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