3. FAN TEST - RESULTS AND DISCUSSIONS
-> continued pg.3
3.5 Variation of performance with wind
speed
The Cp and Ct curves for 3 fans at various fan speeds are shown in Graph
6 and 7 respectively. The ‘high’, ‘medium’, and
‘low’ fan speed settings created wind with average speed of
4.66, 4.03, and 3.66m/s respectively. For each fan speed, 4 sets of collected
data were plotted together.
Graph 6: Ct vs. tsr for 3 fans at various fan speeds
Graph 7: Cp vs. tsr for 3 fans at various fan speeds
The graphs show that when the rotor was subjected
to a higher wind speed, both the Cp and Ct curve ‘spread out’
to become much taller and slightly wider than the curves obtained from
lower wind speeds. However, this behaviour was predicted from graphs with
small distances between curves and considerable amount of data scatter
so additional work will be needed to strengthen this claim.
The highest Cp, which is 6.7% only, was produced
by the highest wind speed at the optimum tsr of 2.6. The optimum tsr for
Cp seems to remain constant with wind speed but the Cp dropped to 4% at
lowest wind speed. The maximum Ct occurred slightly earlier than maximum
Cp at tsr of 2.4.
The rotor self-starting ability became weak under
low wind speed as the Ct prior to the acceleration was very low and some
even recorded negative values. However, the tsr where the rotor acceleration
started to pick up seems to remain constant at around 1.3.
The maximum tsr, like the optimum tsr and ‘start
accelerate’ tsr, remained quite constant at 3.25 regardless of the
wind speed. This trend might not be true as the changes of Re number might
be to small to notice any of the tsr change.
The increase of performance with the wind speed or Reynolds number can
be attributed to the increase of airfoil’s lift to drag ratio with
Reynolds number. This performance trend agrees with the streamtubes model
result by Jesch and Walton (1976), which included the important variable
Re, effects. Figure 1 shows the variation of Cp curves with Re number
from Jesch and Walton result. These computed Cp curves did ‘spread
out’ quite similar to the testing result but the validity of this
model at high solidity with low tsr is questionable. In addition, the
solidity, average Reynolds numbers, and type of airfoil are different.
Figure 1: Variation of Cp with Re number from Jesch and Walton result
(Jesch & Walton, 1980, pg. 332)
3.6 The influences of number of fans
The Cp and Ct curves for different number
of fans but all at maximum fan speed are shown in Graph 8 and 9. The numbers
of fans employed are 1, 2, and 3. These fans produced wind speed of 4.18,
4.39, and 4.46m/s with increasing number of fans. Each number of fans
was tested for 4 times and plotted together.
Graph 8: Ct vs. tsr for different number of fans
Graph 9: Cp vs. tsr at different number of fans
The graphs clearly show that when subjected to
a fewer number of fans with decreasing wind speed, both Ct and Cp curves
‘shifted toward lower tsr’ and ‘contracted to a narrower
and shorter curves’. Although considerable amount of data scatter
exists, this trend is quite clear as the distances between curves are
very evident. It has to be noted that this trend is not result of the
fan number alone but also include the effects of Re number unfortunately.
The optimum tsr and maximum Cp decreased with number
of fans but the decrease in Cp is very probable caused by the accompanying
decrease in wind speed. The optimum tsr decreased from 2.6 to 1.75 while
the maximum Cp from 6.7% to 3.8%. The maximum Ct, which occurred earlier
than Cp, also exhibited similar trend.
The rotor self-starting ability is probably the
most noticeable changes observed during fan number testing. The rotor
under single fan accelerated strongly through the normally slow starting
period. Even the time needed for this rotor, which was under the lowest
wind speed, to reach the maximum rpm was shortened. As expected, the graphs
shows a high Ct value prior to acceleration for single fan even when it
provided the lowest testing wind speed. In addition, the shifting of the
curves brought the acceleration tsr earlier for lower number of fan.
The maximum tsr decreased from 3.3 to 2.6 with
decreasing number of fans. The decrease is most prominent when the fan
number was reduced to one.
The number of fans is definitely influencing
the model performance in term of self-starting ability, optimum tsr, and
maximum tsr. A casual observation of the rotor wake might explain the
performance differences. The wake behind the rotor blew by single fan
seemed to expanded more and moved slower while the 3 fans rotor wake seemed
to be more restricted but moved faster. Another reason might be the area
of coverage by these fans. The single fan only managed to cover an area
not wider than the swept area while the 3 fans provided a wider band of
uniform wind which cover more than 1.5 times of the swept area. Other
possible reasons are the fans direction, gap between fans, and mixing
of wind from fans. Whatever the reason is, extra precautions have to be
made to ensure consistent result can be obtained if fan testing has to
be used. Also, testing with more fans seems to be a much better choice
as the area coverage will be wider which seems to resemble the testing
condition in wind tunnel more.
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