Environmental Engineering Reference
In-Depth Information
2000
1500
1000
500
0
1
0.5
0
r/R
tip
0
−1
x/c
Fig. 8.5 Simulated heat flux requirement (due to convection loss) versus nondimensional chord
position x/c (negative values are for the lower surface and positive numbers are for the upper
surface of the blade) and nondimensional span-wise radius (distance from hub) r/R
tip
. The heat
flux plotted is the amount needed from a de-icing system to compensate for convection loss from
the blade; T
amb
¼
30
C, x
¼
20 RPM, u
w
= 12 m/s, R
tip
= 25 m, AoA = 0, P = 500 KW,
where T
amb
is ambient temperature, x is angular velocity of the rotating blade, u
w
is wind speed,
R
tip
is the span-wise radius at the blade tip, AoA is angle of attack, and P is rated power produced
by the wind turbine
q
conv
¼
h
ð
T
d
T
amb
Þ
ð
8
:
2
Þ
where T
d
is the maximum desired blade temperature and T
amb
is the ambient
temperature.
The thermodynamic behavior of convection loss changes once the flow
becomes turbulent over the blade. This was investigated in [
19
] where it was
shown that the effect of a transition to turbulent flow at the leading edge is to
increase the required heat transfer by an average factor of 2.5. Therefore, laminar
and turbulent flow conditions need to be detected throughout the blade for different
turbine operating conditions for active de-icing.
A de-icing system must provide enough thermal energy to compensate for
convection heat loss, sensible heat to increase the initial temperature of ice to its
melting temperature, and latent heat for the phase change from solid ice to liquid
water. The latent heat flux required for the phase change of ice is dominant
compared to the sensible heat flux even when the initial ice temperature is sig-
nificantly below zero. Calculations show that for ice with an initial temperature of
-30 C, the amount of heat required to change its temperature to 0 C is only
18 % of the heat required for the phase change. Since the phase change of ice
requires much higher thermal flux than sensible heat flux, it is not profitable to
melt the entire ice layer formed on the blade. Thus, one strategy for reducing
thermal energy expenditure is to only melt a thin layer of ice and then shed off the
remaining ice by using centrifugal force from the blade rotation [
17
].
Figure
8.6
shows a plot of the summation of sensible and latent heat flux of ice
required for de-icing versus melting time for a 1 mm soft rime ice at two different
initial ice temperatures. It is seen that this heat flux exponentially decreases when
