Environmental Engineering Reference
In-Depth Information
accuracy within 36 microns). Direct ice detection requires measurement of the
region just outside the blade surface. To accomplish this thickness measurement,
optical signals directed normal to the surface reflect from the upper surface of the ice
to reveal its presence, thickness, and type. Thickness could be measured by time of
flight of a short pulse, referred to as time-domain reflectometry (TDR) of sub-mm
films. TDR requires optical pulses of approximately 100 fs duration which are costly
to generate and tend to disperse rapidly in solids. Conversely, optical frequency
domain reflectometry (OFDR), as shown in Fig.
8.7
, uses continuous laser light with
a swept frequency which is easier to generate and can be directed through optical
fiber [
21
,
22
]. Fiber coupled sources allow for many sensors to be multiplexed on a
single fiber running down the full length of the blade, greatly reducing complexity.
Additionally, recent advances in all-fiber, rapidly tuning laser sources [
21
] promise
to reduce the cost and delicacy of the lasers while increasing the measurement speed.
OFDR can measure multiple simultaneous returns in different time windows. The
optical signal to each sensor is thus delayed by a unique delay such that the entire
array can be measured in a single interrogation.
To demonstrate the efficacy of OFDR for the detection of icing on a blade surface,
we use the instrument whose schematic is shown in Fig.
8.7
to monitor the thickness
of an evaporating water film on a solid substrate, as shown in Fig.
8.8
. Gradient-index
(GRIN) lenses with an antireflective coating are used as our optical ice sensors. Many
such probes can be queried by a single OFDR system, providing spatially resolved
icing data [
14
,
23
]. The transform-limited time resolution of the measurement is
ds
¼
1
=
dm
¼
121 fs which corresponds to a spatial resolution of 36 lm in air.
As shown in Fig.
8.8
, top, the system can easily distinguish both the presence
and thickness of the material on the surface.
We have recently shown [
22
] the use of phase information in the OFDR to
simultaneously measure thickness to ±61 nm precision and the index of refraction
to
2
10
6
. Since variations in the ice type are reflected in their refractive
index, this suggests that both type and thickness of ice can be detected.
The sensor area must be sufficiently large to average over ice crystal size,
entrapped air, and other spatial variations. The 10 lm diameter of a fiber core is
thus too small and will not return a repeatable signal. Thus, we used a fiber
collimator consisting of a graded index collimating lens attached to the end of the
optical fiber. The lens is covered by an anti-reflective coating and we detect water
and ice which sits on top of the sensor. The collimating lens has a circular
detection diameter of 3 mm.
Figure
8.9
presents experimental optical sensing results, demonstrating that the
sensor can distinguish between air, water, glaze ice, and rime ice on the blade. For
the case of air, the measured signal is symmetric with a small peak due to the
existence of the anti-reflective coating (Fig.
8.9
a). In this case, there is no volume
scatter and the peak of the signal corresponds to the airfoil-air boundary. For the
case of liquid water, two smooth peaks are captured with no volume scatter
(Fig.
8.9
b). For the case of glaze ice (Fig.
8.9
c), a smooth peak is captured followed
by a small scattered signal. The first peak corresponds to the airfoil/ice boundary,
