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Figure 4.13.: The dimensionless minimum receding diameter d ∗ rec−min
=
d rec−min /d sp−max on SHSs of different temperatures. The Stefan
number is defined as St = c ice (T m − T B )/L, where T K is contact
temperature, T m is the melting temperature, and L is the latent heat.
4.2.3 Physics of the Rime Ice
The rime ice forms at very low temperatures approaching −20 ◦ C. This type of
ice has a regular shape as shown in Figure 4.14 and its structure is fragile, easy to
be removed from the wing surface. The conventional hypothesis of its formation
is that cloud droplets freeze completely upon impact. In the Messinger Model as
depicted in Figure 2.1, only the impinging mass is considered in the mass balance,
whereas the energy balance is completely ignored. This method leads to accurate
prediction of ice shape, for example by LEWICE [165]. However our experimental
results prove that complete freezing upon impact is impossible because of the slow
second stage of solidification. It is the first stage of nucleation and the resultant
formation of the ice/water mixture which hinders, or prohibits, the motion of the
liquid, leading to such an “apparent” complete freezing. Without supercooling, the
rime ice is an impossible phenomenon.
Another oversimplification of the conventional assumption with the Messinger’s
model is that the drop impacts on dry ice surface. In the icing wind tunnel test of
Tsao et al. [147], rime ice was observed with the MVD of 15 µm MVD, the LWC of
0.5 g m −3 , the impact velocity of 77m/s, and at the temperature of −21 ◦ C. Assum-
118 4. Results and Discussion