462
B. Arizmendi et al.
increases the available convective cooling, which takes a larger amount of latent
heat that the droplets release when they change phase. Thus, lower OAT values
increase the likelihood of the droplets freezing when impacting on the surface. The
requirement for the ice to accrete on the surface is that the surface temperature is
maintained below 0 ◦ C. Some ice protection systems raise the surface temperature
over the freezing temperature to avoid ice accretion. Based on icing encounter
measurements, Politovic [13] stated the common range of temperatures measured
was between 0 ◦ C and −25 ◦ C with an average of −10 ◦ C. However, very few
encounters were found at temperatures below −20 ◦ C and greater than −5 ◦ C.
It is also important to acknowledge that the occurrence of a total temperature
greater than 0 ◦ C alone at the surface of the body does not suffice to ensure that ice
accretion will not take place. For the global computation of the energy balance, the
evaporative heat ˙
Q e for a certain control volume can be calculated as follows:
˙
Q e = ˙
m e L E ,
(15.2)
where ˙
m e is the mass of water evaporated and L E is the latent heat of evaporation.
˙
m e can be calculated as:
˙
m e =
0.7
C p
h c A
p v,s − RHp v,e
p e
,
(15.3)
where C p is the specific heat at constant pressure, p v,s is the saturation pressure at
the surface, RH is the relative humidity, p v,e is the saturation pressure of air, and
p e is the absolute pressure. Several droplets absorb latent heat to evaporate, thus
reducing the surface temperature and allowing ice accretion even at temperatures
greater than 0 ◦ C.
The static air temperature can be measured by means of thermocouples. One of
the challenges to obtain precise readings is that the probe should be sheltered from
any airflow such as the main stream or the exhaust from the motors. The outside
temperature can be also computed from the total air temperature subtracting the
heating provided by the airspeed.
15.2.2.2 Liquid Water Content
In addition to the temperature, the liquid water content (LWC) is a critical parameter.
The LWC measures the mass of liquid water that can be present within certain
volume of a cloud. It is expressed as the grams of liquid water per cubic meter
of air. The heat rate of freezing ˙
Q l , for a certain control volume over a dry surface
can be quantified as:
˙
Q l = ˙
m f r L f ,
(15.4)
B. Arizmendi et al.
increases the available convective cooling, which takes a larger amount of latent
heat that the droplets release when they change phase. Thus, lower OAT values
increase the likelihood of the droplets freezing when impacting on the surface. The
requirement for the ice to accrete on the surface is that the surface temperature is
maintained below 0 ◦ C. Some ice protection systems raise the surface temperature
over the freezing temperature to avoid ice accretion. Based on icing encounter
measurements, Politovic [13] stated the common range of temperatures measured
was between 0 ◦ C and −25 ◦ C with an average of −10 ◦ C. However, very few
encounters were found at temperatures below −20 ◦ C and greater than −5 ◦ C.
It is also important to acknowledge that the occurrence of a total temperature
greater than 0 ◦ C alone at the surface of the body does not suffice to ensure that ice
accretion will not take place. For the global computation of the energy balance, the
evaporative heat ˙
Q e for a certain control volume can be calculated as follows:
˙
Q e = ˙
m e L E ,
(15.2)
where ˙
m e is the mass of water evaporated and L E is the latent heat of evaporation.
˙
m e can be calculated as:
˙
m e =
0.7
C p
h c A
p v,s − RHp v,e
p e
,
(15.3)
where C p is the specific heat at constant pressure, p v,s is the saturation pressure at
the surface, RH is the relative humidity, p v,e is the saturation pressure of air, and
p e is the absolute pressure. Several droplets absorb latent heat to evaporate, thus
reducing the surface temperature and allowing ice accretion even at temperatures
greater than 0 ◦ C.
The static air temperature can be measured by means of thermocouples. One of
the challenges to obtain precise readings is that the probe should be sheltered from
any airflow such as the main stream or the exhaust from the motors. The outside
temperature can be also computed from the total air temperature subtracting the
heating provided by the airspeed.
15.2.2.2 Liquid Water Content
In addition to the temperature, the liquid water content (LWC) is a critical parameter.
The LWC measures the mass of liquid water that can be present within certain
volume of a cloud. It is expressed as the grams of liquid water per cubic meter
of air. The heat rate of freezing ˙
Q l , for a certain control volume over a dry surface
can be quantified as:
˙
Q l = ˙
m f r L f ,
(15.4)