Originally Posted by Freemasm
“2. As others noted, most of the cooling is due to the evaporation of fuel.”
I’d need more data but with a lack of such, this seems doubtful. Low local static pressure levels seem to be the prime driver. Example: Axial Compressors ice at very high ambient temps when inlet guide vanes are less than full open (no fuel evaporation effects). Likewise, carb heat is rarely needed at power levels greater than 80% i.e. throttle valves open/low static pressure loss. Similarly, closed throttles (resulting low local static P added to that of Venturi) where very little fuel flow (very low latent heat losses) are most prone to icing for given conditions.
If someone has related data, it would be appreciated. As of now, I would need some convincing.
From AC 8083-32:
Ice can form in the induction system while an aircraft
is flying in clouds, fog, rain, sleet, snow, or even clear air that
has high moisture content (high humidity). Induction system
icing is generally classified in three types:
• Impact ice
• Fuel evaporation ice
• Throttle ice
Fuel evaporation ice or refrigeration ice is formed because of the decrease in air temperature resulting from the evaporation of fuel after it is introduced into the airstream. As the fuel evaporates, the temperature is lowered in the area where the evaporation takes place. Any moisture in the incoming air can form ice in this area. It frequently occurs in those systems in which fuel is injected into the air upstream from the carburetor throttle, as in the case of float-type carburetors. It occurs less frequently in systems in which the fuel is injected into the air downstream from the carburetor. Refrigeration ice can be formed at carburetor air temperatures as high as 100 °F over a wide range of atmospheric humidity conditions, even at relative humidity well below 100 percent. Generally, fuel evaporation ice tends to accumulate on the fuel distribution nozzle in the carburetor. This type of ice can lower manifold pressure, interfere with fuel flow, and affect mixture distribution.
Throttle ice is formed on the rear side of the throttle, usually when the throttle is in a partially “closed” position. The rush of air across and around the throttle valve causes a low pressure on the rear side; this sets up a pressure differential across the throttle, which has a cooling effect on the fuel/air charge. Moisture freezes in this low pressure area and collects as ice on the low pressure side. Throttle ice tends to accumulate in a restricted passage. The occurrence of a small amount of ice may cause a relatively large reduction in airflow and manifold pressure. A large accumulation of ice may jam the throttles and cause them to become inoperable. Throttle ice seldom occurs at temperatures above 38 °F.
Impact ice is formed either from water present in the atmosphere as snow, sleet, or from liquid water which impinges on surfaces that are at temperatures below 32 °F. Because of inertia effects, impact ice collects on or near a surface that changes the direction of the airflow. This type of ice may build up on the carburetor elbow, as well as the carburetor screen and metering elements. The most dangerous impact ice is that which collects on the carburetor screen and causes a very rapid reduction of airflow and power. In general, danger from impact ice normally exists only when ice forms on the leading edges of the aircraft structure. Under some conditions, ice may enter the carburetor in a comparatively dry state and will not adhere to the inlet screen or walls or affect engine airflow or manifold pressure. This ice may enter the carburetor and gradually build up internally in the carburetor air metering passages and affect carburetor metering characteristics.