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  #21  
Old 01-19-2021, 05:13 PM
Freemasm Freemasm is online now
 
Join Date: Nov 2017
Location: Orlando
Posts: 318
Default Id need more data on that

Regarding;
2. As others noted, most of the cooling is due to the evaporation of fuel.

Id 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.

Thanks.
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  #22  
Old 01-19-2021, 05:49 PM
Bicyclops Bicyclops is offline
 
Join Date: Oct 2012
Location: LA, California
Posts: 363
Default The FAA says:

Quote:
Originally Posted by Freemasm View Post
Regarding;
2. As others noted, most of the cooling is due to the evaporation of fuel.

Id 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.

Thanks.
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

and:
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.

Ed Holyoke
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  #23  
Old 01-19-2021, 06:29 PM
Freemasm Freemasm is online now
 
Join Date: Nov 2017
Location: Orlando
Posts: 318
Default

Im well aware of the different mechanisms of formation/types of induction ice. That was never the question. Ive done plenty of heat xfer and associated cycle analysis in my professional life. I questioned if the latent heat of fuel is the primary driver versus the effects of low local static P. I dont see anything in the previous cut/paste that begins to answer that. My previous examples of when induction heat is recommended and others posters examples of induction ice encounters backs up that hypothesis. If anyone has real data regarding the relative contributions of each associated contributor, I would like to know. So far, there is no contrary evidence to my previous statements.
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  #24  
Old 01-19-2021, 06:48 PM
BobTurner BobTurner is offline
 
Join Date: Dec 2011
Location: Livermore, CA
Posts: 7,290
Default

Quote:
Originally Posted by Freemasm View Post
Regarding;
2. As others noted, most of the cooling is due to the evaporation of fuel.

Id 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.

Thanks.
Evaporation of liquids is a very powerful cooling mechanism - thats how most air conditioners work. As you point out, carb ice is a problem with the throttle closed - when air flow thru the venturi, and hence adiabatic cooling, is minimal. What happens is most of those fuel droplets hit the closed throttle plate, and evaporate right then and there, not spread out thru the induction system. Also, you might note that many (most?) fuel injection systems use a venturi to sense the air flow, but almost none of these systems have a venturi heat system.
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  #25  
Old 01-19-2021, 07:38 PM
Freemasm Freemasm is online now
 
Join Date: Nov 2017
Location: Orlando
Posts: 318
Default

Quote:
Originally Posted by BobTurner View Post
Evaporation of liquids is a very powerful cooling mechanism - that’s how most air conditioners work. As you point out, carb ice is a problem with the throttle closed - when air flow thru the venturi, and hence adiabatic cooling, is minimal. What happens is most of those fuel droplets hit the closed throttle plate, and evaporate right then and there, not spread out thru the induction system. Also, you might note that many (most?) fuel injection systems use a venturi to sense the air flow, but almost none of these systems have a ‘venturi heat’ system.
I am aware, Sir. Sorry if I'm being dense but I'm going to be stubborn here. Bare with me.

For a given RPM, the IC engine is a constant volume machine. (Probably) No one will argue that. The volumetric flow through the venturi (thus ~ fluid speed) does not changep with throttle position alone. I’m aware that usually RPM will not stay that constant Air density on the other hand... Your position assumes that the static P effects are limited to the venturi. That is far from true. At some point of closure, the throttle losses become the predominant pressure loss; very high velocities occur around the butterfly valve. Static P drops proportionately to the square of this velocity.

So I'll ask again. Does anyone have data that supports the previous statement that fuel evaporative effects are the primary driver? I'll state again, the engine OEMs operation recommendations and the experiences posted here by others (at least anecdotally) backs that up as does axial compressors icing, i.e.closing inlet guide vanes increases this risk still without no latent heat effects.

Sorry for the back and forth. Respectfully submitted. I’m really trying to understand the entire proportional relationships.

Last edited by Freemasm : 01-19-2021 at 08:51 PM.
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  #26  
Old 01-19-2021, 10:18 PM
Bicyclops Bicyclops is offline
 
Join Date: Oct 2012
Location: LA, California
Posts: 363
Default

Quote:
Originally Posted by Freemasm View Post
I am aware, Sir. Sorry if I'm being dense but I'm going to be stubborn here. Bare with me.

For a given RPM, the IC engine is a constant volume machine. (Probably) No one will argue that. The volumetric flow through the venturi (thus ~ fluid speed) does not changep with throttle position alone. I’m aware that usually RPM will not stay that constant Air density on the other hand... Your position assumes that the static P effects are limited to the venturi. That is far from true. At some point of closure, the throttle losses become the predominant pressure loss; very high velocities occur around the butterfly valve. Static P drops proportionately to the square of this velocity.

So I'll ask again. Does anyone have data that supports the previous statement that fuel evaporative effects are the primary driver? I'll state again, the engine OEMs operation recommendations and the experiences posted here by others (at least anecdotally) backs that up as does axial compressors icing, i.e.closing inlet guide vanes increases this risk still without no latent heat effects.

Sorry for the back and forth. Respectfully submitted. I’m really trying to understand the entire proportional relationships.
As you point out, a closed throttle causes lowered pressure and causes the moisture in the air to freeze out. This is why throttle icing tends to occur at mostly closed throttle and on the back side of the throttle plate where the pressure is the lowest. Fuel vaporization icing occurs at the discharge nozzle and will be more likely to occur at higher throttle settings because the pressure is low enough in the venturi to draw the fuel from the float chamber and cause it to vaporize. lowering the temperature enough to freeze the moisture in the air. With the throttle closed, there isn't enough delta P to draw fuel through the main jet reliably and so an idle circuit with an outlet right by the edge of the closed throttle is necessary. The idle port could still ice at the same time the throttle is icing for a different reason.

So the question that I see you raising is which effect predominates. It depends on throttle position. I don't think it matters much to the pilot which effect is causing the power loss or making the throttle immovable. Yes, full carb heat is what you need for either cause.

Ed Holyoke

Last edited by Bicyclops : 01-19-2021 at 10:25 PM.
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  #27  
Old 01-20-2021, 03:08 AM
Cumulo Cumulo is offline
 
Join Date: Apr 2015
Location: KHMT
Posts: 76
Default isentropic vs adiabatic expansion?

Quote:
Originally Posted by Freemasm View Post
Regarding;
2. As others noted, most of the cooling is due to the evaporation of fuel.

Id 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.

Thanks.

Expanding a gas through the blading of the inlet guide vanes of an axial compressor, which was one of your points, could possibly produce much more cooling effect than a throttle process..
I suggest that that process will be adiabatic and produce a lower temperature that a throttling process will. I am less certain about the difference compared to the carburetor venturi though.

I don't have the numbers for the relative effect of expansion vs latent heat of fuel, but I suppose some one could grind it out using the different ratios.

Learned to fly in Michigan in the winter and early spring in my then C65 Champ nearly seven decades ago. It was stick/throttle/carb heat most days. Carb icing was expected and pretty much ho-hum. Later, early Bonanzas with PS-5's, a dry carburetor with the fuel introduced down stream, no icing. So, I would have to guess that the temperature drop from fuel evaporation is the more important factor. But again, no numbers.

Something related to fuel cooling effect: In Libia in WW2, the standard beer cooler was an empty drum, usually delidded with prima cord, filled with beer bottles and avgas, then bubbled with an air hose. Presto, cold beer due to the latent heat of gasoline. And that is how to win a war.
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  #28  
Old 01-23-2021, 10:08 AM
Tom @ N269CP Tom @ N269CP is offline
 
Join Date: Oct 2017
Location: Green Cove Springs, FL
Posts: 111
Default

Quote:
Originally Posted by Christopher Murphy View Post
Someday somewhere that carb heat will be needed. If you dont have it available it will become a very high pucker factor experience.

Ive had my 0320 ice up between the hangar and the runway and Ive had airborne battles with carb ice. I know from the previous posts there is a lack of understanding of carb ice and how you deal with it. Flying around with the carb heat on is a bad idea.

Cm
Yup. This past July taking off out of Salida, CO on a chilly Rocky Mountain morning just before sunrise with virga in the sky around Poncha Pass. Barely made it over the low hills around the airport. Around 10.8 gph showing at WOT. Later concluded it was likely carb icing. Was not using carb heat on takeoff. I have a Carb T probe and find I get just a tad more than 10F rise on carb heat...which Im not happy with. Would be interested in designs to get more temp rise.
__________________
Kind regards,

Tom

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