WIND CHILL re: Auto radiators

[BarnieTrk]

Now I know there are some pretty smart folks that frequent this site and I respect their opinions. Please help me confirm or deny my understanding of the condition known as WIND CHILL.

As I know it, WIND CHILL only affects surfaces that are capable of drying out; such as human skin or other damp surfaces. Surfaces that are DRY, will not realize a drop in surface temperature when exposed to moving cold air.

Examples of WIND CHILL can be seen if you stick your BARE arm out the window of your vehicle while traveling at speed. The wind passing over your relatively-moist skin dries your skin. As the cold air removes moisture from your skin this action causes the surface of your skin to become cooler than the actual air temperature. Even though your skin isn't visibly wet, it has moisture in it which keeps it soft and pliable. When it dries out, it shrinks to the point of becoming cracked or chapped. Green wood doesn't look wet when you first cut it, but try and burn it the same day and it'll show you just how 'damp' it really is.

To the point:
In regards to a vehicles' radiator, let's say you don't allow the engine to warm up, you simply start it and get going down the road on this cold Jan day. We know that there isn't any heat is being added to the coolant in the radiator until the thermostat begins to open and then allows warm coolant into the radiator from the engine. So for that first minute or so of operation, as long as the radiator outside surface remains dry - meaning it does not have a coolant leak or is being splashed from the weather or the road - the radiator surface temperature will not cool further than the outside air temperature even though you may be pushing it through the cold winter air at 50-60 mph.

However, if you have rain, sleet, road splash, or DO HAVE A COOLANT LEAK causing the radiator core to become dampened , then the radiator will realize a wind chill cooling affect and it's surface temperature will be lowered to less than the actual air temperature until the thermostat opens and allows warm coolant in.

Thus - in a wet slush-covered road driving situation, your vehicle's radiator will likely be continually sprayed with fine, wet slush, which will dampen the radiator fins, causing the fins to likely be facing a WIND CHILL condition for that first minute or so until the thermostat allows warm coolant in. So, to ensure anti-freeze protection for your radiator during those instances when it'll be sprayed or dampened, you should equip your radiator with coolant to protect it from temperatures that the WIND CHILL will cause, not just the outside air temps. Referring to the chart:

http://www.nws.noaa.gov/om/windchill/

Lesson: On those days when I'm driving on slush-covered roads at 60mph in outside temps of 0*F, I should have my radiator equipped with anti-freeze protection down to the -33*F range to protect it from freezing during that first minute of operation.

Did I explain my question adequately?
If so, is my understanding correct?

Thanks, Guys!
BarnieTrk

[ICEMAN6166]

wind chill does not really have any effect on the cooling system.

metal is non porous, unlike skin. nothing to dry out and water on the outside of the radiator that may possibly block the fins will make the coolant in it warmer as the fan will not be drawing air thru as it should.

my trucks are protected somewhere between -65 and -84 so i dont ever worry about the temp.

your cooling system should have protection below 32F/above 212 F no matter where you live.
anti freeze is also a coolant, water will steam and evaporate so you need it in the summer too.

[gearhead21]

I grew up in the suburbs of Chicago. One thing that really cracked me up was when the temps would get to 0 or below, people would put a blanket over their engine to "keep it warm" so it would start in the morning. This was done at night, to a cold engine. I had even heard TV newscasters telling people to do this. I tried to tell people that did this that an engine is not like a person, where a blanket would keep in body heat to keep you warm. No one would believe me.

I remember winters there where we would go a couple weeks below 0. One winter we had -32 with a -67 wind chill. I had just came out of the detective division to go back to patrol. That night I regreted that decision. One of the reasons I moved to Texas. Now I hear that later this week we will have lows in the lower teens, with near 0 wind chills. At least I'm retired, and won't be working midnights patroling.

[ICEMAN6166]

they make block heaters to keep your engine warm,we plug in at 20F or colder.
really improves the starting, the 1500 watt tank type heater are the best. i dont care for the freeze plug type, they are not nearly as powerful and will collect sediment around the element in the block just like an electric water heater element in your house.

[The Big M]

Examples of WIND CHILL can be seen if you stick your BARE arm out the window of your vehicle while traveling at speed. The wind passing over your relatively-moist skin dries your skin. As the cold air removes moisture from your skin this action causes the surface of your skin to become cooler than the actual air temperature.

The scenario you're describing is convective heat transfer, which takes place between the atmosphere and a surface when there is relative motion present (i.e. the bare arm travelling at speed through still air, or conversely, wind blowing across a stationary object). The rate of convective heat transfer is proportional to this relative velocity.

The windchill factor is a means of quantifying the effect of convection (combined with evaporative cooling), which is a perceived temperature that is lower than the measured ambient temperature. It's not that the surface of the skin is at a lower temperature than the air, it's just that the air feels colder than it actually is because the rate of heat transfer is increased due to the effects of the wind.

Let's say the temperature is -20 deg C, and the windchill factor is -30 deg C. What this tells you is that the wind makes it feel as cold as it would be if it were -30 deg C with no wind. That is, your body would cool at the same rate as it would at the lower temperature.

[mike135]

The body loses heat largely by evaporative cooling and convection. The rate of heat loss by a surface depends on the wind speed above that surface: the faster the wind speed, the more readily the surface cools. For inanimate objects, the effect of wind chill is to reduce any warmer objects to the ambient temperature more quickly. It cannot, however, reduce the temperature of these objects below the ambient temperature, no matter how great the wind velocity. For most biological organisms, the physiological response is to maintain surface temperature in an acceptable range so as to avoid adverse effects. Thus, the attempt to maintain a given surface temperature in an environment of faster heat loss results in both the perception of lower temperatures and an actual greater heat loss increasing the risk to adverse effects such as frostbite and death.

http://en.wikipedia.org/wiki/Wind_chill

[BarnieTrk]

Guys, Thanks for the responses!

If: "The windchill factor is a means of quantifying the effect of convection (combined with evaporative cooling), which is a perceived temperature that is lower than the measured ambient temperature."

Check out this link: http://www.math.montana.edu/frankw/ccp/ ... /text3.htm

According to the way I understand the link info, there is an actual cooling effect measured when moisture is evaporated from a surface.
I'm thinking that evaporation is an exothermic action, thus takes energy or heat out of the surface material, be it a cup (as in the link) or our skin, truck fender or radiator surface.
This would dis-prove the thought that evaporation cooling only is a sensation to our skin, rather it truly does lower the temperature of the surface to a point lower than the surrounding air.

Is that correct or am I missing something still - again?

BarnieTrk

[The Big M]

There is indeed an actual cooling effect, where the droplets of moisture absorb energy from the skin as they evaporate (latent heat of vapourization), but the surface of the skin is not colder than the surrounding air as a result. What does happen is that the rate of heat transfer is greatly increased. So in a way a wet radiator could be affected by windchill, or more accurately evaporative cooling, but as Brian mentioned, a film of water clinging to the fins would most likely decrease the thermal efficiency of the radiator and may actually cause the coolant temperature to rise (if the thermostat were open, that is). Any cooling effect would simply make the coolant reach the ambient temperature quicker. It can not make it colder than the ambient temperature.

Say you're standing outside at -20 deg C with no wind. Any exposed skin will conduct heat to the atmosphere at a certain rate. If the ambient temperature is -30 deg C, your skin will conduct heat at a faster rate and you sense that it feels colder outside. The higher the temperature differential, the higher the rate of heat transfer.

Now, the presence of wind will increase the rate of heat transfer even further due to convection and evaporative cooling. Since the rate of cooling is higher, you sense that it feels colder than it would if there were no wind. The windchill factor quantifies this effect and relates it to an equivalent ambient temperature.

If the windchill factor is -40 deg C, exposed skin will cool at the same rate as it would at -40 deg C with no wind. It doesn't mean that the surface of your skin is -40 deg C. If it were you'd have serious issues!

[BarnieTrk]

Big M,
I think I follow your logic regarding the skin sensing a colder condition.

But what about non-porous materials?

Let's say we have two milk jugs full of 50*F water setting two feet apart on a shelf outside on a windy (45mph), cold (33*F) and dry (10%RH) day.

The outside of the 1st jug is sprayed every minute with a slight mist of water that is the same temp as the water inside the jug; where as, the 2nd jug is kept dry on the outside.

Does the water within the 1st jug (the wetted jug) loose temperature faster than the water within the 2nd (non-wetted) jug?

Will the water within the 1st jug cool below the level of the outside air temperature?

I'm sorry to be so thick-headed on this - I appreciate your patience with me, Big M!

BarnieTrk

[The Big M]

For the sake of this thought experiment, let's say the jugs are suspended such that they're not in direct contact with a cold surface. Therefore heat transfer by conduction between the jug and the shelf isn't a concern.

If the conditions are such that evaporation of the water droplets occurs, then yes, the wetted jug should cool faster. As the water droplets evaporate they would absorb energy from the surface of the jug, thereby cooling it. The rate of conductive heat transfer would then increase between the water in the jug and the outside surface, since the temperature differential would be greater than that of the non-wetted jug. The porosity of the surface isn't really the issue.

That being said, I've probably confused you because I've been approaching this in the context of extremely cold temperatures. I should clarify that evaporation can be used to cool a surface significantly below the ambient temperature in certain cases (hot with low humidity, for instance), but the difference here is that cold air can't hold as much moisture as hot air can, therefore the evaporation rate would be quite small even though the relative humidity of the air is low.

The keys are the dry-bulb and wet-bulb temperatures of the air. It's dependent on relative humidity, and the difference between the two determines the potential for evaporative cooling. The bigger the difference, the more cooling that can be achieved. The surface of the wetted jug would experience the wet-bulb temperature of the air, which is always lower than the dry-bulb temperature (33 deg F in this case).

However, at extremely low temperatures the difference between the two starts to converge. Also in this example it's difficult to say exactly how cold the jug would get because evaporation would cease at 32 deg F, at which point the droplets would freeze. Heat transfer would likely reverse at that point since the jug would be 1 deg colder than the ambient temperature.

I should also mention that a big difference between this example and that of human skin is that the human body is generating heat.

My key point is that the jug will not reach the temperature stated in the windchill factor. I hope that helps!

[charliemccraney]

Pressure must come into play somehow. Wouldn't the pressure across the fins or jugs be lower because of the wind, resulting in a drop in temperature?

It seems like I have been outside when the ambient temperature was slightly above freezing but ice was on the road.

[The Big M]

Hopefully this will help further illustrate my point:



The vertical green lines are the dry bulb temperature, which is what a common everyday thermometer would measure. The red curves denote relative humdity. The wet bulb temperature is in blue at the top left. The dry bulb and wet bulb temperatures are equal at 100% humidity.

In order to determine the wet bulb temperature at a given dry bulb temp. and R.H., follow the vertical green line up until it intersects the R.H. From there you follow a line parallel to the blue line up and to the left to where it intersects the 100% R.H. curve. This is the wet bulb temp. Oh, and 20 deg C = 72 deg F and 0 deg C = 32 deg F, for the metrically challenged.

For example, 20 deg C at 10% humidity should give a wet bulb temp of 7.5 deg C. In this case the potential for evaporative cooling is 12.5 deg C. Compare that to 0 deg C and 10% humidity where the difference is closer to 5 deg C. Note that the chart only goes down to -10 deg C (14 deg F), the R.H. curves begin to converge, and the wet bulb temp lines (blue) become closer together as the dry bulb temp decreases. This suggests that the potential for evaporative cooling (as expressed in degrees) becomes lower in extreme cold.

Charlie, the reason for the ice on the road was likely that the ground was still below freezing even though the air had warmed to above freezing.

EDIT: Here's another interesting link regarding water-absorbent materials at sub-zero temperatures: http://www.natmus.dk/cons/tp/cool/suprcool.htm

[BarnieTrk]

For the sake of this thought experiment, let's say the jugs are suspended such that they're not in direct contact with a cold surface. Therefore heat transfer by conduction between the jug and the shelf isn't a concern.

Agreed. Eventhough I would think that the coduction would be nearly equal for each jug and thus "cancel each other out".


If the conditions are such that evaporation of the water droplets occurs, then yes, the wetted jug should cool faster. As the water droplets evaporate they would absorb energy from the surface of the jug, thereby cooling it. The rate of conductive heat transfer would then increase between the water in the jug and the outside surface, since the temperature differential would be greater than that of the non-wetted jug. The porosity of the surface isn't really the issue.

That being said, I've probably confused you because I've been approaching this in the context of extremely cold temperatures. I should clarify that evaporation can be used to cool a surface significantly below the ambient temperature in certain cases (hot with low humidity, for instance), but the difference here is that cold air can't hold as much moisture as hot air can, therefore the evaporation rate would be quite small even though the relative humidity of the air is low.

The keys are the dry-bulb and wet-bulb temperatures of the air. It's dependent on relative humidity, and the difference between the two determines the potential for evaporative cooling. The bigger the difference, the more cooling that can be achieved. The surface of the wetted jug would experience the wet-bulb temperature of the air, which is always lower than the dry-bulb temperature (33 deg F in this case).

However, at extremely low temperatures the difference between the two starts to converge. Also in this example it's difficult to say exactly how cold the jug would get because evaporation would cease at 32 deg F, at which point the droplets would freeze. Heat transfer would likely reverse at that point since the jug would be 1 deg colder than the ambient temperature.

I should also mention that a big difference between this example and that of human skin is that the human body is generating heat.

My key point is that the jug will not reach the temperature stated in the windchill factor.

Agreed.


I hope that helps! --- YES, Thank you very much, Big M!

The "What happens to water absorbent materials below freezing?" link was also interesting.........

Thanks again!
BarnieTrk

[The Big M]

No problem! It's interesting stuff, with no easy answers. It's possible for liquid water to be supercooled, and for water to sublimate directly to vapour from a solid state. But then again, a layer of ice on the jug would insulate the water inside!

As for conduction, if the first jug was cooled faster due to evaporation on its surface, then the rate of heat transfer via conduction would be higher in the second jug. The reason for this is that the water inside the non-wetted jug would be warmer and therefore a larger temperature differential would exist between it and the cold surface of the shelf. So eliminating conduction would be required to pinpoint the effect of the evaporative cooling.

Something else that should be mentioned is that if you took a third jug and sheltered it from the wind, it would cool slower than both of the other two due to a lack of convection.

[ICEMAN6166]

if you walk down to the spring in the winter at 0*F to get water the water is flowing but as soon as it gets into the container it will begin to freeze, a 1 liter jug will freeze quite fast.

at -40F you can throw water into the air and it will freeze before it hits the ground.
at -50 any moisture in trees and wood in your house framing will begin to freeze and make loud cracking noises as it expands the dry wood.

there is never wind when it gets that cold here, but finally when a slight breeze even 5 mph begins the temp will go up , sometimes as much as 20* in a short time because its pushing the stagnant super cold air away.