Heat transfer question - ice on windshield

[freehat]

Ok, easy one for you.

When its a cold morning and you get in your car, start her up then spray water on the windshield, it freezes (obviously as the windshield is below freezing). You then scrape it off and drive. 5 mins later, while at speed, if you spray water on the windshield it wont freeze.

Why?

My school of thought is that the heat in the car (still very cold) wont be enough to convectivly heat the windscreen, and then conductivly heat through to bring the surface of the outside of the windcreen to non-freezing. I was thinking it would more be due to the fact that the car is a cold sink and so would be colder than the outside air (due to the fact that in the night the air temp was much less than in the morning) and so the convective "heating" of the outside air blowing on the windshield at speed, will result in the water not freezing.

thoughts?

 

[daveh55et]

My thoughts are that in a few minutes the engine has heated the engine compartment and the washer fluid to well above freezing, and if the windshield is still so cold and freezes the fluid, the wind sublimates the ice very quickly.

 

[Bribyk]

It will freeze on the windshield while moving but then it usually evaporates off from its frozen state (sublimation?). I think the high evaporation rate makes it appear that the fluid doesn't freeze.

It would be nice if a block heater heated your windshield washer fluid as well as the coolant so that you don't have to scrape. There's an idea for you automotive guys (if not already patented). I don't want credit, just my next vehicle to have this.

 

[Unotec]

If it doesn't have anti freeze, it will freeze, you will not even be able to use the windshield washer.
However, if the engine has been running for a few minutes and you have but water as a windshield washer (it would have to have melted by now), due to the cold in the air, the vapour pressure of the water, and the delta T and the low moisture content in the air, it will probably evaporate before it actually gets to the windshield.
I've tried to thow a bucket of water to make snow at -30ºC but it all turned to steam

<<A good friend will bail you out of jail, but a true friend
will be sitting beside you saying ” Damn that was fun!” - Unknown>>

 

[Bribyk]

Even the stuff with antifreeze (-40 rate) does freeze due to evaporative cooling. The fluid can reach below its rated freezing point due to the cooling effect of some of the fluid evaporating off, especially while driving.

 

[IRstuff]

Windshield wiper fluid can indeed freeze on the windshield while moving; I've been there, done that, lot's of fun trying to stay on the road with only a ~5"x5" viewport to navigate by.

It's just a matter of energy balances and the temperatures of all substances involved. Unless you can drive your car at Mach 0.5 or better, there will always exist conditions that will freeze the fluid. I can tell you from industry experience that you need about 2 W/in^2 on the windshield to anti-ice at about 100 kts in military cold environments.

See SAE 1168-4 if you're a glutton for punishment or looking for some light bedtime reading. It's an entire book on windshield deicing and antiicing.

TTFN

FAQ731-376

 

[Unotec]

Tried it this morning (-30ºC) without turning the defroster on. It created a thin hazy layer on the first spray, and did the same after about 10min of driving. (Then I turned the defroster on 'cause some places that shouldn't be too cold were freezing!)

<<A good friend will bail you out of jail, but a true friend
will be sitting beside you saying ” Damn that was fun!” - Unknown>>

 

[Dan320]

On a cold night with no wind the car will be colder than the ambient air due to heat loss from radiation. When you drive, the relatively warmer air will thaw the windshield somewhat.

 

[Transient1]

Dan,

Your comment doesn't seem right. What are you assuming the car is radiating to that will make it colder than the still air around it?

 

[LOUDOG]

The car will radiate to the cold night sky. Space must be close to absolute zero.

 

[IRstuff]

IF you were actually in space, then you'd be close to the microwave background emission of 2.7K.

However, you're not, so the actual sky temperature is something like 220 K, which still pretty darn cold.

This assumes that you actually have a large, unobstructed field of view of the sky, of course.

TTFN

FAQ731-376

 

[Transient1]

Okay I'll give you that if it was a clear night in the middle of a large field the car would radiate to the upper atmosphere. However, it would also be radiating to air at warmer temepratures than the upper atmosphere. Lastly, if there was even a slight breeze, the rate of convective heating is comparable to the rate of radiative cooling. So at best I would say that the car would only be slightly cooler than the air.

 

[IRstuff]

Air is relatively transparent, that's why the typical, apparent, sky background is 220 K, which reflects the fact that the air is not completely transparent, but the integrated heat flux from the sky is low enough that it's consistent with a much colder temperature than the ambient temperature.

You're obviously welcome to your opinion, but the reality is that we routinely get frost on the roof and on cars in Southern California on days when the recorded air temperatures are nowhere near freezing.

And, the math says otherwise as well. Assume 273K surface temp and 220 K sky temp with 0.5 emissivity. The heat flow is 163 W/m^2 skyward. Static air has a htc of about 2.5 W/m^2-K, so assuming +10°C air temp, you'd only get 25 W/m^2 heat out of the air.

TTFN

FAQ731-376

 

[Dan320]

Transient1, the point is that the car does not radiate to anything but "empty space". If it radiated to anything like a roof above or a cloud layer the radiation would be reflected, and the car would not be notably colder than ambient air. The thing to remember is that the car, as long as it is warmer than absolute zero, is radiating heat all the time.
Everything around us radiate 100% of the time.
With no wind to supply heat from the air the car just keeps getting colder and colder until the sun is high enough in the sky to give more radiation heat than the car is loosing and the car will become warmer again. Sometimes the frost forms after sunrise and will only go away when the sun is so high it will shine directly on the car.

 

[Transient1]

I wasn't saying it is impossible to get much lower than static air temperature. I just felt it was unlikely, but as IRstuff shows the numbers say otherwise. Thanks for the clarification.

 

[IRstuff]

s'OK, I thought otherwise as well at one point.

But, actual data and the physics convinced me.

TTFN

FAQ731-376

 

[pauljohn]

I feel like a student posting amongt professors. While I can't offer any theoretical explanations or numbers, thermodynamics was not one of my strong points, I have a few observations I would like to run by you to see what you think (By coincidence I was thinking about this same problem a few days ago.)I have seen washer fluid freeze but only in the coldest of circumstances usually I am suprised when it doesn't.

First - if the washer fluid is able to be pumped it either is not at or below 0C or has some impurities or anti-freeze, second - the pump will impart a fairly large amount of energy by shooting it at the windshield which will change to heat when hitting the glass, third - water freezes with a lot more reluctance when moving and it certaily will not be stationary after being shot out of the jets then blown around by the slipstream.

A combination of all these should keep the fluid liquid long enough for the wipers to clear the screen. Does this make sense to anyone else?

 

[IRstuff]

Again, that's only when it doesn't freeze. Obviously, others, myself included, have spritzed wiper fluid, even those fortified with alcohol, only to have it freeze in place.

The basic fallacy of the argument is that the amount of heat energy contain in the less than 1 ounce of fluid is not enough to offset the coldness of a 20 lb sheet of glass, particularly if it's significantly cooler than freezing to start with.

Again, for more details, refer to SAE 1168-4. There is no set "limit" for freezing or not freezing, it's purely a matter of heat flows, starting temperatures, etc. Once you have the numbers, you can readily determine whether it'll freeze or not.

TTFN

FAQ731-376

 

[Surestick]

I submit that the OP is wrong in his assumption that wiper fluid won't freeze when the car is moving.

Being Canadian I have experience with this.

Washer fluid, the winter stuff which is supposed to be good down to -40C, will freeze quite readily when sprayed on the windshield.
Below -10C seems to be the point at which it will start freezing when the car is not moving but even at around -5C on the highway it will freeze if the windshield hasn't warmed up from the defroster.
It's evaporative cooling that causes the freeze as it occurs once the fluid has been spread across the windshield by the wiper blades so that it has a lot of surface area. It will not freeze as readily if sprayed and left there without the wipers at least not until it gets really cold out.
At colder temperatures, say -20C, at highway speed even a windshield that has had the defroster blasting it on full hot for a few minutes will have the windshield washer fluid freeze once the wiper blades spread it out.

This freezing effect does not seem to occur as readily with the rear wiper on my car which backs up the evaporative cooling hypothesis as the rear window is sitting in relatively still air compared to the front windshield. Of course the rear window is heated electrically and the front by hot air so temps may be different and the test is hardly scientific.

 

[DRWeig]

Without getting into the math (though I did so many years ago), I'm siding with Surestick and IRStuff -- radiation to the night sky and evaporative cooling combined can overwhelm convection in cool (but not yet freezing) temperatures.

Good on y'all,

Goober Dave