One of you patient parent types please help me out with this one.

Not exactly an Aerostar topic, but we were riding in one at the time she asked this.

Daughter studied wind chill in science class. The teacher gave her the formula for figuring the wind chill, and according to the prof, at 20° F and 80 mph wind (driving Aero at 60 mpg into a 20 mph headwind) the wind chill is around -55° F. She wants to know why the antifreeze doesn't freeze at that temp. Being an English major, I don't have a clue.

Any help appreciated!

Thanks!

Dave

 

The only real part of the equation is the actual temperature. All the other parts of this is just how it would feel to a human being.

However, if you had, let's say a bucket of water, the time that it would take to get to 20 degrees would be quicker than if there was no wind at all because of the heat being blown away quicker.

In the case of this equation, if you had antifeeze that didn't freeze until it was 19 degrees, you would be okay. (Before somebody says anything, I advocate much stronger antifreeze, BTW)

 

So it's a trick question?
Thanks! Now I don't feel quite so ignorant.

 

Not a complete trick question, but the wind chill factor is for determining how you would feel in the presence of cold wind.

The basics of heat transfer from a hotter object to a colder object is this: The transfer rate is proportional to the temperature difference (or the average thermal energy) between the two objects. Assuming the hot object does not have an active heat source and the cold object does not have an active heat sink, as soon as these two objects are placed together, heat will start transferring from the hotter object to the colder object. The hotter object will start to cool down and the colder object will start to warm up, especially in the immediate regions of contact, the temperature changes of each are based on the specific heat of each object. This will slow down the heat transfer. Heat from other parts of the hotter object will move into the region of contact because it is now cooler, and the region of contact on the colder object will start warming up other parts of that object. This called a gradient, and grows as heat continues to transfer, but it does slow down the rate of transfer.

If the two objects are your body and the cold wind, the blowing wind is constantly exposing you to fresh cold air, so a gradient never forms, so your rate of heat loss is higher. Similarly, if the bucket of water is constantly being blown by cold wind (the active heat sink), it will lose its heat faster. This is also why you want the coolant in your engine to move quickly to pick up heat at as high rates as possible, but move slower when going through the radiator to lose as much heat as possible to the blowing air, which hopefully is colder than the coolant.

 

How is this accomplished? By bigger hoses on the radiator and smaller passages in the engine?

Thanks for the lessons, gents!

 

Hi Dave:

The wind chill situation you described only means this:

With the wind blowing, even though the wind is only 20 deg, your body will feel as cold as standing in -55 deg without wind while trying to maintain the same body temperature, i.e. not allowing your body to cool down. In short, it means you are losing heat to the air at the same rate under both circumstances.

However, a 20 deg stream of air will only cool an object to a temperature of 20 deg, and no lower regardless of the wind chill. That's why the antifreeze won't freeze.

 

I am surprised no-one brought up the cardboard trick to prevent anti-freeze from freezing.

 

Wind chill only happens when there is water that can evaporate. If an object is DRY, the wind can only cool it down to the air temperature, and absolutely no colder

Unless it's leaking, the outside of the radiator and engine are dry.

We humans are wet - water evaporates through our skin, even if we are too cold to sweat. The energy required to make the water evaporate can drop our skin temperature below the air temperature.

The "old school" method of measuring relative humidity used two thermometers in moving air - one "dry bulb" thermometer and one "wet bulb" thermometer. I remember using one when I was a student that had to be twirled around a handle, but that's off topic. The dry bulb measured the ambient air temperature. The evaporating water caused the wet bulb to show a lower temperature than the dry bulb. The relative humidity was then determined by looking up both temperatures on a chart that came with the unit. This is exactly the same effect we "wet" people feel as wind chill on a cold, windy day.

I hope this helps.

Cheers,
Eric

 

The flow rate through the whole cooling system has to be the same, but the linear rate would be determined by the effective diameter of the pipe that the water flows through. To get the water to flow slower through the radiator, you make the effective diameter bigger; ie: get a larger capacity radiator, like one with more rows or tubes. Not only do you spread the coolant across more core tubes that are exposed to more air, but you increase the time that the coolant spends there for a given flow rate.

I'm thinking about those blustery winters in Detroit, when the wind was strong enough to penetrate the coats I wore. I'm not sure how much evaporation was going on from my skin at that time, but it did feel very cold.

You probably also know about how humidity in hot weather makes you feel more miserable than in dry air; it prevents your sweat from evaporating to cool you.

By the way, most people know that the specific heat of liquid water is 1 calorie / gram / degC. But it's like 560 calorie / gram to evaporate water, which is why it can feel so cold to have water evaporate from your skin.

 

Look, sometimes our body feels like -55 dgr when it is -20 only becouse human body heats air under clothe and its temperature is stabil, usually 36.6 dgr C. And our body is not always dry, and when liquids evaporate, temperature decreases.

Formaly from school works for human body only. Anymal's body temperatures differ and formula is not accurate. For example cat's body temperature is about 40 dgr C.

 

copper is spot on with his post about why it does not freeze.

 

I remember that. (Gives away that I am old school as well.) It was called a sling hygrometer.