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Temperature needed to freeze moving water

by starfish99
Tags: freeze, moving, temperature, water
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starfish99
#1
Jul19-11, 08:18 PM
P: 28
There are many examples in nature of frozen moving water(frozen rivers, frozen streams,etc).
How does the movement and turbulence of water affect its freezing point?
If in a cold room kept at -1 degree centigrade we have two bowls:one bowl has one liter of water and the other has one liter of constantly stirred water(one revolution/second).
Initially both bowls are at 20 degrees centigrade, but the surrounding temperature of the room(-1 degree) will cause the unstirred water to freeze solid. Will the stirred water also freeze but take a longer time or will it never freeze?
If the stirred water never freezes is the reason that stirring creates a small amount of friction in the water slightly raising its temperature, so that the water never falls to -1 degree centigrade.
or
Is it simply mechanical? Tiny ice crystals form and try to grow, but they are broken up by the stirrer and the water doesnt have a chance to freeze.
Surely at very low temperatures, -40 degrees centigrade, any stirring at any speed will not prevent freezing.
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AlephZero
#2
Jul19-11, 08:45 PM
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One significant factor is that water has its maximum density at 4 degrees C.

In still water, the whole volume cools down to 4C as the colder water sinks to the bottom. a thin layer at the top surface can then quickly cool to 0C while floating on the denser water below, and start to freeze.

In moving water the whole volume has to cool to 0C before freezing starts.

Because of the high specific heat of water and the latent heat of freezing, it would take a very long time (probably several days) even for still water to "freeze solid" with an air temperature of only -1C.

The temperature rise generated by the energy needed for stirring is unlikely to be significant. Google for the classical experiments on the mechanical equivalent of heat to see how much work you need to do to get a measurable change in temperature.
davidj89
#3
Jul19-11, 09:03 PM
P: 11
Ya I think it would be all mechanical. You are just cooling the water to its ground state so it freezes, when you stir the water you are just bringing the water above its ground state with kinetic energy. Prob wrong but I would look at the first law of thermo dU=dQ+dW, where dU=0? so dQ=-dW where dW would be the stirring of the water? and maybe find what temp you would need using the third law? I would like to hear from someone else to figure out how you would find the temp needed to freeze water being given a certain amount of energy.

cragar
#4
Jul20-11, 02:06 AM
P: 2,466
Temperature needed to freeze moving water

This is probably irrelevant but what if I were moving at a constant speed on a train with a hole cut in the floor and down below was a river moving at the same speed as me. I wouldn't even be able to tell if i was moving and the river would appear to be stationary. When I measure the temperature of the water I could measure it below freezing temp and stationary. Or I could be in a tank of water in free-fall towards earth.
mikeph
#5
Jul20-11, 07:22 AM
P: 1,212
I think it will freeze but the stirring will delay it for the reasons poster number #2 said, the stirring ensures freezing cannot begin until the entire body of water is at 0. Aside from this effect, and the slight heating due to work done by the stirrer, I'd suggest the stirrer could aid freezing by providing an extra surface on which to begin nucleation. And as long as your stirrer is powerful enough you'd just get an icy slush rather than a block of ice.

I had a look at this and it is possible to cool water down to below freezing point without stirring,
http://en.wikipedia.org/wiki/Supercooling
http://www.webcitation.org/5mjJauR7S
An interesting phenomena in fluid physics is the undercooling of liquids. This is the lowering of the temperature of a liquid beyond the freezing temperature and still maintaining a liquid form. Normal freezing occurs when the atoms of the container walls impose an ordering on the liquid atoms causing them to arrange themselves into a crystalline structure and begin to grow. Without the container, the onset of freezing (called nucleation) has a good chance of not happening. But, at some lower temperature nucleation will occur and very rapid freezing takes place (called recalescence).
Droplets of supercooled water often exist in stratiform and cumulus clouds. Aircraft flying through these clouds seed an abrupt crystallization of these droplets, which can result in the formation of ice on the aircraft's wings or blockage of its instruments and probes, unless the aircraft are equipped with an appropriate de-icing system.
AlephZero
#6
Jul20-11, 08:27 AM
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Quote Quote by cragar View Post
This is probably irrelevant but what if I were moving at a constant speed on a train with a hole cut in the floor and down below was a river moving at the same speed as me. I wouldn't even be able to tell if i was moving and the river would appear to be stationary. When I measure the temperature of the water I could measure it below freezing temp and stationary. Or I could be in a tank of water in free-fall towards earth.
The relevant thing is the relative velocity of different parts of the water.

For the river example, the bed of the river is moving backwards relative to you. Because of the viscosity of the water, the water that is in contact with the river bed is not moving relative to the bed. You can see this effect in a real river, by comparing the speed of flow close to the bank with the speed in the middle of the stream. You would measire the same change of flow velocities at different parts of the river relative to the train or relative to the ground. (But of course in your thought experiment you can only see the what is happening on the surface!)

If a tank of water is in free fall (effectively in zero gravity) then there will be no buoyancy effects and the water will not mix as its density changes. So in that case, quite likely the surface could freeze even quicker, while the water below was still above 4C.
starfish99
#7
Jul20-11, 07:48 PM
P: 28
Suppose you had an electric mixer that stirs the water vigorously at room temp(20 degrees centigrade). The temp of the room suddenly drops 50 degrees. Now everything is at -30 degrees. Would the mixer eventually stop working because the ice forms and the blades of the mixer can't move or would the water still be liquid. If it is still a liquid , will the temperature of the water be 30 below zero if you measure its temperature?
mikeph
#8
Jul21-11, 03:22 AM
P: 1,212
Quote Quote by starfish99 View Post
Suppose you had an electric mixer that stirs the water vigorously at room temp(20 degrees centigrade). The temp of the room suddenly drops 50 degrees. Now everything is at -30 degrees. Would the mixer eventually stop working because the ice forms and the blades of the mixer can't move or would the water still be liquid. If it is still a liquid , will the temperature of the water be 30 below zero if you measure its temperature?
The answer to this question must depend on the power of the mixer. But the water can still freeze without becoming a single block of ice, it would just be smaller blocks of ice that are "mixable".
Doomemperor
#9
Nov11-11, 12:51 AM
P: 3
I think that it would not freeze, but at -10 it probably would.


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