# Heat transfer

1. Dec 29, 2013

### jeeva14

how to reduce temperature of a solid bar from 550°c to 450°c by water spaying... someone really help me...??

2. Dec 29, 2013

### phinds

Do you think the shape of the bar would have an effect? How about the size? What do you think WOULD have an effect? You are asking a question without giving any sense of what you know. That's not how we do things here on this forum. Say how YOU think it could be done.

Also, do you think your question has enough information for anyone to give an exact answer?

3. Dec 29, 2013

### dawin

I think water spraying should do it.

4. Dec 29, 2013

### jeeva14

a bar of d=120mm and l=200mm is heated to a temp of 550°c, then it has to be cooled to 450°c by water spraying. The amount of water sprayed need to be calculated and in how much time it would reach the desired temp is also to be find. Both ends of the bar is adiabatic, and the heat transfer is considered over the circumference. I have clearly defined my problem. Now can u help me...?????

5. Dec 29, 2013

### Staff: Mentor

Now it is starting to sound more like homework. Is it? Regardless of if it is or not, it is of a form where you'll need to show some effort in trying to solve it. We don't just spoon-feed answers here, we help people find them for themselves.

6. Dec 30, 2013

### caldweab

Did you even think about the problem? That's not a very difficult problem to solve.

7. Dec 30, 2013

### AlephZero

Actually, it's an impossible problem to solve without making some drastic assumptions about how the water spray interacts with the bar.

If you make some highly unrealistic assumptions, you could turn it into a textbook type of problem. But without knowing more about the book, or what the OP's level of knowledge is, that is just guessing IMO.

8. Dec 30, 2013

### dawin

Well now I feel like I'm missing something. You could easily make that a 1D heat transfer problem and consider the enthalpy of vaporization and surface area to determine the volume of water necessary, no? You'd have to assume properties for the bar, too, but that's not a big deal.

I think since it isn't specified, you have to assume the water sprays uniformly over the surface, and that convective losses from the surrounding air are negligible. That doesn't sound too drastic for this type of problem though (if it's a back of the envelope, or especially a homework).

I agree that it sounds like a homework problem, either way.

9. Dec 30, 2013

### phinds

Do you really think you have defined the problem? Clearly you have not. Do you think it would take the same amount of time and water to cool a bar with very high heat capacity vs. one with very low heat capacity? Say one of depleted uranium vs one of aluminum?

10. Dec 30, 2013

### AlephZero

To get you started thinking about what you are missing, how fast is the water being sprayed? Both interpretation of "fast" are relevant: liters/second and meters/second.

In real life, the water won't just sit on the bar until it boils.

11. Dec 30, 2013

### dawin

Oops, re-reading I see already that I missed the time requirement. I'm sure it's not the only thing I'm missing.

In this case of real life, at least as it would be in the confines of this problem--adiabatic ends and all--the bar is 550°C. If it's steel it's likely glowing red if not very close to it. The droplets aren't hanging out very long.

If it was for some process (and he's just not giving us all the info) and an estimate for the spraying was needed, I still think you could ballpark the spraying and time results using some rudimentary assumptions and realistically bounding the parameters. It it's a homework problem and that's all you've got, well, I agree, have fun.

12. Dec 31, 2013

### caldweab

It looks like one of the problems I solved in my heat transfer class. You do have to make some assumptions. I actually think I still have the assignment somewhere. I don't think he wrote the problem here as it is in the book though. Looks like he is just trying to get something to turn in.

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