heat transfer

[smokey2007]

i am trying to determine if it is better to have laminar or turbalent flow for my problem.

 

i have a hot material (nylon 6) that needs to be cooled. so i will have a jacketed wall (heat exchange plate) that will have water running through it.

 

should i use dimensions that will result in laminar or turbalent flow?

 

also, i am having trouble finding a simple equation to use that involves laminar flow.

 

thanx!

[DeanK2]

Laminar flow - imagine a Liebig condenser

http://www.efm.leeds.ac.uk/CIVE/CIVE1400/Section4/laminar_turbulent.htm provides simple example for equation.

[Bignose]

Laminar flow - imagine a Liebig condenser

http://www.efm.leeds.ac.uk/CIVE/CIVE1400/Section4/laminar_turbulent.htm provides simple example for equation.

 

Dean,

 

We try not to just answer questions in the homework part of the forum.

 

We do help the OPs by giving hints, or pointing in the right direction, or asking leading questions of the OP -- basically all-in-all a Socratic approach.

 

But, we don't just give answers.

 

On top of that, I don't think that you can say with certainty which one is better. Because I don't think that the OP has given enough information. I.e. is it more important to achieve the cooling? or is it more important to keep the flow nicely behaved? And, are we talking about the flow of the nylon, or the flow of the water inside the wall? What about the size/capabilities of the pump? For that matter, nylon being a polymer probably doesn't flow like a typical Newtonian fluid. Turbulence in non-Newtonian fluids isn't anywhere as well behaved as turbulence in Newtonian fluids -- not that turbulence in Newtonian fluids is well behaved in the first place.

 

Furthermore, Dean, your example of a condenser isn't terribly appropriate, since the OP didn't mention needing to condense anything... Condensing something is very, very different from just cooling a fluid down.

 

There are a lot of questions that need answers to fully define the problem. You have to think about some of the fundamental differences between laminar and turbulent flow. In particular, the major difference between the two is the amount of mixing. The question is whether you want that mixing or not. Or can you prevent the mixing/turbulence if you want to?

[smokey2007]

i appreciate your help, but that did not answer my question so i will give more info like you suggest.

 

the most important aspect is achieving optimal cooling (i could care less about whether or not the fluid is well behaved).

 

the fluid inside of the jacketed wall will be the water.

 

i am currently using a pump with 40 gallons per minute. but i can increase this if needed.

 

mixing does not matter to me one way or the other.

 

i just need something that works, and i am not sure if laminar or turbalent is more efficient. somebody with a PhD in polymer science told me i should avoid laminar because it is not as efficient. but in order to do so, i have to increase my pump size as well as other things.

 

if i did use laminar flow, then i am not sure how to solve for the heat transfer coefficient of the watear. with turbalent flow it is:

 

Nu=.023*Reynalds number^(4/5)*Prandtl number^n

 

where:

 

D refers to the "characteristic length" used to calculate the respective parameter

Pr is the Prandtl number

n=0.4 for heating of the fluid, and n=0.3 for cooling of the fluid[2]

 

but i do not think the above is accurate for laminar flow.

 

i have looked up and down google looking for an answer to this, so any help would be greatly appreciated.

[Bignose]

smokey, I'm not going to just tell you an answer. I was hoping that my questions would help you explore the answer on your own.

 

In that regard -- are you sure that mixing is unimportant? (I don't think that I can be any more unsubtle than this - *wink* *wink*)

[smokey2007]

i have seen the proof i seek that turbalent flow is more heat transfer efficient. thanx for the right direction.

[Bignose]

Since you've gotten your answer, I just wanted to make one final comment in case anyone else reads this thread.

 

I think that smokey didn't get my first hint because he only considered mixing to be between two different substances. But mixing is a very general term, and in this case, it is the mixing of the warm and cool fluid. If no other considerations are important, then turbulent heat transfer is always more efficient because of the mixing inherent with turbulence. In this case, the turbulence brings the warm fluid in the center of the vessel to the cooling jacket much more efficiently than a laminar flow would. Because of the mixing, the fluid ends up more uniformly cooled than it otherwise would have.