Heat Transfer Questions & Answers

Question by Student 201312147 Professor, I have a question assignment#3 Q5(a). To solve this problem, we have to need T of Sun?? or can we solve to using $q^{”}_{rad} = q{”}_{gen}$??	04.25.19

You don't have to find $T_{\rm sun}$ if it's not needed to find the answer to the problem.

Question by Student 201527136 Professor, I have question about assignment#5 Q1. In class, You said that power and viscous dissipation are same. But the answers of power and viscous dissipation are different. Why they are different?	04.29.19

You should be able to explain this on your own.

Question by Student 201427135 Professor, I have a question. A5Q2, the question is "What is the rate of heat transfer from the bearing, and how much power is needed to rotate the journal?" Does the rate of heat transfer from the bearing mean rate of heat transfer at journal ? or at bearing? If I took at journal, it's perfectly same with answer but at bearing, it's not. But they said there is no heat transfer into journal. So I'm really wondering. Thank you.

I don't understand your question. As written in the problem statement there is no heat transfer to the journal.

Question by Student 201312147 Professor, i have a question in couette flow. Is the couette flow assumed to be a laminar flow? in class, we din't consider Reynolds numbers. If the couette flow is the turbulent flow, should other conditions be considered?	04.30.19

Couette flow is laminar generally because the short distance between the plates makes it difficult for turbulent eddies to form. In this course, always assume Couette flow is laminar. Other types of viscous flows can be either laminar or turbulent as we will see shortly.

Question by Student 201527143 I have question about A5Q2-b. When I calculate the heat transfer from the bearing, I took Area term as bearing’s area. And I took Area term for power as journal’s area. Then, these results are not same. You neglected 0.25mm? Or any problem for my calculation process?

If you unroll the bearing into $xy$ coordinates, you have to give the same area to the bearing and the journal: otherwise this wouldn't be consistent. If you don't unroll the bearing and solve it in cylindrical coordinates, then you should use two different areas for the bearing and the journal.

Question by Student 201527130 I have a question about adiabatic wall temperature.(EG. H-T over plate with V.D) T_infinity = 290K T_wall = 300K so first, I think heat flux(q”) goes wall to infinity. but using T_aw(=360K), heat flux goes infinity to wall. ( q”=h(300K-360K) ) I think this phenomenon is occurred by viscous heating. Is my think right?	05.19.19

I'm not sure what is confusing you. As you mention, when V.D. is important, $q^"=h(T_{\rm w}-T_{\rm aw})$.

Question by Student 201428239 Professor, I have a question about Assignment 6 Q3. In this problem, there is liquid(water) flowing over plate. In 'Summary of Equations for flow over Flat plates' Tabel, there are only two informations about liquids ( Laminar, local and Laminar-TURB average). Should I only use these two informations?? or It doens't need to consider about it?? Thank you	05.21.19

You can use other correlations too unless they specifically say they are not applicable to liquids.

Question by Student 201527130 professor, I have a question about Tube banks (staggered). Is minimum area lcated at between 1st line to 2nd line? If that’s right, I think Is my thinking right?	05.26.19

You're on the right track, but this is not correct still. Work on it more — the diameter of the tubes should be somewhere in your areas.. Also, use the PREVIEW command and make sure your math show up clearly: your expressions are too close to each other and hard to read. Separate them on different lines.

Question by Student 201428239 Professor, I have a question about Assignment 7 of Q5. In this problem, Can I use correlation equation about vertical plane?? Or Should I use irregular solids?? And also, In your lecture note, there is an example about free conv H-T example(Desktop problem with q=30W). In that problem, we determined surface T first and then free convection heat transfer coefficient. But this problem is opposite. Can I determine free convection heat transfer coefficient without surface Temperature??? Thank you.

05.27.19

Use the most accurate correlation generally. Yes, you can find the $h$ without knowing the surface temperature a priori through an iterative process.

Question by Student 201428239 Professor, I have a question about A7 of Q6. In this problem, I think I should use Nu as flow across a sphere. But in table, there is a expression about $u_w$ (viscosity at the wall)what is it exactly?? Do I go right??

Well, as you mention, $\mu_{\rm w}$ is the viscosity of the fluid at the wall determined at the temperature of the wall.

Question by Student 201428239 Professor, I have a question about A7 of Q6. I don't know how to find $\mu_w$ in this problem. Is it fine to assume this is same as $\mu_\infty$??

Find the wall temperature, and then find $\mu_{\rm w}$ from this wall temperature.

Question by Student 201427135 Professor, I have a question about the class. You taught us about hydraulic diameter, example for river. But I can't understand why it doesn't have friction area on the both sides. I thought river has ground on bottom and on both sides (actually extended ground ) also. Thank you.	06.03.19

True, it could be this way, but I was thinking of a river section where the sides are flowing water.

Question by Student 201527136 Professor, I have a question about heat transfer in pipe. You said $\,\Delta T_{lmtd}$ is almost same with $T_{w}-\frac{1}{2}(T_{b,o}-T_{b,i})$ when $\,\frac{T_{w}-T_{b,o}}{T_{w}-T_{b,i}}\geqslant 0.5$. If $\, T_{b,o}$ is not given, I can't judge$\,\frac{T_{w}-T_{b,o}}{T_{w}-T_{b,i}}\geqslant 0.5$. Then, do I have to use only $ \,\Delta T_{lmtd}$ instead of $ T_{w}-\frac{1}{2}(T_{b,o}-T_{b,i})$?

Solve the problem and check if the simplified expression can be used or not. If it can not be used, re-solve the problem with the full expression.

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