Heat Transfer Questions & Answers  
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.
Question by Student 201428239
Professor, I have a question about A8 of Q1. I need $\mu$ to calculate Reynolds Number. In your lecture note of EG H-T in pipe, we used $\mu$ at average bulk T. What happened if I use $\mu$ at wall Temperature??? Does it cause wrong answer?
If you really want to know, you can try this in the quiz or exams and find out how many points you will lose.
Question by Student 201900067
Hello professor,I have a question about Assignment #8 question #4. In this question, it is said that pipe walls oppose negligible resistance to heat flow. However we must still consider the contact resistance between the pipe and the ground, if I'm not mistaken. The problem is that there is no indication about the material that makes up the pipe, so we shouldn't be able to use the tables to determine the contact resistance. In that case, can we neglect this resistance?
If you can't find the resistance, then list in your assumptions you assume no resistance.
Question by Student 201427129
Prefessor, I have a question on assignment 8 for problem 4. To have shape factor of Isothermal cylinder of radius r buried in semi-infinite medium having isothermal surface, there are three shape factor. Each of them have restrictions. However, given values are D = 2m, r= 0.15m and they satisfy the restriction in each case. Also, they have the same value, 191 m. which shape function should be chosen?
Any one is fine as long as the conditions are satisfied.
Question by Student 201312147
Professor, i have a question in table. In "Summary of convection correlations for internal flow in a circular tube of length L and diameter D", What is the difference between "Fully-developed turbulent flow (smooth and rough tubes)" and "Fully-developed turbulent flow (rough tubes)"? One is "smooth and rough tubes" and the other is "rough tubes", but I don't know the difference.
Well, as is written, one can be applied to either smooth and rough tubes, while the other can only be applied to rough tubes. Explain better what you don't understand.
Question by Student 201542124
Professor, I have a question on assignment 8 for question 1. We don't know $ T_{b2}$ and only know $T_{b1}$. Should I assume average $T = 343 K $ because the wall temperature is $344 K$, pipe is long compared to radius and it is fully developed flow?
No, you have to follow the instructions associated with the correlation. If it's specified the properties need to be determined at the average bulk temperature, then you need to do so.
Question by Student 201312147
For example assignment#8 Q1, pipe is rough pipe. because pipes have eddis. So, I think this problem "Fully-developed turbulent flow (rough tubes)" should be applied to this problem. But in solving this problem, I found that Reynolds number is "0.5 < Pr < 2000, 3000 <Re < 5E6". This condition enables "Fully-developed turbulent flow (smooth and rough tubes)" to be applied to the problem. So I don't know which of the two should apply to this problem.
Whether there are eddies or not has nothing to do with surface roughness. You can have lots of eddies (turbulence) in a tube with perfectly smooth walls.
Question by Student 201312147
So assignment#8 Q1, pipe is having a relative roughness. Then, can I apply either of the equation in the table?
As long as all the conditions specified are met, you can use any correlation you wish.
Question by Student 201327111
Professor I have a question about this assignment 8 question 1. I was able to get right answer(63degree) using correlation for fully developed turbulent flow (smooth and rough tubes) with assumed average bulk temperature of 57degree. But after several iteration process, the bulk temperature of second state converged to 48degree. I wonder if the answer is right. If you don't mind, can you check the answer again?
Right, both answers for A8Q1 were not for this problem. I fixed them: check again.
Question by Student 201527143
Professor I have a question about A8Q5-b hint. I got friction factor value from the hint using momentum eqn. The value is same with F-D Laminar friction factor from the table. However, flow of (b) is turbulent. Which one should I use? F-D turb friction factor from the table or Hint?
The friction factor mentioned in the “hint” is valid for both laminar or turbulent flows.
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