Heat Transfer Questions & Answers  
Question by Student 201227125
I have question of shape factors from heat transfer tables. At Isothermal cylinder of radius r buried in semi-infinite medium having isothermal surface. There are three shape factors that $$ \frac{2 \pi L}{cosh^{-1}{(D/r)}} , \frac{2 \pi L}{ln(2D/r)} , \frac{2 \pi L}{ln\frac{L}{r}[1-\frac{ln(L/2D}{ln(L/r)}]} . $$ Each shape factor has restrictions. But restrictions are overlapped. at assignment 8 - #3, L>>r, D>3r. I can use both $\frac{2 \pi L}{cosh^{-1}{(D/r)}}$ and $\frac{2 \pi L}{ln(2D/r)}$. What should I use in this case? I got the correct answer using $\frac{2 \pi L}{cosh^{-1}{(D/r)}}$. but, at assignment 8 - #3, Should $ \frac{2 \pi L}{ln(2D/r)} $ be used to obtain more accurate values? $$ $$ And I have another question. A>>B means that is A is 10times larger than B? or 100times? I'm not clear that how much larger or lower value makes the sign << or >> .
I don't think switching from one shape factor formula to another will make much of a difference — you should get a very similar result. Just make sure that the restrictions are applicable to your case. I'm not sure what $A\gg B$ means exactly. This is highly case dependent. But I would guess at least 5-10 times larger. 1 point bonus.
Question by Student 201427111
Professor i have a question for Heat treansfer over flat plate EG. $\rho_\infty$ is used to distinguish between laminar flow and turbulent flow. And if $5*10^5<Re_x<10^7$ so we can use $st_xPr^\frac{2}{3}=0.0296Re^{-0.2}$ but this time we use $\rho_f$, we know $\rho_f=\frac{\rho_\infty*T_\infty}{T_f}$. However assignment #6 Question1 $\rho_\infty$ is used to distinguish between laminar flow and turbulent flow. And we choose $\overline{Nu_L}=Pr^\frac{1}{3}(0.037Re_L^{0.8}-871)$. This time just use $\rho_\infty$. What's the difference between using $\rho_\infty$,$\rho_f$
It's better to use $\rho_\infty$ to distinguish between laminar and turbulent flow. This is because what triggers the turbulence are disturbances happening on the edge of the boundary layer (where $\rho=\rho_\infty$). This has little to do with the density midway through the boundary layer. Thus you should also use $\rho_\infty$, not $\rho_f$, to determine whether $5 \cdot 10^5 < {\rm Re}_x < 10^7$ because the lower limit of this Reynolds number range is related to laminar to turbulence transition. This applies also to other correlations for external flow over flat plates. But for external flow around cylinders and spheres and for natural convection, the Reynolds/Rayleigh number ranges are not related to laminar-to-turbulence transition. Thus, for these correlations, use $\rho_f$ not $\rho_\infty$. 1 point bonus.
Question by Student 201327132
Dear professor, I have a question about assignment 7, #5. In (b), I use this relation: ... = .... And we know Left hand side value.$q^"_{radin}=700W/m^2$. Then I put the $h=2.7\frac{W}{m^2 K}$ and $T_s = 319K$ in right hand side. But that is not same $q^"_{radin}$. Should I consider about additional term? I want to know what I missed. Thank you.
There is no mistake in the answers provided. You're missing out on an important aspect of radiation heat transfer. You should think about this more.
Question by Student 201427115
I have question about high speed flow. For high speed flow we use $T_{aw}$. To get $T_{aw}$, I need Pr and $C_p$. Here which temperature should I use to get Pr, $C_p$?
Good question. You can use the free stream values. Pr and $c_p$ don't change too much with temperature so it won't make much of a difference.
Question by Student 201427103
Hello, professor I have a question while studying Prandtl number.

Here's the first question. If there is a flow of fluid in motion of Pr > 1, the point at which the separation occurs will increase the heat dissipation locally?
I am not sure what you mean by the point at which separation occurs. Separation of what? Why would this dissipate the heat? I don't understand. Rephrase your question better. Please write one question per post. I deleted the others.
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