Heat Transfer Questions & Answers  
Question by Student 201327132
Dear, professor. I have a question for calculate temperature unit. How to calculate $$ \frac{K}{°C} $$ I confuse that relation.
04.19.18
I don't understand what you mean. Please explain better.
Question by Student 201327132
Dear Professor. Sorry to my poor explanation. I have no idea how to calculate in assignment 4, question 5, Biot number. $$ \frac{hR}{3k} = \frac{3[W/m^2K]×0.0245[m]}{0.147[W/m°C]×3} =0.167[\frac{K}{°C}]$$ So I don't understand that relationship. $$0.167[\frac{-272°C}{°C}] or [\frac{K}{274K}]$$ Which one is correct? I can't find any solution this relationship. Thank you.
In this case, the Kelvin degrees used for $h$ can be written as Celcius degrees because convection heat transfer is function of temperature difference only and a difference in Kelvin is exactly the same as a difference in Celcius degrees.
Question by Student 201427115
Professor, I have a question about finding thermal Diffusivity. In table of Properties, I can find $\alpha$ directly. But when I use $\alpha = \frac{k}{\rho c}$ I get different answer. Which one should I use?
04.20.18
If you use $k$, $\rho$ or $c$ elsewhere in your problem, then to be consistent you should compute $\alpha=k/\rho c$ and not take it from the tables.
Question by Student 201327132
Dear professor. I wonder about insulated cable design problem(inner radius as small as possible). At previous assignments, we directly use $ P_{elect} = S_{gen}$. But this example, Even though we know $P_{elect}$, we calculate $S_{gen}=R_{elect}×(\frac{P_{elect}}{V}^2 ×L/2\pi r_i^2)$. I don't know difference of two relations. Thank you.
04.28.18
Hm I think I explained this in class. I am not sure what you don't understand. Tell us what exactly is confusing you..
Question by Student 201427115
Professor, I have a question about insulation. At the surface insulated, heat transfer is zero. But in assignment 1 problem #1, even the wall has insulation layer, it has heat transfer inside the insulation part. What’s the difference between two? Thank you.
If an insulation layer is specified without a thermal conductivity, then assume its conductivity is zero (and hence there is no conduction heat transfer). If the insulation layer has a given thermal conductivity, then the heat transfer through the insulation layer will not be zero. 1 point bonus.
Question by Student 201327132
Dear Professor. Today, We learned about Thermal layer. I have a question $$ \frac {\mu}{\rho c} (\frac {\partial u}{\partial y})^2 = \frac {\mu}{\rho c} \frac {(u_\infty - 0)^2}{\delta_t^2}$$ I think that we can't use ($u_\infty - 0$). Because Boundary layer is longer than Thermal layer. So If we use $\delta$, ($u_\infty - 0$) is reasonable. But, If we use $\delta_t$, The velocity at the point have some difference with $u_\infty$. Is it because assume very small difference? Thank you.
04.30.18
Here we were doing an order of magnitude analysis. As long as $\delta$ and $\delta_t$ are within the same order of magnitude (and as will be shown later, they are), the analysis is correct. 1 point bonus.
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$\pi$