Heat Transfer Questions & Answers  
Question by Student 201428239
Professor, I have a question #4 of design 1. Can I use Efficiencies of rectangular fins table?? I think I can't use because it is not insulated tip. I am confused about this.
04.22.19
The efficiency chart in the tables can be used when the tip is not insulated. When the tip is insulated, you can also use the efficiency chart but you need to set $L_c=L$ instead of $L_c=L+t/2$.
Question by Student 201542124
In design #1, problem #4, I have a question about the number of fins. $$ q=\frac{T_{gas}-T_{i}}{R_{conv}} $$ So I substitute values and get $$ q=\frac{1200}{\frac{1}{h_i*2\pi*r_i*1m}} = 7540W $$ There are 8 fins. I am wondering if q should be divided by 8?
04.23.19
Hm, maybe, maybe. But you should be able to deduce this from the problem statement..
Question by Student 201427135
I have a question what you taught us professor. On march 18th, you taught us about convective heat transfer and conductive heat transfer. I thought it can be solved by finite element method and I tried

node 1 is $T_i$, node 2 is $T_s$, node 3 is air's temperature
element 1 is between node 1 and 2 , element 2 is between node 2 and 3 \begin{equation} q_{i} = -kA\frac{T_{i+1}-T_i}{l} = kA\frac{T_i-T_{i+1}}{l} \end{equation} \begin{equation} q_{i+1} = \frac{kA}{l}(T_{i+1}-T_i) \end{equation} $$ \left(\matrix{q_i \cr q_{i+1} }\right) = \frac{kA}{l}\left(\matrix{1 & -1 \cr -1 & 1 }\right)\left(\matrix{T_i \cr T_{i+1} }\right) \rightarrow conduction $$ $$ \left(\matrix{q_i \cr q_{i+1} }\right) = hA\left(\matrix{1 & -1 \cr -1 & 1 }\right)\left(\matrix{T_i \cr T_{i+1} }\right) \rightarrow convection $$ $$ [K]^{1G} = A\left(\matrix{k/l& -k/l & 0\cr -k/l & k/l & 0\cr 0 & 0 & 0}\right)$$ $\rightarrow$ stiffness matrix of element 1 between node 1 and 2 $$ [K]^{2G} = A\left(\matrix{0& 0 & 0\cr 0 &h & -h\cr 0 & -h & h}\right)$$ $\rightarrow$ stiffness matrix of element 2 between node 2 and 3 $$ [K]^{G} = A\left(\matrix{k/l& -k/l & 0\cr -k/l & k/l+h & -h\cr 0 & -h & h}\right) $$ $\rightarrow$ Global stiffness matrix

Applying Boundary conditions

$$A\left(\matrix{1/A& 0 & 0\cr -k/l & k/l+h & -h\cr 0 & 0 & 1/A}\right)\left(\matrix{50\cr T_s\cr 20}\right) = \left(\matrix{50\cr 0\cr 20}\right)$$
$$A(-50k/l+T_s(k/l+h)-20h)=0$$
$$l=0.1m , k=1.37W/m*celcius, h=4.5W/m^2*celcius, A=0.1*0.3m^2$$

So $T_s$ = 42.582 celcius. It's actually correct with what you taught us . Would you check for me whether it's right approaching or not ?

Also, I'm wondering that these equations are really used when heat transfer problems occur like in factory..
04.24.19
This seems ok, but is beyond the scope of this course thus. But thanks for sharing this with us!
Question by Student 201427135
Professor, I have some questions of Assign#1 [Assign#1-1] It's insulated wall but heat is transferred through the insulation wall. Is it not perfectly insulated so they give me conductivity ? I don't know exactly why heat can be transferred. [Assign#1-2] They give me velocity but we don't think about velocity. I think if velocity exists, convective heat transfer is much higher than if velocity doesn't exist. Also they give me convective heat transfer coefficient. Does this coefficient include velocity effect? Thank you.
Hm, 1 question per post only. And keep in mind there are no bonus points for questions that are obviously asked for the sole purpose of getting a bonus. Spend time rather working harder on the assignment problems: this will help you more get a higher grade.
Question by Jaehyuk
Professor, I have a question regarding assignment #3, question #5. There are two sources of heat, which are internal(heat generation) and external(radiation). As the system is in between these sources, how does the direction of the heat flux can be determined when using resistance analogy?
You can set the direction from left to right and if $q$ ends up being negative it means your guess was wrong. What matters is to be consistent. If you draw $q$ from left to right and $q$ is negative in your answer, then keep the answer as negative but make a note that the heat transfer is going from right to left.
Question by Student 201427135
Professor, I have a question A2 Q5-(c).

I should get shape factor for solving insulation thickness.
So I think it's steady state, the insulation is isothermal so I used isothermal cylinder shape factor. It's actually the first thing of shape factor table. But it didn't give me answer so I used hollow shape factor, then it did.

I don't know why shape factor is about hollow cylinder. I think inside of cylinder, it's full of steel cable so it's not hollow cylinder. Thank you
04.25.19
I don't understand. If it has steel cables in it, the cylinder must be hollow. It can not be full.
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