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.

Question by Student 201428239 Professor, I have a question about radiation H-T. Should I always use unit of temperature K instead of Celcius degree??

Yes of course.

Question by Student 201312147 Professor, i have a one question. We said that most of the problems were not radiation. However, I think that radiation occurs in most situations. why we thought no radiation?? Is there an exact reason to assume that we don't have a radition?

Radiation heat transfer coming out of objects at 300 K is generally quite small compared to convection heat transfer. It becomes more important thus when the temperatures are higher because its intensity scales with the 4th power of the temperature. At low temperatures, you can ignore it. But at higher temperatures, you should take it into consideration unless there's not enough information to calculate it.

Question by Student 201312147 Professor, i solve the assignment#4, problem 7 (a) using iterative method. But I don't know if this is the right way to solve it. So, I want to know when to use the iterative method.

Sure, you must use an iterative method for this problem.

Question by Student 201527105 I have a question about Assignment#3 Q5. When i use ‘Shape Factor table’ , there is no case for Hollow sphere with r1,r2,r3. In this case, can i use the table about Hollow sphere with ri,ro?

You should use shape factors here. Think about it more.

Question by Student 201428239 Professor, I have a question about Assignment 3 of Question #5 (c). I don't know the answer about (c). So I don't know if my answer is correct. I got 450K for (c). Is it correct??

There's a reason why this is not given. Think about the problem more.

Question by Student 201428239 Professor, I have a question about Assignment 3 of Q5. Should I calculate T of SUN?? I already calcualted with respect to radiation H-T and I got 381.4K.... I think this is not true in r eal case. Is there anything that I missed??

Explain to us how you can calculate the sun's temperature.

Question by Student 201428239 I calculated the T of sun like this. $$q_{\rm sun}^”={\sigma}{T_{sun}}^4$$ And I continued like this. $$q_{\rm gen}={\sigma}A({T_{Bo}}^4-{T_{sun}}^4)$$ A is equal to $4{\pi}{r_3}^2$ Is there anything that I missed?

How can you find the temperature of the sun? I can't follow you. I see no equation $T_{\rm sun}=..$.

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