Heat Transfer Questions & Answers  
Question by Student 201427115
Professor, I have a question in assignment #5. In question #2, to find heat transfer, I needed to multiply area per unit depth. Here, heat transfer($q_{out}$) occurs in the casing of the journal bearing. Because we assumed no heat transfer into the journal. So I multiplied by $\pi D_o$. But to get right answer I should multiply $\pi D_i$. Why does it use $D_i$?
05.08.18
In the question, it says the journal bearing has a diameter of 75 mm. It's not mentioned whether this is the diameter of the shaft or of the casing. This doesn't really matter here because the difference in diameter is quite small. I chose the diameter of the casing ($D_{\rm o}$) to be 75 mm. 1 point bonus.
Question by Student 201527110
Professor, I have a question about the "Eckert Number." I found that some of the books had defined the Eckert number as the rate of difference of temperatures as following below $$Ec=2\frac{T_0-T_\infty}{T_w-T_\infty}$$ However, in the Table what you offered, the Eckert number had defined as the rate of kinetic energy to Thermo energy difference as following below $$Ec=\frac{u_\infty^2}{C_p\mid T_\infty-T_w \mid}$$ I wonder they have different meaning (Defined for other reasons) or just have different forms?
05.12.18
What is $T_0$ in your second definition of Ec? We need to know how $T_0$ is defined. Write down the definition for $T_0$ and try to to express it as a function of $u_\infty$.
Question by Student 201527110
Ok I see. I found that $T_0$, stagnation temperature can be defined as $$T_0=T_\infty+\frac{u_\infty^2}{2C_p}$$ and then we substitute it in the other with the table one, then both Eckert number have same form. I appreciate for your help.
Great! 1 point bonus for bringing this up.
Question by Student 201327139
Professor, I wonder about 'Summary of eqs for flow over flat plate' table. When flow regime is Laminar, local, we have another 2 restriction, same condition of \( Re_x<5*10^5, 0.6<P_r<50 \) and ' \( T_w=const \)' / ' \( q_w=const \)'. What's the difference of two condition? What should I use between two condition? Thank you.
05.14.18
Well, you should use the ${\rm Nu}_x$ expression that says $T_{\rm w}$ is constant when the wall temperature over the entire plate is a constant. And you should use the ${\rm Nu}_x$ expression that says $q^"$ is constant when the heat flux at any location on the flat plate is constant. 0.5 point bonus.
Question by Student 201427126
I ask a question about assignment 7-2. In this problem, i want to find the density for film temperature. I think there are two ways. First, get through table “properties of air at atmospheric pressure” Second, it is obtained through an ideal gas equation. What is correct? What’s the difference?
05.18.18
In this case there is no difference because the air pressure is 1 atm.
Question by Student 201427126
I have solved both two methods. But there is a difference in value. The difference is not great, but i think that an appropriate choice is needed to reduce the error of the answer. What should i use for this problem, and if the pressure is not 1 atm, which of the two methods is appropriate? And why?
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05.19.18
Please write your equations using , not using a scan. Scans should be used for schematics. Post your complete question again below using and I will answer it.
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