Heat Transfer Questions & Answers  
Question by Student 201700043
Dear professor, You said this morning the Reynolds number on a tube banks is equal to Red=\frac{\rho Umax D}{\mu } . Can we consider the Reynolds number is the same in all the bank? Because the temperature or Umax won’t be exactly the same if we are on the middle or on the top of the bank...
05.16.17
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Question by Student 201700043
Dear professor, You said this morning the Reynolds number on a tube bank is equal to : $$ Red = \frac{ \rho Umax D}{\mu} $$ Can we consider the Reynolds number is the same in all the bank ? Because the Temperature or Umax won't be exactly the same if we are on the middle or on the top of the bank...
Yes, the Reynolds number is function of $U_{\rm max}$ everywhere within the bank. But you're right, $U_{\rm max}$ may vary within the bank but this is taken into consideration by the empirical correlations in the tables. 1.5 point bonus: I would have given 2 if you would have typeset $Umax$ as $U_{\rm max}$.
Question by Student 201327104
Professor, I have a question for Question 2 from Assignment 6. After I figured out that the $Re_{x}$ at one side of the plate is $3.2 \times 10^{5}$, I tried to find appropriate equation for this situation (Laminar,local). But, there are two equations available, and properties also fulfill both condition for the equations. In this case, Can I get same result with two equations? which equation is better for the situation in Question 2 ?
Well, what is the difference between these two equations? The answer lies there probably..
Question by Student 201227127
professor, I wonder about question 1 of assignment 6. to solve it, I checked Reynolds number to know flow is laminar or turbulent. so I found Re>$5X10^5$, it is turbulent flow. in class, you talked laminar flow means same as steady-state. but in problem, find wall temperature in steady-state. but this flow is turbulent. so I wonder in steady-state, can turbulent flow appear?
Strictly speaking, no, a turbulent flow is always unsteady. But, if you take a time average of the flow properties within the turbulent boundary layer over a small amount of time (like 1-10 ms or so), then the turbulent flow can appear steady. In this course we're not concerned with the dynamics of turbulence, we simply consider the averaged properties of turbulence, so we call such turbulent flows “steady”. Good question. 2 points bonus.
Question by Student 201227153
Dear professor, I haver question about laminar and turbulent flow. The criteria Reynold's number is $Re_{x cret}=5*10^5$. If $Re_{x}<5*10^5$, the flow is laminar. Also, if $5*10^5<Re_{x}$, the flow is laminar-turbulent mixed. My question is $Re_{x}$ is only laminar or mixed flow along x. Turbulent flow depends on the point like Q2 of assignment#6. In summary, along x, the flow is laminar or mixed. However, along with the point, the flow is laminar or turbulent. Is it right?
In this course, the flow is considered fully turbulent at a location where the Reynolds number is larger than $5\times 10^5$. The flow is considered “mixed” if integrating both the laminar and the turbulent regions. 1.5 point bonus.
Question by Student 201227127
professor, in A6 Q1, I found $Re_L=1.45x10^7$. To find $T_w$, we need to find h. flow is mixed with laminar & turbulent, so in table, I choose "laminar-Turbulent average" but the restrictions are $T_w = const, Re_L <10^7$ in both 2 options. so I don't know what I choose ?
Well then you can not use these correlations. So you need to find another way..
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