Heat Transfer Questions & Answers

Question by Student 201327111 Professor I have a question about this assignment 8 question 1. I was able to get right answer(63degree) using correlation for fully developed turbulent flow (smooth and rough tubes) with assumed average bulk temperature of 57degree. But after several iteration process, the bulk temperature of second state converged to 48degree. I wonder if the answer is right. If you don't mind, can you check the answer again?

06.04.19

Right, both answers for A8Q1 were not for this problem. I fixed them: check again.

Question by Student 201527143 Professor I have a question about A8Q5-b hint. I got friction factor value from the hint using momentum eqn. The value is same with F-D Laminar friction factor from the table. However, flow of (b) is turbulent. Which one should I use? F-D turb friction factor from the table or Hint?

The friction factor mentioned in the “hint” is valid for both laminar or turbulent flows.

Question by Student 201527130 I have a question about assumptions. $$ m(Cp_2Tb_2-Cp_1Tb_1) = q_{ADDED} $$ For using this equation(m is massflow) and defining bulk temperature, I need to assume that density is constant. But in gass case, density is not constant. So the question is how to use bulk temperature in gass case?	06.06.19

For a gas, you don't need to assume constant density but you need to assume negligible kinetic energy change. I think I mentioned this in class..

Question by Student 201428239 Professor, I have a question about Design set 2 of #2. In this problem, I think I need to fine h which is natural convection H-T coefficient. So I need to select correlations. Can I use correlation about Horizontal cylinders??	06.08.19

Yes, this sounds correct.

Question by Student 201527130 I have a question about entrance region_length. To find the length, I use equations from handout $$ \delta = {4.64x\over{R_E^{1\over2}}} , \delta_t = {4.64x\over1.025R_E^{1\over2}P_R^{1\over3}} $$ and I think “x” is entrance length when 2$\delta$ is D(diameter of duct). on my thinking, I calculate these equations. $$ L_u = 0.108R_E^{1/2}D $$ $$ L_t = 0.11R_E^{1/2}P_R^{1/3}D $$ But in this case, $$ R_E = R_{E_x} $$ so, there is error i think.. Am i going the right way?

I don't understand what you write. What is $L_u$ and $L_t$? How do you calculate this? You need to define these new terms and explain how you got them.

Question by Student 201527130 I am sorry about omission. I use equations from handout. $$ \delta = {4.64x\over{R_E^{1\over2}}} , \delta_t = {4.64x\over1.025R_E^{1\over2}P_R^{1\over3}} $$ and I think x is entrance length when 2$\delta$ is D(diameter of duct). $$ 2\delta = D , 2\delta_t = D $$ If we summarize this equation about x, $$ x = 0.108R_E^{1/2}D = L_u $$ $$ x = 0.11R_E^{1/2}P_R^{1/3}D =L_t $$ Here, $L_u$ is entrance length of velocity and $L_t$ is entrance length of temperature that I think. But in this case, $R_E = R_{E_x}$. I remember that values are determined by diameter, not the length in duct. so, there is error i think.. Am i going the right way? I'm sorry I have always had a scanty question.

You're on the right track!

Question by Student 201527130 I have a question about design projects Q#5. I think the velocity in duct is too high. so, density is too big(10.2kg/m^3). I think it seems to be 20m / s instead of 200m / s when we match the answer.Could you confirm it if it does not work?

There is no problem with the question formulation.

Question by Student 201527136 Professor, I have question about fully-developed flow in pipe. You said the equation $\frac{U}{U{_{b}}}=2\left ( 1-\frac{r^{2}}{R^{2}} \right )$. Is this expression valid for both laminar flow and turbulent flow?	06.09.19

This is valid only for laminar flow.

Question by Student 201428239 Professor, I have a question about A7 of Q5. In this question, I need to use correlation of free convection H-T. I should use vertical plane correlation. In the table, a comment written as "x the distance from the bottom". In this comment, the x means height?? Then, when I use correlation ( $Nu_x = C(Gr_xPr)^m$, should I use x = H??	06.14.19

In this correlation, $x$ is the distance from where the boundary layer starts.

Question by Student 201428239 Professor, I have a question about A6 of Q3. In this question, I know the value of q local and q average.Then, I can get the relation between $Nu_x and Nu_L$ average. To solve (a), Do I need to compare these values and find the flow type??? If so, I can easily get Laminar flow, but in your comments, it could be either laminar or lam-Turb mix. How can find Lam-Turb mix relation??? Thank you

06.15.19

Well, by using a Nusselt number correlation that is suited to the turbulent/laminar regimes.

Question by Jaehyuk Professor, I have a question regarding A7Q5. As far as I believe, it is suitable to use the correlation for a vertical plane with constant heat flux;$Nu_x = C(Gr^*_{x}Pr_{f})^m$. Here starts my problem. In order to find constants(C and m), the range of $Gr_x^*$ has to be set first. However, when $Gr_x^*$ is in between $1E11<Gr_x^*<2E13$, there is no option for constants(C and m). In this case, is it possible to choose any one of two options?

06.18.19

If you can't find a correlation that fit perfectly your situation, then choose the one that is the closest.

Question by Student 201428239 Professor, I have a question about A7 of Q5. In this problem, I should find free convection heat transfer coefficient. But in correlation, I should know $T_s$ first. I can get $T_s$ through iteration process. And then get h. Is this procedure correct?? Because in the problem, the order is to find h first and then find $T_s$.

Yes, exactly. I mentioned this in class.

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