Heat Transfer Questions & Answers  
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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