Heat Transfer Questions & Answers  
Please ask your questions related to Heat Transfer in this thread, I will answer them as soon as possible. To insert mathematics use . For instance, let's say we wish to insert math within a sentence such as $q_{\rm rad}$ is equal to $\epsilon A \sigma T^4$. This can be done by typing \$q_{\rm rad}\$ is equal to \$\epsilon A \sigma T^4\$. Or, if you wish to display an equation by itself out of a sentence such as: $$ q=-\frac{\Delta T}{\sum R} $$ The latter can be accomplished by typing \$\$q=-\frac{\Delta T}{\sum R}\$\$ . You can learn more about on tug.org. If the mathematics don't show up as they should in the text above, use Chrome or Firefox or upgrade MSIE to version 9 or above. Ask your question by scrolling down and clicking on the link “Ask Question” at the bottom of the page.
Question by Student 200727117
Is it something wrong if I already have an ID on website? I've checked if I still have an ID here or not, then I've found that I have it. It is second time to take this course so I registered an ID here before. Also I've clicked on "Subscribe" to Heat Transfer forum. Thank you.
No, you don't need to register again if you already have an account. I can't give you a bonus boost for this question thus because it doesn't involve heat transfer theory ;)
Question by Student 201027128
First, I'm sorry When I don't study properly I ask you a question blindly my question is situation in different temperature wall each other In body-volume situation \begin{equation} q=\epsilon\sigma(T_{wall}^4-T_{body}^4) \end{equation} and then in wall-wall situation $q$ = $A\sigma T_{wall}^4 $ but why we do use only T?? For example ,right wall is black body has a T1 and left wall is gray body has a T2 (BUT T1>T2) I think I use equation in body-volume situation. therefore, I am curious why we doesn't matter different tempature , and then why two equations are different?
In class we covered heat transfer in a room with all the walls at the same temperature. The equation that we derived is not applicable to a situation where the walls are not at the same temperature (if we have time, I may address this situation in the last class, but this is beyond the scope of this course). I'll give you 0.5 point for this question.
Question by Student 201027128
I ask a question about assignment 3 -1. In this problem $T_\infty$ is not maximum temperature but assume $T_\infty$ is very very very higher (like a sun). how do I solve?? I think first direction of heat flux is left side but I should think about heat generation so, heat transfer in c and d slabs is $q_\infty$ -$q_S$ (S means heat transfer for heat generation) and then in 3-2 I think $T_\max$ is in inside slab and solve that problem but problem does not assume $T_\inside$ > $T_\infty$ I think i have to prove to slove problem why $T_\inside$ > $T_\infty$ but I can't think good idea How can I do??
Even if $T_\infty$ is much higher, the maximum temperature in the composite wall will still be higher than $T_\infty$. This is because the left side is insulated and the heat must come out to the environment on the right side. In order for the heat to go out from the wall to the environment, the wall must have a maximum temperature higher than the environment temperature (recall $q^{\prime\prime} \propto -\partial T/\partial x$). Of course, if the environment temperature is a thousand degrees Kelvin or more and the wall is made in plastic, the wall would melt and the problem wouldn't make sense ;) But for this question, we can assume that the wall does not melt at the temperature encountered.. I'll give you 1.5 point for this question. I would have given 2 points if you wouldn't have made a mistake in typing $T_{\rm inside}$ — check carefully with the “preview” command that your post is well typeset.
Question by Student 201027128
I have a question about 2013 mid-term exam problem number 4 This problem is based by assignment 4 number 3 but this problem is given two device at both sides and both sides temperature are unknown so I have to find temperature of sides to find temperature of rod at midway point I assume boundary condition x=0(spot at device 2 ), T=$T_1$ and x=-1(spot at device 1) $ \frac{{\rm d}T}{{\rm d}x}=0\qquad $ because energy of device2 (50W) > energy of device1(30W) so heat transfer direction is left side . In steady state dq(conduction)+dq(convection)=0 dq(conduction)=-k$\pi$$r^2$ $ \frac{{\rm d}T}{{\rm d}x}\qquad $ dq(convection)=2h$\pi$r(T-$T_\infty$)dx so we can have $m^2$$\theta$ =$d^2$$\theta$/d$x^2$ \begin{aligned} m & = \sqrt {2h/kr} \end{aligned} In this condition I solve the equation and find $T_1$ q(x=0)=50W= -k$\pi$$r^2$ $ \frac{{\rm d}T}{{\rm d}x}_\qquad $ (x=0) . When this equation solve, I get the value of $T_1$, $T_1$=429.27$\circ$C But when this value used, answer is wrong . At midway point is x=-0.5 . When I use this conditions . midway of temperture is 189$\circ$C , but unfortunately answer is 291$\circ$C . I don't know where is wrong could give me some hint?
Hmm, I am not sure what could be wrong with your solution.. Make sure to impose the boundary conditions correctly (fixed heat transfer at both ends) and if the arithmetic is done without mistake, you'll get 291$^\circ$C in the center of the rod.
Question by Student 201027128
Hello Dr. parent I ask a about the problem #5 in 2013 final exam This is circumferential fin problem. It is very similary assignment 4-1 problem First, We use free convection to find h and then, solve fin problem . In the circumferential fin problem , We use chart [Efficiencies of circumferential fins of rectangular profile] But diameter of fin is 0.3m ,diameter of pipe is 0.03m and thickness is 0.03m I calculate ratio( $r_2$/$r_1$) . The value is 11. It's very big..... maximum ratio in a chart is 5.... how can I find Fin efficiency?? could you give me some hint??
Using the chart and analyzing the intervals between the curves $r_{2c}/r_1=1$, $r_{2c}/r_1=2$, $r_{2c}/r_1=5$, etc, you can take a guess of where the value $r_{2c}/r_1=11$ would be. Alternately, you could simply state that the closest answer in this case is $r_{2c}/r_1=5$ and mention how much error you would expect this to yield on the fin efficiency (give an estimate). I'll give you 1.5 point bonus for this question. I would have given more if you would have written your paragraph better with proper punctuation.
Question by Student 201027110
Hi, professor. I have a question about #3 of Design Problem set 2. In this problem, Air flows at Mach number of 6 on top steel plate. It's hypersonic flow. So I think that I have to use 'this equation' in tables for high-speed flow over flat plates 'T* = $T_{\rm infinite}$ + 0.5($T_{\rm aw}$ - $T_{\rm infinite}$) + 0.22($T_{\rm aw}$ - $T_{\rm infinite}$)'. Am I right? and High Reynolds number means high speed all the time? Although air flows at a low speed, high $\rho$ and low $\mu$ make high Reynolds number in this special case. So please tell me criterion of high speed.
You have to use the “high-speed flow” correction when the Eckert number is higher than the inverse of the Prandtl number. Sometimes, it's not possible to know this before solving the problem. If you think the high-speed flow correction is not necessary, then solve the problem ignoring the high-speed flow correction, then calculate the Eckert number, and make sure it's low enough. If the Eckert number is too high, recalculate the problem using the high-speed flow correction. I'll give you 2 point bonus boost for this question.
Question by Student 201027128
Hello, Dr. parent I want to know about assignment 8 question number 4 condition of problem is 1m square vertical plate Can I use the following ???
and then plate has two surface so I have to find 2*(convection heat transfer)???
Yes you can use that correlation as long as the Rayleigh number falls in the appropriate range restriction. Also, you should compute the heat loss on both sides of the plate by multiplying by 2. I'll give you 0.5 point bonus boost for this question.
Question by Student 201027128
Professor one more question about free convection first, It is about irregular solids free convection table box is written "The charaterisitic length L corresponds to the distance a fluid particle travels in a boundary layer":
what does mean?? you solve about the irregular solids problem in class the solid has each length 0.02m, 0.05m and 0.04m and you find L =(0.4+0.25)*0.25+(0.4+0.1)*0.75=0.54 How can I do that????
Hm, well, the characteristic length $L_c$ is the average distance a fluid particule would travel while touching the surface of the body.. How to determine $L_c$ depends on the situation.. I explained this in class.
Question by Student 201027128
professor, I want to know about the final exam 2013 question number 6. condition is Friction Force 0.144N and you give a hint friction factor f is equal to (-dP/dx)D/[$\rho$u$_b$^2*0.5] D(Diameter) is given and also u$_b$ is found by mass flow rate. I think (-dP/dx) is getted by Friction force -dP/dx have to positive and dimenssional of -dP/dx is N/m^3 so I use Friction Force/ Volume of pipe -dP/dx$= $61N/m^3 , u$_b$$=$0.2m/s friction factor is 0.0305 almost same 0.03 this pipe is smooth e/D$=$0 We find Reynolds Number if we use moody chart but laminar flow and turblent flow each different value when laminar flow viscous is 0.001kg/ms(answer of question) but turblent flow viscous is 0.0002kg/ms But we don't know this flow is laminar flow or turblent flow but Nusselt Number is different for each flow We want also Prandtl Number so we find C$_p$ and k many value is unknowm so I think it needs to iteration but when I play iteration, i have to iterration each flow laminar and turblent ??? How can I deside sort of flow??
Please typeset the mathematics correctly using , and put proper punctuation.. Then I will answer your question..
Question by Student 201427150
Hello Professor :) I have 2 questions about your previous lecture contents. First, at first time of Heat Transfer lecture, you said there are 2 types of Heat Transfer. These are Conduction and Radiation. However, we studied about 'Convection' today. Does 'Conduction' includes 'Convection' on broad sense?? Also, I have one more question about our handwriting. This is also at first lecture, 'mean free path'. You said 'Z' is the number of collisions by one particle during time 'delta t', and 'N' is the number of particles per unit volume. What I wonder is that why Z equals to N? Actually I don't understand it well... I want to know that what relationship exists between Z and N. Thank you very much!!
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