Convective Heat Transfer Questions & Answers


B. Parent • AE61280 Convective Heat Transfer

Convective Heat Transfer Questions & Answers

Question by Student 201783216

I have a question about when I revised the last week's lecture. When the mean free path is considered, concept of 'cylinder of influence' is appeared. I clearly understand that one particle can affect to other particles. But I can't understand why criterion of one particle's influence is cylinder. Intuitively, I think that criterion of influence should be sphere because the particle travels in 3-D space. Which point I misunderstand about it?

03.18.17

The cylinder corresponds to the volume that the particule goes through during a certain amount of time $\Delta t$. I.e., the radius of the cylinder is the radius of influence of the particule and the length of the cylinder is the particule speed times $\Delta t$.

Question by Student 201793103

Dear Sir, I have a question about Question #3, Assignment #1. Sorry for that I have to give my question as attachment. I cannot figure out the equation code here. Please find the detail in attachment, which is in MS office WORD(docx.).

Question_201793103.docx

03.21.17

I only answer questions written within the post, not within the attachments.. Or, you can come see me in my office.

Question by Student 201793103

Dear Sir I am very sorry that I cannot make it for midterm exam on 26th, though I've voted for 26th. I have forgotten that I had business trips from 13th to 23th and from 26th to 29th. Could I have midterm after 29th? Sorry again.

04.11.17

The exam is already officially scheduled on the department homepage so the date can not be changed anymore. In your case, because you can not attend the midterm exam, the final exam will be worth more points.

Question by Student 201799110

Dear Professor Parent. I want to check my think about driving 2-dimensional energy equation. In 2-Dimensional, I think, stress from environment acts on the 4 surface of fluid element.(for example, x direction) $$ \frac{d \tau_{xx}}{dx} \frac{-d \tau_{xx}}{dx} \frac{d \tau_{yx}}{dx} \frac{-d \tau_{yx}}{dx} $$ And work is consist of stress and relative velocity between environment and fluid element. Regarding velocity, I think that $$ \frac{du'_{yx}}{dx} $$ is equal to $$ \frac{du_{x}}{dx} $$ Is it correct? quotation mark means relative velocity.

04.25.17

Hm, I don't understand fully the question. The stresses acting on the fluid element are $\tau_{xx}$, $\tau_{yx}$, etc and not $d\tau_{xx}/dx$, etc. Also, what is $u_{yx}^\prime$?

Question by Student 201693239

In the case of high speed flow in a flat plate, I wonder whether the total heat produced inside boundary layer is transfered to both direction(to the plate and to the outside of boundary layer) or to just one direction(to the plate). Considering the temperature profile you professor drew in the last class, I guess the total heat is transfered to both direction. Then professor, how to interpret the equation $q_{w}^{"}=h(T_{w}-T_{aw})$? Does it mean heat(or heat flux) transfrering to both direction?

05.16.17

You're correct: not all of the heat generated by V.D. goes to the wall. Some of it goes to heat the free stream (making the thermal layer grow), but this is a small percentage because the thermal layer goes slowly. Nonetheless, keep in mind the equation $q_{\rm w}^"=h(T_{\rm w}-T_{\rm aw})$ is an approximation, not an exact solution.

Question by Student 201883332

Dear professor. I am hard to understand the meaning of "starting at x=x0" : "Restrictions" of the 7th "Flow regime", "Summary of Equations for Flow over Flat Plates" on page 4 of "Convective Heat Transfer Tables". If we are talking about the starting point of the integral when we get the average Nusselt number, we can not acquire integral value at that point. I have one more question. When I asked about the relationship between thermal layer thickness and velocity layer thickness, professor answered that velocity layer thickness is larger than thermal layer thickness. After that, I saw "Unheated starting length" in the book. Does 'velocity layer thickness is larger than thermal layer thickness' means that velocity layer thickness is equal to thermal layer thickness plus unheated starting length? Oh, I think x0, "starting at x=x0" in the first question, is where heat transfer starts.

05.03.18

Yes $x=x_0$ is the location where the thermal layer starts while $x=0$ is where the momentum layer starts. In the derivation we did in class, I mentioned to you that $\delta_t<\delta$ at $x=0$ not that $\delta_t<\delta$ at any $x$. This is because the thermal layer is assumed to start just a bit downstream of the momentum layer. Whether $\delta_t$ is smaller or larger than $\delta$ for a large $x$ depends on the Prandtl number.

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