Heat Transfer between 3 plates of different materials

[Saver2008]

Hi!

I have a problem and it is the following:
There are 3 plates of 20cm x 20 cm x 10cm (for refractory) , 5cm (for insulation), and 1.9cm (for Shell)...as per attached image. In one face, the hotter face, there is a temperature (ti) of 1000°C. Taking into account the following thicknesses of each material...I want to know the final temperature of the coldest face considering that in the coldest face has a initial temperature of 25°C and the air is flowing through that coldest face.
The thermal conductivity of each material is as follows:
(refractory = 0.52 W/m*K, insulation = 0.46 W/m*K, and Shell = 52 W/m*K)
I used SOLIDWORKS SIMULATION to solve this problem. When I give the THERMAL LOADS to each face, I begin with the known temperature and it is the hotter face (1000°C = ti) but for the coldest face I give a CONVECTION ISSUE as thermal load but SOLIDWORKS asks me for two parameters:
-Convection Coefficient and Bulk Ambient Temperature.
I consider that air is flowing through coldest face so I put 25°C in BULK AMBIENT TEMPERATURE but for convection coefficient...I had problems with that part.
I found on engineering toolbox web site of a empirical formula to know the convection coefficient or convective heat transfer coefficient of air when air velocity is from 2 to 20 m/s and it is the following:
- hc = 10.45 - v + 10 v ^1/2. (V = m/s speed)
I used 20 considering standard value and the value was 35.1 W/m2*K.
The software solved the solution and tf (final temperature) was like 90-100°C. I want to know if what I did was correct or what steps I did wrong.....hope you can help me how can I be sure that I am in the correct path


Thankyou

Roland

 

[IRstuff]

20 m/s wind is pretty fast -- 45 mph

In any case, the surface temperature seems high; I get more like 36ºC

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[Saver2008]

@ IRstuff,

But...how did you do it?

Roland

 

[IRstuff]

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[Saver2008]

Hi,

@ IRstuff

It is not supposed that R by conduction = (e1/k1 + e2/k2 + e3/k3)1/A = 1/R = A / (e1/k1 + e2/k2 + e3/k3) and it is more like = 0.13 W/K so adding the R by convection it would be like 0.11 W/K. So the Q of the system ´d be 116 W and the temperatura of the cold face its more like 108°C So...there is something that I dont get...and also how do I choose a correct convective heat coefficient for air taking into account air speed

Hope you can help me

Roland

 

[IRstuff]

Sorry, my bad, I flipped the units. 116.5 W and 108.2 C are correct.

The convection coefficient is dependent on air speed and geometry. There are tables and graphs on the web:

http://www.engineeringtoolbox.com/convective-heat-transfer-d_430.html

so 35 W/m^2-K was pretty much the value for 20 m/s, but that, I think, is specifically for a flat plate.

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[dvd]

Why not make it more realistic and put a convection coefficient and radiation on the hot side? Is the inside of the (whatever it is) at a uniform 1000 C, or is the actual inside wall surface temperature 1000 C with hotter interior? How is it getting heated, by a burner or combustion of some sort? Is not the temperature of the flame hotter than that? One can solve this problem with FEA, or whatever program, based on good or bad assumptions. Is this a real gizmo already, or are you just getting started with the analyses of a new process?

 

[Saver2008]

Hi!

@dvd

The main idea is to check SOLIDWORKS SIMULATION (THERMAL ANALYSIS) is a good software to solve these kind of problems taking into account you feed the input data correctly. I made an analysis using SOLIDWORKS FLOW SIMULATION considering HEAT CONDUCTION IN SOLID. I made an external analysis where air speed is 20 m/s and adding over the hot face a CS plate in order to represent the temperature of 1000°C. (Volume source and a temperatura of 1000°C) This was to indicate to the program that the hot face of the initial 3 plates is 1000°C. Realizing this I checked that the temperatura in the cold face is between 23 to 26°C. The convection coefficient is the matter actually.
@ IRstuff
I found on engineering toolbox (the website you already wrote) web site of a empirical formula to know the convection coefficient or convective heat transfer coefficient of air when air velocity is from 2 to 20 m/s and it is the following:
- hc = 10.45 - v + 10 v ^1/2. (V = m/s speed)
I used 20 considering standard value and the value was 35.1 W/m2*K.
The table is made using above formula but it says is empirical. I think there should be another good way to find out the correct value of HEAT TRANSFER COEFFICIENT OF AIR CONSIDERING SPEED.
Roland

 

[IRstuff]

That particular equation is presumably empirical, but there are other accepted correlations:

http://www.cheresources.com/content...orrelations-for-convective-heat-transfer?pg=3

but there are other correlations that have been developed:

http://www.pathways.cu.edu.eg/ec/text-pdf/part b-9.pdf

For the length of your plate, the equation from the first citation results in 40W/m^2-K

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[IRstuff]

Please refrain from double posting

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