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Old   May 15, 2007, 07:34
Default Natural convection
  #1
Jozef
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Hi, i need to program natural convection in 2D, for example hot plate high 10cm and colder water paralel to it. I know plate and water temperature and all coefficient(Prandtl, Grashof, density, ...) I can calculate heat transfer from plate to water, but i dont know how i should calculate next distance from plate and temperature them. How i can use heat transfer q=h.A.deltaT(h-coefficient, A-area, deltaT- difference between temperatures plate and water) for define it? Or how i can calculate temperature in water in next areas? Thanks a lot, Jozef
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Old   May 15, 2007, 10:48
Default Re: Natural convection
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OPS
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Delta T = T_hot - T_fluid.

Here T_hot is the temperature of the plate. T_hot is also the temperature of the fluid in contact with the plate. T_fluid is known from the calculation. It is the temperature of the ambient fluid. Note that when T_hot = T_fluid, no heat transfer by convection occur.

OPS
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Old   May 15, 2007, 11:27
Default Re: Natural convection
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Jozef
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Thanks OPS, but I wrote my question probably wrong or bad to understand. I know temperature T_hot and T_fluid too. But now, I need to know temperature T1, T2 ... I think that by natural convection temperature is position as i draw. And here is picture of my problem http://www.ukazto.com/img/otazka_forum.jpg or http://img411.imageshack.us/img411/6...kaforumfx3.png Thanks, Jozef
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Old   May 15, 2007, 11:55
Default Re: Natural convection
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OPS
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You need not know T1, T2 etc. By definition,

Q = k(T_hot - T_infinity), (1) and this is the maximum heat than can be transfered by convection. T_infinity is the far field fluid temperature.

Let me put this way.

Heat is conserved. So,

Q = k(T_hot - T1) = k(T_hot - T2) = k(T_hot - T1)

= k(T_hot - T_infinity)

If above equation is satisfied, then T1 = T2 = T_infinity.

OPS
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Old   May 15, 2007, 12:58
Default Re: Natural convection
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Jozef
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Thanks for your time, but still one question. I understand what you wrote, but..

1, I define Prandtl, Grashof number =>heat transfer koeficient for convection k. now Q=k(T_hot-T_fluid). This is between fluid on interface and "next" fluid. But i need to know temperature between this two points.

T1 and T2 cant be the same i think. Yeah, after some time T1=T2, but not immediately. And i need this state. Some idea how calculate temperature with some realistic measurements(eg. 0.01m-T=50C,0.015m-45C)? I need draw it to graph. Thanks, Jozef
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Old   May 15, 2007, 23:14
Default Re: Natural convection
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OPS
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Q=k(T_hot-T_fluid). This is between fluid on interface and "next" fluid.

The above formula is not for the "next fluid". Let me explain:

At time t = 0, Q1=HA(T_hot-T_fluid). This is the maximum amount of heat that will be transfered. Now for any subsequent time t>0, if one take T_fluid as T1 such that T1>T_fluid ( and this is what your are telling) then

Q2 = HA(T_hot - T1).

In this case Q1 > Q2. What happen to the excess heat (Q1-Q2)? So by taking T1, you are not conserving the heat.

And regarding knowing temperature between two points, then simulation will give you the answer. If you are doing simulation, then at each point in the domain, you can get T.

OPS

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Old   May 16, 2007, 08:47
Default Re: Natural convection
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Jozef
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So if i understand:

1, I use formula Q1=H1.L.(T_hot-T_fluid) where L is distance between points with temperature T_hot, T_fluid

2, I calculate Q2=H2.L.(T_hot-T1)

3, As you wrote, deltaQ=Q1-Q2

4, deltaQ=H3.Lx.(T1-T_fluid) where Lx is distance between points with temperature T1, T_fluid which i am looking for

5, Q1= Q2 + deltaQ = H2.L.(T_hot-T1)+H3.Lx.(T1-T_fluid)

H1, H2, H3 are diferent( function of temperature). The heat transfer Q1 is equal, but on different distance. Is it right? Thanks Jozef
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Old   May 16, 2007, 12:13
Default Re: Natural convection
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OPS
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Yes Mr. Jozef, you got it. If fact I had thought of writing the same thing what you have written. I appreciate your quick understanding.

OPS
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Old   May 16, 2007, 12:18
Default Re: Natural convection
  #9
Jozef
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Thanks so much much OPS
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Old   May 16, 2007, 23:56
Default Re: Natural convection
  #10
OPS
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You are most welcome Jozef !!!

OPS
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