# Heat equation

1. Sep 25, 2004

### revenant

im looking for an exact analytical solution for the following

(1/alpha) * dT/dt = d2T/dr2 + (1/r)*dT/dr

where d is actually dou(partial diff.)

subject to the boundary conditions,

a<=r<=infinity
t>=0

T=Ts (constant) at t=0;

-dT/dr + HT = H(Tf) at r=a; where H, Tf are constants

2. Sep 25, 2004

### Tide

I recommend starting with a Laplace transform in time.

3. Sep 25, 2004

### Dr Transport

Try separation of variables, the answer will be apparent.

4. Sep 25, 2004

### Feynman

you can use the harmi\onic solution

5. Sep 26, 2004

### revenant

i tried seperation of vriables, but the solution is a complex one and since i apply to a real physical process, some results are not meaningful... as for laplace transform in time, cud u help me with that.

i want to know if integral methods, similarity solutions can be applied

6. Sep 26, 2004

### HallsofIvy

Why in the world would you want to use such advanced methods for such a simple problem? I don't know why you say "the solution is a complex one". There are no complex numbers in the problem and certainly none will arise by separating variables.

If you take T(r,t)= U(r)V(t) the differential equation becomes
(1/&alpha;)UV''= V(U''+ (1/r)U')
V''/V= &alpha(U''+ (1/r)U')/U. Since the left side depends only on t and the right only on r, to be equal for all r and t they must both be constant:

V''/V= k so V''= kV and &alpha;(U''+ (1/r)U')/U= k so U''+ (1/r)U'= &alpha;kU. Those equations have real solutions.

7. Sep 26, 2004

### Dr Transport

Given HalsofIvys' post, look carefully at the equations he has suggested. The only other suggestion I can make is, instead of the separation constant k, try k^2 (from years of solving these problems, you'll find that it will be easier to work with later). If you can not find the solution come back for another hint, we'll help. He has the answer, just apply the boundary conditions.

8. Sep 27, 2004

### revenant

first of all, thanx

second of all by complex i meant, it was complex method (tedious)

i already tried seperation of variables ages ago and got a solution but i cant use that in the physical process im applying it to because of certain limits to the solution.

im looking at more of an integral methods, similarity approach or any other solution for that matter other than seperation of variables

9. Sep 28, 2004

### Dr Transport

What limitations, the solution should be exact......

10. Oct 15, 2004

### quantitative

Bit of a guess but a different way...

use Feyman-Kac to represent PDE as an SDE.
Then integrate SDE to have a distribution result.

11. Oct 15, 2004

### ReyChiquito

From my experience, i would do the transform in space.

Plus, the solution of your equation is unique (or modulo constant), so you might want to express the fourier series you get in the form of an inegral. You can also transform the known heat solution (in its integral form) for your domain using conformal mappings.

Last edited: Oct 15, 2004
12. Oct 15, 2004

### Feynman

ok we can use the harmonic spheric

13. Oct 15, 2004

### Dr Transport

Why use a spherical harmonic, the original equation was in terms of cylindrical coordinates, the solution is in terms of a Bessel function..........separation of variables is the exact way to solve this problem......

14. Oct 16, 2004

### Feynman

Why Dr?
(you are a doctor on what?)

15. Oct 16, 2004

### Dr Transport

Look at the form of the original equation posted above, cylindrical coordinates, hence a solution is a Bessel function. There is no angular dependence indicated, only radial and time. If I have time tonight, I'll work out the solution and post it, if not, keep trying.

Yes, a dr, PhD Solid State Theory specializing in electronic transport properties of anisotropic semiconducting crystal systems and their optical properties.

16. Nov 13, 2004

### Feynman

But we can use Fourier series Dr?

17. Nov 13, 2004

### Dr Transport

The equations variables are time and radius. If you separate variables, you should get a term which is an exponential for the time solution and the radiual part is a Bessel function of zeroth order.......if my memory serves me correctly. The boundary conditions may impose a series solution. I'll have to take another look and see.

18. Nov 13, 2004

### lalbatros

I think this could inspire you:

You may need "Gradshteyn and Ryshik" to check if it is ok.

19. Feb 5, 2010

### 05me39

i solved this equation using the "separation of variable method". now i have a book "Basic Heat and Mass Transfer" by A. F. Mills. he states, "it might at first appear that the separation of variables solution method can be used once again. As in the slab analysis, the variables are separable in the differential equation,(the same one as mentioned in this thread). However, a necessary requirement for completing the solution is that the boundary conditions of the eigenvalue problem be specified on coordinate surface, and x = infinity (r = infinity in this case) is not a coordinate surface of the Cartesian coordinate system"

then he uses the similarity variable method. now the problem is that the final solution that i have got and the one given in many books is different. I wonder if some one knows this method well and can help me.

20. Feb 8, 2010

### juice34

can i ask what class this is for possibly Transport 2?