Solve a 3-D parabolic PDE problem by reducing the problem to 2-D using coordinate transformation. This example uses the PDE Modeler app. For the command-line solution, see Heat Distribution in Circular Cylindrical Rod.

Consider a cylindrical radioactive rod. Heat is continuously added at the left end of the rod, while the right end is kept at a constant temperature. At the outer boundary, heat is exchanged with the surroundings by transfer. At the same time, heat is uniformly produced in the whole rod due to radioactive processes. Assuming that the initial temperature is zero leads to the following equation:

$$\rho C\frac{\partial u}{\partial t}-\nabla \text{\hspace{0.17em}}\xb7\text{\hspace{0.17em}}\left(k\nabla u\right)=q$$

Here, *ρ*, *C*, and *k* are
the density, thermal capacity, and thermal conductivity of the material, *u* is
the temperature, and *q* is the heat generated in
the rod.

Since the problem is axisymmetric, it is convenient to write this equation in a cylindrical coordinate system.

$$\rho C\frac{\partial u}{\partial t}-\frac{1}{r}\frac{\partial}{\partial r}\left(kr\frac{\partial u}{\partial r}\right)-\frac{1}{{r}^{2}}\frac{\partial}{\partial \theta}\left(k\frac{\partial u}{\partial \theta}\right)-\frac{\partial}{\partial z}\left(k\frac{\partial u}{\partial z}\right)=q$$

Here *r*, *θ*,
and *z* are the three coordinate variables
of the cylindrical system. Because the problem is axisymmetric, $$\partial u/\partial \theta =0$$.

This is a cylindrical problem, and Partial Differential Equation Toolbox™ requires
equations to be in Cartesian coordinates. To transform the equation
to the Cartesian coordinates, first multiply both sides of the equation
by *r*:

$$\rho rC\frac{\partial u}{\partial t}-\frac{\partial}{\partial r}\left(kr\frac{\partial u}{\partial r}\right)-\frac{\partial}{\partial z}\left(kr\frac{\partial u}{\partial z}\right)=qr$$

Then define *r* as *y* and *z* as *x*:

$$\rho yC\frac{\partial u}{\partial t}-\nabla \text{\hspace{0.17em}}\xb7\text{\hspace{0.17em}}\left(ky\nabla u\right)=qy$$

For this example, use these parameters:

Density,

*ρ*= 7800 kg/m^{3}Thermal capacity,

*C*= 500 W·s/kg·ºCThermal conductivity,

*k*= 40 W/mºCRadioactive heat source,

*q*= 20000 W/m^{3}Temperature at the right end,

*T_right*= 100 ºCHeat flux at the left end,

*HF_left*= 5000 W/m^{2}Surrounding temperature at the outer boundary,

*T_outer*= 100 ºCHeat transfer coefficient,

*h_outer*= 50 W/m^{2}·ºC

To solve this problem in the PDE Modeler app, follow these steps:

Model the rod as a rectangle with corners in (-1.5,0), (1.5,0), (1.5,0.2), and (-1.5,0.2). Here, the

*x*-axis represents the*z*direction, and the*y*-axis represents the*r*direction.pderect([-1.5,1.5,0,0.2])

Specify the boundary conditions. To do this, double-click the boundaries to open the

**Boundary Condition**dialog box. The PDE Modeler app requires boundary conditions in a particular form. Thus, Neumann boundary conditions must be in the form $$\overrightarrow{n}\text{\hspace{0.17em}}\xb7\text{\hspace{0.17em}}\left(c\nabla u\right)+qu=g$$, and Dirichlet boundary conditions must be in the form*h**u*=*r*. Also, because both sides of the equation are multiplied by*r = y*, multiply coefficients for the boundary conditions by*y*.For the left end, use the Neumann condition $$\overrightarrow{n}\text{\hspace{0.17em}}\xb7\text{\hspace{0.17em}}\left(k\nabla u\right)=HF\_left=5000$$. Specify

`g = 5000*y`

and`q = 0`

.For the right end, use the Dirichlet condition

*u*=*T_right*= 100. Specify`h = 1`

and`r = 100`

.For the outer boundary, use the Neumann condition $$\overrightarrow{n}\text{\hspace{0.17em}}\xb7\text{\hspace{0.17em}}\left(k\nabla u\right)=h\_outer\left(T\_outer-u\right)=50\left(100-u\right)$$. Specify

`g = 50*y*100`

and`q = 50*y`

.The cylinder axis

*r*= 0 is not a boundary in the original problem, but in the 2-D treatment it has become one. Use the artificial Neumann boundary condition for the axis, $$\overrightarrow{n}\text{\hspace{0.17em}}\xb7\text{\hspace{0.17em}}\left(k\nabla u\right)=0$$. Specify`g = 0`

and`q = 0`

.

Specify the coefficients by selecting

**PDE**>**PDE Specification**or click the**PDE**button on the toolbar. Heat equation is a parabolic equation, so select the**Parabolic**type of PDE. Because both sides of the equation are multiplied by*r = y*, multiply the coefficients by*y*and enter the following values:`c = 40*y`

,`a = 0`

,`f = 20000*y`

, and`d = 7800*500*y`

.Initialize the mesh by selecting

**Mesh**>**Initialize Mesh**.Set the initial value to 0, the solution time to 20000 seconds, and compute the solution every 100 seconds. To do this, select

**Solve**>**Parameters**. In the**Solve Parameters**dialog box, set time to`0:100:20000`

, and*u*(*t*_{0}) to`0`

.Solve the equation by selecting

**Solve**>**Solve PDE**or clicking the**=**button on the toolbar.Plot the solution, using the color and contour plot. To do this, select

**Plot**>**Parameters**and choose the color and contour plots in the resulting dialog box.

You can explore the solution by varying the parameters of the model and plotting the results. For example, you can:

Show the solution when

*u*does not depend on time, that is, the steady state solution. To do this, open the PDE Specification dialog box, and change the PDE type to**Elliptic**. The resulting steady state solution is in close agreement with the transient solution at 20000 seconds.Show the steady state solution without cooling on the outer boundary: the heat transfer coefficient is zero. To do this, set the Neumann boundary condition at the outer boundary (the top side of the rectangle) to

`g = 0`

and`q = 0`

. The resulting plot shows that the temperature rises to more than 2500 on the left end of the rod.