Conjugate Heat Transfer

Conjugate Heat Transfer:

Setteing up the case setup for the conjugate heat transfer problem in a pipe(made of aluminium) with fluid flow inside and a added heat flux into the domain through soild. The case is setup to predict the heat transfer in the fluid and soild regions respectively.

What is Supercycling:

The case can be ran for a stedy-state simulation if we are interested in only the end result but for a detailed results we use transient mode for simulating the flow. The heat transfer in fluid and solids occur at a different rate and we basically solve different equations for both the region. Hence when using a transient mode for simulating we use a specific type of averaging for updating the temperature in the solid region for this we use supercycling by using this converge runs the transient simulation for the fluid region and then the fluid solver is made to freeze preodically till the soild region reaches a steady state. This averaging of the solid region while running a transient for fluid region is know as supercycling.

Following is the geometry for the CHT simuation:

Turbulence Model used - K-Epsilon, Super-cyclying option is enabled

For Grid Dependency test 0.002, 0.003, 0.004 m base grid is used.

A transient simulation is ran for 0.5 seconds.

Change in temperature in the solid region at a particular point is plotted w.r.t time. the co-ordinate of the point os given below

-0.003421(x-axis), 0.012023(yaxis), 0.18(z-axis)

Boundary Conditions:

For inlet we use Neumenn boundary condition for pressure and define velocity for outlet we define static pressure.

In conjugate heat transfer simulation we have an interface boundary i.e. there is a boundary that is shared with both solid and fluid region in this case it is the inner wall of the pipe as shown in the following figure.

So, we provide an interface boundary condition for this and the interface boundary requires us to provide boundary condition for both forward and reverse boundary.

First we figure out which region is for forward and which is for reverse for doing so we look at the normals of the triangles of the interface boundary. The direction in which th normals are pointiong is defined a forward direction and in this the normas are pionting towards inside and inside is the fluid region hence the forward boundary of the interface is in fluid region and the reverse boundary of th interface is in solid region.

The reason for splitting the interface boundary into two parts is that when converge runs it is going to make a copy of the interface boundary and one boundary is going to belong to fluid region and other to the soild region hence the splitting of interface boundary.

Grid Dependency Test Results:

The case was ran for three different grid sizes 0.004, 0.003, 0.002

For grid size - 0.004:

Base grid = 0.004mm

Temperature mgnitude:

The above image is at the last time step.

Velocity Magnitude:

The above image is at the last time step.

Effect of super-cycling averaging:

Temperature animation:

Base grid = 0.003mm

Temperature magnitude:

The above image is at the last time step.

Velocity magnitude:

The above image is at the last time step.

Effect of supercycling averaging:

Temperature animation:

Base grid = 0.002mm

Temperature magnitude:

The above image is at last time step

Velocity magnitude:

The above image is at last time step

Effect of supercycling averaging:

Temperature animation:

Comparision of temperature for different grid sizes:

The outlet temperature converges to a particular value but is dependent on the grid size as it can be seen from the above plot, the temperature converges to a different value for different grid sizes.

Effect of supercycle stage interval:

For the following comparision the case was ran on 0.004mm grig size.

The effect that the supercycling stage interval has on temperature is that by decreasing the stage interval the temperature converges to a steady state sooner with comparision to higher value of stage interval, as it can be seen from the above plot. But the final temperature reached by using different stage interval were same hence it did not had any effect on the final temperature.

The effect on the simulation time for the different stage interval(on 0.004mm grid):

For 0.03 stage interval : 148.76 seconds

For 0.02 stage interval : 144.95 seconds

For 0.01 stage interval : 142.60 seconds

The decrease in the stage interval results in decrease in the simulation time.