Conjugate heat transfer analysis on an exhaust manifold

Objective:-

1. To give a brief description of Conjugate heat transfer analysis & its application.
2. To Perform a CHT analysis on a model of an exhaust manifold. And comparing the solutions for 2 sizes of meshes. The first one will be the baseline mesh and the second one will be a finer mesh on the solid zone and also with inflation layers.
3. And showing the velocity, temperature and heat transfer coefficient plots and explaining if refining the mesh produced any changes to the results
4. To verify if the HTC predictions from the simulations are right? On what factors does the accuracy of the prediction depend on?.

Conjugate Heat Transfer:-

When ever their is a heat transfer from solid volume to fluid volume or vice-versa by exchaning their thermal properties at their interface in order to simulate this case conjugate heat transfer analysis is used.Conjugate heat transfer analysis basically describe the processes which involve variations of temperature within solids and fluids, due to thermal interaction between the solids and fluids.

Applications:-

It helps us to simulate & understand thermal systems like heat sinks,cooler,exhaust port of vehicle,HVAC,ICE etc.

CASE 1:Baseline Mesh

Step 1.Geometry

• Below image is basically the image of exhaust manifold which has 4 inlets & 1 outlet.

• In order to define fluid volume we need to extract volume by using edge extract option.
• Since,in this case we are doing CHT analysis we have to combine solid volume & fluid volume together.
• Also in order to do conformal meshing which basically avoid overlapping mesh we need to on shared topology
• Shared topology basically shared each face of fluid volume with solid volume so,that we get accurate results.This we can see in the below image

Step 2.Meshing

• Before do meshing we need to give name selection(inlet,outlet etc) this we can see in the below image.

• Below image shows baselinemesh of 150mm element size which did on exhaust manifold.

• In the below image we see conformal mesh at interface between solid volume & fluid volume.
• This conformal mesh happen because we have on shared topology in geometry stage.

• Around 149957 elements & 30360 nodes is generate for element size 150mm

Step 3.Setup

In order to setup Conjugate heat transfer analysis following steps is followed

• Type of analysis:Steady-state
• Viscous model:K-epsilon because of turbulent flow
• Material:Aluminium for Solid volume & Air for Fluid volume

Now,we have to define Boundary condition

• Inlet:velocity=5m/s & Temperature=700k
• Outlet:0 gauge pressure
• Outer wall:Convection is taking place therefore we have to define heat transfer coefficient=20 Wm^-2k^-1

After initializing the solution,in the postprocessing menu we can create contour plot to monitor how temperature variation is taking place in the solid & fluid volume,also we can create solution animation of this contour plot to observe it.

Plots:

Residue

• We can see in the above image the solution is run for 350 iteration.

• From above image its quit clear that velocity is maximum at curved surface in the outlet pipe.

• From above image its quit clear that temperature is maximum at curved surface in the outlet pipe.

• From above image its quit clear that heat transfer coefficient is maximum at curved surface in the outlet pipe because of high velocity in that region.

CASE 2:Refined Mesh

Step 1.Geometry

• Below image is basically the image of exhaust manifold which has 4 inlets & 1 outlet.

• In order to define fluid volume we need to extract volume by using edge extract option.
• Since,in this case we are doing CHT analysis we have to combine solid volume & fluid volume together.
• Also in order to do conformal meshing which basically avoid overlapping mesh we need to on shared topology
• Shared topology basically shared each face of fluid volume with solid volume so,that we get accurate results.This we can see in the below image

Step 2.Meshing

• Before do meshing we need to give name selection(inlet,outlet etc) this we can see in the below image.

In this case in order to refine mesh we have refine the mesh below are the mesh detail which we have use

• Element size for solid volume=100mm
• 5 inflation layer is added to fluid volume by using total thickness method=10mm
• Total Elements=227024
• Total Nodes=83074

Below image shows refined mesh of exhaust manifold

• In the below image we see conformal mesh at interface between solid volume & fluid volume.
• This conformal mesh happen because we have on shared topology in geometry stage.
• Also,we can see inflation layer at the interface between solid volume & fluid volume.

Step 3.Setup

In order to setup Conjugate heat transfer analysis following steps is followed

• Type of analysis:Steady-state
• Viscous model:K-epsilon because of turbulent flow
• Material:Aluminium for Solid volume & Air for Fluid volume

Now,we have to define Boundary condition

• Inlet:velocity=5m/s & Temperature=700k
• Outlet:0 gauge pressure
• Outer wall:Convection is taking place therefore we have to define heat transfer coefficient=20 Wm^-2k^-1

After initializing the solution,in the postprocessing menu we can create contour plot to monitor how temperature variation is taking place in the solid & fluid volume,also we can create solution animation of this contour plot to observe it.

Plots:

Residue

• We can see in the above image the solution is run for 350 iteration.

• From above image its quit clear that velocity is maximum at curved surface in the outlet pipe.

• From above image its quit clear that temperature is maximum at curved surface in the outlet pipe.

• From above image its quit clear that heat transfer coefficient is maximum at curved surface in the outlet pipe because of high velocity in that region.

Conclusion

As we refined the mesh frome case 1 to case 2 clearly we can say that case 2 has more accurate results because in this case we have use sizing method for solid volume & also added inlfation layer to fluid volume.

Verification of Heat Transfer Coeficient Results

• Verification of Heat Transfer Coeficient Results can be done by performing grid dependence test.We can run the simulation until it become independent to grid size.
• Also we can verify the Heat Transfer Coeficient Results by calculating Theortical Nusselt number and then equating with numerical result.

Factors affecting the accuracy of predication results

• Quality criteria
• Type of mesh generated
• Total number of element generated & Element size
• Use of inflation layer
• Type of connection between solid volume & fluid volume.