Structural Analysis with imported thermal loads simulated in Simcenter FLOEFD

After performing a CFD analysis with considerable temperatures and heat transfer, one may want to study the impact these loads have on the structural integrity of the piece. FLOEFD has a very streamlined process to do this, where the resulting temperatures from the CFD analysis can be imported as thermal loads for the structural analysis. In this case, an exhaust manifold will be studied to exemplify this process.

An exhaust manifold collects the exhaust gases from a motor’s cylinders after ignition through multiple pipes which converge into one, which leads the ignition gases outside of the car.

 

1. Setting up the CFD simulation

The geometry is loaded, and the project is created with help of the Wizard with the following properties:

The boundary conditions are set at the inlets and outlets:

• Inlets: Inlet Volume Flow, Q=0.117 m^3/s, T=823.15 K.
• Outlet: Environment Pressure.
• Radiative Surfaces: “Steel, cast, smooth, oxidized 1” on the whole body.
• Solid materials: Insulator on inlet and outlet lids.

The appropriate goals are set up:

• Surface goals:
  • Inlet: Average Static Pressure Inlet, Average Temperature (Fluid), Volume Flow Rate.
  • Outlet: Average Temperature (Fluid), Volume Flow Rate, Absolute Total Enthalpy Rate.
• Volume Goals: Average Temperature (Solid) Body, Maximum Temperature (Solid) Body.
• Equation Goal:
  • dP (Pressure drop): Average Static Pressure Inlet surface goal – Environment Pressure boundary condition.
  • dT (Temperature drop): Inlet Volume Flow boundary condition’s temperature – Average Temperature (Fluid) @Outlet.

 

2. Mesh Convergence Test

Now, to ensure the results are accurate, a mesh convergence test is performed (to know more about how the steps to set up an appropriate mesh convergence test, refer to the Mesh Convergence Study for flow in channels article found in the Related Articles section). The refinement is indicated the following way: [Fluid Cells refinement, Partial Cells refinement, Solid cells refinement]:

Looking at the percentual change of every point compared to the previous one it can be seen that the results have converged at the 2,15,3 design point as the following point results do not change much when compared to this one’s, so this is the mesh chosen for the simulation.

3. CFD Results visualization

A Cut Plot showing the chosen mesh is now shown along with an image of the Global Mesh settings previously mentioned.

In this case it wasn't deemed necessary, but the user also has access to creating local meshes if they wish to refine the mesh in a certain zone.

Now, the goals can be analyzed and the results visualized.

• The pressure drop is considerable, as expected, as the gas comes from the combustion process, and it is released into the atmosphere.
• The same cannot be said for the temperature drop as the gas travels fast and thus it does not have much time to exchange heat, and it can only expand until it exits the manifold. As a result, both the average and maximum temperatures of the gas are quite high.
• The situation with the average and maximum temperatures of the body is similar. Most of the heat exchange happens through radiation and convection, but that is not enough to significantly reduce the temperature as the body is in constant contact with hot gas.

In the case of the fluid’s temperature, it remains basically constant on the inlet pipes, and it decreases when the flows converge and reach the outlet, this means that this is where the heat flux is the highest. The velocity field presents a similar behavior, it remains constant in the inlet pipes, and it increases at the outlet where the mass flow equals the sum of the inlet pipes’ mass flow, as the flow is incompressible.

As for the body temperature, it’s at its lowest at the inlets as this is where the fluid’s velocity is the lowest and the body has enough time to cool through convection and radiation, however this doesn’t happen close to the outlet as that’s where the fluid’s velocity is the highest.

• Flow’s Temperature

 

 

 

 

• Flow’s velocity

• Body Temperature

 

4. Structural case set up

A new project is set up by copying the original one, making the following changes to the settings:

 

 

The projects created so far can be visualized on the FLOEFD Projects Tree panel on the left:

 

Next, the constraints are placed:

• Constraints: Fixed at inlets common surface & flange-like structure close to the outlet.

And now, in order to set the thermal loads, Temperature is chosen as a load.

In the menu the whole body is selected, and then the dependency button is clicked.

In the Parameters section the Transferred option is chosen

 In the Dependency menu, in Dependency type the Transferred option is chosen, and  FLOEFD Project is selected.

Finally, the results of the project obtained from the CFD mesh convergence test is selected in the Results file.

Now that the boundary conditions are set the following volume goals are placed on the body:

• Volume (solid).

 

• Average Von Mises Stress.

 

• Maximum Von Mises Stress.

 

• Average Displacement.

 

• Maximum Displacement.

 

5. Structural mesh convergence test

In a similar way to the CFD mesh convergence test, a structural one is performed to ensure an appropriate mesh is chosen, in this case the element size was gradually changed and the goals compared. The results are the following:

Looking at the percentual changes it can be observed that the results have converged at the 1.7 mm element size mesh, as the following size doesn’t change its results much compared to the previous one’s.

6. Structural Analysis Results visualization

The chosen mesh is now shown, it's worth noting that in structural cases, the user also has the option to create local meshes if they wish to refine a certain zone:

After the simulation is completed, the option to show the mesh in its deformed state (scaled to a chosen factor) is also available:

The average and maximum displacements are not great but could be thought as considerable depending on the design objectives, however it must be noted the fluid temperatures are on high side of the working operation spectrum.

On the other hand, the average and maximum stress are quite considerable even if they are in concentrated areas even if the working condition are on the higher side of the spectrum.

As it could be expected, most of the VM stress is located close to the constraints and where the body’s temperature is the highest, meaning where the thermal loads are the greatest. The displacement however, as it could also be expected, is greater the farther away it is from the constraints, meaning in the direction with the least opposition.

• Displacement

• Von Mises Stress