CHT analysis on an Exhaust manifold

Objectives :-

• To perform a CHT analysis on a model of an exhaust manifold.
• Perform a CHT analysis on exaust manifold with 2 different sizes of mesh and compare the results.
• Show the velocity, temperature and heat transfer coefficient plots and explain if refining the mesh produced any changes to the results.
• Describe why and where a CHT analysis is used?
• How would you verify if the HTC predictions from the simulations are right? On what factors does the accuracy of the prediction depend on?

Why and where a CHT analysis is used?

Conjugate heat transfer (CHT) analysis allows the simulation of the heat transfer between Solid and Fluid domains by exchanging thermal energy at the interfaces between them. It requires a multiregion mesh to have a clear definition of the interfaces in the computational domain. Such a mesh can be created with the Hex-dominant parametric operation in the mesh creator.

The CHT approach has an advantage over FE thermal analyses in that wall heat transfer coefficients and their local variations on surfaces are directly calculated within the model rather than based on simplified empirical calculations. The CHT approach therefore has benefits for those applications where heat transfer is either non-uniform or difficult to calculate empirically.

Typical applications of this analysis type is the simulation of heat exchangers, cooling of electronic equipment, and general-purpose cooling and heating systems.

Setup for the CHT analysis in ANSYS

Geometry :-

We perform a CHT analysis on a model of an exhaust manifold.

For this exaust manifold model we have 4 inlets and one outlet.

Case setup :-

1) Volume extract

For the CFD analysis we created fluid domain by the tool Volume Extract inside ANSYS Spaceclaim and rest of the solid body is considered for the CHT.

2) Meshing

Before start meshing process make sure that share topology is ON so the mesh created between solid and fluid region share its edge between them.

Case I : Base mesh

For this case we go through the default mesh generated by ANSYS

• Element size = default (150 mm)

Total cell

• Nodes = 27371
• Elements = 137172

Fig : Meshing on exhaust manifold model both for solid and fluid region

Fig : Mesh quality metrics

Case II : Refined mesh

For this case we increase the quality of mesh so we capture accurate heat transfer between fluid and solid region.

For refining the mesh to the solid body we use face sizing method

• Element size = 10 mm

Inflation layer provided at the interface between solid and fluid

• Method = All Faces in Chosen Named Selection
• inflation option = Total Thickness
• Number of layer = 5
• Growth rate = 1.2
• Maximum thikness = 5 mm

Total cell

• Nodes = 155538
• Elements = 509965

Fig : Meshing on exhaust manifold model both for solid and fluid region

Fig : Inflation Layer at the interface between solid and fluid

Fig : Improved Mesh quality metrics

Simulation Setup :

Setting up the physics for the CHT simulation in ANSYS fluent.

• Steady state simulation
• Pressure based solver
• Absolute velocity formulation
• Operating Pressure = 101325 Pascals
• Calculate the Energy equation
• For Turbulance model follow the standard K-epsilon method

Material :-

Fluid Flow inside the domain :

• We have various options to choose material from which are described in detail under Fluent database. But for this simulation we chosen Air as fluid inside the Exhaust manifold

Solid material Outside :

• For the Solid material also we have various options to choose from which are described in detail under Fluent database. But for this simulation we chosen Aluminium as Solid part of the Exhaust manifold.

Boundary condition :-

Inlet :

• Type - Velocity
• velocity - 5 m/s
• Temperature - 700 K

Outlet :

• Type - Pressure
• Pressure - Gauge pressure (0 Pa)

Outer wall :

• Type - Wall
• Thermal conditions - Convection
• Heat transfer coefficient - 20 W/m^2.K

Solution and Result for different mesh sizes :

Case I : Base Mesh

Fig : Residual plots

Fig : Temperature Contour

Fig : Temperature profile at the outlet of the exhaust port

Fig : Velocity at the outlet side

Fig : Velocity of fluid shown by streamline

Fig : Wall Heat Transfer coefficient

Case II : Refined Mesh

Fig : Residual plots

Fig : Temperature Contour

Fig : Temperature profile at the outlet of the exhaust port

Fig : Velocity at the outlet side

Fig : Velocity of fluid shown by streamline

Fig : Wall Heat Transfer coefficient

Fig : Wall Heat Transfer coefficient at the interface

From the above figure it shown that wall heat transfer coefficient is only capture inside the 1st mesh that are close to that wall. So by refining the mesh at the interface we capture it in more easy way and the result is more acurate.

Lets compare the wall heat transfer coefficient between base mesh and refiened mesh. For the base mesh maximum HTC is 112.7 but for the refiened mesh the maximum HTC is 213.5.

How would you verify if the HTC predictions from the simulations are right? On what factors does the accuracy of the prediction depend on?

For fully developed (hydrodynamically and thermally) turbulent flow in a smooth circular tube, the local Nusselt number may be obtained from the well-known Dittus-Boelter equation.

Dittus-Boelter equation

The Dittus-Boelter equation (for turbulent flow) is an explicit function for calculating the Nusselt number. It is easy to solve but is less accurate when there is a large temperature difference across the fluid. It is tailored to smooth tubes, so use for rough tubes (most commercial applications) is cautioned.

`Nu _D=0.023*Re _D^(4/5)*Pr ^{n}`

where,

• Nu = Nusselt number
• D is the inside diameter of the exhaust port
• Pr is the prandtl number
• n = 0.4 for the fluid being heated
• n = 0.3 for the fluid being cooled.

So Reynolds number and Prandtl number can be calculated by the inlet velocity and fluid properties and put the value to the above equation to calculate Nusselt number.

After calculation of the Nusselt number we use this equation to calculate Heat transfer coefficient and compare our simulation result with this.

`Nu = (h*L)/k`

where,

• h = Heat transfer coefficient
• L = Characteristic length
• k = Thermal conductivity

Accuracy of predition depends on the element quality of the mesh and the structure of the model.

Conclusion :-

• By the above temperature contour it shows that the maximum temperature on the outer wall is at the outlet region of the model because we have 4 inlet and only one outlet so the velocity at the outlet is more. Temperature is proportional to the average kinetic energy of the molecules of the gas. So when temperature increases fluids velocity increases and vice-versa.
• While comparing temperature contour between the base mesh and refined mesh we observed that the temperture will uniformaly distributed in refined.
• The veloicty is greater in the refiened mesh compare to base mesh because in refined mesh we provide a inflation layer so it capture more accurate velocity at the sharp edge.
• The Wall heat transfer coefficient is also greater in the refiened mesh compare to base mesh because the same that we provide a inflation layer in refined mesh so it capture more accurate HTC near the wall.
• So, by the above all comparision we conclude that if we create mesh properly around the model then we definitely got much accurate result.