CHT Analysis on Exhaust port

 

                                                          CHT SIMULATION & ANALYSIS OF FLUID FLOW THROUGH AN EXHAUST MANIFOLD

 

l. OBJECTIVE

1. Simulate & analyze the fluid flow through an exhaust manifold using Conjugate Heat Transfer(CHT) modeling.

2. Plot the velocity, temperature, and heat transfer coefficient contours at the outlet region.

 

ll. INTRODUCTION

1. Conjugate Heat Transfer (CHT)

Conjugate heat transfer is defined as the heat transfer between two domains by exchange of thermal energy. For a system the thermal energy available is defined by its temperature and the movement of thermal energy is defined by its heat flux through the outer walls. Heat transfer in solid happens through conduction and walls by convection and in liquid phase through convection.

CHT provides the temperature prediction and the hotspot regions at the solid-fluid interface and we can also predict the heat transfer accurately for example- Conduction through solids, convection through fluid, and thermal radiation. It also provides the velocity and pressure distribution of fluid moving inside the solid. We can also use CHT in design optimization for improvement for heat transfer and cooling capacity.

2. Advantages

1. The Conjugate Heat Transfer setup replaces the empirical relation of proportionality of heat flux to temperature difference with heat transfer coefficient. The wall heat transfer coefficients & their local variations are directly computed within the model and do not involve the solving of complex empirical relations. Thus the simulations are carried out much faster.

2. As long as the boundary conditions are defined, the solutions are highly accurate due to the simplicity of the CHT model.

3. There is no limit for the number of surfaces and hence can be applied for a large number of regions in a given assembly.

4. Since the CHT model is a combination of heat transfers in both solid & fluid models, it can be applied for all combinations of interactions(i.e Solid-Fluid interaction, Solid-Sold interaction, and Fluid-Fluid interaction).

3. Applications

The CHT method is a powerful tool for simulating & analyzing flows in a variety of different applications such as designing different vehicles, machines & also investigating natural phenomena such as thermal interaction in the atmosphere.

4. Exhaust Port 

In the exhaust port, the hot exhaust fumes come out of the engine head and it transfers the heat from hot fumes to a solid wall. Thus it transfers the heat from the solid surface to the ambient air. The process of heat transfer is known as Conjugate Heat Transfer.

lll. PROBLEM STATEMENT

1. The inlet flow velocity, V= 5 m/s.

2. The geometry of the exhaust manifold is given below-

 

 

lV. SPACECLAIM GEOMETRY

As we are interested in finding the surface heat transfer coefficient on the internal solid surface and the velocity distribution in a fluid, we need to generate two volumes namely fluid volume and solid volume from this model using volume extract toll in SpaceClaim. Using the volume extract tool, we get a computational fluid domain as shown below.

 

Once the fluid volume has been created the topology is set to share such that the data could be shared between the different volumes of fluid and solid body of the exhaust port.

 

 

V. INITIAL TEST - BASELINE SETUP

A.MESH

1.Boundaries

The boundaries of the geometry are generated using the named selection feature of Ansys-

1. Inlets - 4 Inlet Ports

2. Outlet - 1 Outlet Port

3. Outer Wall Convection - the outer body joining the inlets to the outlet

2.1 Baseline Mesh

A basic mesh is generated using the standard values recommended by Ansys. This mesh is used to obtain an initial solution which will help us to determine the location where mesh refinement is required.

1.Element Order: Linear

2.Element size: 150 mm

3.Number of Nodes: 27379

4.Number of Elements: 137268

 

2.2. Baseline Mesh At The Inlet

3. Element Quality

 The quality of some elements is low. However, since the lowest quality of the mesh is above 10%, the generated mesh is accepted.

 

B. SIMULATION SETUP

1.Solver: Steady

2.Type: Pressure Based

3.Turbulence Model: k-epsilon(Standard)

4.Materials:

• Solid- Aluminium
• Fluid- Air

5.Boundaries:

• Inlet:

              Type: Velocity Inlet

              Velocity:5 m/s

              Temperature:700 K

• Outlet:

               Type: Pressure Outlet

               Pressure: 0 Pa(Gauge Pressure)

               Temperature: 300 K

• Outer-Wall-Convection

                 Type: Wall

                 Thermal Conditions: Convection

                 Heat Transfer Coefficient: 20 W/`m^2K`

                 Free Stream Temperature: 300 k

6. Solution

• Method: Hybrid
• Number of Iterations: 350      

7. Simulation Output

7.1. Residuals

 

From the above figure, we can observe that the residuals repeat themselves periodically after a given number of iterations. Hence, the solution can be assumed to have converged.

 

7.2. Temperature Contour

The temperature contour is plotted using a banded color map, to have a proper understanding of the temperature distribution across the exhaust manifold.

We can see that the temperature across the body of the inlets is low as compared to the outlet, where the temperature increases due to the combination of hot fluids.

 

C. POST PROCESSING

1.Temperature Contour

 

2. Velocity Streamlines

The velocity streamlines accurately display the velocity distribution across the exhaust manifold.

There is an increase in the velocity at the point of intersection of the four inlets to meet the requirements of the mass conservation across the model.

3. Velocity Contour Along with The Outlet

Due to the conservation of mass, there is an increase in velocity at the outlet's ports.

From the velocity contour along with the outlets, we can observe that the maximum velocity is in the inner region of the pipe, where the direction of flow changes.

4.1. Wall Heat Transfer Coefficient Contour Along with The Outlet

 

 

Vl.  REFINED SETUP:

CASE 1: Yplus = 0.1

A. MESHING

1.Boundaries:

Apart from the boundaries generated in the baseline mesh. an additional boundary called inflation-layer is added to the inner body joining the inlets to the outlet. This boundary shall be used to generate a body-fitted mesh across the model.

The boundaries of the geometry are generated using the named selection feature of Ansys

1. Inlets: 4 Inlet Ports

2. Outlet: 1 Outlet Port

3. Outer-Wall-Convection: The outer body joining the inlets to the outlet

4. Inflation Layers: The inner body joining the inlets to the outlet

2.1 Refined Mesh

The initial mesh is refined by decreasing the mesh size & creating a body-fitted mesh by adding an inflation layer to the boundary of the inner pipe.

1. Element Order: Linear

2. Element Size: 0.04 m

Body Sizing:

• Location: Fluid Volume
• Element Size: 0.02 m
• Curvature Capture: Yes

Edge Sizing:

• Location: Edges of Inlets and Outlet
• Type: Number of Divisions
• Number of Divisions: 40
• Curvature Capture: Yes

4. Inflation Layer

• Use Automatic Inflations:  All Faces chosen in name selection.
• Location: Inflation Layer
• Inflation Option: First Layer Thickness
• First Layer Height: 5.5602360536833E-06 m
• Number of Layers: 5
• Growth Rate: 1.2

5. Number of Nodes: 130444

6. Number of Elements: 389565

 

 Inflation Layer At The Inlet

We can from the figure that the inflation layers have been appropriately applied to the inner pipe.

 

3. Element Quality.

The element quality is very low for certain areas of the mesh. These areas correspond to the regions where the inflation layers are generated. If we want to increase the quality, then we have to increase the mesh size of the inflation layer or remove them.

However, since we want to observe the wall heat transfer coefficient at the boundaries, we require a small size inflation layer for its accurate representation. Hence, the low-quality inflation layers are accepted. However, a meshed check needs to be performed before setting up the simulation, to avoid problems in the simulations due to mesh quality.

 

B. SIMULATION SETUP

The simulation setup is the same as that selected for the initial mesh. Only the output of the simulation will change because of the change in the mesh size.

1.Solver: Steady

2.Type: Pressure Based

3.Turbulence Model: k-Omega(SST)

4.Materials:

• Solid- Aluminium
• Fluid- Air

5.Boundaries:

• Inlet:

              Type: Velocity Inlet

              Velocity:5 m/s

              Temperature:700 K

• Outlet:

               Type: Pressure Outlet

               Pressure: 0 Pa(Gauge Pressure)

               Temperature: 300 K

• Outer-Wall-Convection

                 Type: Wall

                 Thermal Conditions: Convection

                 Heat Transfer Coefficient: 20 W/`m^2K`

                 Free Stream Temperature: 300 k

6. Solution

• Method: Hybrid
• Number of Iterations: 500 

C. Simulation Outputs

1. Residuals

 

 

 2. Temperature Contour

 

D. POST PROCESSING

1. Temperature Contour

 

2. Velocity Streamlines

3. Velocity Contour Along with The Outlet

 

4. Wall Heat Transfer Coefficient Contour Along with The Outlet

 

CASE 2 : Yplus=0.5

 A. MESH

1.1 First layer Thickness: 2.78011802684165E-05 m

1.2 Number of Nodes: 130297

1.3 Number of Elements: 388619

 

 

B. Mesh Quality

 

C. Simulation Outputs

1. Residuals

 

D. Post Processing

1. Temperature Contour

 

 

2. Velocity Streamlines

 

3. Velocity Contour at the outlet

 

 

4. Wall Heat Transfer Coefficient Contour Along with The Outlet

 

CASE 3: Yplus=1

 A. MESH

1.1 First layer Thickness: 5.5602360536833E-05 m

1.2 Number of Nodes: 165277

1.3 Number of Elements: 456138

 

B. Mesh Quality

C. Simulation Output

1. Residuals

 

D. Post Processing

1. Temperature Contour

2. Velocity Streamline

3. Velocity Contour at the outlet

4. Wall Heat Transfer Coefficient Contour Along with The Outlet

 

CASE 4: Yplus=30

 A. MESH

1.1 First layer Thickness: 1.66807081610499E-03 m

1.2 Number of Nodes: 122008

1.3 Number of Elements: 339902

 

B. Mesh Quality

C. Simulation Output

1. Residual

 

D. Post Processing

1. Temperature Contour

2. Velocity Streamline

3. Velocity Contour at the outlet

4. Wall Heat Transfer Coefficient Contour Along with The Outlet 

 

 

Vlll.Results:

1. Refining the mesh by decreasing the mesh size increases the accuracy of the solution.
2. Generating a body-fitted mesh helps to capture the boundary functions such as the wall heat transfer coefficient accurately.
3. It can be observed that Yplus values of 30, using a k-epsilon standard wall function model, resulted in a very coarse inflation layer and did not capture the boundary effects properly.
4. On the other hand, Plus values of 0.1, 0.5, and 1 (with k-omega model) generated the best results since the low Yplus values allowed more refinement and accurate capturing of boundary effects. 
5. The velocity increases from the region of the junction of the four inlets until the outlet, to ensure mass conservation.
6. The wall heat transfer coefficient is higher at the pipe bend near the outlet due to the increase in Reynolds Number.
7. There is more heat transfer at the bend near the outlet due to the increase in the wall heat transfer coefficient.
8. The temperature on the other hand did not experience a drastic change in the refinement of the mesh
9. This is mainly because the temperature contours were plotted on the outer convection wall which was far away from the inflation layer.

10. Y Plus values	HT coefficient (W/m^2k)
    0.1	9457.37
    0.5	1871.6
    1	978.96
    30	184.524

 

Xl. Verification

1. Verification of the simulation results can be analytically by calculating the Nusselt Number.
2. Nu=0.023*(Re^0.8)) Pr^n-the Reynolds number is known and the Prandtl number can be calculated by taking the ratio of momentum diffusivity and thermal diffusivity.
3. Nu=h.D/k
4. h= wall transfer coefficient can be calculated accordingly for verification

 

X.CONCLUSION

The flow simulation through the exhaust manifold is carried out using Conjugate Heat Transfer Methods. The solution is obtained very quickly & the physics expected from the flow is accurately displayed in the solution contours.