CHT analysis on Exhaust Port

CONJUGATE HEAT TRANSFER ANALYSIS OF EXHAUST PORT

OBJECTIVE:

To perform a CHT analysis on an exhaust port using ansys fluent and study various parameters that effect the rate of heat transfer in the exhaust port.

CONJUGATE HEAT TRANSFER:

The term conjugate heat transfer can be used to describe the processes which invlove a temperature variation within the solids and fluids itself due to the thermal interaction between the solid and the fluid.During CHT the heat transfer can take place from solid media to the fluid media and viceversa.A common example to the conjugate heat transfer can be the cooling or heating of a solid object by the flow of air over them.

CHT analysis is benficial in engineering applications because it shows the heat excahnging capabilities between the two surfaces in the engineering device and various optimization can be done to improve the thermal characteristics of the devices.Thus,this can be used to improve the heat transfer rate wherever required.It alos helps us to iddentify the key parameters that enhance the fluid flow and heat transfer within the system.

Conjugate heat transfer corresponds with the combination of heat transfer in solid and fluid.In solids the conduction often dominates,wherereas in fluids,convection dominates.Efficiently combining heat transfer in fluids and solids is the key to designing effective coolers,heaters or heat exchangers.The forced convection is the most common way to achieve high heat transfer rate.In someother applications,the performance are further improved by combining convction with phase change.Heat transfer in solids and fluids are combined in majority of applications.This is because fluids flow around solids or between solidwalls,and also the solids are mostly immersed in a fluid.Some of the applications where the CHT can be employed are given below.

• CHT can be performed to improve the cooling performance of water jackets and increase the engine life.
• Advancements n cooling of applications such as gas turbine components can be achieved after understanding the compplex heat transfer mechanisnm and interactions between the media
• CHT analysis can be done for bikes,passenger vehicles etc..

CHT Analysis has been done in ansys fluent for an exhaust port and the case has been simulated for two different mesh settings.The procedure for the numerical simulation are as follows.

1.GEOMETRY:

• The exhaust port of an inline 4 cylnder engine has been used for the simulation.

2.MESHING:

The meshing for the simulation has been done in two different cases ie, one with a base line mesh and the other with a more refined mesh.The mesh profile of the exhaust port for the tow cases has been shown below.

CASE-A : BASELINE MESH

• Element Size: 100 mm
• Type of mesh element : Tetrahedral
• Number of Elements : 51,905
• Number of nodes : 78,645

The mesh metric for the element quality has been given below:

CASE-B: REFINED MESH

• Element size : 100 mm
• Maximum Element size : 100 mm
• Type of Element :Tetrahedral
• Inflation Layer : - Number of Inflation layer : 5

- Inflation option: Total Thickness

- Maximum thickness : 5 mm

- Growth rate : 1.2

• Number of Nodes : 108805
• Number of Elements :334750

• The base line mesh has been refined by adding inflation layers to the fluid_volume faces and by reducing the size of elements in body sizing to 20 mm the edge sizing has been done to the inlet and outlet edge to 50 number of divisions.

3.SETUP:

• The case has been simulated with two different cases as mentioned earlier with a base line mesh and the other with a refined mesh and within each case the turbulence model k-epsilon and k-omega SST has been employed for each cases.

ie, CASE-A: A.1 k-epsilon turbulence model

A.2 k-omega SST

CASE-B : B.1 k-epsilon

B.2 k-omega SST

rest of all the setup for both the cases remains the same,and are discussed below:

• Pressure-based solver
• Steady-state simulation

Model : k-epsilon realizable and k-omega SST turbulence model for the aforth mentioned cases.

Material : Fluid - Air

Solid - Aluminium

Boundary Conditions: - inlets : velocity inlet ( 5m/s)

Temperature 700 K

- Outlet : Pressure-outlet ( zero gauge pressure )

- Outer-wall : Convection Boundary condtion with convective heat transfer coefficient `25 W/(m^2K)`

Residual with convergence condition unchecked

• SIMPLE Solver.

4.SOLUTION & RESULTS:

4.1 CASE-A: BASELINE MESH

A.1 k-epsilon Model:

1. Residual Plot:

• The iterations has been run and the flow appears to be converged at about near to 1000 iterations as the residual showed a constant pattern.

2. Temperature Contour:

• The temperature distribution in the exhaust manifold has been shown above

3. Wall heat transfer coefficient contour Plot:

• The wall h.t.c near to the wall of the outlet found to be higher because the more rapid heat transfer tends to take place near to that region as the flow of hot gases converge near to the outlet pipe from all the inlet pipes.

4.Streamline Plot for Velocity:

5.Temperature contour across a plane at the outlet bend:

6.Velocity contour across the plane at the outlet bend:

• The velcity of the hot gases is to be found highest at the cornor.

7.wall h.t.c across the plane:

A.2 k-omega SST Model:

1.Residual Plot:

• While using the k-omega SST model unlike the residual plot obtained from the k-epsilon model,the residuals of velocities and continuity show much divergence eventhough the flow appears to be converged.

2.Temperature contour:

3.Wall h.t.c contour Plot:

4.Streamline Plot for velocity:

5.Velocity Plot along a plane at the outlet:

6.Wall h.t.c contour plot along the plane:

CASE-B: REFINED MESH:

B.1 k-epsilon Model:

1.Residual Plot:

• The flow converged for about 180 iterations.

2.Temperature contour Plot:

3.Velocity Contour Plot:

4.Temperature Plot along the plane at the outlet:

5.Wall h.t.c contour Plot:

B.2 k-omega SST Model:

1.Residual Plot:

2.Temperature Contour Plot:

3.Velcity Plots:

4.Wall h.t.c contour Plot:

Based on the simulations that has been done the parameters obtained from these are tabulated below:

For CASE A:

wall h.t.c maximum velocity Temperature at outlet

k-epsilon `106 W/(m^2K)` 37.37 m/s 678.979 K

k-omega SST `91.6 W/(m^2K)` 37.9 m/s 678.516 K

For CASE B:

k-epsilon `212.7 W/(m^2K)` 38.37 m/s 675.469 K

k-omega SST `225.4 W/(m^2K)` 39.83 m/s 675.372 K

From the above table, it can be summarised that as the mesh is becoming finer and due to the interpolation of more number of elements,the simualtion becomes more practical and the it can be noted that the as the velocity is increased which further rises the Reynolds number and as a result the heat transfer coefficient is increased which leads to more heat transfer rate along the walls of the exhaust manifold.This increase in heat transfer rate can be further verified by checking at the outlet temperature of vehicle exhaust in all the four simulations.We are able to observe that as the h.t.c increased the temperature at the outlet is also reduced.

• The h.t.c obtained from the simulation can be mathematically validated with the help of the following eqautions:

`Nu=0.023*Re^0.8*Pr^0.4` & `Nu=(hD)/k`

The above two equations for the Nusselt Number can be equated and we will be able to obatin the h.t.c for the case.

Here, Re = Reynolds Number

Pr = Prandl Number

h = heat transfer coefficient

D = Hydraulic Diameter

k = Thermal conductivity

Factors effecting the simulations:

• Number of elements in the mesh
• Inflation layers
• Setting shared topology for conformal meshing

CONCLUSIONS:

• k-omega SST turbulence model is best fitted for this problem from the results that are obtained.

Thank You

Arjun Vs