Exhaust Port Challenge

OBJECTIVE: To Carryout Conjugate Heat Transfer Analysis on the Exhaust Port of an I.C Engine.

INTRODUCTION: The analysis type Conjugate heat transfer (CHT) allows for the simulation of heat transfer between Solid and Fluid domains by exchanging thermal energy at the interfaces between them.The term conjugate heat transfer (CHT) is used to describe processes which involve variations of temperature within solids and fluids, due to thermal interaction between the solids and fluids. The exchange of thermal energy between the two physical bodies is called study of Heat Transfer, the rate of transferred heat is directly proportional to the temperature difference between the bodies. A typical example is the heating or cooling of a solid object by the flow of air in which it is immersed and some other example includes conduction through solids, free and forced convection in the gases/fluids and thermal radiation.

Conjugate heat transfer corresponds with the combination of heat transfer in solids and heat transfer in fluids. In solids, conduction often dominates whereas in fluids, convection usually dominates. Efficiently combining heat transfer in fluids and solids is the key to designing effective coolers, heaters, or heat exchangers. Forced convection is the most common way to achieve high heat transfer rate.Heat transfer in solids and heat transfer in fluids are combined in the majority of applications. This is because fluids flow around solids or between solid walls, and because solids are usually immersed in a fluid.

CHT is used in various places such as:

• In an Engine
• In a boiler
• In a microprocessor
• In an Inverter

GEOMETRY CLEAN-UP:

• For the Geometry clean-up, open spaceclaim and import the model.
• From the given model the fluid volume is to be extracted and this can be done using volume extract option available in space claim.

• The rest will be the solid volume which is the outer part.

• Then the model is given the respectives names for easier underastanding.

• Since the geometry clean-up is done, now we can proceed with meshing of the model.

PROCEDURE: 

1.Baseline mesh: In this baseline case we go with the default setting of the mesh i.e. mesh size of 150mm.

Now launch fluent and set the following parameters:

• Velocity = 5m/s
• Heat Transfer Coefficient = 20W/m2K
• Turbulence model = k epsilon
• Temperature = 700K
• Steady State and Pressure based solver
• Initialize the solution using hybrid initialization and set the no.of iterations to 150 and run the simulation.

Residual Plot:

Here we see that though the set iterations are 150,but here we see that the solution has stopped before 130 it means that at this point the solution has converged. 

Temperature distribution:

Temperature distribution on plane:

Velocity ditribution on plane:

Heat transfer coefficient on the inside wall:

HTC on the inside wall (zoomed view):

2. Refined mesh: In the refined case, the solid part is selected and body sizing is done with element size as 11mm.

• Inflation layers are added to the inner body which is the fluid volume with total thickness, 5 layers and maximum thickness of 5mm. With adding inflation and refining the mesh, the no.of elements are 444607.

• Now Fluent is launched and the neccessary parameters are set and calculations are done for 150 iterations.

Residual plot:

Temperature contour:

Temperature distribution on Plane:

Velocity distribution on Plane:

There is a slight increase in velocity in the refined case to that of the baseline mesh. The whole intention of refining the mesh is to get accurate results.

Heat transfer coefficient on the inside wall:

HTC on the inside wall (zoomed view):

The Heat Transfer Coefficient (HTC) value in the refined case is around 224 W/m2k and in the baseline case 103 W/m2k and hence refining the mesh has helped in getting good results. The HTC is highest in the bend region of the outlet pipe. This is also due to mass flow rate,velocity and temperature being higher in that region.

Streamline distribution:

In the above streamline distribution we see that the temperature is highest in the outlet and that too near the bend. This is because the mass flow rate at the outlet in greater than the mass flow rate of the inlets which increases the velocity and there by increasing the temperature.

Y plus value:

y+ is a non-dimensional distance. It is often used to describe how coarse or fine a mesh is for a particular flow pattern. It is important in turbulence modeling to determine the proper size of the cells near domain walls. The turbulence model wall laws have restrictions on the y+ value at the wall.For instance, the standard K-epsilon model requires a wall y+ value between 30 - 300.

From the above picture we see that the Y plus value is in the range of 30-300. Some are failing but since maximum of them are in accordance with mentioned range hence can be considered.

Factors on which accuracy of prediction depends:

1. Correct modelling of the problem.

2. Proper Geometry clean-up.

3. Mesh size should be accurate to yield correct results.

4. Setting-up the right parameters and materials.

5. Grid test to be done to confirm the results.