CHT analysis on a Exhaust Port using ANSYS FLUENT

OBJECTIVE:

1. Application of CHT Analysis

2. To perform a CHT analysis on a model of an exhaust manifold and compare the solutions for 2 sizes of meshes. The first one is the baseline mesh and the second one is a finer mesh on the solid zone and also with inflation layers.

3. How do we 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 Convection Heat Transfer Model describes the heat transfer between a body and a fluid flowing over or inside it as a result of the interaction of two objects.

The physical processes and solutions of the governing equations are considered separately for each object in two different regions. Matching conditions for these solutions at the interface provide the distributions of temperature and heat flux along the body–flow interface, eliminating the need for a heat transfer coefficient.

CHT analysis helps us in thermal management. By coupling the differential form of energy equation with the equations of fluid motion, we can use a CFD code to calculate properties associated with Heat transfer. For instance, Temperature distribution or rate of heat transfer from a solid surface to a liquid or vice versa.

Some Specific applications are:

Cross Flow Heat Temperature, Integrated Circuit Chips, HVAC, Turbocharger, Internal Combustion Engine, Energy Saving applications etc.

CHT Analysis

Geometry Model

In this project, a CHT analysis is been done on a model of an exhaust manifold with 2 different mesh sizes (Case1 and Case2)

These two regions were combined using Share Topology feature.

Here, Steady Pressure-Based solver was used. Energy equations were used and for the viscous turbulence, the k-epsilon model was used.

In Materials, For fluid it is air and for the solid region, it is aluminium.

Boundary Conditions:

Inlet, Velocity=5 m/s and the temperature=700 k.

For the outer wall, convection was used as the thermal condition with Heat Transfer Coefficient=20 w/m^2.k

CASE 1

In this case, an Exhaust Port baseline setup was done with 138491 cells.

Mesh

Residual Plots

The solution gets converged with enough number of iterations provided.

Temperature Contour

The maximum temperature on the solid region is approx. 606k at the outlet throat. Max temp. is 701k and the least temp is 464k.

Streamline Velocity

The Maximum velocity Obtained is 37.11 m/s

Velocity Contour at Outlet throat

Wall Heat Transfer Coefficient

Max. wall heat transfer coefficient produced is 102.5 W/m^2-k

CASE 2

In this case, a finer mesh was been used on the solid region along with the inflation layers. The total mesh cells were 500883.

Mesh

We can see the mesh is finer, and the inflation layers at the boundary interface of 4 inlets.

Residual Plots

The solution gets converged with enough number of iterations provided.

Temperature Contour

The maximum temperature on the solid region is approx. 622k at the outlet throat. Max temp. is 700k and the least temp is 504k.

Animation for Temperature Contour

Streamline Velocity

The Maximum velocity Obtained is 39.78 m/s

Velocity Contour at Outlet throat

Wall Heat Transfer Coefficient

Max. wall heat transfer coefficient produced is 210.7 W/m^2-k

Observations and Conclusions:

• In temperature contour, even our inlet temperature of the air is 700k (for fluid), it doesn't get the transfer to the outer solid body as the temp. remains the lowest and as we go towards the outlet, we see a gradual increase in temperature. As we increased the mesh size, the solid region maximum and minimum temperature also increase.
• For Streamline velocity plot, the velocity at the inlet is zero and as we move towards the outlet we see a gradual increase in velocity and maximum at the curvature of the throat. This happens because of total mass conservation, as there are 4 inlets and a single outlet.
• Here, velocity is directly proportional to temperature, and hence the maximum temperature is also produced at the curvature of the throat on the plane where the maximum heat gets transferred from the fluid region to Solid zone. Therefore, the highest Wall heat transfer Coefficient is obtained at that location.
• For Wall Heat Temperature Coefficient, it raises from102.5 W/m^2-k to 210.7 W/m^2-k as the mesh number of elements increase and meshes size decrease.
• As the no. of elements increases the Velocity, Temperature and Wall Heat Transfer Coefficient increases.

Verifications of Result:

The results can be verified by doing experiments physically i.e. directly comparing the model results with physical measurements.

or it can also be verified by performing the grid dependency test.

In Convection, using Buckingham's Pi Theorem, we obtain the dimensionless number like Nusselts Number, Prandtl Number and Reynold's Number. Well, Reynold's Number deal with fluid flow properties Nusselt's Number deal with the heat transfer convection in the form of Heat transfer coefficient.

Nusselt Number Nu=hL/k = q(convection)/q(conduction)

Factors which affects the simulation:

1. If it is the Conjugate Heat Transfer problem, then Share Topology quality affects the smooth transfer of heat with 2 different regions.

2. Inflation Layer which helps to develop body fitted mesh to get efficient and complete geometry.

3. The number of elements used to mesh the geometry, the finer the mesh the more accurate the results.