Week 4 - CHT Analysis on Exhaust port
Introduction:
Conjugate heat transfer analysis is based on a mathematically structured problem, which describes
the heat transfer between a body and a fluid flowing over or inside it as a result of interaction between two objects.
At the matching interface, the details are provided for temperature distribution and heat flux along the interface
eliminating the need of calculating the heat transfer coefficient. Moreover, the heat transfer coefficient can be
calculated later.
One of the simplest ways to realize conjugation is through numerical methods. The boundary condition for the fluid
and solid interface is set and solved through iteration methods. There are no right guesses for the values of the initial
boundary condition for the convergence except through the hit and trial method.
Application:
The conjugate heat transfer methods have become a more powerful tool for modeling and
investigating nature phenomena and engineering systems in different areas ranging from aerospace and nuclear
reactors to thermal goods treatment and food processing from the complex procedures in medicines to ocean
thermal interaction in metrology.
CHT in recent years has significantly improved the cooling performance of electronic equipment such as the design
of heat sinks and the design of heat exchangers for the waste treatment plant. One such application of CHT is the
exhaust port system.
Solving and modeling approach :-
The exhaust port was modeled into the Spaceclaim.
The exhaust port is a three-dimensional model containing four inlets and one outlet. Despite having isothermal
boundary conditions and a uniform flow rate at the four inlets, the model cannot be cut into two halves to save
computational time because of its asymmetrical nature. Therefore the whole model is discretized and is
considered for meshing.
CHT analysis requires both the solid and the liquid volume under consideration, Spaceclaim gives an option of
volume extract for selecting edges for fluid volume.
Both the solid volume and liquid volume are copied into one component.
Share topology is created.
Spaceclaim gives an option for share coincident topology, when share coincident topology is selected and
executed, the mesh from the fluid volume and the solid volume are said to be conformal with each other which
is a requirement for CHT analysis.
The exhaust port is loaded into the meshing window and then simulated for baseline mesh and the refined
mesh.
The named selections are created (inlet, outlet, outer wall convection)
The necessary contour images and graphs are studied to understand the behavior of temperature, velocity,
and surface heat transfer coefficient at the interface.
Pre-processing and solver setting
Baseline mesh
The geometry is loaded into Spaceclaim
Fluid volume extraction:-
It is process to extarct the fluid from the solid volume.
Share Topology
It is the process of sharing the information from fluid volume to solid volume. It plays the cruicial role.
Meshing:-
Default element size is used = 150 mm
No of elements :- 136431
Name selection:-
Inlet 
Outlet 
Outer wall convection 
Mesh quality
Here we can see that the major elements have higher quality in the range of 0.7 - 1.
Setup :-
In this section, we set up the simulation. Kε standars model is applied for base simulation. Keep the energy on. And then we apply cell zone condition for simulation. we apply different boundary conditions like inlet , outlet boundary conditions, outer wall convection boundary condition. After this initialize the process. Create temperature contour and create the animation for temperature for geometry. Certain No of iterations are set for simulation.
Inlet velocity = 5 m/s at 700K temperature, heat transfer coefficient for outer wall = 20 W/(m^2.K), No of iteration = 300
Solution :-
No of iteration = 300
Residuals
Surface heat transfer coefficient := 21.18 W/(m^2.K)
Animation of temperature
Solution :-
Temperature along the plane located at the exhaust section of the port
Velocity along the plane located at the exhaust section of the port
Temperature on outer wall convection
Velocity streamlines
Refined case :-
Default element size is used = 150 mm
Take name selections as same as base simulation.
Add inflation layer in inner wall of outer solid body.
Apply total thickness about 5 mm and no of layers are 10.
Add sizing in outer body about 10 mm.
Mesh matric
No of elements :- 578675
Setup :-
In this section, we set up the simulation. Kε standars model is applied for base simulation. Keep the energy on. And then we apply cell zone condition for simulation. we apply different boundary conditions like inlet , outlet boundary conditions, outer wall convection boundary condition. After this initialize the process. Create temperature contour and create the animation for temperature for geometry. Certain No of iterations are set for simulation.
Inlet velocity = 5 m/s at 700K temperature, heat transfer coefficient for outer wall = 20 W/(m^2.K), No of iteration = 300
Solution :-
No of iteration = 300
Residuals
Surface heat transfer coefficient := 21.68 W/(m^2.K)
Solution :-
Temperature along the plane located at the exhaust section of the port
Velocity along the plane located at the exhaust section of the port
Temperature on outer wall convection
Velocity streamlines
Analytical solution :-
The Reynolds number for the given flow is 58189.8960 which suggests that the flow is turbulent in nature, the Dittus
Boelter heat transfer coefficient relation can be used to determine the Nusselt number which is described
mathematically as follows
Nu = 0.023.Re^(0.8).Pr^(0.4)
where Pr=μ.Cp/k
where μ is the dynamic viscosity of the fluid in= 1.7894e-5 kg/(m.s)
k is the thermal conductivity of the material =0.0242 W/(m.K)
Cp is the specific heat capacity of the fluid at constant pressure= 1006.43 J/(kg.K)
Therefore Pr = μ.Cp/k = 0.744
n is constant 0.3 heat transfer takes place from fluid to solid.
Therefore Nu = 136.4827
Therefore convective heat transfer coefficient h = Nu.k/D = 19.43 W/(m^2.K)
Comparison for the cases:-
| Analytical values | Values from simulation | % Deviation | |
| Base simulation | 19.43 | 21.18 | 8.2 % |
| Refined case | 19.43 | 21.68 | 10.3 % |
Conclusion :-
The CHT analysis on the exhaust port was simulated successfully on ANSYS-FLUENT.
The pressure-based, coupled scheme solver is used to discretize the whole geometry.
The analytical value of the heat transfer coefficient is 19.43 W/(m^2.K)
The deviation of numerical heat transfer coefficient values from the analytical solution was found to be within 5
% to 10 % for the base line simulation and refined case.
The presence of the boundary layer is quite visible near the inlet of the pipe where the viscous force makes the velocity near the wall to be zero causing a no-slip condition to occur., but as the flow is turbulent in nature (Reynolds number 58189.8960) the velocity profile goes from flat to the flutter and the boundary layer vanishes at the outlet.
The velocity at the outlet is higher than the velocity at the inlet can be attributed to the presence of four inlets
and one outlet. It satisfies the continuity geverning equation.
The heat transfer coefficient values predicted by CFD were validated against the numerical solution depicted the deviation in the range of 5% to 10% which is in compliance with industrial standards.
The values predicted by the baseline mesh and the refined mesh were close to each other this can be attributed to the fact that the boundary keeps on growing and declining from the inlet to the outlet owing to change in velocity, thus the value is predominant at the outlet section of the exhaust port.