Week 4 - CHT Analysis on Exhaust port
Aim: Study state CHT analysis on Exhaust port at an inlet velocity of 5M/sec.
Objective:
* Give the brief description of why and where a CHT analysis is used.
* Maintain the Y+ value according to the turbulence model and justify the results.
* Calculate the wall surface heat transfer coefficient on the internal solid surface and show the velocity and temperature contours in appropriate area.
* How would you verify if the heat transfer coefficient predictions from the simulation are right, on what factor does the accuracy of the prediction depends?
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 inside it as result of intersection between two objects. At the matching interface, the details are provided for temperature distribution and heat flux along the interface the need of calculating the heat transfer coefficient. Moreover, the heat transfer coefficient can be calculated later.
One of the simplest way to realize conjugation is through numerical method .The boundary conditions for the fluid and solid interface is set and solved through iteration method. There are no right guesses for the values of the initial boundary conditions for the convergence expect through the hit and trial method.
Application:
The Conjugate heat transfer method have become a more powerful tool for modelling and investigating nature phenomena and engineering system in different areas from aerospace and nuclear reactors to thermal goods treatment and food processing from the complex procedures in the medicine to ocean thermal interaction in meteorology.
CHT in recent years has significantly improved the cooling performance of electronic equipment such as 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 Meshing Procedure:
* The exhaust port was modeled into space claim
* The exhaust port is a 3D model consists of four inlet and one outlet, Despite having isothermal boundary conditions and a uniform flow rate at the four inlets , the model cannot be cut into two half to save computational time because of its a symmetrical in nature. Therefore the whole model is discretized and it is considered for meshing.
* CHT requires both solid and liquid volume under consideration, Space claim gives an option of volume extract for selecting edges of fluid volume.
* Both the solid volumes and liquid volumes are copied into one component.
* Share topology is created.
* Share gives an option for share coincident topology, where share coincident topology is selected and executed , The mesh from the fluid volume and solid volume are said to be conformal with each other which is the requirement of CHT analysis .
* The exhaust port is loaded into the mesh window and then simulated for base line mesh and the refined mesh.
* The named section are created (inlet, outlet, and 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 space claim.
Fluid volume Extraction:
It is the process of extracting the fluid volume from the solid volume.
Share topology:
It is the process of sharing information from fluid volume to solid volume .It plays important role in CHT analysis
Meshing:
It is the process of discretizing the geometry into a small number of volumes containing nodes. Since the geometry is complex and the mesh is unstructured therefore the finite volume is used as a discretization scheme.
In this case the default element size of an ANSYS -FLUENT is used (0.15m).
Mesh statistics:
Number of Elements;137416
Number of nodes:27374
Named selection:
Inlet boundary condition (velocity inlet) at 5 m/sec
Outlet (pressure outlet) Gauge pressure at 0 pa
Outer wall convection (Heat transfer coefficient 20w/m^2k free stream temperature at 700k)
The flanges around the inlet and the component encompassing the outlet are adiabatic (no heat transfer takes place through the walls).
Setting up of Physics and Boundary Conditions:
The geometry orientation in the setup window. The blue-colored lines are inlet and red -colored lines are outlet.
Set UP:
The different boundary conditions are mentioned below:
* Inlet boundary conditions
- Type: inlet velocity
- Velocity magnitude 5 m/sec.
- Temperature :700k
* Outlet boundary Conditions:
- Type: Pressure Outlet.
- Gauge Pressure:0 pa
The rest boundary conditions values are set to be default in ANSYS -FLUENT
- Energy Equation: Turned on
- Viscous Model: it is set to k-omega SST model
Material Properties:
Fluid:
* Material Name: Air
* Density: 1.225 kg/m^3
* Specific heat: 1006.43 J/kg k
* Thermal conductivity: 0.0242 W/mK
* Viscosity: 1.7894e-05 Kg/m-sec
Solid:
* Material Name: Aluminum
* Specific heat: 871 j/kg k
* Thermal conductivity: 202.4 W/mK
SIMPLE Method:
1.) Residual Plot
2.) Surface heat transfer coefficient
Surface heat transfer coefficient value: 21.972121 W/m^2K
SIMPLEC Method:
1.) Residual Plot
2.) Surface heat transfer coefficient
Surface heat transfer coefficient value: 21.972121 W/m^2K
COUPLED METHOD
1.) Residual Plot
2.) Surface heat transfer coefficient
Surface heat transfer coefficient: 21.995436 W/m^2K
Surface heat transfer coefficient values for three different types of pressure -based solvers
|
Element Size |
Number of Elements |
Number of Nodes |
HTC (SIMPLE) |
HTC (SIMPLEC) |
HTC (COUPLED) |
|
0.15m |
137416 |
27374 |
21.972121 W/m^2K |
21.972121 W/m^2K |
21.995436 W/m^2K |
The above table represents the value of the surface heat transfer coefficient value from the three different pressure -velocity schemes VIZ SAMPLE , SIMPLEC, COUPLED. The value of heat transfer coefficient is exactly SIMPLEC and COUPLED . Both SIMPLE and SIMPLEC are segregated schemes where pressure and velocity coupling are solved sequentially, takes less memory , but involves a lot of computation time, The COUPLED Scheme , on the other hand solves pressure and velocity equation instantaneously saves a lot of computational time, and requires more memory.
CHT analysis of exhaust port with finer mesh
First Refinement:
Element Size: 90 mm
Since the value of the heat transfer coefficient takes the value of the first cell near the wall, so it can be inferred that the Y+ value will be in the viscous sub-layer which is taken as 1.
Inflation
Number of inflation layers: 6
Growth rate: 1.2
Inflation Option: Inflation layer thickness
First layer thickness: 0.0000514162m
Captured curvature is set to yes
1.) Mesh Quality
2.) Residual Plot
3.) Surface heat transfer coefficient
Mesh statistics and surface heat transfer coefficient
Element size: 0.09m
Number of elements:169425
Number of nodes:38412
Y+:1
First cell height:0.0000514162m
Heat transfer coefficient:20.896W/m^2K
Second Refinement
Element size: 0.08m
Number of Inflation Layers:10
First layer thickness:0.0000514162m
Growth rate:1.2
1.) Mesh Quality
2.) Residual Plot
3.) Surface heat transfer coefficient
Mesh statistics and surface heat transfer coefficient
Element size: 0.08m
Number of elements:371011
Number of nodes:151032
Y+:1
First cell height:0.0000514162m
Heat transfer coefficient:19.156W/m^2K
Final Refinement:
Element size: 0.07m
Number of Inflation Layers:15
First layer thickness:0.0000514162m
Growth rate:1.2
1.) Mesh Quality
2.) Residual Plot
3.) Surface heat transfer coefficient
Mesh statistics and surface heat transfer coefficient
Element size: 0.07m
Number of elements:484556
Number of nodes:207216
Y+:1
First cell height:0.0000514162m
Heat transfer coefficient:20.519W/m^2K
Analytical solution
The Reynolds number for the given flow is 58189.06 which suggests that the flow is turbulent in nature .The Dittus Bolter heat transfer coefficient relation can be used to determine the nassult number which is described as follows.
Comparison of all cases
|
Element Size |
Number of Elements |
Number of nodes |
First cell height |
Y+ |
Inflation Layer |
Surface heat transfer coefficient( Numerical Value) |
Surface heat transfer coefficient (analytical value) |
Deviation |
% Deviation |
|
0.15m |
135694 |
27143 |
N/A |
N/A |
N/A |
21.972121 W/m^2K |
- |
2.5666 |
13.2 |
|
0.09m |
169425 |
38412 |
0.0000514162m |
1 |
6 |
18.896W/m^2K |
19.4287W/m^2K |
0.5327 |
-3.32 |
|
0.08m |
371011 |
151032 |
0.0000514162m |
1 |
10 |
20.192W/m^2K |
19.4287W/m^2K |
0.7633 |
3.07 |
|
0.07m |
484556 |
207216 |
0.0000514162m |
1 |
15 |
20.519W/m^2K |
19.4287W/m^2K |
1.09 |
5.01 |
Results:
The temperature along plane-1located at the exhaust section of the port
The Velocity along the plane-1 located at the exhaust section of the port
Temperature at outer convection Wall.
Stream line representation of velocity
Surface heat transfer coefficient
Conclusion:
* The CHT analysis on the exhaust port was simulated 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 19.4287 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 second and final mesh refinement are found to be in compliance with the industrial standards
* The Grid dependence test was also observed during mesh refinement are closer to each other. It is important that we limit ourselves to the second refinement only to save the computational time and cost of the project
* The value of the heat transfer coefficient is located in the cell adjacent to the wall which allows us to choose the value of Y+=1 (Viscous sublayer) and the turbulence model k-omega SST.
* 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 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
* The accuracy of the heat transfer coefficient depends on selecting the suitable value of Y+ and turbulence model k-omega SST model. 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 .However the accuracy of k-omega SST model itself depends on accuracy of the analytical solutions, Flow properties and geometry. The values are predicted by the baseline mesh and the refined mesh where 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.