Week 4 - CHT Analysis on Exhaust port
CONJUGATE HEAT TRANSFER (CHT) ANALYSIS ON EXHAUST PORT
AIM:
To perform the steady-state conjugate heat transfer simulation on the exhaust port when the air enters at the inlets at a speed of 5m/s and at a temperature of 700K.
OBJECTIVE:
- Give a 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 & show the velocity & temperature contours in appropriate areas.
- How would you verify if the HTC predictions from the simulations are right? On what factors does the accuracy of the prediction depend?
THEORY:
CHT analysis:
- Conjugate heat transfer analysis describes heat transfer as a result of interaction between a body and a fluid that is flowing over or inside it.
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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.
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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 of CHT analysis:
- The CHT methods have become a more powerful tool for modeling and investigating natural 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.
Y+ value:
- To calculate the boundary values accurately, the mesh needs to be finner at the boundaries. The y+ value can be used to determine the mesh size to be used at the boundaries.
- It is a non-dimensional distance that is often used to describe how coarse or fine a volumetric mesh is for a flow pattern. The y+ is applicable only for turbulent flow and mesh cells directly in contact with a no-slip wall.
- The y+ values less than 5 are preferred for boundary layer problems, as the solver in the post-process uses viscous sublayer equations for those values.
- The formulae to calculate y+ are as follows,
- There are few opensource online boundary layer calculators available.
SIMULATION PROCEDURES:
- ANSYS- Workbench is used.
a) BASELINE MESH:
Geometry - SpaceClaim:
- The Exhaust port.STEP file is imported to 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.
- The fluid volume is extracted using the volume tool.
- CHT analysis requires both the solid and the liquid volume under consideration, So both the solid volume and liquid volume are copied into one component.
- Share topology is created, which allows information to be shared between the fluid and solid medium.
- To make sure there is only one interface between solid and fluid volume, the Share prep tool is used.
- close SpaceClaim.
Mesh - Meshing:
- The named selections are created as follows,
- For the baseline simulation, we use the default mesh size. Generate Mesh,
- and check if the mesh from solid and fluid volumes are in sync.
- The mesh quality is checked,
Setup - Fluent:
- The meshed geometry is loaded in ANSYS Fluent.
- For this simulation, the Pressure-Based solver is used. The steady state is considered,
- The reference values are updated,
- Although K-Epsilon is not preferred for flow near walls, we use it for baseline simulation. The energy equations are included,
- The default air is used for fluid material and Aluminum is used for the solid material,
- The inlet(velocity boundary) velocity = 5 m/s and temperature = 700 K,
- The outlet is a pressure boundary.
- For the outer surface of the pipe, the convection boundary condition is used.
- The heat transfer coefficient for the outer surface wall, h = 20 W/(m2k) is assigned after verifying with the analytical value,
- The solution method used is,
- Area weighted average of the surface heat transfer coefficient for the internal surface of the pipe is defined,
- The hybrid initialization is performed to enable create a contours tool.
- The temperature contour is created and the animation contour is created for the same,
Simulation - Fluent:
- The simulation is performed for 150 iterations.
Results - PostProcessing:
- The temperature contour and the surface heat transfer coefficient plot are generated here.
RESULTS for Baseline simulation(a):
Residual plot:
Temperature Contour:
Surface heat transfer coefficient on the internal solid surface plot:
- The outer surface heat transfer coefficient is also calculated below,
Surface heat transfer coefficient on the internal solid surface contour:
Velocity streamline plot:
b) Refined mesh (K-Epsilon model):
Geometry - Spaceclaim:
- The geometry is the same as the baseline simulation.
Mesh - Meshing:
- An extra name selection is added for the creation of an inflation layer.
- The inflation layer boundary is the outermost layer of the fluid volume which is selected by hiding the solid surface,
- The mesh element size = 20mm,
- The inflation layer values are calculated for the y+ value of 1 using an opensource boundary layer calculator,
- New Inflation is created accordingly,
- check whether the inflation layers are created only on the fluid volume or also in solid volume, If it is created in both volumes then redo the inflation only for fluid volume using mesh>insert>inflation tool.
- The mesh quality is checked,
Setup and solution generation - Fluent:
- The k-Epsilon turbulence model is used here,
RESULTS for Refined mesh (K-Epsilon model) (b):
Residual plot:
Temperature Contour:
Surface heat transfer coefficient on the internal solid surface plot:
- The outer surface heat transfer coefficient is also calculated below,
c) Refined mesh (K-Omega model):
Setup and solution generation - Fluent:
- The k-omega turbulence model is used here,
RESULTS for Refined mesh (K-Omega model) (c):
Residual plot:
Temperature Contour:
Surface heat transfer coefficient on the internal solid surface plot:
Surface heat transfer coefficient on the internal solid surface contour:
Velocity streamline plot:
Temperature Contour of the outer wall with velocity streamlines:
OBSERVATION:
Temperature contour:
- As the hot air from the 4 inlets is made to pass through a single outlet pipe a high temperature is observed at the outlet.
Velocity plots:
- The velocity is higher at the outlet region than at the inlet region.
- All the air from the 4 inlets needed to exit the system through a single outlet, so due to mass conservation the velocity is high at the outlet.
- At the bend near the outlet, we can observe the highest velocity region.
Heat transfer coefficient at the internal solid surface:
- The baseline simulation inner surface value was negative which is bad, which is fixed after proper refinement of the mesh.
- Due to the high velocity at the outlet region, the Reynolds number is high at that region which means the heat transfer coefficient is also high at the outlet.
- Thus, more heat is transferred between fluid and solid volume at the outlet region. The highest heat transfer coefficient is observed at the bend where velocity is the highest.
Turbulence Model:
- The K-omega model gave the most accurate result of all.
- The K-omega turbulence model is recommended for flow near walls and for flows away from walls k-epsilon model is recommended.
- The difference between those models can be observed clearly in the table above.
CONCLUSION:
- The CHT analysis was explained briefly with its applications and the simulation was performed successfully on the exhaust port.
- The y+ value of 1 was maintained for the refined mesh cases which increased the accuracy of the result.
- The best turbulence model suited for this problem was found to be the K-omega model.
- The surface heat transfer coefficient on the internal solid surface was calculated and the temperature and the velocity contours were plotted appropriately.
- The predicted HTC value for the numerical method is verified with the analytic value.
- The accuracy of the heat transfer coefficient depends on selecting the suitable y+ value and the turbulence model.
REFERENCES:
1) https://skill-lync.com/knowledgebase/convective-external-forced-heat-transfer-validation-2