Conjugate Heat Transfer Analysis on Exhaust Port
Conjugate Heat Transfer Analysis:
- 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. In some applications, the performances are further improved by combining convection with phase change (for example liquid water to vapor phase change).
- 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 analysis can be used to generate increased resolution thermal inputs on the stage wetted surfaces. Whereas the standard approach uses constant heat transfer coefficients, CHT analysis will export individual nodal Heat Transfer Coefficients and Wall Adjacent Temperature.
- In addition, a CHT model enables the opportunity to generate transient thermal loads in support of higher resolution subsequent Damage and Thermal Mechanical Fatigue (DTMF) analysis.
Exhaust Port Geometry:
- The extra edges & other errors in the geometry are corrected and saved for better mesh quality.
- The fluid & Solid bodies have a shared topology of their common faces & edges, for preventing gaps when the meshing is done.
Computational Meshing:
Here we are considering two cases of meshing, for comparing the results.
Case (1) :
- The baseline mesh is taken for the first simulation.
- The mesh quality is good after the corerections are made in the geometry.
Case (2) :
- A further refined mesh is generated for better accuracy.
- The element size is decreased to increase the number of elements.
- Inflation layers are included for smoother results near edges.
Comparing Velocity Results:
- The inlet velocity of the exhaust gas is given as 5 m/s.
- From the four inlets the gas is being fed into the manifold.
- As the gases from all the inlets being ejected out through a single outlet, the velocity of the exhaust air increases at the outlet.
Case (1) :
Velocity Streamline:
Velocity at outlet Plane:
Average Velocity over the solid surface:
Case (2) :
Velocity Streamline:
Velocity at outlet Plane:
Average Velocity over the solid surface:
Observations on Velocity results:
Maximum velocity:
Case (1) : 37.5 m/s
Case (2) : 40.7 m/s
Average Velocity :
Case (1) : 15.8 m/s
Case (2) : 16.75 m/s
The maximum velocity can be found near the curvature of the manifold before the outlet.
Conclusion on Velocty results:
- We can observe similar changes in the velocity profile in both cases.
- Refining the mesh gives more accurate values than the base line mesh.
- The time for convergence is incresed as the mesh is refined.
- Thus base line mesh can just be used to identify the characteristics of the flow and cannot be trusted for calculating result values.
Comparing Temperature results :
- The initial temperature is given as 700 K at the inlet boundary condition.
- As the solid walls conditioned to be convective, the heat is transfered to the solid body.
- Thus the heat is distributed all over the body of the manifold.
Case (1) :
Temperature over the body :
Temperature at outlet plane :
Average Temperature :
Case (2) :
Temperature over the body :
Temperature at outlet plane :
Average Temperature :
Observations On Temperature results:
Maximum temperature:
Case (1) : 700.3 K
Case (2) : 499 K
Average tempertature:
Case (1) : 700 K
Case (2) : 538 K
Conclusion on Temperature Results:
- The heat transfer along the body varies from each case.
- As large amount of gas flows through the outlet pipe, the temperature is high near them than the inlet pipes.
- Thus there is variation in temperature along the solid faces of the body.
- Refining the mesh increases the average temperature of the body which is the accurate average temperature.
Comparing Heat Transfer Coefficient Results:
- As the flow continues inside the solid volume, the solid body keeps on obsorbing the heat from the exhaust gas.
- This convection differs from one point to another through out the body.
- Thus the Heat transfer coefficient also varies accordingly.
- The initial HTC for the outer body is given to be 20 W/(m2K).
- This varies upon increasing itrations.
Case (1) :
HTC over the outer Body :
HTC on Outlet Plane :
Average HTC :
Case (2) :
HTC over the outer Body :
HTC on Outlet Plane :
Average HTC :
Observations on HTC Results :
Maximum HTC:
Case (1) : 114.3 W/(m2K)
Case (2) : 34.5 W/(m2K)
Average HTC:
Case (1) : 270.6 W/(m2K)
Case (2) : 144 W/(m2K)
Conclusion on HTC Results :
- As the mesh is refined, the HTC values is increased.
- Finer mesh increses the convection over the body , which makes the solid surface to absorb heat smoothly.
- Addition of Inflation layers gives us a smoother prediction along the edges of the wall.
- This can get more accurate than the results obtained in case (1).
Verification of HTC predictions :
- The results obtained can be compared with experimental results, which can be reffered from any published documents. By performing the analysis based on the reference paper (i.e) considering the initial conditions & other factors that affect the simulation to be copied from the reference. Then the analytical results can be compared with the experimental results.
- After getting the analytical results from the simulation, we can theoritically calculate the values using standard formulae and compare both the values for better accuracy.
For example in this case we can compare Nusselt Number.
where h is the convective Heat Transfer Coefficient of the flow
L is the Characteristic Length
k is the thermal conductivity of the fluid.