Conjugate heat transfer analysis of exhaust port
The conjugate heat transfer analysis was introduced after the computers came into wide use. Before this, the emperical relation of proportionality of heat flux to temperature difference with heat transfer coefficient which was the only tool in theoretical heat convection since the times of Newton. This model, based on a strictly mathematically stated problem, 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 process and the governing equations are considered for the two seperate sub-domains. 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. Moreover, it may be calculated using these data.
The CHT analytical method is used for various applications such as Electronic cooling, Boilers, Heat exchangers, Engine cooling systems etc.
Analysis of Exhaust manifold:
The analysis was performed for two types of mesh:
1) Base line mesh
2) Refined mesh
The geometry was setup in ANSYS spaceclaim. The exhaust model was setup using volume filling option to setup fluid flow domain for meshing. Then the solid volume and fluid domain were merged using topology sharing option. Due to this sharing feature a conformal mesh was formed between fluid and solid volumes. The formation of conformal was very important to see heat transfer effects at the surface of the solid boundary.
The solver that was used to simulate the model was FLUENT. The pressure based equation was used and for solving turbulent equations k-epsilon model of viscosity was used. To capture the tempreature variations in the fluid and solid domains energy equations were also used. The initial boundary conditions that were used are:
Inlet:
Velocity= 5m/s
Tempreature= 700 K
External solid wall:
Convective heat transfer coefficient= 20 W/m^2 K
Outlet:
tempreature= 300 K
Gauge pressure= 0 pa
Operating conditions:
Absolute pressure= 101320 pa
Baseline mesh:
Cell count: 137926
Cell size: 0.15m
The figure given below is the conformal mesh between the fluid and the solid volumes:
Analysis Results:
Tempreature plot:
Velocity plot:
Wall heat transfer coefficient:
Refined mesh:
Cell size: 0.15m
No of inflation layer: 6
Total thickness: 5mm
Cell count: 360790
The figure below shows the inflation layer that was created to capture heat transfer effects in the walls accurately:
Tempreature plot:
Velocity plot:
Wall heat transfer coefficient:
Coclusions:
From the above figures it can be concluded that the heat transfer coefficient (HTC) increases as the velocity of the fluid increases. Therefore, it can be seen at the inner bend near the outlet the velocity of the fluid is maximum and thus the heat transfer coefficient.
In the first case where the baseline mesh is used the velocity of the simulation changes from 0 to 37.3 m/s. The tempreature changes from 464 K to 700 K. The heat transfer coefficient near the wall is 0 to 100.2 W/m^K.
In the second case where the mesh was refined the velocity of the simulation changes from 0 to 39.32 m/s. The tempreature changes from 504.2 K to 700 K. The heat transfer coefficient near the wall is 0 to 210.2 W/m^K.
It was evident from the results that the values of the simulations changes as the grid changes. Therefore, it can be concluded that the simulation was grid dependent and would require further refinements to make the results independent of the mesh.
Verification and factor affecting HTC:
The HTC can be verified using nusselt number formulae:
Re= Reynold\'s no.
Pr= Prandlt no.
Nu= Nusselt number
h= HTC of fluid
D= Diameter of the pipe
k= Thermal conductivity of the fluid
Using these formulae we can find out heat transfer coefficient by finding out prandlt and reynold\'s number.
mu= Dynamic viscosity of the fluid
Cp= specific heat of fluid
k= Thermal conductivity of the fluid
Therefore, it can be seen that the values of HTC depends upon the fluid and the velocity at which the fluid flows through the pipe. Therefore, varying the velocity of the fluid we can increase HTC.
Also, we can verify the obtained heat transfer coeffecient by inputing all the parameters and one obtained parameter i.e. velocity from the simulation and check the accuracy of HTC value.