Week 4 - CHT Analysis on Exhaust port

REPORT FOR CHALLENGE-4

Aim : To perform the Conjugate heat transfer analysis on a exhaust port for to find the convective heat transfer of the internal fluid for the any two cases of different no.of elements and compare the analytical results with the theoretical results.

Software Required : Ansys Workbench – Fluid Flow vR2-2022

Hardware Required : Desktop, Keyboard, Mouse, CPU.

Theory:

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. Conjugate heat transfer is observed in many situations. For example, heat sinks are optimized to combine heat transfer by conduction in the heat sink with the convection in the surrounding fluid.Conjugate heat transfer model was developed after computers came into wide use in order to substitute the empirical 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 processes and solutions of the governing equations are considered separately for each object in two subdomains. 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.

Reynolds number (Re): It is a dimensionless quantity that helps predict fluid flow patterns in different situations by measuring the ratio between inertial and viscous forces. At low Reynolds numbers, flows tend to be dominated by laminar flow, while at high Reynolds numbers flows tend to be turbulent. The turbulence results from differences in the fluid's speed and direction, which may sometimes intersect or even move counter to the overall direction of the flow.

The Prandtl number is a dimensionless quantity that puts the viscosity of a fluid in correlation with the thermal conductivity. It therefore assesses the relation between momentum transport and thermal transport capacity of a fluid. It is defined as

Pr=Thermal (or heat) transport/Momentum transport​=α/ν​

The Nusselt number(Nu) is a dimensionless number, is equal to the dimensionless temperature gradient at the surface, and it provides a measure of the convection heat transfer occurring at the surface.Thus, the Nusselt number is defined as:

Reynolds number, Y+, Turbulance model all are connected to eachother. So, heigher the reynods number heigher the Y+. So, Keep Y+ below 10 for viscous dominate flows with separation.Use Y+ < 1 for the transition prediction and heat transfer calculations.Use Y+ < 10 for SST model (omega based models in general).Use K-Epsilon model with scalable wall functions (default option in new version of Fluent). It will keep the Y+ above 11.06 so that it does not suffer the limitation of old standard wall functions. Moreover now you can carry the successive refinement of mesh, which was not possible with standard WFs.

Procedure :

1.Open the Ansys Workbench.

2.Double click or drag the Fluid Flow module.

3.Import the required geometry and extract the volume by selecting the required edges and to share topology for tje common node points.

4.Mesh the geometry with the suitable element size in addition to name all the faces which model have.

• Provide the sizing for to assign the fine mesh for the fluid.
• Create inflation layer with the suitable values which are taken from the y+ calculator.
• Assign the Names for all the faces.

5.Solve the equations in the set-up and solution block for required result.

• Set 3D, Double Precision and Assign number of cores.
• Set Steady state.
• Change method to Coupled.
• Set Energy on and choose the K-Omega Turbulance model.
• Assign the Material Properties and Boundary conditions.
• Check and assign all the Reference values properly.
• Create definition plots for htc.
• Create Contours.
• Initialize and Calculate.

6.Then do the post-processing in the CFD solver in the result block.

7.Save and close the file.

Observations :

Geometry:

-Model to be Analize after Cad clean-up.

 

Mesh Details:

On the consideration of the accuracy of the result, we worked on the concept of independent grid generation. So, we analysed the given exhaust port in 2 different cases which are Medium and Fine mesh and all the mesh details were given below,

Case	Inflation	Detais			Total No.of Elements
-	Y+	First layer thickness (mm)	Growth Rate	No.of Layers	-
Medium	3	0.2	1.2	20	4,15,589
Fine	3	0.17	1.3	18	5,02,985

1.Medium Mesh:

 

2.Fine Mesh:

 

Reference values:

Density	1.225kg/m^3
Velocity	25m/s
Area	0.0657m^2
Viscosity	1.78e-5Kg/m-s
Temperature	700k
Convective heat transfer coeff	20W/m^2-k

Boundary Conditions:

The Conjugate heat transfer analysis of the given exhaust port was done in the flow field having the velocity of 5m/s and pipe wall heat transfer coefficient of 20W/m^2-k.

Solution Status:

We analyzed the given body in 2 cases such as Medium and Fine Meshing based on the No.of Elements. The details were given below,

Case-1:

-After the required equations solved, tabulated the results that were obtained.

Plots and Contours:

Case-2:

-After the required equations solved, tabulated the results that were obtained.

Plots and Contours:

Precautions :

1.Carefully solve the problem.

2.Choose the suitable methods carefully for to get the desired results.

3.Note down the values carefully which are required for theoretical calculations.

Result:

Convective heat transfer coefficients for 2 different element sizes was given below,

Case	Total No.of Elements	h-analytical(W/m^2-k)	h-theoretical(W/m^2-k)
Medium	4,15,589	21.29037	20.68
Fine	5,02,985	21.30297	20.68

case-1:

case-2:

Conclusion:

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. An accurate description of heat transfer modes, material properties, flow regimes, and geometrical configurations enables the analysis of temperature fields and heat transfer. Such a description is also the starting point for a numerical simulation that can be used to predict conjugate heat transfer effects or to test different configurations in order, for example, to improve thermal performances of a given application.

It was observed that convective heat tranfer coefficient in either outer or inner walls increases with the increase of the renyolds number.

In this case the result were getting are similar to the theoretical values, so we can conclude that the analytical results are correct. Also we can come to a colleague that the accuracy is always shows the difference in the size of the mesh element.