Week 4 - CHT Analysis on Exhaust port

1. Give a brief description of why and where a CHT analysis is used.
2. Maintain the y+ value according to the turbulence model and justify the results. 
3. Calculate the wall/surface heat transfer coefficient on the internal solid surface & show the velocity & temperature contours in appropriate areas.
4. How would you verify if the HTC predictions from the simulations are right? On what factors does the accuracy of the prediction depend on?

What is CHT Analysis, Why and Where it is used? 

Conjugate Heat Transfer (CHT) refers to the coupling of conduction in solids with the convective and radiative heat transfer in the surrounding fluids. In other words, CHT 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 heaters, coolers, or heat exchangers. Heat transfer in solids and heat transfer in fluids are combined in majority of applications. This is because fluids flow around the solids, or between solid walls, and because solids are usually immersed in a fluid.  

The CHT Analysis is used in 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. 

The physics of conjugate heat transfer is common in many engineering applications, including heat exchangers, HVAC, and electronic component design. 

Application: The conjugate heat transfer methods have become a more powerful tool for modeling and investigating nature 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. 

Solving and modeling approach 

• The exhaust port was modeled into the Space claim. 
• 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.  
• CHT analysis requires both the solid and the liquid volume under consideration, Space claim gives an option of volume extract for selecting edges for fluid volume. 
• Both the solid volume and liquid volume are copied into one component. 
• Share topology is created. 

• Space claim gives an option for share coincident topology, when share coincident topology is selected and executed, the mesh from the fluid volume and the solid volume are said to be conformal with each other which is a requirement for CHT analysis. 
• The exhaust port is loaded into the meshing window and then simulated for baseline mesh and the refined mesh. 
• The named selections are created (inlet, outlet, 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 criteria 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 scheme is used as a discretization scheme. 

CASE 1

In this case, the default element size of ANSYS-FLUENT is used (150mm). 

Named selection 

Inlet boundary condition (velocity inlet) @ 5m/sec 

 
Outlet (Pressure outlet) Gauge pressure 0Pa 

Outer wall convection (Heat transfer coefficient `20 (W/(m^2k))` free stream temperature 300K) 

The flanges around the inlet and the component encompassing the outlet are adiabatic (no heat transfer takes place through these walls). 

 

Set up 

The different boundary conditions are listed below 

• Inlet boundary condition 
• 1 Type: Velocity inlet 
• 2 Velocity magnitude: `5m/sec`
• 3 Temperature: 700K 

• Outlet boundary condition 
• 1 Type: Pressure outlet 
• 2 Gauge pressure: 0Pa 

• Outer wall convection 

1. Type: wall 
2. Thermal condition: convection 
3. Heat transfer coefficient: `20 (W/(m^2k))`

The rest boundary condition values are set up as default values in ANSYS-FLUENT. 

•  Energy equation: It is turned on. 

• Viscous model: It is set to `k−ω SST`

Material properties 

Fluid 

• Material Name: Air 
• Density: 1.225`(kg)/m^3`

• Specific heat: 1006.43`J/(kgK)`
  
• Thermal conductivity: 0.0242`W/(mK)`
• Viscosity: 1.7894e−05`(kg)/(m−sec)`

Solid 

• Material Name: Aluminum 

• Specific heat: 871`J/(kgK)`
• Thermal conductivity: 202.4`W/(mK)`
  

 
 Baseline mesh results

Residual plot 

Heat Transfer coefficient

Temperature

Temperature plot Outer wall convection

Velocity Streamline

Pressure Plane

Velocity

 Wall Heat transfer 

Heat Transfer coefficient

Heat transfer coefficient

21.926374 `(W/(m^2k))`

CASE 2

A.First refinement 

Element size: 90mm 

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 to be 1. 

Steps involved in calculating the first cell height. 

Reynolds number= `(ρ∗v∗L)/(μ)`

                          =`(1.225∗5∗0.17)/(1.7894e-05 )`

                           =`58189.8960`

The characteristics length is taken as the inlet dia. 

Skin friction coefficient, `C_f=0.058/(R_e^(0.2)`

                                           `=0.00646`

Wall shear stress formula, `τ_w=0.5∗C_f∗ρ∗v^2`

                                                     =`0.5∗1.225∗25∗0.00646`

                                                      =`0.0989Pa`

Frictional velocity formula, `u_τ=((τ_w)/ρ)^0.5`

                                                  =`0.2841m/sec`

Now,`Y+=(△y∗u_τ∗ρ)/μ`

Since `Y+=1`

Therefore `△y=μ/(u_τ∗ρ)`

                               =`0.0000514162m`

                               =`0.05141621mm`

Inflation  

Number of inflation layer: 6 

Growth rate: 1.2 

Inflation option: First layer thickness 

First layer thickness: =0.05141621mm 

Captured curvature is set to yes. 

Body Mesh

 

Inflation layers

Mash

Residual plot 

Heat Transfer coefficient

Temperature outer wall convection

velocity

Temperature plane

Velocity

Heat transfer

Heat Transfer coefficient

Heat transfer coefficient

21.137651 `(W/(m^2k))`

B.Second refinement 

Element size: 80mm 

Number of inflation layer: 10 

First layer thickness:0.05141621mm 

Growth rate: 1.2 

Inflation layers

Residual plot 

Heat Transfer coefficient

Temperature outer wall convection

Velocity streamline

Temperature Plane

Velocity plane

Heat transfer

Heat transfer coefficient :

Heat transfer coefficient

17.964092 `(W/(m^2k))`

Conclusion :– 

In case 1  mesh generated has default setting and the results captured are less accurate due to the same. 

In case 2  ‘a’  & ‘b’, inflation created results in finer mesh around solid wall. Leading to a capturing temperature contour and HTC more accurately than case 1. 

From above cases we notice the following results – 

• Velocity tends to increase from inlet to outlet due to mass conservation and mixing of air. 

• Transfer of heat from fluid to solid is high and leads to formation high velocity at the junction resulting in HTC capturing. 
• The internal flow velocity and its characteristics are affected by initial velocity, pressure and physical features of geometry. Thus, CHT analysis is essential prior experimental process for better prediction product life of design component. 
• As shown in the plots, the exhaust gases passing through the manifold transfers some of the heat via convection. The amount of heat transfer can be related directly with the velocity of exhaust. 
• The temperature is lower where flow stagnates or is moving at the velocity. As the velocity increases, so does the Reynold’s number and the amount of heat transfer. 

Most accurate methods to predict results is to do a comparative study of experimental and analytical results on Nusselt’s Number. 

Otherwise, another method to verify is to validate simulation data with published experimental data.