Week 4 - CHT Analysis on Exhaust port

 

Aim: CHT Analysis on Exhaust port.

 

Objective: The objectives will mainly focus on

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?

 

Introduction:

             Conjugate heat transfer is a type of heat transfer analysis between solids and fluid(s). This type of heat transfer includes both convection (between fluids) and conductive (between solids) heat transfer, as well as both forced and natural convection. Common applications and use cases of this thermal simulation type include electronics cooling, heat exchangers, industrial machinery, some AEC cases, and more. 

Conjugate heat transfer analysis can accurately predict heat transfer by simultaneous solving all the relevant solid and flow field heat transfer processes. 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: conduction through solids, free and forced convection in the gases/fluids and thermal radiation.

 

1.Pre-processing and solver setting:

 

Baseline mesh

• The geometry is opened into Space claim

• The next step is extract the fluid volumes,

 

• Enable to share Topology
• Next step is click ''Overlap bodies''

 

 

2.Meshing:

After Overlap bodies see the mesh vertex arrangements,

 

Create Name of the Individual part

 

Inlet :

 

Outlet:

 

Outer wall convection:

 

Generate default mesh:

 

 

No of Elements: 138841

No of Nodes : 33782

Element Size : 0.15m

 

Mesh metrics:

 

 3. Solution:

Physics:

The materials assigned for the fluid and solid are air and aluminum respectively.

Properties of air and aluminum:

Air:

Density	1.225 kg/$m^3$
$C_p$	1006.43 J/kg K
Thermal conductivity	0.0242 W/m K
Viscosity	1.7894e-5 kg/m s

Aluminum:

Density	2719 kg/$m^3$
$C_p$	871 J/kg K
Thermal conductivity	202.4 W/m K

 

• Energy equation: It is turned on.
• Viscous model: It is set to $k-\omega SST$

 

Boundary conditions:

The different boundary conditions are listed below

• Inlet boundary condition

1. Type: Velocity inlet
2. Velocity magnitude : $5\frac{\mathrm{m/}}{\sec }$
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: $20\frac{\mathrm{W/}}{{\mathrm{m^2}}^{}K}$

• The rest boundary condition values are set up as default values in ANSYS-FLUENT.
• Hybrid initialization is done. The simulation is run for 300 iterations.

 

Results:

Residual plot:

Surface Heat Transfer coefficient :

 

Surface heat transfer coefficient =  21.306477 W /m^2 K

Temperature at outer convection wall:

Streamline representation of velocity:

The velocity along plane-1 located at the exhaust section of the port:

The Temperature along plane-1 located at the exhaust section of the port:

 

The  heat transfer coefficient along plane-1 located at the exhaust section of the port:

 

 

The  heat transfer coefficient with velocity streamline along plane-1 located at the exhaust section of the port:

 

CHT analysis of exhaust port with finer mesh:

 

First refinement:

 

Calculating  the first layer cell height:

Inflation 

Number of inflation layer: 5

Growth rate: $1.2$

Inflation option: First layer thickness

First layer thickness: $=0.05141621mm$

Captured curvature is set to yes.

 

Meshing:

 

 

 

 

 

 

 

 

 

 

 

 

 

Heat Transfer Coefficient Contour:

 

 

Mesh statistics and surface heat transfer coefficient

Element size	$0.09$
Number of elements	195914
Number of nodes	63556
Y+	1
First cell height	$0.0000514162$
Heat transfer coefficient	21.505318 W / m2K

 

 

Second refinement:

Element size: 50 mm

Number of inflation layer: $10$

First layer thickness$:0.05141621mm$

Growth rate: 1.2

 

Meshing:

 

Mesh Quality:

 

Results:

Residuals plot:

 

Surface heat transfer coefficient plot:

 

Temperature contour:

 

Temperature-Plane 1 - Contour:

 

Velocity - Plane 1 - contour:

 

 

Mesh statistics and surface heat transfer coefficient

Element size	$0.05$
Number of elements	255343
Number of nodes	93821
Y+	1
First cell height	$0.0000514162$
Heat transfer coefficient	21.664864  W / m2K

 

Final refinement:

Element size: 10 mm

Number of inflation layer: $10$

First layer thickness$:0.05141621mm$

Growth rate: 1.2

 

Meshing:

 

Mesh Quality:

Results:

Residuals plot:

 

Surface heat transfer coefficient plot:

 

Temperature contour:

 

Temperature-Plane 1 - Contour:

 

 

Velocity - Plane 1 - contour:

 

Mesh statistics and surface heat transfer coefficient

Element size	$0.01$
Number of elements	1380939
Number of nodes	458722
Y+	1
First cell height	$0.0000514162$
Heat transfer coefficient	$21.153098$

 

Analytical solution:

 

• The Reynolds number for the given flow is $\mathrm{58189.8960.}$
• $\mathrm{Flow\; is\; turbulent\; in\; nature}$

 

• $\mathrm{The\; Dittus\; Bolter\; heat\; transfer\; coefficient\; relation\; can\; be\; used\; to\; determine}$

$\mathrm{the\; Nusselt\; number\; which\; is\; described\; mathematically\; as\; follows,}$

 

Comparison of all cases of Results:

Element     size	Number       of   elements	Number   of nodes	First cell height	Y+	Inflation layer	Surface heat transfer coefficient.(Numerical value)	Surface heat transfer coefficient. (Analytical solution)	Deviation	% Deviation /error %
$0.15m$	138841	33782	N/A	N/A	N/A	$21.306477$	$19.4287\frac{\mathrm{W\; /}}{{\mathrm{m^}}^{2}K}$	1.8777	4.139
$0.09m$	195914	63556	$0.0000514162m$	1	5	$21.505318$		2.076618	4.178
$0.05m$	255343	93821			10	$21.664864$		2.236164	4.209
$0.01m$	1380939	458722			10	$21.153098$		1.724398	$4.109$

 

Conclusion:

• The CHT analysis on the exhaust port was simulated successfully on ANSYS-FLUENT.

 

• The pressure-based, coupled scheme solver is used to discretize the whole geometry.

 

• The analytical value of the heat transfer coefficient is $19.4287\frac{\mathrm{W\; /}}{{\mathrm{m^}}^2\mathrm{K.}}$

 

• The all deviation of numerical heat transfer coefficient values from the analytical solution was found to be less than  $5$ .

 

• The grid independence test was also observed during mesh refinement. The value of the heat transfer coefficient for the  final mesh refinement are close to each other. It is important that we limit ourselves to the final refinement only to save the computational time and cost of the project.
  

 

• The value of the heat transfer coefficient is located in the cell adjacent to the wall which allows us to choose the value of $Y+=1$ (viscous sublayer) and the turbulent model $k-\omega$.

 

• The presence of the boundary layer is quite visible near the inlet of the pipe where the viscous force makes the velocity near the wall to be zero causing a no-slip condition to occur., but as the flow is turbulent in nature (Reynolds number = $58189.8960$) the velocity profile goes from flat to the flutter and the boundary layer vanishes at the outlet. 

 

• The velocity at the outlet is higher than the velocity at the inlet can be attributed to the presence of four inlets and one outlet.

 

• The accuracy of the heat transfer coefficient depends on selecting the suitable value of Y+ and turbulence model $k-\omega$ model.

 

• The heat transfer coefficient values predicted by CFD were validated against the numerical solution depicted the deviation in the range of less than $5\%$ which is in compliance with industrial standards. However, the accuracy of  $k-\omega$ model itself depends on the accuracy of the analytical solution, flow properties, and geometry. The values predicted by the baseline mesh and the refined mesh were close to each other this can be attributed to the fact that the boundary keeps on growing and declining from the inlet to the outlet owing to change in velocity, thus the value is predominant at the outlet section of the exhaust port.