Week 4 - CHT Analysis on Exhaust port

Aim 

To perform the steady-state conjugate heat transfer simulation on the exhaust port when the air is coming at the inlet at a speed of 5m/sec.

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?

Conjugate Heat Transfer(CHT):

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.

Numerical approaches are one of the simplest ways to realise conjugation. Iterative methods are used to set and solve the boundary conditions for the interface between a fluid and a solid. Other than by using the hit-and-miss approach, there is no way to make accurate assumptions about the values of the initial boundary condition for convergence.

Why CHT :
Understanding of the complex heat transfer mechanisms and the interactions between those mechanisms to improve the performance and increase the lifespan of the components to be analysed.
Conjugate Heat Transfer analysis provides the temperature distribution in solid and fluids, and clear insight on velocity distribution and mechanism of heat transfer of fluid, like in engines.
It is more accurate and fast to simulate the problem.
Can be used in complex scenarios like in automotive components or in power plant components.
Multiple number of regions can be simulated.
All types of region combination can be simulated like solid-fluid,fluid-fluid, etc.

Where is CHT analysis done :

• Analysis of Heat transfer mechanism in turbocharger.
• HVAC systems
• Burners, combusters, electronic devices.
• Automotive, power plants and various other applications.
• Heat transfer in ribbed passages
• Heat transfer in micro-channels 
• Heat transfer in fin arrays
• Heat transfer in mobile phones
• Drying of a porous flat plate

Simulation precedure :

We are going to simulate the exhaust port refine mesh 

Geometry :
1. Import the model of exhaust port.

2. Go in repair and clear the extra edges if any.

3. Then go in prepare and extract the inner volume.

4. After that select solid and fluid component move them to new componet.

5. Finally go in workbench and select shareprep and also share the topology in desing.

            N.B-: Pink colour border tells that solid-fluid volume shared correctly

Mesh :

• Open the mesh section in workbench.
• First name the faces which are inlets, outlet, outer-wall-convection
• Then mesh the geometry as defualt mesh size 
• Then we can add the inflation layer so, can get the better result in the wall surface 

The here given in inflation has

total layer thickness =5mm

maxium layer = 6

growth rate = 1.2

Setup:

• Turn on the energy equation and viscous model.
• Set up the materials, solid and fluid one.
• Then set up the boundary conditions like inlet velocity, outlet pressure, outer wall convection's HTC.
• intialzation the solution
• add the contour plot

 Inlet

velocity-5 m/s

Tempereture- 700°C

Outlet

Gauge presure- 0 pa

Outer wall convection

wall heat transfer co-efficient (h)= 20k

velocity- No slip 

wall-solid-fluid-solid_volume

No slip

Heat flux - 0°C

Adiabatic

Result

How would you verify if the HTC predictions from the simulations are right? On what factors does the accuracy of the prediction depend on?

The typical flow conditions in automotive exhaust systems produce re in the range of 10^3 to 5*10^4. The exhaust flow often enters the region re<2300, especially in exhaust manifold runners. In spite of that, the flow remains actually turbulent since it has flowed through a substantial restriction. The exhaust valves and the persisting unsteady flow pulsation effects don’t favor the transition to the laminar region.

According to the sieder tate relation which correlates the Nu number with Re and Pr

Nu = 0.023*Re^0.8Pr^n

It means Nusselt no. is the function of Reynolds no and Prandtl number

As the turbulent boundary layer in the bend of the outlet port is the thin maximum velocity of flow is observed at bend and hence higher Reynolds number.

Now, Nu = h*L/k

As the length of pipe and thermal conductivity is a constant value of h will increase as Nu increases and Nu increases as Re increases.

As per our result, the heat transfer coefficient is high for a higher value of velocity and hence our results are correct.

Re=Density*velocity*lenght/dynamic viscosity

Re=1.204*5*0.166/1.1813e^-5

Re = 56867.66 

By using the Dittus Bolter heat transfer coefficient relation in natural turbulent flowe the Diltus Bolter HT coefficient is determined by the Nusselt number which is related by the formula of 

Nu(D) = 0.023* Re^0.8*pr^n  here n=0.3   D = diamter of the pipe = characteristics length = 0.166 m

Prandtl number

pr = mu*cp/k    here mu = 1.789*10^-5  cp = 1006.43 J/kg-k and k = thermal conductivity = 0.0242 w/mk

pr = (1.789*10^-5*20)/0.0242 

pr= 0.744

Nusselt number is calculated as

NU = 0.023*56867.66 ^0.8*0.744^0.3

NU= 131.792

Nu= hD/k 

NU=131.792=h*0.166/0.0242

h=19.52 w/m^2k.

Factors affects the quality of prediction:

• More refined mesh leads to the better results.
• Use inflation layers and apply proper Boundary condition.
• Proper selection of turbulence model according to y+ values. 

Conclusions- 

• Here we find the theoretical value and the computational value of the thermal specific heat transfer coefficient to be = 19.52 so the value is really close to the value of h in the computational way which is = 20 w/m^2k.
• The value of the heat transfer coefficient is located in the call adjacent to the wall which allows allows us to choose the value of y+ = 1 for the turbulence model.
• The presence of the boundary layer is quite visible near the inlet of the wall where the velocity near the wall are to be close to be 0 to be the zero slip boundary condition so the viscous force is dominant near the inlet but near the outlet the  velocity is the highest so giving the highest Reynolds number. 
• The velocity at the outlet is high than in the inlet this can be attributed to one outlet and the four inlets.
• The value of the y+ is maintained in the range of 1 for the turbulence model . This is validated with the analytical solution which comes out to be near to it. The value of the  heat transfer coefficien is found out to be 21.05 w/m^2k. which is close to the analytical value.
• The maximum heat transfer  results at bend of exit port, It is because to ensure conservation of mass at exit port velocity should more, which resuts in higher velocity and increse in velocity attain a Reynolds number that is sufficient to make flow trurbulent and higher Nusselt number at pipe bent at exit, which increses Heat transfer co-efficient at that loicaton.