CHT Analysis on Exhaust port

The contemporary conjugate convective heat transfer model was developed after computers came into wide use 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. Moreover, it may be calculated using these data.

The Conjugate Heat Transfer (CHT) analysis type allows for the simulation of heat transfer between solid and fluid domains by exchanging thermal energy at the interfaces between them. Typical applications of this analysis type exist as, but are not limited to, the simulation of heat exchangers, cooling of electronic equipment, and general-purpose cooling and heating systems.

Applications

Conjugate heat transfer problems, where the heat conduction in a solid region is usually coupled with the convection heat transfer in an adjacent fluid region, are of great significance in many practical applications, for example, the electronic circuit cooling, the design of thermal protection system for space vehicles, thermal insulation of nuclear reactor, turbine blade cooling, and solar panel design.

Conjugate Heat Transfer (CHT) is applicable whenever there are two adjacent domains and we wish to analyze the heat transfer between these domains. These domains can either be solid or fluid. One example is the forced or natural convective cooling of a heat-sink attached to active electronics components which generate heat

For many simulations of real-world engineering applications, the predictions of heat transfer properties are as important, if not more important, than the actual flow field. Such scenarios include simulations of heat exchangers, HVAC (Heating, Ventilation and Air Conditioning), combustion/burners, electronics cooling, and many more. In these applications, we are often interested in how heat moves through both the fluid and solid domains, and importantly the transfer of heat across the interface between adjacent domains.

CHT analysis on the Exhaust port by using Ansys fluent:-

GEOMETRY:-

After creating the Fluent cell in the Project schematics, open design modeler to prepare geometry for fluent CFD simulation and import the geometry of the Exhaust Port.

It will generate fluid volume inside the exhaust port as shown in the below figure

We also have to share geometry surfaces. Go to workbench --> select ‘Share’. it will show user-connected faces in red automatically. Simply click on the green checkmark to share faces.

The final model prepared in space claim in space claim will look as shown in the below figure.

MESHING:-

Naming the inlet, outlet and walls, and the inflation layer b/w solid and volume

Turbulence model k-omega 0<y+<5

y+ = 1

First cell thickness = 0.0485 mm

Element Size = 20 mm

Inflation layer

Mesh Quality

No.of Nodes	118928
No.of Elements	345095

Fluent setup(k-omega SST):-

Why k-omega

To calculate wall/surface heat transfer coefficient on the internal solid surface

On the other hand k- omega and k- epsilon are two-equation models. These are semi-analytic models by the way. k- epsilon model predicts well far from the boundaries (wall) and k- omega model predicts well near wall. Even though it depends on Y+.

Materials

Boundary conditions

Definitions

And then click on the initialization it will initialize the entire fluid flow which is hybrid initialization we can see the convergence in scalar as shown in below

we are going to simulate 500 iterations

RESULTS:-

surface heat transfer coefficient plot

wall heat transfer coefficient plot

We can see that there is an increase in velocity at the outlet from below GIF where CHT will be more in this area

As we know that the CHT happens b/w fluid and solid wherein the inflation layer as shown

Inflation layer b/w solid and volume

wall/surface heat transfer coefficient on the internal solid surface

wall heat transfer coefficient on the internal solid surface

surface heat transfer coefficient on the internal solid surface

velocity & temperature contours

K-omega turbulence model 0< y+ <5

Y+	First layer thickness	Re(Renolds number)	Surface HTC W/m^2 K	Wall adjacent HTC W/m^2 K
1	0.0485 mm	7701.60	-18.957	1099.08

We can also compare our results with analytical results

Heat transfer coefficient at the inlet

The heat transfer coefficient can be calculated as

`h = (k)/(D)*Nu`

Dittus-Boelter equation

The Dittus-Boelter equation (for turbulent flow) is an explicit function for calculating the Nusselt number. It is easy to solve but is less accurate when there is a large temperature difference across the fluid. It is tailored to smooth tubes, so use for rough tubes (most commercial applications) is cautioned. The Dittus-Boelter equation is:

where:

is the inside diameter of the circular duct

is the Prandtl number

for the fluid being heated, and for the fluid being cooled

`Nu = 0.023*R_e^0.8*Pr^n`

n = 0.3

Re=7.530e+04 ( as we calculated earlier)

`Pr = (c_p.mu)/(k)`

`Pr = (1075*1.789e-05)`

`Pr=0.7949`

`Nu = 0.023*(7.530e+04)^0.8*0.7949^0.3`

Nu = 139.2182

`h = (0.0242)/(0.022)*27.1182`

h = 29.831 W/m^2 K

At the inlet, the velocity is very less which causes less heat transfer rate.

Heat transfer coefficient at the outlet

The average outlet velocity from the simulation is 25.278964m/s

Therefore, Reynolds's number will be...

`R_e = (rho*U*L)/(mu)`

`R_e = 34122.00`

`Pr = (c_p.mu)/(k)`

`Pr = (1075*1.789e-05)`

`Pr=0.7949`

`Nu = 0.023*(34122)^0.8*0.7949^0.3`

Nu = 90.83

`h = (0.0242)/(0.0177)*90.83`

h = 124.18 W/m^2 K

The above analytical problem refers to the heat transfer coefficient of simple pipe problems which never gives exact results for complicated geometry like the exhaust port

But we can compare the result to our result so that we can know the criteria of our results

Conclusion:-

• The mesh is a very important parameter to get good results, that's why we have to refine it in the important place of our geometry.
• One factor that should be considered which is the most significant is the turbulent boundary layer. If we are using any kind of low Reynolds turbulence model considering enhanced wall treatment (as our K-w SST model), so we need to make sure that we stay in the laminar sub-layer region where velocity profiles are assumed to be laminar and viscous stress dominates the wall shear which means (Y+<5)
• In the fluent software, the value of the convective heat transfer coefficient extremely depends on the reference temperature.
• Mesh/ grid dependency test is very important