Week 4 - CHT Analysis on Exhaust port

CHT Analysis on Exhaust port

Objective :
In this challenge,
1. brief description of why and where a CHT analysis is used.
2. simulate both fluid flow and also the heat transfer to the solid i.e., CHT Analysis on an Exhaust port
3.calculate the wall heat transfer coefficient on the internal solid surface & show the velocity & temperature contours in appropriate areas.
4. verify the HTC predictions from the simulations are right? And mention the factors that the accuracy of the prediction depend on.

Conjugate Heat Transfer (CHT) analysis :
Conjugate Heat Transfer analysis allows for the simulation of heat transfer between solid and fluid domains by exchanging thermal energy at the interfaces between them and transported via conduction within solids and adiditionally via convection within fluid domains.

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 situationsfor example: conduction through solids, free and forced convection in the gases/fluids and thermal radiation.
The three main modes of heat transfer:
‐ Conduction is heat transfer that occurs when a temperature difference exists across a stationary medium.
‐ Convection is the heat transfer between a surface and a moving fluid when there is a temperature difference between the two.
‐ Radiation(thermal radiation) is the emission of energy in the form of electromagnetic waves. In the absence of an intervening medium ,there is net heat transfer by radiation between two surfaces at different temperatures.

Introduction :
Here our objective is to simulate the flow through it as the flow is going through manifold it is going to exchange heat with the solid,we want to simulate that process.We are going to do this by initially assuming that the flow is comming through the exhaust which is basically through all the four inlets and we are giving or going to assume that the fluid isreally very hot and thats going to heat the solid pipe from inside and on the external surface we are assuming that htere is a particular heat transfer.

So inreal world scenario as your engine is air cooled as it is exposed to air and air which is near to the port has particular heat transfer coefficient and tha would be provided as input and that value depends on the external air reynolds number.We can simualate it and find the value but here we are just giving the value as input i.e, heat transfer coefficient for the outer wall boundary condition.Then by aking these assumptions into consideration we start
the simulation.

Solution procedure :
We wil simulate both fluid flow and also the heat transfer to the solid i.e., CHT Analysis on an Exhaust port with the steady state .Then for solving we
follow the below procedure :
1)Geometry setup
2)Meshing
3) Problem setup
4)Solving
5)Post processing
Then we calculate the wall/surface heat transfer coefficient on the internal solid surface & show the velocity & temperature contours in appropriate areas.

1)Geometry setup

In fluent module we have to open spaceClaim and import the geometry.if you look closely to the geometry it has solid part .So first let us extract the fluid volume.

For that we have to go to,
prepare -> volume extract -> select edge selection -> then select all the edges of inlet and outlet -> then click ok.fluid volume will be created .

 we need to share topology.

Now if you open meshing and you will see that there is perfect mesh connectivity and the interface is shared perfectly.This is exactly how we need to make meshes for CHT simulation.
This completes the creation of geometry.Then we close the spaceClaim ,the progress will be saved automatically.

2) Meshing :
For meshing we open Meshing,where we mesh our model so that we can use it to solve our model.
In general it is always good to name the surfaces before meshing. So let us do that. We first select the entity by using the entity selection by using faces so that we can provide name for our entity selection.

steps : select the edge you want to name by using edge selection -> then press N -> name the selection.

In the similar way we name our inlets,outlet,and for outer face just double click on the outer part any where and ANSYS mesher will automatically select the faces we require and we name it ass outer wall convection(Since we are going to use convective BC for it), and dyou can see additional walls like flangs which have not selected you cn name them or even if you dont name what ANSYS mesher will do is it is going to automatically import those into fluent as a wall by default these walls will have adiabatic BC,which is exactly what we want.

Then next thing to do is to mesh.We click the mesh option in the side window and create our base line mesh.We then refine mesh.Let's mesh the geometry .Once the mesh has done we need to see what are the things we have to refine.Let us refine one by one .Let us start with creating inflation layer.

a) Inflation :
Inflation layer helps us to create a body fitted mesh.It means it creates a layer of mesh same as the geometry.
Since at the interface it is very important and we want to calculate the heat transfer at that place we need to take that region and create inflation layers so that flow is captured exactly and the values are calculated exactly.

To do this we first with selecting fluid part and naming it as inflation layer.
In the global mesh select inflation option and in that for use automatic inflation - select all faces in the choosen named selection and for named

selection - select the inflation layer which we have just named.
And for inflation option we use first layer thickness and we give this based on y+ value and the turbulence model we use.Here we use k-omega SST turbulence model so y+ should be in between 0.1 to 5.Lets calculate the first layer thickness.

Now we got the first layer thickness ,let us give the above first layer thickness and maximum of 10 growth layers and with a growth ratio of 1.2 for inflation layers.

Clear the generated data and we have to mesh again.By this we have completed the meshing process then we have to update or generate the mesh
again.You can observe formation of inflation layers.

b) Sizing(Body sizing) :
We are making the mesh of inside fluid domain finer by using sizing.To do this,
Right click on mesh => sizing => in geometry -we select inner fluid volume by hiding outer component and give size 20 mm => Then generate mesh again you will see inner fluid volume is refined finer.

Now we have to check the quality of the mesh.steps : mesh ->quality-> mesh metric-> Element quality
Here our minimum quality is around 15 %,so we can move further for our solution setup.

3) Problem Setup :
To set up our problem we open Fluent and setup inital conditions , boundary conditions material etc.,
We first start with checking the mesh quality using perform mesh check under domain.If no error is seen in console window then we can proceed further.
Then we move move to setting up physics where we select the solver ,viscous model ,initial and boundary conditions etc.,

We use a steady state pressure based solver to solve our model.
Physics :

General :
Solver type : pressure based
time : steady state
velocity formulation : absolute
models :
viscous model : k-omega SST
Material : Fluid : Air
Solid : aluminum
Zones :
Cell zone : In cell zones you will see fluid volume,it is of type fluid with material air and solid volume it is of type solid with material aluminum.

 
Boundary conditions :We will basically give our boundary conditions here such as inlet velocity and outlet conditions.

we give,
Inlet :
velocity : 5 m/sec
temperature : 700K

Outlet :
pressure outlet : gauge pressure = 0 pa

Outer wall convection :
type : wall
Theremal conditions => convection : Heat transfer coefficient = 20 W/m^2 K

Flanges or Support :

type : wall
Theremal conditions =>Heat flux : Heat flux = 0 W/m^2 (Adiabatic walls)

Now if you check other BC they are just wall boundaries we are not changing anything we are going to ignore them.
Now we are almost ready to run the simulation ,but as the solution is being solved we want to moniter different flow quantities while the solution is being generated ,we create contour for this.But before that we have to initialize the domain.We use the Hybrid initialization , this provides a very good initial condition .

Now we want to see the temperature contour.
For contour -> go to solutions tab -> graphics -> contour -> in contour of : select temperature under that select static temperature -> select the outer walls and press save /display.

Then you can see the initial values for temperature for whole domain which is obtained by initialization which is not the final one.
Then we want to create animation so that we can see how the temperature is varying over the domain when problem is being solved.
For that,

steps : Activities -> solution animation -> select temperature contour,which we have created -> preview -> set the required view and use active -> and name it as temperature animation.

4) Solving :
Give iterations such that your solution is converged.
Run the simulation until the steady state is reached.Lets run for 500 iterations ,we can find the solution is converged.
Then we can see that in the residual plot that residuals dropping and reach a steady state after 100 iterations.
Then let us post process the results.

5)Post processing :
Residuals plot :

You can observe from the below plot the residuals has converged after 150 iterations

Temperature contour :
The below plot is the temperature contour on exhaust port.

Then if you think about the results we have,the idea is simple ,we have hot exhaust gases comming in,they are at very high temperature 700k then if you look at the solid metal temperature close to the inlets,the temperature is 474K but if you check in a place where all pipes meet the temperature is around 557K.Now you would have assumed that the temperature of solid is high at the inlet as the fluid enters with high temperature there but actually it is not,the hifh temperature is at the outlet pipe the reason for this is because of convection.To explain more about this let use streamlines.

Streamline plot :
Here in the below streamline plot you can see that streamlines coloured with velocity.

Velocity contour :
In the below contour you can see the velocity is higher in the outlet pipe than inlet pipe

Heat transfer coefficient contour :
In the below contour you can see the heat transfer coefficient is high in the outlet pipe than in the inlet pipe,So thats the reason for high temperature in
the outlet pipe than at the inlet.

Verifying the HTC predictions from the simulations :
The Nusselt number is the ratio of convective to conductive heat transfer across a boundary.

where h is the convective heat transfer coefficient of the flow, L is the characteristic length, k is the thermal conductivity of the fluid.
Then this Nusselt number is related to Reynolds number i.e.,

and u is the flow speed (m/s)

So if you observe heat transfer coefficinet is directly proportional to the Reynolds number (also to the velocity). So if you increase the velocity then there
will be increase in the convective heat transfer coefficient. That's what we have proved above and if you want you can run another simulation by
increasing the reynolds number of the flow then you will observe there will be increase in the overall convective heat transfer coefficient or at a particular
location than in the previous low reynolds number simulation.

Factors that the accuracy of the prediction depend on :
The factors that the accuracy of the prediction depend on are

We need to make sure that share topology is set to share.Only then the meshing is going to be uniform and with perfect connectivity.
We need to select exact turbulent model.As it is wall bounded flow k- omega SST will be very much suitable.And when selecting the turbulence model
selecting exact Y+ value also important.

We need to enable share(share prep) in the work bench and click share .This is very important if you dont do this step you will not see the single
interface between them.So this particular step takes care of creating the interfaces ,so that you will have one interface between your solid volume or
block and fluid block.

We need to give exact heat transfer coefficient for the outer boundary.Then only we get the overall accurate solution.
While giving the values,like for inflation layers or any other values with decimal values,give the exact values or else it may lead to round off error.
Since at the interface it is very important as we want to calculate the heat transfer at that place we need to take that region and create inflation layers so
that flow is captured exactly and the values are calculated exactly