Week 4 - CHT Analysis on Exhaust port

AIM:

To give a brief description of why and where a CHT analysis is used.

To maintain the y+ value according to the turbulence model and justify the results.Calculate the wall/surface heat transfer coefficient on the internal solid surface & show the velocity & temperature contours in appropriate areas.

And to verify whether the HTC predictions from the simulations are right.On what factors does the accuracy of the prediction depends on

DESCRIPTION:

CONJUGATE HEAT TRANSFER (CHT) ANALYSIS:

Cojugate heat transfer analysis allows 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 additionally via convection within fluid domains.CHT can accurately predict heat transfer by simultaneous solving all the relevant solid and flow field heat transfer processes.In solids, conduction often dominates whereas in fluids, convection usually dominates.CHT is observed in many situations like conduction through solids, free and forced convection in gases/fluids and thermal radiation.

The three main modes of heat transfer are

1.Conduction: It is heat transfer that occurs when a temperature difference exists across a stationary medium.

2.Convection: It is a heat transfer between a surface and a moving fluid when there is a temperature difference between the two.

3.Radiation/Thermal Radiation: It is the emission of energy in the form of electromagnetic waves.In the absence of an intervening medium,there is net heat transfer by transfer of radiation between two surfaces at different temperatures.

CONDUCTION:

1.Conduction is thermal energy transmission through solid or fluid by the action of translational, rotational and vibrational motion of atoms and molecules.

2.Molecular collisions transfer energy from more energetic molecules to less energetic ones.Higher temperature=increased atomic/molecular motion.

3.conduction occurs in all fluid and solid media.

4.Due to the 2nd law of thermodynamics, heat must always be conducted from higher to lower temperatures.

5.Conduction problems oftem involve fluid flows at the boundaries of the solid, which serve as a mechanism for heating or cooling the solid.

6.The mechanism of conduction is the same for liquids as the molecules are closely spaced and the molecular interactions are stronger and more frequent than gases.

7.In case of electrical conductor, the translational motion of free electrons contribute to energy transfer.

8.The rate equation for conduction is Fourier's law for one-dimensional plane wall given as

Here qx'' is heat transfer rate per unit area in x-direction (W/m^2).

k is the thermal conductivity (W/mK)

-ve sign indicates the fact that the heat transfer is in opposite direction of the temperature gradient.

CONVECTION:

1.Heat is transferred to/from the fluid via conduction of walls.Fluid carries thermal energy to other parts of the system.

2.Heat transfer only due to bulk motion of the fluid (no diffusion) is reffered to as advection.

3.It is further classified into

Forced convection, where flow is caused by external means such as by fan, pump or atmospheric winds.

Free/natural convection, where flow is induced by buoyancy forces, caused by density differences due to the temperature variations in the fluid.

Mixed convection, where forced and natural convection can also exist if both external driving forces and buoyancy forces exist.

4.Heat transfer by convection is rooted in the idea that the heat transfer rate is proportional to the temperature differnce between an fluid and heated/cooled body.This idea is called Newtons law of cooling.The paramer governing the rate of heat transfer is the heat tranfer coefficient, usually denoted by h

Where q'' is the convective heat flux

Ts is the temperature of the solid surface

T infinity is the temperature of the fluid

5.One of the central problems in convection is determining the heat transfer coefficient for a given flow field.

6.The heat transfer coefficient depends on the conditions in the boundary layer and it is affected by the surface geometry, the nature of the fluid flow and various fluid thermodynamic and transport properties.

RADIATION HEAT TRANSFER:

1.Thermal radiation heat transfer is the emission of energy via electromagnetic waves.

2.All matter at a non-zero temperature emits thermal radiation.

3.This emission is due to changes in the electron configurations of the atoms or molecules, and the energy is transported via electromagnetic waves (photons).

4.Radiation intensity depends on body temperature and surface characteristics.

5.Unlike conduction or convection, radiation does not require the presence of material medium and can occur in a vacuum.

6.Radiation is a important mode of heat tranfer at high temperatures.

7.It is also the only mode of heat tranfer in outer space and this is how solar energy reaches earth.

Heat transfer in solids and heat tranfer in fluids are combined in the majority of applications.This is because fluids flow around solids or between solid walls and because solids are usually immersed in a fluid.

Here our objective is to simulate the flow through exhaust ports as the flow is going through manifold and exchange of heat with the solid, there are four inlets through which the fluid flow occurs and one outlet to pass the fluid flow for heat transfer.

Y+ CALCULATION FOR INFLATION

Inflation layer helps us to create a body fitted mesh.

`Uoo`=free stream velocity =5m/s

`rho`=freestream density =1.225 kg/m3

`mu` = dynamic viscosity=0.000017894 kg/ms

L = diameter of the inlet = 0.162206m

Desired y+=1

Reynolds number `Re=(rhoUooL)/mu`=56896

Skin firction `Cf=(0.026)/(Re)`=0.083314

Firction velocity `Uf=sqrt((tauwall)/(rho))`=(0.083314/1.225)^1/2=0.2607911

Therefore first layer thickness, `Deltas=(y+*mu)/(Uf*rho)`= 0.056mm

CASE-I: SIMULATION OF EXHAUST MANIFOLD WITH DEFAULT MESH ELEMENT SIZE

SPACECLAIM

First load the model in the spaceclaim

By using prepare and volume extract select the edges of the exhaust fold

If there are any unnecessary edges use repair >extra edges select them and remove them . But we don’t have any extra edges and it say no extra edges however

Then selecting the edges of the fold as below

After applying the volume extract the volume looks as below

Calling the volume as fluid volume and solid volume

MESHING

From a distance the meshing looks alright but when we zoom in we see its not meshed properly

Here is the problem in the mesh, both meshes are not connected to each other i.e no conformal mesh(cells connected between fluid and solid) between them so there won’t be a heat transfer between them so this needs to be fixed

For this too fix first we need to do setting for share topology in geometry, but before that we moved the solid and fluid volume to new component ‘final geometry’ and deleted the empty ones

Then we go to workbench and select share trip

This shows in pink colour where it is being shared

Now we see a conformal mesh as follows

So as mesh is fine, we need to name the parts so we delete the generated mesh and start naming it

For inlets

The outlet

Next double clicking the faces it selects all the faces automatically , naming as outer wall convection

We didn’t select the below wall, but ansys automatically takes it as wall and regards as adiabatic condition (no heat transfer)

Checking the element quality we see its >10% which is acceptable

ANSYS FLUENT

Going to the graphics window we see the geometry

We use a pressure based steady slover

Enabling the energy equation and viscous model with k-epsilon for turbulent

And materials for fluids we use Air and for solids we use Aluminium

Defining the boundary conditions

For inlet

For the outer wall convection the boundary conditions are

The wall is set to adiabatic i.e heat flux with 0 heat

And displaying only the below faces as follows

A hybrid initialization is run then create the post-processing results

The temperature contour

The temperature contours is

The residual plots is

Now looking the velocity in the solution Ansys after post-processing

Then adding streamlines , starting from the inlets

The velocity streamlines are

Due to mass conservation velocity at inlets is low and high at outlet, and as only outlet is 1port compared to the inlet, all the air needs to be flow out so velocity is high there.Due to this higher velocity we have higher Reynolds number at outlet which means we need to have higher heat transfer coefficient and more heat transfer from fluid to solid to be there.

Inserting a plane at outlet and checking the wall heat flux

The wall heat is highest at the curve passage this is because the velocity is also highest there as can be seen below

Applying the velocity in plane we can see how high the temperature is with velocity

CASE-II: SIMULATION OF EXHAUST MANIFOLD WITH FINER MESH ELEMENT SIZE

Adding the inflation of 5mm for the mesh

First we select the body and select hide the body

Then selecting the faces and naming it as inflation layer

Generating the mesh the inflation mesh is created

Then we select the show body for the solid part

As there are only quarter million cells we can increase still the number of cells for the body

Then we go to mesh>sizing>and using body selection we select the body and apply

So applying 100mm element size we get as follows

By changing element size to 10mm

Then going to setup, file recorded mesh operations >match zone names and matching the names

And reload the setup

Then running the Hybridization and initialize the solution and then running the calculation

The residuals are as follows

The temperature contour is

But still we see same temperature contours even after refining the mesh, once we will check for the post-processing solutions

Here we see there are improvements in the heat flux and heat transfer is available only in the first cells attached to the walls

Due to creating the inflation layers we see pretty much good heat transfer

TO VERIFY HTC PREDICTIONS FROM THE SIMULATIONS:

The Nusselt number is the ration of convective to conductive heat transfer across a boundary.

`NuL`=Convective heat tranfer/Conductive heat transfer = `(h)/(k/L)`=`(hL)/k`

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 nusslet number is related to Reynolds number i.e

`NuD=0.023Re^(4/5).Pr^n`

where `Re=(uL)/v=(rhouL)/mu`

Where u is the flow speed m/s

So if we observe heat transfer coefficient is directly proportional to Reynolds number (also the velocity).So if you increase the velocity then there will be increase in convective heat transfer coefficient. Thats what we have proved by using different mesh size one with default and another with refinement in mesh size elemental.

FACTORS ON WHICH THE ACCURACY OF PREDICTION DEPENDS ON:

1.We need to make sure that the share topology is set to share.Only then the meshing is going to be uniform and with perfect connectivity.

2.We need to select the exact turbulent model as it is wall bounded flow k-omega SST will be very much suitable and when selecting the turbulence model selecting y+ value is also important.

3.We need to give exact heat transfer coefficient for the outer boundary.Then only we get the overall accurate solution.

4.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.

5.Inflation layer is very much important as heat tranfer flow is captured exactly and the values are calculated exactly.

CONCLUSION:

Thus the analysis for CHT for different scenarios one with default mesh with no inflation and another with mesh refinement with inflation has been studies successfully.

And by observing we see that in case-I the velocity flow of heat transfer is high at the turn of outlet and the heat flux is even high at that area. But whereas in case-II with finer element size and inflation size we observe that the heat transfer is less compared to the previous case at the outlet and heat transfer is across the first layer cells of the outlet.