C.H.T Analysis on an Exhaust Manifold

Introduction : 

The term conjugate heat transfer (CHT) is used to describe processes which involve variations of temperature within solids and fluids, due to thermal interaction between the solids and fluids. The exchange of thermal energy between the two physical bodies is called study of Heat Transfer, the rate of transferred heat is directly proportional to the temperature difference between the bodies.

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. Efficiently combining heat transfer in fluids and solids is the key to designing effective coolers, heaters, or heat exchangers. Heat transfer in solids and heat transfer 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.

Typical examples on which CHT analysis is performed are Exhaust manifold, car radiators, heat exchangers, coolers, chemical reaction vessels, engine cooling etc.

Objective : 

1. Performing CHT analysis on exhause manifold with different sizes of mesh. 

2. Evaluate velocity, temperature & HTC produced in the exhaust manifold and explain results. 

3. Accuracy of HTC predictions in exhaust manifold. 

Geometry Creation & Ansys spaceclaim: 

Theres 4 inlets and 1 Exhaust pipe in the Geometry. 

After uploading it in Ansys spaceclaim, Fluid volume is extracted.

Some edge repairs were done & topology was set to share, and share prep was ran to ensure the transfer of information between the Fluid & Solid body to be smooth & uniform. 

 Meshing :

CASE 1. 

This is a baseline mesh. We first solve using this mesh to see if we need to further refine the mesh or where we need to refine it to obtain better result. 

Elements : 137900

Nodes : 27495

Element size : 150 mm

CASE 2.

In this case, we further refined the mesh 

Element size : Default 150 mm size

Solid body i.e outer wall : 20 mm

Elements : 332627 

Nodes : 118530

Inflation layer is created on the volume body with total thickness of 5 mm for 5 layers & 1.2 gr

This concludes our refined mesh setup

Case Set-up

Case setup is common for both cases .

1. Solver : Pressure Based 

2. Time : Steady

3. Energy : On

4. Viscous model : K epsilon (2eqn)

5. Material of Fluid - Air

6. Material of Solid : Aluminium 

7. Cell zone - Volume : Fluid

                     Solid : Solid

8. B.C :  inlet - 5 m/s Temp : 700k in all 4 inlets

             Outer-wall-convection (Solid body) - Heat transfer Coefficient 20 w/m^2-k (Assumed)

             Outlet : Gauge pressure 0 pascal : 1 atm

9. Hybrid Initialization 

Solution & Post processing the Results :

Case 1. 

Solution was ran for 600 iterations & it converges at around 350 iterations . execution time is 177.70 seconds 

Velocity plot : 

We see the velocity created in a plane in between the exhause manifold, we can see how the velocity is less at the beginning and quickly escalates by almost 10 m/s when it enters the outlet port. 

This happens due to mass conservation, hence when more mass of fluid is accumulated, and since its directily proportional to velocity in momentum equation, there is increase of velocity.

Two planes are created in center of exhaust manifold to obtain this, we can see how velocity increases as it enters the outlet port. we can observe the highest velocity nearing 36 m/s at the inner radius of the curve which means there is high Re there which leads to conclusion that there is more heat transfer in this location.

Temperature Plot : 

The temp at inlet being 700k we can observe that it has most highest temperature, but as the fluid is moving we can see its slightly reducing in temperature at some sections. 

  Another plane view, we can observe the temp remains high in red color nearly everywhere for the fluid since this plane is created in mid sections, we can observe the solid edge of the plane is green at the outlet end which means the temperature there has increased in the solid showing higher heat transfer there.

Increased temperature on the outlet side of the Outer solid body and it increases more as its nearing the exhause outlet.

Heat Transfer Coefficient plot :

 We can observe a highly irregular looking heat transfer coefficient plot, that is due to the baseline mesh, any how we can observe that since the there is a high temperature fluid going through this portion with a high velocity, we can observe the reddness of the plot which shows us heat is being transferred at hight rate in nearly the entire section. which can be observed from the image right above this. 

Case 2 ( Finer mesh).

Solution was ran for 600 iterations & it converges at around 550 iterations. execution time is 450 seconds .

Velocity Plot: 

This plot is much finer and smoother looking visuals and we can see the velocity variations in the outlet port zone much clearly. The first mesh plot gave slightly different velocity variations than this one. The maximum velocity at the curve section in red color is more than from the Baseline mesh.

Temperature Plot : 

There is a difference here as well from the baseline mesh plot, we can clearly observe the smoother looking temperature drop in the High velocity zones, Which actually means the heat transfer is higher in those places to the solid body. This mesh produced finer & better results.

Heat Transfer Coefficient Plot : 

The heat transfer coefficient has nearly doubled in the finer mesh as opposed to baseline mesh, This plot is much finer looking 

This the zoom in of the inflation layer created. We observe a very smoother plot. 

Showing the Inflation layers in Meshing.

HTC Predictions & its factors for accuracy : 

1. HTC varies significantly for different applications, materials, conditions, geometry etc. 

2. Htc predictions depend upon theoretical / analytical calulations to begin with, giving us a certain baseline target to acheive . There are numerous correlations for internal, external flows, Compositions to factor in conduction, convection of geometry . Whether it is forced, natural, Flow is laminar, turbulent, different fouling factors for different applications etc

3. Then there is Experimental data again which is compared with analytical values for specific conditions.

4. And when we perform simulations, we have a general baseline target available, relations, fouling assumptions  & where experimental assesment of htc gets challenging when small fluxes are to be measured heavy computational power can capture it. 

5. We can then predict if our htc is correct & even visualize it. And then again we experiment with simulation data to test an application like this exhaust manifiold coupling it with an engine under supervised conditions & add more weight to our analysis. 

6. As far as factors for htc predictions accuracy goes, its the skill of application engineer that comes handy,  selecting proper turbulence models, Fine meshing, inflation layers at required place in geometries, Transient simulations to predict time accurate data and observe changing htc with time then validate steady conditions. 

7. Observing convergence of Governing equations & Doing some heavy post processing to clearly observe graphs, plots, use vector analysis etc.

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