CHT ANALYSIS ON EXHAUST MANIFOLD
CHT ANALYSIS ON EXHAUST MANIFOLD
OBJECTIVE :
To perform a CHT analysis on exhaust port manifold.
To perform the analysis with different mesh sizes and turbulence models.
Verifying the values of heat transfer coefficient for different cases.
To Verify the HTC predictions.
THEORY :
Exhaust manifold :
The exhaust manifold is a pipe, receives the exhaust gases from the combustion Chamber and leaves it to the atmosphere. Exhaust manifolds are mounted to the cylinder head. V- Type engines have two exhaust manifolds, and an in-line engine usually has one.
This design improves breathing capacity of an engine. Exhaust manifolds are typically made of cast iron or steel, although some latest-model cars use stainless steel manifolds.Cast iron is a good material for exhaust manifolds. Exhaust gas temperature is related to the amount of load on the engine.
why and where a CHT analysis is used :
Conjugate heat transfer analysis is generally used when there is a temperature variation during heating and cooling in the material due to interaction between solid and fluid phases . The analysis of heat transfer of solid and liquid domains by exchange of thermal energy at the interfaces between them . Heat transfers in solids occur by conductions while in liquids it occurs by convection.At solid-liquid interface heat transfer takes place by convection.
CHT analysis is used in simulations of heat exchangers , engine equipments , cooling of electronic equipment and general purpose heating and cooling systems.
Application areas :
- Heat Exchangers
- Cooling of electronic equipment
- Engine equipments
- Coolers
- Heat pipes
- Heat Sink
- HVAC
- Boilers etc…
PROCEDURE :
STEP 1 : GEOMETRY USED
The geometry is taken from the modelling software into the space claim
Solid Surface manifold Fluid surface manifold
Note : Check for share topology between these two bodies for better meshing and to get proper simulation results.
STEP 2 : MESHING
For this analysis, Base line meshinfg and Refined meshing for the different turbulence models .
BASELINE MESHING
Element Quality :
Conformal Mesh :
REFINED MESHING
Element Quality :
Inflation Layers :
Mesh statistics – After getting finer mesh without exceeding cell count 0.5 million , we get total number of cell count 488743
STEP 3 : SET UP & SOLUTION
Fluent Setup
- Solver type : Pressure based
- Steady state solver
- Check the Energy equation
- Turblent Model : K-Epsilon (standard) and K-Omega (SST model)
- Fluid Material : Air
- Solid Material : Aluminum
- Intialization : hybrid
Boundary conditions
- Inlet velocity : 5 m/s
- Inlet temperature : 700 k
- Outlet pressure : 0 pa
- Outer wall Convection
*Convection with Heat transfer coefficient = 20 W/m2K
Expected Results :
- Explaining the convergence plot getting with both the models.
- Showing the velocity, temperature, and heat transfer coefficient plots .
An Over-view for the Various cases used in this simulation :
STEP 4 : RESULTS
BASELINE SETUP RESULTS (A)
CASE 1(A) : K-epsilon Standard Model
Residual Plot :
Temperature Stream-lines & contour plot :
Velocity Stream lines & contour plot :
Wall Heat Transfer Coefficient :
Readings :
CASE 2(A) : K-Omega SST Model
Residual Plot :
Temperature Stream-lines & contour plot :
Velocity Stream lines & contour plot :
Wall Heat Transfer Coefficient :
Readings :
REFINED CASE RESULTS (B)
CASE 1(B) : K-epsilon Standard Model
Residual Plot :
Temperature Stream-lines & contour plot :
Velocity Stream lines & contour plot :
Wall Heat Transfer Coefficient :
Readings :
CASE 2(B) : K-Omega SST Model
Residual Plot :
Temperature Stream-lines & contour plot :
Velocity Stream lines & contour plot :
Wall Heat Transfer Coefficient :
Reading :
OBSERVATIONS :
- The effect of element size can be clearly observed in two cases.
- Results are more accurate and near to exact values with finer mesh.
- In case of refined meshing we introduced "inflation layers" , because of that there is a drastic change in the wall HTC .
- Remaining Values are not that much effected .
- The velocity streamline starting from the inlets is shown , where the velocity is low in the inlet sections and higher at the outlet section , this is because of mass conservation.
From all cases , For the respective parameters , we Tabulated values as following :
- Vc Critical velocity is found to be very high velocity at outlet , this is due the effect of curvature .
*From the above tabulated values , we conclude that K-omega SST turbulence model is more suitable for this problem.
Animation Vedio : Temperature Distribution for refined case
Verify the HTC predictions :
HTC -Heat transfer coefficient is a quantitative characteristic of a convective heat transfer between a fluid medium and the surface(wall) flowed over by the fluid .
Heat transfer coefficient can be verified by using ‘Nusselt number’.
Nusselt number is given by :
where ,
D is the inside diameter of the circular duct
Pr is the Prandtl number
N=0.4 for fluid being heated and n=0.3 for the fluid being cooled.
Nu = Nusselt’s Number
Re = Renold’s Number
Also , Nu = Convective heat transfer / Conductive heat transfer
We know that , Nusselt’s number is a function of Reynold’s number, which in turn is a function of flow velocity.
Therefore, HTC is directly proportional to flow velocity.
From the above results and images ,we can say that both the HTC and velocity are maximum near the outlet curvature.
Also , when velocity increases , heat transfer coefficient will also increases.
Hence , the HTC predictions from the simulation are correct.
Factors affecting the HTC predictions are as follows :
share topolygy – Mesh should be conformal for smooth transferring at solid-fluid interface
Selection of right viscous model.
Inflation layers – It increases the quality of mesh near the boundary interface.
No. of elements – Finer mesh gives more accurate results.
REFERENCES :
http://static.dergipark.org.tr/article-download/cbcf/ed95/9f5d/5dea5b38b0eb6.pdf?