Exhaust Port Challenge

Introduction to conjugate heat transfer

• Conjugate heat transfer is ability to compute heat transfer; Coupling heat transfer in solid that is conduction in solid and it’s coupled with convective heat transfer in fluid that is convection.
• Example ,Consider fluid is flowing through circular tube with cold fluid enters and that tube having small thickness and that is placed in constant it flux or variable heat flux so that outlet temperature of fluids must be increased. So, whenever solid and fluid interaction happen that is all comes under conjugate heat transfer simulation. And yes it’s also to beneficial for predicting thermal stress.
• CHT used in - Heat exchanges such as boiler and condensers design, IC engine water jacket , aerospace industry for proper selection of material, house and building roof design, electronic chip design, coolant and gear interaction so that life of gear must guessed properly, IC engine exhaust port , in combustion chamber design and fins design ,oil and gas industries ,automotive radiator, activation pharmaceutical drugs in vines ,etc.
• Overall by doing this type simulation it will reduce cost and time in real wold so that more effective and efficient design find through computers.
• By doing this type of simulation analyst can visualize where heat transfer coefficient is more or less than that is comes handy while designing the parts of boiler, wings of aeroplane etc.

Exhaust port simulation

• Geometry
• Meshing
• Setup
• Post processing

1 geometry

Fluid zone below image shows the inlet dimensions there are four inlet

Fluid zone below image shows the outlet dimensions and that’s is only outlet

Below image shows solid zone

2- Meshing

M1 – element size 150 mm for both fluid and solid zone and for sizing capture curvature is on

M2- element size for 150 mm for fluid zone for sizing capture curvature is on below image shows M2 mesh

• Element size for 10 mm for fluid zone with curvature capture ticked
• Inflation layer is added on fluid zone for 5 layers so 5 layers on fluid sized and five on solid side

Total thickness of 5mm with increment of 1.2 percent.

3- Setup

• Steady-state pressure based energy solver is used and the k-e model is selected.
• For fluid zone air is flowing and solid zone as aluminium for their default properties provide by as software
• Inlet velocity of 5 /s and temperature of 700k
• Outlet as the atmosphere and outlet temperature as 300k
• Outer wall that is a solid thick wall is selected as a convention and heat transfer coefficient for free stream is 20 w/square meter and free stream temperature as 300k.

4- Post processing

Residuals of M1 and M2 mesh

for M2 mesh

Outlet temperature convergence of M1 and M2 mesh

for M2 mesh

Velocity stream for M1 and M2 mesh

Total pressure variation for M1 and M2 mesh

Wall shear on outer wall for M1 and M2 mesh

Temperature on xy plane 1 for M1 and M2 Mesh

Velocity on xy plane 1 for M1 and M2 Mesh

Wall heat transfer on plane 1 for M1 and M2 mesh

Surface heat transfer coefficient on solid outer convection wall for M1 and M2 mesh

Surface heat transfer coefficient for M1 and M2 mesh

Explanation all above contour plots

It can be observed that L log-shaped near outlet the pressure at the bent of L log is drop though it will withdraw more air that it will increased in velocity because of this wall heat transfer coefficient at that L shaped bend which is attached to outlet get increase; that means it will reduce temperature air which is flowing inside the L-log shaped exhaust port.

Below table shows different outcomes of M1 and M2 mesh size

Mesh size	M1	M2
	KE realizable enhanced wall	KE realizable enhanced wall	KE realizable enhanced wall ; option pressure gradient and thermal effect ticked and viscous heating and curvature correlation also ticked
Number of Elements	137926	473867	473867
Area Average of Wall Heat Transfer Coefficient on Plane 1	14.6205 [W m^-2 K^-1]	18.8835 [W m^-2 K^-1]	17.8806 [W m^-2 K^-1]
Area Average of Temperature on outlet	679.705 [K]	677.115 [K]	677.195 [K]
Area Average of Surface Heat Transfer Coefficient on outer_wall_convection	21.6115 [W m^-2 K^-1]	N/A	27.2266 [W m^-2 K^-1]
Reynolds number at only at one inlet v=5m/s d=0.1662m	5.6*10^4	5.6*10^4	5.6*10^4
Area Average of Velocity on outlet	25.3127 [m s^-1]	N\A	25.3213 [m s^-1]
Area Average of Total Pressure on inlet	477.216 [Pa]	N\A	530.518 [Pa]
Area Average of Total Pressure on outlet	398.793 [Pa]	N\A	403.715 [Pa]
Prandtl number air	0.74	0.74	0.74
Thermal Conductivity of the Fluid (k) air:	0.0242 [W m-k]	0.0242 [W m-k]	0.0242 [W m-k]

• A different method for the above simulation

1- Surpass solid domain of solid in space-claim

2- Than rename all boundaries add 5 inflation layer to fluid domain and name as outer_convection

3- Inlet as 5m/s and temperature as 700k and outlet temperature as 300k

4- For outer convection set as heat flux and select shell conduction of 5 layers re and provide thickness to it and select that thickness material as aluminium with no heat generation within that solid boundary now it may look like below image.

5- By running calculation it will give results.

Prediction of heat transfer coefficient and factors affecting to predict it-

• For RE 5.6*4*10^4 flow is turbulent so entry hydrodynamic entry length as 10D as 1.662m and our fluid is not travelling through this much longer and that is why fluid is in entrance length that is in a hydrodynamic layer; developing range.
• For smooth tube Nu_x = (h*L or Dh)/k=0.023*((Rex)^(0.8))*((Pr)^0.3) and that is correlated to resistive thermal resistance network. Also in our simulation nusselt number is 132.536609943; it gives Heat transfer coefficient as 19.3100 for at inlet diameter as 0.16661 and outlet diameter as 0.15 as 21.3826 And yes that is nearly same we have provided in a simulation that is 20 w/m/k { because we have provided heat transfer coefficient as 20 [w/m/k]
• Also, one can; to validate result dimensionless numbers may take in consideration, also sometimes we have to do real-world experiments and then validate the result.

• Mesh element size and mesh quality effect in calculating the wall heat transfer coefficient
• Y+ is also one of the important factors which have to select properly to calculate wall heat transfer coefficient.
• Adding sub-shell layers in pre-processor or in processor wherever solid-fluid interaction is happening is a very important step in CHT simulation or to predict wall heat transfer coefficient.
• Also, local shape and size of geometry have its own effect on all other fluidic parameters.

Conclusion

Hear in exhaust port simulation objective is to reduce the temperature of exhaust gases and to visualize the effect of how CHT simulation predict wall heat transfer coefficient, Hear it observe that wherever velocity of fluid gets increased it will increase the convective heat transfer coefficient.

Scope

• By selecting proper Y+ , also Perform this simulation by increasing number of inflation number for a same total length of layer and also adding some shell layers of air around the geometry then the interaction will be air(fluid zone 2)-solid (aluminium solid zone)-air (flow –fluid zone 1).
• Also by selecting the different turbulence models such as the k-omega SST model.