Exhaust Port Challenge

• Conjugate heat transfer (CHT) is the form where heat transfer occurs through convection, conduction , radiation or any two of them simaltenously. In real life CHT occurs almost everywhere in the industry where heat transfer occurs.
• For example in Condenser unit the heat is transfered from the fluid to the metallic pipes via convection and from that heat conducts to the atmosphere. Normally this will occur everywhere and depending on the condition insulation is provided for reducing the heat transfer (generally in mechanical cases) while in electronics purpose the major aim is to maximize the heat transfer.

The following steps were performed to perform the analysis.

• Geometry cleanup and Volume extraction
• Mesh generation
• Boundary condition setup
• Solving
• Post processing

Volume extraction and geometry details:-

The following geometry was used for the analysis.Name selection was done in Space claim itself by using Groups tool.

• Shared topology was enabled in order to perform conformal meshing in the mesh generating stage.

Mesh Generation:-

• For mesh generation ansys fluent with fluent meshing was used to mesh the geometry. Gambit meshing was causing certain errors during meshing and a better quality of mesh was performed by using ANSYS fluent with fluent meshing.
• The following polyhydra elements were created in the ANSYS Fluent meshing with minimum orthogonal quality of 0.20.

The mesh parameters for the finest mesh done is given below:-

Mesh details:-	Total mesh = 463320
Body size fluid (m)	0.015
Body size solid(m)	0.01
Surface mesh(m)	0.014min	0.017 max
Inflation layer	y+=3	0.0001044 m
volume mesh(m)	0.019

For other cases the y+ was varied in order to compare the results.

Cases	Y+	First layer thickness (m)	Mesh count
k omega SST	3	0.0001	464083
K EPSILON WITH ENHANCE WALL TREATMENT	3	0.0001	464083
K EPSILON WITH ENHANCE WALL TREATMENT	30	0.001044	333404
K EPSILON STANDARD	100	0.0034	316505
K EPSILON STANDARD	300	0.0104	279240

These models had to be studied for the wall heat transfer coefficent, Temperature, Velocity and Pressure contours.

For each case the minimum orthogonal quality was ensured to be more than 0.2. Due to limitation of student version the mesh size was so selected that it was always lesser than 512k counts.

Boundary condition setup :-

Pressure based solver was used with SIMPLE algorithm to solve the model. Different models were selected in order to compare them with varying y+ values. The details are given in previous table. Air was chosen as fluid and Aluminium was chosen as solid material.

Fluid/ Solid	Density (kg/m^3)	Thermal Conductivity (W/m.K)	Dynamic Viscosity (kg/m.s)	Inlet Velocity (m/s)	Inlet Temperature (K)	HTC (W/m2.K)
Air	1.225	0.0242	1.7894 e-5	5	700	22
Aluminium	2719	202.4

The following results were found during the analysis.

Results and discussion:-

• The temperature, velocity and pressure were foud to be almost independent of the mesh size and were varying in minor limits. The highest variation was seen in Wall Heat transfer co-efficient with change in y+.
• The y+ greatly affects the wall heat transfer coefficient as shown in the results.

Cases	Y+	First layer thickness (m)	Mesh count	Wall heat transfer coefficient (W/m2K)
k omega SST	3	0.0001	464083	495.346
K EPSILON WITH ENHANCE WALL TREATMENT	3	0.0001	464083	498.646
k omega SST	15	0.0005222	344705	124.943
K EPSILON WITH ENHANCE WALL TREATMENT	30	0.001044	333404	81.24
K EPSILON STANDARD	100	0.0034	316505	45.687
K EPSILON STANDARD	300	0.0104	279240	37.83

• With increase in the y+ values the detoriation of Wall HTC began. This is majorly because of the in effectivness of the mesh to capture the turbulence. As indicated in https://skill-lync.freshdesk.com/support/solutions/articles/43000561288-what-is-yplus , it is a good practice to have the y+ less than 10 to accurately capture the physics of boundary layer.
• The Wall HTC for the finest mesh was found to be 495 W /m2K.
• This can be verified by the Dittus-Boelter for circular pipe with Reynolds Number higher than 10000.
• During calculation of reynolds number the velocity was considered to be 24.5 m/s by simple Mass conservation equation.

• In our case the Re = Rho x Velocity x ID of pipe/ Dynamic Viscosity , which comes out to be 250244.2.
• For conformation of the Wall HTC, using Dittus Boelter equation for turbulent flow in a circular pipe.

On equating Nusselet number by hd/k we can get the convective heat transfer coefficient (h).

• The another approach is to equate the heat transfer occuring inside the exhaust port. The convection always occurs on the expense of conduction in the fluid. A simple example is whenever we boil water we often see the bubbles coming upwards, this is due to bouyancy occuring due to density variation.
• During heating the adhesive forces between the molecules of fluid and the surface of the metal is strong and hence it stays stick to the surface and this results into conduction of the heat in the first layer and then convection inside the whole fluid domain.
• This first layer is estimated by the mesh in our case.