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STEADY STATE CONJUGATE HEAT TRANSFER SIMULATION THROUGH AN EXHAUST PORT OF AN IN-LINE 4-CYLINDER ENGINE USING ANSYS FLUENT AIM Our aim is to simulate a steady state fluid flow through an exhaust port of an in-line 4-cylinder engine to study the conjugate heat transfer using ANSYS FLUENT. THEORY/EQUATIONS/FORMULAE…
Ramkumar Venkatachalam
updated on 29 Jan 2022
STEADY STATE CONJUGATE HEAT TRANSFER SIMULATION THROUGH AN EXHAUST PORT OF AN IN-LINE 4-CYLINDER ENGINE USING ANSYS FLUENT
Our aim is to simulate a steady state fluid flow through an exhaust port of an in-line 4-cylinder engine to study the conjugate heat transfer using ANSYS FLUENT.
ANSYS FLUENT academic version CFD package is used to carry out the simulation. It is a user friendly interface which provides high productivity and easy-to-use workflows. Workbench contains all workflow needed for solving a problem such as pre-processing, solving and post-processing.
Structure of ANSYS FLUENT simulations
The basic steps for a simulation are as follows,
Conjugate Heat Transfer
Conjugate heat transfer is a combination of conduction and convection. It’s a heat transfer which involves the interaction of conduction within a solid body and convection between the solid surface and fluid volumes.
Typical example is a heat exchanger as in the figure the cold fluid enters the tubes and takes heat from the hot air flowing around the tube via natural convection. Some of the applications which involve conjugate heat transfers are building roofs, open water, chimney etc.
Heat Transfer Coefficient (HTC)
It’s a measure of convective heat transfer between fluid volume and solid medium around which the fluid flows.
Heat transfer coefficient is defined by the newton’s law of cooling. It is proportionality constant between heat flux (q) and temperature difference (ΔT) between the solid medium and the surrounding fluid. The SI unit of heat transfer coefficient (HTC) is watts per square meter kelvin (W/m2K).
For convective heat transfer coefficient calculation, usually T2 is temperature of the solid surface and T1 is temperature of the fluid around the surface or we can also call it as reference temperature. The choice of reference/ fluid temperature is important as the temperature near and away from the wall would be different depending on the flow due to thermal boundary layer.
There are two heat transfer coefficient for a flow through a pipe i.e. Internal HTC and External HTC
For External flows, fluid temperature will be the free stream temperature.
For Internal flow, fluid temperature will be the mass flow average temperature as the temperature profile inside a tube will be parabolic.
Nusselt Number
It is the ratio of convective heat transfer to the fluid heat conduction heat transfer under the same conditions.
Nu = qconvection /qconduction
Convective heat transfer, qconvection = h*ΔT,
where h = Convective heat transfer coefficient, ΔT = Temperature difference
Conductive heat transfer, qconduction = (k*ΔT)/ L,
where k = Thermal Conductivity of the fluid, ΔT = Temperature difference, L = characteristic length.
So, Nusselt Number, Nu = (h*L)/ k
Nusselt Number is also a function of Reynolds number and Prandtl number.
Dittus-Boelter equation – It’s an equation to calculate the Nusselt number for internal turbulent flow.
Exhaust Port - In-line 4 Cylinder Engine
Fig: Different views for Exhaust Port
Problem – Flow via Exhaust Port
Steady state internal flow simulation through an exhaust port to study the conjugate heat transfer for a flow velocity of 5 m/s
Calculation
Reference values – Exhaust Port with 4 Inlets and 1 Outlet
Cylinder Port Dia = 200 mm
Steady State Internal Flow Simulation
Fluid chosen for the problem – Air, Inlet Temperature – 700 K, Internal ref. temperature – 300 K (Chosen)
Density of Air = 1.225 kg/m3, Dynamic viscosity = 1.7894e-5 kg/ms, Velocity of fluid = 5 m/s,
So, Reynolds Number, Re = 68458.70
As the following analysis is done using two turbulence models i.e. k-omega & k-epsilon, the corresponding Y+ values are chosen.
Calculation of first cell height
For Y+ = 5 (For viscous sub layer, turbulence model = k-omega)
For Y+ = 50 (For turbulence layer, turbulence model = k-epsilon)
Skin Friction Coefficient, Cf
Cf = 0.058 * Re(-0.2) [External Flow]
Cf = 0.079 * Re(-0.25) [Internal Flow]
So, Cf = 0.00488 [using Internal Flow Eq.]
Wall Shear Stress, τw
τw = 0.5Cf ρu2
τw = 0.0747 N/m2
Frictional Velocity, Ut
Ut = (τw / ρ)(1/2)
Ut = 0.25 m/s
Cell Height, y
y = Y+ * ν
Ut
where y = 0.30 mm (for Y+ = 5) and y = 2.92 mm (for Y+ = 50)
Total Thickness Calculation for Inflation layers
We can see from the above table the total thickness value with the growth rate of 1.5 for both the Y+ values are well within the range, Hence can be used for the simulation.
Inputs
External HTC – 20 W/m2k,
External Reference temperature – 300 K,
Inlet Temperature – 700 K,
Internal Reference temperature – 300 K
Output – Internal HTC - ??
Dittus-Boelter equation can be used to find the Nusselt number for internal turbulent pipe flow.
where Re – Reynolds number, Pr – Prandtl Number, D – Inside diameter of the circular duct, n – 0.3(fluid being cooled) / 0.4 (fluid being heated)
By using Pr – 0.71 for air at 700 K , ReD = 68458.70, n = 0.4
NuD = 148.11
Calculate Heat Transfer Coefficient from Nusselt number formula
h = (Nu*k)/L
So, Internal HTC, h = 17.92 W/m2k
3. PROCEDURE
4. NUMERICAL ANALYSIS (Software used – ANSYS 2018 R1)
4.1 Geometric Model
The 3D geometry of Exhaust Port is imported in SpaceClaim and the cleanup is done as per the figure given below.
3D Geometry with the flow domain – Ahmed Body
4.2 Mesh
Fig: Baseline Mesh
Fig: Element Quality
Fig: Inflation Layers
Fig: Size Function and Inflation Layer
Fig: Final Mesh for the complete domain
4.3 Boundaries
Fig: Boundaries for the complete domain
Fig: Inflation Layer Boundary
4.4 Solver Set-up
4. Energy equation was switched on for the analysis process as we are interested in temperature of the system.
5. k-epsilon and k-omega turbulence models were used for the analysis as the Reynolds number used for simulation is in the range of 68458.70
6. The fluid material chosen is air.
7. The surface of the solid volume is chosen as Aluminum.
8. Convergence and monitor are checked for absolute criteria of 0.001 for all the residuals.
9. Solution methods – SIMPLE Scheme used for Pressure-Velocity coupling and the methods for Spatial Discretization are as per the below image.
10. Hybrid initialization is done and numbers of iterations are set for running the steady simulation.
11. Temperature contours are set in order to monitor the variation during run time and also animations are added.
12. Reference Values are set to 700 K and 300 K for k-epsilon and k-omega turbulence model respectively.
Initial Setup and Boundary Condition
Fig. Boundaries
Fig. Inlet Boundary - Velocity
Fig. Inlet Boundary – Temperature
Fig. Wall Boundary – Thermal conditions – Convection
5. RESULTS
Fig: Case 1 Fig: Case 2
Fig: Case 3 Fig: Case 4
Video Link (Exhaust Port Simulation to study Conjugate Heat Transfer) - https://youtu.be/PUWEoNpmpx4
Fig: Case 1 Fig: Case 2
Fig: Case 3 Fig: Case 4
Fig: Case 1 Fig: Case 2
Fig: Case 3 Fig: Case 4
Fig: Case 1 Fig: Case 2
Fig: Case 3 Fig: Case 4
Fig: Case 1 Fig: Case 2
Fig: Case 3 Fig: Case 4
Fig: Case 1 Fig: Case 2
Fig: Case 3 Fig: Case 4
Fig: Case 1 Fig: Case 2
Fig: Case 3 Fig: Case 4
Fig: Case 1
Fig: Case 2
Fig: Case 3
Fig: Case 4
Comparison Study of all Cases
6. CONCLUSION
7. REFERENCES
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