CHT Analysis on Exhaust port (Ansys Fluent)

Objective: To carry out CHT Analysis on the Exhaust port

Give a brief description of why and where a CHT analysis is used.

Conjugate Heat Transfer(CHT):

The Conjugate Heat Transfer (CHT) analysis type allows for the simulation of heat transfer between solid and fluid domains by exchanging thermal energy at the interference between them. A typical application of this analysis type exists as but is not limited to the simulation of heat exchangers, cooling of electronic equipment, and general-purpose cooling and heating systems.

The contemporary conjugate convective heat transfer model was developed after computers came into wide use in order to substitute the empirical relations of proportionality of heat flux to temperature difference with heat transfer coefficient which was the only tool in theoretical heat convection since the times of Newton. This model based on a strictly mathematically stated problem describes the heat transfer between a body and fluid flowing over or inside it as a result of the interaction of two objects. The physical processes and solutions of the governing equations are considered separately for each object in two subdomains. Matching conditions for these solutions at the interface provide the distributions of temperature and heat flux along with the body flow interface, eliminating the need for a heat transfer coefficient.

Why CHT analysis is used:

• CFD and more specifically CHT analysis can accurately predict heat transfer by simultaneously solving all the relevant solid and flow field heat transfer processes, for e.g, conduction through solids, free and forced convection in the gases/fluids, and thermal radiation.
• The CHT approach has an advantage over Finite element thermal analysis in that wall heat transfer coefficients and their local variations on surfaces are directly calculated within the model rather than based on simplified empirical calculations. The CHT approach, therefore, has benefits for these applications where heat transfer is either non-uniform or difficult to calculate empirically.

Where is CHT analysis used:

Starting from simple examples in the 1960s, the CHT method has become a more powerful tool for modeling and investigating natural phenomenon and engineering systems in different areas ranging from aerospace and nuclear reactions to thermal goods treatment and food processing, from the complex procedure in medicine to atmosphere/ocean thermal interaction in metrology, and from relatively simple units to multistage, nonlinear processes. 

Applications:

• Internal Combustion Engines
• Electromobility
• Pumps and Compressor
• After-treatment Systems

Geometry Set-up:

• Load the model into SpaceClaim and change the units into meters.
• Remove the duplicate edges in the model.
• Then extract the fluid volume and give the name inner volume as fluid and outer volume as solid and remove the empty component.
• Enable the shared topology of the main component so that the mesh and physics information can be shared conformal mesh can be achieved.
• Lastly, to achieve conjugate heat transfer between the fluid and wall we have to choose share prep under the workbench tool so that heat transfer is captured throughout the wall from inlet to outlet.

Mesh Set-up:

• Select the faces and name them accordingly.
• Create the name selection for fluid volume so that the inflation layer can be achieved on the body since CHT is between fluid volume and solid wall.
• Create body sizing on the outer wall convection region to achieve the mesh refinement and accurate results can be captured at that region.

Fluent Set-up:

• After updating the mesh open fluent setup, select a pressure-based solver using the steady-state time simulation and absolute velocity formulation under the general settings.
• Under the physic choose the energy option as we are dealing with temperature properties.
• Input the boundary values, 5 m/s as inlet velocity at 700K. Set the outer wall convection boundary to convection type and input constant value of heat transfer coefficient as 20.
• Under materials, choose air as the fluid material and aluminum as solid material.
• Enter the reference value as per the geometry and inlet boundary conditions.
• Initialize the hybrid initialization and the simulation for 300 iterations.

CASE 1: Baseline Simulation

Element size: 150mm

Sizing Element size: 50mm

No. of Element: 238216

Viscous Model: K-epsilon

Inlet Velocity: 5m/s

Inlet Temperature: 700K

Heat transfer coefficient: 20 `W/(m^2K)`

1. Temperature Distribution:

2. Residual Plot:

3. Surface Heat Transfer Coefficient:

4. Wall Heat Transfer Coefficient:

5. Temperature and Velocity Countour:

6. Velocity Streamline Plot:

CASE 2: Refined Simulation

Element size: 100mm

Sizing Element size: 20mm

No. of Element: 332433

Viscous Model: K-epsilon

Inlet Velocity: 5m/s

Inlet Temperature: 700K

Heat transfer coefficient: 20 `W/(m^2K)`

1. Temperature Distribution:

2. Residual Plot:

3. Surface Heat Transfer Coefficient:

4. Wall Heat Transfer Coefficient:

5. Temperature and Velocity Countour:

6. Velocity Streamline Plot:

CASE 3: Refined Simulation

Element size: 100mm

Sizing Element size: 20mm

No. of Element: 332433

Viscous Model: K-Omega

Inlet Velocity: 5m/s

Inlet Temperature: 700K

Heat transfer coefficient: 20 `W/(m^2K)`

1. Temperature Distribution:

2. Residual Plot:

3. Surface Heat Transfer Coefficient:

4. Wall Heat Transfer Coefficient:

5. Temperature and Velocity Countour:

6. Velocity Streamline Plot:

How would you verify if the HTC predictions from the simulations are right? On what factors does the accuracy of the prediction depend on?

According to Bernoulli's mass conservation principle mass is conserved in our model which is clearly explained below;

Let us assume the flow rate from each inlet as Q1, Q2, Q3, Q4 respectively, and Q5 be the flow rate from the exhaust manifold.

As mass is neither created nor destroyed, mass flow rates are also balanced as below;

Q1+Q2+Q3+Q4=Q5     ...........(1)

we have Q= `rho*A*V` ...........(2)

Now equation 1 becomes

A1V1+A2V2+A3V3+A4V4=A5*V5

As it is an incompressible flow to compensate for mass flow, velocity is increased. and the overall mass flow rate is conserved.

As velocities are high, the Reynolds number at the outlet manifold will be high.

Nu=f(Re,Pr) for forced convection application.

Nu= `(hD)/k`

from above formula Nu`directly prop` Re. Hence Nu`prop`h, therefore, Re`prop`h.

From our simulation, we have observed velocities are high at bend section of outlet manifold hence higher Reynolds number which was at high heat transfer coefficient. Therefore predictions from our simulations can be considered correct.

Factors that influence accuracy of predictions:

• Mesh quality greatly impacts the accuracy of prediction value in our simulations which can be observed from above contours and maximum values of related parameters like velocity and heat transfer coefficient.
• An appropriate turbulence model has to be used in accordance with the y+ value which is very important.
• Appropriate y+ value helps in calculating total layer thickness from the wall for the better capturing of effects near wall, which is Heat transfer coefficient in our case.

References:

1. https://skill-lync.com/knowledgebase/choosing-a-right-turbulence-model
2. https://skill-lync.com/knowledgebase/boundary-layer-y-plus-wall-functions-in-turbulent-flows-2