Week 4 - CHT Analysis on Exhaust port
Introduction
Conjugate Heat Transfer (CHT)analysis allows for the simulation of heat transfer between solid and fluid domains by exchanging thermal energy at the point of intersection. A typical application exists but is not limited to the simulation of heat exchangers and general purpose heating and cooling systems.
Contemporary CHT model was developed after computers came into use in order to substitute the empirical relations of the proportionality of heat flux to the temperature difference with what transfer coefficient which is the only tool in theoretical heat convection since the times of Newton.
Why Cht?
The CHT approach has an advantage over Finite element thermal analysis because 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
Aim
To carry out CHT Analysis on the Exhaust port
Geometry Setup
- First, the model is to be loaded in the Space claim, and units are to be changed to meters.
- Duplicate edges are to be removed and then fluid volume is extracted and inner and outer are named fluid and solid respectively.
- Enable the shared topology so that the mesh and other information can be shared.
- To achieve heat transfer between the fluid and wall we have to choose share prep under the workbench tool so that the heat transfer is captured throughout the wall from in to outlet.
Mesh Set-up
- Select the required faces and name them accordingly.
- The name selection for fluid volume is to be created so that the inflation layer can be achieved on the body since CHT is between fluid volume and solid wall.
Fluent-Setup
- 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 the constant value of the heat transfer coefficient as 20.
- Under materials, choose air as the fluid material and aluminum as the solid material.
- Enter the reference value as per the geometry and inlet boundary conditions.
- Initialize the hybrid initialization and the simulation for about 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
1. Temperature Distribution
2. Residual plot
3. Surface heat transfer coefficient
4. Wall heat transfer coeffiecient
5. Temperature and velocity contour
Velocity streamline
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
Temperature distribution
Residual plot
Surface heat transfer coefficient
Wall heat transfer coefficient
Temperature and Velocity coefficient
Velocity streamline
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
Temperature distribution
Residual plot
Heat transfer coefficient
temperature and velocity contour
| Cases | Viscous Model | No. of Element | Surface HT coefficient | Wall HT Coefficient |
| 1 | K-epsilon | 238216 | 27.94 | 205 |
| 2 | K-epsilon(Refined) | 332433 | 26.63 | 194.2 |
| 3 | K-Omega | 362257 | 24.83 | 244.8 |
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=
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.
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 the 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 the wall, which is the Heat transfer coefficient in our case.