Week 6: Conjugate Heat Transfer Simulation

                                                 CONJUGATE HEAT TRANSFER SIMULATION

AIM: to simulate a Conjugate Heat transfer simulation across a pipe.

OBJECTIVE:

Baseline configuration

• Setup a flow simulation through a pipe
• Inlet Reynolds number should be 7,000 
  • Re = (rho*V*D)/(Dynamic viscosity)
• Turbulence modeling - K-Epsilon
• Super cycling parameters - same as in the video

Additional 3D outputs

• Add Y+ to your post.in file

Grid dependence test

• Start with an initial base grid
• Run grid dependence test on 3 grids and show that the outlet temperature converges to a particular value

Effect of supercycle stage interval

• Set supercycle stage interval to 0.01,0.02 and 0.03
• For these three values, how does the total simulation time compare against the baseline configuration

THEORY:

Conjugate heat transfer:

Conjugate heat transfer is used to describe the processes which involve variations of temperature within the solids and fluids, due to thermal interaction between the solids and fluids. 

The CHT analysis is generally used in places where there is heat transfer taking place for example when a fluid flows through a pipe and the heat of the fluid is transferred to the pipe by convection. CHT analysis is very much useful in analyzing the areas where there are two bodies in which heat transfer takes place. when this analysis is used it is easy to calculate the heat transfer coefficient temperature and velocity of a fluid flowing through a certain body.

This type of heat transfer corresponds to the combination of heat transfer in solids and heat transfer in fluids. heat transfer in solids is called conduction and heat transfer in a fluid is called convection.

conduction occurs due to the direct molecular collision. the faster kinetic energy collides with the low-speed kinetic energy and the heat transfer occurs.

The convection process is not happening alone. it is combined with the conduction or radiation. it occurs mostly in the cooling process like Heat sink, Battery cooling, Airflow over the solid object for cooling.

SUPER CYCLING:

Super-cycling is a method used in converge studio for CHT problems with multiphase regions(solids and liquids). The solvers for different fluids cannot run at the same speed for solving fluid it is much faster as compared to solids. If, this is done the solution does not reach convergence. so, super cycling can be used to resolve this issue. This helps solver of fluid domain is paused until the solver for solid converges.

Y+:

 Wall functions are useful in telling the solver how to approach the solution near the wall. this helps in predicting the turbulence models for the coarse grid. no-slip condition is used if the flow is completely laminar that does not depends on grid size. For turbulent flows, the laminar region is very small to get accurate results we need a very small grid size near the boundaries. This is where the Y+ value helps. Y+ is a non-dimensional term that can be used to understand the grid is finer or course. This can be used to determine if wall functions are used or not. In viscous sub-layer Y+ is less than 10 and in a turbulent region, Y+ is greater than 30. 

SIMULATION AND SOLUTION:

In this project, we'll be running a total of 6 cases where 3 are based on mesh size and 3 are based on supercycle stage intervals

Geometry:

Our geometry is created with a base radius of 0.015 m and a thickness of 0.005 of a cylinder.

Setup: 

Application Type	Time-based
MATERIALS:
1. Gas simulation	Standard
2. Global Transport Parameters	Standard
3. Solid simulation
4. Species	1. fluid: O2 and N2  2. Solid Aluminium
SOLUTION PARAMETERS
1. Run Parameters      Steady-state monitor:	Transient >Full hydrodynamic>Compressible
2. Simulation time parameters
3. Solver parameters	Navis-stokes solver scheme: PICO Navis-stokes solver type: density-based
BOUNDARY CONDITIONS
1. Inlet	Where, Re = (rho*V*D)/(Dynamic viscosity)   Given: Re = 7000 rho = 1.161 kg/m^3   D = 0.03 m Dynamic viscosity = 1.845e-5 kg/ms substituting  velocity = 3.708 m/s
2. outlet
3. Outer wall
4. Side-wall
5. Interface
INITIAL CONDITIONS AND EVENTS
1. Regions and initialization
PHYSICAL MODELS
1 Turbulence modeling	RNG- k-epsilon
2. Super cycling
GRID CONTROL
1. Base mesh	CASE 1 dx = 0.004, dy = 0.004 and  dz = 0.004 CASE 2 dx = 0.003, dy = 0.003 and  dz = 0.003 CASE 3 dx = 0.002, dy = 0.002 and  dz = 0.002
2. Fixed embedding
OUTPUT/POST-PROCESSING
1. Post variable selection	Add Y+
2. Output files:

Then Export the Files

Then continuing the setup in Cygwin :

• leading to the folder where the exported files are saved
• Then using mpiexec.exe -n 2 converge.exe restricted to simulate the setup
• After the output is generated, we lead the Cygwin to the output folder, and by copying the post_convert folder and
• Using mpiexec.exe -n 2 post_convert.exe and converting these files for processable output in ParaView.

BASED ON MESH

CASE 1: 

 VELOCITY COUNTER

 PRESSURE COUNTER

 TEMPERATURE COUNTER

 YPLUS

CASE 2

 VELOCITY COUNTER

 PRESSURE COUNTER

 TEMPERATURE COUNTER

 YPLUS

CASE 3

 VELOCITY COUNTER

 PRESSURE COUNTER

 TEMPERATURE COUNTER

 YPLUS

BASED ON SUPER CYCLING:

CASE 1 : Super cycling intervel= 0.01

CASE 2:Super cycling intervel= 0.02

CASE 3: Super cycling intervel= 0.03

FLUID REGION

SOLID REGION

CONCLUSION:

• Refinement of grid results in clearer plots however at the expense of computational time
• Decreasing the SUpercycling time interval results in a reduction of computational time
• Irrespective of grid size, fluid values converge to a particular value.(380 K)