Conjugate heat transfer flow simulation using CONVERGE

SITUATION

        The flow of air through a solid pipe & it is heated through an external heat source in converge studio and post-processes the results in Para View and CYGWIN was used to run the commands for simulation and converting the files for the post-processer. 

TASK

         The simulation is to be run for three different cases with different super cycling values and three different mesh size as shown below.

        Case 1 : dx = 0.0025m,     dy = 0.0025m,      dz = 0.0025m.

        Case 2 : dx = 0.004m,       dy = 0.004m,        dz = 0.004m.

        Case 3 : dx = 0.008m,       dy = 0.008m,        dz = 0.008m.

        Set the supercycle stage interval to 0.01,0.02 and 0.03 by using 0.004 as the base grid for the ease of the simulation.

Run grid dependence test that the outlet temperature converges to a particular value and also for the different value of the supercycle stage the total time vs baseline configuration.

Conjugate Heat Transfer

        It is used to describe processes that involve variations of temperature within solids and fluids, due to thermal interaction between the solids and fluids (i.e) when a heat transfer occurs simultaneously within and also between fluids & solid regions. Converge solves the fluid heat transfer using an energy equation and solid heat transfer using a solid heat equation.

Super cycling 

In general, the heat transfer in the solid occurs more slowly than the convective and diffusive time-scales dictated by the fluid flow in the cylinder. This disparity in time-scales is problematic for CFD simulation because each cycle can be computationally expensive.

CONVERGE offers an alternative approach called super-cycling for overcoming the time-scale disparity. In super-cycling, the fluid solver is frozen periodically while the heat transfer in the solid is allowed to progress to steady-state. CONVERGE solves the fluid and solid phases in the same simulation without requiring you to stop and restart the simulation. Super-cycling significantly reduces the computational cost of the CHT simulation. This uses time-based spatially averaged temperature and heat transfer coefficients to perform steady-state heat transfer calculations – effectively allowing the simulation of the solid materials and the fluids to run on two different timescales concurrently.

ACTION

Workflow for a CONVERGE CFD Simulation

   i) Pre-processing(preparing the surface geometry and configuring the input and data files).

   ii) Running the simulation.

   iii) Post-processing(analysing the *.out ASCII files in the Case Directory and using a visualization program to view the information in the post*.out).

Pre-processing :

          File --> Import --> import STL file, this option is used to import the conjugate heat transfer file.

Boundary dialog box

Model

Select on the Normal toggle option and direction on the normal must be in the direction of the flow of air. The CONVERGE is mainly developed for running the IC engine simulation & other types of flow simulation are considered to be General flow. So General flow is selected.

Since there is an interface surface between solid and fluid in the boundary setup once the interface type is selected then the error of the non manifold problem gets eliminated. 

1. Materials – Select Air as a predefined mixture & unselect reaction mechanism for fluids. For the select the solid simulation as the pipe used is solid.                                                                i) Solid material- select aluminum as the material.                                              ii) Species – Solid – Aluminium.

                                                 Gas – O2 & N2.                                                                                     

1. Simulation Parameter – i) Run Parameters – Transient solver is chosen.                                                          ii) Simulation time parameter – start time – 0.

                                                                                        end time  - 0.5.

                                                                                        Initial time step – 1e-7.

                                                                                        Min time step – 1e-7.

                                                                                        Max time step – 1.

     3. Regions – i) Add a Fluid region & select air under the species area.                                                     ii) Add Solid region & provide the stream ID as 1 & select solid box.

1. Physical models – i) Turbulence modeling - Select RNG k-e model is chosen.                                            ii) Super cycling – Begin storing – 0.05.

                                                             Time length – 0.03.

                                                             Conduction CFL – 100.

                                                             Relaxation factor – 1.4.

                                                           Output points – Measure -->location --> Avg of vertex, to capture the data inside the geometry.

1. Grid control – i) Base Grid – the value of grid depending on the case.

                  Re = (rho*V*D)/(Dynamic viscosity)                                                                              where Re = 7000.                                                                                                                       rho = 1.225 kg/m3.                                                                                                         D = 0.03m (internal dia of pipe since Reynold number is a fluid property).                             D_Visco = 1.81*10^-5 kg m/s.

                           V = 3.45 m/s.        

    6. Boundary –       i) Inlet –  Region – Fluid.

                                              Type – inflow.

                                              Pressure B.C – Zero normal gradient (N.E).

                                              Velocity – 0            0        3.45

                                              Species B.C’s – Air.

                                ii) Outlet – Region – Fluid.

                                                Type – outflow.

                                                Pressure B.C – 101325.

                                                Species B.C’s – Air.

                                iii) Solid outer wall  – Region – Solid.

                                                               Type – wall.

                                                               Surface movement – Slip.

                                                               Temp B.C – Heat flux.

                                                               Heat flux - -10000 W/m2 (-ve sign is due to the flow direction)

                               iv) Solid Thickness  – Region – Solid.

                                                              Type – wall.

                                                              Surface movement – Slip.

                                                              Temp B.C – Zero Normal gradient(N.E).

                               v) Interface – Type – Interface.

                                                    Forward boundary – region – Fluid.

                                                    Reverse boundary – region – Solid.

                                                    Wall treatment – Slip.

                                                    Temp B.C – Specified value (DI) & validate it 

1. Output/post-processing – The Y+ plot is selected under turbulence and validate all the files under it. 

Y+ value

It is a ratio between turbulent and laminar influences in a cell, if Y+ is big then the cell is turbulent, if it is small it is laminar. The importance in many cases of this concerns wall functions which assume that the laminar sub-layer is within the first cell, if Y+ is small then the cell is totally laminar and the next cell in has some laminar flow in it, the wall functions are not applied to this cell and you make bad modeling assumptions. If Y+ is too big then you are not so bad with the laminar/turbulent problem but other assumptions are invalidated.

Running the Simulation

File --> export – To export the case set up for running the simulation using CYGWIN.

Mpiexec.exe -n 4 converge-2.3.26-msmpi-win-64.exe </dev/null>logfile & - The command is used to run the simulation using 4 processors & the following command is used to store the file in the name of the log file and & symbol is used to store the files in the background.

Output of Simulation

i) Mesh size - 0.008.

ii) Mesh size - 0.004.

iii) Mesh size - 0.0025.

iv) Super cycling - 0.01

v) Super cycling - 0.02

vi) Super cycling - 0.03

Results of the plot

Temperature plot at the outlet of the pipe.

i) Mesh size - 0.008

ii) Mesh size - 0.004

iii) Mesh size - 0.0025

iv) Super cycling comparision plot

Post-processing:

Provide a suitable case name & select the Paraview VTK inline binary format as the file type and enter the directory which contains the output files which are to be post-processed & then click all in the files & cell variables and click convert.

Open the Paraview application and open the file using File -- > Open -- > test..vtm.

Click apply & select the slice option & choose the z normal to divide the geometry along the z-axis.

Element mesh

i) Mesh size - 0.008

ii) Mesh size - 0.004

iii) Mesh size - 0.0025

RESULTS 

        From the grid dependence test, it is clear that the outlet temperature converges to a different value for different grid sizes, as the size of the element reduces the temperature value gets reduced. The super cycling stage increases with the decrease in the mesh size as the element size so the time for the converges increases. The next inference from the project is the effect of varying the time length of each stage of the super cycling, the value of the outlet temperature has no effect on time length and also the simulation time it depending mainly on the number of the solution it solves and other factors to it.