Conjugate Heat transfer simulation by CONVERGE CFD

Title: Conjugate Heat transfer simulation by CONVERGE CFD

Objective:  1. Preapare model of hollow pipe with thickness & Setup in CONVERGE Studio.

                 2. Grid indepance test & see effect on pressure, temprature & velocity 

                 3. Effect of supercycle stage interval on computation time & cycle

Theory :

What is CHT analysis

         Computaational fluid dynamic (CFD) and more specifically conjugate heat transfer (CHT) analysis can accurately predict heat transfer by simultaneous solving all the relevant solid and flow field heat transfer processes, for example: conduction through solids, free and forced convection in the gases/fluids and thermal radiation.

        Conjugate heat transfer corresponds with the combination of heat transfer in solids and heat transfer in fluids. In solids, conduction often dominates whereas in fluids, convection usually dominates. Efficiently combining heat transfer in fluids and solids is the key to designing effective coolers, heaters, or heat exchangers. Forced convection is the most common way to achieve high heat transfer rate. In some applications, the performances are further improved by combining convection with phase change.

Problem understanding

     for this challenge hollow pipe with thickness of 0.005m is provided on outer side Aluminium material is provided. the heat flux -1000w/m2k is provided on solid wall through this it transfer to interface & from this it transfer to fluid region (air). 

      For inlet turbulent flow is considered with Reynold No 7000 & velocity is 3.71m/s

Inlet velocity calculation 

`Re= (rho.v.D)/(mu)`

Re is given 7000

D=0.03m

density of air=1.161kg/m3

dynamic viscosity of air= 1.85 Ns/m2

`V= (Re. mu)/(rho.D)`

V=3.71m/s its used for inlet of velocity 

Geometery 

Geometery create 

1. Prepare fluid region

2. Solid Region

3. Delete the intersections

Repari>delete>triangle>Any angle 20

4. Loft the for create the solid region & interface

Create>triangle>loft edge>lof edge by arc

5. Identify the inlet & outlet by patch

Repair>patch>list of edge by arc

             

CASE SETUP

1. Application type> Time based

2. Material>air 

• Turn on Gas simulation & solid simulation (Aluminium material select)
•  Species for Air> N2 & O2     For Solid > Aluminium

3. Simulation time parameter

• solver> Transient , Simulation mode> Full hydrodynamic 
• Start time>0 sec , end time>0.5sec
• Initial & minimum time step=1e-7
• Maximum time step=1
• Navier stroke solver> Density based

4. Boundry name & Region name

1.Solid wall

heat flux apply on solid material -ve sign indicate heat transfer in to the solid body

2. solid thickness

3. Inlet

inlet velocity provide is 3.71m/s

4.outlet

for outlet static press. provided

5. Interface

interface where two region is created for forword side is liquid region & reverse side is for solid side 

 

5. Region & Intialization

1. liquid Region>Pressure>101325pa & material>air

2.Solid region> material>Aluminium

6.Physic model

RNG>K-Epsilon model

7.Super cycling modeling

    The basic idea of super-cycling is that the solver for the fluid domain is paused until the solver for the solid domain converges. This pausing is done in intervals that can be set by the user 

8. Base grid >0.004m for grid dpendance test 0.003m ,0.002m

9.post variable selection > Y+criteria

10 0utput file

Grid Depedancy test

for grid depedancy test 0.004, 0.003, 0.002m mesh size is uses

Mesh 

Mesh size: 4e-3m

                   

Mesh size: 3e-3m

                    

Mesh size: 2e-3m

                    

CASE 1: 4e-3m

Temprature contour

for solid outer wall -1000w/m2k Heat flux & inlet velocity is 3.71 m/s is provided , so we can see that on outer side around 860 degree celcius temp. at the interface of outlet side is 500 degree celcius. 

 

Velocity contour

As we provide the inlet velocity is 3.71m/s , but as air is pass from left to right side velocity is increases & which is higher up to 5m/s its due to the lower static pressure at outlet 

  

Pressure contour

As at inlet neumann Boundry condition is provide for Pressure . so, we can see that at the inlet higher pressure up to 1bar & lower pressure at outlet.

   

Y PLUS

y + criteria is important for capture the good result nearest the solid wall boundry. basically y+ crtieria is decide the mesh should be fine or coarse near wall boundry. if y+ is in the range of 0-10 we can capture laminar flow. if y+ is in the range of 30-100 we can capture the turbulent flow.

   

CASE II: 3e-3m

Temprature contour

due to mesh size is decrease  the y+ criteria is below the 10 , which is good for capture the temp. near the wall region . so, we can see that at the middel section temp is capture 

     

Velocity contour

As we provide the inlet velocity is 3.71m/s , but as air is pass from left to right side velocity is increases & which is higher up to 5m/s its due to the lower static pressure at outlet 

     

Pressure contour

As at inlet neumann Boundry condition is provide for Pressure . so, we can see that at the inlet higher pressure up to 1bar & lower pressure at outlet.

     

Y PLUS

     

CASE III: 2e-3m

Temprature contour

     

Velocity contour

     

Pressure contour

     

Y PLUS

Y+ is below the 7 . so, its good to capture the result near the wall surface.

     

Mean Temprature Solid Region

from the below plot we can see that as mesh size is decrease , it capture the correct temp for .002 m around 700 degree celcius the temp is steady state. There is main resaon for capture good result is due to refine mesh & y+ criteria is less than 10 which is good 

 

     

Mean Temprature Fluid Region

for fluid region the mean temp. is steady at 350 degree celcius. so, for mesh 0.002m capture good result

     

Total cell count

for mesh 0.002m there is highest total cell is 35500 because of as mesh size is decrease the domain size is divide in to the small cell. for mesh 0.004m there is lowest total cell is 5000

    

Temprature animation

                 

Pressure animation

                 

Velocity animation

                 

Grid depedancy test

from below table we can see that as mesh size is decrease the computation time , no. of cycle  & total cell is increase 

         

 

Supercycle stage

The basic idea of super-cycling is that the solver for the fluid domain is paused until the solver for the solid domain converges. This pausing is done in intervals that can be set by the user 

Mean Temprature Fluid Region

as supercycle stage is increase it reaches in to steady state early .

     

Mean Temprature Solid Region

as supercycle stage is increase it reaches in to steady state early .

     

Super cycle stage test

 from below table we can observe that as supercycle stage is increases  the computaton time & no. of cycle is decreases. as pause time increases for fluid region the solid region is easily converge which help for reduction of computation time.  

           

 

Overall conclusion

1. Conjugate heat transfer analysis is uses for analysis the heat transfer from solid to fluid through interface region.

2. Y+ criteria is very important in CHT analysis ,as it help for capture accurate temprature & heat transfer coeff. 

3. It is observed that as supercycle stage is increases the computaton time & no. of cycle is decreases

4. As mesh size is decrease the computation time , no. of cycle  & total cell is increase