Week 1- Mixing Tee

ANSYS FLUENT-CHALLENGE 1

AIM

• To conduct a CFD simulation of air-mixing inside a Mixing Tee

INTRODUCTION

• Air is passed through a mixing tee with two inlets and one outlet
• The mixing tees are of diffeent length
• The air inlet velocity is changed in two cases
• The effect of mixing tee lengths and velocity ratios needs to be studied

THEORY

• These are simple structures used in industries for mixing of subtances (mostly liquids)
• The mixing can be for thermal gains or chemical mixing
• The application can be seen in Automobiles, petrochemical industries etc.
• In HVAC we can see its use in mixing fresh air with the re-circulated air
• In Petroleum industries, they are used to infuse chemicals and steams into pipelines for various processes

CASE 1

• In the first case we take the short mixing-tee
• The hot inlet in x-direction is at a temperature of `36^o`C and a velocity of 3 m/s and Cold inlet in y-direction is of `19^o` C and 6 m/s (momentum ratio 2)

Length : 192.37 mm

Diameter : 33.87 mm

• The working domain is chooses as Fluid Flow (Fluent) as it captures the fluid flow analysis the best
• Then for geometry editing, we use SpaceClaim
• We capture volume of fluid interations using volume extract tool and with the help of edge selection
• Then we supress the shell physics from the analysis

• The next step we close the SpaceClaim and we go to the mesh tool
• In mesh we go for a mesh size of 2.5mm
• To check the mesh, we use section plane, we click and drag to cut the geometry to have a check on the meshes
• To generate the mesh, click generate

Nodes: 12580

Elements: 61186

• Then we click update mesh to use the meshed geometry for further analysis
• Add names to different faces so as to give Boundary conditions to the faces
• With this we have completed the Pre-Processing

Solution

• In the Workbench we then go for the solution, which launches the Fluent
• Turn on the double precesion and start with 4-processor
• Firstly we check for the mesh
• Then we select pressure based solver (Mach No < 0.3, so incompressible fluid fow), absolute velocity formulation and Steady State analysis
• In the physics we select energy as temperature is an input for the analysis
• In the Viscous-Turbulent Model, we select k-epsilon(2-eq) model
• Add air as the material
• In the cell zones add air as the volume region
• And the next step is to add Boundary conditions
• Give the necessary Boundary conditions to both the inlet
• In Solution ----> Definitions (Report tab) ---> New ----> Surface Report ----> Area Weighted Average and Standard Deviation and apply it
• Initialize and then give 350 iterations and give calculate

• The solution stage is now completed
• The residuals obtaines, we need to make sure stead-state is achieved or not, continue the iteration till staedy-state is achieved
• If solution hasnt attained convergence, we should increase the iterations

Residuals

Area-Weighted Average of Velocity magnitude at Outlet

Area-Weighted Average Temperature at outlet

Standard Deviation of temperature at Outlet

Post-Processing (Momentum Ratio 2)

• X-Inlet Velocity = 3 m/s
• Y-Inlet Velocity = 6 m/s
• First we increase the transparency of the body in the results/post-processing
• Then we insert a plane in xy-plane with z=0
• In colors we give the colour as variable temperature and we get the temperature distribution in the plane created
• Solution is not converged even in 550th iteration

Post-Processing (Velocity Ratio 4)

• X-Inlet Velocity = 3 m/s
• Y-Inlet Velocity = 12 m/s
• Attained convergence in 175th iteration

Post Processing - k-omega (2 eq) || VR = 2

• Here the solution has attained convergence in 258th iteration
• In k-omega model the solver stops the iteration once the convergence criteria is attained and the convergence criteria is found to achieve at a faster rate

Post Processing - k-omega (2 eq) || VR = 4

• X-Inlet Velocity = 3 m/s
• Y-Inlet Velocity = 12 m/s
• The solution is converged in 138th iteration
• Steady state is attained at a faster rate when the velocity ratio is increased

Comparison of k-epsilon and k-omega with different velocity ratio for short mixing-tee

VR	k-epsilon || Velocity	k-epsilon || Temperature	k-omega || Velocity	k-omega || Temperature
4
	Converged at 175th iteration		Converged at 138th iteration
2
	No Convergence at 2000th iteration		Converged at 258th iteration

COMPARISON

• As we increase the velocity ratio, solution gets converged faster
• As the VR is increased, the magnitude of temperature and velocity has been more uniform
• As we compary k-epsilon and k-omega, the solution is attained at a faster rate, 175th iteration to 138th iteration for a velocity ratio of 4
• From this, it cam be concluded that for the particular flow condition, k-omega model helps in better solution at a faster computational time

Case 2 - Long Mixing Tee

Length : 268.33 mm

Diameter : 33.87 mm

Volume Extraction - SpaceClaim

Meshing

Mesh Size : 2.5 mm

Nodes : 17624

Elements : 86711

Solution || VR=2

• The solution is converged at 163rd iteration

Post Processing - k-omega (2 eq) || VR = 2

• X-Inlet Velocity = 3 m/s
• Y-Inlet Velocity = 6 m/s
• The solution is converged in 163rd iteration
• As the lenth of the pipe is increased, the solution attained convergence earlier from 258th iteration to 163rd iteration

Temperature Distribution

Velocity Distribution

Solution || VR=4

• The solution attained convergence at 575th iteration

Post Processing - k-omega (2 eq) || VR = 4

• X-Inlet Velocity = 3 m/s
• Y-Inlet Velocity = 12 m/s
• The solution is converged in 575th iteration
• As the lenth of the pipe is increased, the solution attained convergence later due to the increase in temperature

Temperature

Velocity

Conclusion

• As we inclease the length of the pipe, the solution attained convergence at 163rd iteration for VR 2
• For VR of 4, the solution required more iterations and the solution attained convergence at 575th iteration, this may be due to more turbulence set by the higher velocity ratio
• In short the solution is greatly affected by the pipe length and velocity ratio