Steady State Simulations to Compare The Mixing Effectiveness
Objective:-
Steady state simulation of mixing tee and compare the mixing effeciency when hot inlet temperature is `36^oC` and cold inlet temperature is `19^oC` using ANSYS Fluent.
Theoritical Average Outlet Temperature
Formula
for momentum ratio=2
`T_{mixture}=24.667^oC`
for momentum ratio=4
`T_{mixture}=22.4^oC`
Geometry:-
Mixing tee is a flow control device. It has two inlet, one along x axis and another along Y axis. It has an outlet along Y axis. Hot inlet at along X axis and cold inlet is aong Y axis. We are
Boundary Conditions
- Case 1
- Short mixing tee with a hot inlet velocity of 3m/s.
- Momentum ratio of 2, 4.
- Case 2
- Long mixing tee with a hot inlet velocity of 3m/s.
- Momentum ratio of 2, 4.
Momentum ratio = velocity at cold inlet / velocity at hot inlet
Case 1
Meshing
Number of node=21,329
Number of Element=1,05,841
Element size=0.002
Mesh Quality
Mesh quality should be greater than 5%. Here, mesh quality is greater than 25%.
i) K-epsilon Realizable model
a) Momentum ratio =2 and K-epsilon Realizable model
The simulation converge around 140 iteration.
Standard Deviation at oulet is around 1.75.
Average temperature at outlet is around `30.27^oC`.
Result
Temperature Contour
Velocity Contour
Line plot of velocity
Line plot of teperature
b) Momentum ratio =4 and K-epsilon Realizable model
The simulation converge around 175 iteration.
Standard Deviation at oulet is around 1.2.
Average temperature at outlet is around `27.5^oC`
Result
Temperature Contour
Velocity Contour
Line plot for Temperature
Line plot for Velocity
Discussing the effect of momentum ratio for case 1 and K-epsilon model
From above graph, it can be observed that number of iteration required to converged depends upon momentum ratio. Number of iteration is higher for momentum ratio 4 and lower for momentum ratio 2.
ii)K-omega SST model
a) Momentum ratio =2 and K-omega SST model
The simulation converge around 140 iteration.
Average temperature at outlet is around `30.5^oC`
Standard Deviation at oulet is around 2.4.
Temperature Contour
Velocity Contour
Line plot for Velocity
Line plot for Temperature
b) Momentum ratio =4 and K-omega SST model
The simulation converge around 400 iteration.
Average temperature at outlet is around `27.5^oC`
Standard Deviation at oulet is around 1.68.
Result
Temperature Contour
Velocity Contour
Line plot for Temperature
Line plot for Velocity
Discussing the effect of momentum ratio for case 1
From above graph, it can be observed that number of iteration required to converged depends upon momentum ratio. Number of iteration is higher for momentum ratio 4 and lower for momentum ratio 2. Mixing effectiveness is higher for higher momentum ratio.
Comparing K-omega SST model and K-epsilon Realizable model
For momentum ratio=2, more oscillation can be seen in K-epsilon Realizable model than K-omega SST model but both model get converged at around 140. Average temperature is almost same for both the model.
For momentum ratio=4, K-epsilon Realizable model requires 175 while K-omega SST model requires 400 number of iteration. More oscillation with low amplitude can be seen in K-epsilon Realizable model while K-omega SST model has lesser oscillation and high amplitude. Average temperature is almost same for both the model.
Case 2
Meshing
Number of node=28,279
Number of Element=1,40,023
Element size=0.002
Mesh Quality
Mesh quality should be greater than 5%. Here, mesh quality is greater than 27%.
Selection of model for case 2
Various model were tested
K-omega SST model does not get converged in 300 iteration. This model required more number of iteration. Both K-epsilon Standard and Realizable model converge around 225 but amplitude of oscillation is more in case of K epsilon standard model. So, K-epsilon model is selected for simulation.
a) Momentum ratio=2 and K-epsilon Realizable Model
The simulation get converged at 200 number of iterations.
The standard deviation of temperature at oulet is 1.4.
The average temperature at outlet is `30.4^oC`
Result
Temperature Contour
Velocity Contour
Line plot for Temperature
Line plot for Velocity
b) Momentum ratio=4 and K-epsilon Realizable Model
The simulation get converged at 200 number of iterations.
The average temperature at outlet is `27.4^oC`
The standard deviation of temperature at oulet is 0.82.
Result
Temperature Contour
Velocity Contour
Temperature Plot
Velocty Plot
Discussing the effect of length for K-epdilon Realizable model
More no. of iteration is required for longer pipe to get converged solutions. Mixing effectiveness is high for longer pipe as it get sufficient time for mixing before coming out from oulet.
Table for Cell count, Average outlet temperature and Number of iterations for convergence
Mesh independent study for short mixing tee:-
Geometry with element size of 0.0015, 0.0025 and 0.005 are simulated for mesh independent study.
fig :- Element size=0.0015 fig:- Element size=0.0025
fig:- Element size=0.005
Simulation
fig :- Element size=0.0015 fig:- Element size=0.005
fig. Element size=0.0025
Number of iteration required to get converged are almost same for all of three different element size.
Temperature contour
fig:- Element size=0.0015 fig:- Element size=0.0025
fig:-Element size=0.005
Velocity Contour
fig:- Element size=0.0015 fig:- Element size=0.0025
fig:- Element size=0.005
From velocity and temperature contour we can obseve that as the elemet size of mesh decreases less diffused and distinct contour at mixing of two stream can be obtained.