CHALLENGE 2 - MIXING EFFICIENCY

MIXING EFFICIENCY

Aim: To study the effectiveness of a mixing tee for different momentum ratios.

Introduction:

Mixing Tee is an application that is extensively used in applications such as air-conditioning, petrochemical industries, fuel mixing, wastewater management, etc. Usually, a tee is formed by two pipe sections joined at a certain angle or 90 degrees.

Objective:

This challenge is aimed at analyzing the effectiveness of mixing with respect to 3 cases with different geometries and momentum ratios.

Following is the description of cases under consideration:

Case 1: Short mixing tee with a hot inlet velocity of 3m/s.Momentum ratio of 2.

Case 2: Long mixing tee with a hot inlet velocity of 3m/s.Momentum ratio of 2.

Case 3:Short mixing with a hot inlet velocity of 3 m/s.Momentum ration of 4.

Whereas Momentum ratio = velocity at cold inlet/velocity at hot inlet

Methodology: The following methodology is applicable to all cases.

Stage 1: Importing the Geometry

1. Import the required geometry which is in the extension (.STEP) to the ANSYS SpaceClaim.
2. After generating the model to the platform we need to modify the existing model as we are concerned with the fluid flow. For that click on prepare option in the toolbar and select the volume extract and extract the volume through which the fluid is supposed to flow.

Fig 1: Short Tee Geometry used for cases 1 and 3.

Fig 2: Long Tee geometry used for case 2

Stage 2: Meshing the geometry

1. After importing the geometry head to ANSYS workbench and create a FLUENT session with the above geometry. Now, head to the meshing section and generate meshing (this is automated with default mesh sizes).
2. Create named selections such as inlets, walls, and outlet which would be helpful in the latter part of the analysis.
3. Chose mesh metric as element size as it would be helping in analyzing the mesh quality. It can be observed that meshing and mesh metrics for cases 1 and 3 are the same as they possess the same geometry.

Fig 3: Meshing for short Tee geometry used in cases 1 and 3.

Fig 4: Element quality for the meshing in cases 1 and 3.

Fig 5: Meshing for Long Tee geometry for case 2.

Fig 6: Mesh metric for Long Tee geometry for case 2.

Stage 3: Setting up boundary conditions (Pre-processing)

A parallel run of simulation with 4 processes is used for setting up physics and boundary conditions. The following settings are used for all three cases.

• Model: Pressure-based solver.
• The energy equation is turned on in order to capture temperature.
• Gravity is set to -9.81 m/s^2 as it is directed along -ve y-axis.
• Turbulence model: k-e with the realizable model.
• Inlet velocity: 3 m/sec in every case for hot air at 25 C. Cold air is sent at 10 C and velocity is computed from momentum ratio.
• Walls are designated wall boundary conditions by default. Similarly, the outlet boundary condition is set with gauge pressure as zero.
• The standard deviation of outlet temperature and the weighted average temperature at the outlet is computed from the monitors and are plotted accordingly.
• Standard initialization is done and 200 iterations are set for simulation in each case.

Stage 4: Results and observations (Post-processing)

Fig 7: Residuals for case 1 for a simulation run of 200 iterations.

Fig 8: Residuals for case 2 for a simulation run of 200 iterations.

Fig 9: Residuals for case 3 for a simulation run of 300 iterations.

Fig 10: Standard deviation of temperature at the outlet for case 1.

Fig 11: Standard deviation of temperature at the outlet for case 2.

Fig 12: Standard deviation of temperature at the outlet for case 3.

Fig 13: Area-weighted average of temperature at the outlet for case 1.

Fig 14: Area-weighted average of temperature at the outlet for case 2.

Fig 15: Area-weighted average of temperature at the outlet for case 3.

Fig 16: Temperature contour at the midplane parallel to the XY plane for case 1.

Fig 17:Temperature contour at a midplane parallel to XY plane for case 2.

Fig 18: Temperature contour at a midplane parallel to the XY plane for case 3.

Fig 19: Velocity contour at a midplane parallel to the XY plane for case 1.

Fig 20: Velocity contour at a midplane parallel to the XY plane for case 2.

Fig 21: Velocity contour at a midplane parallel to the XY plane for case 3.

Observations:

• It can be observed that from the above simulations is that the momentum ratio plays an important role in obtaining the desired effect of reduced temperature at the outlet.
• The length of the pipe has got no significant effect in reducing the temperature rather it would result in wastage of material.
• So, for improving effectiveness one must consider increasing the momentum ratio.
• Maybe it is too early to judge the effectiveness with a coarse mesh and a lesser number of iterations. So, one can refine the mesh and increase the number of iterations to check the convergence.