Mixing Efficiency

Aim : For the created versions of the mixing tee (i.e. short and long), setting up a steady state simulations to compare the mixing effectiveness when both hot inlet temperatures and cold inlet temperatures are given by using Ansys Fluent.

Given conditions :

• One tee is longer than the other.
• Hot inlet temperature is 36⁰C and the Cold inlet temperature is at 19⁰C.
• To use the k-epsilon and k-omega SST model for the test cases.
• 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.

• Also given that, Momentum ratio = velocity at cold inlet / velocity at hot inlet.
• To compare the test cases by :
  1. Cell count
  2. Average outlet temperature
  3. Number of iterations for convergence

To Find :

• Velocity and temperature contour plots on the cut planes along and across the pipe.
• Velocity and temperature line plots along and across the length of the pipe.
• Perform Mesh independent study for any one case.
• The effect of length and momentum ratio on results.

Introduction:

>> T-joints are important flow control devices. Mixing tee is bascially used for mixing two different flow streams to form the gradients of the combined flow streams.

>> Application of T-joint is mostly used in refregiration systems like Air conditioning system.

>> Here outside air comes through one of the inlets and chilled air comes from the other side.

>> By adjusting the flow rate of the chilled air we can get the desired temperature which is connected to the air conditioner.

>> Here a computational model is developed to investigate the mixixng effectiveness by changing the actual length of the mixing tee and the ratios of velocities of the two inlet flows streams i.e. hot flow and the cold flow.

>> Mixing of hot and cold fluid gives the desired temperature at the outlet. As given, the ratio of the cold flow velocity to hotflow velocity is defined as the momentum ratio of mixing tee.

Represented by:- Momentum Ratio = Inlet velocity of cold flow/Inlet velocity of hot flow.

>> In order to obtain the following results a 3D model or geometry is imported into ANSYS Workbench to simulate the flow.

K-epsilon Turbulence Model :

>> This model is used in CFD to simulate mena flow characteristics for turbulent flow conditions.

>> It is known as the simplest turbulence model for which only initial and boundary conditions needs to be supplied.

>> Relizable k-epsilon model exhibits sperior performance for flows involving rotation, boundary layers under strong adverse pressure gradients.

>> K-epsilon model is ideal for prediciting flow behaviour in regions away from the wall.

K-omega SST Turbulence Model :

>> SST stands for Shear Stress Transport.

>> This model can be used for boundary layer problems, where the formulation works from the inner part through the viscous sub layer untill the walls. Hence this model can be used as low reynolds flow applications without additional damping functions.

>> The k-omega SST model accounts for its good behaviour in adverse pressure gradientsand seperating flow. It produces large turbulence levels in regions with large normail strain. This effect is less with a normal k-epsilon model.

>> The SST models exhibit less sensitivity to free stream conditions (flow outside the boundary layer)than many other turbulence models.

Procedure :

• For geometry manipulations, Space Claim is used.
• Imported geometry by navigating to the location where geometry is stored. As in this case we are performing the flow simulation,so we require only the fluid volume. Hence rest can be elminated. Once the solid geometry is imported then go to PREPARE > VOLUME EXTRACTION > SELECT EDGES (select the following 3 edges of the fluid flow volume)> click TICK.
• In STRUCTURE > MIXING TEE SHORT > UNSELECT SHELL1(hide the solid component)> right click on the same and select SUPPRESS FOR PHYSICS(helps not to load the component for mesh generation). Now it gives required geometry for simulation.
• Now close the geometry part.
• Now double tap on MESH.It opens the mesh interface with geometry.
• ENTITY SELECTION > FACE SELECTION (as per our requirement) from toolbar > select the faces of the geometry to be mentioned as boundaries.
• Now name the boundaries that we have to get the perfect meshing.
• Click CTRL+F > select the Face > RIGHT CLICK on the face > select CREATE NAMED SELECTION or select the Face and use shortcut 'N' to name the FACE.
• Now box opens asking to enter the Structure Name > 'Name the face'. Similarly name all the required faces along the geometry.
• In my case I named them as- inlet-x, inlet-y, outlet, walls.
• Go to OUTLINE > click MESH > we can see the details of the mesh below the outline box.
• Right Click on MESH > GENERATE MESH. The obtained mesh is called as computational mesh.
• Initailly keep the geometry in Y-direction. Now by using SECTION PLANE BUTTON, click and drag at the center to foam a line, so that it cuts the geometry.
• In DETAILS OF THE MESH > select QUALITY > for MESH METRIC > select ELEMENT QUALITY. Now we can c a bar graph(or similar to histogram) showing Element Metric vs Number of Elements.
• After getting the graph, just go to DETAILS OF MESH > check ELEMENT SIZE > scroll down we find STATISTICS at the end, here check the NODES & ELEMENTS.
• It should be noticed that ELEMENT QUANTITY is not lower than 5%.
• If the above criteria is satisfied we can run the simulation successfully.
• Close the Mesh > double tap on the SETUP(opens fluent launcher) > FLUENT LAUNCHER opens. Select DOUBLE PRECISION. Here for doing simulation we have two options i.e. running by single processor (select SERIAL) or running by multiple processor (select PARALLEL) - [For parallel processors, specify the number of processors we want to use] > Press OK.
• Ansys Fluent is opened. In Graphics window > the geometry is shown and a CONSOLE window is displayed. In this window it displays the run time or errors during the simulation.
• Here we can see multiple options in the top menu as well as on the left hans side. Top menu bar is convient as wee have multiple operations clearly in a sequential order. Among them firstly go to SETTING UP DOMAIN > go to MESH > select CHECK. Then in CONSOLE > it shows 'CHECKING MESH…. DONE' that means mesh is ok.
• To Change the Units, go to SETTING UP DOMAIN > click Units and change.
• Go to SETTING UP PHYSICS > in SOLVER > select Time = STEADY, Type = PRESSURE BASED, Velocity Formation = ABSOLUTE.

• In SETTING UP PHYSICS > go to MODELS > select ENERGY. As the flow can be turbulent in this case, select VISCOUS MODEL > Model > select Viscous Model = K-epsilon Model, now select K-epsilon Model= REALIZABLE K-epsilon Model, Near-Wall Treatment = STANDARD WALL FUNCTIONS > press OK. Here the equations solved are the RANS(continuity &momentum) equations and the energy equations.
• To select material go to SETTING UP PHYSICS > add MATERIALS > in this case consider as AIR > press CLOSE.
• In SETTING UP PHYSICS > go to ZONES > CELL ZONES – this is set automatically.
• For assigning boundary conditions go to SETTING UP PHYSICS > select BOUNDARIES > we can see that a TASK PAGE opens. Here it shows all the zones i.e inlets, outlet, walls and interior-volume. Here click on the particular zone and click EDIT > then VELOCITY INLET window opens up showing Momentum, Thermal, Radiation etc.
• As per our consideration enter the values for Momentum, Thermal for both 'inlet-x' and 'inlet-y'. Then click OK.
• As initially we represented the outlet face as 'outlet', the GUAGE PRESSURE (PASCAL) = 0 is automatically entered > Click OK.
• Now move to SOLVING. In SOLVING > we have CONTROLS, REPORTS, INITIALIZATION, RUN CALCULATION.
• Firstly go to REPORTS > go to DEFINITIONS > NEW > SURFACE REPORT > select AREA WEIGHTED AVERAGE. Similarly, go to REPORTS – REPORTS > go to DEFINITIONS > NEW > SURFACE REPORT > select STANDARD DEVIATION. For both the cases, a 'SURFACE REPORT DEFINITION WINDOW'pops up > select the FIELD VARIABLE = Temperature/Pressure/velocity(as required for output ). For this case I have selected two different REPORT TYPE > i.e Area -weighted Average and Standard Deviation. In both cases Temperature is prefered as Field variable. After this select Surface > select 'outlet' (as we need reports at outlet). At left corner in CREATE > select REPORT FILE and REPORT PLOT.
• In order to set the conditions for convergence go to Convergence Conditions > click Add > select the Reports > set the Stop Criterion(in my case 1e-06) > OK
• Now move on to INITIALIZATION > select METHOD = HYBRID > click t=0 INITIALIZE > No of iterations can be observed in CONSOLE WINDOW.
• Now go to RUN CALCULATION > choose NO. OF ITERATIONS and press CALCULATE.
• Now calculation for solution begins. Where Reports can be monitered and CONSOLE displays the iterations, velocities at each iterations, energy values etc. Once calculation is done it shows CALCULATION COMPLETE IN CONSOLE.
• Go to GRAPHICS > Residuals plot, Area -weighted Average of temperature plot, and Standard Deviation of temperature are obtained. Examine or understand the results obtained.
• Close the fluent window completely and go to RESUTS to see the simulation(CFD).
• Double click on RESULTS > directs to the CFD post.
• In volume_volume > select 'wall volume_volume' (hide all inlets and outlets).
• Double click on wall volume_volume > we get 'Details of wall volume_volume'.
• In Details of wall volume_volume > RENDER > adjust the level of TRANSPERANCY > go to color > select White > Apply.
• From the Top Toolbar > INSERT > LOCATION > PLANE > Name it as Plane 1(this is created in XY plane hence z=0) > Apply. A plane is created at centre of the tee.
• Now go to 'Plane 1' > Double click > COLOR > Mode = VARIABLE > In VARIABLE > select TEMPERATURE > Apply.
• Hence we can clearly observe the temperature changes. Further we can mention the range of temperatures so that it can be changed in Legends.
• Following the similar steps Velocity trend can be observed by selecting that term instead of temperature. Hence finally the contours of velocity and temperature at the midplane of the mixing tee is obtained.

Observations :

Case 1 (a)

• Short mixing tee with a hot inlet velocity of 3m/s.
• Momentum ratio of 2.
• Momentum ratio = velocity at cold inlet / velocity at hot inlet

Considerations for inlet-x :

Velocity Magnitude = 3 m/s.

Temperature = 25°C

Considerations for inlet-y :

Velocity Magnitude = 6 m/s.

Temperature = 10°C

Geometry of Mixing Tee :

Mesh Quality :

Residuals plot :

Area-Weighted Average plot :

Standard Deviation plot :

Temperature plot :

Velocity plot :

Case 1 (b)

• Short mixing tee with a hot inlet velocity of 3m/s.
• Momentum ratio of 4.
• Momentum ratio = velocity at cold inlet / velocity at hot inlet

Considerations for inlet-x :

Velocity Magnitude = 3 m/s.

Temperature = 25°C

Considerations for inlet-y :

Velocity Magnitude = 12 m/s.

Temperature = 10°C

Geometry of Mixing Tee :

Mesh Quality :

Residuals plot :

Area-Weighted Average plot :

Standard Deviation plot :

Temperature plot :

Velocity plot :

Case 2 (a)

• Long mixing tee with a hot inlet velocity of 3m/s.
• Momentum ratio of 2.
• Momentum ratio = velocity at cold inlet / velocity at hot inlet

Considerations for inlet-x :

Velocity Magnitude = 3 m/s.

Temperature = 25°C

Considerations for inlet-y :

Velocity Magnitude = 6 m/s.

Temperature = 10°C

Geometry of Mixing Tee :

Mesh Quality :

Residuals :

Area-Weighted Average plot :

Standard Deviation plot :

Temperature plot :

Velocity plot :

Case 2 (b)

• Long mixing tee with a hot inlet velocity of 3m/s.
• Momentum ratio of 2.
• Momentum ratio = velocity at cold inlet / velocity at hot inlet

Considerations for inlet-x :

Velocity Magnitude = 3 m/s.

Temperature = 25°C

Considerations for inlet-y :

Velocity Magnitude = 12 m/s.

Temperature = 10°C

Geometry of Mixing Tee :

Mesh Quality :

Residuals plot :

Area-Weighted Average plot :

Standard Deviation plot :

Temperature plot :

Velocity plot :

Results and Discussions :

Case	Considerations for inlet-x	Considerations for inlet-y	Element Size	Element Count	Avg. Outlet Temperature (°C)	Number of iterations for Convergence
Case 1 (a)	Velocity Magnitude = 3 m/s. Temperature = 25°C	Velocity Magnitude = 6 m/s. Temperature = 10°C	2.e-003 m	105944	20°C	86
Case 1 (b)	Velocity Magnitude = 3 m/s. Temperature = 25°C	Velocity Magnitude = 12 m/s. Temperature = 10°C	2.e-003 m	105944	17.5°C	165
Case 2 (a)	Velocity Magnitude = 3 m/s. Temperature = 25°C	Velocity Magnitude = 6 m/s. Temperature = 10°C	2.e-003 m	140525	20.°C	129
Case 2 (b)	Velocity Magnitude = 3 m/s. Temperature = 25°C	Velocity Magnitude = 12 m/s. Temperature = 10°C	2.e-003 m	140525	17.4°C	151

• Formation of the boundary layer is observed near the walls of the tee due to the no slip conditions for all the simulations.
• For case-1(a), it is clearly observed that the temperature distribution at the outlet is not uniform and also it is noticed that small amount of hot air exists due to the uneven mixing of cold and hot air.
• For case-2(a), the mixing of air is more uniform when compared to case-1(a) and also the same average temperature at outlet is observed.
• For case-1(b) and case-2(b), the uniform temperature at outlet is observed due to the even mixing of hot and cold air.
• By increasing the velocity of cold air by the factor of 2, we can see that we are able to reduce the outlet temparature. This can be observed clearly in the cas-1 by increasing the velocity of cold air.
• The longer tee (case-2) is not much effective and useful as it also gives similar results that we have obtained in case-1.
• Hence from above observations it is concluded that on increasing the velocity for cold air in short mixing tee we get better turbulant mixing.
• We can identify two types of mixing from the contour plots i.e. Primary mixing and Secondary mixing.
• In primary mixing, the maximum mixing is done because of the higher velocity of cold air and the secondary mixing happens because of diffusion. So, the primary mixing is responsible for the outlet temperature.

Conclusion :

This report shows the effectiveness of mixing of two air streems with different velocities for both short and long mixing tee.Hence it is conclude that by varying the momentum ratio(increasing) and keeping the length of mixing tee constant, there is a significant change in the outlet temperature.