Week 1- Mixing Tee

Aim

To study action and Perform simulation of mixing two streams of air at different temperatures and velocities in a T- Joint.

The effect of varying velocity of chilled air stream and length of mixing on temperature obtained is studied.

Introduction

In mixing tee, we are mixing the two streams of air, which are at different temperatures and velocities to get the desired value at the outlet.

Theory

The air conditioning unit has capacity of provding the air at 19 ° C . But the human comfort temperature is around 28 ° C- 30 ° C. Hence air from air conditioning unit is mixed with the atmospheric air having 36° C temperature so that the air having temperature close to human comfort shall be delivered.

In this project we are going to study the Effect of length , Momentum ratio and Mesh Element Size on the outlet temperature.

Turbulence Model Study

1. K - Epsilon , Relizable

2. K - Omega , SST

Starting with Simulation

Selecting Mixing Tee - Short Geometry in spaceclaim

A solid model is genereated.

We do not need the thickness of the geometry. We are only interested in volumes interacting with fluids.

Hence we need to extract fluid volume

Volume Extraction Ansys

Select Prepare - Volume Exctraction - Select Edges

A new volume is generated

We only need the exctracted volume for meshing hence the geometry shell 1 is hidden

We dont want shell1 geometry for meshing hence we use Tool - Suppress for Physics

Spaceclaim is closed.

Mesh tab is opened

Geometry is loaded in Mesh

Defining Faces

Tool - Select Faces - Create Named Selection

Clicking on 'Mesh'

All the parameters are decied by Ansys using ' Bounding Box Method'

Click on ' Generate Mesh' , Mesh is formed

Section Plane View

Setup

Steady

Pressre Based

Velocity - Absolute

Turning Energy Equation

Viscous Model K - Epsilon

Material - Air

Cell Zone Conditions - Volume - Fluid

Boundry Conditons

Hot inlet velocity of 3m/s.

Momentum ratio of 2, 4.

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

CASE 1 - A

Momentum ratio = 2 ,

2 = velocity at cold inlet / 3

velocity at cold inlet = 6 m/s

Inlet-x Velocity = velocity at hot inlet = 3m/s , hot inlet temperature is 36⁰C

Inlet-y Velocity = velocity at cold inlet = 6 m/s , cold inlet temperature is 19⁰C

Outlet - Pressure Outlet

Definitions

Area - Weighted Average - Temperature

Hybrid Initialization

No. of Iterations

Reports

No of Iterations - 150 Weighted Average Temperature

Case No. 1

C on workbench

No of Iterations - 250

Scaled Residuals

Weighted Average Temperature Standard Deviation

Solution is converged at 205th Iteration Computed Values

Velocity and Temperature Contour Plots

Velocity and temperature line plots along and across the length of the pipe.

Inserting new streamline

Starting from Volume , Max. No of Points - 1000

User Specified rage for velocity and temperature

Case No. 2

D on workbench

K-Omega SST Model

Scaled Residuals Weighted Average Temperature Standard Deviation

Solution is converged at 205th Iteration Computed Values

Velocity and Temperature Contour Plots

Velocity and temperature line plots along and across the length of the pipe.

Choosing Turbulence Model

Turbulence model SST K- Omega is reduced to the model Standard K- Epsilon as it moves away from wall.

Which means SST K -Omega model covers the Standard K- Epsilon Model.

Avg. Temp Std. Deviation No.of Cells

k - epsilon 30.216053 [C] 1.5824002 [C] 12756

k - omega 30.267803 [C] 1.5227045 [C] 12756

As there are is turbulence is involved and no separation of flow occurs it is better to use k- epsilon model. Inspite of having many drawbacks it suits the best with the given application and provieds the strandard result

Hence we are going ahead with the k - epsilon model.

Mesh Independant Study

E on workbech

Changing mesh element size

Mesh - Sizing - Element Size - 0.005

Generate Mesh

Mesh is generated

Turbulence Model - K - Epsilon

No of Iterations - 250

Scaled Residuals Weighted Average Temperature

Standard Deviation

Solution is converged at 205th Iteration Computed Values

Velocity and Temperature Contour Plots

Velocity and temperature line plots along and across the length of the pipe.

Details

If we compare the results with the case having same turbulence model and momentum ratio but diferent mesh element size,

1. Element Size - Default

There is no any noticeable difference between two values.

Reducing the mesh element size gives the value close to actual values but it cannot enhance the outlet conditions.

Giving small element size will help in acheiving accurate results.

Case No - 3

F on workbench

Momentum ratio = 4 ,

4 = velocity at cold inlet / 3

velocity at cold inlet = 12 m/s

Inlet-x Velocity = velocity at hot inlet = 3m/s , hot inlet temperature is 36⁰C

Inlet-y Velocity = velocity at cold inlet = 12 m/s , cold inlet temperature is 19⁰C

Suraface Report Definition

Scaled Residuals Area Weighted Average of Temperature Standard Deviation of Temperature

Solution is converged at 151 Iteration Computed Values

Details of Wall

Insert Counter 1

Giving user specified values Default Legend View - Text Parameters - Fixed

Velocity and Temperature Contour Plots

Insert Streamline

Velocity and temperature line plots along and across the length of the pipe.

2nd Case - Mixing Tee - Long

All the procedure till meshing is repeated.

Defining Inlet, Outlet, Walls Mesh Element Size - 0.005 m

No of Cells - 17941

Setup

1. Momentum ratio = 2 ,

2 = velocity at cold inlet / 3

velocity at cold inlet = 6 m/s

Inlet-x Velocity = velocity at hot inlet = 3m/s , hot inlet temperature is 36⁰C

Solution did not converge after 1000 Iterations.

Computed Values

Temperature Variable

Velocity Variable

Pressure Variable

Streamline Plot for Velocity along and across the Mixing Tee

Streamline Plot for Temperature along and across the Mixing Tee

2. Momentum ratio = 4 ,

4 = velocity at cold inlet / 3

velocity at cold inlet = 12 m/s

Inlet-x Velocity = velocity at hot inlet = 3m/s , hot inlet temperature is 36⁰C

Inlet-y Velocity = velocity at cold inlet = 12 m/s , cold inlet temperature is 19⁰C

Solution is converged at 140th Iteration.

Temperature Variable

Velocity Variable

Pressure Variable

Streamline Plot for Velocity along and across the Mixing Tee

Streamline Plot for Temperature along and across the Mixing Tee

Coclusion :

Comparing Values from all the cases

• Effect of length and momentum ratio on results.

1.There is not much difference between outlet temperatures of short and long mixing tees , but for long mixing te the effectivenesss of the mixing is enhanced ( as the standard deviatio is decreased)

Length of tee does not contribute in providing optimized outlet temperature.

2. Momentum ratio plays important role in outlet values

As the momentum ratio is rised to 4 , the averge temperature at outlet is reduced. Also its effectiveness increases and it takes very less no. of iteratins to provide a converged solution.