NUMERICAL OPTIMIZATION AND EXPERIMENTAL VALIDATION OF HYDRODYNAMIC CAVITATION

DEVICES

A. B. PanditInstitute of Chemical Technology

University of MumbaiINDIA

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INTROCUTION

Cavitation in many cases such as propellers and pumps is an undesirable occurrence.

When cavitation takes place, local hot spots and ( upto 10000 K) and shock waves are generated (pressures upto 1000 atm).

Earlier efforts for dealing with cavitation have been directed towards avoiding it.

Such high energy released during cavitation can be harnessed for the positive effects of cavitation.

Applications: Wastewater treatment Water/ wastewater disinfection Size reduction

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INTRODUCTION

Objective of the work

Comparison of numerical simulations of elliptical, rectangular slit

and standard circular venturi on the basis of cavitational efficacy.

Experimental validation by comparing results of rectangular slit

venturi with standard circular venturi and orifice plate.

Advantages of Using Non-Circular Venturi

Higher p/a Ratio; i.e. More length available to produce shear . For

same cross sectional area

More number of cavitational events

Higher Cavitational efficacy

Experimental outcome

Non circular venturis gives higher cavitatonal yield compared to

standard circular venturi and orifice plate.

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CFD Simulations: Simulations of Standard Circular Venturi, Slit

Venturi and Elliptical Venturi Comparison of the three geometries based on

the CFD simulations Experimental Studies

Experimental studies on Slit venturi geometry reported by Bashir et al. (2011)

Comparison of Standard Circular Venturi, Slit venturi and Circular Orifice based on the experimental results

OUTLINE OF PRESENT WORK

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CFD SIMULATIONS Geometries Considered:

Standard Circular Venturi Rectangular Slit Venturi Elliptical Venturi

Softwares Used: Gambit 2.2.30 Ansys Fluent 6.30

MODELING STRATEGY

Equations used by Realizable k-ε Model: Equation for Turbulent Kinetic Energy (k)

Equation for Dissipation Rate (ε)

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MODELING STRATEGY Cavitation Model:

Cavitation Number

“Full cavitation model” by Singhal et al. (2001) Rayleigh-Plesset equation

Second order term is eliminated and is solved on the assumptions of isothermal expansion of the isothermal cavity collapse

Both bubble formation and subsequent collapse are taken into account in the model

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MODELING STRATEGY Equations Used in Cavitational Model:

Vapor Transport Equation

Equation for local static pressure

Equation for turbulence induced pressure fluctuations

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MODELING STRATEGY Effect of non-condensable gases

Equation for Phase change rates

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CFD SIMULATIONS OF STANDARD CIRCULAR VENTURI Geometry:

2D Geometry

QUAD Meshing,

Mesh Number-22500

Turbulence Models-

SST k-ω model and Realizable k-ε model

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CFD SIMULATIONS OF STANDARD CIRCULAR VENTURI Inlet Pressure – 5 bar, Outlet Pressure – 1 bar

Velocity Contours

Pressure Contours

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Pressure and Velocity Profiles

CFD SIMULATIONS OF STANDARD CIRCULAR VENTURI

0 0.02 0.04 0.06 0.08 0.10

100000

200000

300000

400000

500000

0

5

10

15

20

25

30

35

Pressure

Velocity

Length (m)

Pre

ssu

re (

Pa)

Velocity (m

/s)

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Effect of Inlet Pressure

CFD SIMULATIONS OF STANDARD CIRCULAR VENTURI

0 0.02 0.04 0.06 0.08 0.1 0.120

100000

200000

300000

400000

500000

600000

0.2 MPa

0.3 MPa

0.4 MPa

0.45 MPa

0.5 MPa

Length (m)

Pre

ssure

(P

a)

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Effect of Inlet Pressure

As inlet pressure increases cavitation number decreases which indicates increase in intensity of cavitation.

Cavitation activity at the pressures above 5 bar is almost constant. So all further simulations were carried out at the pressure of 5 bar.

CFD SIMULATIONS OF STANDARD CIRCULAR VENTURI

Pin (MPa)

Pout

(MPa)

P2 (MPa) u (m/s)

v (m/s)

Cavitation Number (σ)

ρmin (kg/m3)

Maximum Vapor

fraction fvap (%)

Pressure Recovery

zone length (mm)

0.198 0.1010.0026

71.91

21.20

0.4403 213.24 78.67 12

0.297 0.1010.0026

42.38

25.09

0.3143 191.66 80.83 28

0.396 0.1010.0023

42.77

28.86

0.2377 131.39 86.86 40

0.446 0.1010.0023

42.947

30.58

0.211 106.83 89.31 47

0.496 0.1010.0023

43.117

32.27

0.1901 93.937 90.61 55

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CFD SIMULATIONS OF RECTANGULAR SLIT VENTURI Geometry

3D Geometry HEX Meshing, Mesh Number - 200,000 Turbulence Models- Realizable k-ε model

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Meshing

Slit Venturi Designs

CFD SIMULATIONS OF RECTANGULAR SLIT VENTURI

L/WL at inlet (mm)

W at inlet (mm)

Perimeter of Pipe (mm)

c/s Area of Pipe (mm2)

P/A of

Pipe

L at Throat (mm)

W at Throat (mm)

Perimeter of Throat

(mm)

c/s Area of throat (mm2)

P/A at throat

1:0.2 33.68 6.74 80.83 226.86 0.36 3.96 0.79 9.51 3.14 3.03

1:0.5 21.3 10.65 63.9 226.86 0.28 2.51 1.255 7.52 3.14 2.39

1:1 15.06 15.06 60.25 226.86 0.27 1.77 1.77 7.09 3.14 2.26

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CFD SIMULATIONS OF RECTANGULAR SLIT VENTURI (L/W = 1:0.2) Pressure Contours

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CFD SIMULATIONS OF RECTANGULAR SLIT VENTURI (L/W = 1:0.2) Velocity Contours

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CFD SIMULATIONS OF RECTANGULAR SLIT VENTURI (L/W = 1:0.2) Pressure Zones

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CFD SIMULATIONS OF RECTANGULAR SLIT VENTURI (L/W = 1:0.5) Pressure Zones

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CFD SIMULATIONS OF RECTANGULAR SLIT VENTURI (L/W = 1:1) Pressure Zones

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CFD SIMULATIONS OF ELLIPTICAL VENTURI Geometry

3D Geometry HEX/WEDGE Meshing, Mesh Number - 550,000 Turbulence Models- Realizable k-ε model

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CFD SIMULATIONS OF ELLIPTICAL VENTURI

Meshing

Elliptical Venturi Design

D1/D2 D1 at inlet (mm)

D2 at inlet (mm)

Perimeter of Pipe (mm)

c/s area of Pipe (mm2)

P/A of pipe (mm)

d1

At throat (mm)

d2

At throat (mm)

Perimeter of throat (mm)

c/s area of throat (mm2)

P/A at throat (mm)

1:0.2 38.01 7.6 79.81 226.86 0.35 4.47 0.89 9.39 3.14 2.99

1:0.5 24.04 12.02 58.20 226.86 0.25 2.83 1.41 6.84 3.14 2.18

1:0.8 19.01 15.2 53.88 226.86 0.23 2.23 1.78 6.33 3.14 2.018

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Pressure Zones

CFD SIMULATIONS OF ELLIPTICAL VENTURI (D1/D2=1:0.2)

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Pressure Zones

CFD SIMULATIONS OF ELLIPTICAL VENTURI (D1/D2=1:0.5)

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Pressure Zones

CFD SIMULATIONS OF ELLIPTICAL VENTURI (D1/D2=1:0.8)

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COMPARISON OF CIRCULAR AND NON-CIRCULAR VENTURI

Type of Venturi

p/a Ratio

Pin (MPa)Pthroat

(MPa)Pout

(MPa)u

(m/s)V

(m/s)Cavitation

Number (σ)

Max Vapour volume

fraction fvap (%)

Pressure Recovery

zone length (mm)

Theoretical Number of

Cavities (x1019)

Circular Venturi

2 0.501 0.0023 0.101 0.36 30.83 0.21 90.6 50 5.09

Rectangular Slit Venturi

3.03 0.501 0.0023 0.101 0.35 29.64 0.22 98.45 5 8.12

2.39 0.501 0.0023 0.101 0.37 30.46 0.21 99.9 8 5.58

2.26 0.501 0.0023 0.101 0.37 30.42 0.21 98.9 25 5.62

Elliptical Venturi

2.99 0.501 0.0023 0.101 0.4 29.47 0.22 84.19 5 8.45

2.18 0.501 0.0023 0.101 0.37 30.22 0.21 99.9 8 6.44

2.01 0.501 0.0023 0.101 0.37 30.44 0.21 99.9 22 5.56

As perimeter of throat increases more length is available for shear production, resulting in more number of cavitational events

Slit venturi with p/a ratio of 3.03 and elliptical venturi with p/a ratio of 2.99 show maximum number of cavitational events as shown in table

More number of cavities means they would behave as a cluster rather than a single cavity which increases intensity of collapse.

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Experiments were performed for orange-G dye degradation

Circular venturi, circular orifice and slit venturi were used for the experiments.

The experimental setup used is as shown below:

EXPERIMENTAL STUDIES

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Circular Orifice Geometry

Circular Venturi Geometry

17 mm 2 mm

1 mm

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Slit Venturi Geometry

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COMPARISON OF THE DEVICES ON THE BASIS OF % DECOLORISATION OF DYE

0 50 100 150 200 250 300 350 4000

10

20

30

40

50

60

70

80

90

100

circular venturi at 5 bar inet pressure

slit venturi at 3 bar inlet pressure

orifice plate at 5 bar inlet pressure

Number of passes

% D

eco

lori

sati

on

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Comparison of the devices on the basis of mg of TOC reduced per unit of energy supplied

slit venturi circular venturi orifice plate0.00E+00

1.00E-06

2.00E-06

3.00E-06

4.00E-06

5.00E-06

6.00E-06

mg

of

TO

C r

ed

uc

ed

/ En

erg

y s

up

plie

d

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CONCLUSIONS

Cavitational intensity depends on perimeter to cross

sectional area ratio, and it was found that non-circular

venturi with a higher perimeter to cross sectional area ratio

show more cavitation.

In non-circular venturis more number of cavities are formed

which collapse over a shorter length compared to circular

venturi. However the experimental results suggest that the

collapse is more violent and results in higher cavitational

intensity.

Applications of non-circular venturi can be found in

intensification of processes involving physical as well as

chemical transformations. Examples of such processes are

cell disruption, water disinfection, oxidation and

degradation of pollutants etc.

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Thank you