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NUMERICAL OPTIMIZATION AND EXPERIMENTAL VALIDATION OF HYDRODYNAMIC CAVITATION
DEVICES
A. B. PanditInstitute of Chemical Technology
University of MumbaiINDIA
2
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
3
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.
4
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
7
MODELING STRATEGY Equations Used in Cavitational Model:
Vapor Transport Equation
Equation for local static pressure
Equation for turbulence induced pressure fluctuations
8
MODELING STRATEGY Effect of non-condensable gases
Equation for Phase change rates
9
10
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
11
CFD SIMULATIONS OF STANDARD CIRCULAR VENTURI Inlet Pressure – 5 bar, Outlet Pressure – 1 bar
Velocity Contours
Pressure Contours
12
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)
13
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)
14
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
15
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
18
CFD SIMULATIONS OF RECTANGULAR SLIT VENTURI (L/W = 1:0.2) Velocity Contours
19
CFD SIMULATIONS OF RECTANGULAR SLIT VENTURI (L/W = 1:0.2) Pressure Zones
20
CFD SIMULATIONS OF RECTANGULAR SLIT VENTURI (L/W = 1:0.5) Pressure Zones
21
CFD SIMULATIONS OF RECTANGULAR SLIT VENTURI (L/W = 1:1) Pressure Zones
22
CFD SIMULATIONS OF ELLIPTICAL VENTURI Geometry
3D Geometry HEX/WEDGE Meshing, Mesh Number - 550,000 Turbulence Models- Realizable k-ε model
23
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
24
Pressure Zones
CFD SIMULATIONS OF ELLIPTICAL VENTURI (D1/D2=1:0.2)
25
Pressure Zones
CFD SIMULATIONS OF ELLIPTICAL VENTURI (D1/D2=1:0.5)
26
Pressure Zones
CFD SIMULATIONS OF ELLIPTICAL VENTURI (D1/D2=1:0.8)
27
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
30
Slit Venturi Geometry
31
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
32
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
33
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.
34
Thank you