numerical optimization and experimental validation of hydrodynamic cavitation devices
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NUMERICAL OPTIMIZATION AND EXPERIMENTAL VALIDATION OF HYDRODYNAMIC CAVITATION DEVICES A. B. Pandit Institute of Chemical Technology University of Mumbai INDIA
description
numerical optimization and experimental validation of hydrodynamic cavitation devices. A. B. Pandit Institute of Chemical Technology University of Mumbai INDIA. introcution. Cavitation in many cases such as propellers and pumps is an undesirable occurrence. - PowerPoint PPT Presentation
Transcript of numerical optimization and experimental validation of hydrodynamic cavitation devices
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 7
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
35PressureVelocity
Length (m)
Pres
sure
(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)
Pres
sure
(Pa
)
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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 0.101 0.00267 1.91 21.2
0 0.4403 213.24 78.67 12
0.297 0.101 0.00264 2.38 25.0
9 0.3143 191.66 80.83 28
0.396 0.101 0.00234 2.77 28.8
6 0.2377 131.39 86.86 40
0.446 0.101 0.00234 2.947 30.5
8 0.211 106.83 89.31 47
0.496 0.101 0.00234 3.117 32.2
7 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 DesignD1/D2 D1 at
inlet (mm)
D2 at inlet (mm)
Perimeter of Pipe (mm)
c/s area of Pipe (mm2)
P/A of pipe (mm)
d1At throat (mm)
d2At 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
ecol
oris
atio
n
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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 T
OC
redu
ced/
Ene
rgy
supp
lied
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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
