Institute of Chemical Technology 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. 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

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.

Outline of Present work 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

CFD Simulations Geometries Considered: Softwares Used: 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)

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

Modeling Strategy Equations Used in Cavitational Model: Vapor Transport Equation Equation for local static pressure Equation for turbulence induced pressure fluctuations

Modeling Strategy Effect of non-condensable gases Equation for Phase change rates

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

CFD Simulations of Standard Circular Venturi Inlet Pressure – 5 bar, Outlet Pressure – 1 bar Pressure Contours Velocity Contours

CFD Simulations of Standard Circular Venturi Pressure and Velocity Profiles

CFD Simulations of Standard Circular Venturi Effect of Inlet Pressure

CFD Simulations of Standard Circular Venturi 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. 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 21.20 0.4403 213.24 78.67 12 0.297 0.00264 2.38 25.09 0.3143 191.66 80.83 28 0.396 0.00234 2.77 28.86 0.2377 131.39 86.86 40 0.446 2.947 30.58 0.211 106.83 89.31 47 0.496 3.117 32.27 0.1901 93.937 90.61 55

CFD Simulations of Rectangular Slit Venturi Geometry 3D Geometry HEX Meshing, Mesh Number - 200,000 Turbulence Models- Realizable k-ε model

CFD Simulations of Rectangular Slit Venturi Meshing Slit Venturi Designs L/W L 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 0.28 2.51 1.255 7.52 2.39 1:1 15.06 60.25 0.27 1.77 7.09 2.26

CFD Simulations of Rectangular Slit Venturi (L/W = 1:0.2) Pressure Contours

CFD Simulations of Rectangular Slit Venturi (L/W = 1:0.2) Velocity Contours

CFD Simulations of Rectangular Slit Venturi (L/W = 1:0.2) Pressure Zones

CFD Simulations of Rectangular Slit Venturi (L/W = 1:0.5) Pressure Zones

CFD Simulations of Rectangular Slit Venturi (L/W = 1:1) Pressure Zones

CFD Simulations of Elliptical Venturi Geometry 3D Geometry HEX/WEDGE Meshing, Mesh Number - 550,000 Turbulence Models- Realizable k-ε model

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 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 0.25 2.83 1.41 6.84 2.18 1:0.8 19.01 15.2 53.88 0.23 2.23 1.78 6.33 2.018

CFD Simulations of Elliptical Venturi (D1/D2=1:0.2) Pressure Zones

CFD Simulations of Elliptical Venturi (D1/D2=1:0.5) Pressure Zones

CFD Simulations of Elliptical Venturi (D1/D2=1:0.8) Pressure Zones

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 0.35 29.64 0.22 98.45 5 8.12 2.39 0.37 30.46 99.9 8 5.58 2.26 30.42 98.9 25 5.62 Elliptical Venturi 2.99 0.4 29.47 84.19 8.45 2.18 30.22 6.44 2.01 30.44 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.

Experimental Studies 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:

Circular Orifice Geometry Circular Venturi Geometry 17 mm 2 mm 1 mm

Slit Venturi Geometry

Comparison of the devices on the basis of % decolorisation of dye

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

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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