assign8.pptx

CLL 769: Applications of Computational Fluid
Dynamics Semester I, 2021-2022
CLL 769: APPLICATIONS OF CFD
Department of Chemical Engineering
Indian Institute of Technology-Delhi
Assignment 7: Passive Mixing Simulation in a Rectangular
Bubble Column using Euler-Lagrange Approach
Submitted by
Vasundhara Agarwal
To
Prof. V Buwa

Contents
 Dispersed Liquid Gas Flows
 Geometry for CFD Simulation
 Computational Grid (Mesh Model)
 Physical Properties
 Governing Equation for Euler-Lagrangian Dispersed
Phase, Turbulence Model and species transport
 Initial and Boundary condition
 Numerical Details in ANSYS FLUENT
 Results

Dispersed Liquid Gas Flows
 In fluid mechanics, multiphase flow is the simultaneous flow of
materials with two or more thermodynamic phases. Virtually all
processing technologies from cavitating pumps and turbines to
paper-making and the construction of plastics involve some form of
multiphase flow. It is also prevalent in many natural phenomena.
 These phases may consist of one chemical component (e.g. flow of
water and water vapour), or several different chemical components
(e.g. flow of oil and water). A phase is classified as continuous if it
occupies a continually connected region of space. Whereas a
classification of disperse is applied when the phase occupies
disconnected regions of space. The continuous phase may be either
gaseous or a liquid. The disperse phase can consist of either a solid,
liquid or gas.
 Two general topologies can be identified, disperse flows and
separated flows. The former being those consisting of finite
particles, drops or bubbles distributed within a continuous phase.
The latter is defined as consisting of two or more continuous
streams of fluids separated by interfaces

Common vertical flow regimes - From left to right: Churn
flow, Annular flow and Wispy annular flow

Geometry for CFD simulation
View in X-Y-Z Plane Sketch mode in X-Y-Z Plane View in X-Y Plane
width (W) = 20 cm
depth (D) = 5 cm
height (H)= 90 cm

Computational grid (mesh model)
View in X-Y-Z Plane View in X-Y
Plane
Fluent mode

Physical Properties
 Thermo physical properties used for numerical simulation
 The experiments were conducted using air and water as working fluids
under ambient (isothermal) conditions.
 Both fluids are incompressible
 Initially (at t=0), the entire column (with H=90 cm) is filled with a
quiescent liquid (water).
 Modelling in Eulerian-Lagrangian framework.
 Tracer was patched at 36 second and 48 seconds for the two velocities
respectively

Governing Equation For Euler-Euler
Dispersed Phase, Turbulence Model and
Species Transport
The simulations were carried out using the Euler/Euler two-phase
model in which water was the continuous phase and air was the
dispersed phase. The model equations are given below

Species Transport

Initial and Boundary condition
 The experiments were carried out with continuous air flow at superficial
air velocities (based on column cross section) of 0.16 and 0.73 cm/s. Initial
liquid height it to be taken as 90 cm (corresponding to an H/W ratio of 4.5)
 Since liquid phase is in a batch mode, the liquid velocities ( ) are set equal
to zero.
 The gas inlet is to be modelled as an area of 24 mm x 12 mm located at the
centre of bottom cross section. The bubbles are allowed to enter into the
column through the above-mentioned gas inlet with a vertical velocity of
20 cm/s. mass flow rate of air corresponding to superficial gas velocities of
0.16 and 0.73 cm/s was specified.
 A constant bubble size of 5 mm is to be assumed in the simulation. The
inter-phase momentum exchange (momentum given by the gas phase to
liquid phase and vice a versa) is to be accounted by considering drag force
acting on the bubbles.

Mass flow-rate calculation
 Column cross-section area = (0.2*0.05) m2
 Inlet face area=(24*12)*(10e-6)m2
 Inlet volumetric flow-rate = Superficial gas velocity * Column cross-section area.
 Inlet air mass flow-rate = Inlet volumetric flow-rate * Density
Superficial gas velocity (based on column cross-section area) = 0.16cm/sec =
0.0016m/sec
 inlet volumetric flow-rate (m3/sec ) = 0.0016m/sec * (0.2*0.05) m2
= 1.6*10e-5 m3/sec
 inlet air mass flow-rate(kg/sec) = 1.6*10e-5 m3/sec * 1.225kg/m3
= 1.96*10e-5 kg/sec
Superficial gas velocity (based on column cross-section area) = 0.73cm/sec =
0.0073m/sec
 inlet volumetric flow-rate (m3/sec ) = 0.0073m/sec * (0.2*0.05) m2
= 7.3*10e-5 m3/sec
 inlet air mass flow-rate(kg/sec) = 7.3*10e-5 m3/sec * 1.225kg/m3
= 8.9425*10e-5 kg/sec

Tracer calculation
 Tracer Amount= 100ml
 Tracer Volume= 1*e-4 m^3
 Patched Region:
 X= 0.06 m (from 0.07 to 0.13), Y= 0.09 m (from 0.81to
 0.90), Z= 0.02 m (from 0.015 to 0.035)
 Mass of water= 8.99 kg
 Mass of tracer= 0.134 kg
 Equilibrium Tracer Mass Fraction= 0.134/(8.99+0.134)
 = 0.0146

Numerical Details in ANSYS
FLUENT
 ANSYS STUDENT used for mesh generation and meshing. The numerical
simulation carried out by using a student finite volume method solver
(ANSYS FLUENT).
 Pressure Velocity Coupling –
Semi –implicit Method for Pressure Linked Equation (SIMPLE)
 Solver Settings
• Transient, second order Implicit Incompressible solver with gravity in
–Y direction with magnitude of 9.81 m/s
 Physical Model:
• k-ϵ model to incorporate the turbulence of the continuous phase.
• Discrete random walk (DRW) used to model the turbulent dispersion
of the dispersed phase.
 Wall Boundary Conditions
• The bubbles are allowed to REFLECT from all walls (expect at the
outlet) and allowed to ESCAPE from the outlet.

Types of wall boundary
conditions

 Pressure Velocity Coupling – Per phase Semi –implicit
Method for Pressure Linked Equation (SIMPLE)
 Discretization Scheme or Spatial Interpolation–
Pressure – PRESTO!
Momentum – QUICK (Quadratic Upwind interpolation for
Convective Kinetics)
Turbulent Kinetic Energy.: QUICK
Turbulent Dissipation rate :- QUICK
Convergence Criteria: Convergence criteria for all flow
variable kept 1e-3
 Observations were made at 9 probes
P1 (0.01, 0.6, 0.025), P2 (0.1, 0.6, 0.025), P3 (0.19, 0.6,
0.025)
P4 (0.01, 0.35, 0.025), P5 (0.1, 0.35, 0.025), P6 (0.19,
0.35, 0.025)
P7 (0.01, 0.15, 0.025), P8 (0.1, 0.15, 0.025), P9 (0.19,
0.15, 0.025)

Initial Tracer Mass Fraction Contour in
Different Planes and Alignment
 The First contour is drawn on X-Z plane at Y= 0.85m
 The third contour is drawn on Y-Z plane at X= 0.1m
 The fourth contour is drawn on X-Y plane at Z= 0.025m
 The second contour is drawn on the combination of x-z and y-z plane.
 The fifth contour is drawn on the combination of all planes

Tracer Mass Fraction Contour at different
time instants for Superficial Gas Velocity
0.16 cm/sec.

Y-Velocity Vector on Mid Plane at different
time instants for Superficial Gas Velocity
0.16 cm/sec
.

Tracer Mass Fraction Contour at different
time instants for Superficial Gas Velocity 0.73
cm/sec.

Y-Velocity Vector on Mid Plane at different time
instants for Superficial Gas Velocity 0.73 cm/sec

Tracer Mass Fraction Plot at different
points for Superficial Gas Velocity 0.16
cm/sec.
40 50 60
40 50 60
0.00
0.02
0.04
0.06
0.08
0.00
0.02
0.04
0.06
0.08
Median
0.01487
Time (s)
Time (s)
P1
P2
P3
P4
P5
P6
P7
P8
P9
Tracer
mass
fraction
60
60
0.012
0.014
0.016
0.012
0.014
0.016
Median
0.01487
Time (s)
Time (s)
Tracer
mass
fraction
From close observation all points show a mixing time of nearly 24s (60-
36)

Tracer Mass Fraction Plot at different
points for Superficial Gas Velocity 0.73
cm/sec.
50 60 70
50 60 70
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.00
0.02
0.04
0.06
0.08
0.10
0.12
Median
0.01487
TIME (s)
TIME (s)
P1
P2
P3
P4
P5
P6
P7
P8
P9
Tracer
mass
fraction
67 68 69 70 71 72 73 74 75
67 68 69 70 71 72 73 74 75
0.012
0.013
0.014
0.015
0.016
0.017
0.012
0.013
0.014
0.015
0.016
0.017
Median
0.01487
TIME (s)
TIME (s)
P1
P2
P3
P4
P5
P6
P7
P8
P9
Tracer
mass
fraction
From close observation all points show a mixing time of nearly 24s (72-48)

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