Numerical Investigation for Single-Phase and Two-Phase Flow in Duct Banks with Multi Types of Vortex Generators
Published at : 09 May 2023
Volume : IJtech
Vol 14, No 3 (2023)
DOI : https://doi.org/10.14716/ijtech.v14i3.5811
Ibrahim, A.Q., Alturaihi, R.S., 2023. Numerical Investigation for Single-Phase and Two-Phase Flow in Duct Banks with Multi Types of Vortex Generators. International Journal of Technology. Volume 14(3), pp. 484-500
| Adnan Qahtan Ibrahim | 1. Department of Mechanical Engineering, Faculty of Engineering, University of Babylon, Hilla, Babylon, 51002, Iraq, 2. Automotive Department, College of Engineering/AL-Musaib, University of Babylon, |
| Riyadh Sabah Alturaihi | 1. Department of Mechanical Engineering, Faculty of Engineering, University of Babylon, Hilla, Babylon, 51002, Iraq |
This paper examines the numerical work of the thermal convection over
tube banks with winglets through the heat exchanger in two parts under
turbulent conditions. The first section investigates the influence of two-phase
(water and air) flow on the performance of two kinds of vortex generators
(Delta and Rectangular) winglets across the oval tube banks. The second section
studied the performance of four types of winglets (Delta, Rectangular, Zikzak,
and Sinusoidal wavy), circular and oval tubes, forward and downward
configurations, and various angles of attack (15°, 20°,
and 25°) in
two-phase flow. Delta winglets provide the highest performance at an attack
angle of 15° for oval tube banks in a two-phase flow with a moderate turbulent
flow rate.
Duct Banks; Heat exchanger; Sinusoidal winglets; Zikzak winglets
In numerous industrial processes, heat convection is essential in
heating and cooling operations. At high flow velocities, a two-phase flow is
utilized to minimize pressure losses in the duct. Over the last two decades, researchers have
focused on winglets' ability to generate interconnected vortices or swirling
flow parallel to the flow orientation. These winglets have been studied to enhance the Heat convection effectiveness of various heat
exchangers via air- or gas-side heat transfer. (Fiebig,
1998; Fiebig, Valencia,
and Mitra, 1993) explained flow
with winglets could lead to a massive increase in heat convection coefficient
in the laminar duct flows, increasing transverse vortex generators (VGs). It
was found that an inline tube configuration enhanced the heat convection rate
by 55–65 %, with a comparable pressure drop of 20–45 %. Lau, Meiritz, and
Ram (1999) published the results of an experimental
work investigating the movement in a turbulent duct flow with heat and momentum
using Vortex generators. Oval tubes have several advantages over circular tubes
in lowering pressure drop and reducing the wake zone. As a result, when vortex
generators are combined with oval tubes, it is possible to get increased
thermal performance without considerably raising the pressure drop (He and Zhang, 2012). Compared to the Baseline instance, the vortex generator exhibits
a promising improvement in
The Geometric Model
2.1. Modelling of Flow and Boundary Condition
The characteristics of the problem and
boundary condition for the single and two phase-flows can be shown in Figure 1.
The rectangular duct inlet is represented as the fluid inlet's superficial
velocities. The oval tube wall is exposed to constant heat flux, while the
outlet pressure is represented at the outlet of the rectangular duct. The
remaining portion of the duct walls is put to be adiabatic.
The heat
convection coefficient and the temperature distribution were investigated in
the duct by using various values for the discharges of water and air with
different shapes and positions of the vortex generators. ANSYS-Fluent 19.0 is
used to evaluate the flow characteristics of water flow and mixture (water-air)
flow through the banks of the tubes with vortex generators. Fluent shows temperature
distribution, pressure gradient, and velocity for two-phase and single-phase
flow through the tube banks.
Fluent is a fluid flow simulation software suite that solves fluid flow problems using computational fluid dynamics (CFD). It solves a fluid's governing equations by using the finite volume method. Fluent's fluid problem scheme is defined by momentum, mass, and energy conservation laws. This law defines a finite volume-based discretization of a partial differential equation. These parameters pass in the course of the rectangular duct for fluid flow over oval tube banks with and without winglets. Computational fluid dynamics (CFD) is the branch of fluid dynamics that investigates the issue by calculating and giving practical methods for reproducing natural flow by solving the governing equations numerically (Abdulnaser, 2009).
Figure 1 Boundary
Condition
The boundary conditions of
the single-phase and two-phase flow systems are presented in Table 1.
Table 1 Boundary conditions
|
Zone |
Fluid |
Energy |
|
Inlet |
Velocity |
293 k |
|
Tube walls |
No-slip |
21883.8 w/m2 |
|
Duct walls |
Symmetry |
Symmetry |
|
Winglets |
No-slip |
Adiabatic |
|
Outlet |
Pressure |
Adiabatic |
2.2. The Geometry of the Testing Section
In order to simulate the system, it has
been modeled as a 3-D model using Solid Works 2018 combined with Ansys
Workbench 19.0. The model has been drawn as a rectangular shape, and its
dimensions are (12 cm × 2 cm × 100 cm). The geometry of the testing section is
set to be fluid, as shown in Figure 2.
Using the diameter of the tube (D) as the characteristic length scale,
all dimensions of the duct are calculated as L = 10D, W = 1.2D, and H = 0.2 D,
respectively.
Figure 2 (a)
Computational domain for test section; (b) Vortex generator placement with oval
tube; (c) Four types of vortex
generators
2.3. Mesh Generation
Because there are so many mesh types to
choose from, it is essential to consider factors like flow field, geometry, and
complexity when choosing which mesh to use. The required CPU time, solution
accuracy, and convergence rate are all influenced by the size and kind of mesh (Bakker, 2006). In this work, the meshing
procedure is performed in the Ansys Workbench 19.0 application using
Quadrilateral structured grid elements. The meshing sizes for maximum and
minimum meshing sizes are set to be equal (0.001 m) for oval and circular
tubes, as shown in Figure 3. Table 2 shows how many elements and nodes each
situation in this study contains for oval and circular tubes, respectively.
Table 2 The number of
elements and nodes
|
Case |
Nodes No. |
Elements No. |
|
Without Vortex |
202860 |
181440 |
|
Delta |
124767 |
624620 |
|
Rectangular |
124831 |
624663 |
|
Zikzak |
124826 |
624657 |
|
Sinusoidal |
134436 |
674249 |
Figure 3 The Mesh of
oval tubes
2.4. Grid independence
A grid-independent
solution is required to enhance the precision of the computations. The present
work on grid independence consists of three parts: convergence index, grid
refinement, and General Richardson. Two various winglet positions relative to
the center of each tube are used. For forward (
X = -1, Y = ±4), downward (X =
1, Y = ±4) of each tube with 
= 15° and Re = 3643.5.
3. Governing Equations
The
challenge entails solving the flow field and heat convection problems
associated with a bank of oval and circular tubes fitted with winglets in a
rectangle duct under a transient state. Vortex
generators are added to the duct to improve heat transfer. A computational
examination is necessary because of how their addiction impacts the flow field
and heat convection. The flow field must be resolved to establish the ideal
heater diameter and form for the vortex generators.
3.1. Continuity Equation
3.2. Momentum Equation
3.3. Energy Equation
4. Turbulence Model
Models are utilized to have the capacity for
characterizing and predicting the physics of the multiphase flow. Also, these
models are suitable for different applications that have multiphase flow. Some
demonstrating approaches are the Euler-Lagrange approach, the Volume of fluid
approach, the Euler-Euler approach, and dispersed phase modeling. The
Euler-Lagrange method is computationally expensive and is appropriate for flows
with a small volume percentage of the dispersed phase. The (k-
) standard model is utilized in place of the (k-
) model in this
work due to the (k-) model's poor prediction of rotating and swirling flows,
as well as fully developed flows in the rectangular ducts (Shbeeb and Mahdi, 2016).
Also, the (k-) standard turbulence model will be used to simulate the
flow-through test section. The single and two-phase flows are modeled by
combining the model with various parameters based on the testing factors and the
outcomes of the experiments to compare and validate the CFD results (Vejahati et al., 2009; Fluent, 2006).
5. Performance parameter
where j is
the Colburn factor, and f is the coefficient of friction (Chu, He, and Tao, 2009).
6. Model validation
The computational fluid dynamics model
validation by numerical simulations of flow through the heat exchanger with an
intake of the Reynolds number between 600 and 3000 were compared to the
numerical outcomes of (Fiebig, Valencia, and Mitra, 1993).
The maximum errors between the present model
findings and the numerical outcomes of (Fiebig, Valencia, and Mitra, 1993) are 4.35% for Nu and 5.825% for f. The model
validation results are shown in Figure 4. From the above analyses, the high
degree of agreement between these outcomes illustrates the model's reliability
in precisely forecasting the flow structure and heat convection properties.
Figure 4 Validation of the present model
with the germane work (Fiebig, Valencia, and Mitra, 1993) for various values of Re (a) Nu and (b) f
Figures 5 to 19 show the numerical results of the increased water-air flow rates on the temperature gradient at various locations in the duct with a constant electrical power of (110 W) as a heat flux. In this study, the performance of VGs was investigated based on the temperature gradient between the fluid flow and the surface of the tube bank, which corresponded to the pressure loss inside the duct. The best performance at the lowest temperature difference and the lowest pressure drops, with other parameters remaining kept constant. The Nusselt number (Nu) was directly proportional to the coefficient of heat convection. When the temperature gradient between single or two-phase flow and the surfaces of tube banks increased, the heat convection coefficient decreased, and vice versa. Also, the friction coefficients (f) were directly proportional to the pressure drop in the duct, according to Equation 7.
7.1. Effect of the phase
7.1.1. Single-phase flow
Figure
5 illustrates the temperature gradient between water flow and oval tube
surfaces at various locations in the duct without winglets, with Delta winglets, and with Rectangular winglets for three different water flow
rates (15, 17.5, and 20 L/min). Observed from this figure that the temperature
gradient decreased as the water flow rate increased. These results agree with Chu, He, and Tao (2009), and Haque and Rahman (2020) for reduced temperature
gradient at the water flow velocity increased; therefore, the heat convection
coefficient increased. When the water flow rate increases, the flow velocity
increases, ultimately enhancing the heat convection coefficient.
Figure
6 illustrates the entrance and exit pressure for water flow at many points in
the duct without winglets, with Delta winglets, and with Rectangular winglets
for three different water flow rates (15, 17.5, and 20 L/min). The pressure
loss in the duct raised as the discharge of water increased, and these results
agree with Chu, He, and Tao (2009). When the water flow
rate increases, the pressure drop decreases, which reduces the coefficients of
friction.
Adding
vortex generators to the duct can augment the heat convection coefficient by
raising the velocity of single-phase flow and the turbulence intensity within
the duct. As the flow velocity increases, the temperature gradient reduces.
When flow is oriented toward the oval tube's surface, the temperature gradient
is inversely proportional to flow velocity.
Figure
7a illustrates the fluctuation of a temperature gradient with Reynolds number
in water flow across the bank of oval tubes without winglets generators. Owing
to a decrease in the temperature gradient between the water flow and surfaces
of the oval tube, the heat convection coefficient improved as the Re number
raised.
Figure
(7b) represents the fluctuation of the pressure reduction with Reynolds number
in water flow over oval tube banks without winglets. As the Reynolds number
grew, the duct pressure losses were reduced.
Figure (7c) represents the Reynolds number performance parameter for water flow over oval tube banks without winglets. As the Re number climbed, the performance parameter improved because the pressure in the duct and the temperature gradient between the water flow and the surfaces of the oval tube decreased. The delta winglets' performance is greater than that of the other two vortex generators. These results concur with the reports of Haque and Rahman (2020), and Naik and Tiwari (2020b) which examined the influence of different vortex generator forms on heat flow in the duct and showed that Delta winglets provide the highest performance.
Figure
5
The temperature gradient between water flow and surfaces of an oval tube for
various water flow rates (15, 17.5, and 20 L/min) for single-phase flow (a)
without winglets (b) with Delta winglets (c) with Rectangular winglets
Figure 6 Inlet and
Outlet pressure in the duct with an oval tube for single-phase flow (a) without
winglets (b) with Delta winglets (c) with Rectangular winglets
Figure 7 (a) The temperature
gradient with
Delta and Rectangular winglets for single-phase flow over oval tube (b) Pressure
drop (c) Performance parameter
7.1.2. Two-phase
flow
Figure 8 indicates that at a
constant discharge of water, the temperature gradient between mixture
(water-air) flow and surfaces of oval tubes increased. At these points, the
airflow rate increases (8.33, 16.67, and 25 L/min) with a constant water flow and
heat flux. When the flow rate of water and air flow is increased without
winglets, the heat convection rate drops, and the temperature gradient becomes
less significant. The heat convection coefficient improved as the temperature
difference reduced because the winglets concentrated the working fluid on the
tube surface with Delta winglets and
Rectangular winglets
used in the duct.
Figure 9 represents the
inlet and outlet pressure in the duct for water-air flow at various points. At
these points, the airflow rate increases (8.33, 16.67, and 25 L/min) with a
constant flow rate of water and heat flux. The pressure losses without winglets
in the duct are reduced. Increased pressure drops compared to Delta winglets,
and Rectangular winglets are used in the duct that is not utilized. In
addition, the pressure drops in the duct are minimized due to growth in the
flow rate of water-air Delta winglets. This is in accordance with the findings
of Chu, He, and Tao (2009)
and Haque and Rahman (2020).
In addition, the pressure
drop is reduced when the discharge of mixture (water-air) flow is increased in
the absence of winglets. When rectangular VGs are
used in the duct, the pressure losses are more significant than not. In
addition, when the flow rate of water-air increases with rectangular winglets,
the pressure losses in the duct are decreased. There is good agreement with
that of Haque and Rahman (2020) and Naik
and Tiwari (2020b).
Adding winglets to the duct
can raise the surface velocity of water-air flow, raising the heat convection
coefficient by generating intense turbulence. The temperature gradient
decreases as the flow velocity increases. When the flow is directed toward the
surfaces of the oval tube, the temperature gradient is inversely related to the
flow velocity. The airflow rate increased from (8.33, 16.67, and 25 L/min) with
a constant flow rate of water and heat power of (110 W).
The effect of the Reynolds
number for water-air flow over oval tube banks in a turbulent region on heat
convection rate and pressure reduction is essential for optimal design and
location of vortex formation. Due to the decrease in pressure and temperature
gradient between water-air flow and surfaces of the oval tube in the duct, the
performance parameter was improved when the Re number of water was lowered. The
Re number of air grew as the air flow rate raised (8.33, 16.67, and 25 L/min),
but the water flow rate remained constant.
The variance of the
temperature gradient with Re number in water-air flow over oval tube banks
without winglets, with Delta winglets, and with Rectangular winglets are
represented in Figure 10. At a constant flow rate of water, the heat convection
coefficient increases as the temperature gradient between the water-air flow
and surfaces of the oval tube decreases.
The variance of the drop in
pressure with the Re number in water-air flow over oval tube banks without
winglets, with Delta winglets, and with Rectangular winglets are visualized in
Figure 11. When the Re number of water and air increased, the pressure drop
decreased in the duct with a constant flow rate of water.
The variance between the Re
number and performance parameter in the water-air flow over oval tube banks
with and without winglets is depicted in Figure 12. As water-air flow
increased, the performance parameter improved. Delta winglets offer the most
efficient performance. This is in agreement with the findings by Chu, He, and Tao (2009).
Figures 13,14, and 15
illustrate that the velocity contour for water flow and water-air flow without
winglets, with Delta winglets, and with Rectangular winglets are used in the
duct from Workbench 19.0 (ANSYS - Fluent 19.0) when a flow rate of air
increases (0, 8.33, 16.67, and 25) (L/min) respectively, with a constant flow rate of water.
Figure 8 The temperature gradient between the flow of water and air with oval tube surfaces for two-phase flow (a) without winglets (b) with Delta winglets (c) with Rectangular winglets
Figure 9 Inlet and Outlet pressure in the
duct with an oval
tube for two-phase flow (a) without winglets (b) with Delta winglets (c) with
Rectangular winglets
Figure 10 The temperature gradient with Delta and Rectangular
winglets for two-phase flow over oval tube at (a) Qw= 15 L/min (b) Qw=
17.5 L/min (c) Qw= 20 L/min
Figure 11 Pressure drop with Delta
and Rectangular winglets for two-phase flow over oval tube at (a) Qw=
15 L/min (b) Qw= 17.5 L/min (c) Qw= 20 L/min
Figure 13 The velocity vector without
vortex generators (a) single-phase flow (b) two-phase flow at constant a flow
rate of water 15 Lpm with a flow rate of air 8.33 Lpm (c) 16.67 Lpm (d) 25 Lpm
Figure 14 The velocity vector with
delta winglets as vortex generators (a) single-phase flow (b) two-phase flow at
constant a flow rate of water 15 Lpm with a flow rate of air 8.33 Lpm (c) 16.67
Lpm (d) 25 Lpm
Figure 15 The velocity vector with
rectangular winglets as vortex generators (a) single-phase flow, (b) two-phase
flow at constant a flow rate of water 15 Lpm with a flow rate of air 8.33 Lpm
(c) 16.67 Lpm (d) 25 Lpm
7.2. Effect of the position, an
angle of attack, and the configuration of vortex generators
Four shapes of vortex generators
(Delta, Rectangular, Zikzak, and Sinusoidal wavy) winglets are investigated
numerically. Workbench 19.0 is used to compute the results and analyzes for
turbulent two-phase flow area. Other parameters are used in this investigation
of the oval and circular tube banks, forward and downward configurations, and
the (k-?) standard model is used.
Figures 16 and 17 show the
temperature gradient between water-air flow and surfaces of the circular and oval tube
at various points in the duct. Figures 18 and 19 illustrate the inlet and
outlet pressure in the duct at many angles of attack for circular and oval tube
banks, respectively.
Figures 20 and 21 show the performance parameter at various positions in the duct at many angles of attack for circular and oval tube banks, respectively. The performance of Delta winglets in the forward configuration is higher than the other winglets, and these results agree with, Haque and Rahman (2020), Naik and Tiwari (2020c), and Chu, He, and Tao (2009).
Figure 16 The temperature gradient for circular tube banks in two-phase flow at an angle of attack (a) 15o, (b) 20o,
(c) 25o
Figure 17 The temperature gradient for oval tube banks in two-phase flow at an angle of attack (a) 15o, (b) 20o,
(c) 25o
Figure 18 Inlet and Outlet pressure in the duct for
circular tube banks in two-phase
flow at an angle of attack
(a) 15o, (b) 20o, (c) 25o
Figure 19 Inlet and Outlet pressure in the duct for oval tube banks in two-phase flow at an angle of attack (a) 15o, (b) 20o, (c) 25o
Figure 20 Performance parameter for circular tube banks in two-phase flow at an angle of attack (a) 15o, (b) 20o, (c) 25o
Figure 21 Performance parameter for oval tube banks in two-phase flow at an angle of attack (a) 15o, (b) 20o, (c) 25o
When
water-air flow rates are increased in the absence of winglets, the heat
convection rate reduces due to the temperature gradient for the mixture flow
and the tubes' bank surface raises with less intensity, reducing the pressure
loss in the duct. In water-air flows with delta and rectangular winglets, the
heat convection coefficient increases as the water and air flow rates increase.
As the temperature difference decreases, the pressure loss in the duct also
decreases.
Comparing the absence and
presence of winglets in the water flow across oval tube banks, as the Re number
raised, the performance parameter decreased due to a large fall in pressure
within the duct as the temperature gradient for the water flow and wall of tube
decreased. The performance of delta winglets is greater than that of other
vortex generators. In water-air flows across tube banks with and without vortex
generators, the discharge of air increases while the water discharge remained
unchanged. The performance parameter decreases substantially as the pressure
within the duct is reduced and the temperature gradient for the water-air flow
and tube bank surfaces increases. The performance of delta VGs is higher than
that of other kinds of (VGs). Delta
winglets perform optimum at a 15° angle of attack for banks of oval tubes in
two-phase flow with a moderately turbulent flow rate due to the lowest pressure
loss in the duct relative to the other two angles of attack, which corresponds
to an increase in heat convection. Researchers should continue to develop novel
designs for vortex generators in future work in this field by investigating
their shape, length, height, and position.
I want to express my gratitude to my
supervisor, Dr. Riyadh S. Al-Turaihi, for his assistance throughout my time at
the University of Babylon. Additionally, I would like to express my gratitude
to the faculty and staff of the University of Babylon, Faculty of Engineering,
Mechanical Engineering department for their assistance and education.
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