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ISSN : 1598-6721(Print)
ISSN : 2288-0771(Online)
The Korean Society of Manufacturing Process Engineers Vol.17 No.6 pp.177-185
DOI : https://doi.org/10.14775/ksmpe.2018.17.6.177

# A Numerical Study for Optimum Design of Dust Separator Screen Based on Coanda Effect

Seong-Min Yun*, Yong-Sun Kim*, Hee-Jea Shin**, Sang-Cheol Ko***#
*Graduate School, Department of Mechanical Engineering, Jeonju UNIV.
**Carbon Technology, Jeonju UNIV.
***Mechanical and Automotive Engineering, Jeonju UNIV.
Corresponding Author : scko@jj.ac.kr Tel: +82-63-220-2623, Fax: +82-63-220-3161
26/10/2018 04/11/2018 22/11/2018

## Abstract

There is a need to study dust separator screens with good drainage efficiency while effectively filtering suspended solids and other contaminants entering the intake pumping station, the drainage pumping station and the mediation pumping station, the cooling water inlet of the power plant, and the like.

In this paper, Numerical studies were conducted for the optimal design of the dust separator screen using the Coanda effect. The shape of the dust separator screen is important, such as the right curvature radius R1 at the top of the dust separator screen and the left curvature radius R2 at the top, h is the height difference and shape between the screen and the accelerating plate, and θ is the inclination angle of the screen. A total of 4 shape factors were set and the effects of Coanda and drainage performance of each element were compared and analyzed, the optimum length and size of each shape element were derived by classifying the shape elements into direct and indirect influences. Finally, it was possible to effectively filter foreign matter by narrowing the screen spacing, and the drainage performance was analyzed and optimized through numerical studies of dust separator screen.

# 코안다효과를 이용한 제진기 스크린의 최적설계를 위한 수치적 연구

윤 성민*, 김 용선*, 신 희재**, 고 상철***#
*전주대학교 기계공학과
**전주대학교 탄소연구소
***전주대학교 기계자동차공학과

## 초록

This is an Open-Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

## 1. Introduction

Machinery and equipment can effectively filter large amounts of floating matter and foreign matter flowing into water-intake pumping stations, drainage pumping stations, relay pumping stations, and coolant inlets in power plants when large volumes of foreign matter is generated in rivers or the water flow is increased rapidly by rain. If foreign matter flows into the channels, it can flow into the drai nage pumping stations and power plant coolant inlets. This foreign matter will be sucked into motors or pumps and enter pumps’ impellers and impeller casings, causing the pumps to overload, which may even burn out the pumps. Moreover, if foreign matter enters the drainage pumping station and relay pumping station and proper drainage is not performed, it can cause floods, resulting in enormous property damage.[1] To prevent these problems, particulate separators are used to filter foreign matter from rivers and facilitate smooth drainage. Since particulate separators were first introduced, technical developments have been made to improve their problems and performance. However, the performance of particulate separators has not changed significantly. [2]

Recently, localized torrential rainfalls have developed with South Korea’s changing climate patterns, and the amount of foreign matter has greatly increased due to the large amount of flow over a short period. Therefore, we need to develop particulate separators that can quickly drain a large amount of flow while effectively filtering foreign matter.

A particulate separator is divided into a screen unit and rake unit. The screen filters the incoming foreign matter, but if foreign matter accumulates in the screen, it hinders and can halt the water flow. To prevent the foreign matter accumulating, a rake is installed that removes foreign matter from the screen.[1,3]

Existing studies had problems because if the spacing between the screens was reduced to effectively filter foreign matter, it lowered the drain performance and if the spacing were widened to improve drainage, it lowered the particulate separation performance.

This study reduces the screen spacing by introducing the Coanda effect to optimize the shape of the particulate separator screen, thereby achieving excellent drain performance while improving the particulate separation performance.

## 2. Theoretical Background and Numerical Analysis Method

### 2.1 Coanda effect

The Coanda effect refers to the phenomenon of a fluid flowing along a curved surface. The surface pressure along the curved wall gradually increases to the surrounding pressure, and the curved wall and flow jet are separated in this condition.

Bradshaw described the Coanda effect from the perspective of non-viscous, non-rotating flow through a theoretical hypothesis. Assuming that the flow is initially non-viscous, the formula for the flow’s pressure behavior is derived from the Bernoulli equation as follows[45]:

$p 0 = p ∞ − ρ U 2 a$
(1)

where ρ is the density of the ejected fluid, U is the mean flow rate, and a is the radius of the curvature. Under the condition of Eq. (2), the pressure of the non-viscous fluid is lower than the surrounding pressure.

Here, bis the spacing of the ejected slots.[46]

$ρ U 2 b a ≤ p ∞$

The mean speed decreases while the surface pressure along the wall increases and equals the atmospheric pressure. When the condition of the surface pressure ps is $p s = p ∞$ , flow separation from the curved surface occurs.[4]

The Coanda effect can be explained with a few physical parameters. Through a Coanda flow analysis at a high Reynolds number by Newman in Eq. (3), the flow along a cylinder is described as the following proposed equation[4]:

$θ s e p = f [ ( ( p 0 − p ∞ ) ⋅ b ⋅ a ρ ⋅ v 2 ) 1 2 ]$
(3)

where, $θ s e p$ is the jet flow separation angle, b is the slot width, a is the radius of the curvature, and $( p 0 − p ∞ )$ is the difference between the pressure on the wall surface and the atmospheric pressure. This shows that the flow separation angle $θ s e p$ is a function of the pressure difference, the cylinder’s geometrical characteristics, and the fluid’s characteristics.[4]

### 2.2 Governing equation

The Shear-Stress Transport (SST) k-ω turbulence model of the ANSYS FLUENT 16.0 was used to predict the drain performance and flow of the water flowing into the particulate separator screen and accurately predict the flow near the wall. The SST k-ω model combines the k - ∈ model, which can analyze the flow of a high Reynolds number appropriately and the k-ωmodel, which simulates the low Reynolds number area near the wall. Eqs. (4) and (5) are the transportation equations of the kinetic energ k and the specific dissipation rate ω. In Eq. (5), $D ω$ is a cross-diffusion term. The k-ω model is used to correctly predict the viscous effect near the wall and the k-∈model is used in the flow’s internal area.[79]

$∂ ∂ t ( ρ k ) + ∂ ∂ x i ( ρ k u i ) = ∂ ∂ x j ( Γ k ∂ k ∂ x j ) + G ˜ k − Y k + S k$
(4)
$∂ ∂ t ( ρ ω ) + ∂ ∂ x i ( ρ ω u i ) = ∂ ∂ x j ( Γ ω ∂ ω ∂ x j ) + G ω − Y ω + S ω$
(5)

Here, the viscosity terms Γk and Γω are defined by the strain rate and specific dissipation rate instead of the Reynolds number to reflect the transfer effect of the turbulent stress. Yk and Yare dissipation terms that apply the regression analysis (piecewise) method. In Eq. (4), $G ˜ k$ the generation term of k, is a corrected value of Gkfor the flow strain and the specific dissipation rate ω.[7~9]

### 2.3 Analysis variables and boundary conditions

To verify the Coanda effect according to the shape of the particulate separator screen, analysis variables were set for each part of the screen shape to be applied to numerical analysis. Fig. 1

Fig. 2 shows the shape factors for the particulate separator screen. In total, four variable factors were given: the radius of the curvature R1(0-30mm), that can directly influence the Coanda effect; the radius of the curvature R2(0-20mm), that can indirectly affect the Coanda effect and affect the fluid’s main flow; h(0-15mm), ), which represents the height difference between the acceleration plate that increases the flow rate of water and the screen shape; and the angle θ (10-40°) that influences the flow rate in the fluid entering the screen.[10]

The number of screen shapes was identical at 10, the spacing between the shapes was fixed at 10 mm, and the length of the acceleration plate was 800 mm.

For the flow field and outlet, the atmospheric pressure 101.325 kPa was applied as the initial condition. Fig. 3 shows the boundary conditions and a rough system that was applied in the analysis. Water with a 100 kg/s flow rate flows into the inlet, the water that passes through the screen filter is drained through Outlet and the water that does not pass through the screen is drained through Outlet.

### 2.4 Evaluation of the drain performance for the particulate separator screen

Water flows into the inlet and drains through Outlet and Outlet.

The higher the screen’s drain performance, the larger the flow rate through Outlet1, which confirmed the screen’s drain performance.

The drain performance, which is determined by comparing the water flow rate into the inlet and the flow rate of the water cleaned through the screen draining through Outlet, can be expressed as Eq. (6).[11~12]

$D r a i n a g e P e r f o r m a n c e ( % ) = O u t l e t 1 I n l e t × 100$
(6)

## 3. Numerical Analysis Results

### 3.1 Flow analysis according to the radius of the R1

In the screen shape, R1is a shape factor that can directly influence the Coanda effect. The drain performance of the screen was analyzed while increasing the radius of the curvature R1 from 0 to 30 mm in 5 mm intervals considering the shape fabrication. Table 1 shows the flow rate and drain performance of Outlet and Outlet according to the radius of the curvature R1. Fig. 4 shows the drain performance of the screen according to the radius of the curvature R1.

The analysis results show that the drain performance was 0.52% when the radius of the curvature R1was 0 mm. Starting from 10 mm or higher, a low-pressure zone from the Coanda effect was formed on the surface of the screen shape and the drain performance also increased greatly.

Fig. 5 shows the flow rate and pressure distribution of water according to the curvature radius. The larger the radius of the curvature R1, the more the low-pressure zone develops on the curvature surface and the faster the flow rate of the water draining through the shapes. As the radius of the curvature R1 of the shape increased, the drain performance improved and the pattern of the low-pressure zone changed on the shape surface.

Fig. 6 shows the water speed vector when the radius of the curvature R1 was 30 mm. The development of the water speed vector by the Coanda effect can be seen on the surface of the screen shape.

Fig. 7 shows the pressure distribution when the radius of the curvature R1 was 30 mm. The development of a large low-pressure zone by the Coanda effect can be seen on the shape’s surface, which is consistent with the results of the speed vector in Fig. 6.

### 3.2 Flow analysis according to the radius of curvature R2

The drain performance of the screen shape was analyzed according to the radius of curvature R2. It is expected that R2will have a small effect on the Coanda effect, but can influence the main flow of the incoming water. Since high pressure occurs at R2 the difference needs to be checked. For the screen shape, R1=30mm was used, which showed the best performance in Section 3.1, and the value of R2 was increased from 0 to 20 mm in 5 mm intervals.

Table 2 shows the flow rate and drain performance of the fluid draining through Outlet and Outlet according to the radius of the curvature R2 The flow analysis results showed that the drain performance was the largest at the R2 value of 5mm, and an approximately 1–3% performance difference was observed at each different R2 value.

In the pressure distribution in Fig. 9, the radius of the curvature R2 does not significantly affect the Coanda effect. However, as the R2 value increases, a low-pressure zone was formed at the meeting point of R1 and R2 at top of the screen shape and the pressure rapidly became similar to the atmospheric pressure. Then, a flow separation occurs and the amount of water flowing on the shape surface decreases, lowering the drain performance.

### 3.3 Flow analysis according to h

A flow analysis was performed because h, which represents the acceleration plate to adjust the water speed and the top of the screen shape, can influence the shear stress and Coanda effect at the top of the screen shape. For R1 and R2, 30 mm and 5 mm were used, respectively.

In Table 3 and Fig. 10, as the value of h increased, the drain performance tended to improve, but the drain performance dropped sharply when the value of h became 15 mm.

Fig. 11(a) and (d) show that drainage occurs from near the center of the particulate separator screen. In contrast, the drainage occurs evenly for every shape in (c) and (d) from the start to the end of the particulate separator screen.

### 3.4 Flow analysis according to θ

The water flow velocity accelerates as the angle θ increases. Since the flow velocity can have a large effect on the Coanda effect and pressure distribution, the effect of the flow velocity on the drain performance was observed while changing the size of angle θ .

The dimensions optimized in this study were: 30 mm for R1 , 10 mm for R2 , and 10 mm for h. Table 4 shows the flow rates of Outlet and Outlet according to θ and Fig. 12 shows the drain performance.

The smaller the angle θ , the larger the drain performance. When θ was 10°, the drain performance was 100%. Comparing the flow velocity and pressure distribution on the screen shape surface in Fig. 13 shows that a large flow velocity developed on the shape surface and the pressure of the low-pressure zone decreased further. In contrast, at a θ of 50% where the drain performance was 50%, almost no low-pressure zone formed on the shape surface.

## 4. Conclusion

The screen shape factors of the particulate separator that can influence the Coanda effect were determined and the drain performance was numerically analyzed while changing the sizes of factors.

1. For the radius of the curvature R1, the Coanda effect was caused by the low-pressure zone that formed on the surface of the particulate separator screen and the development of flow velocities in the low-pressure zone, and it had the largest effect on the drain performance.

2. The amount of water that flowed along the shape surface due to the Coanda effect was the largest when the radius of curvature R1 was 30 mm.. The analysis results showed that shape factors other than R1 had small effects on the Coanda effect.

3. The size of θ mainly influenced the water flow velocity. The smaller it was, the lower the flow velocity and the higher the drain performance.

4. When each shape factor was analyzed from an optimum design perspective, the drain performance was highest at 96.73% when R1 was 30 mm, R2 was 5 mm, and h was 10 mm.

## Acknowledgment

This research was supported by Basic Science Research Program through the National Research Foundation of Korea funded by the Ministry of Education (No. 2016R1A6A1A03012069). This work was also supported by the national Research Foundation of Korea grant funded by the Korea government (MSIP) (No. 2017R1 D1A1B03036070).

## Figure

Hydro Intake Screens
Schmatic Diagrams of the Screen Shape Variable
Boundary Condition of Screen Analysis
Drainage Performance According to R1
Stream line and Distribution of Water Flow about R1 (Left : Velocity, Right : Pressure)
Velocity Vector of Water Flow about R1 = 30[mm]
Pressure Distribution of Water Flow about R1 = 30[mm]
Drainage Performance According to R2
Pressure Distribution of Water Flow about R2
Drainage Performance According to h
Stream line and Distribution of Water Flow about h (Left : Velocity, Right : Pressure)
Drainage Performance According to θ
Stream lne and Distribution of Water Flow about θ (Left : Velocity, Right : Pressure)

## Table

Comparison of Flow According to R1
Comparison of Flow According to R2
The Comparison of Flow According to h
Comparison of Flow According to θ

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