2007 Han Vent-Cap Wind

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A Numerical Study on the Improvement of Exhaust Performance

of a Vent Cap by Wind Effects

Hwataik Han1, a

, Suk-Hyo Cho2,b

, Kwang-Young Kim3,c

, Jung-Ho Hwang4,d

and Seong-Hwan Yi5,e

1School of Mechanical Engineering, Kookmin University, Seoul 136-702, Korea

2Graduate School, Kookmin University, Seoul 136-702, Korea3Air-Fine Co., Yoosung-Gu, Taejeon, 305-380, Korea

4Department of Mechanical Engineering, Yonsei University, Seoul 120-749, Korea5Ventopia, Jeongro-Gu, Seoul 110-040, Korea

[email protected], [email protected], [email protected], [email protected],[email protected]

ABSTRACT

Ventilation systems are mandated in new residential apartment buildings in addition to public andcommercial buildings recently in Korea. Mechanical ventilation systems and/or natural ventilationschemes should be implemented to achieve the required air change rate of 0.7 ACH. The energyconsumption by exhaust fans can be minimized by utilizing properly designed ventilation systemswith high efficiency. An air cap with a ventury-type opening are developed so as to improve theexhaust performance by taking advantage of surrounding winds around buildings. Airflow patternsand pressure distributions are simulated numerically around the vent cap using the CFD method.

Results indicate that there are complicated airflow patterns around the vent cap and that there areadditional airflows through exhaust duct induced by the Bernoulli effects. The exhaust performances

are compared with the case of simple opening geometry without any vent cap. The system parametersare wind speed, wind direction, and exhaust duct pressure. Discussions are included on theimprovements of the exhaust performance of the vent cap depending on the system parameters.

KEYWORDS: Ventilation, Air vent cap, Exhaust performance, Wind effect, CFD simulation

1. INTRODUCTION

As there is a growing concern on indoor air quality in Korea recently, the IAQ Act has been legislated

(2003) and the Construction Law has been revised (2006) to meet the social needs. Building energyconsumption is expected to increase due to additional ventilation system operations. The energyconservation is a new issue related to building ventilation. Heat recovery systems would help to saveheating/cooling loads of buildings. Duct systems should be composed of equipments and componentswith high efficiency and low flow resistance to save fan power consumption.

The vent cap is used for termination of ducts or chimneys. The vent cap should be able to preventrain and debris penetrating into the duct systems. Also, it should have proper wind resistance in order not to be affected by surrounding wind conditions especially for combustion applications. Many proprietary vent caps have been designed to stabilize the wind effect and improve the performance.

Tests methods are described in UL standard 441 for gas vent caps (1996). The performance of the ventcaps for ventilation purposes is not so critical as the gas vent caps for combustion appliances. Simple

vent caps are frequently used, which can go well with building exterior design.



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There are only a few previous studies on vent cap performances (Hama and Downing, 1963,Karava et al, 2004). Most of the studies (Evans, 1957) deal with the layout of the locations of exhaustoutlets or supply inlets in terms of the airflow patterns and pressure distributions around buildings.Academic researches on the airflows around air vent caps can be barely found (Sepsy and Pies, 1972),

even though there are commercial vent caps claiming fluid dynamic design.It is the purpose of the present paper to investigate the airflow patterns numerically around a

ventury-type vent cap. Several vent caps are being developed so as to improve the exhaust performance by taking advantage of surrounding winds around buildings. This paper is intended to provide theoretical backgrounds for developing aerodynamic vent caps. The exhaust performance iscompared with that of a simple opening for various system parameters.

2. ANALYSIS

2.1 Vent cap model

The vent cap investigated in this study, which is axis-symmetric, is shown in Figure 1. The ventury plate is held to the base with six cylindrical supports.The base is attached to a vertical building wall. Theduct diameter is 125mm, and the minimum gap betweenthe plate and the base is 125mm. The airflow through

the gap is expected to be nearly parallel to the wallregardless of wind directions. According to theBernoulli equation, the static pressure at the neck decreases as the fluid velocity increases. The pressuredecrease at the neck helps discharging air through theexhaust duct.

2.2 CFD analysis

The overall 3-D calculation domain is 3 x 3 x 3m. A simple circular opening of 125mm in diameter islocated at the middle of the wall. The vent cap is attached to the opening. The cross-section of thecentre plane is shown in Figure 2. Inlet boundary conditions are given along the top and left boundaries, and outlet boundary conditions are given for the rest of the flow boundaries. The direction

of wind, θ, is zero when the free stream velocity is parallel to the building wall. The pressure boundarycondition is applied to the exhaust duct outlet to simulate the exhaust pressure of a duct system.

Grids are set fine near the vent cap and coarse in the far field. The total number of grid is 73000.The grids around the vent cap are shown in Figure 3. In case there is no vent cap, there is no solidobstacle in the calculation domain. This case (Case 0) is used as a base for comparison purposes. Thenumber of grid is 62000 for Case 0.

Figure 2. Calculation domain and boundary conditions for CFD analysis

Figure 1. A model for a ventury-type air cap.

InletOutlet

P=0

Pressure boundary

VVVV

d=125

P0

QQQQ

D=375mm

H=177mm

Inletθ

YL

XL

Wall

h=125mm

Ventury plate

Base

InletOutlet

P=0

Pressure boundary

VVVV

d=125

P0

QQQQ

D=375mm

H=177mm

Inletθ

YL

XL

Wall

h=125mm

InletOutlet

P=0

Pressure boundary

VVVV

d=125

P0

QQQQ

D=375mm

H=177mm

Inletθ

YL

XL

Wall

h=125mm

Ventury plate

Base



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2.3 Governing equations and system parameters

The flow is assumed to be 3-dimensional steady and incompressible. The governing equations arecontinuity equation and three momentum equations. These equations are solved simultaneously along

with the equations for turbulent quantities, such as turbulent kinetic energy and turbulent dissipationrate. The standard k-ε model is applied for turbulence modelling. The SIMPLE (Semi-Implicit Methodfor Pressure Linked Equations) algorithm is used to solve the coupled pressure-linked equations.Details of the governing equations and the numerical procedures are not included in this paper, but can be found in Patankar (1980). A commercial CFD program is utilized for the present study.

The system parameters are wind speed, wind direction, and exhaust duct pressure. The ranges of the parameters are shown in Table 1. The volumetric airflow rate induced by the wind is calculated atthe exhaust opening, which is calculated by integrating the normal velocities multiplied by the mesh

areas for each grid over the exhaust opening.

Table 1. System parameters applied for CFD analysis

System parameters Parameter Ranges

Wind speed (V)

Wind direction (θ)

Exhaust pressure (Po)

0 - 30 m/s

0 - 90o

0 - 100 Pa

3. RESULTS AND DISCUSSIONS

The volumetric airflow rate passing through the exhaust duct outlet is calculated. When the exhaust

duct pressure is given zero, the exhaust airflow rate is generated by wind due to the vent cap.

Figure 4 shows the effect of wind speed on the exhaust airflow rate. As the wind speed increases, theairflow rate increases linearly. This is due to the Bernoulli effects. The flow induction cannot beobserved for Case 0, which results in airflow rates nearly zero. The amount of airflow induced by theBernoulli effects can be expressed with the following equation.

KV Q = (1)

where V is the wind speed (m/s), and Q is the airflow rate (m3/s). The coefficient of flow induction, K,

is approximately 0.0022 for the present case.When the exhaust duct pressure is applied, the airflow occurs due to the combination of the flow

discharge by the pressure difference and the Bernoulli induction. It can be easily expected that theairflow rate would increase as the exhaust duct pressure increases. Figure 5 shows the effect of theduct pressure on the exhaust airflow rate. The dotted lines show the case when there is a singleopening without a vent cap. It can be noticed there is a significant increase in the exhaust flow rateregardless of the duct pressure.

Figure 6 shows the airflow rates for duct pressures of 0, 50, and 100 Pa in comparison with thecases without a vent cap. It shows the airflow rates are composed of the induction by Bernoulli effectsand the discharge by duct pressure. The effect of Bernoulli induction increases as the wind speedincreases linearly regardless of the duct pressure.

Figure 3. CFD grids around the vent cap



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-0.02

0.00

0.02

0.04

0.06

0.08

0.10

0 10 20 30 40

Wind Speed (m/s)Wind Speed (m/s)Wind Speed (m/s)Wind Speed (m/s)

A i r f l o w

R a t e

( m 3 /

s )

A i r f l o w

R a t e

( m

3 /

s )

A i r f l o w

R a t e

( m

3 /

s )

A i r f l o w

R a t e

( m 3 /

s )

Case 0

Case 1

Figure 4. Airflow rates as a function of wind speed induced by the vent cap. (P0=0 Pa)

0.00

0.05

0.10

0.15

0.20

0 20 40 60 80 100

Duct Pressure (Pa)Duct Pressure (Pa)Duct Pressure (Pa)Duct Pressure (Pa)

A

i r f l o w

R a t e ( m 3 / s )

.

A

i r f l o w

R a t e ( m 3 / s )

.

A

i r f l o w

R a t e ( m 3 / s )

.

A

i r f l o w

R a t e ( m 3 / s )

.

Case 0

Induction by Bernoulli effects

Discharge by duct pressure

Case 1

Figure 5. Airflow rates as a function of duct pressure in comparison with Case 0. (V=10 m/s)

Figure 6 Airflow rates generated by Bernoulli effects and by duct pressure.

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0 5 10 15 20 25 30 35 40

Wind Speed (m/s)Wind Speed (m/s)Wind Speed (m/s)Wind Speed (m/s)

A i r f l o w

R a t e

( ( m 3 / s )

A i r f l o w

R a t e

( ( m 3 / s )

A i r f l o w

R a t e

( ( m 3 / s )

A i r f l o w

R a t e

( ( m 3 / s )

Discharge by

duct pressure

with Vent Cap

w/o Vent Cap

Po=50 Pa

Po=100 Pa

Po=0 Pa

Induction by

Bernoulli effects



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The airflow pattern and pressure distribution along the center plane around the vent cap areshown in Figure 7 when the flow velocity is 10m/s. Airflow is parallel to the wall and velocity isslightly lower near the wall due to a no-slip condition. High-pressure regions are formed upstream of the cylindrical supports. It can be observed that the pressure between the supports inside the vent cap

is slightly reduced by the Bernoulli effects, which works as an additional driving force for exhaust.

(a) Airflow distribution (b) Pressure distributionFigure 7. Airflow pattern and pressure distribution around the vent cap. (V=10 m/s, P0=0 Pa)

The effect of wind direction with respect to the wall has been obtained as is shown in Figure 8.The angle of 90 degree means a perpendicular flow with respect to the wall. As the angle increases the

airflow rate decreases. The dynamic wind pressure is converted to the static pressure, which hindersairflow coming out of the duct. When the angle is over 80 degree, the flow direction is reversed. InCase 0, however, the flow reversal occurs at 50 degree approximately. The pressure distributions arenot included in the paper, but the pressure is observed to increase near the wall as the angle increases.It has been also observed that the airflows are nearly parallel to the wall regardless of the wind angle,which creates positive induction effects for different wind angles also.

-0.10-0.10-0.10-0.10

-0.05-0.05-0.05-0.05

0.000.000.000.00

0.050.050.050.05

0.100.100.100.10

0000 30303030 60606060 90909090

Wind Direction (deg)Wind Direction (deg)Wind Direction (deg)Wind Direction (deg)

A i r f l o w R

a t e

( m

3 / s )

A i r f l o w R

a t e

( m

3 / s )

A i r f l o w R

a t e

( m

3 / s )

A i r f l o w R

a t e

( m

3 / s )

Case 0

Case 1

Figure 7. Airflow rate with respect to wind direction (V=10 m/s, P0=50 Pa)

4. CONCLUSIONS

This paper investigated the enhancement of the exhaust performance by a vent cap compared to the

case of a simple opening. Computational fluid dynamic analysis has indicated that the exhaust airflow

rate increases with the wind speed increases. The coefficient of flow induction has been defined toexpress the induction performance of the vent cap quantitatively by the Bernoulli effects. The



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discharged flow rate has been found nearly proportional to the exhaust duct pressure with the windspeed fixed. The total exhaust flow rate was the sum of the flow induced by the Bernoulli effects andthe flow discharged by the duct pressure. CFD results exhibited comprehensive information about theair velocity and pressure distributions around the vent cap. The dependence of the wind direction has

indicated that the flow rate decreases with the angle increases. The exhaust performance by the ventcap should be optimised over wide range of system parameters including wind speed, wind direction,

and exhaust duct pressure. Further researches are necessary to examine other geometries of vent capsand to find out an optimal design conditions.

ACKNOWLEDGMENTS

This research was supported by a grant (06ConstructionCoreB02) from Construction CoreTechnology Program funded by Ministry of Construction & Transportation of Korean government.

REFERENCES

Ministry of Environment, 2003, Indoor Air Act for Public Buildings, Statute 6911.Ministry of Construction and Transportation, 2006, Building Construction Law, Standards for

Building Facilities-Revised, Statute 497.

UL 1996, Gas Vents, Standard 441-96.Hama, G.M. and Downing, D.A., 1963, “The characteristics of weather caps,” Air Engineering

(December): 34.Karava, P., Stathopoulos, T., and Athienitis, A.K., 2005, “Wind driven flow through openings- A

review of discharge coefficients,” Int. J. of Ventilation, Vol. 3, No. 3.

Evans, B.H., 1957, “Natural air flow around buildings,” Texas Engineering Experiment Station

Research Report 59.Sepsy, C.F. and Pies, D.B., 1972, “An experimental study of the pressure losses in converging flow

fittings used in exhaust systems,” Ohio State University college of engineering, Columbus, OH

(December)ASHRAE, 2004, Handbook-Fundamentals, Atlanta, American Society of Heating, Refrigerating and

Air-Conditioning Engineers, Inc.Patankar, S.V., 1980, Numerical Heat Transfer and Fluid Flow, New York, Hemisphere Publishing

Corporation.

2007 Han Vent-Cap Wind

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