Intelligent simulation experimental study on influence of air velocity of air supply hood and exhaust hood with vertical push-pull ventilation

Abstract

The occupational health and safety problems caused by indoor dust and toxic gases in industrial plants are becoming increasingly serious. Vertical push-pull ventilation can effectively control dust and toxic gases and avoid indoor environmental pollution. The velocity of the air supply hood and exhaust hood in the push-pull ventilation intelligent device directly affects the control effect of the dust and toxic gases. However, no scholar has given a reasonable ratio of the air supply and exhaust velocity of the vertical push-pull ventilation device. Therefore, this paper uses the combination of Fluent numerical simulation technology and smoke visualization experiment to explore the best ratio of the air supply and exhaust velocity in the push-pull ventilation flow field. The research shows that the traces of the wind flow obtained by numerical analysis under different k which is the ratio of the air supply and exhaust velocity is consistent with the traces of visualized smoke in the actual experiment; The optimal ratio of air supply and exhaust is k = 1.75, which can effectively guarantee the indoor environment, occupational health and work efficiency of the operators.

Keywords

Dust and toxic gases push-pull ventilation ratio of the air supply exhaust velocity smoke visualization intelligent

1 Introduction

The occupational health and safety problems caused by indoor dust and toxic gases in industrial plants are becoming increasingly serious. Effective control of dust and toxic gases can avoid pollution of the indoor environment, maintain occupational health of workers, and provide work efficiency for operators [1, 2]. Local ventilation is an effective method to control dust and toxic gases [3]. The research shows that vertical push-pull ventilation is a new type of local ventilation, which has obvious advantages compared with local ventilation with only one side exhaust hood. It has better control effect on dust and toxic gases and has been gradually promoted and applied in recent years [4]. The vertical push-pull ventilation device is mainly composed of Air Supply Hood and Exhaust Hood (see Fig. 1), and the wind flow is sent out from the upper hood and exhausted through the lower hood. A push-pull ventilation flow field is formed between the air supply hood and exhaust hood, and the wind velocity of the air supply hood and exhaust hood directly affects the control effect of the dust and toxic gases.

Fig.1

Vertical push-pull ventilation device schematic diagram.

Some scholars have noticed this problem and studied the aerodynamic characteristics of the push-pull ventilation device [5], the collection efficiency of dust and toxic gases [6] and the evaluation methods [7, 8]. Suyan B H found in the study of the regression equation of the push-pull ventilation control distance that the wind velocity of the exhaust hood is an important factor affecting the control effect of dust and toxic gases [9]; Noting the influence of wind velocity on the exhaust cabinet, Chen D J studied the effect of air supply on the airflow of the laboratory exhaust cabinet [10]. In the study of the velocity variation law of the square exhaust hood [11] and the variation law of the axial velocity [12] using the dimensionless method, Chen Jianwu and others found that the wind velocity of the exhaust hood has an important influence on the control effect of the local exhaust hood.

However, the above research only studies a single air supply hood or exhaust hood. The research on the effect of the combined air supply hood and exhaust hood in the vertical push-pull ventilation is not clear. Therefore, in order to explore the reasonable velocity of the push-pull ventilation flow field, it is urgent to study the characteristics of the vertical push-pull ventilation flow field under different ratio of the air supply and exhaust velocity and providing design guidelines and references for efficient control of dust and toxic gases.

2 Methods

2.1 Research condition

The vertical push-pull ventilation device is geometrically modeled by GAMBIT design software. The upper air supply hood is 118.0cm long and 63.0cm wide. The lower exhaust hood is 120.0cm long and 65.0cm wide. The distance between the air supply hood and the exhaust hood is 120.0 cm, and a cubic computing domain of 300.0 cm×300.0 cm×300.0 cm is established to simulate the indoor working environment. Since the push-pull ventilation device is used to control Dust and toxic gases, obstacles in the flow field are inevitable. In order to ensure the authenticity of the simulation results, an obstacle of 30cm×20cm×10cm is placed on the exhaust hood, which is located at the center of the windward side of the exhaust hood. The ratio of the air supply and exhaust velocity is recorded as k. The model of the vertical flow suction ventilation device is shown in Fig. 2. The origin of the coordinate is the center point of the upper surface of the exhaust hood, and the direction of the coordinate axis is as shown.

Fig.2

Vertical push-pull ventilation device model and mesh.

The air supply hood has a fixed wind velocity of 0.4m/s, and the ratio of the air supply and exhaust velocity is set to 0.25 times. That is to say, k = 0.75, 1.00, 1.25, 1.50, 1.75, 2.00 working conditions are selected, and the wind velocity of the exhaust hood is changed from 0.3 m/s to 0.8 m/s, and the simulation is performed by increasing each time by 0.1 m/s. The gravity factor is added to the operating environment settings and the gravitational acceleration is set to 9.81 m/s2.

2.2 Mesh

The configurable ventilator model is divided by a tetrahedral hybrid mesh (Tet/Hybrid), which is dominated by a tetrahedral mesh, a hexahedral mesh is used in some areas, and the meshing density is 0.5. Through the grid-independence test, the mesh of the model boundary is guaranteed to be accurately divided. The model is shown in Fig. 2.

2.3 Control equations, boundary conditions and solution parameters

The flow field solution uses a single-precision 3D solution and a standard k-ɛ dual equation model. The standard k-ɛ double equation model has reasonable precision and is widely adopted in engineering simulation calculation. The specific form is shown in Equations 1 and 2. The solver uses the SIMPLE algorithm (semi-implicit method for pressure-linked equations). The calculation model parameter settings are shown in Table 1.

Table 1
Calculation model setting table

Solve Define

Solver Segregated

Viscous model k-epsilon

Pressure-velocity coupling SIMPLEC

Discretization scheme Second order upwind

Convergence criterion 10-6

Solve	Define
Solver	Segregated
Viscous model	k-epsilon
Pressure-velocity coupling	SIMPLEC
Discretization scheme	Second order upwind
Convergence criterion	10-6

k Equation: $\frac{\partial}{\partial x_{i}} (ρ u_{i} k) = \frac{\partial}{\partial x_{i}} [(μ + \frac{μ_{t}}{σ_{k}}) \frac{\partial k}{\partial x_{i}}] + G_{k} - ρ ɛ$ (1)

Equation:

$\begin{matrix} \frac{\partial}{\partial x_{i}} (ρ u_{i} ɛ) & = & \frac{\partial}{\partial x_{i}} [(μ + \frac{μ_{t}}{σ_{k}}) \frac{\partial k}{\partial x_{i}}] \\ + \frac{C_{ɛ} 1 ɛ}{k} G_{k} - C_{ɛ 2} ρ \frac{ɛ^{2}}{k} \end{matrix}$ (2)

The boundary condition of the air supply hood is Velocity-inlet, the air supply velocity is 0.4m/s, the hydraulic diameter is 0.82m, the turbulence intensity is 4.69%; the boundary condition of the exhaust hood is set to Velocity-inlet, and the exhaust wind velocity is 0.6 m/s; the hydraulic diameter is 0.84m, the turbulence intensity is 4.44%, and the remaining model boundaries are set to WALL Condition. The solution parameters are shown in Table 2.

Table 2

Boundary conditions table

Location	Boundary conditions	Define
Air supply hood	Inlet boundary type	Velocity-inlet
	Inlet velocity magnitude (m/s)	0.40
	Hydraulic diameter (m)	0.82
	Turbulence intensity (%)	4.69
Exhaust hood	Outlet boundary type	Velocity-inlet
	Inlet velocity magnitude (m/s)	–0.60
	Hydraulic diameter (m)	0.84
	Turbulence intensity (%)	4.44

3 Results

3.1 Numerical simulation analysis

The model of the push-pull ventilation device with the ratio of the air supply and exhaust velocity k of 0.75, 1.00, 1.25, 1.50, 1.75, 2.00 is established, and the flow field is solved. Since the push-pull ventilation device has symmetry, the XOZ and YOZ sections are established, and the long side section and the short side section of the device can be observed separately. The velocity vector of the section is shown in Fig. 3. The velocity vector is also the traces of the wind.

Fig.3

Velocity vector under different k conditions, (a) velocity vector in XOZsection, (b) velocity vector inYOZsection.

As can be seen from Fig. 3, in the vector diagram of k = 0.75 to k = 1.25, after the airflow is sent out from the air supply hood, part of it is exhausted through the exhaust hood and partly flows downward from both sides of the exhaust hood. And a clear recirculation zone is formed directly below the exhaust hood and below the working environment. In the vector diagram of k = 1.50 to k = 2.00, it can be seen that the airflow sent from the air supply hood is substantially exhausted through the exhaust hood, and there is no recirculation zone in the working environment. From the process of k = 0.75 to k = 2.00, taking the XOY plane as the origin, the positive direction of the Z-axis is the positive wind velocity, and the wind flow traces on both sides of the device is obviously changed. It can be seen that in the device with k = 0.75 to k = 1.25, the velocity vector direction on both sides of the device visible below the XOY plane is the Z-axis negative direction. That is, the airflow from the air supply hood passes through the exhaust hood and continues to below. In the device with k = 1.50 to k = 2.00, the direction of the velocity vector below the XOY plane changes, from the Z-axis negative direction to the Z-axis positive direction. That is, the airflow from the air supply hood is exhausted through the exhaust hood, and the wind flow in the lower part of the XOY plane also partially enters the exhaust hood.

Since the velocity vector diagram can only reach qualitative conclusions, in order to accurately determine the direction of the wind flow around the exhaust hood, the optimal ratio of the air supply and exhaust velocity is explored, and the wind velocity monitoring line is established 2 cm outside the device. The wind velocity line L1 is located 2cm outside the long side of the device, starting point (0, 37, –30), end point (0, 37, 30); The wind velocity line L2 is located 2 cm outside the short side of the device, starting point (–62, 0, –30) and ending point (–62, 0, 30). The wind velocity change of wind velocity line is shown in Fig. 5. In the figure, the abscissa 0 is located in the XOY plane, and the abscissa greater than 0 means that the part is above the XOY plane of the hood plane, and the abscissa of less than 0 means that the part is below the XOY plane of the hood plane.

It can be seen from Fig. 4 that when the abscissa is greater than 0, the wind velocity above the exhaust hood is all less than 0, that is, the wind flow moves in the negative direction of the Z axis; When the abscissa is less than 0, the direction of the wind velocity below the exhaust hood changes with the change of k. When k ⩽ 1.25, the wind velocity below the exhaust hood is less than 0, which means that the wind velocity direction is the negative direction of the Z axis, that is to say, there is still wind flow downward. When k = 1.5, the wind velocity on the long side of the exhaust hood is close to 0, which means that the wind velocity flows in a horizontal direction; However, the wind velocity on the short side of the exhaust hood is still less than 0, that is, the wind flow at the short side will still flow downward beyond the hood. When k ⩾ 1.75, it can be seen that the wind velocity below the exhaust hood is greater than 0, which means that the wind velocity direction is the positive direction of the Z axis, that is, the wind flow flows from the bottom to the top.

Fig.4

Wind velocity trend chart of wind velocity line, (a) L1 wind velocity trend chart, (b) L2 wind velocity trend chart.

3.2 Experimental verification

The results of numerical simulation analysis show that when k ⩾ 1.75, there is no wind flow moving in the negative direction of the Z axis around the exhaust hood, and all the airflow is exhausted by the exhaust hood. In order to observe the traces of the wind flow around the exhaust hood when k ⩾ 1.75, the smoke visualization technology is adopted, and the k = 1.75 working condition is adopted, and the smoke pipe is used for the smoke test on the long side and the wide side of the exhaust hood. In order to avoid the influence of the initial velocity of the smoke pipe, the smoke pipe is placed vertically, and the initial direction of the smoke injection is the negative direction of the Z axis. As can be clearly seen from Fig. 5, after the downward moving smoke moves a small distance in the negative direction of the Z axis, the smoke is turned to the positive direction of the Z axis and then exhausted through the exhaust hood.

Fig.5

Smoke experiment under k = 1.75 condition.

To further verify the accuracy and reliability of the numerical analysis, experiments were carried out in a vertical push-pull ventilation device. Take the experimental conditions of k = 0.75 and k = 1.75. That is, the wind velocity of the air supply is 0.4 m/s, the wind velocity of the exhaust air are 0.3 m/s and 0.7 m/s, respectively, and a rectangular obstacle of 30 cm×20 cm×10 cm is placed in the push-pull ventilation flow field. An aerosol generating device is set up to visualize the blown airflow for easy viewing of the push-pull ventilation flow field characteristics. As a control group, Fig. 6(a) shows the wind traces when k = 0.75, and it can be seen that the visible smoke flows from the upper surface of the exhaust hood and diffuses in the negative direction of the Z axis around the exhaust hood; Fig. 6(b) shows the traces of the wind flow when k = 1.75. At this time, the visible smoke is completely discharged through the exhaust hood, and thepush-pull ventilation device has a good control effect under the working condition.

Fig.6

Wind traces under different conditions, (a) wind traces under k = 0.75 condition, (b) wind traces under k = 1.75 condition.

The experimental results fully show that the results of numerical simulation analysis are accurate and reliable. When k ⩾ 1.75, it can ensure that the wind flow in the push-pull ventilation flow field is completely exhausted through the exhaust hood.

4 Discussion

Through the simulation analysis of the velocity vector and the wind velocity line, combined with the experimental research, it can be found that when k = 0.75 to k = 1.25, the wind flow will cross the exhaust hood and continue to move in the negative direction of the Z axis on both sides of the exhaust hood. At this time, if there are dust and toxic gases in the push-pull ventilation flow field, that cannot be completely discharged by the exhaust hood, and will diffuse into the working environment with the downward flowing wind flow. It accumulates under the hood and under the working environment to form a recirculation zone as shown in Fig. 3, which can have a serious impact on personnel health and work efficiency in the operating environment. When k = 1.50, it is difficult to see the obvious recirculation zone in Fig. 3. However, the data in Fig. 4(b) indicates that the wind velocity at the short side of the exhaust hood is still negative. This means that although the wind flow at the long side of the hood is no longer flowing in the negative direction of the Z axis, the wind flow at the short side of the hood still moves in the negative direction of the Z axis beyond the hood. This will carry dust and toxic gases under the exhaust hood and in the working environment, which is unfavorable for effective control of dust and toxic gases. When k ⩾ 1.75, Fig. 3 shows that there is no recirculation zone in the working environment; It is found from the data in Fig. 4 that the direction of the wind flow around the exhaust hood is obvious, and the positive direction of the Z axis is turned to the positive direction of the Z axis, indicating that the wind flow no longer flows below the exhaust hood, and flows from bottom to top; Combined with the smoking experiment of Fig. 5 and the wind motion trajectory of Fig. 6(b), it can be seen that the smoke whose initial velocity is negative in the Z-axis direction is also exhausted by the exhaust hood. It shows that even if the dust and toxic gases in the push-pull ventilation flow field are diffused, they will be trapped by the airflow of the exhaust hood and will not spread into the working environment, which can fully ensure the occupational health and work efficiency of the indoor environment workers. Although the numerical analysis results show that the k value exceeds 1.75, the larger the k, the better the trapping effect of the exhaust hood on the surrounding dust and toxic gases. However, in practical engineering applications, the larger the value of k, the greater velocity of the exhaust hood. Taking into account the economic problems caused by high wind velocity noise and energy loss, k is 1.75 is the best ratio of the air supply and exhaust velocity. Under this condition, the indoor environment can be guaranteed to ensure the occupational health and work efficiency of the operators.

5 Conclusion

The simulation analysis and experimental study on six sets of vertical push-pull ventilation devices with different ratio of the air supply and exhaust velocity k indicate:

(1) The traces of the wind flow obtained by numerical analysis under different conditions of the air supply and exhaust velocity k is consistent with the traces of visualized smoke in the actual experiment;

(2) When the distance between the suction and suction fields is 1.2m, the optimal ratio of the air supply and exhaust velocity is k = 1.75.That is to say, the wind velocity of the exhaust hood is 1.75 times of the wind velocity of the air supply hood, which can economical and effectively ensure the indoor environment, the occupational health and work efficiency of the workers.

Footnotes

Acknowledgments

This work was supported financially by the National Key R&D Program of China (2016YFC0801700), the basic research funding of China Academy of Safety Science and Technology (2018JBKY01), and the Key Laboratory of Toxic and Dust Hazards Prevention and Control Technology Fund Project (2018KFKT01).

References

Lunden

M.M.

, Delp

W.W.

, Singer

B.C.

Capture efficiency of cooking-related fine and ultrafine particles by residential exhaust hoods, Indoor Air 2015 25 (2015), 45–58.

Huang

, Wang

, Liu

et al., Reduced-scale experimental investigation on ventilation performance of a local exhaust hood in an industrial plant, Building & Environment 85 (2015), 94–103.

Hughes

Robert T.

, An Overview of Push—Pull Ventilation Characteristics,&, Enviromental Hygiene 05 (1990), 156–161.

Huebener

D.J.

, Hughes

R.T.

Development of Push-pull ventilation, American Industrial Hygiene Association Journal 46 (1985), 262.

Huang

R.F.

, Lin

S.Y.

, Jan

S.Y.

et al., Aerodynamic characteristics and design guidelines of push–pull ventilation systems, Annals of Occupational Hygiene 49 (2005), 1–15.

Betta

, Cascetta

, Palombo

Push-pull ventilation system: A CFD approach for the performance analysis, International Journal of Ambient Energy 28 (2007), 123–134.

Mothilal

, Pitchandi

Influence of inlet velocity of air and solid particle feed rate on holdup mass and heat transfer characteristics in cyclone heat exchanger, Journal of Mechanical Science and Technology 29 (2015), 4509–4518.

Cao

Yingxue

, Wang

, Li

Congcong

et al., A field measurement study of a parallel-flow push–pull system for industrial ventilation applications, International Journal of Ventilation 15 (2016), 167–181.

Wang

, Yang

, Wei

et al., Experimental Investigation on the Flow Characteristics of an Exhaust Hood Assisted by a Jet, International Journal of Ventilation 13 (2014), 89–100.

10.

Baptiste

Déjean

, Berthoumieu

and Gajan

, Experimental study on the influence of liquid and air boundary conditions on a planar air-blasted liquid sheet, Part I: Liquid and air thicknesses, International Journal of Multiphase Flow 79 (2016), 202–213.

11.

Liu

, Wang

, Xi

Orthogonal Design on Range Hood with Air Curtain and Its Effects on Kitchen Environment, Journal of Occupational and Environmental Hygiene 11 (2014), 186–199.

12.

Xiang

L.P.

, Wang

H.Q.

CFD Study on Effect of the Air Supply Velocity on Contaminant Transport within an Road Vehicle Cabin, Applied Mechanics and Materials 295-298 (2013), 532–535.

Intelligent simulation experimental study on influence of air velocity of air supply hood and exhaust hood with vertical push-pull ventilation

Abstract

Keywords

1 Introduction

2.1 Research condition

2.3 Control equations, boundary conditions and solution parameters

Table 1 Calculation model setting table Solve Define Solver Segregated Viscous model k-epsilon Pressure-velocity coupling SIMPLEC Discretization scheme Second order upwind Convergence criterion 10-6

3.1 Numerical simulation analysis

5 Conclusion

Footnotes

Acknowledgments

References

Table 1
Calculation model setting table

Solve Define

Solver Segregated

Viscous model k-epsilon

Pressure-velocity coupling SIMPLEC

Discretization scheme Second order upwind

Convergence criterion 10-6