Performance evaluation of ductless ventilation system in basement space of an underground sewage treatment plant: A scaled model case study

Building and Environment 160 (2019) 106211

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Performance evaluation of ductless ventilation system in basement space of an underground sewage treatment plant: A scaled model case study

T

Xin Zhanga, Lei Donga,b, Yan Xiaoa, Xuehai Zhuc, Lu Fengc, Ran Juc, Naiping Gaoc,* a

Shanghai Municipal Engineering Design Institute (Group) Co., Ltd, China State Key Laboratory of Pollution Control and Resources Reuse, College of Environmental Science and Engineering, Tongji University, Shanghai, 201804, China c School of Mechanical Engineering, Tongji University, Shanghai, 201804, China b

A R T I C LE I N FO

A B S T R A C T

Keywords: Underground sewage treatment plant Ductless ventilation system Unidirectional piston flow Performance evaluation Scale-model experiment

In recent years, the number of underground sewage treatment plants has increased dramatically in densely populated cities. Mechanical ventilation system is necessary to ensure indoor air quality in the enclosed basement space of underground sewage treatment plants. Many disadvantages exist when using conventional overhead duct ventilation system. In this paper, a ductless ventilation system based on the concept of unidirectional piston flow is proposed for such flat large-space in underground sewage treatment plants. The influences of perforated plate porosity, installation of passage partitions, opening state of vehicle passage entrance doors and air supply mode on ventilation performance are investigated by using scale-model experiments. The results suggest that perforated plate porosity is recommended between 0.25 and 0.35 and partitions near vehicle passage entrance need to be installed. With a reasonable design, the unidirectional flow is formed in the whole level. No air leakage from the basement space to ambient environment happens when the pressure at vehicle passage entrance is 200 Pa higher than that at the outlets. However, serious airflow short circuit phenomenon occurs when the pressure difference reduces to −50 Pa in scale model. Present research provides a new option for the design of ventilation system in such kind of large and flat space.

1. Introduction Sewage treatment plants play a key role in urban infrastructure. The typical plants on the ground level are gradually unable to meet the increasingly strict environmental protection standards since that they generate odor and affect the surrounding residents. The quantity of underground sewage treatment plants which can save land resources and improve the environmental quality on the ground level is on the rise in densely populated cities in China. In a common underground sewage treatment plant, the basement space located at the underground first floor is the main inspection area for workers. The basement space connects to the outdoor ambient environment by sloping vehicle passages. For the plant with daily sewage treatment capacity of 500000m3/ day as studied object in this paper, the space size of the basement can be up to 226 m long, 252 m wide and 5.1 m high. It is a very large and flat space. Usually it is empty except for some equipment rooms and structural support pillars. There are several sewage reaction pools under the basement space which are the sources of odors. It is found that sulfur-containing substances and aldehydes are the main pollutants [1]. Relevant studies

*

have found that the risk of workers getting hepatitis A and cancer is greatly increased due to exposure to microorganisms and chemicals when working in basement spaces [2]. The symptom of workers getting nasal inflammation, fatigue and diarrhea is obviously aggravated when airborne toxins concentration exceeds 3.8 ng/m3 [3]. It is necessary to adopt mechanical ventilation system to guarantee the indoor air quality in basement spaces [4,5]. Until now, there are very few investigations on the ventilation system in basement spaces of sewage treatment plants which belongs to typical large flat spaces with a length-to-height ratio and width-toheight ratio greater than 10. Relative studies on the ventilation system of large public garages with similar geometric features could be employed for reference. Natural ventilation, inductive ventilation and mixing ventilation are the three most common ventilation systems adopted in underground garages, as shown in Table 1. Sometimes natural ventilation system is applied for energy saving, but the airflow rate can hardly meet the requirement of air change rate. Ceiling-level jet fans are occasionally amounted as well. However, for a common jet fan with the airflow rate being 1265 m3/h and the diameter of nozzle being 0.08 m, the longitudinal and lateral range of jet flow is going to

Corresponding author. E-mail address: [email protected] (N. Gao).

https://doi.org/10.1016/j.buildenv.2019.106211 Received 19 April 2019; Received in revised form 30 May 2019; Accepted 13 June 2019 Available online 14 June 2019 0360-1323/ © 2019 Elsevier Ltd. All rights reserved.

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Table 1 A brief summary on ventilation systems of underground garages. Reference

Size (m3) 2

Ventilation system

Research purposes Design a new style of garage ventilation

Fan [8]

2730m × 3.6 m

Zhao [9] Cai [10]

156.6 × 62.2 × 3.3 2000m2 × 2.8 m

Natural ventilation and Mixing ventilation Natural ventilation Inductive ventilation

Qin [11] Lu [12] Chan [13] Xue [14]

3463m2 × 3.9 m 80 × 40 × 3.2 150.5 × 113.3 × 2.2 25.2 × 14 × 2.6

Inductive ventilation Mixing ventilation Mixing ventilation Mixing ventilation

Measure the PM concentrations in different seasons Numerically simulate and measure the inductive ventilation system to improve air flow patterns in the garage Compare the conventional ventilation system with the inductive ventilation system Investigate the smoke control capacity of impulse ventilation Compare the ventilation system in six underground garages Simulate the distribution of temperature and carbon monoxide concentration in an underground garage.

Fig. 1. The 1:40 reduced physical model of the basement space in the experiment. (a) Three-dimensional (3D) perspective view; (b) The details of inlet A; (c) The details of outlet; (d) The details of inlet B; (e) The details of passage partitions and openings connecting indoor and outdoor space.

ducts is large which increases the buried depth of the whole plant by 1–2 m, indicating a substantial increase in civil construction cost. In addition, the complex duct system will cause difficulty in installation and maintenance. In order to solve these problems, we propose ductless ventilation system based on the concept of unidirectional piston flow. This system supplies the air at both ends of the basement space and exhausts the air in the middle. Piston flow is widely used in clean rooms [15]. But in

be 16 m and 7.5 m respectively. About 500 jet fans are need to be installed in a real underground sewage treatment plant with 226 m long, 252 m wide and 5.1 m high, resulting in a complex system. Mixing ventilation systems are mostly adopted in the underground garages. Given that the required air change rate of the basement space of underground sewage treatment plants is 6 per hour in the standard [6], the airflow rate could reach as high as 1.8 × 106 m3/h. It will lead to a complex overhead ductwork system [7]. The cross section of the air 2

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present kind of large and flat spaces, to the best knowledge of the authors, there are no reports on the application of piston flow. The main purpose of present study is to evaluate the performance of horizontal piston flow created by the ductless ventilation system in basement spaces. Such ductless ventilation system are now under construction for the first time in a real sewage treatment plant in Shanghai, China. A 1:40 reduced scaled model of the basement space and ventilation system is established in this study. The performance of the new proposed ventilation system is assessed by measuring the air velocity and tracer gas concentration distribution. Moreover, some key factors such as the porosity of perforated plate, the length of air barriers along the piston flow direction, the pressure difference between the indoor and outdoor, different air supply modes are investigated. The conclusions are meaningful for the innovative design of ventilation system in underground sewage treatment plants.

the y direction, as shown Fig. 1 (e). The length of partition A-E is 1.1 m and the partition E is further divided into E−1 and E−2. Partition A-E can be installed or removed freely to achieve various experimental cases for investigation of their effect on forming piston flow. Partition F is always installed since it is in the supply air plenum of inlet A. The passages connect to the outdoor environment by opening A-B, as shown in Fig. 1 (e). The size of openings is 0.1 m × 0.1 m. They could be opened or closed independently. The combination of opening state (open/close) and partitions installation (w/o) can reveal when the air short-circuit from inlet A or B to the opening A or B happens at various indoor-to-outdoor pressure differences. For the sake of analysis, the whole model is divided into two sections with plane B (z-x plane of y = 2.8 m) as shown in Fig. 1 (a), i.e. section I (white region) between inlet A and outlet and section II (orange region) between inlet B and outlet. The ventilation airflow rate in section I is twice of that in section II because the contaminant generation intensity in section I is twice of that in section II in reaction pools of real plant. There are four equipment rooms in the model. Three of them are in section I and the other one in section II. Tracer gas, CO2, is generated to mimic the pollutant. The tracer gas is emitted on the ground level (z = 0 m) with a surface dimension of 0.015 m × 0.015 m. There are fifteen sources, including ten in Section I and five in Section II. The distribution of pollutant sources is shown in Fig. 1 (a), and the detailed coordinates are listed in Table 2.

2. Methodology 2.1. Physical model The real underground sewage treatment plant is located in Shanghai Pudong district. The net basement space is 226 m long, 252 m wide, and 5.1 m high. For conventional overhead mixing ventilation, there should be supply fans, main air supply ducts and branch ducts, exhaust fans, main exhaust ducts and branch ducts, etc. In theory the ventilation efficiency is 1.0 if full mixing is achieved. We propose a ductless ventilation system in place of mixing ventilation by setting the air supplies at the both ends of the large space and outlets in the middle at the ceiling level. The purpose is to remove the huge amount of ducts and achieve the unidirectional flow at the same time whose theoretical ventilation efficiency is 2.0. However, there are equipment rooms and structural support pillars in the basement space which could disturb the piston flow. Different from the case in clean room, the flow path in the space is about 100 m. Can our hypothetical piston flow be truly achieved? In order to comprehensively evaluate this plan a scaled model is set up. Due to the space limitation of laboratory, the prototype is reduced by 40 times. Fig. 1 shows the 1:40 reduced physical model of the basement space and Fig. 2 shows the photograph of the scaled model. The scaled model is almost a replica of the real project. Pillars with a cross-section dimension of 0.015 m × 0.015 m are evenly arranged inside the space. The inlet A (five inlets at one end with red color in Fig. 1 (a) and (b)) and inlet B (ten inlets at the other end with red color in Fig. 1 (a) and (d)) are air supply units. The outlets (ten outlets with yellow color in Fig. 1 (a) and (c)) are located in the middle of model. Such arrangement in the prototype is due to the space constraints at the project site and this design is the result of many discussions with the owner and design institute. In real project, the air at a rate of 1.2 × 106 m3/h is supply through inlet A to three independent air plenums and then the air is uniformed by perforated plates with a size of 159.2 m × 2 m × 0.32 m before it flows into the basement space. The overall flow path is U shape. The remained air at a rate of 0.6 × 106 m3/h is pushed into the basement space by ten axial fans at inlet B. The air is directed by the guide grilles and flows obliquely into the space at an angle of 45°. Ten axial exhaust fans are installed in the middle of the model. Each of them has a flow rate of 0.18 × 106 m3/h. In real situation there are two vehicle passages throughout the longitudinal direction of the basement space for the connection with outdoor space. One kind of reasonable worry is that could the supply air escapes through the passage doors which are usually open before it flushes the flat large-space. A technical solution is to installed partition walls on both sides of the passages to separate them from the large space. Another benefit of these walls is to narrow the space in the width direction and ensure a better piston flow. In order to investigate the effect of partition walls, in the scaled model there are two 0.175 m wide passages which divide the model into three parts completely. Each of the passage partition wall is divided into six parts, partition A-F, along

2.2. Scale-model experiment setup The experiment configuration is shown in Fig. 2 (c). The air velocity is measured by KANOMAX 6006 with an accuracy of ± 0.01 m/s and the pressure measured by Testo 480 with an accuracy of ± 1%. CO2 is released by KONXIN YJ-700C through Teflon tubes with an inner diameter of 14 mm and an outer diameter of 16 mm. The flow rate of CO2 is 0.15 m3/h, corresponding to a release velocity up of 0.27 m/s. The mean y-direction velocity of plane A (z-x plane at y = 4.25 m) um is 0.34 m/s. The ratio of CO2 flow rate to the airflow rate in plane A is lower than 0.01. Sundo uSafe-3000 with an accuracy of ± 2% is selected for CO2 sampling. The sampling tube is a stainless-steel tube with an inner diameter of 3 mm and an outer diameter of 5 mm. The sampling interval of velocity and pressure is 15s and the sampling interval of concentration is 120s. In order to accurately measure velocity and pressure, rectangular ducts with a ratio of length to width greater than 10 are mounted on inlet A, inlet B, outlet and openings (if open). Pressure measuring points are located on each duct with a total number of 29. ΔPbs is defined as the mean pressure difference between inlet A and outlet, i.e. PinletA minus Poutlet. ΔPoto is defined as the difference between mean pressure of opening A-B and outlet, i.e. Popening A-B minus Poutlet. The measuring points of velocity and pollutant concentration are located in the breathing zone plane (z = 0.0375 m in the scaled model and corresponding to 1.5 m height in the prototype) with a total number of 109 and their locations are shown in Fig. 3. The velocity and pollutant concentration are measured at every point. Each red dot represents both the velocity and concentration measurement point. Eight representative lines are selected to study the vertical profile of velocity and concentration in the basement space. Five of them are located in section I and three in section II. The height of lines are set to z = 0–0.05 m corresponding to 0–2 m in real case since that we mainly care about the occupied zone. The main factors affecting the turbulent flow and pollutant dispersion in the basement space are as follows: perforated plate porosity p, installation of passage partition walls, opening state of vehicle passage entrances (open/closed) and air supply modes. Aiming at these factors, 25 experimental conditions in total are designed and they are divided into four groups, as shown in Table 3. Porosity p is defined as the ratio of the area of holes to overall area of the perforated plates. p = 1 means that there is no perforated plate. The smaller the porosity, the better the 3

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Fig. 2. The picture of the scaled model. (a) Internal and external picture of the model; (b) The picture of perforated plates; (c) The sketch of experiment configuration.

air flow uniformity, but higher pressure difference and energy consumption. The purpose of experimental conditions in group 1 is to search for a suitable perforated plate porosity considering the compromise between the pressure difference and air flow uniformity. The narrower the space, the easier the piston flow will form. On the one hand, large space can be divided into several smaller spaces by the passage partition walls. It is beneficial to form the piston flow. On the other hand, the partition walls can reduce the airflow short circuit through vehicle passage entrances when opening A and B are open. In

order to investigate the influence of partition walls on airflow uniformity, the cases in group 2 are designed. The vehicle passage entrances are generally open for long periods of time due to the busy traffic. It will cause fresh air leaking to the ambient from the passage entrance before it reaches the interior of the large space and takes away the pollutant. The air short-circuit problem is evaluated by cases in group 3. One possible design is to remove the present outlets and set inlet B as outlet by reversing the axial fans. Such design is much simpler than current one but the length of flow path is doubled. The effect of

Table 2 Location of pollutant sources in the scaled model.

x (m) y (m)

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

0.3 4.8

1.4 4.8

2.0 4.8

3.2 4.8

3.5 4.8

0.3 3.7

1.4 3.7

2.0 3.7

3.2 3.7

3.5 3.7

0.3 1.7

1.4 1.7

2.0 1.7

3.2 1.7

3.5 1.7

4

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Fig. 3. The distribution of measuring points and lines. (a) x-y plane at z = 0.0375 m; (b) y-z section at x = 0.28 m. The distribution of lines is as follows: line A: x = 0.28 m, y = 4.07 m, line B: x = 1.98 m, y = 4.07 m, line C: x = 3.15 m, y = 4.07 m, line D: x = 3.34 m, y = 1.45 m, line E: x = 1.79 m, y = 1.45 m, line F: x = 0.28 m, y = 1.45 m, line α: x = 1.40 m, y = 4.07 m, line β: x = 3.52 m, y = 4.07 m.

Table 3 Experimental conditions. Group No.

Case ID

Porosity

State of passage partition walls

Opening state of opening A-B

ΔPoto (Pa) in prototype

ΔPoto (Pa) in scaled model

Air supply mode

The airflow rate ratio of two inlets

Flow rate of inlet A (m3/s)

Flow rate of inlet B (m3/s)

1

1-A 1-B 1-C 1-D 2-A (1-A) 2-B 2-C 2-D 2-E (1-D) 2-F 2-G 2-H 3-A 3-B 3-C 3-D 3-E 3-F 3-G 4-A 4-B (1-A) 4-C 4-D 4-E 4-F 4-G (1-D) 4-H 4-I 4-J

0.25 0.35 0.5 1.0 0.25

All installed

closed

N.A.

N.A.

A

2

0.402

0.201

closed

N.A.

N.A.

A

2

0.402

0.201

0.25

All installed C removed B,C,D removed All removed All installed C removed B,C,D removed All removed A,E removed

open

2

0.402

0.201

All installed

closed

200 150 100 50 0 −30 −5.2 N.A.

A

0.25

20.9 15.7 10.5 5.2 0 −3.1 −50 N.A.

A A A A B A A A A B

1 2 3 4 N.A. 1 2 3 4 N.A.

0.302 0.402 0.453 0.484 0.604 0.302 0.402 0.453 0.484 0.604

0.302 0.201 0.151 0.121 N.A. 0.302 0.201 0.151 0.121 N.A.

2

3

4

1.0

1.0

(NCV), mean dimensionless pollutant concentration (Kavg) and maximum dimensionless pollutant concentration (Kmax) in breathing zone plane (z = 0.0375 m) are adopted. They are defined as follows [16]. The smaller the NCV is, the more uniform the velocity field is.

this design is evaluated by cases in group 4. Case 1-A, 2-A and 4-B are the same working conditions and Case 2-E and 4-G are the same as case 1-D. The air supply mode A denotes that both ends of the basement space are inlets and outlets are in the middle. The air supply mode B means that inlets are set at one end and outlets are located at the other end. The air quality in breathing zone plane of the basement space is significant to ensure the health of workers. Therefore, three evaluation indexes which includes the non-uniformity coefficient of velocity 5

V=

u um

(1)

K=

cQa Qp

(2)

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Fig. 4. Variations of dimensionless velocity and pollutant concentration distributions in the basement space: (a) Line A (x = 0.28 m, y = 4.07 m); (b) Line D (x = 3.34 m, y = 1.45 m); (c) Line α (x = 1.40 m, y = 4.07 m) (d) Line β (x = 3.52 m, y = 4.07 m).

NCV =

2.3. Calculation of critical Reynolds number

2 n (Vi − Vavg ) n

∑1

Vavg

(3)

Usually some similar criteria between the prototype and scaled model must be met to ensure that the real physical phenomenon can be truly reflected. Given that the size of the basement space is less than 5 km and thermal effect is not considered here, the Reynolds number (Re) could be the only criterion in this study [17–19]. Generally Re of the prototype and scaled model should be kept equal to guarantee the similarity of the flows [20,21]. However, it is very difficult to meet this requirement if the scale ratio is high. The air velocity of inlet A and B in the prototype is 1.8 m/s and 3.7 m/s, respectively. In order to achieve the same Reynolds number, the air velocity could be as high as 72 m/s

where u is the measured velocity magnitude; um is the mean y-direction velocity of plane A (z-x plane at y = 4.25 m); c is the measured pollutant concentration; Qa is the total airflow rate; Qp is the airflow rate of pollutant; Vavg is the average dimensionless velocity of all measuring points; n is the number of measuring points.

6

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Fig. 5. The variations of evaluation indexes as the perforated plate porosity p changes. (a) Pressure difference from inlet A to outlet (ΔPbs); (b) Non-uniformity coefficient of dimensionless velocity (NCV); (c) Maximum dimensionless pollutant concentration (Kmax); (d) Mean dimensionless pollutant concentration (Kavg); (e) Dimensionless velocity on breathing zone plane (z = 0.0375 m) obtained by interpolation of experimental results under case 1-A (p = 0.25, partitions A-E installed and opening A-B closed, air supply mode is A). (f) Dimensionless velocity contour on breathing zone plane (z = 0.0375 m) obtained by interpolation of experimental values under case 1-D (p = 1.0, partition A-E installed and opening A-B closed, air supply mode is A).

Given that we mainly care about the overall flow regime in the space, the Reynolds number is specified as,

at inlet A and 148 m/s at inlet B. Actually it is not reasonable. In order to solve this problem, the concept of Reynolds number independence (Re-independence) is employed [22]. The flow characteristics will not change as the Reynolds number exceeds a certain value. This value is called critical Reynolds number (Recrit) [23,24]. Since there are few investigations of Re-independence in such a flat large-space, a numerical model with the same size of current scaled model is established.

ReH =

um H υ

(4)

where um is the mean y-direction velocity of plane A (z-x plane at y = 4.25 m), H is the height of the basement space, υ is the kinematic 7

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Fig. 6. The influences of partition walls on evaluation indexes under two conditions of p = 0.25 and p = 1.0. (a) Non-uniformity coefficient of dimensionless velocity (NCV) in section I; (b) Non-uniformity coefficient of dimensionless velocity (NCV) in section II; (c) Maximum dimensionless pollutant concentration (Kmax); (d) Mean dimensionless pollutant concentration (Kavg).

3. Results and discussion

viscosity. Taking case 1-A as an example, the critical Reynolds number (Recrit) is calculated by investigating the variations of velocity and pollutant concentration distribution with the increase of ReH. The numerical results are used to provide reference for air supply velocity of the scalemodel test. The numerical simulation are completed in a commercial computational fluid dynamics (CFD) program, Fluent 17.0. The computational domain is discretized by hexahedral mesh system of 6.4 × 106 cells testified by grid-independence examination. The grid is refined for more accurate numerical results at all openings and the area near the ground and pillars. The turbulence intensity I is set as 4.2% in the simulation. A standard wall function based on the proposal of Launder and Spalding [25] and no-slip boundary condition are applied at all wall surfaces. The predicted results from three k-ε turbulence models: the standard k-ε [16], RNG k-ε [26] and realizable k-ε [27] model are compared with experimental test. It is found that numerical results of standard k-ε model are in the best agreement with the experimental data. This consistent with some literature studies [28–30]. Fig. 4 shows the variation of dimensionless velocity and pollutant concentration distribution in the basement space with the increase of ReH. For the sake of concision of article, only the results of four lines are exhibited. It can be seen that the vertical profile at each line converges gradually as ReH increases. When um reaches 0.7 m/s and the corresponding ReH exceeds 5950, the flow regime and pollutant diffusion can be considered to reach Re-independence. Considering a safety factor, ReH is finally set to be 6800 and the air supply velocity at inlet A is 5.6 m/s and 11.4 m/s at inlet B in the scaled model experiments.

3.1. Effect of perforated plate porosity on pressure difference and flow pattern The porosity of perforated plate, p, is one of the key factors which mainly affect the airflow in the region of section I. Fig. 5 (a)-(d) shows the variation of ΔPbs, NCV, Kavg and Kmax as p reduces from 1.0 to 0.1. The solid lines in the figure are the results by simulation with the standard k-ε model and the red points are experimental values. It can be seen from Fig. 5 (a) that ΔPbs increases with the decrease of p. The increase rate of pressure difference is marginal if p > 0.4 but is remarkable when p < 0.2. Fig.5 (b) - Fig.5 (d) show a consistent reducing pattern of NCV, Kmax and Kavg as p decreases. The change rate of three evaluation indexes are small when p is in the interval of 0.6–1.0. The indexes decrease sharply when p shrinks to 0.25–0.6 and remain basically unchanged when p is further reduced to 0.1–0.25. The results show that the perforated plate can hardly works when p > 0.6. The flow regime of section I is disordered at this time and the pollutant concentration is relatively high. When p is in the range of 0.35–0.6, the pressure difference changes slightly but the flow uniformity varies significantly. When p is less than 0.25, the unidirectional piston flow is basically formed in the entire space. Further decrement of p results in a sharp increase of pressure difference but no obvious help in airflow uniformity and pollutant concentration control. This finding provides a guidance on the selection of perforated plate in the basement space. The porosity of perforated plate can be increased for energy saving when the requirements of air flow uniformity and pollutant concentration are low while it must be strictly controlled if the requirements are high. Comprehensively considering the flow pattern and pressure difference, 8

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p = 0.25 are mounted because the perforated plates make airflow move almost straightforward. However, in the absence of the perforated plates (p = 1), the partitions especially the ones close to opening A and B can significantly uniform the airflow and reduce pollutant concentration. The airflows of different regions will interfere with each other when the partitions are removed. The more partitions, the more uniform the airflow distribution and lower pollutant concentration, but higher the construction costs. It is suggested that the partitions near opening A and B should be installed in view of construction costs. 3.3. Air leakage from opening A and B to the ambient environment It is necessary to ensure that the air change rate meets the design requirements when the ductless ventilation system is running. However, the airflows from inlet A and B may be exhausted directly from opening A and B if partition A and E are removed, forming shortcircuit. The pressure difference between opening A, B and outlet, ΔPoto, plays a key role in such situation. A physical quantity which is called air leakage ratio (ALR) is proposed. It is the ratio of the airflow rate leaking from the opening A and B to the total exhausted airflow rate of the basement space. If ALR is less than 0, it means there is no air leakage and external air enters into basement space from opening A and B instead. If ALR is larger than 0, it means internal air leaks into outdoor environment from opening A and B. The relationship between ALR and ΔPoto is shown in Fig. 7 (a). It can be found that no air leakage occurs when ΔPoto reaches 200Pa (the value for prototype is 20.9Pa) and ALR gradually increases with the reduction of ΔPoto. All the air entering the space will leak out to the ambient through opening A and B when ΔPoto is reduced to −50Pa (the value for prototype is −5.2Pa). The ΔPoto of the scaled model is measured in experiments. The ΔPoto of prototype is estimated according to the data in the scaled model. For the scaled model and prototype, the pressure difference is proportional to the square of velocity magnitude while independent of size. Consequently, the ΔPoto in the scaled model is 9.57 times of that in prototype based on the velocity magnitude relationship and the results are validated by simulations. Fig. 7 (b) displays a dimensionless velocity contour in breathing zone plane in case 3-G. The dimensionless air velocity is the lowest which is close to 0 in the middle of the large space and the highest at the both ends. The position with the highest velocity is located at the vehicle passage entrance. It indicates that airflow short-circuit phenomenon is serious when ΔPoto = −50Pa.

Fig. 7. The air leakage when partition A, E are removed and opening A-B are open. (a) The variation of air leakage ratio (ALR) with the reduction of pressure difference between opening A-B and outlet (ΔPoto); (b) Dimensionless velocity contour in breathing zone plane (z = 0.0375 m) obtained by interpolation of experimental results under case 3-G (p = 0.25, partition B, C, D are installed and partition A, E are removed, opening A-B are open and ΔPoto = −50Pa, air supply mode is A).

p is recommended to be 0.25–0.35 in this paper. Fig. 5 (e) and (f) shows a dimensionless velocity contour in breathing zone plane in case 1-A and 1-D, respectively. It can be seen that the air distribution in section I is more uniform with p = 0.25 than that with p = 1.0. 3.2. Effect of the partition walls on flow pattern

3.4. Effect of air supply mode on flow pattern

The cases in group 2 include four conditions: (1) all partitions were installed; (2) partition C is removed; (3) partition B, C, D are removed; (4) all partitions are removed. Fig. 6 shows the influence of partition walls on NCV, Kmax and Kavg under two conditions of p = 0.25 and p = 1.0. It can be found that the perforated plate with p = 0.25 can greatly improve the uniformity of airflow and reduce pollutant concentration in section I. The partition has no obvious effect on evaluation indexes with p = 0.25. However, when p = 1.0, comparing to case 2-E under which all partition walls are installed, NCV of section I of case 2F, case 2-G, case 2-H increases by 4.35%, 14.49%, 42.03%, Kmax increases by 2.87%, 8.69%, 28.18% and Kavg increases by 2.20%, 7.65%, 23.04% respectively. It is worth of noting that the NCV of section II remains almost unchanged at this time because the airflow distribution of this region is basically uniform. The difference of Kmax and Kavg values under case 2-F and those under case 2-E represent the effect of removing partition C on pollutant concentration. The difference of case 2-G and 2-F represent the effect of partition B, D and the difference of case 2-H and 2-G represent the effect of partition A, E. The results show that installing partitions to divide the airflow has no help on optimizing flow patterns and reducing the pollutant concentration in the basement space when the perforated plates with

Fig. 8 illustrates the results of case 4-A to 4-J. The abscissa represents ventilation mode and the ratio of airflow rate on both ends. For example, “A-2″ represents the air supply mode is A and the ratio of airflow rate at inlet A to inlet B is 2.0. In air supply mode A, the airflow rate ratio of inlet A to inlet B has no obvious impact on NCV of the entire space regardless of whether or not the perforated plate is installed. In mode B, NCV of section II is greatly reduced but NCV of section I keep almost unchanged. Air supply mode B is the most effective in reducing Kmax no matter p = 0.25 or p = 1.0. The Kmax of air supply mode B is 16.63% smaller than that of mode A-2 with p = 0.25 and 63.39% with p = 1. The second effective mode is A-2 and it may be related to the fact that the airflow rate and the pollutant generation rate in section I are twice of those in section II. It is worth noting that Kavg is the largest when mode B is applied since the pollutants in section I and section II have the longest path to outlets. In contrast, the airflow can drive indoor pollutants to the ambient efficiently in a short path under mode A-2. Moreover, mode A-2 can minimize Kavg in all air supply modes. Fig. 8 (e) exhibits the dimensionless velocity contour in breathing zone plane under case 4-J. The airflow in section I is non-uniform due to the absence of perforated plate. However, the unidirectional piston flow 9

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Fig. 8. The influence of air supply modes on evaluation indexes under two conditions of p = 0.25 and p = 1.0. (a) Non-uniformity coefficient of dimensionless velocity (NCV) in section I; (b) Non-uniformity coefficient of dimensionless velocity (NCV) in section II; (c) Maximum dimensionless pollutant concentration (Kmax); (d) Mean dimensionless pollutant concentration (Kavg); (e) Dimensionless velocity contour in breathing zone plane (z = 0.0375 m) obtained by interpolation of experimental values in case 4-J (p = 1, partition A-E are installed and opening A-B are closed, air supply mode is B).

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References

is formed in section II as a result of the presence of partitions. It indicates that mode B can effectively improve the airflow uniformity of section II. In summary, for a basement space where pollutant generation rate in section I are twice of that in section II, air supply mode A-2 and mode B perform best. Mode A-2 is suggested to be applied when the mean pollutant concentration in the basement space need to be control. Mode B is recommended to be applied when Kmax is limited.

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4. Conclusions Ductless ventilation systems for the flat and large basement space in underground sewage treatment plants are proposed and their performance is evaluated by experiments. The conclusions are as followings. (1) Reduction of perforated plate porosity p can improve the airflow uniformity and reduce pollutant concentration. But it will lead to a higher pressure drop and increase system operating costs at the same time. When p is less than 0.25, the airflow uniformity has no significant improvement but the pressure drop between inlet A and outlet (ΔPbs) increases sharply with the further decrease of p. When p is greater than 0.35, the variation of ΔPbs is very small but the airflow uniformity becomes much worse. Weighting both ventilation performance and pressure difference, p should be controlled in the range of 0.25–0.35. (2) Installing partitions has no help on optimizing flow patterns in the basement space when perforated plates with p = 0.25 are mounted. But the partitions close to openings can significantly uniform the airflow and reduce the pollutant concentration when there are no perforated plates. Partitions near opening A and B are recommended to be installed for the best performance and lowest economic cost in real engineering project. (3) Air leakage ratio is related to the pressure difference between opening A, B and outlet (ΔPoto). No air leaks through the vehicle entrance openings when ΔPoto is 200Pa in the scaled model experiments. Air leakage ratio will close to 1.0 when ΔPoto reduces to −50Pa and serious airflow short circuit phenomenon occurs at this time. (4) Both ends of the basement space are set as inlets and outlets are located in the middle in air supply mode A. Inlets and outlets are located at different ends of the basement space in air supply mode B. For a basement space where pollutant generation rate in one section (the area from the inlets located at one side of the basement space to the outlets) are twice of that in the other section, mode A is suggested to be applied and the airflow rate ratio of the inlets located at both ends of the basement space should be 2.0 when the mean pollutant concentration need to be control. . Mode B is recommended when the maximum pollutant concentration is limited. Acknowledgements This work is partially supported by the Fundamental Research Funds for the Central Universities, the 13th Five-Year Key R&D Program under the project number of 2017YFC0702304-01 and National Natural Science Foundation of China under the project number of 51878462.

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