Numerical simulation of air ventilation in super-large underground developments

Tunnelling and Underground Space Technology 52 (2016) 38–43

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Numerical simulation of air ventilation in super-large underground developments Ming Li a,b,⇑, Saiied M. Aminossadati b, Chao Wu a a b

School of Resources and Safety Engineering, Central South University, Changsha, Hunan Province 410083, China School of Mechanical and Mining Engineering, The University of Queensland, QLD 4072, Australia

a r t i c l e

i n f o

Article history: Received 6 May 2014 Received in revised form 22 October 2015 Accepted 17 November 2015 Available online 28 November 2015 Keywords: Ventilation Super-large underground development Carbon monoxide Dust

a b s t r a c t Recent advancements in engineering technology have enabled the construction of super-large underground engineering projects in China. Currently, the ventilation requirements and standards of normal-size underground spaces are used for super-large underground excavating engineering projects in China. For example, the minimum air velocity of 0.15 m/s is the standard velocity for normal-size underground spaces; however, this value is also used as the required air velocity for diluting underground contaminants in super-large underground developments. This paper aims to examine the minimum ventilation requirements for super-large underground developments (S > 100 m2). A threedimensional computational domain representing a full-scale underground space has been developed. The pertinent parameters such as dust concentration, smoke density, oxygen concentration and air temperature have been simulated. The results show that at some specific underground conditions, the ventilation air velocity of 0.15 m/s is sufficient to control the dust level, provide required oxygen concentration and maintain the air temperature at acceptable levels during development; however, it is not sufficient to bring the CO concentration below an acceptable safe limit. This must be considered by the ventilation system designers of super-large underground developments. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction Recent advancement of engineering technology has enabled the construction of large-scale underground engineering projects in China. It is essential for designers of such projects to utilise correct ventilation practices and consider appropriate standard requirements. The ventilation performance of underground tunnels can be examined either by a full-scale experimental investigation, a reduced-scale test, or a numerical simulation. Of all these methods, the full-scale experimental investigation normally produces the most useful data since it is presumably reproducing the operational conditions of real situations. However, this method is costly and time-consuming. A numerical simulation, on the other hand, does not require expensive experimental facilities and instruments. It can be used to regenerate the real physical conditions; and if it is defined and validated properly can repeatedly analyse a problem under various conditions. Many researchers such as Gosman (1999), Parra et al. (2006), Migoya et al. (2009), Colella ⇑ Corresponding author at: School of Resources and Safety Engineering, Central South University, Yuelu District, Changsha, Hunan Province 410083, China. E-mail address: [email protected] (M. Li). http://dx.doi.org/10.1016/j.tust.2015.11.009 0886-7798/Ó 2015 Elsevier Ltd. All rights reserved.

et al. (2011), and Montazeri (2011) have carried out experimental and numerical simulations to study three-dimensional geometries with various flow and thermal conditions. They found a good agreement between their numerical and experimental results. The commercial software package ANSYS-FLUENT has commonly been used by many researchers to study various fluid-flow and heat transfer problems. Yuandong and Zhonghua (2013) demonstrated a good agreement between their numerical results for the airflow and pollutant dispersion and the experimental results obtained by Rafailidis and Schatzmann (1995) on a detailed wind tunnel study. In terms of numerical simulation studies of underground engineering, some researchers have conducted studies on underground transport tunnels (Jain and Kumar, 2011; Kim and Kim, 2009; Juraeva et al., 2013) and underground car parks (Xue and Ho, 2000; Viegas, 2010). Other researchers have conducted numerical simulation of underground mine environments (Gao et al., 2002; Parra et al., 2006; Hargreaves and Lowndes, 2007; Toraño et al., 2009; Zheng, 2011; Torno et al., 2013; Zhongwei and Ting, 2013). Some of the reported numerical studies on operating ventilation systems in underground environments have focused on dust management. Wei et al. (2011) used the gas–solid two-phase flow

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M. Li et al. / Tunnelling and Underground Space Technology 52 (2016) 38–43

theory to predict dust distribution and movement at the work-face in an underground mine environment. They compared their results with the observed data from an actual work-face. Toraño et al. (2011) studied the dust behaviour under two auxiliary ventilation systems by using three-dimensional models. They determined dust concentrations at different cross-sections of an operating coal work-face. Ren et al. (2014) simulated the airflow and respirable dispersion patterns along the belt roadway to assist in the design of a better dust mitigation system. They also discussed ways for reducing the respirable dust concentration. ‘‘Construction specifications on underground excavation engineering of hydraulic structures” developed by the Ministry of Water Resources of the People’s Republic of China (2007) is currently used for the development of normal-size tunnels, roadway excavations and other underground facilities in China. These standards are also used by the designers of super-large underground excavating engineering with huge cross-sectional areas (S > 100 m2) in order to minimise the investment and operational costs of environmental and ventilation control systems. For example, the minimum air velocity of 0.15 m/s, recommended for normal-size tunnels, is used for calculating the ventilation requirements of large-scale underground spaces. The main concern is to whether this velocity can meet the safety requirements for diluting blasting gases, removing dust and controlling the air temperature at the work face. This study has been undertaken to examine the ventilation performance of super-large underground excavating engineering projects and determine the minimum ventilation requirements.

2. Numerical analysis 2.1. Methodology The numerical model used in this study is based on solving the differential equations of conservation of mass, momentum and energy, and is complemented by turbulent flow models. Calculations are performed with the commercial software package ANSYS-FLUENT. This code uses the finite volume method and solves the three-dimensional Navier–Stokes equations on an unstructured grid. The turbulence is simulated with the standard k–e model. The species transport model is used to simulate the variation of the CO and O2 mass faction. The mass fraction of each species, Y i , is calculated using a convection–diffusion equation for the i species. The conservation equation and mass diffusion in turbulent flows can be presented in the following general forms:

8 < @t@ ðqY i Þ þ r  ðq~ mY i Þ ¼ r ~Ji þ Ri þ Si
 lt :~ J i ¼  qDi;m þ Sc rY i  DT;i rTT t

ð1Þ

where Ri is the net rate of production of species i, Si is the rate of creation by addition from the dispersed phase plus any userdefined sources, ~ J i is the diffusion flux of species i, Di;m is the mass diffusion coefficient for species i, DT;i is the thermal diffusion coefficient, Sct is the turbulent Schmidt number, lt is the turbulent vism is the cosity, DT is the turbulent diffusivity, q is the fluid density, ~ average flow velocity, r represents the gradient and r represents the divergence.

5.0m

Ventilation duct 7.5m P6

4.0m 2.0m

P4

3.5m

P5

0.95m 10.0m

6.0m 4.0m 2.0m P1

3.5m

4.0m P2

P3

15.0m

(a) Cross-section 2 Z=75m (Cross-section 1)

X=0m

Ventilation duct Y

Heading face 50m Fresh airflow

Z

Dust and gas generation

X Y=1.5m 50m

100m

25m

(b) Fig. 1. Schematic diagram of underground excavation engineering project: (a) Cross-sectional view of underground development and test points. (b) Three-dimensional view of computational domain.

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Table 1 Environment standards for construction ventilation (Ministry of Water Resources of the People’s Republic of China, 2007). CO mass concentration

O2 mass concentration

Dust mass concentration

Air temperature of work-face

630 mg/m3

P22.1%

62 mg/m3

628 °C

Table 4 Summary of simulation results versus standard construction specifications. CO O2 Dust

Temperature

Table 2 Main ventilation parameters. Area of duct

Net area of crosssection

Required air velocity

Required air flow rate

Velocity of duct inlet

Dynamic pressure of duct inlet

3.07 m2

200.77 m2

0.15 m/s

30.12 m3/s

9.80 m/s

57.57 Pa

Exhaust CO of blasting 45/20 (min) or 250–300/30 (mg/m3) After blasting For workers and engines 35/20 (min) or 0.20/0.221 1800/1950 (m3) Continuous producing dust Instantaneous dust of blasting 1.95/2.0 (mg/m3) 2.0/2.0 (mg/m3) 308 K 313 K 318 K 323 K 298/301 K 299.5/301 K 301.5/301 K 303/301 K

Note: Simulation result/standard value.

Table 3 Validation of simulation model using site data. Category

Items

Simulation results

Site test

Error rate (%)

After blasting Continuous producing dust Air temperature

O2 Dust

20.2% (mass) 0.34 mg/m3

21.3% (mass) 0.375 mg/m3

5.2 9.3

Temperature

290.8 K

292.2 K

0.5

Blasting approximately 100 kg explosive; O2 mass fraction in fresh air is 23.1%; continuous ventilation for 20 min; continuous producing dust; five drillers in operation; surrounding rock temperature is 294 K; and fresh air temperature is 288 K.

8 ~ g ðq qÞ dup > ¼ F D ð~ u ~ up Þ þ qp þ ~ F > > dt p > < 18l C D Re F D ¼ q d2 24 p p > > > > : Re  qdp j~up ~uj l

ð2Þ

where ~ F is an additional acceleration (force/unit particle mass) term, F D ð~ u ~ up Þ is the drag force per unit particle mass, ~ u is the fluid phase velocity, ~ up is the particle velocity, l is the molecular viscosity of the fluid, q is the fluid density, qp is the density of the particle, and dp is the particle diameter, Re is the relative Reynolds number.

Fig. 2. Distribution of CO mass fraction in the plane x = 0 m at different ventilation times: (a) 1 s, (b) 30 s, (c) 60 s, (d) 300 s, (e) 600 s, (f) 900 s, (g) 1200 s, and (h) 1500 s.

1300 1200

CO mass concentration/mg.m-3

The discrete phase model (DPM) model is used to simulate the results of discharging dust. The trajectory of a discrete phase particle is simulated by integrating the force balance on the particle in a Lagrangian reference frame. This force balance equates the particle inertia with the forces acting on the particle, and can be written (for the x direction in Cartesian coordinates) as:

Longitudinal plane x=0m Horizontal plane y=1.5m Cross-section plane z=75m Total volume

1100 1000 900 800

Requirement ventilation time

700 600 500 400 300

Requirement mass concentration

200 100 0 0

5

10

2.2. Computational domain Fig. 1 presents the three-dimensional computational model that includes the end-section of an underground space under development. A ventilation duct, which provides fresh air for ventilating the development face, is also included in the model. The dimensions of the model represent the actual sizes of a super-large underground excavation engineering project. Fig. 1(a) presents the layout and dimensions of the cross-section. The diameter of the ventilation duct is 2 m and the area of the tunnel’s cross-section is 203.77 m2. In order to reduce the mesh distortion and improve the mesh quality, the distance between the centre of the duct and the side-wall is considered 0.95 m. The distance from the centre of the duct to the ground is 8 m. The length of the simulation domain is 100 m and the length of the ventilation duct is 50 m (Fig. 1(b)).

15

20

25

30

35

40

45

50

55

60

Ventilation time/min Fig. 3. Variation of CO mass concentration at different ventilation time.

The simulation uses the standard k–e turbulence model and standard wall functions. The boundary conditions of ‘Mass-flow inlet’ and ‘Injections’ are used to include the CO and dust simulation in the model. The minimum mesh size is 0.0149 m, the number of nodes is 31,227, and the number of elements is 155,743. 2.3. Simulation criteria The main drives of ventilation in tunnel construction and roadway excavation are to exhaust blasting products, diesel particulate matters, fumes and dust, to control air temperature at the workplace and to supply oxygen for workers and machineries. The stan-

M. Li et al. / Tunnelling and Underground Space Technology 52 (2016) 38–43

0.24 0.22

Requirement mass fraction

O2 mass fraction

0.20 0.18 0.16 0.14

Longitudinal plane x=0m Horizontal plane y=1.5m Cross-section plane z=75m Total volume

0.12

0.08 5

10

15

20

25

30

35

40

45

50

55

the concentration of continuous producing dust and the air temperature were measured at six points of P1–P6 (Fig. 1(a)) and two cross sections (Fig. 1(b)). For the comparison study between the simulation and experiments, it was assumed that 100 kg of explosive were blasted, oxygen concentration was 23.1%, the ventilation was continued for 20 min, dust was generated continuously, five drills were in operation, the surrounding rock temperature was 294 K and the fresh air temperature was 288 K. 3.2. Model validation

0.10

0

41

60

Ventilation time/min Fig. 4. Variation of O2 mass fraction at different ventilation time.

The present model is validated against the results of site measurements for oxygen concentration after blasting, dust concentration during continuous producing dust and air temperature (Table 3). It is evident that the results of model simulation agree well with the experimental data. Table 4 presents a summary of the simulation results with respect to the standard construction specifications for CO, oxygen, dust and temperature. 4. Results and discussion

Table 5 Dust concentration for continuous producing dust. Position

Plane x=0m

Plane y = 1.5 m

Plane z = 75 m

Total volume

Average value

1.88 mg/m3

3.40 mg/m3

1.62 mg/m3

1.95 mg/m3

dard value for acceptable levels of CO, O2, dust and temperature are given in Table 1 (Ministry of Water Resources of the People’s Republic of China, 2007). CO concentration is required to meet these standards after continuous ventilation for 20 min. The main ventilation parameters, presented in Table 2, are calculated according to the minimum air velocity for discharging dust (0.15 m/s).

The performance of ventilation system (based on the minimum air velocity of 0.15 m/s) in a large-scale underground space is examined in terms of exhausting blasting products, discharging dust, controlling air temperature and supplying oxygen. The simulation results are presented at three different planes (shown in Fig. 1(b)) to provide a complete understanding of the flow regime in the excavation development. The three planes are: the longitudinal plane (x = 0 m), the horizontal plane (y = 1.5 m) and the cross-section plane (z = 75 m). The plane y = 1.5 m represents respiratory zone of workers and the plane z = 75 m represents the working area of excavating engineering. 4.1. Exhausting blasting products

3. Experimental study 3.1. Test conditions In order to validate the simulation results, a series of tests were designed and carried out. The oxygen concentration after blasting,

(a) Longitudinal plane x=0m

(c) Cross-section plane z=75m

According to the construction specifications established by the Ministry of Water Resources of the People’s Republic of China (2007), one kg of explosives can produce 0.04 m3 of CO. In the present study, the transient model and special transport are used to simulate the variation of CO concentration at the work-face. The

(b) Horizontal plane y=1.5m

(d) Distribution of dust concentration of whole domain

Fig. 5. Distribution of dust mass concentration at different regions and 3D distribution.

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M. Li et al. / Tunnelling and Underground Space Technology 52 (2016) 38–43

Fig. 4 shows the variation of oxygen mass concentration within the 60 min of ventilation time. It is assumed that the oxygen mass concentration of airflow in the ventilation duct is 0.231. The results show that the oxygen mass concentration in the working space reaches only 0.2 after 20 min of ventilation. It was required to ventilate the working space for 35 min till the oxygen concentration reaches the required standard value of 0.221.

55 50

Longitudinal plane x=0m Horizontal plane y=1.5m Cross-section plane z=75m Total volume

Dust concentration/mg.m-3

45 40 35 30 25 20

4.2. Dust removal

15 10

Requirement dust concentration

5 0 -5 0

5

10

15

20

25

30

35

40

45

50

55

60

Ventilation time/min Fig. 6. Variation of dust mass concentration at different ventilation time.

CO is injected at the work-face within the first one second, and the total simulation time is considered to be 60 min. Some of the simulation results (nephograms) are presented in Fig. 2. Fig. 3 shows the average values of CO concentration at the three longitudinal (x = 0 m), horizontal (y = 1.5 m) and cross-section (z = 75 m) planes. The results show that the CO concentration is between 250 and 300 mg/m3 during the required ventilation time of 20 min. This range is significantly higher than the standard value of 30 mg/m3. This means that if the minimum air velocity of 0.15 m/s is used as the required minimum air velocity, the CO concentration does not reach the standard safety limit of 30 mg/m3 within 20 min. In fact, continuous ventilation with an air velocity of 0.15 m/s for more than 45 min is required to bring the CO concentration below the safe limit.

There are mainly two types of dust generated during underground development: continuous producing dust during drilling and shot-concrete operations; and instantaneous dust generated during blasting. In the present simulation, the continuous producing dust is assumed to be 500 mg/s considering 20 mg/s of dust is generated during a normal drilling operation. The instantaneous dust is assumed 54.2 kg/t according to the data obtained from an experimental test in Daye Iron Mine. In this instance, the mass fraction of total suspended particle (TSP) is 10%. Consequently, 2.71 kg dust due to blasting (500 kg) is selected for simulation. The continuous producing dust is considered to be steady and the dust generation due to blasting is considered to be transient. The floor of the model is set as the trap surface and other walls are set as the reflect surfaces. Table 5 presents the average value of concentrations of continuous producing dust at the three longitudinal, horizontal and cross-section planes. It is evident that the concentration of continuous producing dust in the total volume of working space is below the standard value of 2.0 mg/m3; therefore, it meets the ventilation requirement. Fig. 5 presents the dust patterns at the three longitudinal, horizontal and cross-section planes as well as in the whole working space. Fig. 6 presents the variation of instantaneous dust concentration during blasting. The results show that as a result of ventilation with an air velocity of 0.15 m/s, the dust concentration rapidly

(a) 60s

(b) 300s

(c) 600s

(d) 900s

(e) 1200s

(f) 1500s

Fig. 7. Distribution of dust mass concentration of the place x = 0 m at different ventilation time.

M. Li et al. / Tunnelling and Underground Space Technology 52 (2016) 38–43

304 Longitudinal plane x=0m Horizontal plane y=1.5m Cross-section place z=75m Total volume

Temperature of air/K

303 302 301 300

Requirement tempertature

43

of an underground excavating engineering project. The simulation is performed for exhausting CO, discharging dust, controlling air temperature and supplying oxygen. The results of simulation are compared against the standard requirements of construction specifications. It is found that the minimum air velocity of 0.15 m/s meets the requirements for supplying oxygen, discharging dust and controlling air temperature of the work-face. However, the concentration of CO during blasting does not reach the standard level when the ventilation is maintained only for 20 min. To meet the standard of CO, the ventilation time should be extended to 45 min.

299

Acknowledgements 298 297 308K

313K

318K

323K

Temperature of surrounding rock/K Fig. 8. Variation of air temperature under different temperature of surrounding rock.

decreases and reaches 2.0 mg/m3 after 15 min of ventilation. This is significantly different from the time (45 min) required to dilute CO from the working space. Fig. 7 shows the dust patterns at longitudinal plane (x = 0 m) and at different times of ventilation. It can be seen that the working space becomes clear of dust particles after 20 min. 4.3. Temperature control It is assumed that airflow temperature in the ventilation duct is 293 K (20 °C). The simulation is carried out for four different temperatures of the surrounding rock: 308 K (35 °C), 313 K (40 °C), 318 K (45 °C) and 323 K (50 °C). The heat radiation and engine exhaust heat are ignored. The rock material is considered to be CaCO3. The simulation results for temperature show (Fig. 8) that at relatively low surrounding rock temperatures of 308 K (35 °C) and 313 K (40 °C), the ventilation air with a velocity of 0.15 m/s is adequate to maintain the air temperature in the working area below the standard 301 K (28 °C). However, when the surrounding rock temperature is greater than 318 K (45 °C), the ventilation airflow cannot maintain the air temperature at acceptable level. 4.4. Oxygen level According to the construction standards developed by the Ministry of Water Resources of the People’s Republic of China (2007), the required oxygen for one worker is 3 m3/min and for an engine is 4 m3/min kW. In this study, an excavator (1.5 m3, 180 kW) and a truck (6  4, 250 kW) are considered. It is assumed that the maximum volume of required oxygen corresponds to a situation where one excavator and two trucks are operating at the work-face. One truck is driving away from the work-face and another is idle and waiting for loading the materials (convert 20% power). Ten workers are also considered to be present at the work-face during development. This means that 1950 m3/min is required to supply enough oxygen at the work-face. The results of simulation shows that the required airflow of 1950 m3/min is greater than the air quantity of 1800 m3/min (30 m3/s) provided by the air velocity of 0.15 m/s. 5. Conclusions Based on the minimum air velocity for discharging dust (0.15 m/ s), a full-scale three-dimensional CFD model is constructed to assess the ventilation performance of a super-large cross-section

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