Design of Mine Ventilation Networks:Application of CFD

A demonstration prepared by Transoft International (P) Ltd, Bangalore.

•Introduction Objectives of Mine Ventilation Design Mine Structure and Types of Ventilators Pollutant and Heat Sources in Mines

•CFD in Mine Ventilation Design

•CFD Simulation of Mines using Fluidyn-A Demonstration Study

Objectives of the Present Study Problem Definition CFD Solver Parameters Results Suggestions for better Ventilation Design Conclusions

Contents

• Mining companies aim at operating the mine at high level of safety and

productivity. Health and environmental factors considered to be as important

as productivity.

• Mine ventilation design should lead to

– economical operation with low power consumption, and

– a ventilation system capable to sufficiently dilute pollutants to safe

levels.

• Mine Ventilation Study is a strategic approach at the planning/designing

stage of the mine ventilation network, to identify, assess and evaluate the

air quality to safe health/environmental limits.

Introduction: Objectives of Mine Ventilation Design

Mine Crew

Any tool that would be used for mine ventilation design should

consider the following features of tunnel and ventilation network:

• Tunnel network

– Inter-connected network of tunnels of random shape and orientation.

– Dynamic nature - shape, size and location of tunnels changes with

time/operation.

– Flow turbulence due to surface roughness.

• Ventilators

– Pathways for air circulation between tunnels.

– Generation and dissipation of radon, radon progeny and dust concentration in

tunnels and their transport by airflow through ventilators between tunnels.

– Control of working temperature and humidity.

Introduction: Mine Structure

Ventilation design process and tools should also be capable

to analyse different kinds of systems such as:

• Mechanical Ventilation

– Fans and ducting.

– Low pressure, high volume air distribution fans.

– Exhaust fan at exhaust end of underground mine.

• Natural Ventilation

– By temperature/density difference of air/gas mixture.

– By air Pressure and elevation.

• Auxiliary Ventilation

- Mechanical ventilation with local fans.

Introduction: Types of Ventilators

Interconnected Mine Tunnels

Mine surface is very rough (causes

turbulence)

Transport Vehicles Ventilator Opening

Finally, the design tools should have the facility to accurately model the generation and dissipation of different pollutants in the mine:

• Radon generation from mine surfaces, porous backfill, ore pit, and spillage water.

• Dust particles generated from mining, blasting, loading/unloading operations.• CO and CO2 from diesel equipments and vehicles.• Radiation emission from Radon and Radon progeny.• Moisture from spillage water.• Heat generated from mining/blasting operation.

Introduction: Pollutant and Heat Sources

Water acts as Source for Humidity and RadonRadioactive radon gas is

generated from mine surface

CFD for the Design of Efficient Mine Ventilation

Network

CFD in Mine Ventilation System Design: Results from CFD Simulations

CFD tools, such as Fluidyn-VENTMINE, provide following capabilities to help

the design of ventilation systems for mines:• Simulation of air flow patterns (aeroulic studies) for a given mine-ventilation configuration.

• Simulation of pollutant and radioactive gas/dust/particle generation and dispersion.

• Prediction of the radiation dose from radioactive materials.

• Prediction of the oxygen distribution in the mine.

• Identify the pockets/locations where pollutant/radiation level is above permissible limits.

• Redesign the Ventilation system for better air quality.

• For the new design/construction of mine network, CFD simulation provides,

– Total quantity of air flow by exhaust fans to meet air quality requirements

inside the mine.

– Optimal locations of the fans.

– Optimal locations, dimensions and orientation of the ventilators and passages.

– Safety design considering fire/explosion inside the mine.

CFD in Mine Ventilation System Design: Representation of Geometry

To obtain accurate results with reduced computational

expenses, the transient and complex mine tunnel

ventilation network can be represented by a

combination of1. Line mode (1D computational

mesh),

2. 3D mode (3 dimensional

computational mesh)

3. 3D-1D Hybrid mode

Characteristics of the three modes in Fluidyn-VENTMINE are:

• 1D MODE– Flow is one-dimensional; however, effects of the wall roughness can be

considered.– Suitable in regions away from intersections and flow re-circulation

zones.– Computationally least expensive, but 2D and 3D effects such as

boundary layers and flow turnings will not be properly resolved.

• 3D-1D HYBRID MODE– 3D-1D coupled flow.– Proper interpolation at 3D-1D mesh interface. – Computationally economical than full 3D.

• 3D MODE– Full 3D flow.– Most accurate, however, computationally more expensive.

Representation of Geometry (Continued)

Fluidyn-VENTMINE can model the following

physical/chemical processes occurring in a mine network:• Turbulent air flow.

• Radon gas emission from walls and/or water and dispersion in air

stream.

• Radon progeny generation and dispersion.

• Particle generation and dispersion in air.

• Heat emission from different equipments and its dissipation.

• Change in humidity.

CFD in Mine Ventilation System Design: Flow Characteristics

CFD Simulation of Mines using Fluidyn-VENTMINE:A Demonstration Study

Prepared by Transoft International (P) Ltd, Bangalore.

• To simulate the air flow field in a representative mine tunnel ventilation network.

• To simulate radon emission and dispersion from the tunnel walls in the above mentioned flow field.

• To simulate the generation, dispersion, and consumption of radon progeny in the same turbulent air flow field.

• To demonstrate the feasibility of using 1D-3D hybrid computational mesh.

One simulation is done with a fully 3D mesh. Then this simulation is repeated using a 1D-3D hybrid mesh in one

of the tunnels.

Objectives of the Present Study

Problem Definition: Geometry of the Simulated Mine Network

Entity Size (m3) Mesh

Tunnel 800 3 3

500 3 3

Shaft 450 3 3

400 3 3

Ventilator

141 3 1

90 3 3

Extractor

450 1 1

150 3 3This network contains one mine shaft, nine ventilators, four

mine tunnels and one extractor.

• Flow is considered to be incompressible and viscous.

• Turbulence is modeled by standard k- ε model.

• Roughness factor of 1.8E-03m used for tunnels.

• Radon and its progeny are considered as passive scalars.

• Ra 222 emission is modeled by surface sources on the tunnel walls.

• Generation rates of the daughter products Po 218, Pb 214, Bi 214, and Pb 210 calculated using chemical reaction equations based on half-life time.

(Continued…)

Problem Definition: Flow Configuration

Problem Definition: Generation/Depletion Rates for

Radon and its ProgeniesIn the present study we assume that radon decomposes to its progenies as follows:

Rn222 Po218 + Radiation ; rate constant = k1

Po218 Pb214 + Radiation ; rate constant = k2

Pb214 Bi214 + Radiation ; rate constant = k3

Bi214 Pb210 + Radiation ; rate constant = k4

The rate constants are calculated from the half-life periods. In the present study we used the following values:Substance Half-life Period,

sRate constant

(k)

Radon 331776 0.00000208

Po218 186 0.003725

Pb214 1620 0.0004277

Bi214 1194 0.0005804

Pb210 Assumed to be stable because of long half-life

period.

• It is assumed that the air flow in the network is driven by forced circulation induced by the exhaust

fan at the extractor outlet. The flow rate through the exhaust fan is 20 m3/s.

• At the shaft inlet the static pressure is 1 atm.

• Log-law at the wall with a roughness factor of 1.8103 m is used at the tunnel walls.

• Radon emission rate from walls is different for each tunnel.

Tunnel number Emission rate (pCi/m2 s)

1 50

2 40

3 30

4 20

Problem Definition: Boundary Conditions

Numerical Schemes and CFD Solver Parameters

• Fluidyn-VENTMINE uses a 3D, unstructured CFD solver based on finite volume method.

• It incorporates a number of numerical schemes for modeling convection, diffusion, and

unsteadiness.

• It has three schemes for pressure-momentum coupling, namely, SIMPLE, SIMPLEC, and

PISO.

• It also offers different linear equation solvers from which user can select a suitable one.

• In addition, it provides openings for user coding for many aspects including numerical

schemes, source definition, initialisation, boundary conditions etc.

In the present study following schemes were used:

• Convective flux – Upwind Scheme

• Pressure-momentum coupling using SIMPLE scheme.

3D-1D Hybrid Mesh

Green and Red – 3D mesh

Blue – 1D mesh

• Two simulations were done. For the first simulation 3D mesh was used everywhere.

• In the second simulation a combination of 3D and 1D mesh was used in the Tunnel 3. 3D mesh was used near the tunnel-ventilator joints. 3D mesh is used up to about 10 times the ventilator cross-sectional diagonal.

• Total number of computational cells decreased from 35550 in a fully 3D mesh to 32948 in the hybrid 3D-1D mesh. For the present problem If the 3D-1D combination is used in all the tunnels and ventilators the the number of cells could be reduced by about 50%.

Results: Pressure Field

Shaft inlet

Extractor

Tunnels 1 2 3 4

• Extractor generates a low pressure region which drives the flow through the network.

• The pressure and velocity fields obtained were same in both the fully 3D and hybrid 3D-1D simulations.

• At the mine shaft inlet the pressure is 101325 Pa and at the extractor the pressure is 100512 Pa.

Results: Velocity Distribution

Shaft inlet

Extractor

Tunnels 1 2 3 4

• The pressure gradient between the shaft inlet and the extractor induces a flow of air. Air flow rate of 20 m3/s is imposed at the extractor outlet. The average velocity at the extractor outlet is 20 m/s (cross sectional area is 1 m2.

• At the shaft inlet an average of velocity of 2.22 m/s was obtained (the cross sectional area is 9 m2) as expected.

• Inside the network the velocity varies according to the cross sectional area.

• Mass flow rates through different segments of the network is shown in the next slide…

Results: Mass Flow Rates for 3D Mesh

Extractor

Mine Shaft Inlet

20

9.82 8.09 6.64 5.35

20

10.18

6.32

3.87

2.28

1.59

1.73

1.94

0.99

2.58

6.11

3.21

1.44

5.91

1.66

3.97

0.9

3.48

1.29

5.76

1.43

4.60

0.795

4.28

14.65

8.88

Unit of flow rate: kg/s

• The net flow rate at each of the nodes is zero.

• Results with hybrid mesh is shown next …

Tunnels

Ventilators

Results: Mass Flow Rates for 3D-1D Mesh

Extractor

Mine Shaft Inlet

20

9.82 8.11 6.65 5.38

20

10.18

6.32

3.86

2.27

1.59

1.71

1.94

0.99

2.58

6.10

3.22

1.46

5.92

1.64

3.96

0.90

3.47

1.27

5.78

1.41

4.58

0.79

4.26

14.62

8.85

• The flow rates are very close to that with fully 3D mesh.

• Air flow is non-uniformly distributed to mine

tunnels.

• Tunnel away from extractor exhaust fan, in this

case Tunnel 1, gets less air flow.

• 3D-1D hybrid mesh could be used if regions of 1D

and 3D mesh are judiciously selected. Generally,

1D mesh should be used sufficiently away from

the regions of flow turning, such as tunnel-

ventilator joints.

• Results obtained with the fully 3D mesh and

hybrid 3D-1D mesh are nearly same.

Results: Observations

Results: Steady Radon Distribution without Decomposition

Shaft inlet

Extractor

Tunnels 1 2 3 4

Tunnel number Emission rate (pCi/m2 s)

1 50

2 40

3 30

4 20

• Results show steady state distribution of Radon in pCi/kg of air). It is assumed that Radon does not decompose.

• The concentration of Radon is a function of the emission rate and the local velocity.

• Radon is found to accumulate where the velocity is low, in the present case in Tunnel 1.

• Steady state distributions of radon and its daughter products, when there is decomposition of Radon, are shown in the following slides …

Results: Steady Radon Distribution with Decomposition

Shaft inlet

Extractor

Tunnels 1 2 3 4

• As expected the concentration of radon in this case is much less than that when there is no decomposition.

• Steady state distributions of the daughter products, Polonium, Lead-214, Bismuth, and Lead-210, are shown in the following slides …

Shaft inlet

Extractor

Tunnels 1 2 3 4

Results: Steady Polonium Distribution

Shaft inlet

Extractor

Tunnels 1 2 3 4

Results: Steady Lead-214 Distribution

• Concentration of Radon is the highest among all the products because, its half life period (331776 s) is much longer than that of Polonium (186 s) , to which it decomposes.

Shaft inlet

Extractor

Tunnels 1 2 3 4

Results: Steady Bismuth Distribution

Shaft inlet

Extractor

Tunnels 1 2 3 4

Results: Steady Lead-210 Distribution

Shaft inlet

Extractor

Tunnels 1 2 3 4

Results:Transient Radon Distribution• An unsteady simulation

of the emission,dispersion, and generation of radon and its daughter products was also done to demonstrate the effect of the different half-life periods.

• It is assumed that at time t = 0, the steady-state velocity field is established. The emission of radon starts at t = 0 s at the rates used in the previous case.

• Simulation was done up to about 60 minutes by which time the steady-state has been attained.

Click on the picture to see the animation

Results: Transient Polonium Distribution

Click on the picture to see the animation

Results: Transient Lead-214 Distribution

Click on the picture to see the animation

Results: Transient Bismuth Distribution

Click on the picture to see the animation

Results: Transient Lead-210 Distribution

Click on the picture to see the animation

Conclusions

• Simulation of air flow for a typical mine ventilation network has been performed. Emission, generation, dispersion, and accumulation of radioactive substances such as radon and its progenies in this flow field are studied and results presented.

• Use of a fully 3D mesh and capability to use a hybrid 3D-1D mesh has been demonstrated.

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A demonstration prepared by

Transoft International (P) Ltd

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