View from the MG end

Plan view

Side view

1.1 DEVELOPMENT OF CFD MODELS

CFD models have been developed based on the floor contours and layout of the longwall panels at a QLD coal mine (Mine A). Models have been built using commercial CFD software tool ANSYS which comprises of the CAD tool DESIGN MODELLER and the numerical fluid flow solver FLUENT. The CFD models include longwall face geometries with different cutting heights, face equipment, gateroads, and 1.0 km long goaf areas behind the longwall face, covering various 1.0 km long sections of the longwall goaf areas with different seam gradients. The information obtained from the site characterisation/review studies was used for setting up the base-case CFD models and for initial calibration of the numerical models.

1.1.1 CFD MODEL –MINE A

The CFD models were developed based on the dimensions of the longwall panel. Geometric coordinates from the working seam floor contours were used for developing the CFD models. The longwall panel width is 310 m and the roadway width on both maingate (MG) and tailgate (TG) sides of the face is 5.4 m. The longwall extraction thickness is 3.6 m, which also represents the face height in the model. The goaf height up to 80 m above the working seam and the floor strata down to 12 m below the working seam is included in the models. The layout of the CFD model is shown in Figure 1.

Figure 1 CFD model of the longwall panel – Mine A

MG and TG cut-throughs of 5m in width and 75m in length have been incorporated into the CFD models, and these cut-throughs were spaced at 125m intervals along the panel in the models. One rear shaft of 2.1m diameter has also been included in the CFD model, which is located on the TG side at the start-up areas of the models. A number of goaf gas drainage holes of 0.3m diameter at

TG transition shields and TG motor drive

Shearer location at the mid of the face

MG side equipment’s and transition shields

different locations were incorporated into the micro model to investigate the effect of various goaf gas drainage strategies. Each individual goaf hole can be switched ‘ON’ and ‘OFF’ in various simulations, depending on the goaf gas drainage to be investigated.

After development of the basic configuration for the CFD model, all the longwall face equipment was also incorporated into the model, based on the equipment drawings of the longwall shields, MG and TG shields, BSL, and shearer. The CFD models also contain details of the longwall face such as the BSL at the MG side of the panel, shearer located at the mid face, transitional shields at MG and TG sides of the face and the AFC motor drives at MG and TG areas etc., as shown in Figure 2.

Figure 2Longwall equipment configuration in CFD model – Mine A

1.1.2 BOUNDARY CONDITIONS – MINE A

Mine A uses U ventilation system for longwall panels. The ventilation layout and actual airflow quantities of mine are used in CFD simulations, as shown in Figure 3. In the CFD model, the velocity inlet boundary conditions were specified at the MG intake roadway (70 m 3 /s) and also at the TG intake roadways (40 m 3 /s). An outlet velocity condition was specified at the MG roadway (30 m 3 /s) and a free outflow condition was specified at the TG end for the total ventilation quantity of 80 m 3 /s leaving the face. The rear shaft which was incorporated as intake shaft was closed in the modelling

75 m

Roadways3.6m x 5.4m 125 m

1km

300m

simulations. The permeability distribution in the goaf area varies considerably at various locations depending on the stress distribution, and it ranged from 10e 3 millidarcy (md) to 10e 10 md in the goaf area, and goes up to 10e 12 md in the collapsed roadways just behind the face.

Figure 3. Longwall ventilation layout – Mine A

1.2 BASE CASE MODEL SIMULATIONS AND VALIDATIONS

After development of CFD models representing longwall panels of mine A, base-case simulations were carried out with actual panel geometry and operating conditions to compare and validate the CFD modelling results. The details and results of the base case simulations along with field measured data are presented in this section.

1.2.1 BASE CASE MODEL SIMULATIONS AND FIELD DATA –MINE A

In the base case model simulations, actual geometry and operating conditions of the initial longwall panels at Mine A were used. Boundary conditions as discussed in section 1.1.2 are used in the base case simulations. The total goaf gas emissions were around 3,000 l/s of methane, with total goaf gas drainage flow rates of around 3,300 l/s. The results of base case simulations along with field measured values are presented in Figure 4, showing comparison of oxygen and methane gas concentration levels at various locations behind the longwall face.

Methane gas distribution in the goaf at working seam level

CH4 – 4.5 %– 41.7 % (Field data)CH4 – 5.0 %– 45.0 % (CFD results)

X X

CH4 – 15.0 % – 30.0 % (CFD results)CH44 – 17.5 % – 26.5 % (Field data)

X X

O2 – 10.5 % – 1.82 % (Field data)O2 – 10.0 % – 2.0 % (CFD results)

Oxygen distribution in the goaf at working seam level

X X

O2 – 19.5 %– 15.0 % (CFD results)O2 – 19.4 %– 15.4 % (Field data)

X XCH

4 in

retu

rn -1

.2 %

(CFD

)CH

4 in

retu

rn -1

.1 %

(Fie

ld)

Figure 4. Comparison of CFD simulation results and field data –mine A

Simulation results show that oxygen ingress is high on the maingate side, with oxygen concentration levels over 15% even at 500m behind the longwall face (Figure 4(a). Results show high methane gas concentration levels on the tailgate side of the panel due to gas buoyancy effects. Comparison and analyses of the results presented in Figure 4a and Figure 4b shows that simulation results are in close agreement with the field measured data from Mine A.

Pressure distribution in the seam level in Pascals

Velocity distribution in the seam level in m/s

Velocity distribution in the seam level in m/s

Figure 5. Pressure and Velocity distribution from CFD simulation- mine A

The pressure and velocity distribution in the seam level are shown in the above figure 5.

1.2.2 GAS EMISSION SPECIFICATION IN VENTSIM MODEL

The goaf gas emission rate is a function of the space and is given by

Oxygen distribution in the goaf at working seam level

Methane distribution in the goaf at working seam level

Pressure distribution in the goaf at working seam level

˙ch 4= V̇∗0.68∗¿(L∗W∗H )

(1.5−(Z−Zstart ))(Zend−Z start)

¿

Where V̇ is the total gas emission rate in m3/s in the goaf and ˙ch 4 in Kg

m3−sec

L,W,H are the dimensions of the goaf.

1.2.3 SIMULATIONS WITH 1000 L/S AND NO GOAF GAS DRAINAGE

Figure 6. Pressure and Velocity distribution from CFD simulation- mine A

Figures above show the distribution of goaf gases and pressure for goaf gas emission rate of 1000 l/s in the goaf.

1.3 VENTSIM SIMULATIONS

Objective:

Build a 3D goaf model in Ventsim that can produce similar results with CFD (as discussed above).

The results will be showcased in the 2020 ACARP short proposal to be submitted in around 5 May. However, prior to submission, a preliminary version is expected before 20 April for comments of ACARP industry monitors of this project.

Geometry:

Similar dimension with the CFD model described above Face width in the VentSim model should be approximated to the dimension that gives air velocities

between 4-5m/s in the pipe(face pipe) for flow quantity of 80 m3/s. At this stage, Ventsim model does not need to include face equipment, floor strata region, and goaf

gas drainage. The height of the goaf can be tried with 30 m, 50 m, or 80 m. If a smaller height (e.g. 30 or 50m)

produces similar results with a larger height (80m as CFD model), then smaller height is preferred. Consider how to reflect the goaf void volume in different regions (shown in below). M = mining

thickness, in the CFD model above, M = 3.6 m

Gas emission into goaf and other boundary conditions

Goaf gas emission rate of 1000 l/s without goaf gas drainage to be considered, as modelled by the CFD. No need to include bore holes at this stage but need to consider how to specify the boundary conditions for boreholes for future stages.

Pattern of gas emission rate is introduced in 1.2.2. Other boundary conditions are same to the CFD model described above

Calibrations

The results should be calibrated with what are shown in section 1.2.3, including

O2% distribution CH4% distribution Pressure distribution

Ventsim todo:

Some questions and comments in red:

The details that can be included:

Resistances assigned at various regions in the seam level(near working face, in the annular region, away from the face, mid region, and in the start-up area) this will just be your permeability model, along with any of the (few) adjustments we’ve made

Resistance assigned in the goaf at various levels again, this is just the permeability model. I could describe how this plays out in a discrete grid in Ventsim

Resistances in the cut-throughs and the leakages in the cut-throughs Pressure drop across the working face and the velocities across the face Velocities and the flow patterns, say 100 m, behind the face How the gas emissions are specified in the goaf Gas flow patterns in the goaf Leakages occurring through the seals on the MG side Leakages occurring in the start-up seals( 4 cut-throughs at start) is this the four cut-throughs

to the bottom-left? All other steps involved for obtaining the results.

Ventsim Methodology

Underlying equations

The results in this model were obtained using VentSim™ Design (Version 5.4). To simulate the air flows and pressures in the mine network, each airway in the network has an equation formed, based on the Atkinsons Resistance Equation:

∆ P=r Qn , [Pa ] ,

where ∆ P is the pressure drop from one end of an airway to the other, Q is the flow quantity [m3/s] and r is the Atkinson’s resistance. n is usually equal to 2, however for low speed flows n can be made equal to 1. The Atkinson’s resistance is given by:

r= ρρ ref

fLSA3,[ N s2m8 ]

where ρ is the density and ρref is the reference density [kg /m3¿, f is the Atkinson friction factor [kg /m3¿, L is the length [m], S is the perimeter [m] and A is the airway area, [m2].

The other equation is an expression of conservation of mass, and is similar to Kirchoff’s Current Law, which states that the sum of flows into a node must be equal to the sum of flows out of it.

∑ ρQ¿=∑ ρQoutThe user has the option of running Ventsim with constant air densities throughout. This is often done when simulating coal mines, which are generally very flat; however, if considering the effect of buoyancy of different gases or the effect of natural ventilation pressures on the gas flow in the goaf then the compressible flow option should be enabled.

These equations are solved using a method based on the gradient method of Todini & Pilati, 1987 to solve for the flows, pressures and densities throughout the network.

Reference settings for atmospheric density, temperature and pressure are all configurable through the user interface.

Goaf Simulation

For simulating the flow through airways specified as part of the goaf, the exponent used in the Atkinson’s resistance equation is set to 1 and the pipe modelled as a “Darcy” pipe, as outlined earlier TODO. In this way the resistance created by the porous medium of the goaf is represented in the equation in the Ventsim solver. The equation in this case is:

r goaf=μLkA,

where k is the goaf permeability and μ is the dynamic viscosity [Pa. s ]. The goaf permeability is calculated according to the goaf permeability model provided elsewhere in the report. In this way, the resistance of a goaf airway is a function of the permeability model, the viscosity of the air (in turn, a function of the gas composition), and the cross-sectional area of the goaf represented by the individual airway.

Building the goaf grid

In Ventsim, the goaf will be represented by a grid of airways, with airways aligned in various directions to give an approximation, on a macro-scale, of a three-dimensional solver; however, locally, each “grid-point” gives a one-dimensional flow, aligned with the direction of the airway. The clearest starting point is to comprise the grid of airways aligned in three orthogonal dimensions. There is the possibility that airways aligned in other directions could assist in recreating vortical flows, or other flow phenomena; this will be tested as well.

Ventsim is a mature and user-friendly software, with a range of existing methods to create airways; these include the Draw tool, where users can click and drag on the screen to draw an airway. More precise is clicking once on the screen to open a coordinate dialog. However, underground mines are often complex with many thousands of airways, so often users build models automatically from graphical references of airway centreline data generated by mine-planning softwares. However, these options are not suited to building a goaf grid.

The goaf grid will likely consist of hundreds, if not thousands of airways. Furthermore, airway centrelines from mine plans are unlikely to be available; although it will be likely that an outline of the goaf is. For these reasons, goaf-building in Ventsim requires a specific tool for generating the goaf grid. To this end, a Goaf Builder has been created for Ventsim; this builder works on the assumption that the outline of the goaf has been selected; it then fills in the space with airways and applies resistances to each airway according to the equations above. The builder takes in as inputs the following information:

The airways defining the outline of the goaf The height of the goaf The X, Y and Z resolution of the goaf grid

An option has also been added to change the resolution of the grid locally, to provide greater resolution in parts of the goaf where greater changes in air velocity are expected.

A base template model is used (shown below)

This template model is built according to the specifications outlined earlier in the report. To build the goaf, the innermost airways of the model are selected, and then the goaf builder launched. After this, the resolution and height of the goaf can be specified. With this information, Ventsim builds the goaf, adjusting for the size, aspect ratio and alignment of the goaf, as well as any variations in elevation.

An example goaf grid is shown below, of X, Y, Z resolution equal to 160 x 50 x 4 (with reduced resolution in the centre of the goaf), representing a goaf of dimension 1000m x 300m x 100m. Each airway of the grid has been assigned a permeability according to its position in the goaf. This permeability is then converted to a resistance based on the “Darcy” resistance described above.

Gas Injection

For adding methane gas to the goaf, an injection system has been used, to distribute the gas according to the gas distribution model. To do this, an extra “pseudo-airway” is added, starting from each airway intersection in the goaf grid. Each “pseudo-airway” ends at a common point which is arbitrarily set at a distance above the goaf grid. From this common point a surface connection is made and the gas concentration set at 100% methane.

According to the gas distribution function, a fix flow is placed on every “pseudo-airway” to inject the correct amount of methane for the volume that the intersection point represents and to all together give the correct total gas injection rate for the entire goaf. In the images below, at top, a grid is shown with the gas injection “pseudo-airways” hidden; at bottom the “pseudo-airways” are shown converging on a common point, with a rotating green cross on each representing the specific gas injection rate.

In this way, the gas in distributed according to the equation and can be easily controlled and changed with each newly built goaf grid.

Goaf Grid Resolution test

The set of four images below shows a 5m high goaf (resolution 160 x 50 x 1) , at seam level, with a gas injection of 1000 litres. The four after that show the same but with double the resolution. The gas injection is done with the gas injection method which doesn’t add mass. With this method, some gas in very low flow airways is lost. From here we will use the method of direct gas injection, which adds to the airflow quantity. Nonetheless, the results below serve as a useful test of goaf grid resolution. The difference between the results is negligible. The results thereafter use 160 x 50 x 1 resolution.

160 x 50 x 1

Pressure

Velocity (0-0.01m/s)

Methane concentration (0-100%)

Oxygen concentration (0-21%)

320 x 100 x 1

5m high Goaf (resolution 320 x 100 x 1) , at seam level. Gas Injection (no mass addition) = 1000 litres/s

Pressure

Velocity (0-0.01m/s)

Methane concentration (0-100%)

Oxygen concentration (0-21%)

The next set of results shows the sets of results for four tests. Again a single layer (320 x 100 x 1) is used representing 5 m goaf height. The methane injection is varied in each set, with V̇ equal to 1, 0.5, 0.1 and 0.001 m3/sec, distributed according to the gas distribution equation of section 1.2.2.

Pressure

V̇

= 1.000 m3/s

V̇

= 0.500 m3/s

V̇

= 0.100 m3/s

V̇

= 0.001 m3/s

Velocity

V̇

= 1.000 m3/s

V̇

= 0.500 m3/s

V̇

= 0.100 m3/s

V̇

= 0.001 m3/s

Methane

V̇

= 1.000 m3/s

V̇

= 0.500 m3/s

V̇

= 0.100 m3/s

V̇

= 0.001 m3/s

Oxygen

V̇

= 1.000 m3/s

V̇

= 0.500 m3/s

V̇

= 0.100 m3/s

V̇

= 0.001 m3/s

Varying the Maximum Permeability (minimum permeability = 3.5)

The next set of results varies the maximum permeability in the permeability function; the minimum permeability is fixed at 3.5. Gas injection rate is set at V̇ = 0.100 m3/s. The grid is again a single layer of (320 x 100 x 1), representing 5 metres goaf height

Pressure

max_p = 13

max_p = 12

max_p = 10

max_p=9.5

max_p = 9

max_p = 8.5

max_p = 8

max_p = 7.5

Velocity

max_p = 13

max_p = 12

max_p = 10

max_p = 9.5

max_p = 9

max_p = 8.5

max_p = 8

max_p = 7.5

Methane CH4 concentration

max_p = 13

max_p = 12

max_p = 10

max_p = 9.5

max_p = 9

max_p = 8.5

max_p = 8

max_p = 7.5

Varying the Minimum Permeability (maximum permeability = 10)

The next set of results varies the minimum permeability in the permeability function; the maximum permeability is fixed at 10. Gas injection rate is set at V̇ = 0.100 m3/s. The grid is again a single layer of (320 x 100 x 1), representing 5 metres goaf height

Pressure

min_p = 4.0

min_p = 3.5

min_p =3.0

min_p = 2.5

min_p = 2.0

Velocity

min_p = 4.0

min_p = 3.5

min_p = 3.0

min_p = 2.5

min_p = 2.0

Methane CH4 concentration

min_p = 4.0

min_p = 3.5

min_p = 3.0

min_p = 2.5

min_p = 2.0

Varying the Goaf Height (max perm = 10, min perm = 3.5)

The next set of results varies the height of the goaf, still with a single layer; the maximum permeability is fixed at 10, the minimum at 3.5. Gas injection rate is set at V̇ = 0.100 m3/s. The grid is again a single layer of (320 x 100 x 1)

Pressure

goaf height = 5m

goaf height = 10m

goaf height = 20m

goaf height = 50m

goaf height = 75m

goaf height =100m

Methane concentration

goaf height = 5m

goaf height = 10m

goaf height = 20m

goaf height = 50m

goaf height = 75m

goaf height =100m

goaf height =100m, lower res

Comparing one layer to several, max_p = 10

Here, we compare the goaf Height = 100m case with one layer, to the same case, but adding layers, n = 1, 2, 3 and 4. The methane is injected in the same way, distributed evenly vertically. The resolution is dropped to 160 x 50 x n. Maximum permeability is set to 10

Methane Concentration

Comparing one layer to several, max_p = 9

Here, we compare the goaf Height = 100m case with one layer, to the same case, but adding layers, n = 1, 2, 3 and 4. The methane is injected in the same way, distributed evenly vertically. The resolution is dropped to 160 x 50 x n. Maximum permeability is set to 9

Methane Concentration

Permeability across the four layers

Comparing one layer to several, max_p = 9

Here, we compare the goaf Height = 100m case with one layer, to the same case, but adding layers, n = 1, 2, 3 and 4. The methane is injected in the same way, distributed evenly vertically. The resolution is dropped to 160 x 50 x n. Maximum permeability is set to 9. Do V = 0.5m3/s

Methane Concentration

Comparing one layer to several, max_p = 9, but not adding gas on MG side

Here, we compare the goaf Height = 100m case with one layer, to the same case, but adding layers, n = 1, 2, 3 and 4. The methane is injected in the same way, distributed evenly vertically, but no gas is injected within 20 metres of the main gate side. The resolution is 160 x 50 x n. Maximum permeability is set to 9. Gas V = 0.5m3/s

Methane Concentration

Comparing one layer to several, max_p = 9, but not adding gas on MG side

Here, we compare the goaf Height = 100m case with one layer, to the same case, but adding layers, n = 1, 2, 3 and 4. The methane is injected in the same way, distributed evenly vertically, but no gas is injected within 20 metres of the main gate side. The resolution is 160 x 50 x n. Maximum permeability is set to 9. Gas V = 1.0m3/s

Methane Concentration

Comparing with injection distributed evenly and gas injected only at middle layer, max_p = 9

Here, goaf Height = 100m case with 3 layers. The methane is injected in the same way, but injected only at the middle layer. The resolution is dropped to 160 x 50 x 3. Maximum permeability is set to 9. Gas V = 0.5m3/s

Methane

Distributed evenly vertically

Distributed only at the middle layer

Comparing with injection distributed evenly and gas injected only at middle layer, max_p = 9

Here, goaf Height = 60m case with 5 layers. The methane is injected in the same way, but injected only at the second layer from the top. The resolution is dropped to 160 x 50 x 5. Maximum permeability is set to 9. Gas V = 0.5m3/s

Methane

Distributed evenly vertically (optional)

Distributed only at the second layer from the top

TODO

Comparing extra resolution at the cutting face

Here, goaf Height = 100m. The methane is injected evenly vertically. The resolution is 160 x 50 x n. Maximum permeability is set to 9. Gas V = 0.5m3/s. The first results are for the regular grid resolution. The second with a bigger instep of higher resolution at the cutting face of the goaf

Methane

Compare 3 layers to 5 layers, is 3 adequate?

Roadway size/resistance

Here, goaf Height = 100m. The methane is injected evenly vertically. The resolution is 160 x 50 x 3. Maximum permeability is set to 9. Gas V = 0.5m3/s.

The first results are for the regular roadway size of 5.4m x 3.6m.

The second results are for the reduced roadway size of 2.7m x 1.8m, for > 200m into the goaf.

The third results are for the reduced roadway size area of 1m2, for > 200m into the goaf.

This shows that increasing the resistance of the roadway around the goaf leads to less ingress of oxygen into the goaf.

Methane

Level 1 Level 2 Level 3

A new test case

Here, goaf height = 90m, length = 850 m, width = 230m. Only one intake to the goaf is available. The roadway size is 5 x 3 m. There are no other connections to the goaf.

The air flow rate is 2,2720m3/min, or 45.3 m3/s.

The gas injection rate is 30 m3/min, or 0.5 m3/s, slightly more than Qingdong’s model we are comparing to.

U shape

Y shape

This is the same, but with another 520m3/min(8.67m3/s) added at top left and an exhaust at the top right

A new test case, but with 1m2 side roadway after 200m, and no changes to permeability for roadway proximity

Here, goaf height = 90m, length = 850 m, width = 230m. Only one intake to the goaf is available. The roadway size is 5 x 3 m. There are no other connections to the goaf. After 200 metres, the roadway is reduced to 1m2. There are 5 layers at 0, 20, 40, 60 and 90 metres elevation.

The air flow rate is 2,2720m3/min, or 45.3 m3/s.

The gas injection rate is 30 m3/min, or 0.5 m3/s, slightly more than Qingdong’s model we are comparing to.

U shape

0m

20m

40m

60m

Y shape

This is the same, but with another 520m3/min(8.67m3/s) added at top left and an exhaust at the top right