1 INTRODUCTION 

Spontaneous combustion continues to be a significant risk for underground coal mines, particularly in 

U.S. western mines where the coal is generally of 

lower rank. This hazard is exacerbated in mines with 

appreciable levels of methane, due to the potential of 

an explosion ignited by a spontaneous combustion 

fire. The most recent reported data on mine fires in 

the U.S. for the period 1990 – 1999 show that there 

were 15 reported fires caused by spontaneous combustion in underground coal mines, accounting for 

17% of the 87 total reported fires (DeRosa, 2004). 

Although the number of fires is relatively low, 

amounting to an average of 1 to 2 fires per year, the 

potential for catastrophic consequences is very high. 

In fact, three of the mine fires from the reported 

1990 – 1999 period resulted in subsequent methane 

explosions. In addition, although the number of 

spontaneous combustion fires has remained nearly 

constant for the last 35 years, there is the possibility 

that the number of spontaneous combustion fires increases due to growth in the dimensions of longwall 

panels and due to the exhausting of easily-mined 

coal beds in the U.S., which results in increased 

mining of lower rank coals and deeper coal beds. 

Most spontaneous combustion fires have occurred in the gob areas of U.S. coal mines. Gob areas, by the very nature, present accessibility problems regarding detection, fairly precise locating, and 

fire extinguishment. The self-heating of coal occurs 

when the heat produced by low-temperature oxidation is not adequately dissipated, resulting in a net 

temperature increase in the coal mass. Under conditions that favor a high heating rate, a fire ensues. 

The oxidation of coal requires a fuel, the coal, and 

an oxidizer, usually the ventilation air. To reduce the 

amount of oxidation, and thus the amount of heat being produced, either the coal or the air must be removed. On the other hand, if the airflow is sufficiently high, it can remove the heat produced by the 

coal oxidation, and thus prevent acceleration of coal 

self-heating that may result in a fire. The amount of 

coal in a retreat longwall gob can be somewhat controlled by the mining practices, such as pillar design 

and the amount of coal left in the roof and floor. 

Additional coal from un-mined coal bands immediately above or below the mined seam may be present 

in the gob as well. 

The amount of airflow through a longwall gob 

can be reduced by using a bleederless ventilation design. In the U.S., bleederless systems are only perComputational Fluid Dynamics Study on the Ventilation Flow Paths in 

Longwall Gobs 

Liming Yuan, Alex C. Smith and Jürgen F. Brune 

Pittsburgh Research Laboratory 

National Institute for Occupational Safety and Health 

ABSTRACT: To provide insights and assistances for the optimization of ventilation systems for U. S. underground coal mines facing both methane control and spontaneous combustion issues, a computational fluid dynamics (CFD) study is being conducted to investigate the effect of the ventilation scheme on the prevention of 

spontaneous combustion in longwall gob areas. This report focuses on the flow patterns within the gob under 

three different ventilation systems; one-entry and two-entry bleederless systems as well as a three-entry 

bleeder system. The gas flow in the caved gob area is simulated as a laminar flow through porous media while 

the gas flow in ventilation airways is simulated as a fully developed turbulent flow. The air flow patterns are 

visualized using flow path lines. Air velocity contours and vector data are also obtained. The possible location 

of critical velocity zones where the gob is most liable to spontaneous heating is discussed. 

Disclaimer: The findings and conclusions in this report are those of the authors and do not necessarily represent the views of the National Institute for Occupational Safety and Health. 
mitted as a spontaneous combustion control method 

in mines with a demonstrated history of spontaneous 

combustion. However, in U.S. mines with appreciable methane levels, ventilating gob areas with 

bleeder systems to dilute and remove methane has 

been traditionally regarded as the preferred method 

to deal with the methane hazard. Bleeder system design can exacerbate the spontaneous combustion 

hazard in mines with both methane and spontaneous 

combustion issues. 

In order to optimize the airflow in the gob to prevent spontaneous combustion and control the methane content, it is important to understand likely air 

flow patterns inside the gob and their effect on both 

coal self-heating and methane migration. Because 

much of the gob area is inaccessible, it is difficult to 

measure the air flow rate in it. Although some field 

measurement methods such as tracer gas technology 

have been used to determine the air flow pattern, the 

results are limited. Therefore, flows through the 

caved area of the gob and even to some extent along 

the periphery have been mainly conjecture based on 

spotty experiences or investigations. Absent practical methods to gather empirical data, controlled 

modeling techniques are viewed as the only current 

reasonable way to assess the ventilation in gob areas. 

Computational Fluid Dynamics (CFD) modeling 

has been increasingly applied to mine ventilation in 

recent years. CFD is the application of numerical 

techniques to solve the Navier-Stokes equations for 

fluid flow. The Navier-Stokes equations are derived 

by applying the principles of conservation of mass 

and momentum to a control volume of fluid and are 

solved by discretizing the equations using either finite difference or finite element techniques. In Australia, Balusu et al. (2002) and Wendt & Balusu 

(2002) conducted CFD modeling of longwall gob air 

flow dynamics with focus on bleederless ventilation 

systems. In the U.K., Ren et al. (1997) performed 

CFD modeling of methane flow around longwall 

coal faces. All these studies were based on specific 

ventilation schemes, and results were limited to 

those ventilation systems. In the U.S., Wala et al. 

(2003) have used CFD to evaluate aspects of mine 

face ventilation, but no CFD work has been done to 

model flows in the longwall mine gob area. Brune et 

al. (2000) identified the significant changes for U.S. 

longwall mine ventilation and studied the effects of 

different bleeder configurations and ventilation adjustments. Longwall gob leakage was also simulated 

using a ventilation network analysis program, 

VNET, by Brunner (1985) and Banik et al. (1995). 

NIOSH has initiated a new program to develop 

CFD models to evaluate ventilation schemes to control spontaneous combustion and to describe the 

ventilation pathways through the immediate gob and 

to evaluate their effect on gob methane control systems. In this paper, a preliminary CFD study of air 

flow patterns in the gob is presented. 

2 VENTILATION SCHEMES 

The increase of longwall panel lengths and widths in 

the U.S. continues to present challenges to longwall 

mine ventilation systems. Among these challenges is 

the characterization of flow patterns through and 

around the gob, and the effect of these airflows on 

the spontaneous combustion risk. 

Three types of ventilation schemes were investigated to demonstrate the air flow patterns in the gob. 

The simulated gob area in the three studies is 1000 

m long, 300 m wide and 50 m high starting from the 

bottom of the coal seam, representing a typical 

longwall panel layout in the U.S. The caved region 

may have a height of 3 ~ 4 times mining height, but 

a highly fractured region may extend beyond that 

height. The 50 m height was chosen to cover both 

the caving and highly fractured regions. Although 

actual longwall panel lengths extend to 3,000 or 

4,000 m, a 1,000 m section was selected to model 

the flow paths in this study. The ventilation airway 

dimensions are 2 m high by 6 m wide, and the longwall face is 4 m wide by 2 m high. 

The first ventilation scheme is a simple “U” 

bleederless ventilation system with one entry, as 

shown in Figure 1 (a). The second scheme is a twoentry bleederless system as shown in Figure 1 (b). 

For the convenience of simulation, two intake entries were combined into one entry, and only one 

crosscut close to the face was open to the second return entry. The third ventilation system, shown in 

Figure 1 (c), is a three-entry bleeder system. The 

three collapsed entries at the tailgate side were combined into the gob area and were connected to the 

bleeder fan through an open entry. It should be 

pointed out that this treatment may be only valid 

near the end of the panel life. During the earlier 

stages of the panel life, the middle entry remains at 

least partially open and has a different airflow resistance than the collapsed area. It then serves as a primary route for bleeder air. With the retreat of the 

longwall face, the middle entry continues to collapse, and will have a same order of magnitude resistance as other collapsed areas approaching the end 

of the panel life. On the headgate side, the belt entry 

was combined with the intake entry and the flow rate 

in both entries was taken into account. All crosscuts 

between the second and third entries on the headgate 

side were open. Although intact stoppings may be 

maintained between the second and third entry for 
some longwalls for the purpose of methane dilution, 

it will not affect this model significantly. The 

bleeder entries at the back end of the gob were also 

combined into one model entry connecting to the 

bleeder fan. 

Figure 1. Ventilation schemes: (a) one-entry bleederless system; (b) two-entry bleederless system; (c) three-entry bleeder 

system. 

3 ESTIMATION OF GOB PERMEABILITY 

It is generally believed that the permeability inside a 

gob varies in different areas. The global permeability distribution within the gob of a single longwall 

panel was represented by five constant-permeability 

zones, as shown in Figure 2. The division of the gob 

area and the assignment of permeability data were 

based on geotechnical modeling of longwall mining 

and the associated stress-strain changes/rock failure 

using FLAC (Fast Lagrangian Analysis of Continua) 

code (Esterhuizen & Karacan, 2005). In FLAC modeling, mining was simulated in increments, starting 

from one side of the grid and advancing to the other 

side. Extraction of the coalbed was modeled by removing elements over the height of the coalbed. The 

process of gob formation was modeled by first deleting rock elements in the roof of the coalbed, so that 

they are stress relieved, followed by inserting gob 

properties in these elements. Gob properties were 

also inserted in previously mined coalbed elements, 

so that the gob filled the mined void. 

Figure 2. Gob zones for a single longwall panel. 

In the gob caved area, the re-compaction of the 

caved rock has a significant effect on its permeability. Stress changes in the rock in the gob cause 

changes in the fracture apertures which then impact 

the permeability. Immediately behind the advancing 

face, the caved rock is loosely stacked and has a porosity of approximately 0.4. As the mining face advances away from this caved rock, the weight of the 

overburden gradually increases, which re-compacts 

the caved rock. This loading or stress in the caved 

material increases exponentially, until the full overburden load is supported by the caved material. The 

re-compaction, estimated with the FLAC model’s 

assigned strength and mechanical properties of the 

gob material, results in a reduction in the void space 

within the broken rock rubble and, therefore, significant changes in permeability. A simple relationship 

was used to estimate the changes in permeability in 

the caved rock based on the Kozeny-Carmen equation: 

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

− = 2

3

(1 n)

n k f (1) 

where n is the porosity and k is the permeability. 

 Using the results of the FLAC model, the permeabilities for the five zones were determined to be 

1x106

 millidarcies (md) for zone 1, 2x105 md for 

zone 2, 7x104

 md for zone 3, 1x104 md for zone 4, 

and 5x103

 md for zone 5. The longwall face shields 

were simulated as a thin porous media with a permeability of 9x107

 md. Note that the higher permeability in zone 1 provides for lower resistance pathways 

along the periphery of the gob. It should be pointed 

out that the permeability data used in this study are 

different from those used by Brunner (1985), 1x10-7

to 1x10-5 m2

 (1x108 to 1x1010 md), Ren et al. (1997), 

1x10-15 to 1x10-10 m

2

 ( 1 to 1x105 md), and Wendt et 

al. (2002), maximum 1x10-9 m2

 (1x106 md). These 

differences may be caused by different coal seam 

geology, different mine panel layout, and different 

time periods in which the permeability was estimated. 
4 NUMERICAL MODELING 

A commercial CFD software program, FLUENTTM 

(Reference to a specific product is for informational 

purposes and does not imply endorsement by 

NIOSH), from Fluent, Inc., was used to simulate the 

air flow in the gob. FLUENT is a general purpose 

CFD solver for a broad spectrum of flow modeling 

applications. It has the capability to simulate laminar 

and turbulent porous media flow based on the 

known permeability and the inertial resistance factor 

data. The air flow in the longwall mine gob area is 

treated as laminar flow in a porous media. The real 

flow inside a gob can be very complicated, and this 

laminar flow assumption may not be valid for every 

region inside a gob. Further research will be carried 

out that may modify this assumption in the next 

phase of this program. The air flow in the ventilation 

airways was simulated as fully developed turbulent 

flow using standard k-ε equations with k the turbulent kinetic energy, ε the turbulence dissipation rate. 

The k-ε equations are derived from the eddy viscosity theory of turbulent flow, and have been used successfully to model turbulent flow in many different 

situations. 

The physical model and mesh for the CFD simulation were created using the mesh generation software, GAMBIT, from Fluent, Inc. The cell size varies from 1 to 4 m in the model. They were selected 

to resolve the flow patterns both in the gob and inside typical mine airways, as well as to achieve an 

acceptable convergence within a reasonable amount 

of time. This mesh is depicted in Figure 3. The total 

cell number for the three-entry bleeder system was 

644,660, most of which were hexahedral cells as 

shown in the figure. 

 Figure 3. The mesh for the thee-entry bleeder system. 

For the simulations with FLUENT, boundary 

conditions were chosen to represent pressures and 

flow quantities found in typical longwall ventilation 

situations for the geometry modeled. For one-entry 

and the two-entry ventilation systems, the boundary 

condition used was 0.12 kPa (0.5 inches water 

gauge) pressure differential between the intake and 

return entry along the longwall face. To control the 

air flow quantity to the longwall face, the wall 

roughness was adjusted to have a realistic intake airflow rate of 30 m3

/s (64,000 cfm). 

For the three-entry bleeder ventilation system, 

two regulators were placed at the end of the second 

and third intake entry, respectively, as shown in Figure 1 (c), and were simulated as surfaces that can 

have a pre-defined pressure change. No regulators 

were placed at the tailgate side of the gob, and the 

flow was controlled by the permeability value assigned. In actual bleeder ventilation systems, there 

are usually regulators in the entries at the tailgate 

side. In the model this simplification was justified 

since the boundary conditions effectively replace 

these regulators. The boundary conditions used were 

-0.62 kPa (-2.5 inches water gauge) pressure at the 

intake airway inlet on the headgate side of the face, -

0.75 kPa (-3 inches water gauge) pressure at the return entry outlet on the tailgate side of the face, and -

3.7 kPa (-15 inches water gauge) pressure at the bottom of the bleeder fan shaft. The wall roughness was 

adjusted to have the total airflow rate of 43 m3

/s 

(91,000 cfm) for all three intake entries. The pressure drops across the two headgate regulators were 

also adjusted so that the airflow quantity along the 

longwall face was the same as longwall face quantities used with the one-entry and two-entry ventilation systems, i.e. 30 m3

/s (64,000 cfm). 

5 FLOW PATTERNS INSIDE THE GOB 

The air flow inside a gob is expected to be three dimensional with the flow in the vertical direction 

weaker than in the other two directions. In order to 

visualize the flow patterns inside the gob, a virtual 

horizontal reference surface was created 1 m (3 ft) 

from the bottom of the coal seam floor. All results 

reported hereafter are with respect to this horizontal 

reference surface. 

5.1 One-entry bleederless system 

Figure 4 shows the flow path lines for the one-entry 

bleederless system colored by velocity magnitude. 

Figure 4 (a) shows the path lines in the entire gob 

area, and Figure 4 (b) shows the path lines near the 

face. The path lines show that flow through the gob 

itself was mainly concentrated behind the shields. At 

the headgate side, air leaked through the shields but 

some was forced back into the face again through 
the shields near the tailgate side. The air velocity 

ranged between 1.0x10-5 to 1.0x10-4 m/s (0.002 to 

0.02 fpm) near the shields, and below 1.0x10-5 m/s 

(0.002 fpm) farther away from the shields. 

Figure 4. Flow path lines colored by velocity magnitude (m/s) 

for the one-entry system: (a) in the whole gob; (b) near the 

shields. 

5.2 Two-entry bleederless system 

Results of the modeled flow paths for the two-entry 

bleederless ventilation system were similar to the 

one-entry system, except for slightly higher air velocities on the tailgate side created by the second return crosscut inby the face shown in Figure 5. For 

both one-entry and two-entry bleederless ventilation 

systems, the air flow velocity at the back end of the 

gob was very low. Figure 6 compares velocity contours close to the shields and near the tailgate between the one-entry and two-entry ventilation systems. The velocity contours were only slightly 

different between the two systems, indicating that 

the second return entry has little effect on the velocity field close to the shields. Inside the gob and near 

the second return entry inside the gob, the air velocity was about 1.0x10-4 to 7.0x10-4 m/s (0.02 to 0.14 

fpm). The air velocity inside the second return entry 

was about 7.0x10-4 to 1.0x10-3 m/s (0.14 to 0.2 fpm). 

Figure 5. Flow path lines colored by velocity magnitude (m/s) 

for the two-entry system: (a) in the whole gob; (b) near the 

shields. 

Figure 6. Comparison of colored velocity contours (m/s) between the one-entry and two-entry ventilation system: (a) oneentry system; (b) two-entry system. 
5.3 Three-entry bleeder system 

Figure 7 (a) shows the flow path lines in the gob 

with the three-entry bleeder ventilation system. A 

bleeder fan is located at the back of the panel. Gob 

air flow was mainly concentrated in three areas; behind the shields, near the back end of the gob, and 

along the tailgate entry, where the permeability is 

highest compared to other caved zones in the gob. 

The air velocity along the least compacted zone 

(zone 1) at the tailgate side was larger than that behind the shields, but smaller than that near the back 

end of the gob. Figure 7 (b) shows the flow path 

lines near the shields. Air that leaked through the 

shields at the headgate side does not flow back into 

the face at the tailgate side in this ventilation 

scheme, but flows towards the tailgate side and then 

out of the gob through the least compacted zone. 

Some air from the second and third intake entries enters into the gob through the crosscuts near the back 

end of the gob. The air velocity behind the shields 

was between 1.0x10-3 to 7.0x10-3 m/s (0.2 to 1.4 

fpm), much higher than was seen for the one-entry 

and two-entry ventilation systems. Figure 8 (a) 

shows the flow path lines near the back end of the 

gob. The air velocity was about 1.0x10-2 to 3.0x10-2

m/s (2 to 6 fpm), even higher than that behind the 

shields. Figure 8 (b) shows the flow path lines near 

the center of the gob. These path lines were directed 

towards the bleeder fan, but with very low air velocity, about 1.0x10-6 to 7.0x10-6 m/s (0.0002 to 0.0014 

fpm). 

Figure 7. Flow path lines colored by velocity magnitude (m/s) 

for the three-entry bleeder system: (a) in the whole gob; (b) 

near the shields. 

Figure 8. Flow path lines colored by velocity magnitude (m/s) 

for the three-entry bleeder system: (a) near the back end of the 

gob; (b) away from the back of the gob. 

6 CONDITIONS FOR SPONTANEOUS 

COMBUSTION 

In coalbeds that are reactive to oxidation, critical velocity airflow over reactive coal increases the risk of 

coal heatings. Critical airflow is defined as insufficient airflow to remove the heat due to oxidation, 

but sufficient airflow to maintain the oxidation process (Smith et al., 1994). In general, just behind the 

shields and along the edges of the gob (in the 

bleeder case), the air velocity is high, so that any 

heat generated by self-heating is carried away. 

Deeper in the gob, the air velocity is too low to provide sufficient oxygen to support spontaneous combustion. However, there possibly exists a critical air 

velocity zone in the gob where the airflow and oxygen concentration are suitable for the continual 

growth of a self-heating to an eventual fire. The exact range of the critical velocity and its location depend on several factors such as the coal’s particle/fragment size, physical and chemical properties, 

local pressure, temperature, and water vapor concentration. To demonstrate the effect of the ventilation 

scheme on the critical velocity zone, an estimation 

for the minimum velocity to carry away heat generated from the self-heating of a 150 μm coal particle 

was made using the experimental data from Smith & 

Lazzara (1987). 

The self-heating rate of coal under adiabatic 

condition can be expressed as: 
dT / dt = Aexp(−E / RT) (2) 

where dT/dt is the self-heating rate in K/s, A the preexponential factor in K/s, E the activation energy in 

kcal/mol, and T the temperature in K. Smith & Lazzara (1987) conducted a series of spontaneous combustion studies and determined the activation energy 

and the pre-exponential factor for a number of U.S. 

coals. The heat generation rate from the self-heating 

is: 

Q mC (dT / dt) = p (3) 

where Q is the heat generation rate in W, m the mass 

of the particle in g, Cp the specific heat of coal in 

J/(g⋅K) at constant pressure. In order to prevent the 

occurrence of self-heating, the generated heat needs 

to be carried away by the airflow. The convective 

heat transfer rate can be expressed as: 

Q = ShΔT (4) 

where S is the particle surface area for convective 

heat transfer in m2

, h the heat transfer coefficient in 

W/(m2

⋅K), )T the temperature differential between 

the particle and the airflow in K. The heat transfer 

coefficient h is related to a non-dimensional Nusselt 

number Nu as: 

Nu = hl / λ (5) 

where l is the diameter of the particle in m, λ the 

heat conductivity of air in W/(m⋅K). For the laminar 

flow over a sphere, the Nu number correlates with 

the Re number as (Kays, 1966): 

1/ 2 Nu = 0.292Re (6) 

here Re = ul /υ with u the airflow velocity in m/s, ν

the kinematic viscosity of air in m2

/s. 

Using an example of the Wyoming No. 80 seam 

coal particle with l = 0.15 mm, the maximum particle diameter used by Smith & Lazzara (1987), A = 

6.9x106

 K/s, E = 16.8 kcal/mol, T = 60oC (333 K) 

and )T = 40 K for the No. 80 seam coal particle, coal 

specific heat Cp = 1.17 J/(g⋅K), air heat conductivity 

λ = 2.59x10-4 J/(cm⋅s⋅K), and ν = 15x10-6 m

2

/s, the 

upper bound for the critical airflow velocity calculated using equations (2) to (6) was 1.1x10-4 m/s 

(0.02 fpm). The minimum airflow velocity (lower 

bound for critical velocity) required to provide sufficient oxygen to sustain the self-heating will be determined by incorporating the model of kinetic reaction of coal self-heating into the CFD modeling in 

the next stage of this program. 

Figure 9 shows the velocity contour of 1.1x10-4

m/s for the three ventilation systems. The critical velocity zone for the particle size of 150 μm should 

start from this contour towards the lower velocity 

area. Thus, in this example for the No. 80 seam coal 

using a one-entry and two-entry bleederless ventilation system, the critical velocity zone started immediately behind the shields towards the center of the 

gob. Closer to the headgate and the tailgate, the zone 

started a short distance behind the shields. For the 

three-entry ventilation system, the critical velocity 

zone was pushed away both from the shields and 

from the back end of the gob towards the center. 

Under these simulation conditions, the velocity contour line was close to the boundary between zone 1 

and zone 2 as shown in Figure 2. 

 Figure 9. Velocity contour of 1.1x10-4 m/s for the three ventilation systems: (a) one-entry bleederless system; (b) two-entry 

bleederless system; (c) three-entry bleeder system. 

7 CONCLUSIONS 

CFD simulations were conducted to investigate the 

air pathways in the gob area under different ventilation schemes. The air flow patterns in the gob were 
visualized by flow path lines, velocity contours and 

velocity vectors. For the one-entry and two-entry 

bleederless ventilation systems, the flow patterns in 

the gob were very similar. Only a slightly higher velocity zone occurred near the second return entry 

with the two-entry ventilation system. This demonstrates that sudden changes in airflow resistance can 

potentially constitute zones with critical velocities 

that support spontaneous combustion. 

The air velocity was about 1.0x10-5 to 1.0x10-4

m/s (0.002 to 0.02 fpm) near the shields, and 1.0x10-

4

 to 7.0x10-4 m/s (0.02 to 0.14 fpm) near the second 

intake entry. The velocity near the back end of the 

gob was very low. For the three-entry bleeder system, the air velocity was much higher behind the 

shields than with the bleederless systems, about 

1.0x10-3 to 7.0x10-3 m/s (0.2 to 1.4 fpm). There was 

also high velocity air flow near the back end of the 

gob, about 1.0x10-2 to 3.0x10-2 m/s (2 to 6 fpm). 

A preliminary calculation for a coal particle size 

of 150 μm in diameter and similar reactivity to the 

coal used in this study shows the potential critical 

velocity zones may occur immediately behind the 

shields towards the center of the gob for the oneentry and two-entry ventilation systems. For the 

three-entry bleeder ventilation system, the critical 

velocity zone may occur farther away from both the 

shields and the back end of the gob where a permeability gradient exists. 

A gob will have coal particles of variable sizes. 

For any single particle size, there are multiple solutions for particle temperature T and temperature difference between particle and air. Therefore, it is difficult to develop a unique solution. However, the 

goal is to outline areas with spontaneous combustion 

potential. Future work will be done to further investigate the flow patterns and to simulate the effect of 

methane control measures on the air flow patterns 

inside the gob in mines with appreciable methane 

levels. It is also intended to verify the results of the 

CFD model by using tracer gas and quantity/pressure surveys where possible. 

ACKNOWLEDGMENTS 

The authors wish to acknowledge the technical help 

from Drs. Gabriel S. Esterhuizen and Özgen C. Karacan with the estimation of permeability in the gob. 

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