Fire-generated smoke rollback through crosscut from return to

intake – experimental and CFD study

G.F. Friel, L. Yuan, J.C. Edwards, & R.A. Franks

National Institute for Occupational Safety and Health, Pittsburgh Research Laboratory, Pittsburgh, PA, USA

ABSTRACT: Two mine-fire experiments were conducted in the National Institute for Occupational Safety

and Health’s (NIOSH) Safety Research Coal Mine (SRCM) at the Pittsburgh Research Laboratory (PRL) which

demonstrated that smoke from diesel-fuel fires of 500 kW and 660 kW heat-release rates in a return airway

can develop, without causing a complete air flow reversal, into a roof layer that can migrate upwind forming

a counter flow to the primary airflow in a crosscut. Subsequently, smoke can penetrate into an intake airway

and create a hazardous atmosphere in the intake airway upwind from the fire. Visibility conditions less than

13 m were created by the smoke in the intake airway downwind from the crosscut. Modeling of the event with

a three-dimensional, time-dependent, computational fluid dynamics (CFD) program correctly represented the

smoke movement.

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.

1 INTRODUCTION

Smoke rollback in a mine entry from a fire can pose

hazardous consequences for mine evacuation, miner

rescue, or firefighting. Inhalation of fire-generated

and toxic products-of-combustion (POC) can be injurious or fatal to miners. The low visibility associated

with smoke impedes safe miner escape and rescue.

The greatest hazard of smoke rollback is in fighting

fires. Smoke rollback can occur directly upwind in

the entry with the fire, or if it penetrates a crosscut,

it can be transported downwind in the adjoining airway. If a mine rescue team or firefighting brigade

should approach a fire in the fresh air of an intake

airway before the fire intensity becomes sufficient to

push smoke through a breached crosscut into the intake

entry upwind of the mine rescue team or firefighting

brigade, these miners could then become enshrouded

in low visibility and toxic POC.

Initially the thermal buoyancy forces generated by a

fire in a mine entry will produce an ascending plume

of fire smoke. In the absence of ventilation, the hot

smoke will impinge upon the roof and migrate radially

outward. Any entry ventilation will bend the ascending smoke plume. Initially, the dominant transport of

the smoke at the roof will be in the direction of the

ventilation. The smoke will be transported downwind

from the fire along the roof until the smoke is cool

and is mixed over the entry cross-section. However,

once the fire has evolved to sufficient intensity, the

buoyancy forces associated with the temperature of

the smoke plume will overcome the inertial forces of

the ventilation, and some of the smoke will migrate

upwind along the roof counter to the existing ventilation direction. Another hazard of smoke rollback,

noted by Mitchell (1996), is that the smoke can contain combustible gases in a low-oxygen mixture which

are subject to ignition by the mine fire when diluted

with fresh air.

Previous research (Edwards et al. 2005, Hwang &

Edwards 2005) has focused upon smoke rollback in a

single entry and the air velocity required to prevent the

smoke rollback. For a sufficiently intense fire relative

to the airflow, it is possible to develop complete airflow

reversal in an airway that can be modeled accurately by

a network mine-fire simulator such as MFIRE (Chang

et al. 1990).

Smoke rollback in a mine entry can be complicated

by the connectivity of the entry to other mine entries

by open crosscuts and leakage paths. The smoke layer

which accumulates near the roof can migrate into parallel airways depending upon the air velocity and the

amount of leakage between the airways. Permanent

stoppings are used to separate intake and return airways in a coal mine except for the first two connecting

crosscuts outby the working face. A check curtain may

be used at the second crosscut outby the working face

to provide for vehicular traffic movement. Any breach
of the stoppings due to an open man door or stopping

destruction by a fire or explosion, or significant leakage around a stopping or check curtain, creates the

potential for smoke flow from a fire in a return airway

into an intake airway.

The objective of this work was to demonstrate with

in-mine fire experiments the potential for smoke rollback from a fire in a return airway into an intake airway

and to model this experimental work with CFD simulation. CFD provides the opportunity to expand the range

of prediction of smoke spread for a wide spectrum of

airflows, fire intensities, and mine-section inclinations

and layouts.

2 EXPERIMENTAL CONFIGURATION

Experiments were conducted in parallel entries in the

SRCM shown in Figure 1.

The average entry height and width at the location of

the fire in B-Butt were 1.81 and 2.79 m, respectively.

Diesel fuel was selected as the combustible material

for the fires because of its relatively uniform burning rate from containers. The three containers that

were used were horizontally square steel pans with

sides of 46 cm, 61 cm, and 76 cm long. The heights

of the pan sides were about 13 cm. The fire zone was

protected from thermal radiation by lining the roof

and ribs with a ceramic fiber insulation. The smoke

concentration in B-Butt upwind of the fire was measured by an ionization detector at sensor station, S1,

near the middle of B-Butt with its inlet about 20 cm

from the roof and its position at 15.5 m from the

upwind edge of the fire zone and 4.2 m from the

upwind edge of Room 11 (See Figure 1). A thermocouple was also placed near the ionization detector

and about 8 cm from the roof. A one-meter-long, laser,

smoke-obscuration meter (laser-light meter) was also

positioned near the ionization detector and 30 cm from

the roof. Along the rib of B-Butt at S2 and 12.5 m

upwind from the ionization detector was the first of

two steel mine posts which supported a pair of pathaveraging sensors. The second post was 21.8 m from

the ionization detector on the opposite rib of B-Butt.

An infrared smoke sensor, composed of a transmitter

and a receiver, was attached to the posts about 60 cm

from the roof with the infrared beam directed horizontally and diagonally 10.2 m across B-Butt. Two units

of a gas-velocity sonic sensor also were attached to

these two posts. One unit was attached 20 cm from the

roof on the first post and the other unit was attached

30 cm from the floor on the second post. The length

of the path between these units was 9.7 m. The gasvelocity sensor units were attached on the sides of the

posts closest to each other while the smoke sensor units

were attached on the sides farthest from each other.

Figure 1. View of the mine section in the NIOSH SRCM

(Not drawn to scale).

In the crosscut upwind of the fire, Room 11,

and 4.8 m from B-Butt, a one-meter-long, optical,

smoke-obscuration meter (light meter) was suspended

horizontally 30 cm from the roof at S3. A thermocouple was attached 10 cm from the roof above this light

meter. The upwind edge of the crosscut downwind

from the fire zone, Room 13, was 25 m from the fire

zone. Within Room 13, 5.2 m from B-Butt, and at S4

was another light meter 30 cm from the roof. A thermocouple was also placed 10 cm from the roof above

this light meter.

In A-Butt, 2 m downwind from Room 13, and at S5

was placed the first of two posts to hold gas-velocity

sensor units and optical, smoke sensor units similar to

the configuration in B-Butt. The spacing between the

gas-velocity units was 8.9 m and the spacing between

the smoke sensor units was 9.4 m.

In A-Butt, at 25 m from the downwind edge of

Room 11, and at S6 were placed an ionization smoke

sensor and a laser-light meter near the middle of AButt to detect smoke emanating from either or both

of Rooms 11 and 13. The laser-light meter was 30 cm

from the roof and the inlet to the ionization smoke

sensor was about 20 cm from the roof.

3 EXPERIMENTAL PROCEDURE

The air velocities at the six positions on Figure 1

were measured using a vane anemometer. At positions

1, 3, 5, and 6 the average velocity was measured at

each of five points in the cross section of the airway. At positions 2 and 4 the average velocity was

measured at each of 3 points in the cross section of

the doorways next to A-Butt. These velocity averages

were determined by measuring the velocity over at

least one cycle length of atmospheric changes to mine
air velocities which usually lasted about 30 s. Crosssectional areas were calculated from measurements

of height and width using an electronic, ultrasonic

distance-measuring tool.

The fire pans were placed level, next to each other,

and on the floor near the middle of B-Butt in the

fire zone. Attached to a laboratory stand next to the

pans was a thermocouple positioned horizontally and

about 10 cm above the middle of the downwind pan

to indicate the end of combustion. The diesel fuel

was poured into a measuring container before being

poured into each pan. Before ignition of the diesel

fuel using a propane torch, about 250 mL of denatured,

95% ethanol was poured into each pan. The cold diesel

fuel was difficult to ignite without the ethanol. After

ignition, exiting of personnel from B-Butt was through

Room 11 into A-Butt and then along A-Butt to a point

upwind of Room 13. A digital camcorder was attached

to a tripod about 1.5 m from the floor of A-Butt and

opposite Room 13 to record the smoke emanating

from Room 13 and also Room 11 by pivoting the

camcorder 90◦.

4 MEASUREMENT OF ROOM RESISTANCES

The static pressure drops between A-Butt and B-Butt

through Room 11 and Room 13 were measured using

a manometer placed level on a pedestal within the

rooms with tubes extended into A and B-Butts. The

airflow rates through the rooms were calculated from

the average of three velocity measurements at different

heights within doorways in each room using the vane

anemometer and the dimensions of the doorways. The

static pressures were measured using an altimeter. The

temperatures and relative humidities were measured

using a digital psychrometer. The resistances were calculated by dividing the air static pressure drop by the

square of the volumetric airflow rate. The results of

those measurements at mine static air pressures and

temperatures are listed below in Table 1. The resistances can be converted to a standard set of conditions

for use in simulators such as MFIRE.

5 EXPERIMENTAL RESULTS

Table 2 shows the volumetric airflow rates in the mine

network prior to each experiment. Also shown are

the air velocity measurements at position 1 that were

repeated after each experiment because of changes

to the airflow rate into A-Butt during each experiment. These velocity changes were made after a

semi-equilibrium position of smoke had occurred in

Room 13 and to facilitate penetration of smoke into

A-Butt before the diesel fuel was all consumed. These

ventilation decreases were accomplished by opening a

Table 1. Measured resistances through crosscuts.

Static Relative

Room Resistance, Pressure, Temperature, Humidity,

Number Ns2/m8 cm Hg ◦C %

11 0.1709 73.56 2.39 89.8

13 0.1407 73.53 1.78 92.6

Table 2. Airflow rates at locations in Figure 1.

Experiment 1 Experiment 2

Location on (500 kW Fire) (660 kW Fire)

Figure 1 Airflow Rate, m3/s Airflow Rate, m3/s

1 4.2 4.5

2 1.8 1.6

3 3.2 3.5

4 1.7 1.6

5 2.2 2.4

6 1.7 1.8

1, During 3.5 3.7

Experiments

door in a crosscut fromA-Butt to the return and upwind

of the entrance to Room 13. The airflow rates for the

two experiments were similar, although the inaccuracies in the volumetric balances could have been caused

by the vortices produced by the edges of nearby rooms

and leakage around temporary brattices.

In the first experiment, 23.7 L of diesel fuel was

burned in the 46 cm and 61 cm square pans. Based

upon the fire duration and the heat of combustion

for diesel fuel, the fire intensity was estimated to be

500 kW. A large quantity of smoke passed through

Room 11 upwind of the fire and from there downwind

through A-Butt and back through B-Butt and the fire

zone. This fire yielded no penetration of smoke from

B-Butt through Room 13 into A-Butt until a reduction in airflow rate through A-Butt was made. Figure

2 shows the light meter response in millivolts (mV),

which decreased with an increase in the concentration of smoke at the light meter, and the temperature

at the thermocouple within Room 13 during the first

experiment. The POC’s penetrated to the doorway next

to A-Butt. All sensors were sampled every two seconds. Figure 3 shows the light meter and thermocouple

response within Room 11 during the first experiment.

Figure 4 shows the temperature and indicates smoke

intensity at the point ionization smoke sensor in BButt (S1), and indicates the smoke intensity at the

point ionization smoke sensor in A-Butt (S6) located

25 m downwind from Room 11 during the first experiment. About a minute was required for the smoke to

backup 12 m.
Figure 2. Temperature and light meter response in Room 13

for experiment no. 1.

Figure 3. Shows the light meter and thermocouple response

within Room 11 during the first experiment.

Figure 4. Sensor responses in A-Butt and B-Butt for experiment no. 1.

In the second experiment, 33.1 L of diesel fuel were

burned in the 46 cm and 76 cm square pans. The estimated fire intensity was 660 kW. This fire yielded a

brief penetration of smoke from B-Butt through Room

13 into A-Butt. A large quantity of smoke also passed

through Room 11 upwind of the fire and from there

downwind through A-Butt. The quantity of smoke

from Room 13 was much less than the quantity of

smoke emanating from Room 11 into A-Butt. Figure

5 shows the light-meter response to the smoke and the

temperature of the smoke at the thermocouple within

Room 13 during the second experiment. These POC’s

penetrated from Room 13 into A-Butt continuously

during the fire after the flow rate through A-Butt was

reduced. The greater smoke rollback into Room 11

Figure 5. Temperature and light meter response in Room 13

for experiment no. 2.

Figure 6. Temperature and light meter response in Room 11

for experiment no. 2.

was associated with the maximum roof temperature of

78◦C in Room 11 compared with the maximum roof

layer temperature of 42◦C in Room 13. For the first

experiment, the maximum roof-layer temperatures in

Rooms 11 and 13 were 59◦C and 29◦C, respectively.

Figure 6 shows the light-meter response to the

smoke and thermocouple measurement of the smoke

temperature within Room 11 during the second experiment.

Figure 7 shows the temperature and indicates smoke

intensity at the point, ionization smoke sensor in BButt and indicates the smoke intensity at the point,

ionization smoke sensor in A-Butt located 25 m downwind from Room 11 during the second experiment.

The gas temperature may have caused the temporary failure of the thermocouple’s electronic interface

which also was supported near the roof.

6 VISIBILITY

As the light-meter responses in Figures 2, 3, 5, and 6

show, smoke obscuration is total near the roof within

the crosscuts. Smoke movement through Room 11

from the return, B-Butt, into the intake, A-Butt, created a reduced visibility downwind from Room 11 in

A-Butt. The visibility was determined from Jin’s relationship (Jin 1977), which, in turn, was based upon
Figure 7. Sensor response in A-Butt and B-Butt for experiment no. 2.

Figure 8. Smoke optical density and visibility in A-Butt

25 m downwind from Room 11 during experiment no. 1.

the smoke optical density determined from the reflection of light from a reflecting sign – as might occur

when a miner’s cap lamp is focused upon a reflecting mine-escape marker. Figure 8 shows the optical

density (OD) at the laser light monitor located 25 m

downwind from Room 11 and the associated visibility

for the 500 kW fire of the first experiment. The optical density is defined from the light transmission at

the laser light monitor by equation 1:

T is the light transmission between the transmitter and

the receiver in smoke-laden air, T0 is the light transmission in clear air, and d is the distance between the

transmitter and the receiver.

The minimum visibility was 9.4 m. Also shown are

the 4 m and 13 m visibility requirements reported by

Jin (1981) for minimum visibility of someone familiar

and not familiar with the surroundings, respectively.

For the 660 kW fire of the second experiment, the minimum visibility was 5.5 m. Someone unfamiliar with

the SRCM would not have the minimum visibility for

escape from the fire zone through B and A-Butts. A

scenario could exist of a firefighting brigade entering B-Butt through A-Butt upwind of the fire zone to

extinguish the fire. Initially the fire intensity might not

be adequate to force smoke rollback through Room 11

into A-Butt. As the fire intensity increases, the smoke

Figure 9. Smoke density distribution near the roof at 600 s

for the 500 kW fire.

rollback could not be confined to B-Butt but would

penetrate through Room 11 and soon be upwind of the

firefighting brigade.

7 COMPUTATIONAL STUDY

Smoke reversal through crosscuts from a return airway into an intake airway was simulated using the

Fire Dynamics Simulator (FDS), a CFD program

developed by NIST (McGrattan et al. 2002). FDS

is a three-dimensional, large-eddy simulation model

developed for studying the transport of smoke and hot

gases during a fire in an enclosure. It is a widely used

large-eddy simulation model in the field of fire science

and has demonstrated good agreement with experimental data in numerous validation studies. The model

uses finite difference techniques to estimate solutions

of the Navier-Stokes equations for fluid flow with a

mixture-fraction combustion model.

The physical model for the numerical simulation

consists of the intake airway, the return airway, two

crosscuts, and an airway connecting the intake and the

return airways – as shown in Figure 1. In Figure 9

below the connecting airway was not drawn.

Because of the memory limitations and speed of the

computer running FDS, Room 11 was made perpendicular to both intake and return airways to increase

the mesh size of the simulation.The two crosscuts each

have two doors and a plenum between the doors. All

dimensions for the airways and crosscuts were entered

from a mine-section survey. Each diesel fire was modeled using its average heat-release rate estimated in the

experiment. The airflow velocity measured upwind of

Room 13 in A-Butt was entered as the initial flow condition. In both experiments, the airflow velocity was

reduced after about 15 minutes from the ignition of the

diesel fuel. In the simulation, the smoke movement

became quite stable after 10 minutes, so the airflow
Figure 10. Smoke density distribution near the roof at 670 s

for the 500 kW fire.

velocity change was made after 10 minutes. For the

500 kW fire, the airflow velocity was 0.96 m/s at the

beginning and was changed to 0.79 m/s after 10 minutes. For the 660 kW fire, the airflow velocity was

0.99 m/s at the beginning and was changed to 0.80 m/s

after 10 minutes.

A cold airflow simulation was conducted prior to

the fuel ignition. The airflow rates in A-Butt and the

two crosscuts obtained from the simulation agreed well

with the rates calculated from the pre-experiment ventilation survey. However, the measured airflow rate in

B-Butt was smaller than the simulated rate because of

leakage around brattices along one of the ribs in BButt into other crosscuts and the rest of the SRCM.

Simulations were then conducted for the experimental

fires. Figure 9 shows the smoke (soot) density distribution near the roof at 600 s for the 500 kW fire,

immediately before the change of the intake air velocity. Smoke passed through the open door of Room 11

adjacent to B-Butt and filled the Room-11 plenum,

but could not pass through the open door adjacent to

A-Butt. Smoke also passed through the open door of

Room 13 adjacent to B-Butt and only partially filled

the Room-13 plenum. The smoke inside Room 13 was

also less dense than the smoke in Room 11.

Figure 10 shows the smoke density distribution near

the roof at 670 s for the 500 kW fire 70 s after the

reduction of intake air velocity.

Smoke passed through the open door of Room 11

adjacent to A-Butt and leaked into A-Butt but could

not roll upwind in A-Butt. Smoke filled the entire

plenum in Room 13 but could not pass through the

open door adjacent to A-Butt. As time increased, more

smoke leaked into A-Butt from Room 11 and slightly

rolled upwind next to the rib closest to Room 11. A

small quantity of smoke passed into A-Butt through

the open door of Room 13 adjacent to A-Butt. These

results are consistent with the observations in the first

experiment.

Figure 11. Smoke density distribution near the roof at 600 s

for the 660 kW fire.

Figure 12. Smoke density distribution near the roof at 660 s

for the 660 kW fire.

Figure 11 shows the smoke density distribution near

the roof at 600 s for the 660 kW fire and immediately

before the change of the inlet velocity.

Some smoke already passed intoA-Butt from Room

11 through the open door adjacent to A-Butt but was

quickly diluted and could not roll upwind in A-Butt.

Smoke passed through the open door of Room 13

adjacent to B-Butt and filled most of the plenum.

Figure 12 shows the smoke density distribution near

the roof at 660 s for the 660 kW fire 60 s after the

ventilation reduction.

Dense smoke leaked into A-Butt from Room 11

and rolled upwind in A-Butt. Smoke filled the entire

plenum of Room 13 and less dense smoke leaked

into A-Butt. As time increased, more smoke leaked

into A-Butt from Room 11 and rolled upwind against

the intake airflow, while no more smoke leaked into

A-Butt from Room 13. These results also are qualitatively consistent with the observations in the second

experiment.

It can be seen from the FDS simulation that the

smoke leakage from Room 11 into the A-Butt only

occurred near the roof. Close to the floor in Room
11 the ambient air still moved in against the smoke

leakage. This is one advantage of the FDS’s threedimensional simulation. In one-dimensional modeling, one airway is either occupied by clear air or an

air-smoke mixture but cannot be both. In a real situation partial smoke leakage into an airway could occur

and poses a threat to the safety of underground miners

and mine rescue personnel.

8 CONCLUSIONS

The following conclusions were determined from the

experimental results and the CFD simulations.

• The experimental mine fires in a return airway

produced sufficient buoyancy to establish a smokeladen roof layer that flowed through connecting

crosscuts counter to the direction of fresh air from

the intake entry.

• The density of the smoke that leaked into the intake

was shown to yield insufficient visibility downwind

from the last connecting crosscut in the intake entry

for someone unfamiliar with the mine to find their

way out easily.

• The CFD simulations showed good agreement with

the experimental observations of smoke movement.

Smoke rollback along the roof from a fire countercurrent to the cooler airflow near the floor can be

a mechanism for smoke to move from a mine return

into a mine intake in low airflow sections. This movement of smoke can impede firefighting efforts. The

realization of this possibility would not be predicted

from a mine-network ventilation program which is

based only upon unidirectional flow. CFD modeling

is a viable method for analyzing potential visibility

hazards associated with smoke from an in-mine fire.

REFERENCES

Chang, X., Laage, L.W. & Greuer, R.E. 1990. A user’s manual for MFIRE: a computer simulation program for mine

ventilation and fire modeling. U.S. Bureau of Mines IC

9245, 171 pp.

Edwards, J.C., Franks, R.A., Friel, G.F. & Yuan, L.L. 2005.

Experimental and modeling investigation of the effect of

ventilation on smoke rollback in a mine entry. SMEAnnual

Meeting, Feb 28–Mar 2, 2005, Salt Lake City, UT, paper

05-14, 6 pp.

Hwang, C.C. & Edwards, J.C. 2005.The critical ventilation in

tunnel fires – a computer simulation. Fire Safety Journal

40: 213–240.

Jin, T. 1977. Visibility through fire smoke. Journal of Fire

and Flammability 9: 135–155.

Jin, T. 1981. Studies of emotional instability in smoke from

fires. Journal of Fire and Flammability 12: 131–142.

McGrattan, K.B., Forney, G.P., Prasad, K., Floyd, J.E. &

Hostikka, S. 2002. Fire Dynamics Simulator (Version 3)

– User’s Guide. U.S. Department of Commerce, National

Institute of Standards and Technology.

Mitchell, D.W. 1996. Mine fires prevention, detection, fighting: 19-202. Chicago, IL: Intertec Publishing Inc.