SOCIETY FOR MINING, METALLURGY, AND EXPLORATION Vol. 320 • 2006 TRANSACTIONS 149

Smoke reversal interaction with diagonal 

airway — its elusive character

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

Research physicist, chemical engineer, associate service fellow and electronics engineer, respectively, National 

Institute for Occupational Safety and Health, Pittsburgh Research Laboratory, Pittsburgh, Pennsylvania

Abstract

The reversal of smoke products-of-combustion (POC) from a mine fire was determined in a mine section with the airway connectivity of an electrical Wheatstone bridge. Four diesel-fuel fire experiments 

with fire heat-release rates between 504 and 771 kW were conducted in the Safety Research Coal Mine 

(SRCM) located at the National Institute for Occupational Safety and Health’s (NIOSH) Pittsburgh 

Research Laboratory (PRL). Smoke reversal propagated upwind from the fire with significant leakage 

into the upwind diagonal airway and without causing a complete reversal of airflow in the diagonal 

airway. A control measure consisting of a brattice suspended half entry height from the entry roof was 

determined to abate the smoke rollback. Computational fluid dynamics (CFD) analysis of the smoke 

movement agreed with the measurements.1

Introduction

Diagonal airways, or connecting airways between two parallel 

airways, can be subjected to airflow reversal, depending on the 

pressure imbalances between the connected airways. As shown 

by Wala and Stoltz (1999), connections of diagonal airways 

can result in catastrophic events, such as methane explosions, 

if consideration is not given to the ventilation requirements. 

Diagonal airways are part of all mine networks and can serve 

useful purposes such as moving equipment from one longwall panel to another longwall panel. They also facilitate the 

development of mining sections by providing ventilation and 

access to transportation. Unlike ordinary airways to which 

they connect, they cannot be combined into series and parallel 

airways for mine ventilation planning. 

The classification of airways into ordinary and diagonal 

airways is important for mine ventilation simulator applications. 

Within the local plan of a mine section, the diagonal element 

and the airways to which it connects form the equivalent electrical circuit of the Wheatstone bridge in which the connecting 

arm is the diagonal element of the ventilation circuit. From the 

viewpoint of graph theory, this circuit is a Hamiltonian, but not 

an Eulerian connected graph, because it is possible to traverse a 

closed path that includes every node only once but not possible 

to traverse a closed path that includes every airway only once. 

These considerations are important when determining a mesh 

representation for ventilation network computations. 

A ventilation change induced by the movement of equipment, by the opening and closing of doors or by changes in 

temporary stoppings can result in airflow reversals in a diagonal 

airway. The diagonal airway can be unstable with respect to 

airflow direction because its resistance can be less than that 

of the airways it connects. The effect of the release of thermal 

energy in one of the main airways to which the diagonal airway 

is connected will have a more complex effect. 

The thermal expansion of the POC from a fire will increase 

the local airway resistance, which could alter the ventilation 

pattern in the diagonal. Natural ventilation buoyancy forces 

will cause the hot POC to rise toward the mine roof and be 

transported with the imposed ventilation. If sufficient thermal 

energy is released from the fire, the momentum of the buoyant plume will be transferred at the roof into an expansion 

of a smoke roof layer upwind countercurrent to the imposed 

ventilation in the lower cross section of the entry. 

Consideration in this study is given to a fire that occurs 

downwind from the diagonal. This smoke-laden roof layer might 

also migrate into the diagonal entry, and a similar countercurrent flow could result. The floor air layer can be accelerated 

towards the fire as the fire becomes a local low-pressure zone 

responsible for entraining the fresh air. The establishment of 

two countercurrent layers poses a challenge to predict the transport of hazardous fire POC. The problem is three dimensional, 

which is beyond the scope of the traditional, unidirectional 

flow mine-network program. 

The objective of this study was to demonstrate experimentally and model with CFD the consequence of a fire in 

Paper number TP-06-021. Original manuscript submitted online July 2006. Revised manuscript accepted for publication 

October 2006. Discussion of this peer-reviewed and approved paper is invited and must be submitted to SME Publications 

Dept. prior to Sept. 30, 2007. Copyright 2006, Society for Mining, Metallurgy, and Exploration, Inc. 

1 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.
2006 TRANSACTIONS • Vol. 320 SOCIETY FOR MINING, METALLURGY, AND EXPLORATION 150

an intake airway downwind from a diagonal airway from the 

point-of-view of smoke reversal along the roof upwind in the 

intake and its movement into the diagonal airway. This study 

was based on mine fire experiments and analysis with a CFD 

Fire Dynamics Simulator (FDS) (McGrattan et al., 2004). 

FDS is a hydrodynamic simulator with chemical reactions 

represented by a mixture-fraction model. The effectiveness of 

a smoke-control measure that consists of a brattice suspended 

half entry height from the entry roof upwind from the fire was 

also determined experimentally and modeled with CFD. 

Previous research (Eisner and Smith, 1954) has shown the 

utility of a brattice covering the lower two-thirds of the entry 

height as a smoke-control measure for fire fighters advancing 

towards a fire. This method was reported (McPherson, 1993, 

pp. 431 and 825) to be similar to that used for control of 

methane roof layers. In the case of a brattice attached to the 

roof and partially blocking the entry cross section there will 

be two effects. The first effect is to dissipate the momentum 

of the reverse smoke layer. As the smoke is deflected towards 

the floor the hot products-of-combustion (POC), momentum 

and energy will be through recirculation transferred to the 

cooler counter airflow near the floor. The second effect is the 

increased airflow in the open area beneath the brattice. The 

momentum of this airflow will push the deflected roof layer 

into the primary airflow towards the fire. 

Experimental conditions

Four diesel-fuel-fire experiments were conducted in NIOSH’s 

SRCM. There was no significant grade in the section of the 

mine in which the experiments were conducted. The mine 

layout plan is shown in Fig. 1a. Experiments were conducted 

for the air-course configurations shown in Figs. 1b and 1c. 

These configurations were created from the mine plan in Fig. 

1a with brattices used to isolate airways. The entry heights 

were approximately 2 m (6.6 ft). B-Butt had an average width 

of 3 m (9.8 ft), whereas 11-Room and F-Butt widths were 

approximately 4 and 4.5 m (13 and 15 ft), respectively. The 

mine exhaust fan established airflow in the mine from B-Butt 

to F-Butt and G-Butt through connecting rooms under normal 

ventilation conditions. Figure 1b depicts the Wheatstone bridge 

ventilation circuit selected for Experiments No. 1 and 2, and 

Fig. 1c depicts the ventilation circuit for Experiments No. 3 

and 4. 11-Room is a diagonal airway in both mine plans. It 

is connected to the ordinary airways B-Butt and F-Butt. The 

fire source was located approximately 10 m (33 ft) downwind 

from the center of the diagonal entry, 11-Room, for each 

experiment.

Regulator resistance. The selection of the fire site downwind 

from 11-Room required the initial reversal of the airflow from 

F-Butt to B-Butt in 11-Room for the purpose of determining the 

influence of the fire on the airflow in 11-Room. To adjust the 

flow so as to produce an initial low airflow from F-Butt to BButt in the diagonal, 11-Room, two-variable airflow resistances 

were introduced into the network. One variable resistance was 

a regulator with a sliding door constructed in F-Butt between 

11-Room and 13-Room. The other variable resistance was a 

brattice placed across B-Butt between 10-Room and 11-Room 

for Experiments No. 1 and 2 and across B-Butt downwind from 

7-Room for Experiments No. 3 and 4. For each experiment, 

the open width for the 1.60-m- (5.25-ft-) high regulator in FButt is listed in Table 1. The total cross-sectional dimensions 

in F-Butt where the regulator was constructed were 4.88 m 

(16.0 ft) wide and 1.88 m (6.17 ft) high.

Aerodynamic resistances for the regulator were estimated 

from the orifice equation for isothermal flow (Bird et al., 1960). 

There was significant leakage around the brattice used to regulate airflow in B-Butt along the ribs and roof. The estimated 

resistance for the brattice in B-Butt was based upon an airflow 

Figure 1a — Plan view of the experimental mine section 

in the SRCM.

Figure 1b — Schematic diagram of the mine section for 

Experiments No. 1 and 2.

Figure 1c — Schematic diagram of the mine section for 

Experiments No. 3 and 4.
SOCIETY FOR MINING, METALLURGY, AND EXPLORATION Vol. 320 • 2006 TRANSACTIONS 151

balance within the mine section. The estimated resistances for 

the regulator and brattices used in the four experiments are 

listed in Table 1. The resistances for the network airways that 

constitute the Wheatstone bridge are less than 0.1 N s2/m8. 

The result is that the ratios of the resistances in the B-Butt 

segment upwind from the diagonal to the segment downwind 

from the diagonal are greater than 2.5, and the ratios in F- and 

G-Butts of the upwind to downwind segments are less than 

0.2. This condition, in which the airway resistance ratio in 

B-Butt exceeds the airway resistance ratio in F- and G-Butts, 

results in a prefire airflow through the diagonal from F-Butt 

to B-Butt for each case based on a ventilation circuit analysis. 

Only if the fire provided sufficient resistance to the branch it is 

contained in would the airflow reverse in the diagonal. What 

occurs is more complex and must be considered from a threedimensional perspective.

Ventilation. Table 2 lists the prefire airflows for each experiment 

at the specified locations in Figs. 1b and 1c. Airflow measurements were made with a vane anemometer for locations other 

than the diagonal airway. Except for the regulator, ventilation 

station No. 2, where a three-point measurement was made, a 

five-point anemometer measurement was made at each station. The five-point measurement was based on the average of 

the measured airflow at each of the four corners of the cross 

section and at the center of the cross section. The three-point 

measurement was based on the average of the measurements 

at the top, center and bottom of the open section of the regulator. In the diagonal airway at ventilation station No. 5 a smoke 

tube was used to estimate the airflow for experiment Nos. 3 

and 4. This was based on a single time measurement near the 

airway center over a 3-m (10-ft) distance. For Experiments 

No. 1 and 2 the B-Butt airflow measurements were made 

upwind from the split at 10-Room, ventilation station No. 3, 

and for Experiments No. 3 and 4 the airflow measurement was 

made both upwind and downwind from the split at 7-Room at 

ventilation stations 3 and 4.

As expected, the volumetric airflow through the F-Butt 

regulator, ventilation Station No. 2, as listed 

in Table 2, is highest for Experiment No. 1, 

with the comparatively lower resistance, as 

shown in Table 1. Also shown in Table 2 are 

the predicted airflows based on an application of MFIRE (Chang et al., 1990) with 

the inlet airflow specified as the measured 

airflow at ventilation Station No. 3. The 

significant difference between the predicted 

and measured airflow values occurs at the 

regulator and in the diagonal airway. The 

discrepancy at the regulator is possibly due 

to leakage into F-Butt around bratticed-off 

cross cuts, which are connected to a parallel intake airway. The discrepancy in the 

diagonal is due to the inaccuracy of low 

airflow smoke-tube measurement.

For Experiments No. 3 and 4, the volumetric airflow at ventilation Station 5 in the 

diagonal based on the smoke tube measurements compared favorably with the difference between the anemometer measured 

airflows at ventilation Stations 1 and 4 in 

B-Butt. The air quantity measured with the 

smoke tube in the diagonal was 8.4% and 

2.3% less than the difference between the 

anemometer measured airflows at ventilation Stations 1 and 4 in B-Butt for Experiments No. 3 and 4, 

respectively. Insufficient information was available to compare 

the diagonal airflow with F-Butt airflow. 

Fire source. In the experiments the fire source on the floor in 

B-Butt was diesel fuel contained in two fire pans for Experiments No. 1 through 3 and a single pan for Experiment No. 4. 

The fire pans were approximately 8 m (26 ft) downwind from 

11-Room rib. A 0.61-m- (2.0-ft-) square pan was positioned 

downwind from the 0.46-m- (1.5-ft-) square pan for Experiments No. 1 through 3 and a single 0.76-m- (2.5-ft-) square 

pan for Experiment No. 4. Table No. 3 lists the quantity of 

diesel fuel used for each experiment. The fuel quantities for the 

three experiments with two pans were proportioned according 

to pan area. The heat-release rate for each fire was calculated 

from the initial volume of diesel fuel in the pans, diesel fuel 

heat of combustion of 42.3 MJ/kg, fuel density of 876 kg/m3

and fire duration based on thermocouple measurements in the 

fire zone. The calculated heat release rate is listed in Table 3

for each experiment. 

Sensors. Figures 1b and 1c indicate the location of the sensor 

stations denoted by S#. The sensors at each sensor station are 

listed in Table 4. Sensor station S1 was located in F-Butt 5 m 

(16 ft) from the rib of 11-Room for Experiments No. 1 and 2. 

For Experiment No. 3, S1 refers to the location 33 m (108 ft) 

1 1.27 0.4 0.1 

2 0.42 3.6 0.1 

3 0.46 3.0 0.1 

4 0.46 3.0 0.1 

 F-Butt 

Experiment regulator width, Regulator, Brattice, 

No. m N s2/m8 N s2/m8

Table 1 — Estimated airflow resistance at F-Butt regulator and B-Butt brattice for each experiment.

1 1 0.68 3.26 3.11 4.6

1 2 0.91 1.84 1.14 38.0

1 3 0.96 4.26 – –

2 1 0.60 2.84 2.58 9.1

2 2 0.78 0.52 0.32 38.5

2 3 0.65 2.89 – –

3 1 0.76 3.58 3.83 7.1

3 2 0.79 0.58 0.48 17.2

3 3 0.95 4.31 – –

3 4 0.49 2.16 2.19 1.4

3 5 0.15 1.30 1.64 26.1

4 1 0.70 3.29 3.54 7.6

4 2 0.87 0.64 0.44 31.2

4 3 0.88 3.99 – –

4 4 0.49 1.98 2.02 2.0

4 5 0.15 1.28 1.52 18.7

 Air Measured Predicted 

Experiment Location in velocity, air quantity, air quantity, Percent

No. Figs. 1b and 1c m/s m3/s m3/s error

Table 2 — Measured and predicted airflows for each experiment
2006 TRANSACTIONS • Vol. 320 SOCIETY FOR MINING, METALLURGY, AND EXPLORATION 152

upwind from the fire in B-Butt. S2 included several sensors 

in 11-Room positioned from 16 to 28 m (52 to 92 ft) from the 

F-Butt rib. The location of S3 was in F-Butt approximately 2 

m (6.6 ft) upwind from the regulator. Sensor Station S4 was 

located 13 m (43 ft) upwind from the fire. The light monitor 

(LM) is an incandescent light source and photocell separated 

by 1 m (3.3 ft) to measure the smoke obscuration. It was used 

at S3 for Experiments No. 1 and 2. Similarly, the laser light 

monitor (LLM) is a laser red light source and photocell separated by 1 m (3.3 ft). At S2 the light monitor was a laser light 

monitor, LLM1, and at S3 for Experiments No. 3 and 4 the 

light monitor was a laser light monitor, LLM2. Becon (Anglo American Research Laboratories) is an ionization smoke 

sensor. (Reference to a specific product is for informational 

purposes only and does not imply endorsement by NIOSH.) 

Beam (DS240, Detection Systems Inc.) is an optical smoke 

sensor with a transmitter and receiver separated approximately 

9 m (30 ft). FloSonic (ComLink Group Inc.) is an ultrasonic 

airflow measurement sensor with a transmitter and receiver 

separated along a path similar to the Beam smoke sensor path. 

The carbon monoxide (CO) sensors were (Conspec Controls 

Inc.) diffusion mode CO sensors. In each experiment, CO17 

was near the roof, and, except for Experiment No. 2, CO15 

was near the floor. For Experiment No. 2, CO15 was at midheight. The sequence of sensors at S2 in the direction from 

F-Butt to B-Butt consisted of CO15 and CO17, approximately 

16 m (52 ft) from the F-Butt rib, followed within a 2 m (6.6 

ft) distance by LLM1 and followed at another 10 m (33 ft) 

distance by CO4. A thermocouple, TF, was positioned in the 

fire pan to determine the fire duration. At S4, a thermocouple, 

T4, was positioned near the roof. The data were collected by 

an atmospheric mine monitoring system (Conspec Controls, 

Inc.) with a two-second data-retrieval frequency.

A triangular sail was constructed from a thin sheet of polyethylene and suspended by a nearly frictionless sharp-edged 

support in the lower half of the diagonal airway at S2. The cross 

section of the sail was normal to the longitudinal direction of 

the airway. Movement of the sail in the airflow was recorded 

by electrical contact with wires attached to the two sides of 

the sail. For the experiments conducted, the indicated flow 

was from F-Butt to B-Butt in the diagonal airway prior to each 

experiment. Within the duration of the experiments the flow in 

the lower half of the airway was maintained 

in opposition to the roof smoke layer that 

flowed from B-Butt to F-Butt.

Experimental results 

Experiments No. 1 and 2. During Experiments No. 1 and 2, smoke reversal from the 

fire leaked around the brattice positioned 

across B-Butt and into 10-Room, which 

was approximately 15 m (50 ft) upwind 

from 11-Room. This smoke was visually 

observed to migrate along F-Butt and into 

11-Room. Figure 2 shows the response of 

the Becon sensor for Experiment No. 1, 

and Fig. 3 shows the response of CO7 for 

Experiment No. 2 in F-Butt. The measured 

values of these two sensors, which were at 

approximately the same location in F-Butt, 

indicate the early arrival of the smoke from 

10-Room into F-Butt. Visual observations 

during the experiments indicated well-mixed 

smoke over the 10-Room cross section. At 

the same time, a thick roof layer of smoke 

was observed visually to enter 11-Room from B-Butt as a 

reverse smoke roof layer. In the course of Experiments No. 1 

and 2, the laser light monitor LLM1 in 11-Room and either the 

Becon or CO7 in F-Butt responded before CO15 and CO17 in 

11-Room responded. This implies an ambiguity with regard 

to whether the CO17 first sensor response is to the smokeladen roof layer that advances from B-Butt or from the smoke 

that advances from 10-Room. The distance between the light 

monitor and the vertically spaced CO sensors in 11-Room 

was approximately 1 m (3.3 ft) for Experiment No. 1 and 2 m 

(6.6 ft) for Experiments No. 2, 3 and 4. The distance between 

the CO sensors in 11-Room and the Becon and CO sensor in 

F-Butt was approximately 21 m (69 ft) for Experiments No. 

1 and 2.

A comparison of Figs. 2 and 3 indicate a higher concentration 

at the CO sensors in 11-Room during Experiment No. 2 than 

during Experiment No. 1. A comparison of the light monitor 

normalized responses in Experiment No. 1 indicates a denser 

concentration of light-obscuring smoke occurred in 11-Room 

than reached the regulator in F-Butt. Conversely, for Experiment No. 2, more light-obscuring smoke reached the regulator 

than occurred in 11-Room. This is consistent with the lower 

airflow for Experiment No. 2, which permitted more extensive 

smoke reversal past the brattice in B-Butt, and its transport 

with the split air into 10-Room and to the regulator. The ratio 

of the airflow upwind from B-Butt at ventilation Station No. 

3 of Experiment No. 1 to the airflow at the same location of 

Experiment No. 2 is 1.47. The higher volumetric airflow of 

Experiment No. 1 not only diluted the smoke in the reversed 

roof smoke layer, but also enhanced the cooling and reduced 

the thermally induced momentum of the reversed roof layer.

It was observed in Experiment No. 2 that the reversed smoke 

layer in B-Butt, which had passed around the brattice downwind 

from 10-Room, backed up to a doorframe in B-Butt 29 m (95 

ft) upwind from the fire. The frame had a 250-mm- (10-in.-) 

thick roof beam across the mine entry. The roof beam stopped 

the movement of the smoke layer.

Experiments No. 3 and 4. Figures 4 and 5 show the POC 

sensors’ responses for Experiments No. 3 and 4. In these experiments a longer path, approximately 50 m (165 ft) from 7-Room 

to 11-Room, was established for the air split to minimize the 

1 15.1 8.5 23.6 522

2 15.1 8.5 23.6 504

3 18.5 10.4 28.9 559

4 – – 33.7 771

 Diesel fuel Diesel fuel

 in 0.61-m- in 0.46-m- Diesel fuel Heat

Experiment square pan, square pan, total, release rate,

No. L L L kW

Table 3 — Fire conditions for each experiment. 

1 Becon Sail, LLM1, CO15, CO17 LM T4

2 CO7 Sail, LLM1, CO4, CO15, CO17 LM T4

3 FloSonic, Beam Sail, LLM1, CO4, CO15, CO17 LLM2 T4

4 – Sail, LLM1, CO4, CO15, CO17 LLM2 T4

Experiment 

No. S1 S2 S3 S4

Table 4 — Sensors at station S# for each experiment.
SOCIETY FOR MINING, METALLURGY, AND EXPLORATION Vol. 320 • 2006 TRANSACTIONS 153

entry of smoke from F-Butt into the diagonal from its intersection with F-Butt. Figures 4 and 5 show the reverse smoke layer 

in the diagonal resulted in a significantly greater response of 

the light monitor to smoke obscuration in the diagonal airway 

for Experiments No. 3 and 4 than occurred for Experiments 

No. 1 and 2, as shown in Figs. 2 and 3. The optical response 

is both more rapid and indicates total obscuration instead of 

the partial obscuration that occurred in Experiments No. 1 and 

2. This difference in smoke intensity is the result of the more 

intense heat production for Experiments No. 3 and 4 than for 

Experiments No. 1 and 2, as shown in Table 3.

Another difference in sensor response between Experiments 

No. 1 and 2 and Experiments No. 3 and 4 is the response of CO 

sensors CO17 and CO15. For Experiments No. 1 and 2, CO15 

and CO17 in the diagonal airway responded simultaneously. 

The CO sensor responses would indicate the smoke laden air 

from 10-Room, which was observed to be well mixed, had 

reached the 11-Room CO sensors at approximately the same 

time as the reversed smoke layer in 11-Room. This is to be 

contrasted with Experiments No. 3 and 4, which indicate a 

lag in the time of arrival at the roof and floor CO sensors. For 

these latter two experiments, the inability of the smoke roof 

layer to travel to the upwind split at 7-Room precluded smoke 

entering 11-Room from F-Butt.

In Experiment No. 3, CO sensor No. 17 indicated a first 

response at 11:35 am. This time corresponds to the initiation of 

the decrease in the fire intensity as interpreted by a reduction 

in the thermocouple temperature in the fire zone above the 

large pan. When CO17 reached a maximum, the temperature 

in the fire zone has decreased to 325°C (620°F). It was visually 

observed in B-Butt that the smoke layer commenced to retreat 

about 11:33 am, which is also characteristic of decreasing 

fire intensity. For Experiments No. 1, 2 and 4, the CO sensor 

response in 11-Room occurred in the steady state period of the 

diesel fuel burn. This can be attributed to the higher airflow in 

the fire zone for Experiment No. 3 relative to the other three 

experiments. 

Unlike Experiment No. 2, in Experiment No. 3 with 10-

Room isolated with brattices at the B-Butt and F-Butt ends, 

the smoke was only temporarily stopped by the doorframe in 

B-Butt. The layer thickened vertically below the beam and 

formed a recirculation zone below the roof before advancing 

Figure 2 — Sensor measurements from Experiment No. 1.

Figure 3 — Sensor measurements from Experiment No. 2.

Figure 4 — Sensor measurements from Experiment No. 3.

Figure 5 — Sensor measurements from Experiment No. 4.
2006 TRANSACTIONS • Vol. 320 SOCIETY FOR MINING, METALLURGY, AND EXPLORATION 154

farther upwind from the doorframe. The smoke advanced 10 

m (33 ft) upwind from the doorframe. The average airflow 

velocity between the fire and the extent of the reverse layer can 

be estimated from the measured air velocities at the fire zone 

and upwind from the diagonal air split. That average velocity 

is 0.55 m/s (1.8 fps). Based on earlier research (Edwards et al., 

2006), the estimated critical air velocity required to prevent 

reverse smoke layer would be 1.96 m/s (6.4 fps). 

Based on the observation of the temporary stoppage of 

smoke by the doorframe located 29 m (95 ft) upwind from 

the fire in Experiment No. 3, Experiment No. 4 was conducted 

with a brattice secured tightly along the roof and ribs of the 

doorframe and extending 0.91 m (3.0 ft) down from the roof. 

The brattice was set in place prior to the airflow measurements. 

The doorframe dimensions were 1.75 m (5.74 ft) high and 

2.51 m (8.23 ft) wide. It was observed during the experiment 

that a roof smoke layer retreated upwind with an approximate 

speed of 0.19 m/s (0.62 fps) until it reached the partial brattice 

covering at the door frame. This was counter to the average 

airflow of 0.55 m/s (1.8 fps) between the fire and the upwind 

brattice. Based on previous research (Edwards et al., 2006), 

the critical velocity to prevent smoke rollback would be 2.1 

m/s (6.9 fps) for the 771 kW fire, which is greater than the 

0.55 m/s (1.8 fps) in the entry. The smoke layer’s reversal 

was blocked by the brattice. It was observed that the smoke 

recirculation extended less than 0.3 m (1 ft) beneath the bottom of the brattice. Figure 6 shows the brattice and the smoke 

slightly below the brattice.

For all four experiments the light monitor in F-Butt between 

the regulators and 11-Room responded to smoke. An initial 

advancement of a smoke roof layer was visually observed in 

13-Room slightly beyond C-Butt and toward G-Butt for Experiment No. 1. The relatively small response of the light monitor 

upwind from the regulator in F-Butt for Experiments No. 3 

and 4 compared with Experiments No. 1 and 2 indicates that 

the significant responses in the first two experiments were due 

to smoke leakage from 10-Room into F-butt. This illustrates 

the elusive characteristic of smoke when temperature induced 

buoyancy effects compete with forced ventilation.

The buoyancy effect in the reversed smoke layer is seen in the 

temperature near the roof in B-Butt 15 m (50 ft) upwind from 

the fire for Experiment No. 3, as shown in Fig. 7. The smoke 

temperature reaches a temperature of 95°C (200°F), which is 

significantly higher than the ambient air temperature of 4°C 

(39°F). Also shown in Fig. 7 are the responses of the Beam 

optical smoke sensor and FloSonic ultrasonic airflow monitor 

in B-Butt 9 m (30 ft) upwind from the fire. The response of the 

Beam optical smoke sensor is coincident with the increase in 

fluctuations of the FloSonic airflow monitor. These fluctuations 

in the acoustic velocity monitor are due to the refraction and 

absorption of ultrasound waves by the smoke-laden air. This 

effect was discussed in a previous study (Friel and Edwards, 

1996) on the acoustic detection of smoke from a fire.

CFD modeling

CFD modeling can be used to determine the smoke movement, 

concentration and temperature produced by a fire in a mine 

entry. CFD requires a specification of the mine dimensions, inlet 

airflows and fire intensity. The three-dimensional character of 

a CFD application yields a prediction of the extent of smoke 

reversal directly upwind from the fire and into a diagonal 

airway that intersects the airway with the fire. Nonuniform 

pressure imbalances between the ends of the diagonal airway 

can result in partial airflow reversals. This is not available 

from standard mine ventilation simulators for unidirectional 

airflow. The spatial resolution required for CFD three-dimensional modeling imposes practical limitations on the extent of 

the mine section that can be modeled. These limitations are 

determined by the computer memory and processor speed. 

Implicit in the FDS, the CFD program used in this study is 

a rigorous treatment of the throttling and acceleration effects 

imposed by a fire upon the airflow. Beyond the application of 

FDS to a localized mine section, a standard mine ventilation 

simulator can provide information on the spread of smoke and 

heat to the remainder of a mine if the smoke is well mixed over 

the mine entry. This suggests a coupling of the output from a 

CFD application to the input of a mine ventilation simulator 

to model the entire mine.

FDS analysis was applied to model the smoke (soot) concentration and airflow in the entries and connecting rooms. 

For Experiment No. 2, Fig. 8 is a plan view that shows the 

predicted smoke concentration 200 mm (8 in.) below the roof, 

and Fig. 9 is a profile view of the predicted smoke distribution. Figure 8 shows the reversed smoke layer that advanced 

Figure 6 — Brattice blockage of smoke 29 m (95 ft) upwind 

from the fire in B-Butt in Experiment No. 4.

Figure 7 — Sensor response in B-Butt from Experiment 

No. 3.
SOCIETY FOR MINING, METALLURGY, AND EXPLORATION Vol. 320 • 2006 TRANSACTIONS 155

into both 10-Room and 11-Room. The predicted smoke (soot) 

concentration in Fig. 8 is in units of mg/m3. The CFD prediction of the smoke distribution shown in Fig. 8 is in agreement 

with the experimental observation for the smoke abatement 

in Experiment No. 2 at the B-Butt doorframe.

An application of FDS was made to Experiment No. 3 

to model the smoke reversal in B-Butt. FDS computations 

showed a temporary smoke stoppage, but the predicted smoke 

movement was approximately 30 m (100 ft) upwind from the 

doorframe. The experimental observation was 10 m (33 ft) of 

smoke advancement upwind from the doorframe. 

The effectiveness of the blockage of smoke reversal by the 

brattice suspended from the roof in B-Butt during Experiment 

No. 4 was modeled with the FDS program. The result of the 

simulation is shown in Fig. 10. Although the simulation showed 

a small amount of leakage upwind underneath the brattice, 

which was not observed in the experiment, the smoke reversal 

was essentially blocked.

An FDS application to Experiment No. 4 was used to predict 

the airflow and smoke density in the diagonal airway. Figure 11

shows the smoke concentrations and airflow directions along a 

middle vertical plane parallel to the direction of the diagonal 

airway. The arrows indicate the airflow direction. The location 

Y = 0 m is the location of the B-Butt rib and Y = 53 m (175 

ft) is the location of the F-Butt rib. In the upper section the 

figure shows the roof smoke-layer movement from B-Butt to 

F-Butt, and the lower section shows the airflow movement from 

F-Butt to B-Butt. The smoke concentrations indicated by the 

contour lines are not actual concentrations but only relative 

concentrations. As expected, the denser smoke concentration 

occurs near the roof closer to B-Butt. This is in qualitative 

agreement with the CO concentration reported in Fig. 5, which 

shows higher CO concentration near the roof than near the floor. 

The CO concentrations in Fig. 5 are eventually truncated by 

the maximum measurable value of 50 ppm.

During Experiment No. 4, the response of the thermocouple 

in the fire, TF, and the thermocouple, T4, 13 m (43 ft) upwind 

from the fire in B-Butt are shown in Fig. 12 along with the 

values predicted by the FDS program. The CFD simulation 

was terminated after a steady state was achieved. The predicted 

temperature upwind from the fire was in relatively good agreement with the measured temperature. Temperature prediction 

within the fire is affected by the reaction kinetics of the fire. 

In the simulation the heat production is specified in the fire 

zone, which would result in more accurate temperature prediction away from the heat source. One significant difference 

between the measurement and the simulation was the time for 

the hot gases in the reversed smoke layer to reach the upwind 

thermocouple T4. The measured temperature increase at T4 

occurred 80 seconds after ignition of the fire, whereas the 

predicted value was reached 16 seconds after ignition. This 

was a consequence of the treatment of turbulence by FDS. 

The FDS does not account for the mine wall friction, which 

would retard the advancement of the reversed smoke layer. An 

earlier response of the predicted value in comparison with the 

measured value was also seen in the response of the laser light 

monitor LLM1 in 11-Room. The laser light monitor responded 

to smoke 7 min after the initiation of the fire. However, the 

FDS predicted a response about 2.4 min after initiation of the 

fire. There was a roof cavity with a depth near to but less than 

1 m (3.3 ft) in 11-Room between B-Butt and the laser light 

monitor, which would have retarded the advancement of the 

reversed roof smoke layer in 11-Room.

Conclusions

How smoke from a mine fire downwind from a diagonal airway 

can undergo reversal into the diagonal airway without causing 

a complete airflow reversal in the airway was demonstrated 

experimentally and modeled successfully with a CFD program. 

For fire intensities between 504 and 771 kW complete flow 

reversal did not occur in the diagonal airway. Instead of a total 

airflow reversal in the diagonal airway, the roof smoke layer 

flowed counter to the established airflow along the floor. A 

Figure 8 — Predicted smoke density 200 mm (8 in.) below 

roof in Experiment No. 2.

Figure 9 — Predicted smoke density abatement by the 

doorframe in Experiment No. 2.

Figure 10 — FDS simulation of smoke blockage by the 

partial brattice in Experiment No. 4.
2006 TRANSACTIONS • Vol. 320 SOCIETY FOR MINING, METALLURGY, AND EXPLORATION 156

smoke reverse layer extended 8 m (26 ft) upwind from the fire 

to the diagonal and at least 40 m (130 ft) along the diagonal 

counter to the established airflow, as determined by the CO 

sensors in the diagonal. This event in a simple mine airway 

configuration is not modeled by standard mine-ventilation 

network simulators, which are dependent on unidirectional 

airflow in any particular airway. Preventive planning with 

recommendations for miner egress and rescue are complicated 

by the smoke reversal into connecting airways. CFD analysis 

is a viable method to predict the formation of reversed smoke 

layers, and their movement provided the input parameters, 

such as mine dimensions, airflow quantities and fire intensity, 

are sufficiently known. 

It was demonstrated for a 771-kW fire that smoke reversal 

could be abated by a partial brattice coverage extending from 

the roof to approximately half the entry height. The 29 m (95 

ft) distance of the brattice from the fire zone would make the 

ventilation effect localized. This is a possible smoke-reversal 

control measure for mine rescue. The FDS program was shown 

to be useful for determining the extent of fire-generated, roofsmoke reversal and the abatement of the smoke by a brattice 

suspended from the roof to mid-height. The turbulence model 

in FDS did not adequately account for the rate of roof smoke 

reversal. Additional research on the implementation of CFD 

methods needs to be conducted for a variety of conditions, 

which include airway dimensions, brattice cross-sectional 

area restriction, fire intensity and airflow. For each mine-entry configuration and partial entry blockage with a brattice, a 

critical ventilation velocity can be determined as a function of 

fire intensity which prevents smoke reversal. Given the complexities for conducting the experiments, a limited number of 

experiments could be modeled with a CFD application. Based 

on a successful comparison of predictions with experimental 

results, additional CFD applications could be used to determine 

a more comprehensive relationship between critical velocities, 

fire intensity, airway dimensions and smoke-control measures. 

The experimental problem encountered in determining if the 

smoke arrival in the diagonal airway near the F-Butt airway 

was from the smoke reversed layer in the diagonal or from 10-

Room could be resolved with additional POC monitoring in the 

diagonal. This should be incorporated in future research.

As a subsidiary measurement, it was further demonstrated 

how an ultrasonic path airflow monitor can detect smoke from 

a mine fire by its erratic fluctuations. These fluctuations in the 

signal are caused by the attenuation and refraction of sound 

waves by smoke particulates in the airflow.

Acknowledgment

The authors acknowledge Paul Stefko, Joseph Sabo and Jack 

Teatino, who maintain the SRCM at the PRL, for their assistance in preparation of the experiments.

References

Bird, R.B., Stewart, W.E., and Lightfoot, E.N., 1960, Transport Phenomena, J. 

Wiley & Sons, Inc., New York, p. 226.

Chang, X., Laage, L.W., and 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 Informational Circular 9245, 171 pages.

Edwards, J.C., Franks, R.A., Friel, G.F., and Yuan, L., 2006, “Experimental and 

modeling investigation of the effect of ventilation on smoke rollback in a 

mine entry,” Mining Engineering, April 2006, pp. 53-58.

Eisner, H.S., and Smith, P.B., 1954, “Convection Effects from Underground Fires: 

The Backing of Smoke Against the Ventilation,” Safety in Mines Research 

Establishment Report No. 96, 12 p. July

Friel, G.F., and Edwards, J.C., 1996, “Mine Fire Detection by Ultrasonic Ranging 

Systems,” U.S. Bureau of Mines RI 9624, 15 pages.

McGrattan, K.B., Forney, G.P., Floyd, J.E., Hostikka, S., and Prasad, K., 2004, 

Fire Dynamics Simulator (Version 4) User’s Guide, U.S. Dept. of Commerce, 

National Institute of Standards and Technology, NIST Special Publication 1019, 

U.S. Government Printing Office, Washington, D.C., July

McPherson, M.J., 1993, Subsurface Ventilation and Environmental Engineering, 

Chapman and Hall, New York, 905 pp.

Wala, A.M., and Stoltz, J.R., 1999, “Three underground coal mine explosions 

— twenty miners killed — one reason,” Proceedings of the 8th US Mine 

Ventilation Symposium, J. Tien, ed., Univ. of Missouri-Rolla Press, Rolla, 

Missouri, pp. 397-404, June 11-17, 1999.

Figure 11 — Predicted airflow direction and smoke concentration (mg/m3) along the middle, vertical plane parallel to 

the direction of the diagonal airway in Experiment No. 4.

Figure 12 — Measured and predicted temperatures in fire 

and near the roof 13 m (43 ft) upwind from the fire in B-Butt 

in Experiment No. 4.