Small-Scale Experimental Burns: Effect of Ignition Patterns on
Smoke Production, Rate of Spread, and Fuels Consumption
February 11, 2011
Introduction
Smoke management is integral to any prescribed or wildfire activity, and is key to ensuring the safety of the
public, and public support for prescribed burning in the future. When planning a prescribed burn, state legal
requirements for smoke screening and mitigation of potentially negative impacts must be considered. The
prescription parameters for a prescribed burn should take potential smoke effects on Smoke Sensitive Areas and
Critical Smoke Sensitive Areas into account. In order to avoid negative effects on human and ecological
resources, the smoke screening process guides the fire manager towards defining the appropriate conditions
for a burn which limit the potential negative effects of smoke production. Fuel type, weather conditions,
ignition patterns and timing, and mop-up procedures all influence whether a prescribed burn’s smoke is
accounted for. In this lab, you will explore how fire ignition patterns influence key fire behavior
characteristics associated with smoke production.
Objectives
1) To experiment with different ignition patterns to determine smoke production
2) To quantify fire behavior characteristics including rate of spread, flame length, and flaming and
smoldering combustion time
3) To explore the relationship between smoke production and fuels consumption
Directions: Each group will be responsible for running a small-plot experimental burn with the following
ignitions:
Group A: Backing fire. Ignite the plot using one line of drip torch fuel on the downwind side of the plot.
Group B: Head fire. Ignite the plot using one line of drip torch fuel on the upwind side of the plot.
Group C: Ring fire. Beginning downwind from the fire (like a backing fire), ignite the fire around the perimeter
of the plot, making sure to stay outside of the plot.
Group D: Spot/ grid fire. Start downwind, and light a grid of spot fires moving towards the wind.
Assign Group Member Roles
“The Igniter/ fuels consumption monitor”: This person, who has fire boots and nomex pants on, measures litter
depth before and after the burn, and uses the drip torch to ignite the fire.
“The Timer”: This person, who has great timing, times the duration of the burn from first ignition to when the
fire burns across the whole plot, to when flaming ignition ceases, and to when smoldering combustion ceases.
“The Seer”: This person, gifted with impeccable eyesight, assesses visibility to the other side of the fire every 5
seconds and takes notes.
“The Estimator”: This person, who is confident in estimating lengths, estimates flame length every 5 seconds,
and keeps notes.
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Detailed Instructions for Measurements
“The Igniter”: Fuel Consumption
In order to determine the proportion of the fuel consumed during the burning of the fuelbeds we will
simply estimate the proportion of the litter depth that was consumed. All you will need is pre-burn and postburn litter depth measurements. After fuel is loaded into the burning rings, take four litter depth
measurements, one at each cardinal direction and at approximately one-third of the distance from the edge of
the ring. Record the litter depths in the pre-burn column below in the row that corresponds to the cardinal
direction of the sample measurement. After burning is complete take four additional litter depth measurements
at the same locations and record them in the table. Subtract the post-burn measurement from the pre-burn
measurement to get litter depth consumed. Now divide this number (litter depth consumed) into the pre-burn
litter depth measurement to calculate the proportion of litter consumed. Do this for each of the four sample
points and the calculate the average proportion of fuel consumed during burning. Now compare the proportion
of fuel consumed across the four burning scenarios. Are there any differences in fuel consumption that may be
attributed to the type of ignition and subsequent burning behavior?
We can take this one step further to estimate the mass of fuel consumed over a unit area by using the
proportion of fuel consumed and an assumed value of pre-burn fuelbed bulk density. We will do this by using
just the litter depth consumed and an equation that predicts litter mass per hectare from litter depth by
assuming a constant fuelbed bulk density (~23 kg/m3). Use your litter depth consumed values from each of the
four sample points and estimate the mass (Mg)of fuel consumed per hectare using the equation provided
below. Remember most fancy 'smart' phones, and even the non-smart phones, have a calculator on them.
“The Timer” Rate of Spread
Record the time when the igniter starts ignitions. Record how long it takes for the fire to move across the entire
plot. Next, record how long flaming combustion (flames are visible) continues. Finally, record when all smoking
has ceased.
“The Seer”: Visibility
You have a challenging task. Since we don’t have the ability to measure amounts of particulate matter in the
smoke, you will be estimating the only thing we can measure about smoke; visibility. Every five seconds or so,
record how many letters you can see clearly across the fire. Be sure to take a mental “snapshot”. Don’t wait for
the letters to become clear. Calculate the mean number of letters seen during both flaming and smoldering.
“The Estimator”: Flame Length
Flame length is measured from the base of the flaming front to the tip of the visible flame. It is NOT the same as
flame height. Unless there is no wind, flame length will typically be longer than flame height is high. To
estimate flame length, prepare yourself: stand at about 10 feet away from the plot, have someone hold a tape
up at 1 m length (or any measure you are familiar with, say, feet), and use your hand to determine how the
meter distance can be estimated from where you’re standing. Take measurements at about every 5 seconds
and quickly write down the numbers, or have someone record for you. Calculate the mean number.
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DATA SHEET
IGNTION PATTERN:
GROUP MEMBERS:
DATE:
“The Igniter”: Measures Fuel Consumption using a ruler at four litter depth measurement locations, one at
each cardinal direction and at approximately one-third of the distance from the edge of the ring. Group works
together to calculate litter mass consumption as described above.
Sample
Pre-Burn
Litter
Depth(cm)
Post-Burn
Litter Depth
(cm)
Litter Depth
Consumed
(cm)
Proportion
Consumed
North
East
West
South
Mean:
Pre-Burn Litter Depth-Post-Burn Litter Depth=Litter Depth Consumed
Litter Depth Consumed/Pre-Burn Litter Depth = Proportion Consumed
Litter Mass Equation
𝐿𝑖𝑡𝑡𝑒𝑟 𝑀𝑎𝑠𝑠 𝐶𝑜𝑛𝑠𝑢𝑚𝑒𝑑 (𝑀𝑔ℎ𝑎−1 ) =
238+199×𝐿𝑖𝑡𝑡𝑒𝑟 𝐷𝑒𝑝𝑡ℎ (𝑐𝑚)
100
“The Timer”: Rate of Spread
When to measure
Time of Day
Length of time
(subtract from time of
day)
Start of burn
Whole plot burned
Flaming combustion ceases
Smoldering combustion
ceases (no smoke)
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Litter Mass
Consumption
(Mg/ha)
“The Seer”: Visibility
Flaming
Combustion
Measurement
1
Number of letters visible
Smoldering Combustion
Measurement
Number of letters visible
1
2
3
4
5
6
7
8
9
Mean
2
3
4
5
6
7
8
9
Mean
“The Estimator”: Flame Length/ stop measuring once fire has burned across entire plot.
Measurement
1
Flame Length
2
3
4
5
6
7
8
9
Mean
Questions
1)
How did your ignition pattern compare to others in terms of flame length, visibility, consumption, and
rate of spread?
2) What did you expect to see? What could explain differences between your expectations and your
observations?
3) What is the best ignition method for reducing smoke production during flaming and smoldering
combustion? In what cases would you NOT want to use this method?
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