Download Report: Exploratory research into the existence of humidity domes

Wildfire Operations Research
1176 Switzer Drive
Hinton AB T7V 1V3
Exploratory research into the existence of humidity domes created
by wildfire sprinklers
Fire Report - FR
Kelsy Gibos
Steven Hvenegaard
July 2009
©
Copyright 2012, FPInnovations
Introduction
Before delving into the experimental results it might be useful to look into the
relationships between the terms Relative Humidity (RH), Temperature and Absolute
Moisture Content (or Humidity Ratio, W). If you are familiar with these concepts then
skip ahead and nothing will be lost.
Mixtures of air and water can be complex as the amount of moisture that can be mixed
into a given amount of air is a very nonlinear function of temperature – the higher the
temperature the more vapor phase water that can be held. To illustrate this concept,
imagine two containers, each holding 1 cubic meter of air (about 1 kg of air). If we add
the same amount of moisture to each container we know that the absolute moisture
content is the same. Now take one containers and warm it a bit. If one container is at
20°C and the other at 30°C there is still the same amount of water in each but the Relative
Humidity is not the same. Now suppose that 10 grams of water was added to each. The
20°C container has a humidity ratio (usually labeled W) of 10 grams water per kilogram
of air but its Relative Humidity is between 65-70%. The 30°C container has the same
humidity ratio, 10 gwater/kgair but its Relative Humidity is 35-40%. Thus when we
measure RH it is important that we also measure temperature so that we can determine
the absolute moisture content of the gas mixtures.
Relative humidity can be measured using a number of instruments including wet and dry
bulb thermometers, dew point apparatus or solid state sensors (most commonly variable
capacitance sensors – ie. KestrelTM models). In all cases the measure of RH only has
meaning if accompanied by a measure of air temperature. When a given volume of liquid
water evaporates, there is an energy transfer from the surroundings to evaporate the
moisture. Because a significant amount of energy is required to drive the phase change
(liquid to vapor) there is a corresponding decrease in the temperature of the gases. For
example, the energy required to evaporate 10g of liquid water is approximately 22.5
kJoules. If this energy is taken from one cubic meter of gas the resulting temperature
change is approximately 22.5°C. Thus the resulting mixture of air and water vapor would
be about 22°C cooler than the mass of dry air alone – this principle is used in evaporative
air conditioning.
When a sprinkler is used on either an agricultural application or a fire line operation the
objective is to create an area, zone or line of increased moisture. Depending on the type
of system used to distribute the water the moisture may evaporate before reaching the
ground, may land on the ground and low vegetation where it wets the surface and then
evaporates or it may land on surfaces (bole of a tree, low branches) where it evaporates,
stays on the surface or ends up eventually on the ground surface. In wildland fire
operations, (either fire line or structure protection) the question often arises as to how
much and when moisture should be added to provide an effective deterrent to fire spread.
The concept of a “humidity dome” has been around for a while and the experimental
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program outlined in this document is an attempt at showing its existence or alternatively
explaining why it should not exist.
One should also note that in most cases, even in low wind conditions, the air is not
stagnant. Even at very low wind speeds (1 m/s, 3.6 km/hr) there is significant movement
which will convect the air water mixture downwind from the source. A Rainbird
sprinkler has a rotational period of between 60 and 90 seconds (takes 60 to 90 seconds for
one complete rotation). In this time period, moisture-laden air from inside the circle
inscribed by the sprinkler throw will be between 60 and 90 meters downwind of the
sprinkler!
Objectives
1. Measure the change in moisture content of the air mass surrounding a sprinkler
head delivering water over a given time period in an open field.
2. Measure the change in moisture content of the air mass surrounding a sprinkler
head delivering water over a given time period in a closed stand of forest.
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Methods
The experiment was conducted in McBride, BC on July 2 and July 3, 2009. Overall the
methodology was to place temperature and relative humidity data loggers in a grid
around a single sprinkler fed using an engine driven centrifugal pump, monitor
temperature and RH for a period before starting the pump to establish background
conditions and then run the pump/sprinkler combination for a fixed time. A loop system
was set up using a Honda pump and several lengths of 1½ inch (3.8 cm) hose as indicated
in Figure 1.
Figure 1. General schematic of sprinkler system set-up.
A flow meter (Figure 2a) was installed close to the pump to record the amount of water
passing through the system and the total amount of water delivered. A pressure gauge
was used to monitor system pressure and to ensure adequate pressure delivery to the
sprinkler head (Figure 2b).
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2a) Turbine Flow Meter
2b) Line Pressure Gauge
Figure 2. Instrumentation for water flow and pressure measurement.
A single Rainbird model 70GW sprinkler (Figure 3) with an orifice size of 0.25 inch was
used in each trial of the study.
Figure 3. Rainbird sprinkler.
Sprinklers were installed in two locations: in an open field and in a forest stand. Two
open field trials on separate sites were conducted (July 2 and 3) while one forest stand
trial was undertaken on July 3. The open field sites consisted of grasses and oxeye daisy
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species approximately 15 cm tall (Figure 4a). The sites were open to predominant winds
and free from shade. A third setup was installed beneath the forest canopy (Figure 4b).
The forest consisted mainly of mature cedar and birch trees, with a forest floor composed
of needle litter and short understory shrubs.
Figure 4a. Open field study site.
Figure 4b. In stand forest study site.
Figure 4. Study sites.
A weather station recording temperature, relative humidity, wind speed and wind
direction was installed in the open field away from the spray of the sprinklers. Hourly
weather data (Table 2) were recorded for the duration of the experiment using the station
shown in Figure 5.
Figure 5. Weather station installed in the open field.
Data loggers were installed around the sprinkler heads to measure and record relative
humidity and temperature during operation of the sprinkler heads. Each data logger
(MicroMeasurements USB 502 RH/Temp.) was enclosed in a protective PVC tube and
aligned facing away from the direct stream of water to prevent liquid water from
impacting the sensors (Figure 6). A total of nineteen sensors were used in each trial. The
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data loggers were set to record relative humidity and temperature every 10 seconds but
these measurements were subsequently reduced to one minute averages to reduce the
number of data points.
PVC housing
open side and
bottom
USB 502
Data Logger
Figure 6. Relative humidity and temperature sensor in protective PVC casing.
Sets of data loggers were arranged at various heights along vertical poles: 2 feet (61 cm),
5 feet 6 inches (160 cm), 7 feet (213.4 cm) and 9 feet (274.3 cm). Based on the spray
pattern from each sprinkler, poles with combinations of two and three data loggers were
arranged at distances from the sprinkler as shown in Figure 7. The height of each data
logger for the open field data logger configuration is show in Table 1.
Table 1. Data Logger heights in open field configuration.
Data logger
height
9’ - 0”
7’ – 0”
5’ – 6”
2’ 0”
Data Logger Number
3
6
9
12
1
2
15
4
5
7
8
10
11
13
14
16
17
19
20
Water was run through each sprinkler head for approximately two hours while the data
loggers recorded relative humidity and temperature at various heights and distances from
the sprinkler head. Over the course of each trial the sprinkler delivered about 1500-1600
US gal (equivalent to about 5 mm of rainfall).
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Figure 7. Schematics showing the arrangement of nineteen relative humidity and
temperature sensors at each sprinkler head location. Note that the photograph and
diagram for the open field site are oriented west while the photograph and diagram
for the in-stand site are oriented east.
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Results
Weather
Table 2. Weather data for July 2 and 3.
Time
1300
1400
1500
1600
1700
1800
1900
2000
2100
2200
2300
2400
0100
0200
0300
0400
0500
0600
0700
0800
0900
1000
1100
1200
1300
1400
1500
1600
1700
1800
1900
2000
2100
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July 2 Weather
Temperature Relative
Humidity
25.2
21
24.9
21
21.9
20
22.1
27
22.3
35
21.3
33
21.3
36
21.3
34
20.6
39
16.2
58
12.1
71
9.6
78
July 3 Weather
7.8
85
6.6
90
5.8
91
5.3
94
4.1
94
3.8
94
3.9
94
4.9
95
9.9
90
13.9
63
16.2
53
18.2
49
20.1
37
21.9
34
23.7
31
24.4
22
25.3
26
25.6
19
25.7
23
25.7
30
21.7
53
Wind
Speed
0
0
0
0
0
4.8
8
6.4
0
1.6
0
0
Wind
Direction
3.2
1.6
0
0
1.6
0
0
0
0
0
0
8
9.7
4.8
4.8
3.2
0
0
6.4
0
0
SE
E
NNW
WSW
WSW
N
E
SW
W
W
SSE
WNW
W
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Applications
Open Field – July 2, 2009
The initial trial setup was placed late afternoon on July 2 in an open field near McBride
B.C. Nineteen RH/Temperature loggers were placed surrounding a single Rainbird
sprinkler in the pattern shown in Figure 7. The major axis of the configuration was
placed in the same direction as the local wind as indicated with the portable weather
station. Wind speed over the duration of the trial varied between .44 and 3.14 m/s with
an average of 1.58 m/s.
Data were collected at 10 second intervals but later averaged to one minute readings. As
indicated in Figure 8 the sprinkler was started at 7PM PST and operated for about one
hour. Over this time the volume of water delivered was 916 imperial gallons (4164 litres)
over an area of 1800 m2. The sprinkler throw was measured by locating the edge of the
wetted portion of the area while the sprinkler was operating.
RH1
90
RH2
RH3
80
RH4
Relative Humidity (%)
70
RH5
RH6
60
RH7
RH8
50
RH9
RH10
40
RH11
RH12
30
RH13
20
RH14
Sprinkler started 7PM PST,
Flow Rate 14 usgpm, total
flow 1100 us gal
10
RH15
Sprinkler Stopped
RH16
RH17
0
0
20
40
60
80
Time (one minute average)
100
120
RH19
RH20
T1
Figure 8. Relative humidity measured around a sprinkler in an open field (Trial 1).
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Figure 8 is a plot of relative humidity and temperature during the course of the trial.
Ambient temperature, initially at about 25°C, remained relatively constant over the
sprinkler operation period and fell to about 18°C during the hour after the sprinkler was
stopped. As was indicated earlier, relative humidity is a measure of the moisture content
of the air relative to what it is capable of containing given its temperature. Figure 8
indicates an initial decrease in humidity when the sprinkler was turned on followed by an
increase about 20 minutes into the trial. This initial decrease and subsequent rise in
relative humidity closely coincides with the rise and drop in temperature and reflects the
inverse relationship between relative humidity and temperature.
Figure 9 shows the same data converted to absolute moisture content in the air. Note that
for the entire trial period the absolute moisture content decreased with some periodic
increases where the sprinkler water stream struck the enclosures containing the RH/Temp
data loggers.
1.40E-02
W1
W2
1.20E-02
W3
Humidity Ratio (kgw/kgair)
W4
W5
1.00E-02
W6
W7
8.00E-03
W8
W9
W10
6.00E-03
Sprinkler started 7PM PST
Run for approx 1 hour @
14 usgpm (1100 gal total)
4.00E-03
W11
Sprinkler stopped
W12
W13
W14
W15
2.00E-03
W16
W17
0.00E+00
0
20
40
60
80
Time (one minute average)
100
120
W19
W20
Figure 9. Humidity Ratio measured around a sprinkler in an open field (Trial 1).
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Open field – July 3, 2009
A second open field trial was conducted the following day to compare results obtained
with the same sprinkler setup and data logger configuration deployed on a similar site at a
different location. Note that conditions were favorable (very low wind speed) for
minimizing moisture drifting away from the sprinkler center of rotation. If high winds
were present we would expect a plume of moisture laden air to move down wind of the
sprinkler location.
Figure 10 shows the relative humidity measured during the second trial in an open field.
During the period of sprinkler operation (1107 to 1207 PST), the ambient air temperature
steadily increased from around 21°C at the start of the trial to near 28°C at the end. The
relative humidity showed an initial increase until about 20 minutes into the trial where it
fell and continued to fall for the remainder of the trial. This further illustrates the
influence of temperature on relative humidity.
RH1
60
RH2
RH3
RH4
50
Relative Humidity (%)
RH5
RH6
40
RH7
RH8
RH9
30
RH10
RH11
20
RH12
RH13
Sprinkler on
11:07 Pacific
flow 14 usgpm
10
Sprinkler off
12:07 Pacific
RH14
RH15
RH16
RH17
0
0
30
60
90
120
150
180
Time (one minute averages)
RH19
RH20
T1
Figure 10. Relative Humidity measured in an open field (Trial 2).
Figure 11 is a plot of moisture content (humidity ratio) as a function of time. Note that
during the entire trial period the moisture content remained constant with no indication of
moisture addition to the air mass.
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W1
1.20E-02
W2
Humidity Ratio (kg water/kg air)
W3
1.00E-02
W4
W5
W6
8.00E-03
W7
W8
6.00E-03
W9
W10
W11
4.00E-03
W12
2.00E-03
W13
Sprinkler
off
Sprinkler on
11:07 Pacific
flow 14 usgpm
W14
W15
W16
0.00E+00
W17
0
30
60
90
120
150
180
Time (one minute averages)
W19
W20
Figure 11. Humidity Ratio measured in an open field (Trial 2).
In stand – July 3, 2009
The sprinkler head was moved to a location within a stand of trees to gauge the effects of
this environment on the formation of an area of humidity. Figure 7 indicates the
locations of the individual RH/Temperature data loggers for this trial while Figure 12 is a
photograph taken within the stand.
During the in stand trial, water was run through the sprinkler system from 1607 to 1807
PST. Water flow was approximately 14 gallons per minute with a total flow of 1582
gallons.
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Figure 12. Photograph of in stand sprinkler trial.
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RH1
100
RH2
90
RH3
RH4
Relative Humidity (%)
80
RH5
RH6
70
RH7
60
RH8
RH9
50
RH10
RH11
40
RH12
30
RH13
RH14
20
RH15
Sprinkler on
Flow 14 usgpm
10
Sprinkler off
RH16
RH17
0
RH19
0
30
60
90
120
150
180
210
Time (one minute average)
240
RH20
T1
Figure 13. Relative Humidity measured in stand.
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1.60E-02
W1
W2
Humidity Ratio (kg water/kg air)
1.40E-02
W3
W4
1.20E-02
W5
W6
1.00E-02
W7
W8
8.00E-03
W9
W10
W11
6.00E-03
W12
W13
4.00E-03
W14
Sprinkler on
Flow 14 usgpm
2.00E-03
Sprinkler off
W15
W16
W17
0.00E+00
0
30
60
90
120
150
180
Time (one minute average)
210
240
W19
W20
Figure 14. Humidity Ratio measured in stand.
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Discussion
During the open field sprinkler trials, the first trial recorded a rise in relative humidity
while the second trial recorded a drop in relative humidity (Figures 8 and 10). This
inconsistent change in relative humidity demonstrates the typical inverse relationship
between temperature and relative humidity. However, the absolute moisture content
measured during these trials remained almost constant during the sprinkler operation and
declined minimally after the sprinkler was turned off (Figure 9 and 11). The average
wind speeds during these two trials in the open field were 1.58 and 1.17 m/s.
In the closed forest sprinkler trial, the overall increase in relative humidity over the
duration of the sprinkler operation was more pronounced than in the open field trials.
While all dataloggers recorded an increase in relative humidity, the dataloggers that were
closest to the ground measured a greater change in relative humidity with the highest
relative humidity recorded (Figure 13). The average wind speed during the closed forest
trial was 0.79 m/s. The absolute moisture content for the duration of this trial rose very
slightly over the duration of this trial (Figure 14).
Conclusions
The negligible increase in absolute moisture content recorded during sprinkler operation
in all three trials suggests that an air mass is not a good receptive agent for retaining
additional water vapor.
Unless moisture can adhere to or be absorbed by materials (combustible fuels) in the
sprinkler area, air-borne moisture is easily migrated from the sprinkler area. Wind will be
the most influential factor in this movement of air-borne moisture.
Sprinklers are often deployed as part of Wildland Urban Interface (WUI) structure
protection or wildland fire suppression operations. The success of these operations is
often attributed to a rise in humidity in the zone of water delivery and the resulting
reduction in moisture content of combustibles through absorption of water vapor.
However, results from this study indicate that this zone does not have an appreciable rise
in absolute moisture content. Wetting of fuels through water vapor absorption may not be
the most influential moisture trasfer mechanism at play. A more likely moisture transfer
mechanism is through direct contact between moisture and fuels.
With a better understanding of the moisture transfer mechanisms provided by sprinklers,
the limited moisture retaining qualities of an air mass and the ease of moisture migration
with minimal winds, the placement of sprinklers and duration of operation can be
planned to provide cost-effective sprinkler operations.
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