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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 FR-0- 2 | Page 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. FR-0- 3 | Page 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). FR-0- 4 | Page 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 FR-0- 5 | Page 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 FR-0- 6 | Page 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). FR-0- 7 | Page 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. FR-0- 8 | Page 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 FR-0- 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 9 | Page 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). FR-0- 10 | Page 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). FR-0- 11 | Page 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. FR-0- 12 | Page 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. FR-0- 13 | Page Figure 12. Photograph of in stand sprinkler trial. FR-0- 14 | Page 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. FR-0- 15 | Page 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. FR-0- 16 | Page 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. FR-0- 17 | Page