Wind Tunnel Experiments to Study Chaparral Crown Fires

The overall goal of this methodology is to clarify some of the key characteristics that dictate the degree of surface crown layer interaction in chaparral crown fires. This method could help answer key questions in the wildland fire field, such as the conditions dictating successful transition and spread to the crown layer. The main advantage of this technique is that it enables the measurement of mass loss in temperature for both the surface fuel layer and the crown fuel layer.

To begin, modify four C clamps by attaching dual spring gate carabiners through the pinhole at the clamp screw end. Use these carabiners to suspend the crown fuel bed. Using a different set of C clamps, affix each load string gauge cell to the top portion of the wind tunnel frame.

Then, attach the modified C clamps to the free end of the strain gauge cells with the carabiners hanging down. Use chains to connect the platform for the crown fuel bed to a carabiner. Connect their wires to the weed stone bridge, which will be used for data acquisition.

Cover the load cells with fire insulating material, such as the kind used for fire shelters. To obtain the calibration constant, A, hook precision weights to the first load cell. Set the load cell gain to 128 using the input number field.

This corresponds to the maximum value allowed by the device. Then, read the signal output in response to the weight at output zero. Next, unhook the weight to produce a no load signal.

Read the new value displayed in the instrument interface. Calculate the calibration constant based on the equation shown here. Then, in the controller interface, fill in the channel M value for each sensor with the value A.To find the offset value, B, remove all of the weights, read the value in the outputs calibrated G box and multiply this value by negative one.

The resulting number is the constant, B.Type this number into the addition channel zero A box. Repeat these calibration steps for each load cell that is part of the system. From the pile of fuel collected for burning, collect three to four one pint bottles of fuel.

Following the procedures delineated by Countryman and Dean, oven dry the samples to obtain the proper fuel moisture content. Once ready, trim the individual branches from a bundle of recently harvested chamise to remove dead material and branch material greater than a quarter inch diameter. Place the remaining live fuel material in the container for weighing.

Select two kilograms of the trimmed chamise and 0.5 kilograms of excelsior using an electronic scale. Then, place 0.5 kilograms of excelsior onto the surface fuel bed on the wind funnel floor, ensuring that the bulk density is as uniform as possible. Next, pull apart to fluff the compacted excelsior and decrease it’s bulk density so it will burn readily.

Load two kilograms of trimmed chamise onto the platform hanging from the load cells to create the elevated fuel bed. Evenly spread the chamise branches over the entire platform to produce a uniform fuel bed. Connect an array of 16 thermocouples to a data logger with a response time of 0.9 seconds.

Insert six of the thermocouples into the crown fuel layer 20 centimeters apart, while avoiding contact with branches. Then, insert 10 thermocouples into the surface fuel layer. Place these surface fuel thermocouples 10 centimeters apart and avoid contact of thermocouples with branches.

Activate data logging by clicking the start button in the the thermocouple control software interface. Mount a visual reference target that has red marks at 10 centimeter intervals above the wind tunnel window. Next, place cameras for photographic data collection.

Focusing on the wind funnel test area, adjust the camera’s focus so as to capture the entire vertical reference target as well as the fuel bed area. Then, set up a video camera by mounting it with the universal camera wall mount on the wall to provide a full view of the wind funnel test section. Next, set the wind tunnel’s fan speed to one meter per second on the speed controller.

Then, briefly turn on the fan to ensure that it is functioning properly. Close all doors in the building to ensure that the roof vents are the only possible exit for smoke evacuation. Then, turn on the air supply fans to bring in fresh air from outside the building at four level and turn on the exhaust fans to evacuate smoke through the roof vents.

Prior to each experiment, use a wet bulb hygrometer to measure the relative humidity and temperature of the ambient air. Turn on the video camera to record the experiment. Then, speak aloud the experiment number or code, the date, and the experimental configuration so that the microphone on the video camera records this information.

Next, instruct the computer crew to begin data logging by ticking the enable data logging option in the instrument control interface. Prior to burning, make sure all building doors are closed. Once the fuel bed has been ignited, step out of the test section and close the tunnel door.

When instructed to ignite, soak the leading edge of the excelsior surface fuel bed with denatured ethyl alcohol and then place the alcohol bottle away from the ignition zone. Using a butane torch, ignite the end of the surface fuel bed in a line parallel to the leading edge of the fuel bed. If wind is required for the experiment, turn on the wind tunnel fan.

Record the burn for its entire duration, which varies based on the setup and conditions. Using video data, the crown and surface flame height data were obtained. The data shown here represents an experiment that includes wind.

In these types of experiments, the flame starts small, gets larger close to the middle of the fuel bed, and decays with time as flames get closer to the end of the fuel bed. By utilizing the strain gauges, the fuel consumption rates can be determined and show the evolution of the mass loss during the entire burn. The percent of mass loss for both the surface and crown layers are shown here, versus the percentage of total burn time.

Wind increases the mass loss rate for both the surface and crown fuel layers. The no wind cases took an average of four and a half minutes to burn to completion, and the one meter per second wind case took only two and a half minutes to burn. Once mastered, this technique can be completed in 30 to 40 minutes if it is performed properly.

While attempting this procedure, it is important to remember to properly calibrate the crown field mass loss instrument and to trim crown fields so that they are at or below a quarter inch diameter. Following this procedure’s methodology and use could be performed in order to answer additional questions about radiative heat transfer. After watching this video, you should have a good understanding on how to measure some of the key parameters that dictate the surface and crown layer interaction in chaparral crown fires.

Working with fire at wind funnel scales can be extremely hazardous. Precautions such as wearing personal protective equipment, including fire resistant clothing and protective eyewear, while performing experiments should be taken.