Dimensioning of an hydraulic system
for water bombing from an aircraft
Firefighting often involves using air fleet when forest fire is ultra-powerful. Thermal imaging is an innovative technology which can be used to determine the burned-out zones. In that way the pilot is able to determine the moment and the conditions to drop water, and its quantity.
To be efficient, firefighting also requires accurate knowledge of the temperature on soil, in order to drop water necessary to extinguish fire.
Power of fire
Forest fire intensity (power in kilowatt per meter : kW/m) is calculated by the Byram's formula (1959) :
$P = H w R$
- $w$ : the weight of fuel (burned biomass (leaves, needles, twigs...)) consumed per unit area (kg/m2)
- $H$ : the fuel low heat combustion : 18700 kJ/kg
- $R$ : the fire spread rate (m/s)
Considering a scrubland fire that can burn 1 $kg/m^2$ of fuel and with velocity spread of 0.5 m/s (1.8 km/h), its intensity is 9 350 kW/m. As a result, 100 m of this fire emits the same power than a nuclear power plant (about 1 gigawatt).
We can notice that a bulb emits 100 W and a heating more than 1 kW.
When the biomass is homogen, fire spread rate is an efficient indicator to predict the behaviour of fire and its dangerousness. But power of fire remains the best way to evaluate the threat.
Up to 2000 kW/m it is possible for gound-based firefighters to bring under control a forest fire. Beyond we need additional help from trackers. If power of fire exceeds 10000 kW/m it is impossible to put out.
Quantity of water and watering methods
Using water to extinguish fire requires efficient rules. For that, let us consider :
- fire spread rate : $R=0.15 m/s (540 m/h)$
- weight of biomass burned : $w=0.7 kg/m^2$
- power of fire : $P=18700wR=2000 kW/m$
- time of combustion of thin elements : $T=30 s$
- depth of the flame front : $L=RT=4.50 m$
- quantity of heat required to evaporate 1 liter of water initially at 40 °C : $q=2500 kJ$
Evaporation of water is used to stop progress of fire. It absorbs fire energy, so prevent energy tranfer by conduction (in the litter), convection (temperature in humid spray is only about 100°C) or radiation, depending on the situation. This is the result of water spill in T seconds (in liter per meter of fire front), time necessary for the fire to burn the whole biomass.
Firemen are fighting against forest fire by spilling water in jet on burning vegetation (the "red") or on green biomass with fine rain. In practice burned zones are watered at 1.50 m depth (namely a third of L) and non-burned zones on 5 to 6 meters ahead flames.
Some liters of water probably evaporate before they reach the fire, and some seep into the soil, so total amount needed to extinguish the fire can be twice as much as the one calculated. Watering is made continually during the combustion time of fine vegetation (leaves, needles, twigs) and lasts about 30 minutes.
Let us assume that a fire front has a power of 2000 kW/m. The goal is to stop propagation of a fire and to narrow it till it extinguishes.
In theory watering absorbs one third of fire power of fire, which implies concerning the "red" 8 liters per meter of fire front for watering in T=30 s. Given that 1 to 2 liters of water evaporate before reaching the fire, total amount of water to use is then 10 liters. The watering in jet enable to blow flames and reduce losses of water.
To prevent burning of green zones firemen water with fine rain. It let absorb heat produced by thermal radiation by evaporation of water. This prevents vegetation to warm up and to catch fire. The quantity of water to be dropped on these 5 meters depth of “green” is 0.048 liter water per second and per meter of fire front. Once fire front is stopped, its depth and its power progressively decrease till they become null. So it is necessary to use about 0.75 liter water in 30 seconds to prevent “green” from catching fire. Effect of air and burning gases which activate evaporation of water and seeping in soil are taken into account by doubling or tripling theoretical quantity calculated. It gives 1.5 to 2.5 liter of water.
In the end this method requires a dozen liter of water per meter of fire front, 9 to 10 liter in jet in the "red" and 2 liter in fine rain in the "green".
For the "green" zone we water with fine rain to cover the vegetation, using foaming or wetting agent if possible. Compared to watering in "red", the one in "green" is much more economical in terms of water on low power fire. But this advantage is reduced when dealing with a fire storm. Indeed emitted radiation increases with square root of power P and depth of watering with P. As a result quantity of water changes as P to the power 1.5 : when P doubles quantity of water almost triples. Besides when forest fires are severe surface areas are huge because depth of green zone to wet becomes very large.
Source : "Pourquoi les incendies de forêt sont-ils si meurtriers ?" - R.B. Chevrou