Bombardier Beetle Research
The Bombardier Beetle and its Novel Spray Mechanism with application to Fire Extinguishers
An example of a bio-inspired spray system.
The bombardier beetle (Figure 1) exhibits a predator defence mechanism, by which it generates a powerful directional spray of hot, chemically noxious droplets from its rear end, to great effect upon attackers, such as ants, Spiders, frogs or birds. A seminal paper produced by Professor Tom Eisner and his team at Cornell University proved by accurate and painstaking experiments, the method by which the bombardier beetle defended itself .
The investigations of Eisner indicated that the rear internal cavity of the beetle contained an inlet valve which brought in potentially explosive chemical reactants. Further joint work with the Leeds team led by Professor McIntosh [2, 3, 4] showed that there are in fact two valves in the beetle chamber system. Thus not only an inlet valve but also an outlet valve controls the outlet. A spray is generated by heating the liquid mixture and only allowing it to exhaust once a predefined pressure is reached. The spray itself is hot, pulsed at approximately 500 Hz, and is largely water mixed with quinones. The beetle chemically heats the mixture, which when a high enough pressure is reached due to a small amount of vapour being made in the chamber, this then causes a membrane to be raised and the exhaust valve to be opened. The mixture of liquid droplets and steam is then released through the exhaust valve and passes out through a turret which is able to direct the exhaust in any direction it wishes (even over its head as illustrated in the top photograph of Figure 1). The whole mechanism is an example of pulsed combustion as there is a series of explosions (500 per second) which come out of the exhaust tube.
More detail of the valve system operating in the beetle exhaust system is shown in Figure 2 taken from ref. 5:
Before each explosion there is a low pressure part of the cycle and water and the reactants fill the chamber (first image figure 2a). Then as each explosion takes place, the pressure of that explosion closes the inlet valve and for a short time the chamber is a sealed unit (second image figure 2b). There is then a significant pressure increase and as this pressure increases, a passive cusp exhaust valve opens at the increased pressure, and this allows the exit of the spray (third image figure 3a). This inject/close/heat/eject cycle repeats at a very fast frequency of approximately 500 Hz, which forms a distinct high pitched note due to the pulsed ejections.
Closer observation of these pulsed ejections have shown important features highly relevant to the application of this technology to inhaler applications in the pharmaceutical industry :
- The ejected spray is remarkably non-divergent. This means that the mainly aqueous mixture is able to travel relatively long distances to impact a target – some 20 cms (200x the beetle cavity length of 1 mm).
- The emergent spray has an initial velocity of up to 150m/s.
- The spray contains a distribution of droplet sizes which have Dv90 of sub-10µm – that is 90% of the droplets are down to a size of 10 micron (µm). This can be changed such that the size of the droplets are larger as required for different types of applications. For a fire extinguisher application what may be used for sprinklers would be a fine divergent spray, but for fighting a strong fire then a narrow larger droplet ejection of steam and water would be used.
- Mass ejections show that such a system used by the beetle can deliver 2-3 gms every second.
- The beetle is only able to sustain such ejections for a few minutes, before a period of rejuvenation is required.
Experimental rig built by Leeds team – the beetle device for generating sprays
- Applying this same principle of valve construction used by the beetle, a chamber has been built (figure 3) with a typical length of 2 cms (20 X the beetle typical dimension of 1mm) but using water and electrical heating (so no chemistry involved).
- The control system involves an inlet and outlet valve, both of which are actively controlled (this is unlike the beetle which has a passive valve system).
- The exhaust valve diameter can be varied and has a major controllable effect on the spray droplet size and distribution.
- The temperature and pressure of the chamber are readily controlled by electrical power and again these have significant effects on the type of spray emerging from the chamber. This work is described in the open literature – see ref.  and in particular pages 2501-2502.
The application of the beetle device to a Fire Extinguisher system
1) The beetle inspired spray system, using in most cases water and steam, can be applied to a fire extinguisher enabling the ability to fire a mixture of steam and water a number of metres from the fire fighter and to a precise location. The distance will depend on the length of the chamber and the nozzle characteristics. It is known that from a chamber of typical length 2 cms, such a device will eject at least 4 metres. For small chamber sizes the growth is linear, such that for a larger chamber of typical length 5 cms, the distance reached should be approximately 10 metres.
2) The beetle device also has the distinct advantage that one can change the droplet size and throw distance of the spray system which could be particularly important for different types of fires, ranging from application to sprinkler technology to hand held fire fighting capabilities with small water packs carried by firefighters .
3) The heating mechanism is such that a burst of energy is used each time the spray is emitted, so power is not used constantly, but in peaks of high energy intensity.
5 important reasons for using the beetle device for Fire Extinguishers
A) Research has shown (McIntosh and Beheshti 2007, 2008) that it is possible to fire a series of pulses of water and steam over 4 metres from a chamber only 2 cms long. This has clear application to a new generation of fire extinguishers where fire fighters can then use such focused spray systems to precisely and yet remotely target the seat of fires in buildings, aircraft and forest fires and many other civilian firefighting situations.
B) A heating mechanism is needed to raise the saturated pressure within the chamber. Since a small amount of water creates a great volume of steam (the ratio of volume of steam to water is approximately 1700), the application of the beetle device to a fire extinguisher is very effective. The electrical power required is within the range for heating required, such that a battery hand-held nozzle would be feasible.
C) Of particular interest is the beetle system gives dynamic control, meaning that the ejected spray characteristics can be modified over a number of spray parameters. So that a sprinkler application or ejecting steam and water directly from a nozzle held by a firefighter is possible from the same basic equipment.
D) Precision and targeting the seat of such fires is crucial. Coupled with that is the need to protect the firefighter him or herself from getting trapped and overcome by the fire or fumes. This is particularly true in forest fires where a number of fire kernels need to be tackled at one time and where individual fire fighters can be in extreme danger. Therefore the need to have the ability to be mobile and to shoot a spray of water and steam at a distance of many metres to the seat of a fire is very important. With a larger chamber of typical length 5 cms, the distance reached should be approximately 10 metres.
E). Unmanned air systems (UAS) using hyperspectral imaging techniques can be used to pin-point the source of the fire, and, in real time, inform the fire fighter of the GPS location of the fire source. If a series of GPS locations are known for the fire kernels in a forest fire, then a mobile device, equipped with GPS-based software, could then be used to assist a fire fighter in targeting the source. The beetle device with its low use of water volume lends itself for such mobile use.
- Aneshansley, D.J. & Eisner, T., Spray aiming in the bombardier beetle: photographic evidence, Proceedings of the National Academy of Sciences USA, 96, pp. 9705–9709, 1999
- Beheshti, N. and McIntosh, A.C. (2007) “A biomimetic study of the explosive discharge of the Bombardier Beetle.” Int. J. of Design & Nature, 1(1), pp.61-69.
- Beheshti, N. and McIntosh, A.C. “The bombardier beetle and its use of a pressure relief valve system to deliver a periodic pulsed spray” Bioinspiration and Biomimetics (Inst of Physics), 2, pp.57-64, 2007.
- McIntosh, A.C. and Beheshti, N, “Bio-inspiration for new pharmaceutical sprays”, Industrial Pharmacy (Euromed Communications), 17, 12-14, March 2008.
- McIntosh, A.C. and Beheshti, N, “Insect inspiration”, Physics World (Inst of Physics), 21(4), 29-31, April 2008.
- Booth, A., McIntosh, A.C., Beheshti, N., Walker, R., Larsson, L.U. and Copestake, A. Spray Technologies Inspired by Bombardier Beetle, in Bhushan, B. (Ed.), Encyclopaedia of Nanotechnology, Springer, Heidelberg, New York, London, 2012, pp. 2495-2503