The Proceedings of the Eighth International Conference on Creationism (2018)

ejections, similar to the chamber extremities which control flow into the beetle’s chamber through an inlet tube. Opening or closing these valves and the inlet / exhaust valves then controls the flow of liquid (mimicking the bombardier beetle) and results in atomized spray generation. As with the beetle and the CFD simulation work, this is due to the formation (for a few msecs) of a closed volume of liquid at high temperature in the chamber just before the exhaust valve is opened. It is important for this high-temperature volume of liquid in the chamber to be closed off to allow the required temperature rise with minimal associated vaporization. This ensures that the chamber liquid temperature is allowed to rise above its boiling point and that a portion of liquid will flash evaporate once the exhaust valve is opened. This produces liquid atomization at much lower pressures than utilized in most liquid atomization devices (these generally use pressure atomization – forcing the liquid through small holes at high pressure) and is more energy efficient. The fact that flash evaporation is used to achieve fast ejection of the water and steam, combined with appropriate control systems, means the system lends itself to a fine control of spray characteristics, particularly droplet size. This control is further improved by the decoupling of the exhaust valve from the chamber pressure. In an ideal system, there would be no vaporization of the chamber liquid during the heating phase; however, in practice, some vaporization will always occur. This is the major cause for the rise in chamber pressure seen in the bombardier beetle, which in that case triggers the opening of the (passive) pressure relief exhaust valve. Therefore in the bombardier beetle system, this limits the atomization level achieved, since the extent of flash evaporation will be roughly similar at each ejection. The liquid is heated a similar amount and exhausted at a similar time due to the correlation between chamber pressure and chamber temperature. As the exhaust valve on the experimental system is not pressure triggered, but actively controlled electronically, the extent of flash evaporation can be controlled to a much greater degree than that of the bombardier beetle. B. Results from experiments The physical scale of the first experimental system was considerably larger than that of the system found in the bombardier beetle. Whereas the bombardier beetle system has a chamber of approximately 1×10 -3 m in length, the first experimental system has a much larger chamber, 0.02 m in length. This level of scaling is not limited to the chamber of the system and extends also to other parameters, such as the diameter of the feed and outlet tubes. The process in the experimental system however significantly increases the atomization of the liquid as it exits the chamber. The experimental system is not restricted to only atomizing very small liquid volumes like those found in the bombardier beetle, but also much larger volumes with a wider range of practical uses. Some key performance parameters are also scaled with the increase in chamber size, such as the throw ratio. A bombardier beetle can throw liquid 0.2 m, using a chamber 1×10 -3 m in length. This gives a throw ratio of 200 where throw ratio is the distance of ejection divided by the chamber length. Comparatively, it was found that the chamber of 0.02 m in length on the experimental system could also achieve a throw ratio of 200, throwing a mixture of liquid and steam a distance of up to 4 m (Beheshti and McIntosh 2008, McIntosh and Beheshti 2008). Initially single blasts of spray were explored and then repeated blasts. This required a computer-controlled system for opening and closing the inlet valve, the return valve and the exhaust valve. It was found that a wide range of droplet size and temperature of the spray could be achieved depending on the control of the valve system. The minimum droplet size was 0.002 mm and the largest droplet sizes are in the region of 0.1 mm. The temperature of the spray varies from a warm 45° C at a 20 cm distance from the nozzle for large droplets, to room temperature for the smallest droplets. The frequency of ejection on this first experimental facility could be varied from 1 to 20 Hz and the velocity of ejection is typically in the range 5-30 ms -1 . Later experimental facilities have readily reached a frequency of ejection of 100Hz. One of the key features of the experimental facility is that it can produce a large variation in the characteristics of the spray produced. The type of spray produced is controlled by a number of factors including pressure and temperature in the chamber. Consequently, a range of spray characteristics can be achieved which is much wider than possible with other liquid atomization systems. Experimental work has shown that it can generate a very wide range of spray characteristics such as droplet size distribution, the ejection velocity of the spray, the mass ejection rate, and the temperature of the ejected spray. These features are described in more detail in Booth et al. (2012) and fgures 7 and 8 are reproduced here to show the different possibilities of droplet size distribution that can be achieved. In summary, atomization is achieved by heating a liquid past its boiling point, and at a constant volume. This is suddenly allowed to vaporize though the rapid opening of an exhaust valve. This causes a flash evaporation of a portion of the liquid, which generates a very large force, which then ejects vapor and liquid out through the valve. The flash evaporation is such that as the liquid is rapidly McIntosh and Lawrence ◀ Design of the bombardier beetle ▶ 2018 ICC 272 Figure 6 The experimental rig inspired by the bombardier beetle’s combustion chamber. This facility is 2 cm in length and can fire liquid up to distances of 4 m. Shown also is the Malvern laser to measure droplet sizes. The facility can eject very fine mist with droplets 2 μm in diameter as well as much larger droplets 100 μm across.