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Accoustic Cavitation As A Souce Of Ignition And Its Possible Relevance To TWA Flight 800

by: JOHN D. UNSWORTH, MICHAEL SHERAR, PH.D. and BRIAN LATTO, PH.D.

What follows are two passages from Patent Application: U.S. 09/251,958 that introduce the description of the invention:

Acoustic cavitation at or near the surface of liquid fuel that is in contact with an air and fuel vapor mixture could cause ignition of the air fuel vapor. This could occur in the absence of an electrical discharge or other conventional ignition source.

Acoustic cavitation is the production of small bubbles in a liquid exposed to a sound field. These bubbles can oscillate in size in a stable manner (termed stable cavitation) or they can grow rapidly followed by a violent collapse where temperatures of thousands of degrees Kelvin can be produced (termed collapse cavitation). If such a cavitation collapse event occurred at or near the surface of the fuel, the energy produced could be sufficient to ignite the vapor immediately above the surface. The production of collapse cavitation in liquids has been well documented in the scientific literature.

It is possible, but unlikely that the heat produced by a collapsing bubble of fuel vapor that contains no oxygen, could itself ignite the fuel vapor and air above the liquid fuel stored in the tank. However, although the temperature of a collapsing bubble can reach thousands of degrees Kelvin, the heat is concentrated in a very tiny spot for only a brief instant of time. The energy of the collapsing vapor bubble, containing no oxygen, can be no greater than the sound energy that created it, and this amount of energy would likely be quenched by the relatively cool fuel that surrounds the collapsing bubble. For these reasons cavitation has not been considered to be an ignition source.

What has been overlooked is that if the collapsing bubble itself contains the fuel vapor and air mixture, that is it contains oxygen, and it ignites as a result of the heat generated by the collapsing bubble, much more energy is available to ignite the fuel vapor and air mixture above the liquid fuel. This additional energy results from the combustion of the mixture within the collapsing bubble.

A nucleating bubble containing both fuel vapor and air can occur in a fuel tank in which the fuel is sloshing, mixing ambient air (containing oxygen) with the fuel vapor. When the nucleating bubble is subjected to sound having certain characteristics, the bubble will first enlarge and then collapse. As the bubble collapses, the temperature of the vapor in the bubble increases and ignition of the fuel vapor and air mixture may occur. This process is the same as occurs in diesel engines which burn kerosene fuels similar to those burned in jet engines. Diesel engines do not rely on a spark igniting the fuel, but instead rely on the air in the cylinder being heated sufficiently, by the piston compressing the air, to in turn ignite the diesel fuel. The pressure at which combustion typically occurs in a diesel engine is instructive. The pressure just prior to ignition is from 275 to 400 psi (1895 to 2755 kPa), far less that the maximum pressures created by a cavitation event. Like a diesel ignition event, the collapsing bubble of fuel vapor and air (containing oxygen) produces sufficient heat to ignite the fuel vapor and air mixture within the bubble. How this in turn causes the ignition of the main fuel vapor and air chamber above the fuel depends on the varied circumstances that exist within the sloshing fuel and air mixture. Certainly a bubble of combusting fuel and air near the surface of the fuel could have sufficient energy for ignition of the vapor chamber above the fuel. The existence of foam on the surface of the fuel, caused by sloshing, would also promote the spread of combustion into the main fuel vapor chamber on top of the liquid fuel.

In order to calculate the minimum bubble size that upon cavitation could ignite the fuel tank we assume that the energy E = 2 mJ is required. From this assumption, the mass of fuel required (mf) is given by:

where HHV is higher heating value of the fuel. For hydrocarbons of weight similar to kerosene constituents (we assume C8H10), HHV = 44702 KJ/kg. Therefore the mass of fuel required is 4.47 x 10-11 kg. The radius, r, of a bubble that would contain this amount of fuel is determined by the fuel/air mixture in the bubble and the atmospheric conditions.

where r is the bubble radius, r air is the density of air, r f is the density of the fuel and x is the fuel/air ratio. Rearranging gives:

The density of air (r air) is determined by the pressure which was 50KPA at 14,000 feet (or approximately _ atmosphere). Assuming the air and fuel in the tank were at 55 C, this gives the density of air = 0.531 kg/m3. Given that the density of kerosene (r f) is approximately 850kg/m3 and assuming a fuel air/fuel ratio (x) = 15 then this gives an r = 0.67 mm.

The likelihood of collapse cavitation occurring is dependent on a number of factors. These are the ambient pressure, the surface tension of the liquid and the pre-existence of small bubbles in the liquid (termed bubble nuclei) and the amplitude of the sound field in the liquid. If small bubbles exist in the liquid, the threshold sound field pressure amplitude, PT required to produce collapse cavitation is given by the Blake threshold (Neppiras, E.A., Acoustic Cavitation And Cyclic Processes, Ultrasonics 18, 201-209 (1980)).

Where Ro is the bubble radius, s is the surface tension of the liquid, and Po is the ambient pressure. For the case of low sound frequencies and relatively large bubble sizes in the range calculated above this equation reduces to:

In general, the most favorable conditions for collapse cavitation to occur (i.e. when the Blake threshold is low) are when the ambient pressure is low and the surface tension is low. These conditions could occur in the fuel tanks of aircraft.

An example of such conditions would exist in an airplane that has fuel sloshing around in a near empty tank, creating small bubbles from the agitation. Since, typically, the fuel tanks are not pressurized, the pressure in the tanks would be low at elevated altitudes. For example, at an altitude of 14,000 feet the ambient pressure is approximately 0.5 atmospheres. In addition, the surface tension of the fuel (kerosene) is naturally low compared to other liquids such as water. At room temperature, the surface tension of kerosene is 25 dynes/cm or 0.025 Nm-1. Also, the surface tension would be artificially low if the fuel temperature was warm due, for example to its proximity to heat producing equipment such as an air conditioning unit (as temperature increases, surface tension decreases). For this example, if we assume an ambient pressure of 0.5 atmospheres and the existence of bubbles with a radius of 0.67 mm, the Blake threshold pressure is calculated to be 0.5 atmospheres. That is, the threshold pressure amplitude is equal to atmospheric pressure and represents the pressure required to put the fuel into tension. This is equivalent to a sound intensity in the fuel (assuming a velocity of sound in the fuel of 1500 ms-1) of 0.1 W cm-2. It is possible that vibration caused by an electrical motor, transformer or turning shaft in an engine could cause a sound field of this intensity on the surface of the fuel that would be sufficient to cause collapse cavitation, which in turn could ignite the fuel/air boundary.

The danger that acoustic cavitation could cause an explosion in a fuel tank has not until now been appreciated. Measures to prevent such an occurrence is the subject matter of this patent.

 

Also:

The relative speed of the engines may be important and engine synchronizers that bring the operating RPM’s of the engines closely together may cause greater cavitation when the RPM’s are identical or approximately the same, but the phase of the frequency generated is in an out of phase, causing constructive and destructive interference. As the synchronizers attempt to bring the engines into synchrony, the RPM’s of the engines get faster and slower than each other as the identical RPM’s is located. While this is occurring the phase of sound produced goes into and out of phase, which may be the condition with the greatest risk of causing cavitation.

 

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