Self sustained detonation apparatus

Details Self sustained detonation apparatus Self sustained detonation apparatus

1999-01-12

2001-04-10

Johnson, Stephen M. (Department: 3641)

Ordnance

Pneumatic

Explosive charge

Reexamination Certificate

active

06212988

ABSTRACT:

OBJECT OF THE INVENTION
The present invention generally relates to the field of gas detonation coating technology and, more particularly is concerned with increasing the detonation rate of a gas detonation coating apparatus through self sustained detonation.
A self sustained detonation apparatus, like the one described in the present invention, is also related to the “Pulse Combustion Devices”. These have been developed mainly for propulsion applications (from the early “Pulse Jets”, like the German V1 “Buzz Bomb” used in World War II, to the more recent “Pulsed Detonation Engines”, PDE's) but have also been found to be valuable for applications such as drying, smelting, water heating and slurry atomization. This invention is concerned with the development of a particular “Pulse Detonation Device” to be used, specially but not exclusively, as a Detonation Coating Apparatus.
BACKGROUND OF THE INVENTION
Coatings commonly protect substrates from the effects of exposure to severe environmental conditions such as heat, wear and corrosion. A significant factor in the coating's protection ability relates to the manner in which the coating is applied to the substrate. In many industrial applications, coatings are applied via thermal spraying techniques. Two (2) types of thermal spraying apparatus include HVOF (High Velocity Oxygen Fuel) guns and detonation guns.
In a HVOF gun, a continuous high temperature combustion creates a supersonic high energy flow stream. A coating powder interjected into the continuous high energy flow stream, typically within the barrel of the HVOF gun, forms a coating when applied to a substrate. In contrast, the detonation gun, which operates in a pulsed manner, uses kinetic and thermal energy from the detonation of combustible gases to deposit powdered coating materials onto substrates in a pulsed manner. A combustion chamber receives a certain amount of fuel and oxidant gas. A spark plug ignites the combustible gas mixture to initiate combustion which transforms into detonation. The shock wave formed by this detonation travels at a supersonic speed from the combustion chamber into the barrel where a suitable coating powder is typically injected. The shock wave and further expanding detonation products propel the coating powder out of the barrel and deposit it onto a substrate, thereby forming a coating layer. This process repeats until the substrate obtains a sufficient coating thickness. In some detonation spray systems, between successive ignitions, an inert gas, such as nitrogen, is fed into the combustion chamber to halt combustion and prevent backfire into the fuel and oxygen supply, and to purge the combustion chamber and barrel of combustion detonation products.
The mechanics of detonation are key to the operation of the detonation gun. Detonation produces shock waves that travel at supersonic velocities, as high as 4,000 meters per second (m/s), and elevated temperatures, as high as 3,000 degrees Celsius. Detonation within the detonation gun is controlled by the type and amount of fuel (i.e., natural gas, propane, acetylene, butane, etc.), the fuel and oxygen mixture ratio, the initial pressure of the gases in the combustion chamber, and the geometry of the combustion chamber. Cycled ignition of a portion of the combustible mixture creates combustion which increases the entropy within the combustion chamber and, in turn, propagates ignition of the combustible mixture throughout the combustion chamber. With the correct combination of parameters which result in sufficient local pressure and temperature within a given volume, accumulated combustion energy provides transition to detonation.
At a fixed moment in time the detonation wave front is made up of a system of individual detonation cells. The behavior of detonation at the cell level is an important attribute in the control and operation of a typical detonation gun. The detonation cell is a multidimensional structure, which is formed under influence of both the shock wave front and transverse shock waves. The propagation of the shock wave front, created by detonation, is perpendicular to the inner circumference of the combustion chamber and it is directed from the closed end of the combustion chamber to the open end of the combustion chamber. Transverse shock waves also form at the inner circumference of the combustion chamber and move toward and out the central line of the combustion chamber. Under the current description, a detonation wave constitutes the final case of the multidimensional structure of the detonation front that includes a number of traverse shock waves.
The frontal surface of a detonation cell has a convex shape. Behind the frontal surface is a reaction zone where the chemical reactions take place. At the edge of the cell, transverse shock waves form at substantially right angles to the frontal surface of the detonation cell. The transverse waves have acoustic tails that extend from the aft edges of the transverse waves and define the aft edge of the detonation cell. The transverse waves move from cell to cell and reflect off of each other and off of any limiting structure such as the combustion chamber wall. Once detonation has been initiated, the reaction continues in a fairly stable fashion if subsequent detonation cycles are initiated and maintained under similar conditions as the previous detonations.
The shock wave moves from the closed end of the combustion chamber toward the open end of the combustion chamber and into the barrel. It is of particular importance that the combustion chamber be of sufficient length and sufficient diameter to complete the transition from combustion to detonation before entering the barrel, otherwise, the accumulated energy may dissipate within the barrel. It is also important in the operation of a detonation gun to produce a shock wave and direct it to the barrel as efficiently as possible so that a large amount of the kinetic and thermal energy of the gaseous detonation products goes directly to carrying the powder out of the barrel and onto the substrate. However, reflecting transverse waves colliding with other wave structures can collapse, thus diminishing both the speed of the detonation wave and the transfer of detonation energy as it travels through the combustion chamber. These collisions reduce the amount of the energy available to be transferred to the coating powder which decreases the adherence characteristics between the coating and the substrate and lowers the density of the coating itself.
The size of the detonation cell is another important attribute in the control and operation of a detonation gun. Cell size is a function of the molecular nature of the fuel, the initial pressure within the combustion chamber and the fuel/oxygen ratio. The particular cell size for certain conditions can be determined experimentally. The width of a cell, Sc, is measured along the wave front between successive transverse waves. The length of a cell, Lc, is the perpendicular distance from a line tangent to the wave front measured to the intersection point of the acoustic tails from adjacent transverse waves. The typical ratio of cell width, Sc, to cell length is Sc=0.6Lc for the detonable gases under consideration. The physical parameters of a particular detonation gun, such as the geometry and operating pressures, are determined by the cell size of a particular fuel and oxygen mixture.
In a typical detonation gun the components of the detonable mixture are fed into the combustion chamber and, the coating powder is fed directly into the barrel by inert gases ahead of the detonation wave. A certain gas content system and different gases supplied from a continuous source through a valve arrangement of the gun. For example, the operation of the powder valve is coordinated with the firing of the spark plug so that the powder and carrying gases are in position along the barrel to be properly effected by the detonation wave. Typically the gas control valves are opened by mechanical means such as a cam and tappets or a solenoid which

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