Patent Application
20120214105
1. Field of the Invention
The present invention relates generally to hazards classification of energetic materials and other explosive ordinance. More particularly, the present invention relates to a propane fueled combustion device which provides a controlled heat flux environment for hazardous classification of an ordinance system.
2. Description of the Prior Art
In the United States, all ordnance must be hazard classified. Hazards classification of an energetic material (which are used in all ordnance systems) requires a number of tests to determine the type of reaction and level of reaction violence for various potential accident scenarios in transport and storage situations. These tests include shock initiation, sympathetic detonation, and external fuel-fires which are referred to as cook-off tests. Both shock initiation and sympathetic detonation tests have small-scale analog tests that allow for alternative options to expensive full-scale testing.
To date, no small-scale test has or exists or is under development for the external fuel-fire test required for classification of explosive ordnance. A test of this type can be very expensive for rocket motors greater than 11 inches in diameter as it requires the use of three full-sized, production assets or rocket motors in their shipping and storage configuration.
For a liquid fuel/external fire test, a rocket motor is exposed to a liquid fuel fire, which extends a maximum of one meter beyond the edge of the motor. There is also a requirement that the fuel burn for 150 percent of the time required to cause a reaction. The initial cost of the rocket motor, the potential hazards associated with conducting the test, and the amount of land required for a test site are some of the difficulties in performing an external fire test on a solid rocket motor.
For large-scale rocket motors, it is highly likely that performing a full-scale fuel-fire test will be cost prohibitive. Furthermore, the physical nature of a fuel-fire is very difficult to quantify and measure. Understanding how heat flux is coupled from the fuel-fire flames to a specific device, such as a large scale rocket motor is important to experimental and computational modeling efforts in this area.
Currently for hazard classification, the Department of Defense Explosives Safety Board mandates that full-scale external fuel fire tests are performed using a shallow pool of aircraft fuel (JP-5 or JP-8 jet fuel) and a minimal amount of instrumentation to determine the air temperature at several locations. This type of full-scale fuel fire testing is difficult to perform because it requires the use of a full-scale test specimen and large specialized facilities, both of which can be extremely expensive. The full-scale test is capable of providing the necessary thermal stimulus, but it lacks sufficient instrumentation to quantify the stimulus for use in present day computational models. Technology is not currently available to provide the necessary resolution of measured heat flux level or provides a sufficient level of control for the application of constant thermal boundary conditions into a small-scale/smaller than full-scale test specimen for the purpose of observing the resulting response of the energetic material. Accordingly, there is a need for a small-scale external fuel fire test apparatus for hazard classification and also to probe the underlying physical response of energetic materials to fast cook-off in a controlled manner.
The present invention comprises a controlled heat flux combustor which provides a controlled heat flux environment for hazardous classification of an ordinance system. A five horsepower fan motor delivers air through an air duct tube. The fan delivers air at a mass flow rate of approximately one pound/second. Propane fuel is introduced through an aluminum manifold located at the end of the tube via eight fuel injectors.
An array of eight injectors provides for uniform fuel distribution of a propane fuel. The fuel is mixed with air, which provides a flammable mixture. The fuel air mixture then expands into an 18 inch combustion chamber which is approximately six feet in length. This fuel air mixture is then ignited in the combustion chamber, at which time the fuel is consumed in a reaction region within the combustion chamber, generating high temperature gas products. The combustion chamber has an inside diameter that is larger than the air duct. The change in area from the smaller air duct to the larger combustion chamber allows flame stabilization for the reaction region. The amount of fuel and air introduced into the chamber controls the gas temperature and therefore the heat flux generated in the combustion chamber.
Referring to
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The test article 34 contains energetic material for evaluation when subjected to a controlled heat flux. The heat flux is generated by two components in the combustion chamber 24. The first component of the heat flux is the convective high temperature gases Qc from the combustion products, while the second component of the heat flux is from radiation Qr generated from the high temperature wall inner surface 36 of combustion chamber 24. The wall surface 36 of the combustion chamber 24 is heated to the required temperature by the high temperature gases of the reaction region 32, or can be augmented by introducing additional heat Qa to certain areas of the combustor chamber wall 38.
The combustion device 20 comprising the present invention is intended to apply a uniform and constant heat flux level to a small-scale sample of a test article 34. The test article 34 contains energetic material so as to simulate the thermal penetration of heat flux experienced by a full-scale article in a hydrocarbon fuel fire. The combustion device 20 is controllable, tunable, and variable in its rate of heat flux application. Combustion device 20 has reusable and expendable sections and provides for a method of assessing the resulting damage due to fragmentation of the test specimen 34. Test specimen 34 may be a small scale rocket motor containing an energetic material.
Referring to
At this time, it should be noted that other liquid fuels which exhibit the same properties as liquid propane could be substituted for propane as the fuel used in the air propane flammable mixture of the present invention.
The combustion device 20 is to be used as a common diagnostic tool to assist in developing modeling codes using subscale energetic test articles, as an aide in the design and development of IM compliant systems to fast cook-off, and to be part of an alternate test protocol for external fuel-fire hazards classification test.
As shown in
The flammable mixture of propane fuel 28 and air 22 burns in the combustion chamber 24 of the combustion device 20. The result of the reaction in reaction region 32 is high-temperature gases that create a known heat flux based on the composition of the flammable mixture. The heat flux is controlled by a combination of the gas temperature and the total mass flow rate of the air 22 and propane fuel 28. While the convective component of the heat flux Qc, is primarily controlled by the gas temperature, the radiation component Qr, is interdependent on several conditions. The temperature within the surrounding combustion chamber 24 is controlled by the temperature of the contacting gases, the boundary layer at the wall surface 36, and insulation 48 on the outer-side of the chamber 24.
Along with the temperature of the chamber wall 38, the emissivity of the wall surface 36 has a large influence on the magnitude of Qr. To better control the radiation component Qr of the heat flux, surface coatings are applied to the inner surface 36 of combustor chamber wall 38 to increase the emissivity on the combustor chamber wall 38 in the area surrounding the test article 34. A further increase and control of the radiation emitted can be accomplished by augmenting the wall surface temperature and by heat flux Qa addition. This will have a large influence on the total heat flux with only a moderate increase in wall temperature because graybody radiation is a function of the fourth power of temperature.
This temperature increase can be accomplished by injecting additional propane fuel 28 near the chamber wall 38 and allowing the flame to be in close proximity to the surface 36 of chamber wall 38. The chamber wall temperature only needs to be raised at the area surrounding the test article 34 for this to be effective.
The prototype for combustion chamber 20 was constructed and operated at various temperatures and mass flow rates which are calculated to be the required conditions. The nonexpendable portion of the controlled heat flux combustor 20 is shown in
The propane fuel and air mixture expands into an 18 inch (0.46 m) diameter stainless steel combustion chamber 24. The non-expendable portion of the combustion chamber 24 is approximately 6 feet (2 meters) in length.
At this time it should that whenever the heat flux requirements are different the dimensions of combustion chamber 24 can be altered and the mass flow rate can be changed to accommodate the new heat flux requirements.
An additional 6 feet (2 meters) of combustion chamber could be added and would represent an expendable portion of the test apparatus and will act as a witness tube should fragmentation occur during the experiment.
Referring to
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Heat fluxes into devices submersed in pool fires generally range from 40 kW/m2 (kilowatts/square meter) to 400 kW/m2. Using this range of heat fluxes, calculations were performed to determine the feasibility of the controllable heat flux combustion device 20 comprising the present invention. The convective heat flux into a test article 34 was estimated using simple engineering analysis calculations. Bounding calculations were performed with various incoming gas temperatures ranging from 1000 degrees Kelvin to 1500 degrees Kelvin and velocities ranging from 7.6 meters/second to 91 meters/second. Heat fluxes of 16 kW/m2 to 170 kW/m2 were calculated over the design range criteria. These calculations confirmed the probability of the desired flux levels over a specified temperature span.
There is also a requirement for combustion device 20 to provide air with a relatively inexpensive fan and not use compressed air. The air mass flow rate needed is around 0.45 kg/s (kilograms per second) and not exceed 0.90 kg/s. This corresponded to a velocity upper limit of approximately 27 meters/second and provides a more realistic upper bound for the convective heat transfer of 65 kW/m2. The calculated heat flux for combustion device 20 at the desired mass flow of 0.45 kg/s ranged from 20 kW/m2 at 1000 degrees Kelvin air temperature to 38 kW/m2 with an air temperature of 1500 degrees Kelvin.
Realizing that radiation is a major form of energy exchange in the combustion device 20, bounding radiation heat flux calculations into the test article 34 were also performed. Stainless steel was the material used for the outer wall cylinder 38 of the combustion chamber 24 and the air duct 26. The emissivity of hot stainless steel ranges from 0.5 to 0.8 depending on the level of oxidation. The emissivity of the test article 34 may have a similar range for metal cases or be higher for composite cases. Using the same temperature range for the outer wall as the hot incoming gas, which is 1000 degrees Kelvin to 1500 degrees Kelvin, a wall emissivity of 0.65 and an article emissivity of 0.8, the radiative heat flux was calculated to be from 30 kW/m2 to 190 kW/m2. In these calculations, using the low temperatures the contribution of radiation and convection to the heat flux into the test article 34 are similar. Since the radiative heat flux is a function of the wall temperature to the fourth power, the contribution of radiation to the incoming heat flux increased much faster than that of convection. When the upper temperature range is utilized, the radiative heat flux is calculated to be 5 times that of convection.
The initial heat flux calculations are important in that these calculations show that the combustion device 20 will deliver the desired heat flux to a test article 34. The calculations also show that there is considerable flexibility in the amount of control of the heat flux into a test article 34. To control the heat flux into the test article 34 requires the user to change or vary the incoming mass flow rate and the temperature.
