Mitigating ignition of fluids by hot surfaces
A platform, housing, conduit, exhaust duct or other structural element that encloses or supports a hot operating engine or other machinery is described wherein a pattern of micro-cavities is defined on the outer surface of the structure for mitigating ignition of a flammable liquid that comes into contact with the structure, the micro-cavities being sized to minimize seepage into the cavities of the liquid because of its surface tension, thereby preventing wetting of the interior of the cavities by the liquid.
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This application claims priority of the filing date of Provisional Application Ser. No. 60/220,226 filed Jul. 24, 2000, the entire contents of which are incorporated by reference herein.RIGHTS OF THE GOVERNMENT
The invention described herein may be manufactured and used by or for the Government of the United States for all governmental purposes without the payment of any royalty.BACKGROUND OF THE INVENTION
The present invention relates generally to systems and methods for suppressing machinery fires, and more particularly to an economical, lightweight and reliable structure for mitigating the ignition of a flammable fluid leaked from the machinery onto a hot surface.
Powdered machinery may operate at very high exterior temperatures as a result of internal combustion or electrical power, grinding or machinery operations, friction or other causes that characterize the operation. In particular, surfaces near the combustion region, exhaust manifolds, or bleed air/steam ducts of an operating engine can reach extremely high temperatures. Flammable fluids such as fuel, oil or hydraulic fluid in use near such surfaces leaking onto the hot surfaces and igniting has been documented as a frequent cause of fires near hot operating machinery, especially in automobiles. Aboard aircraft, a common cause of engine fires is the leakage of such fluids in the engine nacelle and subsequent ignition of the fluid by the hot engine core or uninsulated bleed air ducts. On-board fire extinguisher systems may be rendered ineffective if the fire is re-ignited by the hot surface after the extinguishant is depleted.
The invention solves or substantially reduces in critical importance problems in the prior art by providing a platform, housing, conduit, exhaust duct or other structural element that encloses or supports an engine (or other hot operating machinery) and which are heated in the course of engine operation, for mitigating ignition of flammable liquids that come into contact with such heated structure. A pattern of micro-cavities is defined on the outer surface of the structure and sized to minimize flammable liquid seepage into the cavities because of surface tension of the liquid, thereby preventing wetting of the interior of the cavities by the liquid. A gridwork of the cavities on the surface of the structure may provide 50% or more reduction of direct surface to liquid contact when the liquid spreads across the surface, which minimizes heat transfer to an vaporization and ignition of the liquid. The cavities also promote formation of nucleate bubbles at the onset of boiling that percolate harmlessly through the liquid, rather than form a superheated vapor film beneath the liquid that could seep from under the liquid pool, mix with air and ignite. The cavity pattern may be formed in the structure surface by machining, stamping, rolling, casting or other conventional process. The invention allows substantially hotter operating surface temperatures for the engine, or delays ignition of flammable liquids contacting the structure, and thereby allows a wider range of operating temperatures for the engine safe from the risk of fire. The invention adds no weight to the machinery, is highly reliable and adds no operating cost after initial fabrication. The invention may be conveniently incorporated into bleed air ducts and engine surfaces of aircraft engines and auxiliary power units, military ground vehicle and ship engine or other machinery compartments, commercial vehicles, marine vessels, ground support and stationary power equipment and other industrial machinery applications where liquid-fueled, oiled or hydraulically controlled equipment is operated near hot components of operating machinery.
It is therefore a principal object of the invention to provide structure and method for suppressing machinery fires.
It is another object of the invention to provide a novel structure for a platform, housing, conduit or other structural form for a hot operating engine, machinery or other hot component.
It is another object of the invention to provide a novel structure for a platform, housing, conduit or other structural form that enclose or support a hot operating engine, machinery or hot component and which mitigate the ignition of flammable liquids contacting the structure.
It is a further object of the invention to provide an inexpensive, maintenance free system for suppressing fires near hot operating machinery.
It is a further object of the invention to provide a means of preventing fires near hot structures without adding additional weight to the structure.
It is a further object of the invention to provide a means to mitigate hot surface-induced ignition of fluids without reducing the ability of the hot structure to expel excess heat under normal operating conditions.
These and other objects of the invention will become apparent as a detailed description of representative embodiments proceeds.SUMMARY OF THE INVENTION
In accordance with the foregoing principles and objects of the invention, a platform, housing, conduit, exhaust duct or other structural element that encloses or supports a hot operating engine or other machinery is described wherein a pattern of micro-cavities is defined on the outer surface of the structure for mitigating ignition of a flammable liquid that comes into contact with the structure, the micro-cavities being sized to minimize seepage into the cavities of the liquid because of its surface tension, thereby preventing wetting of the interior of the cavities by the liquid.
The invention will be more clearly understood from the following detailed description of representative embodiments thereof read in conjunction with the accompanying drawings wherein:
Theoretical considerations and underlying principles of operation of the invention may be found by reference to “Analysis of the Mechanisms of Pool Boiling and Ignition on Heated Surfaces and Proposed Mitigation Techniques,” J. Michael Bennett, UDR-TR-98-00159, (Aug. 1, 2000), the entire teachings of which are incorporated by reference herein.
Referring now to the drawings,
In accordance with a governing principle of the invention, reduced conduction heat transfer from the heated surface to the liquid due to techniques to minimize direct surface-to-liquid contact area mitigates vaporization of a sufficient vapor concentration at the liquid's outer surface for ignition, while preventing formation of a vapor film barrier that permits the superheating of the vapor prior to mixing with air.
Referring now to
Referring now to
Micro-cavities 22 may be distributed over any selected portion of heated surface 23, and may be impressed onto surface 23 by any suitable production process known in the applicable art, including forging, casting, rolling or automated machining process such as laser or hot electrode milling, the specific process selected for any particular application not considered limiting of the invention.
A principal feature of the invention is the minimization of heat transfer from hot surface 23 to flammable liquid 21 as a consequence of less than total liquid 21 to surface 23 contact resulting for a portion of the liquid 21 film being suspended over cavities 22, and heat transfer occurring only from vapor 26 within cavities 22, vapor 26 having substantially smaller heat conductivity than hot surface 23 of the structure. If a substantial portion of the surface 23 area contains cavities 22, a significant decrease in heat transfer from surface 23 to liquid 21 results, and a hotter surface 23 would be required to cause ignition, and a wider range of safe operating conditions for the structure results. For cavities 22 of very small diameter in a densely packed arrangement, a minimal quantity of liquid 21 will contact and be heated by the cavity walls.
Referring now to
Referring now to
Referring now to
Surface coatings can also be added in these processes to inhibit wicking and wetting of the pore or cavity walls. These coatings (such as TEFLON) effectively increase the contact angle between the liquid and the coating material (as opposed to the uncoated surface), which in turn reduces the degree of liquid seepage and heating within the cavities.
Coatings may also be applied to fill the cavities with substances well known to those skilled in the art that have lower thermal conductivities than air to further reduce heat transfer to the liquid suspended over the cavities. These substances also inhibit any seepage of liquid and the associated additional heat transfer, but may inhibit the desired heat expelling capability of the structure (if such is important) during normal operation. The substances could be spread over the surface and allowed to seep inside (possibly by the application of pressure), then wiped off the surface exterior to allow retention of the coating only within the cavity interiors. This process could be performed in an automated process by those skilled in the appropriate art. An example of such materials are the aerogel class of materials such as disclosed in U.S. Pat. No. 6,068,882 by Ryu, the entire teachings of which are incorporated by reference herein. Aerogels are an extremely porous and light form of glass (silica) formed in a special process to result in internal pores of nanometer scale. These materials have roughly a third of the conductivity of air (0.015 w/mK versus 0.055 w/mK for air at 750° K), and less than 10 times the density of air (3.0 kg/m3 versus 0.46 kg/m3 for air at 750° C., but roughly three times as much at 300° K), such weight addition being negligible.
The entire teachings of all references cited herein are hereby incorporated by reference.
The invention therefore provides a structure for housing, enclosing or supporting hot operating machinery that mitigates the ignition of flammable fluids coming in contact with the machinery. It is understood that modifications to the invention may be made as might occur to one with skill in the field of the invention within the scope of the appended claims. All embodiments contemplated hereunder that achieve the objects of the invention have therefore not been shown in complete detail. Other embodiments may be developed without departing from the spirit of the invention or from the scope of the appended claims.
1. A structural element for enclosing or supporting hot operating machinery, comprising:
- (a) a structural member for enclosing or supporting a hot operating machinery; and
- (b) means defining a plurality of micro-cavities of preselected width and or preselected depth less than the thickness of said structural member on the outer surface of said structural member for mitigating ignition of a flammable liquid that comes into contact with said structural member, said micro-cavities being about one millimeter in width to minimize seepage of the liquid into said micro-cavities because of the surface tension of the liquid.
2. The structural element of claim 1 wherein said micro-cavities are in the form of holes of selected shape defined in the surface of said structural member.
3. The structural element of claim 1 wherein said micro-cavities are in the form of grooves defined in the surface of said structural member.
4. The structural element of claim 2 wherein the centerline of said holes is non-perpendicular to said surface of said structural member.
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- J. Michael Bennett “Analysis of the Mechanisms of Pool Boiling and Ignition on Heated Surfaces and Proposed Mitigation Techniques” Aug. 1, 2000.
Filed: Sep 18, 2000
Date of Patent: Feb 7, 2006
Assignee: The United States of America as represented by the Secretary of the Air Force (Washington, DC)
Inventor: Joseph M. Bennett (Huber Heights, OH)
Primary Examiner: Aileen Felton
Attorney: Bobby D. Scearce
Application Number: 09/664,976