Abstract:
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.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   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. 

   
     DESCRIPTION OF THE DRAWINGS 
     The invention will be more clearly understood from the following detailed description of representative embodiments thereof read in conjunction with the accompanying drawings wherein: 
       FIG. 1  is a schematic sectional view of a flammable liquid spreading over a heated surface; 
       FIG. 2  is a schematic sectional view of a flammable liquid spreading over a heated surface having micro-cavities formed in the surface according to a governing principle of the invention; 
       FIG. 3  shows a schematic sectional view of a micro-cavity of the invention in a heated surface illustrating trapped vapor generated within the micro-cavity beneath the flammable liquid; 
       FIG. 4  shows a schematic sectional view of a micro-cavity of the invention in a heated surface illustrating percolation of vapor bubbles through the flammable liquid; 
       FIG. 5  shows a schematic sectional view of a representative micro-cavity shape according to the invention; and 
       FIG. 6  shows in section another representative micro-cavity shape according to the invention. 
   

   DETAILED DESCRIPTION 
   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,  FIG. 1  is a schematic sectional view of a pool of a flammable liquid  11  spreading over a heated surface  13  of a structural element  14 . If the temperature of surface  13  exceeds the boiling point temperature of liquid  13 , a thin vapor barrier  16  may form between surface  13  and liquid  11  as a result of film boiling. The separation between surface  13  and liquid  11  has the beneficial effect of reducing direct conductive heat transfer from surface  13  to liquid  11 , which in turn limits liquid  11  evaporation at outer surface  17  and limits the mixing of sufficient vapor with air to result in ignition. However, because vapor barrier  16  is adjacent heated surface  13 , superheating of barrier  16  results while it is entrapped under liquid  11  until the vapor migrates from under liquid  11  at the leading edge  18  of the pool and there mixes with air and easily ignites at  19 . The pre-heated vapor requires a lower surface temperature (and resultant air temperature above at the point of ignition) to successfully ignite. 
   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&#39;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  FIG. 2 , shown therein is a schematic sectional view of a flammable liquid  21  spreading over a heated surface  23  having a pattern of micro-cavities  22  formed in surface  23  according to the invention. The width of each micro-cavity  22  is sized according to the surface tension and resultant contact angles of liquid  21  to minimize seepage of liquid  21  into cavities  22 . A controlling consideration in the sizing of micro-cavities  22  is the contact angle between the heated surface  23  and the liquid  21 . Within some broad ranges of cavity  22  sizes, therefore, the cavity size must be determined in consideration of the flammable liquid  21  which is anticipated to come in contact with heated surface  23 . The contact angle θ between liquid  21  and the wall surface of micro-cavity  22  is characteristic of the balance between the surface adhesion and gravitational forces acting on liquid  21 , and may be different for each liquid  21 , surface  23  material, and temperature, all of which results in the leading edge  24  of the advancing liquid  21  film taking a form as suggested in  FIG. 2  such that the mouth  25  of micro-cavity  22  is sealed off before micro-cavity  22  is entirely filled with liquid. The effective contact angle θ may be larger than normal due to the distorted shape (bulge) of the advancing leading edge  24  as a result of the momentum of the moving liquid  21  film. For example, the contact angle θ of n-heptane (a representative fuel) in contact with polytetrafluoroethylene (TEFLON) is about 22°, and is somewhat smaller for metallic surfaces. The angle θ for a given combination of liquid  21  and surface  23  must therefore be determined or approximated individually. 
   Referring now to  FIG. 3 , shown therein is a schematic sectional view of a micro-cavity  22  with liquid  21  covering thereover with trapped vapor  26  generated within micro-cavity  22  under the covering layer of liquid  21 . After liquid  21  initially seals off micro-cavity  22 , liquid  21  will stabilize to a level L/2 roughly one half of the initial seepage depth L of the advancing leading edge  24  (FIG.  2 ), with the surface tension/wetting force and hydrostatic pressure of liquid  21  above balanced by the pressure of trapped vapor  26  within cavity  22  (FIG.  3 ). For a cylindrical cavity, geometrical considerations show that the cavity  22  depth should be no less than about (T/2)tan θ, where T is the diameter of cavity  22 . For example, for a contact angle θ of 30°, a 0.5 mm diameter cavity  22  would require minimum depth of about 0.433 mm, and a 0.25 mm diameter cavity would require a cavity  22  depth of about 0.217 mm. As a general proposition, acceptable cavity  22  sizes in contemplation of the invention fall in the range of up to a few millimeters in width or diameter and depth for most flammable liquids  21 , the specific sizing of the cavity determined according to the viscosity of the contacting liquid. 
   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  FIG. 4 , shown therein is a schematic sectional view of a micro-cavity  42  of the invention in a heated surface  43  illustrating another feature of the invention in the percolation of vapor  46  bubbles  47  through flammable liquid  41 . Vapor  46  entrapped within cavity  42  is heated by the hot cavity  42  walls, expands, pushes liquid  41  out of cavity  42  and forms a bubble at mouth  45  of cavity  42 . When bubble  47  expands sufficiently to break free of cavity mouth  45 , it percolates upward through liquid  41 . The liquid  41  pool is generally sub-cooled (below boiling), and therefore cools bubbles  47  and harmlessly dissipates the heat within liquid  41 . The repetitive formation and release of bubbles  47  is similar to nucleate boiling, and promotes the release of vapor  46  formed at the solid surface/liquid interface rather than the formation of a vapor film that has been identified as a means of sustained superheating of vapor which migrates to the liquid  41  pool edge and ignites. 
   Referring now to  FIG. 5 , it is seen that the invention also contemplates micro-cavity shapes other than right circular cylinders or grooves oriented normal to the surface. Any shape for the micro-cavities would be acceptable as would occur to the skilled artisan practicing the invention so long as the surface tension of the contacting liquid substantially prevents seepage into the cavity. For example, cavity  52  may be machined or otherwise impressed onto surface  53  with the centerline of cavity  52  inclined at any selected angle to surface  53  in order to further minimize potential wetting of the cavity  52  walls by liquid  51 , and the angle θ′ illustrated in  FIG. 5  may correspond to the contact angle of liquid  51 . Rectangular slots can also be added of fixed width and length, and if the expected point of initial liquid contact with the heated surface is known, a series of annular ring trenches can be impressed in the surface around that point to entrap vapor within each ring. 
   Referring now to  FIG. 6 , a low cost method to manufacture the desired cavity features into the surface  63  of a structure  60  to be protected is suggested wherein a layer  64  of porous medium, such as thermally sprayed metals including steel, iron or others, is applied to surface  63  by powder metallurgy and sintering techniques well known in the applicable art, which may be precisely controlled to produce a distribution of pores of selectable diameter, within a tightly defined range of variability. Once applied, the outer surface of layer  64  may be machined or polished to expose open pores in the form of cavities  62 . Only a small portion of the cavity  62  outer surface is exposed, and such re-entrant cavities promote minimal seepage of liquid because of the extreme angle of the inner pore walls relative to the contact angle of liquid  61 . Because cavities  62  are needed only on surface  63  exterior, a very thin porous section can be applied in a one-step process to the outer surface  63  of the structure to be protected, or the entire structure  60  may be fabricated in this manner. 
   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/m 3  versus 0.46 kg/m 3  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.