Background of the Invention Many existing fire-suppression systems use fluorocarbons (e. g., bromotrifluoromethane, dibromotetrafluoroethane, bromochlorodifluoromethane, etc.), such as are commonly sold under the trademark Halon. The continued use of fluorocarbons has, however, been discouraged, restricted or banned on a worldwide basis due to environmental concerns (i. e. , depletion of the ozone layer). Consequently, a replacement for fluorocarbon-based fire-suppressions systems is being sought. The United States Federal Aviation Administration, for example, is testing alternatives to fluorocarbons in an effort to certify non-toxic, environmentally-friendly, fire- suppression systems for use on aircraft.

U. S. Pat. No. 5,495, 893 discloses an apparatus and method to control deflagration of gases. According to the patent, the apparatus includes a dispersing means for dispersing a stream of liquid droplets, a sensing means for detecting a condition that is conducive to combustion/deflagration, and an actuating means that actuates the dispersing means when triggered by the sensing means. The patent states that the dispersing means can be a liquid atomizing device. The liquid, which is preferably water, is supplied to the atomizing device at a pressure in the range of about 50 to 150 psi.

A second fire-suppression apparatus and method for an enclosed space is disclosed in U. S.

Pat. No. 6,003, 608. According to this patent, a non-combustible gas is introduced into an enclosed space while air is expelled, thereby smothering the fire. The patent also discloses that a fire- extinguishing dry chemical is introduced to the enclosed space.

The systems disclosed in these patents have a variety of shortcomings. Among other drawbacks, the system disclosed in U. S. Pat. No. 5,495, 893 uses an atomizing device that requires the use of relatively high-pressure liquid. And the system disclosed in U. S. Pat. No. 6,003, 608 lacks a capability for long-duration protection against re-flash fires. Such shortcomings are of particular concern when these systems are to be deployed in aircraft.

Summary of the Invention The present invention provides an apparatus and method that avoid some of the drawbacks of the prior art.

In some variations, a fire suppression apparatus in accordance with the illustrative embodiment of the present invention includes a supply of gas, a supply of liquid, a control arrangement and a pneumoacoustic atomizer. The gas and liquid are supplied to the pneumoacoustic atomizer under the control of the control arrangement. The pneumoacoustic atomizer generates a mist of liquid that is carried by the gas.

The gas/mist suppresses fire by two mechanisms: depriving the fire of oxygen by flooding the area with an inert gas (e. g., nitrogen, etc.) ; and cooling the fire through the evaporation of a liquid (e. g., water, etc.) that is suspended in the gas.

The supply of gas advantageously includes two sources of gas: bottled and generated.

Preferably, two or more bottles or tanks of gas are used. With such an arrangement, the pneumoacoustic atomizer can be fed gas from one of the tanks until it is depleted. The depleted tank is then taken off-line and the atomizer is fed by a second tank. Meanwhile, the gas generator generates gas to re-fill the depleted tank.

The control arrangement includes sensors for detecting a fire condition, and a controller that controls the flow of gas and liquid into the pneumoacoustic atomizer when fire is detected.

The control arrangement advantageously supplies water to the pneumoacoustic atomizer at two or more different pressures during operation. Changing pressure in this fashion can increase fire suppression or conserve fire-suppressing liquid, as appropriate. In some variations, the fire- suppressing apparatus includes multiple, independently-controllable pneumoacoustic atomizers for protecting separate areas of a plane, vessel, etc., and for conserving fire-suppressing liquid and gas.

In some embodiments, the pneumoacoustic atomizers used with the fire-suppressing apparatus operate at very low liquid and gas pressures (i. e., liquid: 2 to 10 psi, gas: 40-60 psi).

Low pressure operation is particularly advantageous for aviation applications.

Brief Description of the Drawings FIG. 1 depicts a simplified block diagram of a fire suppression apparatus in accordance with the illustrative embodiment of the present invention.

FIG. 2 depicts further detail of the fire suppression apparatus of FIG. 1 FIG. 3 depicts a first variation of a pneumoacoustic nozzle.

FIG. 4 depicts a second variation of a pneumoacoustic nozzle.

Detailed Description of the Invention FIG. 1 depicts a simplified block diagram of fire-suppression apparatus 100 in accordance with the illustrative embodiment of the present invention. Fire-suppression apparatus 100 is particularly well suited for any of a variety of applications that require non-toxic, long-term, remote fire-suppression capability. For example, fire-suppression apparatus 100 can be used in aircraft and spacecraft, buildings (e. g., warehouses, manufacturing and storage facilities, hospitals, etc.), off-shore facilities (e. g., oil platforms, etc.) and ships.

As depicted in FIG. 1, fire-suppression apparatus 100 includes gas supply 102, liquid supply 104, control arrangement 106, and one or more pneumoacoustic atomizers 108, functionally interrelated as shown. As described in more detail later in this specification, when a sensor that is associated with control arrangement 106 senses fire, the control arrangement causes an inert, fire-suppressing gas from gas supply 102 and a fire-suppressing liquid from liquid supply 104 to flow to pneumoacoustic atomizers 108. Charged with fire-suppressing liquid and gas, pneumoacoustic atomizers 108 produces a mist of liquid droplets suspended in gas.

With regard to fire-suppressing gas, any gas that is substantially inert to the liquid (from liquid supply 104) can suitably be used; however, the gas is preferably not harmful to humans, at least at moderate concentrations. Suitable gases include, without limitation, nitrogen, carbon dioxide, helium, argon and mixtures thereof. Nitrogen is preferably used.

With regard to fire-suppressing liquid, the liquid should have a heat of vaporization that is sufficient to absorb the heat of a fire. The liquid should have a heat of vaporization that is at least about 500 cal/g. Furthermore, the liquid should have a boiling point that is sufficiently high so that it remains in the liquid phase until vaporized by the heat absorbed from the fire. Based on the foregoing considerations, water, which is cheap, readily available, non-toxic and environmentally friendly, is advantageously used. The liquid can include additives, for any of a number of purposes, as is known to those skilled in the art.

FIG. 2 depicts further detail of illustrative fire-suppression apparatus 100. As depicted in FIG. 2, gas supply 102 advantageously includes two sources of gas-bottled, in tanks 218 (individually denoted as 218A and 218B) and generated, in generator 214. Tanks 218 serve as accumulators to provide an immediate supply of fire-suppressing gas with an adequate rate of flow. Generator 214 generates gas to refill tanks 218 as they empty.

In the variation depicted in FIG. 2, gas supply 102 includes two tanks 218A and 218B.

When two (or more) such tanks are used, one tank is used at a time to feed pneumoacoustic atomizers 108. As the tank feeding pneumoacoustic atomizer 108 depletes, it is switched out in favor of a full tank. Once switched out, the depleted tank is advantageously refilled by generator 214.

In some variations, gas supply 102 includes only one source of gas; however, for long- term delivery of the fire-suppressing mist, the two sources of gas, as described above, are advantageously used.

Tanks 218 can be of any design, with preference given to light-weight designs for use in aircraft or spacecraft. The capacity of tanks 218 is determined by the requirements of a particular application. In particular, tank capacity will vary as a function of the volume of the space being protected and as a function of the period of time (typically specified) that the system delivers fire- suppressing mist.

In the variation of gas supply 102 depicted in FIG. 2, generator 214 is connected in parallel to tanks 218 by three-way control valves 220 (individually denoted as 220A and 220B).

Three-way control valves 220 are operable to (1) feed gas from tanks 218 to pneumoacoustic atomizers 108 and (2) admit gas from generator 214 to tanks 218.

Gas (e. g., nitrogen, etc.) generator 214 can be any such device as is commercially available, with a particular selection taking into consideration the weight, power requirements and volumetric flow capability of the unit for a particular application. The generator must be capable of generating gas that has a purity level that is sufficient to suppress combustion. In one variation of gas supply 102, generator 214 is a nitrogen generator, such as membrane nitrogen generator compressed air pretreatment skid with hydrocarbon removal system and 2200 psig pump (part no.

75700-1-484), commercially available from Whatman Inc. of Tewksbury, Massachusetts. In order to provide generator 214 with air at a sufficiently-high pressure, the inlet of generator 214 is advantageously connected, through control valve 212, to compressed air bleed 210 from turbine engine 208, such as is used for propulsion of the aircraft in which fire-suppression system 100 is situated.

It might be necessary to increase the output pressure of generator 214. To that end, pump 216 is connected downstream of generator 214. Pump 216 can be, for example, a Haskel pump that is powered by compressed air. The compressed air can be bled, for example, from turbine engine 208.

Regulator/pressure control valve 222 reduces gas pressure to a pressure that is in the range of about 40 to about 60 psig. For example, in one embodiment, the nitrogen is maintained at 56 psig. Control valve 224 controls the flow of gas to pneumoacoustic atomizers 108.

As depicted in FIG. 1, fire-suppression apparatus 100 also includes liquid supply 104.

Further detail of a variation of liquid supply 104 is depicted in FIG. 2. For the variation depicted in FIG. 2, liquid supply 104 includes tank 226 for storing a volume of liquid (e. g., water, etc.), pressure control valve 228 and supply lines. Tank 226 is advantageously a dedicated supply of liquid for fire suppression apparatus 100. The size of tank 226 is determined by the design requirements of a particular installation, in known fashion. The liquid in tank 226 can be pressurized in known fashion (e. g., an accumulator, a pump, a connection to compressed air bleed 210, etc.). Control valve 230 controls the flow of liquid into pneumoacoustic atomizers 108.

As depicted in FIG. 1, fire-suppression apparatus 100 also includes a control arrangement 106 that, on a signal from one of its sensors, opens control valves 224 and 230. Opening control valve 224 causes gas from gas supply 102 to flow to pneumoacoustic atomizers 108. Similarly, opening control valve 230 cause liquid from liquid supply 104 to flow to pneumoacoustic atomizer 108.

In the variation depicted in FIG. 2, control arrangement 106 includes controller 232 and one or more sensors 234. Controller 232 can be a microprocessor, programmable logic controller, or other digital/analog combination control system. Sensors 234 are positioned in a variety of locations in the monitored location as is appropriate. Any of a variety of different types of sensors 234 that are capable of detecting a condition that is indicative of fire can suitably be used. For example, sensors 234 can be temperature sensors, smoke detectors, infrared sensors, thermal signature sensors, laser sensors and other known devices. Furthermore, sensors 234 can be a combination of different types of sensors (e. g., temperature sensors and smoke detectors).

In operation, sensors 234 monitor their environment for indications of fire. Sensors 234 advantageously send a signal to controller 232 whether or not a fire condition is detected. The purpose of the continuous signal is to provide an indication to controller 232 that sensor 234 is functioning properly.

In the absence of an indication of fire, control valve 212 (controlling compressed air bleed), control valve 224 (controlling gas flow into pneumoacoustic atomizers 108), control valve 230 (controlling liquid flow into pneumoacoustic atomizers 108) and control valves 220 (controlling tanks 218) remain closed.

In the event that a signal from one or more of sensors 234 indicates a fire, controller 232 activates a fire-suppression response. In particular, controller 232 causes control valves 220,224

and 230 to open so that liquid and gas is admitted to pneumoacoustic atomizers 108. It is advantageous to slightly delay the introduction of liquid to pneumoacoustic atomizers 108 for a brief period of time, such as 1 to 2 seconds, until after the gas is introduced. The delay ensures that the desirable dynamic conditions are established at the atomizer. To that end, a delay circuit is incorporated into the logic of controller 232, or included as a separate device associated with control valve 230.

In some variations, water is supplied to pneumoacoustic atomizer 108 at two different pressures (at different times). In particular, after an initial fire extinguishing period wherein pneumoacoustic atomizer 108 is operating at a relatively lower liquid pressure, the pressure of the liquid is increased in order to maximize the cooling effect of the mist generated by pneumoacoustic atomizer 108. After a predetermined amount of time, or after a signal from sensors 234 reaches a predetermined value, the pressure of the liquid can be reduced to the lower pressure. The lower pressure results in a dryer mist being supplied to the protected area. The dryer mist conserves the supply of fire-suppressing liquid and reduces the possibility of the liquid damaging the protected area and/or its contents. In some variations, the lower pressure is about 2 psig and the higher pressure is about 6 psig. The pressure of the liquid supplied to pneumoacoustic atomizer 108 can be varied by adjusting control valve 228.

A further way to vary the amount of fire suppression provided by fire-suppression apparatus 228 is to change the number of pneumoacoustic atomizers 108 that are operating. For example, after an initial period of operation, the number of activated pneumoacoustic atomizers 108 can be reduced. If the intensity of the fire increases, as indicated by an appropriate signal from sensors 234, the decommissioned pneumoacoustic atomizers can be reactivated.

For the purpose of providing adequate fire suppression using a minimum amount of fire- suppressing materials, in some variations of the illustrative embodiment, controller 232 operates fire-suppression apparatus 100 in a pulsed mode. In the pulsed mode, fire-suppressing gas-infused mist is delivered until a predetermined condition is satisfied (e. g., a period of time elapses, temperature falls to a certain value, etc.) and then stopped. Thereafter, sensors 234 and controller 232 monitor the protected environment for indications of re-flash of the fire. In the event of re- flash, controller 232 re-actuates the fire-suppressing gas/mist. The cycle can be repeated as long as necessary.

The duration of the fire-suppressing gas/mist can be determined as a function of a variety of parameters, including, without limitation, the rate of temperature rise, the rate of temperature reduction and the duration of the time period between detected re-flash events.

In addition to monitoring for indications of fire, the gas pressure in tanks 218 is advantageously monitored. If pressure is low, controller 232 can generate an appropriate alarm.

All active components of fire-suppression apparatus 100 are advantageously supplied with <BR> <BR> a back-up power source (e. g. , battery, etc.) and/or are powered by a power source other than the primary electrical system of the vehicle/structure being protected to assure continued operation in the event of a power outage, such as might be caused by a fire.

It is possible to protect a plurality of separate regions with fire-suppression apparatus 100.

For example, multiple cargo areas of a plane or ship can be protected by providing multiple sensors 234 and pneumoacoustic atomizers 108 and placing them in appropriate locations. In such an embodiment, a plurality of gas control valves 224 and water control valves 230 are advantageously used to individually control the flow of gas and the flow of liquid into each of the multiple pneumoacoustic atomizers 108. Logic or circuitry in controller 232 is connected to receive a signal from each of sensors 234 and can function as a means for detecting a location of a fire as proximate to one or more of pneumoacoustic atomizers 108. Controller 232 is advantageously capable of actuating only the gas control valve (s) 224 and water control valve (s) 230 for pneumoacoustic atomizer (s) 108 proximate to the fire. In addition to providing multiple discrete areas of coverage, this variation of the illustrative embodiment reduces consumption of fire-suppressing materials by delivering them to only the areas in which they are needed.

Suitable pneumoacoustic atomizers 108 provide water droplets having a size that is in the range of about 50 to 90 microns. Two variations of a suitable pneumoacoustic atomizer 108 are shown in FIGS. 3 and 4.

Pneumoacoustic atomizer 108A, depicted in FIG. 3, includes rod 336, inner casing 338, outer casing 346 and head 352, interrelated as shown. Defined within or between these elements are gas nozzle 354, gas feed channel 356, water nozzle 358, water feed channel 360 and resonator 364.

Gas nozzle 354, which is annular in shape, is defined between the exterior of rod 336 and tapered portion 342 of inner wall 340 of inner casing 338. Gas is delivered to gas nozzle 354 via annular channel 356 that is defined between the exterior of rod 336 and the non-tapered portion of inner wall 340 of inner casing 338. Opposing and spaced from gas nozzle 354 is resonator 364, which is an annular channel that is formed between the exterior of rod 336 and wall 362. Gas that flows through gas nozzle 354 is braked by resonator 364.

Liquid nozzle 358, which is annular in shape, is defined between outer wall 344 of inner casing 338 and tapered portion 350 of inner wall 348 of outer casing 346. Liquid is delivered to

liquid nozzle 358 via annular channel 360 that is defined between the non-tapered portion of outer wall 344 and the non-tapered portion of inner wall 348 of outer casing 346.

In operation, gas (e. g., nitrogen, etc.) that passes through gas control valve 224 is directed into annular channel 356 that feeds gas nozzle 354. With sufficient gas pressure, typically at least about 21 psi, gas is discharged from nozzle 354 at the speed of sound. Once discharged, the gas expands and its speed becomes supersonic. The gas is decelerated by resonator 364, which causes intense acoustic oscillations in atomization zone 366 between gas nozzle 354 and resonator 364.

The gas jet pulses and periodic shock waves occur. The oscillations cause the atomization of liquid (e. g. , water, etc.) that is supplied through liquid nozzle 358. A mist of water droplets exits pneumoacoustic atomizer 108A through annular outlet 370.

Pneumoacoustic atomizer 108B, depicted via cross-sectional view in FIG. 4, includes casing 472, central core 486, and cowling 506. Casing 472 has an axially-disposed recess that receives central core 486. Cowling 506 engages the exterior of casing 472. Defined within or between the casing, central core and cowling are: axially-disposed channels 498 and 500, gas chamber 494, gas nozzle 496, radial apertures 502, liquid cavity 504, liquid inlet 502, liquid outlet groove 510, and resonator 512.

Gas is received by pneumoacoustic atomizer 108 via axially-disposed channel 498 in casing 472 and passes to axially-disposed channel 500 in central core 486. Radially-disposed apertures 502 in central core 486 enable gas to pass from axially-disposed channel 500 into gas chamber 494. Gas chamber 494 is defined by walls 474 and 476 of casing 472 and a portion of outer wall 488 of central core 486. Tapered wall 478, which depends from wall 476, and a portion of outer wall 488 of central core 486 define annular-shaped gas nozzle 496. Gas flows from gas chamber 494 through gas nozzle 496.

As described in more detail later in this specification, wall 478 should have a taper (measured relative to opposing wall 488) that is within the range of 60 to 80 degrees. This taper is referred to as the"conicity angle. "Furthermore, gas chamber 494 should have a compression factor, A at gas nozzle 496 that is in the range of 5 to 30, wherein u is given by the relation: Ill u = (dk2-d, 2)/ (dn2-d., 2) where: dk is the diameter of gas chamber 494; ds is the diameter of central core 486; and dn is the diameter of gas nozzle 496.

Opposing and spaced from gas nozzle 496 is resonator 512, which is an annular channel that is formed from walls 488,489 and 490 in central core 486. Gas that flows through gas nozzle 496 is braked by resonator 512.

Liquid is received by pneumoacoustic atomizer 108 via inlet channel 502 that is located at a marginal portion of casing 472. Liquid flows from inlet channel 502 to annular liquid cavity 504. Liquid cavity 504 is defined by walls 480 and 482 of casing 472 and a portion of cowling 506. Depending from liquid cavity 504 is a narrow, annular liquid outlet groove 510 that is defined by wall 484 of casing 472 and wall 508 of cowling 506. Unlike the liquid nozzles of other pneumoacoustic atomizers, which are defined by converging walls, the walls that define annular groove 510 are parallel.

It is known that there exists some threshold sound pressure that corresponds to the beginning of the dispersion of liquid during pneumoacoustic atomization. This threshold depends upon a variety of factors, including the surface tension of the liquid being atomized, the shape of the initial liquid jet, and the presence of a gas flow. In accordance with the illustrative embodiment, a sound pressure level required for efficient dispersion of water is in the range of 160 to 170 dB, which corresponds to a sound intensity in the atomization zone of about 1-10 W/cm.

The atomization process depends not only on the sound pressure level, but also on the frequency of the sound. In particular, the size of the resulting liquid droplets decreases with increasing frequency of acoustic waves. It has been found that to obtain water droplets in the size range of 50 to 90 microns, frequency must be within the range of about 16 to 20 kHz. The frequency of acoustic oscillations is a function of the height H of the resonator and the width W of the gap at the mouth of the gas nozzle (hereinafter"5").

It is known that for a near-wall ring jet, such as occurs in the configuration of pneumoacoustic atomizers 108A and 108B, the unsteady modes that are formed as a result of the deceleration caused by an empty resonator are realized at Strouhal numbers, Sh, that are close to the quarter-wavelength resonance. That is : [2] Sh=A/= 0. 21 to 0. 23 where: A is cell length of the supersonic jet; and X is wavelength and X = c/f (c is the speed of sound in the gas and f is the generation frequency).

The cell length A is proportional to the width of the nozzle gap 6 and also depends upon both the pressure of the supplied gas (usually within 2.5-5 atmospheres) and the transverse curvature of the out-flowing jet. The curvature is determined by the ratio between the diameter ds of rod 340 (or central core 486) and the diameter d, of the gas nozzle 354 or 496. For use in fire- suppression apparatus 100, the curvature parameter R should be within the range of about 0.8 to about 0.9 wherein R is given by the expression:

R°dsd, Given this range for the value of the curvature parameter R, the Strouhal numbers are <BR> <BR> obtained for 8 = (0.03-0. 055) X, and the required droplet dimensions (i. e. , 50-90 microns) can be achieved by using a resonator having depth that is determined by the relation: [4] H= (3 o5) 6 since the necessary sound pressure levels of 160-170 dB can be obtained only for these values of H.

The instability of the out-flowing gas is determined by the occurrence of the transverse component of the speed of the gas. It has been found that this instability is related to the epure of <BR> <BR> speed (i. e. , velocity profile) at the cross-section of the nozzle. The shear is related to conicity<BR> angle of the nozzle. During abrupt changes in flow (i. e. , a large conicity angle), the instability is<BR> increased because of a point of flex (e. g. , an inflection point, etc.) in the epure of speed. Thus, to increase the intensity of the shock waves, it is advantageous to use a nozzle having a conicity angle of between about 60 to 80 degrees. This feature is included in the pneumoacoustic atomizer 108B, which is depicted in FIG. 4.

Pneumoacoustic atomizer 108B having a nozzle conicity angle in the range of 60-80 degrees showed an increase in efficiency of 18 to 26 percent over pneumoacoustic atomizer 108A.

The intensity of the shock waves in the atomization zone increase by 4 dB.

A second advantage of pneumoacoustic atomizer 108B is that, relative to atomizer 108A and relative to prior art pneumoacoustic atomizers, it is constructed of fewer parts. In particular, pneumoacoustic atomizer 108B is constructed of only three parts: casing 472, central core 486 and cowling 506. This reduces cost and improves reliability.

It is to be understood that the above-described embodiments are merely illustrative of the invention and that many variations may be devised by those skilled in the art without departing from the scope of the invention and from the principles disclosed herein. It is therefore intended that such variations be included within the scope of the following claims and their equivalents.