Patent Grant
7721811
This invention concerns devices for emitting atomized liquid, the device injecting the liquid into a gas flow stream where the liquid is atomized and projected away from the device.
Devices such as resonance tubes are used to atomize liquids for various purposes. The liquids may be fuel, for example, injected into a jet engine or rocket motor or water, sprayed from a sprinkler head in a fire suppression system. Resonance tubes use acoustic energy, generated by an oscillatory pressure wave interaction between a gas jet and a cavity, to atomize liquid that is injected into the region near the resonance tube where the acoustic energy is present.
Resonance tubes of known design and operational mode generally do not have the fluid flow characteristics required to be effective in fire protection applications. The volume of flow from the resonance tube tends to be inadequate, and the water particles generated by the atomization process have relatively low velocities. As a result, these water particles are decelerated significantly within about 8 to 16 inches of the sprinkler head and cannot overcome the plume of rising combustion gas generated by a fire. Thus, the water particles cannot get to the fire source for effective fire suppression. Furthermore, the water particle size generated by the atomization is ineffective at reducing the oxygen content to suppress a fire if the ambient temperature is below 55° C. Additionally, known resonance tubes require relatively large gas volumes delivered at high pressure. This produces unstable gas flow which generates significant acoustic energy and separates from deflector surfaces across which it travels, leading to inefficient atomization of the water. There is clearly a need for an atomizing emitter that operates more efficiently than known resonance tubes in that the emitter uses smaller volumes of gas at lower pressures to produce sufficient volume of atomized water particles having a smaller size distribution while maintaining significant momentum upon discharge so that the water particles may overcome the fire smoke plume and be more effective at fire suppression.
The invention concerns an emitter for atomizing and discharging a liquid entrained in a gas stream. The emitter is connectable in fluid communication with a pressurized source of the liquid and a pressurized source of the gas. The emitter comprises a nozzle having an inlet connectable in fluid communication with the pressurized gas source and an outlet. A duct, connectable in fluid communication with the pressurized liquid source, has an exit orifice positioned adjacent to the outlet. A deflector surface is positioned facing the outlet in spaced relation thereto. The deflector surface has a first surface portion oriented substantially perpendicularly to the nozzle and a second surface portion positioned adjacent to the first surface portion and oriented non-perpendicularly to the nozzle. The liquid is discharged from the orifice, and the gas is discharged from the nozzle outlet. The liquid is entrained with the gas and atomized forming a liquid-gas stream that impinges on the deflector surface and flows away therefrom. The emitter is configured and operated so that a first shock front is formed between the outlet and the deflector surface, and a second shock front is formed proximate to the deflector surface. The liquid is entrained at one of the shock fronts. The nozzle is configured and operated so as to create an overexpanded gas flow jet.
The invention also includes a method of operating the emitter, the method comprising:
discharging the liquid from the orifice;
discharging the gas from the outlet;
establishing a first shock front between the outlet and the deflector surface;
establishing a second shock front proximate to the deflector surface;
entraining the liquid in the gas to form a liquid-gas stream; and
projecting the liquid-gas stream from the emitter.
The method may also include creating an overexpanded gas flow jet from the nozzle of the emitter, and creating a plurality of shock diamonds in the liquid-gas stream.
A deflector surface 22 is positioned in spaced apart relation with the nozzle 12, a gap 24 being established between the deflector surface and the nozzle outlet. The gap may range in size between about 1/10 inch to about ¾ inches. The deflector surface 22 is held in spaced relation from the nozzle by one or more support legs 26.
Preferably, deflector surface 22 comprises a flat surface portion 28 substantially aligned with the nozzle outlet 16, and an angled surface portion 30 contiguous with and surrounding the flat portion. Flat portion 28 is substantially perpendicular to the gas flow from nozzle 12, and has a minimum diameter approximately equal to the diameter of the outlet 16. The angled portion 30 is oriented at a sweep back angle 32 from the flat portion. The sweep back angle may range between about 15° and about 45° and, along with the size of gap 24, determines the dispersion pattern of the flow from the emitter.
Deflector surface 22 may have other shapes, such as the curved upper edge 34 shown in
With reference again to
Emitter 10, when configured for use in a fire suppression system, is designed to operate with a preferred gas pressure between about 29 psia to about 60 psia at the nozzle inlet 14 and a preferred water pressure between about 1 psig and about 50 psig in chamber 46. Feasible gases include nitrogen, other inert gases, mixtures of inert gases as well as mixtures of inert and chemically active gases such as air.
Operation of the emitter 10 is described with reference to
Gas 45 exits the nozzle outlet 16 at about Mach 1.5 and impinges on the deflector surface 22. Simultaneously, water 47 is discharged from exit orifices 52.
Interaction between the gas 45 and the deflector surface 22 establishes a first shock front 54 between the nozzle outlet 16 and the deflector surface 22. A shock front is a region of flow transition from supersonic to subsonic velocity. Water 47 exiting the orifices 52 does not enter the region of the first shock front 54.
A second shock front 56 forms proximate to the deflector surface at the border between the flat surface portion 28 and the angled surface portion 30. Water 47 discharged from the orifices 52 is entrained with the gas jet 45 proximate to the second shock front 56 forming a liquid-gas stream 60. One method of entrainment is to use the pressure differential between the pressure in the gas flow jet and the ambient. Shock diamonds 58 form in a region along the angled portion 30, the shock diamonds being confined within the liquid-gas stream 60, which projects outwardly and downwardly from the emitter. The shock diamonds are also transition regions between super and subsonic flow velocity and are the result of the gas flow being overexpanded as it exits the nozzle. Overexpanded flow describes a flow regime wherein the external pressure (i.e., the ambient atmospheric pressure in this case) is higher than the gas exit pressure at the nozzle. This produces oblique shock waves which reflect from the free jet boundary 49 marking the limit between the liquid-gas stream 60 and the ambient atmosphere. The oblique shock waves are reflected toward one another to create the shock diamonds.
Significant shear forces are produced in the liquid-gas stream 60, which ideally does not separate from the deflector surface, although the emitter is still effective if separation occurs as shown at 60a. The water entrained proximate to the second shock front 56 is subjected to these shear forces which are the primary mechanism for atomization. The water also encounters the shock diamonds 58, which are a secondary source of water atomization.
Thus, the emitter 10 operates with multiple mechanisms of atomization which produce water particles 62 less than 20 μm in diameter, the majority of the particles being measured at less than 5 μm. The smaller droplets are buoyant in air. This characteristic allows them to maintain proximity to the fire source for greater fire suppression effect. Furthermore, the particles maintain significant downward momentum, allowing the liquid-gas stream 60 to overcome the rising plume of combustion gases resulting from a fire. Measurements show the liquid-gas stream having a velocity of 1,200 ft/min 18 inches from the emitter, and a velocity of 700 ft/min 8 feet from the emitter. The flow from the emitter is observed to impinge on the floor of the room in which it is operated. The sweep back angle 32 of the angled portion 30 of the deflector surface 22 provides significant control over the included angle 64 of the liquid-gas stream 60. Included angles of about 120° are achievable. Additional control over the dispersion pattern of the flow is accomplished by adjusting the gap 24 between the nozzle outlet 16 and the deflector surface.
During emitter operation it is further observed that the smoke layer that accumulates at the ceiling of a room during a fire is drawn into the gas stream 45 exiting the nozzle and is entrained in the flow 60. This adds to the multiple modes of extinguishment characteristic of the emitter as described below.
The emitter causes a temperature drop due to the atomization of the water into the extremely small particle sizes described above. This absorbs heat and helps mitigate spread of combustion. The nitrogen gas flow and the water entrained in the flow replace the oxygen in the room with gases that cannot support combustion. Further oxygen depleted gases in the form of the smoke layer that is entrained in the flow also contributes to the oxygen starvation of the fire. It is observed, however, that the oxygen level in the room where the emitter is deployed does not drop below about 16%. The water particles and the entrained smoke create a fog that blocks radiative heat transfer from the fire, thus mitigating spread of combustion by this mode of heat transfer. Because of the extraordinary large surface area resulting from the extremely small water particle size, the water readily absorbs energy and forms steam which further displaces oxygen, absorbs heat from the fire and helps maintain a stable temperature typically associated with a phase transition. The mixing and the turbulence created by the emitter also helps lower the temperature in the region around the fire.
The emitter is unlike resonance tubes in that it does not produce significant acoustic energy. Jet noise (the sound generated by air moving over an object) is the only acoustic output from the emitter. The emitter's jet noise has no significant frequency components higher than about 6 kHz (half the operating frequency of well known types of resonance tubes) and does not contribute significantly to water atomization.
Furthermore, the flow from the emitter is stable and does not separate from the deflector surface (or experiences delayed separation as shown at 60a) unlike the flow from resonance tubes, which is unstable and separates from the deflector surface, thus leading to inefficient atomization or even loss of atomization.
Another emitter embodiment 11 is shown in
Emitters according to the invention operated so as to produce an overexpanded gas jet with multiple shock fronts and shock diamonds achieve multiple stages of atomization and result in multiple extinguishment modes being applied to control the spread of fire when used in a fire suppression system.
This application is based on and claims priority to U.S. Provisional Application No. 60/689,864, filed Jun. 13, 2005 and U.S. Provisional Application No. 60/776,407, filed Feb. 24, 2006.
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