Abstract:
An emitter for atomizing and discharging a liquid entrained in a gas stream is disclosed. The emitter has a nozzle with an outlet facing a deflector surface. The nozzle discharges a gas jet against the deflector surface. The emitter has a duct with an exit orifice adjacent to the nozzle outlet. Liquid is discharged from the orifice and is entrained in the gas jet where it is atomized. A method of operating the emitter is also disclosed. The method includes establishing a first shock front between the outlet and the deflector surface, a second shock front proximate to the deflector surface, and a plurality of shock diamonds in a liquid-gas stream discharged from the emitter.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     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. 
     
    
     FIELD OF THE INVENTION  
       [0002]     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.  
       BACKGROUND OF THE INVENTION  
       [0003]     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.  
         [0004]     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.  
       SUMMARY OF THE INVENTION  
       [0005]     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.  
         [0006]     The invention also includes a method of operating the emitter, the method comprising:  
         [0007]     discharging the liquid from the orifice;  
         [0008]     discharging the gas from the outlet;  
         [0009]     establishing a first shock front between the outlet and the deflector surface;  
         [0010]     establishing a second shock front proximate to the deflector surface;  
         [0011]     entraining the liquid in the gas to form a liquid-gas stream; and  
         [0012]     projecting the liquid-gas stream from the emitter.  
         [0013]     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. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]      FIG. 1  is a longitudinal sectional view of a high velocity low pressure emitter according to the invention;  
         [0015]      FIG. 2  is a longitudinal sectional view showing a component of the emitter depicted in  FIG. 1 ;  
         [0016]      FIG. 3  is a longitudinal sectional view showing a component of the emitter depicted in  FIG. 1 ;  
         [0017]      FIG. 4  is a longitudinal sectional view showing a component of the emitter depicted in  FIG. 1 ;  
         [0018]      FIG. 5  is a longitudinal sectional view showing a component of the emitter depicted in  FIG. 1 ;  
         [0019]      FIG. 6  is a diagram depicting fluid flow from the emitter based upon a Schlieren photograph of the emitter shown in  FIG. 1  in operation; and  
         [0020]      FIG. 7  is a diagram depicting predicted fluid flow for another embodiment of the emitter. 
     
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS  
       [0021]      FIG. 1  shows a longitudinal sectional view of a high velocity low pressure emitter  10  according to the invention. Emitter  10  comprises a convergent nozzle  12  having an inlet  14  and an outlet  16 . Outlet  16  may range in diameter between about ⅛ inch to about 1 inch for many applications. Inlet  14  is in fluid communication with a pressurized gas supply  18  that provides gas to the nozzle at a predetermined pressure and flow rate. It is advantageous that the nozzle  12  have a curved convergent inner surface  20 , although other shapes, such as a linear tapered surface, are also feasible.  
         [0022]     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 .  
         [0023]     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.  
         [0024]     Deflector surface  22  may have other shapes, such as the curved upper edge  34  shown in  FIG. 2  and the curved edge  36  shown in  FIG. 3 . As shown in  FIGS. 4 and 5 , the deflector surface  22  may also include a closed end resonance tube  38  surrounded by a flat portion  40  and a swept back, angled portion  42  ( FIG. 4 ) or a curved portion  44  ( FIG. 5 ). The diameter and depth of the resonance cavity may be approximately equal to the diameter of outlet  16 .  
         [0025]     With reference again to  FIG. 1 , an annular chamber  46  surrounds nozzle  12 . Chamber  46  is in fluid communication with a pressurized liquid supply  48  that provides a liquid to the chamber at a predetermined pressure and flow rate. A plurality of ducts  50  extend from the chamber  46 . Each duct has an exit orifice  52  positioned adjacent to nozzle outlet  16 . The exit orifices have a diameter between about 1/32 and ⅛ inches. Preferred distances between the nozzle outlet  16  and the exit orifices  52  range between about 1/64 inch to about ⅛ inch as measured along a radius line from the edge of the nozzle outlet to the closest edge of the exit orifice. Liquid, for example, water for fire suppression, flows from the pressurized supply  48  into the chamber  46  and through the ducts  50 , exiting from each orifice  52  where it is atomized by the gas flow from the pressurized gas supply that flows through the nozzle  12  and exits through the nozzle outlet  16  as described in detail below.  
         [0026]     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.  
         [0027]     Operation of the emitter  10  is described with reference to  FIG. 6  which is a drawing based upon Schlieren photographic analysis of an operating emitter.  
         [0028]     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 .  
         [0029]     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 .  
         [0030]     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.  
         [0031]     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  60   a . 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.  
         [0032]     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.  
         [0033]     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.  
         [0034]     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.  
         [0035]     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&#39;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.  
         [0036]     Furthermore, the flow from the emitter is stable and does not separate from the deflector surface (or experiences delayed separation as shown at  60   a ) 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.  
         [0037]     Another emitter embodiment  11  is shown in  FIG. 7 . Emitter  11  has ducts  50  that are angularly oriented toward the nozzle  12 . The ducts are angularly oriented to direct the water or other liquid  47  toward the gas  45  so as to entrain the liquid in the gas proximate to the first shock front  54 . It is believed that this arrangement will add yet another region of atomization in the creation of the liquid-gas stream  60  projected from the emitter  11 .  
         [0038]     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.