Abstract:
An injector model for atomization of liquid fuel using a low pressure atomized gas. The model is focused to mix a volume of liquid fuel with a volume of corresponding atomizing gas to obtain a pressurized liquid fuel-gas mixture. This liquid-gas mixture is ejected through a discharge orifice into a lower combustion chamber pressure, as a result of which the liquid fuel breaks up into ligaments. The atomized gas emerges from the liquid fuel-gas mixture as a result of pressure jump and further enhances this break-up of liquid fuel into smaller droplets and promotes combustion of these droplets in the chamber.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The invention relates generally to a method to enhance atomization of liquid fuel for combustion. In particular, the atomization is increased by pre-mixing an atomizing gas into the liquid fuel before it is ejected into the combustion chamber. 
         [0002]    The design of this invention may be used to the advantage in internal combustion engines, and especially jet engines, as well as any other reciprocating engine, furnace, turbine, or combustor of liquid fuel where the atomization of liquid fuel is desired before its combustion. 
         [0003]    Modern day aircraft engines also require efficient fuel burning inside the combustion chamber to convert this energy into thrust and produce less toxic emissions. Typically these engines use some sort of high pressurized fuel injectors to spray fuel into the combustion air. High injection pressures and high momentum air flows are required to generate small fuel drops and homogenous mixtures. Thus the fuel, after breaking up and mixing with atomizing gas, forms a fine spray pattern as it is discharged into the combustion chamber. 
         [0004]    As more and more stringent rules are imposed on emissions regulations, efficient atomization of fuel is necessary to produce combustion with required levels of emissions. One key measureable parameter for spray quality is the Sauter Mean Diameter (SMD) of the spray droplets, with respect to which the smaller the SMD, the better the spray quality. Unfortunately, to achieve a spray quality of required SMD the current liquid fuel injection systems demand extremely high atomizing gas pressures so that liquid fuel is atomized to the required level. One such technique is being utilized in the design of an effervescent atomizer, which works on the principle stated above. 
         [0005]    In effervescent atomizers, first introduced in late 1980s, volatile liquid fuels are atomized into a finely divided spray of small droplets due to injection of high pressurized gas upstream of the discharge orifice. Much finer spray patterns are produced by such atomizers than compared with other conventional atomizers. The atomization of fuel to produce homogenous mixture is important for further combustion process efficiency. The required efficiency of the combustion process in engines of the present day has assumed greater importance than in the past due to increased environmental concerns and more stringent regulations on exhaust pollutants due to inefficient combustion reactions. 
         [0006]    The advantages of effervescent atomizers as compared to conventional external or internal atomizers are listed for consideration: (a) Effervescent atomizers have the ability to produce relatively good atomization at much lower gas injection pressures as compared to other atomizers, (b) In comparison to other methods of atomization, much smaller drop sizes are obtained for a given gas injection pressure, (c) The atomizing gas is injected into the liquid at relatively lower velocity to form a bubbly mixture than those employed in most other forms of atomization, (d) For a fixed flow rate, effervescent atomizer gives the luxury of having a larger exit orifice diameter as compared to other atomizers. This prevents orifice erosion in case of liquids having solid suspension. Also this advantage facilitates fabrication and manufacturing processes, (e) Spray characteristics are independent of the fluid properties used. SMD, which is the main performance criteria, is the function of internal design and geometry, (f) Effervescent atomizer has a very durable and simple design. It requires nearly no maintenance and can be operated at low cost 
         [0007]    The gas supply pressure has to be kept at slightly higher pressure than the fluid for injection. It is because of the presence of this pressure difference, the atomizing gas is able to flow through the perforations of the central tube and forms a bubbly flow in the liquid stream. Due to this, a two phase homogenous bubbly mixture is formed just upstream of the discharge orifice. The requirement demands presence of a gas pressurization system (probably a centrifugal pump) for a constant supply to the atomizer. Such a pressurized gas system requires considerable space in any of its application. 
         [0008]    Thus, there exists a need to re-evaluate the existing effervescent atomizer design in a way that it utilizes all the advantages of a twin fluid internal mixed atomizer, to produce the desired degree of atomization, and at the same time overcome its inherent shortcoming of operating at higher injection pressure of atomizing gas as compared to the liquid fuel. 
       BRIEF SUMMARY OF THE INVENTION 
       [0009]    The above discussed drawback and deficiency of the prior art (effervescent atomizer) are overcome and alleviated by a new atomizer design which can produce fine liquid fuel drops and homogenous fuel gas mixtures for combustion thereof, particularly in an internal combustion engine. 
         [0010]    Present invention, exploits the fluid mechanics principle of entrainment of atomizing gas into the liquid fuel at a pressure much lower than that of the liquid. Due to this entrainment of gas, a two phase mixture is formed similar to an effervescent atomizer. An engineering application of this principle lies in the design of Ejector or Jet pumps. 
         [0011]    Ejector pump is a device in which the kinetic energy of one fluid stream (primary or driving fluid) is used to drive another fluid stream (secondary or induced fluid). The secondary fluid is entrained due to the viscous friction at the primary jet periphery and turbulent mixing of both the fluids takes place inside the ejector pump. Therefore, ejector pump principle was utilized for the current invention with few necessary alterations in the design, and is to be used as an effervescent atomizer to overcome its shortcoming. 
         [0012]    The foregoing and other aspects will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawing figures. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  is a cross sectional view of the parts of typical prior art effervescent atomizers. 
           [0014]      FIG. 2  is a cross sectional view of effervescent atomizer of the present invention. 
           [0015]      FIG. 3  is a cross-sectional view of the effervescent atomizer of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0016]    The liquid fuel of an internal combustion engine must be mixed with an atomizing gas so that it can be quickly and cleanly burned to produce power. This invention provides a new means to more efficiently dissociate the liquid fuel into ligaments and drops to enhance evaporation and mixing by using atomizing gas at pressures lower than that of the liquid fuel. 
         [0017]    Turning now to the drawings wherein like parts are referred to by the same number throughout the several views,  FIG. 1  is a schematic representation of a prior art effervescent atomizer, that is used to inject atomized fuel into the combustion chamber in the form of a homogenous mixture. 
         [0018]    Generally, in  FIG. 1 , atomizing gas is supplied from conventional centrifugal pump assembly at a pressure higher than that of the liquid fuel and subsequently liquid fuel is also supplied through another pressurizing system. The gas may be any suitable atomizing gas that can be dissolvable in the liquid fuel, such as but not limited to, a non-polar gas soluble with a hydrocarbon based fuel. However, for purposes of describing an exemplary embodiment of the invention, the gas mixed with the hydrocarbon based fuel is a non-polar gas, such as, but not limited to NO, H 2 , O 2 , N 2 , He and Ar. The gas supply system includes the requisite valves and other conventional flow regulating devices and mechanisms that are adapted to ensure the correct volume of atomizing gas and fuel to the atomizer. Such valves and flow control devices are well known to one skilled in the art and therefore further description of these devices is not required. 
         [0019]    The fuel that is mixed with the atomizing gas may be any suitable combustible fuel such as but not limited to fuel oil, gasoline or diesel fuel. The fuel supply system includes the requisite valves and other conventional flow regulating devices and mechanisms that are adapted to ensure the correct volume of liquid fuel to the atomizer. Such valves and flow control devices are well known to one skilled in the art and therefore do not need to be discussed in further detail hereinafter. 
         [0020]    Turning now to  FIG. 2  and  FIG. 3 , which is an exemplary model for achieving liquid fuel atomization at lower gas injection pressures unlike the conventional effervescent model. The proposed model has a primary nozzle  1 , from where liquid fuel enters. Entertainment of atomizing gas occurs through the gas injection holes  2  on the outside periphery of the secondary nozzle  3  along with liquid fuel. It also has a Mixing Chamber  4 , where both the primary and secondary fluids mix with each other and complete momentum transfer occurs, and then it has a diffuser section  5  in which a homogenous fuel-gas mixture is formed. A viscous flow nozzle  6  is also integrated at the exit of the diffuser section for final discharge of the fuel-gas mixture into fine spray pattern for combustion through a orifice  7 . 
         [0021]    In the primary nozzle  1 , a high-pressure fuel with very low velocity at the primary inlet is accelerated to high velocity jet through a converging nozzle. The supply pressure at the inlet is partly converted to the jet momentum at the nozzle exit according to the Bernoulli equation. The high velocity, low static pressure primary jet induces a secondary flow from the suction port and accelerates it in the direction of the driving jet. 
         [0022]    Secondary nozzle  3  helps in entrainment of the atomizing gas along with liquid fuel into the mixing chamber. The atomizing gas is aerated into the secondary stream of liquid fuel through the gas injection holes  2 . The secondary stream may be input at various angles to the primary stream, depending on the application. The high velocity primary stream at the primary nozzle exit produces a region of low pressure which subsequently entrains the secondary stream and increases its velocity. 
         [0023]    The most important component in the proposed model that effectively characterizes the device is the mixing chamber. This is where the primary-secondary turbulent mixing takes place. 
         [0024]    After complete mixing and momentum exchange inside the mixing chamber the static pressure of the mixed stream is generally low. In order to recover the pressure, a divergent section (diffuser)  4  is attached at the end of the mixing chamber. The diffuser reduces the velocity of the mixed stream and transforms it to static pressure. 
         [0025]    The viscous flow nozzle  5  functions as an exit orifice similar to a conventional effervescent atomizer so that the homogenous fuel-gas mixture is discharged for combustion. When the atomized spray emerges from the orifice for combustion  7 , the liquid fuel and gas mixture droplets experience a sudden pressure drop. This reduction of pressure allows the gas to expand violently out of the mixture, thus further breaking up the droplets of fuel atomized by the atomizer. 
         [0026]    In summary, a significant quantity of gas is introduced into the fuel through its injection holes near to the secondary nozzle outlet. This atomizing gas along with the liquid fuel is aerated by the momentum exchange of the liquid fuel, which is at a higher pressure, from the primary nozzle. The two streams then combine inside a mixing chamber where energy transfer takes place. This low pressure liquid-gas mixture enters into the diffuser section where its velocity head is converted into pressure head. The mixture then passes through a contraction nozzle for final exit of pressurized fuel-gas homogenous mixture in the form of a jet. The sudden pressure drop at the discharge orifice permits the gas entrained in the liquid fuel to expand rapidly. This expansion of gas further breaks up the fuel droplets and result in a finer mist that contributes to the creation of a more homogenous mixture. 
         [0027]    Thus, one skilled in the pertinent art will recognize that one key to this invention is the injection of gas at lower pressures as that of the liquid fuel. The precise location of the gas injection holes is the basic feature that distinguishes this invention from the conventional effervescent atomizer design. It is due to this technique the atomizing gas is able to entrain into the liquid fuel at much reduced pressures. The proposed location and number of gas injection holes, as shown in  FIG. 3 , are one just example of an infinite combinations. However, the uniqueness of this invention/design lies in its ability to entrain the atomizing gas at reduced pressures unlike current effervescent atomizer designs. 
         [0028]    The invention disclosed hereinabove, addresses the major design drawback in case of an effervescent atomizer. The limitation lies in the use of atomizing gas at pressure higher to that of the liquid fuel. The present invention overcomes this design limitation and ensures gas entrainment at lower pressure subjected to careful positioning of the gas injection holes in the secondary nozzle. Due to which combustion is achieved with greater efficiency and stability than with conventional atomizers by producing finer droplets of fuel after discharge from the exit orifice. 
         [0029]    While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.