Abstract:
A nozzle for a fuel injector, in particular for a gas-turbine engine, is provided comprising a planar conductive electrode with a sharp edge forming an aperture, an upper insulation layer above the electrode and a lower insulation layer below the electrode, both insulation layers having apertures, and a swirler arrangement for creating a swirling action in liquid fuel introduced into the nozzle. The axis of swirl is generally perpendicular to the plane of the electrode. In use, the swirling fuel passes through the aperture of the lower insulation layer, the aperture of the conductive electrode and the aperture of the upper insulation layer. As the fuel passes through the aperture of the electrode, the electrode charges the swirling fuel, so that the nozzle supplies charged droplets of atomized fuel from an outlet orifice. The swirler arrangement may be a radial or axial swirler arrangement.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is the US National Stage of International Application No. PCT/EP2007/059320, filed Sep. 6, 2007 and claims the benefit thereof The International Application claims the benefits of Great Britain application No. 0621798.8 GB filed Nov. 2, 2006, both of the applications are incorporated by reference herein in their entirety. 
     FIELD OF INVENTION 
     The invention relates to a nozzle for a fuel injector, and to a nozzle for a fuel injector supplying atomised liquid fuel to a device such as a gas-turbine engine. 
     BACKGROUND OF INVENTION 
     Fuel-injector nozzles for supplying atomised droplets of liquid fuel to a combustion chamber in a gas-turbine engine are already known. One example is described in European patent application EP 1139021, which was published on 4 Oct. 2001 and involves the same inventor as the present application. FIGS. 1-3 of EP 1139021 are reproduced here as  FIGS. 1-3  of this present application. 
       FIG. 1  shows a combustor for a gas-turbine engine, comprising a burner  10 , a swirler  12 , a pre-chamber  14  and a main combustion chamber  16 . The swirler  12  includes a number of vanes  18  (see also  FIG. 2 ) defining intervening passages  20 , which are fed with compressed air from a manifold  22 . The combustor may run off liquid fuel, in which case liquid fuel is introduced through nozzles  24  at the burner face  26 . The nozzles  24  can operate in two different modes depending on the load condition. At high load the feed pressure, and hence the flow through the nozzle, is high enough to achieve good atomization of the fuel without the nozzle being electrically charged. However, at low load the flow is reduced and therefore the atomization is impaired. Hence, as the load is decreased, the voltage applied in the nozzle is increased, giving rise to enhanced atomization. 
       FIG. 2  is a plan view of the swirler  12  and burner  10  and showing the injection nozzles  24  arranged circumferentially around the burner, while  FIG. 3  shows an injection nozzle  24  in greater detail. The nozzle  24  comprises a nozzle body  26  having a circular-section spin chamber  28 . Liquid fuel is fed into the spin chamber  28  through an array of slots  30  and is thrown out through a throat  32  and passage  34 , which is frustoconical in shape, in direction A to an outlet orifice  36 . Due to the strong swirling movement of the fuel in the spin chamber, the fuel tends to keep to the inside surface  38  of the passage  34  and is atomised to faun small droplets as it expands out of the passage  34  into the air stream present in the swirler passages  20 . 
     A tubular, electrically conductive electrode  40  is provided near the outlet end of the nozzle  24 . The electrode  40  has a sharp edge  42 , which extends in the direction of travel of the fuel through the nozzle. Insulating layers  44 ,  46  are provided on respective sides of the electrode  40 . 
     The fuel is subjected to an electrostatic charge at the point where the fuel stream, which keeps to the inside wall  38 , starts to break up into droplets as it exits the outlet  36 . A charge supply and control unit  48  (see  FIG. 1 ) feeds the electrode  40  with a voltage via an annular conductor  50 . 
     Electrostatic charging of the fuel is beneficial mainly when the engine is running at low loads, i.e. when less fuel is being delivered to the nozzles  24 . Such charging then helps to control the atomisation and vaporisation of the fuel, the fuel placement and combustion intensity. By contrast, it may not be necessary to employ electrostatic charging when the engine is running at full load. 
     The fuel-injection nozzle disclosed in EP 1139021 has the drawback that it is complex and thereby costly to manufacture. In addition the volume occupied by the nozzle is quite large, especially in the axial direction. 
     SUMMARY OF INVENTION 
     The present invention seeks to mitigate these drawbacks. 
     In accordance with the invention there is provided a nozzle for a fuel injector for supplying atomised liquid fuel, the nozzle comprising: an electrode comprising a substantially planar electrically conductive member containing an aperture, the edge of the aperture being sharp to enable the electrode to impart charge; first and second insulating members disposed to respective sides of the plane of the electrically conductive member, the first insulating member being disposed on an outlet side of the nozzle, and swirler means for supplying a swirling flow of liquid fuel to the aperture, the axis about which the fuel swirls within the aperture being generally perpendicular to the plane of the electrode, wherein, in use of the nozzle, the electrode imparts charge to the swirling flow of liquid fuel within the aperture such that the nozzle supplies charged droplets of atomised fuel. 
     The first and second insulating members may have first and second apertures, respectively, which are substantially coaxial with the aperture of the conductive member. The second aperture may be larger than the first aperture. Furthermore, the aperture of the conductive member may be smaller than the first aperture. 
     The conductive member may have a thickness, which decreases in a radial direction between the second aperture and the aperture of the conductive member. The decrease in thickness of the conductive member may be substantially linear. 
     The nozzle may further comprise first and second substantially planar members disposed on outer planar sides of the first and second insulating members, respectively, the first substantially planar member comprising an outlet orifice for the supplying of the charged droplets of atomised fuel. The outlet orifice is preferably substantially the same size as the first aperture. 
     The swirler means may be a radial swirler means, which may comprise radial passages provided in the second insulating member and communicating with the second aperture. 
     Alternatively, the swirler means may be an axial swirler means. In this case passages may be provided in the second substantially planar member and communicating with the second aperture, said passages being oriented such as to impart an axial and a tangential component of flow to incoming fuel. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, of which: 
         FIGS. 1 and 2  are sectional views of a known gas-turbine combustion system and 
         FIG. 3  is a sectional view through a known fuel-injection nozzle used in the combustion system of  FIGS. 1 and 2 ; 
         FIG. 4(   a ) is a sectional view through a generalised fuel-injection nozzle according to the present invention and  FIG. 4(   b ) is a plan view of part of  FIG. 4(   a ); 
         FIG. 5  is a perspective view of a first embodiment of the nozzle shown in  FIG. 4(   a ); 
         FIG. 6  and  FIGS. 7(   a ) and  7 ( b ) correspond to the view of  FIG. 5  and illustrate the mode of operation of the nozzle; 
         FIG. 8(   a ) is a perspective view of a second embodiment of the nozzle shown in  FIG. 4(   a ), and 
         FIGS. 8(   b ) and  8 ( c ) are a sectional view and a plan view, respectively, of a lower substantially planar member forming part of the nozzle of  FIG. 8(   a ). 
     
    
    
     DETAILED DESCRIPTION OF INVENTION 
     Referring now to  FIG. 4(   a ), a generalised representation of a fuel-injection nozzle according to the present invention is shown, which comprises a laminar arrangement of components. These components are: an upper, or first, planar member  100 , an upper, or first, planar layer of insulation  102 , a planar conductive member  104 , a lower, or second, planar layer of insulation  106  and a lower, or second, planar member  108 . It is understood that by “planar” is meant that the relevant components are generally, or substantially, flat, and not necessarily completely and uniformly flat. These members and layers are held together in any suitable manner, for example by clamping.  FIG. 4(   b ) is a view of  FIG. 4(   a ) looking down from just above the conductive layer  104  and including solely the central circular portion of the nozzle demarcated by lines  110 . 
     The planar members  100 ,  108  are preferably composed of metal, while the insulation layers are preferably composed of mica or a ceramic material. Silicon-based compounds are not suitable, since they are attacked by hydrocarbons. In order to resist erosion and maintain sharpness over a long period, the conductive member  104  is preferably composed of a hard, heat-resistant material, such as the high-speed tool steel or Stellite 6™ mentioned in EP 1139021. 
     There are provided in one of the lower components, e.g. the lower planar member  108 , a series of holes  112 , which are disposed such as to impart a rotational component of flow to liquid fuel flowing through these holes. The swirling fuel enters the space defined by lines  110 , flows past the conductive member  104  and out through the outlet orifice  114 , emerging as droplets of fuel. Along the way, the fuel picks up electronic charge produced by the application of a suitably high voltage between the conductive member  104  and a reference-potential point (e.g. earth). Since the planar members  100  and  108  are made of metal, it is assumed that they will likewise be held at a reference-potential point, e.g. earth. 
     A first, more practical, nozzle arrangement corresponding to a first embodiment of the invention is shown in  FIG. 5 . In  FIG. 5 , which is a perspective view of the nozzle, the liquid fuel is introduced by way of passages  120  provided in the lower insulation layer. These passages correspond to the passages  20  shown in  FIGS. 1 and 2  and therefore impart a large tangential and a smaller radial component of flow to the incoming fuel. The swirling fuel occupies first the aperture formed in the lower insulation layer  106 , then rises into the smaller aperture formed in the upper insulation layer  102 , passing on the way the sharp edge of the conductive member  104 . The charging action of the conductive member is as explained in connection with  FIG. 4(   a ). Finally, the still swirling fuel passes through the apertures of the upper insulation layer  102  and upper planar member  100 , which are of roughly equal size, and exits the nozzle through the outlet orifice  114 , where it appears as charged droplets. 
     The operation of the nozzle is seen in greater detail in  FIG. 6 . The incoming fuel fills the outer portion  122  of the aperture of the lower insulation layer, while avoiding the inner portion  124 . Thus the outer portion  22  constitutes a spin chamber and the portion  124  remains a void in the nozzle. This action results from the centrifugal force exerted on the fuel by the swirling motion. In the diagram this force is such as to give rise to a direction of rotation  128  of the fuel. As a result a thin film of fuel  126  is formed in the vicinity of the conductive member  104 , upper insulation layer  102  and upper planar member  100 . Thus the fuel is readily charged as it rises past the edge of the conductive member  104 . The emerging atomised fuel can be seen as droplets  130 . 
     The detail of the construction and action of the conductive member  104  is illustrated in  FIGS. 7(   a ) and  7 ( b ).  FIG. 7(   a ) corresponds to  FIG. 6 . The part of  FIG. 7(   b ) highlighted by a broken circle is shown in greater detail in  FIG. 7(   b ). In this diagram, the electron flux from the sharp edge  140  is shown by the dotted lines  142  and the direction of the fuel, which swirls past the sharp edge, is shown by the arrow  144 . Incidentally, it is preferable if the sharp edge of the conductive member  104  does not protrude past the upper insulation layer  102 , in order to avoid the possibility of turbulence being created in this region. 
     The conductive member  104  has a thickness, which decreases substantially linearly between the annulus forming the aperture of the lower insulation layer  106  and the annulus forming the aperture of the upper insulation layer  102 . This assists the flow of the liquid fuel from the spin chamber  122  into the passage formed by the apertures of the upper insulation layer  102  and upper planar member  100 . 
     A second embodiment of a nozzle in accordance with the invention is illustrated in  FIGS. 8(   a )- 8 ( c ). In this embodiment the swirler action is created by an axial arrangement of fuel slots  150 . These slots  150  are formed in the lower planar member  108 .  FIG. 8(   b ) is a sectional view through the lower planar member along lines VIIIb in  FIG. 8(   a ) and shows the angled orientation of the slots through the lower planar member. This angled orientation is in a direction roughly tangential to an imaginary circle  152  running through the slots  150 , as shown in  FIG. 8(   c ). Thus the incoming fuel assumes both axial and tangential components of flow in the spin chamber. The action is similar to that of the radial-swirler version of  FIGS. 5-7 , except that the fuel is accelerated more through the nozzle, due to the axial flow component. 
     When the edge  140  of the electrode  104  is referred to as sharp, this means sufficiently sharp to effectively impart charge to the fuel droplets as they rapidly leave the outlet  114  of the nozzle. Purely as an example, it is considered that this requirement could be met with an edge  140  having an included angle of about one half of a degree, and a radius of not more than about one micron, though these are not hard and fast figures. 
     Although it has been assumed that the electrode  104  will have a bevelled profile at its radially inner extremity, this is not absolutely necessary. It is, however, preferred, as mentioned earlier, in order to improve the flow characteristics of the fuel as it passes from the inlet passages into the aperture region of the electrode  104  and first planar layer  102 . 
     In order to ensure that the electrons discharged from the conductive member can reliably charge the passing fuel, account is ideally taken of the tendency of the electrons to flow to ground through the hydrocarbon fuel, which is usually electrically conductive. This is achieved by arranging for a suitable rate of flow of the liquid fuel past the conductive member. 
     Details on how to determine a suitable flow rate through the nozzle are contained in, for example, the paper “The Electrostatic Atomization of Hydrocarbons” by A. J. Kelly, Journal of the Institute of Energy, June 1984, pp 312-320. According to this paper, most commercial hydrocarbons have an electrical breakdown strength in the region of 2×10 7  V/m. Once charge has been injected into the fuel stream by the charging electrode, it stagnates in the fluid. Subsequently, the charge is acted upon by the fluid flow and the electrical forces which act to attract the charge to the orifice electrode. As mentioned earlier, this orifice electrode (the planar member  100  in the present invention) will be held at a reference potential relative to the potential on the charging electrode (the electrode  104  in the present invention). For commercial oxygenated hydrocarbons, the electrical mobility is commonly in the range of 10 −7 -10 −8  m 2 /V·sec. (The electrical mobility is the ratio of the limiting velocity, to which a particle is accelerated in the presence of an electric field, to the magnitude of that field). Therefore, for a maximum electrical field of 2×10 −7  V/m, the mobility of the charge will be approximately 2 m/s. This means that the fluid should ideally be flushed through the nozzle at a speed &gt;2 m/s in order to reliably retain charge and provide good atomization. 
     It should be noted that the dielectric constant (electrical breakdown strength) for biofuels is approximately 50% higher than that for standard fuels. Consequently, if most commercial fuels have a dielectric constant of 2×10 7  V/m, as mentioned above, then most biofuels will have a dielectric constant of around 3×10 7  V/m. Since it is assumed that the electrical mobility for biofuels is roughly the same as for standard fuels—i.e. approximately 10 −7 -10 −8  m 2 /Vs—then a nozzle flow speed of ˜3 m/s would be required, if the same charging efficiency were to be maintained. 
     In an analogous manner, if a silicone oil were to be employed as the fuel passing through the nozzle, this would have a dielectric constant of about 1.5×10 7  V/m. Again, on the assumption that the electrical mobility for biofuels is of the same order as that for standard fuels, a nozzle flow speed of 1.5 m/s would be suitable.