Abstract:
A premixed fuel and air combustion system includes an anti-flashback electrode configured to repel a charge concentration in a combustion fluid and reduce or prevent the flame from flashing back into a mixer.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims priority benefit from U.S. Provisional Patent Application No. 61/653,722, entitled “LOW NO x  LIFTED FLAME BURNER”, filed May 31, 2012, and U.S. Provisional Patent Application No. 61/669,634, entitled “LOW NOx BURNER AND METHOD OF OPERATING A LOW NOx BURNER” filed Jul. 9, 2012, both of which, to the extent not inconsistent with the disclosure herein, are incorporated by reference in their entireties. 
     
    
     BACKGROUND 
       [0002]    The danger of flame flashback can be present in burner systems that contain premixed fuel and air. Large volumes of premixed fuel and air can present an explosion hazard and a containment hazard. 
         [0003]    What is needed is a technology to reduce the incidence of flame flashback. 
       SUMMARY 
       [0004]    According to an embodiment, a premixed fuel burner includes a body defining a fuel and air mixing volume and a passage configured to allow flow of premixed fuel and air from the mixing volume to a combustion volume as a premixed fuel jet. A charge source is configured to apply a first polarity voltage or charge to a combustion fluid corresponding to the premixed fuel jet. The charge source can be arranged in various ways. The first polarity voltage or charge can be applied to the fuel before mixing, to the air before mixing, to the fuel and air in the mixing volume, to the premixed fuel jet, or to the flame. An anti-flashback electrode is configured to carry a voltage at the first polarity and to electrically repel the first polarity charge in the combustion fluid. Flames have relatively high conductivity compared to the fuel jet. The repulsion of the first polarity charge in the flame (the flame being a portion of the combustion fluid) causes the flame to be repelled from the anti-flashback electrode. The anti-flashback electrode is arranged to repel the flame from flashing back into the mixing volume. 
         [0005]    In a premix burner, fuel and air are at least partially premixed in a mixing volume and the premixed fuel and air is output as a premixed fuel stream. A flame is supported with the premixed fuel stream. The presence of premixed fuel and air can create a hazard of “flashback”, where the flame can travel upstream and ignite the mixed fuel and air in the mixing volume. According to an embodiment, a method for reducing the danger of flashback includes applying a first voltage or charge at a first polarity to a combustion fluid and applying a second voltage to an anti-flashback electrode disposed adjacent to the premixed fuel stream and arranged to repel the first voltage or charge from flowing upstream toward the premixed fuel and air. The first voltage or charge can be applied to various portions of combustion fluid, which includes the fuel, the air, premixed fuel and air, the premixed fuel stream, and the flame. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]      FIG. 1  is a diagram of a premixed fuel burner including a mechanism for preventing flashback, according to an embodiment. 
           [0007]      FIG. 2  is a diagram of a premixed fuel burner including a mechanism for preventing flashback, according to another embodiment. 
           [0008]      FIG. 3  is a diagram of a premixed fuel burner including a mechanism for preventing flashback, according to another embodiment. 
           [0009]      FIG. 4  is a diagram of a premixed fuel burner including a mechanism for preventing flashback, according to another embodiment. 
           [0010]      FIG. 5  is a diagram of a premixed fuel burner including a mechanism for preventing flashback, according to another embodiment. 
           [0011]      FIG. 6  is a diagram of a premixed fuel burner including a mechanism for preventing flashback, according to another embodiment. 
           [0012]      FIG. 7  is a diagram of a premixed fuel burner including a mechanism for preventing flashback, according to another embodiment. 
           [0013]      FIG. 8  is a flow chart illustrating a method for reducing a danger of flashback in a premixed fuel burner, according to an embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0014]    In the following detailed description, reference is made to the accompanying drawings, which form a part hereof. In the drawings, similar symbols typically identify similar components, unless context dictates otherwise. Other embodiments may be used and/or other changes may be made without departing from the spirit or scope of the disclosure. 
         [0015]      FIG. 1  is a diagram of a premixed fuel burner  100 , according to an embodiment. The premixed fuel burner  100  includes a body  102  defining fuel and air mixing volume  104  and a passage  106  configured to allow flow of a premixed fuel jet  108 . The premixed fuel burner  100  includes a charge source  110  configured to apply a first polarity voltage or charge to a combustion fluid  112  corresponding to the premixed fuel jet  108 . Additionally, the premixed fuel burner  100  includes an anti-flashback electrode  114  configured to carry a voltage at the first polarity and to electrically repel the first polarity charge in the combustion fluid  112 . 
         [0016]    The premixed fuel burner  100  includes a voltage source  116  operatively coupled to the charge source  110  and the anti-flashback electrode  114  and configured to output the first polarity voltage. Optionally, the voltage source  116  may include separate voltage supplies for the charge source  110  and the anti-flashback electrode  114 . The voltage source  116  can be configured to output a substantially constant first polarity voltage. For example, in some embodiments, the voltage source  116  is configured to output a positive voltage. 
         [0017]    Alternatively, the sign of the first polarity can vary with time. For example, the voltage source  116  can be configured to output an alternating current voltage. By synchronously modulating the polarity of the voltage or charge applied to the combustion fluid and the voltage applied to the anti-flashback electrode, the anti-flashback electrode repels the instantaneously like (alternating) charges in the flame  124 . 
         [0018]    According to an embodiment, the combustion fluid  112  includes the premixed fuel jet  108 . The charge source  110  is configured to apply the first polarity voltage or charge to the mixed jet after the premixed fuel jet  108  is output through the passage  106 . 
         [0019]    According to an embodiment, a conductive flame-holding electrode  117  is configured to hold the flame  124  by providing an electrical attraction to the first polarity charges in the premixed fuel jet  108  and the flame  124 . The conductive flame-holding electrode  117  can be held at ground voltage as indicated in  FIG. 1 . Alternatively, the conductive flame-holding electrode  117  can be driven to a flame-holding voltage opposite in polarity to the first polarity. 
         [0020]    According to another embodiment, an aerodynamic bluff body can act as the flame holder. A bluff body can be made of a cast or extruded refractory material and/or ceramic. 
         [0021]      FIG. 2  is a diagram of a premixed fuel burner  200 , according to another embodiment. In the embodiment  200 , the charge source  110  is configured to apply the first polarity voltage to the flame  124 . The charge source  110  can be nearly any conductive material or shape, and does not eject charges into a dielectric region, as is done by a charge ejecting electrode depicted in  FIG. 1 . In an experimental apparatus, the charge source was a stainless steel rod partly immersed in the flame, and held at +15 kilovolts DC. 
         [0022]      FIG. 3  is a diagram of a premixed fuel burner  300 , according to an embodiment wherein the charge source  110  includes a charge-ejecting or corona electrode configured to apply the first polarity charge to the fuel  118  before the fuel  118  enters the mixing volume  104 . Typically, the walls of the mixing volume are held at the same polarity voltage as the charge source or are alternatively coated with a dielectric coating to minimize depletion of the charge concentration in the mixed fuel and air. 
         [0023]      FIG. 4  is a diagram of a premixed fuel burner  400 , according to an embodiment wherein the charge source  110  includes a charge-ejecting or corona electrode configured to apply the first polarity charge to the air  120  before the air  120  enters the mixing volume  104 . Typically, the walls of the mixing volume are held at the same polarity voltage as the charge source or are alternatively coated with a dielectric coating to minimize depletion of the charge concentration in the mixed fuel and air. 
         [0024]      FIG. 5  is a diagram of a premixed fuel burner  500 , according to an embodiment wherein the charge source  110  includes a charge-ejecting or corona electrode configured to apply the first polarity voltage or charge to the mixed fuel and air  122  in the mixing volume  104 . Typically, the walls of the mixing volume are held at the same polarity voltage as the charge source or are alternatively coated with a dielectric coating to minimize depletion of the charge concentration in the mixed fuel and air. 
         [0025]    Various embodiments of anti-flashback electrodes are contemplated. As depicted diagrammatically in  FIGS. 1-5 , the anti-flashback electrode  114  can be configured as a ring electrode disposed peripheral to the passage  106  and outside the mixing volume  104 , according to an embodiment. 
         [0026]      FIG. 6  is a diagram of a premixed fuel burner  600  according to an embodiment wherein the anti-flashback electrode  114  includes a flame arrestor disposed as a grid across the passage  106 . 
         [0027]      FIG. 7  is a diagram of a premixed fuel burner  700 , according to an embodiment wherein the anti-flashback electrode  114  includes at least a portion of the body  102  defining the mixing volume  104  including a region  702  of the wall of the mixing volume peripheral to the passage  106 . 
         [0028]    Also shown in  FIG. 7  is insulation on the walls of the mixing volume, as described in conjunction with  FIGS. 3-5 . According to embodiments, the wall of the mixing volume  104  can include a dielectric layer  704  disposed on a surface of the wall contacting the fuel  118  and air in the mixing volume  104 . The dielectric layer  704  can include a ceramic, a glass, a thermoplastic polymer, and/or a thermoset polymer, for example. 
         [0029]    According to various embodiments, the burner can include a side-fired burner, an up-fired burner, or a down-fired burner. 
         [0030]    According to an embodiment, the charge ejecting electrode and a counter electrode can be configured as an ionic wind generator operable to accelerate the premixed fuel jet  108  through the passage  106 . 
         [0031]      FIG. 8  is a flow chart of a method  800  for reducing the likelihood of flashback in a burner, according to an embodiment. Beginning at step  802  fuel and air are premixed in a mixing volume. Continuing to step  804 , the premixed fuel and air is output as a premixed fuel stream. In step  806 , a flame is supported with the premixed fuel stream. 
         [0032]    Proceeding to step  808  a first voltage or charge at a first polarity is applied to a combustion fluid. According to various embodiments depicted above, the combustion fluid to which the first voltage or charge is applied can be the fuel, the air, premixed fuel and air, the premixed fuel stream, or the flame. According to an embodiment, step  808  includes applying a voltage at the first polarity to a charge-ejecting electrode to output charges at the first polarity. A charge-ejecting electrode is particularly appropriate when the charge-receiving combustion fluid is relatively non-conductive. Alternatively, step  808  can include applying a voltage at the first polarity to a non charge-ejecting electrode. A non charge-ejecting electrode is particularly appropriate when the charge-receiving combustion fluid is relatively conductive. The most conductive portion of the combustion fluid is typically the flame, and a non charge-ejecting electrode is most commonly used when the voltage is applied to the flame. 
         [0033]    In one embodiment, Step  808  includes applying the first charge to the fuel before the fuel is mixed with the air. According to another embodiment, step  808  includes applying the first voltage or charge to the air before the fuel is mixed with the air. According to another embodiment, step  808  includes applying the first voltage or charge to the mixed fuel and air. According to another embodiment, step  808  includes applying the first voltage or charge to the premixed fuel stream. According to another embodiment, step  808  includes applying the first voltage to the flame. 
         [0034]    Proceeding to step  810 , a second voltage is applied to an anti-flashback electrode disposed adjacent to the combustion fluid and arranged to repel the first voltage or charge from flowing upstream toward the premixed fuel and air. According to an embodiment, step  810  includes applying the second voltage to a ring electrode disposed peripheral to the premixed fuel jet. According to another embodiment, step  810  includes applying the second voltage to a wall of a mixing volume disposed peripheral to the premixed fuel jet. According to another embodiment, step  810  includes applying the second voltage to a flame arrestor disposed across a passage between a mixing volume and a combustion volume. 
         [0035]    The second voltage is the same polarity as the first voltage, at least instantaneously. As indicated above the first polarity charge or voltage can be a single-sign such as a DC voltage/charge concentration. Positive voltages placed on a flame (directly or as charges delivered from air or fuel) were found to be most effective for flame attraction and repulsion compared to negative voltages. Alternatively, the first polarity can vary in time, such as is produced from an AC voltage waveform. By synchronously varying the voltage placed on the charge source and the anti-flashback electrode, the system maintains instantaneous repulsion of the flame by the anti-flashback electrode. 
         [0036]    According to embodiments, the charge concentration placed in the flame can be measured as about  15  kilovolt flame voltage. 
         [0037]    While various aspects and embodiments have been disclosed herein, other aspects and embodiments are contemplated. The various aspects and embodiments disclosed herein are for purposes of illustration and are not intended to be limiting, with the true scope and spirit being indicated by the following claims.