Abstract:
A burner includes: a body defining an interior cavity; a burning surface located in the body and defining, at least in part, the interior cavity; a defusing surface located on an exterior portion of the body; ports on the body extending through the defusing and burning surfaces and configured to provide fluid communication between the interior cavity and ambient air outside the body; and an opening larger than at least one of the ports, the opening providing fluid communication between the interior cavity and a space outside of the body. A method of burning a gas and reducing acoustic feedback in a combustion device are also described.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates generally to a method and apparatus for stabilizing a gas flame and reducing NOx emissions. More particularly, the present invention relates to a method and apparatus for combusting a air and fuel mixture in a hollow cavity of a burner. 
       BACKGROUND OF THE INVENTION 
       [0002]    Many household and commercial combustion devices, such as furnaces, hot water heaters, gas dryers, boilers, absorption heat pumps, sterling engines and other devices, often use burners to burn an air and fuel mixture to create heat. The fuel can include natural gas, propane, or other suitable fuels. Usually the burner is located in a combustion chamber and a pre-mixture of air and fuel is provided to the burner for combustion. 
         [0003]      FIG. 1  is an example of a typical burner. As shown in  FIG. 1 , the burner  10  is generally cylindrical in shape, although typical burners may include other shapes. On the outside of the cylinder is a burning surface  12  upon which the air and fuel mixture will burn. At one end of the cylinder is an end cap  14 . The other end may include a flange  16  which may include fastener holes  18  through which bolts or other fasteners extend for securing the burner  10  to a supporting structure. 
         [0004]    The burning surface  12  includes ports  20 . The ports  20  are often elongated slits which may be as wide about as much as the wall of the burner  10  is thick. For example, a slit may often be a half a millimeter wide by five millimeters long, but other dimensions are certainly used for the ports. The interior  22  of the burner  10 , includes a diffusing surface which cannot be seen in the angle shown in  FIG. 1 . 
         [0005]    An air and fuel mixture is supplied to the burner  10  in the direction of arrow A and flows into the open end of the burner  10  and into the interior  22  of the burner  10 . The air and fuel mixture then exits through the ports  20  in the directions of arrows B. While the directions of arrows B are only shown in right and left orientations, one skilled in the art will appreciate that the air and gas mixture will extend radially out through all of the ports  20 . An ignition device will ignite the air and fuel mixture and it will burn on the burning surface  12  of the burner  10 . As the air and fuel mixture burns, heat will radiate radially out from the burner  10  in all directions similar to arrows B. Heat also moves by convection as the hot gases created by combustion move in the direction of arrows B. 
         [0006]    If the air and fuel mixture is supplied too quickly, the air and fuel mixture may exit through the ports  20  faster than the burn rate of the fuel. Under this condition, combustion may move off of the burning surface  12  of the burner  10  and the burner  10  can experience blow out, a condition where combustion stops and the flame goes out. Blow out can occur in a localized area at an individual port and, under certain conditions, may spread or also occur at other places on the combustion surface  12 . Therefore, the air and fuel mixture must be carefully controlled in order to achieve desired burn characteristics and avoid blow out. 
         [0007]      FIG. 2  shows a cross-section of a typical burner  10  located within a heat exchanger  54 . The burner  10  may be secured to the combustion device  55  via fasteners  52  connecting to the flange  16 . Fasteners  52  may connect directly to the combustion device  55  or in some embodiments as shown in  FIG. 2  connect to the air/gas supply duct  64  configured to supply the air and fuel mixture to the burner  10 . The air and fuel mixture enters the burner  10  in the direction of arrows A. The air and fuel mixture is diffused by the diffusing surface  24  in the interior  22  of the burner  10  and exits out the ports  20  in the direction of arrows B. The air and fuel mixture is burned on the burning surface  12  generating heat for transferring to the heat exchanger  54 . The heat exchanger  54  includes tubes  56  shown in cross section which are filled with a fluid that flows through the tubes  56 . As the hot gas generated by the combustion of the air and fuel mixture passes over and around the tubes  56 , heat from the hot gases flows through the tubes  56  into the fluid flowing through the tubes  56 . After the hot gases pass over the tubes  56 , they may move in the direction of arrows E and exit through an exhaust port  60 . 
         [0008]    The burner  10  located in the combustion chamber  58  of the heat exchanger  54  is generally spaced from the tubes  56  in order for the tubes  56  to be located outside of the post-combustion zone  62 . While combustion generally occurs on the combustion surface  12 , chemical reactions still occur in the post-combustion zone. For example, in the post-combustion zone, CO is converted to CO 2 . An example of a typical post-combustion zone  62  for a household appliance may be about two inches between the burning surface  12  and the tubes  56 . Of course, the size of the post-combustion zone  62  depends on many factors, including the size and capacity of the burner, the fuel used and rate of fuel bring burned and other factors. 
         [0009]    As one skilled in the art can appreciate, not only does the rate of the air and fuel mixture entering the burner  10  affect the burning characteristics, but also factors such as the temperature at which combustion occurs and the temperature of the post-combustion zone  62 . Another factor affecting the burning characteristics is feedback from acoustic resonance of the heat exchanger  54 . Vibrations or resonance of the combustion chamber  58  sometimes referred to as acoustic feedback, also can affect the chemical reactions that occur in the post-combustion zone  62  and the combustion occurring on the combustion surface  12 . 
         [0010]    The combustion device  55 , heat exchanger  54  and flow path shown by arrows E can be considered as a Helmholtz resonator creating acoustic resonance which can affect the chemical reactions occurring both in the site of combustion and the post-combustion zone. Such resonance can be difficult to control. The resonance can affect the combustion of the air and fuel mixture creating undesirable affects such as more NOx emissions, instability of the flame, or other undesirable burning characteristics. In addition, burners  10  may be tuned or controlled to work despite undesirable feedback rather than being tuned to reduce other undesirable characteristics. 
         [0011]    Today&#39;s competitive and environmental concerns constantly place pressure on products such as combustion devices to enhance efficiency and lower emissions. Increasing these performance parameters are becoming more challenging as it is difficult to reach better performance parameters with present known burner technology. The emission limits and efficiency requirements are becoming more difficult to achieve while maintaining or increasing burner performance characteristics, such as modulation range, stability of flame, ignition characteristics, burner load and absence or reduction of resonance combustion noise. 
         [0012]    Accordingly, is it desirable to provide a burner that can increase performance parameters for the emissions requirements and efficiency, but also maintain desirable burner performance characteristics such as a desired modulation range, flame stability, desired ignition characteristics, desired burner load, and a reduction or absence of resonance combustion noise. 
       SUMMARY OF THE INVENTION 
       [0013]    The foregoing needs are met, to a great extent, by the present invention, wherein in one aspect a method and apparatus is provided that in some embodiments improve burner efficiency, reduce undesirable emissions, and maintain desirable characteristics such as a desired modulation range, increased flame stability, desired ignition characteristics, desired burner load, and a reduction or absence of resonance combustion noise. 
         [0014]    In accordance with one embodiment of the present invention, a method of burning a gas is provided. The method includes: flowing a air and fuel mixture through a hole in a burner into an interior cavity in the burner; burning the air and fuel mixture on an interior surface defining, at least in part, the interior cavity; and moving heated gas from the interior cavity through a hole in the burner to outside the burner. 
         [0015]    In accordance with another embodiment of the present invention, a burner is provided. The burner includes: a body defining an interior cavity; a burning surface located in the body and defining, at least in part, the interior cavity; a defusing surface located on an exterior portion of the body; ports on the body extending through the defusing and burning surfaces and configured to provide fluid communication between the interior cavity and ambient air outside the body; and an opening larger than at least one of the ports, the opening providing fluid communication between the interior cavity and a space outside of the body. 
         [0016]    In accordance with yet another embodiment of the present invention, a method of reducing acoustic feed back to a combustion device is provided: creating a concentrated flow path for hot gases generated in a combustion device; locating a heat exchanger in the flow path for harvesting heat from the hot gases; and locating in the flow path between the combustion device and the heat exchanger one of: a gap, a connector diverging from the combustion device to the heat exchanger, and a connector converging from the combustion device to the heat exchanger. 
         [0017]    There has thus been outlined, rather broadly, certain embodiments of the invention in order that the detailed description thereof herein may be better understood, and in order that the present contribution to the art may be better appreciated. There are, of course, additional embodiments of the invention that will be described below and which will form the subject matter of the claims appended hereto. 
         [0018]    In this respect, before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and to the arrangements of the components set forth in the following description or illustrated in the drawings. The invention is capable of embodiments in addition to those described and of being practiced and carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein, as well as the abstract, are for the purpose of description and should not be regarded as limiting. 
         [0019]    As such, those skilled in the art will appreciate that the conception upon which this disclosure is based may readily be utilized as a basis for the designing of other structures, methods and systems for carrying out the several purposes of the present invention. It is important, therefore, that the claims be regarded as including such equivalent constructions insofar as they do not depart from the spirit and scope of the present invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]      FIG. 1  is a perspective view illustrating a typical burner. 
           [0021]      FIG. 2  is a cross-section view of a typical combustion device including a heat exchanger and burner. 
           [0022]      FIG. 3  is a perspective view of a combustion chamber including a burner in accordance with one embodiment of the invention. 
           [0023]      FIG. 4  is a perspective view of a burner in accordance with another embodiment of the invention. 
           [0024]      FIG. 5  is a cross-section view of a combustion chamber containing a burner in accordance with an embodiment of the invention. The combustion chamber and burner are fluidly connected to a heat exchanger. 
           [0025]      FIG. 6  illustrates a side view of a burner in accordance with the invention connected with a converging pipe to a heat exchanger. 
           [0026]      FIG. 7  illustrates a cross-section view of a burner in accordance with the invention connected with a diverging pipe to a heat exchanger. 
       
    
    
     DETAILED DESCRIPTION 
       [0027]    Embodiments in accordance with the invention will now be described with reference to the drawing figures in which like reference numerals refer to like parts throughout. A burner  30  in accordance with an embodiment of the invention is illustrated in  FIG. 3 . The burner  30  is, in some aspects, similar to the burner shown in  FIG. 1 . However, the burner  30  shown in  FIG.3  has several key distinctions. 
         [0028]    The burner  30  includes an interior burning surface  32 . The interior burning surface  32  may in some embodiments be made of refractory high temperature steel. Alternatively the burning surface  32  could be made of metal fibers or ceramics. Any suitable material may be used to comprise the burning surface  32  in accordance with the invention. The burner  30  is generally cylindrical in shape with one end having a cap  34 . Near the other end of the burner is a flange  36  having fastener holes  38  for attaching the burner  30  to a support structure. 
         [0029]    The burner  30  includes ports  40  located on the cylindrical portion  41  as well as the end cap  34 . The ports  40  may be similarly sized to those described with respect to  FIG. 1 . The width of the ports  40  may be approximately the same as the thickness of the material comprising the side wall  41  of the burner  30 . One example size of port  40  may be one half millimeter by five millimeters, however, any suitable size port  40  may be used in accordance with the invention. The ports  40  may be sized according to the specific needs of a particular application. The ports  40  provide fluid communication between the ambient environment outside of the burner  30  and the interior  42  of the burner. 
         [0030]    On the outside of the burner  30  is a diffusing surface  44 . In some embodiments in accordance with the invention, the diffusing surface  44  may be made of stainless steel, however, any suitable material maybe used in accordance with the invention. 
         [0031]    As shown  FIG. 3 , the burner  30  may be located in a pressure chamber  46 . The pressure chamber  46  shown in  FIG. 3  is merely a representation and is not intended to describe specific geometry of a pressure chamber  46 . Any suitable geometry may be used in accordance with the invention. An inlet  48  provides an air and fuel mixture to the pressure chamber  46 . The air and fuel mixture flows in the direction indicated by arrows C through the inlet  48  into the pressure chamber  46  and through the ports  40  into the interior  42  of the burner  30 . 
         [0032]    According to some embodiments of the invention, the inlet  48  and the pressure chamber  46  are pressurized to a positive pressure in order to drive the air and fuel mixture into the burner  30 . Some embodiments may include a fan or other means for pressurizing the inlet  48  and pressure chamber  46  upstream from the burner  30 . Other embodiments may include a fan or other pressure inducing device downstream from the burner  30 . The burner  30  may be used whether the air and fuel mixture is pushed or pulled through the burner  30 . In embodiments where air and fuel mixture is pulled through the burner  30  one skilled in the art will appreciate that the air and fuel mixture may be supplied without a pressurized chamber  46 . Any suitable means of providing the air and fuel mixture to the burner  30  may be accomplished in accordance with the invention. 
         [0033]    Once the air and fuel mixture has flowed in the direction of arrows C through the ports  40  into the interior  42  of the burner  30 , the air and fuel mixture is burned in the interior  42  of the burner  30 . The air and fuel mixture may be ignited by any suitable igniter system. 
         [0034]    The combustion of the air and fuel mixture occurs within the interior hollow cavity  42  defined by the burner  30 . Once the air and fuel mixture has been burned in the burner  30  it escapes from the burner  30  out the opening  50  in the direction of arrow D. 
         [0035]    As can be seen in  FIG. 3  the interior burning surface  32  may have a much larger surface area than the surface area of the opening  50 . This characteristic is in contrast to the burner shown in  FIG. 1 , where the burning surface  12  is roughly the same size (as it is roughly the same area) as the area of heat release. On the burner  10  shown in  FIG. 1 , heat escapes in the direction shown by arrows B. The ratio of the burning surface  12  and the surface emitting heat  12  is roughly 1 to 1. 
         [0036]    With the burner  30  shown in  FIG. 3 , the burning surface  32  may have a much greater surface area than the area of the opening  50  through which the hot gases and the heat escape the burner  30 . This gives the affect of concentrating the hot gases and heat generated by the burner through the opening  50 . The ratio of the burning surface  32  to the opening  50  is theoretically unlimited. 
         [0037]      FIG. 4  is a perspective view of another burner  30  in accordance with the invention. The burner  30  illustrated in  FIG. 4  is box shaped. The burner  30  has ports  40  similar to those described already above. The air and fuel mixture enters the burner  30  through the ports  40  in the directions illustrated by arrow C. The burner  30  illustrated in  FIG. 4  may contain more than one opening  50  through which hot gases and heat may escape the burner  30 . The openings  50  through which the hot gases and heat may escape the burner  30  are shown in irregular patterns to illustrate that the shape of openings  50  for the burner  30  may be selected to satisfy whatever requirements are presented in individual application or to satisfy a particular desire. In addition, while two openings  50  are illustrated in the burner  30  of  FIG. 4 , any number of openings  50  may be used as desired. 
         [0038]    Burners  30  may be in any shape having an interior cavity. Examples may include burners that are in the shape of a cylinder, a box, a sphere, torus, and a U shape. Other shapes, both regular and irregular, may be used. 
         [0039]    The burner  30  of  FIG. 4  combusts the air and fuel mixture in the interior  42  of the burner  30  and vents the heat through the openings  50  in the direction of arrows D. 
         [0040]    According to some embodiments of the invention, the air and fuel mixture provided a burner  30  includes more air than the stoichiometric required amount to achieve combustion. In some embodiments the amount of air supplied to the burner  30  may be 1.1 to 1.5 times the stoichiometric required amount. In other embodiments other air and fuel ratios may also be used. In some embodiments of the invention, applying 1.2 to 1.3 times the stoichiometric required amount of air produces a blue flame within the burner. Using blue flame can reduce NOx emissions. 
         [0041]    Certain burners  30  may achieve a few advantages by conducting the combustion inside the interior cavity  42  of the burner  30 . For example, the modulation range of the burner  30  maybe increased because when combustion is being conducted within a smaller space, the burner is less likely to experience flame out. If combustion along any portion of the burning surface  32  goes out for whatever reason (such as an anolomity in the delivery in the air and fuel mixture) rather than merely flaming out, the surrounding combustion can reignite the air and fuel mixture with respect to the individual port or portion of the burning surface  32  that was experiencing flame out. 
         [0042]    Such a feature may allow the burner  30  to have increased reliability at higher air and fuel mixture speeds thereby increase its modulation range. At the other end of the spectrum, the modulation range may also be increased by allowing a lower amount of an air and fuel mixture to be provided to the burner  30  and the burner  10  still having reliable operation. The burner  30  is able to sustain combustion when both less and more air and fuel mixture is applied to it, thus increasing its modulation range. For the same reasons, flame stability is also increased within the modulation range. Burners  30  in accordance with the invention may have a modulation range at or beyond 10:1. 
         [0043]    At low air and fuel mixtures where infrared burning occurs, the flame may be stabilized by the temperature profile. At very low air fuel mixture velocity when the flame is blue, flame stabilization occurs through the temperature of the cavity in the burner  30 . 
         [0044]      FIG. 5  is a cross-sectional view of a burner  30  fluidly connected to a heat exchanger  66  in accordance with an embodiment of the invention. The burner  30  is mounted in a pressure chamber  46 . The air and fuel mixture enters the pressure chamber  46  through the inlet  48 . The air and fuel mixture is at a positive pressure within the pressure chamber  46  and moves into the burner  30  via the ports  40 . 
         [0045]    Arrows C illustrate the direction of movement for the unburned air and fuel mixture. The air and fuel mixture is burned within the interior chamber  42  of the burner  30  and moves in the direction of arrows D through the burner outlet  50  through the pathway  72  of the heat exchanger  66 . As the hot gases move through the pathway  72  of the heat exchanger  66 , heat is moved from the hot gases through the side walls  67  into the fluid  68  to be heated. 
         [0046]    In some heat exchanging systems, the heat exchanging pathway such as the pathway  72  shown in  FIG. 5 , may be subject to acoustical vibrations and resonance. Passage ways such as passage way  72  shown in  FIG. 5  can be modeled as a Helmholtz resonator and amplify vibrations and/or acoustical resonance. The vibrations may result in acoustical feedback to a burner  30 . 
         [0047]    In accordance with some embodiments of the invention, a gap  70  exists between the opening  50  of the burner  30  and the pathway  72  through the heat exchanger  66 . The gap  70  may help prevent acoustical feedback to the burner  30 . As hot gases moving through the passage way  72  they can create vibrations and acoustical resonance. These vibrations may travel back towards the burner  30 . The gap  70  between the burner and the pathway  72  may reduce the amount of the acoustical resonance transmitted back to the burner  30 . By reducing acoustical feedback to the burner  30 , combustion can be better controlled within the burner  30 . 
         [0048]    Because the combustion and post-combustion zones are in the cavity  42  the heat exchanger  54  and combustion chamber/pressure chamber  46  have little or no influence on the cavity  42  conditions. 
         [0049]    In accordance with other embodiments of the invention, as shown in  FIG. 6 and 7 , other techniques for reducing acoustical resonance feedback to the burner  30  can include using a divergent or convergent tube to connect the burner  30  to the heat exchanger  66 . 
         [0050]      FIG. 6  illustrates a combustion chamber  46  having a burner  30  where combustion is occurring. Hot gases flow in the direction illustrated by arrows D out of the burner  30  toward the pathway  72  through the heat exchanger  66 . The burner  30  connects via the flange  36  to the pressure chamber  46 . A convergent transition piece or tube  74  connects the interior of the burner  30  to the pathway  72  through the heat exchanger  66 . The convergent transition piece  74  may include flanges  76  and  78  similar to that described with respect to the burner  30  in order to allow the convergent transition piece  74  to connect to the pressure chamber  46  and the heat exchanger  66 . 
         [0051]    Contrast to that shown in  FIG. 6 , the embodiment shown in  FIG. 7  includes a divergent transition piece  80  connecting the interior  42  of the burner  30  to the fluid path  72  within the heat exchanger  66 . The divergent transition piece  80  may be a tube having flanges  82  and  84  similar to those described above. The flanges may include holes for providing fasteners to connect the divergent transition piece  80  to the heat exchanger  66  of the pressure chamber  46 . Alternatively the flange  84  may also allow fasteners to connect to the flange  36  of the burner  30 . Any suitable way of attaching a divergent transition piece  80  or a convergent transition piece  74  as shown in  FIG. 6  may be used in accordance with the invention and flanges are not necessary for all embodiments. 
         [0052]    The convergent transition piece  74  of  FIG. 6  and the divergent transition piece  80  of  FIG. 7  serve similar purpose as the gap  70  shown in  FIG. 5 . The transition pieces  74  and  80  reduce the amount of acoustic resonance generated in the heat exchanger  66  transmitted back to the burner  30 . Gaps  70  or convergent transition pieces  74  or divergent transition pieces  80  have been shown to be effective in reducing the amount of acoustical resonance in comparison to a transition piece where the interior diameter of the transition piece is the same as the fluid path  72  and the interior of the burner. In some respects the gap  70  shown in  FIG. 5  can be considered to be a divergent transition piece of infinite diameter. 
         [0053]    In accordance with some embodiments of the invention, none of the combustion of the hot gases occurs within the fluid path  72  of the heat exchanger  66  but rather the combustion as well as the post combustion zone occurs within the interior  42  of the burner  30 . Keeping the post-combustion zone in the burner  30  can increase the temperature of the hot gases within the post-combustion zone. The post-combustion zone  62  of  FIG. 2  is not contained in the interior  42  of a burner. As such, it may not be as hot due to several factors. 
         [0054]    For example, the ratio of the combustion surface and the surface exiting hot gases in the burner  20  of  FIG. 2  is about  1 : 1 . Thus, the heat and hot gases are not concentrated as described above where the ratio of heat emitting surface and the combustion surface can be much higher. 
         [0055]    In addition, the post-combustion zone  62  as shown in  FIG. 2  may lose heat to the tubes  56  of the heat exchanger before all chemical reactions occur in the post-combustion zone  62 . 
         [0056]    In contrast to the burner of  FIG. 2 , and as shown in  FIGS. 3-7 , a burner  10  in accordance with some embodiments of the invention maintain the combustion and post-combustion zone in the interior  42  of the burner  10 . By keeping the combustion and post-combustion zones in the interior  42  of the burner  10 , the heat and hot gases are concentrated. In addition, the hot gases are not able to lose heat to a heat exchanger until the hot gases have exited the post-combustion zone. By maintaining a relatively hot post-combustion zones, the flame is stable and a more complete conversion of CO to CO 2  may be achieved thereby reducing CO emissions. Under these conditions NOx emissions may be very low. In some embodiments CO emissions may be at or lower than 10 ppm air free and NOx emissions may be at or lower than 10 ng/J. 
         [0057]    A further advantage of a burner  10  according to some embodiments is that the heat and hot gases are easily directed to a desired pathway. The heat can easily be directed to encounter a heat exchanger at a desired angle.