1 FLAT BURNER BACKGROUND OF THE INVENTION 1. Technical Field [0001] The present invention relates to a flat burner for use in a combustion apparatus such as a heat source apparatus for hot water supply and the like. 2. Background Art [0002] Generally, in a heat source apparatus for hot water supply, a plurality of flat burners each having a rectangular flame port portion at an upper end thereof are disposed in parallel with one another in a combustion box which forcibly supplies combustion air by means of a combustion fan. [0003] Conventionally, this kind of flat burner has a mixing tube portion and a distribution chamber portion. The mixing tube portion has: an inlet port of a cross-section elongated in a vertical direction where a longitudinal direction of the flame port portion is defined as a back-and forth direction; and a throat portion which lies adjacent to a back of the inlet port and which has a cross-section smaller than that of the inlet port. The distribution chamber portion introduces, from the mixing tube portion into the flame port portion, a mixture of a fuel gas ejected from a gas nozzle disposed to lie opposite to the inlet port and primary air flowing from the inlet port (see, for example, JP-A-1995-91620). In the conventional flat burner the throat portion is formed such that the center of the cross section (also referred to as "sectional center") of the throat portion is present in a position, as seen in the up-and-down direction (also referred to as the vertical direction), that is the same as the sectional center of the inlet port. [0004] By the way, in order to downsize the heat source apparatus, 2 it is sometimes required that the length of the mixing tube portion of the flat burner be shortened. However, if the length of the mixing tube portion is shortened, the mixing between the fuel gas and the primary air becomes insufficient, thereby giving a bad effect on the combustion performances. SUMMARY Problems that the Invention is to Solve [0005] In view of the above points, this invention has a problem of providing a flat burner which is arranged such that, even if the length of the mixing tube portion is shortened, the mixing of the fuel gas and the primary air can be accelerated. Means for Solving the Problems [0006] In order to solve the above problem, this invention is a flat burner having, at an upper end thereof, a rectangular flame port portion. The flat burner comprises a mixing tube portion and a distribution chamber portion. The mixing tube portion has: an inlet port of a cross section elongated in a vertical direction where a longitudinal direction of the flame port portion is defined as a back-and-forth direction; and a throat portion lying adjacent to a back of the inlet port and having a cross section smaller than that of the inlet port. The distribution chamber portion introduces, from the mixing tube portion into the flame port portion, a mixture of a fuel gas ejected from a gas nozzle disposed opposite to the inlet port and primary air entering the inlet port. The throat portion is formed at a position in which a sectional center thereof is offset in one of the upward and downward directions relative to a sectional center of the inlet port, and the gas nozzle is disposed such that an axis of a nozzle hole of the gas nozzle passes through a vertical position that is offset from the sectional center of the throat portion toward the sectional center of the inlet port.
3 [0007] According to this invention, out of the passage portion between the inlet port and the throat portion, in one half portion on one of the upper and lower sides (i.e., in the direction of offsetting of the sectional center of the inlet port relative to the sectional center of the throat portion), the primary air entering the inlet port flows at an angle that is largely inclined toward the other of the upper and lower sides (i.e., in the direction of offsetting of the sectional center of the throat portion relative to the sectional center of the inlet port). Further, the hole axis of the nozzle hole passes through the half portion of said one of the upper and lower sides of the passage portion between the inlet port and the throat portion. Therefore, the primary air collides at an angle against the fuel gas that flows along the hole axis and, consequently, even if the length of the mixing tube portion is short, the mixing between the fuel gas and the primary air is accelerated. [0008] Further, in case the connecting portion between the mixing tube portion and the distribution chamber portion is positioned backward of a longitudinal center of the flame port portion, the concentration of the fuel-air mixture supplied to the former half of the flame port portion is likely to get thinner. In this case, if the sectional center of the throat portion is offset downward relative to the sectional center of the inlet port, the nozzle hole axis passes through the upper side of the sectional center of the throat portion. As a result, the fuel concentration of the fuel-air mixture that flows from the upper portion of the rear end of the mixing tube portion to the front part of the distribution chamber portion becomes relatively thicker. It is thus possible to prevent the fuel concentration of the fuel-air mixture supplied to the former half part of the flame port portion from getting thinner. BRIEF DESCRIPTION OF THE DRAWINGS 4 [0009] FIG. 1 is a sectional side view of a combustion apparatus equipped with a flat burner of an embodiment according to this invention. FIG. 2 is a perspective view of the flat burner according to this embodiment. FIG. 3 is an exploded perspective view of the flat burner according to this embodiment. FIG. 4 is a sectional view of the flat burner taken along the line IV-IV in FIG. 2. FIG. 5 is a sectional side view taken along the line V-V in FIG. 4. FIG. 6 is a graph showing the results of fuel concentration measurements at various front and rear parts of the flame port portion of a flat burner according to this embodiment and a conventional flat burner. DESCRIPTION OF PREFERRED EMBODIMENTS [0010] With reference to FIG. 1, reference numeral 1 denotes a combustion box. The upper surface of the combustion box 1 is left open and an object to be heated such as a heat exchanger and the like (not illustrated) is disposed on top of the combustion box. Inside the combustion box 1 there is provided a partition plate 4 which partitions the space inside the combustion box 1 into a combustion chamber 2 and an air supply chamber 3 lying thereunder. The air supply chamber 3 has connected to a bottom thereof a combustion fan (not illustrated) through a duct 5 so that air can be supplied from the combustion fan to the air supply chamber 3. The partition plate 4 has formed therein a multiplicity of distribution holes 4a so that the air supplied to the air supply chamber 3 is supplied as secondary air to the combustion chamber 2 through these distribution holes 4a. [0011] A plurality of flat burners 6 according to an embodiment of this invention is disposed inside the combustion chamber 2. A riser 5 portion 41 is formed in a bent manner at the front edge of the partition plate 4. A manifold 7 is mounted at the front side of the riser portion 41 in a manner to block the lower front surface of the combustion box 1. The manifold 7 is provided with gas nozzles 71, 72 which face the inlet ports 65a, 67a of mixing tube portions 65, 67 of each of the flat burners 6. Details thereabout will be described hereinafter. It is thus so arranged that: each of the inlet ports 65a, 67a is supplied with a fuel gas from the respective gas nozzles 71, 72; and that each of the inlet ports 65a, 67a is supplied with primary air from the air supply chamber 3 through the clearance to be defined between the riser portion 41 and the manifold 7. [0012] As shown in FIGS. 2 and 3, the flat burner 6 is provided with a burner main body 61 and a burner cap 62 which is covered on top of the burner main body 61. A rectangular flame port portion 63 which opens upward is formed at an upper end of the burner main body 61. By means of the burner cap 62 there is formed a flame retention port portion 64 which is positioned on both sides of the flame port portion 63. It is thus so arranged: that a lean fuel-air mixture which is leaner in fuel concentration than a theoretical fuel-air ratio is ejected from the flame port portion 63; and also that a thick fuel-air mixture which is thicker in fuel concentration than the theoretical fuel-air ratio is ejected from the flame retention port portion 64, thereby performing so-called thick and thin fuel combustion. [0013] Detailed description will now be made of the construction of the flat burner 6, provided that the longitudinal direction of the flame port portion 63 is defined as a back-and-forth direction, and that the width direction of the flame port portion 63 is defined as a lateral direction. As shown in FIG. 4, the burner main body 61 is made up of a pair of side plates 6 la, 6 la which lie laterally opposite to each other. The side plates 61a, 61a are formed by bending a single piece of plate into 6 a shape of two-ply along a bending line which forms a bottom edge of the burner main body 61. Then, by means of press working of each of the side plates 61a, 61a, there are formed in the burner main body 61: the flame port portion 63 on an upper end; the mixing tube portion 65 at a lower part thereof; and a distribution chamber portion 66 which introduces the fuel-air mixture from the mixing tube portion 65 into the flame port portion 63. [0014] With reference also to FIG. 5, the mixing tube portion 65 is provided with: an inlet port 65a which is positioned at a lower front edge of the burner main body 61 and which has a cross-section elongated in the vertical direction; and a throat portion 65b which lies adjacent to a back of the inlet port 65a and which is smaller in cross-sectional area than the inlet port 65a. The rear end part of the mixing tube portion 65 is bent upward so as to be connected to the distribution chamber portion 66. At the front part of the burner main body 61 there is formed a mixing tube portion 67 for flame retention purpose (referred to as a "flame retention mixing tube portion") at a position between the mixing tube portion 65 and the distribution chamber portion 66. This flame retention mixing tube portion 67 ends up by extending slightly backward from the inlet port 67a that is located at the front edge of the burner main body 61, and a ventilation hole 67b is formed on a side surface of the rear end portion of the flame retention mixing tube portion. [0015] The connecting portion between the mixing tube portion 65 and the distribution chamber portion 66 is positioned backward of the longitudinal center of the flame port portion 63. Further, a constricted portion 66a which is constricted or narrowed in lateral width is formed at an upper part of the distribution chamber portion 66. The lateral width of the constricted portion 66a gradually widens or expands forward from the portion that is positioned right above the connecting portion between 7 the mixing tube portion 65 and the distribution chamber portion 66. According to this arrangement, the flow distribution in the longitudinal direction of the fuel-air mixture entering the flame port portion 63 is made uniform. [0016] The burner cap 62 has: a pair of side plates 62a, 62a which are covered on an outside of the pair of side plates 61a, 61a of the burner main body 61; and a plurality of longitudinally disposed bridge portions 62b which connect the side plates 62a, 62a together at the upper edges thereof. Between the side plate 61a of the burner main body 61 and the side plate 62a of the burner cap 62, there are defined: the flame retention port portion 64 at an upper end; and a passage which introduces the fuel air mixture into the flame retention port portion 64, the fuel-air mixture being discharged from the flame retention mixing tube portion 67 to the outside of the burner main body 61 through the ventilation hole 67b. In addition, at a plurality of longitudinal positions of the side plate 62a of the burner cap 62, there are formed recessed portions 62c which are in contact with the outside surface of the side plate 61a of the burner main body 61 in a manner to longitudinally segregate the flame retention port portion 64. [0017] Inside the flame port portion 63, there is mounted a straightening member 68 having a plurality of straightening plates 68a which are laterally disposed in parallel with one another. The straightening member 68 has contact portions 68b formed at a plurality of longitudinal positions corresponding to the bridge portions 62b of the burner cap 62, the contact portions being formed by bringing the straightening plates 68a into contact with each other, thereby segregating, in the longitudinal direction, the flame port passages to be defined between each of the straightening plates 68a. Further, the flame port portion 63 of the burner main body 61 has formed therein dented (or 8 narrowed) portions 63a which sandwich the straightening plates 68 from laterally both sides thereof at a vertically intermediate position of the burner main body 61. According to this arrangement, there are defined blind clearances 63b, being free from ejection of fuel-air mixture, between those parts of the side plates 61a which lie above the dented portions 63a and the outside straightening plate 68a. [0018] By the way, if the mixing tube portion 65 is shortened in length, the mixing between the fuel gas and the primary air becomes insufficient, whereby there is a possibility that a bad effect will sometimes be inflicted on the combustion performances. In addition, suppose that the connecting portion between the mixing tube portion 65 and the distribution chamber portion 66 is positioned backward of the longitudinal center of the flame port portion 63. In the conventional flat burner in which the sectional center of the throat portion 65b of the mixing tube portion 65 is present at the same vertical position as the sectional center of the inlet port 65a and in which the gas nozzle 71 is disposed such that the hole axis of the nozzle hole 71a passes through the sectional center of the inlet port 65a and of the throat portion 65b, the fuel concentration of the fuel-air mixture to be supplied to the former half part of the flame port portion 63 becomes thinner as shown by line b in FIG. 6. [0019] As a solution, as shown in FIG. 5, according to this embodiment, the throat portion 65b is formed such that the sectional center 02 thereof is offset downward relative to the sectional center 01 of the inlet port 65a, and the gas nozzle 71 is disposed such that the hole axis of the nozzle hole 71a passes through a vertical position which is offset from the sectional center 02 of the throat portion 65b toward the sectional center 01 of the inlet port 65a. In this embodiment, an arrangement has been made such that the hole axis of the nozzle hole 71a 9 passes through the sectional center 01 of the inlet port 65a. Alternatively, it may be so arranged that the hole axis of the nozzle hole 71a may be positioned above or below the sectional center 01 of the inlet port 65a as long as the hole axis of the nozzle hole 71a is above the sectional center 02 of the throat portion 65b. . [0020] According to this embodiment, out of the passage portion between the inlet port portion 65a and the throat portion 65b, in the upper half which is the direction of offsetting of the sectional center 01 of the inlet port 65a relative to the sectional center 02 of the throat portion 65b, the primary air entering the inlet port 65a flows at an angle that is largely inclined downward toward the throat portion 65b. Further, the hole axis of the nozzle hole 71a passes through the half portion on the upper side of the passage portion between the inlet port 65a and the throat portion 65b. Therefore, the primary air collides at an angle against the fuel gas that flows along the hole axis and, consequently, even if the length of the mixing tube portion 65 is short, the mixing between the fuel gas and the primary air is accelerated. [0021] In addition, the hole axis of the nozzle hole 71a will pass above the sectional center 02 of the throat portion 65b. As a result, the fuel concentration of the fuel-air mixture that flows from the upper side at the rear end of the mixing tube portion 65 to the front side portion of the distribution chamber portion 66 as shown in an arrow "a" in FIG. 5 becomes relatively thicker. As shown by line "a" in FIG. 6, the fuel concentration of the fuel-air mixture to be supplied to the former half part of the flame port portion 63 can be prevented from getting thinner. [0022] Further, according to the embodiment of this invention, in order to accelerate the mixing between the fuel gas and the primary air in the flame retention mixing tube portion 67, the rear half part of this mixing tube portion 67 is bent obliquely downward. Even if the flame 10 retention mixing tube portion 67 is bent in this manner, the sectional center of the throat portion 65b is offset downward relative to the sectional center of the inlet port 65a. It is thus possible to secure a required sealing allowance or space (the allowance or space for overlapping the side plates 61a, 61a together) between the mixing tube portions 65, 67. [0023] By the way, in the front surface of the riser portion 41 of the partition plate 4, a damper (not illustrated) is generally mounted so that, by partly clogging the inlet port 65a with the damper, the amount of supply of the primary air to the inlet port 65a may be limited. In this case, if the upper part of the inlet port 65a is clogged by the damper, there will be lost the significance of offsetting the sectional center of the throat portion 65b downward relative to the sectional center of the inlet port 65a. Therefore, it should be so arranged that the damper clogs the lower part of the inlet port 65a. [0024] Descriptions have so far been made of the embodiments of this invention with reference to the figures. This invention is, however, not limited to the above. For example, suppose that the connecting portion between the mixing tube portion 65 and the distribution chamber portion 66 is positioned forward of the longitudinal center of the flame port portion 63 and therefore that there will be no such disadvantage as the thinning of the fuel concentration of the fuel-air mixture at the front half part of the flame port portion 63. Then, the sectional center of the throat portion 65b may be formed so as to be present at a position offset upward relative to the sectional center of the inlet port 65a. In this arrangement, the gas nozzle 71 may be disposed in such a manner that the hole axis of the nozzle hole 71a passes through a vertical position which is deviated downward from the sectional center of the throat portion 65b.
11 [0025] In addition, the flat burner 6 according to the above embodiment is a thick-and-thin combustion type of burner provided with a burner cap 62. This invention can similarly be applicable to a flat burner which is not the thick-and-thin combustion type of burner in which the burner cap is omitted. [0026] It is to be understood that, if any prior art publication is referred to herein, such reference does not constitute an admission that the publication forms a part of the common general knowledge in the art, in Australia or any other country. [0027] In the claims which follow and in the preceding description of the invention, except where the context requires otherwise due to express language or necessary implication, the word "comprise" or variations such as "comprises" or "comprising" is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention.