An air-gas mixture burning appliance with a gas governor that comprises a gas restrictor

Abstract

An air-gas mixture burning appliance 100 has an air-gas mixing unit 110. The air-gas mixing unit includes at least one air-gas mixer 118 that forms a point of mixing 119 for mixing of air and gas to form a combustible air-gas mixture 130. A gas restrictor 250 for restricting the flow of gas 240 in a predetermined flow direction 242 in a gas channel 216 is arranged between a gas valve 215 and the air-gas mixer, the gas restrictor including at least one annular gas flow passage (320 Fig. 3) in the gas channel. In use, a gas governor 210 that includes the gas valve controls gas pressure in the gas channel dependant on an air pressure signal 235 that is indicative of a static air pressure in the air-gas mixing unit air way 212 and the restricted flow of gas from the gas valve gas restrictor that restricts flow of gas from the gas valve to the point of mixing. Providing the gas flow passage in the gas channel in an annular form instead of a conventional orifice-type form enables a greater degree of control over a respectively associated Reynolds number of the gas restrictor.

Description

Background of the Invention The present invention relates to an air-gas mixture burning appliance with an air-gas mixing unit that comprises at least one air-gas mixer that forms a point of mixing for mixing of air and gas to form a combustible air-gas mixture, an air supply with an air way that is connected to the point of mixing for supply of air to the point of mixing, and a gas supply with a gas channel that is connected to the point of mixing and a gas governor that is adapted to control supply of gas to the point of mixing via the gas channel, wherein the gas governor comprises a gas valve that is adapted to control gas pressure in the gas channel dependant on an air pressure signal that is indicative of a static air pressure in the air way, and a gas restrictor that restricts flow of gas from the gas valve to the point of mixing. The present invention relates further to a gas restrictor for such an air-gas mixture burning appliance. From the state of the art an air-gas mixture burning appliance with an air-gas mixing unit is known, wherein e.g. a hydrocarbon gas such as methane or propane, or hydrogen may be used as gas and mixed with air to form a combustible air-gas mixture. The air-gas mixing unit may comprise a Venturi-type mixing nozzle that forms the combustible air-gas mixture with a desired concentration or ratio from separate gas and air streams. The air stream is generally forced into motion by a fan, which may be located upstream or downstream of the Venturi-type mixing nozzle. The Venturi-type mixing nozzle usually comprises an airway with a converging channel in order to accelerate the air stream for creating a decreased air pressure. This decreased air pressure, which may also be referred to as suction pressure, causes a suction effect on the gas stream in an associated gas channel, which causes the gas stream to flow into the Venturi-type mixing nozzle, where the gas stream mixes with the air stream. In the associated gas channel, an upstream side of the gas stream is controlled by a gas valve, which regulates a respective gas pressure relative to an air pressure measured at an air pressure measuring point located usually in the airway, thus, controlling a respective flow rate of the gas stream. In order to further control the respective flow rate of the gas stream, a gas restrictor is placed in between the gas valve and the Venturi-type mixing nozzle. The gas restrictor is used to control a respective gas flow rate in response to a current air flow rate. In particular, the shape of the gas restrictor is designed to tailor an underlying air-gas equivalence ratio (lambda) throughout the operating range of the air-gas mixture burning appliance. The ideal variation of air-gas ratio across the operating range varies depending on application. Typically, a respectively desired air-gas ratio is a trade-off between considerations such as thermal efficiency, combustion quality (complete combustion), and burning unit temperature. In an air-gas mixture burning appliance used e.g. for residential heating, a requested target air-gas ratio is usually leaner than stoichiometric throughout the operating range. Additionally, for safe operation, it is usually desired that the air-gas ratio becomes leaner as heat input reduces. Usually, the gas restrictor has a cross sectional area which is smaller than elsewhere in the gas channel between the gas valve and the Venturi-type mixing nozzle of the air-gas mixing unit. By way of example, EP 4 191 135 A1 describes a commonly known gas restrictor with an orifice that comprises a reduced diameter compared to the gas channel’s inner diameter for forming a single flow opening. Instead of such a single flow opening, multiple parallel flow openings may also be used. Summary of the Invention The present invention relates to a gas restrictor for restricting flow of gas in a predetermined flow direction in a gas channel that is arranged between a gas valve and an air-gas mixer of an air-gas mixture burning appliance, comprising at least one annular gas flow passage in the gas channel. Advantageously, providing the gas flow passage in the gas channel in annular form instead of a conventional orifice-type form enables a greater degree of control over a respectively associated Reynolds number of the gas restrictor. Furthermore, control over the associated Reynolds number of the gas restrictor is enabled without affecting an underlying open area ratio of the gas restrictor. Moreover, the at least one annular gas flow passage in the gas channel may be more effective when reducing Reynolds numbers for a given small open area ratio. Furthermore, the at least one annular gas flow passage in the gas channel is easy and cheap to manufacture, especially compared to multiple parallel flow openings in a gas restrictor with a comparatively small open area ratio, as in the latter case the multiple parallel flow openings would need to be small and, therefore, difficult and expensive to manufacture. Preferably, the at least one annular gas flow passage comprises a circular outer circumference. Forming the at least one annular gas flow passage with a circular outer circumference enables realization of a simple and basic annular design which may be implemented easily with a reduced number of associated design variables. Alternatively, the at least one annular gas flow passage may comprise a polygonal outer circumference. Still alternatively, the at least one annular gas flow passage may comprise a star-like outer circumference. Still alternatively, the at least one annular gas flow passage may comprise an undulated outer circumference. Forming the at least one annular gas flow passage with a polygonal, star-like, or undulated outer circumference enables provision of a flow restriction with an increased perimeter, allowing a lower Reynolds number than a fully circular annular gas flow passage at the same open area ratio. Preferably, the at least one annular gas flow passage is rotationally symmetric and / or axially symmetric. Alternatively, the at least one annular gas flow passage may be asymmetric. Thus, multiple different design variants may be implemented for adapting the gas restrictor in an application-specific manner to achieve respectively required gas flow properties. Preferably, the gas restrictor comprises at least one flow restriction component which is arranged in the gas channel for forming the at least one annular gas flow passage in the gas channel. Thus, the at least one annular gas flow passage may easily and reliably be embodied in the gas channel. Preferably, the at least one flow restriction component is connected to at least one support rod which extends coaxially to the gas channel. By connecting the at least one flow restriction component to the at least one support rod which extends coaxially to the gas channel, performance of the at least one flow restriction component is advantageously consistent all around its circumference. The at least one support rod may be connected to one of an external retainer which is arranged outside of the gas channel, or an internal retainer which is arranged inside of the gas channel and connected to an inner circumference of the gas channel. By positioning a respective connection of the at least one support rod to an external or internal retainer upstream or downstream of the at least one flow restriction component, any disturbance of the gas flow in the region of the at least one flow restriction component due to the external or internal retainer may be minimized. Alternatively, the at least one flow restriction component may be connected to at least one support rod which is arranged inside of the gas channel and connected to an inner circumference of the gas channel, wherein the at least one support rod extends perpendicular to the gas channel. Thus, a basic connection of the at least one flow restriction component to the gas channel with a simple design may be implemented. According to one aspect, the at least one flow restriction component comprises a streamlined body with a length in the predetermined flow direction which amounts at least to a restricting diameter of the at least one flow restriction component in the gas channel. Thus, disturbances of the gas flow in the gas channel which are caused by the at least one flow restriction component may at least be reduced. Preferably, the at least one annular gas flow passage comprises an open area ratio between 0.25 and 0.9 relative to the gas channel, and a Reynolds number between 100 and 2500 evaluated inside the at least one annular gas flow passage and associated with a minimum heat input rate of the air-gas mixture burning appliance. Thus, when used in an air-gas mixture burning appliance with an air-gas mixing unit that forms a combustible air-gas mixture, the gas restrictor allows to improve combustion quality by enrichening the combustible air-gas mixture at minimum heat input operating condition without affecting maximum heat input operating condition. Alternatively, the at least one annular gas flow passage may comprise an open area ratio of at most 0.4 relative to the gas channel, and a Reynolds number of at most 1000 evaluated inside the at least one annular gas flow passage and associated with a minimum heat input rate of the air-gas mixture burning appliance. Thus, when used in an air-gas mixture burning appliance with an air-gas mixing unit that forms a combustible air-gas mixture which is to be burned at an associated burning unit, the gas restrictor allows to improve temperature of the associated burning unit by leaning out the air-gas mixture at minimum heat input operating condition without affecting maximum heat input operating condition. Furthermore, the present invention relates to an air-gas mixture burning appliance with an air-gas mixing unit, wherein the air-gas mixing unit comprises at least one air-gas mixer that forms a point of mixing for mixing of air and gas to form a combustible air-gas mixture, an air supply with an airway that is connected to the point of mixing for supply of air to the point of mixing, and a gas supply with a gas channel that is connected to the point of mixing and a gas governor that is adapted to control supply of gas to the point of mixing via the gas channel. The gas governor comprises a gas valve that is adapted to control gas pressure in the gas channel dependant on an air pressure signal that is indicative of a static air pressure in the air way, and a gas restrictor that restricts flow of gas from the gas valve to the point of mixing. The gas restrictor comprises at least one annular gas flow passage in the gas channel. Advantageously, providing the gas flow passage in the gas channel in annular form instead of a conventional orifice-type form enables a greater degree of control over a respectively associated Reynolds number of the gas restrictor. Furthermore, control over the associated Reynolds number of the gas restrictor is enabled without affecting an underlying open area ratio of the gas restrictor. Moreover, the at least one annular gas flow passage in the gas channel may be more effective when reducing Reynolds numbers for a given small open area ratio. Furthermore, the at least one annular gas flow passage in the gas channel is easy and cheap to manufacture, especially compared to multiple parallel flow openings in a gas restrictor with a comparatively small open area ratio, as in the latter case the multiple parallel flow openings would need to be small and therefore difficult and expensive to manufacture. Preferably, the gas restrictor is exchangeable. By providing the gas restrictor as an exchangeable component for the air-gas mixing unit of the air-gas mixture burning appliance, the latter may easily, quickly and reliably by converted from use with a first type of gas, such as a hydrocarbon gas, to a second type of gas, such as hydrogen gas. Brief Description of the Drawings Exemplary embodiments of the present invention are described in detail hereinafter with reference to the attached drawings. In these attached drawings, identical or identically functioning components and elements are labelled with identical reference signs and they are generally only described once in the following description. Fig. 1 shows a schematic view of an illustrative air-gas mixture burning appliance with an air-gas mixing unit, Fig. 2 shows a schematic cross-sectional view of the air-gas mixing unit of Fig. 1 with a gas restrictor according to the present invention, Fig. 3 shows a schematic view of the gas restrictor of Fig. 2 according to a first preferred embodiment, Fig. 4 shows a schematic view of the gas restrictor of Fig. 2 according to a second preferred embodiment, Fig. 5 shows a first diagram with illustrative Reynolds numbers obtained as a function of open area ratios, Fig. 6 shows a second diagram with illustrative Reynolds numbers obtained as a function of open area ratios, Fig. 7 shows a third diagram with illustrative Reynolds numbers obtained as a function of open area ratios, Fig. 8 shows a schematic view of the gas restrictor of Fig. 2 according to a third preferred embodiment, Fig. 9 shows schematic views of rotationally symmetric variants of the gas restrictor of Fig. 4, Fig. 10 shows schematic views of an axially symmetric variant and an asymmetric variant of the gas restrictor of Fig. 4, and Fig. 11 to Fig. 13 show schematic views of the gas restrictor of Fig. 4 with illustrative support rods. Detailed Description Fig. 1 shows an illustrative air-gas mixture burning appliance 100 which comprises an air-gas mixing unit 110 according to the present invention. By way of example, the air-gas mixture burning appliance 100 may be used in a boiler or, more generally, in a building heating system. According to one aspect, the air-gas mixture burning appliance 100 may be convertible for use with different types of gases and, thus, form a gas-convertible air-gas mixture burning appliance. More specifically, the air-gas mixture burning appliance 100 may, for instance, initially be adapted for use with a first type of gas, such as e.g. hydrogen gas, so that the air-gas mixture burning appliance 100 forms an air-hydrogen gas mixture burning appliance. Furthermore, the air-gas mixture burning appliance 100 may, for instance, be converted to be used with a second type of gas, such as e.g. a hydrocarbon gas, for instance methane or propane, so that the air-gas mixture burning appliance 100 then forms an air-hydro-carbon gas mixture burning appliance. In the air-gas mixture burning appliance 100, the air-gas mixing unit 110 is adapted for mixing of air and gas to form a combustible air-gas mixture 130. Preferably, the combustible air-gas mixture 130 is a homogenous mixture of the air and the gas. The air is supplied to the air-gas mixing unit 110 via an air supply 112, which is illustratively connected to the air-gas mixing unit 110, and the gas is supplied to the air-gas mixing unit 110 via a gas supply 116. By way of example, the air supply 112 includes a fan 114 that may be operated with an adaptable fan speed and / or within predetermined ranges of fan speeds to draw air into the air-gas mixing unit 110. The air supply 112 and the gas supply 116 are interconnected via at least one and, illustratively, via a plurality of air-gas mixers 118 of the air-gas mixing unit 110. At least one and, illustratively, each one of the plurality of air-gas mixers 118 forms an associated discrete point of mixing 119. Preferably, the combustible air-gas mixture 130 is formed at all such discrete points of mixing 119 from a respective air flow 140 supplied via the air supply 112 and a respective gas flow 150 supplied via the gas supply 116. The combustible air-gas mixture 130 may be guided via the plurality of air-gas mixers 118 to a burning unit 120. Illustratively, the burning unit 120 is provided with a burner surface 124 that is arranged downstream of the air-gas mixing unit 110 such that the combustible air-gas mixture 130 that is formed at the points of mixing 119 flows towards the burner surface 124. The combustible air-gas mixture 130 may be burned by the burning unit 120 and, more specifically, at the burner surface 124. By way of example, the burner surface 124 is illustrated with a comparatively small flame 122 which occurs e.g. at a low firing rate of the air-gas mixing unit 110, i.e. ata comparatively low rate at which feed of the combustible air-gas mixture 130 from the air-gas mixing unit 110 to the burning unit 120 occurs, in terms of volume, heat units, or weight per unit time. Such a low firing rate may e.g. be applied to the air-gas mixing unit 110 during an ignition phase of the air-gas mixture burning appliance 100. Fig. 2 shows one of the plurality of air-gas mixers 118 of the air-gas mixing unit 110 of Fig. 1. However, for simplicity and clarity of the drawing only a single air-gas mixer of the plurality of air-gas mixers 118 is shown, which is preferably representative of all air-gas mixers of the plurality of air-gas mixers 118 of Fig. 1, which are preferentially embodied identically, at least within predetermined manufacturing tolerances and with respect to an underlying functioning. This single air-gas mixer is described in detail hereinafter and illustratively referred to as “the air-gas mixer 118”. Thus, a detailed description of each one of the plurality of air-gas mixers 118 of Fig. 1 may be omitted for brevity and conciseness. As described above at Fig. 1, the air-gas mixer 118 is provided for mixing of air supplied by means of the air flow 140 flowing through an air way 212 of the air supply 112 with gas supplied by means of the gas flow 150 via the gas supply 116 at the point of mixing 119 in order to form the combustible air-gas mixture 130. More specifically, the air-gas mixer 118 is preferably connected to the gas supply 116 such that the gas flow 150 may be guided from the gas supply 116 to the point of mixing 119. Illustratively, the gas supply 116 comprises a gas channel 216 that is connected to the point of mixing 119 for guiding the gas flow 150 to the point of mixing 119. By way of example, the air-gas mixer 118 is embodied as a Venturi-type mixing nozzle that comprises a converging channel 214 which is connected to the air way 212. The converging channel 214 is provided to accelerate the air flow 140 in order to create a decreased air pressure, which is sometimes also referred to as “suction (air) pressure”, which causes a suction effect on the gas flow 150, which causes the gas flow 150 to flow into the Venturi-type mixing nozzle and mix with the air stream 140 at the point of mixing 119. The gas supply 116 further comprises a gas governor 210 that is adapted to control supply of gas to the point of mixing 119 via the gas channel 216. The gas governor 210 comprises a gas valve 215 and a gas restrictor 250. The gas restrictor 250 restricts flow of gas from the gas valve 215 to the point of mixing 119 via the gas channel 216. The gas valve 215 is adapted to control gas pressure in the gas channel 216. More specifically, the gas valve 215 is connected to the air way 212 by means of a reference pressure port 230 that is adapted to determine an air pressure signal 235 from the airflow 140 in the airway 212, which is indicative of a static air pressure in the air way 212. Thus, the gas valve 215 is adapted to control gas pressure of an incoming gas flow 220 in the gas channel 216 dependant on the air pressure signal 235 in order to create a flow of gas and, more particularly, an air pressure-controlled gas flow 240, which flows in a predetermined flow direction 242 to and through the gas restrictor 250. The gas restrictor 250 comprises at least one annular gas flow passage, as described below at Fig. 3, Fig. 5, and Fig. 8 to Fig. 13. Preferably, the gas restrictor 250 is embodied to form a gas flow with a predetermined Reynolds number. By way of example, the gas restrictor 250 restricts the air pressure-controlled gas flow 240 and, thus, forms the gas flow 150 which is supplied via the gas channel 216 to the point of mixing 119. More specifically, the gas restrictor 250 preferably forms a reduced cross-sectional flow opening 260 that is embodied with an associated isoperimetric ratio. Illustratively, the reduced cross-sectional flow opening 260 is embodied with a reduced isoperimetric ratio and comprises by way of example a reduced hydraulic diameter 255. At this point, it should be noted that functioning of a gas governor and, more particularly, a gas valve and / or a gas restrictor to perform an air-gas ratio control as such is well-known to the person skilled in the art. Thus, a more detailed description of the functioning of the gas governor 210 and, more particularly, of the gas valve 215 and / or the gas restrictor 250 may be omitted for brevity and conciseness. According to one aspect, the gas restrictor 250 of the gas governor 210 may be interchangeable between at least one first gas restrictor that is embodied to form a gas flow with a first Reynolds number and at least one second gas restrictor that is embodied to form a gas flow with a second Reynolds number. More particularly, the gas restrictor 250 may be interchangeable independent of the gas valve 215. The gas valve 215 may comprise a gas valve regulator with a gas offset pressure setting that is identical for the at least one first gas restrictor and the at least one second gas restrictor. For instance, the at least one first gas restrictor may be configured for exclusive use with a first type of gas in the air-gas mixture burning appliance 100 of Fig. 1, and the at least one second gas restrictor may be configured for exclusive use with a second type of gas in the air-gas mixture burning appliance 100 of Fig. 1. By way of example, the gas governor 210 may be adapted to operate with a hydrocarbon gas, such as methane or propane, by means of the at least one first gas restrictor, or with hydrogen gas by means of the at least one second gas restrictor. More specifically, the at least one first gas restrictor and the at least one second gas restrictor may have different pressure drop behaviors in order to change an underlying range of air-gas ratio across an underlying heat input range for the different gases that are useable with a given gas-convertible air-gas mixture burning appliance, such as the air-gas mixture burning appliance 100 of Fig. 1. These different pressure drop behaviors of the interchangeable gas restrictors may be obtained by ensuring that respective geometric designs of the interchangeable gas restrictors create gas flows with different Reynolds number. Preferably, isoperimetric ratios associated with the interchangeable gas restrictors are adapted such that the interchangeable gas restrictors create gas flows with different Reynolds numbers. By way of example, the at least one first gas restrictor may form a first reduced cross-sectional flow opening that is embodied with a first isoperimetric ratio, and the at least one second gas restrictor may form a second reduced cross-sectional flow opening that is embodied with a second isoperimetric ratio. In any case, it should be noted that a respectively desired operating range for Reynolds number depends on desired characteristics of an associated discharge coefficient of the gas restrictor 250. At high Reynolds numbers (Re » 1000), an associated gas flow rate tends to be dominated by inertia, for which the pressure drop of the gas restrictor 250 (Ap) has a quadratic relationship to the gas flow rate (heat input, Q) according to the Bernoulli equation (Ap oc Q2). In this regime of Reynolds number, the flow rate through the gas restrictor 250 follows an idealized trend, such that the discharge coefficient is nearly constant. If the gas restrictor 250 is designed to operate in this regime of high Reynolds number and constant discharge coefficient, then the air-gas ratio approaches a constant value across the operating range. In contrast, at low Reynolds numbers (Re « 1000) the gas flow rate tends to be dominated by friction, for which the pressure drop of the gas restrictor 250 (Ap) has a linear relationship to the gas flow rate (heat input, Q) according to the Darcy-Weisbach equation (Ap oc Q). In this regime of Reynolds number, the flow rate through the gas restrictor 250 underperforms with respect to the idealized trend, such that the discharge coefficient decreases as the Reynolds number decreases. If the gas restrictor 250 is designed to operate in this regime of low Reynolds number and decreasing discharge coefficient with decreasing flow rate, then the air-gas ratio across the operating range tends to vary from comparatively leaner mixtures at lower heat input to comparatively richer mixtures at higher heat input. At intermediate Reynolds numbers (Re ~ 1000), depending on the geometrical details of the gas restrictor 250, a peak in discharge coefficient may exceed the typical value of discharge coefficient at high Reynolds numbers (Re » 1000). If the gas restrictor 250 is designed to operate in this regime of intermediate Reynolds number and a local peak in discharge coefficient, then the air-gas ratio may exhibit a local extreme with a comparatively richer mixture. Advantageously, the flexibility of the at least one annular gas flow passage may be used to create the gas restrictor 250 such that it operates in a desired operating range of Reynolds number. More specifically, the at least one annular gas flow passage may comprise an open area ratio between 0.25 and 0.9 relative to the gas channel 216, and a Reynolds number between 100 and 2500 evaluated inside the at least one annular gas flow passage and associated with a minimum heat input rate of the air-gas mixture burning appliance 100 of Fig. 1. Alternatively, the at least one annular gas flow passage may comprise an open area ratio of at most 0.4 relative to the gas channel 216, and a Reynolds number of at most 1000 evaluated inside the at least one annular gas flow passage and associated with a minimum heat input rate of the air-gas mixture burning appliance 100 of Fig. 1. Fig. 3 shows a first embodiment of the gas restrictor 250 and the gas channel 216 of Fig. 2 by means of a cross-sectional cut in part (A) and by means of a length cut in part (B). As described above at Fig. 2, the gas restrictor 250 is provided for restricting the flow of gas 240 in the predetermined flow direction 242 in the gas channel 216 that is arranged between the gas valve 215 of Fig. 2 and the air-gas mixer 118 of Fig. 2, and comprises at least one and, illustratively, one annular gas flow passage 320 in the gas channel 216. According to a first embodiment, the annular gas flow passage 320 comprises a circular outer circumference 322. Illustratively, the annular gas flow passage 320 is rotationally and axially symmetric. Preferably, the annular gas flow passage 320 in the gas channel 216 is formed by positioning at least one and, illustratively, one flow restriction component 310 in the gas channel 216. By way of example, the flow restriction component 310 has a circular outer circumference 321, which forms an inner circumference of the annular gas flow passage 320. Accordingly, the reference sign 321 also labels the inner circumference of the annular gas flow passage 320, which is, thus, referred to hereinafter as “the circular inner circumference 321” of the annular gas flow passage 320. Illustratively, the gas channel 216 is tube-shaped with an inner circumference 218. The annular gas flow passage 320 in the gas channel 216 has an inner diameter 319 and an outer diameter 229. The inner diameter 319 of the annular gas flow passage 320 equals an outer diameter of the flow restriction component 310 in the gas channel 216. Accordingly, the reference sign 319 also labels the outer diameter of the flow restriction component 310, which is, thus, referred to hereinafter as “the outer diameter 319” of the flow restriction component 310. The outer diameter 229 of the annular gas flow passage 320, in turn, corresponds illustratively to an inner diameter 219 of the gas channel 216. The flow restriction component 310 restricts the flow of gas 240 in the predetermined flow direction 242 in the gas channel 216 such that a gas flow 340 in the predetermined flow direction 242 through the annular gas flow passage 320 is created. Preferably the flow restriction component 310 comprises a streamlined body 315 with a length 317 in the predetermined flow direction 242. The length 317 preferentially amounts at least to a restricting diameter of the flow restriction component 310 in the gas channel 216, i.e. to the outer diameter 319 of the flow restriction component 310. Fig. 4 shows a second embodiment of the gas restrictor 250 and the gas channel 216 of Fig. 2 by means of a cross-sectional cut in part (A) and by means of a length cut in part (B). The gas restrictor 250 and the gas channel 216 of the second embodiment correspond essentially to the gas restrictor 250 and the gas channel 216 with the inner diameter 219 of the first embodiment according to Fig. 3. Accordingly, at least one annular gas flow passage 420 is provided in the gas channel 216, which illustratively comprises the circular outer circumference 322 and the circular inner circumference 321 of Fig. 3, i.e. the outer diameter 229 and the inner diameter 319 of Fig. 3. The annular gas flow passage 420 in the gas channel 216 is rotationally symmetric and illustratively formed by positioning at least one flow restriction component 410 in the gas channel 216 which defines the circular inner circumference 321. By way of example, the flow restriction component 410 corresponds to the flow restriction component 310 of Fig. 3. However, in contrast to the first embodiment the outer diameter 229 of the annular gas flow passage 320 is now smaller than the inner diameter 219 of the gas channel 216. This is illustratively achieved by positioning at least one other flow restriction component 430 in the gas channel 216. The other flow restriction component 430 has an inner diameter that equals the outer diameter 229 of the annular gas flow passage 420. Accordingly, the reference sign 229 also labels the inner diameter of the other flow restriction component 430, which is, thus, referred to hereinafter as “the inner diameter 229” of the other flow restriction component 430. The outer diameter of the other flow restriction component 430 of the gas restrictor 250, in turn, corresponds illustratively to the inner diameter 219 of the gas channel 216. At this point, it should be noted that the performance of the gas restrictor 250 is generally affected by its characteristic Reynolds number. The Reynolds number (Re [-]) is defined as a function of a respective gas density (p [mass / length3]), flow velocity (U [length / time]), hydraulic diameter of the annular gas flow passage (Dh [length]), and dynamic viscosity of the gas (p [mass / (length*time)]). In this definition, the flow velocity shall be taken as the area-averaged flow velocity in the smallest cross-sectional flow opening in the gas restrictor 250. Furthermore, the hydraulic diameter of the annular gas flow passage shall be evaluated at the location of a smallest cross-sectional flow opening in the gas restrictor 250: D pUDh p Substituting the definition of the hydraulic diameter, Dh=4A / P, where A is the cross-sectional flow area ([length2]) of the gas restrictor 250 and P is the perimeter ([length]) of the gas restrictor 250, into the definition of Reynolds number, the definition of the Reynolds number can be rewritten. The resulting function depends on the fluid viscosity and mass flow rate, which are fixed for a given operating condition, and on the perimeter. In fact, it can be shown that the Reynolds number is inversely proportional to the perimeter (Re oc 1 / P) at a given operating condition. Thus, it is apparent that the Reynolds number of the gas restrictor 250 may be controlled by controlling the perimeter of the gas restrictor 250. In addition to selecting the appropriate range of Reynolds number, the gas restrictor 250 should have an appropriate cross-sectional area (A) to control an associated overall gas flow rate. The cross-sectional area of the gas restrictor 250 can be expressed by an associated open area ratio ( / ?), which is a nondimen-sional parameter relating the cross-sectional area of the gas restrictor 250 to the area of the gas channel 216 surrounding the gas restrictor 250. In the illustrated embodiment of Fig. 4, the open area ratio is df — d2 D2 with d2 being the outer diameter 229 of the annular gas flow passage 420, di being the inner diameter 319 of the annular gas flow passage 420, and D being the inner diameter 219 of the gas channel 216. At this point, it should be noted that the annular gas flow passage 420 may be transformed into the annular gas flow passage 320 of Fig. 3 by equating the outer diameter 229 of the annular gas flow passage 420 with the inner diameter 219 of the gas channel 216, i.e. d2 = D. Fig. 5 shows a diagram 500 with illustrative Reynolds numbers associated with two different air-gas mixture burning appliances. The diagram 500 represents illustrative relative ranges of Reynolds numbers 510 across associated open area ratios 520. More specifically, the diagram 500 represents a first graph 530 that illustrates relative ranges of Reynolds numbers across associated open area ratios of a conventional air-gas mixture burning appliance with a gas restrictor of the orifice-type form that comprises a single orifice, hereinafter referred to as “the conventional orifice-type design”, and a second graph 540 that illustrates relative ranges of Reynolds numbers across associated open area ratios of the air-gas mixture burning appliance 100 of Fig. 1 with the gas restrictor 250 comprising the annular gas flow passage 320 of Fig. 3, hereinafter referred to as “the annulus-type design of Fig. 3”. In both graphs 530, 540, the Reynolds numbers are scaled relative to the limit case of a fully open gas channel, i.e. an open area ratio of 1, where the gas flow passages of the conventional orifice-type design and the annulus-type design of Fig. 3 both have equal shapes and equal Reynolds numbers. In the conventional orifice-type design, the outer perimeter of the single orifice decreases when the open area ratio is reduced, so that the Reynolds number increases. In contrast thereto, in the annulus-type design of Fig. 3 the inner perimeter 321 of the annular gas flow passage 320 of Fig. 3 increases when the open area ratio is reduced, so that the Reynolds number decreases. The comparison shows that the annu-lus-type design of Fig. 3 produces lower Reynolds numbers than the conventional orifice-type design for identical open area ratios. This may already be desirable, but the benefits of the annulus-type design of Fig. 3 may be extended by considering the annulus-type design of Fig. 4 instead. Fig. 6 shows the diagram 500 of Fig. 5 which represents the illustrative relative ranges of Reynolds numbers 510 across the associated open area ratios 520, as well as the graphs 530, 540. In addition to the graphs 530, 540, multiple other graphs 541, 542, 543, 544, 545, 546, 547, 548, 549 are also illustrated. Each one of the graphs 541, 542, 543, 544, 545, 546, 547, 548, 549 represents similar to the graph 540 relative ranges of Reynolds numbers across associated open area ratios of the air-gas mixture burning appliance 100 of Fig. 1 with the gas restrictor 250 comprising the annular gas flow passage 420 of Fig. 4, hereinafter and above referred to as “the annulus-type design of Fig. 4”. More specifically, by way of example each one of the graphs 540, 541, 542, 543, 544, 545, 546, 547, 548, 549 illustrates a particular case of the annulus-type design of Fig. 4 in which the outer diameter 229 of the annular gas flow passage 420 of Fig. 4, i.e. d2, is fixed at a certain proportion (100% to 10% in steps of 10%) of the inner diameter 219 of the gas channel 216, i.e. D. In other words, for the graph 540 the outer diameter 229 of the annular gas flow passage 420 of Fig. 4 illustratively equals the inner diameter 219 of the gas channel 216, i.e. dz = D as described above. This corresponds to a proportion of 100%. For the graph 541, the outer diameter 229 of the annular gas flow passage 420 of Fig. 4 illustratively equals 90% of the inner diameter 219 of the gas channel 216, i.e. dz = 0.9*D. This corresponds to a proportion of 90%. For the graph 542, the outer diameter 229 of the annular gas flow passage 420 of Fig. 4 illustratively equals 80% of the inner diameter 219 of the gas channel 216, i.e. dz = 0.8*D. This corresponds to a proportion of 80%, and so on. As a result, by varying both the outer diameter 229 of the annular gas flow passage 420 of Fig. 4 and the inner diameter 319 of the annular gas flow passage 420 of Fig. 4, the entire region between both graphs 530, 540 may be covered using the annulus-type design of Fig. 4. Accordingly, a greater range of Reynolds numbers can be covered by using the annulus-type design of Fig. 4 compared to using the conventional orifice-type design. Furthermore, using the annulus-type design of Fig. 4 gives flexibility to vary Reynolds numbers independent of the open area ratio, which is not possible when using the conventional orifice-type design. Thus, the annulus-type design of Fig. 4 allows more finetuning over a respective air-gas ratio than the conventional orifice-type design. By using the an-nulus-type design of Fig. 4, the open area ratio may be set to reach a respectively required overall gas flow rate, and independently the Reynolds number may be finetuned to some extent to reach an appropriate discharge coefficient. Fig. 7 shows the diagram 500 of Fig. 5 which represents the illustrative relative ranges of Reynolds numbers 510 across the associated open area ratios 520, as well as the graphs 530, 540. In addition to the graphs 530, 540, multiple other graphs 531, 532, 533, 534, 534 are also illustrated. Each one of the graphs 531, 532, 533, 534, 534 represents similar to the graph 530 relative ranges of Reynolds numbers across associated open area ratios of a conventional air-gas mixture burning appliance with a gas restrictor of the orifice-type form that comprises multiple orifices, hereinafter referred to as “the conventional multiple orifices-type design”. More specifically, by way of example each one of the graphs 531, 532, 533, 534, 534 illustrates a particular case of the conventional multiple orifices-type design in which an associated number N of orifices is provided. In other words, for the graph 531 the conventional multiple orifices-type design is illustratively provided with N = 2 orifices. For the graph 532, the conventional multiple orifices-type design is illustratively provided with N = 4 orifices. For the graph 533, the conventional multiple orifices-type design is illustratively provided with N = 8 orifices. For the graph 534, the conventional multiple orifices-type design is illustratively provided with N = 16 orifices, and for the graph 535, the conventional multiple orifices-type design is illustratively provided with N = 32 orifices. As a result, although both the annulus-type design of Fig. 4 and the conventional multiple orifices-type design enable Reynolds number reduction, they may cover a different range when also considering the open area ratio. More particularly, if a lower Reynolds number is desirable, the annulus-type design of Fig. 4 has more potential for reducing the Reynolds number than the conventional multiple orifices-type design with relatively few (N <4) orifices. Furthermore, especially if the open area ratio is relatively low 0.4), a comparatively great number of orifices (N >8) would be required in the conventional multiple orifices-type design to obtain the same reduction in Reynolds number as in the annulus-type design of Fig. 4. This indicates that in some conditions the capabilities of the conventional multiple orifices-type design may also be realized by the annulus-type design of Fig. 4 with only a single gas flow passage. While the manufacturing complexity and effort increases with the number of orifices in the conventional multiple orifices-type design, an annulus-type design of Fig. 4 is comparatively simpler and achievable with reasonably constant manufacturing effort. This is another benefit of the an-nulus-type design of Fig. 4. Fig. 8 shows a third embodiment of the gas restrictor 250 and the gas channel 216 of Fig. 2 by means of a cross-sectional cut in part (A) and by means of a length cut in part (B). Similar to the second embodiment of Fig. 4, the gas channel 216 comprises the inner diameter 219 and the gas restrictor 250 comprises the annular gas flow passage 420 with the circular outer circumference 322 and the circular inner circumference 321, i.e. the outer diameter 229 and the inner diameter 319 of Fig. 4. However, in contrast to the second embodiment of Fig. 4 the annular gas flow passage 420 in the gas channel 216 is now illustratively formed by positioning at least one flow restriction component 810 in the gas channel 216 which defines the circular inner circumference 321 and which is provided with an additional annular gas flow passage 850 that comprises a circular outer circumference 860 and a circular inner circumference 861. By way of example, the annular gas flow passage 420 and the additional annular gas flow passage 850 are concentric. For instance, the flow restriction component 810 may be sleeve-shaped and an additional cylinder-shaped flow restriction component 820 may be positioned inside of the sleeve-shaped flow restriction component 810 such that the additional annular gas flow passage 850 is formed between the sleeve-shaped flow restriction component 810 and the additional cylinder-shaped flow restriction component 820. Illustratively, the sleeve-shaped flow restriction component 810 has an inner diameter 870 which forms the circular outer circumference 860 of the additional annular gas flow passage 850. The additional cylinder-shaped flow restriction component 820, in turn, has an outer diameter 880 which forms the circular inner circumference 861 of the additional annular gas flow passage 850. Providing two - or more - annular gas flow passages enables to achieve a reduced Reynolds number compared to provision of a single annular gas flow passage having an identical open area ratio. This applies likewise to the use of different shapings for the annular gas flow passage(s) as described hereinafter. Fig. 9 shows in parts (A) and (B) a fourth and a fifth embodiment of the gas restrictor 250 in the gas channel 216 of Fig. 2. Similar to the second embodiment of Fig. 4, the gas restrictor 250 respectively comprises a single annular gas flow passage, i.e. the annular gas flow passage 920 in the fourth embodiment and the annular gas flow passage 970 in the fifth embodiment, both of which are rotation-ally symmetric and axially symmetric. However, in contrast to the second embodiment of Fig. 4 the annular gas flow passage 920 in part (A) illustratively comprises a polygonal outer circumference 922. Likewise, the annular gas flow passage 920 may comprise a polygonal inner circumference 921. The polygonal inner and outer circumferences 921, 922 are illustratively formed by respectively shaped flow restriction components 910, 930. Furthermore, also in contrast to the second embodiment of Fig. 4 the annular gas flow passage 970 in part (B) illustratively comprises a star-like outer circumference 972. Likewise, the annular gas flow passage 970 may comprise a star-like inner circumference 971. The star-like inner and outer circumferences 971, 972 are illustratively formed by respectively shaped flow restriction components 960, 980. Fig. 10 shows in parts (A) and (B) a sixth and a seventh embodiment of the gas restrictor 250 in the gas channel 216 of Fig. 2. Similar to the second embodiment of Fig. 4, the gas restrictor 250 respectively comprises a single annular gas flow passage, i.e. the annular gas flow passage 1020 in the sixth embodiment and the annular gas flow passage 1070 in the seventh embodiment. However, in contrast to the second embodiment of Fig. 4 the annular gas flow passage 1020 in part (A) illustratively comprises a circular outer circumference 1022 and a circular inner circumference 1021 which are arranged eccentrically with respect to each other. Thus, the annular gas flow passage 1020 is merely axially symmetric, but not rotationally symmetric. The circular inner and outer circumferences 1021, 1022 are illustratively formed by respectively shaped flow restriction components 1010, 1030. Furthermore, also in contrast to the second embodiment of Fig. 4 the annular gas flow passage 1070 in part (B) illustratively comprises an undulated outer circumference 1072. Likewise, the annular gas flow passage 1070 may comprise an undulated inner circumference 1071. The undulated inner and outer circumferences 1071, 1072 are illustratively formed by respectively shaped flow restriction components 1060, 1080. By way of example, the annular gas flow passage 1070 is asymmetric. Fig. 11 to Fig. 13 show the gas channel 216 with the gas restrictor 250 which comprises the flow restriction components 410, 430 of the second embodiment according to Fig. 4, for illustrating suitable constructions that enable mounting of the flow restriction components 410, 430 to the gas channel 216. These suitable constructions are likewise applicable to any one of the other embodiments described above at Fig. 3, and Fig. 8 to Fig. 10. Fig. 11 shows the gas channel 216 with the inner circumference 218 and the gas restrictor 250 which comprises the flow restriction components 410, 430 of the second embodiment according to Fig. 4, wherein the flow restriction component 410 is illustratively connected to at least one and, preferably, one support rod 1110 which extends coaxially to the gas channel 216. By way of example, the support rod 1110 is connected to an external retainer 1190 which is arranged outside of the gas channel 216. The flow restriction component 430, in turn, is mounted to the inner circumference 218 of the gas channel 216, or integrally formed therewith. Fig. 12 shows the gas channel 216 with the inner circumference 218 and the gas restrictor 250 which comprises the flow restriction components 410, 430 of the second embodiment according to Fig. 4, wherein the flow restriction component 410 is illustratively connected to at least one and, preferably, one support rod 1210. By way of example, the support rod 1210 is arranged inside of the gas channel 216 and connected to the inner circumference 218 of the gas channel 216. Preferably, the support rod 1210 extends perpendicular to the gas channel 216. Furthermore, the flow restriction component 430 is mounted to the inner circumference 218 of the gas channel 216, or integrally formed therewith. Fig. 13 shows the gas channel 216 with the inner circumference 218 and the gas restrictor 250 which comprises the flow restriction components 410, 430 of the second embodiment according to Fig. 4, wherein the flow restriction component 410 is illustratively connected to the support rod 1110 of Fig. 11, which extends coaxially to the gas channel 216. However, in contrast to Fig. 11 the support rod 1110 is now, by way of example, connected to an internal retainer 1290 which is arranged inside of the gas channel 216 and connected to the inner circumference 218 of the gas channel 216. Furthermore, the flow restriction component 430 is mounted to the inner circumference 218 of the gas channel 216, or integrally formed therewith. It should be noted that the embodiments described above are merely intended to illustrate preferred realisations of the inventive gas restrictor without restricting the present invention thereto. Instead, various modifications are contemplated and may be envisaged. For instance, the respective flow restriction component(s) provided in the gas channel may be movable in streamwise direction, i.e. in the respective flow direction of the gas in the gas channel, to adjust the gas flow pas-sage(s) of the gas restrictor. Thus, the respective flow restriction component(s) and the gas restriction provided thereby may be adjusted for different operational requirements or gas types. Furthermore, the inner and outer circumferences of each described annular gas flow passage are respectively shown and described with an identical shaping. However, the shapings of the inner and outer circumferences of a given annular gas flow passage may also differ from each other, and so on.

Claims

1. A gas restrictor (250) for restricting flow of gas (240) in a predetermined flow direction (242) in a gas channel (216) that is arranged between a gas valve (215) and an air-gas mixer (118) of an air-gas mixture burning appliance (100), comprising at least one annular gas flow passage (320) in the gas channel (216).

2. The gas restrictor of claim 1, wherein the at least one annular gas flow passage (320) comprises a circular outer circumference (322).

3. The gas restrictor of claim 1, wherein the at least one annular gas flow passage (920) comprises a polygonal outer circumference (922).

4. The gas restrictor of claim 1, wherein the at least one annular gas flow passage (970) comprises a star-like outer circumference (972).

5. The gas restrictor of claim 1, wherein the at least one annular gas flow passage (1070) comprises an undulated outer circumference (1072).

6. The gas restrictor of any one of the preceding claims, wherein the at least one annular gas flow passage (320; 920; 970) is rotationally symmetric and / or axially symmetric.

7. The gas restrictor of any one of claims 1 to 5, wherein the at least one annular gas flow passage (1070) is asymmetric.

8. The gas restrictor of any one of the preceding claims, comprising at least one flow restriction component (310) which is arranged in the gas channel (216) for forming the at least one annular gas flow passage (320) in the gas channel (216).

9. The gas restrictor of claim 8, wherein the at least one flow restriction component (410) is connected to at least one support rod (1110) which extends coaxially to the gas channel (216).

10. The gas restrictor of claim 9, wherein the at least one support rod (1110) is connected to one of an external retainer (1190) which is arranged outside of the gas channel (216), or an internal retainer (1290) which is arranged inside of the gas channel (216) and connected to an inner circumference (218) of the gas channel (216).

11. The gas restrictor of claim 8, wherein the at least one flow restriction component (410) is connected to at least one support rod (1210) which is arranged inside of the gas channel (216) and connected to an inner circumference (218) of the gas channel (216), wherein the at least one support rod (1210) extends perpendicular to the gas channel (216).

12. The gas restrictor of any one of claims 8 to 11, wherein the at least one flow restriction component (310) comprises a streamlined body (315) with a length (317) in the predetermined flow direction (242) which amounts at least to a restricting diameter (319) of the at least one flow restriction component (310) in the gas channel (216).

13. The gas restrictor of any one of the preceding claims, wherein the at least one annular gas flow passage (320; 420) comprises an open area ratio between 0.25 and 0.9 relative to the gas channel (216), and a Reynolds number between 100 and 2500 evaluated inside the at least one annular gas flow passage (320; 420) and associated with a minimum heat input rate of the air-gas mixture burning appliance (100).

14. The gas restrictor of any one of claims 1 to 12, wherein the at least one annular gas flow passage (320; 420) comprises an open area ratio of at most 0.4 relative to the gas channel (216), and a Reynolds number of at most 1000 evaluated inside the at least one annular gas flow passage (320; 420) and associated with a minimum heat input rate of the air-gas mixture burning appliance (100).

15. An air-gas mixture burning appliance (100) with an air-gas mixing unit (110), wherein the air-gas mixing unit (110) comprises:at least one air-gas mixer (118) that forms a point of mixing (119) for mixing of air and gas to form a combustible air-gas mixture (130),5 an air supply (112) with an air way (212) that is connected to the point ofmixing (119) for supply of air to the point of mixing (119), anda gas supply (116) with a gas channel (216) that is connected to the point of mixing (119) and a gas governor (210) that is adapted to control supply of gas to the point of mixing (119) via the gas channel (216), wherein the gas governor10 (210) comprises:a gas valve (215) that is adapted to control gas pressure in the gas channel (216) dependant on an air pressure signal (235) that is indicative of a static air pressure in the air way (212), anda gas restrictor (250) that restricts flow of gas from the gas valve (215) to15 the point of mixing (119);wherein the gas restrictor (250) comprises at least one annular gas flow passage (320) in the gas channel (216).

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