Flame arresting

Abstract

A boiler 2 is provided with a mixer 4 to mix together fuel from a fuel source 18 and an oxidiser such as air from an oxidiser source 20, the mixture being conveyed via a mixing pathway 12 for combusti

Description

BACKGROUND The present invention relates to a boiler for mixing of fuel with a source of oxygen (for example, air) and the burning of that mixture. Particularly, but not exclusively, the invention relates to the mixing and burning of hydrogen with a source of oxygen in a boiler. In prior art heating systems, it is known for boilers to mix fuel gas with a source of gaseous oxidiser, such as air, upstream of a burner, where the fuel and oxygen is subsequently burnt. This is known as a pre-mixed burning, as opposed to diffusion burning where the process of mixing fuel and oxygen together occurs simultaneously with the process of burning. A problem with pre-mixed burning in boilers is that the fuel and oxygen mixture present upstream of the burner is susceptible to being ignited and burnt prematurely. This can occur in a flashback condition, when a normally stable flame downstream of a burner surface causes ignition of the supply of unburnt fuel and oxygen mixture present on the upstream side of the burner. Features provided in a boiler to promote and improve the mixing of fuel and oxygen typically do so by introducing swirl or turbulence into the fuel / oxygen fluid flow, or by providing a sufficient flow distance for mixing processes to progress, and this would often accelerate a flashback flame and exacerbate its severity, resulting in damage to components of appliances. The likelihood of flashback is increased when the fuel is hydrogen, rather than the more common natural gas used in appliances such as boilers. This is because hydrogen gas has a much greater flame speed than natural gas and produces greater overpressures in transient ignition events. Flame accelerations can also generate flame speeds approaching or exceeding the speed of sound, which can produce damaging shockwaves. It is an object of the present disclosure to obviate or reduce problems associated with flashback. BRIEF DESCRIPTION OF THE DRAWINGS Some non-limiting examples of the present disclosure will be described in the following with reference to the appended drawings in which: Figure 1 is a schematic side view of a first boiler in accordance with aspects of the present disclosure; Figure 1a is an end view showing an arrangement of venturi outlets in accordance with aspects of the present disclosure; Figure 1b is an end view showing an arrangement of venturi outlets in accordance with aspects of the present disclosure; Figure 2 is a schematic cross-sectional side view of a venturi; and Figure 3 is a schematic side view of a further boiler in accordance with aspects of the present disclosure. DETAILED DESCRIPTION In the following detailed description, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration specific examples in which the disclosure may be practiced. It is to be understood that other examples may be utilized and structural or logical changes may be made without departing from the scope of the present disclosure. The following detailed description, therefore, is not to be taken in a limiting sense, and the scope of the present disclosure is defined by the appended claims. It is to be understood that features of the various examples described herein may be combined, in part or whole, with each other, unless specifically noted otherwise. A first schematic example of the present invention is illustrated in Figure 1 of the accompanying drawings. With reference to Figure 1, an appliance 2 is shown having a mixer 4 to mix together fuel and oxidiser, and a burner 6 to bum a mixture of fuel and oxidiser from the mixer 4. The mixer 4 has a plurality of venturis 8,10 and a mixing flow pathway 12 extending from the plurality of venturis 8,10. The mixing flow pathway 12 is configured to provide fluid communication between the plurality of venturis 8,10 and the burner 6. Each venturi 8,10 comprises a mixing chamber 14, a fuel inlet 16 to provide fluid communication between the mixing chamber 14 and a source of fuel 18, an oxidiser inlet 20 to provide fluid communication between the mixing chamber 14 and a source of oxidiser 22, and a venturi outlet 24 to provide fluid communication between the mixing chamber 14 and the mixing flow pathway 12. The venturi outlets 24 of the plurality of venturis 8,10 open into the mixing flow pathway 12. In Figure 1, the venturi outlets 24 are shown lying in the same plane denoted by imaginary line 23. All of the oxidiser for fuel burnt at the burner is provided through the plurality of venturis. In the example of Figure 1, the oxidiser is air and, accordingly, the oxidiser inlet 20 is an air inlet and the source of oxidiser 22 is a source of air. In the appliance 2, the mixing of the fuel and air provides a mix of fuel and air which is suitable for adequately stable and complete combustion. The appliance 2 is a boiler and the boiler 2 may be a hydrogen fuel boiler. The appliance has an arrangement (not shown) to transfer heat to a supply of water, external and separate to the appliance, which passes through the appliance during use. The source of air 22 may be ambient air. The mixing flow pathway 12 extending from the plurality of venturis 8,10 is a single mixing flow pathway common to the plurality of venturis 8,10. The venturi outlets 24 are uniformly distributed in the mixing flow pathway 12. As shown in Figure 1, the venturi outlets 24 are uniformly distributed across the entire cross-sectional flow area of the mixing flow pathway 12. The venturi outlets 24 are uniformly distributed in that they are evenly spaced across the flow area of the mixing flow pathway 12 to introduce fuel and air from the venturi outlets 24 evenly within the mixing flow pathway 12. This promotes a rapid formation of a fuel and air mixture within the mixing flow pathway 12. In the appliance 2, the venturi outlets 24 are uniformly distributed and located in a single plane. In other examples, the venturi outlets 24 are uniformly distributed and located in one of (i) a part cylindrical shape, and (ii) a part spherical shape. In another example, the venturi outlets are distributed and located in one of (i) a triangular pitch, (ii) a square pitch, (iii) a rotated square pitch, and (iv) a staggered array. The longitudinal pitch is either equal to the transverse pitch or not equal to the transverse pitch. Further arrangements of venturi outlets are shown in Figures 1a and 1 b. In Figure 1a, sixteen venturi outlets are arranged with a first venturi outlet 102 located in a central position relative to the other venturi outlets, with the other venturi outlets being located in two concentric circles centred on the first venturi outlet. The inner concentric circle has five venturi outlets 104, and the outer concentric circle has ten venturi outlets 106. The venturi outlets in each concentric circle are equi-spaced from each other i.e. the spacing between neighbouring venturi outlets is the same along the circumference of the circle. The venturi outlets lie in the same plane. In Figure 1 b, thirteen venturi outlets are arranged with three venturi outlets 108 located in a triangular pitch centred on an imaginary point 110, with the remaining ten venturi outlets 112 being located in a concentric circle which is also centred on an imaginary point 110. The venturi outlets in the concentric circle are equi-spaced from each other i.e. the spacing between neighbouring venturi outlets is the same along the circumference of the circle. The venturi outlets lie in the same plane. The mixing flow pathway 12 is configured to present unobstructed and uncongested passage for fluid flowing between the plurality of venturis 8,10 and the burner 6. In this respect, the fluid has a clear passage through the mixing flow pathway without flowing over or around features (for example, protuberances) in the mixing flow pathway 12 which tend to promote swirl, eddy currents and / or turbulence. The burner 6 is planar and nominally flat. However, in a further example, the burner is curved in one or more axis. The use of a plurality of venturis allows a volume of fuel gas and a volume of gaseous oxidiser (such as air) to be separated into sub-volumes of fuel gas and gaseous oxidiser, wherein each sub-volume is mixed in a different venturi. The mixed subvolumes of fuel gas and gaseous oxidiser leave each venturi and further mix downstream in the mixing flow pathway. The sub-volumes from different venturis also mix with one another in the mixing flow pathway where they form a single volume of gas, which may become a homogenous flow. The boiler 2 has a temperature sensor 40 (or a thermostat) configured to sense the temperature of the plurality of venturis 8,10. In this way, the presence of a stabilised flame on or in the plurality of venturis 8,10 can be determined. A controller (not shown) receives measurement data from the temperature sensor 40 and determines whether or not a temperature threshold (indicative of a stabilised flame at the plurality of venturis 8,10) is exceeded. If the controller determines a stabilised flame is present, then the controller provides a warning to the user and / or takes corrective action to reduce the severity of damage to the boiler, for example, isolating the boiler from the source of fuel. The boiler 2 has a single fuel chamber 42 common to the fuel inlets 16 of the plurality of venturis 8,10 wherein said fuel inlets 16 open into the fuel chamber 42. Furthermore, the boiler 2 has an oxidiser chamber 44 common to the oxidiser inlets 20 of the plurality of venturis 8,10 wherein said oxidiser inlets 20 open into the oxidiser chamber 44. The oxidiser is air, and so the oxidiser chamber 44 is an air chamber. Figure 2 shows a detailed view of a venturi 8 for use in a plurality of venturis of the present disclosure. Fuel and air combine in the venturi 8 and mix together along the mixing distance 27 before becoming a mix of fuel and air suitable for combustion at the burner 6. Each venturi has a circular cross-sectional shape. However, in other examples, each venturi has a non-circular cross-sectional shape, such as an oval or rectangular shape, or some geometric form with multiple degrees of symmetry. The plurality of venturis 8,10 is a flame arrester. A flame arrester functions to stop the passage therepast of a travelling flame. A flame arrester may be effective through the rapid transportation of heat energy away from the flame. The plurality of venturis 8,10 (or fuel inlet 16 and / or the venturi outlets 24 of the plurality of venturis 8,10) have a thermal effusivity of at least 8 000 Js’05m’2K’1. Materials providing such a thermal effusivity include Stainless Steel Series 400. In examples, the plurality of venturis 8,10 (or fuel inlet 16 and / or the venturi outlets 24 of the plurality of venturis 8,10) have a thermal effusivity of at least 10 000 J s’0 5 m’2 K’1. Materials providing such a thermal effusivity include Aluminium and Copper. These comparatively high levels of thermal effusivity allow the plurality of venturis 8,10 to perform the function of a flame arrester. Furthermore, in examples, the hydraulic diameter of the fuel inlet 16 (and / or the venturi outlet) of each venturi is less than or approximately equal to the minimum MESG (Maximum Experimental Safe Gap) for the fuel gas and gaseous oxidiser (for example, air) mixture at any equivalence ratio. This further allows the plurality of venturis 8,10 to effectively perform the function of a flame arrester. A consequence of using a plurality of venturis 8,10 instead of a single venturi is that the size of each venturis (and the associated inlets and outlets thereof) is reduced, as discussed herein. This can assist in allowing the fuel inlet 16 and / or the venturi outlet of each venturi to be reduced to a value less than or approximately equal to the minimum MESG (Maximum Experimental Safe Gap) as mentioned above. The present disclosure also provides a heating system having a boiler with a plurality of venturis, wherein the fuel inlet is in fluid communication with a source of hydrogen fuel. A second example is shown schematically in Figure 3. Common features between the boilers of Figures 1 and 3 are provided with like reference numerals, for example, venturi outlets 24” in Figure 3. In Figure 3, an appliance 2” (specifically a boiler) is provided having a mixer to mix 4” together fuel and oxidiser, and a burner 6” to bum a mixture of fuel and oxidiser from the mixer 4”. The mixer 4” has one venturi 8”, and a single mixing flow pathway 12” extending from the one venturi 8”, the mixing flow pathway 12” being configured to provide fluid communication between the one venturi and the burner. The venturi 8” has a mixing chamber 14”, a fuel inlet 16” to provide fluid communication between the mixing chamber 14” and a source of fuel 18”, an oxidiser inlet 20” to provide fluid communication between the mixing chamber 14” and a source of oxidiser 22”, and a venturi outlet 24” to provide fluid communication between the mixing chamber 14” and the mixing flow pathway 12”. The venturi outlet 24” of the venturi 8” opens into the single mixing flow pathway 12”. A portion of a fluid communication pathway between the mixing chamber 14” of the venturi and the source of fuel is a flame arrester and has a hydraulic diameter less than or equal to the MESG (Maximum Experimental Safe Gap) for the fuel gas and gaseous oxidiser mixture. That is, the maximum dimension of the fuel inlet is less than or equal to the MESG. Where the MESG is a range of values, then the maximum value may be used (this applies to examples described throughout this disclosure). The portion may be a restriction in the fluid communication pathway between the mixing chamber 14” of the venturi and the source of fuel. In the example of Figure 3, the portion is the fuel inlet 16”. In an example, the MESG is for a particular equivalence ratio of the fuel gas and gaseous oxidiser mixture, and more specifically, for the equivalence ratio of the fuel gas and gaseous oxidiser mixture used with the boiler. In another example, the MESG is for fuel gas and gaseous oxidiser mixture at any equivalence ratio. In this way, the venturi 8”, and specifically the fuel inlet 16”, functions effectively to arrest upstream progression of a flame in the event of a flashback. In the example, a flame arresting function is provided by the portion of fluid communication pathway (fuel inlet 16” in relation to Figure 3) being less than or equal to the MESG (Maximum Experimental Safe Gap) for the fuel gas and gaseous oxidiser mixture (as described above in relation to Figure 3) and also by the portion of fluid communication pathway having a thermal effusivity of at least 8 000 Js-0 5m’2K'1. The boiler 2” may be a hydrogen fuel boiler and may be provided in a system having a supply of fuel in fluid communication with the fuel inlets 16” of the boiler 2”, wherein the fuel is hydrogen. Although specific examples have been illustrated and described herein, a variety of alternate and / or equivalent implementations may be substituted for the specific examples shown and described without departing from the scope of the present disclosure. This application is intended to cover any adaptations or variations of the specific examples discussed herein. Therefore, it is intended that this disclosure be limited by the claims and the equivalents thereof.

Claims

1015C\j201. A hydrogen fuel gas boiler comprising a mixer to mix together a hydrogen fuelgas and an oxidiser, and a burner to burn a mixture of the hydrogen fuel gas and the oxidiser from the mixer;wherein the mixer comprises at least one venturi and a single mixing flow pathway extending from the at least one venturi, the mixing flow pathway being configured to provide fluid communication between the at least one venturi and the burner;wherein the or each venturi comprise a mixing chamber, a hydrogen fuel gas inlet to provide fluid communication between the mixing chamber and a source of the hydrogen fuel gas, an oxidiser inlet to provide fluid communication between the mixing chamber and a source of the oxidiser, and a venturi outlet to provide fluid communication between the mixing chamber and the mixing flow pathway;wherein the venturi outlet of the or each venturi opens into the single mixing flow pathway;wherein the hydrogen fuel gas inlet of the or each venturi is a flame arrester and has a hydraulic diameter of less than or equal to the MESG (Maximum Experimental Safe Gap) for the hydrogen fuel gas and gaseous oxidiser mixture;wherein the hydrogen fuel gas inlet has a thermal effusivity of at least 8 000 J s’ 0 5-m-2- K-i; andwherein the Maximum Experimental Safe Gap (MESG) is the minimum Maximum Experimental Safe Gap (MESG) for the fuel gas and gaseous oxidiser mixture at any equivalence ratio.

2. A hydrogen fuel gas boiler according to claim 1, wherein the at least one venturi is a plurality of venturis, and the single mixing flow pathway is common to and extends 5 from the plurality of venturis.

3. A hydrogen fuel gas boiler according to any of the preceding claims, wherein all of the oxidiser for the hydrogen fuel gas burnt at the burner is provided through the or each of venturis.

104. A system comprising a hydrogen fuel gas boiler according to any of the preceding claims.xtCMCM CM

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