Patent Application
20180320895
The present invention relates to an improved integrated gas burner and to cooking and heating devices including the improved integrated gas burner.
An integrated gas burner comprises a gas train and a burner module. The gas train has a connection to a gas supply, an ejector for forming an air/fuel mixture and a diffusor or plenum for expanding the air/fuel flow which is in fluid communication with the burner module. In a radiant integrated gas burner, the burner module comprises a burner face having one or more surfaces which have been coated with a catalyst.
Small-power portable LPG-fuelled appliances include portable soldering irons, hair curling tongs and camping stoves. Flameless radiant-mode gas-fired burners offer safety and emissions advantages over blue-flame burners and facilitate wind-resistant operation, so are sometimes specified for small-power appliances where indoor operation is envisaged. As flameless burners must be fully-aerated, they must be supplied with air/fuel mixture at or above the stoichiometric ratio required for combustion. In small-power portable gas-fuelled appliances fuelled by butane or propane-based blends (LPG fuel), one or more large area-ratio passive ejectors (gas-driven jet pump) are generally used to entrain, compress and mix the required fraction of combustion air with the fuel gas. This mixture must be distributed uniformly across the entrance aperture of the radiant burner face to assure uniform combustion with low CO emissions. Flameless burner heads may variously be of substantially flat, cylindrical or conical shape, are generally sized to deliver a heat flux of less than 400 kW/sq. m, and may bear a catalytic coating to promote surface combustion rather than gas-phase combustion in a flame.
Typically, an appliance has an ejector for forming an air/fuel mixture. A packaging problem arises in expanding the high velocity air/fuel flow discharging from the ejector to fill the entrance aperture of the radiant burner face head, whose area is typically two orders of magnitude greater than that of the mixing tube of the ejector, while delivering a uniform velocity profile.
Usually a diffusor or plenum is used to expand the air/fuel flow. Conventional faired diffusers are prohibitively long and inefficient if used to perform a very large flow expansion. Axial-radial diffusor designs are known, and by folding the flow path can deliver the required flow expansion more space-efficiently, but are susceptible to flow separation and therefore difficult to engineer and manufacture with high pressure recovery. Ejectors can be engineered with no diffuser, discharging flow directly to a suitably sized plenum chamber which performs flow expansion. This is energetically inefficient however, providing a very limited pressure rise of the entrained air for the burner designer to work with, and constraining burner design.
A noise problem also can exist in certain gas-fuelled portable appliances. The expansion of the pressurised fuel jet through the mixing ejector and air entrainment is a turbulent and therefore noisy process. Without some intervening muffling, jet noise can radiate from the burner aperture. Muffling is not straightforward however, as simple resistive attenuators are undesirable due to energy losses.
Various catalytic radiant burners are known in the prior art, such as Japanese patent publication numbers JP 560200016 A, JP 557164214 A, JP 55770309 A and JP 56026210 all of which suffer from one or more problems mentioned. Radiant burners used for cooking duties are often subject to ingress of liquid or particulate foreign matter due to spillages. This poses a risk for delicate gas train components, and for ejector nozzles in particular, where the burner and gas train flow are generally directed upward opposing gravity, and can cause CO emissions to exceed regulatory limits. There is a long held need in culinary service industry for radiant burners which adequately address the problem of axial compactness without compromising reliability, cost, durability or safety.
A further problem concerns the need to comply with emissions regulations concerning uncombusted gas and carbon monoxide emissions. Carbon monoxide emissions occur due to incomplete combustion.
A way of ameliorating these problems has been sought.
According to the invention there is provided a burner module, as set out in the appended claims, for use in combusting an air/fluid fuel flow wherein the burner module comprises a burner face comprising catalytic material for combusting the air/fluid fuel flow and a perforated screen having a plurality of micro-perforations wherein the perforated screen is positioned upline to the burner face to increase combustion. It will be appreciated that the term fluid used in the context of the present invention encompasses gas. The invention provides a compact burner module adapted to perform a large flow expansion between a gas jet inlet and a burner aperture outlet, in a minimum of axial length, in a compact volume, with minimal use of expensive catalytic surface area, while also providing excellent handling qualities in use.
In one embodiment there is provided a burner module for use in combusting an air/fluid fuel flow wherein the burner module comprises a burner face comprising catalytic material for combusting the air/fluid fuel flow and a perforated screen having a plurality of micro-perforations and the perforated screen is positioned upline to the burner face to increase combustion, wherein the perforated screen is formed from a material having a thermal conductivity adapted to reduce thermal bridging from the burner face and reflect incident heat radiation in a direction substantially perpendicular to the burner face. Suitably the perforated screen can be made from a thin material, such as a metallic foil or coated with a metallic foil.
According to the invention there is further provided an integrated gas burner for connection to a pressurised fluid fuel flow wherein the integrated gas burner comprises a burner module and a gas train wherein the burner module comprises a burner face comprising catalytic material for combusting the air/fluid fuel flow and a perforated screen having a plurality of micro-perforations wherein the perforated screen is positioned upline to the burner face to increase combustion; and the gas train comprises:
According to the invention there is also provided a perforated screen shaped for use in an integrated burner according to the invention wherein the screen forms a plurality of micro perforations.
According to the invention, there is further provided an appliance comprising an integrated burner according to the invention.
The present invention provides an integrated burner having an improved rate of combustion, effective flow metering, mixing and expansion of air and fuel flows while avoiding the noise, overheating tendencies and flow instability of known methods of flow expansion in a gas train. As a result of the increased combustion which gives an improved rate of combustion, a reduction in the amount of emissions is envisaged, specifically a reduction in the amount of uncombusted gas and carbon monoxide produced by the burner module or integrated burner in use compared to a known burner used in comparable conditions. Furthermore, the perforated screen minimises ingress of liquid or other foreign matter into the gas train. This enables a compact, low cost close-coupled radiant integrated burner and gas train module to be engineered, with good durability and stability.
In some embodiments, the perforated screen may be positioned adjacent to the burner face, for example a few gas jet diameters upline of the burner face. In some embodiments, the burner face has an aperture which is its upline facing surface which admits the air/fluid fuel mixture. In some embodiments, the perforated screen may be a homogenisation perforated screen which has perforations arranged to improve the uniformity of combustion across the aperture of the burner face.
In some embodiments, the integrated burner or burner module according to the invention may comprise two perforated screens upline of the burner face wherein the perforated screens comprise a throttling perforated screen and a homogenisation perforated screen wherein the homogenisation perforated screen is positioned upline and adjacent to the burner face and the throttling perforated screen is positioned upline of the homogenisation perforated screen; and wherein the perforations on the throttling perforated screen are arranged to provide a predetermined degree of aeration of the air/fuel mixture. It will be appreciated that by setting the overall burner backpressure to passage of air/gas mixture by selectively fitting a throttling screen of the most appropriate combined flow area (pressure drop). The tuned backpressure can then be used to control the ratio of entrained air to driving gas flow.
In some embodiments, the perforations on the throttling perforated screen may be arranged to provide a flattened velocity profile of the air/fluid fuel flow perpendicular to the throttling perforated screen to provide uniform combustion. In some embodiments, the integrated burner may be a radiant integrated burner comprising two perforated screens wherein the throttling perforated screen provides a fully-aerated air/fuel mixture. In some embodiments, the degree of aeration of the air/fuel mixture provided by the throttling perforated screen may be 12-20 parts of air entrained with each part of fuel gas (by weight). The advantages of providing such a degree of aeration include that the air/fuel ratio is greater than stoichiometric and that the need for secondary combustion is minimised such that clean burning of fuel is more likely. In some embodiments, the perforated screen may be shaped to provide one or more supports for the burner face. In some embodiments, the one or more perforated screen supports may have a pointed shape to minimise thermal bridging. In some embodiments, the perforated screen may have one or more ribs to provide axial stiffness.
In some embodiments, the plurality of perforations of the perforated screen may be shaped to increase spill resistance and noise attenuation. It has been found that the burner module and the integrated burner according to the invention are noticeably quieter in use than comparable known burner modules and integrated burners. For example noise attenuation can be achieved using Helmholz resonators for noise abatement (e.g. some types of auto exhaust mufflers). In the context of the burner module of the present invention a succession of high impedance and low inpedence flow channels to a turbulent (noisy) fluid flow. Hence two screens and three plenums provide some noise attenuation.
Fuel pressure-driven catalytic radiant integrated burners for LPG service generally require a gas train with a physically long flow length to efficiently mix and expand separate flows of air and vaporous fuel to a larger area homogenous flow of uniform velocity. This flow length scales approximately with the square root of burner power. The present invention overcomes this problem with gas flow train length.
Gas trains which provide the required flow expansion and air/fuel mixing with poor efficiency can make it impossible to engineer radiant integrated burners which are free of flame-lift and light-back under all operating conditions. This leads to a conflict between compactness and low cost on the one hand and performance and reliability on the other. The present invention overcomes this problem. The advantages of the present invention include:
a. Use of one or more perforated screens having plurality of micro-perforations to:
In some embodiments, the perforations of the perforated screen may be chemically etched perforations. In some embodiments, the perforations may have a cusp. In some embodiments, the perforated screen may be formed from a metal such as aluminium or steel. In some embodiments, the perforated screen may have perforations which have uniform density and diameter across the perforated area such that the perforated screen is a homogenisation perforated screen such that it may bring about intense mixing of chemical species in the boundary layer adhering to a catalytic burner face. In some embodiments, the perforated screen may have a thickness which is from 0.1 mm to 1 mm, for example 0.35 mm. In some embodiments, the diameter of the perforations of the perforated screen may be from 0.1 to 0.5 mm, for example 0.25 mm. In some embodiments, the perforated screen may have a square, rectangular, curved or three dimensional shape (such as a cylindrical, spherical or cuboid shape). In some embodiments, the perforated screen may be reflective.
In some embodiments, the perforated screen may be formed from a thin foil having a low thermal conductivity to reduce thermal bridging from the burner face. In some embodiments, the thin foil may be a metallic foil.
In some embodiments, the perforated screen may be a throttling screen and may have a plurality of perforations in a plurality of perforated areas In some embodiments, the density (number of perforations per unit area) and/or diameter of the perforations may be the same or different in each of the plurality of perforated areas and may vary across each perforated area such that the rate of flow across the plurality of perforated areas is regulated. In some embodiments, each perforated area may have the same or a different shape such as a triangular, square, rectangular, radial, annular, polygonal, curved, sector and/or irregular shape as may be required in order to homogenise the gas flow.
In some embodiments, the perforated screen may be formed from a corrosion-resistant malleable, dimensionally-stable metal sheet material, may be capable of sustaining service temperatures in the range 300-600° C. for the intended life of the integrated burner and/or may be capable of being polished to efficiently reflect infrared radiation, for example radiation in the wavelength range of from 0.5 to 7.5 μm. In some embodiments, the perforated screen may be formed from cold-reduced austenitic or martensitic stainless steel strip or FeCrAL alloy or similar alloy. It can be difficult to avoid radiation of heat from the burner face back into the plenum, causing parts to overheat especially when the burner face is presented to external surfaces which reflect incident radiation efficiently. This is a particular risk in radiant camping stoves, making safe management of burner temperature in all possible conditions of use and the avoidance of light-back difficult to assure. The embodiment having a perforated screen which is capable of reflecting infrared radiation overcomes these problems. This is because the infrared reflecting perforated screen may re-radiate radiant energy from the back surfaces of the burner face back out of the burner module, helping to moderate burner self-heating.
In some embodiments, the perforated screen may be manufactured by hot needle rolling, laser-cutting, waterjet-cutting, CNC machining and chemical milling. In some embodiments, the perforated screen may be electropolished to remove burrs. In some embodiments, the perforations of the perforated screen may have a cusp to provide a sharp-edged orifice to fluid flow, producing a consistent jet diameter with minimal frictional energy losses.
In some embodiments, the perforated screen may be shaped to provide a support for the burner face. Advantages of the perforated screen support include that it provides means of stabilising large spans of a light-gauge radiant flat mesh burner face against slumping or handling damage by supporting the burner face from the rear. In some embodiments, the perforated screen support may have a conical shape in order to minimise any thermal bridging effect.
In some embodiments, the perforated screen may be formed from a smooth material, having high reflective efficiency to IR and/or having relatively low heat conductivity such that much of the air/fuel fluid flow can be baffled from rearward infrared radiation from the burner face, limiting burner temperature rise, risk of light-back, and boosting radiant efficiency. In some embodiments, the mean reflectivity of the perforated screen material to normally-incident radiation in the range 0.5-7.5 μm may exceed 80%. In some embodiments, the thermal conductivity of the screen material may be less than 20 W/m·K.
In some embodiments, the gas train of the integrated burner may comprise a diffusor and/or a plenum. In some embodiments, the plenum may have a cross-sectional area which increases in the direction of the fluid flow. In some embodiments, the diffusor may have a cross-sectional area which increases in the direction of fluid flow.
In some embodiments, the gas train of the integrated burner according to the invention may comprise an axial ejector. In some embodiments, an axial ejector may comprise a co-axial gas injector nozzle and one or more radial air inlets. In some embodiments, the ejector may have a cross-sectional area which decreases in the direction of fluid flow.
In some embodiments, the gas train may comprise a radial diffusor downline of the ejector. In some embodiments, the burner may comprise one or more disc shaped perforated screens. In some embodiments, the gas train provides fluid communication from a gas fuel source and an air inlet to the burner module. In some embodiments, the integrated burner comprises a fuel source, for example a gas fuel source.
In some embodiments, the gas train may comprise a folded air/fuel fluid flow such that the integrated burner is a compact integrated burner. In some embodiments, the gas train may comprise a co-annular folded air/fuel fluid flow. In some embodiments, a compact integrated burner may comprise an ejector, an annular diffusor, an annular homogeniser, optionally an annular plenum and/or a cylindrical or annular burner face. In some embodiments, a compact integrated burner may comprise one or more radially-arranged ejectors feeding a common compact plenum. Use of a folded air/fuel fluid flow path through the compact integrated burner allows the axial length of the gas train to be collapsed and simultaneously to provide a plenum of meaningful volume. In some embodiments, where the gas train comprises an annular diffusor connected to an annular plenum, the annular diffusor may be shaped to provide a degree of swirl where it discharges into the annular plenum, potentially improving mixing further.
In some embodiments, the burner module may comprise a crimp ring for attaching a perforated screen to the burner face. In some embodiments, the crimp ring may comprise a clip for attaching the burner module to a gas train. In some embodiments, the crimp ring may be shaped to provide a radial space to allow the burner module to thermally expand in use. In some embodiments, the crimp ring may prevent slip of the air/fuel mixture past the burner module. In some embodiments, the burner module may comprise one or more axial spacers for providing axial separation between a perforation screen and the burner face or between perforation screens. In some embodiments, the axial separator for placement between a perforated screen and the burner face may be selected to have an axial length which is sufficiently short to reduce the risk of light back and overheating of the perforated screen whilst sufficiently long such that flow from the perforated screen turbulates the burner face. A skilled person would understand how to select a suitable axial length for such an axial separator. The crimp ring accordingly allows an effective catalytic burner face to be engineered. By minimising slip past the burner module, the crimp ring enables the radiant burner face to be supplied with a flow of mixed air/gas mixture across its entrance aperture. The radial space of the crimp ring may allow the burner face to be thermally decoupled from its mounting to avoid cold spots and allows the burner face to thermally expand and contract freely to avoid deformation and/or low-cycle fatigue effects.
In some embodiments, the gas train may comprise an adaptor for connection to a pressurised fluid fuel source. In some embodiments, the burner face may comprise a metal mesh, a porous ceramic monolith and/or an open-cell metal foam treated with a combustion catalyst. A suitable combustion catalyst includes platinum (Pt), palladium (Pd), rhodium (Rh) and/or a rare earth compound. In some embodiments, the burner face may have a sufficient area to fully combust the required fuel to generate the target power output. In some embodiments, the burner face may have a greater catalytic area than may be required to assure complete combustion of fuel to allow for imperfections such as incomplete air/gas mixing, non-uniform mixture velocity across the burner face, or excessive conduction of heat away from the burner face to its mounting.
In some embodiments, the gas train may comprise an axial gas inlet. In some embodiments, the gas train may comprise one or more radially-arranged ejectors.
The compact gas train and radiant burner technology described here can be used in many portable and permanent combustion applications, including heating and combustible gas purification and flaring applications. In some embodiments, an appliance according to the invention may be a food warming device such as a chafing dish heater, a space heater or a stove. In some embodiments, a chafing dish heater may comprise two nesting metal vessels and one or more integrated burners according to the invention. In some embodiments, each nesting metal vessel may have a substantially flat under-surface. In some embodiments, a lower dish contains a shallow layer of water, used as a heat distribution and coupling agent and an upper dish contains solid or liquid foodstuffs to be kept warm or cooked, and placed beneath the nested dishes are one or more integrated burners according to the invention with a pressurised fuel supply. As the integrated burner is radiant, the appliance can be operated if necessary with minimal clearance between the burner face and the underside of the nested dishes, as, being a fully-aerated flameless technology there are negligible flame-quenching effects and carbon monoxide emission levels will comply with regulatory norms under all conditions of service.
In some embodiments, a space heater may comprise a reflective surface, an integrated burner according to the invention and a pressurised fuel source. A space heater may additionally comprise a support, an electrical igniter and/or a handle for positioning of the reflective surface and/or integrated burner. In some embodiments, a stove may comprise an integrated burner, a pressurised fuel supply, a pot support, a pot and a burner shield. The pot may be shaped such that the other components can fit inside it for storage.
Designers of all types of fully-aerated integrated burners for vaporous fuels, but of radiant flameless integrated burners in particular, face the challenges of how to expand a small diameter flow of gas into a fully-aerated thoroughly mixed flow of uniform velocity entering the entrance aperture of an integrated burner. The task becomes difficult where space, weight, noise and cost are at a premium; where emissions of NOx and CO must be controlled to very low levels as in indoor service; and where the only energy available for air/fuel mixing is that of the fuel pressure. The relevance of this invention is that use of the features and method described eases those difficulties
The objective of this method is to provide clean-burning, quiet, long-lived ejector-mixers and radiant integrated burners, which may optionally be close-coupled as a module, for appliances powered by fuel vapour pressure, with minimal bulk and cost.
In some embodiments, the burner face may have a spherical, flat, cylindrical, or conical shape, particularly a flat or cylindrical shape.
To a first approximation, the packaging volume Vb required for a radiant integrated burner whose radiant aperture has an area Ab is given by equation (1):
V
b
=A
b*
LT is the path length of gas flow through the integrated burner. This is given by equation (2):
L
T
=L
e
+L
d
+L
p
+L
b (2)
where Le represents the length of the ejector, Ld represents the length of the diffusor, Lp represents the length of the plenum (where present) and Lb represents the length of the burner module and
L
Tax
In some embodiments, the integrated burner has an arrangement where Vb is minimised for a given heat input rate. Arrangements where Vb is minimised also tend to minimise cost. Because Ab is substantially determined by required heat input rate, this implies minimisation of
An expansion ratio Rf may be defined between the cross sectional area of the mixing tube of the ejector (1) and the area of the exit aperture of the burner (4):
Rf typically is in the range 50-150 for radiant burners in fully-aerated ejector-driven appliances. A difficulty arises in mixing an appropriate fraction of entrained air thoroughly with the driving gas jet, then expanding the mixture flow to enter the burner entrance aperture with uniform velocity and air/gas concentration, while simultaneously minimising LT. This is because ejectors, faired diffusers and plenum chambers need to have substantial length to operate efficiently. Gas-driven ejectors for entraining and mixing sufficient air to combust the fuel gas typically require mixing tube lengths in the range 3.5-5.5 times the tube diameter. Faired (conical diverging) diffusers are typically designed with an included angle between opposing walls of around 5.5 degrees, while good recovery of pressure from the high velocity mixture discharging from the ejector requires a ratio of diffuser discharge area to inlet area of better than 2:1. Plenum chambers must be sufficiently large to allow flow separation or other instabilities at diffuser discharge to dissipate and mixture velocity to become homogenous. Residence time of mixture in a plenum sometimes is necessary for sufficient molecular diffusion and micro-mixing to occur between phases to ensure adequate air/gas homogenisation. Given all these considerations, LT for a typical appliance tends to be large.
The invention will now be illustrated with reference to the following Figures shown in the accompanying drawings which are not intended to limit the scope of the invention claimed:
A first embodiment of a perforated screen according to the invention is generally indicated at 40 on
A second embodiment of a perforated screen according to the invention is generally indicated at 30A on
A third embodiment of a perforated screen according to the invention is generally indicated at 30B on
The perforations 38,48 in the perforated screens 30A,30B,40 have a cusp 39,49 which narrows the radius of the perforation 38,48 by an amount 37,47 as shown in partial cross-section in
The perforated screens 30,40 have a hydraulic diameter which for each perforated screen 30,40 is the sum of the diameters of all of their perforations 38,48. The hydraulic diameter determines the back pressure of the throttling screen 30 and thereby the degree of aeration of an air/fluid fuel flow from the gas train may be selected by setting the hydraulic diameter by the appropriate choice of the number of the perforations 38,48 and their diameters.
Suitable perforated screens 30A,30B,40 are preferably made from a corrosion-resistant malleable, dimensionally-stable metal sheet material, capable of sustaining service temperatures in the range 300-600° C. for the intended life of the burner 100,200,300,400, and capable of being polished to efficiently reflect radiation in the wavelength range 0.5-7.5 μm. Cold-reduced austenitic or martensitic stainless steel strip or FeCr alloy are suitable materials. The size of the required perforations depends in part upon the geometry of the burner 100,200,300,400, for example on the clearance between the perforated screen 30A,30B,40 and the upstream surfaces of a burner face 150,250,350,450. A perforated screen 30A,30B,40 having perforations 38,48 having a smaller diameter has improved noise attenuation and spill resistance as ingress by solid or liquid matter into the gas train or burner module is reduced.
The density of perforations 38,48 can either be fixed or can be varied across the surface of the screen to flatten the velocity profile of mixture entering the burner face 150,250,350,450.
The perforated screens 30A,30B,40 may be manufactured by a number of methods, of which the more convenient include hot needle roller, laser-cutting, waterjet-cutting, CNC machining and chemical milling. For the first four methods, improved tolerances and reduced costs result where sheet material is laminated together during machining and clamped between thicker plates. On completion, screens may be electropolished to remove burrs. The preferred method of manufacture is chemical milling of the sheet material from both sides, with accurate mutual registration of a lithographic mask on each side. According to this process, the minimum hole size that is possible with high yield is given by the material thickness used. Referring to
A first embodiment of an integrated burner according to the invention is indicated generally at 100 on
Burner module 159 comprises the burner face 150, the homogenisation screen 140 and the crimp ring 155. Crimp ring 155 secures the homogenisation screen 140 to the burner face 150. Crimp ring 155 has a lower clip 158 for engagement with the homogenisation screen 140 and an upper clip 156 for securing burner face 150. Crimp ring 155 minimises the risk of air/fuel mixture bypassing the burner face 150.
The throttling screen 130 is supported by lower lip 124 of the diffusor 120. Axial spacer 126 is placed on top of throttling screen 130 to space it from the homogenisation screen 140 of the burner module 159. Burner module 159 is placed on axial spacer 126 and is secured by upper lip 122 of the diffusor 120. In an alternative embodiment, the burner module 159 may comprise the homogenisation screen 140. In a further alternative embodiment, the crimp ring 155 may comprise a clip for engaging with the diffusor 120.
Throttling screen 130 may be a perforated screen 30A according to the second embodiment of the invention as shown in
In the first embodiment of the invention, Σ(Ld+Lp) is minimised by use of the radial diffuser 120 and substantial elimination of a plenum upline of the burner face 150. This is achieved by addition of the perforated screens 130,140 which are formed from a thin heat resistant material. The upline screen 130 acts as a semi-permeable boundary for the radial diffuser 120, ensuring high velocity mixture discharging the ejector 110 negotiates the axial to radial direction change with acceptable energy losses and without severe flow separation, while also enabling the flow to distribute across its aperture, permeate, and progress towards the burner face 150. The axial to radial direction change is required because the aperture of the burner face 150 is greater than that of the mixing tube 118. The downline screen 140 behind the burner face 150 acts to further homogenise the velocity distribution of the mixture flow and to reflect radiation emitted by the burner face 150 back out of the burner face 150. Flow passage through each screen 130,140 is accompanied by considerable microturbulence due to jet action through the perforations 138,148. In this way, the flow length of a conventional plenum can be greatly shortened, while the use of a radial diffuser 120 with semi-porous boundary wall formed by screens 130,140 enables flow expansion in a very short axial length. Quality of air/gas mixing over that delivered by the ejector mixing tube is improved due to the micromixing effects of jetting through the perforations 138,148. By fabricating the screens 130,140 from thin smooth material with high reflective efficiency to IR but relatively low heat conductivity, the gas train can be baffled from infrared radiation radiated from the rear of the burner face, limiting burner temperature rise, hence reducing risk of light-back and boosting radiant efficiency.
A further benefit of the use of thin perforated screens 130,140 in the burner 100 according to the first embodiment of the invention is relevant to cooking applications, where burners 100 are generally inverted and stationed below cooking vessels. The small diameter of the perforations 138,148—typically of the same order as the thickness of the screen material—affords some protection to the gas train from liquid and particulate ingress via the burner face 150.
A yet further benefit of the use of at least one thin perforated screen 130,140 stationed a few microjet diameters upstream of the rear of the burner face 150, is the ability to boost mass transport rate of reactant molecules to a catalytically-coated burner face 150 through the effects of jet impingement. The high levels of turbulence in jet impingement of a gas mixture onto catalytic surfaces thins and turbulates the boundary layer of reactants and products of combustion adhering to the surfaces. This decreases the catalytic area that would otherwise be required to combust a given massflow of mixture to a given standard of completeness.
Computational fluid dynamics (CFD) visualisations of the burner according to the first embodiment of the invention are shown in
A second embodiment of an integrated burner according to the invention is indicated generally at 200 on
The integrated burner 200 according to the invention accordingly has a co-annular arrangement of ejector 210, diffuser 220 and plenum 225 which folds these three elements very efficiently via two flow reversals provided by radial tube 219 and radial outlet 221 from the diffusor 220. This minimises Σ(
A single cylindrical perforated screen 240 is added in close proximity to the inlet surfaces of the burner face 250 to turbulate the catalytic surfaces (not shown) of the burner face 250 and acts as a radiation baffle. The co-annular ejector-diffuser 210,220 arrangement provides excellent mixing of air/gas as is clear from the CFD simulations of
A third embodiment of an integrated burner according to the invention is indicated generally at 300 on
Computational fluid dynamics (CFD) visualisations of the integrated burner 300 according to the third embodiment of the invention are shown in
A fourth embodiment of an integrated burner according to the invention is indicated generally at 400 on
The fourth embodiment of the integrated burner according to the invention shown in
In the integrated burner 400 according to the fourth embodiment of the invention, Σ(Le+Ld) is minimised. A diffusing duct 420A,420B,420C incorporating a 90 degree bend is provided at the discharge end of each ejector 410A,410B,410C. This gas train arrangement can be packaged efficiently with radial-firing or axial-firing burner faces 450, using at least one perforated screen 440 to homogenise flow distribution into the burner face 450 and to turbulate catalytic surfaces.
In an alternative embodiment, the burner face 150,250,350,450 may be a catalytic radiant burner head. In an alternative embodiment, the burner face 150,250,350,450 may comprise a porous ceramic monolith or open-cell metal foam treated with a combustion catalyst such as platinum (Pt), palladium (Pd), rhodium (Rh) and/or a rare earth compound. Burner face 150,250,350,450 has a sufficient area to fully combust the required fuel to generate the target power output. In practice, greater catalytic area may be required to assure complete combustion of fuel in the face of imperfections such as incomplete air/gas mixing, non-uniform mixture velocity across the burner face 150,250,350,450, or excessive conduction of heat away from the burner face 150,250,350,450 to its mounting means.
In some applications such as flat burner types, it is advantageous to prop the burner face at additional points on its aperture in addition to providing continuous edge support. This improves burner face durability in the face of rough handling of a portable gas appliance, for example. Malleable screen materials can easily have bumps formed into them through pressing, after perforation. By minimising the thickness of the screen material and favouring the use of materials of low conductivity, it is possible to provide support bumps at one or more points across the aperture which contact and stabilise the upstream side of the radiant burner face, while minimising thermal bridging to the burner support structure.
Radiant burner faces require mounting methods that minimise heat losses at the support points, minimise the degree of ‘slip’ or bypassing of mixture around the burner perimeter, while enabling substantial thermal expansion and contraction of the burner face without restraint. High temperature gasketing materials of needled quartz fibre are suitable.
The compact gas train and radiant burner technology described here can be used in many portable and permanent combustion applications, including heating and combustible gas purification and flaring applications. Three specific examples of suitable appliances are provided in
There are many advantages of applying this technology in this application. A flameless radiant integrated burner has a better-defined object shape than a blue-flame burner, while a truncated spherical profile is optimal for projecting a uniform heat flux on the image plane. Radiant burners, being fully-aerated, are not susceptible to soot accumulation on the reflective optics over time. The low mass of the compact folded gas train embodiments disclosed here enable the suspended mass to be minimised to the benefit of safety and stability, while the mass of embodied materials and therefore the cost of the integrated burner is very low.
The advantage of applying the technology in this application is the superior packaging efficiency, reduced weight, improved stability and lower cost of this stove when compared with state of the art stoves using other technology. This is achieved without sacrificing other contemporary performance features of modern LPG stove.
In the specification the terms “comprise, comprises, comprised and comprising” or any variation thereof and the terms include, includes, included and including” or any variation thereof are considered to be totally interchangeable and they should all be afforded the widest possible interpretation and vice versa.
The invention is not limited to the embodiments hereinbefore described but may be varied in both construction and detail.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1519449.1 | Nov 2015 | GB | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2016/076611 | 11/3/2016 | WO | 00 |
