WO1999001698A1

WO1999001698A1 - Improvements in burners

Info

Publication number: WO1999001698A1
Application number: PCT/GB1998/001932
Authority: WO
Inventors: Graham John Ball; Gavin John Conibeer; John Martin Thirtle; Neil Alexander Downie
Original assignee: Drax Torches Limited
Priority date: 1997-07-01
Filing date: 1998-07-01
Publication date: 1999-01-14
Also published as: GB9713995D0; AU8227498A; GB2326934A; EP1017956A1; GB2341441A; GB9930747D0

Abstract

A gas burner which can use a hydrocarbon fuel other than acetylene or methylacetylene as the fuel gas and air as the oxidant has means (18) for preheating the oxidant gas and means (14) for preheating the fuel gas which becomes cracked. These gases are delivered into a chamber (67) where they become mixed under highly turbulent conditions, and then pass through nozzle (86) and shroud (100) and form a flame (97) of high intensity and high turbulence. The flame typically produces more than 150 MW/metre3, e.g. 300-500 MW/metre3 for lower temperature operations and at least 500 MW/metre3 for welding or cutting steel. Its turbulence, indicated by its acoustic/thermal ratio, is typically about 6x10-6 and enables air of the boundary layer which insulates the workpiece to be stripped off. In an alternative arrangement configured as a gas cutting torch, preheated air or other cutting gas is supplied to a jet adjacent the main flame. A gas torch or gas cutting torch according to the invention can be fuelled with propane or propylene which are inexpensive to store and use, but can achieve a performance comparable to that of an oxyacetylene torch.

Description

IMPROVEMENTS IN BURNERS

The present invention relates to a gas burner. The invention also relates to a gas torch, gas cutting torch, furnace, crucible heater or the like having a burner as aforesaid. It also relates to a method of heating a workpiece by means of a burner or torch as aforesaid.

A gas burner may be part of an apparatus (referred to as a "gas torch") which provides a flame which may be applied to workpieces of metal, glass or other material to heat, cut, weld or generally work the workpiece. Gas torches may be hand held, operated in a fixed installation or attached to a movable part of a machine. Gas torches designed for cutting (referred to as "gas cutting torches") provide a working flame and have means for directing a jet of a suitable cutting gas onto the workpiece in the area heated by the working flame. The present invention has particular application to gas torches and gas cutting torches . Preheating of either or both of the fuel and oxidant gases is known for burners used to heat furnaces. The usefulness of preheating the gases supplied to a burner has long been thought to be because their additional heat then becomes part of the heat of the flame (see K±rk- Othmer Chemical Technology, vol 4, page 285). The typical oxidant-fuel ratios by volume are about 20:1 or above. Where propane is used as the fuel, for example, the stoichiometric ratio by volume of fuel and air to give only carbon dioxide and water is 1:25, and for propylene the same ratio is 1:22.25. At these oxidant- fuel ratios it would be expected that only marginal benefits would be achieved by pre-heating the fuel gas.

GB-A-0717184 (Burch) discloses a gas torch in which coal gas "Calor" gas, water gas, producer gas or a combustible hydro-carbon vapour is burned in admixture with air to produce an uncovered elongated flame suitable for glass-blowing, soldering, brazing, lead-burning and other analogous purposes requiring localised heating to high temperatures . The outlet of the torch is constituted by a divergent Venturi-type outlet nozzle arranged to be fed with a mixture of gas and air, and auxiliary heating means are provided for pre-heating the gas and air in its passage to the Venturi nozzle. The Venturi nozzle is designed to maintain substantially streamlined flow conditions in the flame which is quiet and non-roaring. Gas speeds high enough to produce a turbulent or roaring flame could be achieved by increasing the flows of gas and air, but the resulting flame, thorough markedly hotter and narrower than the corresponding flame without pre-heat was cooler than the narrower quiet or non-turbulent flame. The requirement for laminar flow in the flame placed limits on the size to which the torch could be increased without deterioration in its properties . The air and gas were fed through pre-heating means at an estimated temperature of 750 to 800°C, which is above the temperature for onset of cracking of the fuel gas, and problems arising from the formation of coke or soot in the fuel gas supply are reported . We have realised that the relative coolness of a turbulent flame reported by Burch is the result of mixing of ambient air with the combustion gas in the long thin flames that he produced. With this flame shape, turbulent flow draws significant quantities of ambient air into the flame and produces a cooling effect. In a more powerful burner with a shorter and fatter flame, the advantages of turbulence in enabling higher flow rates and combustion intensity and in producing better heat transfer to the workpiece overcome the effects of any mixing of ambient air with the combustion gas and result in a more effective flame. Furthermore, we have discovered that the coking problem reported by Burch can be overcome by adding air to the fuel gas .

In one aspect the invention provides a gas burner or torch comprising: a mixing chamber; an inlet for supply of fuel gas to the mixing chamber; a heater for heating fuel gas before it enters the mixing chamber for cracking the fuel gas; an inlet for supply of oxidant gas to the mixing chamber; an outlet from the mixing chamber for a flow of mixed fuel and oxidant gas , the arrangement being such that in operation the flow is turbulent; and a combustion zone for receiving and burning the turbulent mixture of f el gas and oxidant gas .

The invention also provides a method for operating a gas burner which comprises : cracking a fuel gas; feeding the cracked fuel gas and oxidant gas to a mixing zone of the burner; allowing a turbulent mixture of fuel gas and oxidant gas to flow from the mixing zone to a combustion zone; and burning the turbulent mixture .

In a further aspect the invention provides a burner for burning a fuel gas and an oxidant gas comprising a body having a mixing chamber, a shroud, a nozzle which divides the mixing chamber from the shroud, an inlet for fuel gas into the mixing chamber, an inlet for oxidant into the mixing chamber and an outlet for the mixture through the nozzle into the shroud, the outlet being of small size relative to the mixing chamber. Supply of fuel and oxidant to a burner as aforesaid at relatively high flow rates can be arranged to produce turbulent mixing, and the discharge of the mixture through the nozzle into the shroud via a relatively small diameter outlet produces a highly turbulent mixture in the shroud which can burn to form a highly turbulent flame.

The invention further comprises a nozzle for fitting to a burner as aforesaid, said nozzle comprising a disclike body having a relatively small diameter protrusion extending from one face thereof away from the body, a main outlet which is directed longitudinally or axially and extends through said body and through said protrusion, and one or more auxiliary outlets which are directed transversely or radially and extend from the main outlet to a side region of the protrusion. In use the nozzle is subject to deterioration and may be supplied as a separate replaceable item.

The effect of preheating the fuel gas, or both the fuel gas and the air, supplied to a burner has been observed to be an increase in the intensity of the flame. The invention permits the burning in air of fuel of lower chemical energy than acetylene or methyl acetylene which comprises supplying preheated fuel and preheated air to a burner where they become mixed and burning the mixture in a turbulent flame having an intensity (rate of release of heat per unit volume) of more than 150 MW/metre³. A proportion of the input gas is believed to become cracked into more energetic or faster burning gases before it passes to the working flame. The chemical change may be achieved by heating the fuel gas, optionally in the presence of a catalyst, before it is fed to the main burner. The present gas torch may incorporate means for achieving high levels of turbulence in a working flame and also means for stabilising a highly turbulent working flame . In conventional gas cutting torches, cutting gas is supplied at ambient temperature. Conventionally, a cutting gas is selected which contains a high concentration of oxidant, and combustion of the workpiece in the oxidant balances the cooling effect that room temperature cutting gas has on the workpiece. Conventional gas cutting torches use bottled oxygen (containing more than 99% oxygen) as the cutting gas.

We have found that preheating the cutting gas significantly enhances the performance of gas cutting torches, enabling an enhanced cutting effect to be achieved and lower concentrations of oxidant to be used in the cutting gas. Such torches, using air as the cutting gas, can deliver an operating performance sufficient to replace, in some applications, conventional cutting torches using oxygen as the cutting gas. However, the invention is also applicable to the use of pure oxygen or an oxygen - air mixture as the cutting gas . The invention therefore also provides a gas cutting torch having means for preheating cutting gas . Preheating the cutting gas may be used in addition to, or as an alternative to, preheating the fuel and/or oxidant gases as aforesaid.

The burner of the invention may be made with a range of thermal outputs according to user requirements . At the bottom part of the range, the burner might have a diameter of about 8 mm and a thermal output of about 0.5 kW. Larger hand torches could be of diameter about 100mm and thermal output about 200 kW. Typically the torch may be about 30 mm in diameter and of total output 10 to 20 kW of which from 5 to 10 kW is produced in the main burner. This may be compared with the output of the Burch burner which is limited by the requirement to maintain laminar flow to an output of about 0.4 kW. As the diameter of the burner increases the length of the burner will also increase, but by a smaller amount. For example, a burner 30 mm in diameter may be of length 100 mm, whereas a burner 100 mm in diameter may be of length about 130 mm.

The pre-heater may comprise a gas burner arranged to play on a heat exchange coil through which air or fuel to be preheated passes, in which case change in the size and diameter of the burner will result in similar changes in the size and length of the heat exchanger coils.

The fuel supplied to the working flame may be a hydrocarbon vapour or other fuel gas and is preferably propane or propylene because these fuels have high calorific value and a convenient vapour pressure at ambient temperatures (about 7 bar gauge) for use and for storage in low-pressure cylinders, and also exhibit good cracking behaviour. It may also be a propane-butane mixture but butane itself is less preferred because of its low vapour pressure. Methane (natural gas) and ethane can also be used but cannot be stored so conveniently and are more difficult to crack. Higher alkanes and alkenes may also be used in vapour form. The fuel burned in the working flame will usually be the same as that used in a pre-heat burner, but it may be cost- effective to use different fuel e.g. propane or propylene for the working flame and natural gas for the pre-heater. The air supplied to the working flame may be air from a centrifugal fan at a pressure of at least 500 mbar gauge, or compressed air from a conventional compressor having a reservoir tank maintained at a pressure of about 7 bar gauge, or compressed air from a building compressed air supply line.

Preheating may be carried out by any convenient means , including combustion of gases from the same sources as the input gases , by combustion of gases from other sources, by catalytic oxidation of such gases, by electric heating, by the use of waste heat from the working flame, or by staged burning or partial catalytic oxidation of the input gases .

Where the fuel gases are hydrocarbon gas, means may be provided for avoiding sooting, conveniently by providing oxidising gas to the fuel upstream of the zone at which heating and cracking occur and/or by use of cracking catalysts in the fuel pre-heater, particularly when these enable lower preheat temperatures to be employed . Various embodiments of the invention will now be described by way of example only, with reference to the accompanying drawings in which:

Figure 1 is a block diagram of a first form of torch assembly according to the invention; Figure 2 is a more detailed diagram of the torch assembly of Figure 1 showing burners and preheaters forming parts of said assembly in axial section;

Figure 3 is a section on the line 3-3 of a fuel preheater forming part of the torch assembly of Figure 2;

Figure 4 is a partly cut-away perspective view of a burner forming part of the torch assembly of Figure 2, and Figures 5 and 6 are sections of the burner taken on the lines 5-5 and 6-6 of Figure 2; Figures 7A to 7C are diagrammatic sections of the burner of Figure 4 showing respectively the pattern of gas flow, the pattern of flame during normal combustion and the pattern of flame during a temporary interruption of normal combustion; Figures 8A and 8B show diagrammatically the flame from the burner of Figure 4 without fuel preheat and with fuel preheat respectively;

Figures 9A and 9B show a burner, a workpiece and a flame formed under relatively low turbulent conditions and without preheat and under relatively high turbulence with preheat respectively;

Figure 10 is a diagrammatic sectional view of a second form of burner according to the invention, and Figure 10a is a perspective view of a baffle plate assembly forming part of the burner of Figure 10;

Figure 11 is a partly sectioned view of a third form of torch assembly;

Figure 12 is a block diagram of a cutting torch assembly according to the invention; Figure 13 is a more detailed block diagram of the cutting torch assembly of Figure 12 with the burner and preheaters shown partly sectioned;

Figure 14 is a view in axial section of the burner of Figure 13; Figure 15 is a view of the burner of Figure 13 and 14 cutting through a steel plate;

Figures 16 and 17 are views in longitudinal section and transverse section on the lines 17-17 respectively of a second form of burner for a cutting torch; Figure 18 is a partly sectioned view of a third form of burner for a cutting torch;

Figure 19 shows a gas cutting torch arranged for cutting a metal workpiece;

Figure 20 is a diagram of a crucible heater in which heating is by means of the burner according to the invention;

Figure 21 shows diagrammatically a furnace heated by a burner according to the invention;

Figures 22, 23 and 24 show various forms of nozzle and heat shield arrangements for attachment to a burner;

Figure 25 is a plot of workpiece temperature against rate of heat release per unit volume for the burner of

Fig 2;

Figure 26 is a fragmentary sectional view of a further torch assembly; Figures 27a and 27b are respectively a view in longitudinal section and a front view of a further burner for a gas torch;

Figure 28 is a view in longitudinal section of a burner for a gas torch that works by laminar flow;

Figure 29 is a view of the burner of Figure 28 with associated fuel and gas supply lines and pre-heaters ; and

Figure 30 is a view of a gas torch which uses a multiplicity of the burners of Figure 28 together with associated fuel and gas supply lines and pre-heaters.

In Figure 1 of the accompanying drawings , a torch assembly comprises a source of compressed air 10 which may be a compressed air line within a building, a tank of an air compressor, or the output of a fan, a source 12 of fuel gas which is a cylinder of propane, a gas- fuelled preheater 14 for the air, a burner 16, and a gas- fuelled preheater 18 for the fuel.

The compressed air source 10 is connected to an air manifold 20 from which an air line 22 passes to the burner 16, an air line 24 passes to the air preheater 14 and an air line 26 passes to the fuel preheater 18. The flow of air along the lines 22, 24, 26 is shown on gauges

28, 29, 30 e.g. of the Rotameter type and is controlled by valves 31, 32, 33. Propane or other fuel gas from the source 12 passes to a manifold 34 from which a fuel line

36 extends through the fuel preheater 18 to the burner

16, a line 38 passes to the fuel preheater 18 and a line

40 passes to the air preheater 14. Flow through the lines 36, 38 and 40 is shown on gauges 41, 42 and 43, and the flow through those lines can be controlled by respective valves 44, 45 and 46.

In Figure 2, compressed air at a flow rate of about

180 1/min and at a pressure of up to about 5 barg supplied from compressor 10a enters a first heat exchanger 50 forming part of the line 22. The line 22 and heat exchanger 50 are of AISI 316 stainless steel tubing of internal diameter 5mm formed into a helix with 8 coils of diameter 35mm and axial length 125mm. The AISI 316 stainless steel tubing comprises Cr 17.5% and Ni 13.4%, the balance being iron and incidental ingredients. There may also be used AISI 321 stainless steel tubing comprising Cr 18.3%, Ni 10.3%, Ti 0.68%, the balance being iron and incidental ingredients. Other materials which can be used include Nichrome, Nickel, Inconel and Incallory 800, and irridium or nickel plated materails. It is heated from one end by the flame of a first preheat burner 52 having a mixing chamber 54 fed with compressed air from line 24 and fed with about 3 1/min of propane or other through line 40. The burner 52 is provided with an apertured internal baffle and the dimensions and the rates of the air and fuel gas flow are such that a jet of flame impinges on the coils of the heat exchanger 50.

The burner 24 is located about 10 mm from the adjacent end of the heat exchanger 50. A cylindrical baffle 56 of diameter 9mm is located within the heat exchanger 50 so as to extend from the downstream end of the heat exchanger 50 about 2/3 of the length of the heat exchanger 50 in the direction of the preheat burner 52. As best seen in Figure 3, the baffle 56 is hollow and has a tubular side wall 58 and a solid end wall 60 facing the burner 52, but is open at the other end. The hollow structure of the baffle 56 which may be of steel or other flame-resistant material enables the baffle 56 to be made of relatively low heat capacity and speeds up the attainment of steady state temperatures . A tubular steel shroud 62 surrounds the heat exchanger 50 and co-operates with the baffle 56 so that the flame produced by the burner 52 is annular flame and plays about the coils of the heat exchanger 50. The outer surface of the shroud 62 is covered by a layer of refractory insulating material 64 (high temperature ceramic fibre insulation). The downstream part of the line 22 leads from the heat exchanger 50 to a 5mm diameter opening 66 in the side wall of a mixing chamber 67 at the rear of the burner 16. Heating the air flowing through the heat exchanger 50 produces a significant increase in the pressure drop across the heat exchanger, so that air from a compressed air source has to be used. Propane is fed under pressure from a supply bottle 12a via manifold 34 and line 36 to a second heat exchanger 68 also of 316 stainless steel tubing of 8mm internal diameter and of geometry similar to that of the first heat exchanger 50. However, the tubing of the second heat exchanger 68 is formed into a helix with four coils diameter 50mm, and is surrounded by a stainless steel tubular shroud 70 provided on its outer surface with a layer of refractory insulating material 72. A second preheat burner 74 is located 30mm from the end of the second heat exchanger 68 onto which it directs a jet of flame. (This difference in spacing is not shown in Fig 2). The burner 74 has a mixing chamber fed with air through line 26 and fed with propane through line 38. A cylindrical baffle 76 which is a hollow tube closed at the end nearer to the burner 74 is located within the second heat exchanger 68 and extends partway along as shown.

The second heat exchanger 68 is formed as a discrete component of the line 36, to which it is connected at joints 78 and 80, and between these joints the second heat exchanger 68 is packed with granules 82 of a cracking catalyst e.g. zeolite 13X from Union Carbide held in place by plugs of steel or copper wire (not shown) . The stainless steel from which the second heat exchanger 68 is made itself catalyses the cracking of hydrocarbon fuel gas into hydrogen and unsaturated compounds. However, the granules 82 of cracking catalyst catalyse the cracking process, and they are packed into the heat exchanger 68 in such a manner as to permit the required through flow of fuel gas. The degree of cracking of the hydrocarbon fuel gas depends on the flow rate of fuel gas supplied at line 36, and a flow of about one standard litre/minute can be used. The heated and cracked fuel gas passes from the heat exchanger 68 through a back wall 84 of the burner 16 via a nozzle 86 into the mixing chamber 67. The jet 86 has an axially directed orifice 88 of diameter 700μm. The use of a small diameter orifice gives rise to turbulence and hence promotes mixing of the fuel gas and air. The mixing chamber 67 is of diameter 26mm and of length 30mm. As is more clearly shown in Figures 4 and 5, the air from line 22 enters the mixing chamber 67 radially, and the gas from the nozzle 86 enters axially. The fuel and air rapidly mix within the chamber 67 under turbulent flow conditions. The forward end of the mixing chamber 67 is defined by an outlet nozzle 90 having a tapered region 92 leading to a 6mm internal diameter central hole 94. A major proportion of the fuel and air mixture from mixing chamber 67 exits through the central hole 94 which provides the main part of the working flame 97 of the burner which is highly turbulent. A minor proportion of the fuel and air mixture exits the nozzle 62 through smaller diameter side holes 96 (approximately lmm diameter) . The fuel and air are delivered to the mixing chamber 67 at flow velocities which exceed the flame speed of the working flame 97 so as to avoid transfer of combustion into the mixing chamber 67.

A tubular wall 98 of the burner 16 is formed with a step for location of the nozzle 90, and a tubular shroud, 100 of length preferably about 40 to 50mm and of diameter 25mm fits within the burner 16 in front of the nozzle 90, where it serves to retain the nozzle 90 in position and is itself held in place by means of a grub screw 102. An annular flange 104 is formed on the exterior of the nozzle 90 forwardly of the side holes 96. The flange 104, and side holes 96 provide a secondary region 106 of the working flame in a low velocity or recirculating zone adjacent to the main part of the working flame. The secondary part is fed by the side holes 96 of the nozzle, and the flange 104 enhances the stability of this secondary part. The distance between the outlet nozzle 90 and the downstream end 110 of the shroud 100 is sufficient for flame stability while allowing access to the hottest part of the flame, which in this embodiment is approximately 10mm downstream of the shroud 100.

Figures 7a and 7b show the working state of a burner 16 with the working flame 97 formed with preheating of both the gas and air supply. Figure 7c shows the state where there has been a temporary lift-off the flame 97, but where a flame 97a remains as a result of the fuel and air mixture emerging through side nozzles 66. This flame 97a maintains combustion and obviates the danger of explosion following an accumulation of unbumed gases . The second preheat burner 74 is run at lower flow rates than the first preheat burner because of the significantly lower volumetric flow rate of the fuel gas in line 36. A flow of fuel gas in line 36 of 0.71/min was found to give a fuel gas temperature at the outlet of heat exchanger 68 of about 200°C, a flow of about 1 1/min was found to give a temperature of about 250°C, and a flow of about 2.6 1/min was found to give a temperature of about 400°C, but significant coking of the heat exchanger 68 and the jet 86 results. This coking, which is not observed at lower temperatures is evidence of cracking of the fuel gas since carbon is a secondary cracking product of propane . Formation of solid carbon deposits can be reduced by adding a small amount of oxidant (less than 10% stoichiometric) to the fuel in line 36. A laminar flow burner with fuel and air preheat of the type described in GB-A-0717184 also suffers from coking, which can be prevented by adding oxidant to the fuel line.

In operation, the shroud 100 and other parts, particularly the nozzle 90 may become heated to 1000°C or above and are liable to erode during use. For this reason they are designed to be easily replaced.

Preheating the fuel supplied to the burner 16 increases the rate of release of heat per unit volume (intensity) of the working flame. The reasons are believed to be that:

(a) Preheating of either the fuel or the oxidant or both without other change in conditions causes an increase in the effective flame speed (which is a complex function of temperature ) .

(b) Preheating of a hydrocarbon fuel gas can cause the fuel gas to crack into substances many of which are faster reacting than the original fuel, which further increases the effective flame speed.

(c) The products of cracking generally have higher heats of combustion than the original fuel, thus increasing the heat given out by the reaction of fuel and oxidant in the flame.

This additional energy is derived from the preheater but is carried to the working flame as chemical energy rather than thermal energy. For example, at temperatures above 500°C propane cracks into a range of products including hydrogen and acetylene, both of which have higher flame speeds than propane. The cracking of propylene is similar. Catalysis by means of the granules 82 significantly reduce the temperature at which cracking occurs . The effect of the higher flame speed is to alter the geometry of the working flame 97 such that the turbulent flame brush (i.e. the visible outline of the flame) is foreshortened, concentrating combustion into a shorter length, thus increasing the rate of release of heat per unit volume of the flame. In a turbulent flame the visible "flame brush" is composed of a highly folded flame surface which is constantly moving and changing shape too rapidly to be followed by the naked eye. Where the air and propane are mixed under turbulent flow conditions the working flame 97b is relatively large as shown in Fig 8a. Where both the fuel and the oxidant are preheated but the flow conditions are unchanged, there is produced a more compact and intense flame 97c as shown in Figure 8b.

The rate of release of heat per unit volume of the flame can be estimated from flame volume (determined by observation with the eye) and the measured fuel flow rate. Flame intensity can be calculated by multiplying fuel flow rate in kg/s by the published calorific value for the fuel (in J/kg,) obtaining the thermal power, and dividing by the volume .

A torch made as described above when fuelled with a fuel of relatively low chemical energy and reaction speed, e.g. propane or propylene, and using air rather than oxygen as an oxidant, can readily achieve a flame with air of an intensity of more than 500 MW/metre³ and up to 800 MW/metre³ has been regularly achieved. By comparison, a propane/air flame in a burner without highly turbulent mixing or preheating typically produces only 150 MW/metre³ with pressurised air or 100 MW/metre³ with air aspiration (like a Bunsen burner), whilst an oxy-acetylene flame typically provides a flame with volumetric flame intensity of 4000 MW/metre³ .

The connection between achieved workpiece temperature and flame intensity can be demonstrated by heating a test workpiece in successive flames of differing intensity. We heated a 10mm diameter alumina rod with a series of burners of each of diameter 20mm and fuelled by propane/air (which therefore had a similar final gas temperatures of around 2000°C) and measured the temperature achieved. The results are shown in the table below and are plotted in Figure 25 of the accompanying drawings:

Intensity Measured workpiece (MW/metre³) temperature (°C) ordinary burner 100 1100

(air aspirated) burner using 150 1200 compressed air

burner of Fig 2 300 1400 air preheat

(half flow) no fuel preheat

burner of Fig 2 800 1800

(air preheat full flow fuel preheat)

For comparison, a burner according to GB-A-0717184 (Burch) can achieve the melting point of iron (1540°C) on a target of only at most 1.25 mm diameter, i.e. 80 times smaller in cross-section.

It will be appreciated that differing applications require differing flame intensities. For example, lower temperature operations such as brazing with hard alloys typically require a flame producing 300-500 MW/metre³ (thus being beyond the capacity of a conventional propane/air torch); and for welding or cutting steel, a flame producing at least 500 MW/metre³ is required.

The efficiency of heat transfer to a workpiece is dependent not only upon the flame intensity, but also, to a lesser extent, upon the degree of turbulence in the flame. The more turbulent the flame is, the easier it can strip off the insulating boundary layer around a workpiece. In Fig 9a a burner 16 having a laminar flame is applied to a workpiece 101. A boundary layer 103 forms between the workpiece 101 and the flame, which boundary layer is relatively thick and therefore provides a barrier to heat transfer to the workpiece 101. In Fig 9b, the fuel and oxidant are preheated and the flame 97 is highly turbulent and reduced the thermal insulation provided by the boundary layer 103, and heat transfer to the workpiece 101 is speeded up.

The degree of turbulence which can be introduced is limited by flame speed. At the flame speeds found in conventional gas burners, the working flame becomes unstable at high flow rates and high levels of turbulence. The maximum flow velocity that can be used is generally limited by flame blow-off or lift-off. Other things being equal, resistance to flame blow-off improves with increasing laminar flame speed. The increase in laminar flame speed caused by preheating permits the use of higher than normal flow rates and consequently higher than normal levels of turbulence (indicated in our experiments by a 20dB increase in noise). Even higher flow rates can be sustained and performance further enhanced, by providing means (e.g. a built in re-ignition facility as described above) to stabilise such highly turbulent flames. During a start-up period the torch is operated at relatively low flow rates to avoid lift off, and as steady state temperatures are approached the flow rates of air and propane can be progressively increased to their steady-state values .

There is no convenient way of characterising or directly measuring the degree of turbulence but we have found the noise produced by the working flame 97 indicates its turbulence . We measure turbulence from the ratio of the acoustic power output to thermal power output, (which we call the Acoustic/Thermal Ratio of the flame, or 'ATR'). Sound intensity emitted by the burner is measured with a calibrated acoustic pressure meter, and then converted to acoustic power. The ATR is then simply the ratio of the acoustic power divided by the thermal power calculated as above.

The ATR of the propane/air torch of Fig 2 with both input gases preheated has been measured at 6xl0^"6 whilst a similar torch without preheating had an ATR of 6xl0^"8. The extent to which the effectiveness of a flame is affected by the ATR reduces progressively with increasing flame intensity and the difference between the desired workpiece temperature and the temperature of the flame. The ATR is thus of more significance for the burner of Figure 2 of intensity 500-800 MW/metre³ and of little significance to the effectiveness of an oxy-acetylene flame with an intensity of hundreds MW/metre³ and a theoretical adiabatic flame temperature of 3070°C, far above the melting point of iron at 1540°C. Temperatures achieved using the burner of Figure 2 have been measured using an optical pyrometer focused on an alumina emitter located in the hottest part of the working flame 97. Using propane as fuel, alumina emitter temperatures of 1200°C were measured with no preheating of the fuel or air, which is at least 100°C above the temperature achieved with a conventional propane burner at the same flow rates. This increase is believed to be attributable to the high degree of mixing occurring in the premixing chamber 67, which results in a highly turbulent flame which permits a high level of heat transfer to the emitter (see Fig. 9b).

Use of the air preheat burner 14 can produce air at 500°C to 700°C of low flow rates. This results in a further increase of about 100-300°C in the measured emitter temperature, with measured values reaching

1530°C. Use of both the air preheater 14 and the fuel preheater 18 gives measured emitter temperatures of 1700-

1800°C and a flame that can melt steel or even platinum.

The burner of Fig. 2 was tested for its ability to melt through 2 mm mild steel plate, and achieved this result provided that both fuel preheating and air preheating were carried out. The ability to melt through steel plate using propane rather than acetylene as fuel and air rather than oxygen as oxidant at a comparable (though somewhat slower) rate than with an oxy-acetylene flame was a significant and surprising result.

We have calculated the Reynolds numbers for the Burch burner and for the present burner under various flow conditions. The results are as follows:

These figures were calculated using gas (air) density at 600°C of 0.4 kg/m³ and a gas velocity of 4 times 10⁵ Pa s. The normal limit for laminar flow is a Reynolds number of 2000, but laminar flow will only be established at that Reynolds number if there is a substantial length of pipe of a smooth constant bore which gives viscous forces a chance to damp out eddies which give rise to turbulence. In the present burner, the turbulence established by the internal nozzle 94 and the mixing chamber 67 will not be damped out in the relatively short length of the shroud 100, and turbulence will be present at all the Reynolds numbers in the conditions according to the invention. At Reynolds numbers of 8500 or above, the flow will inevitably be turbulent. The effects of fuel combustion on the turbulent. The effects of fuel combustion on the transition between laminar and turbulent flow are believed to be relatively small, but tend to increase turbulence. When pre-heating is switched on, there is an increase of noise, which suggests an increase in the effective Reynolds number in the flame.

The pre-heat coils of heat exchangers 50, 68 can be heated electrically rather than by gas . Mains electricity was stepped down by means of a transformer and applied in turn to each of the heat exchangers 50, 68 the other being gas heated as before. A current 200 amps at 10 volts was passed through the coils of each heat exchanger, which, being stainless steel, have significant electrical resistance. 2 kW of power was applied to the coil, which heated it to red heat and provided the necessary flow of heated fuel gas or air to the mixing chamber 67. If two step down transformers are used, then both the heat exchangers can be heated electrically by direct resistance heating. The burner of Fig. 10 is similar to that of Fig. 2 except that it has a single preheat burner 150 which heats a first heat exchanger 152 and a second heat exchanger 154 disposed coaxially with burner 156 identical to the burner 16 of Fig 2 in a common shroud 158. The amount of heat received by each of the heat exchangers is controlled by a replaceable insert comprising a disc-shaped baffle 159 on the upstream side of and adjacent to an apertured disc 160 and suspended by e.g. steel wires as shown. The shroud 158 is a stainless steel tube surrounded by thermal insulation 161 (high temperature ceramic fibres) and has handles (not shown ) attached .

The flame of a gas torch made in accordance with Fig 10 and fuelled by propane and air has been observed to heat a 10mm diameter cylindrical alumina target to over 1800°C, whereas the temperature of such a target heated by the same gas torch without preheating will not exceed 1200°C. A conventional propane/air burner will not burn through mild sheet steel, whereas the propane gas torch according to Fig 10 will, surprisingly, burn through a 2mm steel sheet in under one minute, which is comparable to the performance of an oxy-propane brazing torch with a burner of similar dimensions. The burner of Fig 10 can be used as a gas torch in many applications where an oxy- acetylene flame would previously have been required. Comparative times for various gas torches to burn through 2 mm mild steel sheet with the burner located 5 mm from and at right angles to a large flat sheet are:

Gas torch according to Fig 10 using compressed air and 10 litres/min of propane: 45-60 seconds

Gas torch according to Figure 10 using compressed air and 20 litres/min of methane: 120 seconds

Conventional oxy-propane burner (consuming 10 litres propane/min) : 90-120 seconds Conventional oxy-acetylene burner (consuming 5 litres acetylene/min) : 30 seconds

The burn-through times quoted above are for the gas torches of Fig 10, not for gas cutting torches.

The temperature heat transfer characteristics of the torch continue beyond the melting point of iron at 1540°C, and a rod of platinum of melting point of 1770°C and diameter 2mm has also been easily melted using the above torch.

The laboratory rig was set up for methane using a cylinder of methane whose regulator was set to about 6 barg outlet pressure. Methane then flowed into the main burner at an indicated flow rate about twice that of propane. The pre-heat burner was not converted to methane, because such conversion would not affect the results achieved. A larger than normal jet (0.8mm diameter) was used to give the greater gas flow. With these changes, it was necessary to adjust the rate of air flow, at least during warm up, since the methane flame was somewhat less stable than a propane flame until full pre-heating was established. However, once the torch had warmed up, it gave high operating temperatures, and could melt through large area samples of steel sheet. The temperatures achieved, and the melt through (key-hole) times seemed slightly less favourable than with propane, although performance was considered satisfactory.

Tests using propylene showed that the burner performance was indistinguishable from that using propane, and the same burn-through times for 2mm steel plates were recorded without alteration of the flow rates and pressures.

The gas torch of Fig. 11 is also generally of stainless steel having gas-tight welded joints . The only construction that has so far been found to give gas-tight joints where the joint is subject to strong heating is an all-welded construction because of differential thermal expansion effects. The torch comprises a tubular shroud 170 of stainless steel with heat insulation 172 of ceramic fibres and with first and second heat exchangers 174, 176 made up of tubing of diameter 6 to 8 mm and 6 mm respectively disposed concentrically within it. A wall 178 at one end of shroud 170 supports a preheater 180 fed with air and fuel through lines 182, 184 and burning the same fuel gas as the working flame. A second wall 186 supports the main burner 188 of the same structure to those previously described. The exhaust gases from the preheater 180 exit through lateral ports 190 formed in the shroud 170 adjacent to the end wall 186, so causing the flame from the preheater 180 to play on the heat exchangers 174, 176. A cylindrical baffle 192 may be provided within heat exchangers 174, 176 to improve the flow pattern of hot gas around the preheater coils .

Thermal insulation also extends over a handle in which there are provided valves 194, 196 controlling the supply of fuel and air to the preheater 180, and 198, 199 controlling the supply of air and fuel to the working flame from a common air supply line 200 and from a common fuel supply line 202. Air in the supply line 200 is at a greater pressure than that in the fuel line 202, ideally in this configuration at 7 barg. The second heat exchanger 176 is made of 316L stainless steel tube which itself catalyses the cracking of hydrocarbon fuel gas into hydrogen and unsaturated compounds and may also contain granules of Zeolite 13X or other cracking catalyst packed into the heat exchanger 176 in a manner such as to permit the required through-flow of fuel gas and held in place by plugs of metal wire. A connecting line 204 between the air line 200 and fuel line 202 contains a check valve 206 which prevents fuel from passing into the air line 200 and a flow control valve 208 which allows the introduction of up to 20% stoichiometric of air into the fuel line of the working flame so as to avoid soot formation in the fuel preheater 176. A gas torch of this embodiment (and also of any of Figs 1, 2 or 10), when fuelled with propane and air, can be operated with gas flow rates in litres per minute corrected to atmospheric pressure as follows:

Air to working flame 120

Fuel to working flame 6

Air added to fuel line for working flame 1.5 Fuel to preheater burner 3

Air to preheater burner 60

If the valves 194, 196 are closed to shut off fuel and oxidant to the preheat burner 180, the flame at maximum flow rates from the main burner 188 is about 100 mm long. With these valves open and the preheat burner 180 in operation the working flame extends about 10mm from the end of the nozzle 100 and has a high intensity and turbulence.

Figures 12 and 13 are similar to Figures 1 and 2 except they show a gas cutting torch. Air from the manifold 20 is fed through cutting air delivery line 220 and cutting air preheater 222 to adjacent the working flame 97, where the cutting air is directed onto the workpiece being heated by the cutting flame 97. Flow of air through the line 220 is shown on gauge 224 and is controlled by a valve 226. The cutting air preheater 222 is also gas fired and includes a burner 228 fed with compressed air from the manifold 20 via gauge 230, control valve 232 and line 234 and fed with fuel from the manifold 34 via gauge 236, valve 238, and line 240. A jet of flame from the preheat burner 228 plays on the coils of a heat exchanger 242 of the same construction as previously described and located in the line 220. The spacing between the burner 228 and the adjacent end of the heat exchanger 242 is 10 mm. As shown in Figure 14, the shroud 100a of the burner 16a is slotted at 244 to accommodate the tip of the line 220 which is provided with a nozzle 246 having a restricted diameter orifice 248 from which the cutting air which has become heated to about 700°C is jetted onto the workpiece being cut. The internal diameter of the tube 220 is typically 4.5 mm and the diameter of the orifice 248 is typically 0.5 mm. In Figure 15 the burner 16a is shown cutting through a 2mm thick sheet 250 of mild steel.

A cutting torch made in accordance with Figures 12 to 14, fuelled by propane burning in air and using a cutting gas of preheated air can achieve a cutting speed of 150 mm/minute through a 2mm steel plate.

The cutting torch of Figures 16 and 17 is similar to that of Figure 14 except that the burner lOd has a cutting gas delivery line 220a which passes axially through the central hole 252 in the nozzle 254 to a jet 256 which is located in the middle of the main flame.

A more detailed construction of cutting torch is shown in Fig 18 which is similar to that of Fig 11 except that there is a branch airline 300 controlled by a valve 302 leading to a further heat exchanger 304 for supplying hot air to the line 220 and nozzle 256. The heat exchangers 174, 304 for the air feed to the mixing chamber and for the air feed to the nozzle 256 are of the same diameter and are coiled together as shown, with the heat exchanger 176 for the fuel gas which is of lesser diameter fitted within them.

Figure 19 shows a gas cutting torch according to the invention arranged to cut steel or other metal sheets in an X,Y cutter. A support 320 has a cantilever arm 322 from which there depends a cutting torch 324 operating as described above fed with preheated fuel gas, preheated compressed air and preheated cutting gas through lines 326, 328 and 330. A table 332 supports a workpiece holder 334 which can be driven in an X-direction by drives 336 and in a Y-direction by drives 338, the workpiece 340 being held on the holder 334 by clamping means (not shown).

Figure 20 shows a furnace 350 having a chamber 352 which is closed by means of a lid 354 in which exhausts 356 are provided, and which enables a crucible 358 removably suspended above a burner 360 to be heated by the flame from that burner. First and second heat exchangers 362, 364 for air and fuel gas respectively are located in the floor of the furnace 350 and are preheated by means of a gas-fired preheater 366. There are two exhaust paths, one of them being beyond the heat exchanger 364 as shown, and the other being between the lid 354 and the walls of the furnace 350. Figure 21 shows a furnace 370 having an internal space 372 in which articles 374 to be heated can be placed. The internal space 372 is heated by means of a burner 376 constructed as previously described which is mounted towards the roof of the space 372 and with the nozzle 100 facing inwardly. Air and fuel reach the burner 376 from lines 378, 380 via heat exchangers 382, 384 which are located in a tunnel 386 leading from the space 372 to the exterior of the furnace. The flame from the burner 376 heats the space 372 and the combustion gases circulate around that space, eventually leaving via the tunnel 386 where their flow heats the incoming fuel and air in the two heat exchangers 382, 384. Baffles may be provided to achieve the required pattern of heating of the incoming gases and to permit the required outflow of exhaust gases from the tunnel 386.

Various modifications may be made to the embodiments described above without departing from the invention. For example, in the preheat burner or burners, fuel under pressure could be fed through a jet, and could suck in air or other oxidant gas on the venturi principle. Air aspiration is not possible for the burner which produces the working flame. However, when used on a portable torch air aspiration has the disadvantage that there is a greater risk of hot gases being emitted towards the operator, and that the airways leading to the venturi may become partly obstructed when the torch is being handled. A further possibility is to arrange the main burner or the preheating burner or burners so that compressed air is fed in by means of a jet, and the venturi effect is used to suck in fuel. This arrangement could be useful where the fuel gas is available only at low pressure e.g. when methane is used as the fuel gas.

Various configurations of heat exchanger can be used for preheating the fuel and oxidant gases and for preheating the cutting gas (for example as described in Applied Heat Transfer, Todd & Ellis (Harper & Row 1982)). However, a tubular heat exchanger having a single flow path is preferred for preheating the fuel and the oxidant gases because of the phenomenon of "runaway" or self- amplifying uneven heating that can occur when heat exchangers with parallel flow paths such as parallel tubing or plate or annular heat exchangers are used. Although it is possible to reduce the diameter of the tubing in the heat exchanger to increase the rate of heat transfer, it is preferred to maximise the diameter of the tubing used because larger tubes give rise to lower pressure drops reducing the input gas pressure needed. Instead of round tube, there may be used flattened tube, which gives better heat exchange characteristics but has a slightly higher pressure drop. Since both the fuel and oxidant gas are supplied under pressure, a small increase in pressure drop may not be a significant problem. Exhaust gases from the pre-heater flame may be directed to suit particular applications. For example, exhaust gases may be deflected away from the workpiece where it is required to heat a target zone of the workpiece strongly while minimising the heat transfer to other regions of the workpiece.

Although the illustrated embodiments of the burner 16 show side jets 96 which create a continuously lit flame 97a (Fig 7c) which reignites the working flame where necessary, the same result may be achieved by other ways. For example, subsidiary burners and jets and holes between the mixing chamber and the shroud may be provided. The preheat flame may be sustained by any convenient means including a pilot light or electric means with provision for sensing flame failure.

Preheating of input gases may be achieved by means other than a gas flame. For example waste heat from the working flame can be used which can increase the overall thermal efficiency of the burner and may be particularly suitable where the burner is to be used in a closed or semi-closed space containing the workpiece (e.g. when melting materials in a crucible as described above with reference to Figure 20). The fuel and oxidant gases may be heated electrically using, for example, NiChrome or silicon carbide based heaters, and this may be convenient for torches of working flame output below about lOkW. Cracking of the fuel gas can also be achieved by electric discharge.

The hottest parts of the burner may be made of materials other than stainless steel. For example, the burner shroud and the internal nozzle may be made of or coated with refractory materials , thus reducing or eliminating the need to replace them periodically.

As shown in Figs 22-24 the burner 16 may be provided with various forms of nozzle to modify the shape of the flame produced. Thus it may be fitted with a flame narrowing nozzle 400, a flame spreading nozzle 402, or with a heat shield 404.

Figure 26 shows a variant of the burner of Figure

11 in which there is on the external surface of the insulation 172 a layer of metal foil 410. The insulation

172 and foil 410 fit within an outer tube 412 of plastics material, with an air space 414 between them, and with concentricity being maintained by spacer lugs 416. Compressed air from the air supply line to the burner can be fed as a bi-pass flow indicated by arrow 418 along the space 414 to keep the plastics outer tube 412 cool, and to protect the user from excessive body or handle temperatures .

Figure 27a shows a modified form of the burner 16 of Figure 2 in which the nozzle assembly is replaced by a foraminous plate 420 which is at the rear end of the shroud 100 and divides the mixing chamber 67 from the combustion space and permits gas outflow from the mixing chamber 67. Satisfactory combustion and a turbulent flame can be obtained under a range of conditions. The plate 420 is formed with a relatively large central aperture 422 and with smaller apertures 424 disposed in a ring as shown in Figure 27b.

Figure 28 shows a burner that can be used under laminar flow conditions as recommended by Burch. A burner body generally indicated by the reference numeral 430 has an inlet 432 for air under relatively high pressure as indicated by the arrow 434 and leads via line 436 to a venturi nozzle 438. A line 440 for fuel gas 442 under relatively low pressure leads to a gas manifold chamber 444 surrounding part of the line 436 and the rear part of the nozzle 438 as shown. Side tubes 446 in the venturi nozzle 438 permit fuel gas to be sucked from the chamber 444 into the current of air passing through the venturi nozzle. Thereby the fuel gas and air become mixed, and pass from the nozzle 438 under generally laminar conditions.

Figure 29 shows a fuel gas and air supply arrangement for the burner of Figure 28. Air 450 passes through needle valve 451 into a line 452 and thence via a pre-heater 454 and supply tube 432 to the burner assembly 430. Fuel gas under relative low pressure at 456 passes through needle valve 458 and line 460 to heat exchanger 462 and thence via line 440 to the burner assembly 430. A by-pass line 464 in which air flow is controlled by needle valve 466 connects the lines 452 and 460, and permits the fuel gas to be mixed with a small amount of air before it enters the heater 462. Reverse flow of fuel gas and air can be prevented by check valves 468, 470 in the lines 460 and 464 respectively, these check valves being optional. The injection of air into the fuel gas before it is pre-heated can overcome the sooting problem reported by Burch .

Figure 30 shows an arrangement similar to Figure 29 where more powerful heat is required to be supplied to the workpiece. For that purpose there are a multiplicity of burners 430a to 430d fed in parallel with pre-heated fuel gas through a manifold 440a and with pre-heated air through a manifold 442a.

Claims

1. A gas burner or torch comprising: a mixing chamber; an inlet for supply of fuel gas to the mixing chamber ; a heater for heating fuel gas before it enters the mixing chamber for cracking the fuel gas; an inlet for supply of oxidant gas to the mixing chamber; an outlet from the mixing chamber for a flow of mixed fuel and oxidant gas, the arrangement being such that in operation the flow is turbulent; and a combustion zone for receiving and burning the turbulent mixture of fuel gas and oxidant gas.

2. The burner of claim 1, wherein the mixing chamber is provided with inlets for fuel gas and oxidant gas from first and second directions, the first and second directions being different.

3. The burner of claim 2, wherein the chamber is cylindrical, the inlet for fuel gas is directed axially and the inlet for oxidant gas is directed radially or laterally.

4. The burner of any preceding claim, wherein the means for supplying fuel gas to the mixing chamber comprises a supply line for fuel gas and a heater for heating a region of the supply line to bring about cracking.

5. The burner of claim 4, wherein the region of the supply line which can become heated by the heater is of a material which catalyses cracking of the fuel gas.

6. The burner of claim 5, wherein at least the region of the supply line which can become heated by the heater is of stainless steel.

7. The burner of claim 4 , 5 or 6 , wherein the region of the supply line which can become heated by the heater contains particles of a cracking catalyst.

8. The burner of claim 7, wherein the cracking catalyst is a zeolite.

9. The burner of any of claims 4 to 8, wherein the heater comprises a gas preheat burner and a heat exchanger forming part of the fuel gas supply line and positioned to receive heat from the gas preheat burner.

10. The burner of any preceding claim, further comprising means for avoiding formation of soot as a result of cracking of fuel gas.

11. The burner of claim 10, wherein said means for avoiding the formation of soot comprises means for introducing oxidant into the fuel gas in advance of the fuel gas cracking means .

12. The burner of any preceding claim, further comprising a supply line for oxidant gas and means for heating oxidant gas as it passes along the supply line.

13. The burner of claim 12, wherein the heating means comprises a preheat burner for oxidant gas and a heat exchanger forming part of the supply line for oxidant gas and positioned to receive heat from the oxidant gas preheat burner.

14. The burner of claim 12, wherein there is provided a preheat burner, and wherein heat exchangers forming part of a supply line for fuel gas and forming part of a supply line for oxidant gas are located so that they can both receive heat from the preheat burner.

15. The burner of claim 14, wherein the heat exchanger forming part of the supply line for fuel gas and the heat exchanger forming part of the supply line for oxidant gas comprise coils of tubing which are arranged coaxially.

16. The burner of any preceding claim, wherein the outlet from the mixing chamber comprises a nozzle.

17. The burner of claim 16, wherein the nozzle has a main outlet which faces away from the mixing chamber and one or more auxiliary outlets formed in a protrusion of said nozzle, said auxiliary outlets being directed obliquely to the main outlet for formation of additional regions of flame.

18. The burner of claim 17, wherein the protrusion has a flange for defining a quiet combustion zone, said flange being located further from the mixing chamber than the auxiliary outlet or outlets.

19. The burner of any of claims 1 to 15 wherein the outlet from the mixing chamber comprises a foraminous plate.

20. The burner of any preceding claim, further comprising a tubular member for receiving the turbulent mixture of fuel and oxidant gas and defining an upstream region of the combustion zone.

21. The burner of any preceding claim, connected to a source of fuel gas and to a source of oxidant gas.

22. The burner of claim 21, wherein the source of fuel gas is a hydrocarbon gas.

23. The burner of claim 21 or claim 22, wherein the source of oxidant gas is a source of compressed air.

24. The burner of any preceding claim, further comprising means for supplying a preheated cutting gas to a workpiece adjacent the main flame to erode or cut the workpiece .

25. The burner of claim 24, connected to a source of oxygen cutting gas .

26. The burner of claim 24, connected to a source of an oxygen-air mixture as cutting gas .

27. The burner of claim 24, connected to a source of air as the cutting gas .

28. The burner of any preceding claim made wholly or predominantly of stainless steel welded at joints thereof .

29. A method for operating a gas burner which comprises: cracking a fuel gas; feeding the cracked fuel gas and oxidant gas to a mixing zone of the burner; allowing a turbulent mixture of fuel gas and oxidant gas to flow from the mixing zone to a combustion zone; and burning the turbulent mixture.

30. The method of claim 29, wherein the fuel gas is catalytically cracked.

31. The method of claim 29 or claim 30, wherein the fuel gas is a hydrocarbon gas.

32. The method of claim 31, wherein the fuel gas comprises methane, propane or propylene.

33. The method of any of claims 29 to 32 wherein the oxidant gas is preheated compressed air.

34. The method of any of claims 29 to 33, wherein the fuel and oxidant are mixed and are supplied at flow rates such as to form a turbulent flame that produces more than 150 MW/metre³.

35. The method of any of claims 29 to 33, wherein the fuel and oxidant are mixed and are supplied at flow rates such as to form a turbulent flame that produces 300 to 500 MW/metre³.

36. The method of any of claims 29 to 33, wherein the fuel and oxidant are mixed and are supplied at flow rates such as to form a turbulent flame that produces more than 500 MW/metre³.

37. The method of any of claims 29 to 36, wherein the gaseous mixture entering the combustion zone has a Reynolds number of more than 2000.

38. The method of any of claims 29 to 37, wherein the fuel and oxidant are mixed and are supplied at flow rates such as to produce a flame having an acoustic/thermal ratio (ATR) at more than 6xl0^"8.

39. The method of any of claims 29 to 37, wherein fuel and oxidant are mixed and are supplied at flow rates such as to produce a flame having an acoustic/thermal ratio of about 6xl0^"6.

40. The method of any of claims 29 to 39, comprising the further step of supplying preheated cutting gas to said burner.

41. The method claim 40, wherein the cutting gas is preheated air.

42. A gas burner having means for producing turbulent mixing of fuel and oxidant gases and means for burning the mixture under highly turbulent conditions.

43. A method of heating, brazing, soldering, melting, preheating, welding, annealing or cutting a material which comprises applying to the material or to a container in which the material is present the flame of a burner or torch as claimed in any of claims 1 28.

44. The method of claim 43, wherein the material is a steel workpiece and the flame is applied to cut the workpiece.

45. A burner for burning a fuel gas and oxidant comprising a body having a mixing chamber, a shroud and a nozzle which separates the mixing chamber from the shroud, an inlet for fuel gas into the mixing chamber from a longitudinal or axial direction, an inlet for oxidant into the mixing chamber from a transverse or radial direction, and an outlet for the mixture through the nozzle into the shroud, said outlet being of small size relative to the mixing chamber.

46. A nozzle for fitting to a burner said nozzle comprising a disc-like body having a relatively small diameter protrusion extending from one face thereof away from the body, a main outlet which is directed longitudinally or axially and extends through said body and through said protrusion, and one or more auxiliary outlets which are directed transversely or radially and extend from said main outlet to a side region of the protrusion.

47. The nozzle of claim 46, wherein the longitudinal or axial protrusion has a flange which is located further from the disc-like body than the auxiliary outlet or outlets .

48. A gas burner or torch comprising means for supplying fuel gas the burner at a temperature such that the fuel gas becomes cracked, means for supplying oxidant gas to the burner, a combustion zone for receiving and burning a mixture of the fuel and oxidant gas, and means for supplying oxidant gas to the fuel gas in advance of cracking to reduce or avoid formation of coke.

49. The burner of claim 48, wherein the fuel and oxidant gas in the combustion zone are under laminar flow conditions .

50. The burner of claim 48, wherein the fuel and oxidant gas in the combustion zone are under turbulent flow conditions.

51. A method for operating a gas burner which comprises cracking a fuel gas, feeding the cracked fuel gas to a burner, feeding an oxidant gas to a burner, and burning the mixture, wherein oxidant gas is supplied to the fuel gas in advance of cracking to reduce or prevent coke formation.