FLAME BURNER AND METHOD FOR FLAME BURNING A METALLIC SURFACE

Abstract

The invention relates to a flame burner having a nozzle (10) disposed in a head, wherein in addition to annularly disposed gas supply channels (11) the nozzle has a central gas supply opening (12), which has a laval nozzle-like region and a stabilizing region having a consistent diameter connected thereto in the flow direction. In order to flame burn a metallic surface the oxygen gas guided through a central nozzle of a flame burner is incited to oscillate in such a manner, such that a pulsating oxygen flow exiting the nozzle mouth is formed at the speed of sound, or at supersonic speed.

Description

The invention relates to a flame burner having a nozzle disposed in a head, wherein in addition to annularly disposed gas-supply passages the nozzle has a central gas-supply duct.

The invention further relates to a method for flame scarfing of a metallic surface by means of the stated flame burner.

In the known flame burners combustion gas is guided to a nozzle head via annularly disposed gas-supply passages and is mixed with the oxygen transported via the central gas supply, and forms the combustion flame. Flame burners are utilized for various application purposes. For example, during the cooling of slabs produced by means of casting, undesired tears often occur, which have to be removed by means of a surface treatment. The same is also true for ridges and burrs, which occur, for example, during cutting in the processing of the slabs. The flame burners are guided along the affected surfaces for removing the surface blemishes, which may be accomplished with a manually manipulated burner or an automatically guided burner where a flame burner is attached to a controllable robot arm.

The processing costs for surface treatment are substantially determined by the processing time and the gas consumption, an adequate surface quality being absolutely required.

The object of the present invention is to provide a flame burner and a method for flame scarfing, in which an optimum surface quality of the workpiece to be treated is attainable with an oxygen consumption and processing time that is as low as possible.

In order to attain this object, the invention provides the flame burner described in claim 1 and the method described in claim 9.

Further improvements of the invention are described in the sub-claims.

The flame burner according to the invention has a nozzle comprising multiple gas-supply passages annularly disposed around a central gas-supply duct. The central gas-supply duct has at least three successive regions, as viewed in the flow direction, that is a first region tapering in to a minimal inner diameter, a second region whose inner surface flares out to a larger diameter than the minimal inner diameter, and a third region of uniform cross-sectional profile, preferably a uniform cylindrical inner diameter. The important factor is the cross-sectional constriction of the inner diameter up to a critical measurement of the diameter, which is followed by flaring. As a stabilizing ring, the last third region having a uniform cross-sectional profile serves to maintain the produced gas flow profile. A pulsating gas flow can be created by means of this design, which has the speed of sound, or supersonic speed, at the nozzle outlet mouth. The ratio of the oxygen pressure upstream of the nozzle and the ambient pressure on one hand, and the ratio of the oxygen pressure at the nozzle outlet surface and the ambient pressure, determine the gas profile. If the pressure at the nozzle outlet surface is below ambient pressure, the exiting gas flow has a narrowing shape in the initial section downstream of the nozzle, whereas with reversed conditions the shape expands in a barrel-shaped manner. If the oxygen pressure upstream of the nozzle and at the exit from the nozzle is equal to the ambient pressure, a straight line envelope of the initial section of the exiting gas is created.

The pulse frequency achievable using the nozzle and the amplitude individually depend on the initial pressure, the degree of tapering, and the degree of expansion. The non-isobaric turbulent supersonic flow created is characterized by strong spatial inhomogeneities of the velocity and pressure fields, which lead to the creation of volatile state changes, that is the pulse-like shocks and layer displacements at high velocity gradients. This flow velocity and pressure pulsation leads to a pulsation spectrum. Starting with a certain value, the gas velocity locally reaches supersonic speed in the described nozzle at the smallest critical nozzle cross-section, upon exceeding of which pulse-like compressed and thinned regions occur as pulses. These types of pulse waves can form a barrel-shaped flow, the strung compressions of which depend on the ratio of the oxygen pressure in the nozzle to the ambient pressure, and on the so-called critical velocity ratio, which is the ratio of the gas velocity at the nozzle outlet surface to the speed of sound. In principle, the flame burner has a nozzle configured in the type of a Laval nozzle, which together with the third region as the “stabilizing ring” forms an oscillating resonator.

The present invention generally provides within its scope that the first and the second regions are disposed in succession, however, short partial parts may be contained in them where the minimal diameter does not change. The flow velocity is maintained in such a short partial part.

In the present invention the central gas-supply duct also ends slightly upstream of the level defined by the openings, in which the annularly disposed gas-supply passages end. Solutions within the scope of the present inventions are also incorporated, whereby multiple rings of coaxially extending gas-supply passages that end at different levels downstream of the outlet opening of the central pipe in a graduated manner.

For technical flow reasons, the length of the first region is preferably smaller than the length of the second region, and is preferably also smaller than the length of the third region. The third region may, depending on the desired pulse characteristics, be selected longer, of the same length, or even shorter than the total length of the first and second regions.

According to a further improvement of the invention the diameter of the third region is smaller than the maximum outlet diameter of the central gas-supply duct at an upstream end of the first region. In order to optimize the effect, the diameter and the lengths of these three regions are coordinated such that gas exits at the nozzle outlet mouth in the form of pulses, preferably having a pulse frequency of between 100 and 650 Hz. Preferably, a maximum gas flow velocity of 2 Ma should be present in the central gas-supply duct at predetermined values of the oxygen and combustion gas pressure.

The nozzle may have a round or concentric cross-section, wherein particularly the central gas-supply duct has an annular cross-section in order to elongate the at least one ring, or possibly two rings, on which additional gas-supply ducts are positioned for the combustion gas.

As generally known according to prior art, the nozzle head is preferably cooled, water in particular being suited as the coolant.

The method according to the invention for flame burning of a metallic surface, such as a slab, is characterized in that oxygen guided via a central nozzle of a flame burner is incited to oscillate such that a pulsating oxygen flow exits the nozzle outlet mouth at the speed of sound, or at supersonic speed. The pulsating oxygen flow consists of longitudinal waves, i.e. a periodic succession of pressure increases and decreases of the gaseous oxygen. Not only is the central oxygen flow made to pulsate by means of this measure, but the peripherally inflowing combustion gas is also made to oscillate. The result is a substantial savings of oxygen consumption and a smooth surface of the metal piece to be processed via flame scarfing. Preferably, the process parameters, particularly the oxygen application pressure by means of which the oxygen flow is entered into the nozzle, are selected as a function of the nozzle shape such that the oxygen flow is distributed into a central flow and peripheral flows. The ratio of the oxygen pressure upstream of the central nozzle to the ambient pressure N=p_o/p_u is preferably between 1 and 200, whereas the ratio of the oxygen pressure p_a at the nozzle outlet surface to the ambient pressure p_u is between 0.1 to 100.

Further embodiment variants and details of the invention are illustrated in the drawings and described below. Therein:

FIG. 1 is a combined side view and a longitudinal section through the nozzle of the flame burner according to the invention,

FIG. 2 is a top view of the nozzle, and

FIGS. 3a to d are cross-sections through the central gas-supply duct having different gas flow shapes.

The core of the flame burner according to FIGS. 1 and 2 is a nozzle 10 disposed in a head, wherein in addition to an annular array of gas-supply passages 11 the nozzle has a central gas-supply duct 12. Here and as detailed in FIG. 2, annular arrays of gas inlet openings 111 and 112 on rings are concentric to the gas-supply duct 12. Their angular spacing a is determined by their number n so it equals 360/n. In the present case the gas-supply passages 111 and 112 open into an annular gas-supply passage 11, as shown in FIG. 1. The passages 112, 111, and 11 carry a combustion gas or a mixture of oxygen and a combustion gas, whereas the central gas-supply passage 12 is provided for feeding oxygen.

The central gas-supply duct 12 is subdivided into sections L₁, L₂, L₄, L₃, and L_K, or L₁, L_c, and L_K, along its length L, the latter regions being of particular meaning. The gas-inlet region L₁ corresponds to the inlet region used in nozzles known from the prior art. However, a Laval nozzle-like shape of the central first supply passage 12, extending along the length L_c, is novel. This nozzle shape is formed by a region in which the nozzle inner diameter tapers up to a minimum critical diameter d_min that is maintained along a length L₄ (also see FIG. 3). In the region immediately downstream in a gas flow direction 13, the inner surface of the gas-supply duct 12 flares smoothly to a larger diameter d_K (see FIG. 3) that is maintained up to the nozzle outlet mouth along the remaining length L_k. The following dimensions were selected in the specific illustrated embodiments: L₁=43 mm, L₂=10 mm, L₃=25 mm, L₄=2 mm, and L_K=72 mm. While L₁, L₂, L₃, and L₄ remain unchanged at given oxygen and combustion gas pressures, the length of L_K may be changed to 65 mm or 25 mm.

FIG. 3 merely illustrates the cross-sectional views of the central gas-supply duct in the Laval nozzle-like equipped region and in the stabilization region. The gaseous oxygen flowing into the Laval-like region has a pressure P₀ and a Temperature T₀. The pressure is P_A at the end of this Laval-like region, i.e. at the mouth of the upstream region L_c. The first region where the nozzle is frustoconically tapered is shown at 121, the adjoining region of the frustoconical nozzle expansion is shown at 122, whereas finally the region of constant diameter is shown at 123, and has the shape shown in FIG. 3. FIGS. a to d illustrate different gas pulsations depending on the initial pressure P_o, which appear as longitudinal waves, in which higher and lower pressures alternate. It is also obvious that depending on the initial pressure P_o selected, the central gaseous oxygen flow surrounded by a peripheral combustion gas flow is constricted in a narrower or broader manner. The length L_K is the decisive factor as to the degree in which the pulsating oxygen flow can be stabilized.

The flame burner according to the invention may be configured either as a manual or an automatic device. The pressures utilized, by means of which the gaseous oxygen is pulled into the central opening, are between 5 and 20 bars. The natural gas utilized as the combustion gas is substantially comprised of methane, and is at a pressure of 1 to 5 bars. Methane is added via the nozzle inlets 111 and mixes with the oxygen entering via the nozzle inlets 112 such that an oxygen/methane mixture flows peripherally to the nozzle outlet mouth via the annular opening 11. The velocity aspired in the central line 112 at the stated application pressure of the oxygen flow should be within a range of supersonic speed, and should be up to 2 Mach at the predetermined values of the oxygen and combustion gas pressure.

The following results have been achieved in tests using flame burners:

Initially, a first flame burner having a common nozzle according to the prior art for flaming a slab was used. Oxygen was introduced via the central nozzle at a pressure of approximately 12×10⁵ Pa, and combustion gas was introduced via the peripherally disposed nozzles at a pressure of 2×10⁵ Pa.

Subsequently, a flame burner having a nozzle according to the invention was used. Due to the resultant pressure pulses, blowback was so great that manual flaming at an oxygen pressure of 12×10⁵ Pa could not be performed. For this reason, the oxygen pressure was reduced to 8×10⁵ Pa, whereas the combustion gas pressure remained unchanged.

Surprisingly, oxygen was in the first case during flaming work at amounts between 370 to 290 m³. For the same flaming work only 90 to 100 m³ was required by the nozzle according to the invention, which illustrates that an enormous gaseous oxygen savings can be achieved.

Claims

1. A flame burner having a nozzle disposed in a head, wherein in addition to annularly disposed gas-supply passages the nozzle has a central gas-supply duct wherein the central gas-supply duct has directly succeeding regions in the flow direction, namely

a first region tapering in to a minimal inner diameter,

a second region whose inner surface flares to a larger diameter than the minimal diameter, and

a third region extending up to the nozzle outlet mouth and having a uniform cross-sectional profile, preferably a uniform cylindrical inner diameter.

2. The flame burner according to claim 1 wherein a length of the first region is smaller than a length of the second region.

3. The flame burner according to claims 1 wherein a diameter of the third region is smaller than a maximum initial diameter at an upstream end of the first region.

4. The flame burner according to claim 1 wherein diameters of the three regions and their lengths are adjusted to one another such that gas exits as pulses at the nozzle outlet mouth.

5. The flame burner according to claim 4 wherein the pulse frequency at the nozzle outlet mouth is 100 to 650 Hz.

6. The flame burner according to claims 4 wherein the maximum gas flow velocity is 2 Ma at predetermined values of the oxygen and the combustion gas pressure.

7. The flame burner according to claim 1 wherein the central gas-supply duct has an annular cross-section.

8. The flame burner according to claim 1 wherein the nozzle head is liquid-cooled, particularly water-cooled.

9. A method for flame burning a metallic surface, the method comprising the steps of:

feeding gaseous oxygen guided through a central nozzle of a flame burner; and

controlling flow of the gaseous oxygen in the burner such that the gaseous oxygen is made to oscillate such that and a pulsating oxygen flow exiting the nozzle outlet mouth is formed at the speed of sound or at supersonic speed.

10. The method for flame burning according to claim 9, further comprising the step of

subdividing the gaseous oxygen flow into a central flow and peripheral flows.

11. The method according to claims 9 further comprising the step of

maintaining the ratio of the oxygen pressure upstream of the central nozzle to the ambient pressure is to between 1 and 200 such that the ratio of the oxygen pressure at the nozzle outlet surface to the ambient pressure is between 0.1 to 100.