GB2255572A

GB2255572A - An apparatus for gas atomising a liquid

Info

Publication number: GB2255572A
Application number: GB9109419A
Authority: GB
Inventors: Marios Sava Christodoulou
Original assignee: Rolls Royce PLC
Current assignee: Rolls Royce PLC
Priority date: 1991-05-01
Filing date: 1991-05-01
Publication date: 1992-11-11
Also published as: GB9109419D0

Abstract

An apparatus (110) for atomising a molten metal to produce metal particles or droplets comprises an annular atomising nozzle (112) which defines an opening (114) through which molten metal flows. The annular atomising nozzle (112) has an annular gas manifold (116) which receives gas from a gas supply. A plurality of apertures (10) extend through the atomising nozzle (112) to direct gas from the gas manifold (116) to intersect the axis of the annular atomising nozzle (112). Instead of the apertures in the atomizing nozzle having sharp edged inlets each aperture is provided with a smooth or contoured inlet end to produce a streamlined flow of atomizing gas from the annular gas manifold 116 into the aperture 120 so that the jets of atomising gas issue from the aperture 120 at a uniform flow rate. The inlet ends (142) of the apertures (120) preferably have a bell mouth shape in cross-section such that the jets of atomising gas issue from the apertures (120) at a uniform flow rate. The apertures (120) are machined to accurate tolerances. This produces more symmetric, predictable and repeatable flow of metallic droplets, or particles, onto the substrate (130) to form an article (132). The apparatus may be used to coat articles with metal. <IMAGE>

Description

AN APPARATUS FOR GAS ATOMISING A LIQUID The present invention relates to an apparatus for gas atomising a liquid, and in particular to an apparatus in which molten metal, or molten alloy, is atomised by the gas to produce metallic particles or droplets.

It is well known in the art to produce molten or semi-molten metallic droplets by an atomising gas, and to either deposit the metallic droplets onto a substrate to produce a metallic article, or to deposit the metallic droplets onto an article to produce a coated article.

British patent GB1379261 discloses an apparatus for producing metallic articles in which the molten metal is atomised by jets of gas directed at a stream of molten metal.

A problem associated with the prior art apparatus is that there is a tendency to produce an asymmetric flow of metallic droplets. This is undesirable because a predictable and repeatable flow of metallic droplets onto the substrate is required.

Accordingly the present invention seeks to provide an apparatus for gas atomising a liquid which produces a more symmetric flow of metallic droplets.

Accordingly the present invention provides an apparatus for gas atomising a liquid comprising an atomising nozzle member defining an opening for the flow of the liquid therethrough, the atomising nozzle member defining a gas manifold, the atomising nozzle member having a plurality of apertures extending therethrough arranged to direct atomising gas from the gas manifold towards any liquid flowing through the opening to atomise the liquid, each aperture having a contoured inlet to produce a streamlined flow of atomising gas from the gas manifold into the aperture such that the jets of atomising gas issue from the apertures at a uniform flow rate.

Preferably the contoured inlet defines a venturi.

Preferably the contoured inlet has a bell mouth shape in cross-section.

Preferably the atomising nozzle member is annular.

Preferably the atomising nozzle member comprises at least two members which define the gas manifold.

The present invention will be more fully described by way of example, with reference to the accompanying drawings, in which: Figure 1 is a sectional view through a prior art apparatus for gas atomising a molten metal to produce metallic particles or droplets.

Figure 2 is a sectional view through an apparatus for gas atomising a molten metal to produce metallic particles or droplets according to the present invention.

Figure 3 is an enlarged sectional view through the apparatus in Figure 2.

A prior art apparatus 10 for atomising a molten metal to produce metallic particles or droplets is shown in Figure 1. The apparatus 10 comprises an annular hollow atomising nozzle 12 which defines an opening 14 through which the molten metal may flow. The annular atomising nozzle 12 defines an annular gas manifold 16 which is arranged to receive gas supplied by a pipe 18 from a gas supply (not shown). The annular atomising nozzle 12 has a plurality of equi-circumferentially spaced apertures 20 which extend therethrough and are angled to intersect the axis of the annular atomising nozzle 12.

A tundish 22 contains a supply of molten metal 24, and the tundish 22 has a nozzle 26 at a lower region to discharge molten metal. The tundish 22 is arranged at a higher elevation than the atomising nozzle 12 and the nozzle 26 is arranged such that a stream of molten metal 28 falling from the tundish 22 flows through the opening 14 through the atomising nozzle 12.

The gas jets issuing from the apertures 20 are directed towards the molten metal 28 flowing through the opening 14 to intersect with and atomise the molten metal to produce metallic particles or droplets which collect on a substrate 30 to form an article 32.

As discussed previously there has been a tendency to produce asymmetric flows of metallic droplets and hence predictable and repeatable flows of metallic droplets onto the substrate has been difficult to achieve.

We have found that the asymmetric flow of metallic droplets is due to differences in flow rates of atomising gas in the gas jets issuing from the apertures 20 and inaccuracies in the direction of flow of the gas jets issuing from the apertures 20 in the atomising nozzle 12.

The apertures 20 in the atomising nozzle 12 have sharp edged inlets, which causes the gas flow from the annular gas manifold 16 into the apertures 20 to separate from the walls of the apertures 20. This flow separation produces recirculation of the gas within the apertures 20 and reduces the flow rate of atomising gas through the apertures 20.

The amount of flow separation is not uniform for all the apertures 20, and thus the flow rate of atomising gas through the apertures 20 is different. The flow rate of atomising gas through each aperture 20 probably varies in an unpredictable manner. Thus the prior art atomising nozzle 12 produces an asymmetric, an unpredictable and unrepeatable flow of metallic droplets.

An apparatus 110, for atomising a molten metal to produce metal particles or droplets according to the present invention is shown in Figures 2 and 3. The apparatus 110 comprises an annular hollow atomising nozzle 112 which defines an opening 114 through which the molten metal may flow. The annular atomising nozzle 112 defines an annular gas manifold 116 which is arranged to receive gas supplied by a pipe 118 from a gas supply (not shown). The annular atomising nozzle 112 has a plurality of equi-circumferentially spaced apertures 120 which extend therethrough and are angled to intersect the axis of the annular atomising nozzle 112.

A tundish 122 contains a supply of molten metal 124, and the tundish 124 has a nozzle 126 at a lower region to discharge the molten metal. The tundish 122 is arranged at a higher elevation than the atomising nozzle 112 and the nozzle 126 is arranged such that a stream of molten metal 128 falling from the tundish 122 flows through the opening 114 through the atomising nozzle 112.

The gas jets issuing from the apertures 120 are directed towards the molten metal 128 flowing through the opening 114 to intersect with and atomise the molten metal to produce metallic particles or droplets which collect on a substrate 130 to form an article 132.

The annular atomising nozzle 112 is shown more clearly in Figure 3, and comprises three annular portions 136,138 and 140 which are joined together to define the annular gas manifold 116. The annular portions 136,138 and 140 are joined together by brazing, soldering, adhesive bonding or other suitable methods. The annular portion 136 has the plurality of equi-circumferentially spaced apertures 120 extending therethrough, and it is to be noted that the inlet end 142 of each aperture 120 is contoured to merge smoothly with the surface 144 of the annular portion 136.

Each aperture 120 is provided with a contoured inlet end 142 to produce a streamlined flow of atomising gas from the annular gas manifold 116 into the aperture 120 such that the jets of atomising gas issue from the aperture 120 at a uniform flow rate. The contoured inlet ends 142 define a venturi, and as shown in Figure 3 the contoured inlet end 142 has a bell mouth shape in cross-section. Contoured inlet ends of other suitable shapes may be used.

The annular atomising nozzle 112 is preferably formed from two or more portions which define the annular gas manifold 116, this enables apertures 120 with contoured inlet ends 142 to be machined in the portion 136 having the apertures 120 before the portions 136,138 and 140 are assembled and joined together. The apertures 120 are all machined accurately within small range of tolerance limits.

The flow rate of atomising gas through all the apertures 120 is substantially the same within limits determined by the tolerances of the apertures 120. Thus, because the flow rate of atomising gas through all the apertures 120 is substantially the same, the flow of metallic droplets or particles is substantially symmetric, predictable and repeatable.

For example twenty four equi-circumferentially spaced apertures 120 are machined in the portion 136, and the apertures 120 are within 0.2 degrees of their true angular position. The diameters of the apertures 120 is 2.7mm within - 0.3mm and all the apertures 120 are the same diameter within - O.Olmm. The diameter of the opening 114 is 33.08mm and the apertures 120 are arranged to intersect 0 the axis of the opening 114 at an angle of 7 The apparatus 110 for atomising a molten metal according to the present invention produces a more symmetric flow of metallic droplets or particles and hence a more predictable and repeatable flow of metallic droplets or particles onto the substrate is achieved.

Although the atomising nozzle has been described as being annular, other suitable shapes may be used.

The apparatus for atomising a molten metal to produce metal particles or droplets may also be used to coat articles with metal.

Claims

Claims:

1. An apparatus for gas atomising a liquid comprising an atomising nozzle member defining an opening for flow of the liquid therethrough, the atomising nozzle member defining a gas manifold, the atomising nozzle member having a plurality of apertures extending therethrough arranged to direct atomising gas from the gas manifold towards any liquid flowing through the opening to atomise the liquid, each aperture having a contoured inset to produce a streamlined flow of atomising gas from the gas manifold into the aperture such that the jets of atomising gas issue from the apertures at a uniform flow rate.

2. An apparatus as claimed in claim 1 in which the contoured inlet defines a venturi.

3. An apparatus as claimed in claim 1 or claim 2 in which the contoured inlet has a bell mouth shape in cross-section.

4. An apparatus as claimed in any of claims 1 to 3 in which the atomising nozzle member is annular.

5. An apparatus as claimed in any of claims 1 to 4 in which the atomising nozzle member comprises at least two members which define the gas manifold.

6. An apparatus for gas atomising a liquid substantially as hereinbefore described with reference to and as shown in Figures 2 and 3 of the accompanying drawings.