We describe the application of a high-speed still imaging technique to the study of close-coupled gas atomisation. A pulsed Nd:YAG laser is used to obtain pairs of still images with an effective 6 ns exposure time, from which velocity maps of the flow of the atomised fluid can be reconstructed. We demonstrate directly that the melt spray cone consists of a jet precessing around the surface of a cone. Further, we demonstrate that the width of this jet is directly related to the geometry of the melt nozzle. By applying Particle Image Velocimetry techniques we are also able to map the flow field in both the primary and secondary atomisation zones, demonstrating an asymmetric recirculation eddy exists at the circumferential edge of the gas-melt interface in the primary atomisation zone. Keywords: Gas atomisation; Discrete Fourier transform; Particle Image Velocimetry IntroductionClose-coupled gas atomisation (CCGA) is an important production technique for fine, spherical metal powders. Such powders have a variety of uses, such as for pigments, catalysts, metal injection moulding (MIM) feedstock, solder pastes for 'flip-chip' type circuit board fabrication and solid rocket propellant. In principle, CCGA is straightforward: high pressure gas jets impinging upon a molten metal stream are used to disrupt the stream, breaking it up into a series of fine droplets. However, in practice the complex interaction between the high velocity gas jets and the metal results in a turbulent, and often chaotic, flow with the result that the details of the process are far from well understood. Consequently, early work into gas atomisation, such as that by Klov and Schfer (1972) or Bradley (1973), focused on empirical correlations between median particle size and process parameters such as gas pressure, gas flow rate and melt flow rate. The most widely quoted of these empirical relationships is that due Lubanska (1970), which correlates particle size with (1+G)
This paper reports the numerical and experimental investigation into the effects of different gas jet mis-match angles (for an external melt nozzle wall) on the back-stream flow in close coupled gas atomization. The Pulse Laser Imaging (PLI) technique was applied for visualising the back-stream melt flow phenomena with an analogue water atomizer and the associated PLI images compared with numerical results. In the investigation a ConvergentDivergent (C-D) discrete gas jet die at five different atomization gas pressures of 1 to 5 MPa, with different gas exit jet distances of 1.65, 1.6, 1.55, 1.5, 1.45 and 1.40 mm from the melt nozzle external wall, was combined with four melt nozzles of varying gas jet mis-match angles of 0, 3, 5, and 7 degrees relative to the melt nozzle external wall (referred to as nozzle types 1 to 4). The results show that nozzle type 1 with the smallest mis-match angle of zero degrees has highest back-stream flow at an atomization gas pressure of 1 MPa and a gas die exit jet located between 1.65 mm to 1.5 mm from the external melt nozzle wall. This phenomenon decreased with increasing mis-match angle and at higher atomization gas pressure. For nozzle type 2, with a mis-match angle of 3 degrees, a weak back-stream flow occurred with a gas exit jet distance of 1.65 mm from the melt nozzle external wall. For a gas pressure of 1 MPa with a decrease in the gas jet exit distance from the external wall of the melt nozzle this phenomenon was eliminated. This phenomenon was not seen for nozzle types 3 and 4 at any gas pressure and C-D gas exit jet distances.
High speed photography coupled with sophisticated image analysis has been used to study low frequency pulsation during close-coupled gas atomization. At high gas pressure the instantaneous melt delivery is described by two superimposed log-normal distributions, one with a high standard deviation but little melt at the atomizer tip, the second with low standard deviation but more melt at the atomizer tip. At low gas pressures the distribution is better described by a single log-normal distribution.
A high speed digital analysis technique has been used to study the atomisation plume of a superheated sample of Ni Al in a close coupled gas atomiser. The atomisation, incorporating a generic melt nozzle and die design was captured using a Kodak high speed digital analyser at a frame rate of 18 k frames per second. The resulting 65 536 frames were then analysed using a specially designed routine, which calculates values of optical brightness and position of the intensity maximum for all frames and performs Fourier analysis on the sequence. The data produced from this analysis show that the plume, pulses at low frequencies (,25 Hz) and precesses at higher frequencies (y360 Hz) around the atomiser's centreline. To aid investigation into the origins of this precession and other phenomena it was decided to conduct further experiments using an analogue system. The analogue atomiser reproduces the important features of the full atomiser but instead of atomising molten metal, the analogue system atomises water, providing a quick and easy way of testing the effects of changing parameters. Using this system it was found that the precession of the melt plume is independent of the atomiser's gas inlet pressure but strongly dependent on both the die and melt nozzle's geometry.
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