Works co-authored by VALIMET Technology Manager Ian McCarthy
High Frame Rate Analysis Of The Spray Cone Geometry During Close-Coupled Gas Atomization
A.M.Mullis, N.J.E. Adkins, Z. Aslam, I.N. McCarthy and R.F. Cochrane
Abstract
The geometry of the spray cone during the atomization of Ni31.5Al68.5 alloy within a close-coupled gas atomizer operating with a generic die and nozzle design has been studied using high speed digital video techniques. A Kodak Ektapro 4540mx high speed motion analyser fitted with high magnification optics has been used to record details of a region extending 5 cm from the spray nozzle at frame rates of up to 18,000 frames per second. The material was sprayed at a temperature of a1830 K (corresponding to a superheat of around 200 K), wherein sufficient thermal radiation was emitted for filming to take place without any additional lighting source. In order to quantitatively analyse the large number of still frames that result (up to 65536), image processing routines capable of automating this process have been developed. These have been used to measure the optical brightness and the position of the optical intensity maximum of the material passing though a narrow window at a fixed distance from the nozzle tip. The results of this analysis show that spray cone consists of a jet that precesses around the centre axis of the atomizer in a very regular manner at a frequency around 360 Hz. In order to understand the origins of this motion further experiments have been conducted with a laboratory scale analogue atomizer which atomizes a water jet. We have found that the frequency of precession is essentially independent of the atomizing gas pressure used but does depend upon the geometry of both the die and nozzle used during atomization.
Close-coupled gas atomization: High-frame-rate analysis of spray-cone geometry
A.M. Mullis, N.J.E. Adkins, Z. Aslam, I.N. McCarthy and R.F. Cochrane
Abstract
The geometry of the spray cone during atomization of Ni 31.5Al68.5 in a close-coupled gas atomizer operating with a generic die and nozzle design has been studied utilizing high-speed digital video techniques. Details of the region extending 5 cm from the spray nozzle at frame rates of up to 18,000 frames/s were recorded. The material was sprayed at a temperature ∼ 1,830 K (corresponding to a superheat ∼200 K), wherein sufficient thermal radiation was emitted for images to be recorded without any additional lighting, In order to quantitatively analyze the large number of still frames that result (up to 65,536), image processing routines capable of automating this process have been developed and used to measure the optical brightness and the position of the optical-intensity maximum of the material passing though a narrow window at a fixed distance from the nozzle tip. The results of this analysis show that the spray cone consists of a jet that precesses around the center axis of the atomizer in a regular manner at a frequency ∼360 Hz. In order to understand the origin of this motion, further experiments were conducted with a laboratory-scale analogue atomizer which atomizes a water jet It was found that the frequency of precession is essentially independent of the atomizing-gas pressure, but does depend upon the geometry of both the die and nozzle.
Rapid prototyping of close-coupled atomiser components using an analogue atomiser and high frame-rate analysis
(Conference paper)
I.N. McCarthy, N.J.E. Adkins, R.F. Cochrane, A.M. Mullis
High speed imaging and Fourier analysis of the melt plume during close coupled gas atomisation
I.N. McCarthy, N.J.E. Adkins, Z. Aslam, A.M. Mullis, R.F. Cochrane
Abstract
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 (∼360 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
Investigation of the pulsation phenomenon in close-coupled gas atomization
A.M. Mullis, I.N. McCarthy, R.F. Cochrane, N.J.E. Adkins
Abstract
High speed photography coupled with sophisticated image analysis has been used to study the low frequency pulsation in the volume of melt being instantaneously delivered to the melt nozzle during close-coupled gas atomization. We find that at low gas pressures the distribution of material at the melt tip can be described by a log-normal distribution. At high gas pressure the distribution is better described by two superimposed log-normal distributions, one with a high standard deviation when there is little melt at the atomizer tip and a second with a lower standard deviation when there is more melt at the atomizer tip. We associate this behavior with the transition between open- and closed-wake conditions in the gas. We suggest that the methodology proposed represents a simple, non-invasive technique for characterising the performance of gas atomizers.
Log-Normal Melt Pulsation in Close-Coupled Gas Atomization
(PDF) Log-Normal Melt Pulsation in Close-Coupled Gas Atomization (researchgate.net)
A.M. Mullis, R.F. Cochrane, I.N. McCarthy, N.J.E. Adkins
Abstract
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.
Numerical and experimental modelling of back stream flow during close-coupled gas atomization
S. Motaman, A.M. Mullis, R.F. Cochrane, I.N. McCarthy, D.Borman
Abstract
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 Convergent–Divergent (C–D) discrete gas jet die at five different atomization gas pressures of 1–5 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° relative to the melt nozzle external wall (referred to as nozzle types 1–4). 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 and 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.