Barnaby Osborne

Instabilities and drop formation in cylindrical liquid jets in reduced gravity

The effects of convective and absolute instabilities on the formation of drops formed from cylindrical liquid jets of glycerol/water issuing into still air were investigated. Medium-duration reduced gravity tests were conducted aboard NASA’s KC-135 and compared to similar tests performed under normal gravity conditions to aid in understanding the drop formation process. In reduced gravity, the Rayleigh–Chandrasekhar Equation was found to accurately predict the transition between a region of absolute and convective instability as defined by a critical Weber number. Observations of the physics of the jet, its breakup, and subsequent drop dynamics under both gravity conditions and the effects of the two instabilities on these processes are presented. All the normal gravity liquid jets investigated, in regions of convective or absolute instability, were subject to significant stretching effects, which affected the subsequent drop and associated geometry and dynamics. These effects were not displayed in reduced gravity and, therefore, the liquid jets would form drops which took longer to form (reduction in drop frequency), larger in size, and more spherical (surface tension effects). Most observed changes, in regions of either absolute or convective instabilities, were due to a reduction in the buoyancy force and an increased importance of the surface tension force acting on the liquid contained in the jet or formed drop. Reduced gravity environments allow better investigations to be performed into the physics of liquid jets, subsequently formed drops, and the effects of instabilities on these systems. In reduced gravity, drops form up to three times more slowly and as a consequence are up to three times larger in volume in the theoretical absolute instability region than in the theoretical convective instability region. This difference was not seen in the corresponding normal gravity tests due to the masking effects of gravity. A drop is shown to be able to form and detach in a region of absolute instability, and spanning the critical Weber number (from a region of convective to absolute instability) resulted in a marked change in dynamics and geometry of the liquid jet and detaching drops.

An experimental investigation into liquid jetting modes and break-up mechanisms conducted in a new reduced gravity facility

Liquid jets, important to many industrial applications including various drop-on-demand processes, are experimentally produced in reduced gravity conditions and analysed to determine the mode the liquid jet is operating in. Three physically different modes of liquid jetting are observed along with their associated break up mechanisms and discussed. Additionally, a) the theoretical transition between jetting and the onset of chaotic dripping is shown to be valid over a range of Re and We numbers in a reduced gravity environment; b) the theoretical transition between a convective and an absolute instability acting on a liquid jet does not appear valid for high Reynolds number jets operating in reduced gravity; c) for the first time, chaotic dripping has been observed and documented in reduced gravity; and, d) a transition is shown to exist in reduced gravity between chaotic dripping and quasi-steady growth. The work has applications to systems operating in both reduced gravity and in select systems operating in normal gravity. The work has been highly successful and has demonstrated the utility of the new Australian test facility in providing adequate test time and reduced gravity conditions to study sensitive phenomena under reduced gravity conditions.

Investigation of Burning Aluminum in Oxygen-Enriched Atmospheres through Microanalysis Techniques

Promoted-ignition testing of 3.2 mm diameter aluminum rods in high purity oxygen was performed. The rod and detached drops of both self-extinguished and water quenched samples were examined using scanning electron microscopy and electron probe microanalysis to analyze the physical structure of the sample and gather compositional data. A comparison of the micrographs of self-extinguished and quenched samples reveals clear differences in the extent of melted and re-solidified (unreacted) material and the thickness of the oxide layer, highlighting the effect of cooling rate on the burning system. A qualitative physical model for the burning of bulk aluminum in gaseous oxygen is presented. The model, incorporating a molten drop growth and detachment cycle, is based on an initial heterogeneous burning phase leading to a second phase of combined heterogeneous and homogeneous burning.

Microanalysis of quenched and self-extinguished aluminium rods burned in oxygen

Promoted ignition tests and quench tests have been conducted and analysed for 3.2 mm aluminum rods in 99.995% oxygen. Tests have been conducted in oxygen pressures varying from 538 kPa to 773 kPa. Samples that self-extinguished or were quenched were selected for further analysis. The microstructure of the selected samples were analysed by electron microscopy, using energy dispersive spectrometry and electron back-scatter techniques, to identify and visualize, respectively, the species present. The grain structures of these samples were etched, viewed and photographed under polarized light by an optical microscope. From the micrographs produced by the post-test analysis, clearly defined boundaries between the oxide and the melted and resolidified metal have been observed. In both the melted and resolidified metal and the oxide layer, significant numbers of gas bubbles, solid inclusions and several diffuse oxide bubbles have been captured during the cooling process. It is concluded that convective movement is occurring within the molten drop and that analysis of quenched samples provides more useful information on the state of the burning droplet than samples allowed to cool slowly to room temperature. Recommendations are made regarding future investigations into aluminum burning, focusing on the transport of reactants through the liquid oxide layer.

Drive system alignment calibration of a microgravity drop tower of novel design

We report here the calibration of the drive system of a new scientific facility for production of microgravity, operating on a novel design of electromagnetically driven platform. The construction achieves the design specification of alignment of the guide rails to better than 0.254mm across the entire guide rail height of 8m, despite a small lean to the right (within tolerance) and it was noted that this alignment is improved by the presence of the trolley that carries the platform.