Browsing by Author "Atkinson, Helen V."

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    Differential scanning calorimetry (DSC) and thermodynamic prediction of liquid fraction vs temperature for two high-performance alloys for semi-solid processing (Al-Si-Cu-Mg (319s) and Al-Cu-Ag (201))
    (Springer, 2017-07-31) Zhang, Duyao; Atkinson, Helen V.; Dong, Hongbiao; Zhu, Qiang
    There is a need to extend the application of semi-solid processing (SSP) to higher performance alloys such as 319s (Al-Si-Cu-Mg) and 201 (Al-Cu-Ag). The melting of these two alloys was investigated using differential scanning calorimetry (DSC) and thermodynamic prediction. The alloys had been processed by magneto-hydrodynamic (MHD) stirring before receipt to produce a microstructure suitable for SSP. The DSC results for the as-received MHD material were compared with those for material which has been taken through a complete DSC cycle and then reheated for a second DSC run. The effects of microsegregation were then analyzed. A higher liquid fraction for a particular temperature is found in the second DSC run than the first. Microstructural observations suggest this is because the intermetallics which form during the first cooling cycle tend to co-located. Quaternary and ternary reactions then occur during the second DSC heat and the co-location leads to enhanced peaks. The calculated liquid fraction is lower with 10 K/min DSC heating rate comparing with 3 K/min at a given temperature. The DSC scan rate must therefore be carefully considered if it is to be used to identify temperature parameters or the suitability of alloys for SSP. In addition, the starting material for DSC must represent the starting material for the SSP. With thermodynamic prediction, the equilibrium condition will provide better guidance for the thixoforming of MHD stirred starting material than the Scheil condition. The Scheil mode approximates more closely with a strongly microsegregated state.
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    Flow visualization and particle dispersion measurements inside an ambulance rear saloon while stationary and in motion
    (SAE International, 2022-04-07) Lawson, Nicholas J.; Blackburn, Kim; Sherwood, Glenn; Brighton, James; Atkinson, Helen V.
    The following paper presents flow field and particle dispersion data from a UK National Health Service (NHS) ambulance, under static and dynamic driving conditions and when using different ventilation modes. Data was recorded using laser sheet flow visualisation, particle image velocimetry and hot wire anemometry, from a common plane positioned about the patient centreline. Results indicated a significant influence of the ceiling fan ventilation system on gross flow field behaviour, with the ventilation fan on extract or intake mode. With either ventilation mode, flow velocities in the patient region were found to double from a quiescent condition, to around 50mm/s – 100mm/s. Particle dispersion data also showed dispersion decay rates over five times faster when using the ceiling fan extraction system. All these results were consistent when the vehicle was stationary or driving at a constant speed of 60mph. However, with the vehicle under dynamic driving conditions, such as acceleration or braking, the regular flow patterns were substantially disrupted, with bulk movement of the flow in the direction of the acceleration or braking action. Under these dynamic conditions, the magnitude of the net velocity change on the fluid exceeded any flow generated from the ceiling ventilation system.