Parametric study of the influence of operating conditions, atomiser geometry and fluid viscosity on effervescent atomisation

摘要

This thesis investigates effervescent atomisation, a liquid fuel atomisation technique with wide industrial applications, and one which offers several important advantages over conventional atomiser types. An “inside-out” type atomiser rated at 2MW equivalent power (based on mass flow rate) was designed and tested using a state-of-the-art 2-D Phase Doppler Anemometry (PDA) system which allowed for simultaneous real-time droplet size and velocity data to be obtained. High quality data was achieved, with data rates up to 10 kHz and validation rates over 90% in 2-D PDA coincident mode in the high density sprays. Droplet diameters up to 600 μm could be measured. The parameters investigated included operating parameters (air-to-liquid by mass ratio, pressure drop across the nozzle), geometric parameters (exit orifice diameter, nozzle length-to-diameter ratio, mixing chamber diameter, mixing length and air injection geometry) and fluid viscosity. The parameter ranges investigated included 1.83-11.11% air-to-liquid by mass ratio, 4.64-7.05 barG pressure drop across the nozzle, 2-2.8mm exit orifice diameter, 60-136 mm mixing length, 20-30 mm mixing chamber diameter, 0.5-2 nozzle length-to-diameter ratio and 1-18 x10-6 m/s2 kinematic viscosity. In addition 3 air injector geometries were studied which allowed the influence of air injector hole radial symmetry and aerating hole diameter to be determined. Water and air were used as the operating fluid and assist-medium, respectively, for the operating parameter and geometric parameter tests. However, the use of water-glycerol mixtures in the fluid viscosity tests allowed the viscosity of the operating fluid to be controlled. Altering the fluid viscosity allowed the production of a range of simulated fuels (that will encompass Bio-Fuels). The effervescent atomiser designed was compared to an industrial type Y-Jet atomiser frequently used in steam-assisted boiler combustion applications. It was found that the Y-Jet atomiser performed slightly better than an effervescent atomiser without any optimisation, but that improvements in effervescent atomiser performance were possible once atomiser geometry had been fully optimised. Comparisons were also made with the droplet SMD, coefficient of discharge and spray angle predicted by correlations from the literature (obtained using earlier versions of the hardware or alternative sampling techniques). These were found to provide poor agreement with the present experimental data. Finally, global spray SMD correlations were developed; these were shown to agree well with the present experimental data.