Unsteadiness And Spatio-Temporal Statistics In Intermediate Effervescent Spray Region

Multipoint statistics theory of the ideal spray was used to investigate temporal spray stability. The analysis focused on intermediate spray region, i.e. between dense and self-similar regions. Arrival time and diameter are the studied characteristics of the droplets. Two size classes, with droplet diameters of 50 and 100 mu m, were selected for steadiness analysis. The two size classes were used to investigate temporal droplet behaviour using their arrival time. Chi-square test was used to provide unsteadiness criterion. Data were collected by dual-phase Doppler anemometry (PDA) at three planes perpendicular to the spray axis (x/D = 20, 40, 60). At each plane, there were several measuring points spanning the spray radius, starting from the spray centreline (r/x = 0) to the spray cone boundary (r/x = 0.5). The effervescent breakup mechanism is shown to produce spray unsteadiness, which is however significant only away from spray axis. In a narrow cone around the spray axis, the spray is steady on a significance level of 5 %. This result provides guidance for assessment of effervescent spray unsteadiness. The data show unexpectedly similar levels of unsteadiness for gas-liquid ratios between 5 and 15 %. This has implications for combustion applications of effervescent spray, where unsteadiness is an undesirable property.