Droplet dynamics in internally mixed twin-fluid spray

abstrakt

angličtina

Effervescent atomizers are based on mixing of gas with liquid prior to discharge. We describe the discharge of two-phase mixture and movement of droplets in gas jet using simple theoretical models, following with elucidation of droplets dynamics using experimental data for an effervescent spray. Discharge of the liquid-gas flow from the nozzle is described using a combination of two discharge models. Depending on operation conditions, 59–64% of the total discharged mass corresponds to Separated Flow Model and the rest to Homogeneous Flow Model. Discharge velocity of the liquid reaches 12–27% of the gas exit velocity. The liquid-gas velocity ratio is negatively correlated with gas-to-liquid mass ratio (GLR) and positively correlated with inlet pressure. Radial profiles of axial droplet velocity, as measured using Phase Doppler anemometry, are axisymmetric bell-shaped with a maximum in the centreline analogous to the profile defined for a simple gas jet which however is more flat near the centreline and decline much faster for higher radial positions. Mean velocity in particular spray position varies with particle size within a range of several m/s typically. This variation is closely related with particle Stokes number, Stk. Variation of mean velocity with operation pressure and GLR can be explained with discharge conditions; higher pressures and GLRs lead to higher discharge velocities that are reflected in the spray downstream. Stokes numbers are generally << 1 for particle sizes Dp up to 10 um so they smoothly follow the gas flow. Stk for size 10 um 10 and very weakly interact with the gas.

droplet dynamics, internally mixed twin-fluid atomization, effervescent atomization, spray structure, dispersed two-phase flow.

JEDELSKÝ, J.; JÍCHA, M.

3. 6. 2013

WIT Press

La Coruňa, Spain

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