Numerical simulation of fluid dynamics and transport phenomena in electrostatically charged volatile sprays

Identificador:  TDX:2370

Handle: https://hdl.handle.net/20.500.11797/TDX2370

Autores:  Arumugham Achari, Ajith Kumar

Resumen:
Electrosprays are constituted of highly charged micro drops moving under the action of electrostatic forces. They are generated as a result of the breakup of a liquid jet subjected to a sufficiently strong electric field. The droplets hence generated are transported under the combined influence of the electrostatic gradient between the emitter and counterplate, the interaction with the spray charge and the aerodynamic drag force. Most of the electrospray applications involve droplet evaporation as a critical aspect in achieving their desired result. When a collection of aerosol particles move with a net velocity relative to the surrounding gas, it exerts a drag force on the gas which can cause the gas to flow. In electrosprays, this gas motion is induced by the highly charged micro-drops moving under the action of electrostatic forces. While many numerical models have neglected induced gas flow in the numerical simulations of electrosprays, experimental evidence shows that the gas speed can be significant locally. Also considering the importance it can have in droplet evaporation in volatile electrosprays, there is a need for a general methodology to include the induced gas flow caused by the droplets in current numerical models of electrospray dynamics. Furthermore, since the gas motion also influences the droplet motion, a formulation that can accurately describe these motions should be fully coupled (i.e., two-way coupled). Such improved models should be able to elucidate the influence of the induced gas flow on variables of practical importance such as the flux deposition pattern on the counterplate, plume spread, droplet number density distribution, and also in the prediction of droplet evaporation. We developed a comprehensive numerical scheme which fully couples the Lagrangian electrospray droplet dynamics with the effects of induced gasflow, Coulomb explosions, and the transport of solvent vapor as well as charge left over by vanishing droplets in volatile electrospray systems. Separate codes for the diverse phenomena were developed. These codes have been run sequentially and in an iterative way until convergence was attained for all variables. This methodology has been applied to compare the evaporation effects in three electrospray systems with solvents of different volatility: acetone, methanol and n-heptane. The droplets were injected into the three systems with unimodal and log-normal distributed diameters with a mean value of 8 μm, and a coefficient of variation of 10%. Regions of intense Coulomb explosion events in form of diagonal bands (in the 2D domain) within the spray are well captured. We observe that the vapor transport in these systems is predominantly by forced convection rather than diffusion. Highest vapor concentration is observed near the injection zone for all the three systems, which rapidly decays thereafter, both radially as well as axially. In all three cases, few or no droplets arrive at the counterplate located 3 cm down the capillary nozzle, highlighting the relevance of accounting for evaporation when simulating these systems.