Study of Evaporation Behavior of Pure and Nanofluid Droplets

Abstract

Evaporation of spherical droplets of pure fluid is a well-known process and in the absence of convective effects can be described by the d2-law. However, the process of droplet evaporation becomes more complicated in the case of nanofluids or when the evaporating droplet contains electrical charges, which may cause deviations from the d2 behavior. In the case of nanofluid droplets, the dispersed nanoparticles diffuse through the liquid medium by Brownian motion (following the Stokes-Einstein model) until reaching the receding surface, which becomes more populated with particles as a function of time. As a consequence, the area for evaporation decreases until the total blockage of the surface. Numerical and experimental work was carried out to determine the quantitative deviation from the d2-law. Experimentally, single droplets are generated using the dripping mode of electrospray and are captured by an electrodynamic balance to freely levitate during their evaporative lifetime. A consequence of electrospray is that the droplets acquire a net electrical charge which permits the levitation of droplets using electric fields. The effects of the charges on the liquid and gas phases are studied. In particular, the coupled effects of electric charges and the hygroscopy of short-chained alcohols were investigated as a function of ambient relative humidity. Electrically charged droplets are susceptible to Coulombic fissions through which charges are released upon reaching a charge limit. High-speed imagery was used to quantify a series of surface deformations that occur before and after the fission. Successive Coulombic fissions in evaporating water droplets and water/alumina nanofluid droplets were studied. In nanofluids, the dynamics of fission are presented as a function of particle concentration. Besides evaporation, combustion was studied on droplets of pure and nanofluids suspended on thin silicon-carbide fibers. Particle mobility in burning droplets were caused by buoyancy driven vortices rather than by diffusion, delaying an early formation of shells. Ignition of metallic aluminum particles occurred as a result of clusters of particles being ejected during microexplosions.