Ferrofluid droplet heating and vaporization under very large magnetic power: A thermal boundary layer model

Abstract

In this work, heating and vaporization of a liquid droplet with dispersed magnetic nanoparticles (ferrofluid) are analyzed. The ferrofluid droplet is in a quiescent inert gas phase with a temperature which is set down equal to, higher and lower than the liquid boiling temperature. Under these conditions, an alternating magnetic field is applied and, as a result, the magnetic nanoparticles generate heat by the Brownian relaxation mechanism. In this mechanism, the magnetic dipoles present a random orientation due to collisions between the fluid molecules and nanoparticles. The magnetic dipoles tend to align to the magnetic field causing rotation of the nanoparticles. Consequently the temperature increases due to the energy dissipated by the friction between the resting fluid and the rotating nanoparticles. Assuming a very large magnetic power and a uniform distribution of nanoparticles, the droplet core is uniformly heated. A thermal boundary layer is established in the liquid-phase adjacent to the droplet surface due to heat flux from the ambient atmosphere. The temperature profile inside the thermal boundary layer is obtained in appropriate time and length scales. In the present model, the ferrofluid droplet is heated up to its boiling temperature in a very short time. In addition, the combination of the heat generated by magnetic nanoparticles and heat conduction from gas phase results in a higher vaporization rate. Under specific conditions, the boiling temperature is achieved not at the surface but inside the thermal boundary layer. Moreover, the results point out that the thermal boundary layer depends directly on the vapor Lewis number but the vaporization rate reciprocally on it.

Influence of magnetic field on evaporation of a ferrofluid droplet

“…When alternating magnetic field is applied magnetic particles in a ferrofluid can be heated by magneto relaxation heating [41]. It was also shown that vaporization rate is higher when alternating field is applied [41]. In the present case we have applied uniform magnetic field hence rotation of particles which is largely responsible to produce thermal diffusion in the fluid does not occur.…”

“…It can be determined by measuring weight loss at different time intervals [40]. When alternating magnetic field is applied magnetic particles in a ferrofluid can be heated by magneto relaxation heating [41]. It was also shown that vaporization rate is higher when alternating field is applied [41].…”

An Experimental Validation Study of Ferrofluid Evaporation

“…A PFPE based ferro uid was prepared and characterized by Black et al [15] , which was found to have low volatility under high temperature.Li et al [16] calculated the evaporation rate of hydrocarbon based and uorocarbon based ferro uid under four different vacuum levels. Cristaldo et al [17][18][19] numerically analyzed heating process of ferro uid droplet under an alternating magnetic eld with a thermal boundary layer model. Bolotov et al derived equations of evaporation rate to estimate the life of a tribounit with ferro uid in the ambience of a vacuum [20] and a gas [21] .…”

A microscopic approach to heating rate of ferrofluid droplets by a magnetic field

“…When magnetic nanoparticles are embedded in these droplets, it is possible to accelerate those processes allowing the construction of more compact chambers. [12][13][14] Ferrofluids contain magnetic particles whose mobility can be controlled by a magnetic field. Regarding the applications in MH, the interest is the absorption of large amounts of energy when the magnetization of the particles is reversed.…”

“…This way, the magnetic field modifies the temperature profile in such a way that the boiling condition is found inside the droplet, which can be interpreted as bubble formation and droplet rupture. 13,14 This way, magnetic hyperthermia may be used as an extra atomization process to control the size of droplets and, accordingly, the vaporization rate of sprays.…”