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

Cristaldo, C. F. C.; Fachini, F. F.
March 2013
Physics of Fluids;Mar2013, Vol. 25 Issue 3, p037101
Academic Journal
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.


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