Electric-field-driven enhancement of heat/mass transfer to a spherical liquid drop of one dielectric fluid from another immiscible dielectric fluid is computationally investigated. The flow field is considered to be in the Stokes regime and the energy (species) conservation equations in the dispersed phase and the continuous phase are solved numerically using a fully implicit finite volume method. Results for flow fields inside and outside the drop, transient temperature distributions, Nusselt number variations, and heat/mass transfer enhancement are presented for Peclet numbers (Pe) varying from 0 to 10000 and dimensionless electric field frequency (ω*) from 10 to 50000. Two heat/mass transfer limits are considered based on the relative magnitude of the heat/mass transfer resistance in the two fluids.
In the first part of this thesis, heat/mass transfer enhancement to a spherical drop suspended in an immiscible fluid is analyzed. When the majority of the transport resistance is in the drop (internal problem), the results show that for low to moderate Peclet numbers (Pe < ~750), the steady uniform electric field provides higher rates of heat/mass transfer compared to time periodic electric fields. However, at high Peclet numbers, the non-uniform time periodic electric field is more effective in enhancement of heat/mass transfer compared to the steady uniform electric field. This enhancement is a function of electric field frequency and the maximum enhancement is obtained when the dimensionless frequency is of the order of Peclet number or (ω*) ~ O(Pe). When the majority of the heat/mass transfer resistance is in the continuous phase (external problem), steady uniform electric field is more effective in promoting heat/mass transfer compared to uniform time periodic electric field. Non-uniform time periodic field is most effective when the continuous phase is less viscous compared to the dispersed phase.
In the second part of this thesis, heat/mass transfer to a translating drop is considered. The effect of the electric field is expressed in terms of L, the ratio of the surface velocity induced by the electric field to that produced by drop translation. The results for the internal problem for unsteady uniform field show that the heat/mass transport in the drop interior increases with Pe but displays a non-monotonic variation with the dimensionless electric field frequency. When L ≫ 1, unsteady non-uniform electric field provides higher heat/mass transfer enhancement compared to the steady and unsteady uniform electric fields. In the limit of the majority of the transport resistance being in the continuous phase (external problem), for the steady and time-periodic electric fields, the heat transfer enhancement increases monotonically with Pe and L. The steady uniform electric field gives the highest average Nusselt number for heat transfer from the drop surface to the continuous phase, followed by the non-uniform time periodic electric field and then the time periodic uniform electric field at higher electric field strength. Therefore whether the steady or the time periodic electric field provides the most enhancement of heat/mass transfer for a conjugate problem will depend on the relative transport resistance in the two phases.