Buoyancy Driven Mixing of Miscible Fluids by Volumetric Energy Deposition of Microwaves

Abstract

An experiment that seeks to investigate buoyancy driven mixing of miscible fluids by microwave volumetric energy deposition is presented. The experiment involves the use of a light, non-polar fluid that initially rests on top of a heavier fluid which is more polar. Microwaves preferentially heat the polar fluid, and its density decreases due to thermal expansion. As the microwave heating continues, the density of the lower fluid eventually becomes less than that of the upper, and buoyancy driven Rayleigh-Taylor mixing ensues. The choice of fluids is crucial to the success of the experiment, and a description is given of numerous fluid combinations considered and characterized. After careful consideration, the miscible pair of toluene/tetrahydrofuran (THF) was determined as having the best potential for successful volumetric energy deposition buoyancy driven mixing. Various single fluid calibration experiments were performed to facilitate the development of a heating theory. Thereafter, results from two-fluid mixing experiments are presented that demonstrate the capability of this novel Rayleigh-Taylor driven experiment. Particular interest is paid to the onset of buoyancy driven mixing and unusual aspects of the experiment in the context of typical Rayleigh-Taylor driven mixing.

On buoyancy driven mixing by volumetric microwave energy deposition

“…The proof-of-concept experiment, detailed in [14], was performed in a CEM Discover benchtop microwave, which was found to have several deficiencies for VED mixing experiments. The most significant drawback of the Discover microwave was that it was designed for efficient chemical synthesis, and therefore had a non-uniform microwave field; fluid heating rates were found to be sensitive to the height within the microwave cavity and total load contained in the cavity.…”

“…This vessel was designed so that the rectangular section transitioned to a 7.0 cm diameter cylindrical lip at the top to minimize spillage when removing the vessel from the microwave cavity, see Figure 2. and accuracy of ±1 o C. Measured response time of the fiber optic sensors and data loggers was tested by alternatingly placing the probes into the ice bath and boiling water and was reported in [14]. Fiber optic probes were chosen over higher frequency cold-wire probes because fiber optic probes are not affected by the electromagnetic interference of the microwaves, and therefore do not require any signal correction.…”

“…Joshi et al [13] recognized that RT mixing can be driven by properly forcing a dielectric fluid with an electric field. Recently, Wachtor et al [14] demonstrated that such mixing is possible through the deposition of microwave energy in two miscible dielectric fluids. The physics of how the development of buoyancy driven mixing is affected by volumetric energy deposition (VED) is poorly understood, and has only recently started to be explored [14].…”

“…Recently, Wachtor et al [14] demonstrated that such mixing is possible through the deposition of microwave energy in two miscible dielectric fluids. The physics of how the development of buoyancy driven mixing is affected by volumetric energy deposition (VED) is poorly understood, and has only recently started to be explored [14]. It is believed that VED can aid material mixing processes through buoyancy (pressure gradient) acting on the variation in density.…”

“…The interaction of two dielectric fluids under microwave heating was examined by Kennedy et al [16,17], however, only immiscible fluids were examined. The work of [14] was the first investigation of mixing of two miscible fluids by VED, and set the stage for the current study. The present experiment was initially setup in a RT stable configuration with a lighter, weakly-polar fluid at rest above a heavier, higher polarity (more microwave absorbing) fluid.…”