Aluminum-Based Promotion of Nucleation of Carbon Dioxide Hydrates.

Gas hydrate formation has several applications in CO2 sequestration, flow assurance, and desalination. Nucleation of hydrates is constrained by very high induction (wait) times, which necessitates the use of complex nucleation promotion techniques to form hydrates. Presently, we report the discovery of a simple, passive nucleation promotion technique, wherein an aluminum surface significantly accelerates nucleation of CO2hydrates. Statistically meaningful measurements of induction times for CO2 hydrate nucleation were undertaken using water droplets as individual microsystems for hydrate formation. The influence of various metal surfaces, droplet size, CO2 dissolution time, and the presence of salts in water on nucleation kinetics was characterized. Interestingly, we observe nucleation initiation only on aluminum surfaces, the influence of which cannot be replicated by salts of aluminum. We discover that the aluminum-water interface is responsible for nucleation promotion. We hypothesize that hydrogen bubbles generated at the aluminum-water interface are responsible for nucleation promotion.

Acoustic detection of electrostatic suppression of the Leidenfrost state.

At high temperatures, a droplet can rest on a cushion of its vapor (the Leidenfrost effect). Application of an electric field across the vapor gap fundamentally eliminates the Leidenfrost state by attracting liquid towards the surface. This study uses acoustic signature tracking to study electrostatic suppression of the Leidenfrost state on solid and liquid surfaces. It is seen that the liquid-vapor instabilities that characterize suppression on solid surfaces can be detected acoustically. This can be the basis for objective measurements of the threshold voltage and frequency required for suppression. Acoustic analysis provides additional physical insights that would be challenging to obtain with other measurements. On liquid surfaces, the absence of an acoustic signal indicates a different suppression mechanism (instead of instabilities). Acoustic signature tracking can also detect various boiling patterns associated with electrostatically assisted quenching. Overall, this work highlights the benefits of acoustics as a tool to better understand electrostatic suppression of the Leidenfrost state, and the resulting heat transfer enhancement.

Electrostatic Suppression of the Leidenfrost State on Liquid Substrates.

An applied electric field can fundamentally eliminate the Leidenfrost effect (formation of a vapor layer at the solid-liquid interface at high temperatures). This study analyzes electrostatic suppression of the Leidenfrost state on liquid substrates. Electrostatic suppression on silicone oil and Wood's metal (liquid alloy) is studied via experimentation, high-speed imaging, and analyses. It is seen that the nature of electrostatic suppression can be drastically different from that on a solid substrate. First, the Leidenfrost droplet completely penetrates into the silicone oil substrate and converts to a thin film under an electric field. This is due to the existence of an electric field inside the substrate and the deformability of the silicone oil interface. A completely different type of suppression is observed for Wood's metal and solid substrates, which have low deformability and lack an electric field in the substrate. Second, the minimum voltage to trigger suppression is significantly lower on silicone oil when compared to Wood's metal and solid substrates. Fundamental differences between these transitions are analyzed, and a multiphysics analytical model is developed to predict the vapor layer thickness on deformable liquids. Overall, this study lays the foundation for further studies on electrostatic manipulation of the Leidenfrost state on liquids.

Metal-Foam-Based Ultrafast Electronucleation of Hydrates at Low Voltages.

The induction time for the nucleation of hydrates can be significantly reduced by electronucleation, which consists of applying an electrical potential across the hydrate precursor solution. This study reveals that open-cell aluminum foam electrodes can reduce the electronucleation induction time by 150× when compared to nonfoam electrodes. Experiments with tetrahydrofuran hydrates show that aluminum foam electrodes trigger near-instantaneous nucleation (in only tens of seconds) at low voltages. Furthermore, this study suggests that two distinct interfacial mechanisms influence electronucleation, namely, electrolytic bubble generation and the formation of metal ion complex-based coordination compounds. These mechanisms (which depend on the electrode material and polarity) affect the induction time to vastly different extents. Coordination compound formation (verified via detection of metal ions in solution) exerts a much greater influence on electronucleation than the mechanistic effects associated with bubble generation. This work uncovers the benefits of using foams to promote electronucleation and shows that foams lead to more deterministic (as opposed to stochastic) nucleation when compared with nonfoam electrodes.

Heat transfer enhancement accompanying Leidenfrost state suppression at ultrahigh temperatures.

The well-known Leidenfrost effect is the formation of a vapor layer between a liquid and an underlying hot surface. This insulating vapor layer severely degrades heat transfer and results in surface dryout. We measure the heat transfer enhancement and dryout prevention benefits accompanying electrostatic suppression of the Leidenfrost state. Interfacial electric fields in the vapor layer can attract liquid toward the surface and promote wetting. This principle can suppress dryout even at ultrahigh temperatures exceeding 500 °C, which is more than 8 times the Leidenfrost superheat for organic solvents. Robust Leidenfrost state suppression is observed for a variety of liquids, ranging from low electrical conductivity organic solvents to electrically conducting salt solutions. Elimination of the vapor layer increases heat dissipation capacity by more than 1 order of magnitude. Heat removal capacities exceeding 500 W/cm(2) are measured, which is 5 times the critical heat flux (CHF) of water on common engineering surfaces. Furthermore, the heat transfer rate can be electrically controlled by the applied voltage. The underlying science is explained via a multiphysics analytical model which captures the coupled electrostatic-fluid-thermal transport phenomena underlying electrostatic Leidenfrost state suppression. Overall, this work uncovers the physics underlying dryout prevention and demonstrates electrically tunable boiling heat transfer with ultralow power consumption.