Growing concern over the scarcity of freshwater motivates the development of compact and economic vapor capture methods for distributed thermal desalination or harvesting of water. We report a study of water vapor condensation on cold liquid beads traveling down a massive array of vertical cotton threads that act as pseudo-superhydrophilic surfaces. These liquid beads form through intrinsic flow instability and offer localized high-curvature surfaces that enhance vapor diffusion toward the liquid surface, a critical rate-limiting step. As the liquid flow rate increases, the bead spacing decreases, whereas the bead size and speed stay nearly constant. The resulting increase in the spatial bead density leads to mass transfer conductances and hence condensation rates per volume that are almost three times higher than the best reported values. Parallel and contiguous gas flow paths also result in a substantial reduction in gas pressure drop and hence electric fan power consumption.
Metal-filled polymers containing micro-powders of highly conductive metals can serve as a starting material to fabricate complex metal structures using economic filament extrusionbased 3D printing and molding methods. We report our measurements of the thermal conductivity of copper samples prepared using these methods before and after a thermal treatment process. Sintering the samples at 980 ℃ leads to an order of magnitude improvement in thermal conductivity when compared with as-printed or as-molded samples. Thermal conductivity values of approximately 30 W/mK are achieved using commercially available polymer-copper composite filaments with a copper volume fraction of 0.4. Over-sintering the samples at 1080 ℃ further enhances the thermal conductivity by more than two folds, but it leads to uncontrolled shrinkage of the samples. The measured thermal conductivities show a modest decrease with increasing temperatures due to increased electron-phonon scattering rates. The experimental data agree well with the thermal conductivity models previously reported for sintered porous metal samples. The measured electrical conductivity, Young's modulus and yield strength of the present sintered samples are also reported.
Membrane distillation (MD) is a membrane-based thermal desalination process capable of treating hypersaline brines. Standard MD systems rely on preheating the feed to drive the desalination process. However, relying on the feed to carry thermal energy is limited by a decline of the thermal driving force as the water moves across the membrane, and temperature polarization. In contrast, supplying heat directly into the feed channel, either through the membrane or other channel surfaces, has the potential of minimizing temperature polarization, increasing single-pass water recoveries, and decreasing the number of heat exchangers in the system. When solar thermal energy can be utilized, particularly if the solar heat is optimally delivered to enhance water evaporation and process performance, MD processes can potentially be improved in terms of energy efficiency, environmental sustainability, or operating costs. Here we describe an MD process using layered composite membranes that include a high-thermal-conductivity layer for supplying heat directly to the membrane-water interface and the flow channel. The MD system showed stable performance with water flux up to 9 L/m 2 /hr, and salt rejection >99.9% over hours of desalinating hypersaline feed (100 g/L NaCl). In addition to bench-scale system, we developed a computational fluid dynamics model that successfully described the transport phenomena in the system.
Jet flows induced by oscillating cantilever plates enable power-efficient cooling enhancement and fluid acceleration. We report a combined experimental and numerical study of the vortex regimes present in the wake of a harmonically oscillating thin cantilever plate in a quiescent fluid and analyze their effect on the flow generation downstream. We use Particle Image Velocimetry (PIV) in conjunction with two-dimensional numerical simulations to investigate the vortex evolution around the trailing edge of the plates with different geometries and vibrational properties. Our observations suggest the existence of three distinct regimes in the wake: nonpropagating, intermediate and propagating. Comparing the temporal decay of the vortex circulation in different regimes shows that different mechanisms are involved in the formation of these vortical patterns. A regime map is proposed next, denoting the incident of each vortex regime as a function of relevant dimensionless parameters. Our analysis of the mean jet on the normal mid-plane, as quantified by the momentum-averaged Reynolds number Rejet, shows that the induced jet downstream the trailing edge is tightly correlated with the identified vortex regimes. The present study improves our understanding of vortex generation and propagation in oscillating cantilevers and facilitates optimized design and operation of piezoelectric fans and similar devices.
Piezoelectric fans offer an intriguing alternative to conventional rotary fans for thermal management of portable and wearable electronics due to their scalability, low power consumption and simple mechanical construct. We report a combined experimental and modeling study to help elucidate power dissipation mechanisms in piezoelectric fans. To analyze contributions from these different mechanisms, mathematical models that account for mechanical hysteresis, dielectric loss and viscous damping from generated air flows are used in conjunction with vibration amplitudes and power consumption data obtained experimentally from piezoelectric fans of different blade lengths, thicknesses and mass distributions. In parallel, we perform experiments on convective heat transfer coefficients and aerodynamic forces acting on surfaces that are oriented perpendicular with respect to fan-induced air flows. These experiments establish that the portion of power dissipation ascribed to air flows correlates well with the heat transfer performance and aerodynamic force. A power ratio, defined as the fraction of the air flow power to the total power dissipation, is then proposed as a useful indicator of the power efficiency of the piezoelectric fans. We show that the power efficiency exhibits a peak at a bias voltage amplitude that balances parasitic power dissipation, due in particular to mechanical hysteresis loss and the dielectric loss, with power dissipation directly linked to actual air flow generation. Lastly, we relate the air flow power to the blade's geometrical parameters to facilitate systematic optimization of the blades for both cooling performance and power efficiency.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.