Confinements regulate capillary instabilities of fluid threads

We study the breakup of confined fluid threads at low flow rates to understand instability mechanisms. To determine the critical conditions between the earlier quasi-stable necking stage and the later unstable collapse stage, simulations and experiments are designed to operate at an extremely low flow rate. Critical mean radii at neck centres are identified by the stop-flow method for elementary microfluidic configurations. Analytical investigations reveal two distinct origins of capillary instabilities. One is the gradient of capillary pressure induced by the confinements of geometry and external flow, whereas the other is the competition between local capillary pressure and internal pressure determined by the confinements

Confinements regulate capillary instabilities of fluid threads

We study the breakup of confined fluid threads at low flow rates to understand instability mechanisms. To determine the critical conditions between the earlier quasi-stable necking stage and the later unstable collapse stage, simulations and experiments are designed to operate at an extremely low flow rate. Critical mean radii at neck centres are identified by the stop-flow method for elementary microfluidic configurations. Analytical investigations reveal two distinct origins of capillary instabilities. One is the gradient of capillary pressure induced by the confinements of geometry and external flow, whereas the other is the competition between local capillary pressure and internal pressure determined by the confinements

Efficient Sensitivity Analysis for Enhanced Heat Transfer Performance of Heat Sink with Swirl Flow Structure under One-Side Heating

Excellent heat transfer performance has increasingly become a key issue that needs to be solved urgently in the development process of large-scale fusion equipment. The study of heat transfer performance improvement to scientifically and reasonably determine the design parameters of the high heat flow (HHF) components of fusion reactors based on the efficient in-depth analysis of the heat transfer mechanism and its sensitive factors is of great significance. In this paper, a liquid-vapor two-phase flow model with subcooled boiling for a large length-diameter ratio swirl tube structure in the HHF calorimeter component is proposed to analyze the effects of key design parameters (such as inlet temperature of cooling water flow, swirl tube structure parameters, etc.) on its heat transfer performance. Then, considering the high computational cost of the liquid-vapor two-phase flow model, and in order to improve the efficiency of the sensitivity analysis of these design parameters, the polynomial response surface surrogate model of heat transfer performance function was constructed based on Latin hypercube sampling. On this basis, by combining the proposed surrogate model, the sensitivity index of each design parameter could be obtained efficiently using the Sobol global sensitivity analysis method. This method could greatly improve the calculation efficiency of the design parameter sensitivity analysis of HHF components in the fusion reactor, which provides vital guidance for the subsequent rapid design optimization of related components

Diagnosis of hydrodynamic regimes from large to micro-fluidized beds

International audienceMicro-fluidized bed (MFB) technology is a newly emerging technique that is receiving increasing interest for various industrial applications. MFB is generally characterized by the use of inner diameters (Dt) of a few millimeters, and a large specific contact surface area between the walls and the fluidization system. These specific features have the advantage of enabling ultra-fast heat dissipation for exothermic reactions, as well as isothermal conditions. However, the wall effect also clearly prevails, leading to complicated hydrodynamic phenomena. In addition, the study of MFB is challenging, as a consequence of the difficulties encountered in its characterization, due to strong probe interference effects. In order to understand the wall effect and to make use of suitable fluidization conditions for practical applications of MFB, the present study first establishes a diagnostic methodology, and then applies this to study the influence of a reduction in Dt from large to micro-fluidized bed scales. Experiments were carried out in six glass columns with Dt ranging between 4 and 100 mm, using spherical Geldart group B particles (glass beads). The methodology described here is based on the analysis of pressure fluctuation measurements, which are used to monitor fluidization simultaneously in the time and frequency domains. Fixed bed, pseudo-homogeneous fluidization, bubbling, slugging, and turbulent fluidization regimes are identified in MFBs. When Dt is reduced from large-to micro-fluidized bed scales, minimum fluidization, bubbling and slugging are delayed, whereas the onset of turbulent fluidization occurs more rapidly. The late stage of turbulent fluidization covers a relatively wide range of gas velocities, and is found to have homogeneous fluidization structures, which are of interest for practical MFB applications such as Fischer–Tropsch synthesis

Hopping Behavior Mediates the Anomalous Confined Diffusion of Nanoparticles in Porous Hydrogels

Diffusion is an essential means of mass transport in porous materials such as hydrogels, which are appealing in various biomedical applications. Herein, we investigate the diffusive motion of nanoparticles (NPs) in porous hydrogels to provide a microscopic view of confined diffusion. Based on the mean square displacement from particle tracking experiments, we elucidate the anomalous diffusion dynamics of the embedded NPs and reveal the heterogeneous pore structures in hydrogels. The results demonstrate that diffusive NPs can intermittently escape from single pores through void connective pathways and exhibit non-Gaussian displacement probability distribution. We simulate this scenario using the Monte Carlo method and clarify the existence of hopping events in porous diffusion. The resultant anomalous diffusion can be fully depicted by combining the hopping mechanism and the hydrodynamic effect. Our results highlight the hopping behavior through the connective pathways and establish a hybrid model to predict NP transport in porous environments