Interfacial Friction Prediction in a Vertical Annular Two-Phase Flow Based on Support Vector Regression Machine

Accurate prediction of interfacial friction factor is critical for calculation of pressure drop and investigation of flow mechanism of vertical annular two-phase flows. Theoretical models of interfacial friction factor based on physical insight have been developed; however, these are inconvenient in engineering practice as too many parameters need to be measured. Although many researchers have proposed various empirical correlations to improve computation efficiency, there is no generally accepted simple formula. In this study, an efficient prediction model based on support vector regression machine (SVR) is proposed. Through sensitivity analysis, five factors are determined as the input parameters to train the SVR model, relative liquid film thickness, liquid Reynolds number, gas Reynolds number, liquid Froude number and gas Froude number. The interfacial friction factor is chosen as the output parameter to check the overall performance of the model. With the help of particle swarm algorithm, the optimization process is accelerated considerably, and the optimal model is obtained through iterations. Compared with other correlations, the optimal model shows the lowest average absolute error (AAE of 0.0004), lowest maximum absolute error (MAE of 0.006), lowest root mean square error (RMSE of 0.00076) and highest correlation factor (r of 0.995). The analysis using various data in the literature demonstrates its accuracy and stability in interfacial friction prediction. In summary, the proposed machine learning model is effective and can be applied to a wider range of conditions for vertical annular two-phase flows.

Mechano-Triboelectric Analysis of Surface Charge Generation on Replica-Molded Elastomeric Nanodomes

Replica molding-based triboelectrification has emerged as a new and facile technique to generate nanopatterned tribocharge on elastomer surfaces. The “mechano-triboelectric charging model” has been developed to explain the mechanism of the charge formation and patterning process. However, this model has not been validated to cover the full variety of nanotexture shapes. Moreover, the experimental estimation of the tribocharge’s surface density is still challenging due to the thick and insulating nature of the elastomeric substrate. In this work, we perform experiments in combination with numerical analysis to complete the mechano-triboelectrification charging model. By utilizing Kelvin probe force microscopy (KPFM) and finite element analysis, we reveal that the mechano-triboelectric charging model works for replica molding of both recessed and protruding nanotextures. In addition, by combining KPFM with numerical electrostatic modeling, we improve the accuracy of the surface charge density estimation and cross-calibrate the result against that of electrostatic force microscopy. Overall, the regions which underwent strong interfacial friction during the replica molding exhibited high surface potential and charge density, while those suffering from weak interfacial friction exhibited low values on both. These multi-physical approaches provide useful and important tools for comprehensive analysis of triboelectrification and generation of nanopatterned tribocharge. The results will widen our fundamental understanding of nanoscale triboelectricity and advance the nanopatterned charge generation process for future applications.

Thermal detonation wave in liquid lead – water mixture

It was developed the model of thermal detonation in a mixture of continuous liquid lead and dispersed steam/water particles. Stationary equations of mass, impulse and energy conservations laws for multiphase continuum are applied to describe internal structure of thermal detonation wave. They are supplemented by closing relations describing interfacial friction, heat transfer, and fragmentation. Conditions at leading shock wave and at Chapman-Jouguet plane are used as boundary conditions.

Effect of interfacial friction on wear behavior and microstructure during extrusion of Pure Nickel N6

This paper combines field extrusion and finite element simulation to study the influence of the friction state on the extrusion deformation of pure nickel. The use of lubricant reduces the shearing force of the billet-die interface, resulting in lower heat generation at the interface, greater forming stress and strain, and reduced uneven deformation of the billet. As a result, the surface quality of the workpiece is relatively good, but uneven lubrication is the main cause of tearing damage on the surface of the billet. The grain morphology, grain boundary distribution, and geometrically necessary dislocations distribution on the surface of the lubricating extruded bar are uniform, the microtexture strength is weak, and the Schmidt factor is large.

Molecular Dynamics Simulations on the Thermal Effect of Interfacial Friction During Asperity Shearing

To explore the thermal effect of interfacial friction at the nano/microscale, a solid-solid contact model of a rough surface with a single peak was established to research single-crystal Fe. The friction characteristics, stress distributions, temperature changes, and energy changes under different indentation depths and lattice orientations during the shearing process were analyzed. From the perspective of temperature and energy, the mechanism of the thermal effect was revealed. The relationship between the friction force, temperature, and energy at the atomic scale was clarified. The results showed that the temperature of the asperities gradually increased during the shearing process, and a stress concentration formed in the shearing zone. After contact, the asperities had undergone unrecoverable plastic deformation, and there was wear at the contact interface accompanied by the loss of atoms from the asperity. At each indentation depth, as the rotation angle of the crystal increased, the friction force, average temperature, and the sum of the changes in thermal kinetic and thermal potential energy all first increased and then decreased; the trends of the three parameters changing with the rotation angle of the crystal were consistent. The average decreases in the friction force, average temperature, and the sum of the changes in thermal kinetic and thermal potential energy were 52.47%, 30.91%, and 56.75%, respectively, for a crystal structure with a rotation angle of 45° compared to a crystal structure with a rotation angle of 0°. The methods used in this study provide a reference for the design of frictional pairs and the reduction of the thermal effect of interfacial friction.

Interfacial Friction Anisotropy in Few-Layer Van der Waals Crystals

Friction anisotropy is one of the important friction behaviors for two-dimensional (2D) van der Waals (vdW) crystals. The effects of normal pressure and thickness on the interfacial friction anisotropy in few-layer graphene, h-BN, and MoSe2 under constant normal force mode have been extensively investigated by first-principle calculations. The increase of normal pressure and layer number enhances the interfacial friction anisotropy for graphene and h-BN but weakens that for MoSe2. Such significant deviations in the interfacial friction anisotropy of few-layer graphene, h-BN and MoSe2 can be mainly attributed to the opposite contributions of electron kinetic energies and electrostatic energies to the sliding energy barriers and different interlayer charge exchanges. Our results deepen the understanding of the influence of external loading and thickness on the friction properties of 2D vdW crystals.