Coefficient of restitution for particles impacting on wet surfaces： An improved experimental approach

The coefficient of restitution is widely used to characterize the energy dissipation rate in numerical simulations involving particle collisions. The challenge in measuring the coefficient of restitution is the strong scatter seen in experimental data that results from varying particle properties, i.e. shape and surface roughness, and from imperfections in the experimental technique. To minimize this scattering, a novel experimental setup was developed based on two synchronized high-speed cameras capturing the collision behaviour of a particle in three dimensions. To measure the wet restitution coefficient, which describes particle impact in the presence of a liquid layer in the contact region, additional accuracy can be achieved by measuring the liquid layer thickness by a high-precision optical confocal sensor. The coefficient of restitution was measured for glass particles with two different diameters, at different relative velocities and liquid layer thicknesses, with a focus on small collision velocities and thin liquid layers, using both the improved （three dimensional） and the conventional （two dimensional） approaches to quantify the improvement of the new method＇s accuracy.

A Control Strategy of Frequency Self-adaptation Without Phase-locked Loop for VSC-HVDC

A control strategy of frequency self-adaptation without phase-locked loop(PLL)underαβstationary reference frame(αβ-SRF)for a VSC-HVDC system is presented to improve the operational performance of the system under severe harmonic distortion conditions.The control strategy helps to eliminate the cross-coupling under dq synchronous reference frame(dq-SRF),and is achieved through two key technologies:1)positive phase sequence(PPS)and negative phase sequence(NPS)fundamental components are extracted from the AC grid voltage with an improved multiple complex coefficient filter(IMCF),and 2)grid instantaneous frequency is rapidly and precisely tracked using a frequency self-adaptation tracking algorithm(FATA)without PLL.The proposed strategy is applied to a point-to-point VSCHVDC system and validated by means of simulations.The results are compared to those with the traditional vector control strategy under dq-SRF.Simulation results illustrate that the proposed strategy results in better system performance than that with the traditional strategy in terms of harmonic suppression under normal and severe operating conditions of the AC system.