On the rigidity of four hundred Pickering-stabilised microbubbles

This study explores the rigidity of Pickering-stabilised microbubbles subjected to low-amplitude ultrasound. Such microbubbles might be suitable ultrasound contrast agents. Using an adapted Rayleigh-Plesset equation, we modelled the dynamics of microbubbles with a 7.6-N m−1 shell stiffness under 1-MHz, 0.2-MPa sonication. Such dynamics were observed experimentally, too, using high-speed photography. The maximum expansions were agreeing with those predicted for Pickering-stabilised microbubbles. Subjecting microbubbles to multiple time- delayed pulses yielded the same result. We conclude that Pickering-stabilised microbubbles remain very stable at low acoustic amplitudes.

The Penetration–Explosion Effects of Differently Distributed Inactive/Active Composite Shaped Charge Jets

An analysis of the penetration–explosion (PE) effects of four distributions of inactive/active composite jets shows that a well-designed inactive/active double-layer liner can promote composite jet damage. Penetration experiments were then carried out for shaped charge jets having a single inactive (Cu) liner or an inactive/active (Cu/Al) double-layer liner with variable liner height. The behaviors and firelight patterns of the different jets were captured by high-speed photography. The perforation, deformation area, and deflection were measured for each plate, showing that the Cu/Al jets have stronger PE effects. Numerical simulation shows that the tip of the composite jet generated from the full-height liner is only Cu, whereas for the other jet, from the double-layer liner, Cu is almost wrapped entirely by Al.

Effect of Nozzle Outlet Shape on Cavitation Behavior of Submerged High-Pressure Jet

A submerged high-pressure water jet is usually accompanied by severe cavitation phenomenon. An organ pipe nozzle can greatly improve the cavitation performance of the jet, making use of the self-excited oscillation of the flow. In order to study the effect of organ pipe nozzles of different nozzle outlet shapes on cavitation behavior of submerged high-pressure jet, in this paper we build a high-pressure cavitation jet experiment system and carried out a high-speed photography experiment to study cavitation cloud characteristics of a high-pressure submerged jet. Two organ pipe nozzles with and without a whistle were compared. The dynamic characteristics of the cavitation cloud was extracted through the POD method, it was found that the result effectively reflect the dynamic characteristics of the cavitation jet. The reconstruction coefficients of mode-1 obtained by the POD can better reflect the periodic time-frequency characteristics of cavitation development. The effect of the nozzle outlet shape on the cavitation behavior of organ pipe nozzle was analyzed based on unsteady numerical simulation, and it was found that the jet generated by the nozzle with a divergent whistle had a larger vorticity in the shear layer near the outlet. Further, stronger small-scale vortex and much severe cavitation occurred from the nozzle with a divergent whistle.

Solidification microstructure in a supercooled binary alloy

The maximum undercooling that has been achieved for Ni-Cu alloy, by using molten glass purification and cyclic super-heating technology, is 270 K. With the help of high-speed photography, the solidification front images of Ni-Cu alloy at various typical undercooling were obtained. Two grain refinements occurred in the range of 60 K< ΔT < 100 K and ΔT > 170 K, the solidification front became smoother, and the solidification position appeared randomly. With the increase of undercooling, the transition from solute diffusion to thermal diffusion leads to the transition from coarse dendrite to directional fine dendrite. At large undercooling, considerable stress is accumulated and some dislocations exist in the microstructure. However, the proportion of high-angle grain boundaries is as high as 89%, with twin boundaries of 13.6% and most strain-free structures, and the microhardness decreases sharply. This indicates that the accumulated stress at large undercooling causes the plastic strains in the microstructure, and in the later stage of recalescence, part of the plastic strains is dissipated by the system and acts as the driving force to promote the recrystallization of the microstructure.

Experimental Study on the Relationship Between Cavitation and Lift Fluctuations of S-Shaped Hydrofoil

In order to study the energy loss of bi-directional hydraulic machinery under cavitation conditions, this paper uses high-speed photography combined with six-axis force and torque sensors to collect cavitating flow images and lift signals of S-shaped hydrofoils simultaneously in a cavitation tunnel. The experimental results show that the stall angle of attack of the S-shaped hydrofoil is at ±12° and that the lift characteristics are almost symmetrical about +1°. Choosing α = +6° and α = −4° with almost equal average lift for comparison, it was found that both cavitation inception and cloud cavitation inception were earlier at α = −4° than at α = +6°, and that the cavitation length at α = −4° grew significantly faster than at α = +6°. When α = +6°, the cavity around the S-shaped hydrofoil undergoes a typical cavitation stage as the cavitation number decreases: from incipient cavitation to sheet cavitation to cloud cavitation. However, when α = −4°, as the cavitation number decreases, the cavitation phase goes through a developmental process from incipient cavitation to sheet cavitation to cloud cavitation to sheet cavitation to cloud cavitation, mainly because the shape of the S-shaped hydrofoil at the negative angle of attack affects the flow of the cavity tails, which is not sufficient to form re-entrant jets that cuts off the sheet cavitation. The formation mechanism of cloud cavitation at the two different angles of attack (α = +6°、−4°) is the same, both being due to the movement of the re-entrant jet leading to the unstable shedding of sheet cavity. The fast Fourier analysis reveals that the fluctuations of the lift signals under cloud cavitation are significantly higher than those under non-cavitation, and the main frequencies of the lift signals under cloud cavitation were all twice the frequency of the cloud cavitation shedding.

Effects of temperature on melt electrospinning with auxiliary heating: experiment and simulation study

In this study, the effect of the heating temperature of the spinneret on the melt electrospinning process under the condition of application of auxiliary heating was investigated, in a systematical and comprehensive way. The temperature distribution of the melt jet during the melt electrospinning process was simulated by finite element software in order to provide a good deal of insight into the experimental results. In addition, high-speed photography was adopted to capture images of jet formation and jet motion during the melt electrospinning process. The experimental results indicated that the cooling rate of the polypropylene jet decreases obviously under the condition of auxiliary heating; in addition, the higher spinneret temperature leads to greater drafting force, a drawing fiber drafting rate, and greater jet whipping motion, which is conducive to secondary drawing and refinement of the jet.

STUDY OF STEEL BARRIER SURFACE AFTER HIGH-SPEED EXPOSURE OF W AND TIC POWDERS

The surface layer of an obstacle made of U8 steel is investigated after high-speed exposure to a flow of powder particles. After analyzing the frames of high-speed photography, the average velocities of movement of particles of tungsten and titanium carbide powders were determined. It is shown that the shock-wave loading of the barrier material and the effect of particles accelerated by the explosion energy provide a change in the physical and mechanical properties of the surface and the volume of the steel barrier material.

Characteristics of pulsed nanosecond discharge excited by compact solid-state pulse forming line at atmospheric pressure air

Non-equilibrium plasma is a promising technology for the generation of ozone and removal of exhausted fuel gases. However, applications of non-equilibrium plasma are restricted by energy utilization efficiency in many industry fields. Discharge excited by nanosecond pulsed power is regarded as one of the most efficient methods. In this study, a compact 5 stages stacked blumlein pulse forming line and photoconductive semiconductor switches-based power source was introduced to generate pulsed plasma. This compact source could achieve over 50 kV with 10.1 ns pulse width and 4.8 ns pulse rising time. Coaxial cylindrical reactor was employed to generate a pulsed streamer discharge driven by the nanosecond pulsed source in atmospheric pressure air. Electrical parameters of the streamer discharge have been obtained in this study, the instantaneous power dissipation exceeds 8 MW and the average energy consumption of each pulse exceeds 56 mJ. Experiments of high speed photography have been conducted to observe the evolution process. It can be found that streamer heads start from the central wire electrode and then head to the grounded cylinder electrode in all radial direction of the coaxial electrode. Triple wire-to-cylinder electrodes discharge shows that all the three coaxial discharges develop synchronously and symmetrically, which shows that is capable of generating large volume non-equilibrium diffusive streamer discharge plasma.

Discharge and post-explosion behaviors of electrical explosion of conductors from a single wire to planar wire array

This work deals with an experimental study of a Cu planar wire array (PWA) in air and water under the stored energy 300–1200 J. A single Cu wire is adopted as a controlled trial. Four configurations of PWA and a wire with the same mass (cross-section area) but the different specific surface areas (15–223 cm2 /g) are exploded. The transient process is analyzed using high-speed photography in combination with the results of optical emission and discharge. Discharge characteristics revealed that PWA always has a higher electric power peak, early but higher voltage peak, as well as faster vaporization and ionization process than the single-wire case. Two to three times stronger optical emission could be obtained when replacing the single-wire with PWA, indicating a higher energy density state is reached. Phenomenologically, in both air and water, single-wire load tends to develop a transverse stratified structure, while PWA is dominated by the uneven energy deposition among wires. Finally, the synchronism and uniformity of the PWA explosion are discussed.