Hydrodynamic Instability Analysis of the Axial Flow Pump in an Ethylene Polymerization Loop Reactor

The hydrodynamic instability of the axial flow pump in a loop reactor has long been a troubling issue to be solved in the polyethylene industry due to the lack of a better mechanismic understanding.Generally,the instability of an axial flow pump can be reflected by the fluctuation of the pump head.In this study,the transient computational fluid dynamics(CFD)simulation is adopted to study the hydrodynamic instability of the axial flow pump used in an ethylene polymerization loop reactor.The results show that the pump head under single liquid phase nearly remains constant while the pump head under slurry phase fluctuates due to the variation of solid volume fraction distribution in the pump.Besides,under the combined effect of the maximum solid volume fraction difference in the pump and the turbulence intensity of the liquid phase,the fluctuation of the pump head under slurry phase increases when the solid volume fraction in the loop reactor increases from 0.10 to 0.29,and the fluctuation decreases,when the solid volume fraction increases from 0.29 to 0.35.Furthermore,there is a negative correlation between the pump head and the solid volume fraction in the pump;with the increase of solid volume fraction in the loop reactor,and the correlation coefficient increases as well.Moreover,a‘spiral particulate band’phenomenon is formed in the ascending leg caused by three mechanisms,viz.:the segregation of particles in all bends,the dispersion of particles by the secondary flow in the ascending leg,and the rotational movement of particles in the pump.

Theoretical analysis of the elastic Kelvin-Helmholtz instability in explosive weldings

By considering the joint effects of the Kelvin-Helmholtz(KH) and Rayleigh-Taylor(RT) instabilities, this paper presents an interpretation of the wavy patterns that occur in explosive welding. It is assumed that the elasticity of the material at the interface effectively determines the wavelength, because explosive welding is basically a solid-state welding process. To this end, an analytical model of elastic hydrodynamic instabilities is proposed, and the most unstable mode is selected in the solid phase. Similar approaches have been widely used to study the interfacial behavior of solid metals in high-energy-density physics. By comparing the experimental and theoretical results, it is concluded that thermal softening,which significantly reduces the shear modulus, is necessary and sufficient for successful welding. The thermal softening is verified by theoretical analysis of the increase in temperature due to the impacting and sliding of the flyer and base plates, and some experimental observations are qualitatively validated.In summary, the combined effect of the KH and RT instabilities in solids determines the wavy morphology, and our theoretical results are in good qualitative agreement with experimental and numerical observations.

Stability of Couette flow past a gel film

We study instability of a Newtonian Couette flow past a gel-like film in the limit of vanishing Reynolds number. Three models are explored including one hyperelastic(neo-Hookean) solid, and two viscoelastic(Kelvin–Voigt and Zener) solids. Instead of using the conventional Lagrangian description in the solid phase for solving the displacement field, we construct equivalent ‘‘differential'' models in an Eulerian reference frame, and solve for the velocity, pressure, and stress in both fluid and solid phases simultaneously. We find the interfacial instability is driven by the first-normal stress difference in the basestate solution in both hyperelastic and viscoelastic models. For the neo-Hookean solid, when subjected to a shear flow, the interface exhibits a short-wave(finite-wavelength) instability when the film is thin(thick). In the Kelvin–Voigt and Zener solids where viscous effects are incorporated, instability growth is enhanced at small wavenumber but suppressed at large wavenumber, leading to a dominant finitewavelength instability. In addition, adding surface tension effectively stabilizes the interface to sustain fluid shear.

Review on hydrodynamic instabilities of a shocked gas layer

Hydrodynamic instabilities induced by a shock wave can be observed in both natural phenomena and engineering applications,and are frequently employed to study gas dynamics, vortex dynamics, and turbulence. Controlling these instabilities is very desirable, but remains a challenge in applications such as inertial confinement fusion. The field of “shock-gas-layer interaction” has experienced rapid development, driven by advances in experimental and numerical techniques as well as theoretical understanding. This domain has uncovered a diverse array of wave patterns and hydrodynamic instabilities, such as reverberating waves, feedthrough, abnormal and freeze-out Richtmyer-Meshkov instability, among others. Studies have shown that it is possible to suppress these instabilities by appropriately configuring a gas layer. Here we review the recent progress in theories,experiments, and simulations of shock-gas-layer interactions, and the feedthrough mechanism, the reverberating waves and their induced additional instabilities, as well as the convergent geometry and reshock effects, are focused. The conditions for suppressing hydrodynamic instabilities are summarized. The review concludes by highlighting the challenges and prospects for future research in this area.

Effects of the initial perturbations on the Rayleigh–Taylor–Kelvin–Helmholtz instability system

The effects of initial perturbations on the Rayleigh–Taylor instability (RTI), Kelvin–Helmholtz instability (KHI), and the coupled Rayleigh–Taylor–Kelvin–Helmholtz instability (RTKHI) systems are investigated using a multiple-relaxation-time discrete Boltzmann model. Six different perturbation interfaces are designed to study the effects of the initial perturbations on the instability systems. It is found that the initial perturbation has a significant influence on the evolution of RTI. The sharper the interface, the faster the growth of bubble or spike. While the influence of initial interface shape on KHI evolution can be ignored. Based on the mean heat flux strength D3,1, the effects of initial interfaces on the coupled RTKHI are examined in detail. The research is focused on two aspects: (i) the main mechanism in the early stage of the RTKHI, (ii) the transition point from KHI-like to RTI-like for the case where the KHI dominates at earlier time and the RTI dominates at later time. It is found that the early main mechanism is related to the shape of the initial interface, which is represented by both the bilateral contact angle θ_(1) and the middle contact angle θ_(2). The increase of θ_(1) and the decrease of θ_(2) have opposite effects on the critical velocity. When θ_(2) remains roughly unchanged at 90 degrees, if θ_(1) is greater than 90 degrees (such as the parabolic interface), the critical shear velocity increases with the increase of θ_(1), and the ellipse perturbation is its limiting case;If θ_(1) is less than 90 degrees (such as the inverted parabolic and the inverted ellipse disturbances), the critical shear velocities are basically the same, which is less than that of the sinusoidal and sawtooth disturbances. The influence of inverted parabolic and inverted ellipse perturbations on the transition point of the RTKHI system is greater than that of other interfaces: (i) For the same amplitude, the smaller the contact angle θ_(1), the later the transition point appears;(ii) For the same interface morphology, the disturbance amplitude increases, resulting in a shorter duration of the linear growth stage, so the transition point is greatly advanced.

Laser pulse shape designer for direct-drive inertial confinement fusion implosions

Pulse shaping is a powerful tool for mitigating implosion instabilities in direct-drive inertial confinement fusion(ICF).However,the high-dimensional and nonlinear nature of implosions makes the pulse optimization quite challenging.In this research,we develop a machine-learning pulse shape designer to achieve high compression density and stable implosion.The facility-specific laser imprint pattern is considered in the optimization,which makes the pulse design more relevant.The designer is applied to the novel double-cone ignition scheme,and simulation shows that the optimized pulse increases the areal density expectation by 16%in one dimension,and the clean-fuel thickness by a factor of four in two dimensions.This pulse shape designer could be a useful tool for direct-drive ICF instability control.