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Pumps as turbines (PATs) are used in petroleum and chemical industries to recover high-pressure residual energy. Multistage PATs allow for a wider energy recovery interval and wider range of applications. However, because multistage centrifugal pumps were not originally designed for turbine conditions, complex pressure fluctuations occur, impacting the stable operation and performance of multistage PAT. Pressure fluctuation is essentially a wave, and by analogy with the wave intensity definition, pressure fluctuations were quantified using the pressure wave energy flow density, and the pressure fluctuation patterns at different stages were investigated. The findings reveal significant differences in the intensity of pressure fluctuations at different locations within the multistage PAT. Specifically, the pressure fluctuation intensity is significantly higher from the second to the final stage, compared to the first stage. The difference in inlet flow conditions is the main reason for this difference in pressure fluctuations between stages. The inlet inflow from the second to the final stage is subject to rotational effects that exacerbate the difference in pressure fluctuation intensity between stages. Pressure fluctuations are found to be negatively related to the distance from the source of fluctuations and positively related to the flow state. Different flow conditions and interaction regions of the impeller affect the distribution of pressure fluctuation intensity and the distribution of pressure fluctuation energy across different frequency domains within the guide vanes. The main source of fluctuations in the shaft frequency and four times the shaft frequency is the impeller inlet interaction region, whereas the fluctuations in the blade passing frequency originate from the impeller outlet interaction region. This paper provides a reference for improving the stable operation of multistage PATs. [ABSTRACT FROM AUTHOR]

The multi-size group (MUSIG) model is employed in this paper to simulate the gas–liquid two-phase flow in pump as turbine (PAT) since the traditional Eulerian–Eulerian two-fluid model is unable to take into account the phenomena of breakup and coalescence of bubbles. First, the simulation of gas–liquid two-phase flow in a square column is compared with the experiment to verify the accuracy of the MUSIG model. Then, the results of gas–liquid two-phase flow in PAT simulated by the MUSIG model are compared with those by the conventional uniform bubble (UB) model and find that the MUSIG model is more favorable to capture the flow pattern at high gas content compared to the UB model. Based on the MUSIG model, the internal flow characteristics, pressure fluctuation, and bubble size distribution of the PAT are analyzed. The rotation of the blades breaks a part of big bubbles into small bubbles in the volute, resulting in a smaller diameter of the bubbles entering the impeller. As the gas content increases, the number and size of vortices in the impeller flow channel increase. The vortex is formed at locations where the gas phase distribution in the impeller flow channel is concentrated. The outlet of the impeller is more prone to bubble consolidation under high gas content conditions. In conclusion, the MUSIG model can well predict the complex flow characteristics of gas–liquid two-phase inside the PAT and identify the key influencing factors of energy acquisition, which can provide support for improving the performance of the PAT design. [ABSTRACT FROM AUTHOR]

A five-stage centrifugal pump is utilized to investigate the interstage flow characteristics of the multistage centrifugal pump as turbine (PAT). The simulation results of performance are verified by comparing with the experimental results. Owing to the distinct structural attributes, significant differences in flow occur between the first stage and the other stages of the multistage PAT. To enhance the understanding of these disparities and explore their repercussions, this study focuses on analyzing the flow within the impellers in the first and second stages by a deterministic analysis. The main conclusions are as follows: The discrepancies in the inflow conditions are the major reason for the dissimilarities in the flow of impellers between stages. The impact loss generated by the misalignment between the positive guide vane outlet angle and the impeller inlet angle leads to flow deviation between impeller passages and affects the internal flow pattern. The unsteadiness under low flow rates is mostly produced by the spatial gradient of the blade-to-blade nonuniformities, which is relevant to the relative position between blades and the positive guide vanes. At high flow rates, especially in the second-stage impeller, the pure unsteady term is the primary cause of flow unsteadiness as a result of the flow separation induced by interactions between the blades and the positive guide vanes. This study can provide some references for the practical operation and performance optimization of the multistage PATs in the future. [ABSTRACT FROM AUTHOR]

Numerical simulations of multiphase flows with both interfaces and discrete particles are challenging because they possess a wide range of length and time scales. Meanwhile, the volume of fluid (VOF) method is suitable for resolving the interface, while the discrete particle model (DPM) under the Lagrangian frame better simulates unresolvable particles; a multiscale VOF–DPM combined model is urgently needed for multiscale multiphase flows. The present work implements a VOF–DPM solver that includes a two-way transition algorithm to model the transformation between discrete and continuous phases for bubbles or droplets using OpenFOAM. The interface-capturing scheme in the solver is based on the interIsoFoam solver, which supports the geometric reconstruction of the interface and adaptive mesh refinement. A connected component labeling approach is used for particle detection and VOF-to-DPM transition for discrete bubbles or droplets produced by interface breakup. Conversely, a DPM-to-VOF transition algorithm for particles touching the interface is incorporated to achieve a two-way transition. In addition, phase change modeling between continuous phases and bubble dynamic modeling for cavitating flow cases are also implemented in the solver. Test simulations are performed for validation, including the gas–liquid two-phase dam break and cavitating flow in a convergent–divergent test section. The results demonstrate that the solver is reasonably accurate and can adequately represent the complex phase structure, including the interface and discrete particles. [ABSTRACT FROM AUTHOR]

High-power microwave-induced low-pressure discharges seriously threaten the reliability of space payload systems. Under extremely low-pressure conditions, the evolution of ionized and secondary electrons at the initial stage of discharge is crucial to figure out the discharge process. Therefore, this paper investigates the development of multiple electrons in the discharge process under a highly low-pressure environment using numerical simulation. A three-dimensional simulation model based on the Monte Carlo algorithm is established by considering various electron-gas collisions and secondary electron emissions from different material surfaces. The evolution characteristics of various electrons' populations, energy, and distribution patterns during the discharge process are analyzed. In addition, the influence of the critical conditions at different air pressures on the electron evolution during the discharge process and the intrinsic causes are also investigated. This study is significant in revealing the transition characteristics between multipactor and low-pressure discharge and exploring their inherent mechanisms. [ABSTRACT FROM AUTHOR]

An improved optimization algorithm, namely, multi-strategy-sparrow search algorithm (MSSSA), is proposed to solve highly non-linear optimization problems. In MSSSA, a circle map is utilized to improve the quality of the population. Moreover, the adaptive survival escape strategy (ASES) is proposed to enhance the survival ability of sparrows. In the producer stage, the craziness factor integrated with ASES is introduced to enhance the search accuracy and survival ability. In the scout stage, the ASES facilitates sparrows successful escape from danger. Besides, opposition-based learning or Gaussian–Chachy variation helps optimal individuals escape from local solutions. The performance of the MSSSA is investigated on the well-known 23 basic functions and CEC2014 test suite. Furthermore, the MSSSA is applied to optimize the real-life engineering optimization problems. The results show that the algorithm presents excellent feasibility and practicality compared with other state-of-the-art optimization algorithms. [ABSTRACT FROM AUTHOR]

High-speed centrifugal pumps are widely used in the aerospace and chemical industries. However, the large axial thrust of a rotor due to high rotating speeds exacerbates the device operation instability. Herein, the axial thrust characteristics of a high-speed centrifugal pump under variable speed conditions are analyzed via numerical simulations and experiments. Results show that at high rotating speeds, although the dynamic reaction of the impeller and axial force on the inducer blade only account for a small proportion of the axial thrust, they are the main sources of axial thrust instability. Moreover, axial thrust exhibits a linear relation with the axial force acting on the front and back surfaces of the impeller, and the values of these parameters are very close. However, huge errors are afforded when these two parameters are interchanged, and many axial thrust estimation models ignore this. Therefore, considering the influence of the impeller dynamic reaction and the axial force of the inducer blade on axial thrust, this study proposes an estimation theory for predicting axial thrust. Comparison of the prediction results of the estimation theory and the numerical simulation results reveals that the maximum and minimum relative errors of these two parameters are 11.1% and 2.6%, respectively, under different rotational speeds, thereby suggesting that the prediction results of the estimation theory are sufficiently accurate. The estimation theory can greatly reduce the calculation cost, shorten the design cycle of the centrifugal pump, and provide certain reference for engineering practice. [ABSTRACT FROM AUTHOR]

Cavitating flows always include micro-bubbles and macro-cavities over a wide range of length scale, and understanding the vorticity force evolution induced by cavitation is also a challenging issue. In the present work, a hybrid multiscale cavitation model incorporated with the large eddy simulation approach is applied for comprehensively revealing the unsteady cavitating flow features. The macroscale cavities are captured by the volume of fluid (VOF) method while the microscale bubbles are modeled using a discrete bubble model (DBM) in the Lagrangian way. Simulations of the cavitating flow on a twist hydrofoil are performed and validated against the experimental result. Good agreement in terms of cavity morphology and shedding frequency is obtained. Applying the multiscale cavitation model, the microscale cavitation bubbles that are unable to be predicted by the VOF method are well represented especially in the region near the trailing edge. To investigate the vorticity force evolution, the force element method is adopted to quantitatively analyze the change of force elements along with the evolution of the cavity. The overall effect of cavitation on the lift and drag forces is also investigated. Finally, the superiority of the multiscale VOF-DBM model is attested compared with the original VOF method. [ABSTRACT FROM AUTHOR]

For understanding tip-leakage cavitating flow features, the present work aims to implement a multiscale model to comprehensively reproduce the complicated phase structure. The volume of fluid (VOF) interface capturing method is applied to simulate macroscale cavities, while a discrete bubble model using the Lagrangian formulation is newly developed to take the microscale bubbles into account. The Schnerr–Sauer cavitation model is incorporated into the VOF model to calculate the mass transfer rate between phases from the macroscale point of view. For microscale bubbles, the simplified Rayleigh–Plesset equation is adopted to simulate the bubble growing and collapsing stages. An algorithm for coupling the approaches simulating macroscale cavities and microscale bubbles is also implemented to achieve multiscale simulation. Unsteady flow features are simulated using the large eddy simulation approach. The results show that an anti-diffusive compression scheme for the spatial discretization of volume fraction equation is relatively accurate for simulating the tip-leakage cavitating flow. Applying the multiscale model, the tip-leakage cavitating flow features with multiple time and space scales including the formation of glass cavity tube and the transport of bubble clouds can be revealed. Suitable model parameters including the coefficient of saturated pressure, and the bubble evaporation and condensation coefficients are studied. [ABSTRACT FROM AUTHOR]

For the tip-leakage cavitating flow, the existence of both interface and microbubbles at scales separated by orders of magnitude makes it difficult to be comprehensively reproduced by numerical modeling. This work aims to develop a multiscale model that directly resolves the large-scale cavities and models small discrete bubbles. The volume of fluid (VOF) method and the Schnerr–Sauer cavitation model are first adopted to represent the phase field of water and vapor from a macroscale point of view. A discrete bubble model (DBM) based on the Lagrangian formulation is then developed to simulate the microscale cavitation bubbles that are smoothed in the macroscale model due to the limited mesh resolution. The transition between DBM and VOF is also implemented to achieve the multiscale simulation. For modeling of the turbulence, the scale adaptive simulation approach is used. The tip-leakage cavitating flows induced by the NACA0009 hydrofoil under the conditions with different gaps according to the reported experiment are simulated. Results show that, using the proposed multiscale model, better agreements can be obtained compared with the macroscale model, and specific phenomena can be well revealed including the bubble nucleation, growth and collapse, the interaction between discrete bubbles and large-scale cavities. [ABSTRACT FROM AUTHOR]

PTFE (Polytetrafluoroethylene) with a low dielectric constant and less microwave loss is widely applied in dielectric microstrip antenna and has a significant prospect on low-cost miniaturization satellite cluster. In contrast, the surface discharging induced by space electron beam irradiation degrade antenna radiation efficiency and generate pulse interference signal, which comes to be the critical challenge for employing in orbit. In this paper, we use an experiment and simulation combined method for avoiding surface discharging current path to figure out the charging states and gas desorption phenomenon. The charging state numerical simulation calculates secondary electron emission and deposited electron-hole pair distribution with the Monte Carlo simulation. In addition, we investigate the surface gas desorption, chemical state, as well as morphology of PTFE film under different irradiation situations in situ. We find that the electron irradiation with a higher energy can result in a more frequent discharging, meanwhile, coming with a lower surface discharging potential yet. In spite of a less incident current, the gas desorption rate under the satisfied discharging irradiation presents to be larger than the none discharging irradiation situation. Under the 20 keV E-beam irradiation, the F1s spectrum translates more remarkably, and the C1s spectrum presents a greater promotion on breakage of C–F bond. The presented results and numerical simulation method in this study can offer a comprehensive insight into researches and developments on dielectric microstrip antenna in space. [ABSTRACT FROM AUTHOR]

The energy of interaction, W12(R), of two ions at separation R in an ionic medium is considered using Debye–Hückel (DH) theory for spheres of diameter a0. The ions are treated as spheres of dielectric constant D’ which may differ from that of the medium, say D; they have radii b and carry equal or opposite point charges. The electrostatic potential φ(r) in the ionic medium satisfies the DH equation ∇2φ=κ2φ. An exact, closed-form expression is obtained for W12(R). In the limit of zero ionic strength (κ2→0) previous asymptotic results for R→∞, indicating [uppercase_phi_synonym]4/R4 +[uppercase_phi_synonym]6/R6+[uppercase_phi_synonym]7/R7+... corrections to Coulomb’s law, are reproduced; by contrast for κb>=0 the leading interaction term when R→∞ varies, as expected, like e-κR/DR but the first correction decays as κ2e-2κR/DR2, i.e., with a squared screening factor, and can be stated for two ions differing in charge, radii, etc. The same calculations show that, within DH theory, modeling the ions with D’≠D has no effects on the predicted thermodynamics. [ABSTRACT FROM AUTHOR]

Treatment of ZnO films in a supercritical fluid (SCF) has been reported to improve the performance of devices in which the treated ZnO films are incorporated; however, the mechanism of this improvement remains unclear. In this paper, we study the transformation of the surface morphologies and emission properties of ZnO films before and after SCF treatment, establishing the relationship between the treated and untreated structures and thereby enabling tuning of the catalytic or opto-electronic performance of ZnO films or ZnO-film-based devices. Both undoped and N-doped ZnO nanostructures generated by SCF treatment of films are investigated using techniques to characterize their surface morphology (scanning electron microscopy (SEM) and atomic force microscopy (AFM)) as well as room-temperature photoluminescence (RT-PL) spectroscopy. The water-mixed supercritical CO2 (W-SCCO2) technology was found to form nanostructures in ZnO films through a self-catalyzed process enabled by the Zn-rich conditions in the ZnO films. The W-SCCO2 was also found to promote the inhibition of defect luminescence by introducing -OH groups onto the films. Two models are proposed to explain the effects of the treatment with W-SCCO2. This work demonstrates that the W-SCCO2 technology can be used as an effective tool for the nanodesign and property enhancement of functional metal oxides. [ABSTRACT FROM AUTHOR]

This paper points out that the realistic sound speed is not only a periodic function of range; there are also many other factors such as random perturbation modulating the periodic perturbation which had been investigated in previous papers. By the calculations of phase portraits, power spectra, Lyapunov exponents, etc., the influence of random perturbation on the dynamic behavior of the ray system is investigated. The study demonstrates that the random perturbation lowers the threshold from quasiperiod to chaos, and reinforces the irregularity of the trajectories which were regular originally. However, for the trajectories which were chaotic originally, the random perturbation makes their irregularity decrease slightly. Finally, the probability distribution of the Lyapunov exponent is given. © 1999 Acoustical Society of America. [ABSTRACT FROM AUTHOR]

A method by which asymmetrical random data can be transformed to equivalent symmetrical or approximately symmetrical data is presented. The symmetrization operation is based on the concept of random thresholding. Information about the probability density or central moments, at least the third-order central moment, is needed to define the constrained optimization problem that specifies the random threshold variable for accomplishing the symmetrization. An example of an application to detection is presented. [ABSTRACT FROM AUTHOR]