1,721,006 research outputs found
Numerical simulation of the effects of protrusion on DC arc anode attachment
No full textAbstract
The attachment of the DC arc on the anode is usually affected by surface morphology such as protrusions due to ablation or melting deformation. A three-dimensional thermodynamic and chemical non-equilibrium model is used to numerically simulate the effect of artificially assumed surface protrusions on the arc anode attachment. The numerical simulation results show that the arc deflects toward the protrusions on the anode and attaches to them in a constricted mode, resulting in an increase in the temperature of the arc attachment region. The analysis shows that the presence of protrusion on the anode surface changes the electric field distribution, intensifies the degree of thermodynamic and chemical non-equilibrium in its vicinity, further influences the chemical kinetic process of the plasma around it, which is the main reason for the deflection of the arc toward the protrusions and the arc anode attachment in a constricted mode. In order to verify the numerical simulation results, verification experiments are also performed using similar size scale anode protrusion, and the results showed that the presence of protrusion can indeed cause the deflection of the arc and even cause the ablation of the protrusion
Experimental study on the life and performance of an improved DC arc plasma torch
No full textA novel DC arc plasma torch is designed in this paper in order to reduce the electrode erosion, and a series of experiments are carried out to investigate how to improve the lifetime and performance of the nitrogen DC arc plasma torch. The analysis of voltage characteristics of the plasma torch indicates that the interelectrode insert can increase the average arc voltage and the sudden expansion structure can reduce the voltage fluctuation, which is helpful to improve the working stability to some extent. The spectrum characteristics at the plasma torch outlet and the cold flow simulations show that the dual shielding gas mainly act near the anode and can effectively cover the entire anode wall. Combining the shielding gas distribution with anode heat transfer processes in argon and nitrogen plasma torch, it is inferred that argon shielding gas plays an important role on reducing the anode heat transfer processes in nitrogen plasma torch, which can effectively suppress the anode erosion. The life testing experimental results find that there is no significant erosion of the cathode, anode, and interelectrode insert after cumulative working time exceeding 20 h. The maximum nitrogen plasma jet length can reach similar to 35 mm with the outlet jet temperature of about 20 000 K at the current of 100 A and nitrogen gas flow rate of 10 slm. The maximum average specific enthalpy and thermal efficiency are respectively about 14 MJ kg-1 and 75% in the nitrogen plasma torch. Therefore, this newly designed DC arc plasma torch not only can suppress the electrode erosion but also has good working performance, which is expected to have excellent application prospects
Numerical simulation study on low-power xenon radio-frequency ion thruster
No full textThe optimization design and accurate performance prediction of a radio frequency ion thruster require coupled analysis of the discharge chamber and grided acceleration. In this numerical simulation study, the three-dimensional grid structure is reduced to an axisymmetric model through the equivalent transmittance method, enabling self-consistent simulation of the overall structure of the thruster. A particle-in-cell Monte Carlo collisions (PIC-MCC) method is implemented to investigate a 45 W xenon RF ion thruster operation, and obtain the spatial distributions of the magnetic field, the electric field, the potential, and the plasma parameters in the discharge chamber and the plume region. This approach conveniently correlates the upstream plasma generation process and downstream beam acceleration. Through combined theoretical analysis and numerical simulation, the electron energy absorption mechanism is elucidated, i.e. the RF coil generates the axial magnetic field through the current of a certain frequency, and further induces the azimuthal electric field, which causes the absorbed power to oscillate at a two-fold frequency due to the phase difference between the induced electric field and the current. The change of the absorbed energy of the electrons from the time-varying electromagnetic field causes the periodic change of the ionization rate and potential. Comparative analysis of the ion velocity and ion energy flux distribution before the screen grid and after the acceleration grid reveals that increasing RF power from 18 W to 45 W enhances discharge chamber plasma density while the degree of ionization at the grid exit rises from 15.37% to 32.83%. Concurrently, the grid sheath potential increases, improving the extracted ion beam occupancy ratio and focusing effect and ultimately enhances the thrust and specific impulse. The mass utilization and thrust efficiency of the thruster increase linearly with RF power
Experimental study of liquid aluminum droplet breakup characteristics based on a Drop-on-demand (DOD) magnetohydrodynamic actuation
No full textIn magnetohydrodynamic (MHD) drop-on-demand (DOD) actuation, Lorentz force generated in different excitation stages are exerted on the liquid metal jet to form a "push-pull" mechanism, which is crucial for generating accurate and repeatable metal droplets. In this study, the important parameters of the excitation current waveform in the magnetohydrodynamic (MHD) process, including the influence of the excitation voltage pulse width ratio and current amplitude on the velocity, energy, volume, and breakup length of aluminum droplets are studied. The conversion process of surface energy and kinetic energy during the formation and fall stage of liquid droplets is analyzed. The results show that the negative and positive excitation voltage pulse width ratio of the MHD pump has a significant effect on the droplet velocity, breakup time, size, and sphericity. The amplitude of the excitation current and Hartmann number has a significant impact on the generation of stable droplets. The results show that the single droplet ejection state can be achieved within the Ha number between 319.5 and 391.1. As the Ha number increases, the volume, length, kinetic energy and surface energy of the droplets at breakup time also increase. The size of droplets can be adjusted by changing the amplitude of excitation waveform
Thermochemical kinetic model for high altitude and low altitude high-speed flow and radiation prediction based on the State-to-State model
No full textThe reasonable thermochemical kinetic model is a key prerequisite for accurately predicting the non-equilibrium flow field and radiation of high-speed atmospheric reentry vehicles. A vibrational and electronic specific State-to-State kinetic model is first established to couple with post-shock flow equations to analyze the thermochemical non-equilibrium flow characteristics. It is found that the vibrational and electronic energy levels non-Boltzmann metric are large at the flight altitude higher than 50 km, which is thus considered a demarcation line for constructing the low- and high-altitude models. Therefore, in this study, two new physics-based thermochemical kinetic models are constructed from the State-to-State model using the multi-group maximum entropy method for low-altitude and high-altitude regions, respectively, across the entire flight velocity domains. By analyzing the differences in post-shock flow physical characteristics under various flight altitudes, single-group and multi-group functions are, respectively, adopted to reconstruct the internal energy levels distributions within the flow field for the low-altitude and high-altitude chemical models. Compared to the State-to-State model, the low-altitude and high-altitude models significantly reduce the number of solved species and chemical reactions, while maintaining computational accuracy of post-shock non-equilibrium flow, with calculated thermodynamic parameters error less than 5% and chemical parameters error below 10%. Finally, the low-altitude and high-altitude chemical models constructed in this study exhibit better predictive capability in reproducing ground experimentally measured electron number density and predicting reentry flight heat flux and radiation compared with the classical Park's and Gupta's models, demonstrating the promising applicability of developed models. This study provides a new, reasonable chemical model for the entire flight altitude and velocity domain that can be used in computational fluid dynamics simulations to predict non-equilibrium flow and radiation in high-speed reentry vehicles
Numerical study on the effect of actuation parameters on the formation characteristics of metal droplets in magnetohydrodynamic drop-on-demand (DOD) jetting
No full textNumerical simulations have investigated the effects of electromagnetic drive waveform amplitude and pulse width on the state of metal droplet generation during drop-on-demand printing of molten metal aluminum by electromagnetic drive. It indicates that positive and negative pulses of magnetohydrodynamic(MHD) actuation applied to the molten metal in the crucible can form a so-called "push-pull" effect, which is the main mechanism affecting droplet generation and breakup. The positive pressure pulse causes the liquid metal to be extruded outward, and the negative pressure pulse promotes the fluid retraction at the nozzle to promoting the thinning of the liquid bridge and the generation of liquid droplets. With the increase in the amplitude of the applied current, the ejected molten aluminum from the crucible experiences three states: no droplet, single droplet, and droplet with satellite droplets. In the single drop state, the droplet formation time is shortened as the current peak increases and the droplet volume increases with the increase of pulse width. The energy conversion and force changes during droplet formation and falling are analyzed. When the peak kinetic energy of the ejected molten metal exceeds its surface energy, the jet breaks up and generates a single droplet. For the actuation with constant amplitude and duration for positive pulses, extending the duration of negative pulses weakens the "pull" effect, leading to a transition from the generation of a single droplet to multiple droplets. The numerical simulation results provide information on the current amplitude, pulse width, and Weber number range necessary for stable single droplet generation.</p
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Experimental Study on the Restrike Mode of a DC Arc Anode Attachment
No full textThe restrike mode is an important arc anode attachment mode under the cross flow in DC arc device, which often causes a large amplitude of arc voltage fluctuations and therefore has an important impact on the performance of arc thermal plasma device. A transferred arc device with planar anode parallel to gas flow direction is used to experimentally study the restrike mode characteristics under different operating conditions. It is found that for the case of a fixed gas flow, the arc restrike frequency increases while the amplitude of arc voltage jump decreases with the increase of arc current. For the case of a fixed arc current, both arc restrike frequency and amplitude of arc voltage jump increase with the increase of gas flow rates. Further analysis shows that the restrike process in one period can be divided into two phases. The first phase corresponds to the generation and development of upstream new arc roots and the disappearance of downstream old root, and its time scale is less than the order of 1 ms. The second phase corresponds to the newly formed, single arc root moving downstream under the combined action of gasdynamic drag force and Lorentz force until new arc root appears upstream. The time-resolved temperature field measurement based on the relative intensity of emission spectrum method shows that the temperature at the junction of the arc column and the anode arc root is relatively low, while it still able to maintain the current conduction, which indicates the significant deviation from thermodynamic equilibrium state
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