Arc-Generated Flow Phenomena in Repetitively Pulsed Gas-Flow Spark Gaps

Flow phenomena occurring in repetitively pulsed gas-flow spark gaps have been investigated experimentally and compared with theory. Optical flow diagnostics were obtained after the current pulse using schlieren photography, a flashlamp, and a framing image converter camera (ICC). Experimental results reveal nearly spherical arc-generated shocks propagating symmetrically with respect to the arc debris center. Supersonic shocks rapidly weaken into acoustic disturbances. Heated arc debris convects at the undisturbed gas velocity, while turbulent debris spreading causes a linear growth of the heated gas region radius at a velocity of 10-40 m/s. At low flow velocity (15 m/s), the upstream edge of the heated arc debris remains at a constant streamwise position until 700 ?s after the arc, and then moves downstream at constant velocity. This delay time before downstream motion begins is reduced to 38 ?s for gas velocities of 35 m/s. Numerical results qualitatively agree with experiments, where heated gas is predicted to convect at the undisturbed gas velocity. The shocks, which initially propagate at high Mach number, rapidly slow to acoustic speed.     

Flow Characteristics in the Exhaust of a Pulsed Megawatt Gas Fed Arc

The transient flow generated by a pulsed, megawatt-level, gas-fed arc with an applied magnetic nozzle has been examined with a new design piezoelectric pressure transducer. Sensor thermal conduction and accelerations have been examined and eliminated in the 500?sec period of plasma flow. Existence of a large magnitude cold gas pressure front of 20?sec duration has been reconfirmed and its relationship to the following plasma flow of about 200?sec duration has been examined for the first time. At a point 30 cm from the arc source, initially near vacuum conditions (typically with an arc current of 11.2 kA and 1 tesla applied magnetic field), a pressure pulse of unionized gas with a magnitude of 104 N/m2 is followed by plasma flows with nearly constant impact pressure of 103 N/m2. Pressure and number density in this plasma region are seen to decrease with applied magnetic field strength. With electron density derived from Thomson scattering measurements (1020 m-3) plasma flow velocities on the order of 5 × 104 m/sec are calculated.     

Two-dimensional gas flows under heterogeneous combustion of solid porous media

Two-dimensional unsteady gas flows in porous media with heterogeneous-combustion centers are investigated under forced filtration and free convection. With the use of numerical methods, it is shown that complex gas flows including vortex ones can arise under the combustion of solid porous media. In the case of forced filtration, the gas tends to flow around the heated portion of an object preferring to flow along cold regions. Under natural convection, the vortex gas flows, which can exist for a reasonably long time and strongly affect the oxidizer inflow into the reaction zone, arise at the initial moment of the process in the combustion zone and in its vicinities.     

The DC Arc in a Supersonic Nozzle Flow

The boundary layer integral method at its second level of approximation has been used to study the DC arc in a supersonic nozzle flow. It is shown that with the inclusion of the arc momentum balance, the critical point of the flow is, generally, not the sonic point of the external flow. The speed, at which a disturbance propagates relative to the external flow, is in general supersonic and is dependent on the arc conditions. The arc model is capable of predicting the axial electric field, the arc size and the axial pressure distribution as a function of current. For affinely related nozzles, the solution is determined by a parameter N, which is related to zt, the stagnation condition and the nominal current density at the throat (I/At). Numerical results are given for a particular nozzle shape although the method of analysis is general. Practical implications as regards nozzle design for a gas blast circuit breaker are briefly discussed.     

Experiments on Arc Perturbation in Turbulent Gas Flow

Electric arcs burning in a strong, turbulent, axial nitrogen gas flow are studied. Time-resolving (framing) picture series of the arc and its deformations are taken through the quartz tube channel wall. Individual perturbations like kinks exist under strong, turbulent flow conditions. They move with constant acceleration in the direction of the cold gas flow. The temperature of the arc core is disturbed at the position of these disturbances, too. This shows up most distinctly, if the decay of the arc after short-circuiting is studied, as has been done additionally.     

Reynolds数22 000的孤立方柱绕流的大涡模拟

方柱绕流是典型的钝体绕流问题,蕴含了丰富的流体力学现象,对这类流动的准确预测面临着诸多挑战.采用自主发展的大涡模拟程序,对来流Mach数M=0.3,Reynolds数Re_D=22 000的绕孤立方柱流动进行了细致模拟,亚格子模型使用动力涡黏模型.对计算结果的分析表明,大涡模拟所得的平均流场及Reynolds应力分布与已有实验数据和直接数值模拟结果均吻合较好,验证了预测结果的可靠性;在此基础上对瞬态流场进行了研究,展示了计算条件下方柱绕流分离转捩及尾迹区旋涡交替脱落形成Karman涡街的全过程,为更细致的流动机理探索奠定了基础.      

Experimental study on high-energy surface arc plasma excitation control of cylindrical detached shock wave

In this study, an electrical parameter test system and a high-speed Schlieren system are used to study the control of a cylindrical detached shock wave through high-energy surface arc plasma excitation. The results show that, when plasma excitation is not applied, the bow shockwave angle around the cylinder is 52°. After the plasma excitation is applied, the arc discharge releases a large amount of heat within a short time, generating a shockwave and a control gas bulb (CGB). As a result, the bow shockwave angle first decreases and then increases, the pressure ratio before and after the shockwave decreases, and the intensity of the bow shockwave weakens. At t = 280 μs, the bow shockwave angle is reduced to a minimum of 46°. The effective interference time of high-energy surface arc plasma excitation on a cylindrical detached shockwave is 820 μs. A high temperature is used to control the heating effect of the bubbles, which will increase the local sound velocity near the wall, reduce the local Mach number, cause the sound velocity to move online, and eventually push the bow shockwave away from the cylinder. Concurrently, heating will accelerate the gas flow, reduce the pressure, and cause the mass flow in the unit flow area of the heated area to decrease, resulting in a strong compression effect, which deforms the bow shockwave. The high-energy surface arc plasma excitation will provide a potential technical means for high-speed aircraft detached shockwave control.     

Control of spray spot shape in cold spray technology. Part 2. Spraying process

The present paper studies high-velocity heterogeneous flows produced with nozzle designs unconventional for cold spray; the process of coating deposition was studied under these conditions. The possibility of using unconventional gas dynamic tools (swirling of main flow, nozzles with permeable profiles and with slots in the supersonic part of nozzle) for control of particle distribution in the supersonic jet is investigated: this might be useful for obtaining a proper shape of the spray spot. These experiments offered a method of gas-dynamic design for the spray spot shape, which extends the possibilities of cold spray technique.     

基于格子Boltzmann方法的超声速非平衡流模拟

基于D1Q4可压缩格子Boltzmann模型,按照流通矢量分裂方法的思路,采用坐标旋转技术构造求解三维带化学反应Navier-Stokes方程对流通量求解器.结合有限体积法求解三维化学非平衡流Navier-Stokes方程,采用时间算子分裂算法解决化学反应刚性问题,数值模拟超声速化学非平衡流的三个经典算例.数值结果表明:在高马赫数下,采用D1Q4可压缩格子Boltzmann模型构造的三维对流通量求解器数值模拟中没有出现非物理解,同时在超声速化学非平衡流场中正确分辨激波、燃烧波等物理现象,精度和分辨率均较高,验证了本文构造的三维对流通量求解器的可靠性,拓宽了D1Q4可压缩格子Boltzmann模型的应用范围,为计算超声速化学非平衡流提供一种新方法.     

Nature of convection-stabilized DC arcs in dual-flow nozzlegeometry. I. The cold flow field and DC arc characteristics

An experimental investigation of the steady-state low current air arcs in a dual-flow nozzle system is presented. The cold flow field with no arc was determined for various nozzle geometries, i.e. two- and three-dimensional and orifice nozzles, and nozzle pressure ratios. Supersonic flow separation and oblique and detached shock waves were observed in the flow field. Using a finite-element computer program, the Mach number contours were determined in the flow field for various nozzle-gap spacings and pressure ratios. In addition, the DC arc voltage and current measurements were made for an electrode gap spacing of ≈5.5 cm and current levels of I≈25, 50, and 100 A for the three nozzle geometries. The arc voltage and arc power increased rapidly as the flow speed increased from zero to sonic velocity at the nozzle throat. The shock waves in the converging-diverging nozzles resulted in a decrease in the overall resistance by about 15%     

Lattice-Boltzmann simulation of particle-laden flow over a backward-facing step

This paper deals with the numerical simulation of gas－solid two-phase flows in an Eulerian－Lagrangian scheme. The particle tracks are calculated using a recently developed exponential Lagrangian scheme, and the approach presently used for the computation of fluid phase is based on a modified Lattice-BGK model. Different from earlier publications, the present study employs a two-way coupling mechanism to handle the interactions between carrier phase and dispersed phase in the model. This new model is applicable to simulating gas－solid two-phase flows. For example, based on the scheme, we have recaptured some phenomena of planar laminar particle-laden flow over a backward-facing step in this research, and found a new interesting phenomenon.     

Visualization of thermal cutting fluid flows

Outside of the fields where flow visualization is traditionally applied, there exist many processes where fluid phenomena are critical. Here, we survey flow visualization work with a focus on two thermal metal cutting processes. These two processes – plasma-arc cutting and gas assisted laser cutting – account for a large fraction of the means by which steel is cut in our world. Plasma-arc cutting utilizes an electric arc transferred between a cathode and the steel being cut to produce a high temperature gas jet that melts and removes metal. In gas assisted laser cutting, the assist jet is often high-pressure supersonic nitrogen for stainless steel, or near-atmospheric pressure, low-speed oxygen for carbon steel. Visualization of these millimeter-range diameter jets helps to understand the different roles that the assist gas has in these cutting processes, particularly with how the jets interact with the metal being cut. We describe experimental techniques for visualization of the arc jet and gas assist jet, as well as the liquid metal flows being removed from the cut and the gas flow in the torch itself. These visualizations overcome the small physical scales of the process, the bright illumination from the arc itself, and harsh high-temperature environment. The results lend perspective and understanding of the physical phenomena important to process control.     

Unstructured Adaptive Grid Flow Simulations of Inert and Reactive Gas Mixtures

Unstructured adaptive grid flow simulation is applied to the calculation of high-speed compressible flows of inert and reactive gas mixtures. In the present case, the flowfield is simulated using the 2-D Euler equations, which are discretized in a cell-centered finite volume procedure on unstructured triangular meshes. Interface fluxes are calculated by a Liou flux vector splitting scheme which has been adapted to an unstructured grid context by the authors. Physicochemical properties are functions of the local mixture composition, temperature, and pressure, which are computed using the CHEMKIN-II subroutines. Computational results are presented for the case of premixed hydrogen–air supersonic flow over a 2-D wedge. In such a configuration, combustion may be triggered behind the oblique shock wave and transition to an oblique detonation wave is eventually obtained. It is shown that the solution adaptive procedure implemented is able to correctly define the important wave fronts. A parametric analysis of the influence of the adaptation parameters on the computed solution is performed.     

微通道内不混溶气液两相二氧化碳界面动力学行为的格子Boltzmann研究

将高精度的二氧化碳状态方程与气液两相流格子Boltzmann方法中的伪势模型耦合,研究微通道内二氧化碳气液两相流动的界面动力学行为,包括二氧化碳气泡和液滴的分裂、合并、变形,以及气液两相二氧化碳在演化过程中的质量交换.研究发现：当分裂和合并行为达到平衡,并且两相之间不发生质量交换时流动达到稳态.稳态时的流型主要依赖于表面张力,惯性力,管道的润湿性,以及初始体积分数.当表面张力较大时,微通道内形成的二氧化碳气泡或液滴会收缩成圆形,此时二氧化碳气泡或液滴会堵塞微通道,形成段塞流;随着表面张力的减小,形成的气泡或液滴不容易收缩,在微通道内更容易发生变形,出现泡状流或环状流.当壁面润湿性为强疏水性时,二氧化碳在微通道中的流动为环状流,其它润湿性下,流型为段塞流.体积分数较小时,二氧化碳两相流动的流型为段塞流,体积分数较大时,流型为环状流.     

Evaluating the efficiency of thermo-electric conversion of heat from gas combustion in a small-scale system with counterflow heat exchange

The efficiency of thermoelectric conversion of heat from gas combustion was evaluated in a small-scale system consisting of two channels with opposing gas flows and thermocouples located in the separating wall. Combustion occurred in the chamber fed with fresh mixture heated by combustion products through heat-conducting walls of the channel. In the channel walls, there were thermoelectric converters. It has been shown that in this system, the maximum conversion efficiency of heat from gas combustion may be close to the maximum efficiency of thermoelectric conversion calculated by the maximum acceptable working temperature of the hot side of the converter. This conclusion is valid in the case when the adiabatic combustion temperature of the gas mixture is below the maximum allowable operating temperature of the hot side of the thermoelectric converter. The considered system is promising for the burning of low-calorific gas mixtures and does not require additional energy for cooling the cold side of the thermoelectric converter.     

Étude de deux cas d'écoulements thermoconvectifs le long d'une plaque verticale non uniformément chauffée dans une cavité

Study for two types of thermoconvective flows along a non-isothermal vertical wall in a cavity. For the study of the natural convection along a non-uniformly heated vertical wall in a cavity, we experimentally defined thermal curves. These curves were corrected by our semi-analytical corrective model. The two configurations, the first one instable, with a warm part lower to a cold part, and the other one, with warm and cold parts inverted are studied. We calculated convective flux densities and we showed a different comportment according to the configuration. We proposed some values for the convective exchange coefficient in the case of regular profiles with no extreme. We recommended some correlations for the stable areas of the flow close to the active wall.     

Compressible Flow and Transonic Shock in a Diverging Nozzle

The paper is devoted to the study of compressible flows and transonic shocks in diverging nozzles in the framework of the full compressible Euler system. Consider a nozzle having a shape as a diverging truncated sector with generic opening angle: if the upstream flow at the entrance is supersonic and is near to an axial symmetric flow, and if all parameters of the upstream flow and the receiver pressure at the exit are suitably assigned, then a transonic shock appears in the nozzle. To determine the transonic shock and the flow in the nozzle leads to a free boundary value problem for a nonlinear partial differential equation. We prove that the receiver pressure can uniquely determine the location of the transonic shock, as well as the flow behind the shock. Such a conclusion was conjectured by Courant and Friedrichs, and is confirmed theoretically in this paper for the divergent nozzles. The main advantage of this paper compared with the previous studies on this subject is that the section of the nozzle is allowed to vary substantially, while the transonic shock is not assumed to pass a fixed point. The situation coincides with the requirement in Courant-Friedrichs’ conjecture. To describe the compressible flow we use the full Euler system, which is purely hyperbolic in the supersonic region and is elliptic-hyperbolic in the subsonic region. Solving the free boundary value problem of an elliptic-hyperbolic problem forms the main part of this paper. In our demonstration some new approaches, including the introduction of a pseudo-free boundary problem and the corresponding relaxation, design of a delicate double iteration scheme, are developed to overcome the difficulties caused by the divergence of the nozzle.     

Near-field acoustics of clustered rocket engines

This paper presents an approach for the prediction and characterization of the near-field acoustic levels from closely spaced clustered rocket engines. The calculations are based on the method proposed by Eldred wherein the flow field from the clustered rockets is divided into two zones. Zone 1 contains the isolated nozzles that produce noise independently and extends up to a distance where the individual flows completely mix to form an equivalent single nozzle flow. Zone 2 is occupied by the single mixed stream starting from the station where the jets merge. The acoustic fields from the two zones are computed separately on the basis of the NASA-SP method developed for a single equivalent nozzle. A summation of the spectra for the two zones yields the total effective sound pressure level for the clustered engines. Under certain conditions of nozzle spacing and flow parameters, the combined sound pressure level spectrum for the clustered nozzles displays a double peak. Test cases are presented here to demonstrate the importance of hydrodynamic interactions responsible for the double peak in the sound spectrum in the case of clustered rocket nozzles, and the role of ground reflections in the case of noninterfering jets.     

A Model of the Multicathode-Spot Vacuum Arc

A model is proposed for the multicathode-spot (MCS) vacuum arc. A zero-order model is filrst constructed, whereby the interelectrode plasma is produced by the multitude of cathode spots, and flows to the anode upon which it condenses. The electron density is calculated by assuming that the plasma is uniform within a cylinder bounded by the electrodes and using expenmental data for the ionic velocities and ion current fraction obtained in single cathode spot arcs. The electron density thus obtained is proportionate to the current density, and is equal to 5 × 1020 m-3 in the case of a 107-A/m2 Cu arc. The model predictions are a factor of 3-4 lower than measured values. First-order perturbations to the zero-order model are considered taking into account inelastic electron-ion collisions, plasma-macroparticle interactions, the interaction of the self-magnetic field with the plasma and electric current flows, and the interaction with the anode. Inelastic collisions tend to increase the ionicity of the plasma as a function of distance from the cathode, in agreement with spectroscopic observations. Macroparticles are heated by ion impact until they have significant evaporation rates. The vapor thus produced is ultimately ionized, and most probably accounts for the discrepancy between the zero-order prediction of electron densities and the measured values. Constrictions near the anode in both the plasma and electric current flows have been calculated. An overabundant electron current supply forces the anode to assume a negative potential with respect to the adjacent plasma.     

Der Einfluß einer radialen Masseneinströmung auf elektrische Lichtbögen

In this paper the influence of a radial instreaming gas on electric arcs is investigated. An axial or an azimuthal mass flow may be superimposed to the radial one. Using the basic equations of the single-fluid model of plasma physics the important properties and parameter dependences are numerically calculated and discussed. The results are compared with those of a corresponding arc without an instreaming radial mass flow. Using an arc heater with radial mass flow it is possible to reach much higher axis temperatures and enthalpy densities. Choosing a set of well selected working conditions nearly the whole electric input may be transformed into axial enthalpy flux of the gas to be heated. The paper illustrates the mutual interaction between the dynamic and thermic functions of an electric arc.