The Feasibility of Applying AC Driven Low-Temperature Plasma for Multi-Cycle Detonation Initiation

Ignition is a key system in pulse detonation engines(PDE). As advanced ignition methods, nanosecond pulse discharge low-temperature plasma ignition is used in some combustion systems, and continuous alternating current(AC) driven low-temperature plasma using dielectric barrier discharge(DBD) is used for the combustion assistant. However, continuous AC driven plasmas cannot be used for ignition in pulse detonation engines. In this paper, experimental and numerical studies of pneumatic valve PDE using an AC driven low-temperature plasma igniter were described. The pneumatic valve was jointly designed with the low-temperature plasma igniter,and the numerical simulation of the cold-state flow field in the pneumatic valve showed that a complex flow in the discharge area, along with low speed, was beneficial for successful ignition. In the experiments ethylene was used as the fuel and air as oxidizing agent, ignition by an AC driven low-temperature plasma achieved multi-cycle intermittent detonation combustion on a PDE, the working frequency of the PDE reached 15 Hz and the peak pressure of the detonation wave was approximately 2.0 MPa. The experimental verifications of the feasibility in PDE ignition expanded the application field of AC driven low-temperature plasma.     

Pressure wave propagation in air-water bubbly and slug flow

Related to nuclear reactor safety problems, such as the loss of coolant accident caused by some small crevasses in nuclear reactor, choked flows after postulated breaks of hot and cold legs of pressurized water reactors and the boiling flow instability in parallel channels, the characteristics of pressure wave propagation were investigated experimentally for the air-water bubbly and slug two-phase flow in a vertical pipe. Pressure wave was generated from the small pressure disturbance by the up-and-down movement of piston in the test section. Air void fraction was up to 0.7 and superficial liquid velocity was up to 1.5 m/s as experimental conditions. The experimental results show that the pressure wave propagation velocity in bubbly flow decreases acutely with the increase of air void fraction from 0 to 0.05. In slug flow, it is constant when the air void fraction is less than 0.5 but increases gradually when the void fraction increases beyond 0.5. The attenuation coefficient of pressure wave increases with the increase of air void fraction in bubbly flow. The dependency of pressure wave propagation velocity on angle frequency ω in air-water flow shows the dispersion characteristic. The propagation velocity and attenuation coefficient increases gradually with the increase of angle frequency. However, the increase vanishes slowly as the angle frequency reaches 250 Hz in bubbly flow. The propagation of pressure wave in bubbly flow is independent of the superficial velocity of fluids in the range of experiment.     

Effect of actuating voltage and discharge gap on plasma assisted detonation initiation process

The influence of actuating voltage and discharge gap on plasma assisted detonation initiation by alternating current dielectric barrier discharge was studied in detail.A loose coupling method was used to simulate the detonation initiation process of a hydrogen–oxygen mixture in a detonation tube under different actuating voltage amplitudes and discharge gap sizes.Both the discharge products and the detonation forming process assisted by the plasma were analyzed.It was found that the patterns of the temporal and spatial distributions of discharge products in one cycle keep unchanged as changing the two discharge operating parameters.However,the adoption of a higher actuating voltage leads to a higher active species concentration within the discharge zone,and atom H is the most sensitive to the variations of the actuating voltage amplitude among the given species.Adopting a larger discharge gap results in a lower concentration of the active species,and all species have the same sensitivity to the variations of the gap.With respect to the reaction flow of the detonation tube,the corresponding deflagration to detonation transition(DDT) time and distance become slightly longer when a higher actuating voltage is chosen.The acceleration effect of plasma is more prominent with a smaller discharge gap,and the benefit builds gradually throughout the DDT process.Generally,these two control parameters have little effect on the amplitude of the flow field parameters,and they do not alter the combustion degree within the reaction zone.     

Air-hydrogen deflagration tests at the University of Pisa

In severe accidents, large amounts of hydrogen may be released in the safety containment of a nuclear plant and the gas mixture may become explosive. The University of Pisa and ENEA have undertaken an experimental program to study the physics of flame propagation in a containment model under accident conditions. Up to now 41 deflagration tests have been performed at the HYDRO-SC facility at ambient pressure and temperature. Concentrations, water spray conditions, ignition source and gas turbulence levels were varied. The vessel volume was 0.5 m³, the ignition sources were an electrical spark discharge and an electrically heated surface (glow-plug), the hydrogen molar fractions were in the range 4–16%, the turbulence was generated by fan or spray and two different spray nozzles were utilized. The experimental data indicate that the peak pressures nearly fit the adiabatic isochoric values at the highest hydrogen concentrations and gas turbulences. Weak pressure waves were observed for H₂ molar fractions greater than 10%. A careful examination of the pressure and temperature transients gave information on the flame path and on the heat transfer process during and after combustion. Scale effects on the peak pressures were not observed by comparison of the HYDRO-SC results with data obtained in other laboratories. The glow plug igniter has proved to be a reliable tool for use in deliberate ignition schemes for hydrogen control in nuclear plants.     

Characteristics of dust acoustic waves in dissipative dusty plasma in the presence of trapped electrons

The formation and propagation of nonlinear dust acoustic waves(DAWs) as solitary and solitary/shock waves in an unmagnetized, homogeneous, dissipative and collisionless dusty plasma comprising negatively charged micron sized dust grains in the presence of free and trapped electrons with singly charged non-thermal positive ions is discussed in detail. The evolution characteristics of the solitary and shock waves are studied by deriving a modified Korteweg–de Vries–Burgers(mKdV–Burgers) equation using the reductive perturbation method. The mKdV–Burgers equation is solved considering the presence(absence) of dissipation. In the absence of dissipation the system admits a solitary wave solution, whereas in the presence of dissipation the system admits shock waves(both monotonic and oscillatory) as well as a combination of solitary and shock wave solutions. Standard methods of solving the evolution equation of shock(solitary) waves are used. The results are discussed numerically using standard values of plasma parameters. The findings may be useful for better understanding of formation and propagation of waves in astrophysical plasma.     

Development of a portable cold air plasma jet device and observation of its photo ionization process

Photo ionization plays a critical role in the formation and propagation of atmospheric pressure plasma jet plumes. But in experiments, it is very difficult to observe the photo ionization due to its relative lower density of photo electrons. In the present study, we develop a portable cold air plasma jet device and observe the ionization wave in a dc spark air plasma jet. The discharge images acquired by an ICCD camera show that the ionization wave front performs as a quickly moving bright ball. Breakdown could take place at another side of the quartz plate or pork tissue layer(6 mm thick), which suggests that the ionization should be attributed to photo ionization.The laser schlieren images indicate there is propagation of a shock wave along with the plasma bullet. Based on the photo ionization theory and the photo-electric measurement, the direct photo ionization and multistage photo ionization are the main factors in charge of generating the cold air plasma jet. In addition, the plasma jet outside of the cathode nozzle is colder than 320 K and can be touched safely by a human. In view of the plasma jet including a large amount of active particles, such as NO, O, OH, emitted photons, etc, the proposed portable cold air plasma jet device could be qualified for plasma bio-medicine applications.     

加热双通道密度波流动不稳定性数值研究

密度波流动不稳定性是影响换热设备安全性和可靠性的重要因素之一,其发生机理十分复杂。本工作基于RELAP5程序对加热通道密度波脉动进行了动态计算分析,揭示了脉动期间流体密度、流量及压降等参数的变化规律,并与两种经典机理进行比较分析。结果表明：密度波脉动期间,通道内流量、密度（空泡）及压降呈周期性脉动,加热通道内轴向不同位置流量不同,进出口流量反相脉动,单向段压降和两相段压降基本反相;加热通道密度波脉动的发生与两相段流量波动传播的延迟性有着密切的关系。     

H_2/O_2爆燃波在冷源真空筒内的传播

若反应堆冷中子源真空筒热交换器中H2冷却剂泄漏与空气混合时,H2与O2发生爆炸,其冲击波将对真空筒完整性构成威胁。采用全尺寸模拟件进行实验,研究爆燃波对真空筒的冲击程度。比较在不同当量比和初压H2/O2混合物,在模拟件内点火引发爆燃,采用压力、火焰测量和数据采集系统,测量各测点的火焰及压力时间曲线。结果表明,低初压和化学当量比两种工况产生的冲击波和火焰均可通过真空筒点火端经狭缝进入到另一端,并形成强爆燃。对富油(即H2过剩)工况,管道只产生低速燃烧,无冲击波特征。     

Impulse-coupling coefficients from a pulsed-laser ablation of semiconductor GaAs

1 Introduction When a pulsed-laser beam with sufficiently high power density is focused onto a solid surface, the area irradiated by the laser beam rapidly vaporizes, ionizes, and generates plasma [1]. The vaporized materials mi- grate from the surface and initiate a shock wave, then a laser-supported detonation (LSD) wave is ignited by either the breakdown of the vaporized material or the actual breakdown of the air above the surface [2, 3]. When the plasma apart from the surface and LSD wa…     

Applying chemical engineering concepts to non-thermal plasma reactors

Process scale-up remains a considerable challenge for environmental applications of non-thermal plasmas.Undersanding the impact of reactor hydrodynamics in the performance of the process is a key step to overcome this challenge.In this work,we apply chemical engineering concepts to analyse the impact that different non-thermal plasma reactor configurations and regimes,such as laminar or plug flow,may have on the reactor performance.We do this in the particular context of the removal of pollutants by non-thermal plasmas,for which a simplified model is available.We generalise this model to different reactor configurations and,under certain hypotheses,we show that a reactor in the laminar regime may have a behaviour significantly different from one in the plug flow regime,often assumed in the non-thermal plasma literature.On the other hand,we show that a packed-bed reactor behaves very similarly to one in the plug flow regime.Beyond those results,the reader will find in this work a quick introduction to chemical reaction engineering concepts.     

Influence of Plasma Pressure Fluctuation on RF Wave Propagation

Pressure fluctuations in the plasma sheath from spacecraft reentry affect radiofrequency(RF) wave propagation.The influence of these fluctuations on wave propagation and wave properties is studied using methods derived by synthesizing the compressible turbulent flow theory,plasma theory,and electromagnetic wave theory.We study these influences on wave propagation at GPS and Ka frequencies during typical reentry by adopting stratified modeling.We analyzed the variations in reflection and transmission properties induced by pressure fluctuations.Our results show that,at the GPS frequency,if the waves are not totally reflected then the pressure fluctuations can remarkably affect reflection,transmission,and absorption properties.In extreme situations,the fluctuations can even cause blackout.At the Ka frequency,the influences are obvious when the waves are not totally transmitted.The influences are more pronounced at the GPS frequency than at the Ka frequency.This suggests that the latter can mitigate blackout by reducing both the reflection and the absorption of waves,as well as the influences of plasma fluctuations on wave propagation.Given that communication links with the reentry vehicles are susceptible to plasma pressure fluctuations,the influences on link budgets should be taken into consideration.     

Application study on plasma ignition in aeroengine strut-cavity-injector integrated afterburner

To increase the thrust-weight ratio in next-generation military aeroengines,a new integrated afterburner was designed in this study.The integrated structure of a combined strut-cavity-injector was applied to the afterburner.To improve ignition characteristics in the afterbumer,a new method using a plasma jet igniter was developed and optimized for application in the integrated afterburner.The effects of traditional spark igniters and plasma jet igniters on ignition processes and ignition characteristics of afterburners were studied and compared with the proposed design.The experimental results show that the strut-cavity-injector combination can achieve stable combustion,and plasma ignition can improve ignition characteristics.Compared with conventional spark ignition,plasma ignition reduced the ignition delay time by 67 ms.Additionally,the ignition delay time was reduced by increasing the inlet velocity and reducing the excess air coefficient.This investigation provides an effective and feasible method to apply plasma ignition in aeroengine afterburners and has potential engineering applications.     

Development of DETAC and its application to the hydrogen detonation analysis

In this study, a numerical analysis code (DETAC, Detonation Analysis Code) for hydrogen detonation during the reactor severe accident was developed using Fortran 90 language, and the simulation was performed for the hydrogen detonation. A global-chemistry model was adopted to simulate the chemical reaction. The Euler equations were solved using third-order Runge-Kutta method with fifth-order weighted essentially non-oscillatory scheme handling the convection flux. Afterward, the hydrodynamics solver was verified by comparison of predicted results and exact solutions of four cases of shock tube problems. A hydrogen detonation in a pipe was simulated to verify this code by comparing the results with the classical C-J theory. Furthermore, this code was applied to the hydrogen detonation analysis in the compartment of BWR building. Two cases with different ignition locations were analyzed in this paper and the maximum pressure of these cases were 7.5 MPa and 8.0 MPa, respectively. The pressure and the temperature during detonation were affected by the ignition location. The results indicated that the possibility of reactor building destruction exists if the hydrogen detonation occurs.     

Gas cloud explosions and resulting blast effects

The design of nuclear power plant structures to resist blast effects due to chemical explosions requires the determination of load-time functions of possible blast waves. Whether an explosion of a hydrocarbon gas in the atmosphere will occur in the form of a deflagration or a detonation is largely dependent on the type of flame acceleration process which is closely related to the rate of energy release. Flame propagations at normal flame velocities in a free explosible gas cloud will certainly not lead to detonation. However, with sufficiently large clouds — particularly under adverse boundary conditions — the flame acceleration could become so high that an initial deflagration changes into a detonative process.Results of recent investigations, which will be discussed in detail, show that in a free cloud with deflagrative ignition (flame, heated wire, sparks) the occurrence of a gas detonation can practically be excluded. Apparently, free gas clouds can only be induced to detonate by a sufficiently strong detonative initiation. Independently of the initiating event in the practice of damage analysis, it has become customary to describe the consequences of an explosion by means of the so-called TNT equivalent. Therefore, it is attempted to specify this equivalent for hydrocarbons by means of energetic considerations including the propagation functions for the case of spherically symmetric detonations. Analogous to the safety distances required in the handling and storage of high explosives, a mass-distance relation of the form could be considered where L is the mass of spontaneously released hydrocarbon and k varies only with the structural shape of the blast loaded buildings.With the inclusion of an empirical relation which relates the quasi-static design pressure for a building with the normally reflected blast pressure of a blast wave, it is further attempted to establish a relation between the structural capacity of a building — i.e. the pressure resistance of a building structure for detonative dynamic loading and for quasi-static loading — and the unit-mass distance .     

Study on the effect of focal position change on the expansion velocity and propagation mechanism of plasma generated by millisecond pulsed laser-induced fused silica

In this work, by controlling the positional relationship between the target and the focal point, the surface damage, shock wave phenomenon and propagation mechanism involved in the plasma generation of fused silica by millisecond pulsed laser irradiation at different focal positions were studied. Laser energy is an important experimental variable. The dynamic process of plasma was detected by optical shadow method, and the influence of surface film damage on plasma propagation and the propagation mechanism at different focal positions were discussed. The study found that the plasma induced by the pulsed laser at the focus position within 0–20 μs exploded, the micro-droplets formed around 20 μs. At the same time, a shock wave is formed by the compressed air, the micro-droplets are compressed under the action of the shock wave recoil pressure, and the micro-droplets channel phenomenon is observed in the micro-droplets. The peak velocities of plasma and combustion wave appear earlier in the pre-focus position than in the post-focus position. This research provides a reference for the field of laser processing using fused silica as the substrate.     

Modeling maximum flame speeds

A simple gold model for flame acceleration in tubes, caused by repeated obstacles, has been developed using a “boxcar” approach. The tube is assumed to consist of a series of chambers separated by obstacles. The feedback mechanism for flame acceleration is modeled by assuming that the effective burning velocity in the nth chamber depends on the same quantity in the (n − 1th) chamber. The equation for flame propagation is shown to be a logical difference equation. The equation predicts the various experimentally observed end results of flame acceleration such as total flame extinguishment after a flame has reached a certain critical flame speed, subsonic steady-state flame propagation, and continuous flame acceleration leading to transition to detonation. This equation models flame acceleration phenomenologically by associating various terms with effects such as flame folding, fine-scale turbulence, quenching and gas dynamics. The predicted maximum flame speeds (subsonic flame propagation) for various mixture compositions, obstacle spacings, obstacle blockage ratios, and initial gas temperatures agree with the experimental results fairly well.     

Dust Acoustic Compressive Waves in a Warm Dusty Plasma Having Non-Thermal Ions and Non-Isothermal Electrons

In this article an investigation is presented on the properties of dust acoustic(DA)compressive solitary wave propagation in an adiabatic dusty plasma,including the effect of nonthermal positive and negative ions and non-isothermal electrons.The reductive perturbation method has been employed to derive the lower degree modified Kadomtsev-Petviashivili(mK-P),3D Schamel-Korteweg-de-Vries equation or modified Kadomtsev-Petviashivili(mK-P) equations for dust acoustic solitary waves in a homogeneous,unmagnetized and collisionless plasma whose constituents are non-isothermal electrons,singly charged positive and negative non-thermal ions and massive charged dust particles.The stationary analytical solutions of the lower degree mK-P and mK-P equations are numerically analyzed,where the effect of various dusty plasma constituents on DA solitary wave propagation is taken into account.It is observed that both the ions in dusty plasma play a key role in the formation of DA compressive solitary waves,and also the ion concentration and non-isothermal electrons control the transformation of the compressive potentials of the waves.     

A Cascaded Discharge plasma—Adsorbent Technique for Engine Exhaust Treatment

A cascaded system of electrical discharges(non-thermal plasma)and adsorption process was investigated for the removal of oxides of Nitrogen(NOx) and total hydrocarbons (THC) from an actual diesel engine exhaust.The non-thermal plasma and adsorption processes were separately studied first and then the casecaded procxess was studied.In this study,different types of adsorbents were used.The NOx removal efficiency processes and the removal efficiency was found almost invariant in time.When associated by plasma,among the adsorbents studied.Activated charcoal and MS-13X were more effective for NOx and THC removal respectively.The experiments were conducted at no load and at 50% load conditions.The plasma reactor was kept at room temperature throughout the experiment,while the temperature of the adsorbent reactor was varied.A relative comparison of adsorbents was discussed at the end.     

Mitigation Technologies for Damage Induced by Pressure Waves in High-Power Mercury Spallation Neutron Sources (III)—Consideration of the Effect of Microbubbles on Pressure Wave Propagation through a Water Test—

The effects of microbubbles dispersed in a liquid on a high-rising-rate pressure wave were experimentally investigated with water. Intense, high-rising-rate pressure waves with a rise time of about 1.5 ms were produced by a spark discharge in water, and gas microbubbles were produced by two different bubble generators. Particular attention was focused on the attenuation effect of microbubbles on propagating pressure waves. The dependence of the attenuation effect on the radius and void fraction of the microbubbles was carefully examined. It was found that when the microbubbles are sufficiently small (e.g., about 50 μm in peak radius), the amplitude of wall vibration induced by the spark-induced pressure wave is dramatically decreased with an increase in void fraction. The present study provides strong experimental evidence that microbubbles can act as a strong absorber for high-rising-rate pressure waves as recently predicted numerically.     

Numerical study on the modulation of THz wave propagation by collisional microplasma photonic crystal

In order to demonstrate the modulation of terahertz wave propagation in atmospheric pressure microplasmas, in this work, the band structure and the transmission characteristics of a onedimensional collisional microplasma photonic crystal are investigated, using the transfer matrix method. For a lattice constant of 150 μm and a plasma width of 100 μm, three stopbands of microplasma photonic crystal are observed, in a frequency range of 0.1–5 THz. Firstly, an increase in gas pressure leads to a decrease in the central frequency of the stopband. When the gas pressure increases from 50.5 kPa to 202 kPa, the transmission coefficient of the THz wave first increases and then decreases at high frequency, where the wave frequency is much greater than both the plasma frequency and the collision frequency. Secondly, it is interesting to find that the central frequency and the bandwidth of the first THz stopband remain almost unchanged for electron densities of less than 10¹⁵ cm^–3, increasing significantly when the electron density increases up to 10¹⁶ cm^–3. A central frequency shift of 110 GHz, and a bandgap broadening of 200 GHz in the first stopband are observed. In addition, an atmospheric pressure microplasma with the electron density of 1 × 10¹⁵–6 × 10¹⁵ cm^–3 is recommended for the modulation of THz wave propagation by plasma photonic crystals.