Nanosecond Research Articles

Numerical study on the pretreatment of ammonia by nanosecond pulsed discharge for combustion enhancement: Effects of pretreatment uniformity

The nanosecond (NS) pulsed discharge for ammonia combustion assistance is numerically investigated with a special focus on the effects of discharge pretreatment uniformity. A method to mimic spatially non-uniform discharge pretreatment is established. Using NH3/O2/He and NH3/O2/N2 mixtures as examples, the ignition delay time (IDT), the laminar burning velocity (LBV), and the extinction strain rate (ESR) of mixtures pretreated by different discharge pretreatment methods were analyzed under a wide range of pretreatment uniformity. The results indicate that for the premixed NH3/O2/He (or N2) pretreated by NS discharge pulses, with the increase of the pretreatment non-uniformity, the IDT initially increases slightly, reaching a maximum and then decreases rapidly by 80 %, while the LBV and the ESR first decreases slowly, reaching a minimum and then increases rapidly by 10 %. The initial increment of IDT with the non-uniformity can be attributed to the deterioration of discharge-induced chemical effect but the rapid decrease of IDT as the discharge pretreatment becomes highly non-uniform is largely determined by the combustion-related chemical effect. The effects of pretreatment uniformity on LBV and ESR can be largely due to the thermal effect. When the oxidant is pretreated by the NS discharge, the minimal IDT, the maximal LBV and ESR can be observed under uniform conditions owing to the chemical effects and thus a uniform discharge pretreatment is preferred. Furthermore, a comparative analysis of the three pretreatment methods under identical energy input reveals that under uniform pretreatment, the direct pretreatment of pure NH3 is optimal, since it can generate more H2 to enhance the combustion. However, under highly non-uniform conditions, the discharge pretreatment of premixed fuel/oxidant mixtures is the most efficient for combustion enhancement.

Hydrophilic surface modification of polypropylene by AC-DBD and NS-DBD

Dielectric barrier discharge (DBD) is a method to modify material surfaces by creating low-temperature plasma at atmospheric pressure. However, in comparison to low-pressure discharge, atmospheric DBD has the drawback of uneven surface treatment due to the ease of forming filamentary channels. This limitation hinders the widespread application of DBD technology in material surface modification. In this study, the surface of polypropylene (PP) was treated using high frequency alternating current (AC) and nanosecond (NS) pulsed argon DBDs to improve surface hydrophilicity. The changes of the PP surface hydrophilicity and the treatment uniformity were characterized by water contact angle (WCA) measurement. The relationships between the DBD characteristics, the treatment effect and the treatment uniformity of the PP surface were investigated using electrical and optical diagnostic techniques. The PP material surfaces before and after plasma treatments were evaluated by the physical morphology and chemical composition measurements to examine the influence of DBD on the treatment effect and uniformity on the PP material surface. The findings show that the modification by AC-DBD reduces the WCA of PP material surface by 40°∼50° and exhibits a variance of 1.77, indicating an improvement in hydrophilicity to some extent. Furthermore, the modification by NS-DBD further decreases the WCA of PP material surface, reducing the variance to 1.14. Compared with AC-DBD, NS-DBD has a better uniformity, a higher concentration of active particles in plasma, a greater surface roughness of PP material after treatment, a better hydrophilicity and a more uniform effect with less energy consumption. The results are meaningful to improve the application of DBD in material surface modification and optimize the power source election and operation parameters.

Laser Powder Bed Fusion of 316L Stainless Steel: Effect of Laser Polishing on the Surface Morphology and Corrosion Behavior

Recently, laser polishing, as an effective post-treatment technology for metal parts fabricated by laser powder bed fusion (LPBF), has received much attention. In this paper, LPBF-ed 316L stainless steel samples were polished by three different types of lasers. The effect of laser pulse width on surface morphology and corrosion resistance was investigated. The experimental results show that, compared to the nanosecond (NS) and femtosecond (FS) lasers, the surface material’s sufficient remelting realized by the continuous wave (CW) laser results in a significant improvement in roughness. The surface hardness is increased and the corrosion resistance is the best. The microcracks on the NS laser-polished surface lead to a decrease in the microhardness and corrosion resistance. The FS laser does not significantly improve surface roughness. The ultrafast laser-induced micro-nanostructures increase the contact area of the electrochemical reaction, resulting in a decrease in corrosion resistance.

Compressor Airfoil Separation Control Using Nanosecond Plasma Actuation at Low Reynolds Number

Flow control effects of three types of nanosecond (NS) dielectric barrier discharge (DBD) plasma actuations in suppressing the flow separation of a compressor airfoil at low Reynolds number are investigated using large-Eddy simulation. It has been found that the NS DBD plasma actuation can effectively suppress the flow separation and two mechanisms behind the flow control effect have been uncovered. First, the NS DBD plasma actuation can induce distorted flow structure (DFS) within the flowfield, and the induced DFS of plasma actuations located upstream of the flow separation zone can bring the Kelvin–Helmholtz instability of the shear layer between mainstream flow and separated flow froward to the separation point. Then, a large-scale spanwise vortex, which promotes the mixing of the main flow and the separated flow, is induced within the flow separation zone, thus resulting in suppression of the flow separation. Second, the plasma actuation located at the blade leading edge can improve the momentum of the laminar boundary layer, owing to the propagations of compressive waves as well as DFS along the blade surface. The plasma actuation located within the separation zone fails to suppress the boundary-layer flow separation effectively because the induced DFS cannot influence the shear layer between mainstream flow and separated flow.

Numerical investigation of control of dynamic stall over a NACA0015 airfoil using dielectric barrier discharge plasma actuators

The control of dynamic stall over a periodically pitching NACA 0015 airfoil using alternating current (AC) and nanosecond (NS) dielectric barrier discharge (DBD) plasma actuators is investigated by means of numerical simulation. This study employs a two-dimensional unsteady Reynolds averaged Navier–Stokes (URANS) approach to resolve the flow control process. The pulsed AC and NS plasma discharges are modeled by an empirical body force model and a sophisticated self-similar plasma formulation, respectively. Our study concentrates on the resolution of detailed control process of dynamic stall under DBD plasma forcing at two Reynolds numbers (Re) as well as on comparison of AC and NS plasma actuations in terms of control mechanism and authority. It appears that the dynamic stall without control at both moderate Reynolds number of Re = 2.5 × 105 and high Reynolds number of Re = 7.5 × 105 can be categorized into the so-called trailing edge stall. The trailing edge stall initiates with flow reversal near the trailing edge. Regarding the dynamic stall control, it is found that the jet flow produced by AC DBD or residual heat of NS DBD is responsible for inducing large-scale spanwise vortices, which, in turn, dominate the flow control. For the moderate Re flow, AC and NS plasma actuators have comparable performance and both achieve good control authority. However, for the dynamic stall control at high Re, the NS DBD achieves surprising success in enhancing lift of the airfoil and reducing aerodynamic hysteresis, whereas AC DBD nearly has no effect on the flow. It is found that the superiority of the NS plasma actuator over other control means is due to the thermal convection characteristic peculiar to NS plasma discharge. This characteristic makes the NS DBD plasma actuator more flexible in extending its influence region and acquiring a better control effect.

Effect of NS-DBD Actuator Parameters on the Aerodynamic Performance of a Flap Lifting Device

The flap lift device is an important part of the conventional configuration of aircrafts and has an important impact on the aerodynamic performance. In this paper, a high-efficiency, simple, and energy-saving nanosecond dielectric barrier discharge (DBD) plasma actuator is placed in the vicinity of the flap lift device to improve the aerodynamic performance of the flap by controlling the flow field. The two-dimensional airfoil GAW-1 and its 29% flap were selected as the research objects, and the nanosecond (NS) DBD actuators were fixed at different locations near the deflection angle of the 10°flap. The excitation frequency, pulse width, and energy density parameters of the pulse discharge were adjusted, and then, the effects of parameter changes on aerodynamic characteristics of the airfoil were studied by numerical simulation. The simulation results show that adjusting the excitation frequency on the aerodynamic drag is weak and that the effect on the aerodynamic lift is obvious. The increase of the discharge pulse width will have a more significant effect on the flow field, i.e., a proper increase of the discharge pulse width can achieve better drag reduction, and increase lift after a stall at a high angle of attack. Although the increase of discharge energy density can strengthen the pulse perturbation effect on the flow field, it also contributes to some adverse effects and has no obvious optimization effect on the control efficiency of lift increase and drag reduction.

Effects of controlled non-equilibrium excitation on H2/O2/He ignition using a hybrid repetitive nanosecond and DC discharge

The present work reports on the effects of controlled non-equilibrium excitation of reactant molecules on low temperature H2/O2/He ignition by numerically modeling a hybrid repetitive nanosecond (NSD) and DC discharge at atmospheric pressure. At first, a detailed plasma-combustion kinetic model of H2/O2/He, including non-equilibrium excitation, is developed and validated by experimental data of a repetitively-pulsed nanosecond discharge. Then, the effects of ignition enhancement by NSD and a hybrid NSD/DC discharge, with controlled electron energy distribution for selective non-equilibrium excitation of vibrationally excited H2(v) and O2(v) as well as electronically excited O2(a1Δg) and O(1D), are compared. The results show that H2(v1) contributes significantly to the H production and OH consumption in the hybrid plasma discharge. Moreover, O2(a1Δg) and O2(v1−4) also contribute to the production O and OH. Uncertainty analysis of H2(v) and O2(a1Δg) elementary reactions on ignition delay time is conducted by using several different kinetic models. The comparison of ignition delay time using different plasma kinetic models indicates the selection of accurate rate constants involving excited species is important for plasma assisted ignition modeling. The results of hybrid discharge assisted H2/O2 ignition show that the optimized ignition enhancement is achieved when both excited species and radicals are produced efficiently at an appropriate DC electric field strength. The present modeling provides useful insight into the plasma-combustion model development and the development of controlled plasma discharge to achieve efficient ignition with optimized non-equilibrium excitation of reactants.

Numerical modelling of nanosecond surface dielectric barrier discharge evolution in atmospheric air

This paper analyses numerical efforts regarding nanosecond (NS) surface dielectric barrier discharge (SDBD) modelling in atmospheric air. Numerical results of the discharge structure for positive and negative applied voltage pulse polarity, and the features of the physical models and boundary conditions used, are discussed. The results of 2D simulations of the quasi-uniform SDBD mode are presented and compared with the results of other research teams, and with experimental data, to reveal the most appropriate approaches. New results of numerical simulations and analytical estimations of the energy into gas deposition due to NS SDBD driven by a single NS voltage pulse are presented. The problems relating to NS SDBD modelling are discussed.

The study of flow separation control by a nanosecond pulse discharge actuator

The basic dynamic characteristic of a nanosecond (NS) pulse discharge actuator in still air is investigated, which introduces viscid and inviscid effects to the flow, namely, heated air and pressure waves, respectively. The efficacy of a dielectric barrier discharge plasma actuator driven by a nanosecond pulse for active flow separation control is investigated experimentally on a NASA SC(2)-0712 aerofoil at Re=0.5×106 (25m/s). The pressure distribution on the aerofoil surface is measured, and the result indicates that the separation is well controlled at a high angle of attack. The PIV test result shows that the forcing frequency selectively affects the flow over the aerofoil, and a higher forcing frequency has a better influence on the leading edge shear layer. Schlieren imaging experiments at Re=0.1×106 (5m/s) show that the main disturbance factor for the nanosecond pulse plasma actuator is the heating effect, which changes the local temperature and density and induces several vortex structures. The vortex structures entrain the outer high-momentum fluid into the shear layer, thereby strengthening the mixing of the shear layer with the main flow and delaying separation or even reattaching a separated flow. The research reveals the major disturbance factor introduced by NS discharge applied to flow separation control. Furthermore, the spatiotemporal disturbance process induced by the pulse discharge is investigated.

0616 ナノ秒パルスプラズマアクチュエータを用いた翼上の流れ制御のための数値モデル

The significant effects of dielectric barrier discharge (DBD) driven by repetitive nanosecond (NS) pulse voltage on lift enhancement on stalled airfoils have been demonstrated. The essential effect of NS plasma is considered as thermal effect. The detailed effect of the thermal excitation on the vortex structures is, however, not fully understood. In the present work, a simple momentum model for incompressible Unsteady Reynolds-Averaged Navier-Stokes simulation was proposed, in which the effect of plasma was implemented as a wall velocity boundary condition on the airfoil. Although this simplified simulation does not take account of thermalization process and shock dynamics, the results showed discrete vortices generated by rolling-up shear layer over the leading edge. It qualitatively consisted with the trend of time-averaged pressure distribution obtained by the corresponding experiment.

Flow Separation Control Using Nanosecond Pulse Driven DBD Plasma Actuators

This work continues an ongoing effort aimed at development and use of dielectric barrier discharge (DBD) plasma actuators driven by repetitive nanosecond pulses for high Reynolds number aerodynamic flow control. These actuators are believed to influence the flow via a thermal mechanism which is fundamentally different from more commonly studied AC-DBD actuators. Leading edge separation control on an 8-inch chord NACA 0015 airfoil is demonstrated at various post-stall angles of attack for Mach numbers up to 0.26 (free stream velocity up to 93 m/s) and Reynolds numbers up to 1.15 X 106. The nanosecond (NS) pulse driven DBD is shown to extend the stall angle at low Reynolds numbers by functioning as an active trip. At post-stall angles of attack, the device is shown to excite shear layer instabilities and generate coherent spanwise vortices that transfer momentum from the freestream to the separated region, thus reattaching the flow. This is observed for all high Reynolds numbers and Mach numbers spanning th...

Influence of excitation voltage waveform on dielectric barrier discharge plasma aerodynamic actuation characteristics

Plasma flow control, based on the plasma aerodynamic actuati on generated by air discharge, is an active field in aerodynamics due to its potential application in performance improvement of future aircraft. In order to better understand the underlying physical mechanism of plasma flow control, it is i mportant to investigate the relationship between the operating parameters and the plasma aerodynamic actuation characteristics. This paper reports the electrical, optical and mech anical characteristics of surface dielectric barrier discharge p lasma aerodynamic actuation excited by microsecond and nanosecond high voltage waveforms. The nanosecond discharge is more diffuser than the microsecond discharge and the discharge current is much larger at the same applied voltage amplitude. The optical emission intensity of the nanosecond discharge plasma is stronger than that of the microsecond discharge plasma, while the rotational and vibrational temperatures of N2 in the nanosecond discharge plasma are less. In addition, the relative intensity of the first negative system of N + 2 (B 2 � + u ! X 2 � + g ) and the second positive system of N2(C 3 �u ! B 3 �g) is much less in the nanosecond discharge plasma. The velocity measurements indicate that the air flow induced by the nanose cond discharge plasma aerodynamic actuation is vertical to the dielectric surface, while that induced by the microsecond discharge actuation is parallel to the dielectric surface.

Femtosecond versus nanosecond laser machining: comparison of induced stresses and structural changes in silicon wafers

Laser micromachining has proven to be a very successful tool for precision machining and microfabrication with applications in microelectronics, MEMS, medical device, aerospace, biomedical, and defense applications. Femtosecond (FS) laser micromachining is usually thought to be of minimal heat-affected zone (HAZ) local to the micromachined feature. The assumption of reduced HAZ is attributed to the absence of direct coupling of the laser energy into the thermal modes of the material during irradiation. However, a substantial HAZ is thought to exist when machining with lasers having pulse durations in the nanosecond (NS) regime. In this paper, we compare the results of micromachining a single crystal silicon wafer using a 150-femtosecond and a 30-nanosecond lasers. Induced stress and amorphization of the silicon single crystal were monitored using micro-Raman spectroscopy as a function of the fluence and pulse duration of the incident laser. The onset of average induced stress occurs at lower fluence when machining with the femtosecond pulse laser. Induced stresses were found to maximize at fluence of 44 J cm −2 and 8 J cm −2 for nanosecond and femtosecond pulsed lasers, respectively. In both laser pulse regimes, a maximum induced stress is observed at which point the induced stress begins to decrease as the fluence is increased. The maximum induced stress was comparable at 2.0 GPa and 1.5 GPa for the two lasers. For the nanosecond pulse laser, the induced amorphization reached a plateau of approximately 20% for fluence exceeding 22 J cm −2. For the femtosecond pulse laser, however, induced amorphization was approximately 17% independent of the laser fluence within the experimental range. These two values can be considered nominally the same within experimental error. For femtosecond laser machining, some effect of the laser polarization on the amount of induced stress and amorphization was also observed.