Reverse Vortex Flow Research Articles

Abatement of Volatile Organic Compounds (VOCs) and Fluorine gases by a Microwave Plasma Torch (MPT)

The use of microwave plasma torch (MPT) in the abatement of potent greenhouse and fluorinated (F-gases) gases is crucial due to their high global warming potential. The purpose of this study was the abatement of volatile organic compounds (VOCs) and carbon tetrafluoride (CF4) using specifically designed reverse vortex flow reactor (RVR) and pillar of fire reactor (POF). The RVR has properties that are crucial for their synergistic effects with the MPT, and the POF configurations showed high-efficiency combustion due to plasma radicals and a large contact area with the MPT. The study used Aspen Plus and COMSOL software to analyse the mass balance involved in the thermal decomposition of isopropyl alcohol (C₃H₈O) and CF4 and to design the RVR and POF reactors. Using MPT without hydrocarbon fuel, low-concentration C₃H₈O and CF4 were successfully destroyed in high-flow streams. The experiment on C₃H₈O showed that using a plasma power of 2kW and 1 cubic meter per minute (CMM) of bulk gas, a destruction removal efficiency (DRE) of 95% was achieved for a polluted C₃H₈O of 410 ppm. In a second experiment, a DRE of 94% was achieved for a polluted C₃H₈O of 370 ppm. The first CF4 experiment achieved a DRE of 93.3% with a 7kW plasma power and 1 CMM bulk gas polluted at 285 ppm. In the second experiment, the presence of highly reactive hydroxyl radicals (OH⁎) in steam plasma improved the DRE to 99.8% at 7kW plasma power and 100 LPM bulk polluted at 180 ppm.

Numerical investigation of energy dissipation and vortex characteristics on the S-shaped region of a reversible pump-turbine

In order to meet the regulation needs of power system, the pump-turbine is prone to operate in the S-shaped region during frequent start-up, leading to unit vibration and grid connection failure. In this paper, the S characteristic experiment was carried out, and the entropy production, unstable flow and vortex structure under turbine, runaway, turbine breaking, reverse pump conditions with optimum guide vane opening are detailed analyzed. The relationship between the energy dissipation and flow field characteristics is clarified. The results show that the runner entropy production is the largest in all components, and the wall entropy production is greater than the fluctuating entropy production under turbine condition. With the decrease in discharge, the fluctuating entropy production and wall entropy production first increase and then decrease, reaching the maximum values in the runaway condition. The circumferential movement trend of water enhances, and forms circumferential flow in the vaneless region. The separation vortex formed on the blade suction surface extends from the blade inlet to the middle of the flow channel, inducing a high entropy production. Under the turbine braking condition, the tangential velocity increases rapidly in the vaneless region. The circumferential flow develops into water ring, blocking the water flows into the runner. Under the reverse pump condition, the water flows in the opposite direction, with negative tangential and streamwise velocities. The absolute flow angle becomes obtuse, and a large-scale reverse vortex is formed. The runner inlet is blocked by the water ring, and the flow channel is almost completely filled with the low-speed region. The proportion of the guide vanes entropy production reaches maximum. Unstable flow leads to the increase in vorticity. Separation vortex, bypassed vortex, and reverse flow vortex leads to an increase in VST, forming a high VST region at corresponding positions. Due to the rotational motion accompanying the development of separated vortex, it leads to the increase in CFT. In addition, the circumferential flow and water ring have higher tangential velocities, which can lead to high CFT regions.

A highly effective N2 fixation method based on reverse vortex flow gliding arc plasma under water

A gliding arc plasma has the advantages of strong reactivity, high energy density, and easy scale-up, so it is very competitive in the field of plasma-based N2 fixation. In this work, we designed an underwater transfer arc discharge plasma N2 fixation system based on a reverse vortex flow gliding arc. In full contact with air and water, the plasma could realize the efficient conversion of N2, O2, and H2O, obtaining gaseous NO, NO2, and activated water rich in NOx*. The experiment showed that an underwater transfer arc discharge was formed when the current was greater than 0.6 A. Under this operating condition, the conversion rate of N2 reached 1.63%, the concentration of NOx reached 2.4%, and the production rate of nitrogen-containing products was about 3.41 mol/h. In addition, the energy consumption was as low as 2.21 MJ/mol, the lowest reported value for atmospheric-pressure rotary gliding arc plasma-based N2 fixation. The generated activated water had abundant NO3− and NO2− ions, which could be directly applied as N2 fertilizer in agriculture, promoting plant germination and growth. This paper describes an environmentally friendly and efficient technical route toward developing industrial N2 fixation/green agriculture, strongly supporting the carbon neutrality concept.

Unsteady Mixed Convection Flows in a Rectangular Duct and Dynamical Behaviors of Flow Channel Insert

Buoyancy-assisted upward MHD flows and dynamical behaviors of flow channel insert (FCI) in the dual-coolant lead-lithium (DCLL) blanket are studied numerically. Based on our internally developed and validated solver, the dynamical behaviors of magneto-thermo-fluid-structure coupled multiphysical field are investigated. A large amplitude, low frequency, and quasiperiodic unsteady reverse flow at high Re (31000), high Gr (3.5×1011), and moderate magnetic field (0.7~1.7T) is found in the DCLL blanket. This intricate phenomenon has been discovered for the first time, representing a combination of separate experimental results from Melnikov et al. (2016) and Khanal and Lei (2012). In our study of this large amplitude, low frequency, and quasiperiodic unsteady reverse flows, the importance of the cold helium gas to the instability of fluid flow in the bulk region and the thermal conductivity of the FCI to convection structure and instability are first found and recognized. Additionally, we take into account the effects of the temperature field and flow field on structural deformation and mechanical behavior of FCI, and we have discovered several intriguing phenomena, such as (1) the stability of fluid flow in the bulk region depends on the strength of the heat source, the magnitude of the magnetic field, and thermal conductivity of FCI; (2) the instability and periodicity of the fluid flow are primarily related to the unsteady reverse flow, which rises up and falls down periodically in the bulk region; (3) the physical mechanism of unsteady flow influenced by reverse flow, pressure drop, and Lorentz force has been concluded. It has been discovered that the breakdown of a reverse flow vortex causes a rapid reduction in pressure drop. (4) To avoid this phenomenon in engineering, a phase map of unsteady and steady flows in the DCLL blanket has been created. (5) The quasiperiodic characteristics of solid (flow channel insert) affected by flow are found and analyzed.

Effects of Flame Arrester Core with Different Thicknesses on Hydrogen/Methane/Air Explosion with Low Hydrogen Ratio

ABSTRACT In order to study the flame retardant performance of corrugated flame arrester core on the explosion characteristics of 9.5% hydrogen/methane/air premixed gas, a self-built experimental platform was used to study the explosive characteristics and propagation rules and the impact of varying thicknesses and different volumes of hydrogen. The results indicated that the corrugated flame arrester core on the explosion flame of premixed gas was observed with two scenarios, quenching and penetration. The flame is dominated by reverse vortex flow after quenching, reversing the axial direction of flame propagation. However, the flame penetration leads to more intense burning reactions as φ increases. And the increase in thickness changes the flame structure forms a spherical flame to a mushroom-shaped flame and finally form a turbulent flame. The velocity of flame decreased significantly when L = 60 mm with φ = 10% and 20% respectively. The quenching effect is more obvious in P2 with hydrogen added, and its better performance with increasing thickness, with 9.76% and 3.70% decrease when L = 40 mm with φ = 10% and 20%, respectively, and with 57.05% and 48.55% decrease when L = 60 mm with φ = 10% and 20%, respectively.

Time-resolved PIV measurement of coherent structures downstream of step blockage in a narrow rectangular channel

Experimental study of coherent structures in a narrow rectangular channel with 50 percent single side entrance blockage was conducted by time-resolved particle image velocimetry (TR-PIV) under Reynolds numbers from 2,180 to 21,800. With error and uncertainty analysis, the maximum standard uncertainty is less than 2% of bulk velocity (Wb) for time-averaged velocity and less than 2% Wb2 for Reynolds stress.The blocked channel accelerates fluid and forms strong streamwise velocity gradient, which dominates the flow field downstream the blockage. The shear-flow generates a large reverse flow vortex downstream blockage, which forms a second reverse flow in the corner of narrow channel near blockage. The streamwise flow reattaches the side channel wall downstream the reverse flow vortex. The vortices shedding from blockage interact with the reverse flow, forming a long and narrow region of high Reynolds stress near the inlet. The time-averaged velocity and Reynolds stress are strong in shear-layer and weak in the reverse flow and second reverse flow. The streamwise velocity gradient and Reynolds stress decay in streamwise direction. With Reynolds number increasing, normalized spanwise velocity and reattachment length increase, while normalized streamwise velocity decreases.The coherent structures downstream blockage has different PSD peak frequencies in shear-layer, reverse flow and second reverse flow. The Strouhal number decreases with Reynolds number increasing. The integral spatial–temporal length scales in the coherent structures are determined by the velocity gradient, which forms large turbulent fluctuations in the shear layer and reverse flow. With Reynolds number increasing, the spatial–temporal length scales increase because stronger velocity gradient generates larger coherent structures downstream blockage. The strong anisotropic turbulent flow in narrow rectangular channel with 50% single side entrance blockage exists in terms of Reynolds stress and spatial–temporal length scales.The experimental findings improve understandings of coherent structures in narrow channel with 50% single side entrance blockage. The experimental data can be used for mechanism study of entrance blockage accident and validations of turbulent models.

Non-thermal plasma removal of naphthalene as tar model compound from biomass gasification

Gasification of biomass is an industrial process utilized to produce product gas that can be used in electrical power generation with internal combustion engines. However, there remain huge challenges with regards to tar and other pollutants, thus their removal before further use is necessary. Non-thermal plasma method can be considered as a novel and efficient way for treatment of tar from product gas. In this work, a laboratory-scale, reverse vortex flow gliding arc plasma reactor was developed and tested for the removal of tar model compound using naphthalene as surrogate. Effects of total gas feed rate in the range of 20–60 L/min, applied voltage in the range of 50–220 V, hence, power input in the range of 300–460 W with initial naphthalene concentration of 610 ± 50 mg/m3 on removal and energy input were investigated. It was found the tar removal efficiency increased with increasing applied voltage. Maximum naphthalene removal efficiency of 85% was achieved at applied high voltage of 15 kV and specific energy input of 0.13 kWh/m3 with energy utilization efficiency of about 4.60 g/kWh. From the findings, non-thermal plasma technology appeared to offer great potential in the removal of biomass tar from gasification. The technology may be promising for possible application to real biomass tar cracking and upgrading of product gas from gasification.

Gliding Arc Discharge for Emission Control in Swirl Fuel-lean Non-premixed Combustion

ABSTRACT A reverse vortex flow gliding arc discharge in a fuel-rich premixed mixture was applied to a high swirl fuel-lean global combustion to accelerate fuel oxidation. Both the discharge and flame were generated in natural gas and air. To evaluate the role of the gliding arc in the process, a gas analysis of the exhaust gas was performed in the same operational conditions with and without plasma. The chemical measurements show that the plasma reduces carbon monoxide and unburned hydrocarbons contents with a low impact on the NOx level. Furthermore, the comparison of the relative decrease of the hydrocarbon emissions shows that the hydrocarbons have different sensitivities to the plasma application.

The Thermal Properties of Gliding arc Plasma Produced by Laboratory Reverse Vortex Flow System

A low-cost reverse flow plasma system powered by argon gas pumping was built using homemade materials in this paper. The length of the resulting arc change was directly proportional to the flow rate, while using the thermal camera to examine the thermal intensity distribution and demonstrating that it is concentrated in the centre, away from the walls at various flow rates, the resulting arc's spectra were also measured. The results show that as the gas flow rate increased, so did the ambient temperature. The results show that the medium containing the arc has a maximum temperature of 34.1 ˚C at a flow rate of 14 L/min and a minimum temperature of 22.6 ˚C at a flow rate of 6 L/min.

Effects of tip clearance size on vortical structures and turbulence statistics in tip-leakage flows: A direct numerical simulation study

Direct numerical simulations of the tip-leakage flow, generated by a gap between a straight National Advisory Committee for Aeronautics 0012 hydrofoil and the end wall of a channel, have been performed to investigate the effects of tip clearance size on vortical structures and turbulence statistics. The tip-leakage vortex, tip separation vortex, and induced vortex are the predominant vortical structures in tip-leakage flow for a relatively large gap (3.33%Ca), while the reverse flow vortex dominates the tip clearance region for smaller gaps (1.67%Ca), where Ca is the truncated chord length of the hydrofoil. Detailed analysis of turbulence statistics reveals that the tip-leakage vortex is caused by the rollup of the tip-leakage jet, while the spanwise inflow interacting with the sidewall of the hydrofoil leads to the formation of a reverse flow vortex. The turbulent kinetic energy contours show an arc-shaped distribution on the suction side of the hydrofoil, but their locations are significantly affected by the tip clearance size. In addition, the investigation of tip-leakage loss indicates that increasing the size of the tip clearance can reduce the tip-leakage loss across the hydrofoil. This can be attributed to the dominant vortical structures in the tip-leakage flow.

Construction and spectral characterization of the gliding arc reverse vortex flow plasma system at atmospheric pressure

In this paper, a low-cost reverse-gliding arc three-dimensional reactor with local materials and a homemade voltage source was proposed. This system works by pumping out argon gas while mixing in atmospheric air. The spectral properties of the resulting arc were investigated, as well as the calculation of electron temperature and density. The process of mixing air with argon gas is a novel method for obtaining nitrogen gas at a low cost and with ease of access. Nitrogen gas has numerous applications, and the findings are promising for future applications.

Caudal autotomy and regeneration of arc in a 3D gliding arc discharge plasma

To gain a better understanding of the mechanism governing arc dynamics in 3D gliding arc discharge (GAD) plasma, the spatio-temporal evolution of GAD was investigated in a reverse vortex flow by a novel reactor with ring (powered electrode, PE) and truncated cone (ground electrode, GE) electrodes. A newly underlying mechanism governing arc evolution in 3D GAD was gained with combination of flow field simulation, synchronous electrical characteristics, intensified charge coupled device images and high-speed photos. The spatio-temporal analysis indicates that, being different from the well-known ignition–gliding–extinction mechanism occurring in traditional GAD, the PE arc root glides continuously in the 3D GAD, but the GE arc root features jumps from the end of the gliding path to the beginning of the next one. By means of the jumping, the arc auto-sheds the caudal part of the arc and a new arc tail is simultaneously generated, rather than rebuilding a new arc channel back to the shortest electrode gap. With this special behavior of caudal autotomy and regeneration, the main part of the arc remains for each jump. This new insight improves the understanding of the discharge mechanism governing arc evolution in 3D GAD and provides a reference for optimization design of gliding arc plasma in a vortex flow.

Numerical investigation on vibration and pressure fluctuation characteristics in a centrifugal pump under low flow rate

Vibration characteristic is an important factor in evaluating operation stability of centrifugal pump. The vibration of main shaft was measured using a laser vibrometer, internal flow field was simulated via the shear stress transport turbulence model, and distribution law of vibration and pressure fluctuation in the impeller were analysed to explore the induction factor of vibration and the inherent relationship with pressure fluctuation in a semi-open centrifugal pump under low flow rate condition. Results of the numerical simulation are consistent with the experimental data. In addition to rotation frequency caused by impeller rotation, vibration frequency also includes characteristic frequency with high amplitude induced by unstable flow. The complex vortex in the impeller is composed of tip leakage vortex (TLV), reverse flow vortex, passage vortex and tip separation vortex. The primary tip leakage vortex (PTLV) formed by the streamline spills from 0 to 0.2λ where λ is the dimensionless distance from leading edge to trailing edge collides with tip leakage flow, the leading edge overflow and reverse flow vortex at the frequency of 1.6 fn ( fn is the rotating frequency) and 2.2 fn appear, respectively. The tip separation vortex formed in the tip clearance induced a frequency of 1.2 fn. The frequency of unstable flow phenomenon was consistent with the vibration frequency of main shaft, which induced the vibration of centrifugal pump.

Effect of mach number on high-subsonic and low-Reynolds-number flows around airfoils

High-subsonic and low-Reynolds-number flow is a special aerodynamic problem associated with near space propellers and Mars aircrafts. The flow around airfoils and the corresponding aerodynamic performance are different from the incompressible flow at low-Reynolds-number, due to complex shock wave-laminar separation bubble interaction. The objective of this paper is to figure out the effect of Mach number on aerodynamic performance and special flow structure of airfoil. An in-house Reynolds-averaged Navier–Stokes solver coupled with [Formula: see text] transition model is employed to simulate the flows around the E387 airfoil. The results show that the lift slope is larger than [Formula: see text] in the linear region. No stall occurs even at an attack angle of [Formula: see text]. With increase of Mach number, lift coefficient decreases when attack angle is below [Formula: see text]. However, once the angle of attack exceeds [Formula: see text], higher Mach number corresponds to higher lift coefficient. In addition, the strength and number of shock waves are very sensitive to Mach number. With increase of Mach number, the region of reverse flow vortex near transition location becomes smaller and finally disappears, while a new reverse flow vortex appears near the trailing edge and becomes larger.

Rapid degradation of PFAS in aqueous solutions by reverse vortex flow gliding arc plasma

Gliding arc plasma discharge is an energy-efficient treatment technology for a broad range of poly- and perfluoroalkyl substances in water.

Effect of transverse ridge microtopography on the surface shear stress distribution and soil wind erosion

Through wind tunnel experiments, we measured the surface shear stress (τs) on a bed surface that contained widely but uniformly spaced non-erodible ridges. We found that when the ridge spacing is larger than 10 times the height (H), the τs between two adjacent ridges could be divided into two sections. In each section, shear stress gradually increases and then decreases. The first section appears to be produced by the reverse-flow vortex that develops close behind upwind ridges and the second appears to result from airflow recovery followed by blockage by the downwind ridge. The mean surface shear stress (τs¯) of the total bed increases with increasing ridge spacing and friction velocity, and decreases with increasing ridge density. The spatial differences in τs lead to a non-uniform distribution of wind erosion between the ridges. Based on the threshold shear stress (τt) of the tested soil, we revealed a regular distribution of effective shear stress (τeff) on the sand bed, and established quantitative relationships among mean effective shear stress (τeff¯), H, ridge spacing (L), friction velocity (u*), and the threshold friction velocity (u*t) for sand entrainment: τeff¯=aLHu*-u*t2.

Removal of biomass tar model compound using reverse vortex flow gliding arc discharge

ABSTRACT Removal of tar derived from biomass gasification using plasma discharge was of great interest. In this work, treatment of biomass tar using a high flow rate, reverse vortex flow gliding arc discharge (RVF-GA) was investigated. Naphthalene was used as a light tar model compound. Investigation was carried out at different inlet tar concentrations, total gas feed rates, and applied energy inputs. The naphthalene concentrations considered were low (≈ 610 mg/m3), moderate (≈ 4,330 mg/m3) and high concentration (≈ 10,330 mg/m3). It was demonstrated that the RVF-GA reactor performed well in removal of tar at high flow rates. The removal efficiency in the range of 75–95%, the specific energy input in the range of 0.11–0.33 kWh/m3 and the energy utilization in the range of 1.4–112 g/kWh could be achieved. The plasma assisted decomposition of naphthalene was found to increase with the applied electrical energyinput.

Reforming of biogas using a non-thermal, gliding-arc, plasma in reverse vortex flow and fate of hydrogen sulfide contaminants

Biogas is being produced everyday around the world due to land filling of organic wastes. Reforming of biogas to hydrogen rich gas offers a green source of energy. This work demonstrates reforming biogas into hydrogen rich gas via a non-thermal gliding arc plasma stabilized in a reverse vortex flow with very low and competitive specific energy requirement. Parametric tests determined the individual effects of power input (140-300 W), steam to carbon ratio (0.0–3.0), and equivalence ratio (0.1–0.7) on reformer performance. Factorial tests identified optimal operating condition based on minimizing the specific energy requirement, determined to be 184 kJ/mol H2 or 1.91 eV/H2 molecule, significantly below the value for conventional steam reforming of methane, 3.37 eV/H2 molecule produced. The optimum operating conditions were found at an equivalence ratio of 0.11, a steam to carbon ratio of 0.14, and an input plasma power of 160 W, resulting in methane conversion of 48.8%, hydrogen yield of 23.4%, hydrogen selectivity of 47.8%, and an efficiency of 25.3%. Hydrogen sulfide as a common contaminant in landfill gas has detrimental effects on downstream facilities. The reactor was also evaluated on synthetic biogas containing hydrogen sulfide at low concentration (21 ppm). About 5.7% of the sulfur input to the system was partitioned to the dry outlet reformate stream with the remainder captured as sulfate in a downstream impinger or recovered as a solid of unknown molecular structure deposited on tubing surfaces between the reformer and the impinger. This reforming technology offers potential to be deployed as a lightweight compact portable system for on-site applications such as landfills, and depending on available fuels, in mobile applications such as ships.

Numerical study on cavitating flow-induced vibration of liquid rocket engine inducer

The cavitating flow-induced vibrations have become one of the major issues for the operation safety and stability of the liquid rocket engine. The objective of this paper is to numerically investigate the cavitating flow around a four-blade inducer, with focus on the cavitating flow-induced vibration characteristics. In the numerical simulation, the curvature correction turbulence model and the Zwart cavitation model is used for the simulation of the flow field. The loose coupled method is adopted for the prediction of the fluid structure interaction, including the calculation of the fluid forces based on the multiphase fluid dynamics, computation of the structural deformation via the governing equation of the structural motion, and then the update the fluid and structure mesh. The results showed that good agreement has been obtained between the experimental and numerical results. The reverse flow vortex cavitation develops and rotates with the blade, but with a much lower rotational speed than that of the blade. The vibration of the hydrocone is mainly affected by the blade rotation of the inducer, and the blade vibration is mainly affected by the reverse flow vortex cavitation

INFLUENCE OF MODE AND GEOMETRIC CHRATERISTICS ON HIGHT-FREQUENCY INDUCTIVE PLASMA TORCH WITH REVERSE VORTEX FLOW

The analysis of aerodynamic and heat structure of flow in high-frequency inductive plasma torch has been carried out. The range of plasma torch power is measured in dozens of kilowatts. The numerical simulation methods of the turbulent flow in the plasma torch affected by high frequency electromagnetic field without considering the chemical kinetics are used during the research. The data of temperature field and induced current density in the plasma torch depending on current amperage and frequency are obtained. Also, these data are obtained depending on the flow scheme in the operated on argon and air plasma torches. The inductive plasma torches can be applied to solve a wide range of tasks such as activation of coal-dust mixture with its further gasification, coating process for the stabilization of combustion processes as well as for the recycling processes at the mobile seaport recycling complexes. The calculations demonstrated convincingly the advantage of the operation of plasma torches with reverse vortex flow over plasma torches with “direct” vortex flow. Moreover the obtained data allow executing the assessment of thermal efficiency of inductive plasma jet and obtaining its optimal operational modes.