Reverse Vortex Research Articles

An experimental investigation of reverse vortex flow plasma reforming of methane

This paper focuses on the reforming of methane into hydrogen rich gas by means of gliding arc plasma stabilized in a reverse vortex flow. Parametric tests utilizing a 42 mm diameter reactor investigated the effects of electrode gap distance, reaction chamber exit diameter, steam input, methane input (fuel to oxygen ratio), and power input. Over the range of conditions tested, reactor performance was most sensitive to methane input. Decreasing the diameter of the reaction chamber exit impeded the performance of the reformer. A set of factorial tests determined the optimal operating conditions of the system to be at flow rates of 2 slpm nitrogen, 0.56 slpm oxygen, 1.25 slpm methane, an electrode gap distance of 34.5 mm, an outlet diameter of 12.65 mm, and a power input of 260 W. At these conditions the system yielded 83.3% hydrogen selectivity, 79.8% methane conversion and efficiency of 43.5%. Physical operating boundaries of the system defined by soot production and arc extinction were identified.

Hydrodynamic investigation of a self-propelled robotic fish based on a force-feedback control method

We implement a mackerel (Scomber scombrus) body-shaped robot, programmed to display the three most typical body/caudal fin undulatory kinematics (i.e. anguilliform, carangiform and thunniform), in order to biomimetically investigate hydrodynamic issues not easily tackled experimentally with live fish. The robotic mackerel, mounted on a servo towing system and initially at rest, can determine its self-propelled speed by measuring the external force acting upon it and allowing for the simultaneous measurement of power, flow field and self-propelled speed. Experimental results showed that the robotic swimmer with thunniform kinematics achieved a faster final swimming speed (St = 0.424) relative to those with carangiform (St = 0.43) and anguilliform kinematics (St = 0.55). The thrust efficiency, estimated from a digital particle image velocimetry (DPIV) flow field, showed that the robotic swimmer with thunniform kinematics is more efficient (47.3%) than those with carangiform (31.4%) and anguilliform kinematics (26.6%). Furthermore, the DPIV measurements illustrate that the large-scale characteristics of the flow pattern generated by the robotic swimmer with both anguilliform and carangiform kinematics were wedge-like, double-row wake structures. Additionally, a typical single-row reverse Karman vortex was produced by the robotic swimmer using thunniform kinematics. Finally, we discuss this novel force-feedback-controlled experimental method, and review the relative self-propelled hydrodynamic results of the robot when utilizing the three types of undulatory kinematics.

Simulation Study on Characteristics of the Vortex Structure in Human Mouth-throat Model in Cyclic Respiratory Pattern

The research on vortex structure and vortex evolution in human upper respiratory tract can deepen understanding of the characteristics of the airflow in human upper respiratory tract and plays a very important role in analyzing the diffusion, transition and deposition patterns of aerosol in human upper respiratory tract. Large eddy simulation was used to simulate the vortex structure and vortex movement in human mouth-throat model in cyclic respiratory pattern, and the vortex structure and vortex evolution in mouth-throat model was discussed. The results show that jet formations in pharynx and laryngeal lead to two vorticity growth regions at cyclic inhalation flow, flat vortex appeared in the throat, a curved vortex like the trachea wall appeared in the anterior wall of trachea, and nearly symmetric reverse vortex pairs appeared in the trachea; a vortex like “3” appeared in the posterior wall of the throat, two curved vortexes like eyebrow appeared in the posterior wall of the pharynx.

Mean airflow patterns upwind of topographic obstacles and their implications for the formation of echo dunes: A wind tunnel simulation of the effects of windward slope

[1] Secondary airflow plays an important role in dune formation and development. In the case of the echo dunes that form upwind of an obstacle, it introduces considerable complexity because both the initiation and development processes are influenced by airflow patterns. In this study, we measured the variations of airflow in front of obstacles with different windward (stoss) slope angles by means of particle-image velocimetry in a series of wind tunnel tests. The windward slope angle was the key factor in determining the secondary airflow patterns. The horizontal velocities decreased and the vertical velocities increased as the airflow approached perpendicular to the obstacle, and a vortex of reversed flow formed in front of obstacles with a stoss slope of 60° or steeper. The positions of airflow separation and of the core of the reversed vortex were a function of the windward slope angle. Depending on the position of the reverse vortex and its significance for the formation of echo dunes, the windward slope could be divided into three groups (65° or less, 70°–75°, and 80° or more). The horizontal velocity profiles deviated from a log linear distribution, resulting in five airflow regions with different rates of change of horizontal velocity. We discuss the significance of these velocity variations for sand accumulation and the effects of each airflow region on echo dune formation.

Aerodynamic and thermal characteristics of a maxwell type vortex tube

Abstract A three-dimensional (3D) computational fluid dynamic simulation of a vortex tube is carried out to examine its flow and thermal characteristics. The aim of this work is to model the performance of the vortex tube and to capture the highly swirling compressible flow behavior inside the tube for an understanding of the well known temperature separation process. Simulations were carried out using the standard k-ɛ, k-omega, RNG k-ɛ and swirl RNG k-ɛk-ɛ turbulence models. An experimental setup was built and tested to validate the simulation results. The RNG k-ɛ turbulence model yielded better agreement between the numerical predictions and experimental data. This model captured well the essential features of the flow including formation of the outer vortex and the inner reverse vortex flow. Flow and geometric parameters that affect the flow behavior and energy separation are studied numerically. Effects of the inlet pressure, with and without an insert in the tube, are examined by numerical experiments.

Offsprings of a point vortex

The distribution engendered by successive splitting of one point vortex are considered. The process of splitting a vortex in three using a reverse three-point vortex collapse course is analysed in great details and shown to be dissipative. A simple process of successive splitting is then defined and the resulting vorticity distribution and vortex populations are analysed.

Dissociation of CO2 in a low current gliding arc plasmatron

The process of CO2 dissociation was studied in a non-equilibrium gliding arc plasmatron (GAP). The GAP was designed not only for efficient reforming but also to ensure significant variability of reactor parameters. The effect of vortex flow configuration on efficiency was also studied in the reactor by comparing forward vortex flow and reverse vortex flow. The maximum thermodynamic efficiency of the dissociation process was determined to be approximately 43%. The high level of efficiency may be attributed to non-equilibrium vibrational excitation of CO2 and a high-temperature gradient between gliding arc and the surrounding gas that results in fast quenching.

Asymmetrical reverse vortex flow due to induced-charge electro-osmosis around carbon stacking structures

Broken symmetry of vortices due to induced-charge electro-osmosis (ICEO) around stacking structures is important for the generation of a large net flow in a microchannel. Following theoretical predictions in our previous study, we herein report experimental observations of asymmetrical reverse vortex flows around stacking structures of carbon posts with a large height (~110 μm) in water, prepared by the pyrolysis of a photoresist film in a reducing gas. Further, by the use of a coupled calculation method that considers boundary effects precisely, the experimental results, except for the problem of anomalous flow reversal, are successfully explained. That is, unlike previous predictions, the precise calculations here show that stacking structures accelerate a reverse flow rather than suppressing it for a microfluidic channel because of the deformation of electric fields near the stacking portions; these structures can also generate a large net flow theoretically in the direction opposite that of a previous prediction for a standard vortex flow. Furthermore, by solving the one-dimensional Poisson-Nernst-Plank (PNP) equations in the presence of ac electric fields, we find that the anomalous flow reversal occurs by the phase retardation between the induced diffuse charge and the tangential electric field. In addition, we successfully explain the nonlinearity of the flow velocity on the applied voltage by the PNP analysis. In the future, we expect to improve the pumping performance significantly by using stacking structures of conductive posts along with a low-cost process.

Reverse flow and vortex breakdown in a shear-thinning fluid

The effect of polymer concentration on the development of reverse secondary flow and vortex breakdown was studied using a viscoelastic solution of polyacrlylamide in water. The fluid was contained in cylindrical containers of two different radii, the top end wall of which rotated at a varying speed, thus, imparting a circulating motion to the fluid. Whereas using a newtonian fluid, streamlines will occupy the entire container, the flow of a shear-thinning fluid may divide into two cells of opposite circulating motion. The curve of critical Reynolds and elasticity numbers (Re, E) values corresponding to the development of reverse flow was obtained over a wide range of Re values. Vortex breakdown was found to occur at extremely low Re values.

Basic Characteristics of Gliding-Arc Discharges in Air and Natural Gas

Gliding-arc discharges have been utilized in plasma-assisted combustion processes, among various other applications, due to their properties of high electron density and chemical selectivity in a transitional regime. However, basic characteristics relative to the relations between the fundamental parameters of discharge, like mass flow rate, breakdown voltage, and frequency of repetition (number of discharge breakdowns per half cycle), have not been completely studied. In this paper, an ac-powered gliding-arc discharge having a reverse vortex flow configuration is built to carry on a basic investigation on discharges in air, natural gas, and mixture of both. Electrical measurements, optical emission spectroscopy, and mass spectrometry are the techniques used for these investigations. The results presented in this paper describe the dependence of the breakdown voltage, frequency of discharges, and conversion rates of methane and molecular oxygen with respect to the variation of the mass flow rate (directly related to the residence time) and discharge current.

Using hyperbolic Lagrangian coherent structures to investigate vortices in bioinspired fluid flows

We use direct Lyapunov exponents to identify Lagrangian coherent structures (LCSs) in a bioinspired fluid flow: the wakes of rigid pitching panels with a trapezoidal planform geometry chosen to model idealized fish caudal fins. When compared with commonly used Eulerian criteria, the Lagrangian method has previously exhibited the ability to define structure boundaries without relying on a preselected threshold. In addition, qualitative changes in the LCS have previously been shown to correspond to physical changes in the vortex structure. For this paper, digital particle image velocimetry experiments were performed to obtain the time-resolved velocity fields for Strouhal numbers of 0.17 and 0.27. A classic reverse von Karman vortex street pattern was observed along the midspan of the near wake at low Strouhal number, but at higher Strouhal number the complexity of the wake increased downstream of the trailing edge. The spanwise vortices spread transversely across the wake and lose coherence, and this event was shown to correspond to a qualitative change in the LCS at the same time and location.

Experimental and numerical study on premixed hydrogen/air flame propagation in a horizontal rectangular closed duct

Hydrogen is a promising energy in the future, and it is desirable to characterize the combustion behavior of its blends with air. The premixed hydrogen/air flame microstructure and propagation in a horizontal rectangular closed duct were recorded using high-speed video and Schlieren device. Numerical simulation was also performed on Fluent CFD code to compare with the experimental result. A tulip flame is formed during the flame propagating, and then the tulip flame formation mechanism was proposed based on the analysis. The induced reverse flow and vortex motion were observed both in experiment and simulation. The interactions among the flame, reverse flow and vortices in the burned gas change the flame shape and ultimately it develops into a tulip flame. During the formation of the tulip flame, the tulip cusp slows down and stops moving after its slightly forward moving, and then, it starts to move backward and keeps on a longer time, after that, it moves forward again. The structure of the tulip flame is becoming less stable with its length decreasing in flame propagation direction. The flame thickness increases gradually which is due to turbulence combustion.

Thermal and power characteristics of plasma torch with reverse vortex

The results of experimental investigations of electrical and thermal characteristics of a vortex plasma torch with a reverse vortex, generated in a hollow blind-end electrode, are presented. It is shown that the reverse vortex essentially improves the performance of the plasma torch and contributes to an increase in the thermal efficiency and enthalpy of the plasma jet.

Transitions in the wake of a flapping foil

We study experimentally the vortex streets produced by a flapping foil in a hydrodynamic tunnel, using two-dimensional particle image velocimetry. An analysis in terms of a flapping frequency-amplitude phase space allows the identification of (i) the transition from the well-known Bénard-von Kármán (BvK) wake to the reverse BvK vortex street that characterizes propulsive wakes, and (ii) the symmetry breaking of this reverse BvK pattern giving rise to an asymmetric wake. We also show that the transition from a BvK wake to a reverse BvK wake precedes the actual drag-thrust transition and we discuss the significance of the present results in the analysis of flapping systems in nature.

Modeling of the Coal Gasification Processes in a Hybrid Plasma Torch

The major advantages of plasma treatment systems are cost effectiveness and technical efficiency. A new efficient electrodeless 1-MW hybrid plasma torch for waste disposal and coal gasification is proposed. This product merges several solutions such as the known inductive-type plasma torch, innovative reverse-vortex (RV) reactor and the recently developed nonequilibrium plasma pilot and plasma chemical reactor. With the use of the computational-fluid-dynamics-computational method, preliminary 3-D calculations of heat exchange in a 1-MW plasma generator operating with direct vortex and RV have been conducted at the air flow rate of 100 g/s. For the investigated mode and designed parameters, reduction of the total wall heat transfer for the reverse scheme is about 65 kW, which corresponds to an increase of the plasma generator efficiency by approximately 6.5%. This new hybrid plasma torch operates as a multimode, high power plasma system with a wide range of plasma feedstock gases and turn down ratio, and offers convenient and simultaneous feeding of several additional reagents into the discharge zone.

Swirling Flow of a Viscoelastic Fluid With Free Surface—Part I: Experimental Analysis of Vortex Motion by PIV

The swirling flows of water and CTAC (cetyltrimethyl ammonium chloride) surfactant solutions (50-1000ppm) in an open cylindrical container with a rotating disc at the bottom were experimentally investigated by use of a double-pulsed PIV (particle image velocimetry) system. The flow pattern in the meridional plane for water at the present high Reynolds number of 4.3×104 differed greatly from that at low Reynolds numbers, and an inertia-driven vortex was pushed to the corner between the free surface and the cylindrical wall by a counter-rotating vortex caused by vortex breakdown. For the 1000ppm surfactant solution flow, the inertia-driven vortex located at the corner between the bottom and the cylindrical wall whereas an elasticity-driven reverse vortex governed the majority of the flow field. The rotation of the fluid caused a deformation of the free surface with a dip at the center. The dip was largest for the water case and decreased with increasing surfactant concentration. The value of the dip was related to determining the solution viscoelasticity for the onset of drag reduction.

Wave-turbulence interaction of a low-speed plane liquid wall-jet investigated by particle image velocimetry

The surface-wave amplitude (free-surface level) and the turbulent velocity field of the liquid phase of a plane wall-jet flow have been simultaneously measured by means of particle image velocimetry, which allows for the investigation of surface waves and wave-turbulence interaction. The Reynolds number, Weber number, and Ohnesorge number of the tested flow, based on the bulk velocity, height of the closed channel, and physical properties of water, were 3.6×104, 1.2×103, and 9.5×10−4, respectively. Based on the measured datasets of the velocity field and free-surface level, the characteristics of the wave-turbulence interaction as well as the statistics of surface waves and turbulent velocity field were studied. The characteristics of the turbulent velocity field near the wavy free surface obtained in this study were different from those obtained previously for open-channel flows with no shear at the interface or negligible deformation of the free surface. It was found that reverse vortex motion was predominant beneath the wave crest to a depth of about 2mm under the tested flow condition at the near field of the wall jet, which was conjectured to be the main feature before the onset of breakup of the free surface.

The dynamic characteristics and migration of a pyramid dune

AbstractThe results of wind tunnel experiments and field observations show that when the intersection angle between airflow direction and dune crest (ridge) line is > 30°, a reverse vortex is formed. Because of the convergence of sand streams from the windward and lee slopes at the crest, sand accumulates in the crestal region, causing vertical growth. Nevertheless, studies also show that the common asymmetry of the two slopes of a dune may significantly influence the evolution of arms of a pyramid dune. The migration rates of pyramid dunes are mediated by the interplay of their arms moving transversely and the vertical growth in response to the variations in wind regimes. Comparing the effects of airflow transverse to a given arm with longitudinal airflow, it is indicated that the transverse airflow is more significant in controlling the arms of pyramid dunes. The whole body of the studied pyramid dune, particularly the upper quarter section, migrated SE direction during the monitoring period. The patterns of wind erosion and deposition change alternately with seasonal variations in wind directions. The W, NE and SE sides undergo constant erosion, deposition and both erosion and deposition, respectively. The results of long‐term monitoring of a pyramid dune show that southerly winds, resulting from a local circulation, markedly affect the transverse migration of the whole pyramid dune.

Gliding arc discharges as a source of intermediate plasma for methane partial oxidation

Experimental study of intermediate plasma catalyzed partial oxidation of methane is completed and results are presented along with numerical simulations. To establish the best plasma parameters for industrial chemical applications like partial oxidation, the stability mechanism of different plasma sources is reviewed. Various kinds of intermediate plasma sources are discussed for their application to plasma-chemical processes. Diagnostics of gliding arc discharge: electron temperature, gas temperature, current-voltage characteristics, etc. is done. Different configurations of gliding arc such as the gliding arc in and the plasma disc to enhance intermediate plasma properties are presented along with their advantages. Reverse vortex or tornado stabilization is used for methane partial oxidation catalysis. For applications where gas velocity is low or gas is stationary, plasma disc can be employed. Analysis of experimental results and comparison with nonplasma system is done to establish clear advantage of intermediate plasma catalysis for the partial oxidation process.

Gliding arc in tornado using a reverse vortex flow

The present article reports a new gliding arc (GA) system using a reverse vortex flow (“tornado”) in a cylindrical reactor (gliding arc in tornado, or GAT), as used to preserve the main advantages of traditional GA systems and overcome their main drawbacks. The primary advantages of traditional GA systems retained in the present GAT are the possibility to generate transitional plasma and to avoid considerable electrode erosion. In contrast to a traditional GA, the new GAT system ensures much more uniform gas treatment and has a significantly larger gas residence time in the reactor. The present article also describes the design of the new reactor and its stable operation regime when the variation of GAT current is very small. These features are understood to be very important for most viable applications. Additionally the GAT provides near-perfect thermal insulation from the reactor wall, indicating that the present GAT does not require the reactor wall to be constructed of high-temperature materials. The new GAT system, with its unique properties such as a high level of nonequilibrium and a large residence time, looks very promising for many industrial applications including fuel conversion, carbon dioxide conversion to carbon monoxide and oxygen, surface treatment, waste treatment, flame stabilization, hydrogen sulfide treatment, etc.