Reverse Vortex Flow Research Articles

Evolutions of Pressure Fluctuations and Runner Loads During Runaway Processes of a Pump-Turbine

The pressure fluctuations and runner loads on a pump-turbine runner during runaway process are very violent and the corresponding flow evolution is complicated. To study these phenomena and their correlations in depth, the runaway processes of a model pump-turbine at four guide vane openings (GVOs) were simulated by three-dimensional computational fluid dynamics (3D-CFD). The results show that the flow structures around runner inlet have regular development and transition patterns—the reverse flow occurs when the trajectory moves to the turbine-brake region and the main reverse velocity shifts locations among the hub side, the shroud side and the midspan as the trajectory comes forward and backward in the S-shape region. The locally distributed reverse flow vortex structures (RFVS) enhance the local rotor–stator interaction (RSI) and make the pressure fluctuations in vaneless space at the corresponding section stronger than at the rest sections along the spanwise direction. The transitions of RFVS, turning from the hub side to midspan, facilitate the inception and development of rotating stall, which propagates at approximately 45–72% of the runner rotation frequency. The evolving rotating stall induces asymmetrical pressure distribution on the runner blade, resulting in intensive fluctuations of runner torque and radial force. During the runaway process, the changing characteristics of the reactive axial force are dominated by the change rate of flow discharge, and the amplitude of low frequency component of axial force is in proportion to the amplitude of discharge change rate.

Plazmovo-ridynna systema zi zvorotnym vykhrovym potokom dlya plazmovo-katalitychnoho reformuvannya

A plasma-liquid system with the reverse vortex flow and a liquid electrode, which was designed for the plasma-catalytic reforming of hydrocarbons, has been studied. Discharge operation modes with the solid and liquid cathodes are compared, including the discharge voltage dependences on the distance between the upper flange and the liquid surface. The influence of the water content in a plasma-forming gas on the average energy of plasma electrons is analyzed.

CFD Analysis of the Runaway Stability of a Model Pump-Turbine

The relations between the runaway stability characteristics and the flow patterns inside the runner of pump-turbine are supposed to be close and should be studied. The runaway processes of a model pump-turbine at four guide-vane openings (GVOs) were simulated by the three-dimensional computational fluid dynamics. The results show that the runaway stability characteristics for the pump-turbine are different at different GVOs. For the small GVOs, the turbine characteristic trajectory undergoes damped oscillations; however, for large GVOs, the turbine characteristic trajectory settles into an un-damping oscillation. The evolution features of the reverse flow vortex structures (RFVS) at the runner inlet during the runaway oscillations have distinct patterns between the small and large GVOs. For small GVOs, the RFVSs only locate at the mid-span; however, for the large GVOs, the location of the RFVSs switches back and forth between the mid-span section and the hub side when the turbine passes in and out the turbine braking mode. The changes of RFVS at the runner inlet dominate the energy transfer among the hydraulic, mechanical and dissipation energies during the transient processes, and therefore affect the stability of hydraulic system.

Quasi‐Neutral Modeling of Gliding Arc Plasmas

The modelling of a gliding arc discharge (GAD) is studied by means of the quasineutral (QN) plasma modelling approach. The model is first evaluated for reliability and proper description of a gliding arc discharge at atmospheric pressure, by comparing with a more elaborate non‐quasineutral (NQN) plasma model in two different geometries – a 2D axisymmetric and a Cartesian geometry. The NQN model is considered as a reference, since it provides a continuous self‐consistent plasma description, including the near electrode regions. In general, the results of the QN model agree very well with those obtained from the NQN model. The small differences between both models are attributed to the approximations in the derivation of the QN model. The use of the QN model provides a substantial reduction of the computation time compared to the NQN model, which is crucial for the development of more complex models in three dimensions or with complicated chemistries. The latter is illustrated for (i) a reverse vortex flow (RVF) GAD in argon, and (ii) a GAD in CO2. The RVF discharge is modelled in three dimensions and the effect of the turbulent heat transport on the plasma and gas characteristics is discussed. The GAD model in CO2 is in a 1D geometry with axial symmetry and provides results for the time evolution of the electron, gas and vibrational temperature of CO2, as well as for the molar fractions of the different species.

Plasma‐driven dissociation of CO2 for fuel synthesis

Power‐to‐gas is a storage technology aiming to convert surplus electricity from renewable energy sources like wind and solar power into gaseous fuels compatible with the current network infrastructure. Results of CO2 dissociation in a vortex‐stabilized microwave plasma reactor are presented. The microwave field, residence time, quenching, and vortex configuration were varied to investigate their influence on energy‐ and conversion efficiency of CO2 dissociation. Significant deterioration of the energy efficiency is observed at forward vortex plasmas upon increasing pressure in the range of 100 mbar towards atmospheric pressure, which is mitigated by using a reverse vortex flow configuration of the plasma reactor. Data from optical emission shows that under all conditions covered by the experiments the gas temperature is in excess of 4000 K, suggesting a predominant thermal dissociation. Different strategies are proposed to enhance energy and conversion efficiencies of plasma‐driven dissociation of CO2.

Non-thermal gliding-arc plasma reforming of dodecane and hydroprocessed renewable diesel

This paper focuses on reforming dodecane and hydroprocessed renewable diesel to hydrogen rich gas in a non-thermal gliding-arc plasma stabilized in a reverse vortex flow reformer. The liquid fuels were directly injected into the reaction chamber using an ultrasonic nozzle and entrained in the reverse vortex flow before passing through the plasma. Initial parametric tests were used to investigate the individual effects of varying power input, steam to carbon ratio, and equivalence ratio on reformer performance. Subsequent factorial tests varied these parameters to identify optimal specific energy requirements. Optimal reforming conditions for dodecane, a model diesel compound, resulted in specific energy requirements of 134.1 ± 1.1 kJ mol−1 H2 produced, a H2 yield of 65.0 ± 0.02%, and an efficiency of 37.0 ± 0.02%. Optimal conditions for hydroprocessed renewable diesel resulted in a specific energy requirement of 176.1 ± 3.8 kJ mol−1 H2 produced, a H2 yield of 64.2 ± 1.7%, and an efficiency of 35.0 ± 1.0% at 95% confidence intervals. Physical operating boundaries due to arc extinction were identified.

Similarity Relations of Power–Voltage Characteristics for Tornado Gliding Arc in Plasma-Assisted Combustion Processes

Gliding arc discharges have been utilized in plasma-assisted combustion processes, among various other applications, due to their chemical properties. In this paper, an ac-powered gliding arc discharge having a reverse vortex flow configuration (tornado) was experimentally studied in air and in air–natural gas mixtures. A new method is proposed for the generalization of power characteristics of this type of discharge, based on similarity theory. The application of this method is demonstrated to be efficient for gliding arc discharges with tornado effect, using dimensional numbers. Regression dependences for discharges in air and in mixtures of air and natural gas were obtained in a form of simple power function equations (using the concept of equivalence ratio), which can be applied for the design of different gliding arc equipments for plasma-assisted combustion and related technologies.

Analysis of influence of mode and geometric char-acteristics on processes of high-frequency inductive plasma torch with reverse vortex flow

Analysis of influence of mode and geometric char-acteristics on processes of high-frequency inductive plasma torch with reverse vortex flow

Volatile Organic Compound Elimination Improved on Inlet Position of Vortex Gas in Microwave Torch Plasma

A simulated experiment on purification of volatile organic compounds (VOCs) was carried out in two vortex flow reactors, which consisted of a microwave plasma torch and a reaction chamber with a fuel injector. One is the reactor with a conventional vortex flow, and the other is with a reverse vortex flow. Injection of hydrocarbon fuels into a high-temperature microwave torch plasma provides a plasma flame. The plasma flame can eliminate the odorous chemical agent diluted in air or purify the contaminated air of a large volume from continuous emission. The reverse vortex reactor eliminated VOCs diluted in an airflow rate of 10000 lpm, showing β-value of 80.39 J/L, while the conventional vortex reactor eliminated VOCs with β-value of 100.26 J/L.

Optimization geometries of a vortex gliding-arc reactor for partial oxidation of methane

The effects of the geometry of gliding-arc reactor – such as distance between the electrodes, outlet diameter, and inlet position – on the reactor characteristics (methane conversion, hydrogen yield, and energy efficiency) have not been fully investigated. In this paper, AC gliding-arc reactors including the vortex flow configuration are designed to produce hydrogen from the methane by partial oxidation. The influence of vortex flow configuration on the reactor characteristics is also studied by varying the inlet position. When the inlet of the gliding-arc reactor is positioned close to the outlet, reverse vortex flow reactor (RVFR), the maximum energy efficiency reaches 50% and the yields of hydrogen and carbon monoxide are 40% and 65%, respectively. As the distance between electrodes increases from 5 mm to 15 mm, both hydrogen yield and energy efficiency increase approximately 10% for the RVFR. The energy efficiency and hydrogen yield are highest when the ratio of the outlet diameter to the inner diameter is 0.5 for the RVFR. Experimental results indicate that the flow field in the plasma reactor has an important influence on the reactor performance. Furthermore, hydrogen production increases as the number of feed gas flows in contact with the plasma zone increases.

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.

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.

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.

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.

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.

A new vortex method of plasma insulation and explanation of the Ranque effect

The efficiency of thermal insulation of a microwave-generated plasma using reverse vortex flow was investigated experimentally and by numerical simulations. Comparison with the conventional vortex method of plasma insulation was made. Changing the location of the vortex inlet to the exit end of the plasma torch leads to a significant decrease of the heat loss to the wall; from 30% to 5%. This result contradicts the traditional explanation of the Ranque effect. A new simple explanation of the Ranque effect of energy separation in the vortex tube is proposed. Energy separation takes place due to radial motion of turbulent micro-volumes with differing tangential velocities in the strong centrifugal field. The new model of energy separation explains such apparently mysterious phenomena as the counter-rotation of the central vortex flow layers observed in some experiments and in numerical simulations. A new approach for consideration of the confined vortex flows is described.

Physical mechanisms for transfer of heat, mass, and momentum in a short low-temperature heat pipe. I. Hydrodynamics of vapor flows

The authors present results of an experimental investigation of vapor flow hydrodynamics under positive pressure gradient conditions leading to boundary layer separation and the occurrence of a reverse vortex flow region of vapor in the condensation zone of a planar heat pipe. The influence of a noncondensible gas on heat and mass transfer and the configuration of the vapor gas front is examined.

Some Experiments on the Reattachment of a Laminar Boundary Layer Separating From a Rearward Facing Step on a Flat Plate Aerofoil

Summary:The results of experiments on the reattachment of a laminar boundary layer, separating from a rearward facing step in a flat plate aerofoil, are correlated with the properties of the short leading edge bubble which forms on thin aerofoils near the stall.The experiments, comprising pressure measurements, Pitot explorations, liquid film and smoke studies, indicate that for all Reynolds numbers above the value given by the Owen-KIanfer criterion the reattachment is turbulent behind a stationary air reverse flow vortex bubble. It is also found that the reattachment is laminar for Reynolds numbers below the critical, which further supports Crabtree's interpretation of the Owen-KIanfer criterion in terms of the condition for the growth of turbulent bursts.