Bidirectional Vortex Research Articles

Parametric study of a vortex-enhanced supersonic inductive plasma torch

The feed gas injection configuration in radio-frequency (RF) inductively coupled plasma (ICP) torches plays a critical role in discharge stability, gas heating, and device thermal management: particularly if a supersonic nozzle is used to subsequently accelerate the hot gas. A novel injection configuration is the bidirectional vortex, which segments the internal ICP flow field into two counter-propagating vortices that can significantly enhance gas heating and reduce heat losses. The diameter of the interface between the vortices (known as the mantle) is expected to be an important dimensional parameter affecting torch operation, especially relative to the nozzle size. In this work, we investigate the effect of nozzle throat diameter on the behaviour and performance of a vortex-enhanced supersonic ICP torch. The system is operated at RF powers and argon mass flow rates between 200–1000 W and 0–400 mg s−1 respectively, and different nozzle diameters ranging from 1.5 to 4 mm are explored. Because of the high-temperature environment, and to prevent disruption of the vortex flow fields, non-invasive diagnostics are used to measure the gas temperature and plasma density, and to infer the torch thermal efficiency and achievable gas specific enthalpy change. The maximum temperature is between 8500–9500 K with the 1.5 mm nozzle giving the highest temperature for a given power and mass flow rate, while plasma densities vary between 1020–1021 m−3 depending on the operating conditions. The thermal efficiency increases from 29% for the 1.5 mm nozzle to just above 70% for the 4 mm nozzle with a similar maximum specific enthalpy of around 1.5 MJ kg−1. These results demonstrate the important coupling between torch properties, and how system optimization can lead to tailored performance of potential interest to several ground and space-based applications.

Bidirectional vortex beam and reflective polarization conversion terahertz metasurfaces

We design a terahertz metasurface with the functions of bidirectional vortex beams and reflection polarization conversion. The unit cell of the proposed metasurface consists of a vanadium dioxide metal mixed layer, a dielectric layer, a vanadium dioxide thin film, a dielectric layer, and a metal open ring. At room temperature, when a left-handed circularly polarized wave is incident, the metasurface generates a reflected mode vortex beam with topological charges of l=−1 and l=−2 at frequencies of 0.33 and 0.34 THz. Simultaneously, the metasurface produces transmitted mode vortex beams with topological charges of l=−1 and l=−2 at frequencies of 0.33 and 0.36 THz. When the temperature changes to 68°C and y-polarized waves are incident, the metasurface generates a reflected left-handed circularly polarized wave in the frequency bands of 0.6–0.65 THz and 1.75–1.93 THz, and a right-handed circularly polarized wave in the frequency band of 0.86–0.9 THz, achieving the function of converting linearly polarized waves into circularly polarized waves. This metasurface provides a new method for designing a single metasurface structure to achieve bidirectional multifunctional terahertz devices for transmission and reflection modes. In addition, the designed metasurface achieves a perfect polarization conversion function that can convert linearly polarized waves into circularly polarized waves.

Bidirectional multi-mode multiple vortex beams generator based on terahertz metasurface

Herein, we propose a vortex beam generator based on vanadium dioxide (VO2), which can switch between two operating modes (reflection and transmission) to generate bidirectional multi-mode multi-vortex beams across a wide frequency band range of 0.8 THz to 1.6 THz. By leveraging the transition of VO2 from the insulating state to the metallic state with temperature changes, the switching between transmission and reflection modes is achieved. Furthermore, through the application of the convolution and superposition theorem, the multiplexing of orbital angular momentum (OAM) is realized. Our work introduces a new method for multi-channel, multiplexed communication and imaging systems, and offers a novel approach to achieve powerful and flexible regulation of terahertz communication systems.

PURE AMMONIA COMBUSTION IN A BIDIRECTIONAL SWIRLING FLOW

The paper reports on the new pure ammonia combustion technology based on a bidirectional swirling flow formation. This technology allows for avoiding the application of such additional efforts as fuel preheating and blending, oxidizer modification, and plasma assistance normally required for ammonia combustion. Experiments showed that pure ammonia combustion in a bidirectional swirling flow is possible at both lean and rich operation modes. The lowest achieved value of the equivalence ratio is 0.503 compared to conventional and swirl combustors where it is equal to 0.8. This became possible due to the toroidal geometry of the bidirectional vortex chamber where convective fuel preheating occurs as a natural process and the flame front surface area is increased. The presented results allow consideration of bidirectional combustors for use in modern power plants where ammonia is applied as a hydrogen-containing fuel.

Effects of various geometric parameters on the computed swirl numbers and flow topology in a bidirectional vortex chamber

Effects of various geometric parameters on the computed swirl numbers and flow topology in a bidirectional vortex chamber

The effect of toroidal geometrical shape on turbulent oscillations in a bidirectional vortex combustor

The paper reports on detailed studies of a bidirectional swirling flow structure in a toroidal vortex chamber under non-reactive and combustion conditions at the Reynolds numbers Re = 65,000 and Re = 10,000. The explanation of reduced velocity fluctuations near the head wall compared to simple cylindrical bidirectional chambers is also given. The studies were carried out using transient numerical simulations DES (Detached Eddy Simulation). The necessary experimental validation based on PIV (Particle Image Velocimetry) and temperature measurements were performed as well. The results show that fluctuations of all velocity components in the toroidal part of the chamber do not exceed 20% in non-reactive conditions and 10% in the combustion case. Additionally, temperature oscillations are less than 10% of the adiabatic stoichiometric temperature of propane-air mixture combustion. The studies of combustion at different equivalence ratios show that the rich combustion ϕ = 1.5 provides the most favorable conditions in terms of mixture ignition. Moreover, a large-scale backflow structure is formed beyond the outlet nozzle at rich modes that leads to an increase in combustion stability. This feature can be usefully applied in studies of mesoscale bidirectional arrays and developing multiport combustion chambers.

Bidirectional vortex stabilization of a supersonic inductively coupled plasma torch

Radio-frequency (RF) inductively coupled plasma (ICP) torches using a supersonic nozzle have many industrial materials processing applications and have also been proposed as novel electrothermal plasma thrusters for space propulsion. The gas injection method in plasma torches plays an important role in both gas heating dynamics and overall discharge stabilization. Here, we investigate reverse vortex gas injection into a supersonic ICP torch for RF powers up to 1 kW, argon mass flow rates between 15 and 180 mg s−1, and plasma torch pressures from ∼270 Pa to ∼50 kPa. In this configuration, gas is injected tangentially just upstream of the nozzle inlet. This produces a bidirectional vortex flow field where gas first spirals upwards along the outer edge of the plasma torch walls, before then reversing direction at the torch end and spiralling back down through the central plasma region towards the nozzle exit. Results are compared to a more conventional forward vortex configuration where gas is instead injected tangentially from the upstream end of the torch, and which forms a unidirectional vortex that spirals towards the downstream nozzle. While performance is similar for gas flows below 80 mg s−1, we show that at higher mass flow rates both the effective torch stagnation temperature and thermal efficiency can be increased by almost 50% with reverse vortex injection. Considering that the measured RF antenna-plasma power transfer efficiency is similar for both configurations, this enhancement occurs because of the unique bidirectional vortex flow field which leads to reduced gas-wall heat losses and consequently an increased enthalpy flow leaving the torch.

An experimental study of a bidirectional vortex combustor with two-stage air supply

An experimental study of a bidirectional vortex combustor with two-stage air supply

An experimental study of syngas combustion in a bidirectional swirling flow

The paper reports on the results of an experimental study of methane and syngas combustion as well as their co-firing in a bidirectional swirling flow. The results confirmed that the bidirectional flow structure provides a significant decrease in the lean blow-off equivalence ratio as well as that of emissions of main pollutants. The combustion intensification becomes more evident when using syngas is as fuel. The composition of the used syngas is as follows (by volume): H2 - 29.42%; CO - 14.32%; CH4 - 3.8%; N2 - 49.11%; H2O - 3.35%. In this case, the lean blow-off is achieved at ϕ < 0.1, NOx emission is halved, while CxHy and CO emissions become 20 times less compared to pure methane combustion. However, according to experimental results, the co-combustion of syngas (volume fraction Vsyn = 15%) and methane is the most appropriate fuel utilization mode. It provides blow-off and emission properties similar to those for combustion of pure syngas, whereas energy consumption for its production is much lower. Moreover, unlike hydrocarbon fuel combustion, that of syngas in a bidirectional swirling flow is characterized by the presence of density stratification. This is accompanied by the flame formation at significantly different locations in the combustion chamber at lean and “ultra-lean” modes of operation. Hydrogen combustion most likely to occur in the core region at near-blow-off modes ϕ < 0.1, whereas normal ‘operating modes in the range 0.2 = ϕ ≤ 0.4 result in the formation of a conical flame surface where CH4 and CO combustion occurs. These new results with respect to the flame structure as well as blow-off and emission properties make it possible to consider bidirectional vortex combustors for application in modern gas turbine power plants in order to meet the strict environmental and energy requirements.

A Study of Walls Thermal Conditions for a Bidirectional Vortex Combustor

Bidirectional vortex combustors are among the most efficient devices in terms of pollutants emission, fuel residence time, an extended range of stable operation. Despite many papers devoted to flow structure in the bidirectional chamber, their heat transfer and wall temperature distributions are not well-studied experimentally as well as most available calculation results are based on adiabatic or isothermal wall conditions. The present paper reports on new experimental and numerical results on flow and flame structures and surface temperature distributions obtained for a combustor with a toroidal vortex chamber. These results show a strong effect of the combustor operation mode and geometry configuration on its thermal conditions. Based on experimental validation, a reliable numerical approach for fluid dynamics, combustion, and heat transfer simulations in the bidirectional vortex chamber is suggested. This approach could assist in determining the most efficient parameters of the bidirectional combustor in terms of energy and environmental indicators as well as wall thermal conditions and service life.

Effects of various inlet parameters on the computed flow development in a bidirectional vortex chamber

We vary the inflow properties in a finite-volume solver to investigate their effects on the computed cyclonic motion in a right-cylindrical vortex chamber. The latter comprises eight tangential injectors through which steady-state air is introduced under incompressible and inviscid conditions. To minimize cell skewness around injectors, a fine tetrahedral mesh is implemented first and then converted into polyhedral elements, namely, to improve convergence characteristics and precision. Once convergence is achieved, our principal variables are evaluated and compared using a range of inflow parameters. These include the tangential injector speed, count, diameter, and elevation. The resulting computations show that well-resolved numerical simulations can properly predict the forced vortex behavior that dominates in the core region as well as the free vortex tail that prevails radially outwardly, beyond the point of peak tangential speed. It is also shown that augmenting the mass influx by increasing the number of injectors, injector size, or average injection speed further amplifies the vortex strength and all peak velocities while shifting the mantle radially inwardly. Overall, the axial velocity is found to be the most sensitive to vertical displacements of the injection plane. By raising the injection plane to the top half portion of the chamber, the flow character is markedly altered, and an axially unidirectional vortex is engendered, particularly, with no upward motion or mantle formation. Conversely, the tangential and radial velocities are found to be axially independent and together with the pressure distribution prove to be the least sensitive to injection plane relocations.

On the generalized Beltramian motion of the bidirectional vortex in a right-cylindrical cyclone with a hollow core

In this work, an exact inviscid solution is developed for the incompressible Euler equations in the context of a bidirectional, cyclonic flowfield in a right-cylindrical chamber with a hollow core. The presence of a hollow core confines the flow domain to an annular swirling region that extends into a toroid in three-dimensional space. The procedure that we follow is based on the Bragg–Hawthorne framework and a judicious assortment of boundary conditions that correspond to a wall-bounded cyclonic motion with a cylindrical core. At the outset, a self-similar stream function is obtained directly from the Bragg–Hawthorne equation under the premises of steady, axisymmetric, and inviscid conditions. The resulting formulation enables us to describe the bidirectional evolution of the so-called inner and outer vortex motions, including their fundamental properties, such as the interfacial layer known as the mantle; it also unravels compact analytical expressions for the velocity, pressure, and vorticity fields, with particular attention being devoted to their peak values and spatial excursions that accompany successive expansions of the core radius. By way of confirmation, it is shown that removal of the hollow core restores the well-established solution for a fully flowing cylindrical cyclone. Immediate applications of cyclonic flows include liquid and hybrid rocket engines, swirl-driven combustion devices, as well as a multitude of heat exchangers, centrifuges, cyclone separators, and flow separation devices that offer distinct advantages over conventional, non-swirling systems.

On the generalized Beltramian motion of the bidirectional vortex in a conical cyclone

This work presents an exact solution of Euler's incompressible equations in the context of a bidirectional vortex evolving inside a conically shaped cyclonic chamber. The corresponding helical flowfield is modeled under inviscid conditions assuming constant angular momentum. By leveraging the axisymmetric nature of the problem, a steady-state solution of the generalized Beltramian type is obtained directly from first principles, namely, from the Bragg–Hawthorne equation in spherical coordinates. The resulting stream function representation enables us to fully describe the ensuing swirl-dominated motion including its fundamental flow characteristics. After identifying an isolated singularity that appears at a cone divergence half-angle of 63.43°, two piecewise formulations are provided that correspond to either fluid injection or extraction at the top section of the conical cyclone. In this process, analytical expressions are readily retrieved for the three velocity components, vorticity, and pressure. Other essential flow indicators, such as the theoretically preferred mantle orientation, the empirically favored locus of zero vertical velocity, the maximum polar and axial velocities, the crossflow velocity, and other such terms, are systematically deduced. Results are validated using limiting process verifications and comparisons to both numerical and experimental measurements. The subtle differences between the present model and a strictly Beltramian flowfield are also highlighted and discussed. The conically cyclonic configuration considered here is relevant to propulsive devices, such as vortex-fired liquid rocket engines with tapered walls; meteorological phenomena, such as tornadoes, dust devils, and fire whirls; and industrial contraptions, such as cyclonic flow separators, collectors, centrifuges, boilers, vacuum cleaners, cement grinders, and so on.

Separation efficiency theoretical model of swirl-vane separator based on bidirectional vortex

Swirl vane separator is a vital component to separate liquid water from gas to provide high quality steam to turbines in liquid water reactors. The gas rotating flow in the riser of a swirl vane separator is a bidirectional vortex. Its tangential velocity is similar with Rankine vortex. Its axial velocity is bidirectional. The gas bidirectional vortex is solved by constructing a Bragg-Hawthorne equation for the stream function. The gas flow velocity predicted by new method agrees quite well with CFD. The maximum velocity differences in axial direction are 6.2%. Based on this model, the droplet trajectory is obtained and a new separation efficiency model has been developed. The average difference between experiment results and the predicted separation efficiency is reduced from 14.7% to 3.1%. Moreover, parametric analysis using new model has been carried out to discuss the effect of inlet velocity, droplet diameter, swirl vane angle and hub diameter.

Transient numerical simulations of a cold-flow bidirectional vortex chamber

Bidirectional chambers are well-studied in terms of the flow structure and influence of their input parameters. However, most of the available studies are based on steady-state or time-averaged research methods and do not allow to obtain data on bidirectional flow dynamics over time. The present paper reports on detailed numerical studies based on detached eddy simulations (DESs) and unsteady Reynolds-averaged Navier–Stokes methods applied for two vortex chambers with different aspect ratios. A comparison of the numerical results with the available experimental data shows that the DES method gives the most accurate results on bidirectional flow structure, turbulent fluctuations, and precessing vortex core (PVC) motion. A notable feature of the studied bidirectional flow is the central recirculation zone (CRZ) formation, which is correctly predicted by the DES method only. The presence of a CRZ in a bidirectional flow has a significant effect on turbulent velocity fluctuations and PVC behavior. It is found that CRZ formation leads to a significant decrease in radial and circumferential velocity fluctuations whereas the axial velocity fluctuations are slightly increased. Additionally, the paper reports new findings on CRZ and PVC interaction in bidirectional flows. PVC motion is almost completely nullified by the presence of a CRZ. This can prove useful in many industrial applications of bidirectional chambers, e.g., vortex thrusters and gas turbine combustors. The bidirectional swirling flow transient properties studied in this paper could assist in determining the most efficient operational modes and geometric configuration of industrial chambers as well as enabling control of turbulent fluctuations, which would allow for reliable ignition and stable combustion.

Effects of nozzle inlet size and curvature on the flow development in a bidirectional vortex chamber

A finite-volume solver is used to describe the cyclonic motion in a cylindrical vortex chamber comprising eight tangential injectors and a variable nozzle size. The simulations are performed under steady, incompressible, and inviscid flow conditions with air as the working fluid. First, we apply a fine tetrahedral mesh to minimize cell skewness, particularly near injectors. Second, this mesh is converted into a polyhedral grid to improve convergence characteristics and precision. After achieving convergence, the velocity components are evaluated and compared to existing analytical solutions. We find that well-resolved numerical simulations can accurately predict the expected forced vortex behavior in the core region as well as the free vortex tail in the outer region. We also confirm that the swirl velocity remains axially invariant irrespective of the outlet radius. Similarly, we are able to ascertain that the axial and radial velocities embody the bidirectional nature of the motion. As for the computed pressure distribution, it is found to agree quite well with both theoretical formulations and experimental measurements of cyclone separators. Then using a parametric trade study, the effect of nozzle variations on the internal flow character, mantle structure, and recirculation zones is systematically investigated. Apart from the exit diameter, we find that the nozzle length and inlet curvature can substantially affect the internal flow development including the formation of backflow regions, recirculation zones, and mantle excursions. Finally, an empirical relation is constructed for the nozzle radius of curvature and shown to effectively suppress the emergence of recirculation and backflow regions.

A study of flame and flow structures and their effect on emission properties in a bidirectional vortex pulverized peat combustor

The paper reports on experimental and numerical studies of flame and flow structures in a bidirectional vortex pulverized peat combustor. This study seeks to develop a new method of burning solid pulverized fuel in a bidirectional swirling flow suggested in prior literature. To this end, reactive and non-reactive modes at different input parameters have been examined. Special attention was given to the effect of flame and flow on pollutant emissions, e.g., CO and NOx. It is numerically and experimentally shown that the bidirectional combustor facilitates environmentally friendly and stable combustion of solid pulverized fuel in a wide range of boundary conditions. However, in terms of flame and flow structures, the most effective operating modes have been observed at n ≥ 5.0. Moreover, a similarity of the reaction fronts and velocity profiles is observed within the same range of values of n. Variations in thermal power cause only quantitative changes in parameter distributions, whereas qualitative characteristics of these distributions remain unchanged.

Criterion analysis and experimental study of combustion mechanisms in a bidirectional swirling flow and their relationship with pollutants emission

SummaryThis paper reports on the theoretical and experimental studies of combustion in a bidirectional swirling flow to develop its criterion base. Based on considering known aero‐thermochemical similarity criteria, turbulent combustion numbers were calculated for the bidirectional vortex combustor for the first time. Theoretical assessment and experimental studies of the probability of the known combustion mechanisms' implementation in a limited bidirectional swirling flow, containing the secondary flows and separation phenomena, were performed. This gave us a possibility to provide the new correlation of combustion mechanisms with emission dependencies. A joint review of these data allowed us to establish that the greatest environmental efficiency is observed in the range of the air‐fuel equivalence ratio of 1.5 &lt; λ &lt; 1.9. At the same time, it was defined that stable combustion of liquid fuel in the bidirectional swirling flow is available up to the value λ = 22. Moreover, criterion equations for lean and rich limits of stable combustion were obtained.

Experimental investigation of burning of pulverized peat in a bidirectional vortex combustor

The paper reports on the results of experimental research of bidirectional vortex combustor operation at different modes. This combustor implements a new idea of burning solid pulverized fuel in a bidirectional swirling flow. The experimental results confirmed the thesis about a wide range of stable operation of the combustor, as well as its environmental friendliness: the values of exhaust emission of CO and NOx are significantly lower than those for the direct-flow combustor. Besides, such combustor can operate in two modes: as an igniter or stabilizer of boiler furnace, where it generates a stable high-enthalpy flame as well as an independent device with low emission values. A summary of research results defines that the most environmentally friendly operating mode is achieved at the net air-fuel equivalence ratio λΣ = 1.3 and the ratio of primary and secondary air mass flow rates n = 2.7.

A numerical simulation of burning of pulverized peat fuel in a bidirectional vortex combustor

Bidirectional (or countercurrent) vortex combustors are some of the most promising vortex-flow devices in terms of the fuel and oxidizer mixing efficiency, the combustion efficiency, the emission characteristics, and the combustion stability. This paper suggests a new arrangement principle for combustion of solid pulverized peat in conditions of vortex countercurrent flow which has never been studied earlier; we developed a combustion unit implementing this principle and carried out a numerical analysis of the combustion process with various operating parameters. The calculation results confirmed the possibility of combustion within a wide range of parameters and showed that varying the air-fuel equivalence ratio from λ = 0.5 to λ = 5.05 results in significant change in both the structure of the limited vortex flow and the distribution of total pressure and temperature, as well as the averaged volume concentration of fuel particles. We calculated the boundaries of lean and rich flame blowouts where the gas-dynamic flow loses its stability and the toroidal vortex is disrupted. At the same time, secondary vortex structures are formed throughout the combustor volume.