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Magnetic field control of combustion dynamics of the swirling flame flow
Abstract
Magnetic field effect on combustion dynamics of the swirling flame flow is studied experimentally with the aim to obtain a cleaner and more effective combustion of a renewable fuel (wood pellets), providing a joint research of magnetic field effects on the swirling flame velocity field formation, processes of heat/mass transfer and combustion of volatiles at different stages of wood fuel burnout. The magnetic field effect on the swirling flame flow formation and combustion of volatiles is also studied experimentally using a pilotscale experimental device consisting of a wood fuel gasifier, water-cooled sections of the combustor with diagnostic sections between them for measuring the swirling flame velocity, temperature and composition field formation as well as the field effect on the heat production rate. The results show that the magnetic field effect on combustion dynamics must be related to the field-induced mass transfer of paramagnetic flame species (oxygen, nitrogen oxide) in the field direction, depending on magnetic force acting on the swirling flow field. Because of the magnetic field-induced magnetic force action, local variations of the flame velocity, temperature and composition fields are detected and discussed. The results show that the magnetic field effect on the swirling flame flow can be used as a tool to ensure a more effective burnout of volatiles and a cleaner heat energy production.
... Therefore, although the parameter of flame height can be measured directly, the volume of the flame based on the integrated flame surface is an alternative indicator for studying magnet-flame interaction. The reason is that the force induced by the magnetic field to both ambient oxygen and the oxygen diffused into the flame is a volumetric force in nature [31,32]. So, it can be suggested to derive the parameter of volume by integration for analysis of magnet-flame interaction. ...
... Compared with the decrease rate of flame height, the volume of the flame bas on the integrated flame surface has a faster rate of decline, which means that the param ter of volume is more sensitive to the magnetic field gradient. Therefore, although t The reason is that the force induced by the magnetic field to both ambient oxygen and the oxygen diffused into the flame is a volumetric force in nature [31,32]. So, it can be suggested to derive the parameter of volume by integration for analysis of magnet-flame interaction. ...
The combustion characteristics of laminar biogas premixed and diffusion flames in the presence of upward decreasing magnetic fields have been investigated in this study. The mechanism of magnet–flame interaction in the literature, in which magnetic fields change the behaviors of laminar flames due to the paramagnetic and diamagnetic properties of the constituent gases, is examined and the results are as follows. The magnetic field has no noticeable effect on premixed flames due to low oxygen concentration of the mixed gas at the injection and the relatively high flow momentum. However, due to the diffusion nature of diffusion flames and paramagnetic property of oxygen in ambient air, oxygen distributions are subjected to the gradient of magnetic flux, thus shortening the height of diffusion flames. Results also show that the flame volume is more strongly varied than flame height. Altered oxygen distributions result in improved combustion and higher flame temperature. In the case of current magnet–flame interaction, the magnetic driving force is combined with gravitational force, and a modified gravity g* as well as gravity modification factor G are derived to characterize the paramagnetism theory of oxygen.
... Therefore, there is a need for addition tools to improve the combustion conditions in the downstream flow. A preliminary experimental study has shown that the swirling flow structure and the combustion characteristics can be improved using the gradient magnetic field-enhanced upstream mass transfer of paramagnetic oxygen to the biomass layer, which is responsible for the field-enhanced upstream airflow formation (magnetic wind effect) with field-enhanced mixing of the flame components [11]. A similar gradient magnetic field effect on the combustion conditions and flame formation is observed for laminar diffusion flames, which allows concluding that the upward increasing magnetic field-enhanced mass transfer of paramagnetic oxygen can be used to control the combustion conditions determining faster and more complete fuel combustion with direct influence on the flame shape and size [12][13][14][15][16]. ...
... With the given magnetic field configuration, the mean magnetic field gradient increases towards the channel walls, where it reaches dB/dz ≈ 1.6-1.8 T/m [11] determining the field-enhanced upstream mass transfer of paramagnetic oxygen along the channel walls. To estimate the main gradient magnetic field effects, which determine the formation of combustion dynamics and swirling flame structure, the experimental study includes complex online measurements of the axial and tangential flow velocities, local measurements of the flame temperature, composition (O2, CO2, CO, H2, NOx) and combustion efficiency. ...
An experimental was conducted with the aim to provide magnetic field control of the gasification / combustion characteristics and heat energy production at thermo-chemical conversion of biomass (wood) pellets. The results show that the gradient magnetic field - enchanced mass transfer of paramagnetic oxygen can be used as a tool to provide control of biomass gasification/combustion processes and heat production.
... One of the options to control the process of the combustion characteristics is the application of external forces (electrical, magnetic) to the flame reaction zone. The previous experimental study has shown [4] and [5] that application of the external forces allows control of the flame formation of the flame velocity, temperature and composition profiles, determining the flame shape and structure, but interpretation of the mechanism of the field effects on the flame dynamics requires further detailed analysis. Over the past twenty years, researchers from Japan have found that the magnetic field can cause significant changes in diffusion flame behaviour, such as changes in emissions from the flames, changes in flame shape and size and the extinction points [6], [7] and [8]. ...
... Over the past twenty years, researchers from Japan have found that the magnetic field can cause significant changes in diffusion flame behaviour, such as changes in emissions from the flames, changes in flame shape and size and the extinction points [6], [7] and [8]. The complex experimental research of the magnetic field effects on the swirling flame flow dynamics during biomass combustion that is carried out in the Institute of Physics, University of Latvia [4] and [5] indicate that application of the gradient magnetic field to the bottom part of the flame reaction zone can be used to provide the enhanced mixing of the flame species with a radial expansion of the inner recirculation zone, increasing combustion efficiency and burnout of the volatiles. The motivation of the present study is to provide a detailed study of the mechanism of the gradient magnetic field on the swirling flame dynamics by varying the magnetic field induction (B) and the external force (F) that is applied at the bottom part of the combustor. ...
The focus of the recent experimental research is to provide control of
the combustion dynamics and complex measurements (flame temperature,
heat production rate, and composition of polluting emissions) for
pelletized wood biomass using a non-uniform magnetic field that produces
magnetic force interacting with magnetic moment of paramagnetic oxygen.
The experimental results have shown that a gradient magnetic field
provides enhanced mixing of the flame compounds by increasing combustion
efficiency and enhancing the burnout of volatiles.
... As the velocity of a chemical reaction is the change of molar concentration of one of the reacting substance i.e. oxygen in diffusion flame, the flow speed is decreases and flame become an expanded flow with increased dimension, because of the change in the chemical reaction velocity as well as the combustion velocity arises. The effect of gradient magnetic field on the local variations of the flame temperature depends on mole fraction of oxygen and magnetic susceptibility of paramagnetic oxygen [35]. According to the Curie law, the magnetic susceptibility of paramagnetic oxygen is inversely proportional to its temperature. ...
... The results of previous investigations (Zake et al. 2010;Suzdalenko et al. 2011) confirm that effects of interaction between flame and inhomogeneous magnetic field can be used in order to achieve additional impact on the combustion process. The effect is based on transfer of paramagnetic oxygen towards the gradient caused by the magnetic field gradient. ...
The aim of the recent research is to provide stable, controllable and effective wood pellets combustion with minimum emissions. Two possibilities were chosen, investigated and analysed: wood pellets co-firing with propane-butane mixture and the use of a permanent magnet. The special pilot device was constructed in the laboratory of Heat and Mass Transfer in Institute of Physics. Two types of experiments were conducted: combustion with propane-butane supply (0.9 kW up to 1.27 kW) of wood pellets with different moisture content (W = 8%, 15%, 20% and 25%); combustion of wood pellets with applied magnetic field by using the permanent magnet, an propane-butane supply also was used. The main conclusion of the research is that co-firing and magnetic field can be used as an instrument to provide more effective burnout of volatiles and cleaner heat production.
First published online: 19 Mar 2014
A kinetic study on the combustion of a renewable energy resource - pelletized biomass - is carried out with the aim to obtain clean and effective energy production by minimizing the impact of the heat production process on the environment. To control the processes of biomass gasification, combustion of volatiles and heat energy production, a gradient magnetic field is applied to the flame base providing experimental study of the gradient magnetic field effect on the processes developing downstream the combustor. The joint investigation of the gradient magnetic field effect on the biomass combustion and heat energy production includes the estimation of the magnetic field effect on the formation of flame velocity and composition profiles, temperature of the flame reaction zone and heat production rate providing analysis of the magnetic field effects applicability to control the pelletized biomass combustion and heat energy production. The experimental results have shown that the gradient magnetic field enhances the mixing of the flame compounds, so increasing the combustion efficiency and completing the burnout of volatiles.
Influence of magnetic fields on the properties of methane laminar combustion was investigated experimentally. A high downward gradient of the square of the magnetic flux density was applied on methane laminar diffusion flame. The flame surface temperature and NOX concentration were measured by high-precision testing instruments. The flame size was analyzed by specific image raster analysis software. The results show that with the magnetic field intensity increasing, the flame dimensionless length decreases while the dimensionless width and flame temperatures increase. The gradient magnetic field exerts a great effect on the production of thermal NOX in the flame region under 35 mm due to the oxygen paramagnetic force. Compared to none gradient magnetic field, the concentration of NOX at least decreases by up to 60% on average.
This paper reviews the studies undertaken on vortex breakdown over the past 45 years. The paper is structured such that the area is considered in three sections — experimental, numerical and finally theoretical, and provides a ‘guide’ to the literature and where necessary directs the reader to more indepth reviews in the specific areas.
Inhomogeneous magnetic fields have been found to promote combustion reactions in diffusion flames. When a fuel gas flowed in the direction of a decreasing field strength, the burning velocity was found to increase. No magnetic effect was observed when it flowed into a homogeneous field. Furthermore, when a large diffusion flame was situated around the center of the magnetic field the intensity of which increased centripetally, the combustion reaction was also found to be promoted. The behavior of gas flows under gradient fields was studied. The magnetic promotion of combustion can be explained by the magnetically induced flow of air. The supply of air to the flame front increased by applying inhomogeneous field. Furthermore, the convection of air was also magnetically promoted. The present results suggest the possibility of controlling combustion in diffusion flames by applying inhomogeneous magnetic fields.
The effects of electric fields on the reattachment of lifted flames have been investigated experimentally in laminar coflow jets with propane fuel by applying high voltages to the fuel nozzle. In case of AC, the frequency has also been varied. Results showed that reattachment occurred at higher jet velocity when applying the AC voltages, thus the stabilization limit of attached flames was extended by the AC electric field. Higher voltage and lower frequency of the AC were found to be more effective. On the contrary, the effect of DC was found to be minimal. To understand the early onset of the reattachment with the AC, occurring at higher jet velocity, the influence of AC electric fields on the propagation speed of tribrachial flame edge was investigated during the transient reattachment processes. The propagation speed increased reasonably linearly with the applied AC voltage and decreased inversely to the distance between the flame edge and the nozzle electrode. Consequently, the enhancement in the propagation speed of tribrachial flame edge was correlated well with the electric field intensity, defined as the applied AC voltage divided by the distance.
Prediction of time-mean velocity and pressure in isothermal turbulent flows can be made provided the turbulent stress tensor τ is specified. Isotropie turbulence has generally been assumed in the past with the constitutive equation τ = 2μΔ, where μ is an effective viscosity and Δ is the mean flow rate of deformation tensor. A method is presented here which allows the distributions of μrz and μrθ, the two significant effective viscosity components in a nonrecirculating swirling flow, to be determined from mean value distributions of υz and υθ the mean axial and swirl velocities. Calculations show that the turbulent stress distribution is nonisotropic and that μrz and μrθ are functions of degree of swirl and position in the flowfield. It is shown that the assumption of an isotropic uniform mixing length parameter distribution is quite feasible for weak swirl but is progressively less valid as the degree of swirl increases.
Upward-increasing magnetic fields of lower intensities increased flame dimensions and liberations of carbons, and decreased radical emissions, temperatures and a bluing tendency of the flame. These phenomena were opposite to those occurring in the upward-decreasing magnetic fields. The upward-increasing magnetic fields of stronger intensities caused an anomalous inverse burning, where the positions of the blue and the yellow-orange regions are reversed, and the highest temperature of the flame appeared at flame surfaces distant from the flame top. The above phenomena are ascribable to the fact that the flame shapes and the flow directions of combustion products, air and the flame were changed due to the magnetic forces originating from magnetic gradient fields, thereby resulting in changes in the mixing and the diffusion conditions.
Experimental investigations of the DC field effect on the swirling flame and the phenomena, which cause the formation of soot nanoparticles (carbon black) and greenhouse emissions (CO_{2}, NO_{x}), are carried out. Field-enhanced variations of the flame shape, structure, temperature profiles and the corresponding emission levels are studied for the conditions of the axial fuel and swirling air supply, with incomplete mixing of the fuel and airflows and the development of the stage fuel combustion along the swirling flame. The electric field-enhanced mass transfer of gas species is found to control the swirl-induced internal recirculation, fuel-air mixing and the temperature field near the burner outlet with a direct impact on the processes of soot formation and carbon sequestration from the swirling flame. In addition, parametric studies of emissions were carried out to investigate the electric field effect on the greenhouse gas emissions and to obtain a parametrically optimized swirling combustion. Figs 14, Refs 23.
Effects of magnetic fields on combustion and gas-flow were studied. Methane, propane and hydrogen gases were burned, and flames of these gases were exposed to gradient magnetic fields up to 1.6 T and 220 T/m. Flames bent so as to escape from magnetic fields of higher intensities. Apart from the combustion experiments, flows of gases such as carbon dioxide and oxygen were exposed to magnetic fields up to 2.2 T and 300 T/m. The flows of these gases with a flow velocity 20-140 ml/min were blocked or modified by the magnetic fields. The changes of flame-shape and gas-flow by magnetic fields are understood to be the result of the role of oxygen. Under the intensities of magnetic fields concerned, oxygen gases as paramagnetic molecules are not concentrated but are aligned so as to make a "wall of oxygen". The wall of oxygen presses back flames and other gases.
The behavior of gas flow in air has been demonstrated visually under gradient magnetic fields in the range 0–40 T/m under 0–1.5 T. The magnetic behavior depended on the concentration of oxygen. Pure oxygen or nitrogen mixed oxygen (≳30%) was observed to be attracted by a magnetic field and act like a magnetic fluid where the magnetic effect on the gas was comparable to that of normal diffusion. In contrast, nitrogen gas escaped from magnetic fields of high intensities, and no magnetic field effect was observed for the flow of air. These phenomena were explained by the difference between the force acting on the gas and that acting on the air surrounding it under gradient magnetic fields. The magnetic force increased with increasing concentration of oxygen. This explanation can be also applicable to the escape of flames from magnetic fields of relatively high intensity or the magnetic quenching of flames inside a pair of columnar poles.
The behavior of laminar jet diffusion flames in the presence of non-uniform magnetic fields has been investigated and the results of this experimental study are presented. It has long been recognized that magnetic fields can influence the behavior of laminar diffusion flames as a result of the paramagnetic and diamagnetic properties of the constituent gases. Using a magnet assembly consisting of neodymium iron boron magnets and gray steel prisms, a non-uniform upward decreasing magnetic field was applied to a laminar jet diffusion flame produced using a slotted burner port. The experimental results show that under certain conditions the application of the magnetic field decreased the flame height, prevented the flame from attaching to the prisms, increased the intensity of the flame, decreased the flow rate for which visible soot inception occurred, and increased the flow rate below which the flame extinguished. It was also observed that the degree to which these phenomena occur is proportional to the product of the magnetic induction times the gradient of the magnetic induction. A discussion as to the reasons for these observations is provided. In addition to traditionally defined dimensionless parameters, the previously defined magnetic Grashof number, the magnetic Froude number, and the ratio of the gravitationally induced buoyancy forces to the magnetically induced body forces are identified as key parameters for determining when a magnetic field will affect diffusion flame behavior. Using these dimensionless parameters, the experimental data was cast in a form that clearly indicated the universal nature of diffusion flame behavior in a non-uniform upward decreasing magnetic field. A power law curve fit to the dimensionless data is presented and discussed.
A gas turbine model combustor for swirling CH4/air diffusion flames at atmospheric pressure with good optical access for detailed laser measurements is discussed. Three flames with thermal powers between 7.6 and 34.9 kW and overall equivalence ratios between 0.55 and 0.75 were investigated. These behave differently with respect to combustion instabilities: Flame A burned stably, flame B exhibited pronounced thermoacoustic oscillations, and flame C, operated near the lean extinction limit, was subject to sudden liftoff with partial extinction and reanchoring. One aim of the studies was a detailed experimental characterization of flame behavior to better understand the underlying physical and chemical processes leading to instabilities. The second goal of the work was the establishment of a comprehensive database that can be used for validation and improvement of numerical combustion models. The flow field was measured by laser Doppler velocimetry, the flame structures were visualized by planar laser-induced fluorescence (PLIF) of OH and CH radicals, and the major species concentrations, temperature, and mixture fraction were determined by laser Raman scattering. The flow fields of the three flames were quite similar, with high velocities in the region of the injected gases, a pronounced inner recirculation zone, and an outer recirculation zone with low velocities. The flames were not attached to the fuel nozzle and thus were partially premixed before ignition. The near field of the flames was characterized by fast mixing and considerable finite-rate chemistry effects. CH PLIF images revealed that the reaction zones were thin (⩽0.5 mm) and strongly corrugated and that the flame zones were short (h⩽50 mm). Despite the similar flow fields of the three flames, the oscillating flame B was flatter and opened more widely than the others. In the current article, the flow field, structures, and mean and rms values of the temperature, mixture fraction, and species concentrations are discussed. Turbulence intensities, mixing, heat release, and reaction progress are addressed. In a second article, the turbulence–chemistry interactions in the three flames are treated.
The thermochemical states of three swirling CH4/air diffusion flames, stabilized in a gas turbine model combustor, were investigated using laser Raman scattering. The flames were operated at different thermal powers and air/fuel ratios and exhibited different flame behavior with respect to flame instabilities. They had previously been characterized with respect to their flame structures, velocity fields, and mean values of temperature, major species concentrations, and mixture fraction. The single-pulse multispecies measurements presented in this article revealed very rapid mixing of fuel and air, accompanied by strong effects of turbulence–chemistry interactions in the form of local flame extinction and ignition delay. Flame stabilization is accomplished mainly by hot and relatively fuel-rich combustion products, which are transported back to the flame root within an inner recirculation zone. The flames are not attached to the fuel nozzle, and are stabilized approximately 10 mm above the fuel nozzle, where fuel and air are partially premixed before ignition. The mixing and reaction progress in this area are discussed in detail. The flames are short (<50 mm), especially that exhibiting thermoacoustic oscillations, and reach a thermochemical state close to adiabatic equilibrium at the flame tip. The main goals of this article are to outline results that yield deeper insight into the combustion of gas turbine flames and to establish an experimental database for the validation of numerical models.
This paper reviews the occurrence of the precessing vortex core (PVC) and other instabilities, which occur in, swirl combustion systems whilst identifying mechanisms, which allows coupling between the acoustics, combustion and swirling flow dynamics to occur.Initially, the occurrence of the PVC in free and confined isothermal flows is reviewed by describing its occurrence in terms of a Strouhal number and geometric swirl number. Phase locked particle image velocimetry and laser doppler anemometry is then used to describe the three-dimensional flow fields, which are generated when swirling flow is discharged into an open environment. This shows the presence of a rotating and precessing off centred vortex and associated central recirculation zone (CRZ), extending up to one burner exit diameter. The presence of axial radial eddies close to the burner mouth, in and around the CRZ, is clearly shown. Typically one large dominant PV is found, although many harmonics can be present of lower amplitude. The occurrence of these phenomena is very much a function of swirl number and burner geometry.Under combustion conditions the behaviour is more complex, the PVC occurrence and amplitude are also strong functions of mode of fuel entry, equivalence ratio and level of confinement. Axial fuel entry, except at exceptionally weak mixture ratios, often suppresses the vortex core precession. A strong double PVC structure is also found under certain circumstances.Premixed or partially premixed combustion can produce large PVC, similar in structure to that found isothermally: this is attributed to the radial location of the flame front at the swirl burner exit. Provided the flame is prevented from flashing back to the inlets values of Strouhal number for the PVC were excited by ∼2 compared to the isothermal condition at equivalence ratios around 0.7. Confinement caused this parameter to drop by a factor of three for very weak combustion.Separate work on unconfined swirling flames shows that even when the vortex core precession is suppressed the resulting swirling flames are unstable and tend to wobble in response to minor perturbations in the flow, most importantly close to the burner exit.Another form of instability is shown to be associated with jet precession, often starting at very low or zero swirl numbers. Jet precession is normally associated with special shapes of nozzles, large expansions or bluff bodies and is a different phenomenon to the PVC. Strouhal numbers are shown to be at least an order of magnitude less than those generated by the PVC generated after vortex breakdown.Oscillations and instabilities in swirl combustion systems are illustrated and analysed by consideration of several cases of stable oscillations produced in swirl burner/furnace systems and two where the PVC is suppressed by combustion. The first cases is a low frequency 24Hz oscillation produced in a 2MW system whereby the PVC frequency is excited to nearly six times that for the isothermal case due to interaction with system acoustics. Phase locked velocity and temperature measurements show that the flame is initiated close to the burner exit, surrounding the CRZ, but is located inside a ring of higher velocity flow. Downstream the flame has expanded radially past the high velocity region, but does not properly occupy the whole furnace. This allows the flame and swirling flow to wobble, exciting instability. The next family of oscillations reviewed occur in a 100kW swirl burner/furnace systems whereby oscillations in the ∼40Hz range are excited with flow fields akin to those found in pulsating combustors where the flow is periodically stopped in the limit cycle of oscillation. The phase locked velocity and temperature measurements show a number of mechanisms that can excite oscillation including substantial variations in shape and size of the CRZ during the limit cycle of oscillation, and wobble of the whole flame and flow as shown by negative tangential velocities close to the centre line.Analysis is then made of a high frequency ∼240Hz oscillation in the same 100kW swirl burner/furnace system, this oscillation being caused by minor geometry changes. The flame was shown to not fully occupy the furnace, allowing irregular wobble and precession of the flow and flame to develop, being especially noticeable close to the outer wall. The addition of an exit quarl to the swirl burner is shown to substantially reduce the amplitude of oscillation by eliminating the external recirculation zone (ERZ), reducing flow/flame wobble and variations in the size and shape of the CRZ. The quarl used was designed to largely occupy the space normally taken up by the ERZ.Two gas turbine combustor units firing into chambers are then considered, strong PVCs are developed under isothermal conditions, these are suppressed with premixing in the equivalence number range 0.5–0.75. PVC suppression is attributed to the equivalence ratios used, the burner configuration, location of the flame front and associated combustion aerodynamics. Other work on an industrial premixed gas turbine swirl burner and can showed the formation of strong helical coherent structures for equivalence ratios greater than 0.75. LES studies showed the PVC contributed to instability by triggering the formation of radial axial eddies, generating alternating patterns of rich and lean combustion sufficient to reinforce combustion oscillations via the Rayleigh criteria.Finally, it was concluded that coupling between the acoustics and flame/flow dynamics occurs through a number of mechanisms including wobble/precession of the flow and flame coupled with variations in the size and shape of the CRZ arising from changes in swirl number throughout the limit cycle. Remedial measures are proposed.