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Reviews SCIENCEFOUNDATION INCHINA Vol.25 No.2 2017 61 New conce swirl flame namics cou lin lasma and articles CUIWei REN YiHua &LI ShuiQing 샮쮮쟥Ke Laborator orThermal Science andPowerEn ineerin Ministr Education De artment ThermalEn ineerin Tsin huaUniversit Bei in 100084 China ReceivedApril 15 2017 accepted May 20 2017 Correspondingauthor.E mail lishuiqing tsinghua.edu.cn Abstract The coupling of swirlflame with discrete plasma or particle phases offers promising opportunitiesin combustion controland nanomaterialsynthesis.Overrecent years studies on the dynamics and stabilization of swirl flames have produced significant progress.Starting fromswirl stabilized combustion wefocus mainly ontworecenttrendsin reviewingthese new conceptsinthefield of combustion related to electrically plasma assisted dynamics control and flame aerosol synthesis of nanomaterials.We organize thematerial by four themes :( unsteadycombustiondynamics andcontrol methods ;( electricallyand plasma assisted combustion dynamics control ;( swirl flame based synthesis of nanocomposites and in situdiagnosticmethods for the complex combustion above. Keywords swirl flame dynamics combustion instability plasma assisted combustion flame synthesis in situdiagnostics doi 10.16262 j.cnki.1005 0841.2017.02.002 Introduction The combustor keycomponentof the industrial burner gas turbine aero engine andevenrocket motor posesmanydifficultchallengesconcerning morereliable ignitionandstablecombustion lower total pressure loss lower pollutant emission and longer lifetime.These strict requirementsmight at times seemcontradictory creating dilemmas in the design of thewhole engine or burner .Driven by the latest requirements new combustion concepts may arise from interdisciplinary areas bridging traditional combustiondynamics namelyoldbranchesof chemical physics andotherdisciplinesat the frontiersof physicssuch as plasma physics condensed matter and material physics. Among the combustion community the newconcepts in combustion research encompass wide range of topics including catalytic combustionfor low pollutantemissions oxy fuel combustionandchemical loopingcombustionforCO capture electricall or lasma assisted combustionfor controlling instabilities ], and lame aerosol nthesisoffunctionalnanomaterials .Inthisreview thesetwolatter interestsaregaining more attentionbecause they are still not so familiar tomanydomestic investigators. The swirl flame whether of gaseous liquid or solid fuels is one common flame structure beingused in variety ofcombustion applications .Thereasonisthatswirling adds benefitsofenhancing mixing improvingflamestability andincreasingresidencetime ofreactants.However therisein stringentNO emission standards pushes the operation regime of the combustor of anengine gas turbine or industrial burner towards the leanblow off limit .In addition formodern aero engines the design of swirl combustors facemore strict design specifications spanningoperating regimes fromsteady cruise states to fluctuation combat states 10 .Low NO combustion technologies suchas those termed lean

New concept swirl flame dynamics - NSFC...New-concept swirl flame dynamics ... opportunities in combustion control and nanomaterial synthesis.Over recent years,studies on the

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Reviews

SCIENCE FOUNDATION IN CHINA  Vol.25,No.2,2017  61   

New-concept swirl flame dynamicscoupling plasma and particlesCUI Wei,REN YiHua &LI ShuiQing(李水清)*

Key Laboratory for Thermal Science and Power Engineering of Ministry of Education,Department of Thermal Engineering,Tsinghua University,Beijing 100084,China

Received April 15,2017;accepted May 20,2017

  * Corresponding author.E-mail:lishuiqing@tsinghua.edu.cn

Abstract  The coupling of swirl flame with discrete plasma or particle phases offers promisingopportunities in combustion control and nanomaterial synthesis.Over recent years,studies on thedynamics and stabilization of swirl flames have produced significant progress.Starting from swirl-stabilizedcombustion,we focus mainly on two recent trends in reviewing these new concepts in the field ofcombustion related to electrically/plasma-assisted dynamics control and flame aerosol synthesis ofnanomaterials.We organize the material by four themes:(a)unsteady combustion dynamics and controlmethods;(b)electrically-and plasma-assisted combustion dynamics control;(c)swirl-flame-basedsynthesis of nanocomposites;and(d)in situ diagnostic methods for the complex combustion above.

Keywords swirl flame dynamics;combustion instability;plasma-assisted combustion;flame synthesis;insitudiagnosticsdoi:10.16262/j.cnki.1005-0841.2017.02.002

1 Introduction

The combustor—a key component of the industrial burner,gas turbine,aero-engine,and even rocketmotor—poses many difficult challenges concerning more reliable ignition and stable combustion,lowertotal pressure loss,lower pollutant emission,and longer lifetime.These strict requirements might at timesseem contradictory creating dilemmas in the design of the whole engine or burner[1].Driven by the latestrequirements,new combustion concepts may arise from interdisciplinary areas bridging traditionalcombustion dynamics,namely old branches of chemical physics,and other disciplines at the frontiers ofphysics such as plasma physics,condensed matter,and material physics.Among the combustioncommunity,the new concepts in combustion research encompass a wide range of topics including catalyticcombustion for low pollutant emissions,oxy-fuel combustion and chemical looping combustion for CO2capture,electrically-or plasma-assisted combustionfor controlling instabilities[2—4],and flame aerosolsynthesis of functional nanomaterials[5].In this review,these two latter interests are gaining moreattention because they are still not so familiar to many domestic investigators.The swirl flame,whether of gaseous,liquid or solid fuels,is one common flame structure being used in

a variety of combustion applications[6—8].The reason is that swirling adds benefits of enhancingmixing,improving flame stability,and increasing residence time of reactants.However,the rise instringent NOxemission standards pushes the operation regime of the combustor of an engine,gas turbineor industrial burner towards the lean blow-off limit[9].In addition,for modern aero-engines,the designof swirl combustors face more strict design specifications spanning operating regimes from steady(cruise)states to fluctuation(combat)states[10].Low-NOxcombustion technologies such as those termed lean

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premixed pre-vaporized(LPP)are susceptibility to combustion instabilities and prone to extinction[11,12].Among the various instability modes,the thermoacoustic instability is the most important because itcouples high-amplitude heat release and pressure oscillations,causing surges,detonation,and shutdown,may undermine blades and walls,and even sabotage the engine system [13].Combustion instabilities,equivalent to combustion dynamics,from a certain point of view [14],may be understood as unsteadybehavior of the swirl flame dynamics exposed to variations in the environment,notably pressure,velocity,and temperature.Thus,after the 2000s,there has been a strong renewed interest,both in Europe and inthe U.S.,in investigating turbulent flame structures and thermoacoustic instabilities using a swirl-stabilized combustor as the basic internal configuration for gas turbines and aero-engines[6,15—23].The first issue among the coupling problems tackled by the combustion community was plasma-or

electrically assisted combustion dynamics control,which aims at further extending or broadening theoperational limit that swirl-stabilized combustors have already achieved.Plasma,especially non-equilibrium discharge,has been widely adopted in practice as an active alternative to enhance both ignitionand combustion[24—29].Indeed,the electric-field/flame interaction was known from the early 20thcentury[30—34](Fig.1).Two recent comprehensive reviews on the Progress in Energy and CombustionScience(PECS)are available.Starikovskiy and Aleksandrov reviewed the influence of non-equilibriumplasma on ignition as well as the extension of the combustion-limit[3];Ju and Sun summarized thekinetics and diagnostic of plasma-assisted combustion[4].As far as industrial applications are concerned,the United Technology Research Center(UTRC)applied the electrodes in the recirculation zone of a swirl-stabilized combustor.In conducting experiments under realistic gas turbine conditions,they found thatoscillations were effectively depressed[35]suggesting that there is plenty to explore in this area.The second coupling problem focused on in this review is the gas-phase flame aerosol synthesis of

functional nanomaterials,e.g.,nanocatalysts or nano-energetic materials,which can in turn improveperformance of propulsion platforms.For instance,in situ-generated nanocatalysts are used both toenhance and control endothermic reactions in the fuel system for thermal management and to enhanceignition and combustion rates[36,37].In the past two decades,flame aerosol synthesis has proved to beone of the most scalable and economical technologies for producing well-controlled nanostructuredmaterials,including single metal-oxide,mixed-oxide nanocomposite,and carbon nanostructures,forwidespread applications in renewable energy utilization or environmental compliance,as reported in arecent comprehensive review in PECS[5].Among different burner techniques,the liquid-fed flame spraypyrolysis(FSP)and the spray flame synthesis(SFS)are of most significance,because they apply to allnanomaterials covering nearly all elements in the periodic table,along with facile doping of two or threeelements during synthesis [38—44].In all these practical processes,the non-uniformity of species,velocities,and temperatures,resulting from turbulent swirl flows in the reactor,creates complicatedtemperature-time histories of precursors—particles that play an important role in determining themorphologies and properties of synthesized nanomaterials[45].Thus,the coupling of turbulent swirlflames with nanoparticle formation are urgently sought for both in situ diagnostics and simulations[46].Here,our motivation is to summarize correlatively the latest progress on two important new combustion

Fig.1 Time line marking the relationship between combustion and the electric/plasma field.

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concept technologies,plasma-assisted combustion and flame aerosol synthesis.More specifically,the focusis on the coupling of the swirl flame dynamics with the discharge/electrics and particles.Section 2summarizes the studies on dynamics of unsteady swirl flames including the theories and methods of passiveand active control.Section 3focuses on the fundamental physics principles and advances in electrically-andplasma-assisted combustion dynamics control.Section 4discusses the utilization of swirl flames innanoparticle synthesis.Section 5collects the diagnostic methods for multiple physics fields includingflame,plasma,and condensed phase.

2 Unsteady swirl flames:dynamics and control

In combustion facilities,swirl flow is the simple,effective,and dominating technology to stabilize aflame[47].The rotational flow induced by an axial vane swirler or tangential jets establishes a centralrecirculation zone in the flame,and provides an adverse pressure gradient area forcing burnt gas torecirculate.This swirl flow structure can stabilize the flame well even when the flow velocity is muchhigher than the flame speed.Because of its intrinsic features,the swirl flame is quite sensitive to anyexternal disturbance and prone to oscillate[48].The mechanisms governing the oscillating combustion mainly include the system instability,the intrinsic

instability,and the combustion chamber instability (also known as thermoacoustic instability)[49].First,system instabilities often occur in the anterior of the combustion chamber.For instance,in aturbine,the inlet-flow distortion or compressor stall can induce an unsteady function within combustors.In particular,in swirl combustors,the swirler may complicate the inlet-flow conditions.Sattelmayerreported that the aerodynamics such as the equivalence ratio fluctuations strongly affect combustorinstabilities[50].Candel et al.systemically studied the flame oscillation caused by a vane swirler[6].The axial and azimuthal components disturb simultaneously the swirl number and the flame angle whenunsteady flow with an axial velocity passes a vane swirler.Paschereit and co-workers further studied theswirl-flame dynamics under fluctuations in the equivalence ratio and flow velocities[21].Second,theintrinsic instability arises from flow dynamics and reaction chemistry.The well-known Darrieus—Landauinstability and the thermal-diffusive instability are main subcategories of the intrinsic instability[51,52].The most important oscillating mechanism is the thermoacoustic instability,which results from positive

feedback between pressure and heat-release fluctuations determined by the combustion chamber resonancemode[53].Lord Rayleigh first proposed for the lateral thermo-acoustic instability a criterion between heatand acoustic energy.He noted that if condense air is heated or abstracted from rarefied air,the vibrationincreases[54].Chu further derived a formula for the Rayleigh criterion[55].To estimate the evolution ofthe high-amplitude instability,Putnam gave a simple integral form of the criterion,

1T∫

0p′q′dt>0, (1)

of which,if positive,the instability is amplified,otherwise,it is damped[56].The criterion casts thesystem into a“thermo-acoustic world”or a“flame-dynamics world”.The former is governed by the lineardisturbance equations including p′,u′,and q′.The inlet flow velocity fluctuation acts on the flame,thenchanges the pressure and releases heat.The thermoacoustic coupling then feeds this pressure oscillationback to the flame.In the latter,the inlet disturbance triggers a heat-release pulsation caused by theinteraction between the vortex and combustion(Fig.2).To date,investigators have primarily looked at the thermo-acoustic world.Candel gave a comprehensive

review of the interaction between the premixed-combustion instability and pressure waves,and describedseveral active closed-loop control models and methods using oscillations as feedback.Lieuwen reviewedswirl combustion instabilities covering both numerical and theoretical aspects,and summed up theinvestigations on flamelets and distributed reaction zones using both linear and nonlinear analyses[57].Liand Sun numerically studied azimuthal instabilities affected by vorticity waves in an annular chamber[58].Generally,the modeling and control of the thermoacoustic coupling so far has proceeded quite well.

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Fig.2 Mechanism of thermoacoustic instability in a combustion system.

Comparatively,the research on the flame-dynamics world is less extensive.Recent work has mainlyfocused on controlling the combustion instability based on a linear flame transfer function or a nonlinearflame-describing function between unsteady heat release and inlet flow fluctuations.The theory regardingthe flame transfer function is based on a time-lag phenomenon between inlet velocity fluctuations u′andheat release fluctuations q′.There are three approaches widely used for studying combustion instabilities depending on assumptions

and simplifications:the acoustic analysis,the full Navier—Stokes equation solution,and the energyequation analysis [59,60].First,in early acoustic analyses,Blackshear discovered that a laminaraxisymmetric flame responds like a low-pass filter[61].Merk proposed initially a new method using thetransfer function to describe the oscillation burning[62].Matsui studied the transfer functions of differentflat burner configurations[63].Besides this work on linear perturbations and laminar flames,researchershave investigated the performance of turbulent flames in real combustors working as a real enginecombustor.Becker and Gunther employed pressure sensors in a chamber to study the transfer function of aturbulent flat flame[64].To reveal the nonlinearity of the combustion oscillation,Ogata and Yanginitially used a describing function to analyze amplitude and frequency of oscillations[65].Palies et al.studied the describing function for a confined swirl flame burner subjected to acoustic perturbations[16].Paschereit et al.studied a saturation phenomenon using the describing function for a premixed swirl flame[17].Both the thermoacoustic coupling method and flame transfer/describing function are all based on theacoustic analysis.Although they are sometimes effective in their predictions,the analysis ignores someinteraction details of the reaction.With the second approach,to explore these details,either LES or DNS simulations is used to solve the

full Navier—Stokes equation.The compressible LES is more suitable than the DNS from the perspective ofcomputational cost.The LES method can combine the flame kinetics with the flow perturbations to studythe combustion instability[18].Huang and Yang studied the swirl-flame dynamics from the steady tounsteady mode using the LES method[19,20].Hermeth used the LES and simple reaction model to studytwo different modes of a realistic swirl combustor[66].Poinsot et al.collaborated with the GermanAerospace Centre(DLR),Turbomeca,and ALSTOM to simulate the instability modes and emissions ofswirl combustors using LES;the result accorded with experiments[59,67].Although the LES revealssome fundamental mechanisms of combustion instability,the complexity of this method still restrict itsapplication.The third approach gives also fresh insight on combustion instabilities by analyzing the energy equation.

This method focuses on the gain process during heat release q′and obtains its dynamics distribution.Thisdistribution may be extracted from either chemiluminescence or fluorescence images of heat release markerradicals such as OH,CH,OH×CH2O,and derives the Rayleigh Index which quantifies the energyfluctuation q′.Lee and Santavicca measured the Rayleigh Index via CH chemiluminescence imaging[68,69].Kang et al.studied toroidal structures in the Rayleigh Index map on a swirl burner[70].Huang

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Fig.3 Determination of the Rayleigh index(Adapted from the literature[60]with permission).

further applied the OH-planar laser-induced fluorescence(PLIF)technique and pressure transducers tocalculate the Rayleigh Index map (Fig.3),and systematically examined the swirl-flame response toacoustic perturbations under diverse pressure conditions [60].The Rayleigh Index method is moreaccessible concerning monitoring the heat release fluctuation,and provides more spatial information thanthe flame transfer/describing function.It may be a promising tool to preform combustion dynamicsdiagnostics and active control.Apart from the above macroscopic studies on combustion instabilities,the correlation between the

microstructures of swirl flames and the mechanisms related to the thermoacoustic instabilities is nowreceiving strong attention[71].Heat release is a key parameter in flame dynamics that determines theflame speed,flame surface area(microstructures),and other characteristics.Reaction chemistry occurs inthe very-thin flame surface that is affected by stretch rate,curvature,and other flow properties.First,thestretch rate or flow strain is found to play an important role in determining the flammability and flamestabilization.Several research groups found that different kind of fuels have distinct sensitivities to thestretch rate in an air atmosphere[72—74].Gases with Lewis number less than 1,such as hydrogen andmethane,are lighter than air;the lean flammability limit can be extended under a high stretch rate,andvice versa.Ghoniema and co-workers established the correlation between macro-and microscopic views in aswirl-stabilized combustor via the Lewis number[75].Lieuwen summarized the relationship betweenstretch rate and flame speed using the Karlovitz number and the Markstein number[76].For light fuels inlean combustion and heavy fuels in rich combustion,in which the Markstein number is negative,higherstretch rates produce faster flame speeds.Clearly,the curvature of the flame surface is significantregarding the flame speed and heat release,and dominates the local flame dynamics[71].If the Karlovitznumber,Lewis number,and Markstein number suffice in describing the microscopic interaction betweenstrain structure and swirl-flame dynamics then a breakthrough in the studies of combustion instabilitieswill definitely follow.To control the unsteady combustion dynamics,the methods widely used are through either passive

control or active control.Passive control seeks to reduce the acoustic susceptibility of the combustionsystem by changing the combustor design,such as modifying the geometric structure of the combustor orthe fuel injection system.In contrast,acoustic dampers,such as Helmholtz resonators[77,78],quarterwave tubes,perforated plates[79]and acoustic liners,have been used in passive control to remove theacoustic energy from combustion systems.Generally,passive approaches tend to be effective only over alimited range of operating conditions;in other extreme conditions,especially in low-frequency regimes,such control methods are usually ineffective.Moreover,changing the design of the combustion system isusually time-consuming and costly.

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Active control provides the alternative route to system pulse suppression.Under the guidance of thecontroller(Fig.2),an external actuator can actively generate antiphase perturbations to counteractoscillations.Paschereit et al.adopted loudspeakers and sensors to install an active control system for theinstabilities of a gas turbine swirl combustor[80].Hanson et al.used a tunable diode laser system todetect temperature oscillations and to run closed-loop suppression in the lean blowout of swirl flames[81].Muruganandamet al.actively manipulated the fuel distribution in a swirl combustor to avoid lean blowoutby detecting specific precursors[82].In these active controls,the mechanical techniques adjust upstreamfuel or air flow;hence the time lag must be considered carefully.Therefore,the key problem of thedynamic control of the flame is to develop predictive models against the delay.A flame-dynamic modelestablishes the relationship between upstream flow perturbation u′and the unsteady heat release q′.Thereare several model versions,such as the simple n-τmodel[83,84],linear flame transfer model(FTF)[85—87],and non-linear flame-describing function(FDF)[88].As a linear model,the flame-transfer-function model has difficulty in predicting strong nonlinear phenomena such as drift frequency and limitcycles;researchers therefore started using FDFs to study such non-linear phenomena.For example,Dowling first modeled unsteady behavior of a ducted flame stabilized in the wake of a bluff center-body[89].Lieuwen[90],Balachandran[91],Noiray[92,93]et al.further improved this method from boththeoretical and experimental aspects,and finally established a unified nonlinear flame dynamic model.

Table 1 Differences between real combustion engines and laboratory studies of flame instability

Technique specifications  Content

Size/volume flow rate  Damkhler and Reynolds Numbers are different

Flame structure  Laboratory and real swirl flame are different

Combustor size  Annular combustion chamber in real engine systems

Premixed/diffusion flame  Partial-premixed flame in real engine systems

Instability mode  Flow and acoustic instability modes are different

Scalability Difficult to use laboratory results to scale-up real combustors

Boundary condition  Inlet and outlet conditions lead to instability difference

Chemical reaction  Limited knowledge of chemical reaction in real systems

However,studies on nonlinear FDFs for realistic combustion system have been scant,and those on thecurrent models remain in the laboratory.In the Hottel Lecture of the 36th International Symposium onCombustion,Poinsot pointed out the differences in combustion instability studies between laboratorysetups and industrial facilities(see Table 1 [18]).To narrow the gap between the two,the GermanResearch Foundation funded from 2002to 2008a“Combustion Noise”project to reduce combustion-systeminstability and noise,especially in swirl combustors[94].This project obtained significant achievementsrelated to combustion instabilities and control strategies,and elevated the German community to thefrontier of combustion dynamics.As for the swirl combustor system,a drawback of traditional controlling methods is the long flow/mix

time.If the perturbations from actuators have the same degree of intrinsic instability,the oscillationamplitudes can amplify unexpectedly.Considering the limitations of current passive and active controls,novel control methods to mitigation combustion dynamics are needed to meet more strict requirements.

3 Coupling of swirl flames with plasma and electrics

Electrically-and plasma-assisted control methods can directly manipulate the flame structures rapidlybecause the response time is negligible compared with the delay time of mechanical actuator and flowmixing in active control.During 2009—2014,the Multidisciplinary University Research Initiative projectlaunched by Air Force Office of Scientific Research focused on plasma-assisted combustion mechanisms,predictive modeling,and potential applications in aircraft engines [2].The project developed the

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fundamentals of plasma-assisted combustion kinetics in a shock tube and rapid compression machine,andinvestigated plasma-assisted ignition/flame-dynamics control for burners of simple geometry(e.g.,thecounterflow burner),but paid little attention to the more-realistic swirl combustors.Electrically-and plasma-assisted methods can bypass the upstream flow fluctuation u′ (Fig.2),and

directly change the reaction rate and the heat-release fluctuation q′at the flame fronts.Ju and Sunoverviewed the progress of plasma-assisted combustion from the viewpoint of flame dynamics andchemistry kinetics[4].Indeed,combustion is a rapid,chemically reactive flow,and hence flow control incombustion plays a key role in flame-dynamics control.Wu and Li summarized the main mechanisms ofplasma flow control and plasma-assisted ignition/combustion,and proposed fusing plasma flow andcombustion control[95].Fig.4illustrates these viewpoints on the mechanism underlying electric-field andplasma-assisted combustion.

Fig.4 Mechanisms for electrically or plasma-assisted combustion dynamics control.

First,as far as the interaction between electric field and flame is concerned,Fialkov summarized thechemi-ionization progress in flame sheet that generates in situ weak plasma,including electrons,H3O+,HCO+and other ions[96].Ganguly et al.studied that the sub-breakdown electric field can drive thesecharged species,and change the flow field using ionic winds[97,98].Moreover,high-frequency electricfields can change the diffusion rates of ions and electrons and then alter the Lewis number of the fuel[99].Second,when the external electric field above the breakdown threshold is added,external plasma can be

generated and influence the flame through the chemical,heat and transport effects(Fig.2).Cha et al.reported electric fields inducing a strong ionic wind,which effectively drives the plasma[100].Lacoste etal.showed that the discharge heats gases fast,can induced wave-vortex structure to modify the swirl flowfield[101].Recently,Li and co-workers studied plasma-assisted ignition mechanisms for solid fuelcombustion[102].Regarding chemical effects,plasma adjusts the combustion chemical reaction pathwaysby generating radicals,excited molecules,and fuel fragments[103].For heat effects,Ohm heating,risesin temperature,and the enhancement in reaction rates accompany discharges or variations in the electricfield[104].Moreover,multilateral interactions among electric fields,plasma,and combustions do exist.For instance,the flame structure and its properties influence the electron number density,which in turn

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determines the discharge property and distribution[105,106].For electrically assisted flame-dynamics control,the ionic winds also play an important role.Carleton

and Weinberg discovered that the electric force under low-gravity conditions can directly replace convection[32].Chung et al.found that an AC electric field can extend the blow-out limit and convert the liftedflame to a burner-stabilized flame [107].Belhi et al.numerically studied the stable point shiftingphenomenon,proposed that the electric body force can change the stabilization position from the liftedmode to burner-stabilized mode[108].Despite the above work,electrically-assisted control for swirlflames is seldom studied.As mentioned in Section 2,flame stretching is a key factor in swirl combustor.Li and co-workers employed a simple strained flame,with high stretch,to investigate the oscillations ofthe flame front driven by an AC electric field.This AC field induces a variation in the flame surfaces,causing fluctuations in heat release,and inducing significant noise(Fig.5)[109].This implies that wecan inversely apply this interesting phenomenon to mitigate the generation of instabilities in high-stretchstrained or swirl-stabilized flames in the future.

Fig.5 Mode transition of flame and sound pressure levels(Adapted from the literature[109]with permission).

Then,as for plasma-assisted flame-dynamics control,different power supplies generate several types ofequilibrium and non-equilibrium plasma.These kinds of plasma are easy to classify depending on thereduced electric field and electron temperature(Fig.6).Here the orange area designates flame weakplasma generated by high-temperature chemi-ionization reactions,the green area marks equilibriumplasma,and the blue areas indicate non-equilibrium plasma,such as microwave discharge,dielectricbarrier discharge(DBD),gliding arc,filamentary discharge,glow discharge,and nanosecond discharge.As the electron energy increases,its kinetic effects on combustion strengthen;as the reduced electric fieldintensifies,the efficiency of energy deposition in plasma increases.Non-equilibrium plasma with highelectron temperature exhibit high chemical activities in the flame,which can significantly enhancecombustion by broadening the blow-out limit and eliminating unsteadiness.Starikovskii etal.employed nanosecond discharge plasma,which dramatically increases the flame blow-

out limit and decreases ignition delay time[110].In the EM2C Laboratory,the lean flammability limit wasextended from 0.41to 0.11applying nanosecond discharges in a laboratory-scaled swirl turbine combustor[111].Flame stabilization with its plasma-enhanced effects has attracted intensive attention amongresearchers.Moeck et al.found that nanosecond-pulsed discharges changed the swirl-flame shape anddistribution,lifted the flame,and stabilized it in shear layers near the recirculation zone[112].The plasmaunder some conditions eliminated combustion oscillations but under others,it amplified the instability.

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Lacoste et al.found that nanosecond discharge plasma notably shifted the swirl-FDF [113].At lowfrequencies,plasma reduced the gain amplitude of the function,,whereas at high frequencies,it enhancedthe instability resulting from heat effects and the induced vortex.However,several plasma-assisteddynamics control studies are rooted in phenomenological methods;the fundamental physical mechanismremains to be investigated thoroughly.

Fig.6 Different types of plasma for combustion enhancement.

Some industrial plasma-assisted combustion technologies supported by government and commercialenterprise are ahead of academic research in some fields.Maxim Kuzyuk,from Aviation EquipmentHolding Company,claims that Russian fifth-generation T-50fighters are equipped with‘oxygen-free’plasma ignitor systems[114].Siemens AG cooperates with Lund University to study diverse methods ofplasma-assisted combustion dynamics control[115,116].NASA and United Technologies ResearchCenter first used a swirl combustor under realistic conditions to study the mechanism of plasma-assisteddynamic control[35,117],aiming to improve combustor instability after the working condition transitsfrom the RQL (Rich-Quench-Lean)mode to the LDI (Lean-Direct-Injection)mode.Results showoscillations to be suppressed efficiently with efficiency increasing on applying a nanosecond plasmadischarge.The plasma anchors the stagnation point in the relatively steady recirculation zone of the swirlcombustor,thus decoupling the flame dynamics from the unsteady flow.The above research suggests thatthe plasma-assisted technique has promise in controlling combustion dynamics.

4 Utilization of swirl flames toward nanoparticle synthesis

Another important new-concept coupling zone arises from directly using swirl flames in the synthesis ofnanostructured metal oxides.Flame aerosol synthesis offers various advantages over wet-chemistry routesin several aspects:high yield,fast one-step processing,and simplicity in collection and further assembly.Scalable and economical manufacturing of nanomaterials,with well-controlled particle sizes and structurestailored to within atomic precision,rely to a great extent on well-designed flame reactors.As summarized in our recent review on Progress on Energy and Combustion [5]and other historical

reviews[127,118,119],flame synthesis reactors can be divided into four categories:free-jet burners,stagnation/counterflow burners,FSP reactors,and more recently developed swirl burners for scalableSFS.lists several important flame aerosol reactors in the early development of flame synthesis.The co-flow diffusion burner,as a typical free-jet reactor,offer high operational flexibility and safety but sufferfrom long residence times and limited choice of precursors.To overcome these two drawbacks in free-jetburners,Hai Wang’s group[120,121]and Pratsinis’s group[122]develop the premixed stagnationburner and the FSP burner,respectively.Aiming at making a stagnation flat burner more robust in scale-up or industrial applications,we have developed a novel swirl-stabilized,premixed stagnation burner(also

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Fig.7 Development of various flame aerosol reactors.

referred to as a stagnation swirl burner).Various nanomaterials,including TiO2,Pd-doped TiO2,and Pd-doped CeO2with high-purity crystallization,relative monodispersity,and well-mixed dopants,have beensynthesized using this type of swirl burner.The stagnation swirl burner,with relatively low swirl number of less than 0.6,benefits from its high

stability in ultra-lean conditions.A swirler component consisting of circumferential annular vanes with atubular central bypass(center panel of Fig.8)is fitted into a premixed tube.Both premixed gas andprecursor impact upon the stagnation plate.The swirl number,which depends on the ratio of the centralbypass flow to the annual flow through the annular vanes,was measured to be 0.25,indicating a low-swirlcombustion mode.During operations,the stagnation swirl flame is in one of two modes:the burner-stabilized mode and the divergence-stabilized mode.With stagnation swirl flames,the TiO2nanoparticles produced range from 5.0nm to 8.4nm in size by

increasing the precursor loading from 0.0125to 0.05g/min(see TEM images,upper-left panel of Fig.8).Remarkably,the stagnation swirl burner can accommodate much higher precursor loading rates and thenproduce the same 6-nm nanoparticles as conventional stagnation flat burners but at a higher materialproduction rate.Benefitting from its ultra-lean operation conditions,the stagnation swirl burner has alower flame temperature while retaining the advantage of large temperature gradients~105 K/m near thewall in other stagnation flames.The lower flame temperature then slows down the coagulation of nascentparticles.Moreover,the large temperature gradient and the cold substrate result in a large quenching rateof the nanoparticles,so that sintering and further growth of TiO2does not occur near the stagnationsubstrate[123].Despite the swirl flame being highly turbulent in the main flow field,the deposition of the nanoparticles

is dominated only by their transport in the boundary layer,as described well in the deposition theorydeveloped by our group[125].The boundary layer of the swirl stagnation flame can be divided into threeregimes from the main flow to the stagnation substrate:the convection-controlled regime,the transitionregime,and the thermophoresis-controlled regime.To compare these mechanisms,a thermophoreticPeclet number,Peth,is defined as the ratio of the thermophoretic velocity times the stagnation layer to theBrownian diffusion coefficient of the particle.For nanoparticles larger than 4nm,Pethis much larger than1so that the deposition flux contributed from diffusion can be ignored.Recently,a theoretical model hasbeen validated by directly measuring the volume fraction of nanoparticles near a stagnation plane using

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Fig.8 Configuration,operation conditions,flow field,and synthesis application of the stagnation swirl flame aerosolreactor.(Upper left)TEM images of TiO2particles with the burner-substrate distance of 12mm and 36mm(Reprinted withpermission from[123]).(Upper center)Operating map(Reprint with permission from[123]).(Upper right)SEM imageof the side view of nanostructured TiO2films(Reprinted with permission from[124]).(Middle left)TEM micrographs(a,b)and electron diffraction pattern(c)of as-synthesized Pd@TiO2catalysts;the catalytic activity of as-synthesized catalystsat different Pd loadings(Reprinted with permission from[43]).(Middle center)Stagnation swirl-flame setup(Reprint withpermission from[123]).(Middle right)TEM and STEM images of Pd-doped CeO2;catalytic behavior of Pd-doped CeO2(Reprinted with permission from[44]).

phase-selective laser-induced breakdown spectroscopy [126].After depositing on the substrate,thenanoparticles then form the morphologically-controlled nanostructured film.On the one hand,when thesubstrate temperature is low,increasing precursor concentration can significantly decrease the specificsurface area(SSA)of the film,which indicates a“in-flame agglomeration”regime.On the other hand,theSSA of the film is not affected by the precursor loading when the substrate temperature is high[124].To extend the availability of precursors from volatile precursors to low-volatile and economical

precursors,the swirl stagnation burner was then developed into a spray flame synthesis(SFS)reactor.Compared with flame spray pyrolysis(known as FSP),the major energy contribution in SFS is mainlyprovided by the combustion field,instead of the precursor solution only[127].In our recent work,PdOxclusters were produced from the precursor in the form of liquid droplets,which are prepared by dissolvingPd(OAc)2in xylene.Again,benefitting from fast quenching and turbulence mixing effects of the swirlflame,the palladium nanoclusters are well dispersed on the surface of supported TiO2nanoparticles(centerright panel of Fig.8).From a time-scale analysis together with the population balance model,small Pdclusters were captured by pre-formed TiO2 nanoparticles and further growth haltered.Therefore,increasing Pd precursors loading rates,to a certain extent,results in an increase in the number of PdOxclusters that disperse on one supported TiO2nanoparticle but not the size of the PdOxclusters.Thisuniformly dispersed structure is well suited to improve the catalytic performance.When Pd loadingincreases(bottom-left panel of Fig.8),the catalytic performance,which is demonstrated usually as a

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characteristic temperature with a methane transition ratio of 20%,improves continuously.Our recentwork has shown that by replacing or partially replacing the supported nanoparticle TiO2[44]by CeO2[128],complex Pd@TiO2-CeO2 nano-catalysts can be synthesized in a one-step SFS process in thestagnation swirl burner.The PdOx clusters transfer part of their charges onto the supported CeO2nanoparticles,which creates a repulsive interaction between two neighboring Pd clusters.Such repulsiveforces can prevent two PdOxclusters from further sintering.To further scale up the swirler-based SFS reactor and improve its robustness for industrial use,we

developed a high-swirl tubular flame.Different from our previous premixed stagnation swirl flame,thistubular flame is a corner tangential-fired rapid-mixed diffusion flame with a swirl number that is muchgreater than 1.The burner is surrounded by six tangential jet flow channels and a central tube at the top(top-right panel of Fig.7).Fuel and oxidizer are injected separately from different tangential flowchannels so that the risk of flashback is eliminated.In particular,two different feeding routes co-exist inthe swirl tubular flame,i.e.,the tangential feeding route and the axial feeding route.By analyzing theirtemperature time history,we find that the axial feeding route possesses a long residence time in the high-temperature zone because its streamlines pass the central low-velocity zone,whereas the tangential feedingroute has a short residence time as its helical streamlines only transit the high-velocity zone.The uniquemultiple temperature-time histories of the tubular flame field guide the optimized design of the SFSreactor.For example,we may feed the thermal-driven precursors through the axial feeding route becausethey need a long residence time in the high-temperature environment to decompose completely;the radical-driven precursors are fed through the tangential feeding route,as they need to pass the flame front toaccomplish hydrolysis reactions.Generally,the understanding of swirl tubular-flame dynamics willcertainly promote the development of high-throughput, flexible-doping SFS of metal-oxidenanocomposites.However,there are several studies on interactions between the turbulence of swirl flamesand nanoparticle formation.These interactions greatly limit the scale-up and industrialization of flamesynthesis reactors.With rapid advances in either computational science or in situ diagnostics,the modelingand predictive simulation of nanoparticle formation in turbulent swirl flames,as well as their validation,are expected in the near future.

5 In situ Diagnostics for complex flame field

The combination of multi-physical fields including unsteady swirl flame fields,electric fields,plasma,and condensed phases(nanoparticles)heightens the challenge in in situ multi-phase measurements.Thecharacteristic time and length cover many orders of magnitude.Accordingly,the experimental researchspans the broadest possible range of diagnostic methods fromin situ probe methods to optical and laser-based non-intrusive diagnostic methods.Accurate real-time observations of heat release are a priority in investigating the detailed mechanism

underlying combustion instability.CO + OH → CO2 + H is the main heat-release reaction in thehydrocarbon fuel burning.Hurle et al.were the first to use photomultipliers(PMTs)to monitor theemission intensify of CH and C2radicals and studied the interaction between noise and the turbulentpremixed flame[129].Haber et al.selected OH* and CH* chemiluminescence signals to identify theareas of heat release [130].The collection of all emission signals without spatial resolution is thedisadvantage of the PMT technique.Anders et al.[131]and Paschereit et al.[21]adopted the intensifiedCCD detectors in detecting heat-release signals from the chemiluminescence radicals OH and CH.The timescales of flame range from 106 s to 102 s(Fig.9),but the repetition rate of commercial ICCDs is usuallyabout 10 Hz. Hence,the ICCD cannot meet the requirements for high-speed flame-dynamicsmeasurements.In contrast,pure high-speed cameras,however,cannot vividly capture the low signal-to-noise ratio chemiluminescence signals.Relative to measuring chemical kinetics and turbulence structures,it is easier to measure the accessible

parameters such as stretch rates and heat-release rates in studying swirl-flame dynamics.The PLIF

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Fig.9 Time scale and common methods of electric field/plasma/combustion coupling system.

technique is an effective tool in flame dynamic studies.Huang obtained the Rayleigh Index maps of a swirlflame based on OH PLIF,and used it to investigate the combustion instability[60].OH radicals have arelatively long lifetime and thus a wide distribution in post-flame regions.With the development of laserand camera devices,Bckle et al.[132]and Gordon et al.[133]elected CH2O×OH as the heat-releasemarker and used the compounded PLIF method to measure precisely the heat-release rate and region.Thetraditional PLIF technique is limited by the repetition rate of pump lasers.This limitation is overcomeusing a high-power kilohertz-pulse laser device.Miller et al.built an optical parametric-oscillator modulewith a kilohertz NdYAG pump laser to set up a CH-PLIF experiment for laminar and turbulent flames[134].Meier and co-workers used a 5-kHz simultaneous OH-PLIF and PIV measurements to study thecombustion instability with flame local strain extinctions and vortex core behaviors[135].Driscoll and co-workers studied a swirl combustor heat release fluctuations and pressure oscillations of premixed swirlflames exploiting the 10-kHz CH2O PLIF [136].These high-repeating imaging methods increase thetemporal resolution diagnostics capacity and provide detailed information of swirl-flame dynamics.Despite the considerable progress of high-repetition laser diagnostics,the complexity of the laser system

restricts its applications.Tomographic methods employing high-speed cameras are of use in reconstructingflame surfaces from chemiluminescence emissions.Ma et al.produced a 3Dflame structure using differentfacilities and validated it by a Mie scattering analysis[137,138].Li and co-workers extracted flame-stretching information from chemiluminescence images and calculated the flame surface distributionaffected by the AC electric field[109].The tomographic technique is relatively simpler to perform inexperimental facilities than the kilohertz laser systems,indicating the potential in flame-dynamicsdiagnostics.As for electric field measurements,Marcum and Ganguly used an intrusive voltage probe to measure the

electric field potential in a flame.The probe not only disturbed the flame flow field,but also the internalelectric fields[99].Dinkelacker et al.simulated Marcum and Ganguly’s experiments,but failed toconsider the effect of the probe[139].Chen et al.used a Langmuir probe to measure the number densityof flame ions[140].Fialkov summarized the advantages and disadvantages of mass spectrometry of flameions[96].Hill introduced the application of ion mobility spectrometry in the flame diagnostic[141].Qi etal.adopted photoionization mass spectrometer with a synchrotron light source in detecting theintermediate ions in flame reactions[142].Mphale et al.monitored the ionization of grassfire by themicrowave interferometric method[143].Until now,most of the electric field and ion diagnostics methodsare intrusive.The in situ microwave methods are limited by low resolution.Therefore,high-accuracy insitu diagnostic techniques of electric fields are needed urgently.Rntgen used the Thomson scattering technique to test the electron density of plasma [144].

Starikovskaia et al.measured the reduced electric field of DBD plasma using emission spectroscopy[145].

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Laux and co-workers investigated the discharge plasma mechanism in flames such as nitrogen associationand shock formation induced by fast heating [146,147].They used cavity ring-down spectroscopy(CRDS),two-photon absorption laser induced fluorescence(TALIF),and emission spectroscopy to gatherthe information of nanosecond repetitively pulsed discharge(NSPD)[148].Starikovskiy et al.used PLIFand CRDS to investigate the evolution of OH radicals produced by discharge[149].Adamovich and co-workers used the plasma emission spectroscopies to analyze the evolution profiles of oxygen atoms andradicals in a premixed gas with nanosecond discharge[150,151].These advanced measurement methodsprovide a solid ground for electric field/plasma-assisted combustion dynamics control.Then,if we extend our discussion to the coupling between swirl flames and high-temperature particle

synthesis,the in situ diagnostic methods become more complex.In particular,because laser-baseddiagnostics of the particle phase rely significantly on light-particle interactions,certain information aboutparticles can be resolved from various levels of the interaction.Laser-induced scattering of particles canindicate particle size,whereas inelastic scattering(or Raman)can provide information about crystallinity.For higher laser energies,laser-induced incandescence can measure the number density and size of the sootparticles by irradiating the soot particles,recording the radiation signal,and extracting the informationbased on certain absorption and radiation models.Finally,when the laser power is strong enough tobreakdown all material to plasma,elements of both gas and particles phases can be quantified byconventional laser induced breakdown spectroscopy(LIBS).Recently,we found that atomic emissions of the particle phase are observable from the stagnation swirl

flame,as mentioned in Section 4 [152].The so-called phase-selective laser induced breakdownspectroscopy(PS-LIBS)is useful in qualitatively detecting the gas-to-particle conversion[153],in situcharacterizing bandgap variation processes of certain compositions[154],and quantitatively determiningvolume fractions of the nanoparticles[155].In physics,PS-LIBS is governed by an absorption-ablation-excitation mechanism(Fig.10),which was recently clarified by our group[156].Rayleigh scattering andtime-scale analyses of the Fokker-Planck equation indicate that atomic emissions start from ablationsinitiated with the laser-beam absorption by conduction-band electrons produced in one-or multi-photonionization.After a nano-sized plasma forms,atoms and ions are excited by energetic electrons beforeatomic emissions are observed.

Fig.10 Schematic of the mechanism for(a)absorption(b)ablation and(c)excitation;(d)experimental setup.(Adaptedfrom ref.[156])

6 Conclusion and outlook

With the rapid progress on swirl flame dynamics in the past two decades,the time is proper to furtherexpand our understanding of swirl flames with two attractive new concepts at the frontier of combustionresearch,i.e.,electrically-or plasma-assisted combustion,and flame aerosol synthesis of nanomaterials.

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On the one hand,the use of electrical field and plasmas have developed into promising methods to achieveactive dynamics control of industrial swirl combustion under realistic conditions,with the development ofin situ diagnostics and predicable modeling.On the other hand,use of the turbulent swirl flame is alsoexpected in achieving high-throughput,flexible-doping SFS of metal oxide nanocomposites(e.g.,nano-catalysts or nano-energetic materials for combustion applications).Accordingly,future researchdevelopments and challenges would entail the following aspects:(1)Elucidating mechanisms and building robust models for electrically -and plasma-assisted

combustion dynamics control.The physical mechanism and macroscopic controlling model still need to befurther investigated.Besides the FDF for thermoacoustic instabilities,the electrical response function of aswirl flame requires attention.The competing mechanism between the electrical response function and theFDF is of significance in controlling unsteady combustion dynamics.(2)Developing in situ optical diagnostics for swirl flames coupled with plasma or particle.Instead of

solely relying on kilohertz laser devices,we prefer the simply reliable,in situ optical diagnostic techniquesthat operate with flexibility in complex turbulent swirl flames under realistic conditions.In our opinion,the combination of high-speed cameras with intensifiers and band-pass filters,benefitting from the highsensitivity of the ICCD and high repetition rate of the high-speed camera,may provide a promisingalternative for in situ real-time diagnostics of unsteady swirl combustion that couples multi-fields andmulti-phases.(3)Building multi-scale models of nanoparticle formation in swirl flames and validating them by in

situ diagnostics data.Considering the current progress in in situ and ex-situ diagnostics as well as thephysical processes demonstrated by Pratsinis,Wang,and Tse’s laboratory groups as well as our own,thetime is appropriate to conduct studies of reactor optimization,processes design,modeling/simulationacross scales,and in situ diagnostics of SFS of well-doped nanocomposites based on swirl-stabilizedcombustors.

Acknowledgements

This work is funded mainly by the National Natural Science Foundation of China(Grant Nos.91641204and 51676109).Shuiqing Li is grateful to Prof.Yiguang Ju at Princeton,Prof.Lin Ma at Virginia Tech,and Prof.Min Suk Cha from KAUST.All authors acknowledge sincerely Prof.Yinghong Li at Air ForceEngineering University,Prof.Qiang Yao and Prof.Yikang Pu from Tsinghua University,Prof.Yun Wuat Xi’an Jiaotong University,Prof.Baolu Shi from Beijing Institute of Technology,and Prof.Yun Huangfrom the Chinese Academy of Sciences.

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Li Shuiqing

Li Shuiqing(李水清)is a Professor in the Department of Thermal Engineeringat Tsinghua University.He obtained a Ph.D.in Engineering Thermophysics

from Zhejiang University.He was a visiting scholar at the University of Leeds

in 2004—2005,at the University of Iowa in 2006,at Princeton University in

2010—2011,and at Yale University in 2014.Dr.Li is a recipient of the

National Award for New Century Excellent Talents.He is now the secretary-of-general of International Symposium on Coal Combustion,and colloquium co-chairperson of solid-fuel combustion in the Combustion Institute.His research

interests include particulate flows,discrete particle dynamics,solid fuel combustion,flame aerosol

synthesis,and combustion dynamics control.