Introduction to investigations of the negativecorona and EHD flow in gaseous two-phasefluids

Jerzy MIZERACZYK1 and Artur BERENDT2

1Department of Marine Electronics, Gdynia Maritime University, Morska 81-87, 81-225 Gdynia, Poland2 Institute of Fluid Flow Machinery, Polish Academy of Sciences, Fiszera 14, 80-231 Gdansk, Poland

E-mail: jmiz@imp.gda.pl

Received 21 December 2017, revised 11 March 2018Accepted for publication 12 March 2018Published 9 April 2018

AbstractResearch interests have recently been directed towards electrical discharges in multi-phaseenvironments. Natural electrical discharges, such as lightning and coronas, occur in the Earth’satmosphere, which is actually a mixture of gaseous phase (air) and suspended solid and liquidparticulate matters (PMs). An example of an anthropogenic gaseous multi-phase environment isthe flow of flue gas through electrostatic precipitators (ESPs), which are generally regarded as amixture of a post-combustion gas with solid PM and microdroplets suspended in it. Electricaldischarges in multi-phase environments, the knowledge of which is scarce, are becoming anattractive research subject, offering a wide variety of possible discharges and multi-phaseenvironments to be studied. This paper is an introduction to electrical discharges in multi-phaseenvironments. It is focused on DC negative coronas and accompanying electrohydrodynamic(EHD) flows in a gaseous two-phase fluid formed by air (a gaseous phase) and solid PM (a solidphase), run under laboratory conditions. The introduction is based on a review of the relevantliterature. Two cases will be considered: the first case is of a gaseous two-phase fluid, initiallymotionless in a closed chamber before being subjected to a negative corona (with the needle-to-plate electrode arrangement), which afterwards induces an EHD flow in the chamber, and thesecond, of a gaseous two-phase fluid flowing transversely with respect to the needle-to-plateelectrode axis along a chamber with a corona discharge running between the electrodes. Thisreview-based introductory paper should be of interest to theoretical researchers and modellers inthe field of negative corona discharges in single- or two-phase fluids, and for engineers whowork on designing EHD devices (such as ESPs, EHD pumps, and smoke detectors).

Keywords: DC negative corona discharge, Trichel pulses, gaseous two-phase flow, air withsuspended particle flow, ESP, EHD

(Some figures may appear in colour only in the online journal)

1. Introduction

Natural electrical discharges, such as lightning strikes andcoronas, occur in the Earth’s atmosphere, which cannot beregarded as a single-phase environment consisting only of air.The majority of the volume of the Earth’s atmosphere is amixture of air (a gaseous phase) and suspended microscopicsolid (solid phase) and liquid (liquid phase) particles. Thusthe Earth’s atmosphere should actually be considered a multi-phase environment. Being very spectacular, lightning in the

Earth’s atmosphere occurs between electrically chargedregions of a single cloud, between two clouds, and between acloud and the solid ground or the water surface (rivers, lakes,seas and oceans). Less conspicuous coronas are observedaround high-voltage transmission line conductors, aroundlightning rods and the masts of ships (the legendary SaintElmo’s fires).

The microscopic solid and liquid matters distributed inthe Earth’s atmosphere are called particulate matter (PM) orparticulates. An air/PM mixture is often referred to as an

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aerosol. The majority of atmospheric PM occurs naturally, theeruptions of volcanoes, fires in forests and grasslands, and saltspray over the oceans being the most common sources ofparticulates in the atmosphere. Anthropogenic PM, generatedby the burning of fossil fuels in vehicles, power and heatplants and in various industrial processes, accounts for about10 percent of the total mass of aerosols in the Earth’satmosphere.

The presence of PM in the Earth’s atmosphere influencesthe generation and properties of the lightning strikes andcoronas generated within it. For example, it was noticed thatvolcanic activity produces lightning-friendly conditions dueto the enormous quantity of pulverised solid material andgases ejected into the atmosphere. It was also observed thatthere are certain conditions under which electric transmissionlines more willingly generate coronas. Wet and humid con-ditions, undoubtedly indicating the presence of a multi-phaseenvironment, increase corona activity around the transmissionlines. Utility companies try to reduce the coronas because ofthe electric power loss and system component damage causedby them. These two examples show that neglecting themultiphase nature of the Earth’s atmosphere, when con-sidering the electrical discharges within it, may be a barrier inthe justification and explanation of their apparent occurrence.This remark also applies to the electrical discharges occurringin multi-phase environments other the Earth’s atmosphere.

This paper is an introduction to electrical discharges inmulti-phase environments. Electrical discharges in multi-phase environments are becoming an attractive researchsubject due to the variety of possible discharges and multi-phase environments to be studied. An example of ananthropogenic gaseous multi-phase environment is theexhaust gas flowing through an electrostatic precipitator(ESP), which is typically a mixture of a post-combustion gas(a gaseous phase) with a form of solid PM such as dust orsmoke, and/or microdroplets (a liquid PM) suspended in it,forming a kind of aerosol (air/solid and liquid PMs).

Considering the electrical discharges (lightning strikes,sparks, coronas) run in laboratory multi-phase environments(which are also called aerosols, colloidal aerosols, multi-phase media or multi-phase fluids), it is convenient to use thenomenclature typical of fluid mechanics. A multi-phase fluid,usually two-phase, is a system of two substances (phases), inwhich one phase of dispersed insoluble microscopic particlesis suspended throughout another phase. The dispersed parti-cles have a diameter between approximately 1 and 1000 nm.The phase in which a dispersed (or disperse) phase is dis-tributed is called a dispersion phase (or a continuous phase).The dispersion phase is formed by a gas or a liquid (in gen-eral, also by a solid). The dispersed phase can be single- ormulticomponent (e.g. microscopic solid and liquid mattersdistributed in air). A single-component dispersed phase canhave the form of solid particulates (a solid dispersed phase),liquid particulates (a liquid dispersed phase) and gaseousparticulates (gaseous dispersed phase). A solid dispersedphase can be formed by microparticles of dust, smoke, etcdistributed in air or water. An example of a liquid dispersedphase is that of microdroplets of water dispersed in air.

Gaseous microbubbles in water can be treated as a gaseousphase dispersed in a liquid phase.

In this introductory paper, we limit our consideration toDC negative coronas in a gaseous two-phase fluid formed byair (a gaseous phase) and solid PM (a solid phase), run underlaboratory conditions.

A sound foundation for studying DC negative coronas ina gaseous two-phase fluid is the knowledge acquired from80-years of intensive experimental and theoretical studies ofnegative coronas in single-phase fluids, mainly in air. Theearly (1938–1962) basic studies, carried out by Trichel [1],Loeb’s group and others [2] concerning mostly regular, cur-rent pulses (named Trichel pulses) of negative coronas in air.Later the most important results of these early studies wereanalytically summarised and experimentally verified in [3].The basic data and results of the negative corona studiesperformed up to the 1980s were also collected in [4, 5]. Sincethen, numerous new contributions to the body of knowledgeof the various regimes of negative coronas (not only theTrichel-pulse regime) in air (i.e. in a single-phase fluid) havebeen published.

Much of the research into negative coronas and theinherent electrohydrodynamic (EHD) flows in two-phasegaseous fluids has been geared towards the performance ofESPs by focussing on their practical aspects (breakdownvoltage, back corona, dust particle collection efficiency, NOx

production, etc). However, there are few basic research intonegative coronas in two-phase gaseous fluids similar to thoseof ESP flue gases.

Experimental investigations of the negative coronacharacteristics in a flowing two-phase gaseous fluid (air withsuspended cigarette smoke particles) simulating an ESP fluegas were presented in [6, 7]. The results of these investiga-tions showed that an increase in the concentration of smokeparticles results in a reduction of the average corona current(at the same operating voltage). This conclusion was alsoconfirmed theoretically in [8, 9].

Recently, basic research into the particle charging andelectrical characteristics of a negative corona discharge in aflowing two-phase fluid (consisting of solid micro-particlessuspended in air) was conducted, and the results published in[10, 11]. These results demonstrated that the suspended par-ticles influence not only the average discharge current but alsothe current waveform.

The basic phenomena of the negative corona discharge(in a needle-to-plate geometry) were experimentally andtheoretically investigated using a two-phase jet (air with waterdroplets) in [12]. The results confirmed earlier observationsthat the EHD flows of the dispersion gas (air, in this case) andthe charged PM (charged water droplets) differ.

Our analysis of the body of knowledge of negative cor-onas and corresponding EHD phenomena in single- and two-phase gaseous fluids (air with PM) suggests that the datashould be separated into two distinct groups, and each groupshould be considered separately. The groups are categorisedbased on the state of motion of the fluid at the moment whenthe negative corona is initiated.

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The first concerns the fluid being still (motionless) beforebeing subjected to the corona (an example is a fluid which ismotionless in a closed chamber before the inception of thecorona). The second concerns fluids which are in forcedmotion before the application of the corona (an example is afluid externally forced to move along a chamber, in which thecorona is run). In this case, we are dealing with flowing fluids.

In line with the suggestion above, the results of theinvestigations presented below of negative coronas and EHDflows in two-phase gaseous fluids are divided into twogroups. The first, involving two-phase fluids initially beingmotionless in a closed chamber before being subjected to acorona [13–16] were found to have a strongly transientcharacter, caused by the EHD flow induced by the negativecorona in the closed chamber and the continuous electrostaticprecipitation of the PM suspended in the dispersion gas phase(air). Due to precipitation of the PM onto the plate and thewalls of the chamber, its concentration and distribution gra-dually changes, influencing the characteristics of the coronadischarge.

The corona discharge in the two-phase fluid closed in thechamber maintains its transient phase for a prolonged periodas the PM gradually precipitates onto the surfaces. When thisprocess is almost complete, the transient phase ends with thefluid essentially having only a single-phase (air). This makesreliably studying the mutual relationship between the coronacurrent and the EHD flows rather difficult.

It is more promising studying the second group, i.e. atwo-phase gaseous fluid flowing along a chamber containinga corona discharge [6, 7]. In this group, there is an interactionbetween the continuously replenished two-phase fluid flow(the primary flow) and the EHD flow induced by the coronainside the chamber (the secondary EHD flow). The two flowsand the corona discharge mutually interact. However, in sucha flowing system, the primary two-phase fluid flow con-tinuously resupplies new PM to the corona chamber. Thus,the continuous loss of particulates in the chamber due toelectrostatic precipitation is balanced by the inflowingparticulates.

As a consequence, the particulates in the flowing systemachieve a generally spatially inhomogeneous, steady-statebalance within a relatively short time of the inception of thecorona. The EHD two-phase flows gradually establish asteady-state structural form (for example, a coherent structuresuch as a vortex) and the corona current stabilises in a flowingsystem. Therefore, in contrast to the strongly transient natureof a corona discharge in a two-phase fluid in a closedchamber, in which the local concentration of particulatesvaries in time, the local particulate concentration in theflowing system remains stable (within the fluctuation limit),although it differs spatially.

Typically, ESP flue gas simulators use a low concentra-tion of dust particles (air with suspended dust particles, with aconcentration in the order of 104–108 particles cm−3 [7, 8]).Due to this, it should be expected that the mechanisms of thecorona discharge forming is similar in ‘pure’ air and in airwith suspended dust particles of low concentration.

When using or citing the results of the comprehensivestudies of negative corona in air, such an assumptionwould help.

2. DC negative corona in an initially motionless two-phase fluid (air-smoke particles) in a finite-volumechamber with a needle-to-plate electrodearrangement

2.1. Evolution of an EHD two-phase flow into a single-phaseflow. Stages of the EHD two-phase flow evolution

After applying a high voltage to a needle-to-plate electrode ingaseous two-phase fluid, first, due to electrostatic precipita-tion, the concentration of the dielectric PM in the chamberwill steadily decrease until all particulates are deposited onthe counter electrode and chamber walls. At this point, thefluid effectively becomes single-phase, meaning that a single-phase steady-state regime has been reached. Second, thecorona current, which depends strongly on the concentrationof PM [7] will change until the single-phase, steady-stateregime is reached. Third, after the corona inception, the EHDmolecular and PM flows will transform through severaltransition structures into their final forms, i.e. the EHDmolecular flow will take the final form typical of the single-phase fluid, while the EHD PM flow will cease to exist. So,after the inception of the corona in the two-phase fluid closedin the finite-volume chamber, we should observe a continuoustransition from a two-phase fluid to a single-phase fluid, andeventually the single-phase steady-state will be reached afterthe precipitation of all of the PM. However, this takes arelatively long time.

The suppositions above were confirmed by the results ofthe investigations presented in [13–16]. These investigationsshowed that a transient EHD two-phase flow regime formedin the initially motionless two-phase fluid (air with suspendedsub-micron smoke particles with an initial concentration up to450×103 particles cm−3) in the needle-to-plate (with aninterelectrode gap of 25 mm) negative corona discharge. Thistwo-phase flow transforming through several structuralstages, with its final form being the single-phase steady-stateregime. These transient structural stages were: the two-phasefree jet stage, the initial stage of a two-phase wall-impingingjet, the development stage of a two-phase wall-impinging jetand the fully developed two-phase EHD jet continuouslyevolving due to the precipitation of smoke particles into thesingle-phase (air) steady-state stage.

Figure 1 shows the instantaneous velocity field maps ofthe EHD two-phase fluid flow as it evolves over time. Itdetails how these changes be divided into the four transient,two-phase structural stages listed above. The flow evolutionends in a single-phase (air) steady-state due to the electrostaticprecipitation of the smoke particulates.

In the first stage of the EHD two-phase fluid flow (calledthe two-phase free jet stage), as shown in figure 1(a), theinitial movement is confined particles moving away from thevicinity of the needle electrode tip. All particles in other parts

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of the chamber have not yet been affected by the electric field,so remain unchanged. As a result, a dark area in the form ofan inverse mushroom cap forms at the needle electrode tip.The dark sphere represents the particles having been pusheddownwards, away from the needle and towards the plate. Thisprocess leaves behind an inverse mushroom cap-shapedstructure, free of particles, which can be seen in the figure.

The particles which have been pushed downwards thenappear as a white layer on the underside of the sphere, or as a

cap on the inverted mushroom cap. It is called a mushroomfront. Such an EHD particle flow structure is called a mush-room-like minijet structure. After the first mushroom-likeminijet, similar flow structures are generated (figures 1(b)–(f)).

Simultaneously, the active region begins to expand andformerly quiescent PM is drawn in by the EHD two-phasefluid flow. These particles replace those already pusheddownwards from the needle tip. They then follow the flowtowards the collecting electrode and are pushed downwards,

Figure 1. Flow images and the instantaneous velocity vector field maps illustrating the transition of an EHD two-phase fluid flow to an EHDsingle-phase fluid flow in a needle-to-plate electrode arrangement after the inception of a negative corona. The vectors show the smokeparticle flow directions; the particle flow velocity magnitudes are shown in the colour bar. The voltage rise rate is 13.5 V ms−1, and theultimate amplitude −12 kV [15].

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expanding the mushroom front. After about 80 ms, the jet-likeflow is just about to impinge on the collecting electrode(figure 1(f)).

The flow images in figures 1(g) and (h) (t=90 ms andt=100 ms, respectively) show the development of the

second stage of a two-phase EHD flow, i.e. the initial stage ofa two-phase wall-impinging jet. Once the jet front impingeson the plate electrode (a stagnation area), the velocity andmomentum of the jet front decreases because the immovableplate is an obstacle. The impinging jet with a higher

Figure 1. (Continued.)

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momentum pushes the low-momentum fluid outwards, alongthe plate electrode. The fronts of the fluid being pushedencounter resistance from the surrounding fluid and roll up tobecome vortex cores.

After a period, the initial stage of two-phase wall-impinging jet gradually evolves into the third stage of anEHD two-phase fluid flow; i.e. the development stage of atwo-phase wall-impinging jet (figures 1(i)–(l), fromt=110 ms to t=383 ms). In this stage, the vortex coresgrow and form into large-scale vortex structures with strongentrainment of the air-particle fluid in the wall jet tip region.The vortexes are pushed away from the impinging area alongthe collecting electrode (figures 1(k)–(m)).

After 650 ms (figure 1(m)), the vortices left the cameraobservation area, increasingly dissipating their initialmomentum far beyond the vicinity of the needle electrode. Atthis point, the fourth stage of the EHD two-phase fluid flow isestablished; i.e. a fully developed, two-phase EHD jet. The

flow evolution ends in a single-phase (air) steady-state due tothe electrostatic precipitation of the smoke particulates(figure 1(n)).

2.2. The first stage of the evolution of the EHD two-phase flow.Mushroom-like minijets. Correlation between the minijets andTrichel pulses

The temporal and spatial recordings of the evolution of theEHD two-phase flow (air with suspended submicron smokeparticles) in its very first stage (the two-phase free jet stage) ina DC negative corona in a closed chamber, together withmonitoring of the negative corona (operating in the Trichel-pulse regime) waveform revealed interesting fundamentalprocesses which occur just after the inception of the corona[14, 15, 17]. The high-speed recordings of the EHD particleflow showed the formation of several EHD particle flowsubstructures simultaneously travelling along the interelec-trode gap during the two-phase free jet stage.

Figure 1. (Continued.)

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The monitoring of the negative corona (operating in theTrichel-pulse regime) waveform showed that the first Trichelpulse appeared when the rising voltage pulse reached a valueof approximately −4.23 kV, and was followed by severalsubsequent Trichel pulses, which formed a pulse train with aduration of 5.5 ms (figure 2). The Trichel pulses then dis-appeared for a period of about 60 ms. At about 66 ms, a newTrichel pulse train formed, and disappeared similarly to thefirst. The third Trichel pulse train formed after a period ofabout 50 ms, and disappeared shortly thereafter. The processof Trichel pulse trains forming and disappearing recurred,as shown in figures 2(a) and (b). At t=1 ms (figure 2(a)),the first Trichel pulse train began, and lasted for 5.5 ms. In theimage captured at t=2.5 ms (at a voltage of −4.23 kV),we noticed a slight movement of the dust particles in theimmediate vicinity of the needle electrode (the blurred imageshowing this is not presented in this paper). This means that

the EHD smoke particle flow had just started. At t=12.5 ms,the smoke particles pushed downwards from the vicinity ofthe needle electrode have already formed a very bright surfaceon the underside of the dark (i.e. smoke particle-free) mush-room-shaped region which is forming at the needle electrodeand growing towards the collecting electrode. The descendingparticles are seen as a bright layer on the lower surface of thedark mushroom cap. The image captured at t=75 ms(figure 2(a)), corresponding to the time interval at which thesecond Trichel pulse train has occurred, shows a secondmushroom cap structure that has been generated at the needletip. Both structures are moving downwards as mushroom-likeminijets.

The experiment presented above is one of the first tem-porally and spatially resolved studies of the behaviour of thenegative corona and EHD phenomena in a two-phase fluid.This experiment showed that, in the very early stage, the

Figure 2. Trichel pulse trains and EHD minijets. The inverse-coloured arrows indicate the Trichel pulse train and the corresponding minijetinduced by the train. The time of 1 ms before onset of the first Trichel pulse is assumed as a reference time t=0 [14].

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negative corona discharge in the two-phase fluid (air withsuspended smoke particles) displays the form of Trichel pulsetrains. Each Trichel pulse train initiates a mushroom-likeminijet, which grows downwards and travels towards theplate electrode. Several minijets can simultaneously be pre-sent, stacked in a series in the gap. The series of mushroom-shaped objects travelling downwards in the interelectrode gaplooks like a series of mushroom-like minijets shot from theneedle electrode vicinity towards the collecting electrode. Themushroom-like minijets in the interelectrode gap in the cor-ona discharge resemble negative-ion-charged clouds simul-taneously traversing the interelectrode gap of the coronadischarge (as predicted in [2, 3, 18]).

3. DC negative corona discharge in a two-phase fluidflowing through a chamber with a needle-to-plateelectrode arrangement. Electrical characteristics ofthe negative corona

As stated above, the transient nature of a corona discharge ina two-phase fluid closed in a chamber hinders the reliablestudy of the mutual relationship between the corona currentand the PM in the two-phase fluid. At this point, a morepromising line of inquiry is the study of a two-phase gaseousfluid flowing along a chamber with a corona discharge in anopen-system arrangement where the two-phase fluid is con-stantly replenished with PM. This is, as justified in theIntroduction, due to the steady-state structural forms of theEHD two-phase flows and stable corona current in the flow-ing system.

The DC negative corona discharge in the two-phase fluidflowing transversely (with a velocity limited to 0.8 m s−1)with respect to the needle-to-plate electrode axis, can have 3forms (modes): the Trichel-pulse mode, the ‘Trichel pulsessuperimposed on a steady current’ mode, and the steady glowmode (figure 3). The prevalence of which depends mainly on

the voltage amplitude between the electrodes [19]. In the two-phase fluid the corona inception voltage is lower and theTrichel pulses are less regular than in air. Their repetitionfrequency is lower and amplitudes are higher.

The average current–voltage characteristics I=I (V ) inthe flowing two-phase fluid, having the concentration ofsmoke particles lower than about 50×103 particles cm−3

(figure 4) are close to those in air (figure 5). This is consistentwith the results presented in figure 6, which provide addi-tional proof that the average discharge current decreases whenthe smoke particle concentration increases [6–9].

The dependencies of both microscopic corona dischargeparameters: the Trichel pulse electric charge and the pulserepetition frequency on the particle-concentration explainthe weak dependence of the average negative corona current onthe particle concentration up to about 50×103 particles cm−3

(as shown in figure 6). This gives better understanding of themicroscopic electrical phenomena occurring in the negative

Figure 3. Waveforms of the discharge current in flowing air forseveral voltage amplitudes. At a voltage of −4 kV (Trichel pulsemode) the pulse frequency was 3.3 kHz (the period 300 μs). The airflow velocity 0.8 m s−1. The air flow velocity up to 0.8 m s−1 doesnot influence the corona current modes in the flowing air [19].

Figure 4. Current–voltage characteristics I=I (V ) in the two-phasefluid flow. Particle concentration was variable. The flow velocitywas 0.4 m s−1 [20].

Figure 5. Current–voltage characteristics I=I (V ) in flowing air.Flow velocity was variable [20].

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corona discharge. As seen in figure 7, the electric charge of theTrichel pulses increases, and their repetition frequency decrea-ses, with an increase in particle concentration. Thus, the averagecorona current being the product of the electric charge and therepetition frequency of the Trichel pulses (taking into accountthe DC current component of 4 μA for the case shown infigure 7) remains constant.

As it is shown in figure 7, close agreement of the mea-sured and calculated average corona currents was achieved.The calculated product of the Trichel pulse electric charge(growing with the particle concentration) and the Trichelpulse repetition frequency (decreasing with the particle con-centration) stays almost constant for particle concentrationslower than about 50×103 particles cm−3, similarly to themeasured average corona current. Furthermore, the calculatedproduct of the Trichel pulse charge and frequency decreaseswhen the particle concentrations are higher than about50×103 particles cm−3. This is confirmed by the measured

decrease of the average corona current. Therefore, the parti-cle-concentration dependencies of the Trichel pulse electriccharge and the repetition frequency, both being the micro-scopic parameters, explain the behaviour of the averagenegative corona current, which is a macroscopic parameter.

4. Summary and conclusions

This paper is an introduction to DC negative coronas andaccompanying EHD flows in a gaseous two-phase fluid formedby air (a gaseous phase) and solid PM (a solid phase). Theintroduction is based on a review of the existing literature.

In this review-based introductory paper, the results of theexperimental research on DC negative coronas in a needle-to-plate electrode arrangement in a two-phase fluid (air, as adispersion fluid, with suspended sub-micron cigarette smokeparticles as a dispersed fluid, the smoke particle concentrationbeing up to 450×103 particles cm−3), resembling the com-bustion gas-PM fluid typical of ESPs, are presented. Theexperimental research was focused on the temporally andspatially resolved imaging of EHD two-phase flows, and onthe correlation of the imaging observation results with thecorresponding DC negative corona characteristics. Two caseswere considered: first, the case of the two-phase fluid beinginitially motionless in a closed chamber before being sub-jected to the corona, and second, the two-phase gaseous fluidflowing (with a velocity limited to 0.8 m s−1, typical of anESP flow) transversely with respect to the needle-to-plateelectrode axis along a chamber with the corona dischargerunning in the chamber.

The experimental research on the DC negative corona inthe initially stationary two-phase fluid in the closed chambershowed the evolution of the EHD two-phase fluid (air withsub-micron, suspended smoke particles) precipitate into asingle-phase fluid (air). This evolution can be divided into fourtransient, two-phase structural stages, i.e. the free jet stage, theinitial stage of a wall-impinging jet, the development stage of awall-impinging jet, and a fully developed EHD jet.

The evolution of the flow terminates as a single-phase(air) steady-state due to the electrostatic precipitation of thesmoke particles onto the plate electrode and the walls ofthe chamber. Thorough investigation of the first stage of thedevelopment of the EHD two-phase flow (the free jet stage)showed that just after the inception of the negative corona (inthe air with the suspended smoke particles), it has the form ofan irregular pulse train, each section consisting of severalTrichel pulses. They initiate inverse mushroom-like smokeparticle minijets travelling downwards through the interelec-trode gap towards the plate electrode.

The steady-state DC negative corona discharge in a two-phase fluid flowing through a chamber with a needle-to-plateelectrode arrangement was investigated from the point ofview of the electrical characteristics of the DC negative-cor-ona. The investigation showed that, similar to flowing air, theDC negative corona discharge (in the two-phase fluid flowingtransversely with respect to the needle-to-plate electrode axis)has 3 modes, the prevalence of which depends mainly on the

Figure 6. Dependence of the negative corona average dischargecurrent on particle concentration in the two-phase fluid flow. Theflow velocity was 0.4 m s−1. The applied voltages: −8, −10 and−12 kV [20].

Figure 7. Electric charge and repetition frequency of the Trichelpulses, the calculated and measured average discharge currents in thetwo-phase fluid flow as functions of smoke particle concentration.The flow velocity 0.4 m s−1. The applied voltage −8 kV [20].

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voltage amplitude in the electrode gap. These modes are: theTrichel-pulse mode, the ‘Trichel pulses superimposed onthe steady current (or on the DC component)’ mode, and thesteady glow (or the steady current or DC) mode.

The shapes of the Trichel pulses in the flowing ‘air withsuspended smoke particles’ fluid and in the ‘pure’ air aresimilar. However, the Trichel pulse amplitudes in the formerfluid are higher than those in the latter one, whereas theTrichel pulse repetition frequency is lower. As a consequence,the average corona discharge current in the two-phase fluid islower. The average discharge current in the two-phase fluidremains almost constant as the smoke particle concentration isincreased up to 50×103 particles cm−3, while the Trichelpulse amplitude (and the electric charge) increases, theTrichel pulse repetition frequency decreases and the Trichelpulse shape remains constant. This shows that a macroscopicparameter of a DC negative corona such as the average cur-rent is almost unaffected by the presence of low concentra-tions of PM in the electrode gap, whereas both microscopicparameters (the Trichel pulse amplitude (and electric charge)and repetition frequency) are sensitive to the presence of PM.The average corona current decreases when the particle con-centrations increase over approximately 50×103 particles cm−3.

The calculations of the average negative corona currentdependence on the particle concentration showed that it canbe explained by the behaviour of the Trichel pulse electriccharge and the repetition frequency.

The practical information for the operation of ESPSsobtained from the experiments is that the average coronadischarge current in the two-phase fluid is not affected by thetransverse fluid flow up to a velocity of 0.8 m s−1.

The review presented above serves as a compilation ofpresent knowledge related to the properties of a DC negativecorona and inherent EHD flows in a two-phase medium: airwith suspended solid particles.

Acknowledgments

This work was supported by the National Science Centre(Grant No. UMO-2013/09/B/ST8/02054).

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Plasma Sci. Technol. 20 (2018) 054020 J Mizeraczyk and A Berendt