CORONA IN A TURBULENT JET WITH CONDENSATION A. B. Vatazhin, V. A. Likhter, and V. I. Shul'gin

UDC 532.517.4:537.523.3:536.423.4

The eteetrogasdynamic (EHD) effects associated with the introduction of corona discharge ions into a vapor-air jet with condensation are investigated. The electrical, acoustic and, moreover, integral and local optical (light scattered by condensate droplets) characteristics of the jet are measured. The time-dependent components of the recorded signals, which provide information about the characteristic fluctuations in the flow, are determined and processed. A new effect -- the existence of a correlation between the electrical (Trichel frequency), acoustic and optical fluctuations in the flow -- is detected and analyzed. 1.

FORMULATION

OF THE PROBLEM

It has been shown experimentally that under certain conditions introducing corona discharge ions (point-to-plane system) into a turbulent vapor-air jet causes a sharp increase in condensation [1]. This process, called "electric condensation," is accompanied by the appearance of a specific condensation jet, starting near the point, with an enhanced (by approximately an order) moisture content and a reduced characteristic dispersed droplet size. The simplest explanation for this effect is that condensation nuclei are formed on the corona discharge ions, as a result of which the effective nucleation rate increases by several orders (Wilson cloud chamber effect). However, in addition to this obvious effect, the electric condensation process has other important aspects which have not previously been investigated: charge exchange between the ionic component and the dispersed phase, which leads to the appearance of large charge carriers - droplets with relatively low mobility - - and modifies the current-voltage characteristic of the corona; the different effects on condensation of negative and positive coronas. Moreover, in experimentally investigating electric condensation the authors have detected a fundamentally new effect, namely, the discontinuous (discrete) structure of the condensation on negative corona ions. In the present study these effects are systematically investigated. The investigation of the disperse structure of the corona discharge required the use of special experimental techniques, in particular for recording the time-dependent electrical, acoustic and optical signals from the electrogasdynamic turbulent condensation jet and their subsequent spectral and correlation analysis. In addition, we developed a method of recording the scattered light signal from an extremely small volume of - 0 . 1 mm 3 occupied by dispersed particles, which made it possible to obtain the local average and fluctuating characteristics. 2.

EXPERIMENTAL

SETUP AND MEASURING

APPARATUS

For carrying out the experiments we used a universal setup intended for obtaining multiphase EHD flows. The system was the same as that described in [2], which made it possible to generate a vapor jet in quiescent air and introduce corona ions into it. The experimental setup and the corresponding apparatus are shown schematically in Fig. 1. Vapor flows from the cylindrical nozzle 1 with diameter 2ro=2.8 ram. The special design of the working section [2] made it possible to vary the temperature of the vapor TO at the nozzle exit at a constant vapor flow rate G - 1 g/see and thereby obtain regimes with different condensation levels. The temperature of the ambient medium T~, which was also regulated during the experiments, has an important influence on the condensation rate [1--4]. On the axis of the vapor jet at a distance l 1 = 17 mm from the nozzle exit we mounted a corona electrode (needle) 2, which enabled us to create a positive or negative corona discharge. Downstream from theneedle at a distance/2= 150 mm from the nozzle exit we mounted a high-transparency grounded grid 3, to form the second corona electrode (needle-to-plane system). In all the experiments the vapor nozzle potential was equal to the needle potential, which ensured that the corona current was carded only downstream from the needle. During the experiments we measured the electrical parameters: the needle potential r the needle current J2, and the grid current "/3; by means of special capacitive decoupling we recorded osciUograms of the variable component of the needle current Moscow. Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No.4, pp. 28-35, JulyAugust, 1992. Original article submitted December 25, 1991. 470

0015-4628/92/2704-0470512.50 9 1993 Plenum Publishing Corporation

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Fig i (for a negative corona), which provided information on the time-dependent characteristics of the corona discharge (Trichel frequencies [5, 6]). Two methods of optical investigation of the dispersed phase in the jet were used. The first, integral method [7, 8], based on the theory of small-angle light scattering from particles, enabled us to determine the mean Sauter particle diameter d32 = (d3)/(d2} and the volumetric moisture content of the droplets c s averaged over the length of the probing laser beam. The second, local method consisted in the following. The jet was probed with a laser beam generated by a one-watt LGN402 argon laser (L 1 in Fig. 1). This power (combined with the optical probing beam focusing and scattered light receiving systems) was sufficient for reliably recording the light scattered from a volume V - 0 . 1 mm 3. The scattered light is was recorded at an angle 3' =90~ to the primary laser beam. In the experiments all the optical systems and the measuring volume V were fixed in space, and the various measuring points in the jet were investigated by displacing the entire nozzle system (including the corona electrodes) in the longitudinal and transverse directions. The condensation jets in question are characterized by turbulent gas dynamic fluctuations within the jet and co~esponding acoustic fluctuations in the surrounding space. The characteristic frequency v of this process in a cross section x of the main part of the jet can be found approximately from the relation Sh = p d / u - 1, where d and u are the width and average velocity of the jet in that cross section. The dimensions of the droplets in the jets investigated varied from a - 10-3--10 -2/xm (nuclei) to a-- 1 ~m (developed droplets). Accordingly, to a considerable extent the droplets track the turbulent fluctuating motion of the medium and the light signal they scatter i s contains average and fluctuating c o m p o n e n t s : i s = ( i s ) + i s. In addition to the electrical and optical measurements, by means of a RFT microphone located outside the jet (M in Fig. 1) we also recorded the acoustic pulsations generated by the jet. The experimental information on the variable signals J~, is, and ia was recorded with a Sony multichatmel magnetograph and then processed by means of an Ono Sokki analyzer. 3.

INTEGRAL

CHARACTERISTICS

For corona discharge in the presence of electric condensation it is necessary to take into account the following new processes: nucleation on negative and positive ions, possibly at different rates; droplet growth and a simultaneous increase in droplet charge Q as a result o f the inductive deposition of corona ions. The first of these effects is illustrated in Fig. 2, where we have plotted the dependence of the moisture content c s in the section x / r o = 4 0 on the temperature TO for ~ = - 12, + 12, and 0 kV (curves 1--3). If the temperature TO is high enough, vapor supersaturation conditions are not present in the turbulent jet and there will be no condensation either when ~ = 0 or when ~ # 0. As TO decreases, when ~ = 0 only a slight increase in c s is observed: hardly any condensation develops. However, switching on the discharge fundamentally changes the situation: the moisture content increases approximately by two orders and the intensity o f the reflected light from the particles in the plane of the knife-edge beam is sufficient to record a distinct dispersedphase jet. It is important to note that the intensification of condensation is greater for negative than for positive corona ions. These data can be explained on the basis of the following points. Firstly, under certain conditions the nucleation rate 1 for ions (of either sign) exceeds the value of I for an electrically neutral vapor mixture. Secondly, because of the polarity of the water molecules on the surface of the nuclei, irrespective of their uncompensated electric charge Qs, an electric double layer with a negative external surface charge is formed. This leads to the free polar water vapor molecules oriented along the field 471

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of the charge Qs being more easily adsorbed by the negative nuclei in the nucleation process and to an increase in the effective nucleation rate (we note that here we have used the rough qualitative explanation of the process given in [9]). Thirdly, it can be shown that for the same voltages the negative corona is characterized by a greater ion concentration than the positive. Thus, the electric condensation effect itself can be explained by the first of these points (compare curves 1 and 2 with curve 3). However, the relative position of curves 1 and 2 is related to the other two processes, which act in the same direction. We also carried out experiments to determine the effect on condensation of the polarity of the corona discharge for the same discharge current J ~ =J~-. While in both cases the condensation is intensified, the moisture content does not depend on the polarity o f the discharge: c + = c s . This can be explained as follows. It can be shown that when J~-=J~- the ion concentration in the negative corona is lower than that in the positive corona. (This result follows, for example, from the onedimensional equations of electrodynamics for a corona discharge when the different ion mobilities b - > b + and the different values of the corona ignition fields E + >E~- are taken into account.) Accordingly, in this case the greater intensification of nucleation on negative ions is compensated by their lower concentration. We now turn to the description of the experiments to investigate the effect of electric charge exchange between the ionic component and the dispersed phase on the integral current characteristics of the corona discharge. We will first give an estimate of this effect. We begin by determining the droplet charge and mobility. In accordance with the experimental data, in the developed condensation zone the Sauter diameter d32, which in the first approximation can be taken as the characteristic physical particle size 2a, is 1--2/zm. The maximum droplet charge Qs, acquired in the corona field as a result of the directional motion of the ions, is equal in order of magnitude to 3a2Eo., where E ~ is the effective corona field. Then, when Ee, = 10 kV/cm, /z = 2" 10-1 g/(cm.see) the droplet mobility bs= Qs/(67rl.ta), where/~ is the dynamic viscosity of the carrier medium, will be of the order of 10 - 2 cm2/(V-sec), which is two orders less than the ion mobility [10, 11]. In reality, the droplet mobility is even less than this, since we have overestimated the charge Qs. The current Ys, transported by the dispersed phase, "Is ~ nsvsQsE, where n s is the droplet concentration, v s is the average droplet velocity, and E is the cross-sectional area of the EHD jet. The 9 quantity n s can be found from the relation cs=nsTrd~2/6, where c s and d32 are experimentally determined quantities; in the first approximation the droplet velocity v s is equal to the velocity of the carrier medium; the quantity E can be determined experimentally or calculated from the theory of turbulent jets. Taking c s - 10 -6, d32-2/zm, Qs=O.75d22Er E ~ = 10 kV/cm, vs - 10 m/sec, and E = 3 cm 2, we obtain "/s= 1/xA. Thus, the dispersed-phase current constitutes a significant and, in some cases, decisive fraction of the corona current. Since the droplet mobility is much lower than the ion mobility, we may expect a considerable decrease in the corona current in the electric condensation regime. Thus, in the experiments, when the electric condensation regime was achieved by reducing the vapor temperature TO at the nozzle exit, the discharge current J2 changed from 3 to 1.6/zA as TO was reduced from 410 to 390~ (T6, =291~ ~o=-11 kV). The transport of part of the discharge current by the charged dispersed phase is most graphically illustrated by the results of measuring the electric current J3 at the grounded grid 3. In the absence of condensation Y3 ~-J2, since the ions, which possess high mobility, are all captured by the grid electrode. However, the charged droplets with their low mobility pass through it together with the gas dynamic flow into the surrounding space and, if I s - J2, the grid current will be significantly less than the needle current. Thus, when T0=391, To, =291~ and r - 7 kV, the ratio I3/52=0.4. As the potential ~ increases, so does this ratio, since the ion concentration in the condensation zone increases and not all the ions become condensation nuclei (there is a limiting equilibrium moisture content) or are "precipitated" on the droplets, and to an ever greater degree the discharge 472-

Fig 4 current is determined by the ion current. 4.

FLUCTUATING

CHARACTERISTICS

The fluctuating characteristics of the electric condensation jet were found by recording and processing the time signals: the fluctuations of the discharge current J~ and the light scattered by the droplets is in the integral measurement system [laser dispersity meter (4) based on the small-angle method] and in the local measurement system, and the acoustic fluctuations ia . Figure 3 shows the frequency vm corresponding to the maximum in the power spectrum of the variable signal i s as a function of the potential ~ of the negative corona (curve 1). The same figure also shows the ~ dependence of the Trichel pulse frequency Vrng for the signal J~ (curve 2). The straight line 3 corresponds to the characteristic gas dynamic frequency of the turbulent jet fluctuations in the corona discharge zone. The experimental material reveals some interesting (and, at first glance, unexpected) features of the processes in question. When the corona discharge is switched on and the overvoltage ~ / ~ . = 1 (~, = 4 kV is the corona ignition potential) in the time realization i s (0 and the corresponding power spectrum low frequencies ( - 100 Hz) appear, whereas the characteristic frequency for the signal i s in the absence of a corona is 2 kHz. It is important to note that the latter value coincides with the gas dynamic frequency (straight line 3). However, as the corona overvoltage rises, the frequency of the signal i s ( t ) increases monotonically, again approaching a frequency of 2 kHz. Another feature of these results is the fact that for small overvoltages the characteristic frequency of the signal is coincides with the Trichel pulse frequency. However, as the parameter ~ / ~ . increases, the Trichel frequencies increase, becoming greater than the characteristic gas dynamic frequency v - 2 kHz. These observations can be explained as follows. At small overvoltages the negative corona is characterized by a low-frequency discontinuous structure, which indicates that the motion of the ions in the interelectrode gap is pulsed (stratified). Therefore the condensation process developing on the ions and hence the signal corresponding to the light flux scattered by the droplets are also discontinuous. Of course, the maximum frequency in the power spectrum must approximately correspond to the droplet cluster repetition frequency, i.e., must be close to the Trichel pulse frequency. As ~ increases, this frequency increases sharply, the distance between the "ion clusters" (and hence droplet clusters) decreases and, as ;,m---,v,the usual turbulent fluctuations of the dispersed-phase begin to predominate. Confirmation of this discontinuous condensation structure is provided by photographs of the condensation jet obtained by means of a powerful "knife-edge" laser beam in the absence (Fig. 4a) and in the presence (Fig. 4b) of "electric" condensation (in Fig. 4b a condensation cluster is clearly visible). Further indirect proof of this physical model is provided by an experiment which showed that intensified condensation also results from introducing into the vapor-air jet smoke particles on which heterogeneous condensation develops. In this case, as in the corona, the value of c s increased sharply and that of d32 decreased; however, the values of v m in the is(t) signal power spectrum remained the same as in the absence of smoke particles. Of no less interest is the investigation of the correlations between the acoustic signal ia picked up by the microphone (see Fig. 1) and the Trichel signal J~. The microphone was fairly large and recorded the acoustic noise emitted by the entire jet. Therefore in the absence of electric condensation the time realization does not have sharply expressed peaks, and the power speclrum lacks a distinctive frequency. 473

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However, when the corona is switched on, the time realization of the ia(t) signal changes radically: sharply expressed pulses following each other at a fixed frequency appear. Figure 5 presents the time realizations of the ia(t) signal for ~o=0 (trace 1) and ~;a0 (trace 2) and the time realization for the discharge current (trace 3) (Trichel realization). The experimental conditions were: T0=391, T= =288~ ~ = 4 . 0 kV. There is no doubt about the existence of a correlation between the signals i a for ~or and J~. This shows that the motion of the condensate droplets in the form of "clusters" following each other at the Trichel frequency PmE generates in the surrounding space an acoustic field whose spectrum is characterized by a clearly expressed frequency that coincides with VinE. However, we still lack a complete physical model of the effect. Of course, the fullest information about the fluctuations in the EHD flow is provided by the results of the local optical measurements (the signal is, t from the light scattered by the small volume V= 0.1 ram3). For this signal we found the variants, the power spectrum and the autocorrelation function at various points in the jet. We also found the cross-correlation R(z) between the signals i's,t(t) and the variable component ~ ( t ) of the needle current with characteristic Trichel frequency Vm/~. As an example, in Fig. 6 we have reproduced the function RO') obtained for the section x/ro=30 at points y = 0 (trace 1) and y=0.96, where 6 is the radius of the gas dynamic jet in that section (trace 2) for To=385, To. =286~ r - 3 . 4 kV. The data clearly illustrate the existence of a clearly expressed correlation between the electrical and optical signals, the correlation peak decreasing from - 0 . 6 on the jet axis to - 0 . 2 on the periphery. These maxima of the R(z) curves correspond to the values Zm=0.59 msec for the point y = 0 and zrn =0.98 msec for the point y =0.96. The fact that ~'ra~ 0 is attributable to the finite time of passage along the jet (from the needle to the section in question) of the ion cluster, which during its motion is transformed into a droplet cluster. Since the velocity on the jet axis is greater than that on the periphery, the time zrn is less for the point y = 0 than for the point y=0.96.

474

5.

CONCLUDING

REMARKS

The experiments convincingly show that by intensifying the condensation process a corona modifies the entire gas dynamic flow: a dispersed phase appears, the temperature of the medium as a whole increases, there are changes in the various fluctuating characteristics, etc. We note that the power N E of the corona discharge is equal to N E = ~ J 2 = 10 - 2 W; in the experiments the initial thermal power of the jet GCpT0 (where G is the mass vapor flow rate, TO is the temperature at the nozzle exit, and cp is the specific heat of the vapor) was approximately 500 W, and the thermal power Nk= GCpAT released in the jet as a result of condensation was - 1 0 W (for an experimental value of the increase in the temperature of the medium A T = 5 - - 1 0 ~ Thus, the ratio NflN E - 10 -3, i.e., the consumption of an extremely small amount of electrical energy leads to a significant energetic effect. In this case the corona serves as a seed mechanism, supplying ions on which nucleation develops. This conclusion can also be illustrated by considering the dimensionless interphase interaction parameters. Under the experimental conditions the EHD interaction parameter IIE=eniE~l/(pv2 ) (where e and n i are the ionic charge and ion concentration, E = and l are the characteristic electric field and the characteristic dimension of the corona system, and p and v are the density and velocity of the carrier medium), characterizing the interaction between the ion component and the carder medium, is a small quantity. When E ~ = 10 kV/cm, l=5 era, ni= 108 cm -3, and v - 10 m/sec, we have HE-. 10 -2. At the same time, for a droplet cluster the parameter characterizing the interaction between the dispersed phase formed and the carrier medium IIs=os/p=47ra3nso~ (where n s is the droplet concentration, 00 is the density of water, and n s can be taken to be approximately equal to ni) has the value IIs=0.4. Accordingly, it is reasonable to expect the dispersed phase to have a certain influence on the turbulent characteristics of the jet. REFERENCES

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.

A.B. Vatazhin, V. A. Likhter, and V. I. Shul'gin, "Turbulent condensation jets and the possibility of controlling them by means of an electric field," in: Problems of Modern Mechanics, Pt. 1 (ed. L. I. Sedov) [in Russian], Izd. MGU, Moscow (1983), p. 113. A.B. Vatazhin, R. S. Valeev, V. A. Likhter, et aI., "Investigation of turbulent vapor-air jets in the presence of condensation and foreign particles introduced into the flow,"Izv. Akad. Nauk SSSR, Mekh. Zhidk. Gaza, No. 3, 53 (1984). A.B. Vatazhin, A. B. Lebedev, and V. A. Mareev, "Mathematical modehng of various condensation regimes in turbulent isobaric jets," lzv. Akad. Nauk SSSR, Mekh. Zhidk. Gaza, No. 1, 59 (1985). A.B. Vatazhin, A. Yu. Klimenko, A. B. Lebedev, and A. A. Sorokin, "Homogeneous condensation in turbulent submerged isobaric jets," Izv. Akad. Nauk SSSR, Mekh. Zhidk. Gaza, No. 2, 43 (1988). N.A. Kaptsov, The Corona Discharge [in Russian], Gostekhizdat, Moscow (1947). A.B. Vatazhin, V. A. Lik.hter, and V. I. Shul'gin, "Frequency and current-voltage characteristics of a corona discharge in a gas flow," Teplofiz~ Vys. Temp., No. 1, 1. A.G. Golubev and V. I. Yagodldn, "Optical methods of measuring aerosol dispersity," Tr. TslAM, No. 828, 21 (1977). V . I . Yagodkin and A. G. Golubev, "Method of determining the dispersity and concentration of atomized liquid droplets from the integral characteristics of scattered light," Tr. TslAM, No. 867, t4 (1979). P.C. Raist, Introduction to Aerosol Science, New York (I984). I.P. Vereshchagin, V. I. Levitov, G. Z. Mirzabekyan, and M. M. Pashin, Fundamentals of the Electrogasdynamics of Disperse Systems [in Russian], t~nergiya, Moscow (1974). A.B. Vatazhin, V. I. Grabovskii, V. A. Likhter, and V. I. Shul'gin, Elecrrogasdynamic Flows [in Russian], Nauka, Moscow (1983).

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