EFFECTS OF DIRECT CURRENT ELECTRIC FIELD ON THE BLOWOFF CHARACTERISTICS OF BLUFF-BODY STABILIZED CONICAL PREMIXED FLAMES

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1 Combust. Sci. and Tech., 177: , 2005 Copyright Q Taylor & Francis Inc. ISSN: print/ x online DOI: / EFFECTS OF DIRECT CURRENT ELECTRIC FIELD ON THE BLOWOFF CHARACTERISTICS OF BLUFF-BODY STABILIZED CONICAL PREMIXED FLAMES A. ATA J. S. COWART A. VRANOS B. M. CETEGEN* Mechanical Engineering Department, University of Connecticut, Storrs, Connecticut, USA An experimental study was conducted on the stability enhancement of conical premixed flames by application of direct current electric fields. Turbulent conical premixed flames were stabilized at the tip of a circular cylindrical bluff-body flame holder. An electric field was set up between a positively charged upper electrode and a grounded flame holder to determine its effects on the lean limit stability characteristics. In these experiments, the flame blowoff equivalence ratios were determined as a function of mixture velocity, electric field strength, and the electrode configuration. It was found that the most pronounced effects were observed at the lowest mixture velocities in this study of about 5.0 m=s with the influence of the electric field virtually disappearing at higher velocities of 10 to 15 m=s. The maximum reduction in blowoff equivalence ratios was 4 to 5% at the low-velocity conditions. These findings are consistent with the estimates of the ionic wind velocities expected Received 7 November 2003; accepted 8 December Our interest in this problem was stimulated with discussions of BMC and AV with Dr. H. Calcote. The funding of this work was provided by a seed grant from the University of Connecticut Research Foundation. A. Ata acknowledges the financial help in the form of teaching assistantship during part of this study. *Address correspondence to cetegen@engr.uconn.edu 1291

2 1292 A. ATA ET AL. in hydrocarbon=air flames and point to the rather weak electric field effect for applications in high-speed premixed flame stabilization. Keywords: premixed flames, blowoff, bluff-body, electric field, ionic wind INTRODUCTION Electrical properties of flames have been studied by a number of investigators since the 1950s. These studies have primarily focused on the presence, identification, and generation mechanisms of ionic species in flames as well as the interaction of flames with imposed electric fields. Flame ionization of chemical species has long been recognized and it has been utilized for measurement of hydrocarbon species. Electrical properties of flames have been studied extensively by Lawton and Weinberg (1969) and Lawton et al. (1968). Calcote (1962, 1963), Green and Sugden (1963), van Tiggelen (1963), Poncelet et al. (1956), and Bulewicz and Padley (1963) have identified in the neighborhood of 50 possible ionic chemical species between the mass numbers of 1 to 67 in hydrocarbon flames. Among these many different chemi-ions, the most likely ones in all flames correspond to CH þ 3,H 3O þ,cho þ, and C 3 H þ 3. The concentration of chemi-ions in flames has been measured and found to peak near the flame front, where exothermic combustion reactions take place. It has been also indicated by Calcote et al. (1967) and Green and Sugden (1963) that only a small concentration of negatively charged chemi-ions are present in hydrocarbon flames with up to 99% of the negative charge being carried by free electrons. The concentration of the positive chemi-ions has been determined to range between 10 9 and ions=cc in premixed hydrocarbon=air flames. The maximum concentrations are a function of the fuel type and the overall combustion stoichiometry. It has been reported by Calcote (1962) that the peak concentration levels are four to five times greater in acetylene flames as compared to methane, propane, and ethylene flames. The peak concentration dependence on the equivalence ratio resembles that of the flame temperature in that peak concentrations occur near stoichiometric conditions and fall off on both fuel-lean and fuel-rich sides. Many suggestions have been made to explain the high levels of ionization in flames as extensively reviewed by Calcote (1957). The proposed mechanisms include thermal ionization, ionization due to translational or electronic excitation, as well as chemi-ionization. Ionization due to

3 EFFECTS OF ELECTRIC FIELD ON FLAME BLOWOFF 1293 collisional energy transfers between high-temperature species have been deemed unlikely based on the momentum exchange considerations. However, the ionization due to exchanges of electronic excitations has been suggested to be one of the more likely mechanisms of ion formation. A third possible mechanism is chemi-ionization, which involves production of ionic species as a part of the chemical reactions. Another advanced mechanism concerns the ionization caused by high-energy electrons in some flames. Although none of these mechanisms are believed to be solely responsible for the ion production in flames, a combination of the most likely mechanisms produces the ion concentrations found in flames. One practical aspect of the presence of positive chemi-ions in the flame zone is the possibility of affecting flame stability by application of electric fields on flames. In some of the early work on electric field flame interactions, Weinberg and coworkers (Lawton and Weinberg, 1969; Lawton et al., 1968; Browser and Weinberg, 1972) and Calcote and coworkers (Berman et al., 1991; Calcote, 1949; Calcote and Berman, 1989; Calcote and Pease, 1951) as well as others (Bradley and Nasser, 1984; Noorani and Holmes, 1985) have found a significant effect when an electric field is applied in the flame stabilization region. For example, the Bunsen burner flames studied by Calcote and Pease (1951) were stabilized at leaner fuel=air stoichiometries in the presence of an imposed electric field. The electric potentials in the range of a few to tens of kilovolts have been applied between a positively charged upper electrode and a grounded burner rim to improve the flame blowoff characteristics. Enabling flame stabilization at leaner fuel=air stoichiometries produces beneficial effects of reducing NO x emissions chiefly by the lower peak flame temperatures. Berman et al. (1991) demonstrated significant reductions in NO x levels from premixed methane=air Bunsen burner flames at electric potentials of a few kilovolts. The objective of the present study was to explore the effects of direct current (DC) electric fields on the lean limit stability of bluff-body stabilized inverted conical premixed flames of propane and air. In the remainder of this paper, the experimental systems are first described followed by the results obtained for two electrode configurations. EXPERIMENTAL SYSTEMS The experiments were performed using an axisymmetric burner, shown schematically in Figure 1. The burner was made out of brass in the shape

4 1294 A. ATA ET AL. Figure 1. Schematic of experimental setup. of a converging cylindrical tube with an inner diameter of 128 mm at the base and 40 mm at the exit within a total height of 225 mm. At the burner exit plane, a stainless steel rim with a height of 19 mm was attached to prevent any damage to the brass burner in case of flame attachment to the uncooled brass burner rim. A cylindrical rod flame holder with a diameter of 6.0 mm protruded from the burner exit plane by 2.0 mm. The flame holder was held in place by a cradle at the base of the burner. Internal to the burner, a honeycomb flow straightener and a layer of stainless steel fine-mesh screen were placed before the contraction section to condition the incoming flow into the burner. The fuel and air were first premixed in a mixing chamber consisting of a cylindrical tube with two fuel jets entering at right angles to the tube axis. Two sets of perforated plates were placed in the chamber to promote vigorous mixing. The fuel=air mixture was fed into the burner through eight radial inlets around the burner body. Air was supplied by a compressor system and its mass flow rate was measured by a set of critical flow orifices. Fuel flow was controlled by two electronic mass flow controllers (Porter 202 series and Tylan FC-280 series). Commercial-grade propane was used as the fuel in all the reported experiments. The composition was stated by the supplier to contain a minimum of 96% C 3 H 8 by volume with the

5 EFFECTS OF ELECTRIC FIELD ON FLAME BLOWOFF 1295 Figure 2. Burner rim and the electrode configuration details. remainder being other hydrocarbons and inerts. Mean flow velocities up to 15 m=s could be attained with this burner configuration and the capacity of the fuel=air supply system. Based on the uncertainties in the measurements of the fuel and air flow rates, the uncertainty in the determination of the equivalence ratio was less than 2%. Two different electrode configurations and two types of bluff-body flame holders were used in these experiments. The two upper electrode configurations are shown schematically in Figure 2. One of them consisted of a metal ring supported by a holder and it was placed concentrically above the cylindrical bluff-body flame holder as shown. This upper-ring electrode was elevated to a high positive voltage with respect to the grounded burner body and the flame holder. In this configuration, it was intended to influence the chemi-ions in the vicinity of the flame anchoring region. However, experiments with this configuration yielded virtually no influence on the blowoff characteristics of the flame with applied electric potentials up to 8.0 kv and various standoff distances between the upper electrode and the flame holder. To concentrate the electric field to the flame anchoring region more effectively, a second upper electrode configuration was employed as shown in Figure 2. In this configuration, the upper electrode is a blunt-tip cylindrical stainless steel rod with a diameter of 3.0 mm. The electrode was placed coincident with the axis of the flame holder and charged to a high voltage. The effectiveness of the electric field on the chemi-ions present in the flame depends not only on the distribution of the electric field intensity but also the electrical properties of the medium occupying the space

6 1296 A. ATA ET AL. Figure 3. Flame holder tip geometries: (a) right circular cylinder and (b) cylindrical cavity. between the electrodes. Because the conductivity of the hot flame products in the recirculation zone behind the bluff-body is typically an order of magnitude larger than that of the cold mixture, a second bluff-body flame holder with a tip cavity design was employed as shown in Figure 3. In this design, the metal part of the flame holder was recessed back with a ceramic cylindrical cavity above it to alter the characteristics of the medium between the two electrodes. In the following section, the results from the interaction of the flame with the applied DC electric fields are presented and discussed. RESULTS AND DISCUSSION Experimental results presented in this section are related to the influence of the DC electric fields on the blowoff characteristics of the turbulent inverted conical premixed flames. For the cold flow conditions, the Reynolds numbers based on the bluff-body flame holder diameter and the gas mixture approach velocity were calculated as 2016, 4200, and 5885 for approach velocities of 5.2, 10.4, and 15.0 m=s, respectively. These Reynolds number values are reduced by a factor of approximately 25 if the Reynolds number is based on the hot wake temperatures of the order of 2000 K. The jet Reynolds number at the burner exit range between 12,700 and 37,000 for the burner exit velocities of 5.2 to 15.0 m=s. The profiles of the mean and turbulent axial velocity were measured using a hot film anemometer 18 mm upstream of the bluff-body tip. The measured profiles shown in Figure 4 exhibit a slight skewness of the p ffiffiffiffiffiffi mean velocity profile toward the bluff body. The turbulence intensity, u 02 =um, is found to be less than 2% in the core of the flow and reaches 12% in the boundary layers near the wall and the bluff-body. Based on these and

7 EFFECTS OF ELECTRIC FIELD ON FLAME BLOWOFF 1297 Figure 4. Distribution of mean velocity and turbulence intensity across the nozzle radius between the nozzle inner wall and bluff-body surface at 18 mm upstream of the bluff-body tip. other similar measurements at higher approach velocities, it was determined that the flow approaching the flame holder and the anchored conical flame is spatially uniform and of low turbulence intensity. Effect of DC Electric Field on the Flame Blowoff Characteristics The blowoff characteristics of conical premixed flames were mapped by determining the mixture equivalence ratio at which the flame detaches and blows off the flame holder at a given approach velocity of the combustible mixture. In these experiments, the fuel flow rate was gradually reduced until the flame blowoff was realized. The mixture equivalence ratio and the mixture approach velocity were calculated at the condition of flame blowoff. It was observed in all cases that the flame lift-off from the bluff-body and blowoff conditions were essentially the same such that under no conditions could a lifted flame be stabilized in this configuration. The blowoff characteristics are shown in Figure 5 for these inverted conical flames stabilized at the tip of a circular bluff-body flame holder.

8 1298 A. ATA ET AL. Figure 5. Blowoff characteristics of turbulent conical premixed flames without and with electric field interaction for the simple flame holder configuration. As expected, the blowoff equivalence ratio is a function of the mixture approach velocity with flame blowoff occurring at higher equivalence ratios with increasing approach velocity. This finding is a well-established characteristic of premixed flame blowoff curves that have been presented for different flame holder geometries and stabilization configurations in the literature. It should be noted here that the blowoff data reported here are for the geometry of the needle-type upper electrode whose tip was placed 18 mm downstream of the bluff-body flame holder as shown in Figure 2. In the absence of the electric field, the blowoff equivalence ratio first increases with increasing mixture velocity, but its variation becomes stronger at high mixture velocities. This is expected because the stabilization of the flame at high velocities becomes more difficult and the flame stabilization region becomes very sensitive to small velocity or equivalence ratio perturbations. Additionally, determination of the blowoff point in turbulent flames is not precise in that repeated determinations of the blowoff equivalence ratio can present significant variations. In this work, the blowoff data were repeated a number of times until the limits of the blowoff equivalence ratio were established.

9 EFFECTS OF ELECTRIC FIELD ON FLAME BLOWOFF 1299 Once the blowoff characteristics were established in the absence of the electric field, the electric field was turned on and the experiments were repeated. Figure 5 shows the effects of the electric field on the blowoff characteristics at 4.7 kv and kv voltages. It is seen that a modest improvement of flame stability (as evidenced by a reduction in blowoff equivalence ratio) can be achieved at low flow velocities. With increasing flow velocities, the electric field effect diminishes and it falls within the limits of fluctuations in the typical measurements. Furthermore, it was found in these experiments that the application of the higher electric fields could result in arcing through the high-electrical-conductivity medium of combustion products. Thus, the magnitude of the applicable electric field is limited by the electrical properties of the medium between the electrodes. In all the reported experiments, the shorting of the electric field by arcing between the electrodes was avoided. The data at higher electric potentials and flow velocities, shown in Figure 5 as kv, thus required adjustment of the maximum applied electric field between 6.1 and 7.8 kv. Specifically, the applied electric fields and the corresponding flow velocities were 6.1 kv at 7.5 m=s, 7.0 kv at 8.7 m=s, 7.5 kv at 11.2 m=s, and 7.8 kv at m=s. Although there appears to be a small systematic reduction in the blowoff equivalence ratio with increased electric field potential at higher flow velocities, the effect is small. A qualitative understanding of these experimental findings can be realized by estimation of the ionic wind velocity based on the previously developed theory of Lawton and Weinberg (1969), Lawton et al. (1968), and Calcote (1962), and comparison of these values with the recirculation zone velocities. In this theory, the migration of positive ionic species from the flame zone (typically H 3 O þ in hydrocarbon flames) induces an ionic wind that can be directed by application of a suitable electric field to enhance flame stabilization. Specifically, for the bluff-body flame holder configuration studied here, the ionic wind is directed from the positively charged upper electrode toward the base of the flame holder aiding the recirculating flow. The maximum ionic wind magnitude can be estimated from a momentum balance (see, for example, Lawton and Weinberg, 1969; Lawton et al., 1968) in the flow where ions drift under the imposed electric field: n induced ¼ ja 1=2 qk

10 1300 A. ATA ET AL. Figure 6. Photographs of the conical V-shaped, bluff-body stabilized flame (a) without electric field, (b) with electric field at 7.8 kv; U a ¼ 15.1 m=s, / ¼ 1:0, electrode separation of 18 mm. Upper positively charged electrode is the glowing circular rod in the flame. where j ¼ðE 2 kþ=ð8paþ is the current density (A=cm 2 ) expressed in terms of electric field strength in esu=cm, 1 k is the ionic mobility in cm 2 =(V sec), a is taken as the electrode spacing in cm, and q is the gas density in g=cm 3. Utilizing these relationships, the theoretical p maximum velocity can be determined from n max ¼ E= ffiffiffiffiffiffiffiffi 8pq. Evaluating the gas density at a wake gas temperature of 2000 K for / ¼ 0.8, the maximum velocities are calculated as 1.3 and 2.2 m=s for the corresponding electric field strengths of 2.6 and 4.3 kv=cm. These values of the field strength represent 4.7 and 7.8 kv potentials of the upper electrode. They can be compared with the typical maximum velocity in the recirculation zone toward the flame holder of about 0.3U a as determined from particle image velocimetry in the wake of rod-stabilized flames in the absence of electric field. For the three approach velocities (U a ) employed in this study, the recirculation zone velocities are 1.6, 3.1, and 4.5 m=s as compared to the maximum ionic wind velocities calculated earlier. This comparison suggests that the electric field effect is 1 In substitution of electric potential into these equations, the conversion factor of 1 esu ¼ V has to be employed.

11 EFFECTS OF ELECTRIC FIELD ON FLAME BLOWOFF 1301 of significance only at low approach velocities and the effect diminishes with increasing approach velocities. This is consistent with the blowoff data presented in Figure 5. Although the maximum decrease in the blowoff equivalence ratio was about 5% at a flow velocity around 5.0 m=s, the visual changes have been found to be present at even higher flow velocity conditions, which exhibited minimal effect on blowoff equivalence ratio. Figure 6 shows two images of the conical flame. In Figure 6a, the flame is stabilized in the absence of the electric field. The image in Figure 6b shows the flame when the electric field was switched on. It is seen that the chemiluminescent flame zone is pulled toward the flame holder, indicating the visual effect of the application of electric field on the flame stabilization zone. Figure 7 shows the experimental data obtained for the cavity-type bluff-body flame holder schematically shown in Figure 3 with the needle-type upper electrode. In this configuration, the spacing between the upper electrode and the metal part of the flame holder is increased by 7.0 mm due to the presence of the ceramic cavity. Thus, application of higher electric potentials was possible in this configuration. For Figure 7. Blowoff characteristics of turbulent conical premixed flames without and with electric field interaction for ceramic cavity-type bluff-body configuration.

12 1302 A. ATA ET AL. example, the maximum voltage applied to the upper electrode was 11 kv as opposed to 7.8 kv in the case of the simple circular metal flame holder. However, this difference is simply due to the increased distance between the electrodes and the maximum electric field intensity for both cases was about 4.4 kv=cm. The blowoff data obtained with this configuration are not significantly different than those shown in Figure 5. In this case, the data indicate that the 4.7 kv potential does not appear to be sufficient for any stability enhancement. However, there is a progressive trend of reducing the blowoff equivalence ratio with increasing electric potential, particularly at low mixture velocities. Once again, it is difficult to quantify the influence of the electric field at higher mixture velocities due to its small magnitude of the same order as the data scatter. Finally, the percentage reduction in the blowoff equivalence ratio, defined as D/ bo ¼ð/ noef bo / EF bo Þ=/noEF bo is shown in Figure 8. It is found that the highest reductions, of the order of 3 to 5%, are obtained at low mixture velocities around 5 m=s. As the mixture velocity increases, the effect is reduced and becomes of the same order as the uncertainty in the determination of the blowoff equivalence ratio. Figure 8. Percent reduction in blowoff equivalence ratios under the application of electric field as a function of combustible mixture velocity.

13 EFFECTS OF ELECTRIC FIELD ON FLAME BLOWOFF 1303 CONCLUDING REMARKS An experimental study was conducted to determine the feasibility of improving the lean limit stability of bluff-body-stabilized inverted conical flames by application of DC electric fields. For example, it has been shown in the literature that flame blowoff characteristics could be improved by electric fields in conical Bunsen burner flames stabilized around the burner rim. In the flames studied in this work, flames were stabilized at the tip of a circular cylinder flame holder about which a DC electric field was setup. The blowoff equivalence ratios were found to be marginally reduced with the application of electric field intensities between 2.6 and 4.4 kv=cm. The maximum improvements were limited to about 5% at mixture velocities of 5 m=s. At higher mixture velocities up to 15 m=s, the influence of the electric field was found to be of the same order of magnitude as the uncertainty in the determination of the blowoff equivalence ratios in these turbulent bluff-body-stabilized flames. Comparison of the estimated maximum ionic wind velocities induced by the applied electric field with recirculation zone velocities suggests that electric field effects are only significant at low approach velocities (5 m=s). This finding is consistent with the experimental results reported in this paper. It is thus concluded that the enhancement of bluff-body flame stabilization by applied DC electric field is limited by the magnitude of the applied field and the resulting ionic wind. The maximum applied electric field is in turn limited by the breakdown voltage at which the arcing ensues in the electrically conductive hightemperature combustion products occupying the space between the electrodes. Finally, cooling of the electrodes placed in the high-temperature combustion products poses another practical problem which may be alleviated by a proper cooling scheme. REFERENCES Berman, C.H., Gill, R.J., and Calcote, H.F. (1991) NO x reduction in flames stabilized by an electric field. ASME Fossil Fuels Combustion Symposium, PD- 33, 71. Browser, R.J. and Weinberg, F.J., (1972) The effect of direct electric fields on normal burning velocity. Combust. Flame, 18, 296. Bradley, D. and Nasser, S. (1984) Electrical coronal and burner flame stability. Combust. Flame, 55, 53.

14 1304 A. ATA ET AL. Bulewicz, E.M. and Padley, P.J. (1963) A cyclotron resonance study of ionization in low pressure flames. Proc. Combust. Instit., 9, 638. Calcote, H.F. (1949) Electrical properties of flames: Burner flames in transverse electric fields. Proc. Combust. Instit., 3, 245. Calcote, H.F. (1957) Mechanisms for the formation of ions in flames. Combust. Flame, 1, 385. Calcote, H.F. (1962) Ion production and recombination in flames. Proc. Combust. Instit., 8, 184. Calcote, H.F. (1963) Ion and electron profiles in flames. Proc. Combust. Instit., 9, 622. Calcote, H.F. and Berman, C.H. (1989) Increased methane-air stability limits by a DC electric field. ASME Fossil Fuels Combustion Symposium, PD-25, Calcote, H.F. and Pease, R.N. (1951) Burner flames in longitudinal electric fileds. Ind. Eng. Chem., 43, 12, Calcote, H.F. Kurzius, S.C. and Miller, J. (1967) Negative and secondary ion formation in low pressure flames. Proc. Combust. Instit., 10, 605. Green, J.A. and Sugden, T.M. (1963) Some observations on the mechanism of ionization in flames containing hydrocarbons. Proc. Combust. Instit., 9, 607. Lawton, J. and Weinberg, F.J. (1969) Electrical Aspects of Combustion, Clarendon Press, Oxford. Lawton, J., Mayo, P.J., and Weinberg, F.J. (1968) Electrical control of gas flows in combusion processes. Proc. R. Soc. Lond. A, 303, 275. Noorani, R.I and Holmes, R.E. (1985) Effects of electric fields on the blowoff limits of a methane-air flame. AIAA. J., 23, 9, Poncelet, J., Berendsen, R. and van Tiggelen, A. (1956) Comparative study of ionization in acetylene-oxygen and acetylene-nitrous oxide flames. Proc. Combust. Instit., 7, 256. van Tiggelen, A., (1963) Shuler, K.E. and Fenn, J.B. (Eds.) Ionization in High Temperature Gases, Academic Press, New York, p. 165.