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1 THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS 345 E. 47th St., New York, N.Y AA;74 The Sodety shall not be responsible for statements or opinions advanced in papers or thicussion at meetings of the Society or of its Divisions or Sections, or printed in its publications. Discussion is minted only if the Paper is published in an ASME Journal. Authorization to photocopy, 0 material for Internal. or personal use under circumstance riot falling within the fair nseprovisions dem Copyright Act is granted by ASME to ' libraries and other users registered With the Ccpnight Clearance Center (CCC)Transactional Reporting Service provided that the base fee of per page is paid directly to the OCC, 27 Congress Street Salem MA Requests for special permission or bulk reproduction should be addressed to the ASMETedvical Publishing Department CopyrightO 1997 by ASME All Rights Resented Minted in U.S.A THE EFFECT OF CO-FLOWING STREAM VELOCITIES ON THE FLOW CHARACTERISTICS OF JETS ISSUING FROM ELLIPTIC NOZZLES , N. Papanikolaou and I. Wierzba Department of Mechanical Engineering University of Calgary Calgary, Alberta 12N 1N4 Canada (403) Fax: (403) ABSTRACT The flow structure of cold and ignited jets issuing into a co-flowing air stream was experimentally studied using a laser Doppler velocimeter. Methane was employed as the jet fluid discharging from circular and elliptic nozzles with aspect ratios varying from 1.29 to The diameter of the circular nozzle was 4.6 mm and the elliptic nozzles had approximately the same exit area as that of the circular nozzle. These non-circular nozzles were employed in order to increase the stability of attached jet diffusion flames. The time-averaged velocity and r.m.s. value of the velocity fluctuation in the streamwise and transverse directions were measured over the range of co-flowing stream velocities corresponding to different modes of flame blowout that are identified as either lifted or attached flames. On the basis of these measurements, attempts were made to explain the existence of an apparent optimum aspect ratio for the blowout of attached flames observed at higher values of co-flowing stream velocities. The insensitivity of the blowout limits of lifted flames to nozzle geometry observed in our previous work at low co-flowing stream velocities was also explained. INTRODUCTION Non-circular jets have been investigated extensively (Trentacoste and Sforza, 1967; ICrothapalli et al, 1981; Ho and Gutmark, 1987; Hussain and Husain, 1989; Schacbw et az 1984; Prabhu and Gollahalli, 1991; Gollahalli et al, 1992). Studies on cold jets conducted by Ho and Gutmark (1987) have shown that the mass entrainment in an elliptic jet with an aspect ratio of 2:1 were three to eight times greater along the potential core than that of an axisymmetric jet. It was suggested that this increase in the mass entrainment was largely due to an axis-switching phenomenon, where at some distance downstream of the point of discharge, the jet switched the orientation of its cross-section. This axis-switching is caused by a much greater jet spread in the plane of the minor axis of the nozzle than in the plane of the major axis. Papanambou and Wierzba (1996) employed elliptic nozzles in an effort to increase the blowout limits of jet diffusion flames issuing into a coflowing stream. It was found that the nozzle geometry did not appear to affect the blowout limits of lifted flames, but it did play a significant role for attached flame stability. Their experimental results showed that there existed an apparent optimum aspect ratio for attached flames issuing from relatively small elliptic nozzles of small aspect ratios (less than ). There is only a limited amount of data on the flow characteristics of small elliptic jet flames issuing into co-flowing air streams having velocities spcmer than 0.7 m/s. Paparulaabou and Wierzba (1997) measured axial and radial velocities, and turbulence intensities within small elliptic jets of crosssectional area 16.4 nun' and of aspect ratios ranging from 1.29 to 1.60 at two axial distances ( of 7.5 and 17 mm, i.e., x/r, = 3.6 and 8.1, respectively). They did not observe axis-switching in the near-field of the nozzles for the conditions employed. However, they did observe different rates of jet deformation relatively close to the exit of the jet Therefore the objeaives of the present work were to examine the flow-field further downstream of the nozzle exit for possible axis-switching as well as to investigate the effect of a co-flowing velocity on the jet flow characteristics. APPARATUS AND EXPERIMENTAL PROCEDURE The tests were performed in a square (127 inm x 127 mm) vertical combustion chamber equipped with two opposing optical windows to facilitate laser-doppler velocimetry and flame visualization studies. A blower supplied the co-flowing air. After passing through a honeycomb flow straightener, the air flow distribution was uniform across the combustion chamber. A schematic diagram of the apparatus is shown Fig. 1. The fuel jet was discharged into the co-flowing air stream from circular and elliptic stainless steel tubes. The nozzles with aspect ratios of 1.38 and 1.60 were made by pressing a 4.6 mm diameter circular pipe (with a discharge area of 16.4 mre) into an oval shape, which resulted in discharge areas of 15.6 and 14.9 min 2 for the nozzles with aspect ratios of 1.38 and 1.60, respectively. The nozzle cross-section was not truly an ellipse but will be referred to as elliptic for simplicity. The lip thickness of all of the nozzles was 0.89 mm. To ensure a fully developed jet flow, the length of the aorta was made fifty times the diameter of the circular jet and fifty times the major axis of the elliptic jets. The jet fuel was methane of technical quality. Velocity field and turbulence intensity measurements were made using Presented at the ASME ASIA '97 Congress & Exhibition Singapore September 30 October Z 1997 Downloaded From: on 07/22/2018 Terms of Use:

2 a two-colour LDV with an estimated control volume of (43, 40, 0.778)mm. The origin of the Cartesian co-ordinate system (x,y,z) was Iced at the centre of the plane of the nozzle. Aluminium oxide particles with an average size of 0.3 gm were employed in seeding both flows. Measurements were performed on both cold and ignited jets at axial distances varying from 1.0 nun to 60 mm from the nozzle. The jet velocity was varied ftom 3.4 m/s to II m/s at co-flowing airstream velocities of 0.65 In/sand 1.21 in/s. EXPERIMENTAL RESULTS AND DISCUSSION Effect of the Co-flowing Stream Velocity lime-averaged Axial Velocity. The time-averaged axial velocity profiles for two different nozzles and two different co-flowing stream velocities are shown in Fig. 2 at three different downstream distances. A cold jet was discharged into co-flowing streams with velocities of 0.65 and 1.21 m/s. A co-flowing stream velocity of 0.65 m/s was used since, for conditions of the experiment, it corresponded to the blowout of lifted flames, while a co-flowing stream velocity of 1.21 m/s corresponded to the blowout of attached flames (Papanikolaou and Wierzba, 1996). No significant effect of the nozzle shape was observed in the blowout of lifted flames; however, the blowout of attached flames was affected significantly by the nozzle geometry with the highest blowout limits recorded for a nozzle with an aspect ratio of 1.38 (Papanikolaou and Wierzba, 1996). The jet discharge velocity was held constant at 7.2 m/s. At this jet velocity and a co-flowing stream velocity, u,, of 1.21 m/s, all of the jets employed experienced blowout except for the 1.38 aspect ratio jet, and thus a comparison of the axial velocity profiles from elliptic jets was made on cold jets. The radial distance from the jet axis, y, is plotted in dimensionless form as y/r, where the equivalent nozzle radius, R is defined as (a b) 1/2 (Hussain and Husain, 1989). For the nozzles with aspect ratios of 1.38 and 1.60, R 2.15 and 8 mm, respectively. The rate of axial decay is a useful, indirect measure of the amount of mixing between the jet and the co-flowing air stream. This decay is shown in Fig. 3, where the time-averaged axial velocity on the jet axis u d, is a function of the downstream dimensionless distance, rjr,, from the nozzle exit As expected, the rate of axial velocity decay is considerably less with the presence of a flame. It is well established that the presence of a flame inhibits the growth of large coherent structures developed in the near-nozzle of the jet (Savas and Gollahalli, 1986), resulting in a slower axial decay in flame jets than in cold jets. Jet Half-Width. The radial distance from the jet axis to the location where the time-averaged axial velocity attains half of its maximum value, 6, is usually used to correlate jet growth (Trentacoste and Sforza, 1967; Gollahalli et al, 1992). The jet half-width variation with the downstream distance for both elliptic jets are compared in Fig. 4 at two different coflowing stream velocities. The jet discharge velocity was held constant at 7.2 rrils. There did not appear to be axis-switching in the near-field of the nozzle (up to x = 30 mm or x/r 14) for all of the conditions tested. However, further downstream at x = 45 mm (or x/r, 21), the jet spread in the minor axis greatly exceeded that in the major axis in cold jets. Therefore, it is reasonable to assume that axis switching occurred at some downstream distance between 30 and 45 mm for cold jets. The jet spread in the minor axis appears to be greatest for the 1.38 aspect ratio jet at the lower co-flowing stream velocity of 0.65 m/s. In the presence of a flame, the jet spread in the minor axis was considerably reduced for both jets and axis-switching occurred further downstream. Turbulence intensity. The effect of nozzle geometry on the turbulence level within cold elliptic jets is shown in Fig. 5 at various axial distances for a jet and co-flowing stream velocities of 7.2 and 0.65 m/s. respectively. The turbulence intensity is defined as u'at, where u' is the local tins value and u is the local time-averaged axial velocity. At axial distances of 1.0 and 30 mm, the turbulence level profiles of the 1.38 and 1.60 aspect ratio jets are very different. However, at a distance of 60 mm, these profiles appear to be approximately the same for both of the elliptic nozzles employed. Moreover, the time-averaged jet axial velocity profile and jet growth were also very similar at this distance. A change in the nozzle geometry did not seem to alter considerably the flow characteristics at this downstream distance. Since lifted flames stabilize at approximately this downstream distance at a co-flowing steam velocity of 0.65 rat's, this may explain why the blowout limits of lifted flames were not affected by a change in the nozzle geometry (Papanikolaou and Wietzba, 1995). Effect of the Jet Discharge Velocity The effect of the jet discharge velocity on the flow characteristics was also investigated at a co-flowing stream velocity of 1.21 m/s. Profiles of the time-averaged axial velocity at a downstream distance of 1.0 mm are shown in Fig. 6 for cold jets and for both elliptic nozzles. These velocity profiles are important since the stabilization region for attached flames is approximately at this distance at this co-flowing stream velocity. The jet discharge velocities were varied from 3.4 to 11 rn/s (Re = ). Ii can be seen that the velocity gradient at the jet border, where the flame is usually stabilized, am too steep and can exceed the critical value in all of the jets employed at a jet discharge velocity of II m/s. None of the jets employed could sustain a flame at this jet velocity, while at a jet discharge velocity of 7.2 m/s, only the jet with an aspect ratio of 1.38 could sustain a flame (Papanikolaou and Wierzba, 1996). The corresponding maximum jet velocity decay and jet half-width growth for these conditions are shown in Figs. 7 and 8 as a function of the downstream dimensionless distance from the nozzle exit. As expected, the rate of axial decay decreased with a decrease in the jet discharge velocity. It can be seen that the value of the jet discharge velocity affects very significantly the flow characteristics. For example, at u j =11 m/s, the jet growth starts immediately after discharge in both the minor and major axis planes. This behaviour did not occur for the lower co-flowing stream velocities of 72 and 3.4 m/s. Far the lower jet velocities, the jet growth was observed mainly in the minor axis direction at further downstream distances, with a much gmaterjet spread for the jet velocity of 7.2 m/s than for that of the 3.4 m/s. CONCLUSIONS Generally, all of the cold elliptic jets tested experienced a higher jet spread in the minor axis than that in the major axis, which could lead to axis-switching, at axial distances, x/r, z 14. The degree of this jet spread was dependent on the operating conditions. The presence of a flame reduced the jet spread in the minor axis and as a result delayed axis-switching. The flow characteristics of jets with different geometries became very similar at a lower co-flowing velocity of 0.65 rri/s at a relatively short distance from the nozzle (of x/r aj = 28.6). This explains why the nozzle geometry does not affect the blowout limits of lifted flames at this stream velocity. ACKNOWLEDGEMENTS The financial assistance of the Natural Sciences and Engineering Research Council of Canada (NSERC), ZONTA International and the Killam Foundation is gratefully acknowledged. 2 Downloaded From: on 07/22/2018 Terms of Use:

3 NOTATION a/b major-to-minor axis; aspect ratio Rol equivalent radius, = (eh)" dme-aventged axial velocity, m/s time-averaged axial velocity at the jet axis, m/s 111 discharge jet velocity, m/s u, co-flowing stream velocity, m/s mu value of axial velocity, m/s axial distance, mm radial distance, mm a velocity-half radius, mm REFERENCES Gollahalli, Sit, Khanna, T. and Prabhu, N., 1992, "Diffusion flames of gas jets issued from circular and elliptic nozzles,' Combustion Science and Technology. Volume 86, 1992, pp Ho, C. M. and Gutinark, E., 1987, "Vortex induction and mass entrainment in a trnall-aspect ratio elliptic jet," Journal of Fluid Mechanics, Volume 179, pp Hussain, F. and Husain. HS., 1989, "Elliptic jets. Part 1. Characteristics of unexcited and excited jets." Journal of Fluid Mechanics, Volume 208, pp Krothapalli, A., Baganoff, D., and ICantmcheti. IC, 1981, "On the mixing of a rectangular jet; Journal of Fluid Mechanics, Volume 107, pp Papanikolsou, N. and Wierzba, L, June 1996a, 'Effect of burner geometry on the blowout limits of jet diffusion flames in a co-flowing oxidizing stream,' Journal of Energy Resources Technology, Transactions of the ASME, Volume 118, pp Papmikolacm, N. and Wierzba, L, 1996b, *The effects of burner geometry and fuel composition on the stability of a jet diffusion flame," Proceedings of the Energy Week Conference & Exhibition, ASME Emerging Energy Technology, Houston, TX, pp Papan&olaou, N. and Wierzba. 1., 1997, 'The structure of jet diffusion flames issuing into a co-flowing air stream," Proceedings of the Energy Week Conference & Exhibition, ASME Emerging Energy Technology, Volume 8, Houston, TX. pp Prabhu, N. and Gollahalli, S.R., 1991, "Effect of aspect ratio on combustion characteristics of elliptic nozzle flarnes,"asme Emerging Energy Technology, Volume 36, 1991, pp Sams, 0. and Gollahalli. SR., July 1986, 'Flow structure in near-nozzle region of gas jet flames,' AIAA Journal, Volume 24, Number 7, pp Schadow, ICC, Wilson, KJ., Lee, MJ. and Gutmark, E., 1984, 'Enhancement of mixing in ducted rockets with elliptic gas-generator nozzles,' Proceedings roan AMA Conference, Paper No Tien Lacoste, N., and Storm, P., May 1967, "Further experimental results for three-dimensional free jets," AIM Journal, Volume 5, Number 5 pp Vranos, A., Taback. ED. and Shipman. CW., June 1986, "An experimental study of the stability of hydrogen-air diffusion flames," Combustion and Flame, Volume 12, pp Wierzba, I., Kar, K., and Karim, G.A., June 1993, "An experimental investigation of the blowout limits of a jet diffusion flame in co-flowing streams of different velocity and composition," Journal of Energy Resources Technology, Volume 115, pp Combustion Chamber 6- LDV Pilot Nozzle Honeycomb Flow Straightener Co-flowing Air Stream Fig. 1 A schematic diagram of the experimental set-up Exhaust Jet fuel 3 Downloaded From: on 07/22/2018 Terms of Use:

4 Time-averaged axial velocity (m/s) Time-averaged axial velocity (m/s) x= 1.0 mm Minor axis Major axis Minor axis Major axis Pe.ar -, Time-averaged axial velocity (m/s) b. 0 - I..... x= 1.0 min Minor axisi 0 65 Major axis I Minoraxis % 1.21 Major axis "trait-t z 7.0 g 3.0 at, cis u = 7 2 m/s aib= 1.38 Radial distance (y/r,,,) Radial distance x = 6 mm 00E09.1_ flr"cnesis w to P Radial distance (y/ft, ) (c) Time-averag ed axial velocity (m/s) u = 7 2 rn /s. a/b = 1.60 Radial distance (y/13,0 Radial distance (y/r ) I z 7.0 x = 6 mm 8 g "" CD 3.0 eta, fs. tat:2n natansi cti 1.0 -nenireet I= Radial distance (y/11,,,) (c) Fig. 2 Time-averaged axial velocity profiles within cold jets for two nozzle aspect ratio. Open symbols: minor axis plane. Filled symbols: major axis plane. Figs. to (c) are for the 1.38 aspect ratio jet and Figs. (d) to (0 are for the 1.60 aspect ratio jet at different downstream distances. 4 Downloaded From: on 07/22/2018 Terms of Use: 1/4

5 1 _ u1 = 7.2 m/s S. u, (m/s) sib o-o 0.65 Cold 1.21 Cold 1.38.tt) Flame Cold a-a 121 Cold 1.60 ' Flame N.* Axial distance (x/r,i) Fig. 3 Axial variation of the centerline velocity in cold and ignited jets issued from elliptic nozzles at two different co-flowing stream velocities. u =.65 O m/s; Li /. 7.2 m/s; a/b = 1.38 tk, = 0.65 m/s; 5.( ominor 1 cold jet p. - Major I Minor) Flame Jet / s--6 Major crg.c 3.0 u = 7 2 m/s - a/b = 1.60 ' Minor} cold Jet Major Minor} Flame Jet Major / ' Axial distance (UR.) Axial distance (x/r,) = 1.21 m/s; u i = 7.2 m/s; a/b = 1.38 = 1.21 m/s; uj = 7.2 m/s; a/b = Cold Jet it Cold Jet 3 il to 1.0 it "" e Axial distance (x/r,q) Axial distance (x/r.) Rg. 4 Axial variation of the jet half-width in cold and ignited jets issued from elliptic nozzles at two different co-flowing stream velocities. Open symbols: minor axis plane. Filled symbols: major axis plane. 5 Downloaded From: on 07/22/2018 Terms of Use:

6 Axial turbulence Intens ity (%) u s = 0.65 m/s; u j = 7.2 m/s; a/b = Axial turbulence intensity (%) us = 0.65 m/s; uj = 7.2 m/s; a/b = x = 1.0 mm 8 o--o Minor axis 6 s- Major axis :t 2? 40, Radial distance (y/r,4 ) Radial distance (y/rsq ) (d) Axial turbulence intensity (%) C o-0 x = 3 mm - o Ci6 <9 0, floss-s_,, Radial distance (y/r, q ) Axial turbulence Intensity (%) I.1) 4.1 I 1, 4_ 6_ x = 3 mm ' M 70 Radial distance (y/r e ) (e) ' Axial turbulence intensity (%) %*41 x = 6 mm %000 S Axial tu rbulence intensity (%) Radial distance (y/r e ) Radial distance (y/r ed (0) (f) Fig. 5 Tunoaveraged axial turbulence levels within cold jets for two nozzle aspect ratio. Open symbols: minor axis plane. Filled symbols: major axis plane. Figs. to (c) are for the 1.38 aspect ratio jet and Figs. (d) to (f) are for the 1.60 aspect ratio jet at different downstream distances. 6 Downloaded From: on 07/22/2018 Terms of Use:

7 Time-averaged axial velocity (m/s) Time-averaged axial velocity (m/s) Time-averaged axial velocity (m/s) Radial distance (y/r) Radial distance (y/1710 ) 1 x = 1.0 mm; us = 1.21 m/s a/b = Radial distance (y/r,,) (c) Fig. 6 Time-averaged axial velocity profiles within cold jets for circular and elliptic nozzles at x = 1.0 mm. Open symbols: minor axis plane. Filled symbols: major axis plant UI e--e 3.4 Maximum velocity, ud us = 1.21 m/s ui (m/s) Cold Jets e-o 11 o- a 7.2 } v-v 11 '.-'< ' 3.4 a Axial distance (x/r a ) 3 Fig. 7 Axial variation of the centerline velocity in cold and ignited jets issued from elliptic nozzles at three different discharge jet velocities. Us = 1.21 m/s; a/b = cc 5 2 to Axial distance (x/r, ) u3 = 1.21 m/s. a/b = Axial distance (x/r) Hg. 8 Axial variation of the jet half-width in cold jets issued from elliptic nozzles at two different jet velocities. Open symbols: minor axis plane. Filled symbols: major axis plane 7 Downloaded From: on 07/22/2018 Terms of Use: