An experimental investigation of the effects of nozzle ellipticity on the flow structure of co-flow jet diffusion flames

An Experimental Investigation of the Effects of Nozzle Ellipticity on the Flow Structure of Co-Flow Jet Diffusion Flames

N. Papanikolaou and I. Wierzba

Department of Mechanical and Manufacturing Engineering

University of Calgary

Calgary, AB T2N 1N4

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 1.60. 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. Measurements of the fuel concentration at the jet centerline indicated that the mixing process was enhanced with the 1.38 aspect ratio jet compared with the 1.60 aspect ratio jet. On the basis of the obtained experimental data, it was suggested that the higher blowout limits of attached flames for an elliptic jet of 1.38 aspect ratio was due to higher entrainment rates.

Introduction

A jet diffusion flame in a co-flowing stream can be extinguished with increasing jet velocity either as a lifted flame or an attached flame depending on the value of the co-flowing stream velocity. The flame blows out as a lifted flame at low co-flowing stream velocities and as an attached flame at higher stream velocities. Generally, the blowout limits of jet diffusion flames decrease with an increase in the co-flowing stream velocity (Fig. 1). Different techniques can be employed to enhance the flame stability characteristics.

Non-circular jets have been investigated extensively in the past [1-14]. It was found that for cold elliptic jets with small or moderate aspect ratios, the mass entrainment in the near-nozzle region was three to eight times (depending on the axial location) higher than that in an axisymmetric jet [1]. Hence, elliptic jets, when used in burners, may produce improvements in the stability limits of flames depending on the co-flowing stream velocity [15-17]. Papanikolaou and Wierzba [16] employed elliptic nozzles in an effort to increase the blowout limits of jet diffusion flames issuing into a co-flowing 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 2.0). In the present work, elliptic nozzles were employed to investigate the flow structure of jet flames using a laser Doppler velocimeter (LDV) and hence provide an explanation for the experimentally observed blowout behaviour.

Nomenclature

a/b = Major-to-minor axes ratio of elliptic jet

R_eq = Equivalent nozzle radius [= (a·b)^½ ], mm

R = Radial distance, mm

u_j = Jet velocity, m/s

u_s = Co-flowing stream velocity, m/s

x = Axial downstream distance, mm

x_LL = Axial downstream distance where the jet fuel centerline concentration is equal to the lean flammability of the methane in air
d = Jet half-width, mm

Apparatus and Experimental Procedure

The tests were performed in a square (127 mm x 127 mm) vertical combustion chamber equipped with two opposing optical windows to facilitate laser-Doppler velocimetry and flame visualization studies (Fig. 2). A blower supplied the co-flowing air. The combustor could be fitted with nozzles of different shapes in the centre of the chamber. The jet nozzles were long circular or elliptic stainless steel tubes of approximately constant cross-sectional area. The elliptic nozzles with aspect ratios, a/b, of 1.29, 1.38 and 1.60 were made by pressing a 4.6 mm (inner) diameter circular tube into an oval shape, which resulted in the corresponding discharge areas of 15.9, 15.6 and 14.9 mm², respectively. The lip thickness of all of these nozzles was kept constant at 0.89 mm. The length of the nozzles was made fifty times the diameter of the circular jet and fifty times the major axis of the elliptic jets to ensure a fully developed jet flow at the nozzle exit. A schematic of a typical nozzle is shown in Fig. 3. Technical quality methane (>98%) was employed as the primary jet fuel.

Axial and radial velocity and turbulence intensity measurements were made using a two-colour LDV with an estimated control volume of size (0.043, 0.040, 0.778) mm. The origin of the Cartesian coordinate system (x,y,z) was located at the centre of the discharge plane of the nozzle. Aluminium oxide particles with an average size of 0.3 mm were employed in seeding both flows. Measurements were performed on cold and ignited jets at axial distances downstream from the nozzle varying from 1.0 mm to 79 mm.

Results and Discussion

As shown in Fig. 1, the nozzle ellipticity did not appear to affect significantly the blowout limits of lifted flames, but it did play a significant role for attached flame stability. It was observed that within the attached flame blowout limits there existed an optimum aspect ratio (i.e., a/b =1.38) for co-flowing stream velocities ranging from 1.0 to 1.45 m/s.

LDV measurements reveal that, in general, the effect of nozzle geometry on the flow structure within a cold jet occurred primarily in the region relatively close to the jet exit. The time-averaged axial and radial velocity profiles were obtained at different downstream distances for jets with two different aspect ratios, a/b, of 1.38 and 1.60 (Figs. 4 and 5). They are plotted in term of dimensionless radial distances, r/R_eq, where the equivalent nozzle radius, R_eq , is defined as (a·b)^½. A co-flowing stream velocity, u_s , of 0.65 m/s corresponded to the region of lifted flame blowout, and a positive radial velocity indicates an outward flow. It can be seen that the jet expands faster and becomes wider in the minor axis plane than in the major plane at a certain distance from the nozzle. This was also evident by the small, and frequently negative, radial velocities in the major axis plane, while the radial velocities in the minor axis plane were always positive and several times higher than those in the major plane. Due to this greater jet spread in the minor axis plane, the jet appears as if it has switched the orientation of its cross-section, i.e., axis-switching has occurred.

The corresponding turbulence intensity level profiles for these elliptic jets are shown in Fig. 6. The turbulence intensity is defined as u'/u, where u' is the local rms value and u is the local time-averaged axial velocity. In general, a higher peak turbulence intensity level was observed in the minor axis plane than in the major axis plane. In the near-field region of the jet, the turbulence intensity level profiles of the 1.38 and 1.60 aspect ratio jets were very different; however, at a downstream distance of 60 mm, the turbulence intensity profiles in both of the elliptic jets became similar. The time-averaged jet axial velocity profiles were also very similar at this downstream distance. Therefore, a change in the nozzle aspect ratio did not seem to have a considerable effect on the flow field at these downstream distances. Similar trends were also observed by Trentacoste and Sforza [2] who conducted experiments with cold free rectangular jets and found that the mass entrainment rates of such jets became relatively indistinguishable from those of an axisymmetric jet in its far-field. These researchers also found that the entire flow approached axisymmetry regardless of the nozzle geometry. Since at this co-flowing stream velocity (of 0.65 m/s), the flame (just before blowout) stabilized at a considerable distance from the nozzle for all of the different nozzles employed, changes in the nozzle geometry would not affect significantly the blowout limits of lifted flames as was established experimentally.

As expected, the presence of a flame can significantly change the flow characteristics of the jet. The variation of the jet half-width, d, usually used to correlate jet growth [1,8], with the downstream distance, x/R_eq, is shown in Fig. 7 (a) for cold and ignited jets issuing from a nozzle of aspect ratio 1.60 with a discharge velocity, u_j , of 7.2 m/s. It can be seen that the axis-switching phenomenon has occurred in both the cold and ignited jets. However, the presence of the flame retarded the jet spread, delayed the axis-switching phenomenon, and decreased the decay of the centerline velocity, u_cl (Fig. 7 (b)). The turbulence intensity was also suppressed in the presence of a flame (Fig. 8). At a downstream distance of 30 mm, for instance, the maximum turbulence intensity decreased from 74% in a cold flow to 21% in a reacting flow for a jet with an aspect ratio of 1.60. This is in agreement with observations reported in literature on large jets in a quiescent environment. Browand and Ho [18] reported that a large part of the fluctuating velocity was contributed by the large coherent structures developed in the jet, and Savas and Gollahalli [19] showed that the growth of these structures in the near-nozzle region of an attached flame was retarded in the presence of a flame.

At a co-flowing stream velocity 1.21 m/s, i.e., in the region of attached flame blowout, a change in the nozzle shape considerably affected the flame blowout limits (Fig. 1). The flame stability, in this case, is governed by the near-nozzle conditions. It can be seen that the blowout limits of the jet with an aspect ratio of 1.38 were significantly greater than those of the circular jet and elliptic jets with a lower aspect ratio of 1.29 and a higher aspect ratio of 1.60 at co-flowing stream velocities ranging from 1.0 to 1.4 m/s. Schadow et al. [15] who conducted experiments on large nozzles also reported the existence of an optimum jet aspect ratio, however, it was with respect to large-scale entrainment and higher combustion efficiency.

The variations of the jet spread with the downstream distance are compared for two different elliptic jets with a jet discharge velocity of 3.4 m/s in Fig. 9. Along the downstream distance up to x/R_eq = 37 for jets issuing from the 1.38 aspect ratio nozzle, it appears that axis-switching occurred twice (at x/R_eq = 21 and x/R_eq = 28). There was no axis-switching observed for the 1.60 aspect ratio jet. These results would indicate indirectly that the 1.38 aspect ratio jet was associated with enhanced air entrainment that resulted in greater blowout limits than those for the 1.60 jet. This was confirmed with gas analysis measurements. The axial distance, x_LL, where the jet fuel centerline concentration is equal to the lean flammability of the methane in air was used as an indication of the rate of mixing within the jet. A shorter distance would signify an enhanced mixing rate. Figure 10 shows that the limit concentration was reached closer to the jet exit with the nozzle of aspect ratio 1.38 than with the circular or the 1.60 aspect ratio jet.

Summary

The effect of the nozzle geometry on the flow structure within cold and ignited jets was investigated at two different co-flowing stream velocities corresponding to the regions of lifted and attached flame blowout. It was found that the effect of the nozzle ellipticity on the jet flow structure is significant in the near-field region of the jet. In the far-field region, where lifted flames stabilize, the flow structure is approximately the same for the two aspect ratio jets investigated. Consequently, the nozzle aspect ratio would not affect as significantly the blowout limits of lifted flames as it would for attached flames which are governed by near-nozzle conditions.

Acknowledgements

The financial assistance of the Natural Sciences and Engineering Research Council of Canada (NSERC) and the Alberta Heritage Fund is gratefully acknowledged.

Presented at DINAME 99 – 8th International Conference on Dynamics Problems in Mechanics, 4-8 January 1999, Rio de Janeiro. RJ. Brazil. Technical Editor: Hans Ingo Weber.

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Publication Dates

• Publication in this collection
  28 Sept 2001
• Date of issue
  2001