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Effect of injection typology on turbulent homogeneous mixing in a natural gas swirl burner
Experimental Thermal and Fluid Science 21 (2000) 162±170
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Eect of injection typology on turbulent homogeneous mixing in a natural gas swirl burner Giulio Solero, Aldo Coghe
*
Dipartimento di Energetica, Politecnico di Milano, Facolt a di Ingegneria Milano Bovisa, Via la Masa, 34, 20156 Milano, Italy Received 20 June 1999; received in revised form 15 November 1999; accepted 1 December 1999
Abstract The three-dimensional gas velocity and fuel concentration ®eld in the turbulent mixing zone of a natural gas swirl burner have been investigated in the isothermal case by means of laser Doppler anemometry (LDA) and laser sheet visualisation (LSV). The two techniques enabled useful insight into the fully turbulent, three-dimensional swirling ¯ow to be gained for various operating conditions and fuel injection typologies. The eect of swirl number and fuel injection mode, radial or coaxial, has been quanti®ed on both the ¯ow®eld structure and fuel concentration distribution. The results indicate that the unmixedness index, U, may be a signi®cant parameter to judge the mixing eciency due to swirl strength and fuel±air momentum ratio in a speci®c burner geometry, at least under isothermal conditions in the near-®eld zone downstream the quarl exit. Ó 2000 Elsevier Science Inc. All rights reserved. Keywords: Injection typology; Homogeneous mixing; Natural gas; Swirl burner; Turbulent mixing zone
1. Introduction Non-premixed swirling ¯ows are widely used in industrial combustion systems, notably, gas turbines, boilers and furnaces, because of safety and stability reasons. Swirl increases fuel±air mixing, improves ¯ame stabilisation and has a strong in¯uence on ¯ame characteristics and pollutant emissions. Although the swirling ¯ows have been extensively used in combustion design and it is known [1] that the swirl can signi®cantly aect the NOx emission, the relationship between the combustion characteristics of swirling ¯ames and pollutant formation still needs to be established [2]. Among the methodologies under development to minimise the environmental impact of this type of combustion systems, the most promising are those based on the improvement and optimisation of the mixing process between the reactants and cold fresh combustible mixture with hot gas products. It is well known [3±5] that, independently of the combustion technology used, any improvement of combustion performance relative to
* Corresponding author. Tel.: +39-02-23998537; fax: +39-0223998566. E-mail address: [email protected] (A. Coghe).
pollutant formation, stability and overall eciency requires a careful study of the mechanism of mixing and entrainment in high turbulent reacting ¯ows. In many combustion devices both the reactants are in the gas phase and for technological reasons the coaxial geometry is commonly used to merge the two streams; the swirl motion of the main ¯ow is used to improve ¯ame stability and enhance mixing process [6,7]. The ®ne structure of the resulting mixture and the mechanism of ¯ame stabilisation, related to the recirculation zone induced by the swirling ¯ow pattern, control the combustion process and pollutant formation. At moderate swirl intensity, the interpenetration process between the two reactants is due to a strong shear that determines the entrainment rate of the slow stream by the fast one and the formation of local stoichiometric conditions. A common feature at high swirl intensity (swirl number, S > 0:6) is a recirculation bubble in the vicinity of the fuel jet outlet. The recirculation regime sustains the entrainment process of the outer stream into the inner one and may induce a low-frequency, precessing mode that is distinct from the jet-preferred mode. The recirculating regime also presents the capability of an ecient mixing between the streams in the region near the fuel outlet, therefore leading to a rapid homogenisation of the combustible mixture and a shortening of the combustion chamber.
0894-1777/00/$ - see front matter Ó 2000 Elsevier Science Inc. All rights reserved. PII: S 0 8 9 4 - 1 7 7 7 ( 9 9 ) 0 0 0 6 7 - 9
G. Solero, A. Coghe / Experimental Thermal and Fluid Science 21 (2000) 162±170
The scaling of pollutant emissions in industrial ¯ames is very dicult, because of the complex geometry of the burner and the many parameters involved. The experimental evidence is that each burner is a unique device and even small geometrical changes can in¯uence the level of emissions. It is also known that swirling ¯ows require detailed measurements of the ¯ow structure and speci®cation of the inlet conditions [8]. The ¯uid dynamic analysis is very useful to provide preliminary information on the mixing process, even if the investigation is performed under isothermal conditions. In fact, one approach is to begin with detailed measurements of the ¯ow structure and speci®cation of the inlet conditions for the isothermal ¯ow, without the complications of chemical reactions, considering that the chemical kinetic time scales are usually shorter than the turbulent time scales. It is also important to know at what distance from the injector outlet the fuel has been completely mixed and diluted in the swirling air stream at the molecular level. More generally, one might be interested in characterising the turbulent ¯ow ®eld and the fuel concentration distribution at several cross-sections downstream of the injector outlet. The primary interest is in the near-®eld region since this area is the location in which most of the mixing and reactions take place. Owing to the high complexity of the ¯ow ®eld, the experimental characterisation has been usually performed through non-intrusive optical techniques capable of quantitative point measurements and qualitative visualisations [9,10]. In the past years, we have investigated dierent fuel injection typologies: transverse radial and coaxial injection into a co-¯owing air stream with variable swirl strength and co-¯ow to fuel momentum ratio [11]. This paper reports on the main results of the experimental characterisation of the isothermal mixing process in the near-®eld of natural gas burners. The study has been limited to this primary mixing region because the main characteristics remain almost the same in the presence and in the absence of chemical reactions, so that an isothermal analysis can provide preliminary information about the nature of the subsequent combustion process. The main parameters investigated were: air swirl, fuel inlet geometry and fuel to air velocity momentum. The
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optical techniques used were all based upon the Mie scattering principle: laser Doppler anemometry (LDA) and laser sheet visualisation (LSV) integrated by quantitative image analysis. The ultimate goal of the research is to provide a data base to test submodels for CFD computations and to acquire an understanding of the three dimensional velocity ®eld and fuel concentration distribution in the primary mixing zone, produced by both, ¯ue gas recirculation and swirl, in a turbulent non-premixed swirling reactor similar to many industrial combustion systems. 2. Experimental set-up The experimental apparatus consists of a laboratory scale of a swirl burner mounted in a fully transparent chamber, relatively large to produce little eect upon the ¯ow pattern near the burner exit, where the isothermal conditions could be assumed reasonably representative. A schematic diagram of the burner is shown in Fig. 1. The burner has a centrally positioned fuel delivery tube coaxial with the swirling air¯ow; the swirl motion is imparted through an axial air plus tangential air entry (Burner A) or a mechanical swirler (Burner B), with a divergent quarl exit similar to those used in many industrial burners. Air was supplied by a compressor and injected by four radial and four tangential inlets located on the wall of the Burner A and swirl was controlled by varying the relative amounts of axial and tangential air¯ow. Burner B is characterised by a constant, moderate swirl strength. For the isothermal analysis air was also supplied through the central pipe, instead of methane. Two dierent fuel injection typologies have been investigated: (a) transverse injection through six holes (1.25 mm inner diameter), inclined by 90° to the co-¯owing air stream and located at the end of the cylindrical section upstream the quarl (Burner A); (b) axial injection through a single-hole nozzle (5 mm inner diameter) with the tip protruding 6 mm into the quarl (Burner B). The relevant dimensions of the burner are summarised in Table 1. In the con®guration A, dierent solutions were also tested in the past: the 90° injection was obtained through radial, co-swirl and counter-swirl
Fig. 1. Schematic view of the investigated burner.
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Table 1 Relevant burner dimensions (mm) Burner inner diameter, D2 Fuel tube diameter, D1 Co-axial nozzle diameter, d0 Radial nozzle diameter, dj Burner quarl exit diameter, Dbq Burner quarl divergence angle Test chamber
35.5 18.5 5 1.25 60
Burner A and B Burner A and B Burner B Burner A Burner A and B
30 (°)
Burner A and B
300 300 450
Burner A and B
orientation of the six holes and the dierent solutions were compared at ®xed fuel±air momentum ratio and variable swirl strength. The results indicated a moderate eect of the orientation of the injection holes on the mixing eciency, particularly at high swirl intensity [12]. In this paper, the reference case with radial injection will be only considered. The velocity measurements have been performed by a dual-beam, two-components LDA system operated in back-scatter with a 5 W Argon-ion laser (Spectra Physics, mod. 2020), which allowed contemporary evaluation of axial and tangential or radial and tangential velocity components. Sensitivity to the ¯ow direction was provided by a 40 MHz light frequency shifting through a Bragg cell. Measurement volume dimensions were around 0.1 mm diameter by 1.0 mm length. Oil droplets of nominal 1 lm size were used as tracers, by adding the particles to the surrounding air stream and/or the gaseous fuel, depending on the measurement objectives. The light scattered by the particles was focused onto preampli®ed photomultipliers and the Doppler signals were monitored by a 58N10 PDA Dantec processor interfaced with a PC through a DMA module. At moderate seeding density, the velocity signals presented rather high validation rates, at several kHz; higher seeding density was necessary for the planar visualisations. Errors in the mean axial and swirl ve-
locities were estimated at about 4%, having increased the number of samples in the more turbulent regions. LSVs were obtained by means of a pulsed copper vapour laser (Oxford Lasers, mod. CU15A) with an average power of 15 W and capable of 10 kHz pulse repetition rate. Each pulse has a duration of 50 ns and a system of cylindrical and spherical lenses and mirrors provided a light sheet about 1 mm thick which intersected the burner axis normally at dierent downstream distances from the nozzle (see Fig. 2). The images, produced by the light scattered by the same oil particles used for LDA, were recorded at 90° by a CCD camera with the exposure time set to 10 ms; each image was thus averaged over 100 consecutive laser pulses. The images were analysed and processed through the Image Pro Plus software, in order to improve image quality and contour de®nition after background subtraction. Further processing was obtained by means of specially developed routines. 3. Experimental results The experimental analysis was performed at dierent axial distances from the exit plane of the burner quarl from X =Rbq 0:1±1, as shown in Fig. 1. The standard operating characteristics of the two burners are summarised in Table 2; further details of Burner A have been already reported [11]. In the table the initial air velocity is referred to the inlet cross-section of the burner quarl, while the fuel injection velocity refers to the nozzle outlet. In both burner geometries an important parameter is the square root of ÿthe momentum ÿ ratio, denoted by M, and de®ned as qf Uf2 = qa Ua2 , where f and a denote fuel and air exit conditions. The quantity Uf is the injection velocity of the fuel at the nozzle exit and Ua is the axial bulk velocity of the coaxial air¯ow at the upstream end of the burner quarl (see Fig. 1). In homogeneous ®elds as is the case in the present isothermal experiments, with air used instead
Fig. 2. Schematic view of the experimental set-up (LDA and LSV).
G. Solero, A. Coghe / Experimental Thermal and Fluid Science 21 (2000) 162±170
with fuel only, while C 0 the case with air only. The above de®nition has the advantage that:
Table 2 Operating conditions of the two burners Parameter
Burner A
Burner B
Initial air velocity (m/s) Fuel injection angle (°) Fuel injection velocity (m/s) Fuel±air velocity ratio Overall equivalence ratio (U) Swirl number, S Co-¯ow, Re 104
28 90 150 5.36 0.83 1 54,000
7 0 40 5.71 0.83 0.5 13,500
of natural gas, the square root of M reduces to the velocity ratio, ru Uf =Ua , reported in Table 2. This ratio is an important parameter that controls the penetration of transverse injection into the main air stream and the mixing of fuel and air in the coaxial geometry. Burner A is designed to operate at high swirl (S 1) and the natural gas is supplied through six holes oriented at 90° with respect to the main air stream and located on the periphery of a central fuel pipe. The injection cross-section is 3 mm upstream the inlet of the burner quarl, to produce a partially premixed ¯ow at the outlet. The results are presented for the high swirl case (S 1), together with the no-swirl reference condition (S 0). Burner B, which is characterised by fuel supply through a single co-axial hole, can be operated at a relatively low swirl (S 0:5), in a condition more representative of many commercial burners where, for technological reasons, the coaxial geometry is commonly used to merge the two streams of reactants and the swirl strength is low to limit pressure losses. This burner con®guration has the practical advantage that the fuel supply pressure can be very low, compared with the Burner A. Swirl number, S, is de®ned as the ratio of the ¯ux of angular momentum to the ¯ux of axial momentum divided by the radius of the burner [13]. The swirl number reported in Table 2 has been evaluated by numerical integration of the axial and circumferential velocity pro®les measured by the LDA along one diameter at the burner quarl inlet plane. The ®rst objective of the experimental investigation was to develop measuring techniques and data analysis procedures allowing the evaluation of the mixing eciency for each burner geometry and operating condition. The selected approach was to evaluate the entrained recirculated air¯ow through LDA measurements and to de®ne, as in [14,15], an unmixedness index, U, which can be evaluated through quantitative analysis of light distribution on the planar images: U
165
c02 ; C 1 ÿ C
where C stands for the mean fuel concentration and c0 is the rms value. The value C 1 represents the condition
U 0 means full mixing c0 0; C 6 0; U 1 maximum variance c0 C 0:5 and thus worst mixing; U 1 presence of only air C 0 or only fuel C 1: The mean concentration C may be determined from the laser sheet images by spatial averaging over a selected area. Our choice was to consider the circular area centred on the burner axis with the radial extension of the quarl exit. By comparing the results at several crosssections downstream the exit plane, the spatial evolution of the mixing process can be analysed and a numerical index used to compare dierent operating conditions. In fact, the value of U is only dependent on the initial calibration of the technique, which requires to maintain constant the volume concentration of tracing particles in the seeded ¯ow and normalise the local concentration to the initial mean concentration C0 measured at the nozzle exit. The second step will be to determine the detailed distribution of the three mean velocity components, with related normal and shear stresses, and try to ®nd correlation between the ¯ow®eld structure and mixing ef®ciency. A preliminary analysis has been completed regarding the mean and ¯uctuating velocity components used to characterise the ¯ow structure of the two burners; these results will be presented ®rst. 3.1. Flow®eld structure The basic ¯ow®eld structure in the primary mixing region is presented in Figs. 3 and 4 for the Burner A for the conditions listed in Table 2 and swirl number S 1. The tangential and axial mean and ¯uctuating velocities are shown for three streamwise locations, X =Rbq 0:14, 0.5 and 1, starting from the exit plane of the burner quarl (X being the streamwise co-ordinate); the abscissa is the normalised radial co-ordinate Y/Rbq , where Rbq is the quarl radius. In the near-®eld zone of a swirling ¯ow like this, mean velocity similarity does not develop; hence, the results are presented normalised to the mean bulk velocity at the exit of the burner quarl, as deduced from the measured mass ¯ow rate. Examination of the radial pro®le of mean axial velocity reveals a well-pronounced central recirculation zone (CRZ), characterised by negative values of the velocity, caused by the adverse pressure gradient induced by the intense swirl. The recirculation zone extends in the streamwise direction beyond one Rbq , while in the radial direction ®lls almost entirely the quarl exit area, decreasing progressively downstream. From the axial velocity pro®les, the recirculated air mass has been estimated to be about 40% of the inlet mass ¯ow at X =Rbq 0:1, reducing to about 10% at X =Rbq 1.
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Fig. 3. Normalised radial pro®les of axial mean and ¯uctuating velocities at dierent distances X/Rbq from the eux (Burner A).
Fig. 4. Normalised radial pro®les of tangential mean and ¯uctuating velocities at dierent distances X/Rbq from the eux (Burner A).
The pro®le of mean circumferential velocity (Fig. 4) shows a concentrated vortex near the burner exit, with a core of solid-body rotation extending to about one fuel pipe radius from the axis. The velocity pro®le remains almost constant in the outer annulus, where it is probably distorted by the interaction with the radial fuel jets. Downstream, the maximum tangential velocity occurs
almost at the same radial distance from the axis as the peak of the axial velocity, showing the classical evolution from solid body rotation to Rankine type vortex, owing to turbulence dissipation eects, progressively going far away from the quarl region. The non-zero values recorded near the axis for all the traverses may arise from a residual alignment error or some instability of the central
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167
Fig. 5. Normalised radial pro®les of axial mean and ¯uctuating velocities at dierent distances X/Rbq from the eux (Burner B).
Fig. 6. Normalised radial pro®les of tangential mean and ¯uctuating velocities at dierent distances X/Rbq from the eux (Burner B).
vortex location, as suggested by the high rms values measured on the axis. The normalised rms axial and tangential components shown in Figs. 3 and 4 characterise the turbulence intensity levels. It is observed that inside the CRZ region the two rms levels are almost constant ( 0:5 Ubq ) and do not decrease axially. The tangential rms component remains uniform over a wider area, while higher values of the axial rms turbulence are located close to the positions of the highest velocity gradient in the inner shear layer of the annular air stream. Burner B has been operated at lower swirl intensity, S 0:5, and has been characterised in the presence and absence of the central fuel inlet. The ¯ow®eld structure shown in Figs. 5 and 6 refers to the case with no fuel loading and conditions listed in Table 2. The pro®les of the mean axial velocity components shown in Fig. 5 indicate that no reverse ¯ow occurs, even in the absence of the central fuel jet. The minimum streamwise velocity on the axis is always positive, indicating that the CRZ is not established at this swirl level and the ¯ow®eld has a wake-like structure induced by the nozzle holder. This conclusion applies to any level of fuel loading and is due to the low swirl intensity. At the
quarl exit plane the spreading of the air stream is not enough to ®ll the quarl cross-section and a small recirculation appears close to the wall. It is also seen that the momentum de®cit from the wake of the nozzle tip persists up to X =Rbq 0:5, as a result of a positive static pressure gradient on the axis, although the axial velocity pro®le is approaching a gaussian shape. Downstream, also the rms pro®les are progressively smoothed to an almost uniform level, which is reached at the cross-section X =Rbq 0:5. The pro®les of mean circumferential velocity in Fig. 6 show again a concentrated vortex, with a core of solidbody rotation extending to a narrow radial zone, of the order of the nozzle holder size, and indicate a limited spreading of angular momentum into the free stream. The non-zero values recorded near the axis for all the traverses, as already observed in Fig. 4, may again arise from a minor alignment error or, more probably, from some instability of the central vortex location, as suggested by the higher rms values measured on the axis and bimodal probability distributions. Considering the rms velocity ¯uctuations, a noteworthy feature is the signi®cant level in the turbulence
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values of the streamwise velocity in the wake of the nozzle and the distinct peak in the corresponding tangential component. Higher values of the axial and tangential rms turbulence are found close to the positions of the highest axial velocity gradient in the outer shear layer of the annular air stream. 3.2. Fuel concentration distribution The fuel concentration distribution was measured by LSV, in a number of cross-sections normal to the burner axis, starting from the quarl exit, after collection of the images by a CCD camera located at 90° (Fig. 2). The fuel line was seeded with oil droplets capable of tracing the gas diusion into the main air stream. Owing to the quasi-monodisperse size distribution of the droplets, the scattered light intensity can be assumed proportional to their number density into the illuminated cross-section. Further, the particle concentration is assumed proportional to the fuel concentration, although the evaluation of the absolute value is a very dicult task [16]. Under favourable conditions, it is possible to normalise to the initial mean concentration C0 at the nozzle exit and de®ne a relative quantity that can be representative of the fuel diusion and mixing. With Burner A, it was found more complex to apply this normalisation, due to the geometry of the transverse injection that complicated the measure of the initial mean concentration. Burner B allowed easier optical access to the nozzle exit and the normalisation was easily applied to the results. The spatially averaged fuel concentration C at each cross-section, normalised by the initial mean concentration C0 at the inner nozzle exit, was extracted from images as those shown in Fig. 7, taken by the planar visualisation system and representing the time averaged concentration ®eld at progressive increasing distances from the eux. Fig. 7 clearly shows the in¯uence of air swirl motion upon the progressive interaction and mixing of the fuel jets. For both burners, it was decided to present the results in terms of the quantity U, already de®ned, which represents the eciency of homogenisation of the fuel into the air stream. The parameter U is independent on the initial absolute concentration of tracing particles, injected into the fuel, provided the initial average value C0 remains constant and the same integration area is used to evaluate the spatially averaged mean and rms concentrations. By comparing the values of U at dierent cross-sections, it may be possible to evaluate the rate of homogenisation of the fuel in the co-¯owing air stream [12]. The results reported in this paper are intended as a preliminary investigation on the reliability of this procedure to characterise the mixing eciency and on the sensitivity to the eect of operating conditions. Table 3 reports the values of U for three cases representative of dierent injection conditions and swirl strengths. The data refer to the ®rst cross-section at the exit of the burner quarl and indicate the degree of homogenisation completed inside the quarl. The recirculating regime produced by the higher swirl (Burner A,
S 1) presents the larger capability of an ecient and precocious mixing between the fuel jets and the main air stream in the vicinity of the most probable combustion region. The reason is that, in this regime, the residence time of the outer stream incorporated in the recirculation zone is of the order of the mixing time, Rbq =u0 , u0 being the rms velocity of the turbulent mixing layers. This residence time is much longer than the transit time, Rbq /V, which characterise the ¯ow without swirl (S 0). In fact, the absence of swirl leads to a less ecient homogenisation on the same distance. The third case, Burner B at moderate swirl, presents a larger value of U, indicating a less ecient mixing for this co-axial geometry. It must be noted that the burner quarl exit plane is only four nozzle diameter downstream the fuel outlet. The above results are qualitatively deducible from the pictures shown in Fig. 7. It is observed that, in case of radial fuel injection without swirl, the six jets have merged in three zones, while they remained clearly distinct in presence of swirl, but show a larger diusion into the main air stream. The third case (Burner B) indicates that the inner central fuel jet destabilises slowly, due to the velocity ratio greater than unity. This observation is also con®rmed by Fig. 8, which reports the radial pro®le of fuel concentration at the ®rst measuring station downstream the quarl exit plane, for Burner B at X =Rbq 0:1. The fuel concentration remains constant along a short distance downstream from the injector, the inner unmixed region corresponding to the potential core of the fuel jet, and then decreases in the mixing zone where the two streams meet on the axis. A shortening of the potential core is expected for lower values of the fuel±air velocity ratio [15]; this eect was observed in the LSV pictures and found in the trend of U as a function of the velocity ratio (Fig. 9). The results indicate that the unmixedness index, U, may be a signi®cant parameter to judge the mixing eciency produced in a speci®c burner geometry by varying the swirl strength or the momentum ratio, at least under isothermal conditions.
4. Practical usefulness of the results and main conclusions The three-dimensional gas velocity and fuel concentration ®elds produced in the near-®eld turbulent mixing zone of a natural gas swirl burner have been investigated under isothermal conditions. The eect of swirl number and fuel injection mode, radial or co-axial, has been quanti®ed by means of LDA and LSV. The preliminary results indicate that the unmixedness index, U, may be a signi®cant parameter to judge the mixing eciency of a speci®c burner geometry, being sensitive to the swirl strength and the fuel±air momentum ratio, at least in the near-®eld zone downstream the quarl exit. The method is based on the LSV technique and its reliability strongly depends on the quality of the planar images and the accuracy of normalisation of the
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Fig. 7. Typical pseudo-color images of ``fuel'' distributions at increasing distances X/Rbq from the quarl eux for the following conditions: (a) Burner A without swirl; (b) Burner A with swirl.
Table 3 Unmixedness results Burner A Burner A Burner B
S0 S1 S 0:5
U 0:22 U 0:10 U 0:28
measured average concentration by the mean concentration at the nozzle exit. Correlations with the turbulent ®eld and shear stresses, measured by the LDA technique, are needed to complete the experimental
investigation under isothermal conditions, before starting the combustion tests. The results obtained could be helpful for a more thorough comprehension of the reactants mixing process in model of industrial burners, providing useful informations for optimization of burner design and operating conditions, aiming to improve combustion eciency and reduce environmental impact of fossil fuel combustion. Moreover, experimental data about the turbulent ¯ow ®eld can be used to validate results from numerical simulation through CFD codes.
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References
Fig. 8. Radial pro®le of ``fuel'' concentration near the burner quarl eux (Burner B).
Fig. 9. Unmixedness parameter U at X =Rbq 0:1 as a function of fuel±air velocity ratio (Burner B).
Acknowledgements The present experimental work has been performed at the CNR-TeMPE laboratories with partial support by MURST, under the national programme ``Combustion.'' The authors would like to thank Mr. G. Brunello for the helpful support in the experiments and image analysis. The assistance of Messrs. Benassi, Manfroi and Resnati in collecting the LDA and LSV data is appreciated.
[1] R.H. Chen, J.F. Driscoll, in: 23rd Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, PA, 1990, pp. 281±288. [2] T.S. Cheng, Y.C. Chao, D.C. Wu, T. Yuan, C. Lu, C. Cheng, J.M. Chang, Eects of fuel±air mixing on ¯ame structures and NOx emissions in swirling methane jet ¯ames, in: 27th Symposium (International) on Combustion, University of Colorado at Boulder, CO, August 1998, pp. 2±7. [3] V.J. Lyons, Fuel±air nonuniformity eect on nitric oxide emissions, AIAA J. 20 (5) (1981) 660. [4] T.F. Fric, Eects of fuel±air unmixedness on NOx emissions, J. Propulsion and Power 9 (5) (1993) 708. [5] J. Tomeczek, J. Goral, B. Gradon, Gasdynamic abatement of NOx emission from industrial natural gas jet diusion ¯ames, Comb. Sci. and Tech. 105 (1995) 55±65. [6] R. Hillemans, B. Lenze, W. Leuckel, Flame stabilisation and turbulent exchange in strongly swirling natural gas ¯ames, in: 21st Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, PA, 1986, pp. 1445±1453. [7] R.H. Chen, J.F. Driscoll, The role of the recirculation vortex in improving fuel±air mixing within swirling ¯ames, in: 22nd Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1988, pp. 531±540. [8] R.E. Charles, J.L. Emdee, L.J. Muzio, G.S. Samuelsen, The eect of inlet conditions on the performance and ¯ow®eld structure of a non-premixed swirl-stabilised distributed reaction, in: 21st Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, PA, 1986, pp. 1455±1461. [9] S.H. Starner, R.W. Bilger, Further velocity measurements in a turbulent diusion ¯ame with moderate swirl, Comb. Sci. and Tech. 63 (1989) 257±274. [10] D. Shen, J.M. Most, P. Joulain, J.S. Bachman, The eect of initial conditions for swirl turbulent diusion ¯ame with a straight-exit burner, Comb. Sci. and Tech. 100 (1994) 203±224. [11] A. Coghe, G. Solero, Analisi del miscelamento turbolento aria/ combustibile nel modello di un combustore a bassa produzione di NOx , Final Enea Report, 1997. [12] G. Solero, G. Brunello, A. Coghe, Eects of injection typology on turbulent mixing in the primary region of a homogeneous combustor, Combustion Meeting Õ98, Ravello, May 1998, pp. 26±28. [13] A.K. Gupta, D.G. Lilley, N. Syred, Swirl Flows, Abacus Press, Tunbridge Wells, UK, 1984. [14] A.D. Birch, D.R. Brown, M.G. Dodson, J.R. Thomas, The turbulent concentration ®eld of a methane jet, J. Fluid Mech. 88 Part 3, 1978, 431. [15] E. Villermaux, Mixing and spray formation in coaxial jets, J. Propulsion and Power 14 (5) (1998) 807±817. [16] Y.C. Chao, J.M. Han, M.S. Jeng, A quantitative laser sheet image processing method for the study of the coherent structure of a circular jet ¯ow, Experiments in Fluids 9 (1990) 323±332.
www.elsevier.nl/locate/etfs
Eect of injection typology on turbulent homogeneous mixing in a natural gas swirl burner Giulio Solero, Aldo Coghe
*
Dipartimento di Energetica, Politecnico di Milano, Facolt a di Ingegneria Milano Bovisa, Via la Masa, 34, 20156 Milano, Italy Received 20 June 1999; received in revised form 15 November 1999; accepted 1 December 1999
Abstract The three-dimensional gas velocity and fuel concentration ®eld in the turbulent mixing zone of a natural gas swirl burner have been investigated in the isothermal case by means of laser Doppler anemometry (LDA) and laser sheet visualisation (LSV). The two techniques enabled useful insight into the fully turbulent, three-dimensional swirling ¯ow to be gained for various operating conditions and fuel injection typologies. The eect of swirl number and fuel injection mode, radial or coaxial, has been quanti®ed on both the ¯ow®eld structure and fuel concentration distribution. The results indicate that the unmixedness index, U, may be a signi®cant parameter to judge the mixing eciency due to swirl strength and fuel±air momentum ratio in a speci®c burner geometry, at least under isothermal conditions in the near-®eld zone downstream the quarl exit. Ó 2000 Elsevier Science Inc. All rights reserved. Keywords: Injection typology; Homogeneous mixing; Natural gas; Swirl burner; Turbulent mixing zone
1. Introduction Non-premixed swirling ¯ows are widely used in industrial combustion systems, notably, gas turbines, boilers and furnaces, because of safety and stability reasons. Swirl increases fuel±air mixing, improves ¯ame stabilisation and has a strong in¯uence on ¯ame characteristics and pollutant emissions. Although the swirling ¯ows have been extensively used in combustion design and it is known [1] that the swirl can signi®cantly aect the NOx emission, the relationship between the combustion characteristics of swirling ¯ames and pollutant formation still needs to be established [2]. Among the methodologies under development to minimise the environmental impact of this type of combustion systems, the most promising are those based on the improvement and optimisation of the mixing process between the reactants and cold fresh combustible mixture with hot gas products. It is well known [3±5] that, independently of the combustion technology used, any improvement of combustion performance relative to
* Corresponding author. Tel.: +39-02-23998537; fax: +39-0223998566. E-mail address: [email protected] (A. Coghe).
pollutant formation, stability and overall eciency requires a careful study of the mechanism of mixing and entrainment in high turbulent reacting ¯ows. In many combustion devices both the reactants are in the gas phase and for technological reasons the coaxial geometry is commonly used to merge the two streams; the swirl motion of the main ¯ow is used to improve ¯ame stability and enhance mixing process [6,7]. The ®ne structure of the resulting mixture and the mechanism of ¯ame stabilisation, related to the recirculation zone induced by the swirling ¯ow pattern, control the combustion process and pollutant formation. At moderate swirl intensity, the interpenetration process between the two reactants is due to a strong shear that determines the entrainment rate of the slow stream by the fast one and the formation of local stoichiometric conditions. A common feature at high swirl intensity (swirl number, S > 0:6) is a recirculation bubble in the vicinity of the fuel jet outlet. The recirculation regime sustains the entrainment process of the outer stream into the inner one and may induce a low-frequency, precessing mode that is distinct from the jet-preferred mode. The recirculating regime also presents the capability of an ecient mixing between the streams in the region near the fuel outlet, therefore leading to a rapid homogenisation of the combustible mixture and a shortening of the combustion chamber.
0894-1777/00/$ - see front matter Ó 2000 Elsevier Science Inc. All rights reserved. PII: S 0 8 9 4 - 1 7 7 7 ( 9 9 ) 0 0 0 6 7 - 9
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The scaling of pollutant emissions in industrial ¯ames is very dicult, because of the complex geometry of the burner and the many parameters involved. The experimental evidence is that each burner is a unique device and even small geometrical changes can in¯uence the level of emissions. It is also known that swirling ¯ows require detailed measurements of the ¯ow structure and speci®cation of the inlet conditions [8]. The ¯uid dynamic analysis is very useful to provide preliminary information on the mixing process, even if the investigation is performed under isothermal conditions. In fact, one approach is to begin with detailed measurements of the ¯ow structure and speci®cation of the inlet conditions for the isothermal ¯ow, without the complications of chemical reactions, considering that the chemical kinetic time scales are usually shorter than the turbulent time scales. It is also important to know at what distance from the injector outlet the fuel has been completely mixed and diluted in the swirling air stream at the molecular level. More generally, one might be interested in characterising the turbulent ¯ow ®eld and the fuel concentration distribution at several cross-sections downstream of the injector outlet. The primary interest is in the near-®eld region since this area is the location in which most of the mixing and reactions take place. Owing to the high complexity of the ¯ow ®eld, the experimental characterisation has been usually performed through non-intrusive optical techniques capable of quantitative point measurements and qualitative visualisations [9,10]. In the past years, we have investigated dierent fuel injection typologies: transverse radial and coaxial injection into a co-¯owing air stream with variable swirl strength and co-¯ow to fuel momentum ratio [11]. This paper reports on the main results of the experimental characterisation of the isothermal mixing process in the near-®eld of natural gas burners. The study has been limited to this primary mixing region because the main characteristics remain almost the same in the presence and in the absence of chemical reactions, so that an isothermal analysis can provide preliminary information about the nature of the subsequent combustion process. The main parameters investigated were: air swirl, fuel inlet geometry and fuel to air velocity momentum. The
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optical techniques used were all based upon the Mie scattering principle: laser Doppler anemometry (LDA) and laser sheet visualisation (LSV) integrated by quantitative image analysis. The ultimate goal of the research is to provide a data base to test submodels for CFD computations and to acquire an understanding of the three dimensional velocity ®eld and fuel concentration distribution in the primary mixing zone, produced by both, ¯ue gas recirculation and swirl, in a turbulent non-premixed swirling reactor similar to many industrial combustion systems. 2. Experimental set-up The experimental apparatus consists of a laboratory scale of a swirl burner mounted in a fully transparent chamber, relatively large to produce little eect upon the ¯ow pattern near the burner exit, where the isothermal conditions could be assumed reasonably representative. A schematic diagram of the burner is shown in Fig. 1. The burner has a centrally positioned fuel delivery tube coaxial with the swirling air¯ow; the swirl motion is imparted through an axial air plus tangential air entry (Burner A) or a mechanical swirler (Burner B), with a divergent quarl exit similar to those used in many industrial burners. Air was supplied by a compressor and injected by four radial and four tangential inlets located on the wall of the Burner A and swirl was controlled by varying the relative amounts of axial and tangential air¯ow. Burner B is characterised by a constant, moderate swirl strength. For the isothermal analysis air was also supplied through the central pipe, instead of methane. Two dierent fuel injection typologies have been investigated: (a) transverse injection through six holes (1.25 mm inner diameter), inclined by 90° to the co-¯owing air stream and located at the end of the cylindrical section upstream the quarl (Burner A); (b) axial injection through a single-hole nozzle (5 mm inner diameter) with the tip protruding 6 mm into the quarl (Burner B). The relevant dimensions of the burner are summarised in Table 1. In the con®guration A, dierent solutions were also tested in the past: the 90° injection was obtained through radial, co-swirl and counter-swirl
Fig. 1. Schematic view of the investigated burner.
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Table 1 Relevant burner dimensions (mm) Burner inner diameter, D2 Fuel tube diameter, D1 Co-axial nozzle diameter, d0 Radial nozzle diameter, dj Burner quarl exit diameter, Dbq Burner quarl divergence angle Test chamber
35.5 18.5 5 1.25 60
Burner A and B Burner A and B Burner B Burner A Burner A and B
30 (°)
Burner A and B
300 300 450
Burner A and B
orientation of the six holes and the dierent solutions were compared at ®xed fuel±air momentum ratio and variable swirl strength. The results indicated a moderate eect of the orientation of the injection holes on the mixing eciency, particularly at high swirl intensity [12]. In this paper, the reference case with radial injection will be only considered. The velocity measurements have been performed by a dual-beam, two-components LDA system operated in back-scatter with a 5 W Argon-ion laser (Spectra Physics, mod. 2020), which allowed contemporary evaluation of axial and tangential or radial and tangential velocity components. Sensitivity to the ¯ow direction was provided by a 40 MHz light frequency shifting through a Bragg cell. Measurement volume dimensions were around 0.1 mm diameter by 1.0 mm length. Oil droplets of nominal 1 lm size were used as tracers, by adding the particles to the surrounding air stream and/or the gaseous fuel, depending on the measurement objectives. The light scattered by the particles was focused onto preampli®ed photomultipliers and the Doppler signals were monitored by a 58N10 PDA Dantec processor interfaced with a PC through a DMA module. At moderate seeding density, the velocity signals presented rather high validation rates, at several kHz; higher seeding density was necessary for the planar visualisations. Errors in the mean axial and swirl ve-
locities were estimated at about 4%, having increased the number of samples in the more turbulent regions. LSVs were obtained by means of a pulsed copper vapour laser (Oxford Lasers, mod. CU15A) with an average power of 15 W and capable of 10 kHz pulse repetition rate. Each pulse has a duration of 50 ns and a system of cylindrical and spherical lenses and mirrors provided a light sheet about 1 mm thick which intersected the burner axis normally at dierent downstream distances from the nozzle (see Fig. 2). The images, produced by the light scattered by the same oil particles used for LDA, were recorded at 90° by a CCD camera with the exposure time set to 10 ms; each image was thus averaged over 100 consecutive laser pulses. The images were analysed and processed through the Image Pro Plus software, in order to improve image quality and contour de®nition after background subtraction. Further processing was obtained by means of specially developed routines. 3. Experimental results The experimental analysis was performed at dierent axial distances from the exit plane of the burner quarl from X =Rbq 0:1±1, as shown in Fig. 1. The standard operating characteristics of the two burners are summarised in Table 2; further details of Burner A have been already reported [11]. In the table the initial air velocity is referred to the inlet cross-section of the burner quarl, while the fuel injection velocity refers to the nozzle outlet. In both burner geometries an important parameter is the square root of ÿthe momentum ÿ ratio, denoted by M, and de®ned as qf Uf2 = qa Ua2 , where f and a denote fuel and air exit conditions. The quantity Uf is the injection velocity of the fuel at the nozzle exit and Ua is the axial bulk velocity of the coaxial air¯ow at the upstream end of the burner quarl (see Fig. 1). In homogeneous ®elds as is the case in the present isothermal experiments, with air used instead
Fig. 2. Schematic view of the experimental set-up (LDA and LSV).
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with fuel only, while C 0 the case with air only. The above de®nition has the advantage that:
Table 2 Operating conditions of the two burners Parameter
Burner A
Burner B
Initial air velocity (m/s) Fuel injection angle (°) Fuel injection velocity (m/s) Fuel±air velocity ratio Overall equivalence ratio (U) Swirl number, S Co-¯ow, Re 104
28 90 150 5.36 0.83 1 54,000
7 0 40 5.71 0.83 0.5 13,500
of natural gas, the square root of M reduces to the velocity ratio, ru Uf =Ua , reported in Table 2. This ratio is an important parameter that controls the penetration of transverse injection into the main air stream and the mixing of fuel and air in the coaxial geometry. Burner A is designed to operate at high swirl (S 1) and the natural gas is supplied through six holes oriented at 90° with respect to the main air stream and located on the periphery of a central fuel pipe. The injection cross-section is 3 mm upstream the inlet of the burner quarl, to produce a partially premixed ¯ow at the outlet. The results are presented for the high swirl case (S 1), together with the no-swirl reference condition (S 0). Burner B, which is characterised by fuel supply through a single co-axial hole, can be operated at a relatively low swirl (S 0:5), in a condition more representative of many commercial burners where, for technological reasons, the coaxial geometry is commonly used to merge the two streams of reactants and the swirl strength is low to limit pressure losses. This burner con®guration has the practical advantage that the fuel supply pressure can be very low, compared with the Burner A. Swirl number, S, is de®ned as the ratio of the ¯ux of angular momentum to the ¯ux of axial momentum divided by the radius of the burner [13]. The swirl number reported in Table 2 has been evaluated by numerical integration of the axial and circumferential velocity pro®les measured by the LDA along one diameter at the burner quarl inlet plane. The ®rst objective of the experimental investigation was to develop measuring techniques and data analysis procedures allowing the evaluation of the mixing eciency for each burner geometry and operating condition. The selected approach was to evaluate the entrained recirculated air¯ow through LDA measurements and to de®ne, as in [14,15], an unmixedness index, U, which can be evaluated through quantitative analysis of light distribution on the planar images: U
165
c02 ; C 1 ÿ C
where C stands for the mean fuel concentration and c0 is the rms value. The value C 1 represents the condition
U 0 means full mixing c0 0; C 6 0; U 1 maximum variance c0 C 0:5 and thus worst mixing; U 1 presence of only air C 0 or only fuel C 1: The mean concentration C may be determined from the laser sheet images by spatial averaging over a selected area. Our choice was to consider the circular area centred on the burner axis with the radial extension of the quarl exit. By comparing the results at several crosssections downstream the exit plane, the spatial evolution of the mixing process can be analysed and a numerical index used to compare dierent operating conditions. In fact, the value of U is only dependent on the initial calibration of the technique, which requires to maintain constant the volume concentration of tracing particles in the seeded ¯ow and normalise the local concentration to the initial mean concentration C0 measured at the nozzle exit. The second step will be to determine the detailed distribution of the three mean velocity components, with related normal and shear stresses, and try to ®nd correlation between the ¯ow®eld structure and mixing ef®ciency. A preliminary analysis has been completed regarding the mean and ¯uctuating velocity components used to characterise the ¯ow structure of the two burners; these results will be presented ®rst. 3.1. Flow®eld structure The basic ¯ow®eld structure in the primary mixing region is presented in Figs. 3 and 4 for the Burner A for the conditions listed in Table 2 and swirl number S 1. The tangential and axial mean and ¯uctuating velocities are shown for three streamwise locations, X =Rbq 0:14, 0.5 and 1, starting from the exit plane of the burner quarl (X being the streamwise co-ordinate); the abscissa is the normalised radial co-ordinate Y/Rbq , where Rbq is the quarl radius. In the near-®eld zone of a swirling ¯ow like this, mean velocity similarity does not develop; hence, the results are presented normalised to the mean bulk velocity at the exit of the burner quarl, as deduced from the measured mass ¯ow rate. Examination of the radial pro®le of mean axial velocity reveals a well-pronounced central recirculation zone (CRZ), characterised by negative values of the velocity, caused by the adverse pressure gradient induced by the intense swirl. The recirculation zone extends in the streamwise direction beyond one Rbq , while in the radial direction ®lls almost entirely the quarl exit area, decreasing progressively downstream. From the axial velocity pro®les, the recirculated air mass has been estimated to be about 40% of the inlet mass ¯ow at X =Rbq 0:1, reducing to about 10% at X =Rbq 1.
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Fig. 3. Normalised radial pro®les of axial mean and ¯uctuating velocities at dierent distances X/Rbq from the eux (Burner A).
Fig. 4. Normalised radial pro®les of tangential mean and ¯uctuating velocities at dierent distances X/Rbq from the eux (Burner A).
The pro®le of mean circumferential velocity (Fig. 4) shows a concentrated vortex near the burner exit, with a core of solid-body rotation extending to about one fuel pipe radius from the axis. The velocity pro®le remains almost constant in the outer annulus, where it is probably distorted by the interaction with the radial fuel jets. Downstream, the maximum tangential velocity occurs
almost at the same radial distance from the axis as the peak of the axial velocity, showing the classical evolution from solid body rotation to Rankine type vortex, owing to turbulence dissipation eects, progressively going far away from the quarl region. The non-zero values recorded near the axis for all the traverses may arise from a residual alignment error or some instability of the central
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Fig. 5. Normalised radial pro®les of axial mean and ¯uctuating velocities at dierent distances X/Rbq from the eux (Burner B).
Fig. 6. Normalised radial pro®les of tangential mean and ¯uctuating velocities at dierent distances X/Rbq from the eux (Burner B).
vortex location, as suggested by the high rms values measured on the axis. The normalised rms axial and tangential components shown in Figs. 3 and 4 characterise the turbulence intensity levels. It is observed that inside the CRZ region the two rms levels are almost constant ( 0:5 Ubq ) and do not decrease axially. The tangential rms component remains uniform over a wider area, while higher values of the axial rms turbulence are located close to the positions of the highest velocity gradient in the inner shear layer of the annular air stream. Burner B has been operated at lower swirl intensity, S 0:5, and has been characterised in the presence and absence of the central fuel inlet. The ¯ow®eld structure shown in Figs. 5 and 6 refers to the case with no fuel loading and conditions listed in Table 2. The pro®les of the mean axial velocity components shown in Fig. 5 indicate that no reverse ¯ow occurs, even in the absence of the central fuel jet. The minimum streamwise velocity on the axis is always positive, indicating that the CRZ is not established at this swirl level and the ¯ow®eld has a wake-like structure induced by the nozzle holder. This conclusion applies to any level of fuel loading and is due to the low swirl intensity. At the
quarl exit plane the spreading of the air stream is not enough to ®ll the quarl cross-section and a small recirculation appears close to the wall. It is also seen that the momentum de®cit from the wake of the nozzle tip persists up to X =Rbq 0:5, as a result of a positive static pressure gradient on the axis, although the axial velocity pro®le is approaching a gaussian shape. Downstream, also the rms pro®les are progressively smoothed to an almost uniform level, which is reached at the cross-section X =Rbq 0:5. The pro®les of mean circumferential velocity in Fig. 6 show again a concentrated vortex, with a core of solidbody rotation extending to a narrow radial zone, of the order of the nozzle holder size, and indicate a limited spreading of angular momentum into the free stream. The non-zero values recorded near the axis for all the traverses, as already observed in Fig. 4, may again arise from a minor alignment error or, more probably, from some instability of the central vortex location, as suggested by the higher rms values measured on the axis and bimodal probability distributions. Considering the rms velocity ¯uctuations, a noteworthy feature is the signi®cant level in the turbulence
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values of the streamwise velocity in the wake of the nozzle and the distinct peak in the corresponding tangential component. Higher values of the axial and tangential rms turbulence are found close to the positions of the highest axial velocity gradient in the outer shear layer of the annular air stream. 3.2. Fuel concentration distribution The fuel concentration distribution was measured by LSV, in a number of cross-sections normal to the burner axis, starting from the quarl exit, after collection of the images by a CCD camera located at 90° (Fig. 2). The fuel line was seeded with oil droplets capable of tracing the gas diusion into the main air stream. Owing to the quasi-monodisperse size distribution of the droplets, the scattered light intensity can be assumed proportional to their number density into the illuminated cross-section. Further, the particle concentration is assumed proportional to the fuel concentration, although the evaluation of the absolute value is a very dicult task [16]. Under favourable conditions, it is possible to normalise to the initial mean concentration C0 at the nozzle exit and de®ne a relative quantity that can be representative of the fuel diusion and mixing. With Burner A, it was found more complex to apply this normalisation, due to the geometry of the transverse injection that complicated the measure of the initial mean concentration. Burner B allowed easier optical access to the nozzle exit and the normalisation was easily applied to the results. The spatially averaged fuel concentration C at each cross-section, normalised by the initial mean concentration C0 at the inner nozzle exit, was extracted from images as those shown in Fig. 7, taken by the planar visualisation system and representing the time averaged concentration ®eld at progressive increasing distances from the eux. Fig. 7 clearly shows the in¯uence of air swirl motion upon the progressive interaction and mixing of the fuel jets. For both burners, it was decided to present the results in terms of the quantity U, already de®ned, which represents the eciency of homogenisation of the fuel into the air stream. The parameter U is independent on the initial absolute concentration of tracing particles, injected into the fuel, provided the initial average value C0 remains constant and the same integration area is used to evaluate the spatially averaged mean and rms concentrations. By comparing the values of U at dierent cross-sections, it may be possible to evaluate the rate of homogenisation of the fuel in the co-¯owing air stream [12]. The results reported in this paper are intended as a preliminary investigation on the reliability of this procedure to characterise the mixing eciency and on the sensitivity to the eect of operating conditions. Table 3 reports the values of U for three cases representative of dierent injection conditions and swirl strengths. The data refer to the ®rst cross-section at the exit of the burner quarl and indicate the degree of homogenisation completed inside the quarl. The recirculating regime produced by the higher swirl (Burner A,
S 1) presents the larger capability of an ecient and precocious mixing between the fuel jets and the main air stream in the vicinity of the most probable combustion region. The reason is that, in this regime, the residence time of the outer stream incorporated in the recirculation zone is of the order of the mixing time, Rbq =u0 , u0 being the rms velocity of the turbulent mixing layers. This residence time is much longer than the transit time, Rbq /V, which characterise the ¯ow without swirl (S 0). In fact, the absence of swirl leads to a less ecient homogenisation on the same distance. The third case, Burner B at moderate swirl, presents a larger value of U, indicating a less ecient mixing for this co-axial geometry. It must be noted that the burner quarl exit plane is only four nozzle diameter downstream the fuel outlet. The above results are qualitatively deducible from the pictures shown in Fig. 7. It is observed that, in case of radial fuel injection without swirl, the six jets have merged in three zones, while they remained clearly distinct in presence of swirl, but show a larger diusion into the main air stream. The third case (Burner B) indicates that the inner central fuel jet destabilises slowly, due to the velocity ratio greater than unity. This observation is also con®rmed by Fig. 8, which reports the radial pro®le of fuel concentration at the ®rst measuring station downstream the quarl exit plane, for Burner B at X =Rbq 0:1. The fuel concentration remains constant along a short distance downstream from the injector, the inner unmixed region corresponding to the potential core of the fuel jet, and then decreases in the mixing zone where the two streams meet on the axis. A shortening of the potential core is expected for lower values of the fuel±air velocity ratio [15]; this eect was observed in the LSV pictures and found in the trend of U as a function of the velocity ratio (Fig. 9). The results indicate that the unmixedness index, U, may be a signi®cant parameter to judge the mixing eciency produced in a speci®c burner geometry by varying the swirl strength or the momentum ratio, at least under isothermal conditions.
4. Practical usefulness of the results and main conclusions The three-dimensional gas velocity and fuel concentration ®elds produced in the near-®eld turbulent mixing zone of a natural gas swirl burner have been investigated under isothermal conditions. The eect of swirl number and fuel injection mode, radial or co-axial, has been quanti®ed by means of LDA and LSV. The preliminary results indicate that the unmixedness index, U, may be a signi®cant parameter to judge the mixing eciency of a speci®c burner geometry, being sensitive to the swirl strength and the fuel±air momentum ratio, at least in the near-®eld zone downstream the quarl exit. The method is based on the LSV technique and its reliability strongly depends on the quality of the planar images and the accuracy of normalisation of the
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Fig. 7. Typical pseudo-color images of ``fuel'' distributions at increasing distances X/Rbq from the quarl eux for the following conditions: (a) Burner A without swirl; (b) Burner A with swirl.
Table 3 Unmixedness results Burner A Burner A Burner B
S0 S1 S 0:5
U 0:22 U 0:10 U 0:28
measured average concentration by the mean concentration at the nozzle exit. Correlations with the turbulent ®eld and shear stresses, measured by the LDA technique, are needed to complete the experimental
investigation under isothermal conditions, before starting the combustion tests. The results obtained could be helpful for a more thorough comprehension of the reactants mixing process in model of industrial burners, providing useful informations for optimization of burner design and operating conditions, aiming to improve combustion eciency and reduce environmental impact of fossil fuel combustion. Moreover, experimental data about the turbulent ¯ow ®eld can be used to validate results from numerical simulation through CFD codes.
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References
Fig. 8. Radial pro®le of ``fuel'' concentration near the burner quarl eux (Burner B).
Fig. 9. Unmixedness parameter U at X =Rbq 0:1 as a function of fuel±air velocity ratio (Burner B).
Acknowledgements The present experimental work has been performed at the CNR-TeMPE laboratories with partial support by MURST, under the national programme ``Combustion.'' The authors would like to thank Mr. G. Brunello for the helpful support in the experiments and image analysis. The assistance of Messrs. Benassi, Manfroi and Resnati in collecting the LDA and LSV data is appreciated.
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