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Effect of hardware alignment on fuel distribution and combustion performance for a production engine fuelinjection assembly
Twenty-Sixth Symposium (International) on Combustion/The Combustion Institute, 1996/pp. 2725–2732
EFFECT OF HARDWARE ALIGNMENT ON FUEL DISTRIBUTION AND COMBUSTION PERFORMANCE FOR A PRODUCTION ENGINE FUELINJECTION ASSEMBLY V. G. MCDONELL, L. ARELLANO, S. W. LEE and G. S. SAMUELSEN UCI Combustion Laboratory University of California, Irvine, CA 92697-3550, USA
The hardware in a production combustor cannot be constrained within strict tolerances due to (1) manufacturing variations and (2) provisions for thermal expansion that often preclude exact alignment of all parts over the operating cycle. The demand for higher performance is directing attention to the relationship of hardware alignment to combustor performance. The present article reports on a systematic characterization of hardware alignment on both the spray structure (nonreacting) and associated combustion performance for a practical fuel-injection/swirler assembly. Planar, liquid laser-induced fluorescence (PLLIF) and phase Doppler interferometry (PDI) are utilized for the nonreacting assessment, whereas emissions and temperatures are used to characterize the reacting case. It is found that misalignment of 2.54 mm has a significant effect on the fuel distribution while having only a modest effect on the gas-phase aerodynamics and overall droplet sizes. Quantification of the fuel distribution using unmixedness indices allows for correlations between the fuel distribution and combustion performance to be established and assessed. It is found that the effect of alignment on the fuel distribution under nonreacting conditions directly translates into an impact on combustion performance in terms of mean pollutant levels and stability as well as the spatial distribution of the patterns of the pollutants at the exit plane. The study also demonstrates that PLLIF has a potential for screening designs relative to PDI due to its efficient application. However, the complex phenomena occurring in liquid-fired combustion systems suggests that PLLIF images are not sufficient in general, since changes in droplet size and aerodynamic structures can affect directly the combustion performance as well as the fuel distribution.
Introduction Efforts to reduce emissions from present day aircraft engines have focused upon the combustor. Geometric and operational features of combustors are, as a result, receiving especially close scrutiny. Currently, trial and error is utilized to optimize the performance of existing designs. Little attempt has been made to develop a mechanistic understanding of the processes occurring within the combustor due to the time and effort required to obtain the necessary detailed measurements. For one particular geometry, illustrated in Fig. 1, some basic understanding of the processes occurring is being provided from detailed studies conducted with state-of-theart diagnostics such as phase Doppler interferometry (PDI) and CARS [e.g., 1–3]. However, these diagnostic tools are time consuming to apply and, as a result, tend to preclude extensive characterization of many geometries and conditions. This shortcoming tends to make the trial-and-error approach appealing in meeting short-term requirements. Once trial and error is applied, improved performance is generally realized. However, a successful design must demonstrate robustness to manufactur-
ing tolerances and other practical considerations. It has been demonstrated previously that performance of the device shown in Fig. 1 is sensitive to the geometry of the various components [4] as are other injector/swirler configurations [e.g., 5–8]. The present study evaluates sensitivity of performance using an injection assembly in which the degree of misalignment can be controlled. The range of the misalignment considered is consistent with that found in practice as the engine cycles through different power settings (“0.1 in.” or 2.54 mm). Figure 1 depicts the nature of the misalignment—the nozzle and primary swirler shift relative to the venturi, secondary swirler, and flare. Phase Doppler interferometry is a natural choice for this study since it can measure gas velocity as well as size, velocity, and local volume flux of droplets. However, it is evident that more efficient diagnostic tools are required for (1) problem solving in research and (2) screening production hardware. In addition, such tools must address the critical aspects of the problem. For example, the role of fuel/air unmixedness in combustion performance has been demonstrated for liquid-fueled direct and prevapor-
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features an upfired arrangement. For the reacting studies, an 80-mm-i.d. quartz tube serves as the combustor liner. In this case, the liner is 5D (D 4 80 mm) in length and features an area contraction of 50% at the exit to mimic backpressure. Diagnostics
Fig. 1. Cross-section schematic of the practical fuel-injection assembly.
ized systems as well as for gaseous-fueled premixed systems [e.g., 9–11]. Of these studies, only Fric [10] explicitly studied the fuel distribution prior to combustion, and the system considered was premixed and gas fueled. Studying the fuel distribution in a liquid-fired combustor is more challenging, especially for a directinjection system. As a result, efforts have been recently directed at development of planar, liquid laser-induced fluorescence (PLLIF) to measure more efficiently the fuel distribution produced by liquid-based systems [12]. PLLIF is applied in the present study to (1) efficiently assess the role of hardware alignment on fuel distribution and (2) develop correlations between fuel distribution and combustor performance. The objectives of the present study are to evaluate the role of hardware misalignment on fuel distribution and combustor performance and to assess the utility of PLLIF as a diagnostic and screening tool. The approach taken is (1) apply PLIFF and PDI in the nonreacting environment to characterize the effect of hardware misalignment, (2) characterize the effect of the misalignment on combustion performance, and (3) apply and correlate indices of mixing to the combustion results. Experiment Facility The facility utilized for the nonreacting studies features a 450- 2 450-mm square duct oriented vertically. The swirl cup is fed by a 150-mm-diameter plenum located centrally within the duct and injects the spray downward. Additional details are described elsewhere [13]. A separate facility is utilized for the reacting flow emissions measurements and
PLLIF is utilized to characterize the planar distribution of the fuel. In this technique, a laser is spread into a sheet that is passed through the spray. The energy in the laser induces fluorescence from molecules in the liquid. In the present case, a small amount of fluorescein is doped into methanol and is stimulated with an argon-ion laser operating at the 0.4880-lm wavelength. As a planar imaging technique, PLLIF is relatively quick to apply. Despite the attractiveness of PLLIF, the complexity of liquid-fueled combustion systems, especially those that are swirl stabilized, precludes “simple” characterization relative to the combustion performance. For example, droplet size and velocity play a critical role in the combustion process and, in the absence of some knowledge of these quantities, any observed role of fuel distribution may be fortuitous. Similarly, the nature of the aerodynamics plays a role. For example, the size and extent of the recirculation zone can result in various residence times that may play as much a role as fuel distribution in combustion performance. As a result, characterization of behavior other than fuel distribution is required in general. Hence, tools (such as PDI) that offer proven information regarding details of the gas and droplet behavior remain necessary in spite of their inherently time-consuming operating process. PDI (Aerometrics 3100-S) is utilized in the nonreacting case to measure the gas-phase velocities and droplet characteristics. For the reacting cases, a set of standard-source, monitoring equipment was utilized (HORIBA, Ltd.). Unburned hydrocarbons, oxygen, carbon monoxide, carbon dioxide, and oxides of nitrogen were measured at the exit plane of the combustor at Cartesian coordinates in 8-mm increments. A 6.5mm-diameter water-cooled stainless steel extractive probe was used in conjunction with a heated sample line to convey the sample to the emissions analyzers. The measurements obtained are reported on a “dry basis.” Temperatures were measured in the same grid locations using a thermocouple (type K). The temperature measurements reported are not corrected for catalytic effects or radiation losses. Conditions The main circuit of the production, duplex fuel injector for this swirl cup was utilized for these studies. Methanol was employed to allow the fluorescein
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Fig. 2. Comparison of gas-phase mean axial velocity (m/s) at Z 4 50 mm.
dye to be utilized with the argon-ion laser-based PLLIF system, as discussed above. In each case, 4% pressure drop relative to the ambient pressure (101 kPa) was utilized. Air preheat of 400 K was utilized in the reacting case (60 K above the methanol boiling point). A methanol flow rate of 9 kg/h was utilized, and an overall equivalence ratio of 0.82 was maintained. Results Although results were obtained for three configurations (misalignment of 0, 1.27, and 2.54 mm), configurations 1 (0 mm) and 3 (2.54 mm) are emphasized. Due to the inherent asymmetry of configuration 3, single radial profiles obtained via PDI do not capture the essential features of the flow. Rather, data acquired in planes have the greatest utility in determining the effect of the misalignment. Hence, the majority of the data presented are from a plane 50 mm downstream of the exit of the flare. PDI measurements were acquired at locations in a grid arrangement in 4-mm increments. To obtain the information presented for each case, 241 point measurements, requiring days to obtain, were required. In contrast, the PLLIF images were acquired in a matter of minutes. The nonreacting cases are presented first, followed by the emissions characteristics. Following the measurements, discussion regarding the correlation between the results is provided. Nonreacting Characterization Aerodynamics Measurements of the effective area revealed differences of less than 2% for the three configurations.
In addition, since the same swirler was used for each case, it was expected that the size and extent of the recirculation zone would be similar. To confirm this, PDI was applied to determine the effect of the misalignment on the gas velocity. The results for the mean axial velocity are presented in Fig. 2 for the two extreme cases. Recall that the nozzle assembly is shifted in the `Y direction for configuration 3. The general structure for configuration 1 is relatively symmetric, and the variation is typical for similar “axisymmetric” practical hardware [1,2,4,6–8]. In configuration 3, the high-velocity region moves in the `Y, 1X direction relative to configuration 1. However, by 50 mm, the recirculation zone has closed for both cases, and the general magnitude of the velocities is similar, confirming that the similar effective areas result in similar general aerodynamic structure. Droplet size The effect of the misalignment on the droplet sizes determined by PDI is shown in Fig. 3. Configuration 1 again reflects a relatively symmetric pattern compared to configuration 3. While a region of locally smaller drops in the 1X region of the spray is observed, the difference in the general droplet sizes is small, suggesting that the basic atomization mechanism is not altered by the misalignment. Fuel distribution Figure 4 presents a comparison of the spatial distribution of the fuel. In this case, data from PDI and PLLIF are presented. The PDI results are shown in Figs. 4a and 4b for configurations 1 and 3, respectively. These results indicate trends similar to the size and velocity shown previously. However, the
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Fig. 3. Comparison of droplet distribution D32 (lm) at Z 4 50 mm.
peak in fuel distribution becomes more pronounced for configuration 3 in a manner consistent with observations for another injector assembly [7]. A peak in the fuel distribution occurs in the `X, `Y quadrant in contrast to the peak in gas-phase velocity and minimum in drop size that occur along the 1X direction. The impact of the misalignment on the fuel distribution is far more substantial than on the drop size or aerodynamics. Figure 4 also presents the fuel distribution results as measured by PLLIF. These results are shown in Figs. 4c and 4d for configurations 1 and 3, respectively. These results are based on the average of 32 frames from the CCD camera which requires significantly less time to obtain than collecting the 200` points via PDI. The images reveal identical structure in the fuel mass distribution. Reacting Characterization For the reacting characterization, measurements of the exit plane emissions are presented. It is expected that the general behavior of the aerodynamics, droplet velocities, and temperatures for configuration 1 are similar to that found elsewhere [3]. Figures 5–7 present contours of [CO], [HC], and [NOx]. The [CO] emissions are presented in Fig. 5 and reveal a systematic increase in asymmetry with increased misalignment. Note that the scale on the contours changes for each configuration. Figure 6 presents the contours of the unburned hydrocarbons. This time, configurations 2 and 3 reveal a region of locally high emissions relative to configuration 1. Finally, Fig. 7 presents contours of the [NOx] levels at the exit. Like the CO, the NOx levels reveal systematically degraded symmetry with increased misalignment.
Correlations To quantify the differences observed, the degree of nonuniformity described by “unmixedness,” U, determined using the expression in Eq. (1) [14], is applied: U4
cvar cavg(1 1 cavg)
(1)
where cvar 4 variance in field c cavg 4 average of c. Equation (1) can be applied to arbitrary distributions and is applicable for either space (“spatial unmixedness,” Us) or time (“temporal unmixedness,” Ut). U ranges in value from 0 to 1, and lower values of U reflect more uniform fields. Since fuel distribution is affected significantly by the misalignment and is known to be a critical factor in combustion performance, Eq. (1) is applied to this quantity. In this case, the PLLIF results are considered with an area of interest shown in Fig. 4 to determine the spatial unmixedness of the fuel. Figure 8 compares the spatial unmixedness, Us, based on the PLLIF images and the lean blow-off limit. The results reveal that Us increases systematically with an increase in the misalignment and that the stability improves in a corresponding fashion. Figure 9 presents the correlation between the mean concentrations (right axis) and the unmixedness (left axis) of the emissions results presented in Figs. 5–7 and Us based on the PLLIF images. The slight dependence of the mean NOx level on Us based on the nonreacting fuel distribution observed is within experimental error and is not strong enough to draw a conclusion. Recalling that this case is op-
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Fig. 4. Comparison of volume flux (cc/cm2 s) from PDI and PLLIF intensity (proportional to liquid volume) for configurations 1 and 3 at axial distance Z 4 50 mm from flare exit.
erating at an overall equivalence ratio of 0.82, the expected NOx dependency is not obvious. Previous work [11] suggested that NOx levels should decrease with higher Us values when the overall equivalence ratio exceeds 0.7. However, the mean levels of CO and UHC exhibit increases corresponding to increases in Us. The left axis in Fig. 9 is used to assess Us (i.e., the patterns) based on the measured emissions quantities at the exit plane, with Us based on the nonreacting fuel distribution. Us, based on CO and UHC correlates well with Us based on the nonreacting fuel distribution. Finally, a weak anticorrelation is observed between Us based on NOx and Us based on the nonreacting fuel distribution. Summary and Conclusions A systematic study of the effect of hardware misalignment on the nonreacting spray structure and
combustion performance for a practical fuel-injection/swirler assembly has been conducted for a single condition. PLLIF and PDI were utilized for the nonreacting assessment, whereas emissions and temperatures were used to characterize the reacting case. The following conclusions are drawn from the work: • The effect of misalignment typically encountered by production hardware during operation on the nonreacting flow field is significant. The fuel distribution is dramatically impacted, whereas the gas-phase structure remains similar with the exception of a shift in the flow direction; the overall droplet sizes remain similar. • A nearly linear relationship between the spatial unmixedness of the fuel ascertained via PLLIF in the nonreacting spray and lean blow off was found, which is consistent with previous studies. • Significant correlation was found between the lev-
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Fig. 5. CO concentration contours at exit plane (ppm).
Fig. 6. UHC concentration contours at exit plane.
Fig. 7. NOx concentration contours at exit plane.
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• The results suggest that improving the robustness in the concentricity of the design can help meet increasingly stringent emissions regulations. Acknowledgments GE Aircraft Engines is gratefully acknowledged for supplying the hardware utilized and support for swirl cup studies and diagnostics development. The assistance of Jason Campbell in the computations associated with the images is appreciated.
REFERENCES Fig. 8. Relation between configuration spatial unmixedness from PLIFF images and lean blow-off limit.
Fig. 9. Relationship between unmixedness for spray, unmixedness for emissions, and mean concentrations of emissions.
els of CO and hydrocarbons produced and the spatial unmixedness based on nonreacting fuel distributions characterized by PLLIF. • Significant correlation was also found between the spatial unmixedness of CO and hydrocarbons at the exit plane and the spatial unmixedness of the fuel based on PLIFF images. • The efficiency of PLLIF makes it an attractive tool for screening fuel distributions produced by practical devices.
1. Wang, H. Y., McDonell, V. G., and Samuelsen, G. S., J. Propulsion Power 10:441–445 (1994). 2. Wang, H. Y., McDonell, V. G., and Samuelsen, G. S., J. Propulsion Power 10:445–451 (1994). 3. Takahasi, F., Schmoll, J., Switzer, G. L., and Shouse, D. T., Twenty-Fifth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1994, pp. 183–191. 4. Wang, H. Y., McDonell, V. G., and Samuelsen, G. S., ASME J. Eng. Gas Turb. Power 117:282–289 (1995). 5. McDonell, V. G. and Samuelsen, G. S., ASME J. Eng. Gas Turb. Power 112:44–51 (1990). 6. Rosfjord, T. J. and Eckerle, W. A., J. Prop. Power 7:849–856 (1991). 7. Cohen, J. M. and Rosfjord, T. J., J. Prop. Power 9:16– 27 (1993). 8. Rosfjord, T. J. and Russell, S., J. Prop. Power 5:144– 150 (1989). 9. Pompei, F. and Heywood, J. B., Combust. Flame 19:407–418 (1972). 10. Fric, T., J. Prop. Power 9:708–713 (1993). 11. Lyons, V. J., AIAA J. 20:660–665 (1982). 12. Igushi, T., McDonell, V. G., and Samuelsen, G. S., An Imaging System for Characterization of Liquid Volume Distributions in Sprays, Proceedings, 6th Annual ILASS-Americas Conference, 1993, submitted to Atomization and Sprays. 13. McDonell, V. G. and Samuelsen, G. S., AIAA J. Prop. Power 7:684–691 (1991). 14. Dankwertz, P. V., Appl. Sci. Res., Sec. A 3:279–296 (1952).
COMMENTS Prof. Sigmar Wittig, University of Karlsruhe, Germany. I was surprised to see relatively small effects of alignment deviations on the droplet size distribution. Do you have an explanation? Author’s Reply. In this device, the breakup of the liquid film deposited upon the venturi is the primary mechanism
of atomization. As a result, it is expected that the droplet size will not be strongly affected if the filming mechanism is not strongly affected. In the present case, the misalignment does not change the location of the simplex nozzle relative to the venturi and, as a result, has little impact on filming mechanism and the subsequent droplet sizes.
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Prof. M. M. ElKotb, Cairo University, Egypt. As we know swirling flow creates hollow conical spays, what is the effect of the surrounding cup on the stabilization and penetration of the spray? Author’s Reply. The general characteristics of the spray fields produced by dome hardware similar to that used in the present study have been documented [1–3]. The larger droplets appear in the “hollow” cone portion of the spray, and penetrate “ballistically” into the chamber. A fraction of the mass from the simplex atomizer is transported directly into the core of the spray. Also in the core, a significant population of fine droplets are backmixed to the cup, and are likely to be a key to stabilization. In addition to providing swirl, the location of the cup relative to the atomizer serves to enhance the mixing of the fuel and air prior to exiting the flare.
● Arvind K. Jasuja, Cranfield University, UK. Could you comment on the extent of attenuation encountered by the laser light sheet in your spray experiments and any corrections made for its compensation? In the context of dense, practical sprays at high pressures, severe attenuation can be encountered, thus potentially undermining the utility of the PLIF technique. Author’s Reply. In the present spray, the attenuation through the spray is less than 10%, thus the impact of “dense spray effects” are small. For “dense” (i.e., optically thick) sprays, efforts are currently underway to account for such effects and are described in detail [1,2].
REFERENCES REFERENCES 1. Wang, H. Y., McDonell, V. G., and Samuelsen, G. S., Twenty-Fourth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1992, pp. 1457–1463. 2. Wang, H. Y., McDonell, V. G., Sowa, W. A., and Samuelsen, G. S., AIAA J. Propulsion Power 10:441–445, (1994). 3. Wang, H. Y., McDonell, V. G., Sowa, W. A., and Samuelsen, G. S., AIAA J. Propulsion Power 10:446–451 (1994).
1. Talley, D. G., Thamban, A. T. S., McDonell, V. G., and Samuelsen, G. S., in Recent Advances in Spray Combustion: Spray Combustion Measurements and Model Simulation, K. K. Kuo, ed., Vol. 171, Progress in Astronautics and Aeronautics, AIAA, pp. 113–142, 1996. 2. Talley, D. G., Verdieck, J. F., Lee, S. W., McDonell, V. G., and Samuelsen, G. S., Paper 96-0469, Accounting For Laser Sheet Extinction In Applying PLLIF To Sprays, presented at 34th AIAA Aerospace Sciences Meeting, Reno, NV, January, 1996, submitted to J. Propulsion Power.
EFFECT OF HARDWARE ALIGNMENT ON FUEL DISTRIBUTION AND COMBUSTION PERFORMANCE FOR A PRODUCTION ENGINE FUELINJECTION ASSEMBLY V. G. MCDONELL, L. ARELLANO, S. W. LEE and G. S. SAMUELSEN UCI Combustion Laboratory University of California, Irvine, CA 92697-3550, USA
The hardware in a production combustor cannot be constrained within strict tolerances due to (1) manufacturing variations and (2) provisions for thermal expansion that often preclude exact alignment of all parts over the operating cycle. The demand for higher performance is directing attention to the relationship of hardware alignment to combustor performance. The present article reports on a systematic characterization of hardware alignment on both the spray structure (nonreacting) and associated combustion performance for a practical fuel-injection/swirler assembly. Planar, liquid laser-induced fluorescence (PLLIF) and phase Doppler interferometry (PDI) are utilized for the nonreacting assessment, whereas emissions and temperatures are used to characterize the reacting case. It is found that misalignment of 2.54 mm has a significant effect on the fuel distribution while having only a modest effect on the gas-phase aerodynamics and overall droplet sizes. Quantification of the fuel distribution using unmixedness indices allows for correlations between the fuel distribution and combustion performance to be established and assessed. It is found that the effect of alignment on the fuel distribution under nonreacting conditions directly translates into an impact on combustion performance in terms of mean pollutant levels and stability as well as the spatial distribution of the patterns of the pollutants at the exit plane. The study also demonstrates that PLLIF has a potential for screening designs relative to PDI due to its efficient application. However, the complex phenomena occurring in liquid-fired combustion systems suggests that PLLIF images are not sufficient in general, since changes in droplet size and aerodynamic structures can affect directly the combustion performance as well as the fuel distribution.
Introduction Efforts to reduce emissions from present day aircraft engines have focused upon the combustor. Geometric and operational features of combustors are, as a result, receiving especially close scrutiny. Currently, trial and error is utilized to optimize the performance of existing designs. Little attempt has been made to develop a mechanistic understanding of the processes occurring within the combustor due to the time and effort required to obtain the necessary detailed measurements. For one particular geometry, illustrated in Fig. 1, some basic understanding of the processes occurring is being provided from detailed studies conducted with state-of-theart diagnostics such as phase Doppler interferometry (PDI) and CARS [e.g., 1–3]. However, these diagnostic tools are time consuming to apply and, as a result, tend to preclude extensive characterization of many geometries and conditions. This shortcoming tends to make the trial-and-error approach appealing in meeting short-term requirements. Once trial and error is applied, improved performance is generally realized. However, a successful design must demonstrate robustness to manufactur-
ing tolerances and other practical considerations. It has been demonstrated previously that performance of the device shown in Fig. 1 is sensitive to the geometry of the various components [4] as are other injector/swirler configurations [e.g., 5–8]. The present study evaluates sensitivity of performance using an injection assembly in which the degree of misalignment can be controlled. The range of the misalignment considered is consistent with that found in practice as the engine cycles through different power settings (“0.1 in.” or 2.54 mm). Figure 1 depicts the nature of the misalignment—the nozzle and primary swirler shift relative to the venturi, secondary swirler, and flare. Phase Doppler interferometry is a natural choice for this study since it can measure gas velocity as well as size, velocity, and local volume flux of droplets. However, it is evident that more efficient diagnostic tools are required for (1) problem solving in research and (2) screening production hardware. In addition, such tools must address the critical aspects of the problem. For example, the role of fuel/air unmixedness in combustion performance has been demonstrated for liquid-fueled direct and prevapor-
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features an upfired arrangement. For the reacting studies, an 80-mm-i.d. quartz tube serves as the combustor liner. In this case, the liner is 5D (D 4 80 mm) in length and features an area contraction of 50% at the exit to mimic backpressure. Diagnostics
Fig. 1. Cross-section schematic of the practical fuel-injection assembly.
ized systems as well as for gaseous-fueled premixed systems [e.g., 9–11]. Of these studies, only Fric [10] explicitly studied the fuel distribution prior to combustion, and the system considered was premixed and gas fueled. Studying the fuel distribution in a liquid-fired combustor is more challenging, especially for a directinjection system. As a result, efforts have been recently directed at development of planar, liquid laser-induced fluorescence (PLLIF) to measure more efficiently the fuel distribution produced by liquid-based systems [12]. PLLIF is applied in the present study to (1) efficiently assess the role of hardware alignment on fuel distribution and (2) develop correlations between fuel distribution and combustor performance. The objectives of the present study are to evaluate the role of hardware misalignment on fuel distribution and combustor performance and to assess the utility of PLLIF as a diagnostic and screening tool. The approach taken is (1) apply PLIFF and PDI in the nonreacting environment to characterize the effect of hardware misalignment, (2) characterize the effect of the misalignment on combustion performance, and (3) apply and correlate indices of mixing to the combustion results. Experiment Facility The facility utilized for the nonreacting studies features a 450- 2 450-mm square duct oriented vertically. The swirl cup is fed by a 150-mm-diameter plenum located centrally within the duct and injects the spray downward. Additional details are described elsewhere [13]. A separate facility is utilized for the reacting flow emissions measurements and
PLLIF is utilized to characterize the planar distribution of the fuel. In this technique, a laser is spread into a sheet that is passed through the spray. The energy in the laser induces fluorescence from molecules in the liquid. In the present case, a small amount of fluorescein is doped into methanol and is stimulated with an argon-ion laser operating at the 0.4880-lm wavelength. As a planar imaging technique, PLLIF is relatively quick to apply. Despite the attractiveness of PLLIF, the complexity of liquid-fueled combustion systems, especially those that are swirl stabilized, precludes “simple” characterization relative to the combustion performance. For example, droplet size and velocity play a critical role in the combustion process and, in the absence of some knowledge of these quantities, any observed role of fuel distribution may be fortuitous. Similarly, the nature of the aerodynamics plays a role. For example, the size and extent of the recirculation zone can result in various residence times that may play as much a role as fuel distribution in combustion performance. As a result, characterization of behavior other than fuel distribution is required in general. Hence, tools (such as PDI) that offer proven information regarding details of the gas and droplet behavior remain necessary in spite of their inherently time-consuming operating process. PDI (Aerometrics 3100-S) is utilized in the nonreacting case to measure the gas-phase velocities and droplet characteristics. For the reacting cases, a set of standard-source, monitoring equipment was utilized (HORIBA, Ltd.). Unburned hydrocarbons, oxygen, carbon monoxide, carbon dioxide, and oxides of nitrogen were measured at the exit plane of the combustor at Cartesian coordinates in 8-mm increments. A 6.5mm-diameter water-cooled stainless steel extractive probe was used in conjunction with a heated sample line to convey the sample to the emissions analyzers. The measurements obtained are reported on a “dry basis.” Temperatures were measured in the same grid locations using a thermocouple (type K). The temperature measurements reported are not corrected for catalytic effects or radiation losses. Conditions The main circuit of the production, duplex fuel injector for this swirl cup was utilized for these studies. Methanol was employed to allow the fluorescein
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Fig. 2. Comparison of gas-phase mean axial velocity (m/s) at Z 4 50 mm.
dye to be utilized with the argon-ion laser-based PLLIF system, as discussed above. In each case, 4% pressure drop relative to the ambient pressure (101 kPa) was utilized. Air preheat of 400 K was utilized in the reacting case (60 K above the methanol boiling point). A methanol flow rate of 9 kg/h was utilized, and an overall equivalence ratio of 0.82 was maintained. Results Although results were obtained for three configurations (misalignment of 0, 1.27, and 2.54 mm), configurations 1 (0 mm) and 3 (2.54 mm) are emphasized. Due to the inherent asymmetry of configuration 3, single radial profiles obtained via PDI do not capture the essential features of the flow. Rather, data acquired in planes have the greatest utility in determining the effect of the misalignment. Hence, the majority of the data presented are from a plane 50 mm downstream of the exit of the flare. PDI measurements were acquired at locations in a grid arrangement in 4-mm increments. To obtain the information presented for each case, 241 point measurements, requiring days to obtain, were required. In contrast, the PLLIF images were acquired in a matter of minutes. The nonreacting cases are presented first, followed by the emissions characteristics. Following the measurements, discussion regarding the correlation between the results is provided. Nonreacting Characterization Aerodynamics Measurements of the effective area revealed differences of less than 2% for the three configurations.
In addition, since the same swirler was used for each case, it was expected that the size and extent of the recirculation zone would be similar. To confirm this, PDI was applied to determine the effect of the misalignment on the gas velocity. The results for the mean axial velocity are presented in Fig. 2 for the two extreme cases. Recall that the nozzle assembly is shifted in the `Y direction for configuration 3. The general structure for configuration 1 is relatively symmetric, and the variation is typical for similar “axisymmetric” practical hardware [1,2,4,6–8]. In configuration 3, the high-velocity region moves in the `Y, 1X direction relative to configuration 1. However, by 50 mm, the recirculation zone has closed for both cases, and the general magnitude of the velocities is similar, confirming that the similar effective areas result in similar general aerodynamic structure. Droplet size The effect of the misalignment on the droplet sizes determined by PDI is shown in Fig. 3. Configuration 1 again reflects a relatively symmetric pattern compared to configuration 3. While a region of locally smaller drops in the 1X region of the spray is observed, the difference in the general droplet sizes is small, suggesting that the basic atomization mechanism is not altered by the misalignment. Fuel distribution Figure 4 presents a comparison of the spatial distribution of the fuel. In this case, data from PDI and PLLIF are presented. The PDI results are shown in Figs. 4a and 4b for configurations 1 and 3, respectively. These results indicate trends similar to the size and velocity shown previously. However, the
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Fig. 3. Comparison of droplet distribution D32 (lm) at Z 4 50 mm.
peak in fuel distribution becomes more pronounced for configuration 3 in a manner consistent with observations for another injector assembly [7]. A peak in the fuel distribution occurs in the `X, `Y quadrant in contrast to the peak in gas-phase velocity and minimum in drop size that occur along the 1X direction. The impact of the misalignment on the fuel distribution is far more substantial than on the drop size or aerodynamics. Figure 4 also presents the fuel distribution results as measured by PLLIF. These results are shown in Figs. 4c and 4d for configurations 1 and 3, respectively. These results are based on the average of 32 frames from the CCD camera which requires significantly less time to obtain than collecting the 200` points via PDI. The images reveal identical structure in the fuel mass distribution. Reacting Characterization For the reacting characterization, measurements of the exit plane emissions are presented. It is expected that the general behavior of the aerodynamics, droplet velocities, and temperatures for configuration 1 are similar to that found elsewhere [3]. Figures 5–7 present contours of [CO], [HC], and [NOx]. The [CO] emissions are presented in Fig. 5 and reveal a systematic increase in asymmetry with increased misalignment. Note that the scale on the contours changes for each configuration. Figure 6 presents the contours of the unburned hydrocarbons. This time, configurations 2 and 3 reveal a region of locally high emissions relative to configuration 1. Finally, Fig. 7 presents contours of the [NOx] levels at the exit. Like the CO, the NOx levels reveal systematically degraded symmetry with increased misalignment.
Correlations To quantify the differences observed, the degree of nonuniformity described by “unmixedness,” U, determined using the expression in Eq. (1) [14], is applied: U4
cvar cavg(1 1 cavg)
(1)
where cvar 4 variance in field c cavg 4 average of c. Equation (1) can be applied to arbitrary distributions and is applicable for either space (“spatial unmixedness,” Us) or time (“temporal unmixedness,” Ut). U ranges in value from 0 to 1, and lower values of U reflect more uniform fields. Since fuel distribution is affected significantly by the misalignment and is known to be a critical factor in combustion performance, Eq. (1) is applied to this quantity. In this case, the PLLIF results are considered with an area of interest shown in Fig. 4 to determine the spatial unmixedness of the fuel. Figure 8 compares the spatial unmixedness, Us, based on the PLLIF images and the lean blow-off limit. The results reveal that Us increases systematically with an increase in the misalignment and that the stability improves in a corresponding fashion. Figure 9 presents the correlation between the mean concentrations (right axis) and the unmixedness (left axis) of the emissions results presented in Figs. 5–7 and Us based on the PLLIF images. The slight dependence of the mean NOx level on Us based on the nonreacting fuel distribution observed is within experimental error and is not strong enough to draw a conclusion. Recalling that this case is op-
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Fig. 4. Comparison of volume flux (cc/cm2 s) from PDI and PLLIF intensity (proportional to liquid volume) for configurations 1 and 3 at axial distance Z 4 50 mm from flare exit.
erating at an overall equivalence ratio of 0.82, the expected NOx dependency is not obvious. Previous work [11] suggested that NOx levels should decrease with higher Us values when the overall equivalence ratio exceeds 0.7. However, the mean levels of CO and UHC exhibit increases corresponding to increases in Us. The left axis in Fig. 9 is used to assess Us (i.e., the patterns) based on the measured emissions quantities at the exit plane, with Us based on the nonreacting fuel distribution. Us, based on CO and UHC correlates well with Us based on the nonreacting fuel distribution. Finally, a weak anticorrelation is observed between Us based on NOx and Us based on the nonreacting fuel distribution. Summary and Conclusions A systematic study of the effect of hardware misalignment on the nonreacting spray structure and
combustion performance for a practical fuel-injection/swirler assembly has been conducted for a single condition. PLLIF and PDI were utilized for the nonreacting assessment, whereas emissions and temperatures were used to characterize the reacting case. The following conclusions are drawn from the work: • The effect of misalignment typically encountered by production hardware during operation on the nonreacting flow field is significant. The fuel distribution is dramatically impacted, whereas the gas-phase structure remains similar with the exception of a shift in the flow direction; the overall droplet sizes remain similar. • A nearly linear relationship between the spatial unmixedness of the fuel ascertained via PLLIF in the nonreacting spray and lean blow off was found, which is consistent with previous studies. • Significant correlation was found between the lev-
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Fig. 5. CO concentration contours at exit plane (ppm).
Fig. 6. UHC concentration contours at exit plane.
Fig. 7. NOx concentration contours at exit plane.
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• The results suggest that improving the robustness in the concentricity of the design can help meet increasingly stringent emissions regulations. Acknowledgments GE Aircraft Engines is gratefully acknowledged for supplying the hardware utilized and support for swirl cup studies and diagnostics development. The assistance of Jason Campbell in the computations associated with the images is appreciated.
REFERENCES Fig. 8. Relation between configuration spatial unmixedness from PLIFF images and lean blow-off limit.
Fig. 9. Relationship between unmixedness for spray, unmixedness for emissions, and mean concentrations of emissions.
els of CO and hydrocarbons produced and the spatial unmixedness based on nonreacting fuel distributions characterized by PLLIF. • Significant correlation was also found between the spatial unmixedness of CO and hydrocarbons at the exit plane and the spatial unmixedness of the fuel based on PLIFF images. • The efficiency of PLLIF makes it an attractive tool for screening fuel distributions produced by practical devices.
1. Wang, H. Y., McDonell, V. G., and Samuelsen, G. S., J. Propulsion Power 10:441–445 (1994). 2. Wang, H. Y., McDonell, V. G., and Samuelsen, G. S., J. Propulsion Power 10:445–451 (1994). 3. Takahasi, F., Schmoll, J., Switzer, G. L., and Shouse, D. T., Twenty-Fifth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1994, pp. 183–191. 4. Wang, H. Y., McDonell, V. G., and Samuelsen, G. S., ASME J. Eng. Gas Turb. Power 117:282–289 (1995). 5. McDonell, V. G. and Samuelsen, G. S., ASME J. Eng. Gas Turb. Power 112:44–51 (1990). 6. Rosfjord, T. J. and Eckerle, W. A., J. Prop. Power 7:849–856 (1991). 7. Cohen, J. M. and Rosfjord, T. J., J. Prop. Power 9:16– 27 (1993). 8. Rosfjord, T. J. and Russell, S., J. Prop. Power 5:144– 150 (1989). 9. Pompei, F. and Heywood, J. B., Combust. Flame 19:407–418 (1972). 10. Fric, T., J. Prop. Power 9:708–713 (1993). 11. Lyons, V. J., AIAA J. 20:660–665 (1982). 12. Igushi, T., McDonell, V. G., and Samuelsen, G. S., An Imaging System for Characterization of Liquid Volume Distributions in Sprays, Proceedings, 6th Annual ILASS-Americas Conference, 1993, submitted to Atomization and Sprays. 13. McDonell, V. G. and Samuelsen, G. S., AIAA J. Prop. Power 7:684–691 (1991). 14. Dankwertz, P. V., Appl. Sci. Res., Sec. A 3:279–296 (1952).
COMMENTS Prof. Sigmar Wittig, University of Karlsruhe, Germany. I was surprised to see relatively small effects of alignment deviations on the droplet size distribution. Do you have an explanation? Author’s Reply. In this device, the breakup of the liquid film deposited upon the venturi is the primary mechanism
of atomization. As a result, it is expected that the droplet size will not be strongly affected if the filming mechanism is not strongly affected. In the present case, the misalignment does not change the location of the simplex nozzle relative to the venturi and, as a result, has little impact on filming mechanism and the subsequent droplet sizes.
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Prof. M. M. ElKotb, Cairo University, Egypt. As we know swirling flow creates hollow conical spays, what is the effect of the surrounding cup on the stabilization and penetration of the spray? Author’s Reply. The general characteristics of the spray fields produced by dome hardware similar to that used in the present study have been documented [1–3]. The larger droplets appear in the “hollow” cone portion of the spray, and penetrate “ballistically” into the chamber. A fraction of the mass from the simplex atomizer is transported directly into the core of the spray. Also in the core, a significant population of fine droplets are backmixed to the cup, and are likely to be a key to stabilization. In addition to providing swirl, the location of the cup relative to the atomizer serves to enhance the mixing of the fuel and air prior to exiting the flare.
● Arvind K. Jasuja, Cranfield University, UK. Could you comment on the extent of attenuation encountered by the laser light sheet in your spray experiments and any corrections made for its compensation? In the context of dense, practical sprays at high pressures, severe attenuation can be encountered, thus potentially undermining the utility of the PLIF technique. Author’s Reply. In the present spray, the attenuation through the spray is less than 10%, thus the impact of “dense spray effects” are small. For “dense” (i.e., optically thick) sprays, efforts are currently underway to account for such effects and are described in detail [1,2].
REFERENCES REFERENCES 1. Wang, H. Y., McDonell, V. G., and Samuelsen, G. S., Twenty-Fourth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1992, pp. 1457–1463. 2. Wang, H. Y., McDonell, V. G., Sowa, W. A., and Samuelsen, G. S., AIAA J. Propulsion Power 10:441–445, (1994). 3. Wang, H. Y., McDonell, V. G., Sowa, W. A., and Samuelsen, G. S., AIAA J. Propulsion Power 10:446–451 (1994).
1. Talley, D. G., Thamban, A. T. S., McDonell, V. G., and Samuelsen, G. S., in Recent Advances in Spray Combustion: Spray Combustion Measurements and Model Simulation, K. K. Kuo, ed., Vol. 171, Progress in Astronautics and Aeronautics, AIAA, pp. 113–142, 1996. 2. Talley, D. G., Verdieck, J. F., Lee, S. W., McDonell, V. G., and Samuelsen, G. S., Paper 96-0469, Accounting For Laser Sheet Extinction In Applying PLLIF To Sprays, presented at 34th AIAA Aerospace Sciences Meeting, Reno, NV, January, 1996, submitted to J. Propulsion Power.