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An Interim Assessment of AEAP's Emissions Characterization and Near-Field Interactions Elements Emissions Characterization The Emissions Characterization element encompasses a wide range of research activities. In addition to a variety of combustor-rig and engine-emission measurement activities, extensive efforts are being conducted to develop advanced emission-measurement methods and associated computational fluid dynamics (CFD) analytical models. As of the end of fiscal year 1996, approximately 68 percent of the funding allocated to this element had been expended or committed. The work planned for Emissions Characterization is scheduled to be completed during fiscal year 2001. The funding available for the remaining five years is not large, considering the breadth of activities and the magnitude of test efforts—on the order of $1.91 M. The three key considerations in the execution of the work scope of this element are the prioritization of the engine-exhaust constituents to be measured, the selection of venues for conducting these measurements, and the selection of measurement methods. EMISSION CONSTITUENTS TO BE MEASURED A prioritization of the constituents to be measured was initially generated in 1992 as a part of the AESA project. This prioritization effort was carried out by a committee of experts, the Engine Exhaust Trace Chemistry Committee (EETCC). that was specifically established to guide and support the work being conducted within this research area. The results of the EETCC's prioritization efforts are summarized in Engine Trace Constituent Measurements Recommended for the Assessment of the Atmospheric Effects of Stratospheric Aircraft (Miake-Lye et al., 1992). A summary version of the prioritized listing contained in that report is presented in Table 1. At the start of the SASS project in 1994, the EETCC determined that this AESA prioritization was also applicable to the new project's emission-characterization efforts. But in light of the recently obtained in-flight measurements of the composi-
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An Interim Assessment of AEAP's Emissions Characterization and Near-Field Interactions Elements TABLE 1 Summary of 1992 EETCC Priorities for Engine Emissions to be Measured First Priority Second Priority Low Priority NOx SO2 H2O Soot particulate SO3/H2SO4 HO2/H2O2 OH CH4 HNO3 Non-methane hydrocarbons Total hydrocarbons CO CO2 Cl Metal oxides tion of the plume of a Concorde aircraft, as described in a paper by Fahey et al. (1995) and in NASA's AESA report (Stolarski et al., 1995), some adjustments of this prioritized listing may now be needed. The particulate-emission measurements obtained in these in-flight tests indicate a high degree of sulfur dioxide (SO2) oxidation to condensed sulfate. This finding is surprising, especially because the data suggest that SO2 oxidation by hydroxyl radicals (OH) is not the dominant cause of the conversion. The 1995 AESA report states that a mechanism that rationalizes the observed degree of SO2 oxidation has yet to be identified. The report also notes that increases in sulfate-aerosol surface area in the lower stratosphere may result in ozone depletion, and that this impact is maximized if the sulfate is formed within the exhaust plume. These effects are cited as sources of uncertainties in predicting the impacts of HSCT fleets on the ozone column. (The topic is also discussed in PAEAN's recently published report on SASS (NRC, 1997).) NASA's Concorde measurements are of considerable significance in that they are the first in-flight measurements made behind a supersonic aircraft. It is of especial importance that the measured nitrogen oxides (NOx) emission indices are in good agreement with those obtained previously in emission tests of a Concorde aircraft engine (Olympus) conducted in an altitude test cell. However, any specific inferences concerning the SO2 oxidation characteristics observed in these tests must be tempered by the fact that the Olympus engines that power the Concorde aircraft embody old technology (circa 1965). The future HSCT engine is likely to have a very different thermodynamic cycle, and thus very different exhaust-gas characteristics and compositions. In particular, the NOx and soot particulate emission levels of such an engine are expected to be considerably different from those of the Olympus engine. Accordingly, the direct relevance of the high degree of SO2 oxidation observed in the in-flight tests is unknown. Nevertheless, in view of the significant atmospheric impacts that could result from such high conversion rates, an improved understanding is needed of the chemical and particulate formation pro-
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An Interim Assessment of AEAP's Emissions Characterization and Near-Field Interactions Elements cesses that occur in engine exhaust plumes, particularly in the near field. These mechanisms are being investigated in other elements of AEAP. As an important contribution to this understanding, however, additional engine tests (described below) are needed to determine the degrees of SO2 oxidation that occur within engines. Such information is necessary as the starting point for studies to determine the degrees of SO2 oxidation that can occur at various stages within the plume. Some data relevant to this question will also be forthcoming from ongoing U.S. and European flight-measurement projects. Some changes to the EETCC's prioritization are thus recommended. Measurements of the SO2 and SO3 (sulfur trioxide) emission levels of engines should be changed from second priority to first. Also, because of its importance in near-field chemistry, consideration should be given to a higher priority for OH measurements. In the AESA-related testing, these measurements should be obtained with engines that are more representative of the eventual HSCT aircraft engine. Supersonic military aircraft engines are suitable candidates, being fairly similar in design and operating features to the currently proposed HSCT engine. VENUES FOR CONDUCTING EMISSION MEASUREMENTS A second issue of major importance in the execution of the Emission Characterization element is the selection of venues for conducting measurements. Engine tests, made either during flight or in altitude test cells, are preferred. Because of the high costs of such testing, however, considerable reliance on combustor-rig tests of various kinds is currently featured in the planned work of this element. While attractive from a cost standpoint, the use of combustor-rig tests for the acquisition of emission data does have some significant limitations. Obtaining representative samples at the combustor-exit plane with gas-extraction methods is difficult because most current subsonic engines have large radial and circumferential concentration and temperature gradients. With non-intrusive measurement techniques, obtaining optical access to the combustor-exit gas flow is usually difficult because of adverse geometries and hostile environments. Also, in the case of some constituents, concentrations measured at the combustor-exit plane may not be representative of those existing at the engine-exit plane. This is certainly true for highly reactive species, such as OH, and may also be true for several other species, such as soot particulate, SO2, SO3, and hydrocarbons. To resolve these latter shortcomings, analytical models need to be used to quantify any chemical changes occurring in the combustion gas as it flows from the combustor-exit plane to the engine-exit plane. The development and validation of such highly complex, sophisticated 2-D and 3-D computational fluid dynamics models are formidable undertakings. The models being developed as part of this AEAP activity are still some way from usable. An assessment of the
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An Interim Assessment of AEAP's Emissions Characterization and Near-Field Interactions Elements progress and expected results of these efforts will be made when additional information on their status is available. In the case of soot particulate, obtaining accurate measurements of total mass, size distribution, and other characteristics at the combustor-exit plane is especially difficult with existing gas-sampling techniques. The combustion-gas pressures at the sampling plane are quite high (from several to tens of atmospheres). Before the characteristics of the sampled particulate matter can be measured, the pressure of the sample of gas must be reduced to essentially one atmosphere. Completing this pressure reduction without simultaneously altering some of the particulate characteristics is a challenge that has not yet been fully met. For these reasons, and given the limited remaining budget for this AEAP element, an increased emphasis on engine testing at cruise-altitude operating conditions and a corresponding decreased reliance on combustor-rig testing are recommended for the present. In view of the funding limitations of this element, aggressive efforts to seek out and capitalize on opportunities for conducting such testing on a piggyback basis—taking advantage of other experiments to add emissions instrumentation—are strongly encouraged. To the extent possible, these tests should be patterned after the excellent engine-test program conducted as a part of this element during 1995 at the Arnold Engineering Development Center (AEDC) at Arnold Air Force Base. In this extensive test series, gaseous and particulate emission data were obtained in piggybacked tests of a modern engine at both sea-level and altitude operating conditions. The large array of extracted-gas-sample and non-intrusive measurement methods used in conducting these tests, together with the test findings, are described in Howard et al. (1996). Additional tests are needed to further quantify the emission characteristics of current-technology subsonic engines. These tests should preferably involve civil subsonic engines, especially newer models recently introduced into operational service. The latter engines will power the bulk of the subsonic aircraft fleet in service for at least the next three decades. Although more advanced subsonic engines will be gradually introduced, it is probably premature at this time to speculate on what their NOx and other emission characteristics might be. The degrees of emission abatement that will have to be incorporated into the combustors of these future engines will, in fact, be in large part determined by the SASS project's assessments. These SASS findings will not be available until 2002, at the earliest. For these reasons, the planned rig testing of candidate Advanced Subsonic Technology Program combustors should be relegated to second-priority status. In the case of the AESA-related activities of this element, primary reliance on engine testing is also recommended. While no HSCT engine prototypes exist at this time, a demonstrator engine equipped with an ultra-low-NOx combustor
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An Interim Assessment of AEAP's Emissions Characterization and Near-Field Interactions Elements will be tested during the 2001–2002 time period as part of the HSRP. The specific purpose of this demonstrator-engine test effort is to evaluate a version of the ultra-low-NOx combustor concept that is being evolved in the HSRP. This engine-test series should provide an excellent opportunity to obtain a quantitative determination of the future HSCT aircraft engine's emission characteristics at cruise. Prior to this dedicated-demonstrator test, engine tests to assess the degrees of SOx oxidation occurring within engines should be conducted. For this purpose, piggy backed tests of an advanced military supersonic aircraft engine are recommended. METHODS OF OBTAINING EMISSION MEASUREMENTS The third key issue is the selection of measurement methods for use in the engine and combustor-rig tests. For some constituents (such as NOx, CO2, CO, hydrocarbons, and soot particulates), extractive sampling and analysis methods are well developed. Accordingly, in any given engine or combustor test series, the concentrations of these constituents can be readily quantified. These measured concentrations can then be used to calculate emission-index (grams per kilogram of fuel) values, using known engine fuel-air ratio data. For other constituents (such as SO2 and SO3), well-established techniques for analyzing extracted-gas samples are not available. For the most part, non-intrusive methods of determining exhaust-gas constituent concentrations are still in the research stage. For these reasons, as a part of the work scope of this AEAP element, efforts have been made to develop improved extracted-gas sample-analysis methods, as well as new and/or improved non-intrusive analysis methods. Excellent progress has been made in both areas. Along with a standard suite of extracted-gas sample-analysis methods, these new and/or improved measurement methods were deployed in the above-described AEDC engine test series, with good success. Further improved and/or new methods for measuring emitted levels of soot particulates, OH, SO2, and SO3 at the engine exhaust-nozzle exit plane are still needed, as they are for characterizing the particulates. Intensified efforts to meet these needs are recommended. In the case of soot-particulate characterization, refinement and validation of the methods evolved to date in this AEAP element should receive greater emphasis. In the case of OH-concentration measurement, further refinement of the UV-absorption method currently being developed by AEAP would be desirable. And for measuring SO2 and SO3 concentrations, suitable new extracted-gas sample-analysis methods and/or non-intrusive sampling and analysis methods are needed. For instance, tunable-diode-laser absorption spectroscopy should be sensitive enough to determine SO 3 (which is so reactive a non-intrusive method is needed) and SO2 concentrations, via a mul-
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An Interim Assessment of AEAP's Emissions Characterization and Near-Field Interactions Elements tiple-pass optical system set up to sample across the engine-exit plane. Efforts to evolve and refine the hydrocarbon-speciation methods currently being pursued as a part of the work scope of this AEAP element should also continue. In all cases, emphasis should be focused on measurements at the engine-exit plane, rather than at the combustor-exit plane.
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