Alternate-Fueled Combustor-Sector Performance: Part A: Combustor Performance Part B: Combustor Emissions

Alternate aviation fuels for military or commercial use are required to satisfy MIL-DTL-83133F(2008) or ASTM D 7566 (2010) standards, respectively, and are classified as drop-in fuel replacements. To satisfy legacy issues, blends to 50% alternate fuel with petroleum fuels are certified individually on the basis of feedstock. Adherence to alternate fuels and fuel blends requires smart fueling systems or advanced fuel-flexible systems, including combustors and engines without significant sacrifice in performance or emissions requirements. This paper provides preliminary performance (Part A) and emissions and particulates (Part B) combustor sector data for synthetic-parafinic-kerosene- (SPK-) type fuel and blends with JP-8+100 relative to JP-8+100 as baseline fueling.


INTRODUCTION
Synthetic and biomass fueling are now considered as nearterm aviation alternate fueling. The major impediment is a secure sustainable supply of these fuels at reasonable cost. Alternate aviation fuels are currently required to satisfy MIL-DTL-83133F(2008) for Fisher-Tropsch-(FT-) equivalent processed ASTM D 7566 (2010) and known as "drop-in" fuel replacements (military and civil, respectively). As in aviation, many land-based and marine power generation systems are elderly, known as the legacy issue. Fueling these systems requires careful compliance to the fuel handling and engine systems for which they were (are) designed. To satisfy a sustainable fuel supply, it will be necessary to accept fuels derived from a variety of feedstocks. Consequently, adherence to alternate fuels and fuel blends requires "smart fueling systems" or advanced fuel-flexible systems, including combustors and engines without significant sacrifice in performance or emissions requirements.
For many diesel systems biomass derived oils are unsuitable because sufficient aromatics and sulfur are lacking which provide lubricity thus reducing design component life. To counter these issues, additives are promoted.
This paper provides preliminary performance, emissions and particulates combustor sector data relative to JP-8+100 as baseline fueling, for SPK-type fuels blends (herein FT-type fuel) and projections for testing of biofuel fuel blends leading to preliminary development of smart fueling (fuel flexible) and combustor systems for the next generation aeronautic and aeronautic-derivative gas turbine engines.
Truly performance and emissions are coupled issues; however, combustor performance will be presented in Part A and combustor emissions as Part B for understanding both in sufficient detail.
Although some general aspects of the fuel delivery system and operations of the facility are similar to that in Hendricks et al. (2004), specific facility modifications and increased capability to handle fuel blending had to be made, including remote alternate test fuel storage/tankage and delivery of the alternate test fuel to the facility fuel pumps, flow meters, and control systems.

Facility Development
Before validation data could be taken, it was necessary to learn what it takes to conduct high-pressure combustor testing of alternate fuels such as FT and biomass feedstock fuels. It is first necessary to establish the combustion parameters required by the study such as operability, performance, durability time-dependent measurements such as flame studies and others. Next, an assessment of the effects of pressure ratios and inlet temperatures on both the combustor sector model and desired data was undertaken as well as most importantly, how to safely blend the fuels. The blending system, while complex, enables operations to establish and stabilize combustor inlet pressures and temperatures of preheated air at the required test condition without the additional complication of simultaneously establishing fuel blend.
To establish the fuel delivery system, questions such as how much fuel and time are required to fully evaluate a typical fuel candidate must be resolved. A 500-gal trailer-mounted fuel tank was chosen for porting alternative fuels with the added feature of coupling to the facility fueling system. The facility has two duplicate fuel systems that provided a means of handling JP-8 fuel with one system to pump, meter, and control the JP-8 fuel; this system is referred to as the main fuel delivery system. The identical system was fed the alternative fuel supply, which is pumped from the trailer into the facility primary fuel system and ultimately blended online with the main fuel system to provide the desired fuel blend from 0% to 100% trailer fuel.

Test Parameters and Data Collection
For this series of testing, the nominal test conditions for pressures and temperatures of blends and the extensive data collection systems have been established. The parameters were chosen to be most representative of engine operations envelope from idle to altitude cruise; however, TO (take-off) pressures are currently beyond the range of this facility.

COMBUSTOR THERMAL PERFORMANCE
The combustion efficiencies for combinations of fuel:air ratio F/A and fuel compositions were of the order of 99.9% (Table. 1), and one is unable to distinguish combustor changes from this single parameter; thus, other parameters will be investigated. For example, the calculated flame temperature (Fig. 2) increases with F/A, with FT about AT = 70 °F (39 K) higher.
Lean blow out (LBO) tests were conducted and found consistent with JP-8+100, and no further tests were undertaken. Altitude restart tests are yet to be conducted.

SURFACE THERMAL MEASUREMENTS
The combustor walls and liners were instrumented for pressure and temperature. In general the pressure drop measurements are sensitive information and will not be presented as such. It should be noted that no inconsistent pressure measurements were found.
The liner and wall surface temperature locations are sensitive information and temperatures are noted as sidewall or liner (i.e., facing inside or outside).

Sidewalls
Figure 3 illustrates that sidewall temperatures (TSW) strongly depend on F/A and weakly depend on fuel blend composition JP-8, FT, and 50:50. FWD represents forward; MID, middle; and AFT, the aft axial position of the thermocouple.

Unwrapped Combustor Liner
Figures 4a, b, and c represent unwrapped liner surface temperatures for three F/A values (0.010, 0.015, and 0.020) and three fueling compositions (JP-8, FT, and 50:50). The twin peaks represent sidewall (largest) and maximum inner liner temperatures, respectively. Figure 4a illustrates the unwrapped liner temperatures for F/A = 0.010 with outer liner temperatures to the left of the peak and inner liner temperatures to the right of the peak. The temperatures are slightly higher for the FT fueling. Figure 4b represents unwrapped liner temperatures for F/A = 0.015 with outer liner temperatures to the left of the peak and inner liner temperatures to the right of the peak. The temperatures are lower for the FT fueling. Figure 4c represents the unwrapped liner temperatures for F/A = 0.020 (JP-8) and 0.019 (FT) with outer liner temperatures to the left of the peak and inner liner temperatures to the right of the peak. The temperatures are lower for the FT fueling. Figure 5 shows that peak inner liner temperatures are nearly independent of fueling composition from 100% JP-8 to 100% FT.

Combustor Inner and Outer Liner
Eliminating the sidewall peak temperature allows visualization of the smaller changes in surface temperatures. Omitting the peak temperature, Figs. 6a through 6f illustrate major portions of the combustor liner axial and circumferential surface temperature variations with F/A and fueling composition. Figures 6a and b illustrate variations of combustor axial and circumferential surface temperatures. Figure 6a shows inner liner temperatures at F/A = 0.010 for fueling composition changes from JP-8 to FT. The temperatures for FT and blended fueling are slightly higher than for JP-8 fueling. Figure 6b shows variations of outer liner combustions temperatures at F/A = 0.010 for fueling composition changes from JP-8 to FT. The temperatures for FT and blended fueling are slightly higher than for JP-8 fueling. Figure 6c shows variations of combustor outer liner temperatures at F/A = 0.015 for fueling composition changes from JP-8 to FT. Figure 6d illustrates variations of combustor inner liner temperatures at F/A = 0.015 for fueling composition changes from JP-8 to FT. Figure 6e shows variations of combustor inner liner temperatures at F/A -0.020 for fueling composition changes from JP-8 to FT. Figure 6f shows variations of combustor outer liner temperatures at F/A -0.020 for fueling composition changes from JP-8 to FT.

COMBUSTION VISUALIZATION
High-speed photographs of the combustion process provide some insights for computational fluid dynamics (CFD) analysts as well as heuristic information for combustor designers: for example, decreased luminosity with FT versus JP-8. The proprietary nature of the combustor being tested makes it difficult to provide photographs and high-speed video of the combustor process requiring most details to be withheld, and it will not be discussed further.

CONCLUSIONS: PART A
Alternate fueling testing is being carried out to determine preliminary performance, emissions, and particulates combustor sector data for SPK-type (e.g., Fischer-Tropsch) fuel blends, relative to JP-8+100 as baseline fueling, and to make projections for testing of biofuel fuel blends leading to preliminary development of smart fueling (fuel flexible) and combustor systems for the next generation aeronautic and aeronauticderivative gas turbine engines. Herein alternate fueling test results for a well-characterized but proprietary combustor are provided for JP-8+100 and a Fischer-Tropsch-(FT-) derived fuel and a blend of 50% each by volume.
The test data presented are part of a more extensive data set where combustion parameters were varied over a range of values. The data herein are for the case of nominal inlet conditions at 225 psia and 800 °F (1.551 MPa and 700 K), and JP-8+100 is taken as baseline. These data provide the following results: 1. Combustor performance efficiencies at 0% FT (JP-8), at 50% blended FT and JP-8, and at 100% FT are nearly identical at about 99.9% 2. Both outer and inner wall temperatures run a. warmer at F/A = 0.010 by at most 9 °F with FT fueling b. cooler at F/A = 0.015 by at most 50 °F with FT fueling c. cooler at F/A=0.02 (0.019) by at most 80 °F with FT fueling 3. Center peak liner temperatures nearly the same to within AT = 10 °F (5.55 K) 4. Rake temperatures show core flow generally higher with FT than JP-8, but one rake thermocouple (TC) was lost during testing, which inhibits conclusiveness. 5. All temperatures increase with F/A. 6. The 50:50 blend test results generally are between JP-8 and FT and somewhat closer to FT. 7. LBO testing results show no change in LBO with FT from JP-8. 8. Altitude relight testing remains to be carried out. 9. High-speed photographs of the combustion process provide some insights for CFD analysts as well as heuristic information for combustor designers. For example, there was decreased luminosity with FT versus JP-8, and clips show enhanced vorticity for the conditions cited in Table. 1.

PART B: COMBUSTOR EMISSIONS
Part B presents gaseous emissions as CO 2 , CO, and NOx (which also includes smoke and luminosity data); particulate emissions including distribution; and a brief comparison to small and large engine testing results from other programs. The emissions data are taken for the same tests and test conditions cited in PART A, nominally 225 psia at 800 °F (1.551 MPa at 700 K) with the sampling probe located at the nozzle exit plane. Emissions have a direct impact on aviation climatic constraints based on life cycle analysis (LCA) of fueling feedstocks, which includes fueling development and engine emissions. Herein the testing is directed toward fuel flex engine combustors, providing basic data for LCA fueling evaluations, where combustor A is one of several to be evaluated in development of fuel-flexible engine combustors.

GASEOUS EMISSIONS
Measurements for NOx were determined from combining NO and NO 2 measurements (Figs. 7a and b). Nitric oxide (NO) with molecular atomic dimension (0.115 nm) (NO), while less than JP-8 at F/A = 0.010, steadily increases to become marginally higher than JP-8 at F/A = 0.020 (extrapolated) (Fig. 6a). Nitrogen dioxide (NO2) (0.221 nm) (ppm) for FT or 50:50 blended fueling is considerably higher than for JP-8 and generally increases with F/A. Combining nitrogen dioxide (ppm) and nitric oxide (ppm), the trend with F/A and fuel composition is similar to that seen for NO; less than JP-8 at F/A = 0.010 and marginally higher than JP-8 at F/A = 0.020 (Fig. 7c) The variation of %CO 2 (0.0116 nm), ppm CO, and %O 2 , (Figs. 8,9,and 10), while strongly increasing with F/A, are marginally consistent with varied dependencies on fuel composition. The %CO 2 appears somewhat consistent with decreased %CO and O 2 with fueling changes from JP-8 to FT, in agreement with flame temperature (Fig. 2). Figure 8 shows the strong variation of %CO 2 with F/A, but it is nearly independent of fuel composition. However, it appears somewhat consistent with decreased %O 2 with fueling changes from JP-8 to FT. Carbon monoxide (0.113 nm) (CO) generally is lower with fueling from JP-8 to FT with some changes at F/A = 0.019, which extrapolated is unresolved (Fig. 9). The decrease in %O 2 (Fig. 10) is consistent with increasing F/A-as well as higher rake temperatures-with FT, indicating increased combustion temperatures with more complete combustion (Fig. 2).

SMOKE AND PHOTO DIODE NUMBERS
The general trend of total hydrocarbon emissions (THC) (Fig. 11) strongly depends on fuel:air ratio F/A and is less dependent on fuel blend except with FT at F/A = 0.015. The reason is not known at this time, nor is it entirely clear that for all intensive purposes why THC is nearly independent of fuel composition because the smoke data do show more distinctive trends with fueling composition at F/A = 0.010 (Fig. 12). For FT fueling, the smoke number is well below that of JP-8 at F/A of 0.01 and 0.02, yet they are nearly the same at F/A = 0.015. FT smoke number increases with F/A, but it is not clear for either JP-8 or 50:50 blended fuel.
Smoke number and THC results reinforce the necessity for good particulate measurements, their distribution, composition, and toxicology. Figure 13 illustrates the change in flame luminosity on a relative basis as the blend of JP-8 and FT fuel is varied. Optical access windows are combustor pressure limited, and the data set shown is at (P,T)inlet (75 psia (0.517 MPa), 500 °F (533 K)) at 3% combustor pressure drop.
The increase in flame luminosity follows the same trends for collected smoke data as shown in Fig. 14. The decline in smoke number with increasing FT fueling is most pronounced at lower F/A ratios. Smoke number consistently increases with F/A independently of fueling yet is lowest at 100% FT fueling. A striking feature is the decrease in relative flame luminosity as illustrated in Fig. 13 with the characteristic clean blue flame at 100% FT fueling. This increase in smoke number and flame luminosity as the fuel blend is increased to 100% JP-8 suggests that the radiative heat load on the combustor increases as well at higher F/A ratios; the wall metal temperatures corroborate this increase. Figure 14 illustrates a decrease in smoke number as combustor pressure changes from 175 to 225 psia (1.207 to 1.551 MPa) (note the anomaly at 175 psia (1.207 MPa)) with consistent increases in smoke number and photo diode emissions with increased F/A from 0.020 to 0.025. In general these trends corroborate the particulate data shown later.

PARTICULATE EMISSIONS
The particulate distribution depends on engine power setting, pressure, F/A and fueling composition, and the chemical nature of the particulates and their toxicity. Such data are necessary for determination of environmental health hazards, cloud formations, and climatic changes.
To demonstrate the operability of the emissions probes, the test F/A ratios were compared with the CO2-based F/A ratios. The 100% FT and 50:50 blends are within +12% to -18% of one-toone correspondence whereas the 100% JP-8 is +8% to -34% with one point at -50%. The general trends are for FT and blends to be consistently higher and JP-8 lower than one-to-one correspondence (Fig. 15). Such evidence may reflect the paraffinic nature of FT and the high aromatic and cyclohydrocarbon content of JP-8.
The nitrogen gas tip-diluted, water-cooled particulate probe is illustrated in Fig. 16. Because of in-plane hardware details, the photo and detail insert are shown rotated out of true exhaust plane. The probe cap outer diameter = 0.075 in. (19 mm) with aperture diameter 0.044 in. (1.12 mm). The probe aperture aspirated exhausted gas steam is quenched by water cooling, which also prevents probe failures from overheating. Both diluted and undiluted probes were positioned at the combustor exhaust plane. For the dilution probe, the exhaust gas is further cooled and diluted with nitrogen gas. Both types are held above condensation temperature of water and organics en route to the instrumentation sampling panel. Details of the facility and gas emissions sampling probes are given in Shouse et al. (2001).
In terms of particle emissions indices EIn, the general trends with both pressure and F/A are higher EI n values (number/kg-fuel burn) for JP-8 and lower values for FT with the 50% blend (50% JP-8 and 50% FT) in between (Fig. 17). At an F/A of 0.015, the FT emissions index EIn -FT is nearly 1/4 that of JP-8 at 175 psia (1.207 MPa); at F/A = 0.020, nearly 1/2 at 225 psia (1.551 MPa); and at F/A = 0.025, nearly 7/8 at 175 psia (1.207 MPa). Note, however, the variability of 50% fuel blend at lower pressures of 75 psia (0.517 MPa). Whereas it is difficult to make a direct comparison with on-wing engine testing, the data trends are consistent where FT particulate emissions are much lower than Jet A at low power (lower engine pressure), yet the difference trend diminishes with increased engine power (higher engine pressure).
Trends with the cleaner paraffinic fuels (FT) are also reflected in terms of particulate size distribution (Fig. 18) but not necessarily in terms of the FT blend, where at 75 psia (0.517 MPa) anomalous behavior is observed, namely the number of particulates (N) of size Dp (equivalent diameter) per cubic centimeter increases beyond that of JP-8. However for FT fueling, the values of the [dN/d(log Dp)] derivative indicate the total particle counts (integrals) are nearly half that of JP-8. Note the peak shift toward smaller diameter particulates, and the smaller (about half the size of the JP-8 peak) particulates making more difficult to detect, isolate, collect and dispose of such particulates. Further, the toxicology requires much study.
Particulate size and to some extent, distribution, are highly dependent on the probe. Effects of probe tip dilution and probe secondary dilution are illustrated in Fig. 19 for combustor pressure of 125 psia (0.862 MPa) and F/A = 0.015. Here the trend with particulate size is not as definite as illustrated in Fig. 18, and the effects of probe dilution diminishes with fuel blending.
Looking again at the anomalous trends at combustor pressure of 75 psia ( 0.517 MPa) and 175 psia (1.207 MPa) shown in Figs. 17 and 18, shows similar trends in particulate distribution (Fig.  20). Whereas the cleaner FT fuel particulate peak is still less than that of JP-8 or the FT blend, the trend is minor by comparison with those shown in Fig. 17 at other pressures. While consistent, the behavioral reasons remain to be explored.
In contrast to the distribution trends at combustor pressure of 75 psia (0.517 MPa) and 175 psia (1.207 MPa) and F/A = 0.025 (Fig. 20), the trends at combustor pressure of 225 psia (1.551 MPa) and F/A = 0.020 are consistent with clean fuel blending; namely JP-8 produces more particulates than the FT blend and far more than FT fueling (Fig. 21). The variation with JP-8 fueling is also illustrated as JP-8(2) on the figure. Less pronounced is the variation in the particulate peaking which is more consistent with that of Fig. 20.
The mean particle diameter at 175 psia (1.207 MPa) decreases with fueling blend from JP-8 to FT (Fig. 22). This trend is not evident in Fig. 20, adding to the complexity of predicting combustor particulate variations.

Other emissions and peformance tests indicate small to no changes in emssions within limits as prescribed in the Jet A fueling specifications (Kinder and Rahmes, 2009).
A collaborative [NASA, AFRL, ARI, UTRC, and P&W] small and large on-wing engine emissions and performance test program provides several needed insights into aviations emissions (Bulzan, 2009;and NASA et al., 2008).

Small Engine Testing
Small-engine test stand observations on a test-stand-mounted PW 308 engine fueled by JP-8. FT, and FT-blended fuels 1. At low power, NOx emissions are within instrument measurement capabilities Lower CO emission with FT/blend may be due to higher H/C ratio 2. At intermediate or high power, Very low CO emissions make ratios irrelevant to evaluate differences between the fuels No significant difference in NOx emissions These tests also revealed negligible unburned hydrocarbons (UHC) at all power conditions for both of the two FT fuels tested. The SO 2 emissions indicate the sulfur content of the blend to be around 50% of that for JP-8, whereas for 100% FT fuel a value of 0.1% indicates contamination. The ~2% fuel flow benefit with 100% synthetic fuel can be attributed to the higher heat content of synthetic fuel.
Approximately 2% fuel flow benefit with 100% synthetic fuel can be attributed to the higher heat content of synthetic fuel Rahmes et al. (2009) provides emissions results for an unspecified fuel that was tested in a Pratt & Whitney small turbine engine (inferred as PW 308 and biofueling). Emissions deviations were small except for core smoke (Fig. 23). The particulate distributions change with both fueling (F/A) and engine power settings, showing decreases in emissions with increses in %FT and increases with engine power setting (Figs. 24 and 25). Figure 26 provides a comparison of mean particulate diameters for JP-8, 50:50 blend, and FT fueling with changes in engine power for the PW 308 off-wing engine testing.

Large Engine On-Wing Test Results
A consortium of agencies are working together to provide onwing engine emissions testing for 100% JP-8 or Jet A, a 50:50 blend with SPK, and 100% SPK engine fueling at various power settings. Here SPK represents different Fischer-Tropsch fuels depending on feedstock and refiner. Future testing will include biomass feedstock fueling (HRJ). For these tests the fuel was either coal-or gas-derived jet fuel. Particulate distributions given by Anderson (2009) at 30% and 65% engine power setting are provided on the left side of Fig. 27. The number of particulates and black carbon values are provided on the right side of Fig. 27.
The data presented herein show a strong dependence on F/A and blend with an implied less dependency on fuel composition. The small-engine test data figures are both normalized and too course to illustrate the dependencies for the data herein. As for the on-wing engine test results, the AAFEX program data are planned to be released in a January 2010 workshop.
These comparisons and test data presented herein imply-yet at this time cannot conclude-that sector test data replicate, at least qualitatively, on-wing test data, providing both detail and insights not gained from on-wing tests. Post AAFEX 2010 Workshop analysis of released data and data herein is warranted.

CONCLUSIONS: PART B
Alternate fueling testing is being carried out to determine preliminary performance, emissions, and particulates combustor sector data relative to JP-8+100 as baseline fueling, for SPK-type (e.g., Fischer-Tropsch, FT) fuels blends and projections for testing of biofuel fuel blends leading to preliminary development of smart fueling (fuel flexible) and combustor systems for the next generation aeronautic and aeronautic-derivative gas turbine engines. Herein alternate fueling test results for a well characterized but proprietary combustor are provided for JP-8+100, a FT-derived fuel, and a blend of 50% each by volume.
The test data presented are part of a more extensive data set where combustion parameters were varied over a range of values.
The data herein are for the case of nominal inlet conditions at 75 psia (0.517 MPa) to 225 psia (1.551 MPa) and 800 °F (700 K), and JP-8+100 is taken as the baseline.
1. The 50:50 blend test performance and emission results generally are between JP and FT and somewhat closer to FT 2. Emissions: CO is lower with FT; CO 2 is about the same; NO is lower with FT; NO 2 is higher with FT fueling F/A; NOx is lower to higher with FT with F/A; O 2 decreases with F/A (consistent with temperature increase), is lower with FT with increased spread from JP-8 with F/A, again consistent with rake temperature; HC generally decreases with F/A, yet FT humps at 0.015. No explanation is provided. 3. Basic emissions show more change with F/A than with JP-8 or FT; the latter being the more significant. These results appear to agree qualitatively to on-wing engine testing. Quantitative agreement requires resolution pending data release. The other aspect is to look at how emissions change with pressure and EXTRAPOLATE those results to core pressure on the ground, that is, at much higher pressures. 4. Comparisons of engine on-wing and combustor-sector test data imply (but not conclude at this time) replication. Post AAFEX 2010 Workshop data analysis is warranted. from Shell gas-to-liquid (GTL) with 0% aromatics and 0% sulfur. The JP-8 cited is 19% aromatics and 1200 ppm sulfur   O; i 0 (F, W) Fig