Energy & Fuels 1995,9, 448-457
448
An In-Depth Evaluation of Combustion Performance Predictors of Aviation Fuels Sooting Tendencies Seetar G . Pande Geo-Centers Znc., Ft Washington, Maryland 20375
Dennis R. Hardy* Navy'Technology Center for Safety and Survivability, US.Naval Research Laboratory, Code 6181,Washington, D.C. 20375-5342 Received November 16, 1994. Revised Manuscript Received January 10, 1995@
The capabilities of combustion performance predictors to reliably predict the sooting tendencies of aviation fuels in jet combustors were evaluated. The test matrix included two primary fuel sets which were representative of current and future aviation fuels and combustor data, which consisted of radiation and soot data from two different combustors at specific levels of operation. Four of the predictors evaluated were based on a single parameter, viz., three current predictors (smoke point, hydrogen content, and total aromatics content), and one new predictor, Shell's Premixed Burner Number. The remaining predictors, which were based on more than one parameter, include Rosfjord's correlation, Chin-Lefebvre's correlation, a smoke point-hydrogen content combination predictor, and, in addition, three new trial predictors, of which two were based on composition and one on a combination of differentiated aromatics content and smoke point. The results indicate two of the new trial predictors exhibited the best overall predictability. These two trial predictors are monocyclic aromatics important extensions of the parameters employed in Rosfjord's and Chin-Lefebvre's correlations, respectively. The better predictabilities of the two trial predictors are likely attributable to better weighting of the compositional contributions to sooting, particularly at the highest power demand.
Introduction We define reliable predictors of aviation fuels' combustion performance as capable of predicting the sooting tendency of compositionally different aviation fuels, in any jet combustor, at various levels of operation. The term sooting tendency, in this context, refers to both the exhaust soot and flame radiation emitted. The ability to predict the sooting tendency of fuels is important from military, environmental, and economic standpoints. For example, soot formation increases exhaust emission, which can significantly compromise combat missions due t o plume visibility. Air pollution caused by soot emission poses health hazards; e.g., certain polycyclic aromatics found in soot are carcinogenic. And, soot formation in combustors contributes to a rise in the combustor liner temperature, thereby increasing maintenance costs. Reliable predictability of combustion performance predictors is consequently a desirable capability. Current Predictors. The combustion quality of aviation fuels is currently controlled by bench tests' specifications. These include smoke point (ASTM D1322), which is defined as the maximum flame height (measured in millimeters) at which no smoke is produced when the fuel is burned in a wick-fed lamp. The higher the smoke point, the lower the sooting tendency of the fuel. The minimum allowed smoke point values range from 19 mm for JP-5 military fuels t o 25 mm for commercial fuels. @
Abstract published in Advance ACS Abstracts, March 15, 1995.
0887-0624/95/2509-0448$09.00/0
A well-acknowledged problem associated with smoke point is that its accuracy appears to be operator dependent. A new method of smoke point determination is the radiant extinction of soot reported by Markstein.' This method appears to offer an improvement over the visual method of detection of smoke point. Nonetheless, Carrier and Wetton2 have critiqued the smoke point method, which is based on a laminar diffusion flame, as not simulating the more complex process of combustion in a gas turbine engine, where the flame is realistically characterized as turbulent diffusion. Because of such limitations, additional criticism can be directed at the inability of smoke point to reliably predict fuels of marginal combustion quality. A recommended alternative to smoke point is luminometer number (ASTM D1740). For commercial fuels, the minimum allowed luminometer number is 45 mm. Alternatives to smoke point and luminometer number are compositional limits. These include the conventional limit of 25 vol % maximum total aromatics content, which is determined using the ASTM D1319 method, and the currently favored minimum hydrogen content of 13.4 wt % determined using the ASTM D3701 method. The effect of fuel hydrogen content on soot formation was first demonstrated by Schirmer et al.3x4 Subsequently, hydrogen content was found, in numerous studies, t o be an effective compositional predictor of (1)Markstein, G. H. Twentieth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1984; p 1055. (2) Carrier, D. M.; Wetton, R. J. Trans ASME J . Eng. Gas Turbines Power 1988,110, 100 (and references therein).
0 1995 American Chemical Society
Sooting Tendencies of Aviation Fuels fuels' sooting tendencies; many of these studies were conducted to investigate the effect of anticipated future fuels on existing Martel and Angello5also found the precision of hydrogen content to be much better than either smoke point or luminometer number and recommended a minimum of 13.5 w t % hydrogen content specification for JP-4, JP-5, JP-8, Jet A, Jet A-1, and Jet B fuels; and a minimum of 14.5 wt % hydrogen for JP-7 fuels. However, other investigators2,20-22 have found hydrogen content does not adequately predict the sooting tendency of fuels containing a high concentration of polycyclic aromatics. And more recently, Chin and L e f e b ~ r have e ~ ~ found fuels of similar hydrogen content varied significantly in their sooting tendencies. Furthermore, none of the current predictors, uiz., smoke point, total aromatics content, and hydrogen content, have been found to be adequate predictorsz4of present day and candidate future fuelsS2 New Predictors. The need for an improved combustion performance predictor is reflected in the ongoing publication of new predictors. These include the following: (1) Rosfjord's ~ o r r e l a t i o n ,a~ ~compositional predictor based on weight percent hydrogen content and volume percent naphthalenes. (2) Shell's Premixed Burner Number2 (PMBN),which is based on a flow rate (3) Schirmer, R. M.; Miller, E. C. Presented before Gas and Fuel Chemistry Session, American Chemical Society, Urbana, IL, May 1958. (4) Schirmer, R. M.; Quigg, H. T. Phillips Petroleum Co. Report No. 3952-65R3,Bartlesville, OK, 1965. (5) Martel, R. C.; Angello, L. C. Air Force Aero Propulsion Laboratory Technical Report, AFAPL-TR-72-103,Wright-Patterson Air Force Base, OH, 1973. ( 6 )Butze, H. F.; Ehlers, R. C. NASA Technical Memorandum, TMX71789, presented at Fall Meeting of Western States Section of Combuskon Institute, Oct 20, 19751 (7) Gleason, C. C.; Bahr, D. W. Final Report. National Aeronautics and Space Administration, NASA CR-134972, Washington, DC, 1976. (8) Blazowski, W. S. Sixteenth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1976 p 1631. (9) Blazowski, W. S. J . Eng. Power 1980, 102, 403. (10) Naegeli, D. W.; Moses, C. A. SAE Paper 781026; Society of Automotive Engineers, Inc. Warrendale, PA, 1978. (11)Gleason, C. C.; Oller, T. L.; Shayeson, M. W.; Bahr, D. W. Air Force Wright Aeronautical Laboratories, AFWAL-TR-80-2092, 1980. (12) Friswell, N. J. Combust. Sci. Technol. 1979, 19, 119. (13) Moses, C. A.; Naegeli, D. W. The American Society of Mechanical Engineers, ASME 79-GT-178. Presented at the Gas Turbine Conference, San Diego, CA, 1979. (14) Moses, C. A,; Karpovich, P. A. AGARD-CP-422, 1988, Paper 15-1, Advisory Group for Aerospace Research and Development Symposium on Combustion and Fuels in Gas Turbine Engines, Greece, 19-23 October, 1987. (15) Russell, P. L. Air Force Wright Aeronautical Laboratories, Technical Report, AFWAL-TR-81-2081, Wright-Patterson Air Force Base, OH, 1980. (16)Bowden, T. T.; Pearson, J. H.; Wetton, R. J . J . Eng. Gas Turbines Power 1984, 106, 789. (17) Bowden, T. T.; Pearson, J. H. J . Eng. Gas Turbines Power 1984, 106, 109. (18)Bahr, D. W. Trans ASME J. Eng. Gas Turbines Power 1984, 106, 96. (19) Olson, D. B.; Pickens, J. C.; Gill, R. J. Combust. Flame 1986, 62, 43. (20)Naegeli, D. W.; Moses, C. A. The American Society of Mechanical Engineers, ASME 80-GT-62. Presented at the Gas Turbine Conference, New Orleans, LA, 1980. (21) Dodge, L. G.; Naegeli, D. W.; Moses, C. A. Proceedings of the Western States Section of the Combustion Institute; Paper WSS 80-9, Spring Meeting, April 21-22, Irvine, CA, 1980. (22)Naegeli, D. W.; Dodge, L. A,; Moses, C. A. J. Energy 1983, 7, 168. (23) Chin, J. S.; Lefebwe, A. H. Combust. Sci. Technol. 1990, 73, 479. (24)Moses, C. A. CRC Panel at the ASTM Section 57 Fall Meeting, 1985. (25) Rosfjord, T. J. NASA Conference Publication 2307. Proceedings of the Conference on Assessment of Alternative Aircraft Fuels, NASA Lewis Research Center, Nov 2-3, 1983, Cleveland, OH; p 31.
Energy & Fuels, Vol. 9, No. 3, 1995 449 method of determining the sooting point, is defined as the time in seconds taken for 0.5 g of the test fuel to produce the same luminosity as a low sooting reference fuel (Shellsol T) flowing at lo&; the lower the PMBN, the lower the sooting tendency of the fuel. (3) The combination of smoke point and hydrogenlcarbon ratio proposed by Gulder et aZ.26,27(4) Chin-Lefebvre's ~ o r r e l a t i o n another ,~~ combination predictor, which is based on smoke point and volume percent naphthalenes. Based on the set of 17 fuels examined, Carrier and Wetton2found PMBN exhibited better correlations with exhaust soot and flame radiation data (obtained using a Rolls Royce Tyne combustor) than aromatics content (vol %), hydrogen content (wt %), or smoke point; hence, they reported PMBN to be a more realistic index of combustion quality than the current predictors. Furthermore, because of the diversity in composition,origin, and refinery processing routes of the fuels employed, the PMBN was suggested to be predictive of traditional and unconventional or future fuels. Our earlier evaluation of the PMBN indicated that certain improvements in the soot detectability aspects of the Burner were necessary.28 This observation was particularly pertinent for fuels that might be classified as marginal in their sooting tendencies. The fuels we used were similar to, but not a subset of, the original fuels used by Carrier and Wetton.2 Private communication with Shell indicates the detectability aspects alluded to have since been addressed. In the quest for a more reliable predictor of combustion performance, we included in our evaluation three new trial predictors that we generated. Like many of the new predictors, they are based on more than one parameter and are designated NCP/CP-MADA, HCMADA, and SP-MADA, where NCPXP refers to the saturates ratio of noncyclo/cycloparaffins; MADA to the weight percent or volume percent monocyclic aromatics (MA) and dicyclic aromatics (DA); HC to the weight percent hydrogen content; and SP to the smoke point. NCP/CP-MADA is a compositional predictor. The rationale for including the NCP/CP ratio was to take into account the greater sooting tendency of the cyclo us the noncyclo paraffins. Such differentiation is particularly important from a quantitative viewpoint since the paraffins comprise the major fraction of the fuel. Inclusion of the cyclo paraffins as contributing to the sooting tendency of fuels dates back to studies by Kewley and M i n ~ h i n and , ~ ~ more recently Friswelll2 and G ~ l d e r .Of ~~ the other two new predictors, HC-MADA is also a compositional predictor and SP-MADA a combination predictor. Both HC-MADA and SP-MADA are based on the same parameters as Ro~fjord's~~ and Chin-Lefebvre'~~~ correlations, respectively, but for the inclusion of the monocyclic aromatics in each case. ~
(26) Gulder, 0. L.; Glavincevski, B.; Das, S. Trans ASME J . Eng. Gas Turbines Power 1989, I l l , 77. (27) Gulder, 0. L.; Glavincevski, B.; Baksh, M. F. Trans ASME J . Eng. Gas Turbines Power 1990, 112, 52. (28) Pande, S.G.; Hardy, D. R. Proceedings ofthe Eastern Section of the Combustion Institute; Paper 142-1, Fall Meeting, Orlando, FL, Dec 3-5, 1990. (29) Kewley, J.; Jackson, J . S. J . Inst. Pet. Technol. 1927, 13, 372. (30)Minchin, S. T. J. Inst. Pet. Technol. 1931, 17, 102. (31) Gulder, 0. L. Combust. Flame 1989, 78, 179.
Pande and Hardy
450 Energy & Fuels, Vol. 9, No. 3, 1995 Table 1. Combustion and Composition Dataa for Shell Fuel Set* normalized exhaust flame soot radn
fuel HT3 HC2 JET A-1 AVCAT HC27 high naphth kero North Sea MGO HC29 ERBS2
0.88 0.96 1.00 1.07 1.19 1.41 1.72 1.89 2.60
Shell PMBN (s)
1.11
1.03 1.00 1.17 1.19 1.28 1.38 NA 1.58
216 215 213 22 1 22 1 227 241 248 ND
smoke pt (mm) 26 28 24 23 21 22 19 16 14
compositional analysisC saturates aromatics (vo1 %) NCP/CP total total monoc dicyc
hydrogen content (wt %) 13.95 14.08 13.80 13.80 13.49 13.62 13.48 12.99 12.85
1.21 1.41 1.63 1.23 0.33 1.40 1.45 0.45 1.28
83.6 87.4 81.0 81.0 84.6 79.5 76.1 70.3 72.4
16.4 12.6 19.0 19.0 15.4 20.5 23.9 29.7 27.6
15.53 12.20 16.00 16.98 15.00 15.48 15.85 27.29 15.84
0.87 0.40 3.00 2.02 0.40 5.02 8.05 2.41 11.76
a Supplied by Shell, unless otherwise stated; NA = not available; ND = not determined. Fuels ranked from best to worst according to normalized exhaust soot. Noncycloparaffidcycloparafin ratio, determined at NRL, is based on a modification of the ASTM D2425 method; total aromatics content was determined using ASTM 1319; the dicyclic aromatics content using ASTM 1840; the monocyclic aromatics content was determined by difference: total - monocyclics.
Table 2. Single Parameter Combustion Predictors and Compositional Data for NAWC Fuels” ~~~~
no.*
smoke ptc (mm)
Shell PMBN (s)
hydrogen content (wt %)
modified D2425 NCP/CP ratio
5 4 3 10 2 8 9
23.3 22.2 20.8 20.6 18.7 18.6 18.6
221 238 235 224 241 240 220
13.79 13.82 13.66 13.70 13.48 13.54 13.49
0.97 nd nd nd 1.17 1.18 1.09
82.5 79.5 74.0 76.4 77.9 79.2 73.9
6 1 7
17.0 17.0 13.0
258 240 267
13.22 13.36 12.83
1.44 1.35 1.14
70.5 67.4 69.4
fuel
~
fuel compositional analysis (wt %) total saturates arom 17.5 20.5 26.0 23.6 22.1 20.8 26.1 29.5 32.6 30.6
parafind noncyclic cyclic 40.6 41.9 nd nd nd nd nd nd 42.0 35.9 42.9 36.3 38.5 35.4 41.6 28.9 38.7 28.7 37.0 32.4
aromatics (HPLC) mono 14.5 18.5 22.0 22.5 18.6 14.4 24.6 11.2 27.5 20.9
di 3.0 2.0 4.0 1.1 3.5 6.4 1.5 18.3 5.1 9.7
a Fuels ranked from best to worst according to smoke point. Fuels: 1-4: Suntech blends; fuel 5 : “low” aromatic JP-5; fuel 6: fuel heating oil No. 2; fuel 7: 20/80 blend of HCGO (Hydro Catalytic Gas OilYfuel9; fuel 8: 50/50 blend of DFM (Diesel Fuel Marine)/fuel 5; fuel 9: “high” aromatic JP-5; fuel 10: oil shale (JP-5). As reported in ref 32. Cyclo paraffins based on NCPICP ratio; nd: not determined.
Experimental Section Fuel Sets. Two primary sets of fuels of diverse composition were used for which extensive engine/combustor data were available (see below). One set was supplied by Shell Research Centre, Thornton, England, and comprised nine fuels, and the other by the Naval Air Warfare Center (NAWC), formerly known a s the Naval Air Propulsion Center. Although the NAWC primary set comprised 1 0 fuels, this set was divided into two subsets that were representative of present day and future fuels (see NAWC fuels, below). In general, the diversity of the fuels examined accurately reflects the composition of present day and future fuels. Shell Fuels. These fuels were derived from diverse processing routes but were not a subset of the 1 7 fuels employed by Carrier and WettonS2 Fuel compositional data, along with bench test and combustor data for the nine fuels, are given in Table 1. Unless otherwise stated, much of this data was kindly provided by Shell. As shown in Table 1, the hydrogen content of seven of the nine fuels ranged from 13.48 to 14.08 wt % and their total aromatics content from 12.6 to 23.9 vol 7%. Thus, these seven fuels all met, and in certain cases were well within, the 13.4 wt % minimum hydrogen content (MIL-T5624N specification) and the 25 vol % maximum aromatics content (ASTM D1655) requirements. The hydrogen contents of the remaining two fuels (12.85 and 12.99 wt %) were below the minimum specification requirement, and their aromatics contents (27.6 and 29.7 vol %, respectively) were above the maximum specification. NAWC Fuels. These were experimental fuels. Some were formulated to be representative of future JP-5 fuels, Le., of broadened specification to include more aromatics. Since in future fuels the middle distillates are likely to be processed
from heavy petroleum crudes, upgraded residual fractions, and shalekoal derived liquid fuels, the NAWC fuels were formulated from furnace oil blending stocks, kerosene, and xylene tower bottoms. The compositional data of these fuels, given in Table 2, were taken from an AeroChem Report.32 Included in Table 2 are smoke point and PMBN data. In the effort to evaluate the predictors for fuel sets t h a t are representative of present day and future fuels, the NAWC fuels were subdivided into a present day and future fuel set; each set comprised seven fuels. The present day fuel set included fuels No. 2-5 and 8-10. The future fuel set included fuels No. 1 , 2 , and 5-9. Common to each set were four present day fuels (uiz., No. 2 , 5 , 8 , and 9); Le., their total aromatics contents were within 26% maximum. The difference between the remaining three fuels in each set was their aromatics content. In the present day fuel set, these three fuels (No. 3, 4, and 10) were within a 26% maximum total aromatics content (uiz., 17.5-26%). The hydrogen content of the present day fuel set ranged from 13.48 to 13.82 wt %. The future fuel set included three fuels (No. 1, 6, and 7) whose aromatics contents exceeded the 25% total aromatics limit by 5-7%. The hydrogen content of the future fuel set ranged from 12.83 t o 13.79 wt %. Saturates Analysis. The saturate fractions of both the NAWC and Shell fuels were first separated using HPLC and subsequently analyzed using gas chromatography/mass spectrometry (GC-MS) a s described below. HPLC separation of the saturates was accomplished using a Whatman M-9 10125 Partisil PAC semipreparative column (chemically bonded (32) Gill, R. J.; Olson, D. B. AeroChem Report TP-451, AeroChem Research Laboratories, Inc., Princeton, NJ, 1985.
Energy & Fuels, Vol. 9, No. 3, 1995 451
Sooting Tendencies of Aviation Fuels alkylamino-alkylcyano) with hexane as the mobile phase at a flow rate of 3.5 m u m i n . The detector was a Waters Prep 500 differential refractometer thermostated at 29.5 "C. GC-MSAnalysis of the Saturate Fractions. GC analysis was performed using a Hewlett-Packard 5890 gas chromatograph and a 50 m cross-linked methylsilicone capillary column, which was held at 120 "C for 4.6 min and then rapidly heated at 70 "C/min to a final temperature of 220 "C for 12-22 min depending on the sample. The carrier gas was helium at a flow rate of 1 m u m i n ; the split was 60:l and the injector temperature 240 "C. Mass spectrometry was performed using a Finnigan ion trap detector. The weight percent of total noncyclo paraffins (NCP), i.e., linear plus branched, and the weight percent of total cyclo paraffins (CP), i.e., monocyclo and dicyclo paraffins, were determined using the algorithms described in ASTM D2425 a s well as certain modifications of these algorithms when i t was deemed necessary. Although ASTM D2425 was developed for determining hydrocarbon types in middle distillates, based on the model mixtures examined, predictability of the D2425 algorithm appears to be dependent on the NCPKP ratio. For NCPICP ratios less than one, the nonmodified algorithm appeared predictive; for NCP/CP ratios greater t h a n 1,modifications of the algorithm appeared necessary.
T56 Combustor (a) Smoke Number vs Inlet Conditions +Fuel 1 *Fuel 2 *Fuel 3 8 F u e l 4 *Fuel 5 .*Fuel 6 eFuel 7 +Fuel 8 +Fuel 9
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(33) Reider, S. B.; Vogel, R. E.; Weaver, W. E. Allison Gas Turbine Operations, General Motors Corp., IN; Final Report, NAPC-PE-88C, prepared for Naval Air Propulsion Center, Trenton, NJ, 1983.
24
Inlet Conditions (g/Kg, MPa2)
(b) Radiation Flux vs Inlet Conditions Crubo
Combustor Predictors Evaluated. Single Parameter Predictors. These include three predictors t h a t are in current use, uiz., smoke point (ASTM D1322), weight percent hydrogen content (ASTM D3701), and volume percent total aromatics content (ASTM D1319), a s well as Shell's PMBN.2 Multiple Parameters Predictors. These include the following: (a) Rosfjord's ~ o r r e l a t i o n HC-1.2( :~~ 100 - DA)-0.4. (b) Chin-Lefebvre's ~ o r r e l a t i o n SP-0.92( :~~ 100 - DA)-0.4. (c) Weight percent hydrogen content and smoke point. This dual parameter predictor is a modification of t h a t proposed which comprised smoke point and the by Gulder et ul.,26927 hydrogen-carbon ratio. (d) Three trial predictors: NCPKP-MADA, HC-MADA, and SP-MADA. (e) Additional predictors. To determine the role of aromatics in predicting the fuel's sooting tendency, additional predictors, as indicated in the Tables 4 and 5, were included. Specifically, the effect of total aromatics us differentiation into monocyclic and dicyclic aromatics as well a s a weighting effect of the aromatics were examined. [DA, the naphthalene content, was determined by ASTM D1840 for the Shell fuels (Table 1)and by HPLC for the NAWC fuels (Table 2); HC is the weight percent hydrogen content; MADA, the weight percent or volume percent monocyclic and dicyclic aromatics; NCP/CP, the ratio of weight percent noncyc1o:cyclo paraffins; and SP, the smoke point in mm.] Combustors. For the Shell fuels, a single Rolls Royce Tyne combustor was used. The Tyne is a tubular combustor fitted with a dual-orifice pressure atomizer. Its operating conditions, which correspond to 83% of full power, include 60:l aidfuel ratio, 1MPa air inlet pressure, 330 "C air inlet temperature, and 1.5 kg s-l air mass flow rate. The combustor data comprised exhaust soot and flame radiation data t h a t were normalized to J e t A-1 of the fuel set. The soot concentration data were average measurements over the combustor exhaust plane; flame radiation was measured using a wide-angle Molltype pyrometer.2 Further details of the combustor and the combustor data measurements are described by Bowden et a1.16 For the NAWC fuels, the combustor was the Allison Model ~ T56-A-14 is a T56. As described by Reider et ~ 1 . :the turboprop engine can-annular combustion system with a 4910 equivalent shaft horsepower rating and is installed in both military and commercial aircraft. For the fuels evaluation, a
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single-can combustor rig was employed. The fuel injectors were the dual-orificed, pressure-atomizing type. The combustor data used in the evaluation were taken from an AeroChem and comprised SAE smoke number and radiation flux (kW m-2) values at four levels of operation: (a) idle, (b) cruise, (c) climb-outf'normal", and (d) sealevel take SAE smoke number (SN) is a measure off (SLT0)l'"ilitary". of the smoke levels in j e t engines and is calculated from the expression: S N = 100 (1 - RJR,),where R, is the absolute reflectance of the sample spot on a n exposed Whatman No. 4 filter paper and R, the absolute reflectance of the clean nonexposed filter paper (Society of Automotive Engineers paper, ARP 1179).34a Radiation flux (the rate of radiative transfer of heat) is a measure of the visible and infrared emission from the combustion zone of the engine.32 The infrared emission was measured using a Leeds and Northrup Thin Film Rayotube, Model 8890 series.33
Results and Discussion
Mode of Results Analysis. The predictabilities of the various predictors, for both Shell and NAWC fuel sets, were evaluated based on R2and the standard error values. For each predictor, regression analysis of combustor predictor us combustor data was performed (34) Lefebvre, A. H. Gas Turbine Combustion;Hemisphere hblishing Corp.: New York, 1983; Chapter 11, (a) p 507; (b) p 474.
452 Energy & Fuels, Vol. 9, No. 3, 1995
Pande and Hardy
Table 3. Evaluation of Current and New Combustion Performance Predictorsa regression analysis no. misranking combustor performance parameterlrange current and new predictorsb of fuels R2 std err frequency normalized exhaust soot range: 0.8-2.6 Rosfjord‘s correln: wt % H, vol % DA 9 0.97 0.10 219 HC-MADA: wt % H, V O ~% MA, DA 0.97 9 0.13 219 Premixed Burner Number (PMBN) 8 0.96 0.08 118 Chin-Lebfevre’s correln: SP, vol % DA 9 0.96 0.12 219 NCPICP-MADA vol % noncyclokyclo paraffins ratio, 0.95 0.16 219 9 mono and dicyclic aromatics SP-MADA: SP, V O ~% MA, DA 9 0.94 0.18 219 SP-HC: SP, wt % H content 9 0.87 0.23 219 smoke point SP 0.87 9 0.22 219 wt % hydrogen content: HC 9 0.87 0.22 419 MADA: vol % mono and dicyclic aromatics 9 0.87 0.24 419 vol % total aromatics 0.73 0.31 9 419 normalized flame radn range: 1-1.58
NCPICP-MADA vol % noncyclo/cyclo paraffins ratio, mono and dicyclic aromatics Premixed Burner Number (PMBN) SP-MADA SP, V O ~% MA, DA ~ MA, DA HC-MADA wt % H, V O % Rosfjord’s correln: wt % H, vol % DA Chin-Lebfevre’s correln: SP, vol % DA SP-HC: SP, wt % H content smoke point: SP wt % hydrogen content: HC MADA vol % mono and dicyclic aromatics vol %total aromatics
8
7 8 8 8
8 8 8 8 8 8
0.94 0.93 0.91 0.90 0.89 0.88 0.88 0.86 0.86 0.83 0.80 0.75
0.06 0.05c 0.04 0.08 0.09 0.07 0.08
018 117 118 118 118
0.08
118 118 118 318
0.10 0.10
218 318
0.08 0.08
a Based on Rolls Royce Tyne Combustor Data for Shell fuel set. Predictors ranked in decreasing order according to R 2values. For correlation based on the regression equation obtained for normalized exhaust soot.
using Lotus 123 software. For the Shell fuels, based on the available combustor data, the misranking frequency of the fuels was also used. For the NAWC fuels, because of the significantly larger test matrix employed, the overall predictability of each predictor was further evaluated on its frequency of predictability. Tyne Combustor Data for the Shell Fuels. Based on regression analysis and misranking frequency, the best predictors include Ro~fjord‘s~~ correlation, ChinL e f e b v r e ’ ~correlation, ~~ Shell’s Premixed Burner Number,2 and our new trial predictors (see Table 3). Furthermore, predictability differences among these predictors appear not to be significant. Their R2 values ranged from 0.94 t o 0.97 for normalized exhaust soot and from 0.88 to 0.94 for normalized flame radiation data. In contrast, R2 values for SP-HC and the current predictors (smoke point, fuel hydrogen content, and volume percent total aromatics) ranged from 0.73 to 0.87 for both normalized exhaust soot and radiation data. Differentiation of the total aromatics into monocyclics and dicyclics appears to improve the aromatics correlation more so with exhaust soot than with flame radiation data. T56 Combustor Data for the NAWC Fuel Set. The smoke numbers and radiation fluxes a t the four operating levels in the T56 combustor are illustrated graphically in Figure 1, a and b, respectively, for most of the fuels examined. [Note: increasing power levels are represented not by the conventional fuellair (F/A) ratio, but by the product of the F/A ratio times the square of the inlet pressure (see Figure 1,a and b). This was necessary for the following reason: according to V0ge1,~~ the T56 is a flat rated engine, and thus, the F/A is relatively constant across all thrust-producing (non-idle) operating conditions.] V0ge1~~ has explained (35)Vogel, R. E. Private communication. Allison Gas Turbine, General Motors Corporation, Indianapolis, IN, 1992.
the high smoke numbers in the T56, at idle, as follows: the main nozzle flow needs to be partially engaged at idle, and this creates the potential for relatively high amounts of smoke at the low power setting. Figure l a also shows the sooting tendencies of the fuels based on smoke number differed from those based on radiation flux (Figure lb), particularly at the highest power level. Results of the regression analyses of combustor predictor us combustion performance (smoke number and radiation flux), for the T56 combustor, at four operating levels are given in Tables 4A and 5A for the present day fuel set and in Tables 4B and 5B for the future fuel set. For identification of the role of aromatics in the prediction of sooting, five additional predictors were examined (see Tables 4 and 5). These include HCtotal aromatics, SP-total aromatics, HC-DA, SP-DA, and M A , where the acronyms are as defined earlier (see Experimental Section). Investigation of HC-DA and SPDA served an additional purpose in that weight percent DA, the proposed weighting factor in Rosfjord’s and Chin-Lefebvre’s correlations, could be compared with the other aromatics contributors examined, for the same fuel set and combustor data. To discern the effects of fuel composition on smoke number and radiation flux for the combustors examined, the correlation results were analyzed with respect to (a) the operative correlation parameter(s1 at the various power levels; (b)the effects of differentiation of the total aromatics into W A and (c) a possible weighting effect of the aromatics either as total or differentiated (MADA). For comparison, the identification and occurrence of these three factors are summarized in Table 6 for the overall matrix examined. T56 SAE Smoke Number. In contrast to the correlation results for exhaust soot in the Tyne combustor, the best correlations with T56 smoke number, a t most operating levels for both the present day (Table 4A) and
Sooting Tendencies of Aviation Fuels
Energy & Fuels, Vol. 9, No. 3, 1995 453
Table 4. For Smoke Number in T66 Combustor: Evaluation of Combustion Predictors/Correlation Parameters: (A) Based on Present Day Fuels and (B) Based on Future Fuels regression analyses at operational levels indicated idle combustor predictor/correlation parameters trials
current
new
R2 stderr A. Based on Present Day Fuels HC-MADA. wt % H, MA, DA 0.62 1.1 HC-total aromatics (wt %) 0.51 1.1 HC-DA w t % H , D A 0.33 1.2 SP-MADA: smoke pt, wt % MA, DA 0.62 1.1 SP-total aromatics (wt %) 0.51 1.1 SP-DA SP,%DA 0.36 1.2 MADA wt%MA,DA 0.62 0.9 wt % total aromatics 0.43 1.0 wt % hydrogen (HC) 0.32 1.1 smoke point (SP) 0.35 1.1 SP-HC: smoke pt, wt % H 0.35 1.2 Rostjord (wt % H, DA) 0.30 1.1 Chin-Lefebvre (SP, w t % DA) 0.35 1.1 PMBN 0.02 1.3
smoke number range for fuel setb trials
current
new
44-47
B. Based on Future Fuels 0.87 1.4 HC-MADA wt % H, MA, DA HC-total aromatics 0.86 1.3 HC-DA: wt % H, DA 0.87 1.3 SP-MADA: smoke pt, wt % MA, DA 0.80 1.8 SP-total aromatics (wt %) 0.80 1.5 SP-DA SP, % DA 0.80 1.5 NCP/CP-MADA wt % NCP/CP, MA, DA 0.88 1.4 MADA. wt % MA, DA 0.58 2.2 wt % total aromatics 0.56 2.1 wt % hydrogen (HC) 0.86 1.1 smoke point (SP) 0.80 1.4 SP-HC: smoke pt, wt % H 0.86 1.3 0.63 1.9 Rostjord (wt % H, DA) Chin-Lefebvre (SP, wt % DA) 0.89 1.0 PMBN 0.53 2.1
smoke number range for fuel setc
44-53
cruise
climb-out
stderr
R2
stderr
R2
stderr
0.70 0.60 0.02 0.53 0.48 0.03 0.39
1.7 1.7 2.2 2.0 2.7 2.2
0.12 0.05 0.03 0.11 0.10 0.10 0.09
1.8 1.6 1.6 1.8 1.6 1.6 1.6
0.60 0.52 0.36 0.59 0.49 0.38 0.56
2.5 2.4 2.7 2.5 2.4 2.7 2.3
0.39 0.01 0.01
1.9 2.5 2.5
0.04 0.01 0.09
1.4 1.5 1.4
0.34 0.01 0.00
2.5 3.0 3.0
0.37 0.01 0.00 0.02
2.2 2.5 2.5 2.5
0.61 0.02 0.08 0.00
1.0 1.4 1.4 1.5
0.06 0.11 0.01 0.50
3.3 2.9 3.0 2.2
2.8
34-40
43-46
37-46
0.83 0.81 0.71 0.83 0.79 0.72 0.89 0.81
2.2 2.1 2.6 2.3 2.2 2.5 1.8 2.1
0.91 0.75 0.85 0.91 0.71 0.87 0.91 0.90
1.4 2.0 1.6 1.4 2.2 1.5 1.4 1.3
0.86 0.85 0.11 0.88 0.87 0.14 0.68 0.68
2.3 2.0 4.9 2.1 1.9 4.8 3.4 2.9
0.75 0.68 0.66
2.1 2.4 2.5
0.65 0.69 0.63
2.1 2.0 2.2
0.64 0.11 0.14
2.8 4.4 4.3
0.69 0.66 0.70 0.45
2.7 2.5 2.4 3.2
0.69 0.84 0.71 0.72
2.3 1.4 2.0 1.9
0.15 0.09 0.11 0.01
4.8 4.4 4.4 4.6
34-45
43-50
a Sea level take off. Set comprised seven fuels with an aromatics content of 26% max; includes fuels No. 2-5 and 8-10. seven fuels, of which, three had an aromatics content within 30-33%; set includes fuels: No. 1, 2, and 5-9.
future fuel sets (Table 4B), were exhibited solely by two of the new trial predictors, viz., HC-MADA, and SPMADA. However, the R2 values of all the predictors examined for T56 smoke number were lower in the present day fuel set than in the future fuel set (cf. Table 4,A and B). Thus, for the present day fuel set (Table 4A), although the correlations of HC-MADA and SPMADA were generally much better than the current and new predictors for three of the four power levels, their R2 values ranged from 0.6 to 0.7. At the third power level, only SP-HC exhibited a moderately high R2 value (0.61, but only a t this level. In contrast, for the future fuel set (Table 4B), HC-MADA and SP-MADA were consistently better than the other predictors at all power levels and their R2 values were 0.8 and greater. Correlation Parameter($ and the Effect of Fuel Compositiofl66 SAE Smoke Number. For the present day fuel set (Table 4A), the operative correlation parameter at the lowest and highest power levels for T56 smoke number appears t o be not total aromatics (R2= 0.3-0.41, but MADA (R2= 0.6). The correlations of the current predictors including hydrogen content were smaller at idle (R2= 0.3-0.4) and smaller yet at the highest power level (R2= 0.0-0.3). At the second power level, the significantly higher R2values of HCMADA, HC-total aromatics, SP-MADA, and SP-total
SLTW
R2
37-50
Set comprised
aromatics us the other predictors may be attributable to a possible aromatics weighting effect (cf. those correlation parameters in Table 4A that do not contain MADA or total aromatics). At the third power level, no compositional correlation parameter appears to be identifiable. At all power levels, the weighting effect of the differentiated aromatics (MADA) us total aromatics improved the correlations of the trial predictors by a small degree (cf. HC-MADA us HC-total aromatics; and SP-MADA us SP-total aromatics). For the future fuel set (Table 4B), in contrast t o the present day fuel set, the operative correlation parameter for T56 smoke number, at idle, appears to be hydrogen content (R2 = 0.8). Smoke point was also similar to hydrogen content (R2 = OB), but the total aromatics correlation was smaller (R2 = 0.6). However, at increasing power levels, the role of the aromatics appears to become increasingly important. Differentiation of the total aromatics into MADA improved the correlations only at the third power level. Furthermore, at the highest power level, significant differences in R2values are identified among the various predictors. At this power level, an aromatics weighting effect that involves the total aromatics appears to be operative. The postulate of a total aromatics weighting effect is
454 Energy & Fuels, Vol. 9, No. 3, 1995
Pande and Hardy
Table 5. For Radiation Flux in T56 Combustor: Evaluation of Combustion PredictordCorrelation Parameters: (A) Based on Present Day Fuels and (B) Based on Future Fuels regression analyses at oDerationa1 levels indicated idle combustor predictor/correlation parameters
cruise
R2 std err A. Based on Present Dav HC-MADA: wt % H, MA, DA 0.82 4.2 HC-total aromatics (wt %) 0.80 3.6 0.21 HC-DA wt % H, DA 7.2 SP-MADA: smoke pt, wt % 0.80 4.4 MA, DA SP-total aromatics (wt %) 0.79 3.7 SP-DA SP, % DA 0.33 6.6 MADA: wt % MA, DA 0.80 3.6 wt %total aromatics 0.79 3.2 wt % hydrogen (HC) 0.18 6.3 smoke point (SP) 0.28 5.9 SP-HC: smoke pt, wt % H 0.36 6.4 0.10 6.6 Rosflord (wt % H, DA) Chin-Lefebvre (SP, wt % 0.22 6.2 DA) PMBN 0.15 6.4
trials
current
new
radiation flux range (kW/m2)for fuel setb trials
97-115
HC-MADA: wt % H, MA, DA HC-total aromatics (wt %) HC-DA wt % H, DA SP-MADA: smoke ut, wt % MA, DA SP-total aromatics (wt 8) SP-DA SP,%DA NCP/CP-MADA: wt % NCPICP, MA, DA MADA: wt % MA, DA wt % total aromatics w t % hydrogen (HC) smoke point (SP) SP-HC: smoke pt, wt % H Rosfjord (wt % H, DA) Chin-Lefebvre (SP, wt 8 DA) PMBN
current
new
radiation flux range (kW/m2)for fuel setC
climb-out
SLTOa
~~~~
R2
R2
std err
R2
std err
6.6 9.2 7.2
0.81 0.69 0.05 0.85
5.8 6.2 10.7 5.3
0.86 0.50 0.52 0.88
3.9 6.0 5.9 3.6
0.72 0.33 0.22 0.08 0.24 0.26 0.26 0.33 0.29
5.9 9.2 9.9 9.3 8.5 8.4 9.7 8.0 8.2
0.69 0.06 0.67 0.62 0.05 0.06 0.06 0.04 0.04
6.2 10.7 6.3 5.8 9.3 9.2 10.7 9.3 9.3
0.63 0.66 0.86 0.42 0.36 0.55 0.75 0.51 0.55
5.2 5.0 3.2 5.6 5.9 4.9 4.3 5.2 4.9
0.44
7.3
0.06
9.2
0.26
6.4
Fuels 0.65 0.65 0.33 0.72
std err 8.1
128-150
B. Based on Future Fuels 0.98 2.3 0.77 0.97 2.5 0.77 0.98 2.0 0.56 0.94 4.2 0.80
137-159
151-170
6.8 5.9 8.1 6.3
0.68 0.68 0.01 0.51
5.2 4.5 7.9 6.5
0.30 0.12 0.20 0.11
8.5 8.3 7.9 9.6
0.88 0.94 0.84
4.9 3.6 6.5
0.79 0.53 0.22
5.7 8.4 12.5
0.48 0.01 0.40
5.7 8.0 7.1
0.02 0.10 0.64
8.7 8.4 6.1
0.71 0.60 0.97 0.88 0.97 0.85 0.94
7.7 8.1 2.2 4.3 2.4 4.9 3.1
0.17 0.08 0.56 0.53 0.56 0.41 0.58
11.2 10.5 7.3 7.5 8.1 8.4 7.2
0.27 0.20 0.01 0.00 0.28
6.8 6.4 7.1 7.1 6.8 7.1 7.1
0.10
8.4
0.02 0.01 0.01 0.43 0.01 0.00
7.8 7.9 7.9 6.7 7.9 7.9
0.76 97-133
6.2
0.73 130-158
5.7
0.08 142-159
0.00 151-170
7.9
0.01 0.01
6.8
a Sea level take off. Set comprised six fuels with an aromatics content of 26% max; includes fuels No. 2-5 and 8-9. seven fuels, of which, three had an aromatics content within 30-33%; set includes fuels No. 1, 2, and 5-9.
Table 6. Comparison of the Operative Correlation Parameters and Aromatics Effect for the T56 Matrix Examined combustor measurement fuel set T56 SN: present day future fuel T56 RF: present day future fuel
operative correln parameters at idle above idle W A HC, SP
W A C MADAPTAd
MADAPTA W A C HC, SP HC," SPC
aromatics effects of differno weightingb
+ + ++c
-
a t 2nd? at 4th at 3rd at 2nd, 3rd?
+ ++
Differentiation of total aromatics into MADA, where: to refers to degree of improvement; and - means no improvement. Apparent weighting effect of total aromatics/MADA at the power levels noted. Only at specific power level(s). Total aromatics.
supported by the following observations for the future fuel set: at the highest power level, R2 for HC-MADA and HC-total aromatics were similarly high; likewise that for SP-MADA and SP-total aromatics; moreover, the correlations of these predictors were significantly higher than those predictors that did not weight the
Set comprised
total aromatics contribution. For example, for hydrogen content or smoke point, R2 = 0.1; and for HC-DA or SPDA which weight only the dicyclic aromatics, their R2 values were unchanged. The significantly higher % monocyclic aromatics us % dicyclic aromatics present in fuel NAWC No. 9 (24.6%MA, 1.5%DA) and in NAWC No. 1(27.5%MA, 5.1%DA; see Table 2) may well have contributed to the poor predictabilities of Rosfjord's and Chin-Lefebvre's correlations, which exclude the monocyclic aromatics in their weighting factor. These results further indicate that the sooting tendencies of the monocyclic aromatics are not adequately factored in by hydrogen content alone. This inadequacy of hydrogen content is also further exemplified in the case of the present day fuels, for which hydrogen content was a poor predictor and the operative correlation parameter appears to be M A . Note, weighting the weight percent hydrogen content with smoke point, a modification of Gulder et d ' s dual parameter predictor26,27offered no improvement over weight percent
Sooting Tendencies of Aviation Fuels
hydrogen content alone for T56 smoke number a t the highest power level for both fuel sets. Thus, based on the correlation results for both present day and future fuel sets for T56 smoke number at most operating levels, HC-MADA and SP-MADA exhibited better overall predictabilities us the trial predictor, NCP/ CP-MADA, the current predictors (which includes hydrogen content) and the published new predictors. In the case of the future fuel set, this improvement is likely attributable to an aromatics weighting effect that includes both the monocyclic and dicyclic aromatics. In the case of the present day fuel set, although this weighting effect does not appear to be operative at the highest power level, it was not detrimental either. T56 Radiation Flux. Unlike the correlation results for flame radiation in the Tyne combustor, the highest correlations with T56 radiation flux, a t most operating levels for both the present day fuel set (Table 5A) and the future fuel set (Table 5B), were again exhibited mainly by HC-MADA and SP-MADA, and to a lesser extent by HC-total aromatics and SP-total aromatics. At most operating levels, the R2 values of HC-MADA and SP-MADA for T56 radiation flux ranged from 0.7 to 20.8 for both fuel sets. However, their significantly lower correlations at the highest power level, for the future fuel set (R2= 0.3 and lower) us the present day fuel set (R2= 0.8) in the same combustor suggest a fuel dependency. Furthermore, the significantly higher correlations exhibited by HC-MADA and SP-MADA for T56 smoke number us T56 radiation flux for the same future fuel set, in the same combustor, at the same highest power level, suggest that other factors in addition to fuel composition contribute to radiation flux. Differences in the sooting tendencies of the fuels based on smoke number us radiation flux (Figure 1) support such a postulate. For T56 radiation fldpresent day fuel set, the correlation results of the current and new predictors generally exhibited low R2 values (most ranged from 0.9). However, at the second power level, except for PMBN (R2= 0.71, their correlations were lower (R2= 0.4-0.6) and significantly lower a t the third level (R2 ranged from 0.01 to 0.2 and includes PMBN). At the highest power level, with the exception of NCP/CP-MADA (R2= 0.6), all the predictors exhibited significantly poor correlations (R2= 0.00.4). Correlation Parameter(s)and the Effect of Fuel CompositionPT56 Radiation Flux. For the present day fuel set (Table 5A), MADA appears to be the operative compositional correlation parameter at most power levels (R2= 0.7-0.8, at the first, third, and fourth power levels); the correlation of the total aromatics though similar to MADA at the first and third power levels (R2= 0.8 and 0.6, respectively), was significantly lower at the fourthhighest level (R2 = 0.4). For the future fuel set (Table 5B), the operative compositional correlation parameter at the first and second power levels appears to be hydrogen content. Comparison between Smoke Number and Radiation Flux. Differences in the operative parameters for T56 smoke number between the present day and
Energy & Fuels, Vol. 9, No. 3, 1995 455
future fuel sets were similar to that for T56 radiation flux at idle, but not at higher power settings (see Table 6). Overall Evaluation Based on the Frequency of Predictability. The above detailed correlation results indicate that the compositional contribution to sooting appears to vary with the combustion performance measurement (smoke number us radiation flux), the combustor type, its level of operation, and their interactive effects with each other. Such specificity is consistent with the poor correlations obtained for T56 smoke number us radiation flux in the same combustor at all power levels except idle. Thus, for a 9-fuel set (includes all the fuels examined except fuel No. 10, for which no radiation flux was available), the R2 values for T56 smoke number us T56 radiation flux at the four operating levels are as follows: 0.75 at idle; 0.06 at cruise; 0.00 at climb-out; and 0.01 at SLTO. In addition to a combustor dependency, there also appears to be a fuel dependency, e g . , differences in correlations between the present day and future fuel sets for the same combustor data, as was discussed earlier. Based on the above collective results, for the combustors examined, an irrefutably reliable predictor is likely an unrealistic expectation. Consequently, from a realistic viewpoint, we subsequently evaluated the predictors on a more meaningful consideration, i.e., their frequency of predictability of smoke number and radiation flux in the T56 combustor at all operating levels for both the present day and future fuel sets. Predictability was based on an arbitrary criterion, uiz., R2 = 0.8 and above. Frequency of Predictability. Results of the regression analyses for the wide range of conditions examined, indicate that based on the %-frequency of overall predictability (see Table 7), two trial predictors uiz., HCMADA and SP-MADA appeared the most promising. Their frequencies of predictability, for the rigorous criterion adopted, were 56% in each case us 19% and lower for the current and new published predictors. In general, the predictors appear to rank in the following decreasing order of predictability: predictors that weighted the differentiated aromatics > those that weighted the total aromatics > those that weighted only the dicyclic aromatics > or, similar to, single compositional parameter predictors > PMBN. The higher frequency of predictability of MADA (25%)us % total aromatics (13%) supports an improvement in predictability on differentiation of the total aromatics into two major types. Aromatics Weighting Effect. The generally higher correlations and frequency of predictability, of HCMADA and SP-MADA, are likely attributable to these predictors weighting the various compositional contributions t o sooting better than for example the current and published new predictors at increasing power levels. For the same future fuel sethest matrix, the lower frequency of predictability of NCPKP-MADA relative to HC-MADA and SP-MADA (50%us 75%)suggests that factoring in the different sooting tendency of the cyclo us the noncyclo paraffins was not as advantageous as the aromatics weighting factor. These results are consistent with Graham et aZ.'s modeP that soot forma(36)Graham, S.C.;Homer, J. B.; Rosenfeld, J. L.J. 20th Int. Shock Tube Symp., Proc., Kyoto, Jpn. 1975,621-631.
Pande and Hardy
456 Energy & Fuels, Vol. 9, No. 3, 1995
Table 7. Comparison of Overall Predictabilities of PredictorslCorrelation Parameters for the TS6 CombustoP frequency of predictability: for four power levels in each category RF for fuel setb frequency SN for fuel setb combustor predictor/correlation parameters present future present future total % HC-MADA: wt % H, MA, DA SP-MADA smoke pt, wt % MA, DA NCP/CP-MADA: wt % NCP/CP, MA, DA HC-total aromatics (wt %) SP-total aromatics (wt %) MADA: wt % MA, DA SP-HC: smoke pt, wt % H HC-DA w t % H, DA SP-DA: smoke pt, wt % DA Rosoord (wt % H, DA) Chin-Lefebvre (SP,wt % DA) smoke pt (SP) HC: wt % hydrogen wt % total aromatics PMBN
0 0
NA 0 0 0 0 0 0 0 0 0 0 0 0
4 4 3 4 3 2 1 2 2
3 3 NA
1 1 1 1 1
0 0 0 0 1 0
0
1 1
2 1 0 0
2 2 1 2 2 0 1 1 1 1 1 1 1
0 1
9/16 9/16 418 7116 6116 4/16 3/16 3116 3/16 2/16 2/16 2/16 2116 2/16 1/16
56 56 5OC 44 38 25 19 19 19 13 13 13 13 13 6
a Based on frequency of R2values = 0.8 and above (ranked from best to worst). Present day fuel set includes: NAWC fuels No. 1 , 2 , and 5-9. N A not available. Future fuel set includes NAWC fuels No. 2-5 and 8-10. For future fuel set only.
tion via condensation of the aromatics is kinetically faster than the polymerization route of the aliphatics. Furthermore, based on the overall results, it appears that some of the variability in hydrogen content as a predictor that has been reported in the literature (see Introduction) is likely related to its innate inadequacy at increasing emission levels. The variable predictability of smoke point may likewise be similarly related to the variable predictability of hydrogen content. Thus, the insensitivity of smoke point at increasing power levels may well be related to a similar compositional effect reported by Kent:37For heavily sooting fuels (this factor becomes increasingly significant with increasing power levels), soot concentration was relatively higher in turbulent diffusion flames (typical ofjet engine combustor^)^ than in laminar diffusion flames, on which the smoke point test is based. However, for moderately sooting fuels, smoke point tests on laminar flames were predictive of turbulent flame characteristic soot volume fraction. Nonetheless, for smoke number and, to a lesser extent, radiation flux the need for a % total aromatics weighting factor, particularly at increasing emission levels, may also further serve to account for the reported varying predictabilities of hydrogen content and smoke point. It is important to note that even when an aromatics weighting effect was not identifiable, the correlation results of predictors such as HC-MADA and SP-MADA remained high. However, a larger data base of present day and future fuel sets is required for verification of the aromatics weighting factor and for establishing regression equations that can be used for predictive purposes. Such a data base should comprise both a greater number of fuels and a greater range in the composition of the compound classes including the monocyclic and dicyclic aromatics. For these reasons, the regression equations for the two top rated predictors have not been given. More importantly, this evaluation focuses on the need to adequately weight both the monocyclic and dicyclic aromatics contributions to sooting. Combustor Effect. The striking differences in predictabilities of the various predictors for the T56 us the (37)Kent, J. H. Combust. Flame 1987,67, 222.
Tyne combustor are likely reflective of the differences in combustor design. Although the T56 does not represent current operation technology, it was particularly useful for examining the sooting tendency of fuels. This aspect is important, even for the more advanced combustors (see below), since future fuels are apt to contain more aromatics. Advances in combustor design have diminished the compositional contribution to sooting. For example, for the modified T56, a lean combustion zone was achieved by redistributing the dilution air;38 likewise for the F10139and 579 combustors.11J8 Advances in combustor desigh include the development of an annular combustor design in which large amounts of the combustor air flow are introduced through swirl cups containing axial flow swirlers which surround each of the fuel nozzles.40 This results in the fuel-air mixture in the primary zone being fuel lean and relatively uniform. Bahr40 has reported that this design approach reduces the smoke emission without significant losses in ground ignition or altitude re-light performance. Airblast fuel injection coupled with leaner combustion in the primary zone have also been reported for decreasing the compositional contribution to s o ~ t i n g . ~ J ~ , ~ ~ The above-mentioned improvements are consistent with the factors that govern soot formation, uiz., the physical processes of atomization and fuel-air mixing, more so, than the kinetic^.^^^,^^ Friswell12 has also found that, in a laboratory-scale gas turbine-type combustor, sensitivity of smoke formation and flame radiation to fuel composition is strongly dependent on combustor pressure and to a lesser extent on flame tube design. Also, sensitivity to fuel composition was greatly diminished at combustor pressures above 1.0 MPa. In the case of flame radiation, Bahrl8s4O has found ad(38) Blazowski, W. s.;Jackson, T. A. Air Force Aero Propulsion Laboratory Technical Report; AFAPL-TR-77-93,Wright-Patterson Air Force Base, OH, 1978. (39) Gleason, C. C.; Oller, T. L.; Shayeson, M. W.; Bahr, D. W. Air Force Aero Propulsion Laboratory; AFAPL-TR-79-2018,Wright Patterson Air Force Base, OH, 1979. (40) Bahr, D. W. In Advisory Group for Aerospace Research and Development (AGARD) Conference Proceedings; Aircraft Pollution by Aircraft Engines; 1973; No. 125, Paper 29. (41) Lefebvre, A. H. Air Force Wright Aeronautical Laboratories, OH, AFWAL-TR-84-2104,Wright Patterson Air Force Base, OH, 1985.
Sooting Tendencies of Aviation Fuels
vanced cooling and structural design features of the combustor liner in the J79-17C decreased flame radiation levels. Note, to overcome the conflicting demands of low soot formation, which requires a low fuellair ratio, and low NO, formation, which requires a high fuellair ratio, . ~ the ~ development of a twoBlazowski et ~ 1 suggested stage combustion system for decreasing the fuel bound nitrogen contribution to NO, formation: the first stage is operated as a fuel-rich stirred reactor, i.e., a t equivalence ratios just below that for “hydrocarbon breakthrough”, and the second stage at fuel-lean conditions. Recommendation. Although the more recent combustors are less sensitive t o the sooting tendency of fuels, based on the aromatics weighting effect observed, the threshold limit of the fuel’s aromatics content needs t o be established for these combustors. A case in point is the result reported for the F100, which is a more advanced engine than the T56 and is currently in fleet operation: Russell15 found for fuels with a hydrogen content range of 11.5-14.5 wt %, as the hydrogen content decreased (which relates to increasing aromatics content), the smoke number and flame radiation were found to increase substantially. Lyon and Anderson43 have proposed an upper limit of 35%total aromatics t o minimize degradation of the elastomers. It is important to add that any revision of the limits for future fuels in modern combustors should independently specify the maximum percent monocyclic and dicyclic aromatics.
Conclusions Of the current and new predictors evaluated, two new trial predictors (HC-MADA and SP-MADA) appear to be the best and most promising in the broad matrix examined, which included representative current and future fuels. HC-MADA is based on weight percent hydrogen and % monocyclic and dicyclic aromatics, and (42) Blazowski, W. S.;Sarofim, A. F.; Keck, J. C. J. Eng. Power, 1981, 103,43. (43) Lyon, T. F.;Anderson, B. A. Aero Propulsion Laboratory, Air Force Wright Aeronautical Laboratories, AFWAL-TR-86-2025, WrightPatterson Air Force Base, OH,1986.
Energy & Fuels, Vol. 9, No. 3, 1995 457
SP-MADA, a combination predictor, is based on smoke point and % monocyclic and dicyclic aromatics. The improved predictabilities of HC-MADA and SPMADA are likely related to their weighting the aromatics contribution to sooting better than the other predictors. Moreover, for fuels where a weighting effect was not operative, the predictabilities of HC-MADA and SPMADA were not adversely affected. In addition, factoring in the different sooting tendency of the cyclo (CP) us the noncyclo (NCP) paraffins as in the trial predictor, NCP/CP-MADA, appears not to be as advantageous as the aromatics weighting factor. Furthermore, the simple parameters on which HCMADA and SP-MADA are based make them attractive improvements over the existing predictors especially since weight percent hydrogen and smoke point are required specification measurements. Regarding the recently developed Shell Premixed Burner Number, although it correlated well in the case of the Tyne combustor data for the Shell fuels, it did not appear promising in the case of the T56 combustor data for the NAWC fuels. The above results also focus on (a) the limitations of a single parameter predictor such as smoke point alone or hydrogen content alone and (b) the lack of an adequate aromatics weighting factor being a likely cause for their apparent variabilities in predictions leading to unreliability. For the same fuel sets, the lower predictabilities of correlation parameters such as HCDA and SP-DA, which are based on two parameters, support the significance of a weighting effect based on both the monocyclic and dicyclic aromatics. An important consequence of this study impacts on the sooting tendency of future fuels in more modern combustors. And, although modedadvanced combustors can better tolerate the sooting tendency of fuels, the aromatics specification limits for future fuels needs t o be established. Moreover, this specification should also include the maximum limits for both monocyclic and dicyclic aromatics. EF940209W
