Quest for a Reliable Method for Determining Aviation Fuel Thermal

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Quest for a Reliable Method for Determining Aviation Fuel Thermal Stability: Comparison of Turbulent and Laminar Flow Test Devices S. G. Pande,*,† D. R. Hardy,‡ R. A. Kamin,§ C. J. Nowack,† J. E. Colbert,§ R. E. Morris,‡ and L. Salvucci§ Geo-Centers, Inc., 4640 Forbes Boulevard, Lanham, Maryland, Naval Research Laboratory, Code 6121, Washington, D.C., and Naval Air Systems Command, Air-4.4.5, Patuxent River, Maryland Received August 17, 2000. Revised Manuscript Received November 8, 2000

Thermal deposition, based on carbon burn off, was evaluated in two turbulent flow and one laminar flow test devices. The two turbulent flow test devices were the Navy Aviation Fuel Thermal Stability Simulator (NAFTSS), developed by Rolls Royce, UK, for the U.S. Navy, and the High Reynolds Number Thermal Stability (HiReTS) bench rig, developed by Shell Global Solutions, UK (formerly Shell Research and Technology Centre). The laminar flow test device was the Tubular Reactor (TR), a bench rig developed by the Naval Air Systems Command Air4.4.5 (NAVAIR). Three jet fuels were used in the comparison of the NAFTSS and the TR, and six fuels (five jet and one diesel) in the comparison of the HiReTS and the TR. Good correlations were obtained between the laminar flow and the two turbulent flow test devices. A likely explanation of these results is that for the fuels examined, Reynolds number does not appear to be a critical factor in predicting thermal stability. These results are significant for they support the use of laminar flow in devices such as the specification test method, i.e., the JFTOT, ASTM D3241. Furthermore, on the basis of an overall analysis of the results, we postulate that reactant depletion is likely attributable to a difference in fuel temperature (arising from the difference in flow rate (residence time)/test temperature) and not to a difference in flow velocity (laminar vs turbulent flow). We further postulate that fuel temperature is likely the critical and predominant factor in predicting jet fuel thermal stability, as long as attention is paid to the residence time and thermal gradients of the hot section.

Introduction Thermal stability is considered to be one of the most critical fuel properties. Consequently, the method used for determining thermal stability should be both reliable and as realistic as necessary to simulate operations. The Jet Fuel Thermal Oxidation Tester (JFTOT, ASTM D3241) is the current specification test method for determining the thermal stability of aviation fuels, which include military jet fuels, viz., JP-5, JP-4 (MILDTL- 5624), and JP-8 (MIL-T-83133D), as well as commercial jet fuels, viz., Jet A and Jet A-1 (ASTM D1655). Nonetheless, criticisms of the JFTOT include the following: (a) the visual tube rating method for evaluating fuels (pass/fail) is well recognized as being nonquantitative and subjective; (b) the flow velocity is laminar and hence not realistic of the turbulent flow regime in aircraft propulsion systems; and (c) the heater tube metallurgy, which is 6061 aluminum, appears to inhibit thermal deposition due to the migration of magnesium (present as an impurity in 6061 aluminum) from the bulk metal to the surface.1,2 †

Geo-Centers, Inc. Naval Research Laboratory. Naval Air Systems Command. (1) Hazell, L. B.; Baker, C.; David, P.; Fackerell, A. D. Surface and Interface Analysis 1986, 9, 507. ‡ §

Of the above-mentioned criticisms, the most easily resolved is the heater tube’s metallurgy, i.e., the use of stainless steel heater tubes, which are commercially available for use in the hot liquid process simulator (HLPS). Responses to the other two criticisms are addressed below. The focus of this paper, however, is on the impact of flow velocity on thermal stability determination since this topic is of ongoing investigation. The need to quantify the heater tube deposits led to the development of several methods, viz., carbon burn off, dielectric breakdown, and optical methods, all of which are related to gravimetrysthe primary method of analysis. Nonetheless, these methods all have limitations as subsequently described. Carbon Burn Off (CBO). CBO3,4 is based on the assumption that the total amount of thermal deposit can be represented by its carbon content. This method is not recommended for aluminum heater tubes for two (2) Clark, R. H.; Thomas, L. An Investigation of the Physical and Chemical Factors Affecting the Performance of Fuels in the JFTOT. SAE Technical Paper Series 881533; Society of Automotive Engineers: Warrendale, PA, 1988. (3) Watt, J. J.; Evans, A., Jr.; Hibbard, R. R. Fouling Characteristics of ASTM Jet A Fuel When Heated to 700 °F in a Simulated Heat Exchanger Tube. Report No. NASA TN D-4958, December 1968. (4) Taylor, W. F. Ind. Eng. Chem. Prod Res Dev. 1974, 13, 133.

10.1021/ef000187f CCC: $20.00 © 2001 American Chemical Society Published on Web 12/29/2000

Aviation Fuel Thermal Stability

reasons: (1) the low melting point of aluminum precludes its use in a standard carbon, hydrogen, nitrogen analyzer, since the catalysts require >1000 °C, whereas 6061 aluminum melts at ∼550 °C; and (2) although aluminum heater tubes can be used in a LECO surface carbon analyzer or a total alloy carbon analyzer, the results are questionable because of high blanks. However, in the case of stainless steel tubes, CBO is generally an acceptable method of quantifying thermal deposits. Dielectric Breakdown.5 This technique in not useful for very thin deposits. Instead, measurements are limited to deposits which are thick enough to provide sufficient electrical insulation. The assumption is that all fuels under all test conditions will produce thermal deposits with the same dielectric constant. Optical Methods Including Ellipsometry6,7 and Interferometry.8-10 Ellipsometry is based on the change in the polarization of light passing through a thin reflective surface such as thermal deposits on JFTOT tube surfaces. Thus, it is not applicable in cases of very thick deposits. Also, to reduce the cost of ellipsometric equipment, the technology currently used to examine JFTOT tubes is based on single-beam optics, and hence the assumption that the refractive index of all deposits is identical. Interferometry suffers from similar limitations as ellipsometry, i.e., the inability to measure the volume of very thick deposits. Furthermore, unlike ellipsometry, no commercial equipment is currently available. The third and ongoing criticism of the JFTOT, as mentioned earlier, stems from the fuel flow velocity being laminar, whereas in jet aircraft systems, the fuel flow regime is turbulent. This criticism of the JFTOT led to the development of several large scale, and one bench scale, turbulent flow devices (see below). Clark and Thomas2 have suggested that reactant depletion, possibly dissolved oxygen, occurs in a laminar flow regime. However, it is interesting to note that Kendall and Mills11 have reported “a correlation between fuel performance in the JFTOT and the Single Tube Heat Transfer Rig (STHTR)”, a turbulent flow device. Furthermore, to account for the poor correlation in thermal deposition between two turbulent flow devices, the STHTR and the Mini Injector Feed Arm Rig (MIFAR), Clark and Stevenson12 suggest that a temperature effect (5) Stavinoha, L. L.; McInnis, L. A. Proceedings of the 5th International Conference on Stability and Handling of Liquid Fuels, Rotterdam, The Netherlands, October 1994 1995, 889-903. (6) Baker, C.; David, P.; Taylor, S. E.; Woodward, A. J. Proceedings of the 5th International Conference on Stability and Handling of Liquid Fuels, Rotterdam, The Netherlands, October 1994 1995, 433-447. (7) David, P.; Mogford, R.; Paduschek, P.; Taylor, S. E.; Woodward, A. J. Proceedings of the 6th International Conference on Stability and Handling of Liquid Fuels, Vancouver, B. C., Canada, October, 1997 1998, 947-955. (8) Darrah, S. D. Performance and Safety Characteristics of Improved and Alternate Fuels. Report No. GC-TR-86-1601; Geo-Centers, Inc.: Newton, MA, December 1986. (9) Morris, R. E.; Hazlett, R. N.; McIlvaine, C. L., III. Proceedings of the 3rd International Conference on Stability and Handling of Liquid Fuels, London, UK, September 1988, 226-239. (10) Morris, R. E.; Hazlett, R. N. Energy Fuels 1989, 3, 262. (11) Kendall, D. R.; Mills, J. S. The Influence of JFTOT Operating Parameters on the Assessment of Fuel Thermal Stability. SAE Technical Paper Series 851871; Society of Automotive Engineers: Warrendale, PA, 1985; p 148 (12) Clark, R. H.; Stevenson, P. A. The Thermal Degradation of Aviation Fuels in Jet Engine Injector Feed Arms: Results from a HalfScale Rig. Prepr.-Am. Chem. Soc., Div. Fuel Chem. 1990, 35, 1308.

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may be involved, due to the multistage heating system in the STHTR versus the MIFAR. In our quest for a reliable method for determining aviation fuel thermal stability, we revisited the flow velocity issue. The objective of the investigation was to determine whether a turbulent flow device was indeed essential to adequately predict aviation fuel thermal stability. Consequently, the approach adopted was to compare thermal deposition in a turbulent versus a laminar flow device. Specifically, three test devices that were developed fairly recently were used, viz., the Tubular Reactor (TR),13,14 a laminar flow bench device developed by the Naval Air Systems Command Air-4.4.5 (NAVAIR), and two different turbulent flow devices, viz., the Naval Aviation Fuel Thermal Stability Simulator (NAFTSS),15 a large scale rig developed by Rolls Royce16 for the U.S. Navy (NAVAIR), and the High Reynolds Number Thermal Stability (HiReTS) rig,17 a bench scale device developed by Shell Global Solutions, UK (formerly Shell Research and Technology Centre). The thermal deposition data for the three devices were generated by NAVAIR. For each test device, quantification of thermal deposition was based on carbon burn off of the deposits on the stainless steel tubes and/or the filters used. Experimental Section Test Devices. Tubular Reactor (TR). Like the JFTOT, the TR is a laminar flow test device. An added feature of the TR is that a section of its heated tube functions as a preheater to simulate the heat a fuel receives as a heat sink from the hot lubricating oil, hydraulic fluid, and the avionics system. Briefly, the TR is an eight-inch long x 0.103” ID (20.32 cm × 0.26 cm), 304 stainless steel tube, which comprises both the preheater and the main heater sections. The effective heated lengths of the preheater and the main heater are each, two inches (5.08 cm). The heater block for the preheater section raises the temperature of the fuel such that when it enters the main heater section, the fuel temperature is 121 °C. The TR is generally operated at a fuel flow rate of 10 mL/ min for 12 h, 500 psi backpressure, and a constant bulk fuel outlet temperature of 204 °C. At these operating conditions, the main heater block temperature can vary from ∼330°-400 °C. Higher fuel outlet temperatures, viz., 218 °C and 232 °C, were also explored. However, plots of thermal deposition versus fuel outlet temperature, indicate that the 204 °C fuel outlet temperature appears to be within the optimum temperature range for maximum thermal deposition in this system (see Figure 1A for the main heater + filter deposits, and Figure 1B for the total deposits). The plots shown in Figures1A,B are based on a somewhat larger fuel data set (i.e., three additional fuels) than that examined in this paper. (13) Salvucci, L.; Colbert, J. Navy Thermal Stability Testing. Presented at the Coordinating Research Council (CRC) Meeting, Alexandria, VA, April 1995. (14) Salvucci, L.; Colbert, J. Navy Aviation Fuel Thermal Stability Program. Presented at the Coordinating Research Council (CRC) Meeting, Alexandria, VA, April 1996 (15) Colbert, J. E.; Nowack, C. J.; Proceedings of the 6th International Conference on Stability and Handling of Liquid Fuels, Vancouver, B. C., Canada, October, 1997 1998, 231-239. (16) Aviation Fuel Thermal Stability Test Unit. Rolls Royce Final Report for the Naval Air Systems Command Air-4.4.5. Report Number NAVAIRWARCENACDIVTRENTON-PE-262C under Contract N00140-90-C-07-81; August 1993. (17) Bauldreay, J. M.; Heins, R. J.; Houlbrook, G.; Smith, J. High Reynolds Number Thermal Stability (HiReTS) Rig for Realistic Rapid Evaluation of Distillate Fuel Thermal Oxidative Stability. Proceedings of the 6th International Conference on Stability and Handling of Liquid Fuels, Vancouver, B. C., Canada, October, 1997 1998, 295-314.

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Figure 1. (A) Thermal deposition based either on the main heater plus filter, or on (B) the total thermal deposits, appears to be a maximum at ∼204-218 °C, for most of the fuels examined. The decrease in thermal deposition at increased fuel outlet temperature in some fuels (see Figure1A,B) may well be due to reactant depletion of dissolved oxygen, in which case, these results indicate that reactant depletion is fuel and temperature dependent. Furthermore, these results focus on the importance of a judicious choice of operating temperature for determining fuel thermal stability. The total residence time in the TR for a flow rate of 10 mL/ min is 3.2 s, and includes both the preheater and main heater sections. The Reynolds number at these flow conditions is 200. The precision of the thermal deposit data is approximately (12%. Naval Aviation Fuel Thermal Stability Simulator (NAFTSS)-Turbulent Flow Test Rig. The NAFTSS, originally called the Aviation Fuel Thermal Stability (AFTS) rig18 by its developers, Rolls Royce,16 UK, was designed to evaluate fuel thermal deposition typically found in various aircraft engine components. These include the following three modules: (a) simulating the low-temperature regime (air frame) is a low-

pressure filter module which is operated at 80 psi (552 kPa) and 38 °C; (b) simulating the intermediate temperature regime (i.e., after the engine cooler and before the fuel nozzles) is a high-pressure filter module, which is operated at 700 psi (4828 kPa) and 149 °C; and (c) simulating hot parts of the fuel system, such as the fuel injector feed arms, or the fuel/oil heat exchanger passages is the heated nozzle module.18 The dimensions of the heated nozzle module are 4 in. long × 0.03 in. i.d. (10.16 cm × 0.076 cm). The wetted wall temperature of the nozzle test section measured in the center of the axial length is 288 °C. After an initial 30 min warmup period, the heat supplied to the nozzle module is constant, although the wall temperature is allowed to rise with test duration. The fuel flow rate is 2.5 gal/h (∼158 (18) Daggert, D.; Veniger, A.; Lewis, C.; Bullock, S.; Kamin, R. The Development of an Aviation Fuel Thermal Stability Test Unit. American Society of Mechanical Engineers, Paper 94-GT-217. Presented at the International Gas Turbine and Aeroengine Congress and Exposition, The Hague, Netherlands, June 13-16, 1994.



Table 1. Comparison of Main Heater Plus Filter Deposits versus Total Thermal Deposits Formed in the Tubular Reactor at Constant Fuel Outlet Temperature: 204 °Ca fuel ID

type

main heater (MH)

filter (F)

main heater + filter

total (MH + F+)b

JAX JPTS Tank 21/23 Tank 20/22 POSF 2827 JAX Tank 6 Red Hill Tank 4 Tank 21/23+ ∼250 ppb Cu NAPC 29

JP-8 JP-5 JP-5 JP-5 Jet A, st.run JP-5 JP-5 JP-5 diesel: 50/50 HHO/DFM

0.06 0.09 0.15 0.26 0.05 0.67 0.67 0.76 0.40

0.17 0.22 0.13 0.18 0.34 0.36 0.51 0.54 2.01

0.23 0.31 0.28 0.44 0.39 1.03 1.18 1.30 2.41

0.32 0.33 0.41 0.47 0.57 1.39 1.39 1.33 2.83

a Flow rate, 10 mL/min for 12 h. Average main heater block temperature for the nine fuels was 357 °C. b Total thermal deposits in the TR includes in addition to the main heater + filter, the deposits formed in the other heated sections, viz., the preheater, thermocouple Tee, thermocouple fitting, and the tube jumper.

mL/min) for varying test durations ranging from ∼10-80 h, the residence time is 0.017 s, and the Reynolds number is approximately 13 000. Note these operating conditions of the NAFTSS refer to those used by NAVAIR. A detailed description of the AFTS is given by Daggett et al.18 High Reynolds Number Thermal Stability (HiReTS)Turbulent Flow Test Rig. The HiReTS was developed by Shell Global Solutions, UK (formerly Shell Research and Technology Centre), as a bench test method “to evaluate jet fuel thermal stability under realistic test conditions.”17 Specifically, in response to the criticisms of the JFTOT (see Introduction), it was developed to: (a) simulate a realistic turbulent flow regime, as opposed to the laminar flow in the JFTOT; (b) quantify the thermal deposits formed; and (c) correct for the tube metallurgy by using capillary tubes made of stainless steel instead of aluminum. A detailed description of the HiReTS, its operation, and the studies conducted have been published by Bauldreay et al.17 Briefly, the test section is a stainless steel capillary HPLC tubing (15.24 cm (6 in.) long × 0.025 cm i.d.) that is heated electrically using bus bars. To improve thermal emissivity for IR measurements, the capillary tubes are coated with a high temperature flat black paint. The operating conditions are 35 mL/min flow rate, 2.0 MPa (290 psi) pressure to prevent the fuel from boiling, and a constant fuel outlet temperature that is maintained at 290 °C during the 2-h test duration. To maintain the 290 °C fuel outlet temperature, additional heat is applied during the test. This added heat is the basis for the HiReTS number, the index used to measure thermal stability. Specifically, the HiReTS number is defined as the sum of the temperature increases across a 2.1 cm portion of the heated tube at the fuel outlet. The fuel’s residence time is 0.0128 s, and the Reynolds number at these flow conditions is approximately 15 000. Test Fuels. The total number of fuels examined was eight. These comprised five JP-5s of which, one was examined with and without added copper, one Jet A straight run, one JP-8, and one diesel fuel, which was a blend of 50/50 Home Heating Oil/Diesel Fuel Marine (HHO/DFM). The compositional and performance properties of these fuels were in accordance with their military, and/or ASTM specifications, i.e., MIL-DTL-5624 for the JP-5s, MIL-T-83133 for the JP-8, ASTM D 1655 for the Jet A, ASTM D 396 for the HHO, and MIL 16884 for the DFM. For convenience, the IDs of the fuels are assigned based on either the provider or the location. Because of limited availability not all fuels were screened in the comparison of the test devices. The fuels employed in the comparisons of the NAFTSS vs the TR, and the HiReTS vs the TR are based on the following subsets of the above-mentioned eight fuels: Comparison of the NAFTSS vs the Tubular Reactor (Turbulent vs Laminar Flow Test Device). The fuel set comprised three fuels, viz., a JP-5 with and without copper, and a JAX JP-8.

Comparison of the HiReTS vs the Tubular Reactor (Turbulent vs Laminar Flow Test Device). The fuel set comprised six fuels, viz., four JP-5s, one Jet A straight run, and one diesel fuel (50/50 HHO/DFM). Postrun Carbon Burn Off. In all cases the test sections were stainless steel, and thermal deposition was based on the postrun carbon burn off of the heated sections using a LECO carbon analyzer, which gives the weight of carbon. The CBO data discussed below were corrected for background carbon on unused test sections (“blanks”).

Results and Discussion Thermal Deposition Data. Before evaluating the thermal deposition data in the various test devices, to determine whether all, or only specific fouled sections should be considered, we examined test data from a somewhat larger number of fuels than that employed in the current evaluation. The larger data sets included all, if not, most of the fuels evaluated in this study. The results follow. Tubular Reactor (TR). Thermal deposits were measured in the following locations, listed in the order of the fuel flow sequence: the preheater, main heater, thermocouple Tee, thermocouple fitting, tube jumper, and the filter. However, as shown in Table 1, most of the thermal deposits are formed on the main heater and filter such that their sum (main heater + filter deposits) is similar to that of the total deposits, i.e., the sum of the deposits on all the heated sections listed previously. For this reason, the sum of the main heater and filter deposits was used to evaluate thermal deposition in the Tubular Reactor. NAFTSS. The specific test sections measured include: the nozzle, hot filter, and hot filter bypass deposits, subsequently referred to, collectively, as the hot deposits. Likewise, the cold filter and cold filter bypass deposits are subsequently referred to, collectively, as the cold deposits. However, as shown in Table 2, for the same initial wetted wall temperature, the concentrations (in mg/L) of the hot thermal deposits were similar to that of the hot plus cold thermal deposits. Consequently, only the hot deposits were used to evaluate the NAFTSS. HiReTS. Thermal deposition was measured based on the HiReTS number, as well as on postrun carbon burn off. In contrast to the HiReTS number, which is a measure of the temperature increases only along the hottest section of the capillary tube (2.1 cm at the fuel outlet section), the postrun carbon burn off is a measure

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Figure 2. Poor correlations when total tube deposits were plotted vs wall temperature rise in the NAFTSS. Table 2. Comparison of the Hot versus Hot + Cold Thermal Deposits Formed in the NAFTSS for Fuel 21/23a initial wetted wall temperature, °C

test duration (h)

204 232 260 260 288 316 316 316 343

31 50.3 24 26.5 28.5 7.8 15.5 15.5 11

a

thermal deposits, mg/L hot hot + cold 0.018 0.008 0.038 0.026 0.034 0.084 0.069 0.080 0.197

0.018 0.011 0.041 0.028 0.036 0.092 0.073 0.084 0.206

Fuel flow rate: 9.45 L/h (∼158 mL/min).

of the tube surface deposits along the entire length of the heated tube as four cut sections of the capillary tube. The thermal deposits on the end ferrules in the CBO data were not included, because in a previous investigation,19 their contribution was found to be insignificant. The six fuels used in this study included five of the 12 fuels examined in the earlier investigation.19 Method of Evaluation. The quest to determine the effect of turbulent versus laminar flow on thermal deposition, is complicated by the variable not being confined to flow velocity, and hence to mass transfer, i.e., of the reactants (fuel reactive species and oxygen), and of the products at the fuel/wall interface. Another variable involves the residence time of the fuel, which then generates a third and critical variable, viz., the fuel temperature. Consequently, in our comparison of the two turbulent flow rigs with a laminar flow test device, it is clear that a direct comparison of flow velocity alone is not possible. Instead the approaches taken are as described below for the specified test devices. Unless otherwise stated, thermal deposition was quantified by carbon burn off (CBO) as milligrams of deposits per liter of fuel (mg/L) that passed through the test device. (19) Pande, S. G.; Hardy, D. R. NRL Letter Report 6120/184; Naval Research Laboratory: Washington, DC, September 1999.

Comparison of the NAFTSS and the Tubular Reactor (TR). The effect of turbulent versus laminar flow on thermal deposition was examined by comparing the test fuels’ performance in the NAFTSS versus the TR. The NAFTSS operates at a constant heat input. However, even though additional heat is not applied during the test, the tube inner wall temperature rises significantly, resulting in an increasing fuel temperature gradient along the fuel flow path during the test. This wall temperature rise has been attributed to an insulating effect18,20 on heat transfer to the fuel. Thermal deposition as a function of temperature was compared in the NAFTSS versus the TR for the three test fuels, viz., a JP-5 fuel, with and without copper (fuel from Tank 21/23), and a JP-8, fuel (JAX JP-8). Total Surface Deposit versus Wall Temperature Rise. Total tube (surface) deposit in the AFTS has been reported16 to correlate directly with wall temperature rise. Consequently, to determine whether wall temperature rise should be used to correlate thermal deposition in the NAFTSS, the appropriate plots were constructed for the three test fuels (Figure 2). Wall temperature rise was based on the rise in the middle inner wall temperature, since this was reported16 to represent generally, “both the relative level and the trends during a run of all inner and outer wall temperatures.” Figure 2 shows poor correlations between the total surface (tube) deposits and the middle inner wall temperature rise for all three test fuels (R2 values ranged from 0.12 to 0.33). Furthermore, since the NAFTSS is operated at constant heat input, the results are not biased by added heat, which can promote further thermal deposition. These NAFTSS results are therefore particularly significant because they question the postulate of assessing/quantifying thermal deposition based (20) Kendall, D. R.; Houlbrook, G.; Clark, R. H.; Bullock, S. P.; Lewis, C. The Thermal Degradation of Aviation Fuels in Jet Engine Injector Feed-Arms. Part I - Results from a Large Scale Rig. Paper 87-Tokyo-IGTC-49. Presented at the Tokyo International Gas Turbine Congress, Tokyo, Japan, October 26-31, 1987.



Figure 3. Significantly improved correlations when total hot deposits were plotted vs initial wall temperature for Tank 21/23 fuel, with and without copper.

on wall temperature rise. Nonetheless, on the basis of the premise that the surface thermal deposits indeed act as a thermal insulator,16,18,20 possible explanations of the poor correlations include the following: (a) the surface deposits are likely nonuniformly distributed and (b) the relationship between wall temperature rise and surface deposit is likely nonlinear. Kendall et al.20 have reported that the relationship between wall temperature rise and the weight of carbon deposited in the Injector Feed Arm Rig (IFAR), a large scale turbulent flow rig, is exponential, i.e., Wc ∝ (∆TIW)z, where Wc is the total weight of carbon, and ∆TIW the tube inner wall temperature rise. However, the z values appear to be fuel dependent, ranging from 2.31 to 2.93 for the three fuels examined. Consequently, unless the z value is factored per test fuel, variability in the value of z may well contribute to poor correlations between inner wall temperature rise and total surface deposits. This variability in the z value may also account for a fuel dependency effect when temperature rise is adopted as the basis for quantifying thermal deposition. Total Hot Deposit versus Initial Wall Temperature. Compared to the poor correlations between total surface deposit and wall temperature rise (Figure 2), when the total hot deposits were plotted versus the corresponding initial wall temperatures, the correlations improved significantly for two of the three test fuels (Figure 3). For example, for the fuel from Tank 21/23 + copper, the R2 value increased from 0.12 to 0.81, and for the same fuel without copper, the R2 value increased from 0.25 to 0.66. For the JAX JP-8 fuel, its poor correlation remained unchanged (R2 value stayed at 0.3). A possible reason for JAX JP-8’s poor correlation is that it is a thermally stable fuel, hence its low thermal deposits even at increased temperature. Comparison of Thermal Deposition in the NAFTSS and TR Based on Initial Temperature. In the com-

parison of thermal deposition in the NAFTSS versus the TR based on their corresponding initial wall/main heater block temperatures, the appropriate plots were constructed using the experimental data listed in Table 3A,B, respectively. Specifically, in the case of the NAFTSS, the initial middle inner wall temperature was plotted versus the total hot deposits (i.e., total hot tube and hot filter deposits). In the case of the TR, the initial main heater block temperature was plotted versus the sum of the main heater and filter deposits. The results for the three test fuels screened in the TR and the NAFTSS are illustrated graphically in Figures 4-6 for linear fit of the data and in Figures 7-9 for exponential fit. These regression plots compare thermal deposition in the NAFTSS and in the TR as a function of wall/main heater block temperature, respectively. The resultant regression coefficients (R2) and the slopes for the linear regression analyses are summarized in Table 4. Analysis of the results follows. Linear Regression Analysis (See Table 4). For the TR plots, the linear correlations were good to excellent for all three test fuels (R2 ) 0.81-0.99). For the NAFTSS, the corresponding linear correlations were also fairly good to good for the fuel from Tank 21/23, with and without copper (R2 values ranged from 0.66 to 0.81). JAX JP-8’s poor correlation in the NAFTSS (R2 ) 0.31) is likely due to its high thermal stability coupled with the NAFTSS operating conditions. The consistently higher magnitude in the slopes of the regression lines for all three test fuels screened in the TR versus the NAFTSS (i.e., even for JAX JP-8) suggest that the operating conditions in the TR are likely more severe than those in the NAFTSS. Exponential Regression Analysis (See Table 4). Compared to linear regression analysis of the data, exponential regression analysis improved the correlations in both the TR and the NAFTSS for two of the three

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Figure 4. Linear regression plots for fuel 21/23.

Figure 7. Exponential regression plots of the same data plotted in Figure 4.

Figure 5. Linear regression plots for fuel 21/23 + copper.


Figure 6. Linear regression plots for fuel JAX JP-8.


fuels. For example, in the TR, the exponential regression R2 values for 21/23, with and without copper, improved from 0.8 to 1.0 for exponential versus linear fit of the data. In the case of the NAFTSS, the range in R2 for the same two fuels improved from 0.7 to 0.8, to 0.80.9. However, for JAX JP-8, whereas the exponential regression correlation did not improve in the NAFTSS, it remained excellent in the TR (R2 ) 1). Further analysis of the exponential regression plots of the data shown in Figures 7-9 suggest the following: (a) Below 350 °C, the slopes appear to be fairly linear in both the TR and the NAFTSS.

(b) The exponential increase in thermal deposition in the TR for all three fuels at the very high main heater block temperatures (>350 °C) likely indicates that the test fuels, which include the less stable, 21/23 copper doped fuel, were not reactant-depleted, i.e., of dissolved oxygen. Note, also, the slopes of 21/23’s exponential regression lines for the TR and the NAFTSS appear similar (see Figure 7). It may be useful to add that at the TR’s maximum heater temperatures shown for the three test fuels, the corresponding fuel out temperature was 218 °C. And, at these operating conditions, thermal


Energy & Fuels, Vol. 15, No. 1, 2001 231 Table 3. Thermal Deposit Data

(A) At Corresponding Initial Wall Temperatures for the NAFTSS at the Specified Test Durationa 21/23 + Cu

21/23

JAX JP-8

temperature (°C)

duration (h)

deposit (mg/L)

temperature (°C)

duration (h)

deposit (mg/L)

temperature (°C)

duration (h)

deposit (mg/L)

204 232 260 260 288 316 316 316 343

31.00 50.33 24.00 26.50 28.50 7.83 15.50 15.50 11.00

0.018 0.008 0.038 0.026 0.034 0.084 0.069 0.080 0.197

204 232 260 288 288 316 343 343

50.50 31.50 26.50 17.16 20.00 10.00 10.00 11.17

0.027 0.027 0.044 0.075 0.042 0.072 0.115 0.136

288 316 316 329 329 343 343

66.02 39.00 22.50 9.93 22.50 15.30 23.50

0.008 0.011 0.021 0.006 0.004 0.033 0.028

(B) At Corresponding Main Heater Block Temperatures for the Tubular Reactorb 21/23 + Cu

21/23

a

JAX JP-8

temperature (°C)

deposit (mg/L)

temperature (°C)

deposit (mg/L)

temperature (°C)

deposit (mg/L)

257 287 326 343 393

0.07 0.14 0.17 0.28 0.73

250 297 335 380 388

0.11 0.25 0.38 1.3 0.98

345 358 415

0.18 0.23 0.73

Fuel flow rate: 9.45 L/h. b Fuel flow rate: 0.6 L/h. Test duration: 12 h in all cases. Table 4. Summary of the Correlations Obtained for the Tubular Reactor versus the NAFTSS Based on Linear and Exponential Regressions of the Data and Slopes for Linear Fit (Laminar versus Turbulent Flow Test Rig) linear R2 a

a

exponential R2b

fuel

TR

NAFTSS

TR

NAFTSS

TR

NAFTSS

21/23 21/23 + 280 ppb Cu Jax JP-8

0.83 0.81 0.99

0.66 0.81 0.31

0.97 0.97 1.0

0.83 0.89 0.16

0.0046 0.0080 0.0081

0.001 0.0007 0.0003

On the basis of the values obtained in Figures 4-6. b On the basis of the values obtained in Figures 7-9.

Table 5. Thermal Deposition in the TR and the NAFTSS at Initial Wall/Main Heater Block Temperature of 280 °C thermal deposit, mg/L

a

slope for linear fita

fuela

tubular reactor

NAFTSS

JAX JP8 21/23 21/23 + Cu

0.00 0.09 0.20

0.00 0.06 0.10

Fuels ranked in decreasing order of thermal stability.

deposition is within the thermooxidative regime and not due to pyrolysis. Effect of Temperature (Figures 4-6 and Figures 7-9). At the lower operational temperatures (approximately 21/23 + Cu (the least thermally stable). At the higher operational temperatures (approximately >350 °C), the concentrations of thermal deposits were significantly higher in the TR than in the NAFTSS, even for the highly thermally stable JAX JP-8 fuel. These overall results further focus on the importance of a judicious choice in operating temperatures for determining thermal stability. Apparent Activation Energies. Possible differences in thermal deposition between the NAFTSS and the TR were further investigated by calculating the apparent

activation energies of the test fuels screened in both these devices. We use the term apparent activation energy (Ea), in acknowledgment that thermal deposition is likely due to a complex series of reactions involving multi fuel components.21a On the basis of the experimental data given in Table 3A,B, the apparent activation energies of the three fuels for the TR and the NAFTSS were calculated from the slopes of their respective Arrhenius plots, i.e., ln(rate of thermal deposit) vs 1/T, where T is the wall/main heater block temperature in kelvin (see Figures 10-12). The results, summarized in Table 6, indicate that the apparent activation energy of the copper doped 21/23 fuel was similar for thermal deposition in the TR and the NAFTSS, thereby suggesting that Reynolds number was not a factor for this relatively less stable fuel. Regarding the remaining two test fuels, at first glance, their calculated apparent activation energies appear higher for thermal deposition in the NAFTSS vs the TR, and considerably so, for the JAX JP-8 fuel. Note, a reactant depletion effect, which has been suggested2 to occur in a laminar flow regime, would effect the opposite trend, i.e., a higher Ea for a laminar vs a turbulent flow regime. However, on factoring in the standard error of the slopes of the Arrhenius plots for 21/23 and JAX JP8, in the TR and the NAFTSS, their corresponding apparent activation energies are not that much different (Table 6). These results are also consistent with our (21) Hazlett, R. N. Thermal Oxidation Stability of Aviation Turbine Fuels; ASTM Monograph 1; American Society of Testing and Materials: Philadelphia, PA, 1991; (a) pp 91-92, (b) p 62, (c) p 50.

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Figure 10. Arrhenius plots of the thermal deposits formed in the NAFTSS and in the TR for fuel 21/23.

Figure 11. Arrhenius plots of the thermal deposits formed in the NAFTSS and in the TR for fuel 21/23 + copper.

Figure 12. Arrhenius plots of the thermal deposits formed in the NAFTSS and in the TR for fuel JAX JP-8.

earlier findings22 in which similar apparent activation energies were obtained for a JP-5 fuel, which was tested in both a laminar flow test device (gravimetric JFTOT)23 and the NAFTSS. The lower R2 values of the Arrhenius plots for 21/23 and JAX JP-8 in the NAFTSS vs the TR (Table 6) reflect the scatter in the NAFTSS’s data. (22) Pande, S. G.; Hardy, D. R. Prepr.-Am. Chem. Soc., Div. Fuel Chem. 1998, 43, 89-93. (23) Beal, E. J.; Hardy, D. R.; Burnett, J. C. Proceedings of the 4th International Conference on Stability and Handling of Liquid Fuels, Orlando, FL, November 1991 1992, 245-259.

Table 6. R2 and Calculated Activation Energy in kcal/ mol For the Three Test Fuels in the TR and NAFTSS tubular reactor fuel

R2

21/23 21/23 + Cu JAX JP-8

0.95 0.95 1.0

NAFTSS

Ea

R2

Ea

11 ( 2 12 ( 2 17 ( 1

0.77 0.95 0.56

17 ( 4 14 ( 1 29 ( 12

Nonetheless, the higher activation energy of JAX JP-8 relative to the other two fuels is consistent with our previous evaluation that JAX JP-8 is more stable.



Table 7. Comparison of the HiReTS versus the Tubular Reactor under Standard Operating Conditions HiReTS: 290 °C/2 h at 35 mL/min fuela ID

type

copper concn, ppb

JPTS Tank 20/22b Red Hill Tank 4 JAX Tank 6 NAPC 29 POSF 2827c,d

JP-5 JP-5 JP-5 JP-5 diesel: 50/50 HHO/DFM Jet A

na 68 na na 68 na

number (avg)

µg

mg/L

tubular reactor: 204 °C/12 h at 10 mL/min main heater + filter deposits (mg/L)

8 2038 137 372 350 2015

10 61 74 116 156 530

0.00 0.01 0.02 0.03 0.04 0.13

0.31 0.44 1.18 1.03 2.41 0.39

carbon burn off

a Fuels ranked according to HiReTS carbon burn off. b Outlier in the correlation of HiReTS number versus HiReTS carbon burn off. Outlier in the correlation of HiReTS carbon burn off versus tubular reactor carbon burn off. d Unpublished results from S. Zabarnick (University of Dayton Research Institute, Aerospace Mechanics Division, Dayton, OH).

c

Overview of Thermal Deposition in the NAFTSS vs the TR. Despite the difference in flow regime (turbulent vs laminar), the results described above indicate good correlations for the various comparisons of thermal deposition in the NAFTSS versus the TR. Specifically, these include similar correlations with temperature, a similar ranking order at a specified temperature, as well as differences in apparent activation energies of the test fuels that are within the statistical analysis of the data. Reactant Depletion/Temperature Dependency. As alluded to earlier, the overall operating conditions in the TR (i.e., laminar flow/residence time/temperature) are likely more severe, i.e., with respect to fuel temperature than those in the NAFTSS (i.e., turbulent flow/residence time/temperature). Thus, because of this fuel temperature difference in laminar vs turbulent flow, reactant depletion, which has been suggested2 to occur in a laminar flow regime, may well be due to a temperature effect rather than to a difference in flow velocity (see (c) below). Supporting this postulate are the results of the following direct comparisons: (a) Thermal Deposition versus Fuel Outlet Temperature in the TR. As discussed earlier, the decrease in thermal deposition of some fuels in the TR at increased fuel outlet temperatures (see Figure 1A,B) is likely indicative of a reactant depletion effect, and that this effect is fuel/temperature dependent. (b) Oxygen Depletion versus Heater Tube Temperature. Morris et al.24 found oxygen depletion in the JFTOT increased with increasing heater tube temperature. Specifically, for a non additized commercial jet A fuel, oxygen depletion was 44% at 250 °C, 89% at 260 °C, and 100% at 310 °C. (c) Thermal Deposition versus Fuel Flow Rate in the TOFT (Thermal Oxidation Fuel Tester) at 350 °C. The TOFT is a laminar flow device similar to the JFTOT that allows the fuel flow rate and test duration to be varied. Using a test temperature of 350 °C in the TOFT, Clark and Thomas2 investigated the effect of flow rate on thermal deposition, and found a flow rate dependence, with a maximum at 8 mL/min. Specifically, thermal deposition increased, with increasing flow rate over the range 1 to ∼8 mL /min, but deposition decreased, when the flow rate was increased further. The decreased deposition at flow rates greater than 8 mL/ min was attributed to decreased residence time of the (24) Morris, R. E.; Hazlett, R. N.: McIlvaine, C. L., III; The Effects of Stabilizer Additives on the Thermal Stability of Jet Fuel. Ind. Eng. Chem. Res. 1988, 27 (8), 1524-1528.

fuel. And, the decreased deposition at flow rates less than 8 mL/min flow rate was attributed to reactant depletion, possibly dissolved oxygen, in a laminar flow regime, since a flow rate dependence was not observed in realistic turbulent flow rigs.2 We concur with Clark and Thomas’s2 explanations regarding a reactant depletion effect and a residence time effect to account for the dependence of deposition on flow rate. However, regarding the cause of reactant depletion, we offer the following explanation. Flow velocity in the TOFT is well acknowledged to be laminar even at 8 mL/min flow rate. Consequently, the more probable explanation for reactant depletion in the laminar regime at flow rates less than 8 mL/min, as described above, is likely a temperature effect arising from the very high operating temperature (350 °C) in conjunction with the longer residence time at lower flow rates. Morris24 et al.’s results on total oxygen depletion in the JFTOT at very high test temperatures (100% at 310 °C for the Jet A fuel examined) further supports this explanation. For reference, although the fuel flow rate in the JFTOT is 3 mL/min, the operating temperature is considerably lower, i.e., 260 °C. Nonetheless, based on Morris24 et al.’s oxygen depletion results (89% depletion at 260 °C), one may speculate that the parameter that needs to be revisited in the JFTOT would be either an increase in the fuel flow rate or a decrease in the heater tube temperature. Flow Velocity Effect. The increased concentration in thermal deposition for the TR versus the NAFTSS is consistent with Hazlett’s review summarizing21b the effect of flow velocity on the rate of deposition. Specifically, Hazlett21b states that “the preponderance of evidence indicates that the rate of deposition is greater at low Reynolds numbers” (italics are ours). The low Reynolds numbers for the studies conducted were well within the laminar flow regime. Comparison of the HiReTS and the Tubular Reactor (TR). (All tests were conducted at standard operating conditions per device, and the fuel data set comprised the same six fuels.) Because HiReTS number is the index adopted for quantifying thermal deposition, it was important to first examine the correlation between HiReTS number and HiReTS carbon burn off (CBO), since it was our goal to ultimately compare HiReTS CBO and TR CBO in this investigation. The results, which are summarized in Table 7, and shown graphically in Figure 13, indicate a poor correlation for HiReTS number vs HiReTS CBO (R2 ) 0.30), and zero

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Figure 13. Shows poor correlation for HiReTS No vs HiReTS CBO, and essentially zero, for TR CBO vs HiReTS No, for the same six fuel set.

Figure 14. Shows excellent correlation between HiReTS No and HiReTS CBO for five of the six fuels examined, in which the excluded fuel was that from Tank 20/22. The correlation between TRCBO and HiReTS was also good, though in this five-fuel set, the excluded fuel was POSF 2827.

correlation for TR CBO vs HiReTS CBO (R2 ) 0.01). However, drastically affecting the R2 values for each correlation was one outlier, which was a different fuel in each case. For example in the correlation of HiReTS number versus HiReTS CBO, the outlier was the fuel fromTank 20/22, and in the correlation of TR CBO versus HiReTS CBO, the outlier was POSF 2827 (see Table 7 and Figure 13). On excluding the respective outlier, wherein the data set then comprised five fuels (Figure 14), the correlation between HiReTS number versus HiReTS CBO changed from poor to excellent (R2 ) 0.98). The correlation between TR CBO versus HiReTS CBO also improved

tremendously, i.e., from zero to good (R2 ) 0.82). This latter correlation for the fuel set examined is particularly significant for it suggests that if the fuel chemistry under the thermal conditions established in the HiReTS is indeed representative of the realistic turbulent flow systems in aircraft engines, then thermal deposition appears to be independent of the overall effect of turbulent versus laminar flow. Explanation for excluding the outlier in each case follows: For HiReTS Number versus HiReTS CBO: Outlier Fuel, Tank 20/22. The high HiReTS number of 2038 for the fuel from Tank 20/22, but relatively small CBO (0.01 mg/L) (Table 7), demonstrates that the correlation


between HiReTS number and HiReTS CBO is not universal. A similar problem of the HiReTS involving a larger fuel set was also reported recently.25 For TR CBO versus HiReTS CBO: Outlier Fuel: POSF 2827. Like the HiReTS, the operation of the Tubular Reactor is based on maintaining a constant fuel outlet temperature, but the TR’s outlet temperature is significantly lower than the HiReTS’s (204 °C versus 290 °C). On the basis of the CBO, thermal deposition in the HiReTS for the POSF 2827 fuel was exceedingly high, relative to other fuels. In contrast, its thermal deposition was low in the TR, relative to other fuels (Table 7). We suggest that the significant difference in CBO between the turbulent and laminar flow test devices for POSF 2827 is likely due to a temperature effect and not to a difference in flow velocity. Supporting this viewpoint is the substantial increase in CBO, when the TR was operated at a higher fuel outlet temperature. Specifically, the concentration of CBO in the TR was 4.7 mg/L at 232 °C fuel outlet temperature, versus 0.13 mg/L at 204 °C fuel outlet temperature. With respect to the HiReTS, despite its considerably shorter residence time (0.0128 s vs 3.2 s in the TR), because the HiReTS operates at a significantly higher fuel outlet temperature, the added heat during the test (see Experimental Section, HiReTS) may well be contributing to thermal deposition. Consequently, to adequately resolve the effect of turbulent versus laminar flow on thermal deposition using the HiReTS device, it is important to operate the HiReTS at a constant heating power level and quantify thermal deposition on the basis of the CBO. Is Turbulent Flow Velocity Critical to Predicting Thermal Deposition? The overall good correlations/agreement obtained on comparison of two turbulent flow test rigs (the NAFTSS and the HiReTS) and a laminar flow test device (the TR) tend to suggest that Reynolds number is not a critical parameter in predicting thermal stability. Furthermore, as discussed earlier, the reactant depletion effect which is suggested2 to occur in a laminar flow regime is likely attributable to a difference in fuel temperature (arising from the difference in flow rate/test temperature) and not to a difference in flow velocity (laminar vs turbulent flow). These overall findings support the current use of laminar flow in devices such as the specification test method, i.e., the JFTOT, ASTM D3241. However, the concern regarding oxygen depletion in the JFTOT may be better addressed by either an increase in the fuel flow rate or a decrease in the heater tube temperature. On the basis of our evaluation of the test devices, we postulate that the critical parameter in predicting thermal deposition is likely the fuel temperature and not Reynolds number. This postulate is consistent with Hazlett’s review21c that temperature is the most important of the factors involved in fuel thermal deposition.


dence time, and temperature are interrelated. For this reason, evaluations were based on comparisons of thermal deposition between the different types of test devices, which then incorporates their overall operating conditions. In the comparison of the turbulent flow test rig, the NAFTSS, and the laminar flow test device, the Tubular Reactor (TR), the correlations of total hot deposits versus initial wall/main heater block temperature were excellent for the TR and good for the NAFTSS. The higher concentrations of thermal deposits in the TR vs the NAFTSS, reflected in the larger magnitude of the slopes of the plots, suggest that the overall operating conditions (residence time and temperature) are more severe for the laminar flow test device (TR) than for the turbulent flow test rig, the NAFTSS. Both devices also ranked the test fuels’ thermal stabilities in the same order at similar low wall/main heater block temperatures. Furthermore, differences in the calculated apparent activation energies of the three test fuels for thermal deposition in the NAFTSS versus the TR are within the statistical analysis of the data, and not due to a difference in flow velocity of the two devices. In the comparison of another turbulent flow test rig, the HiReTS and the laminar flow test device, the Tubular Reactor (TR), for a select number of five fuels, the correlation appears to be good (R2 ) 0.82) between carbon burn off (CBO) generated in the HiReTS, and CBO generated in the TR. The disparities observed in the case of two fuels identified as outliers in this investigation are likely related to the much higher fuel outlet temperature in the HiReTS than in the TR (cf., 290 °C vs 204 °C), in conjunction with the modus operandi of the HiReTS for quantifying thermal deposition. The overall good correlations obtained between the laminar and turbulent flow test devices examined, suggest that Reynolds number does not appear to be a critical factor in predicting thermal stability. The reactant depletion effect, which has been suggested to occur in a laminar flow regime is likely attributable to a difference in fuel temperature (arising from the difference in flow rate (residence time)/test temperature) and not to a difference in flow velocity (laminar vs turbulent flow). The overall findings are significant for they support the use of laminar flow in devices such as the specification test method, i.e., the JFTOT, ASTM D3241. However, the concern regarding oxygen depletion in the JFTOT may be better addressed by either an increase in the fuel flow rate or a decrease in the heater tube temperature. On the basis of our evaluation of the test devices, we postulate that the fuel temperature is likely the predominant and controlling parameter in predicting fuel thermal stability.

Conclusions When evaluating the effect of turbulent and laminar flow on thermal deposition, it is difficult to isolate the impact of Reynolds number, since flow velocity, resi(25) Kamin, R. A.; Colbert, J. US Navy HiReTS Evaluation. Presented at the ASTM Joint Sub-Committee SC 14/J8 Task Force Meeting, Reno, Nevada, 6 December 1999.

Acknowledgment. We thank NAVAIR 4.4.5 for financial support, and Dr. James Fleming (Code 6185, Naval Research Laboratory) for the helpful and valuable discussions. EF000187F

Quest for a Reliable Method for Determining Aviation Fuel Thermal

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