Article pubs.acs.org/EF
Surface Deposition Characteristics of Supercritical Kerosene RP‑3 Fuel within Treated and Untreated Stainless-Steel Tubes. Part 1: Short Thermal Duration Kun Zhu,*,† Zhi Tao,‡ Guoqiang Xu,‡ and Zhouxia Jia‡ †
AVIC Academy of Aeronautic Propulsion Technology, Beijing 103400, People’s Republic of China National Key Laboratory of Science and Technology on Aero-Engine Aero-thermodynamics, Collaborative Innovation Center of Advanced Aero-Engine, School of Energy and Power Engineering, Beihang University, Beijing 100191, People’s Republic of China
‡
ABSTRACT: The thermal oxidation deposition characteristics of kerosene RP-3 have been experimentally studied in the vertical tube at supercritical pressure as a crucial concern for the cooled cooling air (CCA) development. Thermal stressing of the fuel was carried out in a heated tube with stainless steel 321 (SS321, 1Cr18Ni9Ti), pre-oxidized, and electrolytically passivated for 1 h. Under the constant pressure of 5 MPa, all of the experiments were conducted at the fixed inlet and outlet fuel temperatures of 400 and 723 K, respectively, under the same heat flux and flow mass rate. Deposition of the different segments was analyzed using a weighting method to observe the deposition profile of the test section. Moreover, the morphology and components of the surface deposition were examined along each tube, with different surface treatments, to investigate the surface thermal oxidative deposit mechanisms. In this work, it was found that the pre-oxidized and electrolytically passivated treatments could reduce the total deposition about 35.83 and 58.33%, respectively, as a result of the formed passivation layer and reduced surface roughness in the treated progress in contrast to the as-received SS321 tube. On the basis of the scanning electron microscopy (SEM) images and component analysis of the surface deposit, the thermal oxidation deposit on the treated tube surface could be attributed to the adhered deposit formed in the liquid fuel rather than the surface catalytic filamentous deposition on the untreated tubes. an aircraft fuel system.12 Therefore, it is crucial to study the influence of the surface on the fuel deposition characteristics under the supercritical pressure. Several studies have been taken to investigate the hightemperature deposition characteristics of different fuels flowing in the different surface materials.3,9,10,13−20 Eser et al.3 conducted the experiments on the carbonaceous solid deposition of fuel Jet-A and JP-8 in the different substrates at 773 K and 3.445 MPa. The results showed that the higher concentrations of sulfur and heavy alkane in Jet-A will lead to a lower amount of deposition than JP-8 under the same thermal stressing conditions. Furthermore, the two fuels will lead to different amounts, morphologies, and structure order of the carbonaceous deposition. As a result of the considerable deposit catalytic effect of the surface material, a passivating layer,6,21−23 preventing the stainless-steel surface from the heated fuel, would be expected to be an efficient method to reduce the oxidative rate and decrease the mass of the deposit,22 especially for the filamentous (catalytic) carbon deposition.4 It is believed that the silica-based layer could prevent the fuel from the more active stainless-steel surface, resulting in the significant delay oxidation of the fuel and surface deposit reduction of both thermal oxidative and pyrolytic procedures.6 Although previous studies have investigated the coking deposit characteristics of jet fuel JP-8 and Jet-A on the different substrate materials in a high temperature, the relative study on
1. INTRODUCTION In the modern aircraft engine, the turbine components suffer thermal stress and thermal loads, which reduce the turbine performance and lifetime. As the compressor pressure ratio increases, the temperature of the cooling air delivered from the compressor to the turbine vane and blades increases. Consequently, the increased cooling air temperature will lead to the lower cooling efficiency of the thermal components. The cooled cooling air (CCA), which uses the fuel as the coolant to absorb the heat from the turbine coolant air, is a new cooling method and has been widely investigated recently.1,2 The fuel is heated, while the coolant air is cooled. The pressure of a typical aircraft fuel system is about 3.45− 6.89 MPa; this value goes beyond the critical pressure of fuel. When the supercritical pressure fuel flows through the heat exchanger, the fuel temperature continually increases as the fuel absorbs the heat from the cooling air. The temperature has been considered as the most important factor affecting both thermal oxidative and pyrolytic coke deposit formation.3,4 When the fuel bulk temperature goes beyond 435 K, the species in the heated fuel will begin to react with the dissolved oxygen in the air saturated to form a surface deposit.5 At the relatively high-temperature region (for temperatures greater than ∼723 K6), the dissolved oxygen is depleted and the deposit mechanism becomes dominated by the pyrolytic reactions. Both the fuel thermal oxidation and pyrolytic deposition chemistry have been strongly influenced by the fuel temperature,7,8 surface material,5,9 and fuel composition.3,8,10,11 Consequently, the surface deposit collects on the fuel flow path inner surface, fuel controls, fuel nozzles, and other parts of the fuel system, resulting in the potential catastrophic failure of © 2016 American Chemical Society
Received: December 10, 2015 Revised: February 1, 2016 Published: February 18, 2016 2687
DOI: 10.1021/acs.energyfuels.5b02889 Energy Fuels 2016, 30, 2687−2693
Article
Energy & Fuels
Figure 1. Schematic of the experimental system.
Figure 2. Schematic of the surface deposition weighting process. overall test system could be separated into four sub-systems: fuel supply system, heating system, data acquisition system, and cooling system. The fuel supply system prepares the fuel to the desired experimental conditions. Initially, the pressure of the fuel in tank 1 is increased to 16 MPa by a piston pump. Then, the pre-pressed fuel goes through an airbag pulsation to reduce the pressure pulsation below 5% of the test pressure. The fuel out from the airbag is then divided into a major path and an otiose path: the mass flow rate of the major path fuel is controlled by a mass flow control value and measured by a Coriolis force flow meter with an accuracy of 0.2%, and the otiose path fuel, whose mass flow rate is controlled by a backpressure value, is collected for reuse. The major path fuel out from the fuel supply system is then preheated with heat flux by a current power on the stainless-steel tube in the heating system to achieve the required inlet fuel temperature of the test section. The system pressure of the fuel in the tube is held by a back-pressure valve (0−15 MPa). The
Chinese kerosene RP-3, which has unique components with other jet fuels, is limited, especially in the high-temperature and high-pressure region. Moreover, the complicated jet fuel compositions result in considerably different deposition characteristics.20 The objective of this study is to investigate the thermal coking deposition characteristics of kerosene RP-3 on the stainless-steel tube (SS321, 1Cr18Ni9Ti) and inner surface passivation tube for 1 h of thermal stressing time at the temperature range of 400−723 K under a supercritical pressure of 5 MPa.
2. EXPERIMENTAL SECTION 2.1. Test System. The setup experimental facility for thermal stressing of RP-3 is shown in Figure 1. Some details of the system have been described in previous studies.23−27 As shown in Figure 1, the 2688
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Energy & Fuels pressure drop and temperature of fuel at the inlet and outlet are measured using a pressure gauge transducer (Rosemount 3051CA4) and K-type sheathed thermocouples, respectively. After testing, the thermal-stressed fuel is cooled to less than 310 K by a water-cooled condenser and collected in tank 2. 2.2. Test Facility. The experiments are conducted in a stainlesssteel 321 tube (1Cr18Ni9Ti) with 1.78 mm inside diameter (id) and 2.20 mm outside diameter (od), and the inner surface average roughness is 2.1 μm.26 The heated length of the test tube is 1.600 m, which does not include the 100 mm entrance and exit lengths. The tube is installed vertically and covered with Aspen for heat insulation. The fuel is heated in the vertical tube by a constant heat flux. The constant heat flux is achieved by heating the stainless-steel tube directly with a low-voltage direct current. The temperature of the outside surface of the tube is measured using 25 K-type thermocouples (φ = 0.1 mm) along the tube; the accuracy of the thermocouple is calibrated to be 99.4% in the test temperature range. All of the data are recorded using a computer with ADAM4018. The aviation kerosene RP-3 was purchased from China Aviation Fuel Supply Co., Ltd. The components have been evaluated in a previous study.24 Before the test, the air has been injected into the fuel bank for 60 min to achieve the oxygen saturation. The tubes for the pre-oxidize passivation effect tests were put into the furnace, heated to 1073 K at the speed of 12 K/min, and maintained for 12 h. To oxidize the tube inner surface completely, the oxygen was continually conducted in the tubes in excess. After the preoxidation process, the tubes were cooled naturally in the furnace. The tubes for the electrolytic passivation were prepared by Dongqing Metal Surface Treatment Corporation. 2.3. Test Procedure. After the thermal stressing experiments, the tube was cut into 5.0 cm segments and analyzed for the thermal deposition using the weighting method. As shown in Figure 2, the weighting procedure was taken as the following steps: (1) the segments were dried in the oven at a temperature of 393 K for 1 h; (2) the segments were weighed by the microbalance (minimum scale value of 0.1 mg, Sartorius BT224S, Germany) for the first time as mass m1; (3) the segments were put into the ultrasonic cleaning machine with alkaline solution for 6 h; (4) the segments were washed with ethanol to wash off the leftover deposit; and (5) steps 1 and 2 were repeated, and the segment mass was obtained as m2. Each of the weighting steps was repeated twice to limit the system uncertainty. The deposit mass of the each segment will be obtained as m1 − m2 after the above steps. In addition to the total deposit amount, the local deposition rate (in micrograms per centimeter squared) of the thermal stressing time has been analyzed in this study to investigate the thermal deposit characteristics with different fluid temperatures along the test tube. To analyze the formed mechanisms of the thermal deposition, scanning electron microscopy (SEM, CamScan3400, Oxford University, Oxford, U.K.) and energy-dispersive spectroscopy (EDS) were used, respectively, to examine the surface morphology and components of surface deposition. 2.4. Uncertainty Analysis and Repeatability. Table 1 shows the experimental uncertainty of the facilities. After the thermal stressing experiments, the test tube has been cut into nearly 32 segments; therefore, the deposit weight of each segment can be obtained as the following equation:
m(i) = m1(i) − m2(i)
n
M=
measurement uncertainty
tube length, L (mm) tube segment length, L1 (mm) tube inner diameter, d (mm) mass flow rate, ṁ (%) tube segment weight, m(i) (mg) fuel inlet and outlet bulk temperature, Tf (K) tube outer wall temperature, Tw,out (K)
0.1 0.02 0.01 0.5 0.05 0.6 0.6
n
∑ m(i)= ∑ [m1(i) − m2(i)] i=1
i=1
(2)
On the basis of the error propagation equation, the uncertainty of a single segment and total thermal deposit amount can be obtained as eqs 3 and 4, respectively. (Δm1(i))2 + (Δm2(i))2
Δm(i) =
(3)
n
ΔM =
∑ (Δm(i))2 i=1
(4)
When the facility uncertainty is taken into the equation, the single segment and total tube thermal deposit weight are ±0.07 and ±0.32 mg, respectively. With the definition of the local surface deposition per unit area, its combined uncertainty is ±0.07 mg/cm2.
3. RESULTS AND DISCUSSION 3.1. Experiment Repeatability. To confirm the reliability of the experimental facility and method, the 1 h experiments were repeated at the same test conditions for the untreated SS321 tubes. As shown in Figure 3, the repeatability of 1 h
Figure 3. Repeatability tests of the experimental system under the same conditions.
experiments is acceptable and the local surface deposition profile is particularly consistent with the former published literature.28−30 Furthermore, the experimental result manifests that the total deposition amount discrepancy between two tests under the same conditions is less than 4.94%. 3.2. Experimental Results of Untreated and Treated Tubes. Figure 4 presents the total amount of the thermal surface deposition on the three different inner surface substrates after 1 h of thermal stressing. The result shows that the total surface deposition amount reduced by 35.83 and 58.33% through coating the tube inner surface with preoxidized and electrolytically passivated layers, respectively. The deposition decrease could be attributed to the effect of the passivation layer on the inner surface of the tube. It also suggests that the tube surface material plays an important role in catalyzing the thermal deposition procedure.3,13,14,17,21 The mechanisms of the deposition decrease will be analyzed as follows.
Table 1. Uncertainty of the Direct Measurements direct measurement
(1)
where i is the serial number of the segment. Therefore, the total thermal deposit weight of each experiment can be calculated using the following equation:
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Figure 4. Total surface deposition amount of kerosene RP-3 on (1) electrolytically passivated, (2) pre-oxidized, and (3) untreated SS321 tubes for 1 h experiments.
3.2.1. SS321. Figure 5 shows the local surface deposition distribution and inner wall temperature profiles along the tube
Figure 6. SEM micrographs of the SS321 tube surface (A) before the experiment and the surface deposition at different segments after a 1 h duration experiment: (B) x/L = 0.156−0.179, (C) x/L = 0.456−0.475, (D) x/L = 0.506−0.525, (E) x/L = 0.744−0.763, and (F) x/L = 0.95− 0.969.
Table 2. Mass Percentage of the Deposition Components after a 1 h Experiment within a SS321 Tube mass percentage
Figure 5. Local surface deposition rate and wall temperature distribution along the SS321 tube for a 1 h thermal duration experiment.
C O Cr Fe Ni S
in the untreated SS321 for a 1 h experiment. The near-wall fuel has a temperature similar to that of the inner wall of the tube; therefore, the inner wall temperature is more appropriate to evaluate the surface deposition condition. For SS321, as shown in Figure 5, the local deposition rate varies smoothly within low- and high-temperature ranges but dramatically in the middle of the tube (between x/L = 0.5 and 0.68 along the test tube), where the inner wall temperature ranges from 645 to 687 K. The deposition rate peaks at nearly x/L = 0.516 of the tube, where the inner wall temperature varies from 643.7 K at the test beginning to 662.2 K at the end of the 1 h experiment. Figure 6 and Table 2 show the SEM micrographs and inner surface components of the SS321 tube surface, respectively, before the test and deposition at different segments for a 1 h duration experiment. The different phases of thermal oxidative deposition forms by three mechanisms are reported by Albright and Marek.8 It is found that iron, nickel, and chromium are generally used as catalysts during oxidation of hydrocarbons to accelerate the reduction−oxidation reaction.3,11,18,19 When the preheated fuel enters the test tube, it begins to be oxidized by dissolved oxygen. As the fuel temperature increases, the oxidation reaction rate is promoted, resulting in a high
A
B
C
D
E
F
77.32 21.78
81.87 17.54
35.06 12.12 10.73 36.71 4.34 1.3
10.96
19.36 71.68 8.4
8.22 5.02 16.71 62.51 7.53
0.9
0.59
15.1 53.35 7.34 13.25
oxygen-consuming rate and oxidation products, as Figure 6 and Table 2 show. It has been found that the deposition peak displayed in the downstream where the dissolved oxygen is fully consumed in the study by Ervin et al.6 The deposition peak on the tube inner surface attributed to sequential processes: (1) the metal tube inner surface catalyzes the initial deposit on the metal surface; (2) as the metal surface is covered by the deposit, the catalyzing effect decreases; and (3) the deposit formed via non-catalytic pathways in the fuel attaches to the catalyzed deposition. The reduction of deposition appearing after the peak value could be attributed to the reactant consumption in the reaction, although the fuel temperature still increases. 3.2.2. Pre-oxidized Tube. In contrast with the SS321 tube, the pre-oxidized tube has different local surface deposition rate profiles for the same experimental conditions according to Figure 7. First, the overall baseline of the deposition rate is lower than that of the SS321 test, especially in the relatively high-temperature regions. In the second place, the maximum peak value of the deposition rate decreases about 52.9% in the 2690
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Table 3. Mass Percentage of the Deposition Components after a 1 h Experiment within a Pre-oxidized Tube mass percentage C O Al S Cr Mn Fe Ni others
A
B
C
D
E
F
19.06
20.12
79.37 18.25
71.83 18.69
9.77 13.61
10 18.4
0.63 25.09 2.94 46.27 5.64 0.99
25.62 3.02 44.54 5.48 1.22
0.48 0.53
0.82 2.55 0.61 4.14
21.94 3.04 40.19 5.01
0.73
1.35
0.42 26.18 5.07 39.56 4.54 0.85
8A. The chromium percentage in the pre-oxidized tube is higher than the as-received SS321 tube. In addition, the metal Mn has been found in the pre-oxidized tube inner surface compared to the SS321. In the procedure of oxidation, chromium oxides will diffuse toward the top layer of the oxidation surface, resulting in the increased percentage of chromium in contrast to the as-received SS321 tube. Iron and nickel oxides will diffuse to the bottom layer of the surface. In the study by Altin and Eser,21 the oxidations of Inconel 600 and Inconel 750 form NiO, Cr2O3, Fe2O3, NiCr2O4, MnCr2O4, and FeCr2O4 crystals in the oxidized surface. The deposition profiles in Figure 7 and SEM images of the deposit in Figure 8 can explain the behavior of the pre-oxidized surface upon 1 h of thermal stressing. It is believed that the oxidation layer prevents the fuel from the catalytic alloy surface and decreases the thermal deposition for a 1 h experiment in contrast to the as-received SS321 tube. On one hand, the morphology of deposits is mainly characterized as amorphous clusters, whose primary components are carbon and oxygen. Moreover, the morphology and composition of the deposit are roughly similar to those found on the SS321 surface at the same experimental conditions. In the former studies, the similar carbon deposit characteristics had been found via non-catalytic pathways.3,15 It has been found that the oxidation of chromiumcontaining alloys can produce protective oxide layers that limit the carbon deposit formation, especially the filamentous carbon,19 which is not found in our experiments as well. Furthermore, the reduction of the catalytic deposition could limit the secondary deposit proceeding via reactions of reactive species on incipient deposits.15 Figure 9 exhibits the local surface deposition and inner wall temperature profiles along the electrolytically passivated tube for a 1 h thermal stressing experiment. Similar to the preoxidized tube, the tube with an electrolytically passivated layer forms a limited surface deposition compared to the as-received SS321 tube at the same experiment conditions. Especially, the peak value of the deposition has been reduced roughly by 75.5% in contrast to the SS321 test. As a result of the less thermal resistance with deposition in the tube inner surface, the wall temperature takes limited variation from the beginning to the end of the experiment. Figure 10 and Table 4 show the SEM micrographs and inner surface components of the electrolytically passivated tube, respectively, before the test and the deposition at different segments after a 1 h duration experiment. In comparison to the as-received SS321 tube, the electrolytically passivated tube has nearly the same components of the inner wall as shown in column A of Table 4. However, the other components in the
Figure 7. Local surface deposition rate and wall temperature distribution along the pre-oxidized tube for a 1 h thermal duration experiment.
middle of the test tube. Moreover, the variation between the inner wall temperature of 0 and 60 min is smaller than that of the as-received SS321 experiment, which means that less thermal resistance in the heat-transfer process resulted from the deposition. Figure 8 and Table 3 show the SEM micrographs and inner surface components of the pre-oxidized tube surface, respectively, before the test and the deposition at different segments for a 1 h duration experiment. It shows that there are plenty of tiny metal particles on the tube inner surface in Figure
Figure 8. SEM micrographs of the pre-oxidized tube surface (A) before the experiment and the surface deposition at different segments after a 1 h duration experiment: (B) x/L = 0.156−0.169, (C) x/L = 0.45−0.4625, (D) x/L = 0.556−0.569, (E) x/L = 0.756−0.769, and (F) x/L = 0.956−0.969. 2691
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Table 4. Mass Percentage of the Deposition Components after a 1 h Experiment within an Electrolytically Passivated Tube mass percentage C O S Cr Fe Ni
A
20.66 71.31 8.04
B
C
D
E
F
4.54
78.64 20.68 0.68
77.94 8.19 0.5 2.62 9.53 1.23
36.3 15.95 4.83 8.52 30 4.4
30.94 5 7.12 11.18 40.95 4.8
17.92 68.26 9.28
polished inner surface limits the adherence of the soot-like tar droplet in forming the surface deposit; (2) the smooth surface will decrease the trapping effect of the tube surface “cavities” for the flowing fuel, which may experience a relatively high temperature and residence time, consequently leading to the reduction of the deposit rate at the constant heat flux condition; and (3) the passivation layer of chromium oxides could separate the fuel and catalytic metals in the tube surface and restrict the metal catalytic oxidation reactions. In the study by Venkataraman and Eser,16 they found that the solid deposit, which had SEM morphology and components similar to that in this work, could be attributed to the soot-like deposit formed in the liquid fuel.
Figure 9. Local surface deposition rate and wall temperature distribution along the electrolytically passivated tube for a 1 h thermal duration experiment.
4. CONCLUSION The thermal deposition characteristics of aviation kerosene RP3 on the different tube metal surfaces have been experimentally investigated for a 1 h stressing time at supercritical pressure conditions. The SEM and EDS examinations of the stressed tube surface provide the deposition morphology and components for further analysis. The following conclusions may be obtained: (1) The pre-oxidized and electrolytically passivated treatments could reduce the total deposition about 35.83 and 58.33%, respectively, in contrast to the as-received SS321 tube. (2) The pre-oxidized treatment will form the layer of metal oxides and limit the surface catalytic filamentous deposition, which were consequently not found in this study. (3) The electrolytically passivated treatment could reduce the surface roughness and form the passivation layer against the contact between fuel from the catalytic surface metal. As a result of the physical and chemical limitations for coking deposition of the electrolytically passivated treatment, it leads to less surface deposition after a 1 h test compared to the preoxidized treatment.
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Figure 10. SEM micrographs of the electrolytically passivated tube surface (A) before the experiment and the surface deposition at different segments after a 1 h duration experiment: (B) x/L = 0.156− 0.169, (C) x/L = 0.45−0.4625, (D) x/L = 0.556−0.569, (E) x/L = 0.756−0.769, and (F) x/L = 0.956−0.969.
AUTHOR INFORMATION
Corresponding Author
*Telephone: +86-10-56680655. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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passivation layer before the experiment have not been detected as a result of the very limited mass percentages. According to the traditional electrolytically passivated treatment, the passivated surface will be electrolytical polished and paasivated with a layer of iron and chromuim oxide products.31 As shown in Figure 10A, the polishing treatment could decrease the tube inner surface roughness, which will consequently affect the surface deposit rate.32 On the basis of the deposit profile and SEM images in Figures 9 and 10, respectively, the deposit reduction could be attributed to the following three aspects, especially in the peak region: (1) the
ACKNOWLEDGMENTS The authors acknowledge the financial support provided by the Aeronautical Science Foundation of China (Grant 2015ZBN3011).
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NOMENCLATURE D = diameter (m) id = inside diameter (mm) L = tube length (m) DOI: 10.1021/acs.energyfuels.5b02889 Energy Fuels 2016, 30, 2687−2693
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Energy & Fuels m = mass flow rate (g/s) m(i) = deposit weight of the ith segment (mg) M = total deposit weight (mg) od = outside diameter (mm) P = pressure (kPa) T = temperature (K) τ = time (min) x = location position (m)
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Subscripts
f = fuel in = inlet o = outlet w = wall
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