Article pubs.acs.org/EF
Flow Pattern Effects on the Oxidation Deposition Rate of Aviation Kerosene Xinyan Pei, Lingyun Hou,* and Zhuyin Ren School of Aerospace Engineering, Tsinghua University, Beijing 100084, People’s Republic of China ABSTRACT: The thermal stability characteristics of aviation kerosene were investigated using a heated tube at a constant heat power of 1087 W, mass flow rate of 1 g/s, and supercritical pressure of 3 MPa. Deposition rates on the tube walls were measured by weight. Each test lasts for 105 min. The inner diameter of the reactor was varied at 2, 4, and 6 mm to simulate different flow patterns, including residence time, Reynolds number, and temperature. Various flow states in local sections were taken to investigate the effect of Reynolds numbers on the deposition rate, including sub- and supercritical temperatures and laminar, transition, and turbulent flow. It is shown that the influences of Reynolds numbers on the deposition mainly depend upon the thickness of sublayer, where the oxidative deposit precursors are formed. The deposition rate correlation is modified by the experimental data in the laminar flow regime. Turbulent pattern and supercritical temperature enhance heat transfer of fluid and the mass transfer of oxidative deposits to the wall.
1. INTRODUCTION Gums and deposits will form when jet fuel is thermally decomposed within the fuel cooling system, which can block the engine nozzles, cause deterioration of atomization, and even damage the combustor.1 Excessive deposit accumulation affects the passage of the injector, which, in turn, impedes the fuel−air mixing and increases the emissions of soot and unburned fuel. Thermal oxidative deposits result from fuel reactions with the dissolved oxygen in fuel and occur as fuel temperatures ranged from 150 to 450 °C.2 Because the flow of fuel changes over a wide range of Reynolds numbers with the variation of the fuel temperature in an air−fuel exchanger, it is crucial to investigate the impact of the fuel pattern, such as Reynolds numbers and fuel temperature, on the deposition rates.3 Although a number of experimental studies4−13 have been reported, the influence of the flow pattern on the thermal decomposition is not yet conclusive. The earliest investigation on the deposition and Reynolds number was conducted by Vranos and Marteney4 on different fuels. A decrease in the deposition rate was reported with an increase in the flow rate range from 2.14 to 22.2 g/s and with outlet Reynolds numbers from 6000 to 34 290. They attributed the results to a decrease in the residence time of the fuel in the heated region. Peat5 considered the effects of the flow velocity on deposition in a heated tube at flow velocities from 1.06 to 5.72 m/s, maintaining the flow in the turbulent regime. It was concluded that the total deposit in the tube sharply decreased as the flow velocity increased. However, with the same apparatus, Kendall et al.6 found no difference in the deposition rate over a 2-fold change in the flow rate with Reynolds numbers of 10 000 and 20 000. In the investigation of Giovanetti et al.7 on aviation fuel RP-1 in a heated copper, it was found that, under the wall temperature below 376 °C, the rate of carbon deposition decreased with increasing the fuel inlet velocity in the range from 6 to 30 m/s. However, for the wall temperature in the range of 376−427 °C, the rates of carbon deposition increased as fuel velocity increased. The opposite results were ascribed to the fact that the rate of © 2015 American Chemical Society
deposit formation was limited by the kinetics of the carbon deposition processes at low wall temperatures. Moses8 studied the role of heat and mass transfer in the deposition process when fuel is passing over heated surfaces. It was demonstrated that the deposition rate decreases with increasing the Reynolds number ranging from laminar flow (500−2200) to turbulent flow (5000−20 000). Nevertheless, when the fuel was preheated, the results showed an increase in deposition with an increased Reynolds number, which was ascribed to the chemical reactions for precursor formation to occur within the core flow. Edwards and Krieger9 have conducted the tests at differing flow rates and tube diameters to examine the impact of residence time and flow velocity on deposition. It was noted that the deposition rates [parts per million (ppm) designed as dividing the carbon deposit weight (μg) by the total amount of fuel used in the test (g)] for Jet A 2926 were relatively independent of the flow rate and tube diameter over a wide range of conditions. Edwards and Joseph10 found that, in the tests with the flow rate from 12 to 200 mL/min, the deposition rate increased as the flow rate increased. In the study of Chin and Lefebvre,11 the variation in Reynolds number was accomplished by both varying the fuel flow rate and tube diameter. It was found that the deposition rate rose with an increase in Reynolds number, which was attributed to the higher heat transfer between the wall and the fuel and the higher transverse velocity components, which were more effective in transporting material from the mainstream fuel flow to the tube walls. Moreover, Vranos et al.12 observed that the coking rate in the specimenlocated section rose as the Reynolds number increased in the isothermal test. Marteney13 investigated the effect of the flow velocity on the thermal decomposition of JP-5 in long duration tests. It was found that the deposition rate in a high flow velocity of 0.3 m/s was about 10 times greater than that at the Received: May 27, 2015 Revised: August 17, 2015 Published: August 18, 2015 6088
DOI: 10.1021/acs.energyfuels.5b01180 Energy Fuels 2015, 29, 6088−6094
Article
Energy & Fuels
Figure 1. Experimental setup.
Table 1. Characteristics of RP-3 Aviation Kerosene property
critical pressure (MPa)
critical temperature (°C)
density (g/cm3)
flash point (°C)
distillation range (°C)
relative molecular weight (g/mol)
averaged molecular formula
value
2.39
372.5
0.7913
50
163−212
148.33
C10.5H22
deposition to be formed, with the constant mass flow rate of 1 g/s for all of the experiments. Wall temperatures along the tube were measured by 18 K-type thermocouples that were equally stuck to the outside of the tube. The fuel temperatures at the inlet and outlet were measured by a tee that enabled thermocouple inserted into the flow. The temperature measurement error was ±1.5 °C. In the flowing system, the three common methods to measure the deposition are carbon burnoff,17 pressure difference method,18 and weight method.19 In the carbon burnoff method, a multiphase carbon analyzer is applied to determine the products of the burned deposition. In the pressure difference method, a differential pressure transmitter is used to measure and track in time the reactor pressure drop, which is indicative of coke deposition. In the weight method, the reactor is cut into segments, which are measured by a high-precision balance before and after each test. The accuracy of the burnoff method is the highest, but it is very complex and needs a long period. The pressure difference method enables the investigation of the coking process with time, while this method is suitable for a large amount of deposition, in that it is widely used in the pyrolysis coke rather than the oxidation deposition. The deposit weight of each section allows for an estimate of the formation rate as a function of the length, which was comprehensively considered and used in the study. A commercial software, i.e., Ansys Fluent,20 was applied to solve the steady and three-dimensional flow as well as heat transfer in the straight tube. The turbulent model of standard k−ε was used because of the transition from laminar to turbulent flow. The thermal physical properties of the aviation kerosene were determined using a threecomponent surrogate model by the previous work of the authors.21 Reynolds number is the key dimensionless parameter for the determination of the flow pattern in pipe and defined as Re = 4ṁ / πdμ for a round pipe, where ṁ is the mass flow rate (kg/s), d is the inner diameter (m), and μ is the viscosity of flow (Pa s).
lower flow velocity of 0.076 m/s under the wall temperatures below 260 °C. It is obvious that there are conflicting results in the literature for the effect of Reynolds number on the deposition. The reasons for the different conclusions in the literature may be ascribed to different experimental devices, flow conditions, various fuel samples, and the way to change the Reynolds number. The Reynolds number can be modified by changing the mass flow rate or velocity, tube diameter, and fuel temperature. Because many interactions can occur, the study should isolate individual effects. Furthermore, previous experimental studies mostly investigated the global total deposition or deposition rate under a certain condition for an isothermal heating period. The non-isothermal heating condition and the local deposition received less attention, which are common in actual heat exchangers and nozzles on aircraft. It is important to better understand the role of different flow patterns on the local heat transfer and deposition. In this study, the effects of variations in flow patterns on the local deposition rates were examined, including residence time and Reynolds number changed by the different inner diameters and heating of the reactor tubes.
2. EXPERIMENTAL SECTION Numerous devices have been constructed and used to examine various parameters of thermal oxidation stability. The apparatus was designed for specification purposes, listed as ASTM coker modification,14 nearisothermal flowing test rig (NIFTR),15 and jet fuel thermal oxidation tester (JFTOT).16 Because the study focused on the local properties of the reactor, the experimental setup was designed as Figure 1. An indirect electrically heated tube was used to simulate the heat exchanger. Chinese No. 3 (RP-3) kerosene was selected as the fuel. The properties of RP-3 are given in Table 1. A straight 316 stainless-steel tube with a length of 450 mm and three different inner diameters of 2, 4, and 6 mm were used in the coking experiments. The parameter of the experiments is given in Table 2. The typical aircraft engine-operating condition was simulated by the back-pressure valve to maintain the supercritical pressure of 3 MPa. The heating power of 1087 W was controlled to be uniform, and the heating lasted for 105 min, guaranteeing enough oxidation
3. RESULTS AND DISCUSSION Physical factors, including the temperature, pressure, and Reynolds number, play a major role in the deposition and heat transfer of fuel during the heating. Thus, the experiments were in the same flow rate of 1 g/s and employed three different inner diameters of the tube, listed as 2, 4, and 6 mm, corresponding to the different inlet Reynolds number given in Table 3. The Reynolds number was calculated on the basis of the viscosity coefficient and density that are varied with the fuel temperatures.21 As the heating process, a variation of the flow state along the tube in each case occurs, including sub- and supercritical temperature conditions and laminar, transition, and turbulent flow regimes that have an obvious effect on heat
Table 2. Experimental Parameters parameter
heating power (W)
heating period (min)
inlet mass flow rate (g/s)
pressure (MPa)
value
1087
105
1
3 6089
DOI: 10.1021/acs.energyfuels.5b01180 Energy Fuels 2015, 29, 6088−6094
Article
Energy & Fuels Table 3. Total Coke Deposition at Different Cases
case 1 case 2 case 3
inner diameter (mm)
average residence time (s)
Re at the inlet (35 °C)
average Re
total mass of deposition (mg)
deposition rate (mg cm−2 h−1)
2 4 6
0.7915 2.6705 6.262
865 432 250
14479.09 1927.605 1623.523
5.13 9.691 13.56
0.10343 0.0977 0.09143
reactions, physical factors, including high fuel velocity, varying Reynolds numbers, and non-uniform wall and fuel temperatures, restrict the availability of reactants in the boundary layer. An increase in the flow velocity and Reynolds numbers serves to replenish the reactant concentration and, thus, to increase the reaction and deposition rate within the boundary layer in unit area. The deposition rate depending upon the Reynolds number implies that the deposition reaction is partially diffusion-controlled. In Figure 3, the deposition rates of three cases were expressed as a function of the axial distance and fitted by a
transfer. In these conditions, it is more convenient to analyze the effects of local properties on the deposition rate. In Table 3, the residence time in the experiments changes significantly with the enlargement of tube diameters and the entrance Reynolds number is reduced accordingly. As shown in Figure 2, the residence time also decreases sharply along the
Figure 2. Residence time profiles along the axis of the tube.
tube as a result of the drop in fuel density as the temperature increases. The fluid residence time in the tube at the beginning of the experiment in case 2 is almost 4.5 times longer than that at the end of the experiment. In addition, the residence time decreased quickly at the entrance section. Although, beyond the entrance sections, the residence time of the three cases stays stable along the tube, there is an accelerated decrease in residence time after the location of 300 mm in case 1 as a result of the effect of transition from the sub- to supercritical temperature introduced afterward. In Table 3, there is just an inverse relationship between the total amount of deposition and the Reynolds number defined at the tube inlet. Because the mass flow rate is constant, the increase of the Reynolds number influences the residence time. Thus, the total mass of deposition increases with the residence time. The amount of deposition in case 3 is about 2.6 and 1.4 times as much as that of cases 1 and 2, respectively. In this condition, more fuel involves the oxidation deposition reaction with an increase in the inner surface area as a result of the mass flow rate controlled as a constant for all cases. It means that, in the real cooling system, more deposition precursors are produced in the bigger diameter tube, moreover, inducing a great abundance of deposit in the nozzle. However, the deposition rate (mg cm−2 h−1) is slightly enhanced as the inlet Reynolds number increases. In comparison to case 1, the deposition rate decreases by 5.5 and 11.6% in cases 2 and 3, respectively. It implies that a reduction in tube diameter causes an increase in deposit thickness. For the oxidation deposition
Figure 3. Coke deposition rates along the axial for various inlet Re.
polynomial. First of all, it is observed in that there are more oxidative deposits located at the inlet section of the tube (
