Experimental and Numerical Study on Oxidation Deposition

In the fuel cooling system of an engine, the heating of aviation kerosene causes it to exhibit complicated, ..... Power 1993, 9 (1), 5– 9, DOI: 10.2...
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Experimental and numerical study on oxidation deposition properties of aviation kerosene Xinyan Pei, Lingyun Hou, and William L. Roberts Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b01250 • Publication Date (Web): 08 Jun 2018 Downloaded from http://pubs.acs.org on June 8, 2018

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Experimental and Numerical Study on Oxidation Deposition

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Properties of Aviation Kerosene

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Xinyan Peia†, Lingyun Hou*b‡, William L. Robertsa†

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a

Clean Combustion Research Center, King Abdullah University of Science and Technology Thuwal 23955-6900, Saudi Arabic b

6 7 8 9

School of Aerospace Engineering, Tsinghua University Beijing 100084, China

Tel.+86-10-62772157. E-mail: [email protected]. Abstract

10

In the fuel cooling system of the engine, the heating of aviation kerosene exhibits complicated,

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unsteady physicochemical processes and forms the undesirable coke deposition. To understand

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these processes better, we investigated the coupling relationship between turbulent flow, heat

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transfer, autoxidation, and deposition reactions of fuel in a cooling heat exchanger. The experiments

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were performed to investigate the whole process within 105 min, separated into five continuous

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phases of 20 min, 40 min, 60 min, 80 min and 105 min, with a heat flux of 38.6 kW/m2. Based on

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the experimental results, we established a three-dimensional model to study the influence of

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kerosene’s heat-transfer process on oxidation deposition in a long, straight, horizontal pipe under

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supercritical pressure condition. A modified six-step, pseudo-detailed chemical kinetic and global

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deposition mechanism has been incorporated into the numerical model with particular attention to

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temperature variation. The model was validated based on the quantity of deposition and dissolved

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oxygen consumption rate under different experimental temperatures and heating times. We then

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analyzed the fluid dynamics profiles and physical parameters of density, specific heat, viscosity and

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Reynold number, species, and deposition rates along the reactor, micrographs and surface elements

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of deposition at various temperatures to understand the coupling effect between heat transfer and

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coke deposition. Results indicated that supercritical characteristics of both the fuel and deposition

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affect the local heat-transfer characteristics, resulting in some instabilities in the wall temperature

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distribution. The fuel temperature determines the regime of the chemical reactions in the flow and

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that the flow conditions and wall temperature determine the deposition rate at the local position of

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the inner surface.

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Nomenclature Pre-exponential factor A Specific heat at constant pressure Cp d E

Diameter Activation energy for the reaction

r Ji

Diffusion flux of species i

p

Effective conductivity Pressure

Qr

Heat of the surface reaction

R

Universal gas constant

SE

Heat source term

Si

Species source term

T x

Temperature Axial coordinate

Yi

Mass fraction of species i

r v

Velocity vector

ke

__

Pa

J/kmol∙°C

°C m

m/s

τ

Stress tensor

ρ

Density Temperature exponent

β

J/kg∙°C m J/kmol

kg/m3

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ω& r ,s 31

Rate of the surface reaction

1. .Introduction

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With advances in the power of aircraft, gas turbine engines must now operate at a higher

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compression ratio and higher turbine inlet temperatures. Aircraft engines therefore experience

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greater heat load and operate in complex thermal environments that include heat effects from the

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combustion system, aerodynamics, hydraulics, and electronics. These engines require elaborate

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cooling systems to deal with the heat load. Fuel-cooled technologies can be used in an indirect

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cooling system, such as a cooling cooled air (CCA) system,1 the cooled air takes the heat from the

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hot parts, passes it to the fuel and then cools other thermal components. In comparison with air-

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cooling systems, fuel-cooled cooling systems are more efficient due to the supercritical

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characteristic of aviation kerosene and the heated fuel from the fuel-cooled system can also return

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thermal energy to the engine’s thermodynamic cycle after being burnt in the combustor. However,

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when hydrocarbon fuel is heated, it experiences deposition reactions that form insoluble particles on

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surfaces, which can decrease the heat transfer and even block the cooling passage. Hydrocarbon

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fuels follow two processes during thermal decomposition, including autoxidation and pyrolysis.

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Autoxidation reactions require dissolved oxygen in the fuel and occur at or above 150°C. When the

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temperature increases above 450°C, pyrolysis reactions dominate, involving the breakdown and

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recombination of hydrocarbon chains.2, 3

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Many experimental

4-6

and numerical investigations

7-10

have been performed with the aim of

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understanding the deposit formation and oxidation processes of hydrocarbon fuels. The formation

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of deposition coke in fuel during the heat-transfer. In general, amount of dissolved oxygen,11, 12

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cooling channel geometries,13-15 operational conditions,16,

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transfer characteristics,19 additives,6 heating time,5,

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characteristics of a system. In a limited oxidation deposition mechanism study, a nine-step model

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was developed by Katta et al.22. Ervin et al.23 modified this model by focusing on surface

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deposition in the cooled regions. Zabarnick et al.24 and Ervin et al.25 further extended the model to a

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17-step, pseudo-detailed mechanism by considering the effects of oxygen consumption on

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deposition. To improve dissolved-oxygen consumption predictions, Doungthip et al.26 calculated

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catalytic deposition reactions into the mechanism. Kuprowicz et al.27 refined the model to include

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21 steps by including reactions of sulfur species, phenols, hydroperoxides and dissolved metals.

20

17

fluid dynamics,13 buoyancy,18 heat

and temperature,4,

21

affect coking

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Most studies, including mechanisms investigation, of the thermal stability of hydrocarbon fuels

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are conducted under isothermal conditions. However, the fuel coolant passes through various

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geometries and thermal conditions in the fuel supply and cooling system. Under conditions of

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incomplete oxygen consumption and non-thermal equilibrium, the interactions among the

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consumed dissolved oxygen, heat transfer and deposition rate are complex but knowledge of these

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interactions, especially when coupled with temperature, is necessary to design fuel cooling systems.

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Therefore experimentally and numerically studies were conducted to investigate the coupling

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effects between heat transfer process and autoxidation in the coke deposition formation under

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different heating times and temperatures. In the experimental study, these processes were

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investigated coupling fluid flow dynamics, heat transmission, autoxidation chemical reactions, and

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oxidation deposition over 105 min. Then, our experimental results were used to compute a three-

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dimensional simulation of the fluid dynamics and heat-transfer characteristics of fuel in a circular

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tube under supercritical pressure condition. The global autoxidation mechanism and the kinetic

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deposition model were developed to simulate the oxidation deposition process of aviation kerosene.

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2. Experimental Apparatus

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In Table 1, Chinese No. 3 aviation kerosene (RP-3) was used as the fuel in our experiments.28

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A small-scale test rig was designed to heat the fuel through a heat-exchanger under different

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temperatures and heating times, which was to stimulate the heat transfer process in the cooling

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passage.

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In Figure 1, the present experimental system includes a fuel supply subsystem with a fuel

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container and a plunger metering pump, a heating subsystem, a parameter control and measurement

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subsystem, and an online sampling analysis subsystem. The detailed experimental parameters are

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given in Table 2.The inlet mass flow rate was kept constant at 1 g/s for all conditions. The SS316

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tube (the inner diameter of 2 mm, the wall thickness of 0.5 mm, the total length of 450 mm) was

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applied as the heat exchanger and reaction section in the experiment. The tube was prepared to be

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flushed by the water and N2 and dried before the experiment to clean manufacturing residues. The

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test tube was in the horizontal arrangement, and an indirect electrical heater simulated the heat flux. 5 ACS Paragon Plus Environment

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The following fuel pressure was monitored by two pressure gauges at inlet and outlet of the test

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section and maintained using a pressure damper and a backpressure regulator at 3 MPa under the

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supercritical pressure condition. The outer-wall temperatures of the pipe were measured using 18 K-

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type thermocouples spot-welded at 2.5 cm intervals along the tube. The fuel temperatures of inlet

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and outlet were measured using K-type thermocouples inserted into the center of the tube. The

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dissolved oxygen concentration was measured and monitored before and after the tests via two

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continuous oxygen sensors (InPro 6850i) based on the polarographic method with the accuracy of

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±1 % +6 ppb.11, 29 The oxygen sensor was separated from the sample by a membrane, which was

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only permeable to oxygen but prevented detrimental components from affecting the measurement.

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The oxygen was electrochemically measured as a current to calculate the oxygen partial pressure at

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the cathode. The current output is proportional to the concentration of oxygen. A polarization

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voltage was applied between the anode and the cathode to obtain the entirely linear relationship

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over the whole measurement range from 0 to 100% oxygen concentrations.

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After heating, the fuel was cooled using a counter-flow water-fuel exchanger and collected for

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GC/MS analysis. The tube was blown by nitrogen to clean the residual fuel in all pipes before

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drying. After each experiment, the tube of the reaction part was cut into 11 equal sections by a wire-

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electrode cutting for weighing the deposition. The repeatability test and the more detailed

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experimental setup information can be referred to our previous work.11, 15

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In the heat-transfer experiments, the fuel reached thermal equilibrium after some time.

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Therefore, five heating times was performed at different times to guarantee the flow to heat balance

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and sufficient deposit sample for the test. For the investigation of the effect of heat transfer process

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on the deposition process, five heating times, i.e., 0-20 min, 0-40 min, 0-60 min, 0-80 min and 0-

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105 min, were conducted in our experiments. The selection of heating time was based on our

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previous heat transfer experiments that the fuel comes to be thermal equilibrium after 105 min

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under the same conditions in Table 2.30

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For all the experiments, the inlet mass flow rate was set at 1 g/s; the initial fuel temperature

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was 20°C; the dissolved oxygen concentration was 10 ppm; the heat flux was 38.6 kW/m2. These

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parameters remained the same under the supercritical pressure of 3 MPa for all heating times. In

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this way, each heating time corresponded to a specific outlet fuel temperature listed in Table 2. We

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used a scanning electron microscope (SEM, FEICO95-001) to study the deposition microstructure

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morphologies of samples from each experiment, and an X-ray photoelectron spectroscopy (XPS,

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Thermo Fisher ESCALAB 250Xi) to measure the surface elements of the coke deposits, and gas

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chromatography/mass spectrometry (GC/MS, Agilent 7890B–5975A) to determine the composition

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of the residual fuel. Results of the error analysis on the measured parameters and transfer formula

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are presented in Table 3.

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3. Numerical Methods

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The Ansys Fluent package 31 was used in conjunction with a user-defined function to simulate

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the fluid-thermal-solid interactions coupled with chemical reactions. The governing equation in the

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fluid region is: 7 ACS Paragon Plus Environment

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r ∂ ( ρφ ) (1) + div(ρ uφ )=div(Γφ gradφ )+Sφ ∂t where φ are generalized variables, which can stand for a variety of physical quantities, such as

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velocity, enthalpy, and turbulence parameters. Γφ is the generalized diffusion coefficient and a

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representation for the transport properties such as viscosity or conductivity. And Sφ is the

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generalized source term such as heat generation in a fluid, production of a chemical species in a

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reaction, and the generation of turbulence kinetic energy.32 In the solid region, the thermal

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conduction equation is solved as ∇⋅ (λ∇T ) = 0 . The surface boundary conditions of the species and

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energy equations are set as follows: ρ Di

λs

∂Yi ∂n

∂T ∂n

= Wi ∑ (bir − air )ω& r ,s

(2)

= ∑ Qrω& r ,s

(3)

r

w

w

r

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The temperature-dependent thermal properties of the fuel were defined by the three-component

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surrogate model,28, 30 including density, specific heat (Cp), viscosity and thermal conductivity. The

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convection terms were discretized in a second-order upwind scheme. The RNG k-ε model,33 was

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used for simulation of the turbulent flow, which predicted the convective heat transfer under

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supercritical conditions well.34, 35 In Figure 2a, the inner diameter of the circular reactor tube is 2

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mm with a wall thickness of 0.5 mm and the total length of the tube is 450 mm. The outer wall

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temperature profiles obtained from the experiments were used as the outer surface boundary

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condition along the tube, and the inner surface was set as a no-slip wall. The mesh of the cross-

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section is shown in Figure 2b with the compressed mesh in the solid region, as the oxidation

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deposition is produced on the inner surface of the tube. The total number of the mesh cells is

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488,920 and the boundary layer of the inter-surface between solid and liquid zone uses 100,076 8 ACS Paragon Plus Environment

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cells. Steady simulation results at different times were used to describe the whole heat-transfer and

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deposition process. The residence time and the heat-transfer process indicated by thermal

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conductivity of the tube wall in the experiments are given in Figure 3. The thermal conductivities

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were calculated from the experimental data such as fuel and wall temperatures and heat flux. Then

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the calculated thermal conductivities were utilized in the setting of the boundary conditions of the

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tube interface for the heat-transfer simulation between the inner surface and the fuel. In Figure 3,

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the residence time of the fuel in the tube gradually decreases as the temperature increases. The

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density of the fuel decreases as the fuel temperature increases, which increases the velocity of the

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fuel in the tube. In addition, the residence time is also affected by the thickness of the deposition

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layer increasing with the heating time due to the change of the inner diameter. But the effects of the

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deposits on the flow were neglected since the thickness of the deposition layer were very thin,

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which was observed in the morphological investigation on deposition.13, 19 The difference in the

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wall thermal conductivity between the ideal condition of Stainless Steel 316 and that at the end of

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the experiment indicates the accumulated effect of deposition on heat transfer.

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The composition of the kerosene is very complicated, and the detailed reaction process is not

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very clear at present. The modified pseudo-detailed deposition mechanism model of supercritical

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RP-3, based on our previous mechanism,9 is given in Table 4, where fuel, precursor, bulk solubles,

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bulk insolubles, and deposition represent five pseudo components. Dissolved oxygen (O2)

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stimulates the oxidation chain reactions. The detailed kinetic parameters of the model are listed in

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Table 4. The main modification is focused on the temperature exponent. The reaction rate constant

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is based on the Arrhenius expression of k = AT β exp( - E ) at each component step, and β is the

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temperature exponent.

RT

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During the heat-transfer process, the fluid fuel experiences a range of different liquid and wall

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temperatures. Thus, the model in this study was developed with particular focus on temperature by

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adding the temperature exponent in the reaction-rate expression comparing to the previous model.

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The mechanism was divided into three bulk fuel reactions that occur in the liquid phase and three

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surface reactions at the wall. The total deposition rate was calculated by adding the three wall

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reactions together.

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4. Results and Discussion

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The numerical simulation model is verified by assessing the experimental results capturing the

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total mass of the deposition and the dissolved oxygen concentration at the outlet in relation to the

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variation in fuel temperature in Figure 4. The numerical results based on the reaction mechanism

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described here are compared with a previous model. In comparison with the previous model, the

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modified model focuses on temperature by adding the temperature exponent in the reaction-rate

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expressions. In this way, we found that the consumption of dissolved oxygen is sensitive to changes

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in temperature in the modified model. The predicted results with the modified model follow the

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same trend as the experimental data. The total mass of the deposition follows a nonlinear increasing

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trend with increasing fuel temperature in both the measured and predicted results. The dissolved 10 ACS Paragon Plus Environment

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oxygen concentration in the fuel follows a decreasing trend with increasing fuel temperature due to

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continued consumption during deposition reactions. This oxygen consumption profile means that

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the increased oxygen consumption rate enhances the rate of autoxidation reactions in the liquid

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phase. Nearly 70% of dissolved oxygen is consumed when heating time lasts 105 min. The

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concentration of oxygen stimulates the chain-reaction oxidation processes. The fuel molecules react

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with dissolved oxygen to form alkyl-peroxides, and in turn, these alkyl-peroxides develop into

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deposition precursor species. Ultimately the precursor grows into particles that form deposits. The

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deposition rate and oxygen consumption both sharply increase above 300°C at 60 min due to the

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effects of temperature and the supercritical properties of the fuel as it will be discussed later while

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looking on the computed fuel properties.

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Temperature is a critical factor in the autoxidation process including both fuel and wall

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temperatures, which in turn affect oxidation deposition reactions. However, the influence of

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temperature on the deposition process is very complicated because the temperature simultaneously

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affects the flow conditions and the fuel’s properties and compositions. There is a strong coupling

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relationship between oxidation deposition and convection heat transfer in the presence of chemical

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reactions. Profiles of the wall and fuel temperatures along the reactor in relation to heating time are

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given in Figure 5 to demonstrate the influence of temperature on deposition during the heat-transfer

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process. The simulated fuel temperatures at the outlet at different times are in acceptable agreement

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with the measured data within about 1% error. The centerline fuel temperatures are almost identical

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to the bulk average temperatures of the cross-section of the tube and are significantly lower than the

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measured wall temperatures. This result indicates the different heat-transfer characteristics of the

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fluid in different locations. The wall temperatures rapidly increase within 50 mm from the inlet.

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However, the fuel temperature remains relatively low. At 150 mm from the inlet, the outer wall

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temperatures increase only a little along the axis of the test section in all cases, but the fuel

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temperature continuously increases along the tube and follows a similar trend in each case. The only

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difference is the rate of increase over time. Wall temperatures along the tube rise gradually to the

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thermal equilibrium in relation to the heating time. The supercritical characteristics of the fuel and

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deposition affect the local heat-transfer characteristics, resulting in some fluctuation in the

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distribution of the wall temperature.11 Therefore, the fuel temperature defines the regime of the

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chemical reactions in the flow and the flow conditions and wall temperature define the deposition

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rate at a specific location at the inner surface of the tube.

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As discussed above, the consumption of dissolved oxygen can reflect changes in autoxidation

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and deposition processes. In addition, the precursor is critical to the formation of deposits10. The

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profiles of oxygen and the deposition precursor are shown in Figure 6a. It is noted that by

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comparing Figure 4 and Figure 6a that the oxygen consumption rate is not directly correlated with

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the temperature. Low oxygen consumption rates near the inlet (within 150 mm) are reasonable since

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temperatures in this location are lower than the autoxidation temperature range for all cases. The

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low oxygen consumption rate and the low precursor production rate at 20 min corresponding to the

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outlet fuel temperature at 160°C are expected because the temperature is at an initial temperature of

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the autoxidation regime. The consumption of oxygen gradually becomes evident after 40 min. The

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autoxidation reaction rate depends on both amounts of the radical species and temperature. Rapid

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oxidation occurs as the number of radical species becomes substantial. These results indicate that

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the oxygen concentration and the amount of precursor are in a trend of a slow reaction rate followed

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by a gradually fast oxidation rate along the axial direction of the tube. The oxygen is consumed with

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an increase in the precursor. The deposition precursor is believed to form solubles at higher

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temperatures and only starts to precipitate from the bulk fuel onto the wall as it reaches a specific

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temperature. The profile of precursor at 105 min become gradual after 350 mm at the axial of the

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tube.

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The physical properties of fuel under supercritical pressure determine the distribution of species

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and affect the deposition. The properties of the supercritical fluid are sensitive to temperature,

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especially in the pseudo-critical region. In Figure 6b, the specific heat of the fuel increases at 105

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min and the fuel temperature increases to the pseudo-critical temperature zone. The heat transfer of

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the flow increases significantly after 350°C. The Reynolds number was mainly defined by the basis

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of the viscosity coefficient and density with the variation of fuel temperature in Figure 5. As

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presented in Figure 6c, the viscosity profiles decrease and the Reynold numbers of the flow increase

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along the axial of the tube for all cases with increasing temperature. The flow is laminar within 60

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mm of the axial of the tube. After 20 min, the flow is fully developed and mainly turbulent as the

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heating process. The Reynold number at 105 min increases sharply at 300 mm of the tube and

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reaches more than 40,000 at the exit of the tube with significant changes in the density and viscosity

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because the fuel temperature reaches the pseudo-critical value.

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After each experiment, the residual fuel was collected at the outlet of the tube. And

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compositions of the residual fuel samples, analyzed by GC/MS, at different temperatures are given

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in Figure 7. The composition of original RP-3 can be divided into four typical classes as straight-

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chain alkanes (54.4 wt%), cycloalkanes (21.3 wt%), aromatic hydrocarbons (21.5 wt%) and others

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(2.7 wt%). As the fuel temperature increases into autoxidation-reaction range, the amount of straight

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chain alkanes decreases by nearly 50% in all cases. There are no significant changes in the

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proportion of aromatic hydrocarbons, cycloalkanes, and olefins with different fuel temperatures. In

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general, naphthenic compounds have higher thermal stability compared with other components.

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Moreover, hydrogen donors such as naphthalene compounds are considered as high-temperature

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thermal stabilizers in jet fuel. There is a significant increase in oxygen-containing compounds in all

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cases in comparison with the original RP-3. Most of the straight chain alkanes are transformed into

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oxygen-containing compounds. In the detailed classification of components, these oxygen-

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containing compounds consist of acid esters, alcohols and ketones. In the autoxidation process,

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oxygen-containing compounds are critical in chain-propagation reactions and forming precursor

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species. In addition, it should be noted that fuel degeneration reactions do not influence deposit

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formation. Autoxidation and pyrolysis reactions occur in the liquid phase of fuel. Only a relatively

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Energy & Fuels

small quantity of fuel involves in oxidation reactions that finally form insoluble and solid deposits.

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In Figure 8, the deposition rates in all locations of the test section increase with heating time.

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The higher amounts of deposition near the inlet are ascribed to the entrance effect that is determined

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by the temperature gradient and boundary layer.11 In the surface elemental analysis by XPS in

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Figure 9, there are relatively high levels of carbon and oxygen, which account for 70-80% and 20-

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30%, respectively. The proportions of other elements are tiny and do not change much respecting

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temperature. Furthermore, there are similar apparent changes from 310°C to 360°C in the residual

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fuel composition as indicated in the SEM analysis during deposition due to the supercritical

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properties of the fuel.

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SEM micrographs of deposition samples at the same position of the test section (x=325 mm from

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the inlet) corresponding to different heating times are presented in Figure 10. The inner surface of

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the experiment at 0 min exhibits random ravines and embossments. The complicated structures

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enlarge the contact surface area and promote deposition from the fuel. The deposition at 20 min and

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40 min is not recognizable in the SEM images. The deposit obtained from a long heating time is

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denser than that from a short one. After 60 min, deposits significantly increase in the micrographs

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and are composed of amorphous and flocculent particles. Similar fluffy, cauliflower-like deposits

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were also observed by Ervin25 and Tao36. The deposition rate visibly accelerates from 60 min to 80

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min. The significant increases in the deposition rate and oxygen consumption after 60 min in Figure

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4 correspond to the slight increase in the oxygen-containing compounds with the decrease in the

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straight-chain alkanes in the residual fuel at 360°C. These results indicate that the morphology of

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deposited carbon is strongly affected by the operating temperature.

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5. .Conclusions

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Detailed experimental and numerical studies of the oxidation deposition characteristics of

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aviation kerosene during the heat-transfer process were conducted to evaluate the critical effects of

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the temperature and heating time. The CFD model with a modified six-step autoxidation deposition

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mechanism, considering flow dynamics, heat-transfer process, chemical reactions of the fuel and

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the heat conduction in the tube wall, was used to obtain detailed information about the flow field

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and species distribution along the tube. The predicted fuel temperatures, total amount of deposits,

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and dissolved oxygen consumption during the heat-transfer process are conforming to the

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experimental results. The wall thermal conductivity and Reynolds number of the flow are

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significantly increased above a temperature of 360°C at 80 min due to the coke deposition and the

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properties of the supercritical fuel. Both the deposition rate and oxygen consumption rate sharply

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increase above a temperature of 300°C at 60 min. An apparent turning point is found in the

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micrographs and surface elements of deposition. Although the wall temperatures after 60 min are

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out of the range of oxidation reactions, the deposition process falls in oxidation deposition reaction

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regime, which is demonstrated by the SEM micrographs of the deposition and residual fuel

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components. Therefore, the regime of the chemical reactions in the flow depends on the fuel’s

295

temperature. The flow conditions and wall temperature determine the deposition rate at specific

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locations.

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Acknowledgment

298

This research described in this paper was supported by King Abdullah University of Science

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and Technology (KAUST). In addition, financial assistance provided by the National Natural

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Science Foundation of China (91641114) is gratefully acknowledged.

301

References

302

1. Walker, A. D.; Bharat, K.; Liang, G.; Peter, B.; Marco, Z., Impact of a Cooled Cooling Air System on the

303 304

External Aerodynamics of a Gas Turbine Combustion System. J. Eng. Gas Turbines Power 2017, 139, (5),

305

2. Hou, L. Y.; Dong, N.; Sun, D. P., Heat transfer and thermal cracking behavior of hydrocarbon fuel. Fuel

306 307

2013, 103, (Supplement C), 1132-1137.

308 309

Reforming for Hydrocarbon Fuel. J. Propul. Power 2012, 28, (3), 453-457.

310 311

Propul. Power 1992, 8, (6), 1152-1156.

312 313

Thermal and Oxidative Stability. J. Propul. Power 1993, 9, (1), 5-9.

314 315

Propul. Power 2008, 24, (2), 206-212.

316

Algorithm Optimization of a Chemistry Mechanism for Oxidation of Liquid Hydrocarbons. AIAA Journal

317 318

2005, 43, (10), 2259-2261.

319

a Range of Temperatures and Dissolved Oxygen Concentrations with Pseudo-Detailed Chemical Kinetics.

320 321

Fuel 2004, 83, (13), 1795-1801.

322 323

Energy & Fuels 2017, 31, (2), 1399-1405.

324

Nozzle Sections within Autoxidation Temperature Regime. University of Toronto , Toronto, Ontario, Canada,

325

2014.

326

11. Pei, X. Y.; Hou, L. Y., Effect of Dissolved Oxygen Concentration on Coke Deposition of Kerosene. Fuel

327 328

Process. Technol. 2016, 142, 86-91.

329

Spectra of Thermally Stressed Commercial Jet A-1 Aviation Fuel in the Autoxidative Regime. Energy &

330

Fuels 2012, 26, (4), 2191-2197.

051504-13.

3. Hou, L. Y.; Jia, Z.; Gong, J. S.; Zhou, Y. F.; Piao, Y., Heat Sink and Conversion of Catalytic Steam 4. Chin, J. S.; Lefebvre, A. H., Experimental Techniques for the Assessment of Fuel Thermal Stability. J. 5. Heneghan, S. P.; Locklear, S. L.; Geiger, D. L. I.; Anderson, S. D.; Schulz, W. D., Static Tests of Jet Fuel 6. Brown, S. P.; Frederick, R. A., Laboratory-Scale Thermal Stability Experiments on RP-1 and RP-2. J. 7. Wade, A. S.; Kyne, A. G.; Mera, N. S.; Pourkashanian, M.; Ingham, D. B.; Whittaker, S., Genetic-

8. Kuprowicz, N. J.; Ervin, J. S.; Zabarnick, S., Modeling the Liquid-Phase Oxidation of Hydrocarbons over

9. Pei, X.; Hou, L.; Ren, Z., Kinetic Modeling of Thermal Oxidation and Coking Deposition in Aviation Fuel. 10. Liu, Z. J. Chemical Kinetic and Thermal Numerical Simulation of Coking Process of Jet Fuels in Thin

12. Commodo, M.; Fabris, I.; Wong, O.; Groth, C. P. T.; Gülder, Ö. L., Three-Dimensional Fluorescence

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Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

331

13. Hua, J.; Ervin, J. S.; Zabarnick, S.; West, Z., Effects of flow passage expansion or contraction on jet-fuel

332 333

surface deposition. J. Propul. Power 2012, 28, (4), 694-706.

334

pressure in helical tubes. Appl. Therm. Eng. 2018, 128, (Supplement C), 1186-1195.

335 336

15. Pei, X. Y.; Hou, L. Y., Secondary flow and oxidation coking deposition of aviation fuel. Fuel 2016, 167,

337

16. Chin, J. S.; Lefebvre, A. H., Influence of Flow Conditions on Deposits from Heated Hydrocarbon Fuels.

338 339

J. Eng. Gas Turbines Power 1993, 115, (3), 433-438.

340 341

Kerosene. Energy & Fuels 2015, 29, (9), 6088-6094.

342 343

J. Thermophys. Heat Transf. 1995, 9, (1), 159-168.

344 345

Oxidation of Jet Fuel. J. Thermophys. Heat Transf. 2013, 27, (4), 668-678.

346 347

Temperature. J. Propul. Power 1986, 2, (5), 450-456.

348 349

Turbines Power 1992, 114, (2), 353-358.

350 351

Sci. Technol 1998, 139, (1-6), 75-111.

352 353

Regions in a Flowing System. Ind. Eng. Chem. Res. 1996, 35, (11), 4028-4036.

354 355

oxygen, and temperature effects. J. Eng. Gas Turbines Power 1996, 118, (2), 271-277.

356 357

1996, 35, (3), 899-904.

358 359

catalysis on the thermal oxidation of jet fuel. Energy & Fuels 2004, 18, (2), 425-437.

360

with chemical kinetic modeling for the prediction of autoxidation and deposition of jet fuels. Energy & Fuels

361 362

2007, 21, (2), 530-544.

363 364

Journal of Tsinghua University (Science and Technology) 2017, 57, (7), 774-779.

365

coefficient and oxygen uptake rate in a stirred tank reactor for uranium ore bioleaching. Annals of Nuclear

366 367

Energy 2013, 53, 280-287.

368 369

Aviation Kerosene. J. Aerospace Power 2015, 30, (9), 2122-2128.

14. Fu, Y.; Xu, G.; Wen, J.; Huang, H., Thermal oxidation coking of aviation kerosene RP-3 at supercritical

68-74.

17. Pei, X. Y.; Hou, L. Y.; Ren, Z. Y., Flow Pattern Effects on the Oxidation Deposition Rate of Aviation 18. Katta, V. R.; Blust, J.; Williams, T. F.; Martel, C. R., Role of Buoyancy in Fuel-Thermal-Stability Studies. 19. Jiang, H.; Ervin, J.; West, Z.; Zabarnick, S., Turbulent Flow, Heat Transfer Deterioration, and Thermal 20. Giovanetti, A. J.; Szetela, E. J., Long-Term Deposit Formation in Aviation Turbine Fuel at Elevated 21. Chin, J. S.; Lefebvre, A. H.; Sun, F. T. Y., Temperature effects on fuel thermal stability. J. Eng. Gas 22. Katta, V. R.; Jones, E. G.; Roquemore, W. M., Modeling of deposition process in liquid fuels. Combust. 23. Ervin, J. S.; Williams, T. F.; Katta, V. R., Global Kinetic Modeling of Aviation Fuel Fouling in Cooled 24. Zabarnick, S.; Zelesnik, P.; Grinstead, R. R., Jet fuel deposition and oxidation: dilution, materials, 25. Ervin, J. S.; Williams, T. F., Dissolved oxygen concentration and jet fuel deposition. Ind. Eng. Chem. Res. 26. Doungthip, T.; Ervin, J. S.; Zabarnick, S.; Williams, T. F., Simulation of the effect of metal-surface 27. Kuprowicz, N. J.; Zabarnick, S.; West, Z. J.; Ervin, J. S., Use of measured species class concentrations

28. Pei, X. Y.; Hou, L. Y., Effect of different species on physical properties for the surrogate of aviation fuel. 29. Zokaei-Kadijani, S.; Safdari, J.; Mousavian, M. A.; Rashidi, A., Study of oxygen mass transfer

30. Pei, X. Y.; Hou, L. Y.; Mo, C. K.; Dong, N., Thermo-physical Properties for Surrogate Models of 31. Ansys, Fluent 14.0 Theory Guide. In ANSYS inc, ANSYS inc: Canonsburg, USA., 2011; pp 390-391.

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Page 19 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

370

32. Johnson, R. W., Handbook of fluid dynamics. Crc Press: 2016.

371

33. Yakhot, V.; Orszag, S. A., Renormalization group analysis of turbulence. I. Basic theory. J. Sci. Comput.

372 373

1986, 1, (1), 3-51.

374 375

characteristics of China RP-3 aviation kerosene at supercritical pressure. Appl. Therm. Eng. 2011, 31, (14-

376

35. Wang, N.; Pan, Y.; Bao, H.; Zhou, J., Numerical investigation on supercritical heat transfer of RP-3

377 378

kerosene flowing inside a cooling channel of scramjet. Adv. Mech. Eng. 2014, 6, 11.

379

3 surface coke deposition under stable and vibration conditions. Energy & Fuels 2015, 29, (3), 2006-2013.

34. Li, X. F.; Huai, X. L.; Cai, J.; Zhong, F. Q.; Xuejun, F.; Zhixiong, G., Convective heat transfer 15), 2360-2366.

36. Tao, Z.; Fu, Y. C.; Xu, G. Q.; Deng, H. W.; Jia, Z. X., Thermal and element analyses for supercritical RP-

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Table 1 Properties of Aviation Kerosene.28

380

Properties Critical pressure Critical temperature Density Values

2.39 MPa

372.5°C

Flash point Distillation range Relative molecular weight

0.7913 g/cm3(20°C)

50°C

381 382

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163-212°C

148.33 g/mol

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Table 2 Experimental Parameters. Heating Time (min)

Outlet Fuel Operation Heat Flux Mass Flow Temperature (°C) Pressure (MPa) (kW/m2) Rate (1g/s)

Inlet Dissolved Oxygen (ppm)

20 40 60 80 105

160 260 310 360 410

10

3

38.6

1

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Inlet Fuel Temperature (°C)

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Table 3 Error Analysis. Items Error (%) Heat Flux 5.1 Temperature Difference 7.1 Mass Flow Rate 0.2 Pressure 1 Deposition Rate 6.6

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Table 4 Kinetic Parameters of the Oxidation Deposition Reactions of RP-3. Reaction No. Pre-Exponential A Activation Energy E Reaction Order α Bulk Fuel Reactions Wall Reactions

3

k1

1.8e-05

5e+07

[T] [F][O2]

k2 k3 k4

500 2 2e-11

4.1861e+07 8.4e+06 8.6e+06

[P] [P] [F][dT/dX]

k5

10

5e+07

[T]

k6

480

8.5e+07

[T]

0.04

[IN]

0.015

[P]

390 391

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Figure1. Schematic Diagram of the Experiment.

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Figure 2. (a) Computational Configuration of Convection Heat Transfer with Oxidation Deposition of RP-3 (Left) and (b) Cross-Section of the Grid (Right).

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Figure 3. Residence Time and Thermal Conductivity of the Tube in Relation to Temperature.

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Figure 4. Comparison between Experimental and Numerical Results of the Total Mass of Deposition and Dissolved Oxygen Concentration.

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Figure 5. Comparison of Experimentally and Numerically Derived Wall and Fuel Temperatures.

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Figure 6. Profiles of Simulation Results along the Axial of the Tube with Time: (a) Precursor and O2; (b) Cp and Density; (c) Viscosity and Re.

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Figure 7. Variation in Residual Fuel Components in Relation to Temperatures Determined by GC/MS.

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Figure 8. Experimental Deposition Rates along the Axial in Relation to Time and Temperature.

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Figure 9. Surface Deposition of Coke Elements Variation in Relation to Temperature.

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437 438 439

20 min

40 min

60 min 80 min 105 min Figure 10. SEM Micrographs at Different Heating times.

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