Experimental and Numerical Study on Oxidation Deposition Properties

Jun 8, 2018 - In the fuel cooling system of an engine, the heating of aviation ... The global autoxidation mechanism and the kinetic deposition model ...
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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

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School of Aerospace Engineering, Tsinghua University Beijing 100084, China

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

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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

13

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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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

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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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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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