A prediction model and a prediction method for oxidation coking reaction of hydrocarbon fuel

Abstract

The application discloses a prediction model and a prediction method for a carbon hydrogen fuel oxidation coking reaction, and relates to the field of carbon hydrogen fuels.The carbon hydrogen fuel oxidation coking process is simplified into two steps, a first step of simplifying the chain free radical reaction of the carbon hydrogen fuel into a total package reaction of the carbon hydrogen fuel directly generating a coking precursor, and a second step of simplifying the carbon deposition reaction of the coking precursor at a wall surface and the deposition process of the coke into a wall surface reaction of the coking precursor directly generating the coke at the wall surface.The reaction rates of the two steps are given in the form of the Arrhenius formula.The unknown parameters in the reaction rate calculation formula are solved based on a pseudo-microelement method, the prediction model for the carbon hydrogen fuel oxidation coking reaction is obtained, and the prediction model is embedded into a CFD solver to predict the carbon hydrogen fuel oxidation coking reaction.The application only comprises two chemical reaction processes, the prediction precision is improved by 20%, and accurate prediction of the carbon hydrogen fuel oxidation coking is realized at low calculation cost.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of hydrocarbon fuel, and particularly relates to a prediction model and a prediction method for oxidation coking reaction of hydrocarbon fuel. BACKGROUND

[0002] Hypersonic vehicles have high flight speed and wide flight envelope, and are of great significance in the fields of national defense, economy, science and technology, etc. With the continuous increase of flight Mach number, the wall temperature of the combustion chamber is continuously increased, and the thermal protection problem becomes the key to inhibit the further development of hypersonic vehicles. The regenerative cooling technology of cooling the outer wall of the combustion chamber by using the carbon hydrogen fuel carried by the hypersonic vehicle can not only achieve the cooling demand by using the high heat sink of carbon hydrogen fuel, but also preheat the carbon hydrogen fuel to improve the combustion efficiency, which is an effective means to solve the thermal protection problem of hypersonic vehicles. However, during the heat exchange and temperature rise process of carbon hydrogen fuel, when the temperature reaches 150 DEG C or above, the carbon hydrogen fuel will undergo oxidation coking reaction to generate a coke layer with low thermal conductivity attached to the wall surface, resulting in heat exchange deterioration and flow resistance increase, and even causing channel blockage to cause local over-temperature and over-limit. Therefore, the accurate prediction of the oxidation coking reaction of carbon hydrogen fuel is related to the successful implementation of the regenerative cooling technology, and is of great significance to the cooling scheme design of hypersonic vehicles.

[0003] The oxidation coking reaction of carbon hydrogen fuel contains hundreds of elementary chemical reactions, and the main method for predicting the oxidation coking of carbon hydrogen fuel is to embed a simplified kinetic model into CFD (Computational Fluid Dynamics) calculation. The existing models that can accurately predict the oxidation coking reaction generally contain more than 6 chemical reactions, such as the model disclosed in document I (Pei X, Hou L, Ren Z. Kinetic modeling of thermal oxidation and coking deposition in aviation fuel. Energy Fuels 2017; 31: 1399-1405.) which contains 6-step reactions, and the model disclosed in document II (Liu Z, Tang S, Li Z, Qiu Z, Qin Z, Yuan S, et al. An improved kinetic model for deposition by thermal oxidation of aviation hydrocarbon fuels. Fuel 2019; 258: 116139.) which contains 34-step reactions. The process is complicated and consumes a large amount of computing resources. SUMMARY

[0004] In order to solve the problems of too many chemical reactions, complicated process and large consumption of calculation resources in the application of the existing prediction kinetics model of oxidation coking of hydrocarbon fuel to the actual prediction process, the present application provides a prediction model and a prediction method of oxidation coking reaction of hydrocarbon fuel. The prediction method of oxidation coking reaction of hydrocarbon fuel provided by the present application only contains two chemical reaction processes, realizes accurate prediction of oxidation coking of hydrocarbon fuel at low calculation cost, and has important theoretical guiding significance for the design of the regenerative cooling scheme of a hypersonic aircraft.

[0005] The technical scheme adopted by the present application to solve the technical problems is as follows:

[0006] The prediction method of oxidation coking reaction of hydrocarbon fuel provided by the present application comprises the following steps:

[0007] The oxidation coking process of hydrocarbon fuel is simplified into two-step reactions, the first step is a total package space reaction, that is, the chain free radical reaction of hydrocarbon fuel is simplified into a total package reaction of direct generation of coking precursors by hydrocarbon fuel, and the second step is a wall reaction, that is, the carbon deposition reaction of coking precursors at the wall surface and the deposition process of coke are simplified into a wall reaction of direct generation of coke by coking precursors at the wall surface; the reaction rates of the above two-step reactions are given in the form of Arrhenius formula; unknown parameters in the reaction rate calculation formula are solved by a method based on the pseudo-element method to obtain a prediction model of oxidation coking reaction of hydrocarbon fuel, and the prediction model of oxidation coking reaction of hydrocarbon fuel is embedded into a CFD solver for prediction of oxidation coking reaction of hydrocarbon fuel.

[0008] Further, the reaction equation of the two-step reaction is:

[0009]

[0010]

[0011] The formula (1) is the reaction equation of the first step reaction, the formula (2) is the reaction equation of the second step reaction, F represents hydrocarbon fuel, a represents oxygen concentration index, O2 represents dissolved oxygen, P represents coking precursor, IN represents insoluble intermediate product, and D represents coke; τ represents reaction time; k'1 represents reaction rate constant of the first step reaction; k1 represents reaction rate constant of the first step reaction after ignoring the concentration change of hydrocarbon fuel itself; k2 represents reaction rate constant of the second step reaction.

[0012] Further, the calculation formula of the reaction rate is:

[0013]

[0014] wherein, k1 represents the reaction rate constant of the first step reaction, A1 represents the pre-exponential factor of the first step reaction, and represents the influence of the density of the hydrocarbon fuel on the transport process of the coking precursor to the wall surface; p b represents the average density of the hydrocarbon fuel; E a1 represents the activation energy of the first step reaction; R represents the molar gas constant; t b represents the qualitative temperature of the hydrocarbon fuel; k2 represents the reaction rate constant of the second step reaction; A2 represents the pre-exponential factor of the second step reaction; E a2 represents the activation energy of the second step reaction; represents the influence of the wall temperature gradient on the coking deposition process, wherein T w represents the wall temperature, and B represents a dimensionless parameter for the influence degree of the reaction wall.

[0015] Further, the specific process of solving the unknown parameters in the reaction rate calculation formula by the method based on the pseudo-element method is as follows:

[0016] The hydrocarbon fuel oxidation coking experiments under different lengths of flow pipelines are carried out, and the hydrocarbon fuel components and concentrations at the outlet position of the pipeline are measured first under the condition that other conditions are unchanged. Then, the hydrocarbon fuel components and concentrations at the outlet position of the short pipeline are regarded as the hydrocarbon fuel components and concentrations at the middle position of the long pipeline, so that the hydrocarbon fuel components and concentrations at each position in the middle of the pipeline can be obtained. Finally, each unknown parameter is solved by solving the reaction process at different positions in the long pipeline, and the prediction model of the hydrocarbon fuel oxidation coking reaction is obtained.

[0017] Further, the specific process of embedding the prediction model of the hydrocarbon fuel oxidation coking reaction into the CFD solver for predicting the hydrocarbon fuel oxidation coking reaction is as follows:

[0018] In one time step, for one grid, it is judged whether it belongs to the coking temperature range. If it does not belong to the coking temperature range, the oxidation coking reaction is not solved, and the process is directly ended. If it belongs to the coking temperature range, the first step reaction is solved, the local aviation kerosene concentration, dissolved oxygen concentration and temperature of the grid are read, the coking precursor concentration is solved, and then it is judged whether the grid belongs to the near-wall grid. If it does not belong to the near-wall grid, the process is directly ended. If it belongs to the near-wall grid, the second step reaction is solved, the local coking precursor concentration, temperature and temperature gradient of the grid are read, the coke generation rate is solved, and the coke generation mass and thickness are further solved. Further, the coke generation thickness is read to realize the grid movement simulation coke layer generation and deposition by the node spring method, and finally the precise prediction of the hydrocarbon fuel oxidation coking process is realized under low calculation cost.

[0019] Further, the specific method for further solving the coke generation mass and thickness is to calculate the coke generation mass by integrating the coke generation rate, and then to calculate the coke generation thickness by dividing the coke generation mass by the coke density.

[0020] This invention provides a predictive model for the oxidation and coking reaction of hydrocarbon fuels, including a two-step reaction equation and a formula for calculating the reaction rate. The two-step reaction equation is as follows:

[0021]

[0022]

[0023] The formula for calculating the reaction rate is:

[0024]

[0025] Formula (1) is the reaction equation for the first step reaction, and Formula (2) is the reaction equation for the second step reaction. F represents hydrocarbon fuel, α represents the oxygen concentration index, O2 represents dissolved oxygen, P represents coking precursor, IN represents insoluble intermediate product, and D represents coke; τ represents reaction time; k'1 represents the reaction rate constant of the first step reaction; k1 represents the reaction rate constant of the first step reaction after neglecting the change in the concentration of hydrocarbon fuel itself; A1 represents the pre-exponential factor of the first step reaction, which represents the effect of hydrocarbon fuel density on the transport process of coking precursor to the wall; ρ b E represents the average density of hydrocarbon fuels. a1 The value represents the activation energy of the first step reaction; R represents the molar gas constant; t b The qualitative temperature of the hydrocarbon fuel is represented by k2; the rate constant of the second-step reaction is represented by A2; the pre-exponential factor of the second-step reaction is represented by E. a2 This indicates the activation energy of the second-step reaction; This represents the effect of the wall temperature gradient on the coke deposition process, where T w The value represents the wall temperature, and B represents a dimensionless parameter used to reflect the degree of influence of the wall.

[0026] The beneficial effects of this invention are:

[0027] This invention provides a method for predicting the coking reaction of hydrocarbon fuel oxidation, which involves only two chemical reaction processes. Under 15 different operating conditions within the range of Reynolds number 5000-50000, pressure 2.5-3.5 MPa, temperature 200-450℃, and dissolved oxygen concentration 2-20 ppm, the deviation between the predicted and experimental values ​​of coking amount does not exceed 10%. Compared with existing hydrocarbon fuel oxidation coking kinetic prediction models, this invention improves the prediction accuracy by 20%, achieving accurate prediction of hydrocarbon fuel oxidation coking reaction with low computational resource consumption. It improves prediction efficiency while ensuring prediction accuracy and has important theoretical guiding significance for the design of regenerative cooling schemes for hypersonic vehicles. Attached Figure Description

[0028] Figure 1 This is a solution path diagram for a predictive model of the carbon dioxide oxidation reaction of hydrocarbon fuels according to the present invention.

[0029] Figure 2 The graph shows a comparison between the prediction results of the model of this invention, the prediction results of existing models, and experimental values ​​under 15 different working conditions. Detailed Implementation

[0030] In a first aspect, the present invention provides a method for predicting the coking reaction of hydrocarbon fuel oxidation.

[0031] The present invention provides a method for predicting the coking reaction of hydrocarbon fuel oxidation, the specific implementation process of which is as follows:

[0032] Step S1: Construct a predictive model for the carbonization reaction of hydrocarbon fuels;

[0033] S1.1: Simplified steps in the carbonization process of hydrocarbon fuels;

[0034] First, the carbonization process of hydrocarbon fuel oxidation is simplified into two steps. The first step is a packaged spatial reaction, which simplifies the chain radical reaction of hydrocarbon fuel into a packaged reaction in which hydrocarbon fuel directly generates carbonization precursors. The second step is a wall reaction, which simplifies the carbonization reaction of carbonization precursors at the wall and the coke deposition process into a wall reaction in which carbonization precursors directly generate coke at the wall.

[0035] The reaction equations for the two steps described above are as follows:

[0036]

[0037]

[0038] Formula (1) is the reaction equation for the first step reaction, and Formula (2) is the reaction equation for the second step reaction. Where F represents hydrocarbon fuel, α represents the oxygen concentration index, O2 represents dissolved oxygen, P represents coking precursor, IN represents insoluble intermediate product, and D represents coke; τ represents the reaction time; k1 represents the reaction rate constant for the first step reaction after neglecting the change in hydrocarbon fuel concentration; and k2 represents the reaction rate constant for the second step reaction.

[0039] The reaction rates of the two steps described above are given using the Arrhenius equation, and the specific calculation formula is as follows:

[0040]

[0041] Where k1 represents the reaction rate constant of the first step reaction, A1 represents the pre-exponential factor of the first step reaction, and ρ represents the effect of hydrocarbon fuel density on the transport process of coking precursors to the wall; bE represents the average density of hydrocarbon fuels. a1 The value represents the activation energy of the first step reaction; R represents the molar gas constant; t b The qualitative temperature of the hydrocarbon fuel is represented by k2; the rate constant of the second-step reaction is represented by A2; the pre-exponential factor of the second-step reaction is represented by E. a2 This indicates the activation energy of the second-step reaction; This represents the effect of the wall temperature gradient on the coke deposition process, where T w The value represents the wall temperature, and B represents a dimensionless parameter used to reflect the degree of influence of the wall.

[0042] S1.2: Solve for the unknown parameters in the reaction rate calculation formula to obtain a predictive model for the carbonization reaction of hydrocarbon fuels;

[0043] The reaction rate calculation formula given above contains unknown pre-exponential factors, activation energy, and reactant concentration exponents, which can be solved using a method based on the quasi-infinite element method. The specific solution steps are as follows: Conduct hydrocarbon fuel oxidation and coking experiments under different flow pipe lengths, keeping other conditions constant. First, measure the hydrocarbon fuel composition and concentration at the pipe outlet. Then, treat the hydrocarbon fuel composition and concentration at the outlet of the short pipe as the hydrocarbon fuel composition and concentration at the middle position of the long pipe to obtain the hydrocarbon fuel composition and concentration at each position in the middle of the pipe. Finally, solve the unknown parameters by solving the reaction process at different positions in the long pipe to obtain a predictive model for the hydrocarbon fuel oxidation and coking reaction.

[0044] Step S2: Predicted steps of the carbonization reaction of hydrocarbon fuels;

[0045] The prediction model for the carbon dioxide oxidation reaction of hydrocarbon fuels obtained above is embedded into the CFD solver, and the solution path is as follows: Figure 1 As shown, within a time step, for a given grid, it is determined whether it falls within the coking temperature range. If not, the oxidative coking reaction is not solved, and the process ends directly. If it does, the first step of the reaction is solved, the local aviation kerosene concentration, dissolved oxygen concentration, and temperature are read, and the coking precursor concentration is calculated. Then, it is determined whether the grid belongs to the near-wall grid. If not, the process ends directly. If it does, the second step of the reaction is solved, the local coking precursor concentration, temperature, and temperature gradient are read, and the coke formation rate is calculated. The coke formation mass and thickness are then calculated. Specifically, the coke formation mass can be calculated by integrating the coke formation rate, and the coke formation thickness can be calculated by dividing the coke formation mass by the coke density. The coke formation thickness is further read, and the grid movement is simulated using the nodal spring method to simulate coke layer formation and deposition. Ultimately, accurate prediction of the hydrocarbon fuel oxidative coking process can be achieved with low computational cost.

[0046] Secondly, the present invention provides a predictive model for the carbonization reaction of hydrocarbon fuel oxidation.

[0047] This invention provides a predictive model for the oxidation and coking reaction of hydrocarbon fuels, specifically including a two-step reaction equation and a formula for calculating the reaction rate. The two-step reaction equation is as follows:

[0048]

[0049]

[0050] The formula for calculating the reaction rate is:

[0051]

[0052] Formula (1) is the reaction equation for the first step reaction, and Formula (2) is the reaction equation for the second step reaction. F represents hydrocarbon fuel, α represents the oxygen concentration index, O2 represents dissolved oxygen, P represents coking precursor, IN represents insoluble intermediate product, and D represents coke; τ represents the reaction time; k′1 represents the reaction rate constant of the first step reaction; k1 represents the reaction rate constant of the first step reaction after neglecting the change in the concentration of hydrocarbon fuel itself; A1 represents the pre-exponential factor of the first step reaction, which represents the effect of hydrocarbon fuel density on the transport process of coking precursor to the wall; ρ b E represents the average density of hydrocarbon fuels. a1 The value represents the activation energy of the first step reaction; R represents the molar gas constant; t b The qualitative temperature of the hydrocarbon fuel is represented by k2; the rate constant of the second-step reaction is represented by A2; the pre-exponential factor of the second-step reaction is represented by E. a2 This indicates the activation energy of the second-step reaction; This represents the effect of the wall temperature gradient on the coke deposition process, where T w The value represents the wall temperature, and B represents a dimensionless parameter used to reflect the degree of influence of the wall.

[0053] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0054] Specifically, for RP-3 aviation kerosene, oxidation and coking experiments were conducted in pipelines with lengths of 600mm-1000mm (one group per 50mm). Other conditions were as follows: inlet oil temperature 20℃, system pressure 3MPa, mass flow rate 1g / s, and average actual heating heat flux density 206kW / m³. 2With an inlet oxygen concentration of 10 ppm and an experimental duration of 2 hours, a two-step oxidative coking reaction prediction model was obtained, where the reaction rate of the first step is denoted by (mol / L). -0.91 / s, oxygen concentration index is 1.634, pre-exponential factor is 8.45×10 7 The activation energy is 8.44 × 10⁻⁶. 4 J / mol; the reaction rate of the second step is dimensional in kg / (m²). 2 The temperature gradient exponent is 0.0769, and the pre-exponential factor is 1.08 × 10⁻⁶. 2 The activation energy is 1.29 × 10⁻⁶. 5 J / mol, as shown in Table 1.

[0055] Table 1. Parameters of the two-step oxidative coking reaction prediction model

[0056]

[0057] The two-step oxidation coking reaction prediction model was embedded into ANSYS Fluent software using the User Defined Function (UDF) method to perform transient simulation of the oxidation coking process of RP-3 aviation kerosene. The specific simulation process is as follows: mesh the model; set the CFD solver solution method, turbulence model and boundary conditions; and load UDF to solve the transient oxidation coking process. The UDF solution path for the oxidative coking process is as follows: Within one time step, for a given grid, determine whether it falls within the coking temperature range (150℃-450℃). If not, do not solve the oxidative coking reaction and terminate directly. If it does, begin solving the first step reaction, read the local RP-3 aviation kerosene concentration, dissolved oxygen concentration, and temperature of the grid, solve for the coking precursor concentration, and then determine whether the grid belongs to the near-wall grid. If not, terminate directly. If it does, begin solving the second step reaction, read the local coking precursor concentration, temperature, and temperature gradient of the grid, solve for the coke formation rate, and further solve for the coke formation mass and thickness. Specifically, the coke formation mass can be calculated by integrating the coke formation rate, and then the coke formation thickness can be calculated by dividing the coke formation mass by the coke density. The coke formation thickness is further read using the nodal spring method to simulate coke layer formation and deposition by implementing grid movement.

[0058] Fifteen different operating conditions were selected within the range of Reynolds number 5000-50000, pressure 2.5-3.5 MPa, temperature 200-450℃, and dissolved oxygen concentration 2-20 ppm for prediction. The results are as follows: Figure 2(a) shows the predicted coking amount under different dissolved oxygen concentrations, (b) shows the predicted coking amount under different inlet hydrocarbon fuel mass flow rates, (c) shows the predicted coking amount under different pressures, and (d) shows the predicted coking amount under different heat flux densities. As shown, the predicted coking amount deviates from the experimental value by no more than 10%, which is 20% higher than the prediction results of the calculation model in Reference I (Pei X, Hou L, Ren Z. Kinetic modeling of thermal oxidation and coking deposition in aviation fuel. Energy Fuels 2017; 31:1399-1405.).

[0059] This invention discloses a predictive model and method for the coking reaction of hydrocarbon fuel oxidation. Those skilled in the art can refer to the content of this document and appropriately modify the process parameters to achieve the desired result. It should be particularly noted that all similar substitutions and modifications are obvious to those skilled in the art and are considered to be included in this invention. The product of this invention has been described through preferred embodiments, and those skilled in the art can clearly modify or appropriately change and combine the product described herein without departing from the content, spirit, and scope of this invention to realize and apply the technology of this invention.

Claims

1. A method of predicting oxidation coking reactions of hydrocarbon fuels, characterized by, Comprise the following steps: The carbon hydrogen fuel oxidation coking process is simplified into two step reactions, the first step is a total package space reaction, that is, the chain free radical reaction of the carbon hydrogen fuel is simplified into a total package reaction of the carbon hydrogen fuel directly generating a coking precursor; the second step is a wall reaction, that is, the carbon precipitation reaction of the coking precursor at the wall surface and the deposition process of the coke are simplified into a wall reaction of the coking precursor directly generating the coke at the wall surface; the reaction rates of the above two step reactions are given in the form of the Arrhenius formula; unknown parameters in the reaction rate calculation formula are solved through a method based on the quasi-microelement method to obtain a prediction model of the carbon hydrogen fuel oxidation coking reaction, and the prediction model of the carbon hydrogen fuel oxidation coking reaction is embedded into a CFD solver to predict the carbon hydrogen fuel oxidation coking reaction; The reaction equation of the two step reactions is: Equation (1) is a reaction equation for the first step reaction, Equation (2) is a reaction equation for the second step reaction, F represents a hydrocarbon fuel, a represents an oxygen concentration index, O2 represents dissolved oxygen, P represents a coking precursor, IN represents an insoluble intermediate product, and D represents coke; τ represents a reaction time; k ' 1 represents a rate constant for the first step reaction; k1 represents a reaction rate constant for the first step reaction, ignoring a change in the concentration of the hydrocarbon fuel itself; and k2 represents a reaction rate constant for the second step reaction. The calculation formula of the reaction rate is: where A1represents the pre-exponential factor of the first step reaction, and represents the effect of the density of the hydrocarbon fuel on the transport process of the coke precursors to the wall surface; p b represents the average density of the hydrocarbon fuel; E a1 represents the activation energy of the first step reaction; R represents the molar gas constant, t b represents the characteristic temperature of the hydrocarbon fuel; A2represents the pre-exponential factor of the second step reaction; E a2 represents the activation energy of the second step reaction; represents the effect of the wall temperature gradient on the coke deposition process, where T w represents the wall temperature, and B represents a dimensionless parameter for the effect of the reaction wall. The specific process of solving the unknown parameters in the reaction rate calculation formula through the method based on the quasi-microelement method is: Carry out the carbon hydrogen fuel oxidation coking experiment under different flow pipeline lengths, ensure that other conditions remain unchanged, first measure the carbon hydrogen fuel components and concentrations at the outlet position of the pipeline; then take the carbon hydrogen fuel components and concentrations at the outlet position of the short pipeline as the carbon hydrogen fuel components and concentrations at the middle position of the long pipeline, so that the carbon hydrogen fuel components and concentrations at each position in the middle of the pipeline can be obtained; finally, each unknown parameter is solved by solving the reaction process at different positions in the long pipeline to obtain the prediction model of the carbon hydrogen fuel oxidation coking reaction.

2. The method of claim 1, wherein the method is characterized by: The specific process of embedding the prediction model of the carbon hydrogen fuel oxidation coking reaction into the CFD solver to predict the carbon hydrogen fuel oxidation coking reaction is: In one time step, for one grid, it is judged whether it belongs to the coking temperature range, if not, the oxidation coking reaction is not solved, and the process is directly ended; if it belongs, the first step reaction is solved, the local aviation kerosene concentration, dissolved oxygen concentration and temperature of the grid are read, the coking precursor concentration is solved, then it is judged whether the grid belongs to the near-wall grid, if not, the process is directly ended; if it belongs, the second step reaction is solved, the local coking precursor concentration, temperature and temperature gradient of the grid are read, the coke generation rate is solved, and the coke generation mass and thickness are further solved; the coke generation thickness is further read, the grid motion is simulated by the node spring method to realize the coke layer generation and deposition, and finally the accurate prediction of the carbon hydrogen fuel oxidation coking process is realized under low calculation cost.

3. The method of claim 2, wherein the method is characterized by: The specific method for further solving the coke generation mass and thickness is to calculate the coke generation mass by integrating the coke generation rate, and then calculate the coke generation thickness by dividing the coke generation mass by the coke density.

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