Novel flame surface turbulence combustion model

Abstract

The invention belongs to the field of engine combustion and computational fluid mechanics, and discloses a novel flame surface turbulence combustion model. A model establishment process: solving a control equation of the diffusion flame surface turbulent combustion model to obtain a diffusion flame surface database; solving a control equation of the premixed flame surface turbulent combustion model to obtain a premixed flame surface database; solving a transport equation of speed, turbulence, mixing fraction and reaction progress variables to obtain flow field parameters; according to the mixing score and the reaction progress variable corresponding to each grid point of the flow field, temperatures and components are obtained in the diffusion and premixing flame surface databases in a table look-up interpolation mode; calculating a weighting function according to the flow field parameter of each grid point; performing weighted averaging on the obtained temperature and components through a weighting function; updating the original temperature and components of the flow field by using the weighted average temperature and components; and iteration is repeated until the flow field converges. The model gives full play to the advantages of diffusion and premixed flame surface turbulent combustion models, and has engineering applicability.

Description

Technical Field

[0001] The present invention belongs to the fields of engine combustion and computational fluid dynamics, and particularly relates to a new flamelet turbulent combustion model. Background Art

[0002] The turbulent combustion process in an engine is very complex. Turbulence affects the average chemical reaction rate through enhanced mixing, while the chemical reaction heat release process affects turbulent combustion. It is very difficult to achieve accurate numerical simulation of turbulent combustion. Up to now, a variety of turbulent combustion models have been developed. Among them, the flamelet turbulent combustion model decouples the turbulent flow and chemical reaction, greatly reducing the computational amount and has been widely applied in engineering practice.

[0003] There are two types of flamelet turbulent combustion models: one is the diffusion flamelet turbulent combustion model, which establishes a diffusion flamelet database by solving the one-dimensional laminar counterflow flame; the other is the premixed flamelet turbulent combustion model, which establishes a premixed flamelet database by solving the one-dimensional laminar premixed flame. During numerical simulation, by querying the flamelet database with local flow field parameters, parameters such as components can be obtained. The two types of models are respectively applicable to diffusion combustion and premixed combustion, while the combustion in an engine is neither purely diffusion combustion nor purely premixed combustion, but partially premixed combustion. To address this problem, it is necessary to improve the existing flamelet turbulent combustion model so that it can improve the simulation ability of partially premixed combustion and enhance its engineering applicability.

[0004] Currently, there is an urgent need to develop a new flamelet turbulent combustion model. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a new flamelet turbulent combustion model to overcome the defects of the prior art.

[0006] The new flamelet turbulent combustion model of the present invention is established as follows: Step1. Solve the control equations of the diffusion flamelet turbulent combustion model to obtain the diffusion flamelet database; Step2. Solve the control equations of the premixed flamelet turbulent combustion model to obtain the premixed flamelet database; Step3. Solve the transport equations of velocity, turbulence, mixture fraction, and reaction progress variable to obtain the flow field parameters; Step4. Obtain the temperature and components by table lookup and interpolation in the diffusion and premixed flamelet databases respectively according to the mixture fraction and reaction progress variable corresponding to each grid point of the flow field; Step5. Calculate the weighting function from the flow field parameters of each grid point; Step6. Perform weighted averaging on the temperature and components obtained in Step4 through the weighting function; Step7. Update the original temperature and composition of the flow field with the weighted average temperature and composition; Step8. Repeat Steps 3 to 7 until the flow field converges.

[0007] Furthermore, the governing equation of the diffusion flamelet turbulent combustion model in Step 1 is: ; where, ρ is the density, T is the temperature, c p is the specific heat at constant pressure, f is the mixture fraction, χ is the scalar dissipation rate, q R is the heat loss; ω i , Y i and h i are the chemical production rate, mass fraction and enthalpy of component i respectively; Under different scalar dissipation rates χ conditions, solve Equation (1) to obtain: ; where, is the temperature or composition of the diffusion flamelet turbulent combustion model; Define the reaction progress variable C as a linear combination of components to obtain: ; Perform probability density integration on Equation (3) to obtain: ; where, is the probability density integration of , and are the mean and variance of the mixture fraction respectively, and Equation (4) is the diffusion flamelet database.

[0008] Furthermore, the governing equation of the premixed flamelet turbulent combustion model in Step 2 is: ; where, λ is the thermal conductivity, is the density of the unburned gas, is the laminar flame speed, y is the spatial coordinate; and are the isobaric specific heat and diffusion coefficient of the components i respectively; Solve Equation (5) under different equivalence ratios Ф to obtain: ; where is the temperature or component of the premixed flamelet turbulent combustion model; equivalence ratio Ф and mixture fraction f have the following relationship: ; where is the mixture fraction under stoichiometric conditions; Combine Equation (6) and (7) to obtain: ; Perform transformation and probability density integration to obtain: ; where is the probability density integration of, and Equation (9) is the premixed flamelet database.

[0009] Furthermore, the weighted average in Step 6 is based on the Damköhler number Da Define the weighting function F , and combine the diffusion flamelet database and the premixed flamelet database; ; where Da is the Damköhler number, is the flow time scale, is the chemical reaction time scale, is the laminar flame thickness, ε is the turbulent dissipation rate, k is the turbulent kinetic energy, is the integral value of the temperature or component of the diffusion flamelet turbulent combustion model, is the integral value of the temperature or component of the premixed flamelet turbulent combustion model, and Equation (14) is the flamelet database of the new flamelet turbulent combustion model.

[0010] The new flamelet turbulent combustion model of the present invention gives full play to the advantages of the diffusion flamelet turbulent combustion model and the premixed flamelet turbulent combustion model. By combining the diffusion flamelet database and the premixed flamelet database through a weighting function, it effectively solves the problem that the original flamelet turbulent combustion model is only applicable to diffusion combustion or premixed combustion alone, and has engineering applicability. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Figure 1 Schematic diagram of the establishment process of the new flamelet turbulent combustion model of the present invention; Figure 2 Schematic diagram of the Cabra partially premixed flame in the embodiment; Figure 2 In, H is the height of flame lift; D is the inner diameter of the fuel nozzle; Figure 3a Comparison curve of the calculated and experimental temperature values of the typical cross-section of the Cabra partially premixed flame obtained by the new flamelet turbulent combustion model of the present invention ( x =D); Figure 3b Comparison curve of the calculated and experimental temperature values of the typical cross-section of the Cabra partially premixed flame obtained by the new flamelet turbulent combustion model of the present invention ( x =15D); Figure 3c Comparison curve of the calculated and experimental temperature values of the typical cross-section of the Cabra partially premixed flame obtained by the new flamelet turbulent combustion model of the present invention ( x =30D); Figure 3d Comparison curve of the calculated and experimental temperature values of the typical cross-section of the Cabra partially premixed flame obtained by the new flamelet turbulent combustion model of the present invention ( x =70D); Figures 3a - 3d In x is the axial direction, and R is the radial direction. Specific embodiments

[0012] The present invention will be described in detail below with reference to the drawings and embodiments.

[0013] As Figure 1 shown, the establishment process of the new flamelet turbulent combustion model of the present invention is as follows: Step1. Solve the control equations of the diffusion flamelet turbulent combustion model to obtain the diffusion flamelet database; Step2. Solve the control equations of the premixed flamelet turbulent combustion model to obtain the premixed flamelet database; Step3. Solve the transport equations of velocity, turbulence, mixture fraction, and reaction progress variable to obtain the flow field parameters; Step4. Obtain the temperature and components by looking up tables and interpolation in the diffusion and premixed flamelet databases respectively according to the mixture fraction and reaction progress variable corresponding to each grid point of the flow field; Step5. Calculate the weighting function from the flow field parameters of each grid point; Step6. Weight-average the temperature and components obtained in Step4 through a weighting function; Step7. Update the original temperature and components of the flow field with the weight-averaged temperature and components; Step8. Repeat Steps 3 to 7 until the flow field converges.

[0014] Further, the governing equation of the diffusion flamelet turbulent combustion model in Step1 is: ; where, ρ is the density, T is the temperature, c p is the specific heat at constant pressure, f is the mixture fraction, χ is the scalar dissipation rate, q R is the heat loss; ω i , Y i and h i are the chemical production rate, mass fraction and enthalpy of component i respectively; Under different scalar dissipation rates χ conditions, solve Equation (1) to obtain: ; where, is the temperature or component of the diffusion flamelet turbulent combustion model; Define the reaction progress variable C as a linear combination of components to obtain: ; Perform a probability density integration on Equation (3) to obtain: ; where, is the probability density integration of , and are the mean and variance of the mixture fraction respectively, and Equation (4) is the diffusion flamelet database.

[0015] Further, the governing equation of the premixed flamelet turbulent combustion model in Step2 is: ; where, λ is the thermal conductivity, is the density of the unburned gas, is the laminar flame speed,y is the spatial coordinate; and are the isobaric specific heat and diffusion coefficient of component i respectively; Solve Equation (5) under different equivalence ratios Ф to obtain: ; wherein, is the temperature or component of the premixed flamelet turbulent combustion model; The equivalence ratio Ф and the mixture fraction f have the following relationship: ; wherein, is the mixture fraction under stoichiometric conditions; Combine Equation (6) and (7) to obtain: ; Perform transformation and probability density integration to obtain: ; wherein, is the probability density integration of, and Equation (9) is the premixed flamelet database.

[0016] Furthermore, the weighted average in Step 6 is based on the Damköhler number Da Define the weighting function F to combine the diffusion flamelet database and the premixed flamelet database; ; wherein, Da is the Damköhler number, is the flow time scale, is the chemical reaction time scale, is the laminar flame thickness, ε is the turbulent dissipation rate, k is the turbulent kinetic energy, is the integral value of the temperature or component of the diffusion flamelet turbulent combustion model, is the integral value of the temperature or component of the premixed flamelet turbulent combustion model, and Equation (14) is the flamelet database of the new flamelet turbulent combustion model.

[0017] Example: This example tests the new flamelet turbulent combustion model of the present invention through the Cabra partially premixed flame.

[0018] Figure 2Figure 0 shows a schematic diagram of the Cabra partially-premixed flame, which consists of a central fuel jet and a surrounding coflow. The inner diameter D of the nozzle of the fuel jet is 4.57 mm, and it is composed of a mixture of methane and air, with a temperature of 310 K and a jet velocity of 100 m / s; the diameter of the coflow is 210 mm, and it is composed of a mixture of water and air, with a temperature of 1355 K and a coflow velocity of 5.4 m / s.

[0019] Figures 3a - 3d Figure 4 shows the comparison curve of the calculated and experimental temperature values of the typical cross-section of the Cabra partially-premixed flame obtained by the new flame-surface turbulent combustion model of this embodiment. It can be seen that the calculated temperature values are in good agreement with the experimental values, indicating that the new flame-surface turbulent combustion model of the present invention has high prediction accuracy for partially-premixed combustion and has engineering practical value.

[0020] Although the embodiments of the present invention have been disclosed as above, they are not limited to the applications listed in the specification and embodiments. For those skilled in the art, without departing from the principle of the present invention, all the features disclosed in the present invention, or all the steps in the disclosed methods or processes, except for mutually exclusive features and / or steps, can be combined in any way. The present invention is not limited to the specific details and the illustrated examples described herein.

Claims

1. A new flame surface turbulent combustion model, characterized in that: The establishment process of the novel flame surface turbulent combustion model is as follows: Step 1. Solve the control equations of the diffusion flame surface turbulent combustion model to obtain the diffusion flame surface database; Step 2. Solve the control equations of the premixed flame surface turbulent combustion model to obtain the premixed flame surface database; Step 3. Solve the transport equations of velocity, turbulence, mixing fraction and reaction progress variables to obtain flow field parameters; Step 4. Obtain the temperature and component by table lookup and interpolation in the diffusion and premixed flame surface databases based on the mixture fraction and reaction progress variables corresponding to each grid point in the flow field; Step 5. Calculate the weighting function based on the flow field parameters of each grid point; Step 6. Perform weighted averaging of the temperature and components obtained in Step 4 using a weighting function; Step 7. Use the weighted average temperature and components to update the original temperature and components of the flow field; Step8. Repeat steps Step3 to Step7 until the flow field converges.

2. The novel flame surface turbulent combustion model according to claim 1 is characterized in that: The control equation of the diffusion flame surface turbulent combustion model in Step 1 is: ； in, ρ is the density, T is the temperature, c p is the isobaric specific heat, f is a mixed fraction, χ is the scalar dissipation rate, q R is heat loss; ω i , Y i and h i The components i Chemical generation rate, mass fraction and enthalpy of; At different scalar dissipation rates χ Under these conditions, solving equation (1) yields: ； in, is the temperature or component of the diffusion flame front turbulent combustion model; Define reaction progress variables C is a linear combination of the components, we get: ； By integrating the probability density of equation (3), we can obtain: ； in, for The probability density integral of and are the mean and variance of the mixture fraction respectively, and equation (4) is the diffusion flame surface database.

3. The novel flame surface turbulent combustion model according to claim 2 is characterized in that: The control equation of the premixed flame surface turbulent combustion model in Step 2 is: ； in, λ is the thermal conductivity coefficient, is the density of the unburned gas, is the laminar flame propagation speed, y is the spatial coordinate; and The components i Isobaric specific heat and diffusion coefficient; At different equivalence ratios Ф Solving equation (5) under these conditions, we obtain: ； in, is the temperature or component of the premixed flame surface turbulent combustion model; Equivalence ratio Ф and mixed fractions f There is the following relationship between them: ； in, is the mixture fraction under chemically appropriate ratio conditions; Combining equations (6) and (7), we obtain: ； Performing transformation and probability density integration, we obtain: ； in, for The probability density integral of , equation (9) is the premixed flame surface database.

4. The novel flame surface turbulent combustion model according to claim 3 is characterized in that: The weighted average in Step 6 is based on the Damke number Da Defining the weighting function F , combining the diffusion flame surface database and the premixed flame surface database; ； in, Da is the Damker number, is the flow time scale, is the chemical reaction time scale, is the laminar flame thickness, ε is the turbulent dissipation rate, k is the turbulent kinetic energy, is the integral value of the temperature or component of the diffusion flame surface turbulent combustion model, is the integral value of the temperature or component of the premixed flame surface turbulent combustion model. Equation (14) is the flame surface database of the new flame surface turbulent combustion model.

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