A Database Accelerated Retrieval Method Applicable to the Flamelet Model

Abstract

The present invention belongs to the fields of engine combustion and computational fluid dynamics, and discloses a database acceleration retrieval method applicable to a flamelet model. The retrieval method includes obtaining retrieval parameters of a local grid from a flow field calculation; determining an initial value of the retrieval parameters and a retrieval method according to retrieval values of adjacent grids and time steps; obtaining final retrieval values of the retrieval parameters of the local grid; obtaining mass fractions of each component of the local grid by interpolation; and traversing all grids until the retrieval of all grid points is completed. The database acceleration retrieval method applicable to the flamelet model of the present invention is based on the characteristics that adjacent grids and adjacent time steps have similar physical characteristics, and determines the initial value of the retrieval variable according to the retrieval values of adjacent grids and adjacent time steps, greatly narrowing the range of variable retrieval, reducing the retrieval time, and significantly improving the calculation efficiency of the flamelet combustion model.

Description

Technical Field

[0001] The present invention belongs to the fields of engine combustion and computational fluid dynamics, and particularly relates to a method for accelerating the retrieval of a database applicable to a flamelet model. Background Art

[0002] The key to numerical simulation of an engine combustion chamber is the turbulent combustion model. Due to its high calculation accuracy and efficiency, the flamelet combustion model is most widely used in numerical simulation of an engine combustion chamber.

[0003] Before calculating the flow field, the flamelet combustion model needs to establish a flamelet database based on different retrieval variables. During calculation, the mass fractions of each component are retrieved and interpolated according to the flow field parameters of the local grid. There are typically 4 retrieval variables, namely the mean and variance of the mixture fraction, and the mean and variance of the reaction progress variable. With the continuous development of the flamelet model, the number of retrieval variables also increases. For example, considering unsteady flamelets requires adding the scalar dissipation rate, considering radiation requires adding enthalpy, and considering compressibility requires adding pressure.

[0004] When calculating with the flamelet combustion model, all grids need to be traversed, and each retrieval variable needs to be retrieved sequentially at each grid. The more retrieval variables there are, the longer the retrieval time required. Therefore, it is necessary to improve the existing method for retrieving the flamelet database to enhance the calculation efficiency of the flamelet combustion model.

[0005] Currently, there is an urgent need to develop a method for accelerating the retrieval of a database applicable to the flamelet model. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to provide a method for accelerating the retrieval of a database applicable to the flamelet model to overcome the defects of the prior art.

[0007] The method for accelerating the retrieval of a database applicable to the flamelet model of the present invention has , , and four retrieval variables:

[0008] ;

[0009] wherein, is the mass fraction of each component, and are respectively the mean and variance of the mixture fraction, and are respectively the mean and variance of the reaction progress variable;

[0010] When establishing the database, the discrete numbers of each retrieval variable are respectivelyNf , Nvarf , NC , NvarC , that is:

[0011] ;

[0012] Since the adjacent grids on the computational nodes are physically adjacent, according to the locality and continuity characteristics of the grids, the adjacent grids have similar physical characteristics. At the same time, the same grid also has similar physical characteristics in the current time step and the previous time step;

[0013] The database acceleration retrieval method applicable to the flamelet model includes the following steps:

[0014] S10. Obtain the retrieval parameters of the local grid from the flow field calculation;

[0015] Obtain the mean value of the mixture fraction of the local grid from the flow field calculation f m and variance varf m as well as the mean value of the reaction progress variable C m and variance varC m ;

[0016] S20. Determine the initial values of the retrieval parameters and the retrieval method according to the retrieval values of the adjacent grids and time steps;

[0017] The retrieval values of the mean mixture fraction in the previous time step are respectively ipref 0 and ipref 1, and the retrieval values of the previous grid point are respectively iupf 0 and iupf 1. Then, assign the initial values to the retrieval parameters according to the preset method and perform the retrieval;

[0018] S30. Obtain the final retrieval values of each retrieval parameter of the local grid;

[0019] Retrieve the mean value and variance of the mixture fraction, and the mean value and variance of the reaction progress variable respectively according to the preset method in S20 to obtain the final retrieval values, so that the finally obtained retrieval values satisfy , and ; , and ; , and ; , and ;

[0020] S40. Obtain the mass fractions of each component of the local grid by interpolation;

[0021] The retrieval values obtained from S20 and S30 are used to obtain the mass fractions of each component in the local grid through interpolation. ;

[0022] S50. Traverse all grids until the retrieval of all grid points is completed;

[0023] Repeat S10 - S40 for each grid point until the retrieval of all grid points is completed.

[0024] Furthermore, the preset method in S20 assigns initial values to the retrieval variables and performs retrieval according to the following rules; for the mixture fraction:

[0025] S21. If , then let , and perform retrieval in ascending order;

[0026] S22. If , then let , and perform retrieval in descending order;

[0027] S23. In the remaining cases, then let , , and perform retrieval according to the bisection method.

[0028] The database acceleration retrieval method for the flamelet model of the present invention is based on the characteristics that adjacent grids and adjacent time steps have similar physical characteristics. The initial values and retrieval methods of the retrieval variables are determined according to the retrieval values of adjacent grids and adjacent time steps, greatly reducing the range of variable retrieval, reducing the retrieval time, and significantly improving the calculation efficiency of the flamelet combustion model. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 is the flowchart of the database acceleration retrieval method for the flamelet model of the present invention;

[0030] Figure 2 is the schematic diagram of the Bunsen burner flame in the embodiment;

[0031] Figure 2 In d p is the diameter of the pilot flame; D is the inner diameter of the central jet; x is the axial direction; r is the radial direction; U is the axial velocity; V is the radial velocity;

[0032] Figure 3a is the comparison curve of the typical cross - section temperature distribution (x = 6.5D) obtained by using two flamelet database retrieval methods;

[0033] Figure 3bIt is a comparison curve of the temperature distribution of a typical cross-section obtained by using two flamelet database retrieval methods (x = 8.5D). Detailed implementation manners

[0034] The present invention will be described in detail below with reference to the accompanying drawings and embodiments.

[0035] Embodiment: As Figure 1 shown, for the database acceleration retrieval method applicable to the flamelet model in this embodiment, a flamelet database has 、 、 and four retrieval variables:

[0036] ;

[0037] Among them, is the mass fraction of each component, and are respectively the mean and variance of the mixture fraction, and are respectively the mean and variance of the reaction progress variable;

[0038] When establishing the database, the discrete numbers of each retrieval variable are respectively Nf , Nvarf , NC , NvarC , that is:

[0039] ;

[0040] Since the adjacent grids on the computing node are physically adjacent, according to the locality and continuity characteristics of the grids, the adjacent grids have similar physical characteristics. At the same time, the same grid also has similar physical characteristics in the current time step and the previous time step;

[0041] The database acceleration retrieval method applicable to the flamelet model includes the following steps:

[0042] S10. Obtain the retrieval parameters of the local grid from the flow field calculation;

[0043] Obtain the mean f m and variance varf m of the mixture fraction and the mean C m and variance varC m of the reaction progress variable of the local grid from the flow field calculation;

[0044] S20. Determine the initial value of the retrieval parameter and the retrieval method according to the retrieval values of the adjacent grids and time steps;

[0045] The retrieved values of the mixture fraction mean at the previous time step are respectively ipref 0 and ipref 1, and the retrieved values at the previous grid point are respectively iupf 0 and iupf 1. Then, the initial values of the retrieval parameters are assigned according to the preset method and retrieval is performed;

[0046] S30. Obtain the final retrieved values of each retrieval parameter for the local grid;

[0047] Retrieve the final retrieved values of the mixture fraction mean and variance, reaction progress variable mean and variance respectively according to the preset method in S20, so that the finally obtained retrieved values satisfy , and ; , and ; , and ; , and ;

[0048] S40. Obtain the mass fractions of each component in the local grid by interpolation;

[0049] According to the retrieved values obtained in S20 and S30, obtain the mass fractions of each component in the local grid by interpolation ;

[0050] S50. Traverse all grids until the retrieval of all grid points is completed;

[0051] Repeat S10~S40 for each grid point until the retrieval of all grid points is completed.

[0052] Furthermore, the preset method in S20 is to assign initial values to the retrieval variables according to the following rules and perform retrieval; for the mixture fraction:

[0053] S21. If , then let , and retrieve in ascending order;

[0054] S22. If , then let , and retrieve in descending order;

[0055] S23. In the remaining cases, then let , , and retrieve by the bisection method.

[0056] Comparative example: The retrieval method of the flamelet database in this comparative example is the bisection method, and the specific steps are as follows:

[0057] Step1. Obtain the mean of the mixture fraction of the local grid from the flow field calculation f m and variance varf m as well as the mean of the reaction progress variable C m and variance varC m ;

[0058] Step2. Set the initial search values of the mean and variance of the mixture fraction, and the mean and variance of the reaction progress variable to be if 0 = 1, if 1 = Nf , ivarf 0 = 1, ivarf 1 = Nvarf , iC 0 = 1, iC 1 = NC , ivarC 0 = 1, ivarC 1 = NvarC ;

[0059] Step3. Obtain the final search values by looking up the table using the bisection method, such that , and ; , and ; , and ; , and ;

[0060] Step4. According to the search values obtained in Step3, obtain the mass fractions of each component of the local grid by interpolation ;

[0061] Step5. Repeat Step1~Step4 for each grid point until the search of each grid point is completed.

[0062] Next, the database acceleration search method applicable to the flamelet model of the present invention and the bisection method of the comparative example are tested by a Bunsen burner flame.

[0063] The Bunsen burner flame is shown in Figure 2 , the central jet is a methane / air premixed gas with an equivalence ratio of , the inner diameter D = 12 mm, the average velocity of the jet is U0 = 30 m / s, and the Reynolds number Re of the central jet is approximately 24200. Surrounding the jet is a methane / air burned gas with an equivalence ratio of as the pilot flame, and the diameter of the pilot flame is d p= 68 mm, and the average speed is 1.5 m / s. Water cooling is provided on the periphery of the pilot flame, so that the inlet temperature of the pilot flame is lower than the corresponding adiabatic flame temperature. The periphery of the burner is an air coflow with an average speed of 0.22 m / s.

[0064] To reduce the computational load, the computational domain is one-fourth of the cylinder, that is, a sector cylinder with a 90° bottom surface, and the number of grids is about 140,000. The flamelet combustion model and the methane Grimech 2.11 reaction mechanism are used for the calculation. The retrieval variables of the flamelet database include the mean and variance of the mixture fraction and the mean and variance of the reaction progress variable. The database acceleration retrieval method applicable to the flamelet model of the present invention and the dichotomy method of the comparative example are respectively used for the retrieval.

[0065] Figure 3a 、 Figure 3b The comparative curve of the typical cross-section temperature distribution obtained by the database acceleration retrieval method applicable to the flamelet model of the present invention and the dichotomy method of the comparative example is given. It can be seen that the calculation results obtained by the two retrieval methods are completely coincident and in good agreement with the experimental results, indicating that the database acceleration retrieval method applicable to the flamelet model of the present invention has no influence on the accuracy of the numerical simulation. At the same time, Table 1 gives the time comparison of the numerical simulation under the two retrieval methods. It can be seen that the calculation time using the database acceleration retrieval method applicable to the flamelet model of the present invention is shortened by about 24% compared with the dichotomy method of the comparative example, and the calculation efficiency is greatly improved.

[0066] Table 1 Calculation time comparison table

[0067]

[0068] Although the embodiments of the present invention have been disclosed as above, they are not limited to the applications listed in the specification and the 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 the mutually exclusive features and / or steps, can be combined in any way. The present invention is not limited to the specific details and the examples shown and described herein.

Claims

1. A database acceleration retrieval method applicable to a flame surface model, characterized in that, A flamelet database has , , and four retrieval variables: ； wherein, is the mass fraction of each component, and are the mean and variance of the mixture fraction respectively, and are the mean and variance of the reaction progress variable respectively; The discrete quantities of each retrieval variable when establishing the database are respectively Nf , Nvarf , NC , NvarC , that is: ； Since the adjacent grids on the computational node are physically adjacent, according to the locality and continuity characteristics of the grids, the adjacent grids have similar physical characteristics. At the same time, the same grid also has similar physical characteristics at the current time step and the previous time step; The database acceleration retrieval method applicable to the flamelet model includes the following steps: S10. Obtain the retrieval parameters of the local grid from the flow field calculation; Obtain the mean of the mixture fraction of the local grid from the flow field calculation f m and variance varf m as well as the mean of the reaction progress variable C m and variance varC m ; S20. Determine the initial values of the retrieval parameters and the retrieval method according to the retrieval values of the adjacent grids and time steps; The retrieval values of the mixed fractional means at the previous time step are respectively ipref 0 and ipref 1, and the retrieval values at the previous grid point are respectively iupf 0 and iupf 1. Then, the initial values of the retrieval parameters are assigned according to the preset method, and the retrieval is performed; S30. Obtain the final retrieval values of the retrieval parameters of the local grid; Retrieve the final retrieval values for the mixed fraction mean and variance, and the reaction progress variable mean and variance respectively according to the preset method of S20, so that the finally obtained retrieval values satisfy , and ; , and ; , and ; , and ; S40. Obtain the mass fractions of the components of the local grid by interpolation; Based on the retrieval values obtained from S20 and S30, the mass fractions of each component in the local grid are obtained by interpolation ; S50. Traverse all grids until the retrieval of all grid points is completed; Repeat S10 to S40 for each grid point until the retrieval of all grid points is completed.

2. The database acceleration retrieval method applicable to the flame surface model according to claim 1, characterized in that, The preset method in S20 assigns initial values to the retrieval variables according to the following rules and conducts retrieval; for the mixture fraction: S21. If , then let and retrieve in ascending order; S22. If , then let , and retrieve in the descending order; In the remaining cases, let , and perform a search according to the dichotomy method.

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