Polymer shear degradation evaluation method based on FENE-P model

Abstract

The invention discloses a polymer shear degradation evaluation method based on an FENE-P model, and belongs to the field of polymer turbulence drag reduction. The method comprises the following steps: carrying out a polymer degradation influence factor experiment based on a polymer degradation loop experiment device, and calculating a degradation rate; carrying out a viscous shear rheological experiment aiming at solutions at different degradation stages; fitting rheological experiment data, and establishing a rheological equation; determining viscoelastic parameters in the FENE-P model, correcting a momentum equation and a molecular deformation transport equation, and compiling a related UDF file; loading the UDF file into a source item, a transient item and a convection item of a fluid domain of FLUENT software, and changing relevant viscoelastic parameters of an FENET-P model to realize polymer fluid degradation flow field simulation; and analyzing the flow field information difference between the Newtonian fluid and the FENE-P solution, evaluating the shear degradation behavior of the polymer flowing from a macroscopic flow field to a microcosmic flow field, and explaining the shear degradation mechanism of the polymer. The method provided by the invention provides a theoretical basis for optimization of the polymer drag reducer in actual fluid turbulence transportation.

Description

Technical Field

[0001] The invention relates to the field of polymer turbulence drag reduction, and in particular to a polymer shear degradation evaluation method based on a FENE-P model. Background Art

[0002] High molecular polymers are commonly used agents in turbulent drag reduction technology. Adding a dosage of one part per million in liquid transportation can significantly reduce energy dissipation in turbulence, and has the characteristics of small dosage and high efficiency. However, in the process of pipeline transportation, due to the existence of strong turbulent pulsation and inevitable mechanical elements, long-chain linear high molecular polymers in the solution are very likely to be shear-degraded, resulting in drag reduction failure, resulting in the inability to ensure the timeliness of drag reduction. Therefore, conducting shear degradation evaluation of polymers in turbulent drag reduction is of great significance for the accurate prediction and engineering application of turbulent drag reduction technology.

[0003] At present, the research on the degradation of polymers in turbulent flow mainly adopts a combination of experimental and theoretical methods, wherein the experimental method adopts a flat plate or pipeline experiment, and the polymer drag reduction rate at different times is calculated to evaluate the degradation of the polymer. The theoretical method adopts a viscoelastic theoretical model (such as Oldroyd-B model, Giesekus model, FENE-P model) to describe such polymer fluids, in view of the viscoelastic characteristics of polymer solutions, wherein the FENE-P (finite extension non-Gaussian chain) model can better capture the coupling effect of elastic stress and viscous stress on the flow field of polymer molecules during the flow process. However, the theoretical research based on the FENE-P model currently only focuses on the turbulent drag reduction behavior of polymers, and does not study the shear degradation behavior of polymers during turbulent drag reduction. Therefore, the present invention proposes a polymer shear degradation evaluation method based on the FENE-P model, which is of great significance for the flow field analysis, degradation behavior evaluation and degradation mechanism research of the degradation process of polymers under different polymer concentrations, different polymer molecular weights and different flow rates during turbulent drag reduction, and provides a theoretical basis for the optimization of polymer drag reducers in actual fluid turbulent transport. Summary of the invention

[0004] In view of the above problems, the purpose of the present invention is to provide a polymer shear degradation evaluation method based on the FENE-P model, so as to realize the degradation evaluation of polymers under different polymer concentrations, different polymer molecular weights and different flow rates during turbulent drag reduction.

[0005] The technical solution of the present invention is as follows:

[0006] S1. Build a polymer degradation loop experimental device. The device geometry diagram is shown in Figure 2 As shown;

[0007] S2. Based on the polymer degradation loop experimental device, the polymer degradation influencing factor experiments of polymer concentration, polymer molecular weight and flow rate were carried out, and the polymer degradation rate was obtained by calculating the pressure difference in the pipeline before and after the addition of the agent under constant flow conditions;

[0008] S3. In the polymer degradation influencing factor experiment of S2, samples were taken from solutions at different degradation stages to conduct viscosity-shear rheology test;

[0009] S4, fitting the viscosity-shear rheology experimental data of S3, and establishing the rheological equations of the polymer solution at different degradation stages under different polymer concentrations, different polymer molecular weights and different flow rates;

[0010] S5. Determine the solute viscosity, relaxation time and maximum stretching length parameters in the FENE-P model based on the rheological equation obtained by fitting in S4, modify the momentum equation and molecular deformation transport equation and compile the relevant UDF files;

[0011] S6. Establish a numerical model for turbulent drag reduction simulation of viscoelastic polymers. Load the UDF file determined in S5 into the source term, transient term, and convection term of the fluid domain of the FLUENT software. By changing the relevant viscoelastic parameters of the FENE-P model, the numerical simulation of the shear degradation flow field of the polymer drag reduction fluid under various working conditions can be achieved.

[0012] S7. Analyze the differences in flow field information between Newtonian fluid and FENE-P solution, evaluate the shear degradation behavior of polymers from macroscopic flow to microscopic flow field, and explain the shear degradation mechanism of polymers.

[0013] Compared with the prior art, the present invention has the following advantages: the method can analyze the real-time distribution of flow field information such as near-wall velocity, turbulence intensity, turbulent kinetic energy and turbulent dissipation rate of polymers at different degradation stages in turbulent drag reduction, thereby realizing the evaluation of the degradation behavior of polymers in the process of turbulent drag reduction. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 It is a technical flow chart of the method of the present invention;

[0015] Figure 2 A diagram of the polymer degradation loop experimental device used to evaluate the turbulent drag reduction degradation behavior of polymers;

[0016] Figure 3 This is a diagram of the local mesh encryption structure of the near-wall region according to an embodiment of the present invention;

[0017] Figure 4 It is a flow field boundary layer velocity distribution diagram of an embodiment of the present invention;

[0018] Figure 5 4 is a turbulence intensity distribution diagram of an embodiment of the present invention. Specific implementation methods

[0020] The present invention will be further described in detail below in conjunction with the embodiments and drawings.

[0021] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. The described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0022] The present invention will be further described below:

[0023] A polymer shear degradation evaluation method based on the FENE-P model, such as Figure 1 As shown, the following steps are included:

[0024] S1. Build a polymer degradation loop experimental device. The device geometry diagram is shown in Figure 2 As shown;

[0025] Specifically, the polymer degradation loop experimental device is a visual small-scale loop composed of multiple sections of detachable acrylic tubes, with an inner diameter of 32mm and a length of 18.3m. The range of the pipeline pressure gauge is 0-0.15MPa, and the range of the flow meter is 0-20m 3 / h, the liquid storage tank capacity is 80L.

[0026] S2. Based on the polymer degradation loop experimental device, the polymer degradation influencing factor experiments of polymer concentration, polymer molecular weight and flow rate were carried out, and the polymer degradation rate was obtained by calculating the pressure difference in the pipeline before and after the addition of the agent under constant flow conditions;

[0027] Specifically, in this embodiment, the operation process is as follows:

[0028] The experiment uses water-soluble polyacrylamide as the experimental reagent, the solution concentration range is 25-300ppm, the polymer molecular weight range is 6-22 million, and the designed experimental flow rate is 1-4.2m / s. To ensure the accuracy of the experimental test, the solvent used in the experiment is deionized water. After the prepared polymer solution is left to stand for 24 hours, the polymer degradation influencing factor experiment is carried out.

[0029] At the beginning of the experiment, the polymer solution that had been standing for 24 hours was added to the storage tank, and the valve was gradually opened to allow the solution to flow into the pipeline, and the air in the pipeline was discharged from the three-way valve. After the fluid in the pipeline was in a full flow state, the centrifugal pump was turned on and the motor frequency was fixed.

[0030] a) to test the degradation rate of polymer solutions with different concentrations, the solution concentration range is set to 25ppm, 50ppm, 150ppm, 200ppm, and 300ppm, and the degradation rates of polymer solutions with different concentrations at different times are measured according to the implementation of the polymer degradation influencing factor experiment;

[0031] b) to test the degradation rate of polymer solutions with different molecular weights, the molecular weights of the added polymers are 6 million, 10 million, 14 million, 16 million, and 22 million, and the degradation rates of the polymer solutions with different molecular weights at different times are measured according to the implementation method of the polymer degradation influencing factor experiment;

[0032] c) to test the degradation rate of polymer solutions at different flow rates, the motor power of the centrifugal pump is set to 10 Hz, 20 Hz, 30 Hz, 40 Hz, and 50 Hz, and the degradation rates of polymer solutions at different flow rates at different times are measured according to the implementation method of the polymer degradation influencing factor experiment;

[0033] Specifically, the degradation rate is calculated by this formula:

[0034]

[0035] Where DR(t), DR max They are respectively the drag reduction value after shearing at time t and the maximum turbulent drag reduction efficiency without shearing for the solution with the same concentration of additive under the same experimental conditions, %.

[0036] S3. In the polymer degradation influencing factor experiment of S2, samples were taken from solutions at different degradation stages to conduct viscosity-shear rheology test;

[0037] During the experiment of factors affecting polymer degradation, 30 ml of the fluid in the pipeline was sampled at 0 min, 5 min, 1 h, and 2 h, and added to the Anton Par MCR302 rotational rheometer. The polymer solution was subjected to a viscosity-shear rheology test using a coaxial cylinder system. The experimental temperature was set at 20 °C and the shear rate was set at 0 to 5000 s. -1 .

[0038] S4, fitting the viscosity-shear rheology experimental data of S3, and establishing the rheological equations of the polymer solution at different degradation stages under different polymer concentrations, different polymer molecular weights and different flow rates;

[0039] Using SPSS software, based on the cross rheological model (Formula (1)), the viscosity-shear rheological experimental data of the degradation solution under different working conditions were fitted to determine the zero shear viscosity, relaxation time and power law index. In this embodiment, the viscosity-shear rheological experimental fitting results of the polymer solution with a molecular weight of 6 million and a concentration of 25 ppm are shown in Table 1:

[0040]

[0041] Where μ0 is the zero shear viscosity, Pa.s; λ is the relaxation time, s; n is the power law exponent.

[0042] Table 1 Fitting results of the solvent-viscosity shear rheological experiment of polymer with molecular weight of 6 million and concentration of 300 ppm

[0043]

[0044]

[0045] S5. Determine the solute viscosity, relaxation time and maximum stretching length parameters in the FENE-P model based on the rheological equation obtained by fitting in S4, modify the momentum equation and molecular deformation transport equation and compile the relevant UDF files;

[0046] The FENE-P model is a constitutive equation that can describe the combined effects of viscosity and elasticity of polymer solutions, specifically:

[0047]

[0048]

[0049]

[0050] In the formula, is elastic stress, Pa; μ p is the solute viscosity, Pa.s; c ij is the deformation rate tensor; f(r) is the Peterlin function; r is the trace of the deformation rate tensor; L is the maximum stretching length.

[0051] Some compiled code is as follows:

[0052]

[0053] S6. Establish a numerical model for simulating turbulent drag reduction of viscoelastic polymers. Load the UDF file determined in S5 into the source term, transient term and convection term of the fluid domain in the FLUENT software. By changing the relevant viscoelastic parameters of the FENE-P model, the numerical simulation of the shear degradation flow field of the polymer drag reduction fluid under various working conditions can be achieved.

[0054] (1) Based on the polymer degradation loop experimental device, ANSYS ICEM software was used to establish a 1:1 structural geometric model of the P3 to P4 segment of the visualized small-scale loop. The inner diameter of the pipeline model was 32 mm and the length was 6.1 m;

[0055] (2) Define the names of the pipeline parts, and name the pipeline inlet, pipeline outlet, wall, and fluid domain as inlet, outlet, wall, and fluid, respectively. The physical model is meshed, and the mesh near the wall is encrypted, such as Figure 3 As shown, generate the mesh file .msh and import it into ANSYS FLUENT;

[0056] (3) Use the Define-User-Defined-Function-Compiled command in ANSYS FLUENT to load the UDF file of the FENE-P equation. In this embodiment, a total of three custom scalar transport equations are set, that is, the number of Scalars in the User-Defined module is 3. By opening source terms in the fluid domain, the source terms and convection terms of the momentum equation and the transport equation are loaded into the fluid domain. The transient terms are defined in the Flux Function in Scalars, and the dissipation terms are defined by entering the artificial viscosity term coefficient in the material panel. For Newtonian fluids, the standard k-ε model is used, and the wall function uses enhanced wall treatment. The polymer solution uses the UDF loading method to load the FENE-P model. Both types of fluid flows use the same velocity inlet (v=4.2m / s) and pressure outlet (P=0Pa).

[0057] (4) After initialization, set the number of iterations to 1000 and the iteration interval to 0.1s, and perform iterative calculations. During the calculation process, the actual number of iterations is determined based on the residual. If the residual is lower than the set value during the iteration, stop the iteration and proceed to step 5.

[0058] (5) After the calculation is completed, the data is processed. The velocity, turbulence intensity, turbulent kinetic energy and turbulent dissipation rate distribution of the pipe near the wall in the flow field are dimensionless. The wall distribution data of velocity and turbulence intensity are shown in the figure below. Figure 4 , Figure 5 shown.

[0059] S7. Analyze the differences in flow field information between Newtonian fluid and FENE-P solution, evaluate the shear degradation behavior of polymers from macroscopic flow to microscopic flow field, and explain the shear degradation mechanism of polymers.

[0060] The present invention solves the problem of lack of complete evaluation of shear degradation behavior of polymers in turbulent drag reduction technology applications. It realizes the evaluation of polymer shear degradation under various working conditions, from macroscopic flow to microscopic flow field. It is of great significance for the evaluation of shear degradation behavior of polymer turbulent drag reduction, the study of degradation mechanism and the design of polymer drag reducer injection scheme.

Claims

1. A polymer shear degradation evaluation method based on the FENE-P model, characterized in that: The following steps are involved: S1. Build a polymer degradation loop experimental device; S2. Based on the polymer degradation loop experimental device, the polymer degradation influencing factor experiments of polymer concentration, polymer molecular weight and flow rate were carried out, and the polymer degradation rate was obtained by calculating the pressure difference in the pipeline before and after the addition of the agent under constant flow conditions; S3. In the polymer degradation influencing factor experiment of S2, samples were taken from solutions at different degradation stages to conduct viscosity-shear rheology test; S4, fitting the viscosity-shear rheology experimental data of S3, and establishing the rheological equations of the polymer solution at different degradation stages under different polymer concentrations, different polymer molecular weights and different flow rates; S5. Determine the solute viscosity, relaxation time and maximum stretching length parameters in the FENE-P model based on the rheological equation obtained by fitting in S4, modify the momentum equation and molecular deformation transport equation and compile the relevant UDF files; S6. Establish a numerical model for turbulent drag reduction simulation of viscoelastic polymers. Load the UDF file determined in S5 into the source term, transient term, and convection term of the fluid domain of the FLUENT software. By changing the relevant viscoelastic parameters of the FENE-P model, the numerical simulation of the shear degradation flow field of the polymer drag reduction fluid under various working conditions can be achieved. S7. Analyze the differences in flow field information between Newtonian fluid and FENE-P solution, evaluate the shear degradation behavior of polymers from macroscopic flow to microscopic flow field, and explain the shear degradation mechanism of polymers.

2. The method according to claim 1, characterized in that In S2, the polymer degradation influencing factor experiment has a solution concentration range of 25 to 300 ppm, a polymer molecular weight range of 6 to 22 million, and a designed experimental flow rate of 1 to 4.2 m / s.

3. The method according to claim 1, characterized in that: In S3, 30 ml of the fluid in the pipeline was sampled at 0 min, 5 min, 1 h, and 2 h, respectively, and added to the rotational rheometer. The polymer solution was subjected to a viscosity-shear rheological test using a coaxial cylinder system. The experimental temperature was set at 20 °C and the shear rate was set at 0 to 5000 s. -1 .

4. The method according to claim 1, characterized in that In S6, a numerical model for turbulent drag reduction simulation of viscoelastic polymer solution is established based on ANSYS ICEM software, and the numerical model for turbulent drag reduction simulation of viscoelastic polymer solution is set in a 1:1 ratio with the test pipe section of the polymer degradation loop experimental device described in S1.

5. The method according to claim 1, characterized in that In S6, the standard k-ε model is used for Newtonian fluid, the wall function uses enhanced wall treatment, and the polymer solution is loaded into the FENE-P model using the UDF loading method; both types of fluid flows use the same velocity inlet and pressure outlet.

6. The method according to claim 1, characterized in that S6 includes the following steps: Based on the FENE-P model under various working conditions, UDF is written and imported into FLUENT for iterative calculation, and then the calculation results are output.

7. According to the method of claim 1, by analyzing the real-time distribution of flow field information such as near-wall velocity, turbulence intensity, turbulent kinetic energy and turbulence dissipation rate of the polymer at different degradation stages in turbulent drag reduction, the degradation behavior of the polymer in the turbulent drag reduction process can be evaluated.

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