Polymer shear degradation evaluation method based on FENE-P model

Abstract

The application discloses a polymer shear degradation evaluation method based on a FENE-P model and belongs to the field of polymer turbulent flow drag reduction. The method comprises the following steps: based on a polymer degradation loop experimental device, polymer degradation influencing factor experiments are carried out, and a degradation rate is calculated; rheological experiments are carried out on solutions in different degradation stages; rheological experimental data are fitted, and a rheological equation is established; viscoelastic parameters in the FENE-P model are determined, a momentum equation and a molecular deformation transport equation are corrected, and relevant UDF files are compiled; the UDF files are loaded into source terms, transient terms and convection terms of a fluid domain of FLUENT software, FENE-P model related viscoelastic parameters are changed, polymer fluid degradation flow field simulation is realized; differences between Newton fluid and FENE-P solution flow field information are analyzed, polymer shear degradation behaviors from macro flow to micro flow field are evaluated, and polymer shear degradation mechanisms are explained. The method provides a theoretical basis for optimization of polymer drag reducers in actual fluid turbulent flow transportation.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of polymer turbulent flow drag reduction, and particularly to a polymer shear degradation evaluation method based on a FENE-P model. BACKGROUND

[0002] Polymer is a common agent in the turbulent flow drag reduction technology, and a dosage of one ten-thousandth can significantly reduce the energy dissipation in turbulent flow in liquid transportation, and has the characteristics of small dosage and high efficiency. However, in the pipeline transportation process, due to the existence of strong turbulent fluctuation and inevitable mechanical elements, the long-chain linear polymer in the solution is prone to shear degradation and thus drag reduction failure, so that the timeliness of drag reduction cannot be guaranteed. Therefore, the shear degradation evaluation of polymer in turbulent flow drag reduction has important significance for accurate prediction and engineering application of turbulent flow drag reduction technology.

[0003] At present, the research on polymer degradation in turbulent flow mainly adopts the method combining experiment and theory, wherein the experiment method adopts a method of calculating the polymer drag reduction rate at different times in a flat plate or pipeline experiment, so as to evaluate the degradation of the polymer. The theoretical method adopts a viscoelastic theoretical model (such as Oldroyd-B model, Giesekus model and FENE-P model) for describing the viscoelasticity of the polymer solution, wherein the FENE-P (finite extension non-Gaussian chain) model can better capture the coupling effect of the elastic stress and viscous stress of the polymer molecules on the flow field in the flow process. However, the current theoretical research based on the FENE-P model only focuses on the turbulent flow drag reduction behavior of the polymer, and does not carry out research on the shear degradation behavior of the polymer in the turbulent flow drag reduction process. Therefore, the present application proposes a polymer shear degradation evaluation method based on the FENE-P model, which has important significance for flow field analysis, degradation behavior evaluation and degradation mechanism research of the degradation process of the polymer in the turbulent flow drag reduction process under different polymer concentrations, different polymer molecular weights and different flow rates, and provides a theoretical basis for optimization of the polymer drag reduction agent in actual fluid turbulent transportation. SUMMARY

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

[0005] The technical scheme of the present application is as follows:

[0006] S1, a polymer degradation loop experiment device is built, and a device geometry schematic diagram is shown in Figure 2 ;

[0007] S2, based on the polymer degradation ring track experimental device, the polymer degradation influence factor experiment of polymer concentration, polymer molecular weight and flow rate is carried out, and the degradation rate of the polymer is obtained by calculating the pressure difference in the pipeline before and after the agent under the condition of constant flow rate;

[0008] S3, in the polymer degradation influence factor experiment of S2, the solution of different degradation stages is sampled respectively, and the shear rheological experiment test is carried out;

[0009] S4, the shear rheological experiment data of S3 are fitted, and the rheological equation of the polymer solution at different degradation stages under different polymer concentrations, different polymer molecular weights and different flow rates is established;

[0010] S5, the rheological equation obtained by fitting S4 is used to determine the solute viscosity, relaxation time and maximum stretching length parameters in the FENE-P model, the momentum equation and the molecular deformation transport equation are revised, and the related UDF file is compiled;

[0011] S6, the numerical simulation model of viscoelastic polymer turbulent flow drag reduction is established, the UDF file determined by S5 is loaded into the source term, transient term and convection term of the fluid domain of FLUENT software, and the shear degradation flow field numerical simulation of polymer drag reduction fluid under various working conditions can be realized by changing the related viscoelastic parameters of FENE-P model;

[0012] S7, the difference between the flow field information of Newtonian fluid and FENE-P solution is analyzed, the shear degradation behavior of polymer from macro flow to micro flow field is evaluated, and the shear degradation mechanism of polymer is explained.

[0013] Compared with the prior art, the method has the following advantages: the method can analyze the real-time distribution of the near-wall velocity, turbulent intensity, turbulent kinetic energy and turbulent dissipation rate of the polymer at different degradation stages in the turbulent flow drag reduction, and realizes the degradation behavior evaluation of the polymer in the turbulent flow drag reduction process. BRIEF DESCRIPTION OF DRAWINGS

[0014] Figure 1 The technical flow chart of the method of the application is shown in the figure;

[0015] Figure 2 The polymer degradation ring track experimental device for evaluating the degradation behavior of polymer turbulent flow drag reduction is shown in the figure;

[0016] Figure 3 The local grid encryption structure diagram of the near-wall region of the embodiment of the application is shown in the figure;

[0017] Figure 4 The flow field boundary layer velocity distribution diagram of the embodiment of the application is shown in the figure;

[0018] Figure 5 The turbulent intensity distribution diagram of the embodiment of the application is shown in the figure. Detailed implementation method

[0019] The application will be further described below in conjunction with the embodiments and drawings.

[0020] To make the objectives, technical solutions and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be described clearly and completely below in conjunction with the drawings in the embodiments of the present application. The described embodiments are part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative work fall within the protection scope of the present application.

[0021] The present application will be further described below:

[0022] A polymer shear degradation evaluation method based on a FENE-P model, as shown in Figure 1 , includes the following steps:

[0023] S1, build a polymer degradation loop experimental device, and the device geometric diagram is shown in Figure 2 ;

[0024] Specifically, the polymer degradation loop experimental device is a visual small-scale loop composed of multiple detachable acrylic pipes, the inner diameter of the pipe is 32 mm, and the pipe length is 18.3 m. The pipeline pressure gauge range is 0-0.15 MPa, the flow meter range is 0-20 m 3 / h, and the liquid storage tank capacity is 80 L.

[0025] S2, based on the polymer degradation loop experimental device, carry out polymer degradation influence factor experiments of polymer concentration, polymer molecular weight and flow rate, and calculate the degradation rate of the polymer by the pressure difference in the pipeline before and after adding the agent under the condition of constant flow;

[0026] Specifically, in this embodiment, the operation process is as follows:

[0027] The experiment uses water-soluble polyacrylamide as the experimental reagent, the solution concentration range is 25-300 ppm, the polymer molecular weight range is 6-22 million, and the designed experimental flow rate is 1-4.2 m / s. To ensure the accuracy of the experimental test, the solvent used in the experiment is deionized water. The polymer degradation influence factor experiment is carried out after the prepared polymer solution is placed for 24 hours.

[0028] At the beginning of the experiment, the polymer solution placed for 24 hours is added to the liquid storage tank, the valve is gradually opened to make the solution flow into the pipeline, and the air in the pipeline is discharged from the three-way valve. After the fluid in the pipeline is in full flow state, the centrifugal pump is started and the motor frequency is fixed.

[0029] a) To test the degradation rate of polymer solution with different concentrations, the solution concentration range is set to 25 ppm, 50 ppm, 150 ppm, 200 ppm, 300 ppm, and the degradation rate of polymer solution with different concentrations at different times is measured according to the embodiments of the polymer degradation influencing factor experiment;

[0030] b) To test the degradation rate of polymer solution with different molecular weights, the added polymer molecular weight is 6 million, 10 million, 14 million, 16 million, and 22 million, and the degradation rate of polymer solution with different molecular weights at different times is measured according to the embodiments of the polymer degradation influencing factor experiment;

[0031] c) To test the degradation rate of polymer solution with different flow rates, the centrifugal pump motor power is set to 10 hz, 20 hz, 30 hz, 40 hz, and 50 hz, and the degradation rate of polymer solution with different flow rates at different times is measured according to the embodiments of the polymer degradation influencing factor experiment;

[0032] Specifically, the degradation rate is calculated by the following formula:

[0033]

[0034] In the formula, DR(t), DR max are the drag reduction values of the same concentration additive solution after shearing for t minutes and the maximum turbulent drag reduction efficiency without shearing, respectively, under the same experimental conditions, %.

[0035] S3, in the polymer degradation influencing factor experiment of S2, the solution at different degradation stages is sampled, and a shear rheological experiment is carried out;

[0036] During the polymer degradation influencing factor experiment, 30 ml of fluid in the pipeline is sampled at 0 min, 5 min, 1 h, and 2 h, and is added to an Anton Par MCR302 rotary rheometer, a coaxial cylinder system is used to carry out a shear rheological experiment on the polymer solution, the experimental temperature is set to 20℃, the shear rate is set to 0-5000 s -1 .

[0037] S4, the shear rheological experiment data of S3 is fitted, and a rheological equation of the polymer solution at different degradation stages under different polymer concentrations, different polymer molecular weights, and different flow rates is established;

[0038] Using SPSS software, based on the cross rheological model (Equation (1)), the visco-shear rheological experimental data of the degraded solution under different operating conditions were fitted to determine the zero-shear viscosity, relaxation time, and power-law exponent. In this embodiment, the fitting results of the visco-shear rheological experiment of the polymer solution with a molecular weight of 6 million and a concentration of 25 ppm are shown in Table 1:

[0039]

[0040] In the formula, μ0 is the zero shear viscosity, Pa·s; λ is the relaxation time, s; and n is the power law exponent.

[0041] Table 1. Fitting results of solution-viscosity-shear rheology experiments for polymers with a molecular weight of 6 million and a concentration of 300 ppm.

[0042]

[0043]

[0044] S5. Based on the rheological equations obtained from S4, determine the solute viscosity, relaxation time, and maximum stretching length parameters in the FENE-P model, correct the momentum equation and molecular deformation transport equation, and compile the relevant UDF files.

[0045] The FENE-P model described above is a constitutive equation that can describe the combined effects of viscosity and elasticity in polymer solutions, specifically:

[0046]

[0047]

[0048]

[0049] In the formula, For elastic stress, Pa; μ p C is the viscosity of the solute, Pa·s; 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 stretch length.

[0050] Part of the compiled code is shown below:

[0051]

[0052] S6. Establish a numerical model for turbulent drag reduction simulation of viscoelastic polymers. Load the UDF file determined in S5 into the source, transient, and convection terms of the fluid domain in the FLUENT software. By changing the relevant viscoelastic parameters of the FENE-P model, the shear degradation flow field of the polymer drag-reducing fluid under various working conditions can be numerically simulated.

[0053] (1) Based on the polymer degradation ring track experimental device, the ANSYS ICEM software is used to establish a 1:1 structure geometric model of the visual small-scale ring track P3 to P4 section. The inner diameter of the pipeline model is 32 mm, and the length is 6.1 m;

[0054] (2) Define the pipeline parts name, and name the pipeline inlet, pipeline outlet, wall surface and fluid domain as inlet, outlet, wall and fluid respectively. The physical model is meshed, and the near-wall region is meshed, as shown in Figure 3 , the mesh file.msh is imported into ANSYS FLUENT;

[0055] (3) The UDF file of FENE-P equation is loaded in the Define-User-Defined-Function-Compiled command of ANSYS FLUENT. In this embodiment, three custom scalar transport equations are set, that is, the number of Scalars in the User-Defined module is 3. The source terms of the momentum equation and the transport equation are loaded into the fluid domain by opening the source terms in the fluid domain. The transient term is defined through the Flux Function in Scalars, and the dissipation term is defined by inputting the artificial viscosity coefficient in the material panel. For Newtonian fluid, the standard k-ε model is selected, and the wall function is selected as the enhanced wall surface. The polymer solution adopts the UDF loading mode to load the FENE-P model. The same velocity inlet (v=4.2 m / s) and pressure outlet (P=0Pa) are adopted for the flow of the two types of fluids.

[0056] (4) After the initialization, the iteration step number is set to 1000 steps, and the iteration interval time is 0.1 s. The actual iteration number is judged based on the residual during the calculation process: if the residual is lower than the set value during the iteration process, the iteration is stopped and step 5 is entered.

[0057] (5) After the calculation is completed, the data is processed. The velocity, turbulence intensity, turbulent kinetic energy and turbulent dissipation rate distribution of the near-wall region of the pipeline in the flow field are processed. The wall surface distribution data graph of the velocity and turbulence intensity is shown in Figure 4 , Figure 5 .

[0058] S7, analyze the differences of the flow field information of Newtonian fluid and FENE-P solution, evaluate the polymer shear degradation behavior from macro flow to micro flow field, and explain the polymer shear degradation mechanism.

[0059] The application solves the problem of lacking of integrity evaluation of shear degradation behavior of polymer in application of turbulent flow drag reduction technology, realizes evaluation of shear degradation of polymer from macro flow to micro flow field under various working conditions, and has important significance for evaluation of shear degradation behavior of polymer in turbulent flow drag reduction, research on degradation mechanism, and design of polymer drag reducer injection scheme.

Claims

1. A method for evaluating polymer shear degradation based on the FENE-P model, characterized in that, Includes the following steps: S1. Construct a polymer degradation loop experimental setup; S2. Based on the polymer degradation loop experimental device, experiments were conducted on the factors affecting polymer degradation, including polymer concentration, polymer molecular weight, and flow rate. The degradation rate of the polymer was calculated by 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 in S2, samples were taken from the solutions at different degradation stages and visco-shear rheological tests were conducted. S4. Fit the viscosity-shear rheological experimental data of S3 to establish rheological equations for polymer solutions at different degradation stages under different polymer concentrations, different polymer molecular weights and different flow rates. S5. Based on the rheological equations obtained from S4, determine the solute viscosity, relaxation time, and maximum stretching length parameters in the FENE-P model, correct 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, transient and convection terms of the fluid domain in FLUENT software. By changing the relevant viscoelastic parameters of the FENE-P model, the shear degradation flow field of the polymer drag-reducing fluid under various working conditions can be numerically simulated. S7. Analyze the differences in flow field information between Newtonian fluid and FENE-P solution. Specifically, by analyzing the real-time distribution of near-wall velocity, turbulence intensity, turbulent kinetic energy, and turbulent dissipation rate of polymer at different degradation stages in turbulent drag reduction, evaluate the polymer shear degradation behavior from macroscopic flow to microscopic flow field, and elucidate the polymer shear degradation mechanism.

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

3. The method according to claim 1, characterized in that, In S3, 30 ml samples of fluid were taken from the pipe at 0 min, 5 min, 1 h, and 2 h, and added to a rotational rheometer. A coaxial cylindrical system was used to conduct visco-shear rheological experiments on the polymer solution. The experimental temperature was set at 20℃, and the shear rate was 0–5000 s⁻¹. -1 .

4. The method according to claim 1, characterized in that, In S6, a numerical model for turbulent drag reduction of viscoelastic polymer solution is established based on ANSYS ICEM software. The numerical model for turbulent drag reduction of viscoelastic polymer solution is set up at a 1:1 ratio with the test tube 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 for selecting Newtonian fluids is... k-ε The model uses reinforced wall functions, and the polymer solution is loaded using the UDF loading method described above for the FENE-P model; 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, a UDF was written, imported into FLUENT, and iterative calculations were performed. The calculation results were then output.

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