Multi-scale conversion method for cavitation risk of top of impeller passage of tubular pump

Abstract

The invention belongs to the technical field of hydraulic mechanical engineering, and provides a multi-scale conversion method for cavitation risks at the top of an impeller passage of a tubular pump, which comprises the following implementation processes: firstly, defining'characteristic cavitation concentration 'to represent the strength of the cavitation risks at the top of the impeller passage; secondly, calculating a concerned prototype geometric scale ratio by taking a standard model geometric scale as a reference; utilizing a core conversion formula to solve the characteristic cavitation concentration under the geometric scale ratio; and finally outputting a result which is used for quantifying the intensity difference of the cavitation risk between the concerned prototype scale and the model reference scale. According to the method, the gradient rule that the vapor phase volume fraction at the top of the tubular pump impeller channel is amplified along with geometric similarity is given for the first time, and the cavitation variation degree of the scale concerned by a user compared with the model reference scale under the nominal primary cavitation number condition can be rapidly and accurately predicted; therefore, the cavitation risk of the large-scale water pump prototype can be evaluated by comparing the experimental result of the scale model in engineering.

Description

Technical Field

[0001] The invention belongs to the technical field of hydraulic machinery engineering, and in particular relates to a multi-scale conversion method for cavitation risk at the top of a flow channel of a crossflow pump impeller. Background Art

[0002] A tubular pump is a type of pump with a horizontal axis and a horizontal structure. There is a gravity-induced hydrostatic pressure difference between the top and bottom of its impeller flow channel. This pressure difference will continue to increase with the increase of the geometric scale, and will significantly increase the cavitation risk at the top of the flow channel under large-scale conditions. In engineering, the magnitude of the vapor phase volume fraction when cavitation occurs is often used to characterize the strength of the cavitation risk. However, under the same cavitation number, it is currently unclear what kind of gradual change law the vapor phase volume fraction at the top of the impeller flow channel of the tubular pump will produce with geometric similarity enlargement. Since it is difficult to directly carry out large-scale water pump prototype performance testing, if this multi-scale conversion method is not mastered, it is impossible to evaluate the cavitation risk of the water pump prototype after geometric similarity enlargement through the scaled model experimental results in the project, and thus it is impossible to determine the reasonable water pump installation elevation or backflow depth. With the construction of important projects in my country, it is urgent to solve this problem. Summary of the invention

[0003] (I) Technical Problems to be Solved by the Present Invention

[0004] The present invention aims to solve the problem that, under the same cavitation number conditions, the vapor phase volume fraction at the top of the impeller channel of a crossflow pump will produce a gradual change law as the geometric similarity is magnified. However, it is not clear at present. Since it is difficult to directly carry out performance testing of large-scale water pump prototypes, if this multi-scale conversion method is not mastered, it is impossible to evaluate the cavitation risk of the water pump prototype after geometric similarity magnification through the experimental results of the scaled model in the project, and thus it is impossible to determine a reasonable water pump installation elevation or backflow depth.

[0005] (II) Technical solution

[0006] The technical solution of the present invention is a multi-scale conversion method for cavitation risk at the top of the impeller flow channel of a tubular pump, and the implementation process includes the following steps:

[0007] Step 1: With the gravity direction as a reference, the vapor phase volume fraction at the top point of the impeller flow channel of the crossflow pump is defined as the "characteristic cavitation concentration" Ω to characterize the strength of the cavitation risk at the top of the impeller flow channel;

[0008] Step 2: Take the impeller diameter D m =0.3m is the reference scale of the standard model, and the impeller diameter D after the geometric similarity enlargement that the user is concerned about is entered. p , the corresponding geometric scale ratio θ=D is calculated p / D m ;

[0009] Step 3: Solve the following formula to obtain the characteristic cavitation concentration Ω under the geometric scale ratio θ;

[0010]

[0011] Among them, β1 is the lower limit value, β2 is the upper limit value, β3 is the slope value, and β4 is the mileage value;

[0012] Step 4: Output the Ω value and compare it with the nominal incipient cavitation number to determine the difference in cavitation risk between the scale of interest to the user and the reference scale of the standard model.

[0013] For the multi-scale conversion method of cavitation risk at the top of the impeller flow channel of a through-flow pump provided by the present invention, in the above step 3, the values ​​of the parameters in the expression of the characteristic cavitation concentration Ω under the geometric scale ratio θ are: β1=0.05, β2=1, β3=0.319, β4=4.857.

[0014] For the multi-scale conversion method for cavitation risk at the top of the impeller flow channel of a throughflow pump provided by the present invention, in the above step 4, the nominal primary cavitation number refers to the value of the impeller diameter D m = The cavitation number corresponding to the primary cavitation state at the top of the impeller flow channel of the crossflow pump under the standard model scale of 0.3m.

[0015] (III) Compared with the prior art, the beneficial effects of the present invention are:

[0016] The present invention proposes a multi-scale conversion method for the cavitation risk at the top of the impeller flow channel of a crossflow pump. The method has the advantages of giving, for the first time, the law of variation of the vapor phase volume fraction at the top of the impeller flow channel of a crossflow pump with geometric similarity amplification, and being able to quickly and accurately predict the difference in the degree of cavitation between the scale of concern to the user and the reference scale of the standard model under the condition of the nominal incipient cavitation number, thereby facilitating the evaluation of the cavitation risk of a large-scale water pump prototype by comparing the experimental results of the scaled model in the engineering. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The present invention will be further explained below in conjunction with the accompanying drawings and specific embodiments.

[0018] Figure 1 This is the location diagram of the "top point of the impeller flow channel of the crossflow pump" described in step 1.

[0019] Figure 2 The relationship curve between characteristic cavitation concentration and geometric scale ratio and its comparison with the measured value.

[0020] Figure 3 A flow chart of the implementation steps of the multi-scale conversion method for cavitation risk at the top of the impeller channel of a crossflow pump provided by the present invention. DETAILED DESCRIPTION

[0021] In order to make the purpose, technical solutions and advantages of the implementation of the present invention clearer, the relevant technical solutions will be described in more detail below in conjunction with the accompanying drawings. In the accompanying drawings, the same or similar reference numerals throughout represent the same or similar elements or elements with the same or similar functions. 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 making creative work and without departing from the essence and spirit taught by the present invention belong to the scope of protection of the present invention.

[0022] like Figure 1 The figure shows the position of the "top point of the impeller flow passage of the throughflow pump" described in the following step 1 of the embodiment of the present invention. Figure 3 As shown, an embodiment of the present invention provides a multi-scale conversion method for cavitation risk at the top of a throughflow pump impeller flow passage, comprising the following steps:

[0023] Step 1: With the gravity direction as a reference, the vapor phase volume fraction at the top point of the impeller flow channel of the crossflow pump is defined as the "characteristic cavitation concentration" Ω to characterize the strength of the cavitation risk at the top of the impeller flow channel;

[0024] Step 2: Take the impeller diameter D m =0.3m is the reference scale of the standard model, and the impeller diameter D after the geometric similarity enlargement that the user is concerned about is entered. p , the corresponding geometric scale ratio θ=D is calculated p / D m ;

[0025] Step 3: Solve the characteristic cavitation concentration Ω under the geometric scale ratio θ according to the following formula;

[0026]

[0027] Among them, β1 is the lower limit value, which is used to determine the minimum value of the characteristic cavitation concentration Ω; β2 is the upper limit value, which is used to determine the maximum value of the characteristic cavitation concentration Ω; β3 is the slope value, and β4 is the mileage value, which together reflect the speed of the change of the characteristic cavitation concentration Ω with the change of the geometric scale ratio θ;

[0028] Step 4: Output the Ω value and compare it with the nominal incipient cavitation number to determine the difference in the strength of the cavitation risk between the scale that the user is concerned about and the reference scale of the standard model. That is, by obtaining the difference in the strength of the cavitation risk at the top of the impeller flow channel of the tubular pump between the scale that the user is concerned about and the reference scale of the standard model under the condition of the nominal incipient cavitation number, the cavitation risk of the large-scale water pump prototype is quantified.

[0029] In step 3 provided in this embodiment, the values ​​of the parameters in the expression of the characteristic cavitation concentration Ω under the geometric scale ratio θ are: β1=0.05, β2=1, β3=0.319, β4=4.857.

[0030] In step 4 provided in this embodiment, the nominal primary cavitation number refers to the impeller diameter D m = 0.3m, the cavitation number corresponding to the primary cavitation state at the top of the impeller flow channel of the crossflow pump is used as the unified comparison condition under different geometric scales.

[0031] like Figure 2 The figure shows the relationship curve between characteristic cavitation concentration and geometric scale ratio, and the comparison between the curve and the measured value. It can be seen from the analysis that under the nominal primary cavitation number, the characteristic cavitation concentration Ω and the geometric scale ratio θ present a special correlation relationship, and do not obey the traditional cavitation similarity theory. Before this invention, the engineering was not clear about this variation law, that is, this invention is the first to give the variation law of the vapor phase volume fraction at the top of the impeller flow channel of the crossflow pump with the geometric scale ratio. m =0.3m is the model reference scale, and the operation condition of the tubular pump is set to the primary cavitation condition. It is found from actual observation that the top of the impeller channel reaches the primary cavitation state at this time, and the corresponding vapor phase volume fraction is about 0.1, while the characteristic cavitation concentration Ω predicted by the present invention is about 0.103, and the relative error between the two is about 3%. Taking this as the unified "nominal primary cavitation number" after geometric similarity amplification, when the geometric scale ratio of the tubular pump is 7, the impeller diameter of the water pump prototype is D p =2.1m, actual observations show that significant cavitation occurs at the top of the impeller flow channel but has not yet reached a pure vaporization state, and the corresponding vapor phase volume fraction is about 0.8, while the characteristic cavitation concentration Ω predicted by the present invention is about 0.837, and the relative error between the two is about 5%; when the geometric scale ratio of the tubular pump is 14, the impeller diameter of the water pump prototype is D p =4.2m, actual observations show that the top of the impeller flow channel is in a completely vaporized state at this time, and the corresponding vapor phase volume fraction is 1.0, while the characteristic cavitation concentration Ω predicted by the present invention is about 0.999, which is almost completely consistent. In contrast, according to the traditional cavitation similarity theory, the original and model have similar cavitation states under the nominal primary cavitation number condition, that is, after the pump is geometrically similarly magnified by 7 times and 14 times, the characteristic cavitation concentration Ω is about 0.1, which is obviously a serious deviation and will greatly underestimate the cavitation risk of large-scale water pumps.

[0032] In summary, the conversion relationship provided by the present invention can quickly and accurately predict the difference in the degree of cavitation between the scale of concern to the user and the reference scale of the model, thereby facilitating the evaluation of the cavitation risk of large-scale water pump prototypes by comparing the experimental results of the scaled model in the project.

[0033] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the protection scope of the technical solutions of the embodiments of the present invention.

Claims

1. A multi-scale conversion method for cavitation risk at the top of a tubular pump impeller flow channel, characterized in that: The method comprises the following steps: Step 1: With the gravity direction as reference, define the vapor phase volume fraction at the top point of the impeller flow channel of the crossflow pump as the "characteristic cavitation concentration" Ω to characterize the strength of the cavitation risk at the top of the impeller flow channel; Step 2: Take the impeller diameter D m =0.3m is the reference scale of the standard model, and the impeller diameter D after the geometric similarity enlargement that the user is concerned about is entered. p , the corresponding geometric scale ratio θ=D is calculated p / D m ; Step 3: Solve the characteristic cavitation concentration Ω under the geometric scale ratio θ according to the following formula; Among them, β1 is the lower limit value, β2 is the upper limit value, β3 is the slope value, and β4 is the mileage value; Step 4: Output the Ω value and compare it with the nominal incipient cavitation number to determine the difference in cavitation risk between the scale of interest to the user and the reference scale of the standard model.

2. A multi-scale conversion method for cavitation risk at the top of a throughflow pump impeller flow passage according to claim 1, characterized in that: In step 3, the values ​​of the parameters in the expression of characteristic cavitation concentration Ω under the geometric scale ratio θ are: β1=0.05, β2=1, β3=0.319, β4=4.

857.

3. The multi-scale conversion method for cavitation risk at the top of the impeller flow passage of a tubular pump according to claim 1, characterized in that: In step 4, the nominal incipient cavitation number refers to the impeller diameter D m = The cavitation number corresponding to the primary cavitation state at the top of the impeller flow channel of the crossflow pump under the standard model scale of 0.3m.

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