Hydraulic design method for hump-free impeller of low-specific-speed fire pump

Abstract

The invention relates to the technical field of fire pumps, in particular to a hydraulic design method for a hump-free impeller of a low-specific-speed fire pump, which comprises the following steps of: S1, designing geometric parameters of the impeller according to a design formula according to parameter requirements of flow Q, lift H and rotating speed n corresponding to a design working condition of the fire pump; s2, calculating and determining a flow lift curve hump value A and a flow lift curve critical hump value A0 according to the geometric parameters of the impeller in the S1; s3, comparing the flow lift curve hump value A with the flow lift curve critical hump value A0, if Agt; if A0, determining that the geometric parameters of the impeller do not meet the hump-free design, and returning to the step S1 to modify the geometric parameters of the impeller; and if A is smaller than or equal to A0, the geometric parameters of the impeller meet the hump-free design. According to the method, the two theoretical hump values are compared, so that the final geometric parameters of the designed impeller are determined, the technical requirement for no hump is met, the operation stability of a fire pump is guaranteed, and the service life of equipment is prolonged.

Description

Technical Field

[0001] The invention relates to the technical field of fire pumps, in particular to a method for hydraulically designing a humpless impeller of a low specific speed fire pump. Background Art

[0002] Low specific speed fire pump refers to a centrifugal pump with a specific speed of n=30-80. It has the characteristics of low flow and high head and is widely used in engineering sites such as fire water supply systems, fire hydrant systems, and water sprinkler fire extinguishing systems.

[0003] However, the flow-head curve of low-speed ratio fire pumps is prone to a "hump" phenomenon. In the hump area, there will be two flow points for one head, resulting in surging phenomena with sometimes small flow and sometimes large flow when the fire pump is running. This cannot meet the water supply demand and does not meet the technical requirements of the "Technical Specifications for Fire Water Supply and Fire Hydrant Systems". At the same time, the fire pump has poor stability when operating in the hump area. This negative effect is constantly accumulated and amplified, and in severe cases it will cause shutdown and affect the service life of the pump. Summary of the invention

[0004] In order to solve the above-mentioned deficiencies in the prior art, the present invention provides an impeller hydraulic design method that meets the requirement of a low specific speed fire pump having a hump-free flow head curve.

[0005] The technical solution of the present invention is: a method for hydraulically designing a humpless impeller of a low specific speed fire pump, comprising the following steps: S1. According to the flow rate Q, head H, and speed n parameter requirements corresponding to the design working conditions of the fire pump, the geometric parameters of the impeller are designed according to the design formula, including the impeller inlet diameter D1, the impeller outlet diameter D2, the blade inlet width b1 and the blade outlet width b2; S2. According to the geometric parameters of the impeller in S1, the hump value A of the flow head curve and the critical hump value A0 of the flow head curve are calculated and determined. The specific calculation method is as follows: The calculation formula for the hump value A of the flow head curve is as follows: Where: β1—blade inlet placement angle; Fc—water-passing cross-sectional area perpendicular to the axial streamline at the blade inlet, m 2 ; The calculation formula for the critical hump value A0 of the flow head curve is as follows: Where: β2—blade outlet placement angle; g—acceleration due to gravity, 10m / s 2 φ2—blade outlet crowding coefficient; S3. Compare the hump value A of the flow head curve with the critical hump value A0 of the flow head curve. If A>A0, the geometric parameters of the impeller do not satisfy the hump-free design, and return to step S1 to modify the geometric parameters of the impeller; if A≤A0, the geometric parameters of the impeller satisfy the hump-free design.

[0006] Further preferably, the water flow cross-sectional area Fc perpendicular to the axial streamline at the blade inlet in step S2 is calculated as follows: The calculation formula of blade inlet placement angle β1 is as follows: Where: R c1 —Radius of the centroid of the blade inlet edge, m; Δβ—blade inlet angle of attack; φ1—Blade inlet crowding coefficient.

[0007] Further preferably, the blade inlet crowding coefficient φ1 is calculated as follows: The calculation formula of blade outlet crowding coefficient φ2 is as follows: Where: Z—number of blades; S u1 —Thickness of impeller inlet circumference, m; S u2 —Thickness of impeller outlet circumference, m.

[0008] Further preferably, in step S1, the impeller inlet diameter D1, the impeller outlet diameter D2, the blade inlet width b1 and the blade outlet width b2 are calculated according to the following formulas: Where: k1—the first coefficient of the inlet diameter, which is 0.8-1; k0—the second coefficient of the inlet diameter, which is 3.5-5.5; k D2 —D2 correction coefficient, take 1.038-1.175; k b2 —b2 correction factor, take 1.326-1.94; d h —Impeller hub diameter, m.

[0009] Further preferably, the impeller inlet circumferential thickness S u1 The calculation formula is as follows: Impeller outlet circumferential thickness S u2 The calculation formula is as follows: Where: δ—real thickness of blade, m; λ—the angle between the blade and the flow surface.

[0010] Further preferably, the impeller blade wrap angle Ψ and the number of blades Z satisfy the following relationship: Where: k Ψ —Blade wrap coefficient, take 1.4-1.8.

[0011] Further preferably, the blade outlet placement angle β2 is 22°-30°, the blade inlet attack angle Δβ is 5°-15°, the angle λ between the blade and the flow surface is 60°-90°, and the blade number Z is 4-6.

[0012] The beneficial effects of the present invention are as follows: the present design method uses the calculated impeller geometric parameters to determine the flow head curve hump value A and the flow head curve critical hump value A0 according to a specific theoretical calculation formula, and compares the two hump theoretical values ​​to determine the final geometric parameters of the designed impeller, so that the designed fire pump meets the technical requirements of the "Technical Specifications for Fire Water Supply and Fire Hydrant Systems", increases its operating stability, and extends the equipment life. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 It is a schematic diagram of the design process of the present invention; Figure 2 It is a cross-sectional view of the impeller axial surface of the fire pump in the present invention; Figure 3 It is a plane cross-sectional view of the impeller of the fire pump in the present invention; Figure 4 It is a front view of the fire pump in the present invention; Figure 5 is a side view of the fire pump of the present invention; Figure 6 It is a comparison diagram of flow head curves of different embodiments of the present invention. DETAILED DESCRIPTION

[0014] In order to enable those skilled in the art to better understand the technical solution in the present invention, the technical solution in the present invention is clearly and completely described below in conjunction with the accompanying drawings. Other embodiments obtained by those skilled in the art without making any creative work should all fall within the scope of protection of the present invention.

[0015] like Figure 1-Figure 5As shown, the present invention provides a method for hydraulic design of a humpless impeller of a low specific speed fire pump, comprising the following steps: S1. According to the flow rate Q, head H, and speed n parameter requirements of the fire pump under the design working conditions, the geometric parameters of the impeller are designed according to the design formula, including the impeller inlet diameter D1, impeller outlet diameter D2, blade inlet width b1, blade outlet width b2, impeller inlet circumferential thickness S u1 and the impeller outlet circumferential thickness S u2 , the calculation formula of the geometric parameters of the above impeller is as follows: Where: Q—design operating flow, m 3 / s; H—design working head, m; n—design operating condition impeller speed r / min; k1—the first coefficient of the inlet diameter, which is 0.8-1; k0—the second coefficient of the inlet diameter, which is 3.5-5.5; k D2 —D2 correction coefficient, take 1.038-1.175; k b2 —b2 correction factor, take 1.326-1.94; d h —Impeller hub diameter, m; β1—blade inlet placement angle; β2—blade outlet placement angle; λ—the angle between the blade and the flow surface; δ—real thickness of blade, m; The impeller blade wrap angle Ψ and the number of blades Z satisfy the following relationship: Where: k Ψ —Blade wrap coefficient, take 1.4-1.8; S2. Calculate and determine the hump value A of the flow head curve and the critical hump value A0 of the flow head curve based on the geometric parameters of the impeller in S1. The specific theoretical basis steps are as follows: S201, given head and flow function H = f (Q) Among them, f1(Q) is the impeller inlet impact hydraulic loss, and its calculation formula is: f2(Q) is the friction hydraulic loss on the impeller surface, and its calculation formula is: Where W1 is the relative speed of the blade inlet and outlet, and its calculation formula is: f3(Q) is the friction hydraulic loss on the volute surface, and its calculation formula is: Where Vm is the sum of the blade inlet axial velocity and the diffuser inlet axial velocity, and its calculation formula is: f4(Q) is the diffusion hydraulic loss, and its calculation formula is: Where W2 is the difference between the blade inlet relative velocity and twice the blade outlet relative velocity, and its calculation formula is: f5(Q) is the impact hydraulic loss at the diffuser inlet, and its calculation formula is: f6(Q) is the hydraulic loss of the diffusion tube, and its calculation formula is: Where: φ2—blade outlet crowding coefficient; g—acceleration due to gravity, 10m / s 2 ; Fc—water-passing cross-sectional area perpendicular to the axial streamline at the blade inlet, m 2 ; F2—water flow cross-sectional area perpendicular to the axial streamline at the blade outlet, m 2 ; L—flow channel length, m; D4—volute throat diameter, m; β3 is the axial inlet angle of the diffuser; b4 is the inlet width of the diffuser, m; k—average velocity coefficient of the volute throat, ranging from 0.81 to 1.21; S202, derive the function in S201: Record df1(Q) / dQ=A+BQ, df2(Q) / dQ=CQ, df3(Q) / dQ=DQ, df4(Q) / dQ=EQ, df5(Q) / dQ=FQ, df6(Q) / dQ=GQ respectively; S203, distinguish and calculate the hump phenomenon of the flow head curve: Let df (Q) / dQ=0, , the geometric meaning of the above formula is: the head corresponding to Q0 is the maximum head in the flow head curve. When Q0≤0, it can ensure that the flow head curve is strictly monotonically decreasing, that is, the curve has no hump, and the following results are obtained: S204, according to the hump phenomenon in S203, the hump value of the flow head curve is determined and set as A, and the critical hump value of the flow head curve is set as A0. The specific calculation method is as follows: S3. Compare the hump value A of the flow head curve with the critical hump value A0 of the flow head curve. If A>A0, the geometric parameters of the impeller do not satisfy the hump-free design, and return to step S1 to modify the geometric parameters of the impeller; if A≤A0, the geometric parameters of the impeller satisfy the hump-free design.

[0016] The calculation formula for the water flow cross-sectional area Fc perpendicular to the axial streamline at the blade inlet in step S2 is as follows: The calculation formula of blade inlet placement angle β1 is as follows: Where: Δβ—blade inlet angle of attack; R c1 —Radius of the centroid of the blade inlet edge, m; φ1—blade inlet crowding coefficient; The calculation formula of blade inlet crowding coefficient φ1 is as follows: The calculation formula of blade outlet crowding coefficient φ2 is as follows: The blade outlet placement angle β2 is 22°-30°, the blade inlet attack angle Δβ is 5°-15°, the angle λ between the blade and the flow surface is 60°-90°, and the blade number Z is 4-6.

[0017] Example 1 In the design working condition, the flow rate Q is 72m 3 / h, design working head H is 150m, and design working impeller speed n is 2970r / min. The calculation process of hydraulic design of impeller 1 is as follows: The specific speed is calculated according to the formula The calculation is 35.7, and the coefficients are selected as follows: k1=1, k0=5.5, k D2 =1.175, k b2 =1.94, k Ψ =1.8; d h The actual measured value is 36mm, the actual measured value of δ is 5.5mm; the value of Δβ is 15°, the value of β2 is 30°, and the value of λ is 70°; the value of Z is 6, and the value of R c1 The actual measurement is 30mm.

[0018] After calculation, the geometric parameters of the impeller in this embodiment are as follows: D1=(5.5 2 (72 / 3600 / 2970) 2 / 3 +(36 / 1000) 2 ) 0.5 =110mm; D2=9.35*1.175*(35.7 / 100) -0.5 *(72 / 3600 / 2970) 1 / 3 =350mm; b1=(110-36) / 2=37mm; b2=0.64*1.91*(35.7 / 100) 5 / 6 *(72 / 3600 / 2970) 1 / 3 =10mm; S U1 =5.5 / sin70°=6mm; S U2 =5.5(1+cot 2 30° / sin 2 70°) 0.5 =12mm; φ1=1-6*6 / 3.14*110=0.9; φ2=1-6*12 / 3.14*350=0.9; β1 = arctan(60 * 72 / 3600 / (3.14 2 * 0.11 2 * 2970 * 0.037 * 0.9)) + 15° = 21°; Ψ = 1.8 * 360 / 6 = 108°.

[0019] The operation results are obtained according to the calculation formulas of the peak value of the flow - head curve and the critical peak value of the flow - head curve as follows: A = (3.14 * 2970 * 0.11 * tan21°) / 60(2 * 3.14 * 0.03 * 0.037 * 0.9) = 1046; A0 = 2970 / (60 * 10 * 0.01 * 0.9 * tan30°) = 953; According to the comparison of the above operation results, it is found that the peak value A of the head - flow curve is greater than the critical peak value A0 of the head - flow curve, which does not meet the requirements of the present invention. As Figure 6 shown by curve a in, the head - flow curve of the impeller in Example 1 shows an obvious hump. Therefore, it is necessary to modify the geometric parameters of the impeller so that the peak value A of the head - flow curve is less than the critical peak value A0 of the head - flow curve.

[0020] Example 2 On the basis of Example 1, the geometric parameters of the impeller are modified. By reducing the value of Δβ, the calculation result of A is reduced, and by reducing the value of β2, the calculation result of A is increased; specifically as follows: the value of Δβ is 12°, and the value of β2 is 28°. The operation results of judging the hump phenomenon by setting the peak value of the flow - head curve and the critical peak value of the flow - head curve in Example 2 are as follows: A = (3.14 * 2970 * 0.11 * tan18°) / 60(2 * 3.14 * 0.03 * 0.037 * 0.9) = 910; A0 = 2970 / (60 * 10 * 0.01 * 0.9 * tan28°) = 1034; A < A0, which meets the requirements of the present invention. As Figure 6 shown by curve b in, the head - flow curve of the impeller in Example 2 does not show an obvious hump. The modified geometric parameters are the final geometric parameters of the impeller. The low - specific - speed fire pump designed with these parameters meets the technical requirements of the Technical Code for Fire Water Supply and Fire Hydrant Systems.

Claims

1. A method for hydraulically designing a humpless impeller of a low specific speed fire pump, characterized in that: The following steps are involved: S1. According to the flow rate Q, head H, and speed n parameter requirements corresponding to the design working conditions of the fire pump, the geometric parameters of the impeller are designed according to the design formula, including the impeller inlet diameter D1, the impeller outlet diameter D2, the blade inlet width b1 and the blade outlet width b2; S2. According to the geometric parameters of the impeller in S1, the hump value A of the flow head curve and the critical hump value A0 of the flow head curve are calculated and determined. The specific calculation method is as follows: The calculation formula for the hump value A of the flow head curve is as follows: Where: β1—blade inlet placement angle; Fc—water-passing cross-sectional area perpendicular to the axial streamline at the blade inlet, m 2 ; The calculation formula for the critical hump value A0 of the flow head curve is as follows: Where: β2—blade outlet placement angle; g—acceleration due to gravity, 10m / s 2 ; φ2—blade outlet crowding coefficient; S3. Compare the hump value A of the flow head curve with the critical hump value A0 of the flow head curve. If A>A0, the geometric parameters of the impeller do not satisfy the hump-free design, and return to step S1 to modify the geometric parameters of the impeller; if A≤A0, the geometric parameters of the impeller satisfy the hump-free design.

2. The method for hydraulic design of a low specific speed fire pump impeller without a hump according to claim 1, characterized in that: The calculation formula for the water flow cross-sectional area Fc at the blade inlet perpendicular to the axial streamline in step S2 is as follows: The calculation formula of blade inlet placement angle β1 is as follows: Where: R c1 —Radius of the centroid of the blade inlet edge, m; Δβ—blade inlet angle of attack; φ1—Blade inlet crowding coefficient.

3. The method for hydraulic design of a low specific speed fire pump impeller without a hump according to claim 2, characterized in that: The calculation formula of blade inlet crowding coefficient φ1 is as follows: The calculation formula of blade outlet crowding coefficient φ2 is as follows: Where: Z—number of blades; S u1 —Thickness of impeller inlet circumference, m; S u2 —Thickness of impeller outlet circumference, m.

4. A method for hydraulically designing a humpless impeller of a low specific speed fire pump according to any one of claims 1 to 3, characterized in that: In step S1, the impeller inlet diameter D1, the impeller outlet diameter D2, the blade inlet width b1 and the blade outlet width b2 are calculated according to the following formulas: Where: k1—the first coefficient of the inlet diameter, which is 0.8-1; k0—the second coefficient of the inlet diameter, which is 3.5-5.5; k D2 —D2 correction coefficient, take 1.038-1.175; k b2 —b2 correction factor, take 1.326-1.94; d h —Impeller hub diameter, m.

5. The method for hydraulic design of a humpless impeller of a low specific speed fire pump according to claim 3, characterized in that: Impeller inlet circumferential thickness S u1 The calculation formula is as follows: Impeller outlet circumferential thickness S u2 The calculation formula is as follows: Where: δ—actual thickness of blade, m; λ—the angle between the blade and the flow surface.

6. The method for hydraulic design of a low specific speed fire pump impeller without a hump according to claim 5, characterized in that: The impeller blade wrap angle Ψ and the number of blades Z satisfy the following relationship: Where: k Ψ —Blade wrap coefficient, take 1.4-1.

8.

7. A method for hydraulically designing a humpless impeller of a low specific speed fire pump according to claim 5 or 6, characterized in that: The blade outlet placement angle β2 is 22°-30°, the blade inlet attack angle Δβ is 5°-15°, the angle λ between the blade and the flow surface is 60°-90°, and the blade number Z is 4-6.

Images