A mixed-flow centrifugal pump and its design method

Abstract

This application relates to a mixed-flow centrifugal pump and its design method, belonging to the field of pumps. The pump includes a pump body with a pump chamber and an inlet. A rotating shaft is located within the pump chamber, and an inducer and an impeller are mounted on the shaft. The inducer extends into the inlet. This application has the advantage of reducing the probability of air binding.

Description

Technical Field

[0001] This application relates to the technical field of pumps, and in particular to a mixed-flow centrifugal pump and its design method. Background Technology

[0002] A centrifugal pump is a type of pump that uses the centrifugal force generated by the rotation of an impeller to transport liquids. Its working principle is based on the centrifugal motion of the water flow caused by the impeller's rotation. Before starting the centrifugal pump, ensure the pump casing is filled with water. Then, start the motor to drive the pump shaft to rotate, which in turn causes the impeller and water to rotate at high speed. Under the action of centrifugal force, the water flow is thrown to the outer edge of the impeller and enters the pump's discharge pipe through the flow channel of the volute casing.

[0003] If a centrifugal pump is not filled with water before starting, or if it is pumping a liquid containing gas (where the density of the gas is less than that of the liquid), the centrifugal force will be weak, and a sufficient low pressure will not be formed at the center of the impeller. This prevents the liquid below the pump from being drawn in under pressure, causing the pump to malfunction. Summary of the Invention

[0004] In view of the shortcomings of the existing technology, one of the objectives of this application is to provide a mixed-flow centrifugal pump and its design method, which has the advantage of reducing the probability of air binding.

[0005] The above-mentioned objective of this application is achieved through the following technical solution:

[0006] A mixed-flow centrifugal pump includes a pump body, a pump chamber and an inlet on the pump body, a rotating shaft inside the pump chamber, an inducer and an impeller on the rotating shaft, and the inducer extending into the inlet.

[0007] In a preferred embodiment, the present application may be further configured such that: the pump body is further provided with an auxiliary shaft, the auxiliary shaft is hollow, the rotating shaft extends out from the auxiliary shaft and is concentric with the auxiliary shaft, the auxiliary shaft is provided with guide vanes, and the guide vanes are provided with receiving cavities for accommodating impellers.

[0008] In a preferred embodiment, this application may be further configured such that the depth of the receiving cavity is 1 / 4 to 3 / 4 of the width of the impeller.

[0009] In a preferred embodiment, the present application may be further configured such that the rotation direction of the impeller is the same as the rotation direction of the guide vane, and the rotation speed of the impeller is not less than 1.5 times the rotation speed of the guide vane and not more than 3 times the rotation speed of the guide vane.

[0010] In a preferred embodiment, this application can be further configured such that the liquid flow direction at the outlet of the inducer is consistent with the flow direction at the inlet of the impeller, and the gap between the blades of the inducer and the wall of the inlet is no greater than 3 mm.

[0011] In a preferred embodiment, this application may be further configured such that the blades of the guide vane and the blades of the impeller are arranged radially, and the number of blades of the guide vane is not less than the number of blades of the impeller.

[0012] In a preferred embodiment, this application may be further configured to include a drive device for driving the auxiliary shaft and the rotating shaft to rotate. The drive device includes a first transmission component, a second transmission component, and a power source. Both the first and second transmission components are connected to the power source. The first transmission component is used to drive the auxiliary shaft to rotate, and the second transmission component is used to drive the rotating shaft to rotate.

[0013] In a preferred embodiment, the present application may be further configured such that: the transmission component one includes a meshing transmission gear one and a transmission gear two, the transmission gear one being driven by the power source, the transmission gear two being mounted on the auxiliary shaft; the transmission component two includes a meshing transmission gear three and a transmission gear four, the transmission gear three being driven by the power source, and the transmission gear four being mounted on the rotating shaft.

[0014] In a preferred embodiment, this application can be further configured such that the impeller speed n1 and the guide vane speed n2 satisfy the following formula: ,in n is the pump's rated flow rate, n1 is the impeller speed, n2 is the guide vane speed, and k is a correction factor, where 0.02 ≤ k ≤ 0.05.

[0015] This application also discloses a design method for a mixed-flow centrifugal pump, comprising the following steps:

[0016] Determine the type of fluid to be transported (viscosity μ, gas content α) and the target flow rate Q;

[0017] Select the depth of the depression area based on the fluid type: Define "depth coefficient λ = When the conveying medium A satisfies μ≤0.001Pa・s and α≤5%, the depth is 1 / 4 of the impeller width; when the conveying medium B satisfies gas content≥5% or viscosity≥0.005Pa・s, the depth is 3 / 4 of the impeller width; if the medium is between medium A and medium B, Where k1 and k2 are the weighting coefficients for the influence of gas content and viscosity on depth, and k1=k2=0.5 can be taken to achieve equivalent influence linear interpolation;

[0018] Calculate the minimum speed difference: the kinetic energy of the fluid ejected by the impeller. υ is the fluid linear velocity, which is positively correlated with the rotational speed. Based on the target flow rate Q, the impeller rotational speed n1 and the guide vane rotational speed n2 satisfy the following formula: ,in Where n is the pump's rated flow rate, n1 is the impeller speed, n2 is the guide vane speed, and k is a correction factor, 0.02≤k≤0.05;

[0019] Define the proportion of loop flow , , For through-flow flow, For circulating flow rate;

[0020] The flow field distribution was verified by numerical simulation. If the proportion of circulating flow was less than 20%, the rotational speed difference or the depth of the sinking region was increased until the anti-air binding requirements were met.

[0021] This application has the following advantages:

[0022] The inducer breaks up the gas in the liquid, allowing the liquid to be smoothly fed into the impeller. The liquid forms a through flow and a circulating flow in the impeller, which effectively solves the problem of gas binding. Even if the inlet water level is low, gas binding will not occur. Attached Figure Description

[0023] Figure 1 This is a schematic diagram of the internal structure of this application.

[0024] Figure 2 This is a schematic diagram of the inducer, impeller, and guide vane structure of this application.

[0025] Figure 3 This is a schematic diagram of the impeller and guide vane structure of this application.

[0026] Figure 4 This is a schematic diagram of the water flow in this application.

[0027] Reference numerals: 1. Pump body; 11. Pump chamber; 12. Inlet; 2. Drive unit; 21. Transmission component one; 22. Transmission component two; 3. Auxiliary shaft; 41. Inducer wheel; 42. Impeller; 43. Guide vane; 431. Receiving cavity; 5. Rotating shaft; A: Through flow; B: Circulating flow. Detailed Implementation

[0028] The present application will be further described in detail below with reference to the accompanying drawings.

[0029] Reference Figures 1-4 This application discloses a mixed-flow centrifugal pump, comprising a pump body 1 and a drive unit 2. The pump body 1 is provided with a pump chamber 11 and a water inlet 12. An auxiliary shaft 3 and a rotating shaft 5 are provided inside the pump chamber 11. The auxiliary shaft 3 is hollow, and the rotating shaft 5 and the auxiliary shaft 3 are rotatably connected by a sliding bearing. The auxiliary shaft 3 and the pump body 1 are rotatably connected.

[0030] The drive unit 2 includes a first transmission component 21, a second transmission component 22, and a power source, which is an electric motor. Both the first transmission component 21 and the second transmission component 22 are connected to the power source. The first transmission component 21 drives the auxiliary shaft 3 to rotate, and the second transmission component 22 drives the rotating shaft 5 to rotate. The first transmission component 21 includes two meshing transmission gears, the first of which is driven by the power source, and the second transmission gear is mounted on the auxiliary shaft 3. The second transmission component 22 includes two meshing transmission gears, the third of which is driven by the power source, and the fourth transmission gear is mounted on the rotating shaft 5. The first and third transmission gears are bevel gears, and the second and fourth transmission gears are spur gears.

[0031] A guide vane 43 is installed on the auxiliary shaft 3. The guide vane 43 is provided with a receiving cavity 431. The front cover plate of the guide vane 43 extends towards the water inlet 12. The end of the front cover plate near the water inlet 12 is supported on the pump body 1 by a bearing.

[0032] The depth of the receiving cavity 431 is 1 / 4 to 3 / 4 of the width of the impeller 42. The blades of the guide vane 43 and the impeller 42 are arranged radially, and the number of blades in the guide vane 43 is not less than the number of blades in the impeller 42. The rotation direction of the impeller 42 is the same as that of the guide vane 43, and the rotational speed of the impeller 42 is not less than 1.5 times and not more than 3 times the rotational speed of the guide vane 43. The inducer 41 is located on the side of the impeller 42 near the inlet 12, extending into the inlet 12. The liquid flow velocity direction at the outlet of the inducer 41 is consistent with the inlet flow velocity direction of the impeller 42, and the gap between the blades of the inducer 41 and the wall of the inlet 12 is not greater than 3 mm.

[0033] This application also discloses a design method for a mixed-flow centrifugal pump, characterized by comprising the following steps:

[0034] Determine the type of fluid to be transported (viscosity μ, gas content α) and the target flow rate Q;

[0035] Select the depth of the depression area based on the fluid type: Define "depth coefficient λ = When the conveying medium A satisfies μ≤0.001Pa・s and α≤5%, the depth is 1 / 4 of the impeller width 42; when the conveying medium B satisfies gas content≥5% or viscosity≥0.005Pa・s, the depth is 3 / 4 of the impeller width 42; if the medium is between medium A and medium B, Where k1 and k2 are the weighting coefficients for the influence of gas content and viscosity on depth, and k1=k2=0.5 can be taken to achieve equivalent influence linear interpolation;

[0036] Calculate the minimum speed difference: kinetic energy of the fluid ejected by impeller 42 υ is the fluid linear velocity, which is positively correlated with the rotational speed. Based on the target flow rate Q, the rotational speed n1 of impeller 42 and the rotational speed n2 of guide vane 43 satisfy the following formula: ,in Where n1 is the pump's rated flow rate, n2 is the impeller speed at 42, n2 is the guide vane speed at 43, and k is a correction factor, 0.02≤k≤0.05;

[0037] Define the proportion of loop flow , , For through-flow flow, For circulating flow rate;

[0038] The flow field distribution was verified by numerical simulation. If the proportion of circulating flow was less than 20%, the rotational speed difference or the depth of the sinking region was increased until the anti-air binding requirements were met.

[0039] More specifically, during centrifugal pump operation, the fluid ejected by impeller 42 needs to simultaneously achieve "high-efficiency discharge of the main flow (through flow A)" and "retention and breakup of gas-containing fluid (circulating flow B)" within the guide vane. According to the fluid continuity equation and the theory of rotating flow resistance: if the depth of the recessed area of ​​guide vane 43 (the dimension of impeller 42 embedded in guide vane 43) is too small (<1 / 4 of impeller 42 width), the rotation space of circulating flow B is narrow, and the frictional resistance of the fluid against the guide vane 43 wall increases sharply, making stable circulation impossible. If the depth is too large (>3 / 4 of impeller 42 width), the overlapping area of ​​impeller 42 and guide vane 43 will interfere with the main flow direction of through flow A, causing the fluid to "collide" at the inlet, leading to vibration and cavitation. Specifically, the following steps are included:

[0040] Determine the type of fluid to be transported (viscosity μ, gas content α) and the target flow rate Q;

[0041] Select the depth of the sinkhole area based on the fluid type: Define the "depth coefficient" through dimensional analysis. Based on statistics of the critical air-binding conditions of centrifugal pumps (more than 70% of air-binding failures are caused by unreasonable flow channel depth), it is determined that when conveying clean water with low air content (μ≤0.001Pa・s and α≤5%), the depth should be set to 1 / 4 of the impeller width. At this point, the flow resistance is minimal, and the through-flow A can efficiently discharge over 95% of the main fluid; when conveying high-gas-content / high-viscosity fluids (gas content ≥ 5% or viscosity ≥ 0.005 Pa·s), the depth is set to 3 / 4 of the impeller width. ), utilizing a larger rotational space to extend the residence time of the gas-containing fluid and break up bubbles; if the fluid condition is between the two, according to (k1 and k2 are the weighting coefficients of the influence of gas content and viscosity on depth. K1=k2=0.5 can be used to achieve equivalent influence.) Linear interpolation.

[0042] Calculating the minimum speed difference: Impeller 42 and guide vane 43 need to form a velocity gradient through the speed difference to provide continuous kinetic energy for the circulating flow B. According to Bernoulli's equation, the kinetic energy of the fluid thrown out by impeller 42... (υ is the fluid linear velocity, positively correlated with the rotational speed) The energy loss (including gas coalescence energy and fluid viscous drag energy) of the circulating flow B during rotation needs to be offset. If the speed difference is too small (the impeller 42 and guide vane 43 have similar speeds), insufficient kinetic energy will cause the circulating flow B to become "unstable," the low-pressure zone at the center of impeller 42 to expand, and air binding to be triggered. Based on the target flow rate Q, the rotational speed n1 of impeller 42 and the rotational speed n2 of guide vane 43 satisfy the following formula: (in The rated flow of the pump is given by n1, the impeller speed is 42, the guide vane speed is 43, and k is a correction coefficient (k=0.02 for clear water and k=0.05 for gas). The speed parameters are determined to ensure the stable formation of through flow and circulating flow.

[0043] Define the proportion of loop flow ( For through-flow flow, (Circulating flow rate). Based on the critical condition for air binding in centrifugal pumps (air binding occurs when the impeller center pressure < the fluid saturated vapor pressure), combined with the fluid dynamics low-pressure zone formation mechanism: when At that time, the "gas replenishment / gas breaking" capacity of the circulating flow B is insufficient, and the low-pressure area in the center of impeller 42 cannot be "filled" by the circulating flow, making it easy for gas to accumulate. This can help avoid air binding.

[0044] The flow field distribution was verified by numerical simulation. If the proportion of circulating flow was less than 20%, the rotational speed difference or the depth of the sinking region was increased until the anti-air binding requirements were met.

[0045] The implementation principle of this embodiment is as follows: Fluid enters the pump chamber 11 from the inlet 12. Through the action of the inducer 41, the gas in the liquid is broken up. The liquid can be smoothly sent into the impeller 42 through the inducer 41. Under the rotation of the impeller 42, the fluid near the top of the impeller 42 forms a through flow A due to the large centrifugal force, which drives the fluid into the inlet of the guide vane 43. The fluid in the middle of the impeller 42 forms a circulating flow B. The guide vane 43 rotates and continuously draws the through flow liquid from the impeller 42 to make centrifugal motion. Since the fluid is passively transported by the impeller 42, and since the rotation speed of the impeller 42 is greater than the rotation speed of the guide vane 43, the pressure of the liquid entering the pump body 1 increases, which further increases the output flow rate of the guide vane 43 and reduces the probability of air binding during the transport process of the guide vane 43.

[0046] The embodiments described in this specific implementation are preferred embodiments of this application and are not intended to limit the scope of protection of this application. Therefore, all equivalent changes made in accordance with the structure, shape and principle of this application should be covered within the scope of protection of this application.

Claims

1. A design method for a mixed-flow centrifugal pump, characterized in that: The mixed-flow centrifugal pump includes a pump body, a pump chamber and an inlet, a rotating shaft inside the pump chamber, an inducer and an impeller on the rotating shaft, the inducer extending into the inlet; the pump body also includes an auxiliary shaft, which is hollow, and the rotating shaft extends from the auxiliary shaft and is concentrically arranged with the auxiliary shaft, the auxiliary shaft having guide vanes, and the guide vanes having receiving cavities for accommodating the impeller, with a speed difference between the guide vanes and the impeller, including the following steps. Determine the type of fluid to be transported and the target flow rate Q. The parameters involved in determining the type of fluid to be transported are: viscosity μ and gas content α. Select the depth of the depression area based on the fluid type: Define the "depth coefficient". When the conveying medium A satisfies μ≤0.001Pa·s and α≤5%, the depth is 1 / 4 of the impeller width. When the conveying medium B satisfies gas content≥5% or viscosity≥0.005Pa·s, the depth is 3 / 4 of the impeller width. If the medium is between medium A and medium B, λ=0.25+0.5×[(α / 5%)×k1+(μ / 0.005)×k2] where k1 and k2 are the weighting coefficients of the influence of gas content and viscosity on the depth. k1=k2=0.5 can be taken to achieve equivalent influence linear interpolation. Calculate the minimum speed difference: the kinetic energy of the fluid ejected by the impeller. υ is the fluid linear velocity, which is positively correlated with the rotational speed. According to the target flow rate Q, the impeller speed n1 and the guide vane speed n2 satisfy the following formula: n1=1.5×n2+n2×(k×Q / Q0), where Q0 is the rated flow rate of the pump, n1 is the impeller speed, n2 is the guide vane speed, and k is a correction coefficient, 0.02≤k≤0.05; Define the proportion of loop flow Q A For the throughflow flow rate, Q B For circulating flow rate; The flow field distribution was verified by numerical simulation. If the proportion of circulating flow was less than 20%, the rotational speed difference or the depth of the sinking region was increased until the anti-air binding requirements were met.

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