【A Long Journey Begins with a Single Step – Centrifugal Pumps】1.1 Understanding the Concept: Viscosity and Viscosity Conversions

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This post was last edited by 3983596_FPPZ on 2018-11-14 at 15:38. 1.1.10 Viscosity and Viscosity Conversions: The media we come into contact with are not ideal fluids; they all possess a certain degree of viscosity. Many people become confused as soon as they hear the term “viscosity,” their thinking gets stuck on this concept, and discussions are limited to qualitative analysis rather than quantitative one. Viscosity is a measure of a fluid’s viscosity; it represents the force exerted by fluid flow against internal friction, and it is the ability to resist deformation under shear forces. One can think of viscosity as the frictional force of a liquid, which is just as essential as the frictional force of solids in everyday life. The relationship between the two viscosities: There are two common ways to express viscosity: dynamic viscosity and kinematic viscosity. Dynamic viscosity is the force required to produce a unit flow rate in a liquid layer of unit area over a unit distance; it is a type of absolute viscosity. The viscosity referred to in everyday language is actually dynamic viscosity, which is denoted by μ. In the International System of Units, the unit for dynamic viscosity is pascal-second (Pa·s). Since the Pa·s unit is too large, we commonly use 1 millipascal-second (1 mPa·s) to express it; 1 Pa·s equals 1000 mPa·s. Other units for dynamic viscosity include poise (P) and centipoise (cP), where 1 P equals 100 cP, which is equivalent to 0.1 Pa·s, while 1 mPa·s equals 1 cP. At 20°C, the dynamic viscosity of common liquids is as follows: water – 1 cP ； Alcohol 1.2cP ； Benzene 0.6cP ； Diethyl ether 0.2cP ； Yogurt 152cP ； Milk 3cP ； Ketchup: 1000cP; Soybean oil: 65cP ； Beer 1.1cP ； Juice 55-75cP ； Honey 3000cP ； Glycerin 1500cP ； Methanol 0.2cP. Kinematic viscosity: In the flow of a fluid, the viscous force that resists shear deformation is proportional to the velocity gradient and the contact area of the fluid. Kinematic viscosity is used as a measure of the internal frictional force when a liquid flows under the influence of gravity; it is equal to the ratio of the dynamic viscosity at the same temperature to the density of the fluid. Kinematic viscosity is denoted by ν. In the International System of Units, it is expressed in stokes (St) (m2/s). *The unit commonly used is centistokes (cSt); 1 m2/s = 100 cSt, and 1 mm2/s = 1 cSt = 10–6 m2/s. The conversion between dynamic viscosity and kinematic viscosity is given by μ = ν·ρ, where μ represents dynamic viscosity in mPa·s, ν represents kinematic viscosity in mm2/s, and ρ represents the density of the liquid in kg/m3. Generally, the higher the temperature of the liquid, the lower its viscosity. Therefore, to ensure that the lubricant has an appropriate viscosity, higher-viscosity lubricants should be used in summer or under high-temperature conditions to maintain its lubricating capacity. In winter, when the ambient temperature is low, lubricants with lower viscosity should be used (lubricants are generally classified according to their viscosity values). The viscosity has a significant impact on the performance of pumps; especially for centrifugal pumps, which rely on centrifugal force to function, an increase in viscosity leads to a significant reduction in pump flow rate, head, and efficiency, as well as an increase in shaft power. When the viscosity of the medium reaches a certain level, various performance parameters of the pump decline significantly; it becomes uneconomical to continue using a centrifugal pump for transportation, and it is necessary to consider using pumps with different designs as alternatives. Therefore, in the product catalogs of many centrifugal pumps, an upper limit for the viscosity of the media that can be transported by these pumps is specified. How to convert the initial parameters of a pump – flow rate Q, head H, and efficiency η – into the parameters for a pump used to transport viscous media, namely Qγ, Hγ, and ηγ? The simplest method for this is to use the statistical charts provided in professional literature to find the correction factor k corresponding to each pump parameter; by doing so, it is possible to determine the parameters of the pump when it is used to transport viscous media, and thereby obtain the corresponding performance curves. Qγ = kq*Q, Hγ = kH*H, ηγ = kη*η. These conversions are valid only for homogeneous media; they are not applicable to heterogeneous media such as those containing fibers or those with sedimentation properties. This correction coefficient k. Within a certain range of viscosity, the pump flow rate remains relatively accurate within a specific range. The charts and lookup methods for the correction factor k can be found on page P738 of Teacher Guan Xingfan’s \"Modern Pump Technology Manual\" and on page P64 of the Shen Pump Research Institute’s \"Vane Pump Design Manual\". After-class questions (multiple choice - ABDF): Assuming the medium has no viscosity, which of the following statements is correct? A. The pump head increases and the flow rate increases. B. The pump efficiency improves significantly. C. The power required to drive the pump shaft will **decrease**. D. The liquid vaporizes more easily. E. The motor speed increases significantly. F. There are no frictional losses or local losses in the piping, so the fluid transfer occurs with zero losses. ABDF