How to calculate the flow coefficient of low Reynolds number liquid


Reynolds number Re is a dimensionless number that indicates the state of fluid flow in a pipe. The characteristics of fluid flow in a pipe depend on the combined action of four parameters (pipe diameter, fluid viscosity, density and velocity). The Reynolds number can determine whether the liquid flow state is laminar or turbulent. The flow coefficient Kv is measured at an appropriate Reynolds number for turbulent flow. With the increase of Reynolds number Re, Kv value has little change. However, as the Reynolds number goes down, the effective Kv value goes down. In extreme cases where the Reynolds number is very low, such as for very viscous fluids, the flow of the liquid has become laminar and its flow is proportional to the valve pressure drop rather than to the reciprocal of the square root of the valve pressure drop. If Kv is calculated according to the above formula, the error must be large. Therefore, for the fluid with low Reynolds number (Re<104), the calculation of Kv should be corrected. The revised flow coefficient K 'v =Kv/FR (3-28), the revised flow coefficient K 'v ------; Kv--------- The flow coefficient calculated according to the above formula, namely the turbulent flow condition; FR---------- Reynolds number correction coefficient can be obtained from FIG. 3-11 according to the magnitude of Reynolds number Re. Reynolds number Re can be obtained from the following formula based on the structure and viscosity of the valve. For regulating valves with two parallel flow paths, such as straight double seat valves, butterfly valves, eccentric rotary valves, the Reynolds number is (3-29). For regulating valves with only one flow path, such as straight single seat valves, sleeve valves, ball valves, Angle valves, diaphragm valves, etc., the Reynolds number is (3-30) the viscosity of v------ liquid at flow temperature, mm2/s. As can be seen from the curve in Figure 3-11, when the Reynolds number Re is larger than 3500, the correction amount is not large, so when the Reynolds number is larger than 3500, there is generally no need to carry out low Reynolds number correction.