➤ calculate cavitation number
➤ calculate local pressure
➤ calculate fluid vapor pressure
➤ calculate fluid density
➤ calculate characteristic flow velocity
calculate cavitation number
calculate local pressure
calculate fluid vapor pressure
calculate fluid density
calculate characteristic flow velocity
a dimensionless number that characterizes the cavitation state of fluid flow. the cavitation number is often used to measure whether cavitation occurs in liquid flow and the degree of cavitation development. the expression of cavitation number (σ) is
in the formula, p is the absolute pressure of the reference point; v0 is the flow rate of the undisturbed reference point; ρ is the density of the liquid; pv is the saturated vapor pressure of the liquid at the corresponding temperature.
the physical meaning of the above formula is the water flow parameter (p-pv) that inhibits cavitation, that is, the pressure difference between inside and outside cavitation, and the water flow parameter that promotes cavitation, that is, flow velocity. the values of cavitation number (σ) are different under different cavitation states. the larger the σ value, the less likely the liquid flow is to produce cavitation; otherwise, the liquid flow is more likely to produce cavitation.
the main factors that affect the occurrence and development of cavitation in liquid flow include: the form and size of the flow boundary, the gas content in the liquid flow and the distribution of gas nuclei, pressure gradient, turbulence degree, etc. incoming flow rate, viscosity and surface tension of the liquid, sand content and impurities in the liquid flow, roughness and wettability of the side wall, and thermodynamic factors of cavitation, etc. cavitation number takes into account only two factors: pressure and flow. therefore, this method of expressing cavitation still lacks sufficient theoretical foundation and comprehensiveness, and many conditions must be attached in practice.
when a small number of tiny holes begin to appear in the liquid flow, that is, the cavitation number when cavitation occurs is called the primary cavitation number (σi). this is the critical state of cavitation, which is very important for the study of cavitation phenomena. when the cavitation number σ>σi at a certain place in the liquid flow, cavitation will not occur there; when σ<σi, the cavitation range at that place in the liquid flow will continue to expand. at present, due to theoretical flaws, the σi value under specific conditions is mostly determined through decompression tests. in addition to being mainly affected by the shape of the flow field boundary, the σi value is also affected by the incoming flow characteristics and water quality. during the research process, it was found that due to various unknown reasons, the σi values obtained from the decompression test under the same conditions were scattered and had poor repeatability. for example, after cavitation occurs during a test, the pressure in the cavitation zone is increased again. when the cavitation phenomenon is observed to disappear, the cavitation at this time is called vanishing cavitation, and its corresponding cavitation number (σd) is called vanishing cavitation. cavitation number. usually σd>σi, and the repeatability of σd is good. this phenomenon that σd is not equal to σi is called cavitation residue (hysteresis).
the cavitation number can characterize the similarity of cavitation phenomena between two liquid flow systems under certain conditions. that is to say, when the reynolds number, froude number and other similar quasi-numbers are equal, if the cavitation numbers of the two liquid flow systems are equal, the cavitation phenomena can be considered to be the same; this is only theoretically correct based on force comparison. , but in fact, since the cavitation number itself does not include other factors that affect cavitation, the cavitation phenomena between two liquid flow systems are usually not completely similar.