A dimensionless number that characterizes the cavitation state of a fluid flow. The cavitation number is often used to measure whether cavitation occurs and the development degree of cavitation in a liquid flow. The expression for the cavitation number (σ) is
In the formula, p is the absolute pressure of the reference point; v0 is the undisturbed reference point flow rate; 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 contrast between the water flow parameter (p-pv) that inhibits cavitation, that is, the pressure difference inside and outside the cavitation, and the water flow parameter that promotes cavitation, that is, the flow velocity. The value of cavitation number (σ) is different in different cavitation states. The larger the value of σ, the less likely the liquid flow is to be cavitated; otherwise, the liquid flow is more likely to be cavitated.
The main factors that affect the occurrence and development of cavitation in liquid flow are: the form and size of the flow boundary, the gas content in the liquid flow and the distribution of gas cores, the gradient of pressure, the turbulence of the incoming flow, the viscosity and surface tension of the liquid , the sand content and impurities in the liquid flow, the roughness and wettability of the side walls, and the thermodynamic factors of cavitation, etc. The cavitation number only considers two factors: pressure and flow rate. Therefore, this method of expressing cavitation still lacks sufficient theoretical basis and comprehensiveness, so 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 a critical state of cavitation, which is very important in the study of cavitation phenomena. When the cavitation number σ>σi of a certain place in the liquid flow, no cavitation will occur at this place; when σ<σi, the range of cavitation at this place in the liquid flow will continue to expand. At present, due to theoretical deficiencies, the σi value under specific conditions is mostly determined by decompression tests. In addition to being mainly affected by the boundary shape of the flow field, 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 unclarified reasons, the σi values obtained through the decompression test under the same conditions are scattered and have poor repeatability. For example, after cavitation occurs during the 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 better. This phenomenon that σd is not equal to σi is called cavitation residue (Hysteresis).
The cavitation number can indicate 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 based on The comparison of forces is correct, but in fact, since the cavitation number itself does not include other factors that affect cavitation, the cavitation phenomena between the two liquid flow systems are usually not completely similar.