Study On Mechanism Of Incipient Cavitation And Scale Effect Of Cavitation

Based on summaries of predecessor's research findings, this paper theoretically analyzes the effects of gas nucleus and fluctuating pressure in water on cavitation inception; experimentally studies incipient cavitation of sudden enlargement type backward facing step flow with different inlet velocity and different height of step; numerically simulates incipient cavitation of sudden enlargement type backward facing step flow by two different methods, mixture model and k ~ ε model combined Non-constant density equation; analyzes and summaries necessary measures when conducting cavitation experiments. Main research contents and findings are as follows: 1. Gas nucleus should follow λ3 l similarity rule before gas nucleus in water don't reach absolute saturation state. After reaching absolute saturation state, density of gas nucleus is not a important parameters and not in accordance with λ3 lsimilarity rule, and similarity between model and prototype is automatically achieved. 2. Establishing empirical relationships of initial radius of gas nucleus and tensile strength of water body; suggesting that effects of tensile strength of water body should be considered when calculating cavitation numbers; analyzing effect of air content in water on eddy cavitation and deriving relationship between incipient cavitation number and gas content in water. 3. Studying the effects of number of gas nucleus on different cavitation type and incipient cavitation number; deriving that gas nucleus state has more effects on incipient cavitation number of bubble cavitation, gas nucleus content has few effects on incipient cavitation number of sheet cavitation, and incipient cavitation number of eddy cavitation is very sensitive to gas nucleus state; analyzing the effects of gas nucleus in water on cavitation phenomena and attained that different component of gas nucleus will result in different cavitation process because development of gas nucleus relates to gas density in bubble, multipartite exponent, diffusion coefficient, etc. 4. Studying influential mechanics of fluctuating pressure on cavitation inception; deriving that fluctuating pressure make cavitation beforehand occur and educing generalized model of cavitation number calculation formula which considering fluctuating pressure term. 5. Deriving model similarity criteria of fluctuating pressure from N-S equations, similarity criteria of fluctuating pressure meets gravity similarity law in strong turbulent motion free flow area, but not meets it for flow fluctuation at smooth boundary. 6. First carrying out normal pressure cavitation experiment for sudden enlargement body form under high flow velocity (from 20m/s to 40.0m/s) and deriving cavitation empirical formula of cavitation scale under different step height and different flow velocity conditions. Findings show that for incipient cavitation number of such body form under high flow velocity condition, adopting flow velocity modified formula suggested by Prof. Keller is not so suitable. 7. First adopting turbulence model to numerically simulate cavitation flow for sudden enlargement body form and the attained incipient cavitation number tallying with experimental data. Findings show that applied method can preferably simulate cavitation inception, whereas density of gas nucleus has larger influence on cavitation inception. Therefore, reasonable density of gas nucleus must be selected when adopting such model. 8. Numerical results for sudden enlargement flow show that the effects of cavitation or non-cavitation are few when inlet flow velocity is larger than critical value ( V > 5. 0m/s for this body form), and distributions of velocity and flow line atsudden enlarging location are basically same. Eddy range behind step of sudden enlargement section is only related with height of step, and eddy dimension is proportional to square of step height. Minimum pressure point of sudden enlargement flow emerges at step peak of sudden enlargement and eddy center behind step. 9. Adopting combined Non-constant density equation of k ~ ε turbulent flow model to simulate cavitation inception of sudden enlargement flow. Results show that incipient cavitation number attained by this method tallies with experimental data and range of incipient cavitation can be qualitatively analyzed. 10. Simulate outcomes of incipient cavitation number show that it is not velocity scale effect. 11. advancing some basic principle & measures which should be followed when conducting cavitation experiments.