Investigation On Characteristics Of Cavitating Flow And Thermodynamic Effects In High Temperature Water

In cavitating flow,liquid adjoining the phase interface provides latent heat required for vaporization,causing a temperature drop in the local cavitation zone,which is known as the thermodynamic effects.Accordingly,the value reduction of saturated vapor pressure changes the condition for cavitation occurrence and its development.It also leads to variations in thermal properties,such as density,surface tension,and dynamic viscosity.The value of saturated vapor pressure and density ratio of vapor to liquid are large and sensitive to temperature variation in high-temperature water.And once cavitation occurs,there will be significant thermodynamic effects and consequently influence the cavitating flow.Therefore,revealing the characteristics of cavitation considering thermodynamic effects in high-temperature water has become the key prerequisite and foundation to solve the problems related to fluid mechanical cavitation.Based on numerical simulation and visualization experiment,the following work were carried out:Firstly,a corrective model considering thermodynamic effects,based on the classic ZGB cavitation model,was proposed.By considering the saturated vapor pressure of the liquid in local cavitation zone is related to the phase transition process and cavitation intensity,the changing law is derived from the Stepanoff-B factor(the ratio of vapor volume to liquid volume in cavitation zone.),characteristic temperature drop and Craberone equation.The pressure term influenced by thermal effects and turbulent kinetic energy effects were added to transport equations of the modified cavitation model.ANSYS Fluent solver is adopted to validate the corrected model.The pressure coefficient predicted by the modified model is more approximate to the test value compared with the classic ZGB cavitation model.The value of temperature drop and local cavitation number grow as the mainstream temperature increases,and consequently the maximum vapor volume fraction decrease.Secondly,with the modified cavitation model,the characteristics of hydrofoil cavitation at different temperatures of water were numerically investigated.Distribution of vapor phase morphology and mass transmission rate were obtained.The variation laws of vapor phase fraction,thermodynamic parameters,and cavity scale were also analyzed.The results show that thermal effects,inlet Reynolds number,and static pressure converge to act on cavitating flow.As the mainstream temperature increases,the ratio of saturated pressure to far-field pressure,inlet Reynolds number,and thermodynamic parameters increase,leading to an increment of mass transfer rate and temperature drop.But the maximum vapor-phase fraction decreases.The thickness of the attached bubble continues to enlarge,but the length expands first and then shortens.Under the constant mainstream temperature,the value of temperature drop and local cavitation number rise with the reduction of the far field cavitation number.And the maximum vapor phase fraction,turbulent kinetic energy,cavity thickness,and length of attached cavity augment consequently.However,the growth rate of the maximum vapor integral flattened.When the mainstream temperature is elevated from 323.15 K to 373.15 K,the evolution period of unsteady cavitating flow increases.The distribution of vapor volume enlarges but vapor volume diminishes.The development of the vortex adhere to the attached cavity closure position and the motion of the re-entrant jet are limited apparently.Finally,numerical studies on venturi cavitation at different temperatures were implemented to enrich the characteristics of high-temperature water cavitation flow under different structures of the flow domain.Moreover,a multi-phase flow test platform,running at high temperature and pressure,was built to visualize the process of venturi cavitation in conjunction with high-speed photography technology.Numerical results show that the value of temperature drop and local cavitation number grow as the mainstream temperature increases,and consequently the maximum vapor volume fraction decrease.The vapor volume fraction at the tail is less than that of the throat.It is shown that the modified model can also effectively predict the venturi cavitation flow characteristics.The increasing cavitation intensity strengthens the inhibition of thermodynamic effects.And,experimental researches also declare inhibition effects on the evolution of cloud cavitation.As the temperature increases,The maximum length of the attached bubble enlarges but the increment percent weakens,and the cavitation cloud survives longer.And the separation between the cavitation cloud and the expansion surface is more obvious.