Experimental Study Of The Two-phase Structure And Dynamics Of Cavitation Flow

Hydrodynamic cavitation is a complex flow phenomenon involving mass and heat transfer between liquid and vapor phase at nearly constant temperature.It typically occurs in some widely-used hydraulic machines,such as pumps and propellers,when the local pressure is reduced below the vapor pressure.In contrast to relatively stable super-cavitation,the instability of partial cavitation appears as the shedding of vapor structures and the resultant cavity length oscillations.These unsteady behaviors are often responsible for undesired effects like performance degradation,material erosion,noise and vibration.Therefore,deep insights into the internal flow structures of partial cavities and the dynamics of cavitation unsteadiness are fundamental to control these detrimental effects.Advances in the understanding of the physical processes of cavitating flows are challenging,mainly due to the lack of quantitative experimental data on the two-phase structures and dynamics inside the opaque cavitation areas.In this thesis,partial cavitation developed in small convergent-divergent(Venturi)channels was studied experimentally in detail for a better knowledge of the physical mechanisms governing the cavitation structures and instabilities.This was achieved by using a state-of-the-art synchrotron X-ray imaging technique aided with conventional high speed photography and Particle Image Velocimetry.The main contributions of the present study are summarized as follows:(1)Due to high penetrability and weak interaction with matter,the use of X-rays instead of visible light solves cavitation opacity related issues.The fast synchrotron X-ray imaging,based on a combined mechanism of absorption and phase contrast,captures the fine two-phase morphological features inside the sheet cavity and visualizes clearly the seeded tracer particles.A wavelet-decomposition-based image processing method is developed originally and applied to separate the particles from the vapor structures,which enables a simultaneous acquisition of time-resolved velocity and void fraction fields in the cavitating flow.(2)The unprecedented data obtained from the X-ray imaging experiments reveals,for the first time,the complex diphasic flow structures of sheet cavitation,which is essentially divided into 6 characteristic parts.Distinct from the current mainstream view,the globally steady sheet cavitation is found to be characterized by a low-speed re-entrant flow existing continuously underneath the cavity.The presence of the re-entrant flow in sheet cavitation does not cause a large cavity detachment although it can pierce the entire cavity.The quasi-stable state of sheet cavitation is attributed to that the weak re-entrant flow does not have sufficient momentum to break off the cavity when it impinges on the cavity interface.(3)The complex effect of cavitation on turbulent velocity fluctuations is revealed.The presence of vapor phase due to cavitation strongly suppresses turbulent fluctuations,which might be attributed to two mechanisms:(i)the presence of the vapor phase modifies the vortexstretching process;(ii)the cavitation compressibility damps out the turbulent fluctuations.The collapse of small vapor structures in the studied sheet cavitation does not result in an evident increase of streamwise and cross-stream velocity fluctuations,but increases the shear stress noticeably.(4)Identification of three distinct mechanisms responsible for the transition of sheet-tocloud cavitation.The two well acknowledged mechanisms(re-entrant jets and condensation shocks)initiating the periodic cloud cavitation are observed in the experiments.In addition to these two mechanisms,the third independent mechanism,i.e.collapse-induced pressure waves is identified.A detailed discussion on the differences of the three mechanisms is presented based on the experimental results.(5)The observed scale effect on Venturi cavitating flows is analyzed.The Venturi throat height and the contraction ratio from entrance to throat are found to both play a crucial role to the Venturi cavitation dynamics.A smaller throat height causes a larger resistance acting on the re-entrant jet and a smaller contraction ratio results in a weaker driving force.Both of them would suppress the full development of the re-entrant jet and thus tent to stabilize the cavity.The Venturi geometrical parameters can only limit the cloud shedding induced by the re-entrant jet,while as the cavitation number decreases to a certain degree,sheet-to-cloud transition would skip the re-entrant jet mechanism and be directly triggered by condensation shocks.