1、1,Chapter 3: Review 1,1.Whats cavitation ? cavitation is the phenomenon of cavity formation of the air, water vapor or nothing in flow field due to pressure drop. 2.Whats cavitating jet ? a jet in which the cavitation is artificially introduced or modulated to increase jet impact capability.,2,Revie

2、w 1,3. Cavitating jet generally experiences three phases: 1) Incipiency of cavitating; 2) Growth of bubbles; 3) Collapse of bubbles and generation of micro-jet,3,Review 1,4. True or false selection A) Cavitation happens only in liquid. On any occasion, it will never appear in air or solid B) Cavitat

3、ion results from the local pressure decreasing, and is generally found in the flow region where strong turbulence and vortex intensity exists. C) Cavitation is a dynamic phenomenon; There wont be cavitaion in a laminar flow.,4,Review 1,5. Identify the cavitation modes: Fixed cavitation rear flow cav

4、itation swirling cavitation shear flow cavitation resonating flow cavitation,5,Review 1,5. Identify the cavitation modes: Fixed cavitation rear flow cavitation swirling cavitation shear flow cavitation resonating flow cavitation,Requirement preferable for vortices combination and growth:,It is calle

5、d Strouhal number,6,Review 1,6. Give a interpretation of the following definition and analyze the cavitation tendence in dependence on the right hand parameters. When the K is less than 0.5, the jet would cavitate,7,Review 1,7. True or false selection (1) the lower the ambient pressure, the easier t

6、he cavitation; (2) the higher the temperature, the easier the cavitation; (3) the higher the relative velocity of the jet, the easier the cavitation,8,(4) Gas content , the number of bubble and the area of the cavitating region , but the intensity of the collapse violence of individual bubble (5)Sur

7、face intensity will inhibit cavitation.,Review 1,9,Review 2,Explain the functions of polymer additives on jet cutting Micro-mechanism linear structure and the aggregates of water reduce the entraining of surrounding fluid Macro-mechanismmacrostructure entrains less surrounding fluid Bombardment of m

8、acromolecules on target material Why do some polymers have no effect on jet performance at extremely high pressure jetting ?,10,Chapter 3,Advanced jet characteristics -Cavitating, polymer solution and star-shaped jets,for 4 hours,11,ROAD MAP OF THE CHAPTER 3,1. Cavitating jet 2. Polymer solution jet

9、 3. Star-shaped jet,12,1. Cavitating jet,1.1 Definition and general description of cavitation 1.2 Cavitation incipiency modes 1.3 Cavitation number and parameter effects 1.4 Study methods of cavitating jet 1.5 Effect characteristics of Cavitation 1.6 General observations on cavitation erosion,13,1.1

10、 Definition and general description of cavitation,1)Definition:Defined simply, cavitation is the formation of air or water vapor bubbles in flow field. When fluid is heated or the pressure decreased below its saturation vapor pressure, the air dissolved in the fluid will separate out and thus bubble

11、s appear. If the process is resulted from temperature rising, it is usually called “ boiling”; while resulted from pressure drop or velocity variation, the process is called “ cavitation.”,14,2) Phenomena: Cavitation in water jet generally experiences three phases: (A) Incipiency of cavitating (B) G

12、rowth of bubbles Cavitating incipiency and the growth are determined by many factors,of which the ratio of ambient pressure to the jet pressure, temperature, and dissolved gas and solid particles in jet fluid dominate the process.,1.1 Definition and general description of cavitation,15,A vorticity f

13、ield in the jet symmetric plane,m/(sm),V,16,(C) Collapses of bubbles and micro-jet generation In cavitating jet, the bubble collapse mainly results from pressure increase in jet flow: (a) When in air, jet pressure will go up a little at some distance downstream the nozzle exit.,1.1 Definition and ge

14、neral description of cavitation,17,C) Collapses of bubbles and micro-jet generation (b) when jet flows down in submerged mode, the vortex intensity generally decreases in jet shear layer, and therefore the low pressure in the eddies will increase subsequently;,1.1 Definition and general description

15、of cavitation,18,Pressure increase in the jet acts on bubbles immediately, and leads to the deformation and rapid collapses of the bubbles. During collapsing of bubbles, micro-jets will be generated of the intrusion water particles with extremely high velocity. The micro-jet generally creates a ultr

16、a-high impact pressure on a solid target,if there is one blocking the way of jet flow.,1.1 Definition and general description of cavitation,19,A possible collapse of a bubble and the generation of micro-jet,Deformation,Collapse There wont be cavitaion in laminar flow.,1.1 Definition and general desc

17、ription of cavitation,21,1.2 Cavitation incipiency modes,Fixed cavitation incipiency rear flow cavitation incipiency swirling cavitation incipiency shear flow cavitation incipiency resonating flow cavitation incipiency,22,1) Fixed cavitation incipiency,Bubble or cavity fixes on the solid surface or

18、in a fixed region. Because of the fluid mixture or fluid back-flow at the end region of cavitation area, fixed bubble may grow and collapse there. Strong oscillation will appear.,Fixed cavitation,C=0.80,23,2) Rear flow cavitation incipiency,Flowing around solid object, cavitaton is usually generated

19、 in the rear flow region Transient bubbles may come into being, grow, and collapse in downstream. This transient traveling bubble may appear at the low-pressure area along the solid surface, or in the core of moving vortex in high turbulent shearing field.,Why are many objects and creatures in water

20、 usually seen in a form of streamline ? Because of the survival of the fittest.,24,3) Swirling cavitation incipiency,This is one kind of cavitation detected earliest by people with naked eye. It always appears at the tips of ships propellers and is sometimes called “tip cavitation”. Only when the bu

21、bble of this cavitation collapses at or near the solid surface, can strong cavitation erosion happen.,Low pressure is generated by hub eddy,25,Swirling flow and rear flow incipiency,Swirl flow cavitation incipiency,rear flow cavitation incipiency,26,4) Shear flow cavitation incipiency,Cavitation, wi

22、th continuous and wide spectrums of amplitude and frequency, characterizes this type .,Vortex ring,Center of small eddy with strongest vorticity,Submerged jet,27,5) Resonation flow cavi. incipiency,28,6) Resonation flow cavi. incipiency,When resonating jet is modulated in a frequency range suitable

23、for the combination and growth of vortexes, the pulsation will be helpful to excite strong cavitation in shear flow. When the Strouhal number meets the requirement: The vortex wave will be self-excited to increase.,29,1.3 Cavitation index and parameter effects,Cavitation number: an index of probabil

24、ity and extent that cavitation appears. For jet: Pa: ambient pressure, Pv: vaporizing pressure; Vj: jet velocity relative to ambient flow. When the cavitation begins to happen, K=Ki , named cavitation incipiency number; when the cavitation further develops, K Ki = 0.4-0.5,30,1.3 parameters effect,1)

25、 Flow and environment parameters (1) ambient pressure: the lower, the easier (2) temperature : the higher, the easier (3) relative velocity: the higher, easier,31,2) gas content and surface intensity Gas content , the number of bubble and the area of the cavitating region , but the intensity of the

26、collapse violence of individual bubble Surface intensity will inhibit cavitation.,1.3 parameters effect,32,On the p2.26,1.4 Cavitating jet study methods,Cavitating initiated in the center of jet,Swirling jet,1) Erosion test method,33,Vortex ring,Vortex in shearing layer,Center of small eddy,One Cent

27、er of three small eddies,Submerged jet,2)CFD method,1.4 Cavitating jet study methods,34,3) Flow field test,1.4 Cavitating jet study methods,4) Acoustic measurement,35,1.5 Effects characteristics of Cavitation,Ultra-High pressure Strong and acute acoustic noise Localized high temperature emission Adv

28、antages The less gas content in the fluid, the bigger pressure is produced.,By Conn Cut width is substantially wider than a continuous jet, thus efficient for cleaning; Performance is enhanced underwater versus continuous jets. Disadvantages: Large kerf width Limited standoff distance capability Ine

29、rtial flow characterizes the high speed flow.,62,2.6 Types of Applications of SUPER-WATER,SUPER-WATER increases the efficiency of high-pressure water blasting in: cleaning lancing切割 decontamination 排污 underwater operations drilling cutting fracturing 破碎、壓裂 descaling 除垢,63,3 Star-shaped jet(星形噴嘴射流),3

30、.1 Back ground and aim 3.2 Comparison research method 3.3 Nozzle design 3.4 Experiment methods 3.5 Experiment results 3.6 Conclusion To give out a method of jet research,64,3.1 Background,Before 1990 the research efforts on water jet nozzles focused mainly on round ones specifically in contraction m

31、ode, surface finish and the modul-ation of desired jet performance. The theory and applications of round jets are rather complete and common.,65,3.1 Background,2. But the round jets are not absolutely more efficient over other shaped ones especially in jet cutting at high pressure. 3. In recent year

32、s some new nozzles with noncircular exit cross sections have shown their high efficiency in water jet cleaning industry.,66,3.2 Comparison research method,1. In Non-submerged mode： to measure standoff distance of incipiency of atomization 2. In submerged mode： to measure velocity field,67,3.3 nozzle

33、 design,Principles： 1）Only change to the star-shaped nozzle is to be made in the shape of cross section of exit section, with other parameters equal to those of round nozzle; 2）Cross section of nozzle exit is to be axisymmetric.,68,3.3 Nozzle design,Cross section area of the star-shaped nozzle： Sche

34、matic of the star-shaped nozzle:,69,3.3 nozzle design,Nozzle parameters,70,3.4 Experiment methods,In non-submerged mode： Two nozzles are connected in parallel to a symmetrically designed upstream entrance pipes, and the tests are conducted simultaneously.,71,3.4 Experiment methods,In submerged mode： with PIV (Particle Image Velocimeter) technology，measuring velocity fields of the two nozzle jets at common both upstream and downstream flow conditions, including exi