水射流教学课件英文版：Oral presentation

1、1 Oral presentation nTo describe the characteristics of jet velocity profile according to the division of jet regions. nTo compare a submerged jet with a non submerged one based on the property of continuous free jet. nTo tell the difference between such pressures as instantaneous pressure and avera

2、ge pressure; and between stagnation pressure, static and dynamic pressures. 2 How to write a reasoning composition Generally contain an Introduction, Development and Conclusion. 1.Propose a point of view first; 2.Then give 2 or 3 examples of supports; 3. write your conclusions. 3 What requirements s

3、hould be met for a good upstream flow condition ? 1)Absence of bend and radial flow in (10-50) 0D the length to the nozzle, or (10-14) 0D if a smoother (a grid like honeycomb) is used. 2)Large diameter (at least 5 times Id of the nozzle ) 3)Symmetric, circular cross section 4)Good surface finish: ac

4、hieve hydraulically smooth state right or false choice 4 2.For a jet used for cutting, Leach 2) length of straight exit section L=(24)d0; 3) entry diameter D1=(510) d0 right or false ? 5 right or false ? There can exist three kinds of pressure resulted from jetting. nThe first is the stagnation pres

5、sure converted from jet velocity when the flow is completely stopped; nThe second can be water hammer pressure, which is produced by shock waves in a slug of water when it impact with solid surface. nThe third would be cavitating pressure exerted by micro-jets at the collapse of cavitating bubbles.

6、6 Lecture 2 Basic mechanics in jetting system For 4 hours 7 Basic mechanics in jetting system 1. Features of non-submerged jet flow 2. Velocity profiles of submerged jet 3. Considerations for jet system design 4. Fluid mechanics in jet flow 5. Basic fluid dynamics of jetting system 6. Review and Hom

7、e work 8 1. General Features of Non- submerged Continuous Jet Flow 9 1. General Features of Non-submerged Continuous Jet Flow As approaching downstream in air, water jet will dynamically entrain the surrounding air so that it tends to atomize and spread with its velocity slowing down gradually. Char

8、acteristics: 1) Atomization 2) Long core region and jetting distance versus submerged jet 10 1. General Features of Non-submerged Continuous Jet Flow According to V-profile characteristics, jet flow is divided into: a) initial region or core region, b)transient region c)essential region Or into: a)

9、Developing zone b) Developed zone-essential section nc dx)1000( 11 2.Features of Velocity Profiles of Submerged Continuous Free Jet virtual Geometric self-similarity of the flow velocity in essential section essential initial transient 12 (1) Initial Region /Potential Core nDefinition: the section w

10、here velocity the jet centerline remains homogeneous. nCharacteristics: (a) Due to Entraining surrounding liquid, jet flow gets wider, and the area of potential flow with common V0 becomes thinner and thinner and finally disappears. (b) For a round jet, potential core is typically a tapered one. 13

11、(1) Initial Region /Potential Core For a finely manufactured nozzle under good upstream flow conditions, the length of initial core of a submerged round jet is generally 3-8 times of D0. Or else, it may extremely be 0. Compared with that in non-submerged jet ! 14 (2)Transient section: Definition: th

12、e region between core region and essential region, where flow is turbulent but not full developed turbulent. (3) Essential region (Developed zone ): Definition: The zone of jet in the downstream of transient zone, where jet velocities attenuate self-similarly. 2.Features of Velocity Profiles of Subm

13、erged Continuous Free Jet How is a flow called a self-similar flow ? 15 Velocity Distribution on Different Cross- sections of Submerged Continuous WJ Velocity distribution varies at different standoff distance. Lx ry 16 Self-similarity in the Developed Zone of Submerged Continuous Jet Flow Velocitie

14、s of the flow coincide with one standard distribution curve or are expressed in one form of equation. Lx ry 17 Centerline Velocity of Submerged Jet nIn initial region: nIn essential region: L = standoff distance off the nozzle exit LC= length of core region C0 LL VVc C LL 1 L Vc 18 Axial Mean Veloci

15、ty Profile in Essential Region =r/(L-L0) Similarity of axial mean velocity 19 Radial Mean Velocity Profile Similarity of radial mean velocity Why so discrete ? 20 3. Considerations for jet system design 3.1 For continuous jet cutting 3.2 For specific application 3.3 Cavitating jet generation 3.4 Pul

16、sed jet generation 21 3. Considerations for Jet System Design Function of Nozzle Basic function of a nozzle is converting the potential energy of high pressure fluid into the kinetic energy of high velocity flow of the jet. The nozzle contraction ratio (dn/Di)2 is generally so small that nozzle exit

17、 flow velocity is quite uniform across whole cross section and that the turbulence intensity of flow in nozzle exit section is also suppressed to a great extent. 22 Though jet tends to spread and slow by nature, it can remain more coherent and more effective through improvements in the phases: n ups

18、tream flow condition n nozzle design n addition of some polymers 3.1 For Continuous Jet Cutting 23 nGood Upstream Flow Conditions It means a fully developed, stable flow created when a feeding pipe meets the requirements: 1)Absence of bend and radial flow in the length of (10-50) ID to the nozzle, o

19、r (10- 14) ID if a smoother (a grid like honeycomb) is used. 2)Large diameter (at least 5 times dn) 3)Symmetric, circular cross section 4)Good surface finish: achieve hydraulically smooth state 3.1 For Continuous Jet Cutting 24 nNozzle design 1950s Roses study suggested that nozzle should be avoidin

20、g roughness and abrupt discontinuity of flow. 1960s Farmer experimentally suggested the importance of following streamline design. 1964, Leach 2) L=(24)dn; 3) D1=(510) dn 3.1 For continuous jet cutting 25 3.1 For continuous jet cutting 3.1 For continueous jet cutting 26 3.1 For continuous jet cuttin

21、g 3.1 For continueous jet cutting 27 nTwo functions of Additive polymer in jet system I.Friction Reduction: to reduce friction in pipe line so as to get higher jet pressure at fixed pump pressure. 3.1 For Continuous Jet Cutting II.Improving Jetting: to make the jet smoother and stronger so as to jet

22、 farther 28 nFor cutting and drilling with small diameters (1d034.5MPa), the L But in jet techn., d2/d1 0.447 Water jet 50 How to get a true “C” of a nozzle? (4) )(1 683. 2 (5) 4 1 2 21 2 2 d d PP dQ QCQa Based on theories: Determining of Discharge Coefficient 51 In testing C of a nozzle, there are

23、some musts: 1)Make sure there is a uniform entry flow condition; 2)give a small pressure increment; if it is the first run, then begin with Pn= 0. 3)When the flow has got steady,Measure the Qa and the corresponding Pn accurately; 4)Calculate Qideal and C with Eq. (4), (5); Determining of Discharge Coefficient 52 5)Mark the point in Pn-C coordinates plane with the calculated C and Pn, and connect the last two points with a line. If the line interval becomes horizontal, the last C wil