Study of Particle Velocities in Water Driven Abrasive Jet Cutting Lab Report Essay
Abrasive jet cutting involves a situation where abrasive particles are driven at high velocities due to high pressure from water jet. In experimental work concerning the nature of abrasive behavior, there is application of conventional garnet sand in mixture with magnetic particles of the same size. Initial stages of the experiment involve injection of magnetic particles into conventional water jet. The action leads to cutting stream that is purposefully directed towards pair of current –carrying coils stretched at varying distances (Swanson et. al 103). There is creation of magnetic current through induction and the process finally recorded on a digital transient recorder. The particle velocity is indicated through measurement of time between signal responses from each of the coils.
The experiment requires use of small diameter in accelerating high velocity water towards target point. This application is utilized in the processes of cutting soft materials. However, inclusion of abrasive materials makes it easier for the process to be used in cutting other materials. The process is referred to as erosion process that involves the science of abrasion and fraction. The differences between the two are deduced from angles of particle impact (Swanson et. al 103).
Background theory
Water Jet is a process used in utilizing high-pressure water for the purposes of cutting soft materials. The pressurized water is usually forced through an orifice and normally approximated at 410 MPA (60ksi). The particle velocity is normally projected at very high speed. Abrasive Water jet is produced when abrasive particles are entrained and mixed through mechanical action into high velocity water jet.
Experimental setup
Operation involving water-jet is set-up in form of a pump capable of pressurizing water of up-to 410 MPa. The pipe is set to deliver water through small orifice of approximate diameter of between 0.13 mm to 1.32mm. The water is delivered in a continuous stream of water at high velocity of 825m/s affecting target material. With such impact, erosion is caused at high rate depending on mass and velocity of water and in the same process yield strength of target material considered. Other factors considered in this experiment are the stress on the equipment as well as precision machining for reliability purposes. Abrasive water jet utilizes the concept of water jet augmented with use of abrasives. The two are first mixed in a mixing tube made of carbide or ceramic, then afterwards the mixture discharged towards the target. The abrasive used are accelerated by water at very high velocity and impacted upon target material. According to research, specific energy (c) of any material is defined as the amount of energy necessary for the purposes of removing unit volume of any material, through abrasive wear (Swanson et. al 105). A good example used is the specific energy for cement given as;
c = ᴀ y2/E
ᴀ- constant
y- Elastic load limit (strength) of cement
E- Elastic modulus of cement
This case reveals the cutting activity as a function of total mass of abrasive particles as well as the square of their velocity. There are measurable quantities such as mass of particles and velocity of water jet that is derived from the relationship between pressure and volume. Despite calculations, actual velocity of various particles should be inferred.
Experimental measurements of velocity
This requires development of designed equipment, where cutting stream is channeled through director nozzle encircled by two small coils of wire attached with sensor and timer. Magnetic particles are mixed with abrasive material and hence accelerated through same stream. There is resultant electrical signal repeated as particles pass through other coil. The value obtained as transit time, spacing distance between the two fixed coils are measured and thence used in calculating individual particle velocity. Water jet velocity is calculated theoretically from Bernoulli’s equation, thus; (Swanson et. al 106).
Vw=2P/p whereby P represents pressure drop across orifice
At the same time force produced by water jet stream is measured using load cell, of which value obtained used in calculation of water jet velocity, thus the following equation used; (Swanson et. al 106)
Vw = F/pA
Vw= water velocity
F = Force of water het as measured
P = density of water
A = represents the axial surface area of water jet
Overall results of the experiment on particle velocity are revealed through graphical representation. This is where signals from first coil are presented on top trace of the graph while bottom trace represents signal results from second coil. Particle in the first coil at time zero T=0 triggers the overall record as shown in the graph. The graph represents two particles with different calculated velocities as well as temporal features. Exact nature of the particles is observed in the event that sweep time is increased for purposes of revealing detailed individual signals. Additionally, rotational characteristics and velocities are revealed with no particular physical significance.

  • An assumption is made on experimental determination of actual particle velocity.
  • Rotational velocities and characteristics of particles have no particular significance in the experiment
  • There is wide variation on individual particle velocities placed under same pressure and flow conditions (106)

Future work to improve on research done
Improvements should be made to ensure that particles are injected directly in the center of produced jet for the purposes of obtaining full jet velocity within slightest distance of travel (Swanson et. al 107).
Work cited
Swanson, Kilman, Carvin and Tarver. Study of Particle velocities in Water driven abrasive jet cutting. San Antonio, Texas, 2013, pp 103-107.

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