CUTTING IMPROVEMENTS BY ADDING DILATANTS FLUID
Effect of addition of viscoelastic additive on the stability and precision enhancement of the abrasive waterjet is studied. Cornstarch is chosen to be added to the mixture of water and abrasive for it is readily available in large quantities at low cost. Yet it shows major nonlinear properties valuable for waterjet machining. It is shown that the normal stresses developed in the nonlinear viscoelastic cornstarch remain substantially unchanged throughout effective jet length resulting in an almost completely prismatic jet, most desirable for precision and straight machining. Furthermore, the jet becomes more stable upon increasing the cornstarch percentage. The additive also causes the jet to produce less friction with the surrounding air avoiding possible jet disintegration. Clearly, due to the increase of elastic as well as viscous effects, there is restriction to the pump delivery upon adding the dilatant cornstarch. Different percentages of the additive are therefore examined. It is found that, a 22% additive results in the best performance based on the precision, available pump power and stability of the jet. The experiment was carried out on three products; marble, aluminum and glass. In all cases, kerf angle was reduced significantly. Simulation of the problem is in good agreement with the experimental observations.
Review of stability of liquid jets and the influence of nozzle design
Abstract An attempt has been made to form a qualitative correlation between the nozzle shape and issuing jet shape. The following points can be made: 1. (i) The major requirement of a nozzle is the efficient conversion of potential energy to kinetic energy. This is best achieved by a sudden, smooth contraction of the flow area from the supply line to the desired nozzle diameter. 2. (ii) The question of the angle of convergence appears uncertain. There would seem to be an optimum somewhere between 15° and 100°. It is possible that this optimum value might be a function of system dynamics and fluid properties. 3. (iii) The nozzle aspect ratio L/d has a highly significant effect on the initial jet velocity profile and subsequent jet surface shape up to L/d values where the shear stress and static pressure gradient are fully developed. In fact, the variability of L/d offers a means of control of the jet surface roughness over a considerable range. 4. (iv) There is some evidence favouring the rounding and polishing of the nozzle internal surfaces in order to achieve maximum performance. 5. (v) The theoretical hydrodynamic approach, involving numerical methods, should provide reasonable plug flow nozzles for systems where the liquid viscosity is low, viz. “water-like”.
Shear induced changes in the viscoelastic behaviour of heat treated potato starch dispersions
Abstract The viscoelastic behaviour of native potato starch dispersions was studied using dynamic viscoelastic measurements. The influence of various shear and heat treatments of gelatinized starch pastes were compared in the concentration range 4–10%. Heating had a minor effect compared to the effect of shear treatment. The complex modulus (G∗) decreased to about 5% of its maximum value when the starch pastes were sheared during the heating cycle, whereas heating alone resulted in a decrease to about 60% of the maximum G∗ value. Shear also caused an increase in the phase angle and characteristic changes were observed with regard to frequency and strain dependencies. When samples were prepared in a Brabender viscograph, the shear induced changes took place early in the pasting cycle. The characteristics of the concentration dependence differed due to the shear history of the sample. Peculiar concentration dependencies, some even negative, could be obtained from samples with different shear histories. The results imply that even moderate shear treatment gives rise to structure changes which are reflected by a transition in the type of the viscoelastic behaviour.