5,274 research outputs found
Continuous direct ink jet printing.
No full textPhDThis thesis describes the preparation and continuous printing of zirconia ink under different
conditions, as well as the development of silver inks, for the same purpose.
The dispersion of sub-micrometer zirconia powder in industrial methylated spirit using other
additives such as dispersant and binder was investigated with different mixing methods and
at varying powder and binder contents. The use of high shear mixing by triple roll milling
followed by ultrasonic disruption as well as adequate sedimentation and filtration produced a
homogeneous and stable ink of 2.5 vol. % ZrO2. The ink could be printed directly and
continuously on a commercial jet printer without interruption of any kind and the phenomena
occurring during printing were investigated. The optimum modulation frequency for
printing was determined with the generation of pear-shaped and symmetrical droplets.
Printing was made on substrates of surface free energies lower and higher than the surface
tension of the Zr02 ink. Powder migration was observed within a relic of the printed dot on
the second type of substrate. Layers were also overprinted on the second type of substrate
by varying the following: print resolution, printing interval, print area, drying conditions and
ink powder loading. These series of prints were accompanied by the appearance of ridges,
spattering and non-vertical walls and the effects were investigated.
The wettability and shrinkage of droplets of the ceramic ink was also studied in-situ by
monitoring the evolution of contact angle, width of ink-substrate interface and droplet height
with a video camera. The shape of the droplet experienced different dynamics on different
types of substrate.
Lastly, the sedimentation behaviour of ethanol-based silver inks dispersed with different
types of dispersant was investigated with respect to the sediment volume and half-value
time. Deflocculated ink was obtained at a low dispersant level and powder loading
Parametric study of dust acoustic wave destabilization in the JET scrape-off layer
No full textPublisher Copyright: © 2024 Author(s).Sudden inflows of W particulate are routinely observed in correlation with plasma instabilities in JET-ITER-Like-Wall experimental campaigns. Dust acoustic wave destabilization by micrometer-size tungsten dust released in such events is simulated here using realistic parameters describing the JET scrape-off layer. Such low-frequency waves may serve as a monitorable signal for dust detection in the scrape-off layer on a pulse-by-pulse basis.Peer reviewe
Ultra-fast escape of a deformable jet-propelled body
No full textIn this work a cephalopod-like deformable body that fills an internal cavity with fluid and expels it to propel an escape manoeuvre, while undergoing a drastic external shape change through shrinking, is shown to employ viscous as well as mainly inviscid hydrodynamic mechanisms to power an impressively fast start. First, we show that recovery of added-mass energy enables a shrinking rocket in a dense inviscid flow to achieve greater escape speed than an identical rocket in a vacuum. Next, we extend the shrinking body results of Weymouth & Triantafyllou (J. Fluid Mech., vol. 702, 2012, pp. 470–487) to three-dimensional bodies and show that three hydrodynamic mechanisms must be combined to achieve rapid escape performance in a viscous fluid: added-mass energy recovery; flow separation elimination; and an optimized energy storage and recovery. In particular, we show that the mechanism of separation elimination achieved through rapid body shrinking, coordinated with the mechanism of recovering the initially imparted added-mass energy, is critical to achieving a high escape speed. Hence a flexible, collapsing body can be vastly superior to a rigid-shell jet-propelled body
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Dependence of Drop Speed on Nozzle Diameter, Viscosity and Drive Amplitude in Drop-on-Demand Ink-Jet Printing
No full textResults of recent experiments and numerical simulations are presented, which have been used to establish empirical rules for
the dependence of drop speed on nozzle diameter and drive amplitude for Newtonian and non-Newtonian fluids printed with a
range of different ink-jet print-head technologies. Experiments were carried out with Xaar, MicroFab and Spectra Dimatix print heads and with solutions of polystyrene in diethyl phthalate as model fluids. These results are compared with
predictions from recent numerical codes developed by collaborators in the University of Leeds, and from simple models for drop-on-demand fluid jetting resulting from physical law
Experiments and DNS of a round jet with turbulent inlet
No full textExperimental and Direct Numerical Simulation data of a turbulent round jet fed by a turbulent pipe are compared in the near field. The Reynolds number achieved in both the experiment and the simulation, Re = 16000, allows a direct comparison of both the average and the fluctuating velocity statistics. In the experiments the jet is fed with olive oil droplets with a Stokes number St ' 1 whose dynamics is compared against the corresponding DNS simulation to asses the ability of particles to reproduce high order turbulence statistics and to asses the accumulation properties of inertial particles in the near field
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Jetting, In-Nozzle Meniscus Motion and Nozzle-Plate Flooding in An Industrial Drop-on-Demand Print Head
No full textThe state of the ink film at and near the nozzles of a drop-ondemand(DoD) print head during jetting has a direct impact on
printing performance and reliability. We have developed highspeed imaging apparatus and analytical techniques to investigate
the ink film dynamics on an industrial print head nozzle-plate in real-time. In addition to a direct correlation between the jet
emergence velocity and drive voltage, drive-dependent variations in the oscillation of the ink meniscus in adjacent nozzles were also observed. Using a ray-tracing model to analyze the meniscus shape, the meniscus oscillations for both printing and nonprinting nozzles were found to be complex and involve elements such as pre-oscillation and high-order surface waves. The flooding of non-firing nozzles, deliberately caused by the application of maximum drive voltage to a neighboring nozzle, has been recorded and analyzed dynamically. The build-up of fluid in an annulus around the nozzle (flooding rate) has been characterized and compared with models for the net ink flow
through the nozzle
Passive scalar mixing of a turbulent jet emitted into homogeneous, isotropic turbulence
No full textAlthough most jets, whether they be natural or industrial in origin, are emitted into a turbulent environment, almost all previous research on turbulent jets has dealt with jets emitted into quiescent or laminar background flows. The present work extends the work of Khorsandi, Gaskin and Mydlarski, J. Fluid Mech., 2013 – who studied the effect of background turbulence on the velocity field of a turbulent jet emitted into turbulent surroundings – to the study of passive scalar mixing of a jet released into a turbulent flow. To this end, the experiments described herein use planar laser-induced fluorescence to study the mixing of a (high-Schmidt-number) passive scalar within a turbulent jet that is emitted into a quasi-homogeneous, isotropic, zero-mean-flow turbulent background. We examine herein statistics of the jet’s scalar field, and compare them to those of a jet emitted into a quiescent background
Turbulence in shallow jet flows
No full textThe general flow pattern of an open channel flow, downstream of a width restriction by two artificial dams, is analysed. A physical Froude-scaled model, under hydraulic rough conditions, with a significant large Reynolds number is used to ensure turbulent flow. Upstream of the dams the flow is uniform in transverse direction, in between and downstream of the narrow part a jet is formed. On both sides of the jet large eddies are formed bounded by the wall, the jet and the dams. Due to the large velocity gradient in transverse direction a mixing layer develops at both sides of the jet. The width of the mixing layer, as expected, grows with the downstream distance and exceeds the water depth. 2D structures are clearly visible by injecting dye. In the mixing layer besides the macro time and spatial scales, the small Taylor and Kolmogorov scales are present. Whereas the macro scales are well represented in the measured data, the small scales are impossible to mark due to limitations of the Doppler device. When there is initial no net momentum in transverse direction present the jet is expected to appear symmetrical. However the jet is aligned to one of the sides every time the model starts to run. The preference for one or the other side seems to be random and cannot be related to momentum in transverse direction in between the dams. During measurements the position of the jet is stationary. The fixed position of the jet during measurements can be related to the Coand? effect. When the flow is disturbed and transverse momentum is added to the upstream flow, the jet can be deflected. The position of the jet and the evolving mixing layers can be related very well to the measured velocities upstream. Due to the limitations of the used momentum balance equation and use of the mean velocity in the bottom friction calculation the measured head loss is large compared to the calculated dissipative terms (bottom friction and Carnot loss).Additional ThesisEnvironmental Fluid MechanicsCivil Engineering and Geoscience
Large eddy simulation of fuel injection and spray combustion in an engine environment
No full textThis thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University.Large eddy simulation of spray combustion in an HSDI engine is carried out in this thesis. The implementation of the code was performed in logical steps that allowed both assessment of the performance of the existing KIVA-LES and later development. The analysis of the liquid annular jet confirmed existence of typical, annular jet exclusive structures like head vortices, stagnation point and recirculation in the inner zone. The influence of the swirl in the ambient domain was found to have profound impact on the development, penetration and radial spreading of the jet. Detailed results were reported in Jagus et al. (2008).
The code was further validated by performing an extensive study of large eddy simulation of diesel fuel mixing in an engine environment. The reaction models were switched off in order to isolate the effects of both swirl and the different numerical treatment of LES. Reference RANS simulation was performed and significant differences were found. LES was found to capture much better the influence of the swirl on the liquid and vapour jets, a feature essentially absent in RANS results. Moreover, the predicted penetration of the liquid was much higher for the LES case and more in accordance with experimental measurements.
Liquid penetration and subsequent evaporation are very important for prediction of heat release rates and encouraging results formed a good basis to performing a full simulation with models for ignition and combustion employed. The findings were analyzed in the paper by Jagus et al. (2009).
Further modifications were introduced into the LES code, among them changes to the combustion model that was originally developed for RANS and calculation of the filter width. A new way of estimating the turbulent timescale (eddy turnover time) assured that the incompatibilities in the numerical treatment were eliminated and benefits of LES maximized. The new combustion model proved to give a very good agreement with experimental data, especially with regard to pressure and accumulated heat release rates. Both qualitative and quantitative results presented a significant improvement with respect to RANS results and old LES formulation. The new LES model was proved to give a very good performance on a spectrum of mesh resolutions. Encouraging results were obtained on a coarse mesh sets therefore proving that the new LES code is able to give good prediction even on mesh sizes more suitable for RANS.
Overall, LES was found to be a worthy alternative to the well established RANS methods, surpassing it in many areas, such as liquid penetration prediction, temperature-turbulence coupling and prediction of volume-averaged data. It was also discovered that the improved LES code is capable of producing very good results on under-resolved mesh resolutions, a feature that is especially important in industrial applications and on serial code structure
Reynolds number effect on 3D turbulent offset jet reattaching to a free surface
No full textExperimental study was carried out to investigate the effect of Reynolds number on 3D offset jet reattaching to above free surface. Sharp edged square nozzle was used to produce the jets, and the measurements were performed at the following six different Reynolds numbers: 2300, 3700, 5100, 7900, 10300 and 11900. Detailed velocity measurements were made in the symmetry plane. From the PIV data, the mean velocity and turbulence statistics were obtained to study the effects of Reynolds number on the salient features of the jet flow. Preliminary results on streamwise mean velocity decay along the nozzle centerline, contours of streamwise mean velocity and Reynolds shear stress are presented herein
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