Portion Of Jet Research Articles

Fine characterization of the capillary instability of free viscoelastic jets

The experimental approach presented in this article investigates low-velocity small-diameter cylindrical viscoelastic jets. It consists in producing and analyzing sets of jet portion images taken at different positions along the jet axis, i.e., from the nozzle producing the jet down to the spray region. A very significant part of the work was dedicated to develop analyzing tools: a proper orthogonal decomposition to identify the onset of the capillary instability, a procedure to determine the mean jet breakup length and a multi-scale tool to describe the jet local shape and its evolution along the jet axis. Used in a similar previous study, the multi-scale analysis has been extended in the present work. These tools are shown to be well suited to the experimental study conducted here, and in particular to meet the need for statistical analysis that free jets impose. They are applied to study the capillary instability of free jets of dilute polyethylene oxide solutions produced from a micrometer nozzle. The parameters of the study are the molar mass of the polymer and the velocity of the jet. Results on the detection of the capillary instability onset, the jet evolution dynamics towards the Bead On A String (BOAS) pattern, the local extensional relaxation time and the terminal elongational viscosity of the solutions, the bead coalescence mechanism and its influence on the size of the spray drops are obtained. Analysis of these results reveals that the polymers undergo two episodes of mechanical degradation during the process: the first one at injection, the second one during the elasto-capillary regime of the jet breakup. Moreover, the scenario in the literature suggesting a sufficiently relaxed axial tension of the flow for the capillary instability to occur is found to be not always valid. It is also shown that the average size of the drops correlates with the intensity of the bead coalescence mechanism that takes place on the BOAS. These results demonstrate the richness of the adopted approach and in particular of the multi-scale analysis to study the development of a fluid interfacial instability.

Gap effect in electrochemical jet machining

Electrochemical jet machining (EJM) is a young-generation micromachining and surface texturing technique with multiple merits. The materials removal function in EJM has traditionally been considered to be Gaussian-like and unaffected by changes in inter-electrode gap when maintaining a constant machining current. Nonetheless, phenomena that contradict this widely accepted perception are sometimes observed. In this paper, to clarify the inter-electrode gap effect in EJM, its impacts on flow field, electric field, and machined shape were studied by both experiments and simulation. With varied gaps ≥0.4 of nozzle diameter the jet profile keeps almost same and only its length changes, while narrowing the gap from 0.4 to 0.2 of nozzle diameter causes thinning of the jet-surrounding-film. Jet profile required for EJM cannot form with an overly tiny gap. In addition, the gap ≥0.6 of nozzle diameter features sufficiently long cylindrical jet portion that uniformly distributes the pass-through current density and eliminates the cathode-shape-determined electric field near nozzle to act on workpiece, resulting in constant materials removal function even though gap changes. With smaller gap the cathode-shape-determined electric field affects the anodic current density distribution and the materials removal function can be modulated by adjusting the gap, allowing EJM to generate geometry other than Gaussian-like profile. Furthermore, the influence of inter-electrode gap when using the specially-shaped nozzle was also investigated. With a small gap this method performs its ability in modulation of materials removal distribution by designing nozzle shapes, while a large gap such as ≥ nozzle diameter enables it to operate as a conventional nozzle.

Jet evolution and fiber formation mechanism of amylopectin rich starches in centrifugal spinning system

AbstractThis article reports the jet evolution process and fiber formation mechanism of the amylopectin rich starch solution in centrifugal spinning system, and the linear polyvinylpyrrolidone (PVP) is applied to compare with starch solution. The results indicate that jet evolution processes of starch solution include steady state and Rayleigh–Taylor instability over the whole concentration range for the spinnability investigation, which are due to the hyperbranched molecular of amylopectin. As a compression, the linear molecular chain PVP solution expresses a steady state jet under the spinning concentration. The spinnability results show that the obtained starch fibers are always containing the beads due to the Rayleigh–Taylor instability of jet, but can be effectively controlled by solution concentration and amylose/amylopectin. Instead, the PVP fibers show a rapid decreasing of beads till to almost disappear with the increasing of solution concentration, which due to the improvement of chain entanglement increase the steady state portion of jet with the increasing of concentration. The thermal properties of obtained fiber show that fibers obtained from amylose with more thermal stability.

Transitional flow at standard sluice gate

ABSTRACTGates are one of the most important hydraulic structures extensively investigated by physical and two-dimensional (2D) numerical research in the past. However, the hydraulics of free gate flow features of engineering interest, including the water surface profile, the force on the gate, or the bottom pressure distribution, were overlooked in the literature, so that the current knowledge is incomplete. In this work this gap is filled in using a higher order one-dimensional model based on a Boussinesq-type closure. The proposed model provides the free surface profile of the approach flow as a cnoidal wave, and the free jet portion as a generalized solitary wave, which compare well with 2D results. The model further provides as a part of the theoretical computation the pressure distribution on the gate. Bottom pressure features were also determined resulting in a good agreement with potential flow data. Boussinesq-type equations for submerged jets are presented and solved for the submerged hydraulic jump.

Reply to Discussion by Duarte et al. of “CFD analysis of the effect of nozzle stand-off distance on turbulent impinging jets”

The authors would like to thank the discussants for their interesting suggestions. The discussants have carried out experiments on impinging jets and have compared their results with the CFD results presented in the paper. In addition, discussants have also presented a solution for the differences observed between the CFD and experimental results in the core portion of the jet. The discussants’ experiment includes measurements on impinging water jets with a 72 mm diameter (D) circular nozzle anda stand-off distanceofH/D=9.58. Issuance velocities ranged from 5 to 22m/s. The discussants have also questioned the use of Reynolds StressModel (RSM) formodeling the turbulence in thepaper.Herein, the reasons for choosing this turbulence model in our simulations are explained. Based on the H/D ratio, different regimes occur in the flow field of impinging jets (Fig. 1 from Shademan et al. 2013). For H/D 8), Beltaos and Rajaratnam (1974, 1977) classified the flow into three different regions: the free jet portion (region I, Fig. 1 in Shademan et al. 2013), the impingement zone (region II, Fig. 1 in Shademan et al. 2013), and the axisymmetric wall jet portion (region III, Fig. 1 in Shademan et al. 2013). As a result, each region has specific characteristics and the objective of the study was to best capture the features in all the regions with a single turbulence model. The purpose of the paper was also to identify some of the issues and difficulties associated with previously used turbulence models in solving turbulent flow in impinging jets. Numerical simulation of a circular impinging jet using RANS simulations has been of interest to many researchers. As a result, these simulations formed part of the 2nd ERCOFTAC-IAHRWorkshop on Refined Flow Modelling in 1993. Later, Craft et al. (1993) published their research on impinging jets using different turbulence models. They reported that due to the weakness associated with the eddy viscosity stress–strain relationship in the turbulence models, the results were not in good agreement with the available experimental data. Similar issues were reported by Cooper et al. (1993), who reported that the k– model over-predicts the turbulent kinetic energy near the stagnation point. To sort out the issues reported by previous researchers in modeling impinging jets, different turbulence models were implemented in the current study. Models that take into account the low-Re effects in the flow were selected. In addition, the selected models take into consideration the anisotropic nature of the turbulent viscosity, which is ignored during Reynolds averaging. The Realizable k– model (Shih et al. 1995), the k– SST (Shear Stress Transport) model (Menter 1994), and the Reynolds Stress Model (RSM) (Launder et al. 1975) were implemented in the simulations. Each of the commonly used turbulencemodels has some advantages and disadvantages. Considering that RSM is a fully anisotropic turbulencemodel, which solves the transport equations for all three fluctuating velocity components, the issues resulting from applying the Boussinesq approximation is removed. Basically, the strength of this model compared to the Boussinesq based turbulencemodels is its capability in producing the fluctuating velocity components in the results. The other reason for using the RSM model is the analysis of wall jet region, which was of interest to the authors. Following impingement, the flow undergoes a large deflection, which is associated with the initiation of the wall jet portion of the flow. This causes the turbulence components to be highly anisotropic in this part of the flow. Therefore, irrespective of the minor weakness observed in the RSM results (Fig. 3 in Shademan et al. 2013), the authors decided to implement this model for the analysis of the entire domain to benefit from both an anisotropic turbulence model and also extracting the fluctuating velocity components. These would not have been possible by choosing either the k– SST or the Realizable k– turbulence models. The authors appreciate the idea of the new length scale (xc) for synchronizing results from different numerical and experimental studies. Indeed this method can help future researchers for comparing the results in an appropriate way. As the discussants have shown, this way of comparison solves the ambiguity observed in the results in the free jet region. However, as noted in the paper and by others, the nozzle design always plays a significant role on the development of the flow in the core and in the free jet region of the flow (Xu and Antonia 2002).

Investigation of the Optimum Electric Field for a Stable Electrospinning Process

Electrospinning is an easy and inexpensive process that produces continuous nanofibers through an electrically charged jet of polymer solution consisting of sufficiently long chain molecules. As reported in the literature, the entire electrospinning process is governed by the external electric field caused by the applied voltage between the electrodes and the induced electric field caused by free and induced charges on the fluid surface. Therefore, the electric field is the most critical parameter in electrospinning. In this work, a comprehensive analysis was carried out to investigate the effects of external and induced electric fields on the electrospinning process, which include the Taylor cone formation, the straight jet portion, and the unstable or whipping jet region. It was observed that all regions are highly influenced by the applied and induced electric fields, which results in a considerable variation in the morphology of the nanofibers.

Deflector-jets affected by pre-aerated approach flow

Jets generated at chute ends move a potential scour zone away from the dam, and dissipate flow energy to reduce the scour. Usually, so-called flip buckets or ski jumps are provided to generate jets, yet they are rarely issued by deflectors. The application of a theoretical mass point parabola is limited to predict the jet features, because effects related to the take-off angle and the disintegration process affect the jet air transport and trajectories. This research discusses the effect of pre-aeration of the approach chute flow on the jet features. A comparison with a former study indicates that mainly the upper jet portion is affected, that the black water jet core becomes shorter, and that the air transport increases. Although the lower trajectory is less affected and thus remains more coherent, the small effect of pre-aeration is sufficient to reduce the total jet throw length, so that pre-aerated jets tend to be shorter as compared with black-water approach flow jets, in agreement with prototype observations.

Emission of impinging swirling and non-swirling inverse diffusion flames

The overall pollutants emission from impinging swirling and non-swirling inverse diffusion flames (IDFs) was evaluated quantitatively by the ‘hood’ method. The results of in-flame volumetric concentrations of CO and NO x and overall pollutants emission of CO and NO x in terms of emission index were reported. The in-flame volumetric concentrations of CO and NO x were measured through a small hole drilled on the impingement plate. In comparison with the corresponding open flame, the CO and NO x concentrations for the impinging swirling IDF are greatly lowered due to the entrainment of much more ambient air which is related to the increased flame surface area. For the swirling and non-swirling IDFs, the EINO x increases as the nozzle-to-plate distance ( H) increases because more space is available for the development of the high-temperature zone in the free jet portion of the impinging flame, which favors the thermal NO formation. The variation of EICO with H is different for the impinging swirling and non-swirling IDFs because they have different flame structures. For both flames, the EICO is high when their main reaction zone or inner reaction cone is impinged and quenched by the copper plate. The parameters of air jet Reynolds number, overall equivalence ratio and nozzle-to-plate distance have significant influence on the overall pollutants emission of the impinging swirling and non-swirling IDFs and the comparison shows that the swirling IDF emits less NO x and CO under most of the experimental conditions tested. Furthermore, it is found that compared with the open flames, the impinging flames emit lower level of NO x and higher level of CO.

Development of a Simple Impulse Turbine for Nano Hydropower

The aim of this work is to provide an impulse type hydraulic turbine to utilize unexploited water resources as nano hydropower in the mountainous area. The turbine is simplified to make cheap energy and uses inexpensive components for widespread utilization. The turbine model is tested experimentally to reveal the power characteristics. The flow visualization and numerical simulation are conducted to clarify the behavior of free surface flow in the runner with different nozzle positions. The experimental results show that the maximum runner efficiency of the prototype turbine is 0.56. Numerical simulation shows that the output power depends on the nozzle positions with the impingement of the tail of the jet portion to the backside of the blade. This study gives the fundamental information of the turbine performance to acquire a guideline for future practical applications.

Comparison of Far-Field Turbulent Structure of a Rectangular Surface Jet to Three-Dimensional Free and Wall Jets

The turbulence structure of a rectangular surface jet is compared to that of the three-dimensional free and wall jets. The surface jet turbulence quantities are mapped using laser Doppler velocimetry. In general, the turbulence structure of these three jets is found to be significantly different. For the surface jet, the free surface kinematic condition has a predominant effect on the whole structure, while for the wall jet, the influence of wall kinematic constraint is contained in the wall layer. A surface current with a higher lateral spreading rate than the submerged portion of the jet is developed, which does not exist for the wall jet because of the no-slip boundary condition. Unlike free jets, the submerged portion of the rectangular surface jet is characterized by two length scales. The Prandtl hypothesis with constant eddy viscosity provides a good estimate for the shear stresses in the lateral direction, but fails in the vertical direction, where the velocity profiles are much flatter, due to the free surface condition, than those for the free and wall jets.

ITAL]Chandra[/ITAL] Discovery of a 100 kiloparsec X-Ray Jet in PKS 0637−752

The quasar PKS 0637-752, the first celestial X-ray target of the Chandra X-Ray Observatory, has revealed asymmetric X-ray structure extending from 3'' to 12'' west of the quasar, coincident with the inner portion of the jet previously detected in a 4.8 GHz radio image (Tingay et al. 1998). At a redshift of z = 0.651, the jet is the largest (100 kpc in the plane of the sky) and most luminous (~1044.6 ergs s-1) of the few so far detected in X-rays. This Letter presents a high-resolution X-ray image of the jet, from 42 ks of data when PKS 0637-752 was on-axis and ACIS-S was near the optimum focus. For the inner portion of the radio jet, the X-ray morphology closely matches that of new Australian Telescope Compact Array radio images at 4.8 and 8.6 GHz. Observations of the parsec-scale core using the very long baseline interferometry space observatory program mission show structure aligned with the X-ray jet, placing important constraints on the X-ray source models. Hubble Space Telescope images show that there are three small knots coincident with the peak radio and X-ray emission. Two of these are resolved, which we use to estimate the sizes of the X-ray and radio knots. The outer portion of the radio jet and a radio component to the east show no X-ray emission to a limit of about 100 times lower flux. The X-ray emission is difficult to explain with models that successfully account for extranuclear X-ray/radio structures in other active galaxies. We think the most plausible is a synchrotron self-Compton model, but this would imply extreme departures from the conventional minimum energy and/or homogeneity assumptions. We also rule out synchrotron or thermal bremsstrahlung models for the jet X-rays, unless multicomponent or ad hoc geometries are invoked.

Ignition Process and Delay of Sprays Injected into a Hot-Air Stream.

A portion of the heat accumulation-type radiant-tube burner, which has a pair of gas burners with a honeycomb regenerator on both ends, was utilized as a test rig for spray ignition. The effects of various parameters on the ignition process were examined by perpendicularly injecting a liquid fuel (gas oil) into the hot-air stream immediately after cutting off the city-gas supply to the burners. If a cavity is embedded in the wall confronting the injection nozzle, only the jet portion of the spray is exposed to the hot stream and ignited, while the tip portion chills in the cavity. With no cavity, on the other hand, ignition takes place in the tip portion just before impinging on the wall, or the secondary spray generated by the impingement is ignited within the wall boundary layer. The data for both cases and our former shock-tube data were compared with those of other researchers and examined. As a result, the reason why ignition delay data were so widely different from researcher to researcher was elucidated to some extent.

Effect of nozzle asymmetry on jet-edge oscillations

Strongly coherent oscillations of a jet impinging upon an edge persist when one lip of the nozzle is axially displaced with respect to the other, even by as much as one wavelength of the jet instability. This persistence of the oscillation is due to rapid stream wise adjustment of the flow in the region between the two nozzle lips, forced by upstream influence from the downstream edge; immediately downstream of the nozzle, cross-stream distributions of fluctuation amplitude and phase are very similar to those occurring in an initially symmetrical jet, as evidenced by comparison with linear theory. In the region between the upstream and downstream nozzle lips, “pre-amplification” of the disturbance occurs in the form of exponential disturbance growth, followed by another (slower) exponential growth in the free jet portion of the flow. These features of the flow structure occur in conjunction with the dipole-like upstream influence from the jet-edge interaction, the strength of which is determined by the pressure loading at the edge. In fact, the pressure fluctuations at the tip of the downstream impingement edge can have amplitudes approximately double those of the corresponding jet from a symmetrical nozzle. This increase in magnitude of the source of upstream influence is due to a slight transverse shift of the minimum amplitude and phase jump of the fluctuating velocity field approaching the edge, relative to those of the jet from a symmetrical nozzle. In the event that the nozzle lip axial ofiset is very large, coherent oscillations still persist, but in a different mode; the frequency is much higher, dictated by the most unstable frequency of the wall jet that forms between the nozzle lips.

Weft Insertion by Water Jet

In order to make clear a mechanism of weft yarn insertion in a water-jet loom, the flow velocity of the jet, the static pressure on the weft surface, and the driving force acting on the weft in the vicinity of the nozzle exit have been analyzed by utilizing the matched asymptotic expansion technique on the basis of results of the authors' previous paper. The present results are as follows. The flow of jet near the weft surface is perturbed by using the small parameter (the reciprocal of the square root of the Reynolds number). The flow velocity of the jet near the weft surface gradually decelerates, and hence the driving force acting on the weft gradually decreases. The static pressure on the weft surface of the jet portion rapidly decreases to atmospheric pressure.

Apparatus for studying the kinetics of rapid reactions in the liquid phase by the stopped-jet method

1. An apparatus for investigating the kinetics of rapid reactions in solution by the stopped-jet method, permitting the recording of reactions with half-conversion times greater than 1 msec according to the change in the optical density in the ultraviolet or visible region, was described. The jet portion is made as an attachment to an SF-4 spectrophotometer. 2. An example of measurement of the rate of complex formation between molybdate and propyl gallate on this apparatus was cited.