Jet Mode Research Articles

Evaluation of the Effect of Tube Pitch and Surface Alterations on Temperature Field at Sprinkled Tube Bundle

Water flowing on a sprinkled tube bundle forms three basic modes: It is the Droplet mode (liquid drips from one tube to another), the Jet mode (with an increasing flow rate droplets merge into a column) and the Membrane (Sheet) mode (with further increasing of falling film liquid flow rate columns merge and create sheets between the tubes. With sufficient flow rate sheets merge at this state and the tube bundle is completely covered by a thin liquid film). There are several factors influencing the individual mode types as well as heat transfer. Beside the above mentioned falling film liquid flow rate they are for instance tube diameters, tube pitches in a tube bundle or a physical condition of a falling film liquid. This paper presents a summary of data measured at atmospheric pressure at a tube bundle consisting of copper tubes of 12 milimeters diameter and of the studied tube length one meter. The tubes are positioned horizontally one above another with the tested pitches of 15, 20, 25 and 30 mm and there is a distribution tube placed above them with water flowing out. The thermal gradient of 15–40 has been tested with all pitches where the falling film liquid’s temperature at the inlet of the distribution tube was 15 °C. The liquid was heated during the flow through the exchanger and the temperature of the sprinkled (heater) liquid at the inlet of the exchanger with a constant flow rate about 7.2 litres per minute was 40 °C. The tested flow of the falling film liquid ranged from 1.0 to 13.0 litres per minute. Sequences of 180 exposures have been recorded in partial flow rate stages by thermographic camera with record frequency of 30 Hz which were consequently assessed using the Matlab programme. This paper presents results achieved at the above mentioned pitches and at three types of tube bundle surfaces.

0804 超音速ジェットにおけるヘリカルモードペア成長に対するレイノルズ数の影響

0804 超音速ジェットにおけるヘリカルモードペア成長に対するレイノルズ数の影響

Numerical design and analysis of a multi-DBD actuator configuration for the experimental testing of ACHEON nozzle model

Two dimensional numerical simulations of plasma actuator flow control of the ACHEON nozzle are conducted to give insight on the design of an experimental setup. Three configurations of the plasma actuators with single and multi AC-DBD actuators are used in steady mode of operation. AC-DBD actuators in standard mode (forward forcing mode), reverse mode (backward forcing mode) and plasma synthetic jets mode were used. Three different main groups of test cases were investigated by varying the reference velocity at the inlet of the nozzle stream from 4, 5 and 6 m/s. Moreover, each group includes four velocity ratios VR=1,1.5,2,2.5. The locations of the flow separation points are obtained numerically for all these cases and the plasma actuators are placed slightly upstream of these points leading to a system of seven DBD plasma actuators in the forward forcing mode over the Coanda surface. The induced thrust of the AC-DBD plasma actuators was estimated using a phenomenological model which considers the maximum achieved voltage and frequency from the experiments. Using an excitation voltage with maximum amplitude of 12 kVpp and frequency of 20 kHz, ionic wind was formed with 2.4 m/s velocity. The effects of plasma actuator are presented through change of the thrust and velocity angle and thrust vectoring efficiency. Preliminary results of the experimental set-up correlate well with the numerical design values.

Wintertime cyclone/anticyclone activity over China and its relation to upper tropospheric jets

In this study, the wintertime cyclone/anticyclone activity and its variability over China are examined based on the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis data from 1948 to 2007. The climatology of the source, path and lysis regions for cyclones/anticyclones is investigated using an automatic tracking algorithm. Apparent asymmetries in source, lysis and path regions are observed between cyclones and anticyclones.The 1948–2007 data exhibit an upward trend in the annual number and a downward trend in the cyclone and anticyclone intensity. The leading empirical orthogonal function (EOF) mode of the cyclone transit counts (CTC) for the 1948–2007 period indicates an increase in cyclone activity over northeastern East Asia since the late 1970s that becomes significant in the mid-1980s. The first EOF mode of the anticyclone transit counts (ATC) is a monopole over northern East Asia, centred west of Lake Baikal, which has increased since 1970. The CTC variability dominates the ATC variability, which corresponds well with the storm track variability.Two distinct variability modes in the upper tropospheric jets over East Asia are also observed. The first mode describes an oscillation in the subtropical jet position; the second mode describes the polar-front jet strength variation. Moreover, the second mode is closely linked to the cyclone/anticyclone activity variability.

Flow behavior around a square cylinder subject to modulation of a planar jet issued from upstream surface

The flow behavior in the up- and downstream regions of a square cylinder subject to the modulation of a planar jet issued from the cylinder׳s front surface was studied using the laser-assisted smoke flow visualization method and hot-wire anemometer measurement. Reynolds numbers were from 1628 to 13000. The drag force experienced by the square cylinder was obtained by measuring the surface pressures on the up- and downstream faces. The temporally evolving smoke flow patterns in the up- and downstream regions were synchronously revealed through the smoke flow visualization. The frequency characteristics of the instability waves in the up- and downstream regions were synchronously detected by the two hot-wire anemometers. Four characteristic flow modes were observed within the different ranges of the injection ratios. At the low injection ratios (IR<1), the ‘swinging jet’ appeared. The jet swung periodically leftward and rightward and formed a fluid bubble on the front surface. The fluid bubble contained a pair of counter-rotating vortices and presented a periodic variation in its height. At moderately low injection ratios (1<IR<4.3), the ‘deflected oscillating jet’ appeared. The jet was deflected in either the left or the right direction and wrapped around one of the edges of the square cylinder. Both the swinging and oscillating motions of the jet in the swinging jet and deflected oscillating jet modes were induced by the periodic feedback pressure signals generated by the vortex shedding in the wake. At the moderately high (4.3<IR<8.3) and high (IR>8.3) injection ratios, the ‘deflection jet’ and ‘penetrating jet’ appeared. The jet detached from the cylinder׳s front surface and penetrated a long distance into the upstream region due to large jet momentum. Neither periodic jet oscillation in the upstream region nor vortex shedding in the wake was observed. The drag coefficient was found to be decreasing quickly with increasing the injection ratio.

Phase-Resolved Spectroscopy Synchronized to Low-Frequency Self-Organized Mode of an Atmospheric Pressure Plasma Jet

Phase-Resolved Spectroscopy Synchronized to Low-Frequency Self-Organized Mode of an Atmospheric Pressure Plasma Jet

An inviscid modal interpretation of the ‘lift-up’ effect

AbstractIn this paper, we give a modal interpretation of the lift-up effect, one of two well-known mechanisms that lead to an algebraic instability in parallel shearing flows, the other being the Orr mechanism. To this end, we first obtain the two families of continuous spectrum modes that make up the complete spectrum for a non-inflectional velocity profile. One of these families consists of modified versions of the vortex-sheet eigenmodes originally found by Case (Phys. Fluids, vol. 3, 1960, pp. 143–148) for plane Couette flow, while the second family consists of singular jet modes first found by Sazonov (Izv. Acad. Nauk SSSR Atmos. Ocean. Phys., vol. 32, 1996, pp. 21–28), again for Couette flow. The two families are used to construct the modal superposition for an arbitrary three-dimensional distribution of vorticity at the initial instant. The so-called non-modal growth that underlies the lift-up effect is associated with an initial condition consisting of rolls, aligned with the streamwise direction, and with a spanwise modulation (that is, a modulation along the vorticity direction of the base-state shearing flow). This growth is shown to arise from an appropriate superposition of the aforementioned continuous spectrum mode families. The modal superposition is then generalized to an inflectional velocity profile by including additional discrete modes associated with the inflection points. Finally, the non-trivial connection between an inviscid eigenmode and the viscous eigenmodes for large but finite Reynolds number, and the relation between the corresponding modal superpositions, is highlighted.

Study of supersonic wave components in high-speed turbulent jets using an LES database

Near-field characteristics of supersonic wave components in turbulent jets are investigated using the well-validated large eddy simulation (LES) database by Bodony and Lele (2005) [1]. Three unheated (constant stagnation temperature) jets with a jet Mach number ranging from 0.51 to 1.95, and one heated (Tj/T∞=2.3) transonic jet are considered. The Reynolds number based on the exit diameter ranges from 79,000 to 336,000. The supersonic wave components of the flow variables are decomposed from the full flow-field using wavenumber–frequency domain filtering. The spatial structure of the fluctuating pressure field is obtained by the proper orthogonal decomposition (POD) of the full and filtered data. POD modes of unheated subsonic jets reveal large scale-disparity between the full and supersonic components. For the supersonic jet at Mj=1.95, the energetic structures of the pressure field also contribute significantly to the supersonic components, and scale disparity is absent. The variance of the subsonic pressure components from unheated jets scales as Uj4, where Uj is the jet exit velocity (i.e., |p˜2|~Uj4), which is the expected scaling for turbulence-associated hydrodynamic-pressure fluctuations. In contrast, supersonic pressure variance, which peaks within the turbulent flow region, scales with Uj8, coinciding with the far field noise intensity scaling associated with Lighthill׳s analogy. In the acoustic near-field, even higher exponent of the jet velocity scaling is found for the positive phase velocity supersonic pressure component (|p˜2|~Uj10), which is consistent with the scaling of the far-field noise at shallow exit angles [2,3]. The filtered velocity components also show a similar pattern, i.e., supersonic velocity variances scale with a higher power of the jet velocity, but the scaling exponents in jet-region are different for different velocity components and depend on the azimuthal mode. Previous investigations have presented two distinct spectral peaks due to the hydrodynamic and acoustic pressure components in the near-acoustic field. Using filtered components, it is found that density and velocities also show such clear hydrodynamic to acoustic transition, albeit with different transition points. The strongly directive noise radiation from subsonic jets is explained using POD projection of supersonic disturbances.

Breakup of a conducting drop in a uniform electric field

AbstractA conducting drop suspended in a viscous dielectric and subjected to a uniform DC electric field deforms to a steady-state shape when the electric stress and the viscous stress balance. Beyond a critical electric capillary number $\def \xmlpi #1{}\def \mathsfbi #1{\boldsymbol {\mathsf {#1}}}\let \le =\leqslant \let \leq =\leqslant \let \ge =\geqslant \let \geq =\geqslant \def \Pr {\mathit {Pr}}\def \Fr {\mathit {Fr}}\def \Rey {\mathit {Re}}\mathit{Ca}$, which is the ratio of the electric to the capillary stress, a drop undergoes breakup. Although the steady-state deformation is independent of the viscosity ratio $\lambda $ of the drop and the medium phase, the breakup itself is dependent upon $\lambda $ and $\mathit{Ca}$. We perform a detailed experimental and numerical analysis of the axisymmetric shape prior to breakup (ASPB), which explains that there are three different kinds of ASPB modes: the formation of lobes, pointed ends and non-pointed ends. The axisymmetric shapes undergo transformation into the non-axisymmetric shape at breakup (NASB) before disintegrating. It is found that the lobes, pointed ends and non-pointed ends observed in ASPB give way to NASB modes of charged lobes disintegration, regular jets (which can undergo a whipping instability) and open jets, respectively. A detailed experimental and numerical analysis of the ASPB modes is conducted that explains the origin of the experimentally observed NASB modes. Several interesting features are reported for each of the three axisymmetric and non-axisymmetric modes when a drop undergoes breakup.

Fast imaging of intermittent electrospraying of water with positive corona discharge

The effect of the electrospraying of water in combination with a positive direct current (dc) streamer corona discharge generated in air was investigated in this paper. We employed high-speed camera visualizations and oscilloscopic discharge current measurements in combination with an intensified charge-coupled device camera for fast time-resolved imaging. The repetitive process of Taylor cone formation and droplet formation from the mass fragments of water during the electrospray was visualized. Depending on the applied voltage, the following intermittent modes of electrospraying typical for water were observed: dripping mode, spindle mode, and oscillating-spindle mode. The observed electrospraying modes were repetitive with a frequency of a few hundreds of Hz, as measured from the fast image sequences. This frequency agreed well with the frequency of the measured streamer current pulses. The presence of filamentary streamer discharges at relatively low voltages probably prevented the establishment of a continuous electrospray in the cone–jet mode. After each streamer, a positive glow corona discharge was established on the water filament tip, and it propagated from the stressed electrode along with the water filament elongation. The results show a reciprocal character of intermittent electrospraying of water, and the presence of corona discharge, where both the electrospray and the discharge affect each other. The generation of a corona discharge from the water cone depended on the repetitive process of the cone formation. Also, the propagation and curvature of the water filament were influenced by the discharge and its resultant space charge. Furthermore, these phenomena were partially influenced by the water conductivity.

Research on Hole Making Mechanism of High Pressure Hydraulic Perforation

In this paper, we integrated use hydraulics, seepage flow mechanics, rock mechanics, and finite element simulation analysis and other methods to study the rock fragmentation mechanism of high pressure water jet. We make tensile stress - crack expansion comprehensive rock fragmentation model for the screw drilling of high pressure water jet. We make finite element simulation according to the mechanism of integrated model of high pressure water jet process, to analysis the internal rock stress distribution and external rock stress distribution of the fluid, and come to the reasonable number of high-pressure water jet nozzle hole. It is verified by the high pressure water jet breaking rock inside experiments of tensile stress - comprehensive rock fragmentation fracture expansion model, summarizes the law of high pressure water jet breaking rock, and we get to know reasonable drilling mode of the high pressure water jet is screw drilling with pitch of 120mm. At present there are two main types of the micro mechanism of the high pressure water jet. One is stress and tensile damage, because of the action produced by stress wave of the high pressure water jet impacting on rock, which mainly makes the tensile failure of rock; another one is crack expansion damage, under the effect of quasi static pressure radiation of water jet, the coupling effect between water shooting jet and rock pore skeleton, which make the rock pore, throat, and micro cracks expanding gradually, eventually the macro damage.

Coherent structure and sound production in the helical mode of a screeching axisymmetric jet

AbstractThe structure of a screeching axisymmetric jet in the helical C mode at a nozzle pressure ratio of 3.4 issuing from a convergent nozzle is studied using high-resolution particle image velocimetry. Proper orthogonal decomposition (POD) is used to extract the dominant coherent structures within the jet. The first two modes produced by the POD are used to reconstruct a phase-averaged data sequence. A triple decomposition into mean, coherent and random velocity components is performed. The embedded shock structures within the jet are shown to strongly modulate the coherent axial stresses within the shear layer and to weakly modulate the random axial stresses. Analysis of the third and fourth moments of the velocity probability density function is used as an indicator of possible regions of shock–vortex interaction and thus screech tone generation. Peaks of kurtosis (flatness) occur at the second, third and fourth shock–boundary intersection points, with the radial position shifting towards the centreline with increasing downstream distance. Analysis of the coherent component of vorticity shows that the largest fluctuations in coherent vorticity occur at the high-speed side of the shear layer in an area extending from the second to the fourth shock cell. With reference to prior literature, the argument is made that it is this increased magnitude of coherent vorticity fluctuation that is the primary factor in the determination of which shock cells act as dominant screech sources.

Emergence and Equilibration of Jets in Beta-Plane Turbulence: Applications of Stochastic Structural Stability Theory

Abstract Stochastic structural stability theory (S3T) provides analytical methods for understanding the emergence and equilibration of jets from the turbulence in planetary atmospheres based on the dynamics of the statistical mean state of the turbulence closed at second order. Predictions for formation and equilibration of turbulent jets made using S3T are critically compared with results of simulations made using the associated quasi-linear and nonlinear models. S3T predicts the observed bifurcation behavior associated with the emergence of jets, their equilibration, and their breakdown as a function of parameters. Quantitative differences in bifurcation parameter values between predictions of S3T and results of nonlinear simulations are traced to modification of the eddy spectrum which results from two processes: nonlinear eddy–eddy interactions and formation of discrete nonzonal structures. Remarkably, these nonzonal structures, which substantially modify the turbulence spectrum, are found to arise from S3T instability. Formation as linear instabilities and equilibration at finite amplitude of multiple equilibria for identical parameter values in the form of jets with distinct meridional wavenumbers is verified, as is the existence at equilibrium of finite-amplitude nonzonal structures in the form of nonlinearly modified Rossby waves. When zonal jets and nonlinearly modified Rossby waves coexist at finite amplitude, the jet structure is generally found to dominate even if it is linearly less unstable. The physical reality of the manifold of S3T jets and nonzonal structures is underscored by the existence in nonlinear simulations of jet structure at subcritical S3T parameter values that are identified with stable S3T jet modes excited by turbulent fluctuations.

The triggering mechanism and properties of ionized outflows in the nearest obscured quasars

We have identified ionized outflows in the narrow line region of all but one SDSS type 2 quasars (QSO2) at z<~0.1 (20/21, detection rate 95%), implying that this is a ubiquitous phenomenon in this object class also at the lowest z. The outflowing gas has high densities (n_e>1000 cm-3) and covers a region the size of a few kpc. This implies ionized outflow masses M~(0.3-2.4)x1e6 Msun and mass outflow rates M(dot)<few Msun yr-1. The triggering mechanism of the outflows is related to the nuclear activity. The QSO2 can be classified in two groups according to the behavior and properties of the outflowing gas. QSO2 in Group 1 (5/20 objects) show the most extreme turbulence, they have on average higher radio luminosities and higher excess of radio emission. QSO2 in Group 2 (15/20 objects) show less extreme turbulence, they have lower radio luminosities and, on average, lower or no radio excess. We propose that two competing outflow mechanisms are at work: radio jets and accretion disk winds. Radio jet induced outflows are dominant in Group 1, while disk winds dominate in Group 2. We find that the radio jet mode is capable of producing more extreme outflows. To test this interpretation we predict that: 1) high resolution VLBA imaging will reveal the presence of jets in Group 1 QSO2; 2) the morphology of their extended ionized nebulae must be more highly collimated and kinematically perturbed.

A thermodynamic basis for predicting falling-film mode transitions

Horizontal-tube, falling-film heat exchangers are used in many air-conditioning and refrigeration systems. Depending on the tube diameter and spacing, the flow rate, and fluid properties, when a liquid film falls over a series of horizontal tubes three distinct flow patterns can be manifested. These flow patterns are the droplet mode, the jet mode, and the sheet mode. A thermodynamic analysis is undertaken to predict the transitions between these modes. By seeking the condition corresponding to thermodynamic equilibrium between two neighboring modes, a scaling relation is developed for the transitional Reynolds number. This theoretical framework is used to provide the first explanation for the relationship between the transitional Reynolds number and modified Galileo number which has been previously based solely on experimental observations. This approach offers insight into the prevailing physics, and it suggests a tube-spacing effect on the mode transitions which has not previously been anticipated. Using limited data and prior results from the literature it is found that this effect is likely to exist. The implications of this thermodynamic approach to predicting two-phase flow patterns are discussed in terms of entropy generation minimization and transition hysteresis, as is its incompleteness.

Ascent modes of jets and plumes in a stationary fluid of contrasting viscosity

Using multi-phase numerical experiments based on the Volume of fluid (VOF) methods we show the ascent behavior of a fluid injected into another immiscible fluid with higher viscosity. Our VOF models reveal that the injected fluid forms jets characterized by a large head trailing to a slender tail, which do not break up, but ascend in two principal modes, Mode 1: continuous and Mode 2: pulsating. In Mode 1, their heads grow continuously either by volume expansion, forming balloon-shaped geometry (Mode 1a) or by curling, forming mushroom-shaped geometry (Mode 1b). In contrast, Mode 2 jets ascend in pulses, forming multiple heads, leading to pinched-off geometry. In this study we show the viscosity ratio (R) between the injecting and the ambient fluids as a crucial parameter in controlling the ascent modes, and provide an estimate of critical R for the Mode 1–2 transition under varying fluid injection rates (non-dimensionalized as Reynolds number, Re). From the entrainment velocity field we explain that the transition of continuous to pulsating process results from the flow convergence at several locations along the upwelling direction. We have also discussed the effects of buoyancy on the ascent modes in case of plumes.

Time and spatial resolved optical and electrical characteristics of continuous and time modulated RF plasmas in contact with conductive and dielectric substrates

Cold atmospheric pressure plasma jets are often used as a remote plasma source for substrate treatments. However, this substrate acts as an electrode and this additional electrode can induce effects on plasma parameters such as the dissipated power, gas temperature, etc. In this work the influence of substrates of different conductivity and permittivity in direct contact with three different operational modes of atmospheric pressure RF plasma jets is investigated. Two different electrode configurations (creating either a linear or a cross electric field) and, for the linear field configuration, two voltage modulations (continuous RF and kHz pulsed RF) have been studied. Electrical and optical diagnostic methods have been performed in order to get quantitative data of the change in plasma dissipated power and gas temperature, when the plasma is in direct contact with the substrate. In all three investigated cases the power dissipation and gas temperature, significantly increase when the plasma is in direct contact with a conductive substrate. The increase of power is attributed to a change of the equivalent electrical circuit, leading to a more favourable matching between the power input and the plasma source.

High Throughput Synthesis of Uniform Biocompatible Polymer Beads with High Quantum Dot Loading Using Microfluidic Jet-Mode Breakup

Uniform polymer microbeads with highly loaded quantum dots (QDs) are produced using high-throughput coherent jet breakup of a biocompatible poly(ethylene glycol) diacrylate (PEGDA) prepolymer resin, followed by in-line photopolymerization. A spiraling and gradually widening channel enables maximum absorption of radiated UV light for the in-line photopolymerization without coalescence and clogging issues. Although the dripping mode in general provides superior uniformity to the jet mode, our nozzle design with tapered geometry brings controlled jet breakup leading to 3% of uniform particle size distribution, comparable to dripping-mode performance. We achieve a maximum production rate of 2.32 kHz, 38 times faster than the dripping mode, at a same polymer flow rate. In addition, the jet-mode scheme provides better versatility with 3 times wider range of size control as well as the compatibility with viscous fluids that could cause pressure buildup in the microsystem. As a demonstration, a QD-doped prepolymer resin is introduced to create uniform biocompatible polymer beads with 10 wt % CdSe/ZnSe QD loading. In spite of this high loading, the resulting polymer beads exhibits narrow bandwidth of 28 nm to be used for the ultrasensitive bioimaging, optical coding, and sensing sufficiently with single bead.

고점도 전도성 잉크 패터닝 기술을 이용한 고성능 미세전극 패턴 구현

EHD (electro-hydro-dynamics) patterning was performed under atmospheric pressure at room temperature in a single step. The drop diameter smaller than nozzle diameter and applied high viscosity conductive ink in EHD patterning method provide a clear advantage over the piezo and thermal inkjet printing techniques. The micro electrode pattern was printed by continuous EHD patterning method using 3-type control parameters (input voltage, patterning speed, nozzle pressure). High viscosity (1000cps) conductive ink with 75wt% of silver nanoparticles was used. EHD cone type nozzle having an internal diameter of 50μm was used for experimentation. EHD jetting mode by input voltage and applied 1st order linear regression in stable jet mode was analyzed. The stable jet was achieved at the amplitude of 1.4~1.8 kV. 10μm micro electrode pattern was created at optimized parameters (input voltage 1.6kV, patterning speed 25mm/sec and nozzle pressure -2.3kPa).

Laser diagnostics of an evaporating electrospray

An electrospray atomizer generates monodisperse, dilute sprays when working in the cone–jet mode. Evolution of an electrospray with droplet diameter below 10 μm is studied with phase Doppler particle analyzer (PDPA) and the exciplex-PLIF technique. The evaporation rate constant is determined from droplet velocity and diameter measured with a PDPA and is found to sharply increase with the velocity slip and the coflow temperature. Fluorescence around 400 nm, usually referred to as TMPD fluorescence, is calibrated with a heated, laminar, coflow vapor jet diluted with nitrogen. The TMPD fluorescence yield nonlinearly increases with temperature up to 538 K and then declines. Single-shot images show that fluorescence around 400 nm is mainly generated from TMPD vapor and that from droplets can be neglected as a first analysis; however, fluorescence around 490 nm, usually referred to as exciplex fluorescence, is generated from both droplets and fuel vapor immediately around droplets. Exciplex fluorescence is correlated with PDPA measurements and TMPD fluorescence. Effects of temperature, fuel composition, overlap of fluorescent spectra, and chemical equilibrium for exciplex formation are discussed. Technical challenges for quantitative exciplex-PLIF measurements are highlighted.