Elliptical Nozzle Research Articles

Axis-switching and breakup of low-speed elliptic liquid jets

Theoretical and experimental investigations are conducted to analyze the instability of a low-speed liquid jet emerging from an elliptic nozzle. The complexity of viscous free surface flow analysis for an asymmetric geometry is simplified using an approach based on the Cosserat theory (also called director theory) which reduces the exact three-dimensional equations to a system depending only on time and on a single spatial variable. This work is mainly focused on the spatial instability analysis to examine the key characteristics of an elliptic jet such as jet profile, axis-switching and breakup length. In the experimental part, both natural (free) and excited (forced) breakup behaviors are studied. In the natural breakup, the effects of nozzle’s ellipticity and length to diameter ratio are examined. In the forced breakup case, disturbances are applied to the jet, by modulating the jet exit velocity using a piezoelectric actuator with given sinusoidal perturbations. The spatial evolution of the jet shape is captured with a high speed camera. Liquid jet instability is studied for various nozzle geometries over a specific range of jet velocities and excitation frequencies. Results are compared with conventional circular nozzles which can be considered as a special case of an elliptic jet.

Simulating radiative shocks in nozzle shock tubes

We use the recently developed Center for Radiative Shock Hydrodynamics (CRASH) code to numerically simulate laser-driven radiative shock experiments. These shocks are launched by an ablated beryllium disk and are driven down xenon-filled plastic tubes. The simulations are initialized by the two-dimensional version of the Lagrangian Hyades code which is used to evaluate the laser energy deposition during the first 1.1 ns. Later times are calculated with the CRASH code. CRASH solves for the multi-material hydrodynamics with separate electron and ion temperatures on an Eulerian block-adaptive-mesh and includes a multi-group flux-limited radiation diffusion and electron thermal heat conduction. The goal of the present paper is to demonstrate the capability to simulate radiative shocks of essentially three-dimensional experimental configurations, such as circular and elliptical nozzles. We show that the compound shock structure of the primary and wall shock is captured and verify that the shock properties are consistent with order-of-magnitude estimates. The synthetic radiographs produced can be used for comparison with future nozzle experiments at high-energy-density laser facilities.

Numerical Investigation of Lateral Jets over a Body of Revolution in Supersonic Cross-flow

DOI: 10.2514/1.B34171 In this paper, the flowfield induced by a sonic lateral jet in a supersonic crossflow of Mach number 3.3 around a bodyofrevolutionisnumericallystudied.Thenumericalsimulationis firstvalidatedforthecaseofacircularnozzle. Shock and vortical structures are examined and compared with those of jet in crossflow over a flat plate. It is found that horseshoe vortices are generated from the flow separation region before the jet orifice. They are shed laterally alongthefootprintofthejetbowshock,wrappingthebodyofrevolution.Thiswrappingeffectisabsentinthecaseof the crossflow over a flat plate. Effects of the jet nozzle shape on the flow structure are further investigated and analyzed. These nozzles, including circular, elliptical, dropletlike, and inverse dropletlike shapes, have equivalent cross-sectional area. The elliptical nozzle with its semimajor axis in the lateral direction has the largest force amplification factor and is thus the most effective. The detailed flow structures also indicate that the low-pressure regionaftofthejet isnot onlyclosely related withthesurfacetrailingvortices,butalsowiththe indentandreflection lines of the leeward barrel shock.

원형 노즐과 타원형 노즐에서 발생되는 캐비테이션 유동

본 연구의 목적은 원형과 타원형 노즐에 발생되는 캐비테이션 유동에 대해 수치 계산을 통해 각각의 특성을 파악하는 것이다. 수치해석을 위해 상용코드인 FLUENT 6.2 을 사용하였고, 실험결과의 캐비테이션 형태와 수치계산에서 얻은 결과를 비교함으로써 계산결과의 타당성을 확인하였다. 주목할만한 결과로는 원형 노즐 내부의 캐이테이션 형상은 노즐 축방향으로 대칭적인 원통형을 가지는 반면, 타원형 노즐의 경우 장축의 캐이테이션 길이가 단축의 캐이테이션 길이보다 긴 말발굽 (horseshoe) 형태가 관찰되었다. 또한, 타원형 노즐의 경우 오리피스의 반경방향 속도는 오리피스의 장, 단축에서 확연히 다른 분포가 관찰되었다.

Jet characteristics from a submerged combustion system

The characteristics of a combustor operating under submerged conditions are affected by the two phase interaction of exhaust gas jet from the combustor with surrounding liquid. The characteristics of combustion gases are simulated with air and helium to represent combustor operation under different conditions. The exhaust gas signatures under submerged conditions are examined using different nozzle exit cross-sections (circular, square, triangular and elliptical with aspect ratio of 1.5 and 2.5) for their effect on sound pressure levels and pressure fluctuations in the combustion chamber. High-speed cinematography is used to examine the two-phase region and the associated instabilities by the gas jet. Dynamic pressure sensor is used to study the effect of submerged jet on the pressure fluctuations in the upstream gas chamber. The sound pressure level from the elliptical nozzle is found to be lower than the circular, square and triangular nozzles. The frequency of jet instabilities is observed to increase with increase in gas jet momentum but independent of nozzle exit cross-section. The pressure fluctuation in the gas chamber is closely coupled with two phase instabilities downstream of the jet region. At lower jet momentum bubbling regime is present but it transitions to more jet like behavior with increase in the jet momentum, representing deep water and shallow water propulsion applications, respectively. These studies provide valuable fundamental information for range of applications in energy systems extending from underwater propulsion, evaporator, heater, desalination and waste water treatment.

Capillary instability of elliptic liquid jets

Instability of a liquid jet issuing from an elliptic nozzle in Rayleigh mode is investigated and its behavior is compared with a circular jet. Mathematical solution of viscous free-surface flow for asymmetric geometry is complicated if 3-D analytical solutions are to be obtained. Hence, one-dimensional Cosserat (directed curve) equations are used which can be assumed as a low order form of Navier-Stokes equations for slender jets. Linear solution is performed using perturbation method. Temporal dispersion equation is derived to find the most unstable wavelength responsible for the jet breakup. The obtained results for a circular jet (i.e., an ellipse with an aspect ratio of one) are compared with the classical results of Rayleigh and Weber for inviscid and viscous cases, respectively. It is shown that in the Rayleigh regime, which is the subject of this research, symmetric perturbations are unstable while asymmetric perturbations are stable. Consequently, spatial analysis is performed and the variation of growth rate under the effect of perturbation frequencies for various jet velocities is demonstrated. Results reveal that in comparison with a circular jet, the elliptic jet is more unstable. Furthermore, among liquid jets with elliptical cross sections, those with larger ellipticities have a larger instability growth rate.

Axis-switching of non-axisymmetric microdroplet ejected from inkjet with an elliptical nozzle

Dynamic behaviors of the microdroplet ejected from the inkjet printhead with an elliptical nozzle are investigated using a high-speed micro-imaging system. The elliptic geometry of the nozzle induces elliptical cross section of the ejected droplet and the periodic switching of its major and minor axes. From the temporally resolved measurement of droplet configuration based on the reproducibility of drop formation, the frequency responses and the viscous damping rates of the droplet oscillations are analyzed. The discrepancies in the dominant frequency and the decaying time constant between the axis-switching and axial oscillations are maintained in the measurement section, despite the effects of the nonlinearity and mode coupling.

On the vortical structures and behaviour of inclined elliptic jets

A study has been carried out to clarify vortical structures and behaviour resultant from imposing inclined exits along either the major or minor plane of an elliptic nozzle. Laser-induced fluorescence (LIF) flow visualizations show production of inclined vortex roll-ups along the inclined planes, with corresponding narrowing of jet columns along the non-inclined planes. Minor-plane inclined nozzles result in significant growths in the jet spread along the inclined plane, while major-plane inclined nozzles produce little variations. Formation of rib structures is observed to be suppressed in minor-plane inclined nozzles and linked to braid vortices inducing the formation of streamwise vortices along the minor plane. Particle-image velocimetry measurements show that increasing the incline angle in major-plane inclined nozzles reduce the strengths of the discrete vortex roll-ups, while the opposite occurs in minor-plane inclined nozzles. Although Reynolds shear stress variations correspond well with changes in incline angle and vortex roll-up strength in major-plane inclined nozzles, they demonstrate a non-monotonic relationship in minor-plane inclined nozzles. LIF visualizations further clarify how strong asymmetric interactions between the inclined vortex roll-ups and braid vortices lead to suppression of axis-switching in major-plane inclined nozzles but not in minor-plane inclined nozzles. The more complex flow behaviour in the latter is responsible for the non-linear relationship in Reynolds shear stress levels observed earlier. Comparisons of the half-jet width profiles confirm the suppression of axis-switching in major-plane inclined nozzles only, while momentum thickness profiles show significant variations in the mixing layer characteristics between major- and minor-plane inclined nozzles.

Effect of primary jet geometry on ejector performance: A cold-flow investigation

The following cold-flow study examines the interaction of the diffracted shock wave pattern and the resulting vortex loop emitted from a shock tube of various geometries, with an ejector having a round bell-shaped inlet. The focus of the study is to examine the performance of the ejector when using different jet geometries (primary flow) to entrain secondary flow through the ejector. These include two circular nozzles with internal diameters of 15 mm and 30 mm, two elliptical nozzles with minor to major axis ratios of a/ b = 0.4 and 0.6 with b = 30 mm, a square nozzle with side lengths of 30 mm, and two exotic nozzles resembling a pair of lips with axis ratios of a/ b = 0.2 and 0.5 with b = 30 mm. Shock tube driver pressures of P 4 = 4, 8, and 12 bar were studied, with the pressure of the shock tube driven section P 1 being atmospheric. High-speed schlieren photography using the Shimadzu Hypervision camera along with detailed pressure measurements along the ejector and the impulse created by the ejector were conducted.

Experimental and Numerical Study of Jets from Elliptic Nozzles with Conic Plug

The study presents results concerning jets exhausting from elliptic nozzles with conic centerbody plugs. A nozzle with 3:1 ratio of exit heights is surveyed experimentally using particle image velocimetry for jet Mach numbers from 0.24 to 1.0, heated and unheated. At the exit, the nozzle inner surface has a slope of zero in the major axis plane, while in the minor axis plane the slope is steeper than that for a corresponding round nozzle. The conic plug causes splitting of the initial elliptic jet into two jets with their centerlines in the major-axis plane of the elliptic nozzle. Increasing M j or total temperature causes the bifurcated potential cores of the individual jets to be slightly smaller and to diverge from one to another at a slightly greater angle from the nozzle centerline. Large eddy simulations of the experimental nozzle and two alternate nozzles are analyzed to find how the flow features are influenced by not forcing the jet towards the conic plug in the minor axis plane and by changing the ratio of exit heights to 2:1. Changing nozzle shape caused reduction in the spreading rate in the major axis and increase in spreading in the minor axis. Bifurcation occurs for all nozzles studied.

EFFECT OF INTERNAL FLOW STRUCTURE IN CIRCULAR AND ELLIPTICAL NOZZLES ON SPRAY CHARACTERISTICS

An experimental study was performed to investigate the atomization characteristics of a circular nozzle and elliptical nozzles of small diameter under high injection pressure, which has a hydraulic flip condition for the nozzle internal flow structure. The flow rate and drop size characteristics were measured for various injection pressures. Numerical simulations were attempted to investigate the internal flow structure in the circular and elliptical nozzles because the experimental study was limited in its measurements of flow velocity distributions, pressure distributions, and streamlines in the relatively small orifices. This study showed that the disintegration characteristics of the liquid jet of the elliptical nozzles were very different from those of the circular nozzle. In the case of the elliptical nozzles, the liquid jet became more unstable at the same injection pressure, unlike that of the circular nozzle. Surface breakup was observed at the jet issued from the elliptical nozzles with the increase of injection pressure. Furthermore, the numerical simulations informed that the internal flow structure of the elliptical nozzle was quite different from that of the circular nozzle. In the case of the circular nozzle, as with much of the literature on the internal flow structure of the hydraulic flip, the flow detached from the orifice wall. However, the internal flow structure of the elliptical nozzle in hydraulic flip was reattached to the orifice wall of the minor axis, unlike the flow in the circular nozzle. It has been concluded that the internal flow structure of the elliptical nozzle has influence on the disintegration characteristics of the liquid jet issued from the elliptical nozzle.

Computational analysis of the effect of nozzle cross-section shape on gas flow and particle acceleration in cold spraying

In cold spraying, the spraying of certain complicated surfaces may require nozzles with special cross-sections. In this study, numerical investigation is conducted to study the effect of nozzle cross-section shape on gas flow and particle acceleration in cold spraying. The comprehensive comparison between rectangular nozzles and elliptical nozzles indicates that rectangular nozzles result in slightly lower mean particle impact velocity than elliptical nozzles. However, for rectangular nozzles, more particles may achieve relatively high velocity due to the larger sectional area of their potential core. Furthermore, it can also be found from the numerical results that the mean particle impact velocity increases gradually with the decrease in Width/Length ratio (W/L) of the cross-section because of the diminishing bow shock size. However, when reducing the W/L to 0.2, the mean particle impact velocity begins to decrease steeply, which may be attributed to the rather small area of the potential core for the case of W/L = 0.2. Moreover, the systematic study on the powder release position shows that releasing particles from the nozzle inlet can ensure that particles achieve a high impact velocity and temperature.

노즐 오리피스 형상에 따른 Discharge Coefficient와 Cavitation에 관한 실험적 연구

본 연구는 타원형 노즐과 원형 노즐 내부에서 발생되는 cavitation의 발생 및 성장을 실험적으로 관찰하였다. 원형 노즐과 타원형 노즐의 cavitation 특성을 가시화 하기위해 투명한 아크릴로 노즐을 제작하였다. 실험에 사용된 노즐들은 같은 단면적으로 제작되었으며, 타원형 노즐의 경우 형상비(a/b)를 다르게 하였다. 분사압력의 증가에 따라 노즐내부 유동은 no cavitation, cavitation, hydraulic flip 영역으로 나뉘어졌다. 노즐의 형상에 상관없이 no cavitation과 cavitation 영역에서는 분사압력의 증가에 따라 유량은 증가하며, 유출계수는 감소하는 경향을 나타냈다. 그러나 hydraulic flip 영역에서의 유량계수는 일정한 값을 나타냈다. 타원형 노즐은 원형 노즐에 비해 높은 cavitation number에서 cavitation이 성장, 발달하였다. 특히 타원형 노즐에서는 장축의 cavitation length가 단축보다 길게 나타났다.

Shock wave configurations and flow structures in non-axisymmetric underexpanded sonic jets

An experimental and numerical study of underexpanded free sonic jet flows issuing from rectangular, elliptical and slot nozzles has been undertaken. Aspect ratios (AR) of 1, 2, and 4 are described at pressure ratios (exit plane pressure to ambient pressure), of 2 and 3. There is good qualitative agreement between the experimental observations and the numerical predictions. In the case of rectangular jets, a complex system of shock waves forming the incident shock system is identified. This shock wave system originates at the corners of the nozzle exits, and proceeds downstream. Mach reflections are found to occur on the incident shock wave surface as well as the presence of a Mach disk terminating the first jet cell. This Mach disk has the shape of a square, a hexagon, or an octagon depending on the nozzle shape. For slot and elliptical jets, the formation of the incident shock wave was not observed along the minor axis plane of the nozzle for AR > 2. The incident shock wave was observed to originate downstream of the nozzle exit in the major axis plane. This wave system undergoes a transition to Mach reflection as it propagates downstream of the nozzle exit. In all cases tested, the shape of the jet boundary is significantly distorted. In rectangular jets, the narrowing of the jet boundary along the diagonal axis of the nozzle exit is observed, and in the case of the elliptical and slot jets axis switching is noted.

Experimental study on a jet issuing from an elliptic nozzle with cylindrical tabs

An elliptic jet with mixing tabs was investigated using the digital particle-image-velocimetry and two-component hotwire anemometry. Two tabs of cylindrical configuration were placed on the minor-axis sides of a 2:1 elliptic nozzle. Because of their sideways pressure-relieving effect, a cylindrical tab relieves the constraint on the flow relative to a thin tab with flat surfaces which can have important implications on tab drag reduction but with similar mixing characteristics. Measurements were made for a jet exit velocity of 20 m s−1 and for Reynolds number based on the nozzle equivalent diameter (De ) of 5.08 × 104. Furthermore, the effect of tab height (h/d= 1.0 and 1.67) is also investigated. The study shows the evolution of a mushroom structure behind each tab that distorts the jet flow into a two-finger structure. The tab wake mainly comprises of base vortices and tip vortices that are enveloped from the sides by spanwise vortical structures. While the strong upwash from the base vortices causes an...

Compressible vortex loops: Effect of nozzle geometry

Vortex loops are fundamental building blocks of supersonic free jets. Isolating them allows for an easier study and better understanding of such flows. The present study looks at the behaviour of compressible vortex loops of different shapes, generated due to the diffraction of a shock wave from a shock tube with different exit nozzle geometries. These include a 15 mm diameter circular nozzle, two elliptical nozzles with minor to major axis ratios of 0.4 and 0.6, a 30 × 30 mm square nozzle, and finally two exotic nozzles resembling a pair of lips with minor to major axis ratios of 0.2 and 0.5. The experiments were performed for diaphragm pressure ratios of P 4 / P 1 = 4 , 8, and 12, with P 4 and P 1 being the pressures within the high pressure and low pressure compartments of the shock tube, respectively. High-speed schlieren photography as well as PIV measurements of both stream-wise and head-on flows have been conducted.

An experimental study on jets issuing from elliptic inclined nozzles

This paper reports on an experimental flow visualisation and digital particle image velocimetry investigation on forced jets exhausting from aspect ratio equal to three elliptic nozzles with exits inclined at 30° and 60°. Flow images show that shear layer instabilities and subsequent vortex roll-ups are formed parallel to the inclined nozzle exits at 30° incline and that rapid re-orientation of the vortex roll-ups occurs at 60° incline. Flow observations also show that strong axis-switching occurs in a non-inclined elliptic nozzle. However, 30° and 60° elliptic inclined nozzles produce significant distortions to and suppression of the axis-switching behaviour, respectively. As a result, flow stresses and turbulent kinetic energy distributions become increasingly asymmetric. Their coherency and magnitudes along the shorter nozzle lengths also vary significantly. This can be attributed to the dissimilar formations of vortex roll-ups and rib structures, as well as unequal mutual interactions between them as the incline-angle increases. Lastly, results also show that unlike circular inclined nozzles, elliptic inclined nozzles do not produce serpentine-shaped jet columns nor lead to significant lateral jet-spread at large incline-angles.

Relationship between Shock-cell Length and Noise of Jets from Rectangular and Elliptic Disk Nozzles

Relationship between Shock-cell Length and Noise of Jets from Rectangular and Elliptic Disk Nozzles

Experimental investigations of compressible vortex loops

The present study involves the shock wave and consequent vortex loop generated when a shock tube with various nozzle geometries is employed. It aims to provide quantitative and qualitative insight into the physics of these compressible phenomena. The geometries included two elliptic nozzles with minor to major axis ratios of 0.4 and 0.6, a 15 mm circular nozzle and a 30×30 mm2 square nozzle. The experiments were performed for driver gas (air) pressures of 4, 8 and 12 bars. Schlieren, shadowgraphy, and particle image velocimetry techniques were employed to visualize and quantify the induced flow field.

Mixing in Circular and Non-circular Jets in Crossflow

Coherent structures and mixing in the flow field of a jet in crossflow have been studied using computational (large eddy simulation) and experimental (particle image velocimetry and laser-induced fluorescence) techniques. The mean scalar fields and turbulence statistics as determined by both are compared for circular, elliptic, and square nozzles. For the latter configurations, effects of orientation are considered. The computations reveal that the distribution of a passive scalar in a cross-sectional plane can be single- or double-peaked, depending on the nozzle shape and orientation. A proper orthogonal decomposition of the transverse velocity indicates that coherent structures may be responsible for this phenomenon. Nozzles which have a single-peaked distribution have stronger modes in transverse direction. The global mixing performance is superior for these nozzle types. This is the case for the blunt square nozzle and for the elliptic nozzle with high aspect ratio. It is further demonstrated that the flow field contains large regions in which a passive scalar is transported up the mean gradient (counter-gradient transport) which implies failure of the gradient diffusion hypothesis.