Elliptic Jet Research Articles

Characteristics of Sheet Formed by Collision of Two Elliptical Jets at Short Impact Distance

The research on characteristics of impinging jet has a long history and focuses mainly on the circular jets, whereas the impingement of noncircular jets, such as elliptical jets, receives much less attention. This paper investigated liquid sheet resulting from the oblique collision of two elliptical jets at short impact distance. The elliptical liquid jets contract and collide obliquely at impact point, forming a sheet in the form of a leaf bounded by a thicker rim. An improved theoretical model, taking jet contraction into account, for two elliptical impinging jets is established. The sheet features are obtained by combining the conservation equations between the liquid jet and sheet with the force balance equations of the sheet rim. The calculated sheet shapes are compared with the experiments, and the results show good agreement. The experimental results also indicate that the liquid sheet formed by elliptical jets tends to be larger and more unstable than that formed by circular jets. Based on the model, the effects of axial ratio and impact distance on the sheet characteristics, such as sheet shape and thickness, are also studied.

Staging behaviour in screeching elliptical jets

Staging behaviour in screeching elliptical jets

Staging Behaviour in Screeching Elliptical Jets

The staging behaviour of jet screech in an elliptical nozzle with with an aspect ratio AR = 2.0 is examined across a range of pressure ratios. Acoustic measurements and high resolution imaging are used to assess the staging behaviour. Based on the frequency of the dominant screech tone, the jet staging behaviour can be separated into five distinct modes over the range of pressures studied. A statistical analysis of the schlieren images suggests that the first mode is varicose, the next three modes are flapping instabilities, and the final mode is helical in nature. A comparison of characteristic length scales in the jet is undertaken to explain the mode switches. A shift from the classical “weakest link” feedback model to an acoustic waveguide model, associated with a shift in characteristic length scale of the jet, is used to explain the discontinuous jumps in screech tone frequency. The results indicate that the different acoustic feedback mechanisms need not necessarily be associated with a particular instability mode shape.

Characteristics of Controlled Mach 2 Elliptic Jet

Mixing characteristics of a Mach 2 elliptic jet, with rectangular tabs placed along the major and minor axes are studied, at different levels of nozzle expansion. At all the pressure ratios, the jet with tabs along the minor axis experiences better mixing than the jet with tabs along the major axis. Also, at all the nozzle pressure ratios, the tabs along the major axis retard the mixing promotion, leading to core longer than the uncontrolled jet. For the range of nozzle pressure ratios studied, the mixing promotion caused by the tabs along the minor axis is the most effective for the highest studied overexpansion level corresponding to nozzle pressure ratio 4, leading to a core length reduction of 65%. The least mixing promoting efficiency for this orientation of tabs is found at the marginally underexpanded level of nozzle pressure ratio 8, at which the core length reduction caused is just 5%. The mixing promoting capability of the tabs along the minor axis is found to decrease with decrease in the adverse pressure gradient at the nozzle exit. The contour plots reveal that the tabs along the minor axis bifurcates the jet into two parts, causing faster spreading of the jet along the major axis than along the minor axis. As a result, there is no evidence of axis switching in the jet with tabs along the minor axis. The shadowgraph pictures show that the waves present in the core of the jet with tabs along the minor axis is weaker than the waves in the jet with tabs along the major axis and the uncontrolled jet. Also, the bifurcation of the jet with tabs along the minor axis is clearly captured in the shadowgraph images.

Development of a cylindrical burner comprising multiple pairs of opposing partially premixed or inverse diffusion flames

A cylindrical burner was developed to combine extensive firing limits, high combustion efficiencies, and low NOx emissions from partially premixed or inverse diffusion flames by setting multiple pairs of circumferentially opposing jets (of concentric fuel and air streams). Normal strain rates as high as 1600 s−1 in addition to transverse strain rates of 640 s−1 were combined with a cross-flow impact and elliptical jet dispersion such that a turbulent kinetic energy peak of 15.4 m2/s2 was produced. The fuel/air mixing was enhanced and the peak temperature was reduced due to the stimulation of cross-flow wake zones and the axis switching of the elliptical jets. Partially premixed methane/air combustion of 12 opposing mixtures at Φ=1.5 led to NOx reduction to 0.36 g/kg fuel via controlling the flow residence time and adjusting the separation between the premixing and diffusion zones of each flame. As elliptical ports are used, the NOx concentrations decreased to 4 ppm with an aspect ratio of 3.0. Increasing the ratio between the diameter of the jets and their axial separation decreased the CO and HC emissions respectively to 489 ppm and 0.019%. Upon having opposing inverse diffusion flames, the blowout stability limit reached a maximum value of 53.2 m/s. With a separation of 4.0 cm, the NOx emissions reached a minimum level of 25 ppm by increasing the momentum flux ratio between the cross-flow air jet and the opposing jets to 1.5. By optimizing the cross-flow/primary air velocity ratio and the primary air/fuel jet diameter ratio, the CO and HC emissions were respectively minimized to 721 ppm and 0.039%. The velocity fluctuations indicated that increasing the opposing jets increases the turbulent kinetic energy and decreases the smallest scales of turbulence.

Design and testing of low-divergence elliptical-jet nozzles

A novel approach was developed to design and fabricate nozzles to produce high-pressure low-divergence fluid jets. Rapid-prototype fabrication allowed for myriad experiments investigating effects of different geometric characteristics of nozzle internal geometry on jet divergence angle and fluid distribution. Nozzle apertures were elliptical in shape with aspect ratios between 1.00 and 2.45. The resulting nozzle designs were tested and the lowest elliptical jet divergence angle was 0.4 degrees. Nozzle pressures and flowrates ranged from 0.32 to 4.45 MPa and 13.6 to 37.9 LPM, respectively. CimCool CimTech 310 machining fluid was used in all experiments at a Brix concentration of 6.6 percent.

Twin inclined jets in crossflow: experimental investigation of different flow regimes and jet elevations

The present experimental study is dedicated to unfolding the mixing process generated by double-inclined, variably elevated jets in crossflow. Twin tandem jets in crossflow are very common in the industry and are closely dependent on several parameters. Detailing the induced interactions in such a model would ultimately enhance our understanding and help optimize related applications. The jets handled in the present work are placed three diameters apart, arranged in line with the oncoming crossflow and sent at variable levels (h = 0, 1, 2, 5 cm) from the ground of a wind tunnel and are discharged from elliptic discharging cross sections. Elliptic jets are of particular interest in applications like industrial and boat chimneys, and are practically easily obtained by razing circular cylinders at the desired inclination and height. Particle image velocimetry and coupled charge device CCD camera were used. The dynamic and turbulent behavior of the resulting flowfield was characterized in terms of streamlines, velocity components and vortical structures. The obtained data helped highlight the impact of jets’ elevation over their mixing mechanism among the surrounding free stream: establishment of the resulting flowfield dynamics and settlement of the induced vortical structures. The mixing process evolution was also considered under different flow regimes. For the matter an injection ratio (defined as jets to the mainstream’s velocity ratio) raging between 0.67, 1 and 1.29 was considered. These cases correspond to jets dynamically dominated by, equivalent to or more dominant than the mainstream. The impact of jets’ elevation combined to that of the injection ratio affects both the developed vortical structures and the established dynamic fields, which in turn highly affects the induced mixing process.

Experimental study of elliptical jet from supercritical to subcritical conditions using planar laser induced fluorescence

The study of fluid jet dynamics at supercritical conditions involves strong coupling between fluid dynamic and thermodynamic phenomena. Beyond the critical point, the liquid-vapor coexistence ceases to exist, and the fluid exists as a single phase known as supercritical fluid with its properties that are entirely different from liquids and gases. At the critical point, the liquids do not possess surface tension and latent heat of evaporation. Around the critical point, the fluid undergoes large changes in density and possesses thermodynamic anomaly like enhancement in thermal conductivity and specific heat. In the present work, the transition of the supercritical and near-critical elliptical jet into subcritical as well as supercritical environment is investigated experimentally with nitrogen and helium as the surrounding environment. Under atmospheric condition, a liquid jet injected from the elliptical orifice exhibits axis switching phenomena. As the injection temperature increases, the axis switching length also increases. Beyond the critical temperature, the axis switching is not observed. The investigation also revealed that pressure plays a major role in determining the thermodynamic transition of the elliptical jet only for the case of supercritical jet injected into subcritical chamber conditions. At larger pressures, the supercritical jet undergoes disintegration and formation of droplets in the subcritical environment is observed. However, for supercritical jet injection into supercritical environment, the gas-gas like mixing behavior is observed.

Multimodal Instability in the Weakly Underexpanded Elliptic Jet

Particle image velocimetry and schlieren imaging are used to study the interrelationship between the hydrodynamic and acoustic fields of a screeching underexpanded elliptical jet. An elliptical jet with an aspect ratio equal to two is studied at a nozzle pressure ratio of 2.2. Two-point correlation analysis of the schlieren image sets suggests that the flow is dominated by a flapping mode about the major axis plane, however, a proper orthogonal decomposition of the transverse velocity field suggests that there is also a helical mode that becomes dominant further downstream. The flapping mode appears to grow, saturate, and decay more rapidly and is associated with the production of discrete acoustic tones in the form of jet screech, whereas no discrete tone sound production is observed from the helical mode. It is suggested that the lack of tonal generation by the helical mode is linked to its slower growth; it does not produce coherent fluctuations in shear layer vorticity in the vicinity of the jet shock cells.

Mixing of a non-circular jet into a counterflow

An elliptic jet and a square jet flowing into a counterflow with different jet-to-current velocity ratios are investigated by using realizable k-ɛ model. Some computed mean velocity and scalar features agree reasonably well with experimental measurements, and more features are obtained by analyzing the computed results. After fluid issues from a nozzle, it entrains ambient fluid, and its velocity and concentration on the centerline decay with the distance downstream from the potential core (l0). The decay ratio increases with the decreasing jet-to-current velocity ratio α. For an elliptic jet, the evolution of the excess velocity half-width b and the concentration half-width bc merely remains constant near the jet exit on major-axis plane while they increase linearly on the minor-axis plane. However, the half-widths on the major-axis and minor-axis plane become proportional to the axial distance downstream after equaling each other. For a square jet, b and bc increase linearly with the distance downstream from the jet exit, but the spread ratio is larger on the middle plane than that on the diagonal plane before they equal each other. The radial extent of the dividing streamline rs or the mixing boundary rsc increases linearly downstream, and decreases exponentially after reaching a peak at xb. The ratio on the minor-axis plane is larger than that on the major-axis plane for an elliptic jet. The characteristics are the same for the square jet. b, bc, rs, and rsc on two corresponding planes become equal to each other more rapidly for the square jet than for the elliptic jet, because the sharp corner of the square nozzle induces secondary structures that are more intense. The distributions of the excess axial velocity and scalar concentration exhibit self-similarity for either the elliptic jet or square jet in the region of l0 < x < xb. On the cross section, four counter-rotating pairs of vortices, which enhance the entrainment between the jet and counterflow, form at the four corners of the square jet or at the two ends of the major-axis plane of the elliptic jet. The recirculation pattern formed by these axial vortices is more complex for the square jet than that for the elliptic jet. The turbulent kinetic energy k have large value in the region near the jet exit and stagnation point. The maximum value of k for the square jet is larger than that of the elliptic jet near the jet exit. This results in the square jet mixing more strongly than the elliptic jet.

Mixing Characteristics of Underexpanded Elliptic Sonic Jets from Orifice and Nozzle

The mixing characteristics of elliptical sonic jets issuing from an orifice and a convergent nozzle with an identical elliptical exit of aspect ratio of two were studied experimentally in the presence of favorable pressure gradients corresponding to nozzle pressure ratios of two to five, in steps of one. To quantify the aerodynamic mixing of the free jets with the fluid entrained from the surrounding environment, the jet centerline pitot pressure distribution and radial pitot pressure profiles along the major and minor axes directions of the jet were measured. The centerline pitot pressure variation of the orifice and nozzle jets shows that the decay of the jet from the nozzle is faster than the jet from the orifice at nozzle pressure ratios more than two. But at a nozzle pressure ratio of two, the decay of both the jets are close to each other. The pressure profiles demonstrate that the axis-switching location shifts upstream with an increase of nozzle pressure ratio for both the jets, implying that the near-field mixing is enhanced by the increase in the favorable pressure gradient. The waves prevailing in the jet field were visualized with the shadowgraph technique.

Elliptic jet control with cross-wire

The effect of cross-wire on the aerodynamic mixing of circular and elliptic sonic jet flow fields is studied experimentally in the presence of favourable pressure gradient at the orifice exit. The cross-wire is found to reduce the amplitude of pitot pressure oscillation drastically, and thus weakens the shock cell structure in the jet core. In the case of elliptic jets, the effectiveness of cross-wire is found to be dependent on its location at the orifice exit. For lower aspect ratio jets (AR 2), cross-wire along the major axis proves to be more effective in enhancing mixing, but as the aspect ratio increases to higher values (AR 6), cross-wire along the minor axis becomes more effective in promoting the jet mixing. The pressure profile results indicate that the cross-wire located on the major axis advances the axis-switching phenomenon, while the cross-wire along the minor axis shifts the axis-switching location further downstream compared to uncontrolled jet. The jet bifurcation caused by the cross-wire has been captured in the shadowgraph pictures.

Instability of elliptic liquid jets: Temporal linear stability theory and experimental analysis

The instability dynamics of inviscid liquid jets issuing from elliptical orifices is studied, and effects of the surrounding gas and the liquid surface tension on the stability behavior are investigated. A dispersion relation for the zeroth azimuthal (axisymmetric) instability mode is derived. Consistency of the analysis is confirmed by demonstrating that these equations reduce to the well-known dispersion equations for the limiting cases of round and planar jets. It is shown that the effect of the ellipticity is to increase the growth rate over a large range of wavenumbers in comparison to those of a circular jet. For higher Weber numbers, at which capillary forces have a stabilizing effect, the growth rate decreases with increasing ellipticity. Similar to circular and planar jets, increasing the density ratio between gas and liquid increases the growth of disturbances significantly. These theoretical investigations are complemented by experiments to validate the local linear stability results. Comparisons of predicted growth rates with measurements over a range of jet ellipticities confirm that the theoretical model provides a quantitatively accurate description of the instability dynamics in the Rayleigh and first wind-induced regimes.

Shifting and breakup instabilities of squeezed elliptic jets

Liquid jets are inherently unstable and breakup into drops due to capillary instability driven by surface tension. Elliptic jets have been studied and reported to be more unstable than circular jets. The reason for their increased instability has been often ascribed to the axis-switching of elliptic jets. We theoretically elucidate, for the first time, that even in the absence of axis-switching, an elliptic jet exhibits greater instability compared to circular jets solely due to interfacial curvature effects. For this purpose, we analyse the stability of a jet with elliptic cross-section subject to a non-uniform ambient pressure. This azimuthally dependent pressure distribution in the ambient fluid helps to prescribe a static base state for the elliptic jet and maintain the azimuthally varying curvature of the interface. Physically, this mathematical treatment implies that the jet is squeezed into an elliptic configuration by a non-uniform pressure distribution in the passive ambient fluid and therefore cannot exhibit axis-switching. The jet is assumed to be incompressible and inviscid. The stability of such a jet is analyzed using Rayleigh’s Work Principle (RWP) and linear stability theory. It is also shown that in addition to capillary breakup, analogous to that of classical circular jets, squeezed elliptic jets are found to exhibit a shifting mode instability, in which the entire jet translates parallel to its major axis. Both capillary breakup and shifting modes are rendered more unstable by an increase in ellipticity. For low ellipticity, the breakup mode exhibits higher growth rates as compared to the shifting mode. We show that there exists a critical ellipticity beyond which the shifting mode starts to dominate the capillary breakup mode. Predictions from both RWP and linear stability theory converge to Rayleigh’s classic results for a circular cylindrical jet.

Inverse diffusion and partially premixed flames with elliptical/swirling- and cross-flows

The combustion performance of inverse diffusion and partially premixed flames was optimized by employing concentric elliptical ports with variable relative angular orientation, co-swirling, and cross-flow opposing jets of different inclination angles. The flexible and favorably active control of combustion was pronounced by changing the nozzle relative angular orientation. The optimum angular shift was found to be 20° where the flame length was shortened by 39% from the case of 0° and the NOx levels were reduced to about 0.51 g/kg of fuel for the inverse partially premixed flames. At such optimum angular shift, the minimum CO and NOx emissions respectively decreased to 17.1 and 1.52 g/kg of fuel for the inverse diffusion flames. The elliptical jets increased the size of the swirling flow recirculation zone by about 28%. Increasing the inner aspect ratio consistently increased the combustion efficiency, where the CO emissions reach minimum values of 12.3 and 6.22 g/kg of fuel respectively for the inverse diffusion and partially premixed flames. The correspondingly minimum NOx emissions were thus 0.89 and 0.51 g/kg of fuel at a cross-flow momentum flux ratio of 2.0. Splitting the opposing cross-flow air jets by increasing their inclination angle above 0° duplicated the stagnation zone vortical impact to enrich the average turbulent energy at the major axis tips. The inclination angle of 15° was the optimum at no swirl while increasing the outer swirl angle shifted the optimum inclination angle to 25°. The inner/outer co-swirl of 45°/60° thus provided a minimum CO emission of 11.1 g/kg of fuel for the inverse diffusion flame and 2.65 g/kg of fuel for the inverse partially premixed flame.

Surface Tension Measurements of Calcia-Alumina Slags: A Comparison of Dynamic Methods

The surface tension of oxide melts is difficult to measure. Even for binary slags with a low degree of polymerisation such as calcia-alumina, there seems no benchmark value for the surface tension available. Published data exhibit considerable scatter even for similar compositions which prevents to build a reliable and consistent database of frequently used physical properties. This study presents an attempt to use three different measurement techniques in the same high-temperature furnace to obtain the surface tension of a binary calcia-alumina slag close to eutectic composition, namely the drop weight method, the pendant drop method, and, for the first time, the oscillating jet method (for circular and elliptic jets) as a function of temperature. The methods used in the present context are explained, and the experimental results evaluated with respect to previously published data. A consistent and coherent trend is observed when the drop weight and the oscillating jet methods are compared.

Acoustic and heat transfer characteristics of an impinging elliptical synthetic jet generated by acoustic actuator

Abstract The acoustic aspects and average heat transfer characteristics of an elliptic synthetic jet impinging on a heated flat plate were experimentally investigated and compared with circular, rectangular and square orifices having same equivalent diameter. The aim of the study is to investigate the effect of orifice shape, aspect ratio and operating parameters of the synthetic jet at which optimum acoustic and heat transfer performance is obtained. It is found that irrespective of the shape of the orifice, the synthetic jet generates the minimum sound pressure level when operated at its resonance frequency, which is equal to the diaphragm frequency in the present case. The maximum heat transfer enhancement is obtained at the resonance frequency for elliptical orifice of aspect ratio 1.4 at the jet to plate spacing (z/de) of 3, where z is the axial distance and de is the equivalent diameter of the orifice. The aspect ratio of an orifice is a sensitive parameter as with increase in the aspect ratio, reduction in the Nusselt number is observed. For low jet to plate spacing ratios (z/de

Mixing Characteristics of Elliptical and Rectangular Subsonic Jets with Swirling Co-flow

AbstractThis paper presents a computational analysis of effects of swirling co-flow and non-circular subsonic compressible inner jets on centerline velocity decay, mass entrainment and jet spreading rate. Three different exit shapes of elliptical, rectangular and circular inner jets were compared for three different co-flow conditions such as no co-flow, straight co-flow and swirling co-flow. Co-flow is issuing from a circular annular duct. Swirling co-flow is created in the co-flow duct by introducing a swirler with stationary angular vanes of 50° oblique to the jet axis. Reynolds number of inner jet is calculated based on its equivalent diameter as 200342. It is found that the swirling co-flow has strong influence on the boundary condition of inner jet and alters the major features of the jet such as jet potential core length, centerline velocity decay rate and jet spread rate. Streamwise corner vortices of different jet conditions have been captured using velocity vector plot to show the effect of swirling co-flow on the jet flow field. Swirling co-flow with elliptical inner jet exhibits higher velocity decay rate and jet spreading rate than the equivalent area circular and rectangular jet.

Exit Geometry Effect on Jet Mixing

An elliptical nozzle of 3:1 aspect ratio (AR) issuing a Mach 1.5 jet has been used to investigate its mixing promoting efficiency under over-expanded and under-expanded conditions at the nozzle exit. Azimuthal asymmetry of the elliptical shape is interpreted as a benefit to the mixing process, which is significantly enhanced in comparison to an equivalent circular jet under identical conditions. The elliptical jet decays faster than its circular counterpart throughout the jet field. It is found that continuous variation in the size of the mixing promoting vortices shed from the nozzle exit, as a result of its azimuthal asymmetry, is responsible for faster decay of elliptical jet. The visualization of elliptic jet corroborates the fact that the waves prevailing in the elliptical jet are significantly weaker than those in the circular jet.

Experimental and numerical investigation of geometry effects on multiple impinging air jets

The effect of jet geometry on the flow and the heat transfer characteristics was investigated experimentally and numerically for elliptic and rectangular impinging jet arrays. Detailed heat transfer measurements over a smooth surface by elliptic and rectangular impinging jet arrays were obtained using thermal infrared camera. Velocity measurements were performed with a Laser-Doppler Anemometry (LDA) system. The aspect ratios (AR) of elliptic and rectangular jets for 1.0, 2.0 and 0.5, jet Reynolds numbers ranging from 2000 to 10,000, and jet-to-target spacings ranging from 2 to 10 were considered to investigate impingement heat transfer performance. The Nusselt numbers for the elliptic jets in the impingement regions were larger than that for a circular jet. This may be caused by the large entrainment rate and large scale coherent structure of the elliptic jet. Best heat transfer performance was obtained with the elliptic jet arrangements. Correlations have been developed for the average Nusselt numbers for elliptic and rectangular jet geometries. Additionally, simulations have been performed and velocity distributions were obtained. Axial, radial velocity distributions and turbulent kinetic energy distributions have been calculated.