PIV Measurements in the Near and Intermediate Field Regions of Jets Issuing from Eight Different Nozzle Geometries

An experimental study was conducted to investigate the effect of nozzle geometry on the mixing characteristics and turbulent transport phenomena in turbulent jets. The nozzle geometry examined were round, square, cross, eight-corner star, six-lobe daisy, equilateral triangle as well as ellipse and rectangle each with aspect ratio of 2. The jets were produced from sharp linear contoured nozzles which may be considered intermediate to the more widely studied smooth contraction and orifice nozzles. A high resolution particle image velocimetry was used to conduct detailed velocity measurements in the near and intermediate regions. It was observed that the lengths of the potential cores and the growth rates of turbulence intensities on the jet centerline are comparable with those of the orifice jets. The results indicate that the decay and spreading rates are lower than reported for orifice jets but higher than results for smooth contoured jets. The jets issuing from the elliptic and rectangular nozzles have the best mixing performance while the least effective mixing was observed in the star jet. The distributions of the Reynolds stresses and turbulent diffusion clearly showed that turbulent transport phenomena are quite sensitive to nozzle geometry. Due to the specific shape of triangular and daisy jets, the profiles of mean velocity and turbulent quantities are close to each other in their minor and major planes while in the elliptic and rectangular jets are considerably different. They also exhibit more isotropic behavior compared to the elliptic and rectangular jets. In spite of significant effects of nozzle geometry on mean velocity and turbulent quantities, the integral length scales are independent of changes in nozzle geometry.     

Effect of Sudden Expansion on Entrainment and Spreading Rates of a Jet Issuing from Asymmetric Nozzles

Turbulent free jets issuing from five different nozzle geometries; smooth pipe, contracted circular, rectangular, triangular, and square, are experimentally investigated by using TSI 2-D laser Doppler velocimetry (LDV) to assess the effect of nozzle geometry and quarl (i.e. a cylindrical sudden expansion) on jet entrainment and spreading. The centerline mean velocity decay and the jet half-velocity width, which are indicators of jet entrainment and spreading rates, are determined for each nozzle’s flow configuration, i.e. with and without sudden expansion. Furthermore, turbulence quantities, such as the flow mean velocities and their mean fluctuating components, as well as Reynolds shear stresses, are all measured along the centerline plane of the jet to facilitate understanding the extent of the effect of nozzle’s geometry (i.e. nozzle’s orifice shape and sudden expansion) on jet’s entrainment and spreading. The main results show that the jet flow with the presence of sudden expansion exhibits higher rates of entrainment and spreading than without. In addition, these results reveal that sudden expansion exercises a greater effect on the asymmetric jet characteristics, especially for the triangular and rectangular nozzles compared to their axisymmetric counterparts (i.e. circular contracted nozzle).     

Particle image velocimetry investigation on mixing enhancement of non-circular sharp edge nozzles

An experimental study using Particle Image Velocimetry (PIV) on free jets issuing from different orifice plate (OP) nozzles is reported. Mean velocity, turbulence intensity and higher order profiles relevant for large and small scale mixing are considered in the near field and interaction zone (0 < X/D < 20). This is done to determine mixing enhancement due to rectangular, squared, elliptic and triangular nozzles in comparison to circular nozzle results in two orthogonal planes. The effect of Reynolds number on the differences among the nozzle shapes is also considered by performing measurements just after laminar–turbulent transition (Re = 8000) and in the fully turbulent regime (Re = 35,000). The results at low Reynolds number show two classes of jets, i.e. at one side, those closer to axial-symmetric conditions, as circular, square and triangular jets, whereas on the other side those with elongated nozzles as rectangular and elliptic. The reason for the different behavior of the latter is connected to the phenomenon of axis-switching which allows a rearrangement of turbulence over the different velocity components and directions. However, for the highest Reynolds number investigated, all nozzles show similar behavior especially in the jet far field (X/D > 10), thus suggesting a significant Reynolds number dependence of the results.     

Breakup of liquid jets from non-circular orifices

The purpose of this investigation is to study the effect of the orifice geometry on liquid breakup. In order to develop a better understanding of the liquid jet breakup, investigations were carried out in two steps—study of low-pressure liquid jet breakup and high-pressure fuel atomization. This paper presents the experimental investigations conducted to study the flow behavior of low-pressure water jets emanating from orifices with non-circular geometries, including rectangular, square, and triangular shapes and draws a comparison with the flow behavior of circular jets. The orifices had approximately same cross-sectional areas and were machined by electro-discharge machining process in stainless steel discs. The liquid jets were discharged in the vertical direction in atmospheric air at room temperature and pressure conditions. The analysis was carried out for gage pressures varying from 0 to 1,000 psi (absolute pressures from 0.10 to 6.99 MPa). The flow behavior was analyzed using high-speed visualization techniques. To draw a comparison between flow behavior from circular and non-circular orifices, jet breakup length and width were measured. The flow characteristics were analyzed from different directions, including looking at the flow from the straight edges of the orifices as well as their sharp corners. The non-circular geometric jets demonstrated enhanced instability as compared to the circular jets. This has been attributed to the axis-switching phenomenon exhibited by them. As a result, the non-circular jets yielded shorter breakup lengths as compared to the circular jets. In order to demonstrate the presence of axis-switching phenomenon in square and triangular jets, the jet widths were plotted along the axial direction. This technique clearly demonstrated the axis switching occurring in square and triangular jets, which was not clearly visible unlike the case of rectangular jets. To conclude, non-circular geometry induces greater instabilities in the liquid jets, thereby leading to faster disintegration. Thus, non-circular orifice geometries can provide a cheaper solution of improving liquid breakup and thus may enhance fuel atomization as compared to the precise manufacturing techniques of drilling smaller orifices or using costly elevated fuel injection pressure systems.     

Mixing characteristics of turbulent water vapour jets measured using an isokinetic sampling probe

 Horizontal turbulent water vapour (steam) jets were discharged into ambient air from a circular convergent nozzle under unchoked/choked and saturated/superheated nozzle exit conditions, resulting in two-phase (liquid and vapour), two-fluid (air and water) condensing free jets. Flow properties and mixing characteristics have been measured with the aid of an isokinetic sampling probe arrangement. Radial and axial profiles of air and steam mass flow rates and mass fractions were measured from which entrainment, centreline decay and half-width spreading rates were calculated and compared with data from the literature. Overall, the mixing characteristics of the condensing jets are very similar to those of non-condensing jets extensively reported in the literature. Received: 30 September 1996 / Accepted: 19 May 1997     

Flow field characterization of coaxial conical and serrated (chevron) nozzles

PIV measurements were performed to provide insight into the effect of serrated (chevron) nozzles on the flow field of a coaxial circular jet. The serrations were tested on the primary nozzle. Mean flow results showed that the chevron effectively redistributes momentum from the high velocity center stream outward to the lower velocity secondary stream by creating lateral jets. This leads to a more rapid decay of the peak jet velocity and a consequent reduction in the length of the jet potential core. Local increases of up to 65% in the outer stream velocity were measured. The interaction of the secondary jets with the lower velocity outer stream produces increases in turbulent kinetic energy (TKE) near the center nozzle lip. These flow field effects correlate with the jet’s acoustic emissions: Reduction of low-frequency noise due to large scale mixing and potential core shortening, and increased high-frequency noise due to increased near-field turbulence.     

Measurements in the near flow field of an isosceles triangular turbulent free jet

The near field mean flow and turbulence characteristics of a turbulent jet of air issuing from a sharp-edged isosceles triangular orifice into still air surroundings have been examined experimentally using hot-wire anemometry and a pitot-static tube. For comparison, some measurements were made in an equilateral triangular free jet and in a round free air jet, both of which also issued from sharp-edged orifices. The Reynolds number, based on the orifice equivalent diameter, was 1.84×10⁵ in each jet. The three components of the mean velocity vector, the Reynolds normal and primary shear stresses, the one-dimensional energy spectra of the streamwise fluctuating velocity signals and the mean static pressure were measured. The mean streamwise vorticity, the half-velocity widths, the turbulence kinetic energy and the local shear in the mean streamwise velocity were obtained from the measured data. It was found that near field mixing in the equilateral triangular jet is faster than in the isosceles triangular and round jets. The mean streamwise vorticity field was found to be dominated by counter-rotating pairs of vortices, which influenced mixing and entrainment in the isosceles triangular jet. The one-dimensional energy spectra results indicated the presence of coherent structures in the near field of all three jets and that the equilateral triangular jet was more energetic than the isosceles triangular and round jets.     

Flow visualisation studies on growth of area of deflected jets

 Laser light sheet visualisation, coupled with image processing, was utilised to understand the effect of exit geometry on the integral properties of jets in cross flow. The study involved jets emanating from circular and rectangular nozzles of different aspect ratios deflected by a uniform free-stream. The investigation considers incompressible momentum jets with exit Reynolds number in the range of 4400–9200, the velocity ratios being 3.9, 5.9 and 7.8. In contrast to a deflected circular jet, those jets emanating from blunt configurations tend to have higher growth rates initially and are devoid of the horse-shoe or the bound vortex system in their cross section. Received: 30 May 1996 / Accepted: 27 November 1996     

In-Nozzle Measurements of a Turbulent Opposed Jet Using PIV

Turbulent opposed jet burners are an excellent test case for combustion research and model development due to the burners’ compactness, relative simplicity, and the good optical access they provide. The flow-field in the flame region depends strongly on the turbulence generation inside the nozzles, so that realistic flow simulations can only be achieved if the flow inside the nozzles is represented correctly, which must be verified by comparison to suitable experimental data. This paper presents detailed particle image velocimetry (PIV) measurements of the flow issuing from the turbulence generating plates (TGP) inside a glass nozzle. The resulting data is analyzed in terms of first and second moments, time-series, frequency spectra and phase averages. The measurements show how individual high velocity jets emerging from the TGP interact and recirculation zones are formed behind the solid parts of the TGP. Vortex shedding is observed in the jet’s shear layer were high levels of turbulent kinetic energy are generated. Time series measurements revealed periodic pulsations of the individual jets and implied a coupling between adjacent jets. The peak frequencies were found to be a function of the Reynolds-number.     

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.     

Liquid jet breakup for non-circular orifices under low pressures

This study uses a high-speed visualization technique to investigate the breakup process and flow behavior of low pressure water jets issued from non-circular orifices including square, triangular, and rectangular shapes. These orifices have approximately the same sectional areas. Stability curve and Ohnesorge chart are employed to make a comparison with circular jets discharged from a circular orifice of the same sectional area. The analysis is carried out for gauge pressures varying from 0.1 psi to 70 psi with small pressure steps corresponding to a range from 0.7 kPa to 482.6 kPa in metric units. Axis-switching phenomenon is observed and analyzed through calculating the axis-switching wavelength and oscillation frequency for rectangular jets. It is found that results for circular jets agreed well with classic theory. Non-circular jets demonstrate enhanced instabilities as a whole compared to circular jets. The different behaviors of non-circular jets are reasonably explained by Rayleigh’s oscillation theory. Axis-switching and aspect-ratio effect in rectangular jets is found to slow down the increase of breakup-length in the Rayleigh breakup regime. Square and triangular jets are more susceptible to wind effects and they are more unstable especially at higher pressure conditions. This can be concluded from the shorter breakup-length and narrower transitional region from the Rayleigh regime to the wind-induced regime as compared to the circular and rectangular jets. Axis-switching wavelength of the rectangular jets is found to increase linearly with increasing jet velocity and oscillation frequency decreases correspondingly.     

An experimental investigation of underexpanded jets from oval sonic nozzles

 Underexpanded jets from oval sonic nozzles were experimentally studied for various pressure ratios up to 20.3. The results revealed that the barrel type of shock structure was present only in the major axis plane except at low aspect ratios. The results also revealed that the jet spreading rate in the minor axis plane of the nozzle was much higher compared to that in the major axis plane, resulting in axis switching of jets. The cross sectional area of these jets were considerably higher compared to the axisymmetric jets indicating higher interface area for viscous mixing in the near field region and increased mixing in the far field region clear of shock structure. Received: 7 January 1997 / Accepted: 7 August 1997     

Vortex Behaviour and Velocity Characteristics of Jets Issuing from Hybrid Inclined Elliptic Nozzles

An experimental study on elliptic nozzles with hybrid flat- and inclined-sections is reported here. The hybrid flat and inclined sections are imposed along either the major- or minor-plane of the nozzles (HIN?A and B nozzles respectively). For HIN?A, results show prevalent pairings between adjacent vortex filaments and induced vortex-loops in the immediate vicinity of the flat-section to produce coalesced vortex roll-ups. Once they detach entirely from the nozzle, they proceed to undergo flow changes resembling that of conventional elliptic jets. HIN?B also leads to near-field vortex pairings but produce discrete inclined vortex roll-ups instead, with accompanying delay in rib structure formations. The roles of induced vortex-loops are significantly more limited in the present elliptic HIN than circular HIN investigated previously, due to the dominance of elliptic braid vortices. HIN?A produces significantly larger centerline velocity decay, as well as higher turbulence levels in the near vicinity of the nozzle exit. Vectoring of axial jet momentum is more apparent for HIN?B, where cross-stream entrainment is also relatively larger. Half-jet width results also demonstrate that both nozzle types eventually produce elliptic jets that undergo axis-switching. Lastly, momentum thickness results suggest that the present nozzle lip-modifications significantly enhance mixing characteristics along the plane upon which they are imposed in both HIN?A and B.     

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.     

Influence of nozzle geometry on the near-field structure of a highly underexpanded sonic jet

Detailed near-field structures of highly underexpanded sonic free jets have been investigated with the help of computational fluid dynamics. Two-dimensional, axisymmetric Euler equations have been chosen to predict the underexpanded jets, and the third-order total variation diminishing finite-difference scheme has been applied to solve the system of governing equations numerically. Several different nozzles have been employed to investigate the influence of the nozzle geometry on the near-field structures of highly underexpanded sonic free jets. The results obtained show that the distance from the nozzle exit to the Mach disk is an increasing function of the jet–pressure ratio, which also significantly influences the shape of the jet boundary. The diameter of the Mach disk increases with the jet–pressure ratio, and it is further significantly influenced by the nozzle geometry, unlike the distance of the Mach disk from the nozzle exit. However, such a dependence on the nozzle geometry is no longer found when an effective-diameter concept is taken into account for the flow from a sharp-edged orifice. A good correlation in the diameters of the Mach disk is obtained, so that the near-field structure of highly underexpanded sonic free jets is a unique function of the pressure ratio, regardless of the nozzle geometry.     

Mixing of high speed coaxial jets

In this study, five different supersonic nozzles – conical, elliptical, tabbed, radially lobed and two-dimensional lobed – are compared experimentally for their mixing performance. With the background of studies by various groups conducted on the above nozzles, the present paper aims to provide an experimental comparison of their respective mixing performances with that of a conventional conical nozzle under identical operating conditions. The mixing of the supersonic stream coming from such nozzles with a coaxial sonic stream is investigated. The investigation is performed non-intrusively, using digital image processing of planar Mie-scattering images of the flow field. The results of these investigations reveal the superiority of mixing performance of the two-dimensional lobed nozzle over conventional circular and other non-conventional nozzles. Received: 15 July 1999/Accepted: 3 July 2000     

Special characteristics of the propagation of three-dimensional viscous jets

Numerical investigations were made of the propagation, in a supersonic wake, of uncalculated jets, flowing out of nozzles of square and rectangular cross section, and of lumped jets, made up of from two to nine individual jets; the special characteristics of their flow were investigated in the initial, transitional, and main sections. Specifically, for lumped jets, the possibility of replacing them by a single axisymmetric jet, equivalent in mass-flow rate, is discussed. To calculate a three-dimensional unexpanded supersonic jet, flowing out into a wake, in [1] it was proposed to use a numerical method for solving a simplified system of Navier-Stokes equations for steady-state flow, and numerical investigations were made of the three-dimensional interaction of four jets in a supersonic wake, at small distances from the outlet cross section of the nozzle, i.e., mainly in the initial sections of the jets, where the mixing layers along the boundaries of the jets are still not closed. Here the method of [1] is used to study the special characteristics of three-dimensional viscous jets at large distances from the outlet cross section of the nozzle in the region of the main section, where the mixing layers have come together and a single three-dimensional jet has been formed. The system of equations, the boundary conditions, the numerical method, the system of coordinates, and the nomenclature used are the same as in [1].     

Structure of three-dimensional turbulent jets

The results are presented of an experimental investigation of the aerodynamics of three-dimensional turbulent jets in natural and artificially heightened conditions of turbulence. On the basis of measurements of mean and pulsation values, a study is made of the characteristics of the development of jets formed when an incompressible fluid issues from a rectangular nozzle and from an obliquely cut circular nozzle. The experimental results are generalized on the basis of the method of equivalent problems of the theory of thermal conductivity and of pulsation-energy balance.     

Studies on freejets from nozzles for high-speed mixing applications

In this study four different supersonic nozzles – circular, elliptical, tabbed, and radially lobed nozzles are compared experimentally for their freejet mixing performance. With the background of studies by various groups conducted on elliptical, tabbed, and radially lobed nozzles, the present paper aims at a comparative experimental study to compare their mixing performance with that of a conventional circular nozzle under identical operating conditions. The investigation is performed non-intrusively, using digital image processing of the planar Mie scattering images of the flow field. The results of these investigations reveal the superiority of the mixing performance of the lobed nozzles over conventional circular and other non-conventional nozzles. Received: 29 July 1998/Accepted: 5 November 1999     

Liftoff of a Co-Flowing Non-Premixed Turbulent Methane Flame: Effect of the Fuel Nozzle Orifice Geometry

This paper reports an experimental study on the effect of the fuel nozzle orifice geometry on the stability of turbulent non-premixed methane flame. Different internal geometries (orifice equivalent diameter, length to diameter ratio and contraction angle) of a circular and a rectangular nozzle with an aspect ratio of 2 were examined. The strength of the co-airflow was also varied to assess its impact on the jet flame stability. The experimental data revealed that the level of turbulence in the jet near-field is, in general, higher for the rectangular nozzle in comparison with the circular nozzle. This high level of turbulence was found to accelerate the liftoff transition of the attached flame. The results revealed also that there is a clear interplay between the flame liftoff height and the jet flow characteristics. That is, a rectangular jet, which spreads faster along the minor axis and generates higher near-field turbulence, results in a flame base sitting closer to the nozzle exit in comparison with that of its circular nozzle counterpart. Finally, the presence of a moderate co-airflow resulted in a higher flame liftoff velocity and height. It also led to the appearance of a hysteresis phenomenon in the low jet velocity range regardless of the exit orifice shape of the fuel nozzle.