Noncircular Jets Research Articles

Numerical Study on Mixing Characteristics of Circular and Non Circular Jets

The study of non-circular jet flows has become important due to their various applications such as aircraft exhaust, combustion chambers and injectors. The goal of present paper is to examine the impact of elliptical, square, and triangular shaped orifices on the mixing characteristics of a free jet with Mach number 0.8. Numerical simulations using an SST K-ω two-equation turbulence model were conducted with a Reynolds number of 3.46×105 for all cases. The mean velocity, decay rate, half-velocity width, spread rate, and turbulence intensity of the jet were analysed. The results showed that the triangular orifice provided the best mixing efficiency, with a shorter jet core length. The decay rate was found to be lowest for the square jet and highest for the triangular jet, which matches with the previous research. The asymmetric jets experienced two axis-switching points, while the square jet experienced a 450 rotation of its axes but no axis-switching. The core region had lower turbulence levels, while the highest turbulence levels were in the shear layer.

Numerical investigation of the velocity and spreading characteristics of non-circular turbulent jets at different Reynolds numbers

The flow field characteristics of non-circular turbulent jets are investigated numerically. A steady numerical simulation is conducted using the k–ε turbulence model in Ansys Fluent software. A hotwire anemometer is utilized to collect velocity data along the jet centerline for velocity validation. The jet exit Reynolds numbers vary from 200 to 5000, covering both laminar and turbulent regimes. The mean flow field characteristics, such as mean velocity, turbulent intensity, velocity decay, and half-jet spread width, are examined. The simulated results depict that mean velocity profile decay reveals a universal decay pattern, and the lateral velocity distribution shows a top-hat-like velocity profile in the near field. It significantly changed into peak-shaped and parabolic-shaped at a far downstream distance. The turbulent intensity profile reveals that shear layer growth begins near the jet exits, and entrainment becomes strong with the Reynold number. The potential core region increases with Reynolds numbers. The jet spread width shows a linear increment with the jet exit Reynolds number. The presented numerical results agree well with the measured experimental results and are consistent with published data.

Direct numerical simulation of turbulent jet in regular waves: A comparison between circular and non-circular cross-sections

Direct numerical simulations of circular and non-circular jets (square and rectangular) when released into regular waves are performed and reported in this article. In case of a rectangular jet, two different orientations of the nozzle have been simulated i.e major axis parallel (case 1) and perpendicular (case 2) to wave advancement direction. An analysis of the leading vortex ring reveals that axis switching in a rectangular jet takes place irrespective of nozzle orientation with respect to wave advancement direction. The axis switching is absent in the square jet. Further, the roll-up frequency of the non-circular jets is found less than the circular jet. Also, the roll-up frequency is independent of the orientation of the rectangular jet with respect to the wave direction. The braid region shows that the number of counter-rotating pairs are four for all the cases, however, the lateral jets are three for circular, square and case 1 of rectangular jets. The lateral jets in case 2 of the rectangular jet remain four due to change in orientation of the lateral jets owing to change in the nozzle orientation. The time-averaged quantities suggest increased mixing and entrainment in the rectangular jet as compared to circular jet. The highest mixing and shortest potential core are observed in case 2 of the rectangular jet. Therefore, for better mixing, the rectangular nozzle should be placed in the ocean outfall such that its major axis remains perpendicular to the direction of wave advancement.

Drop size characteristics of sprays emanating from circular and non-circular orifices in the atomization regime

Traditionally, circular orifices have been used for generating aerosols, however in recent times non-circular orifices are being considered due to their superior atomization and mixing features. In this work, we experimentally investigate spray morphology and drop size characteristics of kerosene (Jet A-1) jets issuing from three non-circular orifices geometries (elliptic, triangular and, square) and one circular orifice with the same exit cross sectional area. Our results show an unexpected and yet unreported coarsening of atomization for non-circular orifice jets quantified by an increase in the Sauter Mean Diameter (SMD) at all tested exit velocities represented by the liquid Weber number, Wel. We attribute this to two distinct spray morphologies: filament and core breakup which generate large size liquid structures identified as filaments and ligaments, noticeable in non-circular orifices jets compared to circular orifice jets. On exploring this further by undertaking a detailed examination of the drop size volume probability distribution at lower Wel corresponding to the location marking the end of primary breakup we see a bimodality due to a dominant distribution of fragments of small and larger sizes owing to the spray morphology. At higher Wel and larger distances from the injector exit we observe the bimodality converts to a unimodal distribution for all orifice jets with a single peak situated at lower drop diameters. Filament breakup is reasoned to be the cause of higher number of smaller drop sizes in triangular sprays among all non-circular orifice jets while thinner filaments in circular orifice sprays lead to smaller drop sizes compared to their triangular counterparts showing the same breakup morphology. We expect our results to help formulation of atomization strategies based on only changing the orifice geometry which heretofore have been achieved by complicated means thereby beneficially impacting applications as diverse as engine fuel combustion, pharmaceutical sprays and CO2 capture by NaOH sprays.

Studies of Jet A1 Fuel Atomization Through Non Circular Orifices


 
 
 The performance of the liquid rocket engine depends on the atomization behavior of the fluid being injected into the combustion chamber. Generally, a plain injector with a circular orifice has been used in the injector, but it has the disadvantage of having a low spray cone angle. The breakup length, mean droplet diameter, and Sauter mean diameter is also higher. Thus, to overcome these drawbacks, non-circular orifices have been utilized in the present study. The shapes used for non-circular orifices are semi-circular and plus. The results obtained with the non-circular orifice is compared with the circular orifices of the same area ratio. The working fluid used for the studies is Jet A1 fuel. Studies were also conducted with different L/D ratios by choosing an effective orifice length to reduce the upstream losses. The axis-switching phenomena were observed with the semi-circular as well as with the plus jets. The mean droplet size of the circular jets was more prominent compared to non-circular jets, and the Sauter mean diameter of non-circular jets droplets was smaller than that of the circular jet droplet. The spray cone angle has increased by 290% for plus jets and 30% for semi-circular jets compared to circular jets.
 
 

Computational investigation of highly heated homogeneous subsonic turbulent jets issuing from plane and circular nozzles

In this paper, dynamic and thermal aspects of highly heated subsonic plane and circular jets flowing into a weak co-flow stream are numerically investigated. The two studied nozzle exit shapes have identical cross-sectional area and so equal equivalent diameter. Simulations are performed using the second order Reynolds Stress Model (RSM). The concordance between code predictions and experimental results is reasonably good. The centerline evolutions of velocity and temperature field are analyzed. It is shown that the mean centerline velocity and temperature decays are more rapid, the fluctuating quantities grow faster and the momentum and thermal half-widths are higher for the plane jet than for the circular jet. Moreover, higher values of entropy generation rate and Merit number correspond to non-circular jet. These results show that the breakdown of the axisymmetry at the exit of the jet together with heating the homogeneous jet enhance the mixing and the entrainment rates between the jet and its surrounding.

Lagrangian analysis on structure evolution and mass transport of circular and noncircular turbulent synthetic jets

Lagrangian analysis on structure evolution and mass transport of circular and noncircular turbulent synthetic jets

Enhancing Jet Array Heat Transfer: Review of Geometric Features of Nozzle and Target Plates

Adequate cooling of electronic components and packages is an ever-increasing challenge as devices become more powerful, compact, and portable. Highly integrated and high-performance solutions are sought across a wide range of industrial and commercial sectors, requiring new methods and means of cooling beyond the traditional fan-fin heat sink. Impinging jets, when arranged in arrays, can achieve very high local and surface-averaged heat transfer coefficients whilst providing sufficient surface area coverage for a broad range of electronic devices. As such, they are a high-potential candidate for next generation heat exchangers for cooling high heat flux electronics. This review article focusses primarily on the means by which the heat transfer of impinging jet arrays can be enhanced by straight-forward alterations of the geometric features of the jet orifices, nozzle plate, and impingement surface, as compared with the baseline case of round jets impinging on flat surfaces. From non-circular jets, to patterned micro-roughness, the review aims to collate and synthesize the state-of-the-art, highlighting advantages and pitfalls, to aid thermal engineers in designing high performance jet impingement heat exchangers.

Shock-Structure Formation in Circular and Non-Circular Sonic Jets at Underexpanded Conditions

The generation of shock waves and their repercussions in high-speed vehicles are inevitable. Particularly, the hot plume from the aircraft exhaust ejecting at a high speed, as well as the emitted aeroacoustic noise, have several consequences. Besides, the occurrence of the supersonic core length and the emission of high screech noise are mostly due to the shock cells, prevailing in high-speed jets. Therefore, understanding the shock cell structures developed at the exit of aircraft or rocket nozzles is vital in improving mixing and thereby the noise characteristics. Essentially, non-circular nozzle shapes are well known for enhancing entrainment characteristics and mitigating the noise due to their differential spreading over the nozzle's perimeter. The current study examines the shock structures of circular, elliptic, and square jets at various sonic underexpansion levels. In this investigation, the nozzle geometries are considered to have the same exit area. The Nozzle Pressure Ratio (NPR) was adjusted to 3, 4, and 5 to achieve moderate and highly underexpansion conditions. The shadowgraph visualization method is used to study the development of shock cells from axisymmetric and asymmetric nozzles. It is interesting to observe that the incident and reflected shock structures exist only at moderate and high underexpansion levels. Besides, the elliptic and the square jets have distinctive flow patterns along their different axis planes. The intercepting shock appears on the elliptic jet in the minor axis rather than the major axis direction. The curvature of the intercepting shock wave was found to be greater for the elliptic jet than that for the circular jet. In addition, the square jet in the symmetry plane diverges from the jet centerline, but the jet in the diagonal direction converges. Moreover, the estimated shock cell lengths using shadowgraph images were compared to a theoretical model where the experimentally obtained results are in good agreement with the theoretical values.

Study on dynamic behaviors from non-circular dense granular jet impact

Dynamic behaviors of non-circular dense granular jets impacting a sufficiently large target are studied by experiment and DEM simulation. Effects of the granular jet velocity, particle diameter, and aspect ratio on the anisotropic pattern were investigated. Results show that the anisotropy of particle concentration of the granular sheet is enhanced with increasing aspect ratio. Interestingly, despite the significant differences in flux, the particle velocities are approximately consistent along the long and short axes. Unlike liquid, a dead zone is present in the impact zone, and the axial-to-radial transformation of the velocity is achieved by similar circular motion along the dead zone. Because the dead zone is adaptively adjusted with the cross section, the particle fluxes are anisotropic. Frequent collisions and frictions contribute to the homogenization of velocity in azimuths. Increasing particle diameter causes the enhancement of velocity fluctuations, the circular motion cannot be maintained and particles become scattered and isotropic. • The anisotropy in velocity and flux of particles from non-circular dense particle jet impact is characterized. • The effects of the velocity, aspect ratio, and particle diameter on anisotropy are assessed. • The mechanism of the anisotropy of the granular sheet is simulated by DEM.

Role of non-circular jets in the performance of Hartmann whistles

Non-circular jets impinging on open-closed type cylindrical cavity are investigated. Five different orifice shapes, namely, circle, ellipse, rectangle, square, and triangle, are used to produce under-expanded jets. For a given orifice shape, experiments were done at four different Nozzle Pressure Ratios (NPRs), 3 to 4.5 in steps of 0.5. The Sound Pressure Level (SPL), the Overall Sound Pressure Level (OASPL) and the pressure inside the cavity are also monitored. In addition, the corresponding high-speed schlieren images are acquired. Circular orifice jet and cylindrical cavity combination emitted the highest noise levels. These devices produced tonal sounds around 130 dB. At low NPRs, there was not much difference in the performance of the Hartmann whistle for different shaped jets. However, at high NPRs, a substantial dip in OASPL value was found at the vertex compared to the span of different shaped orifice jets (rectangle, square, and triangle).

CFD prediction of mixing performance for circular and non-circular jet mixing tanks

CFD prediction of mixing performance for circular and non-circular jet mixing tanks

Experimental investigation on flow characteristics and unsteady heat transfer of noncircular impinging synthetic jets

This study investigates the flow characteristics and unsteady heat transfer of noncircular synthetic jets impinging on a heated plate. Thermographic phosphor thermometry and time-resolved particle image velocimetry are used to acquire quantitative wall temperature and flow field information, respectively. At a constant orifice-to-wall distance (H/De = 7.5), five orifice configurations (circular, square, elliptic with aspect ratio (AR) of 3, and rectangular with ARs of 3 and 5) are examined at a wall temperature of 80 °C. The orifice configuration has a significant effect on the heat transfer and vortex behavior of the impinging synthetic jets. The square orifice displays better stagnation cooling performance with 42% increase in the maximum stagnation cooling coefficient relative to the circular case due to the larger impinging velocity and deeper penetration into the wall shear layer. Axis-switching of the vortex rings is detected for both the elliptic and rectangular orifices before they impinge onto the wall. The axis-switching enhances the near-wall mixing and turbulent kinetic energy in the elliptic and rectangular orifices, and thus heat transfer compared with the circular case. Additionally, stronger vortices and deeper penetration into the wall shear layer are observed in the major-axis plane than in the minor-axis plane, leading to enhanced heat transfer in the major-axis direction. In particular, the rectangular orifice with an AR of 5 induces strong secondary structures in the trailing jet following the primary vortex ring, and these are responsible for the double-peak distribution of wall temperature in the major-axis direction. This is the first time a double-peak temperature distribution has been reported for noncircular impinging synthetic jets. The rectangular orifice with an AR of 5 also yields a larger cross-stream velocity in the minor-axis direction, indicating a stronger wall jet. Thus, among the cases considered in this study, the rectangular orifice with AR = 5 achieves the optimal cooling performance and the maximum time-averaged cooling area reaches more than 14 times the jet orifice area.

Numerical analysis of non-excited and excited jets issuing from non-circular nozzles

This paper concentrates on the application of passive and active flow control methods to jet flows. The passive control is obtained by shaping a nozzle cross-section (hexagonal, hexagram, square, triangular) and the active one by an external excitation. The research is performed by applying the large eddy simulation method (LES). The obtained results are thoroughly validated by comparisons with theoretical, experimental and numerical data for non-excited jets. The presented results comply well with the reference data, both in terms of the mean velocity profiles, velocity fluctuations profiles and the Strouhal number of the preferred oscillation frequency (Stp). Based on its value, the excitation frequencies (Stf) are chosen such to cover the range of Stp and its subharmonics. The applied excitation is defined as a sinusoidal function, which enforces the axial velocity oscillations at the level of 10% of the bulk velocity. It is shown that the dynamics of the excited non-circular jets is significantly different from the dynamics of their non-excited counterparts. It is demonstrated that by changing Stf one may modify both the instantaneous as well as their time-averaged behaviour, i.e., the centreline velocity decay, turbulence intensity, directional spreading rate and the entrainment rate. It is found that not all jets are prone to excitations to the same extent. In the far-field, the dependence of the velocity decay rate on Stf is the most pronounced in the jets issuing from circular and hexagonal nozzles. In the near-field, regardless of Stf the excitation causes a faster velocity drop, shortening of the potential core and increase of the centreline fluctuations in all jets. However, in the hexagram jet, the fluctuations turn out to be the least sensitive to change of Stf. Apparently, in this case, the complex nozzle shape determines the velocity oscillations so strongly that a relatively low amplitude excitation has only a small influence. In this respect, the jets issuing from square and triangular nozzles show the opposite, as for them the impact of the excitation is the strongest. Depending on Stf three qualitatively different scenarios of the fluctuations growth are identified by either manifesting their smooth increase or the occurrence of local maxima and minima. The entrainment in the square and triangular jets also turned out to be more dependent on the excitation rather than by the nozzle geometry. In the near-field, the excitation with Stp/2<Stf<Stp leads to almost doubling the entrainment maximum found for the non-excited jets.

Scaling law for supersonic core length in circular and elliptic free jets

We present here the experiments demonstrating the scaling law for supersonic core length in circular and elliptic free jets. A proper scale for the supersonic core length of free jets is found as Lc*AR e−0.11γMj2, where Lc* is the core length normalized by nozzle throat diameter Dth, AR is the aspect ratio of the elliptic nozzle, γ is the specific heats ratio, and Mj is the ideally expanded jet Mach number. The scaling law developed for supersonic core length shows a close dependence with nozzle expansion ratio pe/pa and agrees well with the results reported in open literature, including the data reported for γ = 1.28–1.67. In spite of their uniqueness, the present results refer to circular and elliptic jets only. Thus, other similar studies on rectangular and square jets for a wide range of parameters are needed for a full comprehension of the universality of the scaling law for non-circular jets.

Axis-Switching Behavior of Liquid Jets Issued from Non-Circular Nozzles Under Low-Intermediate Pressure

HighlightsThe flow behavior of water jets discharged from different orifices was investigated.High-speed photography (HSP) was used to obtain surface structures and spread characteristics of water jets.The deformation process in axis switching related to the corner vortices effect of non-circular jets was researched by numerical simulation.The axis switching of non-circular jets enhances entrainment ability of the jet.ABSTRACT. Low-intermediate pressure sprinkler irrigation systems are important research topics in the field of water-saving irrigation. Non-circular nozzles improve spray uniformity at lower pressures and are key components of sprinkler irrigation systems. In this article, the behavior of discharged water jets from nozzles with circular, square, and equilateral triangular orifices designed with the same flow rate was investigated. High-speed photography (HSP) was used to capture jet characteristics in the near field (z&amp;lt;20D). The largest spread angle was obtained for the square jet, which was on average 37% larger than that of the circular jet. In addition, numerical simulations were performed to analyze the axis-switching process using the large-eddy simulation (LES) method and the coupled level-set and volume of fluid (CLSVOF) method. The axis-switching phenomenon was observed in non-circular jets, in which surrounding air mixed with the jet and promoted the formation of thin diaphragm structures. The deformation process that occurs in axis switching is described according to the simulated vorticity and velocity fields. The research results suggest the axis-switching phenomenon is induced by corner vortex motions produced by the polygonal orifice, which accelerate the decay of the axial velocity and increase the jet entrainment rate. Thus, the effect of corner vortices should be considered in the design of polygonal nozzles. Keywords: Axis switching, High-speed photography, Liquid jet, Low-intermediate pressure sprinkler irrigation, Non-circular nozzle, Numerical simulation.

Turbulent flow characteristics for enhanced thermal mixing in square and triangular jets

In this study, turbulent flows and the associated mixing characteristics are numerically investigated for noncircular jets. The selected nozzle shapes are round, square, and equilateral triangular, and a passive scalar is adopted to quantify the flow mixing. To achieve a constant mass flow rate, the same cross-sectional area is maintained, and the Reynolds stress model (RSM) is applied to show the turbulence-driven secondary flows. For Re = 21,000, the flow fields of noncircular jets are compared to those of the round jet. After 5 < x/De < 10, the half-widths of the round, square, and triangular jets increase linearly. This self-similar structure of the jet flow is rapidly developed for the noncircular jets. Compared to that for the round jet, the spreading rate for square jet increases by 20% and 27% for the triangular jet. From the mixedness values, the mixing is estimated to be enhanced by 16.7% for the square jet and 22.9% for the triangular jet. To elucidate the flow characteristics related to this feature, the scatter plots and streamlines of the secondary flows, vorticity transport equation, and anisotropy-invariant map are discussed. From the results, a balanced relationship between the entrained flow of ambient fluid and the outward spreading flow is observed, and the related turbulence structure is confirmed.

Control of sonic jets by fluidic injection

The present experimental study aims at investigating the effect of fluidic injection by means of air tabs on the core of circular and square sonic jets issuing from orifice at different levels of underexpansion. The experiments were conducted by varying the injection pressure ratio (IPR) from 3 to 5 for a fixed nozzle pressure ratio (NPR) 3, 4 or 5. Shadowgraph visualization was used to quantify core length of jet with and without fluidic injection. The injection in both circular and square sonic jets resulted in reduction of core length at all the NPRs. For each NPR, the maximum and minimum core length reductions occurred at IPRs 5 and 3, respectively, showing that the mixing promotion in circular or noncircular sonic jets is influenced by momentum of the injected jet. Among the NPRs, the reduction in core length was maximum at NPR 3 and minimum at NPR 5 for both circular and square jets at all the IPRs. The injection caused a highest reduction in the core of square jet than the circular jet all NPRs except NPR 5. A highest reduction of about 50% was achieved at NPR 3 and IPR 5 for square jet.

Passive control of coaxial jet with supersonic primary jet and sonic secondary jet

The mixing enhancement of a coaxial jet with a Mach 1.4 primary jet and sonic secondary jet, at different convective Mach numbers, is presented in this study. Rectangular tabs of aspect ratios (AR = h/w, where h and w are the tab height and width, respectively) 2.0 and 0.75 were employed to manipulate the primary and secondary jets, respectively. The primary jet (C0) and coaxial jet without control or manipulation (C1) are studied in order to decouple the effects of rectangular tabs on jet mixing. Four different tab configurations were studied, viz., tabs placed in the primary jet (C2), tabs placed in the secondary jet (C3), and tabs placed in both primary and secondary jets with the relative orientation between them of 0° (C4) and 90° (C5), to document the effect of tabs on mixing. The supersonic core length Lc* of the manipulated jet was used as a measure to quantify the mixing performance of the manipulated jet. The secondary flow reduces the growth rate of the primary shear layer and elongates the supersonic core length. This study reveals that the manipulated jet emulates the characteristic of a non-circular jet. Primary tabs are highly effective in reducing the supersonic core length of the coaxial jet than secondary tabs, and hence, the jet mixing increases. Two different flow categories of the manipulated jet have been identified. The physical reason behind the observed jet mixing and flow categories have been presented based on arguments related to changes in the flow field and shock structure.

Visualization of two-dimensional liquid sheets issued into subsonic gaseous crossflow

Flow dynamics of two-dimensional liquid sheets discharged into low-speed gaseous crossflow were experimentally investigated. The flow characteristics of liquid sheets were visualized by taking advantage of diffused backlight shadowgraphy and high-speed photography. Three injectors with an equal thickness of 0.35 mm and aspect ratios of 30, 60 and 90 were manufactured and tested at different flow conditions. A full discussion about the flow characteristics of two-dimensional liquid sheets in the presence of transverse airflow is provided. Visualizations revealed that the liquid sheets represented a unique concave-like structure that was named as inflated sheet. This special characteristic was not previously seen on any other circular or non-circular liquid jets and therefore made the flow dynamics of liquid sheets in subsonic crossflow very distinguished. The inflated sheet was found to transform from an enclosed structure into an open structure. The open inflated sheet was disturbed by different breakup mechanisms including sheet rupture, bag breakup, and Rayleigh–Taylor instability. Based on the observed phenomena, the flow was grouped into five regimes including biconvex, enclosed inflated sheet, open inflated sheet, bag breakup/sheet rupture, and multimode breakup. Furthermore, it was found that the droplet region was bifurcated due to the different breakup mechanisms acting simultaneously upon the sheet. Measurements of sheet trajectory were performed and the effects of momentum ratio and Weber number were studied. It was found that Weber number was only effective at low values, while momentum ratio remarkably impacted the trajectory.