Elliptic Jet Research Articles

The effect of nozzle geometry on the flow and heat transfer of pulsed impinging jet on the concave surface

The main aim of this paper is to investigate the effect of nozzle geometry on the flow and heat transfer from a pulsed jet into a concave surface. Experiments have been carried out for pulsed circular jet and numerical simulations were done for circular, elliptical, rectangular and square geometries. Numerical solution was performed for the frequency ranges of 25Hz to 100Hz, the jet Reynolds number of 7000, the dimensionless nozzle distance to concave surfaces of 2 and 5. Numerical results show a consistent agreement with experimental results and previous works. Accordingly, the geometry of nozzle directly affects the air entrainment ratio. In addition, with the increase of aspect ratio of the nozzle, average Nusselt number (Nuave) decreases. By pulsating the inlet jet with pulse frequency of 100Hz, the average Nu number for circular/square and elliptical/rectangular jets increases 22% and 15%, respectively. Generally, the pulsation causes a reduction and an increase of the Nuave at low and high frequencies, respectively, compared to that of the steady jet. At low frequencies, the Nuave of circular jet decreases significantly in comparison to that of the square and elliptical jets. However, at high frequencies, circular jet shows higher Nuave compared to other ones. As the distance between the jet and the concave surface increases, the effect of the nozzle shape on Nuave decreases.

Mixing Enhancement of Inverse Jet Flame with Circumferential Fuel Ports

ABSTRACT Despite more than a decade-long effort to understand the flow structure in Inverse Jet flame (IJF) configuration, the performance enhancements on such configurations are sparsely investigated. A passive technique to improve the air-fuel entrainment in IJF configuration is implemented in this study by altering the central-air port geometry with an elliptical port. The flow and mixing characteristics are studied using a three-dimensional Favre-averaged method for the elliptical central air-port IJF compared to its circular counterpart. The numerical model is validated using the standard k-ε turbulence model to capture the complex flow physics and comprehend the mixing characteristics of IJF in this burner configuration. The elliptical central port brought the differences in the spreading and entrainment rates. The present work emphasizes flow dynamics and mixing characteristics, including the features of the recirculation zone, turbulence kinetic energy, and velocity distribution. The (omega) Ω vortex identification method and turbulent kinetic energy distribution provided more profound insights into the role of vortex structures on air-fuel mixing. The vortex structures are prominent in the novel elliptical port IJF compared to the circular counterpart, thus affecting the flame structure. The vortex strength Ω was found to be 0.94 in elliptical IJF and 0.88 for the circular central port IJF. This work further quantifies the enhanced air-fuel mixing as observed with the elliptical inverse jet diffusion flame based on mixture fraction mixing index, which can be applied to any IJF configurations. The mixing index at the axis switching location was greater than 80% for the elliptical IJF compared to 45% in the circular IJF configuration.

Near Field Decay of Mach 2 Elliptical Jet in the Presence of Tangentially Injected Secondary Swirling Flow

This investigation explores the mixing promoting capability of tangentially injected secondary swirling flow on a Mach 2 elliptical jet issuing from an elliptical nozzle of aspect ratio 2. A detailed three-dimensional numerical analysis was carried out for the combination of different levels of pressure gradient at the nozzle exit and five swirl numbers. Reynolds-Averaged Navier-Stokes equation along with Shear Stress Transport model was solved to predict the flow filed. The presence of secondary swirling flow created large pressure gradient in the direction perpendicular to jet axis, resulting in enhanced mass entrainment leading to high spread of the controlled jet compared to the uncontrolled jet. Also, the secondary jet increased the turbulence level in the flow field for all the cases studied compared to that of free jet case, leading to a reduction in jet core length. The swirl number and the pressure gradient at the nozzle exit is found to have a strong effect on the core length reduction. A maximum of about 70% reduction in core length was achieved for nozzle pressure ratio 4 with secondary flow of swirl number 5.

Topological growth of multi-deck vortex structures above an elevated cross jet

The present numerical study provides a detailed topology-based understanding of the three-dimensional vortical flow interaction process for an elevated square cross jet of low-to-moderate velocity ratio (0.25 ≤R≤ 4.0). For R≤ 0.5, a steady inner-vortex (Iv) formed in the jet pipe owing to the leading-edge jet shear layer roll up as a spiral node. The kinematics of the limiting streamlines and separation-attachment patterns along the jet shear layer confirm the presence of the Iv. For 0.5 < R≤ 1.2, the Iv partly escapes, as its front side remains attached to the pipe hole and the azimuthally extended lee side moved out in a tilted fashion. Moreover, the continuous vortex shedding from the stack created a dominant von Karman-like street that grew along the lower side of the jet wake. Distinctly for R = 4, the intruded convective crossflow pulled up the spiral front node/Iv out of the pipe, which also shifted the onset of the Kelvin–Helmholtz instability upward. Significantly, the pulled-up node/Iv facilitated the above-orifice anti-kidney vortical evolution of the issuing jet shear layer. For the first time, the study displays the triple-deck growth of the kidney and anti-kidney vortices above an elevated square cross jet for 2.5 ≤ R ≤ 4.0 and 1000 ≤ Re ≤ 2000. For 0.5 ≤ R ≤ 1.2, the double-decked kidney vortices involving the primary and the secondary pairs grew above the central jet column. However, the stronger primary pair fast entrained the secondary one close to the orifice edge. With R ≥ 2.5, the lateral jet shear layer experienced an unexpected windward concave warping and restructured to evolve as the anti-kidney third-deck situated above the mid-deck primary kidney vortices. The topological shear layer folding and created kidney/anti-kidney vortices above the elevated square cross jet, for 0.25 ≤R≤ 4.0, appear consistent with the past measurements for a high aspect ratio elliptic flush jet. The anti-kidney vortical growth though was not detected above a square flush jet. The dominating above-orifice topological node in the high-pressure area ensured the anti-kidney vortical growth via the generated local pressure-gradient induced flow acceleration.

Effects of circular and non-circular nozzle exit geometries on subsonic and supersonic jet propagations

The mixing enhancement and core length reduction of a jet without significant loss of thrust are essential for reducing infrared radiation, mitigating aeroacoustic noise, improving combustion characteristics, and thrust vectoring. The jet mixing can be improved by manipulating the flow behavior. In subsonic and sonic jets, the flow manipulation may be achieved by utilizing nozzles with non-circular geometries that shed vortices of varying size due to their non-uniform azimuth curvatures. Non-uniform vortices generate differential spreading along the nozzle’s perimeter, causing axis switching and improving entrainment characteristics. Therefore, the present study examines the effects of two non-circular nozzle exit shapes (elliptic and square) on the mixing augmenting efficacy at subsonic and sonic flow conditions. The circular nozzle is tested for comparison. Both quantitative and qualitative analyses evaluate the efficacy of nozzles with non-circular exit geometries. Among the configurations investigated, the elliptic nozzle is superior in shortening the potential core length and enhancing the jet spread. A maximum reduction of 18.75% in core length with rapid jet decay was accomplished with the elliptic nozzle. The measurement of pressure profiles at different streamwise locations reveals that the spread rate is greater for elliptic and square jets than their circular counterpart. The elliptic jet exhibits the highest spread along the minor-axis direction compared to the major-axis direction. The differential jet spread rate in the elliptical jet causes an early axis-switching––direct evidence of mixing augmentation. Shadowgraph images show the asymmetric pattern of shock cell structures and differential spreading in elliptic and square jets.

Design of Fluidic Injector for Supersonic Jet Manipulation

A Mach 2.0 round jet at design and off-design condition is experimentally manipulated using two steady radial injectors to investigate the effect of fluidic injection on jet mixing. A new scaling law for , the supersonic core length of manipulated supersonic free jets, is found as , where is normalized by the nozzle throat diameter , is the specific heats ratio, is the ideally expanded jet Mach number, is the momentum ratio of the individual injector jet to the main jet, and is the expansion ratio of the main jet. The discussion pertaining to the scaling law provides an important insight into the choice of diameter ratio for efficient jet manipulation, where and are the injector and nozzle exit diameters, respectively. Such insight can yield valuable information on the practical usability of fluidic injectors for the full-scale applications. It is also documented that fluidic injectors can provide an effective flow control mechanism for jet mixing applications in comparison to the elliptic jet.

Flow fields around spanwise-inclined elliptical jets in supersonic crossflow

Motivated by favorable effects in separation control with air-jet vortex generators in aerospace applications, we performed implicit large-eddy simulations to analyze the flow physics of spanwise-inclined elliptical jets in supersonic crossflow and reveal the influence of aspect ratio. Orifices with similar hydraulic diameters and aspect ratios of 0.5, 1.0, and 2.0 were studied at Mach number M=2.5 and momentum-thickness Reynolds number Reθ=7000. The jet injection results in the formation of asymmetric flow features (bow and barrel shocks, upstream and downstream separation regions, and vortical structures), which are qualitatively similar for all cases. Compared with the more commonly-studied wall-normal injection, the axis-switching phenomenon and its effects are very mild for spanwise-inclined injection. For the low-aspect-ratio orifice, the clockwise-rotating major vortex strengthens and vice versa for the high-aspect-ratio orifice. Flow penetration increases with aspect ratio; however, the influence of the injection is limited within the boundary layer and the outer layer is undisturbed. In the far field, the enhanced momentum transfer makes the boundary layer more resistant to separation for all orifices. Combined with the amplified vorticity for elliptical cases, elliptical jet orifices become more effective for separation control purposes than circular orifices.

Effect of Nozzle Outlet Type on the Flow Field Velocity and Impact Pressure of High-Pressure Water Jet Peening

Abstract Water jet peening can effectively improve the fatigue strength of metal materials, and the outlet shape of nozzle greatly affects the effect of water jet peening. In this paper, the effects of nozzle outlet shape on water jet velocity and impact pressure are studied by numerical simulation, and the jet velocity and dynamic pressure for different standoff distances are also discussed. The results show that the water jets of square, circular, and triangular nozzles are highly concentrated, and the water jet of elliptical nozzles is the most divergent. The axial velocity attenuation of the square nozzle along the axis is slower than that of the other three nozzles. The water axial velocity of the elliptical nozzle attenuates fastest, and the length of the core segment of the water jet is the smallest. Within a certain axial distance, the dynamic pressure area in the central area of the elliptical water jet is obviously larger than that of the other three nozzles, and the effective treatment range is large, which is more suitable for the welding surface strengthening operation.

Far-field pressure measurements of elliptic jets discharged close to a wing

This paper presents a preliminary experimental investigation into the acoustics of two elliptical jet nozzles installed close to a wing model. Acoustic pressure data is obtained for a range of observer polar angles mounted in the far field of the jets. Three nominal jet Mach numbers, namely 0.4, 0.6 and 0.8 are studied. Results suggest that the elliptic jets surveyed provide a noise reduction of the jet-surface installation noise source. The noise reduction is maximum in the forward arc and in the order of 1 dB for the fully-corrected overall sound pressure level data. Additionally, the noise benefit exists only when the minor axis of the elliptical nozzle is mounted parallel to the wing trailing edge. It is hypothesised that the reduction in the jet plume cross-section width limits the scattering of the near pressure field by the wing trailing edge to a lower frequency range. The jet mixing noise source, however, is seen to increase with decreasing nozzle exit-plane aspect ratio. The three jet velocities surveyed suggest the consistency of the key results discussed in the paper. Investigation of the jet turbulent flow structures and jet near pressure field is under way.

A new calculation formula to describe the dynamic pressure of water jet peening with elliptical nozzle for high-efficiency treatment

Water jet peening is a good potential method to control welding residual stresses. The water jet with elliptical nozzle can improve the treatment efficiency due to its large treatment area. In this article, the water jet velocity and dynamic pressure for different elliptical nozzle dimensions and standoff distances are discussed by numerical simulation. The results show that when the axial distance is 10 mm, the effective impact diameter of the elliptical nozzle a/b=8–12 is about 2 times or more than that of the circular nozzle. The length of the jet core of the elliptical nozzle is only related to the outlet structure and is independent of the inlet pressure. The correlation between the dimensionless core length of the elliptical water jet and its long and short axes is derived. When the ratio of the major axis to the minor axis is between 7 and 13, the core length of the elliptical water jet is 7–7.5 times that of its minor axis. Combining the suitable treatment area and dynamic pressure, the elliptical nozzle with an axis ratio of 8 is recommended to control the welding residual stress. Finally, a new formula for calculating dynamic pressure distribution is proposed for the elliptical nozzle water jet at different stages.

Effect of nozzle geometry on the dynamics and mixing of self-similar turbulent jets.

The effect of nozzle geometry on the dynamics and mixing of turbulent jets is experimentally investigated. The jets with a Reynolds number of 13,000 were issued from four different pipes with circular, elliptical, square and triangular cross sections. The velocity field was measured in the self-similar region of the jets using an acoustic Doppler velocimeter. Statistical parameters, such as the mean velocities, velocity variances, spreading rates, mass flow rates, and entrainment rates are presented. The results show that despite having approximately similar decay rates for the mean centerline velocities, the radial profiles of the axial mean velocity varied in jets with different nozzle cross sections and were widest for elliptical jets and narrowest for the triangular ones. On the other hand, velocity variances were greatest for the triangular jet when compared to the jets released from cross sections of other geometries. Furthermore, the spreading rate, mass flow rate, and entrainment rate were highest for the elliptical jet, and lowest for the triangular jet. From this it can be inferred that the elliptical jet has the highest mixing and dilution. The results of this study could help to improve the initial mixing of pollutants by optimizing the initial conditions.

Heat transfer characteristics of flat and concave surfaces by circular and elliptical jet impingement

ABSTRACT The present paper reports the thermal behavior of flat and curved surfaces with impinging jets employing circular and elliptical nozzle for identical equivalent diameter (de). Tests are performed with the Reynolds number varying from 11,250–22,500, varied range of plate to nozzle distance (z/d = 1–6), various nozzle aspect ratios (AR = 1–4). A comparison of thermal behavior between flat and concave surfaces (d/D = 0.05) is discussed in this study. The maximum enhancement in the stagnation Nusselt number found to be 27.4% and 24.8% with the increase in AR = 1 to 4 for flat surface, curved surface, respectively.

Scaling law for shock-cell length and its correlation with shock-associated noise of circular and elliptic supersonic free jets

A supersonic core length Lc and an average shock-cell length Ls¯ are experimentally investigated for the acoustic properties of supersonic jets. Following Kumar and Rathakrishnan [“Scaling law for supersonic core length in circular and elliptic free jets,” Phys. Fluids 33(5), 051707 (2021)], a proper scale for the average shock-cell length Ls¯ of circular and elliptic supersonic free jets is found as Ls*¯AR0.2e−0.15γMj2, where Ls*¯ is the average shock-cell length normalized by nozzle throat diameter Dth, AR is the nozzle aspect ratio, γ is the specific heat ratio, and Mj is the ideally expanded jet Mach number. The scaling law developed for Ls*¯ exhibits a close similarity with the scaling law for supersonic core length Lc*AR0.5e−0.11γMj2, where Lc* is the nozzle throat diameter normalized supersonic core length. The scaling laws Ls*¯AR0.2e−0.15γMj2 and Lc*AR0.5e−0.11γMj2 are valid across a wide range of parameters, such as γ from 1.28 to 1.6, the nozzle design Mach number Md from 1 to 2, AR from 1 to 6, and Mj corresponds to the absence of Mach reflection. Discussion conducted on the scaling laws provides important insight into the choice of AR. Furthermore, the relationship between Lc* and Ls*¯ with the intensity of shock-associated noise is also discussed; hence, new correlation functions are developed for Lc* and Ls*¯ that provide an analogous measure of the shock-associated noise.

An Artificial Neural Network for the Prediction of Penetration Height of Aerated Elliptical Liquid Jets in Gaseous Crossflows

An Artificial Neural Network for the Prediction of Penetration Height of Aerated Elliptical Liquid Jets in Gaseous Crossflows

Dynamic mode decomposition of elliptical liquid jets in crossflow

The dynamics of round and elliptical liquid jets in subsonic crossflow was studied using high-speed imaging technique. The experiments were performed at constant gaseous Weber number and liquid–gas momentum flux ratios of 6.45 and 17.87, respectively, with orifices of different aspect ratios (AR) having an equivalent diameter of 0.43 mm. All of the cases were carried out inside an open-loop subsonic wind tunnel with a test section of 100 mm × 100 mm × 750 mm. For each case, dynamic modes were generated directly from the snapshots using a variant of the Arnoldi method known as the dynamic mode decomposition (DMD). The DMD results indicate that elliptical liquid jets have more small-scaled patterns with higher frequencies compared with the round liquid jets. As the first attempt to investigate the dynamics of elliptical liquid jets in crossflow, this study captures the dominant spatiotemporal structures. We also found that the orifice AR can significantly alter the jet wavelengths. The data from this study contribute to understanding the behavior of elliptical liquid jets exposed to gas crossflow in an enhanced capillary breakup regime.

Experimental investigation of the effect of cross wire on the flow field of elliptic jet

The effectiveness of cross wire in controlling the mixing characteristics of a circular and an equivalent elliptic jet is investigated experimentally. While circular jets are conventional, elliptic jets have gained attention due to their better mixing characteristics and faster decay. To further explore and augment the capabilities of elliptic jets for practical utility, it is investigated whether using an elliptic jet with cross wire control gives additional benefit in terms of mixing enhancement over an axisymmetric jet. Experiments are performed for subsonic and choked flow conditions with nozzle pressure ratios ranging from 1.2 to 7.0. Time-averaged pitot pressures and schlieren visualization is used for diagnosis. The jet bifurcation can be seen in controlled elliptical jets at all nozzle pressure ratios (NPRs). Core length is reduced to as much as 70% in the elliptical jet and 84% in the case of the circular jet. The core length values estimated from the present data are compared with the previous investigations.

The influence of elliptical and circular orifices on the transverse jet characteristics at supersonic crossflow

The using of an elliptical nozzle has been considered as a potential method for improving the air-fuel mixing and atomization quality. The present paper focuses on the transverse jet breakup behaviors and droplet size distribution for circular and elliptical orifices in supersonic crossflow on a pulse wind tunnel test platform, and the laser based pulsed laser background imaging method and high-speed spray particle size analyzer were adopted. The results show that the jet penetration depth of elliptical 4 (Aspect ratio of 4) orifice is always smaller than that of the circular orifice. Another interesting finding is that the difference of the jet penetration depth between the circular and elliptical 4 orifices decreases with the increase of injection pressure. Moreover, the jet surface wave number of the elliptical orifice is more than that of the circular jet, and the elliptical jet surface wave length is smaller than that of circular one. Additionally, the spray Sauter Mean Diameter (SMD) of elliptical 4 nozzle is 14.5% smaller than the circular spray at injection pressure of 5 MPa, and the elliptical 4 spray also has the smallest Dv50 and Dv90 among other orifices. Because elliptical 4 jet held larger windward surface area, which is helpful to increase the shear action of supersonic crossflow. Furthermore, the elliptical 4 spray produces the smallest droplet diameter at the largest volume fraction, and also the elliptical 4 spray has the smallest span of droplet diameter distribution than other orifices.

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.

Effect of tab parameters on the near-field mixing characteristics of a Mach 1.5 elliptic jet

The effect of delta-tabs with its apex leaning downstream on the jet mixing characteristics of a Mach 1.5 elliptic jet of aspect ratio (AR) 3.37 is reported in this paper. The delta-tabs were placed along the minor and major axes of the elliptic nozzle, referred to as tab configuration C1 and C2, respectively. For each tab configuration, four different orientation angles varying from θ= 90∘ to 150∘ were studied, where θ= 90∘ is the tab positioned normal to the nozzle exit and θ= 150∘ is the tab oriented in the downstream direction at the nozzle exit. The tab effects on jet manipulation were investigated via shadowgraph visualization, and the mean Pitot pressure was measured along the jet centerline and along the orthogonal planes of the jet. The non-dimensonalized supersonic core length Lc*, average shock-cell spacing Ls¯*, and the axis-switching location xas* were examined in detail to understand the jet characteristics of the natural and manipulated jets. The mixing capability of the manipulated jets quantified based on the reduction in supersonic core length ΔLc* depends strongly on the ideally expanded jet Mach number Mj, tab configuration (C1 and C2), and θ. As such, the ΔLc* for the manipulated jet at θ= 135∘ is the largest, revealing the optimal tab orientation for jet mixing enhancement, as also observed for the circular jet [Zaman et al., “Control of an axisymmetric jet using vortex generators,” Phys. Fluids 6(2), 778–793 (1994)]. The relationship between ΔLc* and ΔLs* is presented to understand the correlation between the jet mixing and noise characteristics of the manipulated jet. Discussion is conducted based on the dependence of ΔLc* and ΔLs* with θ, which provides useful insight into the jet flow physics. With suitable placement of tabs C1 and C2, the axis switching could be either stopped or augmented. The superior mixing characteristics of an elliptic jet relative to a circular jet, which were found in previous works on the supersonic over-expanded jet [Kumar and Rathakrishnan, “Characteristics of a supersonic elliptic jet,” Aeronaut. J. 120(1225), 495 (2016)] prevail in the supersonic jet under-expanded jet as well.

Stability of Capillary Waves of an Arbitrary Symmetry on a Jet in a Uniform Electrostatic Field

The dispersion relation for capillary waves of an arbitrary azimuthal symmetry on an elliptic cross-section jet in a uniform electrostatic field which is perpendicular to the axis of symmetry of the jet is derived. The stability of the first three azimuthal modes is investigated. It is shown that the stability of capillary waves on the jet reduces with increase in the external electrostatic field strength and the related eccentricity of transverse jet cross-section, as well as with increase in the polarization charge on the jet surface. Comparison with a jet in a radial electrostatic field is carried out. In the case of the transverse electrostatic field the instability growth rates of the capillary waves of any symmetry on the jet surface are turned out to be significantly higher than those in the case of the radial electrostatic field.