Jet Exit Plane Research Articles

A new burner for studying auto-ignition in turbulent dilute sprays

This paper introduces a new burner to study the auto-ignition characteristics of dilute turbulent spray flames. The spray is formed upstream of the exit plane and carried with air or nitrogen into a hot co-flowing stream of vitiated combustion products. The fluid dynamics is kept intentionally simple and the boundary conditions are well-defined such that the burner lends itself easily to computations. The stability characteristics are presented here for a range of liquid fuels as a function of the controlling parameters. For each fuel, three types of flames are identified based on their visual appearance. These flames are all lifted but differ in the shape of the leading edge and heat release zones. These characteristics are similar for all fuels and three flames, one from each type, are selected for further study. Measurements of mean temperature, velocity and droplet fields as well as high-speed LIF images of OH, Mie scattering from droplets and chemiluminescence of CH ∗ are made in the selected flames of ethanol. It is found that the leading edge of the flame, which is a hardly visible light-blue zone close to the jet exit plane, is marked by the presence of OH (but not CH ∗), and slight increases in peak temperature above the levels of the co-flow. Further downstream, there exists an intense blue region that is characterized by higher OH levels, significant emission of CH ∗, and temperatures that are much higher than those of the co-flow. This is the region where the bulk of the heat release is taking place. Pockets of CH ∗ are seen to appear in this region and grow as they are advected further downstream.

Buoyancy Dominated He–O2 Separated Jet Mixing in a Tubular Reactor

Mixing of two variable-density jets, one of which is essentially a negatively buoyant jet (He) and the other a nonbuoyant jet (O2) is studied experimentally using digital particle image velocimetry technique for separated nozzle geometry. Two jets are separated from each other, i.e., He jet exit plane is located upstream of the O2 jet exit plane. Experiments were carried out keeping the nonbuoyant O2 jet at constant Reynolds number (Re2=245) and varying the Richardson number (Ri1=1.8,4.7,16.4) of the buoyant He jet. The interaction between two jets as a function of buoyancy strength of helium jet is investigated. The flow visualization images clearly demonstrate the growth and shape of buoyant jet as a function of Richardson number. Mean velocity and vorticity field results provide quantitative picture about the mixing and interaction between the two jets. The stream trace results show the flow structures, i.e., recirculation zone and foci of vortex structures as a function of Richardson number. The mixing between two jets takes place at far downstream region for low Richardson number (Ri1=1.8). At high Richardson number (Ri1=16.4), the buoyant He jet is located near its exit plane without any direct interaction with the O2 jet. At the intermediate Richardson number (Ri1=4.7), the buoyant jet encroaches the nonbuoyant O2 jet at a favorable penetration distance and a good amount of mixing between the two jets take place. The jet growth results based on the y0.5-location development in the streamwise direction clearly demonstrate the interaction between the He and O2 jets in the interjet region. The entrainment coefficient, vorticity magnitude, and turbulent kinetic energy magnitude are maximum at the intermediate Richardson number (Ri1=4.7) demonstrating direct relationship among each other. The mixing between buoyant He jet and nonbuoyant O2 jet is a function of both shear between the two jets and the strength of buoyant plume.

Vortex Formation Process of a Starting Square Jet

This paper presents results of an experimental study on starting square jets at Reynolds numbers ranged from 2000 to 5000 (based on the jet diameter and the bulk mean velocity on the jet exit plane) using planar laser induced fluorescence for flow visualization and particle image velocimetry for velocity measurements, respectively. Similar to starting a circular jet at Reynolds numbers below 3000, vortex leapfrogging was found to appear, whereas pinch-off for the leading vortex ring was observed at higher Reynolds numbers. The imperfect leapfrogging between the third and the fourth vortex rings promoted growth of instability, which had led to the transition to turbulence for the entire jet. The maximum circulation that a vortex ring can attain during its formation is reached at the nondimensional time (t* = tW 0 /D e ) within the range of 1.6 to 2.4. The pinch-off of the leading vortex ring appeared shortly after the jet initiation for Reynolds numbers above 3000. Axis switching was found to play an important role for the vortex development in starting square jets. The interaction between the axis-switched vortex ring and the square-shaped jet stem facilitated the generation of contrarotating streamwise vortices at the corners of the jet stem. They were actually rotating in the opposite sense as those responsible for axis switching, and their existence had facilitated the thinning of the jet stem, causing a fast pinch-off for the leading vortex ring.

Imaging of diluted turbulent ethylene flames stabilized on a Jet in Hot Coflow (JHC) burner

The spatial distributions of the hydroxyl radical (OH), formaldehyde (H 2CO), and temperature imaged by laser diagnostic techniques are presented using a Jet in Hot Coflow (JHC) burner. The measurements are of turbulent nonpremixed ethylene jet flames, either undiluted or diluted with hydrogen (H 2), air or nitrogen (N 2). The fuel jet issues into a hot and highly diluted coflow at two O 2 levels and a fixed temperature of 1100 K. These conditions emulate those of moderate or intense low oxygen dilution (MILD) combustion. Ethylene is an important species in the oxidation of higher-order hydrocarbon fuels and in the formation of soot. Under the influence of the hot and diluted coflow, soot is seen to be suppressed. At downstream locations, surrounding air is entrained which results in increases in reaction rates and a spatial mismatch between the OH and H 2CO surfaces. In a very low O 2 coflow, a faint outline of the reaction zone is seen to extend to the jet exit plane, whereas at a higher coflow O 2 level, the flames visually appear lifted. In the flames that appear lifted, a continuous OH surface is identified that extends to the jet exit. At the “lift-off” height a transition from weak to strong OH is observed, analogous to a lifted flame. H 2CO is also seen upstream of the transition point, providing further evidence of the occurrence of preignition reactions in the apparent lifted region of these flames. The unique characteristics of these particular cases has led to the term transitional flame.

Numerical study of a transitional synthetic jet in quiescent external flow

The flow associated with a synthetic jet transitioning to turbulence in an otherwise quiescent external flow is examined using time-accurate three-dimensional numerical simulations. The incompressible Navier–Stokes solver uses a second-order accurate scheme for spatial discretization and a second-order semi-implicit fractional step method for time integration. The simulations are designed to model the experiments of C. S. Yaoet al. (Proc. NASA LaRC Workshop, 2004) which have examined, in detail, the external evolution of a transitional synthetic jet in quiescent flow. Although the jet Reynolds and Stokes numbers in the simulations match with the experiment, a number of simplifications have been made in the synthetic jet actuator model adopted in the current simulations. These include a simpler representation of the cavity and slot geometry and diaphragm placement. Despite this, a reasonably good match with the experiments is obtained in the core of the jet and this indicates that for these jets, matching of these key non-dimensional parameters is sufficient to capture the critical features of the external jet flow. The computed results are analysed further to gain insight into the dynamics of the external as well as internal flow. The results indicate that near the jet exit plane, the flow field is dominated by the formation of counter-rotating spanwise vortex pairs that break down owing to the rapid growth of spanwise instabilities and transition to turbulence a short distance from the slot. Detailed analyses of the unsteady characteristics of the flow inside the jet cavity and slot provide insights that to date have not been available from experiments.

Effects of Various Modeling Schemes on Mist Film Cooling Simulation

Numerical simulation is performed in this study to explore film-cooling enhancement by injecting mist into the cooling air with a focus on investigating the effect of various modeling schemes on simulation results. The effect of turbulence models, dispersed-phase modeling, inclusion of different forces (Saffman, thermophoresis, and Brownian), trajectory tracking, and mist injection scheme is studied. The effect of flow inlet boundary conditions (with/without air supply plenum), inlet turbulence intensity, and the near-wall grid density on simulation results is also included. Simulation of a two-dimensional (2D) slot film cooling with a fixed blowing angle and blowing ratio shows a 2% mist (by mass) injected into the cooling air can increase the cooling effectiveness about 45%. The renormalization group (RNG) k-ε model, Reynolds stress model, and the standard k-ε turbulence model with an enhanced wall treatment produce consistent and reasonable results while the turbulence dispersion has a significant effect on mist film cooling through the stochastic trajectory calculation. The thermophoretic force slightly increases the cooling effectiveness, but the effect of Brownian force and Saffman lift is imperceptible. The cooling performance deteriorates when the plenum is included in the calculation due to the altered velocity profile and turbulence intensity at the jet exit plane. The results of this paper can provide guidance for corresponding experiments and serve as the qualification reference for future more complicated studies with 3D cooling holes, different blowing ratios, various density ratios, and rotational effect.

Interaction of a free jet with a perpendicular control jet

The effects of the upstream location of a single continuous control jet (i.e., within the main jet nozzle) on an axisymmetric free jet, at Reynolds numbers of 6600 and 26 500, were investigated experimentally using PIV. The control jet was oriented perpendicular to the main jet flow and its momentum coefficient was varied for each Reynolds number. The resultant mean flow field and the turbulent stresses were examined in two planes, along and across the control jet's line of action. When the control jet is located near the main jet exit plane, the main jet is vectored away from the control jet due to the direct impact of the control jet. Conversely, with the control jet located farther upstream (i.e., into the nozzle), the main jet is drawn back toward the control jet, which may be as a result of a virtual modification of the nozzle shape. Furthermore, the direction of the main jet can also be controlled by varying the momentum coefficient of the control jet at a fixed upstream location. Finally, the effectiveness of the control jet in altering the main jet flow was found to be more pronounced when the interaction occurs at a lower Reynolds number, which is more susceptive to disturbances.

Three-dimensional interactions of a free jet with a perpendicular synthetic jet

Three-dimensional interaction of an axisymmetric free jet with a single synthetic jet that is oriented perpendicular to the free jet was investigated experimentally using PIV. The main objective was to investigate the effect of the upstream location of the synthetic jet, inside the nozzle of the main jet ( Re U e = 6600). The interaction location of the synthetic jet (for a range of momentum coefficients) was varied from 0.4 to 1.4 orifice diameters upstream of the main jet exit plane. The synthetic jet was driven at a frequency of 1000 Hz, which corresponds to a Strouhal number of 0.16 (within the range of unstable modes of the free jet). To better explore the complex flow field resulting from the interaction, a reduction technique was used where 3D flow renderings were calculated from multiple 2D measurement planes. The interaction is affected by two mechanisms: (1) direct impact of the synthetic jet into the main jet, and (2) amplification of unstable modes of the main jet. When the momentum coefficient is increased, the synthetic jet penetrates deeper into the main jet flow creating larger streamwise vortical structures and increasing the main jet width. Increasing the distance between the synthetic jet exit and the main jet nozzle exit the effectiveness of the synthetic jet's impulse decreases, resulting in smaller jet vectoring, weaker streamwise vortical structures, and decreased turbulence levels. When compared to a continuous control jet of the same momentum coefficient and upstream location, the effects are significantly different.

On the universality of global modes in low-density axisymmetric jets

Experiments conducted in axisymmetric low-density jets reveal that the transition to global instability and the frequency selection of the global mode depend on the operating parameters of density ratio, momentum thickness, and Reynolds number. The onset of the global mode was mapped in the Reynolds number-momentum thickness operating domain for density ratios S=ρ j /ρ ∞ ranging from 0.14 to 0.5. The results provide convincing evidence of the universality of global oscillations in low-density jets and indicate that conditions in the jet exit plane are responsible for driving the global instability.

Numerical Study of the Evolution of Strongly Forced Axisymmetric Laminar Cold-Flow Jets

Periodic jet forcing presents interesting opportunities for jet mixing for a variety of applications. In this work, simulations are performed to study the effect of high-amplitude forcing on laminar jets (Re = 100) with net mass flux. For the cases presented in this work, we look at the effects of simulating the internal nozzle and show that assumptions about the nozzle flow have a large effect on the downstream flow evolution. Studies are performed on strongly forced axisymmetric jets in two geometries: 1) jets issuing perpendicularly from a flat wall and 2) jets issuing from a straight tube (nozzle). The amplitude and frequency of the forcing function are varied to study vortex creation and subsequent evolution downstream of the jet. For example, cases in which the peak jet velocity is three to four times the mean jet velocity are examined. The near-nozzle region was of particular interest due to the strong mixing processes occurring there. We discuss how nozzle flow processes modify the creation of downstream large-scale vortical structures. An interesting result of this work is that the strongest forcing cases possess some striking similarities to synthetic jets. For such cases, the flow reversal processes at the jet exit plane are investigated.

Measurements of Surface-Roughness Effects on the Development of a Vortex Produced by an Inclined Jet in Cross-Flow

This study examines the effects of surface roughness on the streamwise development of a vortex created by an isolated circular jet injected at 45 deg pitch and 90 deg skew into a crossflow. The study is motivated by the typical surface conditions encountered on in-service turbine blades of gas-turbine engines. Detailed measurements of the velocity field have been performed with a miniature four-wire probe at the jet exit plane, in the oncoming cross-stream boundary layer, and in a series of planes that capture the streamwise development of the vortex in the crossflow boundary layer up to about 15 jet-discharge diameters downstream of the jet. The paper presents the effects of surface roughness on the structure of the dominant streamwise vortex created by the interaction of the inclined jet with the mainstream, and documents the changes in the location, streamwise rate of change of circulation, and streamwise rate of diffusion of this vortex. Through these results, the change in the effectiveness of the vortex in energizing the boundary layer in the presence of surface roughness can be quantified.

Pdf calculations of turbulent lifted flames of H2/N2 fuel issuing into a vitiated co-flow

This paper presents detailed calculations of the flow, mixing and composition fields of a simple jet of hydrogen–nitrogen mixture issuing into a vitiated co-flowing stream. The co-flow contains oxygen as well as combustion products and is sufficiently hot to provide an ignition source for a flame that stabilizes at some ten diameters downstream of the jet exit plane. This configuration forms a good model problem for studying lifted flames as well as issues of auto-ignition. The calculations employ a composition probability density function (PDF) approach coupled to the commercial CFD package FLUENT. The in situ adaptive tabulation method is adopted to account for detailed chemical kinetics. A simple k–ε model is used for turbulence along with a low Reynolds number model for the walls. Calculations are optimized to obtain a numerically accurate solution and are repeated for two different H2 mechanisms, each consisting of ten species. The flame is found to be largely controlled by chemical rather than mixing processes. The mechanisms used yield different lift-off heights and compositions that straddle the data. Ignition delays are found to be extremely sensitive to the chemical kinetic rates of some reactions in the mechanisms.

Microjet cooling devices for thermal management of electronics

This research is an effort to demonstrate the applicability of miniaturized synthetic jet (microjet) technology to the area of thermal management of microelectronic devices. Synthetic jets are jets which are formed from entrainment and expulsion of the fluid in which they are embedded. Design issues of microjet cooling devices are discussed along with characterization of excitation elements and the turbulent synthetic jets produced thereby. Geometrical parameters of the microjet cooling devices were empirically optimized with regards to cooling performance. The cooling performance of the optimized microjets was compared with previous theoretical and empirical studies of conventional jet impingement. The cooling performance of the microjet devices has been investigated in an open environment as well as in a vented and closed case environment. In such experiments, the synthetic jet impinges normal to the surface of a packaged thermal test die, comprising a heater and a diode-based temperature sensor. This test assembly allows simultaneous heat generation and temperature sensing of the package, thereby enabling assessment of the performance of the synthetic jet. Using microjet cooling devices, a thermal resistance of 30.1/spl deg/C/W has been achieved (when unforced cooling is used, thermal resistance is 59.6/spl deg/C/W) when the test chip is located at 15mm spacing from the jet exit plane. In order to more directly compare and scale the cooling results, a preliminary study on heat transfer correlations of the microjet cooling device has been performed. Finally, a comparison of the performance of the microjet cooler with standard cooling fans is given.

Experimental and computational visualization and frequency measurements of the jet oscillation inside a fluidic oscillator

PIV measurements and computational simulations (2D, unsteady Navier-Stokes) are performed to visualize the inherently unsteady jet oscillation inside a fluidic oscillator. Both the measurements and computations are obtained for a jet exit Reynolds number of 321, based on the maximum velocity and the nozzle width at the jet exit plane. The computed jet oscillation frequency is in close agreement with the measured PIV frequency. Formation of a pressure gradient across the jet is observed from the computations. The variation of the jet oscillation frequency with jet exit Reynolds number is also determined by single sensor hot-wire measurements inside the oscillation chamber.

Turbulent flow field structure of initially asymmetric jets

The near field structure of round turbulent jets with initially asymmetric velocity distributions is investigated experimentally. Experiments are carried out using a constant temperature hot-wire anemometry system to measure streamwise velocity in the jets. The measurements are undertaken across the jet at various streamwise stations in a range starting from the jet exit plane and up to a downstream location of twelve diameters. The experimental results include the distributions of mean and instantaneous velocities, vorticity field, turbulence intensity, and the Reynolds shear stresses. The asymmetry of the jet exit plane was obtained by using circular cross-section pipes with a bend upstream of the exit. Three pipes used here include a straight pipe, and 90 and 160 degree-bend pipes. Therefore, at the upstream of the pipe exit, secondary flow through the bend and mean streamwise velocity distribution could be controlled by changing the curvature of pipes. The jets into the atmosphere have two levels of initial velocity skewness in addition to an axisymmetric jet from a straight pipe. In case of the curved pipe, a six diameter-long straight pipe section follows the bend upstream of the exit. The Reynolds number based on the exit bulk velocity is 13,400. The results indicate that the near field structure is considerably modified by the skewness of an initial mean velocity distribution. As the skewness increases, the decay rate of mean velocity at the centerline also increases.

Inverted-profile coaxial jet flows relevant to Astovl applications

AbstractAn experimental investigation has been performed into the free jet and wall jet characteristics of high pressure ratio coaxial jets operating with inverted velocity profiles and impinging onto a ground plane. Pitot-static probes have been used to record free jet properties at various distances from the jet exit plane, and wall jet properties of the impinging jets at various radial positions. Jet properties such as velocity decay and spreading rates were calculated and compared to published literature. Wall jet development was investigated and recorded. Inner and outer nozzle pressure ratio (NPR) were altered to give a range of jet exit conditions. The results obtained showed variations in the wall jet properties as jet exit conditions were altered, but wall jet self-similarity was found and varying either nozzle height above the ground plane or NPR did not alter this. Free jet data showed large variations in behaviour with changes in NPR. The inverted profile jets behaved similarly to annular jets, showing similar characteristics. Inner jet pressurisation was observed for inverted profile jets and also significant acceleration of the flow from the inner, lower NPR, jet when this was subcritical.

The formation and evolution of synthetic jets

A nominally plane turbulent jet is synthesized by the interactions of a train of counter-rotating vortex pairs that are formed at the edge of an orifice by the time-periodic motion of a flexible diaphragm in a sealed cavity. Even though the jet is formed without net mass injection, the hydrodynamic impulse of the ejected fluid and thus the momentum of the ensuing jet are nonzero. Successive vortex pairs are not subjected to pairing or other subharmonic interactions. Each vortex of the pair develops a spanwise instability and ultimately undergoes transition to turbulence, slows down, loses its coherence and becomes indistinguishable from the mean jet flow. The trajectories of vortex pairs at a given formation frequency scale with the length of the ejected fluid slug regardless of the magnitude of the formation impulse and, near the jet exit plane, their celerity decreases monotonically with streamwise distance while the local mean velocity of the ensuing jet increases. In the far field, the synthetic jet is similar to conventional 2D jets in that cross-stream distributions of the time-averaged velocity and the corresponding rms fluctuations appear to collapse when plotted in the usual similarity coordinates. However, compared to conventional 2D jets, the streamwise decrease of the mean centerline velocity of the synthetic jet is somewhat higher (∼x−0.58), and the streamwise increase of its width and volume flow rate is lower (∼x0.88 and ∼x0.33, respectively). This departure from conventional self-similarity is consistent with the streamwise decrease in the jet’s momentum flux as a result of an adverse streamwise pressure gradient near its orifice.

Ultrasound scattering by a swirling jet

Recent analytical work has shown that when an acoustic plane wave propagates through a rotational flow field there is a linear relationship between the Fourier component of the scattered acoustic pressure and the Fourier transform in space and time of the vorticity component that is normal to the plane defined by the wave vectors of the incident and scattered acoustic waves. Hence, ultrasound scattering can be used as a non-intrusive spectral probe of vorticity and potentially as a tool for direct measurements of vorticity distributions. Some aspects of this technique have been tested in a swirling air jet emanating from a 2.54 cm diameter nozzle where the swirl is generated upstream of the jet nozzle by a rotating paddle. For a given exit volume flow rate, swirl numbers up to 0.4 are realized. Radial distributions of the streamwise and tangential velocity components downstream of the jet exit plane are measured using two-component hot-wire anemometry and the corresponding distributions of streamwise vorticity are computed. A nominally plane ultrasonic wave field is generated normal to the jet axis by a transmitter having a 16 cm square aperture. The scattered ultrasound in the radial direction is measured at a number of streamwise and azimuthal stations. In accord with the theory, the normalized amplitude of the scattered acoustic wave is a linear function of the magnitude of the centerline vorticity at the exit plane of the jet, and is independent of the intensity of the incident wave field. Fourier components of the vorticity distribution are directly measured by varying the scattering angle and are in good agreement with theoretical predictions.

Manipulation of a jet in a cross flow

Abstract The effect of forcing on the mixing and velocity field of a jet in a cross flow is investigated for a jet-to-cross flow velocity ratio of R=1.0 using reactive-Mie-scattering flow-visualization and two-color Particle Image Velocimetry (PIV) techniques. Reactive-Mie-scattering images are used to assess the effect of forcing on molecular mixing, whereas double-exposed two-color PIV images are employed to obtain instantaneous velocity and corresponding vorticity distributions. The jet emanates from a square conduit and is manipulated by four piezoelectric actuators – one mounted along each side of the jet conduit near the jet-exit plane. Time-and phase-averaged distributions of velocity and rms velocity fluctuations in the x–y plane z=0 are also obtained. A novel approach is employed for processing jet and cross-flow fluids based on the size of the seeding particle. The present results show that when the jet is unforced, the “wake” of the jet contains a domain in which the magnitude of the velocity is very low compared to that of the cross-flow velocity, suggesting that the fluid within this domain is almost stagnant. When the jet is excited, the low-velocity domain in its wake is substantially reduced through vigorous mixing. Manipulation of the upstream and downstream segments of the jet shear layer also leads to an increase in the jet penetration into the cross flow (up to 30% as compared to the unforced case) and substantial mixing enhancement. Unlike in streamwise forcing where the jet fluid remains mainly in the central portion of the spanwise surface, spanwise forcing induces strong spanwise jet undulations.

Heat Transfer in the Forced Laminar Wall Jet

The mean and fluctuating characteristics of a plane, unsteady, laminar, wall jet were investigated experimentally for a constant wall-temperature boundary condition. Temperature and streamwise velocity profiles, including the downstream development of the thermal and hydrodynamic boundary layer thicknesses, were obtained through simultaneous hot and cold wire measurements in air. Even at relatively low temperature differences, heating or cooling of a floor surface sufficiently altered the mean velocity profile in the inner, near-wall region to produce significant effects on the jet stability. Selective forcing of the flow at the most amplified frequencies produced profound effects on the temperature and velocity fields and hence the time-averaged heat transfer and shear stress. Large amplitude excitation of the flow (up to 2 percent of the velocity measured at the jet exit plane) at a high frequency resulted in a reduction in the maximum skin friction by as much as 65 percent, with an increase in the maximum wall heat flux as high as 45 percent. The skin friction and wall heat flux were much less susceptible to low-frequency excitation.