Flow structure and heat transfer characteristics of an unconfined impinging air jet at high jet Reynolds numbers

The flow and heat transfer characteristics of an unconfined air jet that is impinged normally onto a heated flat plate have been experimentally investigated for high Reynolds numbers ranging from 30,000 to 70,000 and a nozzle-to-plate spacing range of 1–10. The mean and turbulence velocities by using hot-wire anemometry and impingement surface pressures with pressure transducer are measured. Surface temperature measurements are made by means of an infrared thermal imaging technique. The effects of Reynolds number and nozzle-to-plate spacing on the flow structure and heat transfer characteristics are described and compared with similar experiments. It was seen that the locations of the second peaks in Nusselt number distributions slightly vary with Reynolds number and nozzle-to-plate spacing. The peaks in distributions of Nusselt numbers and radial turbulence intensity are compatible for spacings up to 3. The stagnation Nusselt number was correlated for the jet Reynolds number and the nozzle-to-plate spacing as Nu _st∝Re ^0.69(H/D)^0.019.     

LDA measurements in the turbulent round jet

In the present paper, LDA was used to measure the velocity field of turbulent round air jet flows. Two cases were investigated; a jet issuing vertically upward and freely in the laboratory surrounding environment, and a jet issuing vertically upward but out of wall section setting flush horizontally at the nozzle exit. Data were collected for three exit Reynolds numbers of 1.32 × 10⁴, 2.64 × 10⁴ and 3.96 × 10⁴, which correspond to exit velocities of 10 m/s, 20 m/s and 30 m/s respectively. For each Reynolds number, profile measurements of the mean velocity, turbulence intensity, skewness and flatness factors were made at 8 downstream stations up to 30 nozzle-exit diameter. The relative influence of using a wall at the jet exit plane on the jet behavior and characteristics is the objective of the present study. The experimental results indicate that the wall, placed at the exit plane, limits the interaction of the jet flow with the surroundings, and consequently results in a reduction in the velocity spread rate, kinematic momentum flux, and kinematic mass flux. Further, the flatness and skewness factors distributions across the jet flow registered relatively higher values in the outer region of the jet when the wall was used. This indicates a more intermittent behavior of the jet flow in that region due to the existence of the wall.     

Experimental study of shock waves from the interaction of a supersonic plasma jet with an ambient gas

Preliminary results of the interaction of a supersonic, radiatively cooled plasma jet with an ambient gas are presented. The experimental setup consists of a radial foil, a mum-thick aluminium disc held between two concentric electrodes and subjected to a 1.4 MA, 250-ns current pulse from the MAGPIE generator. The plasma flow, with typical velocities of ~70?C90?km/s, is produced by the J?× B force acting on the plasma ablated from the foil. A jet is formed from the convergence of this ablated plasma on the axis of the system. A new setup allows the jet to interact with an argon ambient (particle density N ~10^16-17 cm^?3) from a supersonic gas nozzle (Mach ~9). First results are characterised by the presence of several (previously unseen) shock structures, which are formed from the interaction of the jet with the argon ambient.     

An experimental study of vertical turbulent jet with negative buoyancy

A jet discharged upward into an ambient of higher temperature than that of the jet fluid was investigated experimentally. The width of the upflow spread linearly to the distance from the nozzle exit, whereas the width of the whole jet was almost constant. The height of top of jet varied in proportion to the square root of the discharge Froude number. The time-averaged velocities and temperatures as well as the intensities of their fluctuations at the jet axis were well correlated with the same scaling law as that used for the buoyant jet. The radial distributions showed no similarity profiles for the timeaveraged velocities and temperatures, and the intensities of their fluctuations.     

Horizontal bubble flow

An experimental investigation of cocurrent bubble flow in 0.0254 m and 0.0508 m diameter horizontal pipelines has been performed. Gas and liquid mass velocities ranged from 0.00955 to 0.675 and 2720 to 6040 kg/m² sec, respectively, and gas-phase holdups or void fractions ranged from 0.13 to 7.59%.High speed motion pictures revealed that the gas, introduced into the liquid with a concentric nozzle, emerged in the form of a rough jet which was ultimately sheared into 1 times; 10^minus;3 to 3 times; 10^minus;3m diameter bubbles. Approximately 4 meters downstream from the nozzle, a well developed bubble flow was observed where bubble number density and axial velocity were constant with respect to axial position in the pipeline. Bubble velocities ranged from 0.001 to 0.57 m/sec greater than the average liquid velocities. Bubble radial and circumferential spatial distributions were found to be a strong function of the degree of turbulence in the liquid phase. Because of these turbulent flow conditions, bubble shapes were much different than those of equivalent diameter bubbles rising in stagnant liquids. A sphere-ellipsoid of revolution model was developed for characterization of bubble shape and computation of gas-liquid interfacial area and two-phase pressure drop.     

Dynamics of heavy particles in two swirling jets

We report flow visualisations and laser Doppler anemometry (LDA) velocity measurements in the near field of two swirling jets. The Reynolds number based on jet diameter and bulk velocity at the nozzle exit is 1.4 × 10⁵. In the first jet, a small recirculation region is formed around the jet axis, while, in the second, the streamwise velocity remains positive and overshoots near the jet centre. In both cases, flow visualisations show that the vortex core of the jets is depleted of seeding particles. By using time-averaged distributions of the streamwise and tangential velocities measured at the nozzle outlet, the dynamics of the particles is simulated, by integrating their simplified equations of motion. The particles trajectory thus computed agrees well with that observed in the flow visualisations. Although the turbulence intensity is substantially different in the core of the two jets, its effect on the seeding concentration is localised near the edge of the core.     

Behavior of probability density functions in a binary gas jet

A helium/air mixture free round jet into still air was investigated using a laser Doppler anemometer and a hot-wire type concentration probe. The jet Reynolds number was 4,300 and the jet-to-ambient fluid density ratio was set at 0.64. Simultaneous measurements of the mixture density and the axial and radial velocities were carried out in both the near and far fields of the jet. A detailed analysis of the turbulent mass transfer and jet characteristics has been presented by So et al. (1990). This paper reports on the higher order statistics and the characteristics of the single and joint probability density distributions of the mixture density and the axial and radial velocities. The behavior of these distributions across and along the jet is analyzed and compared with other single and joint probability density distributions.     

Simulations of vertical jet penetration using a filtered two-fluid model in a gas–solid fluidized bed

The influence of a vertical jet located at the distributor in a cylindrical fluidized bed on the flow behavior of gas and particles was predicted using a filtered two-fluid model proposed by Sundaresan and coworkers. The distributions of volume fraction and the velocity of particles along the lateral direction were investigated for different jet velocities by analyzing the simulated results. The vertical jet penetration lengths at the different gas jet velocities have been obtained and compared with predictions derived from empirical correlations; the predicted air jet penetration length is discussed. Agreement between the numerical simulations and experimental results has been achieved.     

PIV measurements of an air jet impinging on an open rotor-stator system

The current work experimentally investigates the flow characteristics of an air jet impinging on an open rotor-stator system with a low non-dimensional spacing, G?=?0.02, and with a very low aspect ratio, e/D?=?0.25. The rotational Reynolds numbers varied from $0.33\times10^5$ to $5.32\times10^5$ , while the jet Reynolds numbers ranged from 17.2?×?10³ to 43?×?10³. Particle image velocimetry (PIV) measurements were taken along the entire disk diameter in three axial planes. From the obtained PIV velocity fields, the flow statistics were computed. A recirculation flow region, which was centered at the impingement point and possessed high turbulence intensities, was observed. Local peaks in root-mean-square fluctuating velocity distributions appeared in the recirculation region and near the periphery, respectively. Proper orthogonal decomposition analysis was applied to the cases of the jet impinging on the rotor with and without rotation to reveal the coherent structures in the jet region.     

The preheated Airy wall jet

The Airy jet is a wall-bounded flow belonging to the similarity class of the well known free jet but, in contrast to the latter, its far field behavior is an algebraically decaying rotational flow. The velocity and temperature distributions of a preheated Airy jet flowing over an insulated wall are investigated using both analytical and numerical methods, and are compared with those of the classical (preheated) exponentially decaying wall jet. For the same value of the dimensionless skin friction parameter, the maximum of the similar velocity profile of the Airy jet exceeds that of the classical wall jet by approximately 20%. The dimensionless temperature along the insulated wall scales for large values of the Prandtl number with Pr^2/3 for both jets, while for small values of the Prandtl number the temperature scales with Pr^1/3 for the Airy jet and goes to 1 for the classical wall jet.This work is dedicated to Michael B. Glauert who passed away on June 14, 2004     

Vortical structure evolutions and spreading characteristics of a plane jet flow under anti-symmetric long-wave excitation

The vortical evolutions and spreading characteristics of a low-speed plane jet under anti-symmetric long-wave excitations are investigated experimentally. The perturbation is introduced with two oscillating strips located at the nozzle exit. The experiments were operated at Reynolds number of 8.2 × 10³ based on the nozzle exit height. Mixing and spreading properties are influenced obviously by long-wave excitation after the end of potential core. The increments of half-width, momentum thickness, and volume flow rate depend on the excitation frequency. The results of flow visualization also reveal the dependence of excitation frequency. The power spectra of fluctuating velocities shows that the evolution of coherent structure is significantly influenced by the long-wave excitation in the downstream, but it is similar to the natural jet in the near field. The long-wave excitation at certain frequencies can promote large-scaled anti-symmetric vortical structures in the far field.     

An Experimental Investigation of the Turbulence Structure of a Lifted H2/N2 Jet Flame in a Vitiated Co-Flow

Measurements of mean velocity components, turbulent intensity, and Reynolds shear stress are presented in a turbulent lifted H₂/N₂ jet flame as well as non-reacting air jet issuing into a vitiated co-flow by laser doppler velocimetry (LDV) technique. The objectives of this paper are to obtain a velocity data base missing in the previous experiment data of the Dibble burner and so provide initial and flow field data for evaluating the validity of various numerical codes describing the turbulent partially premixed flames on this burner. It is found that the potential core is shortened due to the high ratio of jet density to co-flow density in the non-reacting cases. However, the existence of flame suppressed turbulence in the upstream region of the jet dominates the length of potential core in the reacting cases. At the centreline, the normalized axial velocities in the reacting cases are higher than the non-reacting cases, and the relative turbulent intensities of the reacting flow are smaller than in the non-reacting flow, where a self-preserving behaviour for the relative turbulent intensities exists at the downstream region. The profiles of mean axial velocity in the lifted flame distribute between the non-reacting jet and non-premixed flame both in the axial and radial distributions. The radial distributions of turbulent kinetic energy in the lifted flames exhibit a change in distributions indicating the difference of stabilisation mechanisms of the two lifted flame. The experimental results presented will guide the development of an improved modelling for such flames.     

Control of diffuser jet flow: turbulent kinetic energy and jet spreading enhancements assisted by a non-thermal plasma discharge

An axisymmetric air jet exhausting from a 22-degree-angle diffuser is investigated experimentally by particle image velocimetry (PIV) and stereo-PIV measurements. Two opposite dielectric barrier discharge (DBD) actuators are placed along the lips of the diffuser in order to force the mixing by a co-flow actuation. The electrohydrodynamic forces generated by both actuators modify and excite the turbulent shear layer at the diffuser jet exit. Primary air jet velocities from 10 to 40 m/s are studied (Reynolds numbers ranging from 3.2 to 12.8 × 10⁴), and baseline and forced flows are compared by analysing streamwise and cross-stream PIV fields. The mixing enhancement in the near field region is characterized by the potential core length, the centreline turbulent kinetic energy (TKE), the integrated value of the TKE over various slices along the jet, the turbulent Reynolds stresses and the vorticity fields. The time-averaged fields demonstrate that an effective increase in mixing is achieved by a forced flow reattachment along the wall of the diffuser at 10 m/s, whereas mixing enhancement is realized by excitation of the coherent structures for a primary velocity of 20 and 30 m/s. The actuation introduces two pairs of contra-rotating vortices above each actuator. These structures entrain the higher speed core fluid toward the ambient air. Unsteady actuations over Strouhal numbers ranging from 0.08 to 1 are also studied. The results suggest that the excitation at a Strouhal number around 0.3 is more effective to enhance the turbulence kinetic energy in the near-field region for primary jet velocity up to 30 m/s.     

Round turbulent jet in a crossf low

We examine the discharge of a turbulent jet from a round opening into an infinite uniform crossflow and find the form of the jet centerline and the distribution of the maximum velocities in the jet along the axis. It is shown that the calculated jet axes and velocity distributions agree well with the experimental values for different ratios of the velocity at the source exit to the crossflow velocity and for different angles of entry of the jet into the crossflow. The study [1] formed the basis for the proposed semiempirical theory.     

Coherent structure interactions in an unexcited coaxial jet

Based on a double triggering criterion, the axisymmetric and azimuthal structures within the initial region of an unexcited coaxial jet of mean velocity ratio of 0.3 (inner to outer) were recovered. Outer vortices, two trains of inner vortices and their amalgamation are found in the outer and inner mixing regions. Bifurcation of the amalgamated inner vortices occurs within the first two diameters of the jet and is responsible for the occurrence of the azimuthal structures in the inner jet region further downstream. Co-dominance of the symmetrical and azimuthal structures having different convection velocities is observed in the fully merged zone. Spatial separation between vortical structures and their relative vorticities are important parameters in affecting their convection velocities, growth and decay.The work was partly supported by a donation from Dr. Haking Wong and by a grant from the Hong Kong Research Grants Council.     

Plane jet excited by disturbances with spanwise phase variations

Evolution of the near-field structures of a plane jet excited by temporal periodic disturbances with spanwise phase variations was investigated with stereoscopic particle image velocimetry. The three-dimensional vorticity distributions were reconstructed by using Taylor’s frozen field hypothesis. When ?, the temporal phase difference of disturbances in the spanwise direction was π; chain-link-fence type structures were formed. The $\Uplambda$ vortices in the chain-link-fence structures were then distorted into an $\Upomega$ shape, and the head of the vortex was detached and reconnects to form a vortex ring, or reconnects to the adjacent V-shaped vortices to form an A-shaped vortex. After the reconnection stage, the flow field was occupied by uniformly distributed fine scale eddies. Here, the overall turbulent kinetic energy and shear stress were suppressed, and the jet width was narrower than that of the unexcited case and other forced cases. In the case of ? = π/2, spanwise rollers and rib structures were formed near the nozzle exit after the first vortex pairing. However, further vortex pairing did not occur downstream, and the rate at which the jet widened was reduced.     

Effect of different initial conditions on a turbulent round free jet

Velocity measurements were made in two jet flows, the first exiting from a smooth contraction nozzle and the second from a long pipe with a fully developed pipe flow profile. The Reynolds number, based on nozzle diameter and exit bulk velocity, was the same (䏪,000) in each flow. The smooth contraction jet flow developed much more rapidly and approached self-preservation more rapidly than the pipe jet. These differences were associated with differences in the turbulence structure in both the near and far fields between the two jets. Throughout the shear layer for x<3d, the peak in the v spectrum occurred at a lower frequency in the pipe jet than in the contraction jet. For x́d, the peaks in the two jets appeared to be nearly at the same frequency. In the pipe jet, the near-field distributions of f(r) and g(r), the longitudinal and transverse velocity correlation functions, differed significantly from the contraction jet. The integral length scale L_u was greater in the pipe jet, whereas L_v was smaller. In the far field, the distributions of f(r) and g(r) were nearly similar in the two flows. The larger initial shear layer thickness of the pipe jet produced a dimensionally lower frequency instability, resulting in longer wavelength structures, which developed and paired at larger downstream distances. The regular vortex formation and pairing were disrupted in the shear layer of the pipe jet. The streamwise vortices, which enhance entrainment and turbulent mixing, were absent in the shear layer of the pipe jet. The formation of large-scale structures should occur much farther downstream in the pipe jet than in the contraction jet.     

A turbulent plane offset jet with small offset ratio

 Mean velocities and turbulence characteristics of a turbulent plane offset jet with a small offset ratio of 2.125 have been studied using laser Doppler anemometry (LDA). Static pressure measurements highlight the importance of side plates in enhancing two-dimensionality of the jet. The spatial distributions of turbulence intensities and Reynolds shear stress show a high turbulence recirculating flow region close to the nozzle plate between the jet and the offset plate. The LDA results have been used to examine the capability of three different turbulence models (i.e. k–ɛ, RNG and Reynolds stress) in predicting the velocity field of this jet. While all three models are able to predict qualitatively the recirculation, converging and reattachment regions observed experimentally, the standard k–ɛ turbulence model predicts a reattachment length that best agrees with the experimentally determined value. Received: 11 September 1996/Accepted: 30 May 1997     

Turbulence control in wall jets

A new method for the control of mixing of a plane turbulent wall jet has been investigated. A thin wire, mounted in the vicinity of the wall-jet nozzle, changes the formation of the shear-layer structures in the early stages of the development of the wall jet. The wire is operated in two ways: (1) a still wire inhibits the natural shear layer roll-up and reduces the size of the turbulent structures and thereby the mixing; (2) a self-excited oscillating wire introduces large coherent structures and thereby enhances the mixing. The size of these structures does not depend on the shear-layer instability but rather on the wire frequency.Measurements of the mean and fluctuating velocities have been performed with hot-wire anemometry as well as measurements of the skin friction by means of Preston tubes, surface fences and wall hot-wires. The Reynolds number based on the slot width was Re_j=10000.     

Effects of the jet-to-crossflow momentum ratio on a sonic jet into a supersonic crossflow

Numerical investigation of a transverse sonic jet injected into a supersonic crossflow was carried out using large-eddy simulation for a free-stream Mach number M = 1.6 and a Reynolds number Re = 1.38 × 10⁵ based on the jet diameter. Effects of the jet-to-crossflow momentum ratio on various fundamental mechanisms dictating the intricate flow phenomena, including flow structures, turbulent characters and frequency behaviors, have been studied. The complex flow structures and the relevant flow features are discussed to exhibit the evolution of shock structures, vortical structures and jet shear layers. The strength of the bow shock increases and the sizes of the barrel shock and Mach disk also increase with increasing momentum ratio. Turbulent characters are clarified to be closely related to the flow structures. The jet penetration increases with the increase of the momentum ratio. Moreover, the dominant frequencies of the flow structures are obtained using spectral analysis. The results obtained in this letter provide physical insight in understanding the mechanisms relevant to this complex flow.