Influence of streamwise pitch on the local heat transfer characteristics for in-line arrays of circular jets with crossflow of spent air in one direction

Influence of the streamwise pitch on local heat transfer distribution due to a rectangular in-line array of circular air jets of length-to-diameter ratio (l/d) of 1.0 is studied experimentally. The flow, after the impingement, is constrained to exit in one direction. Mean jet Reynolds number is varied from 3000 to 10000 and jet-to-plate spacing from d to 3d. Streamwise jet-to-jet distances of 3d, 4d and 5d and a constant spanwise pitch of 4d are considered. A flat target surface is made of thin stainless steel metal foil. The local temperature distribution on a target plate is measured using thermal infrared camera. The jet exit pressures are measured to estimate the cross-flow velocities and individual jet velocities. The streamwise distribution of the jet-flow and the cross-flow is least influenced by the streamwise pitch variation for the range of parameters investigated. Heat transfer characteristics are explained partially on the basis of flow distribution. The cooling performance, based on strip averaged Nusselt number per unit mass flow rate of coolant per unit area of cooled surface, deteriorates for lower streamwise pitch and higher jet-to-plate distance.     

Heat transfer due to impinging co-axial jets and the jets’ fluid flow characteristics

This investigation had multiple goals. One goal was to obtain definitive information about the heat transfer characteristics of co-axial impinging jets, and this was achieved by measurements of the stagnation-point, surface-distribution and average heat transfer coefficients. These results are parameterized by the Reynolds number Re which ranged from 5000 to 25,000, the dimensionless separation distance between the jet exit and the impingement plate H/D (4–12), and the ratio of the inner diameters of the inner and outer pipes d/D (0–0.55). The d/D = 0 case corresponds to a single circular jet. The other major goal of this work was to quantify the velocity field of co-axial free jets (impingement plate removed). The velocity-field study included both measurements of the mean velocity and the turbulence intensity.It was found that the variation of the stagnation-point heat transfer coefficient with d/D attained a maximum at d/D = 0.55. Furthermore, the variation of the local heat transfer coefficient across the impingement surface was more peaked for d/D = 0 and became flatter with decreasing d/D. This suggests that for cooling a broad expanse of surface, co-axial jets of high d/D are preferable. On the other hand, for localized cooling, the single jet (d/D = 0) performed the best. In general, for a given Reynolds number, a co-axial jet yields higher heat transfer coefficients than a single jet. Off-axis velocity peaks were encountered for the jets with d/D = 0.105. The measurements of turbulence intensity yielded values as high as 18%.     

Visualization and structure of confined, milliscale, unsteady impinging slot jets and associated vortices

Flow characteristics of confined, laminar milliscale slot jets are investigated from visualizations, as they impinge upon a flat target plate, with a fully developed velocity profile at the nozzle exit. The effects of Reynolds number Re and normalized nozzle-to-plate distance H/B are considered for a nozzle width B of 1.0 mm. Transition from a stable symmetric jet to an unsteady oscillating jet is observed as the Reynolds number increases (with H/B constant), where the Reynolds number associated with this transition decreases as the normalized nozzle-to-plate distance H/B increases. Instantaneous visualizations show unsteady lateral distortions of jet columns at experimental conditions corresponding to the presence of continuous sinusoidal oscillations, intermittent oscillating motion of the jet column, and jet flow fluctuation/flapping motion. Also apparent in flow visualization sequences are smoke signatures associated with instantaneous vortex structures, which form as secondary flows develop in fluid which, initially, is just adjacent to and within the jet column. Associated jet and vortex structural changes are described as different modes of unsteadiness are present, including characterization of jet column unsteadiness using jet column oscillation frequency, and lateral and streamwise extents of jet distortion.     

Three-dimensional vortex dynamics and convective heat transfer in circular and chevron impinging jets

This paper describes an experimental investigation at Reynolds number equal to 5000 on circular and chevron impinging jets by means of time-resolved tomographic particle image velocimetry (TR-TOMO PIV) and infrared (IR) thermography. TR-TOMO PIV experiments are performed at kilo-hertz repetition rate in a tailored water jet facility where a plate is placed at a distance of 4 diameters from the nozzle exit. Using air as working fluid, time-averaged convective heat transfer is measured on the impinged plate by means of IR thermography with the heated-thin-foil heat transfer sensor for nozzle-to-plate distances ranging from 2 to 10 diameters. The circular impingement shows the shedding and pairing of axisymmetric toroidal vortices with the later growth of azimuthal instabilities and counter-rotating streamwise vortices. In the chevron case, instead, the azimuthal coherence is replaced by counter-rotating pairs of streamwise vortices that develop from the chevron notches. The heat transfer performances of the chevron impingement are compared with those of the circular one, analyzing the influence of the nozzle-to-plate distance on the distribution of Nusselt number. The chevron configuration leads to enhanced heat transfer performances for all the nozzle-to-plate distances hereby investigated with improvements up to 44% at the center of the impinged area for nozzle-to-plate distance of 4. Such enhancements are discussed in relation to the streamwise structures that, compared with the toroidal vortices, are associated with an earlier penetration of turbulence towards the jet axis and a higher arrival speed.     

Local heat transfer distribution between smooth flat surface and impinging air jet from a circular nozzle at low Reynolds numbers

An experimental investigation is performed to study the effect of jet to plate spacing and low Reynolds number on the local heat transfer distribution to normally impinging submerged circular air jet on a smooth and flat surface. A single jet from a straight circular nozzle of length-to-diameter ratio (l/d) of 83 is tested. Reynolds number based on nozzle exit condition is varied between 500 and 8,000 and jet-to-plate spacing between 0.5 and 8 nozzle diameters. The local heat transfer characteristics are obtained using thermal images from infrared thermal imaging technique. It was observed that at lower Reynolds numbers, the effect of jet to plate distances covered during the study on the stagnation point Nusselt numbers is minimal. At all jet to plate distances, the stagnation point Nusselt numbers decrease monotonically with the maximum occurring at a z/d of 0.5 as opposed to the stagnation point Nusselt numbers at high Reynolds numbers which occur around a z/d of 6.     

The effect of sidewalls on rectangular jets

An experimental study is presented regarding the influence of sidewalls on the turbulent free jet flow issuing from a smoothly contracting rectangular nozzle of aspect ratio 15. “Sidewalls” are two parallel plates, flush with each of the slots’ short sides, practically establishing bounding walls extending the nozzle sidewalls in the downstream direction. Measurements of the streamwise and lateral velocity mean and turbulent characteristics have been accomplished, with an x-sensor hot wire anemometer, up to an axial distance of 35 nozzle widths, for jets with identical inlet conditions with and without sidewalls. Centreline measurements for both configurations have been collected for three Reynolds numbers, Re_D = 10,000, 20,000 and 30,000. For Re_D = 20,000 measurements in the transverse direction were collected at 13 different downstream locations in the range, x = 0–35 nozzle widths, and in the spanwise direction at three different downstream locations, x = 2, 6 and 25 nozzle widths.Results indicate that, the two jet configurations (with and without sidewalls) produce statistically different flow fields. Sidewalls do not lead to the production of a 2D flow field as undulations in the spanwise mean velocity distribution indicate. They do increase the two-dimensionality of the jet increasing the longevity of 2D spanwise rollers structures formed in the initial stages of entrainment, which are responsible for the convection of longitudinal momentum towards the outer field, establishing larger streamwise mean velocities at the jet edges. In the near field, up to 25 nozzle widths, lower outward lateral velocities in the presence of the sidewalls are held responsible for the decrease of turbulent terms including rms of velocity fluctuations and Reynolds stresses. Skewness factors increase monotonically across the shear layers from negative values to positive forming sharp peaks at the outer edges of the jet, illustrative of the presence of well defined 2D roller structures in the jet with sidewalls.     

Skin-friction drag reduction in a high-Reynolds-number turbulent boundary layer via real-time control of large-scale structures

While large-scale motions are most energetic in the logarithmic region of a high-Reynolds-number turbulent boundary layer, they also have an influence in the inner-region. In this paper we describe an experimental investigation of manipulating the large-scale motions and reveal how this affects the turbulence and skin-friction drag. A boundary layer with a friction Reynolds number of 14 400 is controlled using a spanwise array of nine wall-normal jets operated in an on/off mode and with an exit velocity that causes the jets in cross-flow to penetrate within the log-region. Each jet is triggered in real-time with an active controller, driven by a time-resolved footprint of the large-scale motions acquired upstream. Nominally, the controller injects air into large-scale zones with positive streamwise velocity fluctuations; these zones are associated with positive wall-shear stress fluctuations. This control scheme reduced the streamwise turbulence intensity in the log-region up to a downstream distance of more than five times the boundary layer thickness, δ, from the point of actuation. The highest reduction in spectral energy—more than 30%—was found for wavelengths larger than 5δ in the log-region at 1.7δ downstream of actuation, while scales larger than 2δ still comprised more than 15% energy reduction in the near-wall region. In addition, a 3.2% reduction in mean skin-friction drag was achieved at 1.7δ downstream of actuation. Our reductions of the streamwise turbulence intensity and mean skin-friction drag exceed a base line control-case, for which the jet actuators were operated with the same temporal pattern, but not synchronised with the incoming large-scale zones of positive fluctuating velocity.     

Measurements in the vicinity of a stagnation point

This paper presents measurements of a plane jet impinging onto a normal flat plate placed up to five jet widths from the jet outlet. The small spacing ensured that the stagnation streamline remained in the potential core of the jet. The plate shear stress distribution compared well to that from an analytical solution for the laminar development of the plate boundary layer whose external velocity was determined from the measured pressure. By comparing the shear stress measured under the present low level of free stream turbulence (0.35%) at the jet exit with that of Tu and Wood [Exp. Thermal Fluid Sci. 13 (1996) 364–373] made at about 4%, it is concluded that the turbulence level at the nozzle exit has only a second-order influence on the surface shear stress around the stagnation point. Some spanwise non-uniformity was observed in the plate shear stress, but this was confined largely to the transition region. The mean velocity, Reynolds stresses, and fluctuating pressure were measured along the stagnation streamline using a fast-response pressure probe. A significant increase in the streamwise normal stress and the mean square of the pressure fluctuations occurred before they were eventually attenuated by the plate. This increase occurred in the region where the streamwise velocity was decreasing close to the plate causing extra energy production through the normal stresses. Spectra of the velocity and pressure fluctuations showed that the increase in level was mainly due to the low frequency motion, whereas the subsequent decrease occurred at higher frequencies.     

UV-mediated coalescence and mixing of inkjet printed drops

In the present study, the characteristics of supersonic rectangular microjets are investigated experimentally using molecular tagging velocimetry. The jets are discharged from a convergent–divergent rectangular nozzle whose exit height is 500 μm. The jet Mach number is set to 2.0 for all tested jets, and the Reynolds number Re is altered from 154 to 5,560 by changing the stagnation pressure. The experimental results reveal that jet velocity decays principally due to abrupt jet spreading caused by jet instability for relatively high Reynolds numbers (Re > ~450). The results also reveal that the jet rapidly decelerates to a subsonic speed near the nozzle exit for a low Reynolds number (Re = 154), although the jet does not spread abruptly; i.e., a transition in velocity decay processes occurs as the Reynolds number decreases. A supersonic core length is estimated from the streamwise distribution of the centerline velocity, and the length is then normalized by the nozzle exit height and plotted against the Reynolds number. As a result, it is found that the normalized supersonic core length attains a maximum value at a certain Reynolds number near which the transition in the velocity decay process occurs.     

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.     

Effect of jet-to-plate spacing in laminar array jets impinging

The rate of heat transfer from a plate due to impinging of an array of jets was investigated. The effect of jet-to-plate spacing in a confined array of impinging laminar square jets was investigated numerically through the solution of Navier Stokes and energy equations. The simulation is carried out for the jet-to-plate spacing between 2 B and 20 B and for jet-to-jet spacing of 4 B, where B is the jet width. Five in-line jets subjected to across-flow were used in this investigation. Also, six different ratios of jet to cross-flow velocity are simulated (0.5, 1.0, 2.5, 5, 7.5 and 10) for the jet Reynolds number of 200. The predicted results show a formation of one or two ground horseshoe vortices between the jets. In addition, a horseshoe vortex forms at different position between the orifice and impinging plates due to the interaction of two jets before they combine. The number of the ground horseshoe vortex and its size are strongly affected by the jet-to-plate spacing and by jet to cross-flow velocity ratio. The effect of jet-to-plate spacing and jet to cross-flow velocity ratio on heat transfer is presented and discussed.     

Computational flow and heat transfer of a row of circular jets impinging on a concave surface

A computational investigation is carried out to study the flow and heat transfer from a row of circular jets impinging on a concave surface. The computational domain simulates the impingement cooling zone of a gas turbine nozzle guide vane. The parameters, which are varied in the study include jet Reynolds number (Re _d = 5000–67800), inter-jet distance to jet diameter ratio (c/d = 3.33 and 4.67) and target plate distance to jet diameter ratio (H/d = 1, 3 and 4). The flow field, predicted with K-ω turbulence model and using Fluent 6.2.16, is characterized with the presence of a pair of counter rotating vortices, an upwash fountain flow and entrainment. The local pressure coefficient and Nusselt number variations along the concave plate are presented and these values are found to under predict the available experimental data by about 12%.     

Experimental investigation of axisymmetric coaxial synthetic jets

Measurements were made in the near field of piston driven axisymmetric coaxial synthetic jets emanating from an orifice and a surrounding annulus of equal exit areas and cavity volumes. Piston velocity, amplitude, radial spacing between the orifice and the annulus, and exit angles had a strong influence on the dominant features of the flow. Flow visualization revealed three distinct topologies of the jet consisting of expanding, contracting and recirculating regions and doubling of the number of foci inside of the cavity compared to jet from the orifice alone. The direction of the swirl/rotation imposed on the mean flow was also dependent on the direction of the rotation of dominant foci. Interaction between flow from the orifice and the annulus amplified the azimuthal instability of ring vortices due to the periodic axial stretching and compression of the streamwise vortex filaments. Bifurcation of ring vortices into elliptical lobes reported earlier [S.V. Gaimella, V.P. Schroeder, Local heat transfer distributions in confined multiple air impingement, ASME Journal of Electronic Packaging 123 (3) (2001) 165–172] for single cavity jet was also observed in the coaxial jet. The number of cellular structures however was considerably larger than the single jet case. Large excursions of the jets from the plane of symmetry were observed. Power spectra exhibited sub-harmonic distribution of energy due to coalescence of the vortices. Growth of jet width and decay of centerline velocity were strongly influenced by the spacing and forcing frequency.     

An experimental investigation on flow structures of confined and unconfined impinging air jets

The flow characteristics of both confined and unconfined air jets, impinging normally onto a flat plate have been experimentally investigated. The mean and turbulence velocities, and surface pressures were measured for Reynolds numbers ranging from 30,000 to 50,000 and the nozzle-to-plate spacings in range of 0.2–6. Smoke-wire technique is used to visualize the flow behavior. The effects of Reynolds number, nozzle-to-plate spacing and flow confinement on the flow structure are reported. In the case of confined jet, subatmospheric regions occur on both impingement and confinement surfaces at nozzle-to-plate spacings up to 2 for all Reynolds numbers in consideration and they lie up to nearly the same radial location at both surfaces. However, there is no evidence of the subatmospheric region in unconfined jet. It is concluded that there exists a linkage among the subatmospheric region, turbulence intensity and the peaks in heat transfer coefficients for low spacings in impinging jets.     

微型射流涡流器对湍流边界层控制的数值研究

运用数值方法,模拟出展向分布的同向倾斜微型射流列与平板湍流边界层相互作用形成流向涡列的流场结构,验证了利用其来对湍流边界层进行控制的可能性.随射流间距减小,流向涡列控制作用流向渗透能力增强,但作用区域减小;随射流速度提高,流向涡列控制作用增强,但过大的射流速度反而会导致流向涡列在局部区域内控制作用的下降;随射流俯仰角减小、倾斜角增大,流向涡列初始控制作用增强,但过小的俯仰角、过大的倾斜角会导致流向涡列流向控制区域明显缩小.要保证流向涡列具有较强的湍流边界层控制作用,必须通过合理配置射流列各主要参数,在保证各流向涡具有一定强度的同时,还要确保各流向涡在形成时部分嵌入边界层内部.     

Parametric analysis of a round jet impingement on a heated plate

In this study, the flow and heat transfer characteristics of a round air jet have been experimentally investigated in details using two techniques: Particle Image Velocimetry (PIV) and the Laser Doppler Velocimetry (LDV). The measurement of the mean velocity components are compared, and agree well with the experimental data obtained by Baydar (1999). The distributions of the velocity, turbulence quantities and temperature profiles are analyzed in the main characteristic regions of the jet where the heat transfer occurs. Parametric variations were conducted to produce information about the influences of the Reynolds number (Re = 1000, 2000, 3000), the distance between the pipe exit and the flat impingement plate (h/d = 1 and h/d = 2) and the temperature of the plane (T_p = 22 °C, 54 °C, 96 °C) on the impinging jet flow field.     

Single-phase convection and boiling heat transfer: Confined single and array-circular impinging jets

In the present study, the effects of diverse situations of confinement on heat transfer from single and array-circular jet impingements are carefully investigated over various heat transfer regimes of single-phase convection and fully developed nucleate boiling. For the single, circular, unconfined free-surface jet, the transition to turbulence was observed to start around x/d = 5.5 and end around x/d = 9. For the array-circular jet, however, the wall jet structure yielded no transition to turbulence for all the tested cases, instead monotonically decreasing the convection coefficient. Conversely, the single-circular jet experienced the transition for V ? 6.1 m/s. For the confined submerged jet, the transition length was very short due to the vigorous mixing driven by lateral velocity components, and the locus of the secondary peak moved downstream as velocity increased. The temperature distributions of the confined array-circular jet were fairly uniform over the whole heated surface. The averaged single-phase convection coefficients indicated that the confined jet provided the most uniform convection in the lateral direction.     

High-resolution imaging of turbulence structures in jet flames and non-reacting jets with laser Rayleigh scattering

A comparative study of the length scales and morphology of dissipation fields in turbulent jet flames and non-reacting jets provides a quantitative analysis of the effects of heat release on the fine-scale structure of turbulent mixing. Planar laser Rayleigh scattering is used for highly resolved measurements of the thermal and scalar dissipation in the near fields of CH₄/H₂/N₂ jet flames (Re _d  = 15,200 and 22,800) and non-reacting propane jets (Re _d  = 7,200–21,700), respectively. Heat release increases the dissipation cutoff length scales in the reaction zone of the flames such that they are significantly larger than the cutoff scales of non-reacting jets with comparable jet exit Reynolds numbers. Fine-scale anisotropy is enhanced in the reaction zone. At x/d = 10, the peaks of the dissipation angle PDFs in the Re _d  = 15,200 and 22,800 jet flames exceed those of non-reacting jets with corresponding jet exit Reynolds numbers by factors of 2.3 and 1.8, respectively. Heat release significantly reduces the dissipation layer curvature in the reaction zone and in the low-temperature periphery of the jet flames. These results suggest that the reaction zone shields the outer regions of the jet flame from the highly turbulent flow closer to the jet axis.     

Study on the radial jet velocity distribution of two closely spaced opposed jets

The experimental and theoretical researches on the radial jet of two opposed jets have been carried out in this paper. The radial velocities of opposed jets with various exit velocities, nozzle diameters and nozzle separations were measured experimentally by a hot-wire anemometer (HWA). The results show that, the normalized radial velocities are self-similar across various radial sections at r ? 1.5D and the radial velocity profiles can be described by a Gaussian distribution function. The half-width increases linearly with increasing radial distance at r ? 1.5D, and spreading rates of radial jet are about 0.121. The normalized radial velocity at impingement plane increases firstly, and then decreases with the increasing normalized radial distance. The normalized radial velocity is independent on nozzle diameter, nozzle separation and exit velocity. The maximum radial velocity at impingement plane is proportional to the exit velocity, and it is inversely proportional to the 0.551th power of the normalized nozzle separation. The position of the maximum radial velocity increases with the nozzle separation at L/D < 1, and keeps invariant at L/D ? 1.     

Numerical modeling of multi micro jet impingement cooling of a three dimensional turbine vane

This paper reports a numerical investigation on the prediction of the thermal and hydrodynamic flow fields of multi micro jet impingement cooling of three dimensional turbine vanes. A three dimensional vane is modeled with an in-line array of impinging jets of diameters 0.5 and 0.25 mm. The numerical model consists of the steady, Reynolds-Averaged Navier–Stokes equations and the Kω SST Turbulence model. The governing equations are solved using a finite volume method. The crossflow mass velocity (G _c ) to jet mass velocity (G _j ) ratio, and the average and local heat transfer distributions are analyzed with varying mass velocity and jet-to-target spacing. It is found out that a significant decrease in crossflow ratio occurs with the smaller diameters. Due to the lower crossflow and higher exit velocities of the smaller jets, the penetration into the crossflow is much higher. Moreover, at a constant mass flow, the use of micro-jets enhanced the overall average heat transfer coefficient by 63%, while at a fixed pressure drop across the vane instead of the mass flow, the smaller diameters will still yield an enhancement of 34.3% in the overall average heat transfer coefficient.