Laboratory experiments of a jet impinging on a stratified interface

 The entrainment rates of vertical and inclined jets impinging on a stratified interface are measured in water tank experiments. At moderate Richardson number, the entrainment rate of the vertical jet is proportional to Ri ^-1/2, independent of Reynolds number. The inclined jets are tilted at 15° from the vertical. In one case, the jet nozzle is rotated about a vertical axis, so that the inclined jet precesses, while in the other, it is stationary. The inclined jets entrain at a rate proportional to Ri ^-3/2, whether precessing or not. This behavior is consistent with a new model of stratified entrainment which accounts for vortex persistence. Received: 15 October 1996/Accepted: 19 December 1996     

Effects of the surface roughness on heat transfer of perpendicularly impinging co-axial jet

Current work presents the comparison of the cooling characteristics of roughened and smooth heated surfaces subjected to co-axial impinging jet. The work fluid is air and the data runs are performed for jet Reynolds numbers for 10,000, 20,000 and 40,000, and non-dimensional surface to jet exit spacing, H/D, from 1 to 10. The co-axial jet configuration is based on a fully developed pipe flow encountering a double-pipe arrangement and splitting between the two pipes. The inner to outer diameter ratio is 0.5. A straight pipe without inner section is used as the circular jet. The impingement of circular jets to the roughened and smooth surfaces is also performed for comparison. Average Nusslet numbers were obtained to show the heat transfer enhancement from the surface. A good agreement between the literature and present paper was obtained. As a result, average Nusselt number with co-axial jet impingement to the roughened surface increased by up to 27% comparing to the circular jet impingement. In addition, the average Nusselt number increased with roughened surface by up to 6% over the whole surface area, comparing to the smooth surface.     

An experimental study on the vortical structures and behaviour of jets issuing from inclined coaxial nozzles

An experimental study on inclined coaxial jets using laser-induced fluorescence and particle image velocimetry is presented here. The Reynolds numbers of the inner primary jet and outer secondary jet were Re = 2,500 and between Re = 500 and 2,000 (based on gap size), respectively, which corresponded to secondary-to-primary jet velocity ratios (VR) of VR = 0.5–2.0. The secondary-to-primary jet area ratio was 2.25, and 45° and 60° incline-angles were studied. Flow visualizations show that relatively independent inclined primary and secondary jet vortex roll-ups were formed at VR = 0.5. At VR = 1.0, regular pairings and mergings between primary and secondary jet vortex roll-ups led to large-scale entrainment of secondary jet and ambient fluids into the primary jet column and conferred a “serpentile”-shaped outline upon it. While the “serpentile”-shaped outline continued to exist at VR = 2.0, it was a result of stronger secondary jet inner vortex roll-ups which “pinched” the primary jet column regularly. These flow behaviours are observed to intensify with an increase in the incline-angle used. Velocity measurements demonstrate that inclined coaxial nozzles promoted vectoring of the primary jet momentum towards the longer nozzle lengths when velocity-ratio and/or incline-angle were increased. Lastly, peak velocity and higher turbulence intensity levels due to augmented vortical interactions are also detected along shorter nozzle lengths.     

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%.     

COMPUTATIONAL STUDIES OF IMPINGING JETS USING K-ϵ TURBULENCE MODELS

This paper reports numerical modelling of impinging jet flows using Rodi and Malin corrections to the k–ϵ turbulence model, carried out using the PHOENICS finite volume code. Axisymmetric calculations were performed on single round free jets and impinging jets and the effects of pressure ratio, height and nozzle exit velocity profile were investigated numerically. It was found that both the Rodi and Malin corrections tend to improve the prediction of the hydrodynamic field of free and impinging jets but still leave significant errors in the predicted wall jet growth. These numerical experiments suggest that conditions before impingement significantly affect radial wall jet development, primarily by changing the wall jet's initial thickness.     

Some observations in the vortex-turning behaviour of noncircular inclined jets

An experimental comparison was conducted for Re = 2,500, free elliptic and rectangular jets inclined at 30° and 60° along major or minor planes. Regardless of the jet base geometry, minor-plane inclined jets produced vortex roll-ups that remain inclined. In contrast, major-plane inclined jets produce significantly stronger vortex-roll-up turning behaviour. Interestingly, major-plane inclined rectangular jets exhibit strong vortex-overturning behaviour, where the vortex-roll-up inclination exceeds the 0° incline angle considerably. Vortex-turning extents and rates are compared between major-plane inclined elliptic and rectangular jets here and support present qualitative observations. Closer inspections reveal that the lack of axis-switching phenomenon in major-plane inclined rectangular jets allows vortex-overturning behaviour. In addition, jet centreline deflection is most sensitive in minor-plane inclined jets, where increasing the incline angle leads to a decrease and an increase in the elliptic and rectangular jet deflection, respectively.     

Heat transfer characteristics of premixed butane/air flame jet impinging on an inclined flat surface

 A series of experiments were carried out to determine the heat transfer characteristics of a round, premixed butane/air flame jet impinging upwards on an inclined flat plate, at different angles of incidence. The flame was fixed with an equivalence ratio of 1.0, a Reynolds number of 2500 and a plate-to-nozzle distance of 5d, while the inclination angles chosen for investigation were 57°, 67°, 80° and 90°. It was found that the location of the maximum heat flux point would be shifted away from the geometrical impingement point by reducing the angle of incidence. Decreasing the angle of incidence also enhanced the maximum local heat flux, while reduced the average heat transfer. The present study presented the effect of angle of incidence on the heat transfer characteristics of an impinging butane/air flame jet, which had been rarely reported in previous similar studies. Received on 11 October 2000 The authors wish to thank The Hong Kong Polytechnic University for the financial support of the present study.     

The impingement of sonic and sub-sonic jets onto a flat plate at inclined angles

The flow field associated with a jet impinging onto a surface at an inclined angle is investigated using particle image velocimetry (PIV). The results indicate that as a free jet impinges on a flat surface at an inclined angle the jet is turned by and spread laterally onto the impingement surface. The impingement angle of the jet is the dominant parameter in determining the rate of turning/spreading for the jet. The stagnation point is located using the PIV data and is found upstream of the geometric impingement point and upstream of the location of maximum pressure. The location of the stagnation point is a strong function of impingement angle and a weak function of impingement distance and pressure ratio. The location of the stagnation point is compared with the location of maximum pressure and compared to a curve fit for the location of maximum pressure based an exact solution of the Navier–Stokes equations for a non-orthogonal stagnation flow.     

A numerical and experimental investigation of the interactions between a non‐uniform planar array of incompressible free jets

The interaction between multiple incompressible air jets has been studied numerically and experimentally. The numerical predictions have been first validated using experimental data for a single jet configuration. The spreading features of five unequal jets in the configuration of one larger central jet surrounded by four smaller equi‐distant jets, have been studied, for different lateral spacing ratios of 1.5, 2.0 and 2.5 and a central jet Reynolds number of 1.24×105 (corresponding to a Mach number of 0.16). Flow of five equal jets has also been simulated, for the sake of comparison. The jet interactions commence at an axial distance of about 3–4 diameters and complete by an axial distance of about 10 diameters for the lowest spacing ratio of 1.5. For larger spacing ratios, the length required for the start and completion of jet interaction increase. Peripheral jets bend more towards the central jet and merge at a smaller distance, when their sizes are smaller than that of the central jet. The entrainment ratio for multiple jets is higher than that for a single jet. Excellent agreement is observed between the experimental data and theoretical predictions for both mean flow field and turbulent quantities, at regions away from the jet inlet. The potential core length and initial jet development, however, are not predicted very accurately due to differences in the assumed and actual velocity profiles at the jet inlet. Copyright © 2004 John Wiley & Sons, Ltd.     

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.     

Experiments on dynamic behavior of near-field region in variable property jet with swirling flow

The dynamic behavior of the near-field region in a coaxial variable property jet has been experimentally investigated under a swirling flow produced by rotating cylindrical inner and outer tubes, focusing on how the swirl of the outer jet affects the formation of a stagnation point in the swirling inner jet. The inner and outer jets rotate in the same direction. Air, CO₂, or He is issued from the inner tube as a variable property jet, and air is issued from the outer tube in this work. In the case of a CO₂ jet (a high-density, low-viscosity gas jet), a stagnation point flow is more easily formed than in the case of an air jet, and the stagnation point location is significantly lower than in that of the air jet. When the swirl of the outer jet is introduced, a stagnation point flow is more easily formed than in the case of a nonswirling outer jet, and the stagnation point location is much lower than in the case of a nonswirling outer jet. In the case of a He jet (a low-density and high-viscosity gas jet), the inner jet does not have a stagnation point flow, and its overall behavior remains nearly unchanged even under high swirl numbers of the inner and outer jets. These results clearly show that the density and viscosity differences between the inner and outer jets have a significant impact on the dynamic behavior of the near-field region in the coaxial swirling jet. The significant lowering of the stagnation point location can be physically explained by considering the theoretical equation obtained in this work.     

Heat transfer from an obliquely impinging circular, air jet to a flat plate

A series of experiments was conducted for the measurement of local convective heat transfer coefficients for an obliquely impinging circular air jet to a flat plate. In the experiments, the oblique angles selected were 90°, 75°, 60° and 45°, with 90° being a vertical jet. Two different Reynolds numbers of 10,000 and 23,000 were considered for the purpose of comparison with previous data available in the literature. Another parameter varied in the measurements was the dimensionless jet-to-plate distance, L/D. Four values of L/D(2, 4, 7, and 10) were considered in the experiments. The experiments were conducted using the preheated wall transient liquid-crystal technique. Liquid-crystal color changes were recorded with a video system. Local convective heat transfer coefficients were obtained through the surface transient temperatures that were related to the recorded color information. Detailed local heat transfer coefficients were presented and discussed in relation to the asymmetric wall jet upon impingement of the jet flow. Results of experiments show that, for a given flow situation, the point of maximum heat transfer shifts away from the geometrical impingement point toward the compression side of the wall jet on the axis of symmetry. The shift is more pronounced with a smaller oblique angle (larger jet inclination) and a smaller jet-to-plate distance. Comparisons of experimental results with existing heat transfer data for both obliquely impinging jets and vertical impinging jets are made. The effect of oblique angles on heat transfer was assessed.     

Droplet size and velocity characteristics of water-air impinging jet atomizer

A water-air impinging jets atomizer is investigated in this study, which consists of flow visualization using high speed photography and mean droplet size and velocity distribution measurements of the spray using Phase Doppler Anemometry (PDA). Topological structures and break up details of the generated spray in the far and near fields are presented with and without air jet and for an impinging angle of 90°. Spray angle increases with the water jet velocity, air flow rate and impinging angle. PDA results indicate that droplet size is smallest in the spray center, with minimum value of Sauter mean diameter (SMD) of 50 µm at the air flow rate of Q_m = 13.50 g/min. SMD of droplets increases towards the spray outer region gradually to about 120 µm. The mean droplet velocity component W along the air-jet axis is highest in the spray center and decreases gradually with increasing distance from the spray center. SMD normalized by the air nozzle diameter is found firstly to decrease with gas-to-liquid mass ratio (GLR) and air-to-liquid momentum ratio (ALMR) and then remain almost constant. Its increasing with aerodynamic Weber number indicates an exponential variation. The study sheds light on the performance of water-air impinging jets atomizers providing useful information for future CFD simulation works.     

Interaction of plane turbulent jets in a confined space: mean flow characteristics

The present study deals with the experimental investigations of static pressure and mean velocity fields obtained as a result of the interaction of two plane turbulent jets at impingement angles of α equal to 30° and 45°, with an additional central jet in a confined space. The investigation is carried out for the velocity ratios of U _c/U _o=1.0, 2.0 and 3.0, where U _c and U _o are the velocities in the central plane at the exit of the central jet and the outer jets, respectively. The introduction of the central jet alters the various recirculation zones present in the flow field for all the cases considered above. Also, the change in the velocity ratio U _c/U _o has a significant effect on the pressure and mean velocity flow fields. Flow visualisation results are presented which give a better physical insight into the flow field considered. Received: 26 July 1999/Accepted: 14 February 2000     

Opposed round jets issuing into a small aspect ratio channel cross flow

Round jets (diameter D) discharging into a confined cross flow (dimension 3.16D × 21.05D) are investigated experimentally. Two configurations are considered: (1) a single jet (momentum flux ratio, J = 155) and (2) two opposed jets with two different momentum flux ratios (J = 60, and 155). A two-component laser-Doppler anemometer is used to make a detailed map of the normal stresses and mean velocities in the symmetry plane of the jets. In addition, smoke-wire and laser-sheet visualization are used to study the flow.

The rate of bending of the single confined jet is found to be higher than the rate of bending of an unconfined jet with the same momentum flux ratio. In the far field, the jet centerline velocity is observed to decay more slowly than the unconfined jet, indicating poor turbulent diffusion of linear momentum. Annular shear layer vortices are visualized on the upstream edge of the jet in the near field. In the far field, the flow visualization suggests that the jet loses its integrity and fragments into independent regions that are convected by the cross flow.

In the opposed jet configuration at the high momentum flux ratio (J = 155), the jets impinge in the center of the duct, and a pair of vortices is observed upstream of the impingement region. The flow visualization implies that the impingement vortices form quasi periodically and have a finite life span. In the impingement region, the jets are observed to penetrate alternately beyond the symmetry plane of the duct. In the two-jet configuration with J = 60, the jets do not impinge on each other owing to the higher rate of bending. Instead, the flow visualization indicates that the shear layers of the jets penetrate to the central region and periodically pinch off regions of the potential-like cross-flow fluid where they meet. The pinch-off regions of cross-flow fluid are convected by the turbulent flow for large distances, yet remain essentially unmixed.     

Optimization of mesh screen for enhancing jet impingement heat transfer

The variations of flow structure and heat transfer characteristics of impinging air jets with respect to mesh solidity are compared for two mesh screen locations at small nozzle-to-plate spacings. Results show that the uniform incoming flow structure produces higher heat transfer rates in the impingement region. The heat transfer enhancement largely depends on nozzle-to-plate spacing, mesh solidity, and jet Reynolds number. The mechanism of heat transfer enhancement is analyzed in light of the field synergy principle.     

Stereo-PIV measurements of vapor-induced flow modifications in confined jet impingement boiling

A single subcooled jet of water which undergoes boiling upon impingement on a discrete heat source is studied experimentally using time-resolved stereo particle image velocimetry (PIV). The impinging jet issues from a 3.75 mm diameter sharp-edged orifice in a confining orifice plate positioned 4 orifice diameters from the target surface. The behavior at jet Reynolds numbers of 5,000 and 15,000 is compared for a constant jet inlet subcooling of 10 °C. Fluorescent illumination allows for simultaneous imaging of both the flow tracers and the vapor bubbles in the flow. Flow structure, time-averaged velocities, and turbulence statistics are reported for the liquid regions within the confinement gap for a range of heat inputs at both Reynolds numbers, and the effect of the vapor generation on the flow is discussed. Vapor generation from boiling is found to modify the liquid velocities and turbulence fluctuations in the confinement gap. Flow in the confinement gap is dominated by vapor flow, and the vapor bubbles disrupt both the vertical impinging jet and horizontal wall jet flow. Moreover, vapor bubbles are a significant source of turbulence kinetic energy and dissipation, with the bubbly regions above the heated surface experiencing the most intense turbulence modification. Spectral analysis indicates that a Strouhal number of 0.023 is characteristic of the interaction between bubbles and turbulent liquid jets.     

Finite volume gravity currents impinging on a stratified interface

An experimental study of two-dimensional gravity currents impinging on a stratified interface in a two-layer stratified environment is presented. The gravity currents are created by the release of a fixed volume of dense fluid along a 6° slope. The effect of the stratified interface on the entrainment and mixing processes is quantified by the use of planar laser induced fluorescence. Moreover, to understand the entrainment process and to quantify the effect of the impingement on the internal structure of the gravity current, particle image velocimetry measurements are performed. Both instantaneous velocity and vorticity fields are quantified and averages are computed over 0.2 s. The measurements are centered on the impact region between the gravity current and the stratified interface. In previous experiments we have determined the entrainment rate and studied the internal structure for a gravity current created by a continuous source under the same experimental conditions. A thorough comparison of the two cases is provided.     

Pressure measurements on the impingement surface of sonic and sub-sonic jets impinging onto a flat plate at inclined angles

The flow field associated with a jet impinging onto a surface at an inclined angle is investigated using pressure-sensitive paint (PSP) and particle image velocimetry. The PSP yields continuous measurements of pressure on the jet impingement surface. The jet footprint on the impingement surface is visualized using the half-maximum pressure contour. The results indicate that the impingement angle of the jet is the dominant parameter in determining the footprint of the jet on the impingement surface. This contour is similar in shape to an ellipse that is created by projecting the nozzle through the impingement surface. The ellipse is centered at the location of maximum pressure and the width of the minor axis is just over one jet diameter. The location of maximum pressure is found upstream of the geometric impingement point and this location is a strong function of the impingement angle. A curve fit for the location of maximum pressure can be constructed using an exact solution of the Navier–Stokes equations for a non-orthogonal stagnation flow. The maximum value of pressure is a function of impingement angle and varies as the sine of the impingement angle squared; the maximum pressure is also a function of jet impingement distance. Using these results, a simple procedure for predicting the overall structure of the jet on the impingement surface is presented.