Impingement Wall Research Articles

Wall Geometry Effects on Atomization Characteristics of an Impingement-Jet Type Jet Engine Fuel Injector

Effects of a wall impingement jet on atomization characteristics of a jet engine fuel injector were investigated experimentally. Liquid is injected from 0.2 mm-diam nozzles and the liquid columns impinge onto a wall. The liquid forms thin films on the impingement wall and fine droplets can be produced. Several kinds of impingement wall geometries, such as slope, parallel and cylinder types, were examined and the droplet size and atomization mode were investigated. The droplet size was measured with an LDSA. For the slope type, two kinds of atomization mode, namely, “air blast atomization” and “pressure atomization” modes were observed. The pressure atomization mode is preferable, since it leads to the atomization improvement of a jet engine in low load operations. For parallel types, the atomization characteristics were similar to that for the slope type. For cylinder types, pressure atomization mode can be realized with a small liquid injection pressure. And the droplet size in the pressure atomization mode is smaller when compared with the parallel types.

2噴口不足膨張衝突噴流に関する研究

Twin underexpanded impinging jets are experimentally and numerically studied in this paper. The experiments were performed by two dimensional wall static pressure measurements and by the schlieren method. Numerical calculations were also conducted by solving three dimensional compressible Navier-Storkes equations in generalization curvilinear coordinate. The main parameters for the twin jets are the nondimensional distance between the two nozzle centers H/D covering 1.5 and 2.0, the nozzle to plate separation L/D 2.0 and 3.0 and the pressure ratio defined by p0/p 2.5–5.0, where D is the diameter of each nozzle exit, p0 the stagnation pressure and p the back pressure. It is found that, so called, a fountain flow which occurs at the middle position between the two jets breaks jets boundaries due to its unsteadiness. Comparison between the experiments and the calculations shows that in case of shorter distancce between two nozzle and higher pressure ratio, two jets interact before they reach the impinging wall causing clear pressure peak at the middle position between the jets.

Atomization of a High Speed Liquid Jet by Wall Impingement

Atomization characteristics of a liquid film were investigated, experimentally. A liquid jet emerging from a straight-type hole nozzle is impinged onto a solid wall, forming a liquid film. Effects of three independent parameters were investigated, namely, injection pressure P0, impingement angle α and impingement wall diameter d. The liquid film velocity V2 was measured with a PDPA. The SMD of the droplets was measured with an LDSA. It is shown that the SMD decreases linearly with the increase in the liquid film velocity V2, and that the gradient of the line decreases with the increase in the impingement angle α. For the case that the impingement angle α is 30 deg, the SMD of the droplets is proportional to V2-0.63. For the case that the impingement angle α is 90 deg, the SMD of the droplets is proportional to V2-0.51. For the case that V2<300m/s, the SMD of the droplets is smaller than that for a liquid jet. It is shown that by use of a liquid film, atomization is enhanced compared with a liquid jet.

Direct numerical simulation of turbulent heat transfer in plane impinging jet

The objectives of this study are to investigate the structures and characteristics of flows and heat transfer for plane turbulent impinging jets in a confined space, where a wall-jet type of flow is influenced by both lower (impingement) and upper walls, and to obtain turbulent statistics for the construction of a turbulent heat transfer model. For these purposes, we have performed direct numerical simulations (DNS) of impinging flows with heat transfer. To obtain the effects of an impingement distance on heat transfer, the value of H is changed in calculations, where H is the distance between the impingement wall and the upper wall. It is found from the present DNS that the Nusselt number increases with a decrease in the distance H similar to the experimental results. In addition, the Nusselt number of the case with the highest distance decreases monotonously in the wall-jet development direction, while in shorter distance cases the second peak of the Nusselt number is clearly observed away from the stagnation point, due to the increase in wall-normal turbulence intensities in the region away from the stagnation point.

Enhanced Impingement Heat Transfer: The Influence of Impingement X/D for Interrupted Rib Obstacles (Rectangular Pin Fins)

Impingement flat wall cooling, with 15.2 mm pitch square hole arrays, was investigated in the presence of an array of interrupted rib obstacles. These ribs took the form of rectangular pin-fins with a 50% blockage to the cross flow. One side exit of the air was used, and there was no initial cross flow. Three hole diameters were investigated, which allowed the impingement wall pressure loss to be varied at constant coolant mass flow rate. Combustor wall cooling was the main application of the work, where a low wall cooling pressure loss is required if the air is subsequently to be fed to a low NOx combustor. The results showed that the increase in surface average impingement heat transfer, relative to that for a smooth wall, was small and greatest for an X∕D of 3.06 at 15%. The main effect of the interrupted ribs was to change the influence of cross flow, which produced a deterioration in the heat transfer with distance compared to a smooth impingement wall. With the interrupted ribs the heat transfer increased with distance. If the heat transfer was compared at the trailing edge of the test section, where the upstream cross flow was at a maximum, then at high coolant flow rates the increase in heat transfer was 21%, 47%, and 25% for X∕D of 4.66, 3.06, and 1.86, respectively.

Direct Numerial Simulation of Turbulent Heat Transfer in Plane Impinging Jet (Effects of Impingement Distance on Heat Transfer in Confined Space)

The objectives of this study are to investigate the structures and characteristics of flows and heat transfer for plane turbulent impinging jets in a confined space, where a wall-jet type of flow is influenced by both lower (impingement) and upper walls, and to obtain turbulent statistics for the construction of a turbulent heat-transfer model. For these purposes, we have performed direct numerical simulations (DNS) of impinging flows with heat transfer. To obtain the effects of an impingement distance on heat transfer, the value of H is chasged in calculations, where H is the distance between the impingement wall and the upper wall. It is found from the present DNS that the Nusselt number increases with a decrease in the distance H similar to the experimental results. In addition, the Nusselt number of the case with the longest distance decreases monotonously in the wall-jet development direction, while the second peak of the Nusselt number is clearly observed away from the stagnation point in shorter distance cases, due to the increase in wall-normal turbulence intensities in the region away from the stagnation point.

Parametric Study of Heat Transfer in Turbulent Gas−Solid Flow in Multiple Impinging Jets

Heat transfer in a dilute gas−particle suspension flow in multiple-slot impinging jets has been numerically investigated using a new Eulerian−Lagrangian method that accounts for conduction heat transfer due to particle−wall collisions. The numerical results are compared with available experimental data. Good agreement between experiment and simulation is obtained only if the additional conduction heat transfer due to particle−wall collisions is considered in the model. The effects of particle diameter, particle material properties and loading ratios, jet Reynolds number, impingement wall temperature, and direction of the gravitation vector on heat transfer are discussed.

Annular synthetic jet used for impinging flow mass-transfer

An annular synthetic jet was investigated experimentally, both with and without an opposing impingement wall. The experiments involved smoke visualization and mass transfer measurement on the wall by means of naphthalene sublimation technique. Two qualitatively different flow field patterns were identified, depending upon the driving amplitude level. With small amplitudes, vortical puffs maintain their identity for a relatively long time. If the amplitudes are large, breakdown and coalescence of the vortical train is much faster. Also the resultant mass transfer to the impingement wall is then much higher. Furthermore, a fundamental change of the whole flow field was observed at the high end of the investigated frequency range, associated with radical reduction of the size of the recirculation bubble.

Numerical Investigation of Impingement Cooling in Ribbed Ducts Due to Jet Arrays

A numerical investigation of impingement heat transfer and fluid flow in a rectangular two-pass duct was carried out. The impingement wall was smooth or with periodically mounted square ribs. A multiblock parallel CFD code was employed for the calculation. The turbulence modeling was treated by applying a low Reynolds number turbulence eddy viscosity model (LEVM) and a V2F model. First, the calculations were validated against available experimental data in a two-pass duct where the impingement wall was smooth. Then, jets impinging on ribbed walls were simulated. The distance from the nozzle to the impingement wall was fixed at four nozzle diameters (d). The square rib width was set as 0.5 d. The duct inlet Reynolds number was fixed at 10,000. Results show that the presence of ribs was favorable for the heat transfer enhancement on the impingement wall, but not for the side wall.

Mean flow and turbulence measurements of the impingement wall jet on a semi-circular convex surface

The detailed mean flow and turbulence measurements of a turbulent air slot jet impinging on two different semi-circular convex surfaces were investigated in both free jet and impingement wall jet regions at a jet Reynolds number Rew=12,000, using a hot-wire X-probe anemometer. The parametric effects of dimensionless circumferential distance, S/W=2.79–7.74, slot jet-to-impingement surface distance Y/W=1–13, and surface curvature D/W=10.7 and 16 on the impingement wall jet flow development along a semi-circular convex surface were examined. The results show that the effect of surface curvature D/W increases with increasing S/W. Compared with transverse Reynolds normal stress, \( \overline {v^2 } /U_{\rm m}^2 \) , the streamwise Reynolds normal stress, \( \overline {u^2 } /U_{\rm m}^2 \) , is strongly affected by the examined dimensionless parameters of D/W, Y/W and S/W in the near-wall region. It is also evidenced that the Reynolds shear stress, \( - \overline {uv} /U_{\rm m}^2 \) is much more sensitive to surface curvature, D/W.

Large Eddy Simulation of a plane impinging jet

Large Eddy Simulations of a plane turbulent impinging jet have been carried out using the dynamic Smagorinsky model. The statistical results are first validated with the measurements from the literature: mean and turbulent quantities along the jet axis and at different vertical locations are presented. This study is completed by the analysis of the wall shear stress at the impingement wall. The effect of the jet Reynolds number (3000⩽Re⩽13500) on the kinematic development of the jet is also discussed. To cite this article: F. Beaubert, S. Viazzo, C. R. Mecanique 330 (2002) 803–810.

Two-dimensional laser induced fluorescence measurement of spray and OH radicals of LPG in constant volume chamber

It is recognized that alternative fuels such as liquid petroleum gas (LPG) have less polluting combustion characteristics than diesel fuel. In this study, LPG was injected into a high pressure and temperature chamber to reproduce the stratification processes in an engine. The spray images were taken by the use of a PLIF method with Nd:YAG laser to analyze their penetration and evaporation characteristics. Also the characteristics of combustion were investigated by simultaneous visualization of OH radical and flames. The results show that the mixture moves along the impingement wall that reproduced the piston bowl and reaches the ignition spark plug. Also, OH fluorescence rises sharply and then decreases gradually when the combustion is carried out actively.

Conjugated Heat Transfer on a Horizontal Surface Impinged by Circular Free-Surface Liquid Jet.

Analytical research was conducted to study the heat transfer from horizontal surfaces to normally impinging circular free-surface jets under arbitrary-heat-flux conditions. General expressions of heat transfer coefficients and recovery factor were obtained in the stagnation region and boundary layer region as well as viscous similarity region by the integral method. Then by using the analytical results, the conjugated heat transfer between a water jet and the impinged wall was numerically solved. Finally, an experimental study was also performed to characterize conjugated heat transfer coefficient on a thick copper target base impinged normally by circular free-surface jet. The present predictions of the mean conjugated heat transfer coefficient were good agreement with the experimental data.

Near Wall Measurements for a Turbulent Impinging Slot Jet (Data Bank Contribution)1

The velocity field in the vicinity of a target surface with a turbulent slot jet impinging normally on it is examined. The impingement region is confined by means of a confinement plate that is flush with the slot and parallel to the impingement plate. The distance H of the impingement wall from the slot is varied from 2 to 9.2 slot widths. Jet Reynolds numbers (based on slot width B) of 10,000–30,000 are considered. Mean velocity and root mean square velocity measurements are carried out using hot-wire anemometry. A boundary layer probe is utilized in order to obtain measurements at a wall distance as close as 110 microns 0.0028B. This corresponds to a distance of approximately y+∼2-4 in wall units and is found to be adequate in order to permit an estimate of wall shear under most conditions. The problems of hot wire interference with the wall and calibration at low velocities are solved by calibrating the probe in a known Blasius flow. With the exception of the stagnation region where shear could not be evaluated, it is found that velocity profiles follow a linear behavior in the viscous sublayer everywhere along the wall. Results indicate that the peak in normal stress occurs at y/B∼0.025 to 0.04 at a distance six to eight jet widths away from the jet-axis.

Characteristics of collision spray of fuel jet using ringed impingement wall head

Characteristics of collision spray of fuel jet using ringed impingement wall head

Study of Several Jets Impinging on a Plane Wall: Visualizations and Laser Velocimetry Investigations

Different experimental techniques have been used in order to describe the flow pattern generated by fifteen jets impinging on a plane wall. The spreading over method revealed the jet influence on the impinged surface, particularly reattachment and detachment lines. The laser sheet visualizations show complex vortical structures such as the ground vortex and secondary rolling-ups near the target plate have been deduced. Velocity measurements have been realised to confirm these observations and to specify the flow pattern. A schema of the flow near the impinged wall is thus drawn.

Numerical Analysis of an Axisymmetric Impinging Jet in the Impingement Region.

An axisymmetric impinging jet onto a flat wall has studied using a numerical model of incompressible viscous steady flow, especially close to the impingement wall. The Navier-Stokes equations are solved in terms of stream function and vorticity with the combined upwind and central finite difference method of an alternating direction implicit way. Distributions of velocity and pressure of the impinging jet are calculated at several large Reynolds numbers using non-uniform grids in the axial direction of the jet for three kinds of fully developed velocity profile at the inlet of the impingement region. Calculated distributions of velocity and pressure agree well with experimental data of impinging air jets. More over, the effect of the inlet velocity on the flow and the flow structures of impinging jets are discussed.

CHARACTERISTICS OF A DEVELOPING DROPLET - LADEN JET IMPINGING ON A NORMAL WALL

The size-velocity characteristics of a droplet-laden air jet impinging on a normal wall is investigated using a phase-Doppler anemometer. The results of measurements conducted for two jet-exit-to-impingement-wall distances are presented. Experimental data are obtained across the jet at several axial stations covering the first ten jet-exit diameters. The radial variation of droplet mean velocity, rms velocity and skewness coefficient indicates that similarity is not achieved. The rate of spreading of the jet is higher than that noticed in both single-phase flow and previous two-phase flow studies. The presence of the impingement wall does not have a significant effect on the mean velocity distributions for Z/Zi &lt; 0.8. There is an increase in droplet size with increasing distance from the nozzle for Z/D &gt; 4.0. In the early regions of the jet, the water droplet size and velocity are strongly correlated, especially in the core regions. With increasing Z/D, there is no association between size and velocity.

Thermofluid experiments on ingress of coolant event

One of the serious thermofluid safety issues in a fusion experimental reactor is an ingress of coolant event (ICE), when the water in a coolant pipe is ejected into the vacuum vessel (VV) if an accident occurs, such as a runaway electron impact on the coolant pipes. If the VV cannot withstand the high pressurization caused by water evaporation in the VV, and on the high-temperature plasma-facing components, the steam containing activated dusts could be transferred from the VV to the environment. Therefore, it is important to understand the entire thermofluid characteristics during ICE conditions. This paper describes the first set of ICE experimental results, such as pressure transients in VV and transient temperature distributions on a water-jet impingement wall. It was found that pressurization under vacuum conditions, as an initial pressure condition in VV, is very different from that under atmospheric pressure conditions when the VV is filled with air at 0.1 MPa. The reasons for this discrepancy and some heat transfer characteristics on the target wall are discussed.

Behavior of Impingement Spray. 1st Report. Wall jet Behavior and Internal Structure of Spray.

In this paper, behavior of a diesel spray impinging on an inclined wall was experimentally investigated in a pressurized vessel. In order to clarify the wall effect on a diesel spray structure, a relative angle of the inclined wall to the spray axis was varied. Spray penetration along the wall was observed optically and compared with that of a free spray. To observe an internal structure of the spray, it was visualized by a YAG laser sheet light and its tomographic image was captured on a film. The photo-image on a film was taken into an image analyzing computer using a high resolved image scanner. High density ridges in the tomographic image were extracted to clarify the internal structure of an impingement spray. Further, the growth characteristics of impingement spray was observed by a high speed photograph. To evaluate various spray motion quantitatively, a spray path penetration which described a development of a spray tip along the wall was used. As the result, the slip flow region of spray was found near the impingement point on the wall. It seemed that the injected fuel does not make a mixture and slipped on the wall surface in this slip region. The length of this region was measured in various impingement wall condition.