Exit Profile Research Articles

Effect of nozzle geometry and semi-confinement on the potential core of a turbulent axisymmetric free jet

The effects of nozzle geometry and confinement on the potential core and subsequent axial development of a turbulent axisymmetric air jet at a Reynolds number of 22 500 have been studied. Four jet exit conditions, namely, flat and fully developed velocity profiles for unconfined and semi-confined cases were investigated. Mean velocity and turbulence profiles were measured using laser-Doppler anemometry. Liquid crystal thermography used in steady state enabled optimal nozzle to plate spacing to be established for maximum heat transfer. Preliminary results presented here indicate that the length of the potential core is greater for the fully developed jet exit profile and is further extended by semi-confinement. The semi-confinement reduces the stagnation point heat transfer by up to ten per cent.

Effect of Nozzle Geometry on Air-Jet Texturing Performance

This paper systematically investigates the effect of a number of geometric parameters on the texturing performance of air-jet texturing nozzles. In order to facilitate the research, an air-jet texturing nozzle with a rectangular cross section has been developed. The texturing performance of the nozzles is assessed by means of process observations and on-line measurement of stabilizing zone tension, and also by measuring the in creased linear density of the yams on textured yarn samples. Furthermore, instability, elongation at break, and tenacity are measured, and texturing quality is judged by visual inspections and examination of scanning electron microscopy images of the textured yarns. Tension in the stabilizing zone, increase in linear density, and to a somewhat lesser extent instability are reliable measures of texturing quality. The best texturing comes from nozzles with a slightly diverging main channel and a single air inlet hole located far from the nozzle exit. A curved diverging exit profile is essential for successful texturing. The results of the tests to determine the effect of air inlet angk are inconclusive and require further investigation.

Optimisation of pyrolysis chamber design for filament pyrolysers

Improvements in the design of the pyrolysis chamber for filament pyrolysers are described. Model calculations of pyrolysis product diffusion characteristics have been used to identify some of the most important design factors involved. These are: minimal cross-sectional area over the filament exit region; minimum volume; minimum distance between the the filament and the chamber exit; smooth chamber exit profile; and a sharp taper between the filament tip and chamber exit. They should also be operable at ‘high’ pressures and constructed from materials that do not react with pyrolysis products.

Turbulent plane jet with and without confining end walls

THE published data on turbulent plane jets show variations in both the mean and turbulence quantities that in most instances have been attributed to differences in experimental conditions. The major influencing factors are the inlet geometry and aspect ratio of the nozzle, the nature of the exit velocity profile and the associated magnitude of the turbulence intensity, the Reynolds number at the nole exit, and the state of the ambient environment.1'2 However, one fundamental difference among plane jet apparatus that is not well documented concerns the use of confining end walls and/or a wall section at the plane of exit. This paper describes the characteristics of a plane jet issuing from a wall with and without confining end walls. The objectives are to provide a perspective for the comparison of existing data under these conditions and to identify the relative merits of the two configurations for the design of future experiments. Contents The plane jet flow facility consists of a centrifugal blower that supplies air via a 1.0-m diffuser section to a 400 mm x 600 mm x 12.5 mm settling chamber, followed by a corner section containing 90-deg turning vanes, a second shorter settling chamber, and finally a 40:1 two-dimensional contraction (based on a cosine curve) that ends as a 600-mm long x 10-mm wide (D) slot. The design of the nozzle insures that the exit turbulence intensity is low [(u2)l/2/U0 = 0.008] and that the wall boundary layers at the exit will be laminar. The jet issues vertically from a horizontal wall section extending along the length of the slot and 1.1 m to each side and may be confined by removable vertical walls located at each end of the slot and extending 1.1 m to the sides and 1.85 m downstream, and vertical screens of open air ratio 0.8 spanning the ends of the vertical walls. The jet exit velocity U0 was 11.0 m/s for all of the test runs, and the Reynolds number based on U0 and the slot width D was 7230.3 Mean velocities were measured independently with a pitot tube (connected to an electronic micromanometer) and also simultaneously with a pitot tube and a normal hot-wire (HWA) separated by 5 mm in the spanwise (z) direction. In addition to near-field pitot tube measurements that confirmed the symmetry of the exit profile, detailed lateral velocity profiles were obtained in the far-field regions of the free and the confined jets to identify any structural differences and to determine the extent of two-dimension ality. The far-field profiles for the free jet show considerably more twisting and variability along the length of the slot than those of the confined jet. The walls of the confined jet achieved the expected effect of extending the region of two-dimension ality. For the streamwise region between x/D = 50 and 100, the standard deviation of the growth rates of the confined jet, based on

The Self-Purification Capability of Karst Underground Watercourses with Respect to the Pollution of the Mediterranean Sea

This paper has reviewed the results of the organic selfpurification process that occurs in karst underground watercourses. The investigations were carried out in two test reaches between 1974 and 1984. The first reach comprised the underground section of a river in an area of shallow karst in Slovenia, while the second one was in the deep karst area of Eastern Herzegovina. The fundamental studies of the change in the BOD loadings showed the value of the oxygen consumption rate coefficient in karst groundwaters, transferred to a temperature of 20° C, to be k1 (r) = 0.160 to 0.210. To determine the change in loadings, the Upstream - downstream method was applied, while the travel time was defined by using a tracer. An identical value of the deoxygenation coefficient was found when observing the changes in the daily loads at the exit profile only. This knowledge enables the practical application of coefficient k1 to estimate the input organic load in karst catchment areas, and also to define the required level of karst springs and coastal sea protection.

Nonlinear finite element analysis of the edge drop effect for cold rolling

A three-dimensional finite element model is presented in order to calculate the exit profile of a cold rolled strip in its edge domain. The method and the results are given. Conclusions concerning the edge drop effect are drawn.

Coannular jets—Are they really quiet and why?

The noise reductions from both normal- and inverted-velocity-profile (IVP) shock-free coannular jets are critically assessed on the basis of existing data. The fully mixed equivalent (FMEQ) single jet, defined as having a uniform exit profile and the same thrust, mass flow rate and exit area as the coannular jet, is used throughout as the reference for noise comparisons. It is shown that uninverted-velocity-profile coannular jets are consistently noisier than FMEQ jets, whereas IVP jets are slightly quieter. The practical implications are discussed in detail. The “inverted velocity” effects are isolated experimentally from the “inverted temperature” effects and the resulting noise reductions or increases are explained by using the measurements of mean flow, turbulence structure, and source location in coannular jets.

Numerical Prediction of the Flow Field and Impingement Heat Transfer Caused by a Laminar Slot Jet

The effects of uniform suction and nozzle exit velocity profile on the flow and heat-transfer characteristics of a semiconfined laminar impinging slot jet were investigated numerically. The full Navier-Stokes and energy equations were solved using a hybrid or upwind finite-difference representation of the equations cast into their vorticity-stream-function form. The importance of the nozzle exit profile is shown by comparison of the computed heat-transfer distribution with the available experimental data in the laminar range. Application of suction at the impingement surface is shown to enhance the local heat-transfer rates by a constant amount. The nondimensional heat-transfer coefficient and skin friction at the plate are computed as functions of the nozzle Reynolds number, the suction rate, and the nozzle velocity profile. The effect of temperature-dependent physical properties is included in the analysis.

Ghost peaks in a sector-focusing magnetic β-ray spectrometer

Zero-, single- and double-loop trajectories in a magnetic sector field, H = H φ ∼ 1 r , have been investigated. The focusing of single- and double-loop trajectories has been calculated for a K = 0.82 β-ray spectrometer with a symmetric entrance and exit profile shape. The trajectories are represented graphically. The counting ratio N 1/ N 0 of single-loop electrons/zero-loop electrons has been measured depending on the spectrometer entrance angle ψ s and the detector width Δz. An important decrease of N 1/ N 0 was achieved by a radial baffle S r placed in the central plane z = 0 of the spectrometer gap. The maximum length Δr max ( Δz, ψ s) of S r was found from the condition that the transmission T 0 ( Δz, ψ s) of the zero-loop electrons is unaffected by S r. Minimum obtainable values (N 1/N 0) min = ƒ min (Δz, ψ s ) are reported.

Jet Reattachment to Walls of Various Shapes

Velocity fields for jets reattaching to walls of various shapes (curved and inclined straight), setbacks, and control port conditions were examined experimentally for the case of undeveloped velocity profiles at the nozzle exit. A canting of the nozzle exit profile occurred for cases where the wall was set back from the lip of the nozzle exit. Significantly different growth characteristics were observed for the case of the inclined straight wall, relative to the curved walls, due to the sharp turning angle which the jet was required to negotiate in the vicinity of the nozzle exit. For each jet, a well-defined settling length was noted by plotting the position of the maximum velocity versus arc length from the nozzle exit. Whenever possible, results were compared with previous investigations for fully developed conditions at the nozzle exit. Off-midplane total pressure profiles were taken for the inclined straight wall and a curved wall at several arc lengths, and three-dimensional effects were found to be significant for arc lengths and nozzle aspect ratios of interest in digital flueric amplifiers.

Effects of Gravity and Surface Tension Upon Liquid Jets Leaving Poiseuille Tubes

A method is developed for approximating the transverse velocity component in the incompressible two-dimensional boundary layer equations. The method is restricted to flows that are symmetrical in the transverse coordinate, and it facilitates easy numerical integration in such problems as the prediction of jet and wake flows. The prediction of velocity profiles and diameters for free exiting Poiseuille flows, under the influence of both gravity and surface tension, is then undertaken. Analytical results obtained by the present method are found to agree very closely with experiments. The experiments also show that, in low Weber number situations, contact angle at the exit plane can dominate the early relaxation of the exit profile.