Single Round Jet Research Articles

Characteristics of Radial Jets and Mixing under Buoyant Conditions

This paper reports on measurements of radial jets using a laser-doppler anemometer and compares the mixing efficiency of a single radial jet with that of a single round jet under buoyant conditions based on experimental data. Dimensional analysis yields two parameters for the calculation of initial dilutions of jets and buoyant plumes. It is noticed that initial dilution of radial jets is much higher than that of round jets having the same source discharge, source buoyancy flux, outlet velocity and area because of intensive entrainment of the radial jet in the zone near the discharge point. Theoretical analysis indicates that the central velocity of the radial jet decays with jet path at a rate of −n ( n > 1), the value of which is proportional to the relative turbulence intensity and hence is related to the geometry of the radial jet. Experimental results show that in most situations, the value of n is approximately 1.5. Measurements also show that the ratio of half-width to distance from the virtual origin of the radial jet is given by b w /r0= 0.101 which is close to that of round jets. However velocity profiles of the radial jet are not completely similar, although the entrainment coefficient is basically a constant of 0.102. These results can help to predict correctly the flow field of the radial jet and its initial dilution.

COMPUTATIONAL STUDIES OF IMPINGING JETS USING K- 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.

COMPUTATIONAL STUDIES OF IMPINGING JETS USINGK-ε TURBULENCE MODELS

We report numerical modelling of impinging jet flows using Rodi and Malin corrections to the k-e 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.

Computational analysis of a single jet impingement ground effect lift loss

A study was carried out on single round jet ground effect lift loss. The jet exit Mach number and velocity were 0.71 and 240 m/s, respectively. The effects of ground height, baffle plate edge, and jet exit turbulent intensity were assessed, and Navier-Stokes equations were solved using computational fluid dynamics. Turbulence closure was achieved using a k-e model, and the result compared with calculations obtained with a differential stress model. Three baffle plate edges were tested (rounded, squared, and chamfered), and the ground heights varied from rj = 0.15 to 0.8. It was found that flow mechanisms varied significantly with ground heights. A coherent vortex existed between the baffle plate and the ground at the low ground heights (17 0.25), the vortex disappeared and separation at the plate edge played an important part in determining the lift loss. The baffle plate edge was found to account for as much as 14% of the ground effect lift loss. The stress model was found to improve the accuracy of the prediction.

Study of double concentric jets with different geometric configurations

The flow field of two different geometric configurations of a double concentric jet has been studied. This jet consisted of a round central jet emerging from a circular nozzle and an annular jet emerging from an annular nozzle which encircled the former. In the basic configuration, the annular jet issued parallel to the circular jet. In the other configuration, which is called the inclined configuration, the annular jet issued at an inclining 15° to the circular jet. Hot wire anemometers were used to study the structure of the combined flow. The measured quantities included mean velocity, and longitudinal and lateral turbulence intensities. It was found that the combined flow of both the central and the annular jets can be simulated by the flow of an equivalent single round jet. The study indicated that the inclined jet developed at a faster rate than the basic jet. The mean velocity profiles of the basic jet showed a good agreement with a typical single round jet. Some agreement with a single jet was observed for the longitudinal turbulence intensity profiles of the inclined jet. Study of the lateral turbulence intensity profiles of both jet configurations showed poor agreement with those of a single jet. No agreement was found between the turbulent kinetic energy profiles and those for a single jet.

Prediction of stagnation point heat transfer for a single round jet impinging on a concave hemispherical surface

This paper deals with a systematic procedure for assessment of fluid flow and heat transfer parameters for a single round jet impinging on a concave hemispherical surface. Based on Scholkemeier's modifications of the Karman-Pohlhausen integral method, expressions are derived for evaluation of the momentum thickness, boundary layer thickness and the displacement thickness at the stagnation point. This is followed by the estimation of thermal boundary layer thickness and local heat transfer coefficients. A correlation is presented for the Nusselt number at the stagnation point as a function of the Reynolds number for different non-dimensional distances from the exit plane of the jet to the impingement surface.

Planar imaging of jet mixing in crossflow

AUNIQUE experiment has been developed for the study of jet mixing in crossflow. The scalar mixing of a round jet discharging normally into a ducted flow is studied through planar nephelometry. Simultaneous, multiple-point fluid concentration measurements are made for a variety of flow conditions in a simulated gas turbine combustor. The field measurements are in good agreement with the point-to-point measurements of other investigators. The potential for studying more practical, multijet configurations is evident. Contents Jet engine turbine lifetimes depend critically on the gas temperature distribution at the turbine inlet. The gas leaving the combustor primary zone must be cooled and the temperature profile flattened in order to preserve turbine integrity. Cooling is usually achieved by injecting compressor discharge air normal to the hot flow through holes in the combustor liner. The size, geometry, and location of these holes control the combustor exit temperature distribution. An improved understanding of mixing in crossflow is, therefore, of significant practical importance. A new approach to studying mixing in crossflow is described below. A generic experiment consisting of planar digital imaging of a single round jet discharging into a uniform crossflow is described. The study of more complex, multijet configurations is in progress. Experiment The configuration chosen for study represents a sector of an annular gas turbine combustor. In these initial experiments, the geometry and flow conditions at the boundaries are simplified to facilitate the interpretation of the data and to allow comparison of the results with those of other investigators. The apparatus is shown schematically in Fig. 1. The working section consists of three parallel contiguous ducts of rectangular cross section, simulating a sector of an annular gas turbine combustor. Sector width is 30.5 cm. The inner combustor duct is 10 cm high, and side viewing is allowed through a 30.5 cm X 10 cm x 0.16 cm pyrex window. The outer ducts, which supply the injectant gas and simulate gas turbine combustor shroud passages, are each 2.54 cm high and fed by a two-dimensional 2:1 ratio contraction nozzle to provide a more uniform approach flow. These are separated from the inner duct (combustor) by removable 3-mm-thick flat-plates. The injectant is fed to the combustor through holes of various sizes and shapes that are machined into the plates. In the present experiment, the injectant (air) was fed through a single round hole in the upper plate only. The hole was beveled inward at 40 deg and brought to a sharp edge at the discharge plane. The jet velocity was varied independently of the shroud velocity by regulating pressure drop across the

Calibration studies of the drum impactor

Calibration studies were performed on the Davis Rotating-drum Universal-size-cut Monitoring (DRUM) sampler utilizing fluorescent monodisperse aerosols of polystyrene latex. The DRUM sampler is an eight-stage Lundgren-type rotating drum cascade impactor utilizing a series of single round jets for each stage including low-pressure stages. The DRUM sampler operates continuously for up to 2 weeks with particle collection on slowly rotating, greased substrate-coated cylindrical drums. The resulting narrow linear deposit for each stage can be evaluated with time resolution as short as 4 h utilizing Particle Induced X-ray Emission (PIXE) spectroscopy and other analytical methods. The DRUM sampler is particularly useful for evaluating environmental aerosols for size-related chemical properties. Aerosols of monodisperse fluorescent polystyrene latex spheres were generated by nebulization from dilute aqueous suspensions, dried with a diffusion drier, and reduced in electrostatic charge with a krypton-85 discharger. The aerosols were characterized by electron microscopy and with a Climet-208 light scattering particle counter; they were quantified utilizing the fluorescent label. DRUM samples were collected at selected flow rates up to the critical 18.3 cm 3 s −1, and the square root of the non-dimensionalized Stokes' number was plotted vs collection efficiency for the impactor stages. The results were in reasonable agreement with the expected effective cut-off aerodynamic diameters based on the non-dimensionalized theory of Marple and with the data of Mercer. All the stages demonstrated dynamic similarity when the Stokes' numbers were calculated at the reduced pressure and temperature at orifice outlets with orifice areas corrected for contraction at sonic and near sonic speeds. The measured cut-off values for another low pressure impactor were verified with these results. A wall loss study was conducted using polydisperse respirable aerosols of ammonium fluorescein generated by nebulization with a Lovelace nebulizer, dried with a diffusion drier, and reduced to Boltzmann equilibrium with a krypton-85 discharger. The results show wall losses to be about 10.7% of the sample.

Acoustic and Flow Characteristics of Cold High-speed Coaxial Jets

Far-field noise of model coaxial supersonic cold jets exhausting from a coplanar convergent two-nozzle coaxial configuraton was investigated. The pressure ratio (velocity) of the outer annular jet was maintained higher than the inner round jet. For each outer-jet pressure ratio, the inner-jet operating pressure ratio for the minimum radiated noise was established. Experimental results on the noise reductions and the modifications of the shock structure and velocity distribution of coaxial jet flows at the minimum noise conditions are presented and are compared with those of a single convergent underexpanded round and annual jet at a wide range of operating pressure ratios.