Free Jet Research Articles

Comparison of flow characteristics of plane jet impingement on a solid plate and on a sand bed

As compared to the well-researched case of a plane jet impinging on a solid plate, relatively rare attention was paid to the impingent on an erodible sand bed, which induces continuous bed transformations and interactively affects the jet development. The present study measured the flow of an impinging plane jet on a solid plate and on an erodible sand bed, respectively, by using particle image velocimetry technology, and then comparatively investigated the flow structure, main jet development and downstream wall jet development for the two cases. The results revealed that the jet impingement on the sand bed has a longer free jet region than that on the solid plate due to enlarged separation distance induced by localized scouring. The width of the plane jet impinging on the sand bed is larger than that on the plate by as high as five times the nozzle width, due to intensified interactions with the complex vortical structures in the concave scour hole. The impinging angle even decreases to negative values near the sand bed due to upward deflection of flow induced by bedform transformation. For both cases, however, the normalized streamwise mean velocity profiles exhibit universal self-similarity at different zones of jet development: specifically, the main jet and the wall jet satisfy exactly the same exponential function and the power law function, respectively.

Effect of inflow disturbances in Pelton turbine distributor lines on the water jet quality

The proper design of the manifold, distributing the flow to the individual nozzles, is crucial for the Pelton turbine performance. The generation of both energy losses and secondary flows must be reduced to a minimum to provide optimal water jet quality and, hence, efficiency. However, the inflow state due to the penstock is commonly disregarded at the design stage. By numerical flow simulation, we show how such penstock perturbations affect the water jet cross-section and axis deviation by considering different pipe bends upstream of the manifold. The two-phase flows resulting in a typical Pelton turbine manifold are predicted using the Volume-Of-Fluid (VOF) approach in conjunction with the k−ω SST turbulence closure model. Pipe bends are attached five manifold inlet diameters upstream of the manifold itself. The consequences on the secondary flows in the nozzles and the free water jet quality are reported quantitatively. The analysis shows that even small inflow perturbations into the manifold due to penstock piping significantly affect the water jet quality. A yet unreported mechanism is revealed, where the secondary flows mitigate the impact of the flow rate imbalance across the nozzle on the water jet deviation.

Optical temperature measurement in unsteady plasma free jet

An Argon plasma free jet is investigated using spectrally narrow bandpass filtered high-speed imaging. The images were captured at 16 kHz with an exposure time of 3.9 μs and then calibrated for absolute radiance. The free jet exhibited behaviour consistent with turbulent free shear flow and maintains an axisymmetric shape. Significant local fluctuations were observed over time, growing in strength and size as the flow convected downstream. Assuming local thermodynamic equilibrium and self-similar free-jet temperature profiles, the flow radiance is used to determine the local plasma temperature and the jet width. Then, both steady and unsteady flow models were applied to account for the jet fluctuation. In regions of low fluctuations near the nozzle exit, both models show good agreement for centreline temperatures, measuring approximately 11 200 K. In regions of significant fluctuations, the assumption of steady flow leads to an overestimation of 32% for temperature, 18% for jet width, and 41% for total jet power. The unsteady analysis approach results in lower temperatures and smaller jet widths while simultaneously satisfying momentum and energy conservation.

Numerical Simulation Study of Energy Separation in Jet Shear Layer

In order to investigate the rational utilization of energy in the fluid jet process, a free dual jet of parallel air is used as a research object. Simulation of turbulence was carried out based on the realizable k-epsilon equations with pressure-velocity coupling, PISO method, discrete method with second-order windward format, and first-order implicit transient solution. The energy separation phenomenon within the jet shear layer and its influencing factors were investigated under dual-jet Reynolds number differences of 12,800, 19,200, 25,600, 32,000, 41,500, and 51,100, respectively. The simulation is in transient format and the jet inlet velocity is given by udf. Calculations show that the pressure perturbation in the air shear layer, which gives rise to the pressure work exchange between the jet and the surroundings, is the main reason for the coexistence of high- and low-temperature regions formed within the jet shear layer, and the larger the Reynolds number, the stronger the energy separation effect. The center streamline path of a parallel air free dual jet can be fitted by a circular arc, whose radius of curvature increases with the Reynolds number, and the curvilinear development path of the dual jet can offset the energy separation effect to some extent. This paper confirms the existence of energy separation in the shear layer of a parallel air free double jet, reveals the mechanism of energy separation in the fluid domain, and explores the flow characteristics of two parallel jets.

Numerical modeling of the two-dimensional free and impinging jets with solid and erosion bed using FLOW-3D

ABSTRACT This study simulates the flow properties of a plane-layered jet in free and impinging jets on a horizontal surface using Flow-3D and validates them with previous research and experimental data. The flow characteristics, such as velocity and shear stress distribution, are compared with theoretical relationships. The simulations vary the outlet velocities for the free jet and the surface types and slip conditions for the impinging jet. The results show that the free jet flow follows the self-similar theory, except for a faster decay of numerical velocities at the end of the jet’s path. The results also reveal a virtual origin (λ) before the nozzle outlet, ranging from 7.6 to 8.4 mm. The impinging jet results match the experimental wall shear stress distribution diagram, with the peak at y/H = 0.14. The transverse velocity profiles and the wall shear stress distribution of the impinging jet are influenced by the slip condition and the surface type, but the difference between smooth and granular surfaces is less than 3%. This study offers valuable insights into the behaviour of free and impinging jets on a horizontal surface and shows the effectiveness of the Flow-3D numerical model.

Comparative research and experimental verification of <?A3B2 pi6?>adjustable nozzle technology for free jet

Comparative research and experimental verification of <?A3B2 pi6?>adjustable nozzle technology for free jet

Small clusters formation in supersonic jets: Theory and experiment

Computational and experimental study capabilities of clusterization in rarefied free argon jet are discussed. Several small cluster formation mechanisms and corresponding computational models are proposed. Simulation results for free jet flows with and without cluster formation are presented and computational and experimental results comparison possibility is discussed.

Experimental study of breakup and atomization characteristics of liquid carbon dioxide jet under high-pressure environment

Considering the working conditions of a Mg/CO2 rocket engine for Mars exploration, an experimental investigation was carried out to study the breakup and atomization properties of a liquid CO2 jet in a high-pressure environment. It is shown that the pressure ratio has a significant effect on the location and process of jet evaporation, resulting in significant difference in jet morphology and breakup characteristic. Two dividing points were identified, one near the CO2 triple point and the other near the saturation point. As the pressure ratio increases, the thermodynamic driving force for jet breakup weakens, leading to larger droplet sizes. The breakup length of the free round jet and Sauter mean diameter are functions of the Weber number and Jacob number, which are influenced by vapor content, injection velocity, and gas-liquid density ratio, in the mass flowrate range of 5.9–9.1 g/(mm2·s). A thermodynamic mechanism dominates when the ambient pressure in the atomize room is low enough, resulting in an unimodal distribution of droplet sizes. When the ambient pressure is in the range of 1–2.5 MPa, both thermodynamic and mechanical breakup mechanisms are active, resulting in a bimodal distribution of droplet sizes.

The influence of gas heating on the calculated supersonic parameters of gas-powder flow at slag blower in the converter. Message 1

The article shows that one of the priority tasks in steel production technology is to increase the durability of the lining of oxygen converters. It is shown that the slag blower is a radical way to increase the durability of the lining of oxygen converters. When blowing the melt with oxygen, the MgO content in the slag is 6-8%. To increase the chemical affinity of the slag and the lining, it is proposed to blow the converter slag with jets of nitrogen or powder gas. To achieve optimal chemical affinity of the lining and slag, it is recommended to modify the latter by increasing the content of magnesium oxide in it to 12-14%. Increased durability of the lining makes it possible to solve the problem of replacing wasteful wastes of coal slag and changing environmental concerns on the excess media. It is shown that an attempt to increase the efficiency of modeling very complex processes of interaction of supersonic jets with converter slag has not yet been solved a. A critical analysis of existing methods for constructing mathematical models based on the use of known gas-dynamic laws of interaction of free turbulent jets with the melt is carried out. It has been established that only supersonic jets with their characteristic shock-wave structure always enter the slag melt. An integral method for calculating the parameters of a gas-powder flow in a supersonic nozzle taking into account the powder concentration is presented. It is determined how heating of the gas suspension affects the required pressure in front of the nozzle block, the speed and flow density in the outlet section of the supersonic nozzle. The gas dynamics system is connected to the heat exchange of the flowing gas-dispersed mixture in the Laval expansion nozzles to establish the heating effect in front of the nozzle block of the two-phase gas suspension to a temperature that will ensure The increase in the fluidity of the gas flow is doubled and its kinetic energy is increased threefold. The method of expanding the parameters of the gas-powder flow between the Laval expansion nozzle involves the infusion of 10 physical effects into 14 parameters of the gas-powder flow. It is shown that numerical expansions allow the tuyere body to be used as a heat exchanger for heating the gas suspension, as well as for increasing the pressure of the gas-powder jet that flows from the nozzles

Pelton turbine jet flow investigations using laser Doppler velocimetry

Hydropower is a sustainable and clean renewable energy source. The Pelton turbine is an impulse type of hydraulic turbine and operates under high head and low discharge. It comprises an injector, which converts high-pressure water into kinetic energy as a high-speed free surface water jet that impinges on the runner generating power coupled with an electric generator. The water is highly accelerated in the nozzle, which produces turbulent kinetic energy in the jet flow. Before impinging on the runner, the surrounding air is entrained into the jet, which causes the jet dispersion. The effective conversion of water energy into mechanical energy and the quality of a jet depend significantly on the uniformity of the jet. However, the jet encounters instabilities and non-uniformity caused by secondary flow and turbulence. Jet surface and velocity distribution do not remain symmetric. Investigation of the jet's velocity profile and turbulence intensities is important to understand the flow behavior and improvements in the design of injector. In this study, the experimental investigation of the jet has been performed using the laser Doppler velocimetry (LDV) system. The turbulent intensity and instantaneous velocity of the jet are analyzed at different locations along the jet flow. The velocity wake zone occurs around the center of the jet, which diminishes in the axial direction of the jet, with non-dimensional jet velocities values 0.91, 0.92, and 0.94 at x/Do of 0.5, 1, and 2 for ∼98% nozzle opening. The secondary flow velocity magnitude is larger for 90°–60° injector jet flow than for 90°–50°. The jet velocity profile obtained from the LDV has been compared with computational fluid dynamics analysis.

Effect of velocity ratio and Mach number on thin lip coaxial jet

Abstract The effect of nozzle lip thickness and velocity ratio on coaxial subsonic jet mixing, at different Mach numbers, has been studied experimentally and numerically. Decay of coaxial subsonic jets emanating from coaxial nozzles of lip thickness 0.7, 1.7 and 2.65 mm with velocity ratio (VR) from 0.2 to 1.0 at primary jet exit Mach numbers of 0.6, 0.8 and 1.0 has been studied. Free jet without co-flow (VR0) was also studied for comparison. Jet centerline Mach number decay, turbulence and velocity variation in the radial direction are analyzed. The results show that mixing the coaxial jet at a low-velocity ratio is better than a high-velocity ratio, at all Mach numbers of the present study. The nozzle lip thickness has a significant influence on the secondary jet. Mixing of the jet in the presence of VR0.2 coaxial jet is found to be the highest. Characteristic decay of Mach 0.8 and 1.0 jet for lip thickness 1.7 and 2.65 mm is faster than lip thickness 0.7 mm. For a given lip thickness, increasing of velocity ratio is found to retard the mixing between primary and secondary jets.

Experimental study of the Reynolds stress distribution and its self-similarity in a semi-confined impinging jet

This paper investigates the Reynolds stress distribution and its self-similarity of a semi-confined impinging jet using the two-dimensional particle image velocimetry (2D-PIV) technology. The confinement is achieved by placing a plate parallel to the impinging plate at the nozzle exit. Nine sets of measurements are conducted at three impingement distances (4, 5, and 6 times the nozzle diameter) and three Reynolds numbers (5084, 8179, and 13,926). The spatial distribution and the self-similarity of Reynolds stresses are obtained. The results indicate that high turbulence occurs mainly in the free jet shear layer and the wall jet shear layer. The variation of Reynolds stresses with impingement distances and Reynolds numbers are determined. The Reynolds stresses in the radial wall jet region do not exhibit self-similarity in the outer-scaled coordinates. Scaling parameters based on Reynolds stresses are proposed, and the Reynolds stress velocity scales decay by a power-law function in the self-similar region, while the Reynolds stress length scales increase linearly. The Reynolds stresses in the radial wall jet region show extremely strong self-similarity using the Reynolds stress scaling parameters normalization. Finally, the corresponding scaling laws for Reynolds stresses are obtained.

Confinement Effects on Coaxial Jet Diffusion Flame

ABSTRACT A combined experimental and numerical study has been conducted to investigate the characteristics of confined coaxial jet diffusion flames in air coflow, using methane as fuel. The main concerns were focused on the unequal fuel velocity (0.1–72 m/s)/air velocities (0.045–0.53 m/s) and confinement ratio (δ*, defined as the ratio of the confinement tube diameter and the jet diameter), and most of the flow remains laminar while the ranges of δ* and gas velocities vary widely. The flame height linearly increases with the central fuel jet velocity, until the flame tip-opening or liftoff occurs, which is similar to that of the free jet flame. The critical fuel jet velocity for flame liftoff decreases with the ratio of air velocities to δ * . The critical fuel jet velocity of flame tip-opening increases with δ*, and the critical volumetric flow rate of air increases to a maximum, then decreases, indicating a combined effect of the oxygen supplement and the mixing with the fuel. Under smaller δ* with high jet flow velocity, a circular plane flame is observed before flame extinction. The formation of a recirculation zone in a confinement with combustion reaction has a significant influence on flame behaviors. A critical nondimensional Craya-Curtet number is used to predict the onset of flow recirculation with combustion reaction. At a certain coflow air velocity, the critical number initially increases and then remains constant with δ*, while at a certain δ*, the critical number decreases with air velocity. When the Craya-Curtet number is higher than the critical point, recirculation cannot be established, resulting in the formation of a tip-closed flame. With the decrease of the Craya-Curter number, the recirculation evolves to the upstream of the flame, then, the flame may be lifted off, along with the recirculation zone moving downstream. With the effect of recirculation, the stoichiometric mixture fraction contour shrinks toward the central axis where there is a higher local velocity. Therefore, the flame will be lifted off under a lower jet velocity in a smaller δ * . With the increase of the jet velocity, the Craya-Curtet number further decreases, and the recirculation becomes much stronger than the upstream inflow, resulting in the flame being compressed into a circular plane flame at a small δ* with a relatively high jet flow velocity or blowout directly at a relatively large δ * .

Investigation on the unsteady ventilation performance of oscillating jet air supply through both spontaneous vortex street effect and active modulation

Ventilation is one of the most vital measures to create a healthy and comfortable indoor environment, significantly influencing indoor air quality and building energy consumption. The conventional steady-state airflow field of a ventilated room, if not reasonably configured, is prone to cause air stagnation in certain regions, whereas the unsteady airflow improves the mixing effect, which favors the dispersion and purging of indoor air pollutants. This paper proposes a method to induce an unstable air flow field utilizing the spontaneous Karman vortex street effect on the air-supply jet and swinging modulation. The quasi-periodicity of the airflow field and its influence on the indoor pollutant purging effect were investigated. The effects of air velocity oscillation and pollutant concentration decay in a ventilated room were studied through experimental measurements and computational fluid dynamics analysis. The results show that the transverse oscillation jet has a dominant period of 1.6 s and the longitudinal oscillation jet has a dominant period of 2.987 s, and both jets have a width about twice that of the free jet. The airborne pollutant discharge efficiency of the oscillating jet was higher than that of the free jet at the occupancy level, and the longitudinal oscillating jet purged pollutants faster than the transverse oscillating jet. This study reveals that oscillating jets can provide an option to create periodic flow fields and improve the efficiency of ventilation.

Effect of tab shape, length, and placement on the over-expanded free jet at Mach 2.0

Abstract Experiments have been carried out to investigate the over expanded free jet at Mach number 2.0 without and with tabs of different cross-sectional shapes, tab length penetration into the jet core defined as blockage ratio, and symmetrical and asymmetrical placement of tabs at the jet exit. The cross-sectional shapes such as triangular, square, and circular have been studied with the penetration depth of 3 %, 7 %, and 11 % blockage. The Pitot tube and Schlieren flow visualization were carried out in experiments. The basic features of the jet with Mach disk, shock cell, and its distortion with the adoption of tabs at various nozzle pressure ratios indicate a definite influence that helps in the reduction of the core jet length. This characteristic is also influenced by changes in tab cross-section and length of penetration. Since the basic jet has three-dimensional structures, the asymmetric placement of tabs is beneficial compared to the symmetric orientation of tabs. The jet width increases with adoption of these tabs and different shapes. Whereas, a maximum core jet length reduction of the order of 60 % could be achieved using the present techniques.

Effect of tab shape, length, and placement on the over-expanded free jet at Mach 2.0

Abstract Experiments have been carried out to investigate the over expanded free jet at Mach number 2.0 without and with tabs of different cross-sectional shapes, tab length penetration into the jet core defined as blockage ratio, and symmetrical and asymmetrical placement of tabs at the jet exit. The cross-sectional shapes such as triangular, square, and circular have been studied with the penetration depth of 3 %, 7 %, and 11 % blockage. The Pitot tube and Schlieren flow visualization were carried out in experiments. The basic features of the jet with Mach disk, shock cell, and its distortion with the adoption of tabs at various nozzle pressure ratios indicate a definite influence that helps in the reduction of the core jet length. This characteristic is also influenced by changes in tab cross-section and length of penetration. Since the basic jet has three-dimensional structures, the asymmetric placement of tabs is beneficial compared to the symmetric orientation of tabs. The jet width increases with adoption of these tabs and different shapes. Whereas, a maximum core jet length reduction of the order of 60 % could be achieved using the present techniques.

Fine characterization of the capillary instability of free viscoelastic jets

The experimental approach presented in this article investigates low-velocity small-diameter cylindrical viscoelastic jets. It consists in producing and analyzing sets of jet portion images taken at different positions along the jet axis, i.e., from the nozzle producing the jet down to the spray region. A very significant part of the work was dedicated to develop analyzing tools: a proper orthogonal decomposition to identify the onset of the capillary instability, a procedure to determine the mean jet breakup length and a multi-scale tool to describe the jet local shape and its evolution along the jet axis. Used in a similar previous study, the multi-scale analysis has been extended in the present work. These tools are shown to be well suited to the experimental study conducted here, and in particular to meet the need for statistical analysis that free jets impose. They are applied to study the capillary instability of free jets of dilute polyethylene oxide solutions produced from a micrometer nozzle. The parameters of the study are the molar mass of the polymer and the velocity of the jet. Results on the detection of the capillary instability onset, the jet evolution dynamics towards the Bead On A String (BOAS) pattern, the local extensional relaxation time and the terminal elongational viscosity of the solutions, the bead coalescence mechanism and its influence on the size of the spray drops are obtained. Analysis of these results reveals that the polymers undergo two episodes of mechanical degradation during the process: the first one at injection, the second one during the elasto-capillary regime of the jet breakup. Moreover, the scenario in the literature suggesting a sufficiently relaxed axial tension of the flow for the capillary instability to occur is found to be not always valid. It is also shown that the average size of the drops correlates with the intensity of the bead coalescence mechanism that takes place on the BOAS. These results demonstrate the richness of the adopted approach and in particular of the multi-scale analysis to study the development of a fluid interfacial instability.

Acoustic characteristics of jets impinging on permeable plates

This article explores the acoustic characteristics and the relevant flow features of jets impinging on permeable plates. Noise generated due to the interaction of the jet with permeable plates is compared with jets impinging on an impermeable plate and the corresponding free jet. This study systematically measures various parameters, including pore size, porosity, and pressure drop, to precisely quantify the permeability of the plates using the Forchheimer equation. The focus is on investigating the impact of permeability on noise reduction. An acoustic study is performed by carrying out blow-up and blow-down tests to find the effect of permeability at different nozzle pressure ratios and different nozzle-plate spacings. An extensive directivity study is conducted to find the directionality of acoustic emissions and calculate acoustic power. It is found that the overall sound pressure level is lower for the jets impinging on the permeable plates compared to the impermeable plates in subsonic cases. It is also observed that the insertion of the permeable plates in supersonic jets generates less noise compared to the corresponding free jet. It is found that most of the tones are absent in the case of permeable plates for supersonic jets, whereas the tones are present with lesser amplitude compared to jets impinging on impermeable plates for subsonic and transonic jets. Finally, flow measurement and flow visualization studies are carried out to understand the flow physics responsible for the noise variance. This study illuminates that the passage of flow through the porous plate results in the reduction of wall-jet velocities, thereby suppressing the turbulent mixing noise. The absence of shock oscillations in front of the permeable plate is identified as the cause of the mitigation of impinging tones.

Acoustic characterization of an installed elliptical jet

This work demonstrates a preliminary acoustic characterization of an installed elliptical jet with an aspect ratio of 2. This configuration is compared against the nominal axisymmetric installed jet case of equivalent diameter. A baseline case for the elliptical and axisymmetric nozzle geometries are obtained in the uninstalled configuration. For all cases, the acoustic Mach number is varied between 0.4 and 1.0, the azimuthal observer angle between 0° and 90°, and the axial distance from the jet exit to the observer between 0 and 30 nozzle equivalent diameters. The latter of these is an analogue for polar angle variation by keeping the radial distance from the jet to the observer constant. A flat metal plate is used for the installed case, creating a jet–plate interaction mimicking that of an aircraft engine’s exhaust and its wing. Using the same variations as the free jet, parameters such as plate height, trailing-edge distance and, for the elliptical jet, nozzle exit rotation angle are investigated to give a broad characterization of the installed elliptical jet and enable direct comparison to the axisymmetric jet case. The key comparison metric for this work is the overall sound pressure level and polar directivity of the acoustic fields.

Experimental and numerical investigation of the thermal and dynamic behavior of a heated vortex multijet system impacting a flat plate

This study concerns the experimental and numerical study of the thermal and dynamic behavior of a configuration of a system of vortex jets impacting a flat plate, The objective of this study is to study the behavior of the thermal and dynamic field of vortex blowing of hot air from a multi-jet system impacting a flat plate. The experimental test bench comprising a support of three diffusers of diameter D, impacting the perpendicular plate. A uniform inlet temperature (T, T, T) is imposed such that the impact height H = 4D. The vortex is obtained by a vortex generator made up of 12 fins arranged at 60° from the vertical, placed just at the outlet of the diffuser. A thermo-anemometer device, to measure the blowing temperature at the point in question. The system was numerically simulated by the fluent code using a k-ε RNG turbulence model. It should be noted that the multi-jet system first appears as a free jet: going from the injection orifice to the impact zone, the axial velocity weakens, the jet undergoes considerable deflection, this is the deflection zone the velocities become mainly radial and the thickness of the boundary layer increases radially: this is the parietal flow zone, the structure of the velocity field has two zones of intense deflection with a wall jet on both sides other, favoring a good development of the resulting jet. The results show that this configuration (T, T, T) gave a better optimized distribution of temperature and velocity on the surface of the plate. This homogenization of the temperatures results from a better thermal transfer of the plate.The k-ε RNG model gave acceptable results, which coincide with those of the experimental results.