Jet Flow Field Research Articles

Characteristics of Flow Field for Supersonic Oxygen Multijets with Various Laval Nozzle Structures

The Laval nozzle is used widely in steelmaking processes to increase the velocity of an oxygen jet up to approximately 2 Mach numbers. The present research is aimed at determining the effect of the Laval nozzle structure on a supersonic oxygen jet flow field. Both one-dimensional isoentropic flow theory and a characteristic-line method were used to design five-hole oxygen lances of two types and, applicable to oxygen multijets, to analyze their flow field characteristics by a series of numerical simulations and experimental studies. When compared to the conventional oxygen lance designed according to the one-dimensional isoentropic flow theory, the new oxygen lance designed with employment of the characteristic-line method is more effective in both suppressing the shock wave generation and improving the initial axial velocity of oxygen jets under the same test conditions. As a result, a larger impaction cavity was generated by the new oxygen lance and the mass transfer within oxygen, liquid slag, and molten steel was enhanced.

Modelling 2-D Supersonic Jet from a Convergent-Divergent Nozzle using k-ε Realizable Turbulence Model

The present study focuses on the numerical investigation of the Mach 1.86 supersonic jet from the 2-D convergent-divergent nozzle for NPR (nozzle pressure ratio) values of 5 to 8 with a step size of one, covering the overexpansion, correct expansion and the underexpansion conditions using Realizable k-ε turbulence model under the steady state condition. The computational model is constructed using the design modeler of ANSYS 16.0 workbench. The structured grids are generated into the ICEM module of the ANSYS workbench and the fluid flow analysis is done by FLUENT solver. The plot of non-dimensional total pressure along the jet centerline with respect to the non-dimensional downstream distance of the nozzle exit along the jet centerline identify the extent of jet mixing. The contours of Mach number with different NPRs visualize the effects of shock cells and shock strength prevail into the jet flow field due the favorable and adverse pressure gradients at the nozzle exit.

URANS study of pulsed hydrogen jet characteristics and mixing enhancement in supersonic crossflow

In this paper, three-dimensional pulsed hydrogen jet in supersonic crossflow (PJISC) is investigated by the unsteady Reynolds Averaged Navier-Stokes (URANS) simulations with the k-ω shear stress transport (SST) turbulence model. The numerical validation and mesh resolution have been carried out against experiment firstly. The effects of the pulsed frequency and amplitude on the jet flow field and mixing performance in supersonic cross-flow are all addressed. It significantly changes the distribution of the hydrogen jet flow by comparing with the steady jet in supersonic crossflow. The fuel jet penetration, mixing efficiency, decay rate of the maximum hydrogen mass fraction and total pressure losses are used to quantitatively analyze the mixing performance. The mixing of fuel and incoming air flow is enhanced by the pulsed jet, especially for the case of 50 kHz, which is the optimal pulsed frequency while considering the effects of jet excitation frequency in the present simulations. The decay rate of the maximum mass fraction of hydrogen in the far field downstream is related to the frequency of the pulse jet. Moreover, the pulsed frequency and amplitude have little effects on the total pressure recovery coefficient for the cases studied in the present simulations.

Flow field features of chevron impinging synthetic jets at short nozzle-to-plate distance

The flow field of a chevron synthetic jet in impinging configuration at Reynolds number Re=4500, dimensionless stroke length L0/D=28 and nozzle-to-plate distance H/D=2 is experimentally investigated by means of Stereoscopic Particle Image Velocimetry. This study is motivated by the need of explaining the heat transfer behaviour of the chevron synthetic jet highlighting its differences with respect to the circular synthetic jet. Two different experiments are carried out. The first experiment is devoted to the characterisation of the flow field evolution on the impingement plate. The second experiment is a three-dimensional reconstruction of the entire flow field at three characteristic phases. The results show that the circular synthetic jet presents a circular velocity pattern on the plate, while the chevron exit leads to the acceleration of the fluid, thus to a larger impinging velocity with a star-shaped pattern. Furthermore, in both configurations a secondary vortex ring is formed on the wall at a radial position of about 2 diameters. For the circular synthetic jet, the intense turbulence levels are located in two annular regions: the inner region is due to the turbulent wall jet generated by the impinging trailing jet and the outer region is due to the effect of the generation of the secondary vortex ring. For the chevron synthetic jet, the intense turbulence level is produced by the turbulent wall jet along preferential paths. These preferential paths are caused by the star-shaped distribution of impinging velocity and the impinging streamwise coherent vortices. These features characterizing the impinging flow field are fundamental to understand and explain the mechanisms behind the heat transfer behaviour of such devices.

Differential Characteristics of the Overexpanded Gas Jet Flow Field in the Vicinity of the Nozzle Edge

A parametric study of the features of the flow field of a plane and axisymmetric overexpanded ideal gas jet in the vicinity of the nozzle edge has been conducted over the entire theoretically admissible range of determining parameters (nozzle divergence angles, exhaust Mach numbers, jet incalculabilities, and gas adiabat indicators). The exhaust parameters that correspond to the extremes of the differential characteristics of a shockwave falling (descending) from the edge and the flow field behind it have been revealed. A significant difference in the character of changes in the characteristics of the shockwave and the flow field behind it depending on the type of symmetry of the gas jet has been found and studied.

Influence of shock structure on heat transfer characteristics in supersonic under-expanded impinging jets

In this study, the relationship between the shock structure of impinging jet flow and target wall heat transfer characteristics was investigated. In the case of a high Reynolds number impinging jet that can enter the supersonic flow regime, the stagnation temperature of the impinged surface is affected by the jet structure as well as other factors, such as the nozzle to plate distance and radial distance. When the jet flow velocity becomes supersonic, shock structures form at the downstream of the nozzle exit. Complicated shock structures, such as the Mach shock disk and plate shock, are expected to affect the heat transfer characteristics of the impingement surface. In this study, the cooling performance of a supersonic nitrogen (N2) jet is investigated by measuring the impinged surface temperature, and the flow of the jet is visualized by schlieren imaging. The visualized image and surface temperature are compared to clarify the shock structure-related heat transfer characteristics. Under experimental conditions where the nozzle pressure ratio changes to a moderate and highly under-expanded regime, the surface temperature fluctuates according to the changes of jet structures in each condition. According to a position of Mach crossing point, which is an intersection of oblique shock structures, a behavior of jet fluid is decided to be trapped inside the recirculation zone or to be spread to wall jet region. In case that the jet fluid is trapped, the cooling performance of the supersonic jet is significantly reduced and vice versa. Therefore, the heat transfer characteristics can be explained by roles of the jet flow field structures including the recirculation flows and shock locations.

Baffle jetting: CFD analysis of plain jets impinging on fuel rods

The risk of baffle jetting exists mainly in pressurized water reactors, which are designed with a counter flow configuration in the core bypass region. Baffle plates are foreseen between core barrel and core, providing the core's lateral restraint and allowing a core bypass flow. A pressure differential establishes between the coolant flow in the core baffle and the upward flow through the core. A part of the coolant bypass flow is directed through the gaps of the baffle plates towards the core. Coolant cross flow through enlarged baffle gaps can result in high velocity jetting in the case of significant pressure differences between core and baffle region. These water jets impact onto the fuel rods and may induce rod vibration that can lead to fuel rod failure.Modelling baffle jetting induced vibrations of fuel rods is still a challenge in the field of fluid structure interaction. The present work aims to extend the understanding of the unsteady flow in rod bundles to situations where baffle jetting occurs. As a first step, the used LES modelling method is evaluated on the example of a well documented test case related to fuel rods in cross flow: the flow over a circular cylinder at Re = 3900. In the successive step, the validated modelling method is applied to the CEA PANACHET experiment, mimicking baffle jetting related phenomena. This analysis helped to understand better the flow field of a corner baffle jet and further validated the modelling approach.The flow field in the core baffle corner is finally analysed for realistic reactor conditions. A plain narrow slot jet is injected parallel to a baffle wall through a 1 mm width gap perpendicular to the axial flow of a corner assembly. The analysis of the flow field showed that: (i) Swirling flow exist close to the corner rod. The calculated force coefficients lift and drag of this rod show a dominant fluctuation frequency of about 80 Hz. (ii) The baffle jet impacts on the third rod in jet direction (passing the first two rods without impact). The force coefficients of the third rod do not show a dominant oscillation frequency but show increased oscillation amplitudes. The distinguished fluctuation frequency detected for rod n°1 and the direct impact of the baffle jet on rod n°3 might explain the rod failures that have been detected for these rods.

Modeling and Analytical Solution of Near-Field Entrainment in Suddenly Started Turbulent Jets

The evolution of the shear-layer flowfield of a round transient turbulent jet is analytically investigated within the near-field of the jet over which the velocity potential core decays. The model results are verified and expanded by performing a large-eddy simulation of the suddenly started jet. Unlike quasi-steady approaches, the present work aims at solving the momentum conservation while preserving the time-dependent term. The governing equation is integrated with a mixing-length model to account for the turbulent mixing in the shear layer. The solution is asymptotically applicable to the shear layer of the circular jet and excludes the head vortex mixing. A reasoned calibration of the model parameters for moderate Reynolds numbers results in acceptable agreement for the streamwise entrainment both with experimental data and with large-eddy simulations. The validity limit of the present model is examined by outlining the characteristic length of the velocity potential core as well as the restricting effects of the jet dynamics on the founding assumptions of the model. The study will be instrumental in developing a hot-jet ignition model in which the rate of mass entrainment into the jet influences the prediction of ignition based on the temperature and species distribution.

PIV Experimental Study on Flow Field near Lateral jet under the Condition of Vegetation

The high-frequency PIV was applied to measure the flow field near the lateral jet under the condition of rigid vegetation and flexible vegetation. Both the Single row vegetation and broadleaf grass were analyzed. The results showed that different vegetation arrangements have different effects on the jet flow field. The vegetation arrangement and vegetation resistance cause significant changes. The changes in flow velocity with changes in water depth displayed “S” type and anti-“S” type distributions, and the flow velocity of free layer was approximately logarithmic. Due to vegetation resistance and jet pressure differences, the lateral jet trajectory in water with vegetation also have different distributions. The lateral jet trajectory in water with broadleaf grass was more likely to bend than in water with rigid vegetation Compared to the influence of two vegetation arrangements on the water flow near the jet, the flexible vegetation has a better deceleration effect on the flow field near the lateral jet.

Numerical Study of Turbulent Air and Water Flows in a Nozzle Based on the Coanda Effect

In the present work it is performed a numerical study for simulation of turbulent air and water flows in a nozzle based on the Coanda effect named H.O.M.E.R. (High-Speed Orienting Momentum with Enhanced Reversibility). The main purposes of this work are the development of a numerical model for simulation of the main operational principle of the H.O.M.E.R. nozzle, verify the occurrence of the physical principle in a device using water as working fluid and generate theoretical recommendations about the influence of the difference of mass flow rate in two inlets and length of septum over the fluid dynamic behavior of water flow. The time-averaged conservation equations of mass and momentum are solved with the Finite Volume Method (FVM) and turbulence closure is tackled with the k-ε model. Results for air flow show a good agreement with previous predictions in the literature. Moreover, it is also noticed that this main operational principle is promising for future applications in maneuverability and propulsion systems in marine applications. Results obtained here also show that water jets present higher deflection angles when compared with air jets, enhancing the capability of impose forces to achieve better maneuverability. Moreover, results indicated that the imposition of different mass flow rates in both inlets of the device, as well as central septum insertion have a strong influence over deflection angle of turbulent jet flow and velocity fields, indicating that these parameters can be important for maneuverability in marine applications.

Flow Statistics and Visualization of Multirow Film Cooling Boundary Layers Emanating From Cylindrical and Diffuser Shaped Holes

The research presented in this paper strives to exploit the benefits of near-wall measurement capabilities using hotwire anemometry and flowfield measurement capabilities using particle image velocimetry (PIV) for analysis of the injection of a staggered array of film cooling jets into a turbulent cross-flow. It also serves to give insight into the turbulence generation, jet structure, and flow physics pertaining to film cooling for various flow conditions. Such information and analysis will be applied to both cylindrical and diffuser shaped holes, to further understand the impacts manifesting from hole geometry. Spatially resolved PIV measurements were taken at the array centerline of the holes and detailed temporally resolved hotwire velocity and turbulence measurements were taken at the trailing edge of each row of jets in the array centerline corresponding to the PIV measurement plane. Flowfields of jets emanating from eight staggered rows, of both cylindrical and diffuser shaped holes inclined at 20 deg to the main-flow, are studied over blowing ratios in the range of 0.3–1.5. To allow for deeper interpretation, companion local adiabatic film cooling effectiveness results will also be presented for the geometric test specimen from related in-house work. Results show “rising” shear layers for lower blowing ratios, inferring boundary layer growth for low blowing ratio cases. Detachment of film cooling jets is seen from a concavity shift in the u′rms line plots at the trailing edge of film cooling holes. Former rows of jets are observed to disrupt the approaching boundary layer and enhance the spreading and propagation of subsequent downstream jets. Behavior of the film boundary layer in the near-field region directly following the first row of injection, as compared to the near-field behavior after the final row of injection (recovery region), is also measured and discussed. The impact of the hole geometry on the resulting film boundary layer, as in this case of cylindrical verses diffuser shaped holes, is ascertained in the form of mean axial velocity, turbulence level (u′rms), and length scales profiles.

A study of a conductive laminar jet flow in an applied magnetic field

This study investigates steady two-dimensional laminar confined jet flow in the presence of an applied magnetic field. The magnetohydrodynamic equations with the format of the stream function and vorticity formulation of the fluid flow are solved numerically. A numerical method was developed by using a first-order upwind scheme at the boundaries and a second-order finite control volume scheme in the flow field. The results show that the expansion region of the jet is moving downstream while the channel width and the Reynolds number are increasing. The vortex and the recirculation zone are stretched with increased Hartmann number, and the jet expansion region is moving upstream while the vortex and the recirculation zone are reduced. The channel width and the Reynolds number for the jet development are positive efforts and the Hartmann number has a suppressed effect in the present confined jet flow field.

Patterns of plasma jet arrays in the gas flow field of non-thermal atmospheric pressure plasma jets

Schlieren photography, the state-of-art to visualize the invisible flows, has appealed gigantic attention of various researchers in the plasma community. Here, this technique is utilized to address the behavior of the plasma jet arrays in the gas flow field. The goal of this study is to probe the signatures of different parameters and their response in the gas flow field. It is concluded that every parameter exhibits its sensitivity to the plasma in the gas flow field. However, frequency has a significant impact on the reduction of the laminar flow. Furthermore, it is suggested that the flow of the higher momentum region to the lower region is the cause in establishing the instabilities. The compression and rarefaction at the rising and falling edges of the discharge pulses play the dominant role. Plasma jet arrays can be a handy tool for industrial applications unless proper parameters are selected.

Numerical Study of the Velocity Decay of Offset Jet in a Narrow and Deep Pool

The present study examines the configuration of an offset jet issuing into a narrow and deep pool. The standard k-ε model with volume-of-fluid (VOF) method was used to simulate the offset jet for three exit offset ratios (OR = 1, 2 and 3), three expansion ratios (ER = 3, 4 and 4.8), and different jet exits (circular and rectangular). The results clearly show significant effects of the circumference of jet exits (Lexit) in the early region of flow development, and a fitted formula is presented to estimate the length of the potential core zone (LPC). Analysis of the flow field for OR = 1 showed that the decay of cross-sectional streamwise maximum mean velocity (Um) in the transition zone could be fitted by power law with the decay rate n decreased from 1.768 to 1.197 as the ER increased, while the decay of Um for OR = 2 or 3 was observed accurately estimated by linear fit. Analysis of the flow field of circular offset jet showed that Um for OR = 2 decayed fastest due to the fact that the main flow could be spread evenly in floor-normal direction. For circular jets, the offset ratio and expansion ratio do not affect the spread of streamwise velocity in the early region of flow development. It was also observed that the absence of sudden expansion of offset jet is analogous to that of a plane offset jet, and the flow pattern is different.

Analysis of the flow characteristics of the high-pressure supercritical carbon dioxide jet

The supercritical carbon dioxide (SC-CO2) jet has a wide application prospect for rock breaking and jet fracturing in the development of the unconventional shale energy, due to its special physicochemical properties. To investigate its jet flow characteristics, the high-speed photography is used in experiments and the simulations through a compressible numerical model are carried out. It is shown that the jet flow field can be divided into three typical regions, the mixing layer possesses the same characteristics as a gas/gas turbulent mixing layer, with the divergent angles evidently smaller than those of the incompressible jets. The predicted results by the numerical model are in a good agreement with those of the experimentations. Through the dimensionless analysis, the potential core shows a length of about 9d and an increasing trend with the increase of the inlet pressure while the decay rate shows a decreasing trend, and the radial profiles consistent well with the “Ue - η” normalized method. In addition, a significant temperature drop of the SC-CO2 is observed between the nozzle inlet and the exit. A simple and convenient semi-empirical equation for calculating this temperature variation is deduced. Finally, the flow characteristics suggest that the SC-CO2 jet should be treated as a compressible jet.

Experimental investigation on plasma jet deflection with magnetic fluid control based on PIV measurement

This paper is devoted to experimentally investigating the influence of magnetic field intensity and gas temperature on the plasma jet deflection controlled by magneto hydrodynamics. The catalytic ionization seed CS2CO3 is injected into combustion gas by artificial forced ionization to obtain plasma fluid on a high-temperature magnetic fluid experimental platform. The plasma jet was deflected under the effect of an external magnetic field, forming a thrust-vector effect. Magnesium oxide was selected as a tracer particle, and a two-dimensional image of the jet flow field was collected using the particle image velocimetry (PIV) measurement method. Through image processing and velocity vector analysis of the flow field, the value of the jet deflection angle was obtained quantitatively to evaluate the thrust-vector effect. The variation of the jet deflection angle with the magnetic field intensity and gas temperature was studied under different experimental conditions. Experimental results show that the jet deflection angle increased gradually with a rise in gas temperature and then increased substantially when the gas temperature exceeded 2300 K. The jet deflection angle also increased with an increase in magnetic induction intensity. Experiments demonstrate it is feasible to use PIV test technology to study the thrust vector under magnetic control conditions.

Expansion characteristics of a plasma jet in the stepped-wall liquid chamber

The stepped-wall liquid chamber is an effective way to control the interaction instability during the expansion process of a plasma jet in the working fluid. The experiment was carried on to observe the plasma-liquid interaction in the stepped-wall liquid chamber. Based on the experiment, a two-dimension unsteady calculated model of a plasma jet expanding in liquid was established to investigate the jet expansion and flow field characteristics. The plasma jet gradually fills the steps of the liquid chamber, and its axial expansion is faster than the radial expansion. The vortex formed internal the plasma jet splits into two small vortices as time goes on. The low-pressure vortex is formed at the corner of the step, which increases the radial expansion of the plasma jet. Increasing the wall structure factor (n), the turbulent mixing is increased, the axial expansion ability of the plasma jet is decreased, and the pressure of the head high-pressure area and the thermal diffusion range of the plasma jet are decreased. The formula for estimating the axial displacement of a plasma jet is obtained for n is 0.4∼0.8.

Development and field application of a pulse-jet hydraulic impactor

The current studies on hydraulic pulse jet mainly focus on the pulse jet flow field and its effect, but have never extended to the collaboration of hydraulic impact and pulse jet for rock breaking. In this paper, both hydraulic impact and pulse jet were combined effectively to develop a pulse-jet hydraulic impactor for drilling after analyzing the working principles and realization conditions. The rock-breaking capacity of this tool was verified through laboratory experiments and field tests. The following results were obtained. First, the tool can run when the weight of the impactor body is less than 60 kg. Second, the rock-breaking capacity of the drilling borehole assembly (BHA) under the synergistic action of hydraulic impact and pulse jet is obviously better than that of other drilling tools, and the tool is much more efficient than other tools in ROP enhancement. Third, the impact effect is dependent on the weight and impact frequency of the impactor and the impactor with the weight of 30 kg is better in impact effect. Fourth, the larger the impulse jet, the higher its rock-breaking capacity is. The pulse jet can be increased by reducing the diameter of the tool's nozzle. Fifth, hydraulic impact can help accelerate the breaking of high-hardness rocks, and the breaking of less-cemented rocks can be greatly enhanced by increasing the pulse jet. Field application results show that the ROP of the drilling tool based on the collaboration of hydraulic impact and pulse jet is 2.52 m/h, which is 72.5% higher than that of conventional BHAs. It is concluded that this developed pulse-jet hydraulic impactor provides a new idea to solve the problems in deep wells and horizontal wells, such as low drilling speed, obvious chip hold down effect and difficult cuttings removal.

Behaviours of supersonic oxygen jet with various Laval nozzle structures in steelmaking process

ABSTRACTThe top-blowing supersonic oxygen jet is now used widely in steelmaking and metal refining processes. However, the ambient temperature and oxygen flow rate is changed during top-blowing process, making the flow field of supersonic oxygen jet unstable. Hence, it is very important to research the behaviour of supersonic oxygen jet in high ambient temperature. In the present study, the supersonic coherent jet flow fields with 2 kinds of Laval nozzle structures were analysed at various ambient temperature conditions. The total temperature and axial velocity were measured by experiment to verify simulation results. Based on the results, the design method of characteristic-line equation could be more effective in the control the velocity vector of oxygen jet, compared with the one-dimension isoentropic flow theory. As a result, the Laval nozzle designed by characteristic-line equation could suppress the forming of shock wave, reduce the radial velocity and increase the stirring ability of oxygen jet under various ambient conditions.

On noise generation in low Reynolds number temporal round jets at a Mach number of 0.9

Two temporally developing isothermal round jets at a Mach number of 0.9 and Reynolds numbers of 3125 and 12 500 are simulated in order to investigate noise generation in high-subsonic jet flows. Snapshots and statistical properties of the flow and sound fields, including mean, root-mean-square and skewness values, spectra and auto- and cross-correlations of velocity and pressure, are presented. The jet at a Reynolds number of 12 500 develops more rapidly, exhibits more fine turbulent scales and generates more high-frequency acoustic waves than the other. In both cases, however, when the jet potential core closes, mixing-layer turbulent structures intermittently intrude on the jet axis and strong low-frequency acoustic waves are emitted in the downstream direction. These waves are dominated by the axisymmetric mode and are significantly correlated with centreline flow fluctuations. These results are similar to those obtained at the end of the potential core of spatially developing jets. They suggest that the mechanism responsible for the downstream noise component of these jets also occurs in temporal jets, regardless of the Reynolds number. This mechanism is revealed by averaging the flow and pressure fields of the present jets using a sample synchronization with the minimum values of centreline velocity at potential-core closing. A spot characterized by a lower velocity and a higher level of vorticity relative to the background flow field is found to develop in the interfacial region between the mixing layer and the potential core, to strengthen rapidly and reach a peak intensity when arriving on the jet axis, and then to break down. This is accompanied by the growth and decay of a hydrodynamic pressure wave, propagating at a velocity which, initially, is close to 65 per cent of the jet velocity and slightly increases, but quickly decreases after the collapse of the high-vorticity spot in the flow. During that process, sound waves are radiated in the downstream direction.