Jet Inlet Research Articles

Application of the environmentally safe operating schemes to the cooling systems of ship power plant

The possibility of using the ecologically safe operation on a closed-loop circuit instead of an open-loop cooling system is considered. For this purpose, it was proposed to use the ballast tanks on the ship. To ensure effective heat removal, it is proposed to apply a jet inlet of coolant to the hull shell of a ship. The flow regimes of the impact fluid jet were visualized. The critical Reynolds number is determined. Heat transfer coefficients and generalizing calculation dependences are determined by experiments.

Modal Analysis of Acoustic Directivity in Turbulent Jets

The directivity of noise from three large-eddy simulations of turbulent jets at Mach 0.7, 0.9, and 1.5 is investigated using spectral proper orthogonal decomposition (SPOD). The most energetic patterns of acoustic radiation are extracted using the far-field pressure 2-norm. Specialization of the norm to the far field is accomplished through localized spatial weighting. Radiation patterns to specific jet inlet angles are isolated by further restricting the spatial weighting to small rectangular regions in the far field. The most energetic radiation pattern for all cases and relevant frequencies is a single superdirective acoustic beam in the downstream direction. The source region of these beams is traced back to the end of the potential core for low frequencies and the shear-layer region for higher frequencies. In the sideline direction, to low angles, the acoustic patterns consist of beams that propagate upstream or perpendicular to the jet axis. The sideline radiation patterns are found to originate from the same source locations as the dominant superdirective beams. Inspection of the SPOD modes reveals that sideline radiation is directly linked to directive downstream radiation. Within the restricted radial extent of the computational domain, these results indicate that the sources of sideline and downstream radiation are intimately linked.

Local analysis of absolute instability in plasma jets

Abstract

Far-field noise radiation characteristics of an afterburning military jet aircraft

Because of the potential noise impacts of tactical jet aircraft, detailed far-field measurements are valuable in developing and validating models. This paper describes far-field measurements taken of the T-7A Red Hawk trainer aircraft. Data were taken at five microphone arcs, over six engine runups from idle to afterburner. The arcs were of varying resolution, but generally spanned 30 to 160 deg (relative to the jet inlet) at distances from 19 to 230 m. The spectra, sound pressure level, and skewness of the time derivative of the pressure waveform are analyzed according to microphone position and engine condition. Measurement consistency over the different runups and across all conditions validates the dataset for future analysis. [Work supported by AFRL.]

An experimental and numerical investigation into scaling considerations for moderator circulation experiments

The moderator system in a CANDU reactor provides the unique ability to provide emergency cooling to the fuel in postulated severe accidents during the early phases of the transient. A key criteria which dictates the effectiveness of heat removal during these events is the integrity of the fuel channel assembly. This in turn relies on the prevention of dryout on the calandria tube. The main tool used to demonstrate adequate margins to dryout on the calandria tubes is Computational Fluid Dynamics (CFD). While several experimental datasets are available for validation of these tools, the historical facilities lack the specific geometrical features present in some of the older CANDU designs. The historic tests were typically scaled based on the so-called Archimedes number and non-dimensional heat flux. To extend the validation database two unique 1/16th scale (based on vessel diameter) experiments are performed which include features found in the moderator systems of two operating CANDU designs. The primary goal of these new experiments was to generate thermalhydraulic data for validation of CFD codes, but several additional tests were performed to examine the potential impact of scaling on the results. This paper demonstrates a consistent conservative bias in the CFD predictions with respect to maximum temperature in the experiments over the range of conditions tested. The combination of CFD and experimental observations concludes that the likely cause of the bias is due to CFD’s over prediction of inlet jet dissipation leading to less cold fluid penetration in the heated core of the test section. In terms of scaling, a combination of experimental evidence and supporting CFD simulations indicate that the historical scaling parameters do provide some level of qualitative similarity in the mixed-convection regimes in the vessel, but some revision to account for geometrical and flow pattern deviations may be necessary.

Additional criteria for MILD coal combustion

We proposed a theoretical basis for Moderate or Intense Low-oxygen Dilution (MILD) coal combustion based on the turbulent scalar energy spectra. This is motivated by the hypothesis that smallest scalar mixing length scales should be on the order of the particle size or smaller to ensure that mixing can occur to prevent formation of diffusion flames. Our proposed criterion is evaluated using several experimental datasets from the literature for coal combustion in both MILD and traditional combustion regimes. The experimental results confirm that the smallest mixing length scales should be of the order of or smaller than the particle diameter, ηmix⪅dp, to breakup the heat and mass transfer boundary layers around particles in MILD coal combustion. Results indicate that poor mixing of species with small Schmidt numbers around small particles leads to the high luminous intensity in the reactor. The effects of inlet velocity and jet diameter on the mixing length scales are analyzed. Higher inlet velocity and smaller jet diameter are expected to reach MILD regime. The proposed criterion can be used to guide experimental design to achieve MILD conditions for coal combustion.

A numerical study of localized swirling injection of oxidizer for homogeneous combustion with oxygen enrichment

Conceptually similar to MILD, FLOX, CDC, HiTAC, etc., Homogeneous Combustion (HC) continues to be pursued as an appealing technique towards minimizing NOx emissions. HC combustors are generally driven by high-momentum inlet jets which enable intense dilution of the reactants. This work numerically studies the effectiveness of localized swirling injection in enhancing reactant dilution for an HC combustor running with enriched oxidizers (XO2>21%). Even though localized and low intensity, swirling injection is found to have far-field effects (over ~75 diameters). Effects of swirling injection on the flow field (near and far field) and on NOx emissions are explained. While swirl does help in reducing NOx, there exists an optimal swirl intensity beyond which NOx emissions increase. A mutual competition is seen between swirl assisted and entrainment driven dilution; and at higher swirl intensities, the reduction in the latter overwhelms the gains accrued by the former (in terms of NOx emissions). Along expected trends, thermal NOx is deduced to be the dominant pathway of NOx formation for oxy-enriched cases. Damköhler numbers in the reaction zone are low even for the highest oxygen content tested here (XO2=40%). Volumetric standard deviation of Heat Release Rate (HRR) is seen to perform well as a measure of the tendency of transition to the conventional mode of combustion.

Thermal response of high-aspect-ratio hydrogen cylinders undergoing fast-filling

Fast-filling can lead to excessive heating of a hydrogen cylinder. Gas cylinders with large length-to-diameter aspect ratios are prone to developing hot-spots that may cause the temperature of the structure locally to exceed specified limits. The hot-spots develop because convective heat transfer into the cylinder wall is not uniform along the length of the cylinder. Computational fluid dynamic simulations of fast-filling reveal that the flow within large aspect ratio cylinders is characterised by two distinct flow patterns. The region near to the inflow nozzle exhibits an axi-symmetric recirculating flow pattern that extends to approximately three cylinder diameters downstream from the inlet. Wall heat transfer in the recirculation region of the flow is driven strongly by the jet of turbulent fluid from the nozzle. The wall heat transfer in the recirculation region rapidly develops quasi-steady behaviour in which the Nusselt number is approximately proportional to the inlet jet Reynolds number. Downstream of the recirculation zone there is a region of axial flow, in which wall heat transfer occurs at a slower rate, driven by decaying turbulence transported from the recirculation zone. The reduced heat transfer in the end gas leads to a higher gas temperature but also to a lower structural temperature downstream of the recirculation zone. Analysis shows that wall heat transfer can be enhanced and peak gas temperatures reduced by dividing the hydrogen injection between separate nozzles. Directing the nozzles axially, ideally at intervals of around 4 cylinder diameters, increases heat transfer and prevents the formation of hot regions of stagnant fluid. This approach allows more hydrogen to be stored for a given pressure or gas temperature limit. However the analysis across various length-to-diameter ratios and nozzle configurations reveals substantially different trends for the gas temperatures appearing in ISO safety specifications, and the temperature of the structural materials they seek to safeguard.

Flow regimes and mixing characteristics in non-aligned T-jets reactors

Flow regimes and mixing characteristics in non-aligned T-jets reactors were investigated by Planar Laser Induced Fluorescence (PLIF) at 30 ≤ Re ≤ 400. The misalignment of inlet jets in non-aligned T-jets reactors leads to a steady engulfment flow at low Reynolds numbers. With the increase of Re, the unsteady engulfment flows with different oscillation behaviors occur. Unlike the vortex-merging oscillation in reactors of ws/h = 0 and 0.5 (ws and h are the misalignment degree and height of inlet jets), an S-shaped self-excited oscillation is identified in reactor of ws/h = 1. Mixing in non-aligned T-jets reactors is quantified by time-averaged Intensity of Segregation (IOS) and the mixing mechanism is revealed. The mixing is significantly enhanced in non-aligned T-jets reactors at low Re due to the earlier emergence of the steady engulfment flow. Both the vortex-merging oscillation in reactors of ws/h = 0 and 0.5 and the S-shaped self-excited oscillation in reactor of ws/h = 1 contribute to the mixing enhancement.

Jet breakup regimes in liquid–liquid Taylor vortex flow

Optical experiments were used to identify breakup regimes for a liquid jet injected into in a second immiscible liquid undergoing turbulent Taylor vortex flow. Four jetting breakup behaviors were observed and these were compared with analogous jetting regimes in quiescent liquid-liquid systems as well as liquid-gas systems with a gaseous crossflow. Several jet breakage regime maps for liquid–liquid Taylor vortex flow were generated using jet Reynolds number, Ohnesorge number, crossflow Weber number, and momentum flux ratio as organizing parameters. However, selection of the breakup mechanism was found to depend primarily on the jet Reynolds number and was largely independent of parameters characterizing the fluid crossflow, such as the azimuthal Reynolds number and crossflow Weber number. This finding is consistent with previous observations that the downstream droplet size distribution in Taylor vortex flow is relatively insensitive to parameters other than the jet Reynolds number. Lastly, the transitions between the four identified jet breakage regimes were found to be coincident with changes in the mean droplet size observed downstream from the inlet jet.

Discrete particle behavior in solid-liquid concave-wall jet

The aim of this paper is to reveal the solid-liquid separation mechanism in concave-wall jet. The discrete particle experiments and numerical simulations were performed to study the large particle behavior near concave-wall. The results show that the particle behavior is divided into two stages by the first collision point of particle-to-wall and the wall shear stress at the collision point quickly changes from zero to peak. The iterative formulae for particle position coordinates are used and 27.88° is obtained as the maximum of circumferential collision position when the jet width is 20 mm. Before the first collision, the particle tangential velocity is constant and the wall shear stress is 0. After the first collision, the wall shear stress and tangential velocity decrease with time. The distance of particle-to-wall is within 0.1 times of jet width. Low-frequency contact of particle-to-wall is the collision and the particle Reynolds number often exceeds 400. The large particle behavior is affected by the shape of jet inlet cross-section. Comparing to the square inlet cross-section, the particle cumulative probability of rectangle inlet cross-section is lower at the same residence time, the local maximum of turbulent kinetic energy and wall shear stress decrease 5.3 % and 5.9 %, respectively.

Numerical study on flow and heat transfer in a multi-jet microchannel heat sink

A multi-jet microchannel (MJMC) heat sink with coolant flowing through alternative inlet and outlet jets in the direction normal to the heated surface is studied. Three dimensional flow and heat transfer processes in the MJMC are numerically simulated using the SIMPLE-type finite volume method (FVM). Compared with traditional microchannels, the MJMC combines the advantages of impinging jet flow and entrance effects of microchannels, and thus its cooling performance overwhelms showing less pumping power, lower thermal resistance and improved uniformity of temperature at the bottom surface. Effects of various geometrical parameters including jet numbers, channel aspect ratio, the fin width to channel width ratio and the width of the outlet on the performance of the MJMC are analyzed in detail. It is found that the MJMC with more jets, wider outlet and smaller fin width to channel width ratio offers better cooling performance. While the cooling performance exhibits a non-monotonic trend with the channel aspect ratio, the optimum structure is obtained with an aspect ratio around 6. Under the range of parameters studied, the MJMC heat sink with 7 jets, aspect ratio of 6 and fin width to channel width ratio of 0.5 obtains the best cooling performance.

Investigation on the thermal energy storage characteristics in a spouted bed based on different nozzle numbers

This paper presents the results of the simulations of the thermal storage and thermal release processes in a spouted bed using a self-developed computational fluid dynamics–discrete element method (CFD–DEM) model to analyze the thermal energy storage (TES) characteristics of the spouted bed with varying nozzle numbers. The thermal storage capacity, thermal release capacity and thermal storage efficiency were analyzed. Additionally, the effects of gas inlet jet number and gas flowrate on the thermal storage characteristics are discussed. The results showed that a higher number of nozzles resulted in a faster change of particle temperature, higher gas phase temperature field update rate and a higher thermal storage capacity. However, when the number of nozzles is increased from 1 to 2 and then 3, the rate of increase in total thermal storage and total thermal release decreases, and the increase in thermal release is always higher than the increase in thermal storage. When the number of nozzles is increased from one through three, the thermal efficiency increased from 52.9% to 75.6%, establishing that a higher number of nozzles improves the thermal efficiency.

Aerodynamic Studies on Non-Premixed Oxy-Methane Flames and Separated Oxy-Methane Cold Jets

Both cold and flame jets find numerous applications in different fields, ranging from domestic applications to aerospace and space technology. Indeed, the applications of isothermal and non-isothermal jets in the flame heating industry fascinated the researchers to gain an in-depth understanding. Nevertheless, these benefits are not standalone, rather, they are associated with major disadvantages such as improper jet mixing and flame instabilities that require careful remedies. In the present investigation, three-inline jets, having methane jet at the center and two oxygen jets at the periphery, are studied computationally in a three-dimensional domain, with and without considering the effects of combustion. To study the mixing characteristics of cold jets, the radial velocity distributions at different streamwise locations have been analyzed at the jet inlet velocity of 27 m/s. However, for oxygen and methane flame jets, inlet velocities are varied as 27 m/s and 54 m/s. Moreover, to investigate the effects of temperature variation on mixing characteristics at a particular jet velocity, the inlet temperatures of reactants are varied as 300 K, 500 K, and 700 K, at the jet inlet velocity of 27 m/s. Combustion is found to have a profound impact on the mixing characteristics. At the inlet temperature of 300 K, a higher centerline velocity decay is observed for non-reactive jets as compared to reactive flame jets. Moreover, the turbulent kinetic energy distribution is relatively higher in the case of non-reactive jets, which is the direct evidence of an augmented mixing. As is obvious, the turbulent kinetic energy at the jet shear layer is maximum due to the formation of large-scale coherent eddies. The decay in centerline velocity is found to be increasing with an increase of inlet temperature. Additionally, with an increase of jet velocity, the radial velocity profiles become more asymmetrical, consequently yielding an unstable flame.

An Efficient 1-D Thermal Stratification Model for Pool-Type Sodium-Cooled Fast Reactors

Investigating thermal stratification in the upper plenum of a sodium fast reactor (SFR) is currently a technology gap in SFR safety analysis. Understanding thermal stratification will promote safe operation of the SFR before its commercial deployment. Stratified layers of liquid sodium with a large vertical temperature gradient could be established in the upper plenum of an SFR during a down-power or a loss-of-flow transient. These stratified layers are unstable and could result in uncertainties for the core safety of an SFR. In order to predict the occurrence of the thermal stratification efficiently, we developed a one-dimensional (1-D) transport model to estimate the temperature profile of the ambient fluid in the upper plenum. This model demands much less computational effort than computational fluid dynamics (CFD) codes and provides calculations with higher fidelity than historical system-level codes. Two flow conditions were considered separately in the current study depending on if in-vessel components are presented in the upper plenum. For the condition where in-vessel components, specifically the upper internal structure, are presented, we assumed that the impinging sodium was evenly dispersed in the ambient fluid within the distance between the bottom of the in-vessel component and the jet inlet surface. For the condition where no in-vessel components are presented, we assumed that the impinging sodium was evenly dispersed in the ambient fluid within the jet length, which was determined through data-driven trainings. The newly developed 1-D model showed similar performance with the CFD model in both cases. However, due to the assumption of flat profiles of the impinging jet axial dispersion rate, nonnegligible discrepancies between the 1-D prediction and the measured data were observed.

Direct Numerical Simulations on Jets during the Propagation and Break down of Internal Solitary Waves on a Slope

Jet flows often have an important role in the water environment. The aim of this research is to study the dilution of jets due to complex velocity fields induced by internal solitary waves in stratified water. Direct numerical simulations are used to study vertical jet flows during the propagation and breaking of internal solitary waves (ISWs) with elevation type on a slope. Energy analysis shows that the internal interface is able to absorb kinetic energy from the jet and that for Re < 10,000 with Ri > 3.7, the ISWs can stay stable during the propagation within the presence of jet flows. The vortices jointly induced by the jets and the ISWs are observed at the bottom behind the ISW’s crest. The transport of the jet’s emitted scalar by the ISWs can be divided into two parts; some is transported by the moving interface and the rest by the bottom vortices. The ultimate transport length scales of two types are defined, and it is found that when the center of the jet inlet approaches the slope, the extension of the bottom vortices into the slope will lead to strong mixing. That causes increasing scalar concentration over the slope of the scalar that originated from the jet.

Experimental study of swirling flow characteristics in a semi cylinder vortex cooling configuration

This study utilised planar Particle Image Velocimetry (PIV) to investigate the flow characteristics in a semi cylindrical confinement with 2 jet inlets. The water based experimental study aims to understand the basic flow behaviour in a gas turbine leading edge vortex cooling configuration. The time averaged and fluctuating flow fields were collected at 3 cross sections and 1 longitudinal section at 3 different inlet Reynolds numbers. The snapshot based Proper Orthogonal Decomposition (POD) was applied to extract the coherent flow characteristics. Results showed that the vortex flow consists a large-scale vortex in the chamber with a small recirculating corner vortex. The core vortex is similar to the Rankine vortex with a near solid body rotating vortex, surrounded by a jetting vortex layer and a boundary layer along the chamber wall. The jetting vortex layer featuring a proportional decline of circumferential velocity is different to the potential layer in a typical Rankine vortex. The jetting vortex at a high velocity level is mainly responsible for the high heat transfer rate for vortex cooling. A turning region at θ≈170° separating the circumferential velocity behaviour was noticed, which provides a flow dynamic explanation to the heat transfer intensity along the surface wall for vortex cooling. The region at the interface between the solid body rotating vortex and the jetting vortex features with relatively low velocity magnitude but strong shear and large fluctuating velocity intensity. The longitudinal section has a low average velocity (approximately 10% of that in the cross sections in magnitude), indicating the vortex flow is strongly rotational but weakly helical. However, the fluctuating velocity intensity has a comparative level to cross sections. POD analysis reveals that the first 4 modes contain about 23.2% of the fluctuating velocity energy. In nozzle cross sections, the coherent vortex near the surface wall dominates the fluctuation energy and is responsible for the heat transfer enhancement. In the cross section between nozzles, the coherent flow structure displays a vortex pair in the core and a minor corner vortex.

Analysis of the Forces Driving the Oscillations in 3D Fluidic Oscillators

One of the main advantages of fluidic oscillators is that they do not have moving parts, which brings high reliability whenever being used in real applications. To use these devices in real applications, it is necessary to evaluate their performance, since each application requires a particular injected fluid momentum and frequency. In this paper, the performance of a given fluidic oscillator is evaluated at different Reynolds numbers via a 3D-computational fluid dynamics (CFD) analysis. The net momentum applied to the incoming jet is compared with the dynamic maximum stagnation pressure in the mixing chamber, to the dynamic output mass flow, to the dynamic feedback channels mass flow, to the pressure acting to both feedback channels outlets, and to the mixing chamber inlet jet oscillation angle. A perfect correlation between these parameters is obtained, therefore indicating the oscillation is triggered by the pressure momentum term applied to the jet at the feedback channels outlets. The paper proves that the stagnation pressure fluctuations appearing at the mixing chamber inclined walls are responsible for the pressure momentum term acting at the feedback channels outlets. Until now it was thought that the oscillations were driven by the mass flow flowing along the feedback channels, however in this paper it is proved that the oscillations are pressure driven. The peak to peak stagnation pressure fluctuations increase with increasing Reynolds number, and so does the pressure momentum term acting onto the mixing chamber inlet incoming jet.

Defect Analysis and Detection of Cutting Regions in CFRP Machining Using AWJM.

The use of composite materials with a polymeric matrix, concretely carbon fiber reinforced polymer, is undergoing further development owing to the maturity reached by the forming processes and their excellent relationship in terms of specific properties. This means that they can be implemented more easily in different industrial sectors at a lower cost. However, when the components manufactured demand high dimensional and geometric requirements, they must be subjected to machining processes that cause damage to the material. As a result, alternative methods to conventional machining are increasingly being proposed. In this article, the abrasive waterjet machining process is proposed because of its advantages in terms of high production rates, absence of thermal damage and respect for the environment. In this way, it was possible to select parameters (stand-off distance, traverse feed rate, and abrasive mass flow rate) that minimize the characteristic defects of the process such as taper angle or the identification of different surface quality regions in order to eliminate striations caused by jet deviation. For this purpose, taper angle and roughness evaluations were carried out in three different zones: initial or jet inlet, intermediate, and final or jet outlet. In this way, it was possible to characterize different cutting regions with scanning electronic microscopy (SEM) and to distinguish the statistical significance of the parameters and their effects on the cut through an analysis of variance (ANOVA). This analysis has made it possible to distinguish the optimal parameters for the process.

Mathematical Simulation of a Twisted Inlet Jet at Variable Mode with Using Various Turbulence Models

The article is devoted to the solution of actual task – the efficiency increasing of the air distribution using twisted and wall air jets to provide normative air parameters in premises . The purpose of the paper is to determin e the characteristics of air flow that is formed by twisted and flat wall jet s at alternating air flow and to obtain analytic equations for determining of needed dynamic parameters of the air jets, and also to design the mathematical model of incoming air flow at the alternating air flow . The mathematical model of air supply with twisted and wall air jets is improved. It is shown that for reaching the maximal efficiency of air distribution it is necessary to supply air by jets that intensively decay before entering into a working zone . Simulation of air flow was carried out using the computational fluid dynamic software Ansys FLUENT. Simulation of the flow are carried out by one-parameter Spalart-Allmaras turbulence model and two-parameter k –e turbulence model. Good coincidence between the results of simulation using both tur b ulence models is obtained. The graphical and the analytical dependences on the basis of the conducted experimental research, which can be used in subsequent engineering calculations, are shown. Dynamic parameters of air flow that is formed by t he twisted and wall air jets at alternating air flow has been determined. New 4D method of researches is proposed for researching of jets with alternating parameters of the air. Results of experimental investigations of air supply into the room by air distribution device which creates twisted air jets for obtaining more intensive turbulization of air flow in the room are presented. Obtained results of these investigations give possibility to realize engineer ing calculations of air distribution with twisted and wall air jets.