Flow Behavior Of Jet Research Articles

Numerical simulation of an impinging jet array in a square channel covered by a porous layer

AbstractIn many heat transfer applications, the design of the flow field should warrant high heat transfer rates and, meanwhile, reduce the thermal stresses so that hot spots along the heat transfer surface be avoided. A combination of multiple jets impinging on a channel bed together with the surface covered by a porous layer is proposed to satisfy both objectives in the current study. A three‐dimensional numerical simulation using a finite volume method with the second‐order discretization has been applied to visualize the multiple‐impinging jet flow behavior. The impacts of the porous medium parameters, including the thickness, permeability, and porosity, on the magnitude and distribution of the heat transfer along the channel bed have been evaluated. It is shown that the overall heat transfer performance of the proposed flow is significantly improved in comparison with the conventional case of the fluid flowing parallel to the channel bed. The presence of the porous layer leads to a more even spread of the fluid on the target surface, which reduces the thermal stresses and prevents the large differences between the highest and lowest values of heat transfer coefficients. They also found that both the porosity and particularly the permeability of the porous layer enhance the effect of the crossflow along the flow associated with the multiple‐impinging jet setup. For a certain thickness of the porous layer, it is possible to reduce the amplitude of the Nusselt number oscillations effectively, while keeping the overall Nusselt number desirably high.

Experimental analysis of lengthwise/transversal thermal characteristics and jet flow of large-format prismatic lithium-ion battery

• Thermal runaway characteristics of prismatic batteries with lengthwise and transversal placements are investigated. • The jet flow behavior of the battery during thermal runaway is analyzed in detail. • A method is proposed to estimate the jet velocity based on mass loss rate. Lithium-ion batteries (LIBs) are among the most promising technologies for electric vehicles and electric energy storage systems, while the safety accidents associated with thermal runaway happen frequently. Thermal characteristics and the jet behavior of prismatic LIBs under overheating conditions are still unclear. In this study, the thermal runaway characteristics of LIBs under lengthwise and transversal placements, such as battery surface temperature, voltage and mass loss are investigated through the overheating experiments. Results show that the center and bottom of the LIB surface of lengthwise placement have higher temperatures during thermal runaway. Compared with lengthwise placement, transversal placement accelerates the occurrence of thermal runaway under overheating conditions. The onset temperature of the safety valve opening is 129 °C, which is not affected by electrical energy stored inside the battery and placements. To comprehensively understand the prismatic LIB jet flow, a method is proposed to determine the jet velocity at the safety valve based on mass loss using mathematical deductions. The jet behavior, internal pressure and gas generation rate are also quantitatively characterized by experiments. The maximum jet velocity occurs with a value of 42.05 m/s occurs at the moment when the safety valve opens. This work further reveals thermal abuse characteristics and jet behavior of prismatic LIBs, the research outcomes of which provide guidance for the safe design and thermal hazard prevention in battery storage systems.

Investigation on the Gas Jet Flow Performance Confined in Round Pipe

In this study, the round jet flow behavior arose by the confinement was investigated experimentally and numerically. The confinement characteristics based on multi-scale characterizations in the atmospheric gas jet flow, which generated from a circular symmetrical subsonic nozzle and flowed into a confined round pipe, was used to studied. The studied region near the nozzle possess really short axial length (within 12d), providing initial conditions and boundaries that affected the flow behaviors in this region. A wide range of inlet velocities (0.98 m/s~84.72 m/s) and confined space sizes (1~20) were involved for simulated and quantitative discussions. The velocity profile evolution was recorded by simulations and characterized by a laser Doppler anemometer (LDA) with a resolution of 0.01 m/s. The confinement characteristics were systematically presented to elucidate the performance under the studied confined conditions with different metrics, such as centerline velocity decay (VR), entrainment rate (MR), pressure coefficient (Cp) and length of recirculation region (LR). The results indicated that the recirculation fluid action mainly contributed to the promoted initial velocity profile evolution with the introduction of the confined space. Theoretically, the confined space could reduce the maximum absolute entrainment mass flux, initiating a peak at the center of the recirculation region, which was controlled by the inlet velocity and the confined space size collectively. The remarkable effect of confined space size further contributes to the confinement characteristics of gas jet flow, representing a shortened flow distance despite similar process as that by reducing the diameter ratio (dR). In some cases, because of the miniaturization of confined space size, the dispersion of initial velocity profile evolution was not significant displayed throughout the variable inlet velocity range, especially when the dR was less than 2. This study gives a new insight in the performance of jet flow confined in round pipe, and such knowledge will be helpful to provide great potential and reference to applied fluid mechanics.

Recent advances in understanding propeller jet flow and its impact on scour

This paper presents an extensive review of the characteristics of a propeller jet-induced flow field and its impact on scour formation. As the driving force of scour, the jet flow behavior is first explored, including both the free and bounded jets under various boundary conditions. Accordingly, their resulting scour, namely, unconfined and confined scour, are expounded in terms of the temporal and spatial development of the scour depth. Hitherto, existing methods for predicting the scour depth are principally developed empirically, which exclude in-depth scrutiny into the mechanism governing the scouring process. With the detailed flow fields obtained in recent research, this paper aims to piece together a state-of-the-art understanding of the scouring mechanism governing confined propeller scour, that is, the combined effect of jet flow and junction flow on the scour hole development. By comparing the similarities and differences between the scouring mechanisms in open and closed quays, this paper provides a comprehensive insight into the scouring process associated with bounded propeller jets. Moreover, in terms of the near-bed flow variations during the scouring process, implications for sediment transport under highly turbulent flow are discussed. Finally, future research needs are identified.

Analysis of the factors influencing the airflow behavior in an impinging jet ventilation room

A validated CFD model is employed to predict the airflow behavior in an impinging jet ventilation (IJV) room with cool, isothermal or warm jets. By using the numerical results, the influences of jet discharge height, supply grille shape and room height on the jet flow behavior as well as the draught discomfort are analyzed for IJV operating in heating scenarios. The results indicate that the warm supply jet of IJV rises upward to the ceiling after spreading along the floor for a certain distance due to thermal buoyancy, resulting in a limited dispersion area, while the cool and isothermal jets of IJV always spread along the whole floor. When IJV is used for space heating, the lower the jet discharge height, the farther the supply air spreads along the floor, and the supply grille shape and room height almost have no effect on the air dispersion area. The results also show that the energy-efficient advantage of IJV in its heating mode compared to the mixing ventilation (MV) system is more remarkable in higher rooms. Moreover, there is a risk of draught discomfort in IJV heating rooms and it is recommended to wear socks to avoid this discomfort.

Transport and deposition behaviors of vapor coating materials in plasma spray-physical vapor deposition

The plasma spray-physical vapor deposition (PS-PVD) technology offers great potential to support the development of non-line-of-sight deposition. However, many researches indicated that the shadowing effect is a significant mechanism of PS-PVD for the coating growth and formation of the micro-structure. There seems to be an irreconcilable contradiction between the two theories. In this study, both the transport and deposition behavior of PS-PVD processes are researched from a macroscopic and microscopic perspective, respectively. The deposition processes of vapor coating materials are separated by the last collision position. The process before reaching the last collision position is defined as transport behavior, and the process after the last collision position is defined as deposition behavior. The results of simulation and experiment indicated that the flow behavior of plasma jet and the frequent collisions between plasma gas and vapor coating material provide opportunities for the vapor coating material to deposit in shadowed surfaces or non-line-of-sight positions. The shadowing effect under micro line-of-sight deposition is a major factor in the growth of columnar structures, and it accompanies the entire process of coating growth. This study divides the deposition process of vapor coating materials into the macro non-line-of-sight transport and micro line-of-sight deposition, illuminates the macro-non-line-of-sight transport behavior of vapor-phase materials, and sheds light upon the micro-line-of-sight deposition mechanism of the PS-PVD process.

Numerical simulation of flow behavior of topped gas-particles jet in a bubbling fluidized bed

Flow behavior of topped gas-particles jet is simulated by means of gas–solids multi-fluid model in a bubbling fluidized bed. The finer particles from the topped gas-particles jet are ejected into a fluidizing coarse particles bed. The interactions of finer particles and coarse particles are simulated by means of the kinetic theory of binary particles with unequal component granular temperatures. The two-equation k-ε model is used to model gas turbulence of the topped gas-particles jet. The effects of the top gas-particles jet velocity and fluidizing gas velocity on the distributions of volume fraction, velocity and component granular temperatures are analyzed along radial and axial directions of the bed. Simulated volume fractions and axial velocities of finer particles show that the fluidized bed is divided into three regions as (a) the diffusion region near the bottom, (b) the concentrating region and (c) the transport region along bed height. The circulation of particles is formed with the down-flow at the center and the up-flow between the center and the walls. Numerical simulations show that the coarse particles volume fractions decrease and the finer particles volume fractions increase with the increase of topped gas-particles jet velocity and fluidizing gas velocity in the bottom of the bed. The component granular temperatures are larger in the concentrating region than that in the diffusion and transport regions. The penetration length of the topped gas-particles jet with the change of topped gas-particles jet velocity and fluidizing gas velocity is discussed.

Evaluation of heat transfer in quenching processes with impinging liquid jets

Liquid jets and sprays often are used in metal-working processes like hot rolling, heat treatment or casting for intensive cooling of hot metal specimen and surfaces. Due to the potential evaporation of the quenching liquid during boiling, the local fluid flow as well as the heat transfer are extremely dynamic processes. Therefore, a numerical simulation model has been developed for flow and heat transfer to analyze the entire cooling process in the jet quenching process including all boiling regimes.In this contribution the focus is on quenching of hot metal surfaces with impinging water jets. The impingement liquid jet flow behavior and the wetting front behavior of the jet on the surface of a plate have been analyzed. As result of the simulations, the local surface temperatures and the temperature gradients at the cooled surface are evaluated as well as the local heat transfer rates.The model validation is done by means of different experimental approaches. Therefore, the progression of the wetting front at the quenched surface and the temperature distribution on the back side of thin metal plates during jet quenching has been evaluated.

Numerical simulation of flow behavior of top-gas jet in a gas-particles bubbling fluidized bed

Flow behavior of top-gas jet and particles is simulated by means of gas-solids two-fluid model incorporating the kinetic theory of granular flow (KTGF) in a top-gas jet bubbling fluidized bed. The RNG k-ε model is used to model gas turbulence of the top-gas jet. The effects of the top-jet gas velocity and fluidizing gas velocity on distributions of volume fractions and velocities of gas and particles are evaluated, and the results are quantified in terms of top-gas jet penetration length. Numerical simulations show that the fluidized gas-particle mixture along bed height is divided into three regions as (a) the top-gas jet region in the freeboard, (b) the concentrating region near the bed surface and (c) the diluting region near the bottom in terms of the axial distribution of solids volume fraction. By increasing the top-jet gas velocity, particles circulate through the down-flow at the center from the concentrating region to the diluting region and the up-flow near the walls from bottom to bed surface along bed height. The distributions of volume fraction and velocity of gas and particles considerably change with the variation of top-jet gas velocities. Moreover, the top-gas jet penetration length with the change of top-jet gas velocity and fluidizing gas velocity is identified and discussed.

Calibration of a silver thin film gauge for short duration convective step heat load

Thin film gauges (TFGs) are a promising candidate for measuring transient heat fluxes in the applications involving very short duration of heating environment. They are basically resistance temperature detectors (RTDs) having the capability of responding in the range of few microseconds. In the present study, a silver thin film gauge (STFG) is fabricated and calibrated in-house with a view to assess the performance of STFGs in dynamic environment. Convective heat load is supplied by a hot-air gun where the heated air jet strikes the gauge and its response is obtained through voltage signal. Subsequently, the surface heat fluxes are estimated by using one dimensional heat conduction modeling. The similar experimental environment is studied to obtain the flow behavior of hot-air jet emanating into atmosphere by using numerical simulations. The self-similar velocities are plotted as well as the interference of outer domain into the experiment parameters has been studied. Ultimately, the surface heat fluxes obtained from various methods are compared to analyse the performance of this hand-made STFG. This study reveals the ability of STFGs to be used in practical short duration transient situations.

OpenFOAMを用いた高速フレーム溶射の粒子挙動の数値シミュレーション

OpenFOAMを用いた高速フレーム溶射の粒子挙動の数値シミュレーション

Electrohydrodynamics of a Cone–Jet Flow at a High Relative Permittivity

We have proposed a new solution of the electrohydrodynamic equations describing a novel cone–jet flow structure formed at a conductive liquid meniscus in an electric field. Focusing on the liquids characterized by a high relative permittivity and using the slender body approximation, the cone–jet transition profiles and their characteristic radii are predicted in relation to the material parameters. The stable value of the cone angle is obtained using the Onsager’s principle of maximum entropy production. Three different regimes of the cone–jet flow behavior are identified depending on the relative importance of capillary, viscous and inertial stress contributions. The presented complete analytical solutions for the cone–jet transition zone and the far jet region yield several different laws of algebraic decrease for the radius, surface charge, and electric field of the jet.

Flow and Heat Flux Behavior of Micro-bubble Jet Flows Observed in Thin, Twisted-Wire, Subcooled Boiling in Microgravity

Thin wire, subcooled boiling experiments were performed onboard an aircraft flying a parabolic trajectory to provide microgravity conditions for improved observation of jet flow phenomena and their behavior in the absence of buoyant forces. A new type of nucleation jet flow was observed in microgravity. This new micro-bubble jet flow is seen at medium to high heat fluxes and is characterized by a region of the wire that forms multiple jet columns which contain micro-bubbles. These columns flow together and penetrate tens of millimeters into the bulk fluid. Bubble behavior on the wire was observed to progress from a dominance of larger isolated bubbles on the wire to a dominance of micro-bubble jet flows on the wire as heat flux was increased. There was also a transient transition from a few large isolated bubbles to micro-bubble jet flow dominance for a set heat flux. A cross correlation calculation provided velocities of micro-bubbles in the flow, which were in the range of 4-14 mm/s. These velocities were used with convection correlations to show that fluid flows induced by jet flows are a significant contributor to the subcooled boiling heat transfer in microgravity, but are not the primary contributor. Additionally, a relative bubble area analysis approximates the direct contribution of these jet flows to the overall heat dissipation. These micro-bubble jet flows, which are only observed on thin wires (not flat surfaces), and the convection currents they induce, have the potential to allow for sustained fluid motion to occur in microgravity.

Influence of the choice of the inlet turbulence intensity on the performance of numerically simulated moderate Reynolds jet flows – Part 1 – the near exit region of the jet

A real problem when trying to develop a numerical model reproducing the flow through an orifice is the choice of a correct value for the turbulence intensity at the inlet of the numerical domain in order to obtain at the exit plane of the jet the same values of the turbulence intensity as in the experimental evaluation. There are few indications in the literature concerning this issue, and the imposed boundary conditions are usually taken into consideration by usage without any physical fundament. In this article we tried to check the influence of the variation of the inlet turbulence intensity on the jet flow behavior. This article is focusing only on the near exit region of the jet. Five values of the inlet turbulence intensity Tu were imposed at the inlet of the computational domain, from 1.5% to 30%. One of these values, Tu= 2% was the one measured with a hot wire anemometer at the jet exit plane, and another one Tu= 8.8% was issued from the recommendation of Jaramillo (1). The choice of the mesh-grid and of the turbulence model which was the SST k-ω model were previously established (2). We found that in the initial region of the jet flow, the mean streamwise velocity profiles and the volumetric flow rate do not seem to be sensitive at all at the variation of the inlet turbulence intensity. On the opposite, for the vorticity and the turbulent kinetic energy (TKE) distributions we found a difference between the maximum values as high as 30%. The closest values to the experimental case were found for the lowest value of Tu, on the same order of magnitude as the measurement at the exit plane of the jet flow. Mean streamwise velocity is not affected by these differences of the TKE distributions. Contrary, the transverse field is modified as it was displayed by the vorticity distributions. This observation allows us to predict a possible modification of the entire mean flow field in the far region of the jet flow.

0205 ゲル粒子を含む非ニュートン流体中の密度噴流の挙動(OS2-1 混相流の多次元可視化計測,OS2 混相流の多次元可視化計測)

0205 ゲル粒子を含む非ニュートン流体中の密度噴流の挙動(OS2-1 混相流の多次元可視化計測,OS2 混相流の多次元可視化計測)

Numerical study of flow and dilution behavior of radial wall jet

The radial wall jet is a flow configuration that combines the radial jet and the wall jet. This article presents a simulation of the radial wall jet by applying the transition Shear-Stress Transport (SST) model. Tanaka's experimental data are used for validation. The computed velocity profiles agree well with the experimental ones. The distributions of the velocity on cross-sections show a similarity in the main region and the profiles are different with those of the free radial jet or the wall jet, because the presence of the wall limits the expansion of the jet. By introducing the equivalent nozzle width, the maximum velocity decays and the half-width distributions are normalized, respectively. In addition to compare the flow field with experiments, this paper also analyzes the dilution effect of radial wall jets in terms of the concentration distributions. The concentrations on the wall keep constant within a certain distance from the nozzle. And the concentration distributions also show a similarity in the main region. Both the decays of the maximum concentration and the distributions of the concentration half-width fall into a single curve, respectively. The dilution effect of radial wall jets is thus verified.

Synchronal measurement of flow structure and heat transfer of impingement jet

This paper will present the characteristics of flow behavior and thermal field of both free and impingement jet issued from a circular orifice nozzle at Re = 8900. The flow behavior of a single round jet and impingement jet was observed by smoke flow visualization recorded by a high speed camera using 5000 frame per second. Heat transfer coefficient on the impingement surface was measured by means of infrared camera (TVS-8500, Avio) with a two-dimensional array of Indeum-Antimony (In Sb) sensors varying in the separation distance between the nozzle and the target plate. The heat transfer coefficient changes in time and spatial. Therefore, the root mean square distribution of the heat transfer was obtained from the data. As a result, it was confirmed that the longitudinal vortex was observed outside of the ring vortex, and then the longitudinal vortex was penetrated in the jet flow. Moreover, the high value of root mean square of the heat transfer coefficient has spread radially in stripy manner, which is caused as the results of the longitudinal vortexes flowing in the radial direction on the impingement plate.

Simulation of a 3D submerged jet flow around a pile

The submerged 3D turbulent jet flow behavior around a pile on a rigid bed and on a scoured bed was studied experimentally and numerically. ADV was used to obtain the jet velocity distributions and Realizable k–ε turbulence model was used for the prediction of flow field around a pile. The jet flow area was three-dimensional and thus numerical model was a three-dimensional model. The numerical results were used to predict the velocity close to the pile and bed shear stress on the bed. In general, the results indicated that the flow field was also in agreement with the findings of previous experiments in literature and the related principles in the subject area. The experimental results demonstrated that Acoustic Doppler Velocimeter (ADV) measurements were almost identical with the Realizable k– ε turbulence model results for turbulence intensity I=10%.

Presumed PDF modeling of reactive two-phase flow in a three dimensional jet-stabilized model combustor

The objective of the present work is to investigate the modeling of a two-phase reactive flow concerning a diesel oil/air flame in order to predict the turbulent flow behavior and temperature distribution in a three dimensional jet-stabilized model combustion chamber. A Finite Volume staggered grid approach is adopted to solve the governing equations. The second-order upwind scheme is applied for the space derivatives of the advection terms in all transport equations. An Eulerian–Lagrangian formulation is used for the two-phase (gas–droplet) flow. The presumed PDF is taken on to model the heat release and the Realizable k– ε turbulence model is applied for the flow predictions. The thermal radiation model for the gas-phase is based on the Discrete Ordinates Method, adopting its S 4 approximation. Comparisons of present numerical predictions with available experimental data and also with another numerical solution employing different combustion and turbulence models reveal that the Realizable k– ε model predicts jet flow behavior more accurately than the standard k– ε model. Also, the presumed PDF model predicts the temperature distribution better than the eddy dissipation model, especially in the near wall region. Negligence of thermal radiation mode results in a failure to predict the concentration of NO species.

Design analysis of a piezoelectrically driven synthetic jet actuator

Technological advancement is being realized by using piezoelectric synthetic jet actuatorsto generate managing forces and moments with zero-net-mass-flux oscillatory jets forvarious air flow control applications. This paper firstly explores the synthetic jet flowbehavior for a dual-diaphragm piezoelectrically driven synthetic jet actuator. In theexperimental study, a flow visualization system was utilized to acquire the particle streakimages scattered from red fluorescent spheres for examining the synthetic jet flow. Thecenterline velocity of the jet was measured with a hot-wire anemometer. For exploring theformation progression of synthetic jets, the numerical analysis implemented unsteadythree-dimensional conservation equations of mass and momentum with a standardk–ε two-equation turbulent model adopted for turbulence closure. The moving boundary wasalso treated to represent the motion of the piezo diaphragm under actuation.For a complete sinusoidal actuation cycle at an operating frequency of 648 Hz,the synthetic jet flow pattern was simulated and compared with the visualizedimage and measured centerline velocity distribution to validate the computersoftware. In general, the far-field flow structure was fairly similar to a commoncontinuous turbulent air jet; whereas, the predicted time-recurring formation ofa vortex pair was observed in the near field. The surrounding air close to theslot was also drawn into the cavity of the actuator when vortex pairs advectedsufficiently downstream. Numerical experiments were then extended to assess theperformance of synthetic jet actuators by systematically varying the driving voltage,relative phase delay of frequency, width of the slot and depth of the actuatorcavity.