Confined Jet Flow Research Articles

Heat Transfer Characteristics of Confined and Unconfined Submerged Air Bubble Jet Impingement

Heat transfer characteristics of two types of jet impingement, confined and unconfined is investigated using air and water as the working fluids. The effects of volumetric quality are evaluated on the lateral variation of the Nusselt number at low nozzle-to-plate spacing. The results show that the lateral Nusselt number of the unconfined submerged air bubble jet is similar to that of the confined submerged air bubble jet in Region I (bubble collision region) due to the effect of bubble collision. In Region II (bubble sliding region), however, the lateral Nusselt number of the unconfined jet was found to be 30%–40% lower than that of the confined jet flow due to the buoyancy effect that causes air bubbles to rise in Region I. In addition, from the flow visualization, high velocity bubble movement was observed from the midpoint of the heating surface to the periphery of the confinement plate in the confined jet set-up. This movement enhances the lateral Nusselt number in Region II. In the unconfined jet set-up, air bubbles rise past the heating surface in Region I and thus neither slide across the heating surface nor increase the lateral Nusselt number in Region II.

Temporal and spatial evolution of cavitating bubbles and coherent structures in a confined submerged jet shear layer

PurposeThe study aims to display the bubbles' evolution in the shear layer and their relationship with the pressure fluctuations. Furthermore, the coherent structures of the first six modes are extracted, in order to provide insight into their temporal and spatial evolution and determine the relationship between cavitating bubbles and coherent structures.Design/methodology/approachIn the present study, numerical simulations of submerged jet cavitating flow were carried out at a cavitation inception condition inside an axisymmetric cavity using the large eddy simulation (LES) turbulence model and the Schnerr–Sauer (S–S) cavitation model. Based on snapshots produced by the numerical simulation, dynamic mode decomposition (DMD) was performed to extract the three-dimensional coherent structures of the first six modes in the shear layer.FindingsThe cavitating bubbles in the shear layer are deformed to elongated ellipsoid shapes by shear forces. The significant pressure fluctuations are induced by the collapse of the biggest bubble in the group. The first mode illustrates the mean characteristics of the flow field. The flow in the peripheral region of the shear layer is mainly dominated by large-scale coherent structures revealed by the second and third modes, while different small-scale coherent structures are contained in the central region. The cavitating bubbles are associated with small size coherent structures as the sixth or higher modes.Practical implicationsThis work demonstrates the feasibility of LES for high Reynolds number shear layer flow. The dynamic mode decomposition method is a novel method to extract coherent structures and obtain their dynamic information that will help us to optimize and control the flow.Originality/value(1) This paper first displays the three-dimensional coherent structures and their characteristics in the shear layer of confined jet flow. (2) The relationship of bubbles shape and pressure fluctuations is illustrated. (3) The visualization of coherent structures benefits the understanding of the mixing process and cavitation inception in jet shear layers.

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.

Synthesis of Aluminum Oxynitride Nanopowders in a Plasma Reactor with a Confined Jet Flow

Experimental studies on the synthesis of aluminum oxynitride nanopowders in a reactor with a confined plasma jet by the interaction of disperse aluminum with ammonia and oxygen in a flow of nitrogen plasma generated in an electric arc plasma torch are performed. Preliminary calculations of the equilibrium compositions and thermodynamic characteristics of the multicomponent Al–O–N system are carried out. An optimal design of the reaction prechamber of the reactor is performed. Powders having a cubic structure and consisting of aluminum oxynitride phases with an average particle size in the range from 20 to 200 nm are obtained. It is established that the specific surface area of the obtained powders increases from 20 to 71 m2/g, and the nitrogen content in the nanopowders increases from 3.6 to 14.7 wt % with an increase in the flow rate of quenching gas from 1.8 to 6.0 m3/h. At the same time, the oxygen content decreases from 35.5 to 25.5 wt %. At a minimal flow rate of the quenching gas, the metallic aluminum and its oxide phases are present in the obtained powders. With an increase in the amount of quenching gas, the conditions for the mixing of aluminum vapors with oxygen and atomic nitrogen from decomposing ammonia are improved, which leads to the formation of aluminum oxynitride and aluminum nitride. The variation of the synthesis parameters allows us to obtain aluminum oxynitride nanopowders having a specific surface area ranging from 28 to 50 m2/g containing from 1 to 11 wt % of nitrogen and from 25 to 40 wt % of oxygen.

Synthesis of titanium nitride and carbonitride nanopowders in confined-jet flow plasma reactor

The synthesis of titanium nitride and carbonitride nanopowders from titanium tetrachloride vapor in a stream of hydrogen–nitrogen plasma, generated by an arc torch, in confined-jet flow reactor has been experimentally studied. Single-phase nanopowders with a NaCl-type cubic crystal lattice as assemblies of preferably cube-shaped nanoparticles of a 20–150 nm size and aggregates based on them have been obtained in the experiments. By varying the synthesis parameters, it has been possible to prepare titanium nitride nanopowders with a specific surface area in the range of 11–39 m2/g containing 18.8–22.5 wt % nitrogen, which corresponds to the empirical formula TiN0.79–TiN0.99. The titanium carbonitride nanopowders had a specific surface area of 13–23 m2/g and carbon and nitrogen contents of 7.5–13.6 and 13.5–5.1 wt %, respectively.

Computational study of a turbulent gas-solid confined jet flow

In the present work, numerical simulation of a confined two-phase particulate jet emanating from a circular nozzle has been performed using Euler-Euler approach and incorporating four-way coupling. The effects of particle size (in the range 50–200μm) and solid loading ratio (in the range 1–5) on the gas phase flow field as well as turbulence have been studied at different co-flow velocities (i.e. 2m/s, 5m/s and 10m/s). It has been observed that the particles have a strong effect on the flow field and the turbulence of the gas phase. As a consequence, gas phase velocity decay rate has been reduced along the center line, and turbulence has been attenuated in the near field region due to the presence of the particles. For a single-phase confined jet, the radial velocity decay is observed in the near-field region (X/D<10). In the presence of a co-flow, the radial velocity has increased beyond X/D=10. In the two-phase jet, the gas phase velocity is decayed in the radial direction throughout the domain. Also, jet half width is reduced with decreasing particle diameter and increasing solid loading ratio (SLR).

Pressure Distribution in Confined Jet Flow

A momentum jet injected into a confined container breaks up to “diffusive turbulence” after traveling a critical distance. It has been argued that an adverse pressure gradient developing within the container, acting against the jet momentum flux, is responsible for this break up (Risso and Fabre, 1997,“Diffusive Turbulence in a Confined Jet Experiment,” J. Fluid Mech., 337, pp. 233–261; Voropayev et al., 2011, “Evolution of a Confined Turbulent Jet in a Long Cylindrical Cavity: Homogeneous Fluids,” Phys. Fluids, 23, 115106). Experimental evidence for this adverse pressure gradient is presented in this paper, supplemented by a control-volume analysis to explain the results. The rise of pressure from the jet-injection level to a location beyond the jet break up xb is shown to be proportional to the jet momentum flux. The overall (integrated) sidewall friction on a control volume is negligible, compared to the increase of pressure, if the flow control volume extends beyond xb. For smaller lengths of the control volume, the side wall drag is not negligible compared to the pressure rise. The Reynolds number similarity was evident for jet Reynolds numbers above 6000. This work was motivated by its applications to degassing of crude oil stored in the U.S. Strategic Petroleum Reserves, which are slender salt caverns. To improve its quality, periodically oil is cycled through a degassing plant and injected back to the cavern as a jet, and the degassing time is critically dependent on jet dynamics.

Heat-and-mass transfer in a confined-jet flow plasma reactor during nanopowder manufacturing processes

The formation of layers of copper and tungsten nanoparticles on the surface streamlined by a flow of products of plasma synthesis in a confined-jet flow reactor has been investigated. The distribution of heat and mass fluxes onto the surface of the plasma reactor has been studied for various values of the enthalpy, thermal-plasma flow velocity, and reactant concentrations.

The Effect of Spanwise Width on Rectangular Jets With Sidewalls

A CFD study on the effect of spanwise width on a rectangular jet with sidewalls is conducted using a standard k-ɛ model with wall functions. An order of magnitude analysis reveals the role played by spanwise turbulent shear terms, arising from the wall bounded flow, as the aspect ratio is decreased at high streamwise distances. A comparative study involving experimental data and other turbulence models is also presented to validate the k-ɛ model for this confined jet flow. It is found that the effect of bounding walls is negligible up to a streamwise distance of at least 105 jet diameters for an aspect ratio of 40, however this distance, within which the flow can be approximated as two-dimensional, decreases with decrease in aspect ratio.

Turbulent mixing and fast chemical reaction in the confined jet flow at large Schmidt number

Turbulent mixing with fast chemical reaction was modeled in the confined jet flow at large Schmidt number (Sc∼1000). The Reynolds-Averaged-Navier–Stokes approach was employed to describe the turbulence–chemistry interaction. Numerical results were validated against data obtained by the two-color planar laser-induced fluorescence method. Based on these data the dissipation rates of the mixture fraction and the product reaction concentration were calculated. Their comparison showed essential differences both in dynamics and values. The influence of different-order finite-difference schemes for scalar gradient determination and of the noise-signal ratio on the dissipation rates was estimated. More accurate higher-order schemes caused both noised and corrected dissipation rates to increase. Eliminating the noise leveled essentially the order-effect of finite-difference schemes.The analysis of the mixing models was demonstrated that the mixing model with the constant mechanical-to-scalar time ratio R and the Multi-Time-Scale model overestimated the mixture fraction variance σf in comparison with experiment. To predict the dissipation in the jet flow at Sc∼1000 the low-Reynolds-number effects were considered: in the transport equation for σf the ratio R and the turbulent Schmidt number Scσ were the functions of turbulent Reynolds number. The accuracy of the used standard k–ε model was improved by coordinating the values of the model constants Сμ and Cε2 with the jet velocity decay and the expansion radius (Сμ=0.06 and Cε2=1.87).The examination of the known reaction rate models for the gas flow (Sc∼1) (the segregation intensity approach, the EDC-model or the PDF method) showed that the reaction rate was overestimated by these models in comparison with the one obtained from the measurements in the liquid flow. The proposed modification of the EDC-model took into account the specific micromixing in vortex structures of flow at Sc⪢1.

Numerical analysis and control of the recirculation bubble strength of turbulent confined jet flow using inlet swirl

Numerical investigation of the turbulent jet flows, both central and annular type of jets has been carried out with the introduction of swirl at the inlet using the modified κ −ε model. It was observed that the recirculation bubble generated by the central jet without swirl diminishes in size due to increase in swirl number, while in the case of annular jet the recirculation bubble size grows gradually by the same. Keywords: swirl, turbulent jet, modified κ −ε model, recirculation bubble

Entropy generation analysis of two-dimensional high-temperature confined jet

In high-temperature systems, thermal radiation becomes the dominant mode of heat transfer. The analysis of entropy generation mechanism is very important to optimize the second-law performance of these energy conversion devices. In this paper, the entropy generation in a two-dimensional high-temperature confined jet flow is analyzed. The computation of combined radiation and convection heat transfer is carried out with the help of a CFD code, and the entropy generation due to heat transfer and fluid friction is calculated as post-processed quantities with the computed data of velocity, temperature and radiative intensity. Numerical results show the entropy generation due to radiative transfer cannot be omitted in high-temperature systems such as boilers and furnaces, in which thermal radiation is one of the main modes of heat transfer. In the case that the temperatures of the inlet gas and the top and bottom are not changed, the total entropy generation number decreases with the increase of jet Reynolds number and Boltzmann number, respectively. For enhancing heat transfer and advancing energy conversion efficiency, large jet Reynolds number and Boltzmann number should be selected.

Destabilization of Laminar Wall Jet Flow and Relaminarization of the Turbulent Confined Jet Flow in Axially Rotating Circular Pipe

Destabilization and relaminarization phenomena have been investigated in an axially rotating circular duct. Standard k-ε model with modification for streamline curvature has been used in the numerical study. The laminar and turbulent velocity distributions at inlet have been observed to become turbulent and laminar, respectively, toward the exit of the pipe. A local velocity profile with parabolic or nearly uniform variation has been considered as the characteristic of laminarlike or turbulent flow, respectively, and changeover of flow from former to the later variation or vice versa has been taken to characterize destabilization and relaminarization, respectively. The predicted azimuthal velocity component was found to be reasonably accurate near the wall and not so encouraging in the core region of the swirling flow. The recirculation bubble generated by a central jet flow at the wall has been observed to reduce in size due to rotation of the pipe confirming the relaminarization phenomenon, whereas with laminar wall jet waspredicted recirculation bubble growing with rotation rate manifesting the destabilization effects.

Numerical investigation of a confined jet flow in a rectangular cavity

We present the results of investigation of a low-Reynolds number uncompressible fluid flow in a rectangular two-dimensional cavity. It has been shown that the behavior of the flow may be controlled by the cavity length and width. Depending on the ratio between the length and width of the chamber the flow is either steady or non-steady. It has been found that for small chamber lengths the flow is steady and geometrically symmetric. Increased chamber lengths lead to unsteady flows with oscillating and hardly predictable jet flow pattern. Further increase of the chamber length leads again to steady but asymmetric flow. These observations are useful for possible control of the jet behavior under various conditions. This research is relevant for both basic jet-flow investigations as well as for industrial purposes. The sensitivity of the solution to perturbation of simulation parameters is identified as an important issue to be attacked in future by both experimental and numerical means.

A numerical study on the fluid flow and heat transfer in the confined jet flow in the presence of magnetic field

The present study numerically investigates two-dimensional fluid flow and heat transfer in the confined jet flow in the presence of applied magnetic field. For the purpose of controlling vortex shedding and heat transfer, numerical simulations to calculate the fluid flow and heat transfer in the confined jet are performed for different Reynolds numbers in the absence and presence of magnetic fields and for different Prandtl numbers of 0.02 (liquid metal), 0.7 (air) and 7 (water) in the range of 0 ⩽ N ⩽ 0.05, where N is the Stuart number (interaction parameter) which is the ratio of electromagnetic force to inertia force. The present study reports the detailed information of flow and thermal quantities in the channel at different Stuart numbers. As the intensity of applied magnetic fields increases, the vortex shedding formed in the channel becomes weaker and the oscillating amplitude of impinging jet decreases. The flow and thermal fields become the steady state if the Stuart number is greater than the critical value. Thus the pressure coefficients and Nusselt number at the stagnation point representing the fluid flow and heat transfer characteristics also vary as a function of Stuart number.

자기장이 인가된 충돌제트의 열전달 특성에 관한 수치적 연구

The present study numerically investigates two-dimensional fluid flow and heat transfer in the confined jet flow in the presence of applied magnetic field. For the purpose of controlling vortex shedding and heat transfer, numerical simulations to calculate the fluid flow and heat transfer in the confined jet are performed for different Reynolds numbers in the absence and presence of magnetic fields and for different Prandtl numbers of 0.02 (liquid metal), 0.7 (air) and 7 (water) in the range of 0≤Ν≤0.05, where Ν is the Stuart number (interaction parameter) which is the ratio of electromagnetic force to inertia force. The present study reports the detailed information of flow and thermal quantities in the channel at different Stuart numbers. As the intensity of applied magnetic fields increases, the vortex shedding formed in the channel becomes weaker and the oscillating amplitude of impinging jet decreases. The flow and thermal fields become the steady state if the Stuart number is greater than the critical value. Thus the Nusselt number at the stagnation point representing the heat transfer characteristics also vary as a function of Stuart number.

자기장이 인가된 충돌제트의 유동 특성에 관한 수치적 연구

The present study numerically investigates two-dimensional fluid flow in the confined jet flow in the presence of applied magnetic field. Numerical simulations to calculate the fluid flow and heat transfer in the confined jet are performed for different Reynolds numbers in the absence and presence of magnetic fields in the range of 0≤ N ≤ 0.05, where N is the Stuart number (interaction parameter) which is the ratio of electromagnetic force to inertia force. The present study reports the detailed information of flow in the channel at different Stuart numbers. As the intensity of applied magnetic fields increases, the vortex shedding formed in the channel becomes weaker and the oscillating amplitude of impinging jet decreases. The flow fields become the steady state if the Stuart number is greater than a critical value. Thus the pressure coefficients at the stagnation point also vary as a function of Stuart number.

Experimental verification of the quasi-steady approximation for aerodynamic sound generation by pulsating jets in tubes.

Voice production involves sound generation by a confined jet flow through an orifice (the glottis) with a time-varying area. Predictive models of speech production are usually based on the so-called quasi-steady approximation. The flow rate through the time-varying orifice is assumed to be the same as a sequence of steady flows through stationary orifices for wall geometries and flow boundary conditions that instantaneously match those of the dynamic, nonstationary problem. Either the flow rate or the pressure drop can then be used to calculate the radiated sound using conventional acoustic radiation models. The quasi-steady approximation allows complex unsteady flows to be modeled as steady flows, which is more cost effective. It has been verified for pulsating open jet flows. The quasi-steady approximation, however, has not yet been rigorously validated for the full range of flows encountered in voice production. To further investigate the range of validity of the quasi-steady approximation for voice production applications, a dynamic mechanical model of the larynx was designed and built. The model dimensions approximated those of human vocal folds. Airflow was supplied by a pressurized, quiet air storage facility and modulated by a driven rubber orifice. The acoustic pressure of waves radiated upstream and downstream of the orifice was measured, along with the orifice area and other time-averaged flow variables. Calculated and measured radiated acoustic pressures were compared. A good agreement was obtained over a range of operating frequencies, flow rates, and orifice shapes, confirming the validity of the quasi-steady approximation for a class of relevant pulsating jet flows.

Studies on cooling by two-dimensional confined jet flow of high heat heat flux surface in fusion reactor

Divertor surface of a magnetic confinement fusion reactor is exposed to strong radiative heating. According to standard design of the ITER, maximum heat flux on the divertor surface becomes locally near 30 MW m −2. To cool such high heat flux surface by water flow, it is necessary to establish a cooling method which enhanced the critical heat flux (CHF). We proposed a cooling by a planar impinging jet with free surface in the previous report. In the jet cooling on flat surface, high CHF was obtained in the limited region where the jet flow hits directly. As apart from the region, the CHF decreases abruptly with the distance from the center. To overcome this difficulty, it was proposed that the planar jet is applied to cool concave surface where the centrifugal force is efficiently used to enhance the CHF. In this study, the CHFs were investigated in the confined jet flow which was guarded by a wall on the other side of the heated wall, because the guard wall works to protect splash of water from liquid film by violent boiling and expects further enhancement of the CHF. In this study, the CHFs were investigated in the confined flow of two-dimensional jet on flat and concave surfaces in the various flow conditions and got a correlation for the CHF. Applicability of this cooling for divertor surface was assessed by using the experimental results.

Characterization of the laryngeal jet using phase Doppler interferometry.

The purpose of this study was to characterize the effects of the laryngeal jet on inhalation air flows in the trachea, and to extend these ideas to further an understanding of aerosol deposition in this region. Phase Doppler Interferometry was used to characterize axial velocity and turbulence intensity contours in the tracheal section of a cadaver-based larynx-trachea model. An array of 30 measurements was made at each of 6 downstream planes within the tracheal portion of the model (immediately downstream of the larynx, and at 0.5, 1, 2, 3, and 4 diameters further downstream). The flow was characterized for steady state flow at three Reynolds numbers (1250, 1700, and 2800). The Re = 1250 case approximates the inhalation of a 6-year-old child. Reverse flows with significant velocities were noted in the anterior trachea within one diameter downstream of the larynx, for all three flow cases. The cross sectional area of the reverse flow regions was larger for the lower Reynolds number cases. These reverse flows are a consequence of the nearly triangular shape of the lumen between the vocal folds in the larynx and constitute a potential deposition mechanism. High levels of axial turbulence intensity were noted near the anterior/left tracheal walls within one diameter downstream of the larynx. This indicates the potential for deposition due to turbulence in this region. Turbulence levels were still significant after four downstream diameters, indicating the potential for turbulent deposition at positions further downstream, including the bronchial tree where passage diameters are smaller. Contrary to expectations, turbulence levels were approximately 20% higher for the Re = 1250 case compared to the Re = 2800 at the furthest downstream locations (with 99% confidence). This is likely due to the complex nature of the confined jet flow.