Jet Zone Research Articles

Impact zone dynamics of dilute mono- and polydisperse jets and their implications for the initial conditions of pyroclastic density currents

The fluid dynamics at the impact zone of an impinging jet dictate the initial conditions for the resulting downstream flow. Current understanding of this impact zone is limited to small scale, single phase, or monodisperse impinging jet experiments with a focus on industrial and engineering applications. Here, we present multifield numerical modeling results of mono- and polydisperse impinging jets with length scales and physical parameters relevant to large explosive volcanic eruptions. Our modeling shows that particle behavior is sensitive to whether a jet is monodisperse or polydisperse. For a monodisperse jet, the downstream flow behavior can be predicted by the coupling between the particles and gas, which we characterize by a form of the Stokes number (Stimp) where the time scale of changes in fluid motion is defined by the length scale and velocity change associated with vertical deceleration as a mixture approaches an impact surface. We find that the length scale of deceleration is sensitive to the mixture Mach number, as high Mach number flows produce standing shocks upstream of the impact site. For low Stimp monodisperse cases, the particles make the transition from axial to radial flow easily, and the flow continues downstream relatively unimpeded and well mixed. In contrast, in situations where the monodisperse particles are sufficiently poorly coupled (high Stimp), the particles rebound and/or concentrate at the impact zone, which results in a radial acceleration of gas as it is expelled from the concentrating mixture. In polydisperse jets, larger particles become better coupled to the gas in the free jet zone when in the presence of smaller, well-coupled particles due to particle-particle drag. However, the larger particles still lose significant momentum in the impact zone, which results in a lag effect where the smaller particles and gas are expelled and advance radially at a greater velocity. Although the simulations are simplified compared with real volcanic eruption scenarios, the results suggest that processes in the impact zone contribute directly to the formation of different types of gas-particle flows (concentrated versus dilute) that move outward across the ground.

PIV experimental study of the large-scale dynamic airflow structures in an aircraft cabin: Swing and oscillation

The dynamic behaviors of airflows, which are common in aircraft cabins with mixing ventilations, significantly influence the thermal sensation of passengers and contaminant transmissions. In this paper, to analyze the large-scale dynamic characteristics in aircraft cabins, a series of long-term large-scale particle image velocimetry (PIV) measurements were performed in an aircraft cabin mockup to provide necessary basic data. Two typical airflow fields, jet zones characterized as large-scale circulations and collision zones featured as interactions of two lateral jets, were selected as the research objects of the dynamic behaviors in the cabin. In collision zones, the temporal characteristics of the swing motions were presented via proper orthogonal decomposition (POD) method, and the swing periods were 25.2 s–29.8 s and 13.0 s–19.8 s under isothermal and cooling conditions, respectively. By analyzing the occurrence probability of swings, the swing-induced contaminant transmission regularity was presented. In jet zones, the dynamic behavior of large-scale circulation was revealed by identifying the circulation center of each instantaneous flow field. However, the center oscillated in a small range near the breath zone of the passenger seating in the middle of one side, and the plumes under cooling conditions further reduced the range, which explained the lock-up phenomenon founded in contaminant transmission studies in the cabin. Finally, several vortex identification methods with raw and reconstructed instantaneous airflow fields were compared, and their practicalities were discussed.

Multi-zone model for diesel engine simulation based on chemical kinetics mechanism

The main purpose of this study was to develop a new multi-zone model for simulating the diesel engine’s closed loop. The proposed multi-zone model is based on chemical kinetics and uses a semi-detailed chemical kinetics mechanism containing 76 species and 327 reactions to calculate the fuel burning rate at each time step. This chemical kinetics mechanism contains 6 reactions to simulate soot formation and 14 reactions to simulate NOx formation. Prior to fuel injection, the combustion chamber is divided into three zones: inner zone, boundary layer zone, and crevice zone. The model considers the heat and mass transfers between the zones. Convective and radiation heat transfers are considered between the boundary layer zone and combustion chamber walls. When fuel injection begins, the spray is modeled and a zone is formed that contains the fuel jet. The geometry of the fuel jet zone is estimated using the spray-cone angle, and the length of this zone (fuel jet) is estimated using the Higgin’s correlation. The spray-cone angle is calculated using the Reitz and Bracco’s correlation. Fuel spray penetration into other zones is calculated using the Wakuri’s relation. Fick’s law is used to calculate the diffusion rate of different species in each zone. The model results are in good agreement with experimental data in predicting the in-cylinder pressure, the start of the combustion time, combustion duration, and emissions. The maximum error of model for predicting soot and NOx are 17% and 12%, respectively.

PIV measurements on physical models of bottom blown oxygen copper smelting furnace

ABSTRACTA 1/5 scaled physical slice model of a bottom blown oxygen copper smelting furnace was established to investigate the flow field characteristics in the pool and in the freeboard above the pool. Particle image velocimetry technique was applied to measure the flow field in different depths of the pool, installation angle of the nozzle and gas flow rate. It was found that two circulations were divided by the jet zone above the nozzle. Above the nominal liquid surface level, there is a fountain zone where splash and liquid droplets occur. With the growth of pool depth, the distribution of velocity tended to become uniform. The area of dead zone decreased and mean velocity became larger. Different installation angles of nozzle will cause very different flow patterns. As the increase of gas flow rate, the area of dead zone decreased and the mean velocity increased.

On the relationship between the QBO/ENSO and atmospheric temperature using COSMIC radio occultation data

In this paper, the spatial patterns and vertical structure of atmospheric temperature anomalies, in both the tropics and the extratropical latitudes, associated with the El Niño-Southern Oscillation (ENSO) and quasi-biennial oscillation (QBO) in the upper troposphere and stratosphere are investigated using global positioning system (GPS) radio occultation (RO) measurements from the Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC) Formosa Satellite Mission 3 mission from July 2006 to February 2014. We find that negative correlations between the atmospheric temperature in the tropics and ENSO are observed at 17–30km in the lower stratosphere at a lag of 1–4 months and at a lead of 1 month. Out-of-phase temperature variation is observed in the troposphere over the mid-latitude band and in-phase behaviour is observed in the lower stratosphere. Interestingly, we also find that there is a significant negative correlation at a lag of 1–3 months from 32km to 40km in the mid-latitude region of the Northern Hemisphere. The atmospheric temperature variations over mid-latitude regions in both hemispheres are closely related to the QBO. There are also two narrow zones over the subtropical jet zone where the QBO signals are strong in both hemispheres, approximately parallel to the equator. Finally, we develop a new robust index to describe the strength of the ENSO and QBO signal.

Topographic, optical and chemical properties of zinc particle coatings deposited by means of atmospheric pressure plasma

In this research, topographic, optical and chemical properties of zinc oxide layers deposited by a cold plasma-spray process were measured. Here, zinc micro particles were fed to the afterglow of a plasma spark discharge whereas the substrates were placed in a quite cold zone of the effluent plasma jet. In this vein, almost closed layers were realised on different samples. As ascertained by laser scanning and atomic force microscopic measurements the particle size of the basic layer is in the nanometre scale. Additionally, larger particles and agglomerates were found on its top. The results indicate a partial plasma-induced diminishment of the initial particles, most probably due to melting or vaporisation. It is further shown that the plasma gives rise to an increased oxidation of such particles as confirmed by X-ray photoelectron spectroscopy. Quantitative analysis of the resulting mixed layer was performed. It is shown that the deposited layers consist of zinc oxide and elemental zinc in approximately equal shares. In addition, the layer's band gap energy was determined by spectroscopic analysis. Here, considerable UV blocking properties of the deposited layers were observed. Possible underlying effects as well as potential applications are presented.

Maximum energy dissipation to explain velocity fields in shallow reservoirs

ABSTRACTShallow reservoirs are often used as sediment traps or storage basins, in which sedimentation depends on the flow pattern. Short rectangular reservoirs reveal a straight jet from inlet to outlet with identical recirculation zones on both sides. In longer reservoirs, the main jet reattaches to the side of the reservoir leading to small and large recirculation zones. Previous studies have found an empirical geometric relation describing the switch between these two flow patterns. In this study, we demonstrate, with a simple analytical model, that this switch coincides with a maximization of energy dissipation in the shear layer between the main jet and recirculation zones: short reservoirs dissipate more energy when the flow pattern is symmetric, while longer reservoirs dissipate more energy with an asymmetric pattern. This approach enables the prediction of the flow patterns without detailed knowledge of small scale processes, potentially useful in the early phase of reservoir design.

Trains of African Easterly Waves and Their Relationship to Tropical Cyclone Genesis in the Eastern Atlantic

AbstractIn this study, the relationship between trains of African easterly waves (AEWs) and downstream tropical cyclogenesis is studied. Based on 19 summer seasons (July–September from 1990 to 2008) of ERA-Interim reanalysis fields and brightness temperature from the Cloud User Archive, the signature of AEW troughs and embedded convection are tracked from the West African coast to the central Atlantic. The tracked systems are separated into four groups: (i) systems originating from the north zone of the midtropospheric African easterly jet (AEJ), (ii) those coming from the south part of AEJ, (iii) systems that are associated with a downstream trough located around 2000 km westward (termed DUO systems), and (iv) those that are not associated with such a close downstream trough (termed SOLO systems).By monitoring the embedded 700-hPa-filtered relative vorticity and 850-hPa wind convergence anomaly associated with these families along their trajectories, it is shown that the DUO generally have stronger dynamical structure and statistically have a longer lifetime than the SOLO ones. It is suggested that the differences between them may be due to the presence of the previous intense downstream trough in DUO cases, enhancing the low-level convergence behind them. Moreover, a study of the relationship between system trajectories and tropical depressions occurring between the West African coast and 40°W showed that 90% of tropical depressions are identifiable from the West African coast in tracked systems, mostly in the DUO cases originating from the south zone of the AEJ.

Three-dimensional model of air speed in the secondary zone of displacement ventilation jet

This paper presents a new three-dimensional model for the distribution of air speed in the secondary zone of a displacement ventilation jet, combining both physical analysis and correlations-based models. For this purpose the measurements on two wall-mounted DV diffusers are used along with other published data. The maximum air speed at the end of primary zone is predicted by using the supplying conditions at the outlet, the thickness of air jet at the end of the primary zone, and the air entrainment in the jet. The longitudinal profile of maximum air speed in secondary zone is modeled using a new normalization and a generalized profile developed using correlation-based models obtained from experimental data. The vertical and transversal air speed profiles are also presented and analyzed. Finally all air speed profiles in the secondary zone are combined into a three-dimensional air speed model in the DV jet.

Modeling and Process Features of Plug Flow Reactor with Internal Recirculation for Biomass Pyrolysis

A new fluidized bed design is considered for the pyrolysis of biomass, in a plug-flow with internal-recirculation (PFIR) arrangement, where the movement of solids from feed zone to product zone and the circulation within a closed loop are achieved, by the use of directional high-speed tuyeres that induce both the suspension and horizontal movement of the charge. Separation between feed and product zones is provided through one or more underflow weirs. Wet and hard-to-fluidize biomass is targeted, where fluidization and pyrolysis of the feed is achieved by the internal hot sand recirculation. Plug flow can allow for both stages (pyrolysis and heating) to occur inside a single vessel, but at separate zones. Computer simulation of the PFIR reactor is demonstrated to verify that plug-flow with internal- recirculation can be achieved under reasonable operating conditions. A commercial software (CPFD-Barracuda) is used, which shows promising results, including: solids circulation due to directional tuyeres; large solids mass transport rates relative to reactor size; and controllability of the circulation rate via tuyere velocity. It is shown that the underflow weir(s) offer little resistance compared to the frictional losses and, therefore, multiple zones within a single reactor can be achieved with little impact on the solids recirculation rate. The horizontal mass flux is shown to occur predominantly near the bottom Tuyeres (inclined jet zone), hence enabling the underflow weir to effectively separate the feed and product zones, with a small bottom passage for the solids.

Numerical analysis on the impact of the flow field of hydrothermal jet drilling for geothermal wells in a confined cooling environment

Hydrothermal jet technology is a novel drilling method expected to be suitable for the exploitation of deep geothermal resources, in which the rocks are broken coupled by thermal spallation effect with high velocity impact. Because of the high temperature return fluid, the cooling of the drill string and wellbore is one major problem in hydrothermal jet drilling. This paper presents two cooling configurations in downhole conditions: the lateral configuration and downward configuration. The investigation on the impact of the flow field of the hydrothermal jet in a confined cooling environment is estimated using CFD methods. The influences of jet velocity, jet temperature, cooling water velocity and standoff distance are analyzed. For both cooling configurations, the impact of the flow field of a hydrothermal jet in downhole conditions is divided into three zones: the jet zone, the return zone and the eddy zone. The hydrothermal jet impinges on the bottom rock, and then returns from the annulus mixed with the cooling water. The temperature in the jet zone stays constant, while it decreases in the return and the eddy zone. The temperature around the wellbore is higher due to the return flow of the cooling water. There is a high bottomhole temperature region, while the annulus is cooled by the cooling water in the lateral cooling configuration. In addition, the jet velocity, the cooling water velocity and the standoff distance have a large influence on the pressure and temperature fields, while the jet temperature has a small influence. For the downward cooling configuration, the length of the potential core of the hydrothermal jet is positively correlated to the maximum axial velocity. Finally, two cooling configurations are compared. The lateral cooling plan is suitable for reaming operation. The lateral cooling configuration is better in the cuttings carrying capacity due to the higher annular return velocity. Results in this paper could guide for parameters design of hydrothermal jet drilling technology.

Unified Analysis of Multi-Chamber Contact Tanks and Mixing Efficiency Evaluation Based on Vorticity Field. Part II: Transport Analysis

Mixing characteristics of multi-chambered contact tank are analyzed employing the validated three-dimensional numerical model developed in the companion paper. Based on the flow characterization, novel volumetric mixing efficiency definitions are proposed for the assessment of the hydrodynamic and chemical transport properties of the contact tank and its chambers. Residence time distribution functions are analyzed not only at the outlet of each chamber but also inside the chambers using the efficiency definitions for both Reynolds averaged Navier–Stokes (RANS) and large eddy simulation (LES) results. A novel tracer mixing index is defined to characterize short circuiting and mixing effects of the contact system. Comparisons of the results of these indexes for RANS and LES solutions indicate that mixing characteristics are stronger in LES due to the unsteady turbulent eddy mixing even though short circuiting effects are also more prominent in LES results. This result indicates that the mixing analysis based on the LES results simulates the mixing characteristics instantaneously, which is more realistic than that in RANS. Since LES analysis can capture turbulent eddy mixing better than RANS analysis, the interaction of recirculation and jet zones are captured more effectively in LES, which tends to predict higher turbulent mixing in the contact system. The analysis also shows that the mixing efficiency of each chamber of the contact tank is different, thus it is necessary to consider distinct chemical release and volumetric designs for each chamber in order to maximize the mixing efficiency of the overall process in a contact tank system.

Unified Analysis of Multi-Chamber Contact Tanks and Mixing Efficiency Based on Vorticity Field. Part I: Hydrodynamic Analysis

Multi-chamber contact tanks have been extensively used in industry for water treatment to provide potable water to communities, which is essential for human health. To evaluate the efficiency of this treatment process, flow and tracer transport analysis have been used in the literature using Reynolds averaged Navier–Stokes (RANS) and large-eddy simulations (LES). The purpose of this study is two-fold. First a unifying analysis of the flow field is presented and similarities and differences in the numerical results that were reported in the literature are discussed. Second, the vorticity field is identified as the key parameter to use in separating the mean flow (jet zone) and the recirculating zones. Based on the concepts of vorticity gradient and flexion product, it is demonstrated that the separation of the recirculation zone and the jet zone, fluid-fluid flow separation, is possible. The separation of the recirculation zones and vortex core lines are characterized using the definition of the Lamb vector. The separated regions are used to characterize the mixing efficiency in the chambers of the contact tank. This analysis indicates that the recirculation zone and jet zone formation are three-dimensional and require simulations over a long period of time to reach stability. It is recognized that the characteristics of the jet zones and the recirculation zones are distinct for each chamber and they follow a particular pattern and symmetry between the alternating chambers. Hydraulic efficiency coefficients calculated for each chamber show that the chambers having an inlet adjacent to the free surface may be designed to have larger volumes than the chambers having wall bounded inlets to improve the efficiency of the contact tank. This is a simple design alternative that would increase the efficiency of the system. Other observations made through the chamber analysis are also informative in redefining the characteristics of the efficiency of the contact tank system.

Erosion of a confined stratified layer by a vertical jet – Detailed assessment of a CFD approach against the OECD/NEA PSI benchmark

Recently, a blind CFD benchmark exercise was conducted by the OECD/NEA (2013–2014) based on an experiment in the PANDA facility at the Paul Scherrer Institute (PSI) in Switzerland, investigating the turbulent erosion of a stratified helium rich layer in the upper region of the test vessel by means of a vertical air-helium jet impinging from below. In addition to the ‘classical’ pointwise measurements available for similar experiments conducted in the past, significant additional efforts were spent on the experimental characterization of the underlying flow field and turbulent quantities by means of particle image velocimetry (PIV) for the benchmark. This data is well suited for a detailed assessment of the driving jet flow and its interaction with the stratified layer. Both are essential in order to avoid elimination of different errors, which is possible if validation is performed in a global manner.Different impacts on the simulation results, in particular on the jet profile and on the mixing progress, are discussed in this paper. A systematic validation is carried out based on measured and derived quantities. It is identified that e.g. the mesh resolution in the jet and mixing zone has only a minor impact, while small changes in turbulence modeling strategy or the chosen model constants, like Sct, significantly affect the simulation results. Finally, the chosen unsteady RANS model represents mixing process consistently in the transient progression and instantaneous flow variables, while an unexpected difference between the k–ε-SST and the standard k–ε model in predicting the jet flow was identified.

Modelling mass transfer in gas‐liquid two‐phase flow in a jet zone loop reactor

Experimentally determined mass transfer rates of the gas‐liquid multiphase flow in a jet zone loop reactor are compared to several existing model approaches. The experimental data deviate due to insufficient consideration of the local hydrodynamic effects in the models. It is identified that bubble surface movement and its impact on the turbulence are not yet appropriately pictured. A new approach for describing the mass transfer coefficient in the riser of the jet zone loop reactor is derived, including the effect of mass transfer enhancement by bubble‐induced turbulence.

The Phenomenon of Bubbles Negative Relative Velocity in Vertical Bubbly Jets

Many experiments demonstrate that the bubble relative (slip) velocities in vertical turbulent sheared bubbly flows are significantly lower than those in quiescent infinite fluid. Moreover, vertical bubbly jet experiments performed by Sun and Faeth (1986, “Structure of Turbulent Bubbly Jets-1. Methods and Centerline Properties,” Int. J. Multiphase Flow, 12(1), pp. 99–114) indicate that bubble slip velocities have negative values in the high sheared zone near the injector. The present analysis shows that the phenomenon of the slip velocity inversion is associated with the effect of the turbulent part of the interfacial force. A new formulation of the turbulent contribution of the added mass force is proposed. This formulation is analyzed using the vertical bubbly jet experimental data. The results provide evidence that the turbulent contribution of the added mass force is at the origin of the slip velocity reduction and could explain the appearance of the negative values observed in bubbly jet experiments. As a whole, the turbulent contribution of the added mass force which comprises two terms (a nonlinear turbulent term and a convective acceleration term associated to the drift velocity) opposes the action of the gravity and their effect may be high enough to produce negative slip velocities. Taken separately, the two turbulent terms cannot explain the reversal and the reduction of slip through the entire section in the near injection zone of the bubbly jet. The combined effect of the two turbulent terms makes it possible to reproduce slip velocity profiles as observed in the near injection zone.

Helical Structures in the Near Field of a Turbulent Pipe Jet

We perform a finely resolved Large-eddy simulation to study coherent vortical structures populating the initial (near-nozzle) zone of a pipe jet at the Reynolds number of 5300. In contrast to ‘top-hat’ jets featured by Kelvin-Helmholtz rings with the non-dimensional frequency S t≈0.3−0.6, no high-frequency dominant mode is observed in the near field of a jet issuing from a fully-developed pipe flow. Instead, in shear layers we observe a relatively wide peak in the power spectrum within the low-frequency range (S t≈0.14) corresponding to the propagating helical waves entering with the pipe flow. This is confirmed by the Fourier transform with respect to the azimuthal angle and the Proper Orthogonal Decomposition complemented with the linear stability analysis revealing that this low-frequency motion is not connected to the Kelvin-Helmholtz instability. We demonstrate that the azimuthal wavenumbers m=1−5 contain the most of the turbulent kinetic energy and that a common form of an eigenmode is a helical vortex rotating around the axis of symmetry. Small and large timescales are identified corresponding to “fast” and “slow” rotating modes. While the “fast” modes correspond to background turbulence and stochastically switch from co- to counter-rotation, the “slow” modes are due to coherent helical structures which are long-lived and have low angular velocities, in agreement with the previously described spectral peak at low S t.

Characteristics of quasi-liquid metal layer in the cumulative jet and barrier contact zone

According to the known experimental data on explosive metal processing, as well as the theoretical and experimental studies on the sintered metal powders, we estimated the minimum pressure and temperature values at which this layer and its thickness are formed. The study used a hypothetical model of forming a quasi-liquid metal layer between the leader of the cumulative jet, and a barrier at the time of its destruction.

Interplay between electrical and rheological properties of viscoelastic inks

We investigated the effect of the interaction between the electrical and rheological properties of the ink in electrodydrodynamic (EHD) printing process by designing model systems that control both the electrical conductivity and the viscoelasticity of the ink to observe how they affect the Taylor cone jet formation. The results demonstrate that the initial voltage at which the Taylor cone jet first appears and the final voltage at which the jet becomes unstable increase at higher electrical conductivity as the conical shape with large surface area is formed and grants stability at higher conductivity. Increased viscosity and elasticity also lead to the similar result: increase in the initial and final voltages, which can be attributed to the slower charge transport that minimizes the stabilizing effect of the inks’ electrical conductivity. In addition, we use two dimensionless variables (dimensionless flow rate and dimensionless voltage) to make an operating window map of the EHD process. Through this map, the processing condition for the Taylor cone jet zone can be predicted with respect to the effect of the interplay between the electrical and rheological properties of the ink.

Cyclones, windstorms and the IMILAST project

By way of introduction to the TELLUS thematic cluster on outcomes of the IMILAST project (Intercomparison of MId-LAtitude STorm diagnostics), this paper presents the results of new research that is fundamental for the correct interpretation of IMILAST results. Specifically we investigated the mesoscale structure of cyclonic windstorms, and the representation of those windstorms in re-analysis data. The paper concludes with an overview of the project itself. Twenty-nine historic windstorms are studied in detail, using wide-ranging observational data, and on this basis a conceptual model of the life cycle of a typical windstorm-generating cyclone is developed. The model delineates three wind phenomena, the warm jet, the sting jet and the cold jet, and maps out the typical damage footprint left by each. Focussing on the boundary layer, the physical processes at work in each jet zone are investigated. These include the impact of near-surface stability and exposure on gust strength. Based on numerous cases, a generic description of the sting jet is provided, with many new features highlighted. This phenomenon looks to be unique in that exceptional gusts can be realised well inland because destabilisation is activated from above. We next investigate how well the widely-referenced ERA-Interim re-analysis, that has been a primary data source for IMILAST, can represent windstorms. In many ways, performance is suboptimal. Compared to a benchmark manually-analysed dataset, windstorm-generating cyclones generally do not deepen rapidly enough. In part, this is a resolution limitation. For one medium-sized cyclone, it is shown, using other models, that horizontal resolution of order 20 km or better is required to capture the most damaging winds. In the context of IMILAST, which has used data at resolutions ≥80 km, this is a fundamental result. For this and other reasons, caution is clearly needed when inferring storm behaviour and severity from model-based metrics.