Jet Geometry Research Articles

Near-zero emissions with high thermal efficiency realized by optimizing jet plume location relative to combustion chamber wall, jet geometry and injection timing in a direct-injection hydrogen engine

Thermal efficiency was substantially improved and NOx emissions were reduced to a level at a single-digit ppm with PCC combustion by optimizing such characteristics as the direction, number and diameter of the injected jet and controlling the injection timing and also by combining with combustion of lean mixture. Output power declined by lean mixture was recovered by supercharging in keeping NOx emissions remained at the same level, while thermal efficiency was improved furthermore by slightly re-optimizing jet conditions. As a result, hydrogen engine which does not emit any CO2 and particulate matter in principle is worth to be called near-zero emission engines in both name and reality.

Wear Mechanism of Abrasive Gas Jet Erosion on a Rock and the Effect of Abrasive Hardness on It

The existing erosion models of abrasive gas jet tend to neglect the effects of the rebounding abrasive. To address this shortcoming, abrasive wear tests were conducted on limestone by using an abrasive gas jet containing different types of particles and with different standoff distances. The results indicate that erosion pits have the shape of an inverted cone and a hemispherical bottom. An annular platform above the hemispherical bottom connects the bottom with the side of the pit. The primary cause of the peculiar pit shape is the flow field geometry of the gas jet with its entrained particles. There is an annular region between the axis and boundary of the abrasive gas jet, and it contains no abrasive. Particles swirling around the axis form a hemispherical bottom. After rebounding, the abrasive with the highest velocity enlarges the diameters of both the hemispherical bottom and erosion pit and induces the formation of an annular platform. The surface features of different areas of the erosion pit are characterized using a scanning electron microscope (SEM). It can be concluded that the failure modes for different locations are different. The failure is caused by an impact stress wave of the incident abrasive at the bottom. Plastic deformation is the primary failure mode induced by rebounding particles at the sides of the hemispherical bottom. The plastic deformation induced by the incident abrasive and fatigue failure induced by the rebounding abrasive are the primary failure modes on the annular platform. Fatigue failure induced by rebounding particles is the primary mode at the sides of the erosion pits. The rock failure mechanism that occurs for particles with different hardness is the same, but the rock damaged by the hard abrasive has a rougher surface.

Sweeping jet impingement heat transfer on a simulated turbine vane leading edge

With the development of additive manufacturing technology, it is now possible to design complex and integrated internal cooling architecture for a gas turbine engine. In search of a spatially uniform heat transfer at the leading edge of a turbine nozzle guide vane, a sweeping jet impingement cooling strategy was proposed. Experiments were conducted in a low-speed wind tunnel to investigate sweeping jet impingement cooling in a faired cylinder leading edge model at an engine-relevant Biot number (Bi). Sweeping jets were generated with additively manufactured fluidic oscillator and steady jets were produced by a cylindrical orifice (with length to diameter ratio of 1). Both sweeping and steady jets were studied at varying mass flow rates, jet-to-wall spacing (H/D), jet pitch (P/D), and freestream turbulence. The effect of varying aspect ratio (AR) of the sweeping jet geometries was also studied. The overall cooling effectiveness of each configuration was estimated using infrared thermography (IR) measurements of the external surface temperature of the leading edge model. The sweeping jet provided higher overall cooling effectiveness values compared to steady jet in specific configurations. The pressure drop across each jet was also measured for each geometry, and the sweeping jet shows comparable pressure drop to steady jet.

Uniform surface modification of polyethylene terephthalate (PET) by atmospheric pressure plasma jet with a horn-like nozzle

This work reports on surface modification of polyethylene terephthalate (PET) polymer by an atmospheric pressure plasma jet (APPJ) operated with argon. A distinguishable feature of this device is that it terminates with a conical horn-like nozzle. Three different nozzles diameters were employed with the purpose to obtain uniform surface modification over large area. Treatments in small 3D objects that fit inside the conical horn were also conducted. In this study, water contact angle (WCA) measurements and X-ray photoelectron spectroscopy (XPS) were performed to assess the samples wettability and the surface elemental composition, as well as, their radial distribution. Plasma-induced changes on the polymer surface morphology were evaluated by Atomic Force Microscopy (AFM). Electrical characterization of the plasma and investigation of the effect of the gas flow rate on the discharge power were carried out. After the plasma treatment PET surface became more hydrophilic over the entire area covered by the nozzle. This effect is caused by the incorporation of oxygen containing polar groups on the surface. It was also observed that depending on process parameters, the plasma treatment can extend even outside the area of the conical horn. The degree of surface modification depends on plasma dose while the treatment uniformity is determined mostly by the distance to the sample. Overall, a quite uniform surface modification was obtained over the entire area covered by the jet nozzle. Thus, the results suggest that by simply changing the jet geometry and choosing the right treatment parameters one can achieve a uniform treatment over an area whose size is determined by the horn diameter.

Parameterizing multi-vent activity at Stromboli Volcano (Aeolian Islands, Italy)

The crater terrace of Stromboli Volcano (Italy) hosts several active vents which have evolved and migrated through time within three main vent areas: south-west (SW), central (C), and north-east (NE). Frequent, jet-like explosions typically take place, episodically interrupted by larger-scale paroxysms, which can substantially modify the morphology of the crater terrace and vent geometries. However, the link between the time-space evolution of vent activity and the shallow conduit system are still a matter of debate. In this work, we analyze the vent position and explosion parameters (jet duration and geometry) of 4296 events at Stromboli in five 72-h-long time-windows between 2005 and 2009, as recorded by an infrared surveillance camera. Vent locations illustrate the resilience of the shallow conduit system, which controls explosive activity at different time scales and depths. At the shallowest depth, where slugs burst, conduit branching and merging determines the evolution of simultaneous or alternating twin vents, while vent shape and slug size control local explosion parameters. These processes show variability on an hourly to daily time scale. Below the depth of the slug burst, the conduit system feeding each vent area controls which specific vent will host the explosions and also, possibly, the size of the slugs. Several observations suggest that the C and SW vent areas may be connected at this depth. The deeper conduit system, common to all vent areas, sets the overall explosion rate of the volcano and maintains a balance of this rate between the NE and the combined SW and C vent areas.

Impact of plasma jet geometry on residence times of radical species

Numerous electrode geometries and power supplies, both commercial and in-house, have been employed for the generation of low-temperature atmospheric plasma jets. In this work, the development and operation of a 12 jet nonthermal atmospheric plasma system is presented. The study is based on optical spectroscopy as a diagnostic method due to its nonintrusive nature. A key focus of this study was the material selection (conductive and nonconductive), with several polymers screened for the jet design leading to polyacetal as the choice material. Their results are compared with other atmospheric plasma jet systems. The results show a significant increase in residence time and the spatial homogeneity for ambient air's main species, including: OH, O I, O2, O3, N2, and N2+. Their densities are studied with respect to treatment time, distance, duty cycle, and discharge frequency, as well as the jets' carrier gas chemistries (argon and helium). For their plasma jet system, the bulk of the chemical reactions occur in the surrounding atmosphere and not in the jet nozzle, which is different from most other plasma jet systems. The electron energy distribution function, for the given chemistries, is also reported.

Effects of Effusion and Film Cooling Jet Momenta on Combustor Flow Fields

The effects of effusion and film cooling momenta on combustor flow fields are investigated. Steady, compressible three-dimensional (3D) simulations are performed on a single-swirler combustor using Reynolds-averaged Navier–Stokes (RANS) with flamelet generated manifold and Lagrangian–Eulerian multiphase spray, while accounting for dome and liner cooling. Two simulations are performed on the same mesh. One simulation is conducted using a parallelized, automated, predictive, imprint cooling (PAPRICO) model with dynamic flux boundary conditions and downstream pressure probing (DFBC-DPP). PAPRICO involves removing the cooling jet geometry from the dome and liner while retaining the cooling hole imprints. The PAPRICO model does not require a priori knowledge of the cooling flow rates through various combustor liner regions nor specific mesh partitioning. The other simulation is conducted using the homogenously patched cooling (HPC) model, which involves removing all the cooling jets. The HPC model applies volumetric sources adjacent to the combustor wall regions where cooling jets are present. The momentum source, however, becomes negligible. The HPC model is not predictive and requires tedious ex situ mass flow measurements from an auxiliary flowbench experiment, afflicted with systematic errors. Hence, the actual in situ air flow splits through the several combustor regions is not known with absolute certainty. The numerical results are compared with measurements of mass flow rates, static pressure drops, and path-integrated temperatures. The results demonstrate that it is critical to account for the discrete dome and liner cooling momentum to better emulate the reacting flow in a combustor.

The University of Michigan Centimeter-Band All Stokes Blazar Monitoring Program: Single-Dish Polarimetry as a Probe of Parsec-Scale Magnetic Fields

The University of Michigan 26-m paraboloid was dedicated to obtaining linear polarization and total flux density observations of blazars from the mid-1960s until June 2012, providing an unprecedented record tracking centimeter-band variability over decades at 14.5, 8.0, and 4.8 GHz for both targeted objects and members of flux-limited samples. In the mid-1970s through the mid-1980s, and during the last decade of the program, observations were additionally obtained of circular polarization for a small sample of radio-bright (S > 5 Jy), active sources. Key program results include evidence supporting class-dependent differences in the magnetic field geometry of BL Lac and QSO jets, identification of linear polarization changes temporally associated with flux outbursts supporting a shock-in-jet scenario, and determination of the spectral evolution of the Stokes V amplitude and polarity for testing proposed models. Recent radiative transfer modeling during large flares supports a jet scenario with a kinetically-dominated, relativistic flow at parsec scales with embedded turbulent magnetic fields and dynamically-weak ordered components which may be helical; the circular polarization observations are consistent with linear-to-circular mode conversion within this turbulent jet environment.

Jet creation in post-AGB binaries: the circum-companion accretion disk around BD+46°442

We aim at describing and understanding binary interaction processes in systems with very evolved companions. Here, we focus on understanding the origin and determining the properties of the high-velocity outflow observed in one such system. We present a quantitative analysis of BD+46$^{\circ}$442, a post-AGB binary which shows active mass transfer that leads to the creation of a disk-driven outflow or jet. We obtained high-resolution optical spectra from the HERMES spectrograph, mounted on the 1.2m Flemish Mercator Telescope. By performing a time-series analysis of the Halpha profile, we dissected the different components of the system. We deduced the jet geometry by comparing the orbital phased data with our jet model. In order to image the accretion disk around the companion of BD+46$^{\circ}$442, we applied the technique of Doppler tomography. The orbital phase-dependent variations in the Halpha profile can be related to an accretion disk around the companion, from which a high-velocity outflow or jet is launched. Our model shows that there is a clear correlation between the inclination angle and the jet opening angle. The latitudinally dependent velocity structure of our jet model shows a good correspondence to the data, with outflow velocities at least higher than 400km/s. We show that BD+46$^{\circ}$442, is a result of a binary interaction channel. The origin of the fast outflow in this system can be attributed to a gaseous disk around the secondary component, which is most likely a main sequence star. Our analysis suggests the outflow to have a rather wide opening angle instead of being strongly collimated. Similar orbital phase-dependent Halpha profiles are commonly observed in post-AGB binaries. Post-AGB binaries provide ideal test bets to study jet formation and launching mechanisms over a wide range of orbital conditions.

Photospheric emission in gamma-ray bursts

A major breakthrough in our understanding of gamma-ray bursts (GRB) prompt emission physics occurred in the last few years, with the realization that a thermal component accompanies the over-all nonthermal prompt spectra. This thermal part is important by itself, as it provides direct probe of the physics in the innermost outflow regions. It further has an indirect importance, as a source of seed photons for inverse-Compton scattering, thereby it contributes to the nonthermal part as well. In this short review, we highlight some key recent developments. Observationally, although so far it was clearly identified only in a minority of bursts, there is indirect evidence that a thermal component exists in a very large fraction of GRBs, possibly close to 100%. Theoretically, the existence of a thermal component has a large number of implications as a probe of underlying GRB physics. Some surprising implications include its use as a probe of the jet dynamics, geometry and magnetization.

Laser cutting using off-axial supersonic rectangular nozzles

Laser cutting is a well-established process in the industry, mainly for cutting metallic materials such as steels. However, the utilization of this laser technique to process advanced materials, such as ceramics or some aluminium alloys, is restricted due to the formation of a large heat affected zone (HAZ), rough cut edges, presence of dross, or thermal cracks. These defects are closely related to the performance of the assist gas during the removal of molten material. Previous works have demonstrated the impressive improvement in cut quality with the utilization of off-axial supersonic axisymmetric nozzles to inject the assist gas. This is due to the higher removal of molten material. However, these results can be even improved if the geometry of the gas jet is tailored to the cut kerf. In the present work, a cutting head with an off-axial supersonic rectangular nozzle was developed to improve the efficiency of the current assist gas injection systems. The performance of this system was compared with that of a converging (coaxial) nozzle, and an off-axial supersonic axisymmetric nozzle during the processing of Al2024-T3 sheets. Results demonstrated the superior performance of the new assist gas injection system in terms of cut quality and productivity (cut edge roughness was 6.5 times lower, and the cutting speed is 1.98 times higher as compared to the results found for a converging coaxial nozzle).

The impact of dilatation, scrambling, and pressure transport in turbulent premixed flames

Premixed turbulent flames feature strong interactions between chemical reactions and turbulence that affect scalar and turbulence statistics. The focus of the present work is on clarifying the impact of pressure dilatation/flamelet scrambling effects with a comprehensive second-moment closure used for evaluation purposes. Model extensions that take into account flamelet orientation and molecular diffusion are derived. Isothermal pressure transport is included with an additional variable density contribution derived for the flamelet regime of combustion. Full closure is assessed by comparisons with Direct Numerical Simulations (DNSs) of statistically ‘steady’ fully developed premixed turbulent planar flames at different expansion ratios. Subsequently, the prediction of lean premixed turbulent methane–air flames featuring fractal grid generated turbulence in an opposed jet geometry is considered. The overall agreement shows that ‘dilatation’ effects contribute to counter-gradient transport and can also increase the turbulent kinetic energy significantly. Levels of anisotropy are broadly consistent with the DNS data and key aspects of opposed jet flames are well predicted. However, it is also shown that complications arise due to interactions between the imposed pressure gradient and combustion and that redistribution is affected along with the scalar flux at the leading edge. The latter is strongly affected by the reaction rate closure and, potentially, by pressure transport. Overall, the derived models offer significant improvements and can readily be applied to the modelling of premixed turbulent flames at practical rates of heat release.

Numerical investigation of gas-disperse jet flows created by coaxial nozzles during the laser direct material deposition

The paper presents the results of modelling of powder transport during the laser direct material deposition, involving standard coaxial nozzles made by Trumpf and Precitec and radiation of a СО2-laser with the power up to 4kW. The patterns of particle trajectories positions are shown in the space for various geometries of coaxial nozzles (the double and triple ones). It is shown that collisions of particles and walls of the transport channel impose the governing effect on the particle trajectories, geometry of the powder jet, and density of particle's distribution in the flow and on the substrate. Inelastic collisions with walls cause velocity decrease after the reflection, which has a positive effect on the powder flow profile and focusing. Heating of particles depends on the position of their trajectories and residence time in the high-intensity radiation area. It is also shown that the triple coaxial nozzle has wider options to control the gas-disperse flows than the double coaxial nozzle. The preferable position of the substrate, 20±5mm, is found by calculations; here, the focusing of the powder jet into the laser spot and heating of the super-alloy Ti-6Al-4V particles in the radiation field are optimal. The results can be used to improve the laser cladding and direct material deposition process.

Planar liquid jet: Early deformation and atomization cascades

The temporal evolution of three-dimensional instabilities on a planar liquid sheet segment is studied using direct numerical simulation, and the level-set and volume-of-fluid methods for the liquid-gas interface tracking. Three atomization cascades are distinguished at early breakup, which are well categorized on a gas Weber number (Weg) versus liquid Reynolds number (Rel) map. These atomization processes include lobe stretching that occurs at low Rel and low Weg, hole and bridge formation that occurs at moderate Rel and high Weg, and lobe corrugation occurring at high Rel and low Weg. Qualitative comparison between the sizes of the ligaments and droplets that result from each process is presented. A transitional region between the prescribed atomization domains is found. At high Rel, the transitional boundary is a constant Ohnesorge line defined based on gas We and liquid Re (Ohm≡Weg∕Rel). At low Rel, the transitional region follows a hyperbolic line on the Weg–Rel plot. These atomization processes are qualitatively independent of the jet geometry—seen in both planar and round liquid jets. At a constant density ratio, characteristic times for the hole formation and for the lobe and ligament stretching are different—the former depending on surface tension and the latter on liquid viscosity. In the transitional region, both characteristic times are of the same order.

Extreme jet ejections from the black hole X-ray binary V404 Cygni

We present simultaneous radio through sub-mm observations of the black hole X-ray binary (BHXB) V404 Cygni during the most active phase of its June 2015 outburst. Our $4$ hour long set of overlapping observations with the Very Large Array, the Sub-millimeter Array, and the James Clerk Maxwell Telescope (SCUBA-2), covers 8 different frequency bands (including the first detection of a BHXB jet at $666 \,{\rm GHz}/450\mu m$), providing an unprecedented multi-frequency view of the extraordinary flaring activity seen during this period of the outburst. In particular, we detect multiple rapidly evolving flares, which reach Jy-level fluxes across all of our frequency bands. With this rich data set we performed detailed MCMC modeling of the repeated flaring events. Our custom model adapts the van der Laan synchrotron bubble model to include twin bi-polar ejections, propagating away from the black hole at bulk relativistic velocities, along a jet axis that is inclined to the line of sight. The emission predicted by our model accounts for projection effects, relativistic beaming, and the geometric time delay between the approaching and receding ejecta in each ejection event. We find that a total of 8 bi-polar, discrete jet ejection events can reproduce the emission that we observe in all of our frequency bands remarkably well. With our best fit model, we provide detailed probes of jet speed, structure, energetics, and geometry. Our analysis demonstrates the paramount importance of the mm/sub-mm bands, which offer a unique, more detailed view of the jet than can be provided by radio frequencies alone.

General relativistic study of astrophysical jets with internal shocks

We explore the possibility of formation of steady internal shocks in jets around black holes. We consider a fluid described by a relativistic equation of state, flowing about the axis of symmetry ($\theta=0$) in a Schwarzschild metric. We use two models for the jet geometry, (i) a conical geometry and (ii) a geometry with non-conical cross-section. Jet with conical geometry is smooth flow. While the jet with non-conical cross section undergoes multiple sonic point and even standing shock. The jet shock becomes stronger, as the shock location is situated further from the central black hole. Jets with very high energy and very low energy do not harbour shocks, but jets with intermediate energies do harbour shocks. One advantage of these shocks, as opposed to shocks mediated by external medium is that, these shocks have no effect on the jet terminal speed, but may act as possible sites for particle acceleration. Typically, a jet with energy $1.8~c^2$, will achieve a terminal speed of $v_\infty=0.813c$ for jet with any geometry. But for a jet of non-conical cross-section for which the length scale of the inner torus of the accretion disc is $40\rg$, then in addition, a steady shock will form at $\rsh \sim 7.5\rg$ and compression ratio of $R\sim 2.7$. Moreover, electron-proton jet seems to harbour the strongest shock. We discuss possible consequences of such a scenario.

MOJAVE – XIV. Shapes and opening angles of AGN jets

We present 15 GHz stacked VLBA images of 373 jets associated with active galactic nuclei (AGN) having at least five observing epochs within a 20 yr time interval 1994-2015 from the MOJAVE programme and/or its precursor, the 2 cm VLBA Survey. These data are supplemented by 1.4 GHz single-epoch VLBA observations of 135 MOJAVE AGNs to probe larger scale jet structures. The typical jet geometry is found to be close to conical on scales from hundreds to thousands of parsecs, while a number of galaxies show quasi-parabolic streamlines on smaller scales. A true jet geometry in a considerable fraction of AGNs appears only after stacking epochs over several years. The jets with significant radial accelerated motion undergo more active collimation. We have analysed total intensity jet profiles transverse to the local jet ridgeline and derived both apparent and intrinsic opening angles of the flows, with medians of $21.5\deg$ and $1.3\deg$, respectively. The Fermi LAT-detected gamma-ray AGNs in our sample have, on average, wider apparent and narrower intrinsic opening angle, and smaller viewing angle than non LAT-detected AGNs. We have established a highly significant correlation between the apparent opening angle and gamma-ray luminosity, driven by Doppler beaming and projection effects.

Abrasion erosion modeling in particulate flow

Solid particle erosion is of great importance to many industries including oil and gas production, drilling, process and transportation of minerals including oil sands. The particles that may cause erosion are of various sizes, shapes and hardnesses. These particles may impact the surface at various speeds and angles, and the influence of these parameters is characterized to some extent in the literature. Experimental and numerical studies have shown that when particles are transported by liquid (e.g. slurry transport in the pipe) or dense gas, the particle impact angles are very low. The impact angles in these cases are sometimes less than the smallest value that can be obtained in a direct impingement erosion test in gas. In this work, the mechanistic erosion equation developed previously is extended to near zero impact angles for sharp particles. The abrasion erosion equation is developed by introducing an initial penetration of the sharp particle in the equation of the motion. This initial penetration may be attributed to the sharp particle rotation and/or turbulent flow fluctuations of the carrier fluid near the wall. The empirical constants are obtained from submerged erosion experiments in liquid. The equation has been implemented in a commercially available Computational Fluid Dynamics (CFD) code (ANSYS FLUENT) to calculate erosion for a submerged impingement jet geometry, and the result is compared with the experiment. It is shown that by neglecting the abrasive term in the erosion equation, the specimen mass loss does not match the experimental measured mass loss. While by including the abrasive term, the total mass loss of the specimen agrees well with the experimental data. Moreover, the erosion pattern becomes closer to the experimentally measured pattern especially farther from the center of the highly eroded area where abrasive wear is expected to dominate.

Faraday rotation in GRMHD simulations of the jet launching zone of M87

Non-VLBI measurements of Faraday rotation at mm wavelengths have been used to constrain mass accretion rates ($\mdot$) onto supermassive black holes in the centre of the Milky Way and in the centre of M87. We constructed general relativistic magnetohydrodynamics models for these sources that qualitatively well describe their spectra and radio/mm images invoking a coupled jet-disk system. Using general relativistic polarized radiative transfer, we now also model the observed mm rotation measure (RM) of M87. The models are tied to the observed radio flux, however, electron temperature and accretion rate are degenerate parameters and are allowed to vary. For the inferred low viewing angles of the M87 jet, the RM is low even as the black hole $\mdot$ increases by a factor of $\simeq100$. In jet-dominated models, the observed linear polarization is produced in the forward-jet, while the dense accretion disk depolarizes the bulk of the near-horizon scale emission which originates in the counter-jet. In the jet-dominated models, with increasing $\mdot$ and increasing Faraday optical depth one is progressively sensitive only to polarized emission in the forward-jet, keeping the measured RM relatively constant. The jet-dominated model reproduces a low net-polarization of $\simeq1$ per cent and RMs in agreement with observed values due to Faraday depolarization, however, with $\mdot$ much larger than the previously inferred limit of $9\times10^{-4}\,\mdotu$. All jet-dominated models produce much higher RMs for inclination angles $i\gtrsim30^\circ$, where the line-of-sight passes through the accretion flow, thereby providing independent constraints on the viewing geometry of the M87 jet.

Use of dual planar jets for the reduction of flow-induced noise

The planar jet has been proposed to reduce the flow-induced noise. However, the self-noise emission from a single planar jet is a limiting factor on the performance based characteristics of this technology. Therefore, this research paper reports an alternative dual planar jets geometry in an attempt to minimize the self-noise of the single planar jet, thus, further enhancing total noise reduction in the application. Tandem cylinders are considered as the generic bluff body, which represents the source of flow-induced noise. Tests are conducted to capture noise reduction effects when using a single planar jet geometry. Subsequently, two different configurations of the dual planar jets geometry were tested in comparison. It is observed that the dual jets geometry achieves more noise reduction when compared to the single jet. Moreover, conclusions are made on the most efficient configuration in the dual jets geometry.