Jet Core Research Articles

Stagnation Point Heat Transfer to an Axisymmetric Impinging Jet at Transition to Turbulence

Abstract A round air jet issuing from a long straight pipe and impinging perpendicularly onto a heated flat plate was investigated experimentally. The Reynolds number (Re) covered the fully laminar, transitional, and fully turbulent regimes—Re = 850–15,400. The main focus of this investigation was the transition regime, which occurred at Re = 2250–3010. Various measurements were recorded during the experiments using a hot-wire anemometer, an infrared thermometer, and a thermopile heat flux sensor; the mass transfer was measured using the naphthalene sublimation technique. The stagnation point heat transfer was correlated to the laminar and turbulent regimes in the form of the stagnation point Nusselt number, Nu0 = CRemPr0.4, where the exponent m = 0.50 and 0.55, respectively. The Nu0–Re relationship exhibited nonmonotonic behavior (decrease) in the transition regime. Two counteracting mechanisms occur during transition—jet core shortening and an increase in velocity fluctuation; the former reduces Nu0, whereas the latter increases it.

Heat Transfer and Fluid Dynamics Study in Solar Air Heater Employing Impinging Circular Air Jet Array for Effective Jet Stability

Abstract This experimental and numerical investigation is intended to increase heat transfer, reduce pumping power, and present important guidelines on the use of confined submerged air jet impingement in solar air heater designs that employ an array of circular air jets for high thermohydraulic performance. In order to increase thermohydraulic performance, a novel approach is adopted, i.e., by improving jet stability and consequently jet deflection that gives origination to four solar air heater designs for this investigation. The obtained numerical results revealed that thermohydraulic performance improves significantly by stabilizing the air jet array, which means lowering the jet deflection. The thermohydraulic performance of design-I is high compared to design-II, design-III, and design-IV, both at equal mass flow rate and Reynolds number. Moreover, design-I presents 20%, 33%, and 52% relative improvement in deviation angle of the jet core compared to design-II, design-III, and design-IV, respectively, at a minimum mass flow rate of 0.01 kg/s and Reynolds number of Re=2000, respectively. In addition, correlations to define heat transfer and fluid dynamics characteristics are established. An important guideline upon the use of solar air heater with jet impingement is that a solar air heater of short length and height (jet and plate spacing), and large width should be used for high thermohydraulic performance.

Effect of bypass ratio on sonic underexpanded co-flow jets with finite lip thickness

Abstract The characteristics of a sonic under-expanded coaxial jet with lip thickness 1.5D p (where D p is the exit diameter of primary jet equals 10 mm) with the primary jet operating at nozzle pressure ratio (NPR) of 3, 4 and 5. For NPR 3 operating primary jet, the secondary jet operates at NPR 2.5, 1.4 and 1.27. For primary jet NPR 4, the secondary jet operating NPR is 3.2, 1.6 and 1.4. For NPR 5 primary jet, the secondary jet NPR is 3.8, 1.89 and 1.52. The study is performed using a co-flow nozzle of bypass ratio (BR) 6.4, 1.4 and 0.7. The core length of the primary jet is used as a measure to quantify the mixing of the primary jet in the presence of coaxial jet. The shock structure present in the near field was viewed using shadowgraph technique. Centreline pitot pressure distribution, radial spread and waves present in the jet core were analyzed. The results show that the mixing associated with the high bypass coaxial jet is superior to the low bypass coaxial jet. This mixing superiority associated with high bypass coaxial jet prevails all levels of expansion.

Piston bowl shape and biodiesel fuel effects on combustion and emission of diesel engines

This study aims to investigate the effects of piston bowl shape and biodiesel fuel on combustion and emissions of a direct injection (DI) diesel engine. Three configurations of piston bowl geometry are modeled: hemispherical, toroidal, and rectangular. Biodiesel fuel derived from soybean oil is being considered. An experimental work was conducted on a diesel engine with a hemispherical piston bowl, and a better validity was attained. The study showed that, diesel engine with toroidal piston bowl delivered the largest power than other shapes, where turbulent regions are formed inside the toroidal piston bowl with higher intensity, which resulted in better combustion characteristics and lower emission concentrations. It is observed from the combustion of biodiesel fuel that, the rapid combustion stage began earlier than conventional diesel fuel by about 5o CA, heat release rate (HRR) decreased by 18.88%, emissions of carbon monoxide (CO), and unburnt hydrocarbon (UHC) reduced but nitric oxide (NO) emission increased by about 17.78% at 55o CA, as compared to diesel fuel. During the injection process, the jet core of biodiesel fuel appeared thicker and longer than conventional diesel fuel. For optimization, preheating with exhaust gas recirculation (EGR) of biodiesel fuel contributes to reduce the emissions of nitrogen oxides (NOx) and improve the atomization rate.

Surface profile evolution model for titanium alloy machined using abrasive waterjet

The surface profile evolution model, which was initially developed for glass and polymers, can accurately predict a channel profile cross-section produced by abrasive jet (AJ) machining. In this study, the model is modified and applied for estimating the profiles of a Ti-6Al-4V alloy eroded by an abrasive waterjet (AWJ). First, the velocity and mass fraction distributions of the gas–liquid–solid phases in the AWJ at the nozzle exit were derived and compared, and several improvements were proposed, such as considering the divergence angle of the jet and particles, as well as the length of the jet core area, to precisely construct a theoretical connection of the erosion efficiency distribution before impacting the workpiece. Computational fluid dynamics (CFD) simulations were then performed to investigate the behaviour of the erosion jets during surface evolution. The results revealed that the jet diffusion provoked by the stagnation zone effect became more pronounced as the surface profile depth deepened, which led to jet directional deflection and suppressed the erosion capacities of the AWJ. Therefore, a central erosion depth function was introduced to correct this detrimental effect with the intention of obtaining an accurate channel profile. In addition, a second-order single-step fitting function was suggested to eliminate the fluctuations caused by uneven abrasive particles and the problem of reduced erosion efficiency due to channel depth variation. Finally, based on the determination of the parameters affecting the channel profile, a normalised centre erosion rate function, which only depends on the channel depth and is isolated from the material properties and the standoff distance, was recommended to simplify the calculation. The erosion function conforming to a Gaussian surface was fitted using MATLAB (R2019b, MathWorks, USA). The results demonstrated that the channel profiles predicted by the surface evolution model were consistent with the measured profiles, with an average error of 11.4%.

Investigating the Effect of Cooling Air Passage on the Temperature Distribution in Turbine Blades

This paper was carried out to study the effect of the velocity ratio of cooling air on the temperature distribution in the turbine blade passage0The experimental and computation method gives a clear picture of the penetration area and the temperature distribution in the passage. All the calibration of the instrument and the laws of the five hole probe may be notes in the section test rig. Several cases have been studied in this research by using three-velocity ratio (VR=0.5, 1and 2), and three different locations (99% ,90% and 75%) from the end wall with the jets angle θ= 450.the hole diameter (d= 0.1 cm). The procedure solves the governing equation numerically by using the finite volume method semi implicit method for pressure-linked equation. Therefore the present results show an accepted agreement if compared with experimental work. The maximum temperature occurred of VR=0.5 and the jet location (99%) from the end wall .Therefore the temperatures are fluctuated at the core, and decreased gradually from the jet core to outside. The temperatures are increase with decrease the velocity ratio at the same the jut location from the axial chored.

Diffusive separation in rarefied plume interaction

In the present study, we propose the use of a light, inert carrier gas to support deposition uniformity and rate in continuous physical vapor deposition, in which closely spaced slots or nozzles are required to achieve a sufficiently high deposition rate. Interaction shocks between the emerging rarefied plumes cause undesired nonuniformities in the deposited coating. The present work evaluates the effect of adding a carrier gas on the interaction shock. We study the interaction between two sonic plumes consisting of a binary mixture, i.e., silver as coating material and helium as a light inert carrier gas, by direct simulation Monte Carlo. While the inlet Mach and Knudsen numbers were kept constant, the fraction of carrier gas was varied to single out the effect of species separation. The influence of rarefaction on species separation was also studied. Species separation produces a high carrier-gas fraction in the periphery and an accumulation of the heavier species in the jet core. The resulting change in the speed of sound alters the local expansion characteristics and, thus, shifts the shock location and weakens the shock. These phenomena intensify with the degree of rarefaction. It is shown that adding a light carrier gas increases deposition rate may enhance uniformity and reduce stray deposition.

Optimal position of air purifiers in elevator cabins for the improvement of their ventilation effectiveness

Ventilation in confined spaces is essential to reduce the airborne transmission of viruses responsible for respiratory diseases such as COVID-19. Mechanical ventilation using purifiers is an interesting solution for elevator cabins to reduce the risk of infection and improve the air quality. In this work, the optimal position and blowing direction of these devices to maximize ventilation and minimize the residence time of the air inside two cabins (large and small) is studied. Special attention is devoted to idle periods when the cabin is not used by the passengers, in order to keep the cabin ambient safe and clean, avoiding that the trapped air in the cabin (after its use) could suppose a reservoir for contaminants. CFD numerical models of two typical cabin geometries, including the discretization of small slots and grilles for infiltration, have been developed. A full 3D URANS approach with a k-epsilon RNG turbulence model and a non-reactive scalar to compute the mean age of air (MAA) was employed. The CFD results have been also validated with experimental measurements from a home-made 1:4 small-scale mock-up. The optimal position of the purifier is on the larger sidewall of the cabins for a downward blowing direction (case 1 of the database). Flow rates in the range of 0.4–0.6 m3/min, depending on the size of the cabin, are sufficient to assure a correct ventilation. Upward blowing may be preferable only if interaction of the jet core with the ceiling or other flow deflecting elements are found. In general, the contribution of infiltrations (reaching values of up to 10%), and how these secondary flows interact with the main flow pattern driven by the purifier, is relevant and not considered previously in the literature. Though an optimal position can improve ventilation considerably, it has been proven that a good choice of the purification flow rate is more critical to ensure an adequate air renewal.

Effect of corrugation on limiting arc-tab mixing performance

The effect of corrugation on the mixing promoting performance of a limiting arc-tab has been assessed by studying the decay of a Mach 1.76 jet emerging from a convergent-divergent nozzle, with a limiting arc-tab, with and without corrugation, running along a diameter at the nozzle exit, operated at nozzle pressure ratios of 4.5 and 8. Uncontrolled jet was also studied for comparison. The physics behind the difference between the mixing caused by the uncorrugated and corrugated tabs in the presence of adverse and favorable pressure gradient is addressed using the shadowgraph images of the jet core as the main source and the centerline pitot pressure decay as the supporting material. The results show that the mixing promotion caused by the tabs leads to the shortening of the third and fourth shock cells in the jet core. This can be regarded as an advantage from the screech suppression point of view.

Experimentally validated high-fidelity simulations of a liquid jet in supersonic crossflow

Utilizing recent advancements in computational schemes for compressible, multiphase flows , this work features a parametric study of a pure liquid jet in supersonic crossflow that involves simulating the atomization process for four values of momentum-flux ratio. These simulations are validated against experimental results measured with high-speed X-ray imaging, which confirm the accuracy of the numerical approach. Also, the effect of numerical resolution on some flow behavior is investigated, revealing convergence of the jet shape and surface instability wavelength. Analysis of the resulting sprays includes statistical descriptions of the liquid distribution, liquid structures created through breakup, interfacial instabilities, and dominant flow features. As the flowrate increases, the spray penetrates further, it becomes more disperse, and less liquid impacts the wall, but the droplet size distribution changes little. The wavelength of instabilities on the windward side of the jet diverges from measured trends in subsonic crossflows. In a visualization of the time-averaged flow, counter-rotating vortices are observed along the jet core and in the wake, affecting the process of primary atomization and early droplet trajectories. • Large Eddy Simulations of liquid jet in supersonic crossflow at high Weber number. • Quantitative validation against experimental results using equivalent path length. • Shear-driven instability wavelengths differ from subsonic crossflow analogy. • Analysis of spray statistics and mean flow features.

On the scalar turbulent/turbulent interface of axisymmetric jets

The effect of a zero-mean-flow turbulent ambient on the geometry of the turbulent/turbulent interface (TTI) of an axisymmetric jet is investigated and compared with the traditional turbulent/non-turbulent interface (TNTI). The ambient turbulence is generated using a random jet array. Orthogonal cross-sections of the jet subjected to different levels of ambient turbulence intensities and length scales are captured using planar laser-induced fluorescence. The enhanced radial transport of concentration from the jet core towards the interface and the increased scalar fluctuations within the jet caused by the ambient turbulence result in steeper mean and root-mean-square scalar conditional jumps across the TTI layer compared with those of the TNTI, respectively. When compared with the quiescent ambient, the mean effect of background turbulence is to stretch and corrugate the interface surface area as evident from the wider probability density of the interface radial position, the lowered occurrence of zero-curvature surface elements, the greater misalignment between the radial and normal unit vectors of the interface, the increased tortuosity and the increased magnitude of the fractal exponent.

Structure and arrangement optimization of non-diagenetic gas hydrate jet crushing nozzles based on CFD simulation

To optimize the optimal nozzle structure and multiple nozzle arrangement for hydrate jet crushing and to promote the development of solid fluidization extraction technology for shallow unconformity hydrate in China’s oceans, the submerged jet flow field of six commonly used nozzle structures in the downhole in-situ jet crushing process was analyzed on the basis of the solid fluidization extraction process in double-layer pipes, and the effect of the jet hole arrangement on the overflow performance of hydrate slurry in the outer annulus of double-layer pipes was also investigated. It was demonstrated that tapered straight nozzles were preferably selected as the nozzle type for hydrate solid fluidization mining process based on jet core stage length, jet energy dissipation rate, and jet fluid axial velocity. In the meantime, the optimum arrangement of the nozzles was preferred on the basis of the annular pressure drop and the flow resistance coefficient, with the number of single circle not higher than 3 and the axial spacing of the nozzles not lower than 50 mm. This study can provide a theoretical basis for nozzle selection and tool design for the solid fluidization jet crushing process of marine unconformable hydrates.

Flow field characteristic of water jet with air ring protective layer under submerged environment and the orthogonal optimization of nozzle structure

AbstractA water jet with an air ring protective layer is a promising rock‐breaking and cutting rate improvement method in submerged environment, such as oil–gas drilling. The jet flow field characteristic determines the jet impinge performance and is affected by nozzle structure parameters. In this study, the water jet flow with air ring under submerged environment was numerically simulated. The jet flow field structure with air ring was obtained by analyzing the distributions of jet velocity and air content at different cross sections. The effects of several critical nozzle structure parameters on jet isokinetic core length and axial velocity were investigated by adopting the orthogonal design method. The results show that the jet flow field structure with air ring in radial direction includes a high‐speed water jet zone, air ring protective layer, and air–water mixed zone, having the advantages of concentrated energy in axis region, long core section length, and low‐velocity attenuation. The influence degrees of nozzle structure parameters on jet core section length and axial velocity at X = 0.3 m are: length‐diameter ratio > outlet diameter > conic angle > air nozzle diameter > water‐air nozzle spacing and outlet diameter > length‐diameter ratio > conic angle > water‐air nozzle spacing > air nozzle diameter, respectively. The jet core section length and axial velocity both increase exponentially with the increasing outlet diameter, both first increase rapidly and then slowly decreases with the increase of conic angle and water–air nozzle spacing. There is the optimal conic angle of 15° and appropriate water–air nozzle spacing of 3–5 mm. With the increase of length–diameter ratio, they both first decrease rapidly and then slow down, indicating that conical nozzle is more suitable for water jet with air ring. And 0.9–1.2 mm is proper for the air nozzle diameter.

Optical superluminal motion measurement in the neutron-star merger GW170817.

The afterglow of the binary neutron-star merger GW1708171 gave evidence for a structured relativistic jet2-6 and a link3,7,8 between such mergers and short gamma-ray bursts. Superluminal motion, found using radio very long baseline interferometry3 (VLBI), together with the afterglow light curve provided constraints on the viewing angle (14-28 degrees), the opening angle of the jet core (less than 5 degrees) and a modest limit on the initial Lorentz factor of the jet core (more than 4). Here we report on another superluminal motion measurement, at seven times the speed of light, leveraging Hubble Space Telescope precision astrometry and previous radio VLBI data for GW170817. We thereby obtain a measurement of the Lorentz factor of the wing of the structured jet, as well as substantially improved constraints on the viewing angle (19-25 degrees) and the initial Lorentz factor of the jet core (more than 40).

Assessment of trends in an integrated climate metric—analysis of 200 mbar zonal wind for the period 1958–2021

Using two reanalysis datasets on latitude bands of nearly equal area, we analyze the trend of 200 mbar zonal wind anomalies for the period 1958–2021 as well as annual-mean latitude of the zonal mean jet stream. We do not find much change of 200 mbar zonal wind in both two datasets although the increasing trend over the high latitude bands from 45 to 61°S for both datasets, and the decreasing trend in 0–5°N and 0–10°S for ERA5 are significant with small magnitudes. In addition, the polar jet core moves poleward in the Northern Hemisphere, but the movement is not statistically significant, while no poleward shift at all has occurred in the Southern Hemisphere. This work provides us a way to look for the long-term changes and variability in atmospheric circulation.

Insight into liquid jet atomization in a swirling crossflow airblast injector: Application of a multi-directional imaging technique

This paper aims to investigate the mechanism of breakup of a transversely injected liquid jet due to swirling cross-stream flow of air in a model airblast injector. The focus is especially on examining the differences in the jet atomization in the absence and presence of air swirl within the injector. Multi-directional imaging of the liquid jet core is achieved by application of the Optical Connectivity (OC) technique for simultaneous visualization of the jet core from the side- and front-view. In addition, visualization of the jet core from the bottom view is also possible. Apart from the jet penetration and breakup length (obtained from the side-view image), the present technique facilitates measurement of jet width and spread rate (from the corresponding front-view image) at the same time. The airblast injector is operated for a wide range of operating conditions. The results highlight significant influence of swirling the air flow on the jet breakup morphology as well as primary breakup parameters. The measurements provided some new insight into the physics of the primary atomization within an atomizer, where the air flow is confined, in contrast to the widely studied jet-in-crossflow in wind-tunnels. In the present case, prior to its breakup into ligaments and droplets, the liquid jet undergoes jet-to-sheet transformation. Also, the liquid sheet deflects in the direction of air swirl and disintegrates in a complex manner. The addition of swirl to the air flow leads to reduction in the mean jet breakup parameters, while the fluctuations are higher.

Characteristics of the Turkana low-level jet stream and the associated rainfall in CMIP6 models

The Turkana low-level jet stream (TJ) is important to climatic conditions over northern Kenya and East Africa. The representation of the TJ in climate models varies due to the TJ interaction with Turkana channel that is influenced by model resolution and influences the model representation of the regional climate. This study compares features of the TJ in CMIP6 AMIP model simulations with ERA5. Models reveal climatological wind speeds that match those of the reanalysis from the ERA5 at the jet entrance (13 m/s) but lower magnitudes of wind speed and vertical shears compared to ERA5 within the Turkana channel. The models with slowest wind speeds, have a flattened Turkana channel and fail to exhibit the terrain constriction at 37° E which otherwise aids in accelerating winds to form a jet core. Furthermore, they fail to represent the narrowing of the channel as in ERA5, thereby forming blocking walls in the channel, forcing vertical ascent and mixing, and weakening shear. This boosting of ascent motion promotes rainfall formation and enhances wet anomalies at the exit of the TJ when the jet stream is weaker. By applying a new narrowing index, we demonstrate the need to improve topography details in the CMIP6 models, particularly those with resolution coarser than 1.5°, in order to properly simulate the TJ and the observed rainfall over the northwestern areas of eastern Africa.

Continuous supercritical hydrothermal combustion experimental and burner structure optimization simulation study

Supercritical hydrothermal combustion is the rapid exothermic oxidation reaction occurring in supercritical water, which can be utilized in the treatment of high-concentration organic wastewaters. In this study, preliminary ignition experiments of continuous hydrothermal combustion are conducted. It is found that using hydrogen peroxide as oxidant can decrease the ignition temperature, but recirculation zone cannot be established due to the increase in the momentum ratio between oxidant and fuel. Furthermore, the coaxial-jet and jet-in-crossflow combustors are simulated to investigate the effect of geometry on the flame characteristics. For the coaxial-jet configuration, the moderate increase in recirculation zone can improve the flame stability, and the optimized Craya-Curtet number is around 0.17. For the jet-in-crossflow configuration, the oxygen-rich recirculation zone is more liable to accumulate heat than the fuel-rich one, and hence it does more favors for the flame stability. A new jet-in-crossflow combustor with oxygen as the core jet is proposed and demonstrates to be able to decrease the extinction temperature by ∼100 K lower than the existing devices.

Effect of tab length on supersonic jet mixing

An experimental investigation has been carried out to assess the efficiency of rectangular tabs of 5% blockage with three different aspect ratios of 1, 1.5, and 2, in promoting the mixing of a Mach 1.73 axisymmetric free jet, in the presence of adverse and favorable pressure gradient, by varying the operating nozzle pressure ratio (NPR) from 4 to 8, in steps of 1, covering all the three levels of expansion, namely, the over-, correct-, and underexpaned states at the nozzle exit. For NPR 4, the Mach 1.73 jet is overexpanded with an adverse pressure gradient of about 23%. At NPR 5, the nozzle is almost correctly expanded with an adverse pressure gradient of only about 3%. At NPRs 6, 7, and 8, the nozzle is underexpanded with favorable pressure gradients of 16%, 35%, and 55%, respectively. It is found that as high as about 88% reduction in the core length is achieved by the tab with an aspect ratio of 1.5 at NPR 7. However, the reduction caused by tabs with aspect ratios of 1 and 2 is only 73.3% and 42.3%, respectively. It is to be noted that though the mixing promotion caused by the tab with an aspect ratio of 1.5, at NPR 7, leads to the core length reduction to the tune of about 88%, it is at the cost of momentum thrust loss of about 5% and the drag caused by the tabs. It is also found that except at NPR 4, the mixing caused by the tab with an aspect ratio of 1.5 is better than tabs with aspect ratios of 1 and 2. The shadowgraph pictures for the uncontrolled and controlled jets demonstrate the effectiveness of the tabs in weakening the waves present in the jet core.

Perspectives for multimessenger astronomy with the next generation of gravitational-wave detectors and high-energy satellites

The Einstein Telescope (ET) is going to bring a revolution for the future of multimessenger astrophysics. In order to detect the counterparts of binary neutron star (BNS) mergers at high redshift, the high-energy observations will play a crucial role. Here, we explore the perspectives of ET, as a single observatory and in a network of gravitational-wave (GW) detectors, operating in synergy with future γ-ray and X-ray satellites. We predict the high-energy emission of BNS mergers and its detectability in a theoretical framework which is able to reproduce the properties of the current sample of observed short GRBs (SGRBs). We estimate the joint GW and high-energy detection rate for both the prompt and afterglow emissions, testing several combinations of instruments and observational strategies. We find that the vast majority of SGRBs detected in γ-rays have a detectable GW counterpart; the joint detection efficiency approaches 100% considering a network of third-generation GW observatories. The probability of identifying the electromagnetic counterpart of BNS mergers is significantly enhanced if the sky localization provided by GW instruments is observed by wide-field X-ray monitors. We emphasize that the role of the future X-ray observatories will be very crucial for the detection of the fainter emission outside the jet core, which will allow us to explore the population of low-luminosity SGRBs in the nearby Universe, as well as to unveil the nature of the jet structure and the connections with the progenitor properties.