Jet Region Research Articles

Spectral and magnetic properties of the jet base in NGC 315

The dynamics of relativistic jets in inner parsec regions are deeply affected by the nature of magnetic fields. The level of magnetization of the plasma as well as the geometry of these fields on compact scales have not yet been fully constrained. In this paper, we employ multi-frequency and multi-epoch very long baseline interferometry observations of the nearby radio galaxy NGC 315. We aim to derive insights into the magnetic field properties on sub-parsec and parsec scales by examining observational signatures such as the spectral index, synchrotron turnover frequency, and brightness temperature profiles. This analysis was performed by considering the properties of the jet acceleration and collimation zone, which can be probed thanks to the source vicinity as well as the inner part of the jet conical region. We observed remarkably steep values for the spectral index on sub-parsec scales ($ -2$, $S_ which flatten around $ -0.8$ on parsec scales. We suggest that the observed steep values may result from particles being accelerated via diffusive shock acceleration mechanisms in magnetized plasma and subsequently experiencing cooling through synchrotron losses. The brightness temperature of the 43\,GHz cores indicates a dominance of the magnetic energy at the jet base, while the cores at progressively lower frequencies reveal a gradual transition toward equipartition. Based on the spectral index and brightness temperature along the incoming jet and by employing theoretical models, we derived that the magnetic field strength has a close-to-linear dependence with distance going from parsec scales up to the jet apex. Overall, our findings are consistent with a toroidal-dominated magnetic field on all the analyzed scales.

Exploring Magnetic Fields in a Merging Galaxy: Comparing Polarization and Velocity Gradient in the Centaurus Galaxy

In this study, we apply the velocity gradient technique to the merging Centaurus galaxy. We compare gradient maps derived from the PHANGS-Atacama Large Millimeter/submillimeter Array survey using CO emission lines with magnetic field tracings from dust polarization data obtained via the HAWC+ instrument. Our analysis reveals a strong correspondence between the directions indicated by these two tracers across most of the galactic image. Specifically, we identify jet regions as areas of antialignment, consistent with previous reports that gradients tend to rotate 90° in outflow regions. Statistically, we find that the alignment of magnetic fields, as revealed by polarization, is most accurate in regions with the highest signal-to-noise ratios. Our findings underscore the utility of velocity gradients as a valuable complementary tool for probing magnetic fields and dynamical processes in merging galaxies. This proves the general utility of velocity gradients for mapping magnetic fields in astrophysical objects with complex dynamics.

PKS 1424-418: A persistent candidate source of the mm--gamma -ray connection?

We present a long-term strong correlation between millimeter (mm) radio and gamma -ray emission in the flat-spectrum radio quasar (FSRQ) PKS\,1424$-$418. The mm --gamma -ray connection in blazars is generally thought to originate from the relativistic jet close to the central engine. We confirm a unique long-lasting mm --gamma -ray correlation of PKS\,1424$-$418 by using detailed correlation analyses and statistical tests, and we find its physical meaning in the source. We employed sim 8.5\,yr of (sub) mm and gamma -ray light curves observed by ALMA and Fermi -LAT, respectively. From linear and cross-correlation analyses between the light curves, we found a significant, strong mm --gamma -ray correlation over the whole period. We did not find any notable time delay within the uncertainties for the mm --gamma -ray correlation, which means zero lag. The mm wave spectral index values (S$_ nu alpha between the band 3 and 7 flux densities indicate a time-variable opacity of the source at (sub)mm wavelengths. Interestingly, the mm wave spectral index becomes temporarily flatter (i.e., alpha \,$>$\,$-$0.5) when the source flares in the gamma -rays. We relate our results with the jet of PKS\,1424$-$418 and we discuss the origin of the gamma -rays and opacity of the inner (sub)parsec-scale jet regions.

Exploring γ-Ray Flares from High-Redshift Blazar B3 1343+451 at GeV Energies

We study the temporal and spectral variability properties of the high-redshift blazar B3 1343+451 utilizing Fermi-LAT data from 2008 to 2022 in the energy range of 0.1–300 GeV. We identify six major flares with many substructures and analyze their temporal and spectral properties in detail. The fastest rise and decay timescales are found to be 4.8 ± 0.48 h and 5.28 ± 0.72 h, respectively. The size of the emission region is constrained to be R ∼ 5.18 × 1015–1.56 × 1016 cm with the typical Doppler factors of δ ∼ 10–30. Most of the peaks from the flares exhibit a symmetric temporal profile within the error bars, implying that the rise and decay timescales are dominated by the disturbances caused by dense plasma blobs passing through the standing shock front in the jet region. We also find that four flares are better fitted with a log-parabolic distribution, while two flares are better fitted with a power-law distribution. Our results indicate that the emission regions vary from one flare to another, which is consistent with earlier results.

Effects of Viscosity and Shear-thinning Characteristics of a Liquid Jet in Air Crossflow

In the present study, we performed numerical simulations of a single liquid jet with varying viscosities and non-Newtonian characteristics in uniform gas crossflows at different velocities, to investigate the effects of liquid properties on jet deformation and breakup behavior. By utilizing air as the gas and examining both Newtonian fluids with different viscosities and a shear-thinning fluid as the liquid, we aimed to uncover the intricate dynamics governing liquid jet deformation and breakup under crossflow conditions. At lower air velocities, the liquid jet bent and elongated without undergoing breakup, regardless of the viscosity and shear-thinning characteristics. Notably, in the case of the shear-thinning fluid, the effect of the viscosity was more pronounced in the outer regions of the liquid jet than at its center. In contrast, at higher air velocities, viscosity and shear-thinning characteristics significantly affected jet breakup; the high-viscosity Newtonian liquid jet resisted breakup, while the low-viscosity jet broke up into threads, with a decrease in apparent viscosity leading to increased penetration height.This study introduces a novel method for quantifying the energy transfer from gas to liquid jet, using pressure differences between a deformable liquid jet and an equivalent rigid cylinder as a reference. This approach allows for the separation of bending and breakup energy components, offering new quantitative insights into the forces driving jet deformation and breakup. These insights advance the current understanding of fluid-structure interactions in crossflow conditions.

Multi-objective geometric optimization of protrusion, droplet ribs, and inclined upper plate of a slit jet impingement heat sink to enhance its thermal performance

To meet the needs of heat transfer and temperature uniformity in the heat dissipation of high-heat-flux electronic devices, a novel slit jet impingement heat sink with a protrusion in the stagnation region, droplet ribs in the wall jet region, and an inclined upper-plate (PDI-SJIHS) is proposed, the coolant is Al2O3-H2O nanofluid. The influences of the structural parameters of the above three components and the Reynolds number on the heat transfer and flow of the PDI-SJIHS are studied numerically. Using the comprehensive heat transfer and temperature standard deviation as objective functions, a multi-objective optimization of PDI-SJIHS is conducted using non-dominated sorting genetic algorithm-Ⅱ (NSGA-Ⅱ). The results indicate that the combination of the three components enhances the thermal performance of the PDI-SJIHS, compared with the slit jet impingement heat sink with only one of the above components. Rising the Reynolds number can enhance the thermal characteristics of the PDI-SJIHS. For a Reynolds number of 8000 and nanofluids with a 3% volume fraction, the average convective heat transfer coefficient, friction coefficient, and comprehensive heat transfer coefficient of the optimized PDI-SJIHS are increased by 182%, 153%, and 108%, respectively, and the standard deviation of temperature is 91% less than that of F-SJIHS.

The putative center in NGC 1052

Context: Many active galaxies harbor powerful relativistic jets, however, the detailed mechanisms of their formation and acceleration remain poorly understood. Aims: To investigate the area of jet acceleration and collimation with the highest available angular resolution, we study the innermost region of the bipolar jet in the nearby low-ionization nuclear emission-line region (LINER) galaxy NGC\,1052. Methods: We combined observations of NGC\,1052 taken with VLBA, GMVA, and EHT over one week in the spring of 2017. Our study is focused on the size and continuum spectrum of the innermost region containing the central engine and the footpoints of both jets. We employed a synchrotron-self absorption model to fit the continuum radio spectrum and we combined the size measurements from close to the central engine out to $ to study the jet collimation. Results: For the first time, NGC\,1052 was detected with the EHT, providing a size of the central region in-between both jet bases of $43\ perpendicular to the jet axes, corresponding to just around $250\,R_ S $ (Schwarzschild radii). This size estimate supports previous studies of the jets expansion profile which suggest two breaks of the profile at around $3 10^3\,R_ S $ and $1 10^4\,R_ S $ distances to the core. Furthermore, we estimated the magnetic field to be 1.25\,Gauss at a distance of $22\ from the central engine by fitting a synchrotron-self absorption spectrum to the innermost emission feature, which shows a spectral turn-over at $ 130\,$GHz. Assuming a purely poloidal magnetic field, this implies an upper limit on the magnetic field strength at the event horizon of $2.6 10^4\,$Gauss, which is consistent with previous measurements. Conclusions: The complex, low-brightness, double-sided jet structure in NGC\,1052 makes it a challenge to detect the source at millimeter (mm) wavelengths. However, our first EHT observations have demonstrated that detection is possible up to at least 230\,GHz. This study offers a glimpse through the dense surrounding torus and into the innermost central region, where the jets are formed. This has enabled us to finally resolve this region and provide improved constraints on its expansion and magnetic field strength.

Advances in predicting surface shape changes of mirror blanks through elliptical nozzle gas jet forming

To facilitate the formation of bifocal aspheric optical surfaces, we present an approach for predicting the surface profile of bifocal aspheric surfaces formed by the gas-liquid interface when an elliptical nozzle gas jet is employed. Through an analysis of the gas flow field morphology emanating from the elliptical nozzle, we inferred the impact of gas jet parameters on the gas-liquid interface surface shape within the core region of the gas jet. By analyzing the variation in the gas flow field morphology emitted from an elliptical nozzle, we deduced the influence patterns of gas jet parameters on the gas-liquid interface surface shape within the gas jet's core region. Theoretical analysis is substantiated by numerical simulations, confirming regular changes in the vertex curvature and conic constant of mirror blanks concerning variations in jet initial velocity and nozzle aspect ratio. A comparison between experimental data and numerical simulation results reveals an average prediction deviation of 0.0083 mm−1 for the vertex curvature and a prediction deviation of 10.7 % for the conic constant, challenging to rectify within numerical simulations. Hence, an empirical model, incorporating jet parameters, is developed based on experimental data to predict the vertex curvature and conic constant of mirror blanks. This model demonstrates an average prediction error of 2.901 × 10−3 mm−1 for the vertex curvature and 7.64 % for the conic constant, surpassing the predictive accuracy of the numerical simulation model.

Estimation and research of the temperature field models of large space spherical mesh shell structures under localized fire

This paper firstly investigated fire dynamics characteristics and the temperature field distributions of large space spherical mesh shell (SMS) structures under localized fire in FDS, and divided the temperature fields into plume region, smoke accumulation region and ceiling jet region. Based on the classical axisymmetric plume model, the predictive models for the steady-state and transient temperature fields in different fire regions for large space SMS structures were proposed in this paper. Thereafter, the proposed temperature models were compared with the simulated results and experimental test results by other scholars, and they were in good agreement. In order to further discover the realistic fire development progress and temperature distribution of large space fire, and validate the proposed temperature field models, this paper designed and conducted three full-scale large space fire tests (Test-1 ∼ Test-3) with different fire source powers in a large space building. Results showed that temperature fields of large space fires could be consisted of near-fire region (Flame region) and far-fire region (Plume region and Ceiling jet region). The steady-state fire source powers were tested for 443 kW, 886 kW, 1090 kW for Test-1, Test-2 and Test-3, respectively, while their maximum temperatures of the roof reached 75 °C, 105 °C and 120 °C. Based on McCaffrey plume model, we further proposed the transient temperature model in flame region for large space structures, and its accuracy was verified by the test results. Moreover, the proposed steady-state and transient temperature fields for large space SMS structures were compared with the tested temperature field data, and the tested and predicted temperature curves were generally in good agreement in plume and ceiling jet regions. Finally, the test results aimed to provide a scientific basis and reference for the fire safety and structural fire-resistant design of large space buildings.

Influence of the Western Disturbance on Winter Precipitation: Observations over the Central Himalayas using VHF Radar

This study investigates the influence of the Western Disturbance (WD) on atmospheric wind patterns and associated precipitation over the Himalayan region utilizing observations carried out using the ARIES ST Radar (ASTRAD) at Nainital [29.36° N, 79.46° E, 1800 m] during March 2024. The observations reveal that during the WD events, the wind direction was predominantly south westerly/westerly with varying speeds, reaching up to 67 m/s. The HYSPLIT model back-trajectory analysis confirmed that the moisture-laden air masses originated from the Atlantic Ocean. Vertical wind measurements indicated significant atmospheric instability and convection, characterized by updraft and downdraft motions in three distinct height regions between 8 km and 10 km, an updraft between 6 km and 7 km, and another downdraft between 4.5 km and 5 km. The core height region of the subtropical westerly jet (SWJ) embedded in WD and shift in height with maximum wind speed are also identified.

Structured Jet Model for Multiwavelength Observations of the Jetted Tidal Disruption Event AT 2022cmc

AT 2022cmc is a recently documented tidal disruption event that exhibits a luminous jet, accompanied by fast-declining X-ray and long-lasting radio and millimeter emission. Motivated by the distinct spectral and temporal signatures between the X-ray and radio observations, we propose a multizone model involving relativistic jets with different Lorentz factors. We systematically study the evolution of faster and slower jets in an external density profile, considering the continuous energy injection rate associated with time-dependent accretion rates before and after the mass fallback time. We investigate time-dependent multiwavelength emission from both the forward shock (FS) and reverse shock (RS) regions of the fast and slow jets, in a self-consistent manner. Our analysis demonstrates that the energy injection rate can significantly impact the jet evolution and subsequently influence the lightcurves. We find that the X-ray spectra and lightcurves could be described by electron synchrotron emission from the RS of the faster jet, in which the late-time X-ray upper limits, extending to 400 days after the disruption, could be interpreted as a jet break. Meanwhile, the radio observations can be interpreted as a result of synchrotron emission from the FS region of the slower jet. We also discuss prospects for testing the model with current and future observations.

Investigation of combustion mode conversion driven by fuel flow variation in a cavity-based scramjet

This work aimed to investigate combustion mode conversions by rapid variation of the fuel flow rate in a cavity-based scramjet combustor. The experiments were carried out on a direct-connected facility with an inflow condition of Mach number 2.52, a total pressure of 1.35 MPa, and a total temperature of 1650 K. The fuel injector consisted of two injection ports: fuel was continuously injected from one port while the other controlled the fuel flow for mode conversions by switching it on or off. Simultaneous schlieren and CH* imaging techniques were used to characterize the dynamics of combustion mode conversions. It was recognized that the combustion modes characterized by different flow field structures and heat release distributions can be classified into three types: the shear-layer mode, the transition mode, and the jet-wake mode. During the combustion mode conversion, the mixing region of the transverse jet and air became thicker with the increase in fuel flow rate, and the gradient of the flow field density and the flame area increased, making the flame more likely to propagate upstream. The combustion suppression induced by rapid fuel addition was observed at low equivalence ratios. It was speculated that the weak heat supply was insufficient to provide adequate heat for the rapid ignition of the added fuel. Furthermore, it was found that the flame-flow matching process with frequent flame propagation upstream occurred during the combustion mode conversion. This process was attributed to the mismatch between the increasing heat release and the original flow field structure.

Flow and Heat Transfer Characteristics of Swirling Impinging Micro-Slot Jets by Adopting Circular Ribs in the Wall Jet Region

The flow and heat transfer behaviors of swirling impinging micro-slot jets using circular ribs in the wall jet region have been numerically studied. In this study, the total spiraling length was defined as 290 mm due to the two complete revolutions of the helix generated at a slot nozzle thickness of 1.0 mm. The range of circular rib radius ratios varied from 1 to 4 under the jet-to-target spacing ratio at 2 and 4. Predictions were made using the turbulent kinetic energy-epsilon model at Reynolds numbers ranging from 5000 to 20000. The results showed that the heat transfer enhancement on the target wall depended on the circular rib radius ratios. The jet flow at a low circular rib radius ratio generated recirculating ambient fluid behind the rib zone and a minuscule vortex within a circular rib region. At a high ratio, the jet’s potential flow directly affected the top rib surface and generated secondary flows in the circular rib. The heat transfer rate tended to increase as the Reynolds number increased. The average heat transfer value at a circular rib radius ratio of 3 became the best, up to 18.1% for jet-to-target spacing of 2. Whereas above 14.8% average value at a circular rib radius ratio of 1 in the case of jet-to-target spacing of 4 gained the highest.

Flow and heat transfer behavior of acoustically excited pulsating air jet impinging on a flat surface

Experiments are carried out to understand the flow and thermal behavior of a pulsating jet. The pulsating jet is generated using acoustic excitation. The study considered variations in Reynolds number (Re = 2800, 4900, and 6800), Strouhal number (St = 0–0.84), pulsation amplitude (A = 0–60 %), and nozzle-to-surface distance (z/d = 1–8). Findings revealed that the potential core length of the pulsating jet is shorter compared to the steady jet. The potential core length initially decreases with an increase in St up to 0.42, then begins to increase. Pulsating jets improve thermal performance in the wall jet region due to greater entrainment and mixing from the surrounding fluid. Results demonstrated that pulsating jets could increase average heat transfer rate by up to 58 % at Re = 2800 compared to the steady jet. Although heat transfer rates are higher in pulsating jets, changes in pulsation frequency or amplitude had a minimal effect. The enhancement in average heat transfer rate diminishes as the Reynolds number increases for the same Strouhal number. Each tested Reynolds number showed at least a 10 % improvement in heat transfer in pulsating jet over steady jet. The improved thermal performance of the acoustically pulsating chamber offers the potential for enhanced thermal management in various applications.

Jet array impingement heat transfer in a rectangular cavity with effusion holes

Various researchers have studied jet array impingement heat transfer in impingement/effusion cooling systems. However, there is a lack of research on impingement/effusion cooling systems installed within rectangular cavities that focus on the impact of the proximity of the jet hole to the cavity sidewalls on cooling performance. The main objective of this study is to investigate the flow and heat transfer characteristics of jet array impingement with effusion holes in a rectangular cavity, considering various spacings between the cavity sidewalls and the outermost jet hole. The design parameters in this study include the ratio of the jet hole pitch to jet hole diameter of 7.1, 10.0, and 16.7, and the ratio of the distance between the jet and impingement plates to jet hole diameter of 2, 6, and 10, with the Reynolds number based on the jet hole diameter ranging from 2500 to 15,000. Heat transfer characteristics in the stagnation region and wall jet region were examined using local Nusselt number distributions on the impingement surface, measured by liquid crystal thermography. The local Nusselt number was high in the stagnation region and decreased radially from the stagnation region as the wall jet region formed. The closer the outermost jet hole is to the sidewall, the higher the Nusselt number on the impingement surface near the sidewall. Moreover, the flow structure in the rectangular cavity was numerically investigated, and the velocity vectors and streamlines showed that primary and secondary vortices were generated in the middle of two neighboring jets and near the sidewall, respectively. This study also assessed previous average Nusselt number correlations. Based on experimentally determined average Nusselt number data with 54 center unit cells and 1296 side unit cells, new correlations to predict the average Nusselt number on the impingement surface in a rectangular cavity with effusion holes were developed.

Effects of axisymmetric confinement on vortical structures emanating from round turbulent jets

Confined jets occur in many engineering applications including combustion chambers, jet pumps, and chemical reactors. The effects of axisymmetric confinement on the vortical structures identified in a turbulent jet are investigated using large eddy simulation at a Reynolds number of 30 000 (based on nozzle exit conditions) and expansion ratio (chamber-to-nozzle diameter ratio) of five. The results obtained from the confined jet are compared with those of a free jet under the same nozzle exit flow conditions. A prominent recirculation zone forms between the expanding jet and the confining wall, resulting in early shear layer distortion and a shorter interaction length in the confined jet (0.85 jet diameters) compared to the free jet (1.15 jet diameters). Using the λ2 criterion for vortex identification, two dominant structural modes are identified in the near-exit region of the free jet: ring and helical modes. However, in the confined jet, the helical mode is absent, and the turbulent confined fluid accelerates the breakup of the ring vortices. The interaction of the secondary line vortices with the breaking structures leads to the formation of new hairpin-like vortices, which also contribute to further vortex breakup. These results explain the enhanced mixing performance of confined jets as the mixing is directly tied to the breakup of large vortical structures. Proper orthogonal decomposition modes are also presented to identify the structures/events with the highest contribution to the total turbulent kinetic energy in both flow fields.

Integrated Study of X-Ray Spectrum and Time Lags for HBL Mrk 421 within the Framework of the Multiple-zone Leptonic Model

We present the timing analysis of 10 archived XMM-Newton observations with an exposure of >40 ks of Markarian 421. Mrk 421 is the brightest high-frequency-peaked BL Lac object emitting in X-rays produced by electrons accelerated in the innermost regions of a relativistic jet pointing toward us. For each observation, we construct averaged X-ray spectra in 0.5–10 keV band, as well as 100 s binned light curves (LCs) in various subbands. During these observations, the source exhibited various intensity states differing by close to an order of magnitude in flux, with the fractional variability amplitude increasing with energy through the X-ray band. Bayesian power spectral density analysis reveals that the X-ray variability can be characterized by a colored noise, with an index ranging from ∼ −1.9 to −3.0. Moreover, both the standard cross-correlation function and cross-spectral methods indicate that the amount of time lags increases with the energy difference between two compared LCs. A time-dependent two-zone jet model is developed to extract physical information from the X-ray emission of Mrk 421. In the model, we assume that the jet emission mostly comprises a quasi-stationary component and a highly variable one. Our results show that the two-zone model can simultaneously provide a satisfactory description for both the X-ray spectra and time lags observed in different epochs, with the model parameters constrained in a fully acceptable interval. We suggest that shocks within the jets may be the primary energy dissipation process responsible for triggering the rapid variability, although magnetic reconnection cannot be excluded.

Experimental heat transfer analysis of conical pin configurations in jet impingement cooling with elongated nozzle holes

BackgroundThis study investigates the impact of jet impingement cooling with varying nozzle lengths on the heat transfer performance of smooth and conical pinned target surfaces under turbulent flow conditions. The study focuses on the Reynolds numbers (Re) of 13,000, 26,000, and 39,000, using the liquid crystal thermography (TLC) method to collect experimental data. The dimensionless conical pin heights (Hc/d = 0.00, 0.67, 1.00, 1.33) and target surface-nozzle distances (G/d = 1.0, 2.0, 3.0, 6.0) were analyzed to understand their effects on heat transfer. MethodsThe experimental setup involved measuring Nu numbers and pressure losses in models with elongated nozzles and conical pins. The study examined how the interaction between pin heights and nozzle lengths influences heat transfer in a channel-confined impinging jet flow. The analysis included evaluating heat transfer performance in different jet regions and assessing pressure loss coefficients for various configurations. Significant FindingsResults indicated that for smooth target surfaces, elongated nozzles increased heat transfer in the first two jet regions but decreased it in the last jet regions due to cross-flow effects. In contrast, conical pinned surfaces showed a significant increase in heat transfer, particularly in the stagnation and last jet regions, with lower G/d and higher Hc/d values. Conical pinned surfaces enhanced overall heat transfer by at least 5 % compared to smooth surfaces, with a maximum Nu number increase of 21.87 %. However, configurations with G/d < 2.0 and Hc/d ≤ 0.67 negatively impacted heat transfer. Pressure loss analysis revealed that using conical pins and extended jets together increased pressure loss, with a maximum drop of 5.67 kPa at Re = 39,000. The Thermal Performance Criterion (TPC) ranged from a minimum of 0.97 at Re = 26,000 (G/d = 1.0, Hc/d = 1.00) to a maximum of 1.18 at Re = 26,000 (G/d = 6.0, Hc/d = 1.33).

Evolution of the Termination Region of the Parsec-scale Jet of 3C 84 Over the Past 20 yr

We present the kinematics of the parsec-scale jet in 3C 84 from 2003 November to 2022 June observed with the Very Long Baseline Array at 43 GHz. We find that the C3 component, a bright feature at the termination region of the jet component ejected from the core in 2003, has maintained a nearly constant apparent velocity of 0.259 ± 0.003c over the period covered by observations. We observe the emergence of four new subcomponents from C3, each exhibiting apparent speeds higher than that of C3. Notably, the last two subcomponents exhibit apparent superluminal motion, with the fastest component showing an apparent speed of 1.22 ± 0.14c. Our analysis suggests that a change in viewing angle alone cannot account for the fast apparent speeds of the new subcomponents, indicating that they are intrinsically faster than C3. We identify jet precession (or reorientation), a jet–cloud collision, and magnetic reconnection as possible physical mechanisms responsible for the ejection of the new subcomponents.

Under-expanded jet and diffusion characteristics for small-hole leakage of hydrogen-blended natural gas in high-pressure pipelines

Hydrogen blending in natural gas pipelines is an economical method of hydrogen transportation, but it can increase explosion risks from leakages. Current leakage models struggle to capture under-expanded jet structures near leakage holes due to limited computational resources and cannot accurately describe explosion hazardous distances of subsequent diffusion. A two-stage model for simulating the leakage of hydrogen-blended natural gas (HBNG) pipelines is introduced, comprising jet and diffusion models. The source strength determined in the jet model serves as the inlet boundary condition for the subsequent diffusion model. The results show that considering the under-expanded jet structure can more accurately reflect the pressure fluctuation changes near the leakage hole, particularly at the location of the Mach disc, xm. The pressure at the 10xm section of the jet region stabilizes at atmospheric pressure and utilizing this value as the inlet boundary for the diffusion model prevents supersonic and oscillatory regions. Subsequently, the impact of pipeline operating pressure, leakage hole diameter, and hydrogen blending ratio (HBR) on the far-field diffusion characteristics is analyzed. Notably, the leakage hole diameter has a more pronounced effect on the explosion hazardous distance compared to other factors. This research provides insights for the prevention and management of HBNG leakage accidents.