Two-zone Model Research Articles

Revisit the γ-ray flare associated with blazar Mrk 421

Abstract A Very High-energy (VHE) flare was observed by Major Atmospheric Gamma
 Imaging Cherenkov Telescopes (MAGIC) on MJD 57788. This VHE flare was characterized
 by increased VHE flux and short timescales. In this study, we used one-zone synchrotron
 self-Compton (SSC), two-zone, and Spine/Layer models to explore this VHE flare’s origin.
 The results indicate that this γ-ray flare can be explained by Inverse Compton Scattering
 (ICS) radiation from the layer contributed gamma radiation and results in the γ-ray flare in a
 different way than the two-zone model. The difference between the two models lies in the
 performance of spectral energy distribution (SED) after γ-ray flare as well as a possible
 difference in the time scale of the flare.

Thermodynamic Modeling and Analysis of Boil-off Gas Generation and Self-Pressurization in Liquefied Carbon Dioxide Tanks

<i>The importance of the safe transport of liquefied carbon dioxide (LCO<sub>2</sub>) is increasing owing to environmental issues. When transporting a low-temperature liquid, boil-off gas generation and self-pressurization occur due to heat ingress, affecting the holding time of a low-temperature liquid tank. This study developed and compared three thermodynamic self-pressurization models to estimate the holding time of LCO<sub>2</sub>: Thermal homogeneous model (THM), Thermal two-zone model (TTZM), and Thermal multi-zone model (TMZM). Thermodynamic differential equations were solved for THM, and software was used for TTZM. For TMZM, the parameters were optimized using experimental data to determine the heat ratio parameter f and heat transfer parameters K<sub>1</sub> and K<sub>2</sub>. THM and TTZM estimated an unreasonably long holding time, approximately 42 days. The TMZM, however, showed a satisfactory holding time of 12–13 days. These results can help predict the self-pressurization in the storage tanks of LCO<sub>2</sub> and be applied to actual LCO<sub>2</sub> carrier cargo handling systems, with the modeling results indicating that the 12–13 days of LCO<sub>2</sub> self-pressurization based on the TMZM appears to be the most suitable.</i>

Development and validation of a predictive combustion model for hydrogen-fuelled internal combustion engines

Internal combustion engines (ICEs) fuelled with hydrogen can play a major role in the short-term future transportation sector since they abate all criteria pollutants at engine-out reducing tailpipe CO2 emissions to near-zero levels. However, optimizing hydrogen ICEs is a challenging task that can be addressed through the development of a robust simulation tool capable to predict the H2 combustion process. In this study, a previously developed two-zone combustion model has been updated considering different laminar flame speed computations, both based on a detailed chemistry scheme: a polynomial correlation function and a tabulated approach. The predictive capabilities of the combustion model have been validated against experimental data coming from a 0.5L PFI single-cylinder engine under several operating conditions. The tabulated approach for laminar flame speed definition proved to be the best solution, leading to a combustion duration average error lower than 3 deg over a dataset containing more than 45 different operating conditions.

Cosmic-Ray Feedback on Bistable Interstellar Medium Turbulence

While cosmic rays (E ≳ 1 GeV) are well coupled to a galaxy’s interstellar medium (ISM) at scales of L > 100 pc, adjusting stratification and driving outflows, their impact on small scales is less clear. Based on calculations of the cosmic-ray diffusion coefficient from observations of the grammage in the Milky Way, cosmic rays have little time to dynamically impact the ISM on those small scales. Using numerical simulations, we explore how more complex cosmic-ray transport could allow cosmic rays to couple to the ISM on small scales. We create a two-zone model of cosmic-ray transport, with the cosmic-ray diffusion coefficient set at the estimated Milky Way value in cold gas but smaller in warm gas. We compare this model to simulations with a constant diffusion coefficient. Quicker diffusion through cold gas allows more cold gas to form compared to a simulation with a constant, small diffusion coefficient. However, slower diffusion in warm gas allows cosmic rays to take energy from the turbulent cascade anisotropically. This cosmic-ray energization comes at the expense of turbulent energy which would otherwise be lost during radiative cooling. Finally, we show our two-zone model is capable of matching observational estimates of the grammage for some transport paths through the simulation.

Early warning signals of flashover in compartment fires

There have been efforts to predict the occurrence of flashover. Due to its sudden development and the difficulty in discerning warning signs during the induction phase, the approach to flashover prediction is still under investigation. This research tests a new approach for detecting flashover events within a compartment through single-signal processing of the heat release rate (HRR). A set of ordinary differential equations aligned with a two-zone model are formulated and transformed into stochastic differential equations, subsequently solved through a numerical method. Based on the noisy HRR readings, a dynamical marker is constructed as a product of two quantities: the smoothed HRR and the standard deviation of the noise component. The dynamical marker was found to increase prior to a rise in the HRR signal on its own, confirming its superiority as a sign of flashover detection. To assess the practical applicability of the dynamical marker, we computed it to detect flashover incidents using HRR obtained from an FDS simulation and the fire calorimetry database provided by NIST; the dynamical marker exhibited a significant rise before the transition to flashover, confirming its potential as an early warning signal.

A new perspective on the diffuse gamma-ray emission excess* *Supported by the National Natural Science Foundation of China (12105292, 12175248, 12393853)

The Large High-Altitude Air Shower Observatory (LHAASO) recently published measurements of diffuse Galactic gamma-ray emission (DGE) in the 10−1000 TeV energy range. The measured DGE flux is significantly higher than the expectation from hadronic interactions between cosmic rays (CRs) and the interstellar medium. This excess has been proposed to originate from unknown extended sources produced by electron radiation, such as pulsar wind nebulae or pulsar halos (PWNe/halos). In this paper, we propose a new perspective to explain the DGE excess observed by LHAASO. The masking regions used in the LHAASO DGE measurement may not fully encompass the extended signals of PWNe/halos. By employing a two-zone diffusion model for electrons around pulsars, we find that the DGE excess in most regions of the Galactic plane can be well explained by the signal leakage model under certain parameters. Our results indicate that this signal leakage from known sources and contributions from unresolved sources should be considered as complementary in explaining the DGE excess.

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.

Guldborgsund Arson House Fire Experiment and Numerical Investigation

AbstractThis paper describes the Guldborgsund arson house fire experiment performed in Denmark and the subsequent numerical investigation. Gas temperatures were measured with four thermocouple trees, and smoke detector activation times were recorded in all rooms. A two-step approach was used to perform the numerical modelling for reproduction of the fire scene. The measured temperatures in the room of fire origin were used as an input for back calculating the Heat Release Rate (HRR) with the two-zone model Argos. As a next step, this HRR was used in the Fire Dynamic Simulator (FDS) to predict the temperatures and the smoke detectors’ activation times in other rooms. The FDS model was partly build using output files from the laser scanning. A sensitivity analysis is presented, where the effect of nine input parameters was investigated, including HRR, the material properties, the height of the fuel bed, the fire area, level of geometrical detail of the first item ignited etc. This study showed that the measured soot deposition heights on the walls differed from the heights of measured sharp temperature gradients used to indicate the hot smoke layer. The numerical simulations resulted in less than 50% error for most of the temperature measurement points during the fuel-controlled stage of the fire and results were the most sensitive to the input HRR. Material properties in FDS had a significant influence on the computed upper-layer gas temperatures at late stages of the fire.

Theoretical evaluation of high-permeability wellbore skin effect on aquifer response under pumping

Skin zones with distinct hydraulic properties often form around wellbores during drilling operations. Conventional skin-factor models, developed for low-permeability (positive) skin, assume discontinuous hydraulic heads between the wellbore and aquifer. However, these models may be inadequate for high-permeability (negative) skin scenarios resulting from intensive well development. This study derives a new skin-factor approach for negative skin approaches based on the two-zone model approach. We derive a new skin-factor approach that maintains continuous heads between the wellbore and aquifer while allowing for discontinuous fluxes. Using Laplace and finite cos-Fourier transformations, we develop a groundwater flow model for aquifer pumping under a partially penetrating well, incorporating this new approach. An asymptotic expansion method is employed to obtain a large-time solution. The proposed model is also compared to the existing two-zone model. Sensitivity analysis reveals that high-permeability skin significantly affects drawdown, particularly when there is a sudden increase in porosity within the skin zone. This revised approach provides a more accurate framework for modeling various skin effects, offering valuable insights for groundwater management and well hydraulic modeling in both hydrogeology and petroleum engineering applications.

A γ-Ray-emitting Blazar at Redshift 3.64: Fermi-LAT and OVRO Observations of PKS 0201+113

High-redshift (z > 3) γ-ray blazars are rare, but they are crucial for our understanding of jet evolution, γ-ray production and propagation, and the growth of supermassive black holes in the early Universe. A new analysis of Fermi-LAT data reveals a significant (5σ), spectrally soft (Γ ≃ 3.0) γ-ray source in a specific 4 month epoch, cospatial with PKS 0201+113 (z = 3.64). Monitoring of PKS 0201+113 at 15 GHz by the Owens Valley Radio Observatory 40 m telescope from 2008 to 2023 shows a prominent flare that dominates the radio light curve. The maximum of the radio flare coincides with the γ-ray flare, strongly suggesting an association (p-value = 0.023) between the γ-ray and the radio sources. PKS 0201+113 is only the third γ-ray blazar to be identified with z > 3.5, and it is the first such object to be identified by the detection of quasi-simultaneous γ-ray and radio flares. The jet properties of this peculiar blazar have been investigated. A detailed study of a two-zone leptonic model is presented that fits the broadband spectral energy distribution. An alternative scenario is also briefly discussed.

Simulation of melting characteristics of thermal washing of the wax layer in the pipeline based on lattice Boltzmann method

ABSTRACT The escalation of transportation costs and the obstruction of pipelines due to wax deposition significantly impact the secure and efficient production of oil fields. The thermal washing method, characterized by its simplicity and cost-effectiveness, is widely employed for dewaxing crude oil gathering pipelines. The investigation of the melting mechanism of the wax layer during thermal washing is extremely important. In this paper, the two-zone composite model of multiphase flow-porous medium in the mushy zone is introduced into the solid-liquid phase change lattice Boltzmann model based on the enthalpy method, and the problem of circumferential inhomogeneity in the melting of the wax layer and the important influence of flow and heat transfer in the mushy zone on the thermal washing are systematically investigated. The demarcation point (γ tr) between the high and low liquid fraction zones is measured to be 0.81 by microscopic experiment. The impacts of varying γ tr and Ste numbers on wax melting at different circumferential locations are analyzed in detail. The results show that the melting efficiency of wax exhibits a nonlinear enhancement from − 90° to 90°. The melting efficiency of wax decreases with increasing γ tr for wax layers between 90° and − 45°. The melting efficiency of wax increases with higher Ste numbers at all angles. The Ste number increases from 0.42 to 5.0, and the melting efficiency of the wax layer at angles ranging from 90° to − 90° increases by factors of 2.16, 2.16, 2.20, 2.61, and 3.33 respectively. The growth of the Ste number improves the warming rate and shortens the generation time of convection at the center point of the wax layer. The research findings serve as a fundamental reference for formulating a rational and efficient thermal washing scheme.

Optimizing phased-evacuation strategy for high-rise buildings in fire

Evacuation in high-rise buildings is a critical safety issue due to multiple floors and high occupant density. Phased evacuation strategies can mitigate this challenge. This research focuses on developing optimal phased evacuation strategies to enhance occupant safety during high-rise building fires. The study demonstrates these strategies in a 25-story building, housing 2400 individuals, across various scenarios. To simulate fire and smoke development, a two-zone model is employed to analyze behavior within the building. Pedestrian evacuation scenarios are simulated using Pathfinder and a mathematical model, providing insights into individual movements and congestion during evacuation to analyze different phased evacuation strategies. Findings underscore the importance of smoke flow and pedestrian motion in optimizing phased evacuation strategies. The modeling results show that implementing the optimal strategy can lead to a 65 % reduction in staircase congestion time. This study provides a quantitative analysis of high-rise building fires, considering dynamic fire and evacuation evolution. It emphasizes the importance of phased evacuation strategies and advocates for their real-life application. This research enhances the understanding of high-rise building evacuations to improve safety measures and provides a robust framework for optimizing evacuation strategies by integrating fire modeling. This study demonstrates the effectiveness of phased evacuation and paves the way for future applications to ensure occupant safety in fire emergencies.

Thermal characteristics of gasoline engine powered by petrol and LPG under variable speed, compression ratio

ABSTRACT This study involves the evaluation of the thermal characteristics of gasoline engines fuelled with LPG instead of gasoline fuel using the simulation software Diesel-RK. The feasibility of using LPG is investigated via parameters (combustion, performance, and emissions) at different engine speeds and compression ratios. A two-zone combustion model is used. The results revealed that the maximum pressure obtained was 82 bar for gasoline compared to 78 bar for LPG at 5000 rpm. There is a noticeable reduction in BSFC with the use of LPG relative to gasoline for all ranges of compression ratio and engine speed used. The NOx emissions were significantly reduced by 8.60% using LPG rather than gasoline. Studies conducted by different researchers validate the obtained results.

Effect of Biochar on NO3--N Transport in Loessial Soil and Its Simulation

The objective of this study was to explore the effects of different amounts of biochar on the migration process and characteristics of NO3--N in loessial soil. In this study, six groups of mixed soil samples with biochar and loessial soil mass ratios of 0% (T0), 1% (T1), 2% (T2), 3% (T3), 4% (T4), and 5% (T5) were used as research objects. NO3--N was used as the tracer. Through the indoor soil column solute transport simulation tests, the effects of different biochar application amounts on the NO3--N transport process in loessial soil were simulated and studied. The results showed that the breakthrough curve of NO3--N in loessial soil shifted to the right with the increasing of biochar application, and the peak value gradually decreased. The initial penetration time, complete penetration time, and total penetration time increased with the increasing of biochar application amount. The total penetration time of NO3- in the T1, T2, T3, T4, and T5 treatments was 1.26, 2.31, 2.72, 3.22, and 3.57 times that of T0, respectively. The R2 was > 0.997 and RMSE was < 2.083 of the two-zone model (TRM). Compared with the convection-dispersion equation (CDE), the TRM model had higher fitting accuracy and could better simulate the NO3--N migration process in loessial soil after the application of different contents of biochar. The analysis of the fitting parameters of the TRM model showed that the average pore velocity, hydrodynamic dispersion coefficient, and water content ratio in the movable zone gradually decreased with the increasing of biochar application, whereas the dispersion and mass exchange coefficient showed an increasing trend. The results showed that biochar application could effectively enhance the ability of loessial soil to fix NO3--N, reduce the leakage of NO3--N to groundwater, and play an important role in maintaining soil fertility and preventing groundwater pollution.

Correction to: Two-Zone Model of Laser-Induced Plasma

Correction to: Two-Zone Model of Laser-Induced Plasma

Magnetar as the Central Engine of AT2018cow: Optical, Soft X-Ray, and Hard X-Ray Emission

AT2018cow is the most extensively observed and widely studied fast blue optical transient to date; its unique observational properties challenge all existing standard models. In this paper, we model the luminosity evolution of the optical, soft X-ray, and hard X-ray emission, as well as the X-ray spectrum of AT2018cow with a magnetar-centered engine model. We consider a two-zone model with a striped magnetar wind in the interior and an expanding ejecta outside. The soft and hard X-ray emission of AT2018cow can be explained by the leakage of high-energy photons produced by internal gradual magnetic dissipation in the striped magnetar wind, while the luminous thermal UV/optical emission results from the thermalization of the ejecta by the captured photons. The two-component energy spectrum yielded by our model with a quasi-thermal component from the optically thick region of the wind superimposed on an optically thin synchrotron component well reproduces the X-ray spectral shape of AT2018cow. The Markov Chain Monte Carlo fitting results suggest that in order to explain the very short rise time to peak of the thermal optical emission, a low ejecta mass M ej ≈ 0.1 M ⊙ and high ejecta velocity vSN≈0.17c are required. A millisecond magnetar with P 0 ≈ 3.7 ms and B p ≈ 2.4 × 1014 G is needed to serve as the central engine of AT2018cow.

Lobe-dominated γ-Ray Emission of Compact Symmetric Objects

The γ-ray emitting compact symmetric objects (CSOs) PKS 1718–649, NGC 3894, and TXS 0128+554 are lobe-dominated in the radio emission. In order to investigate their γ-ray radiation properties, we analyze the ∼14 yr Fermi/LAT observation data of the three CSOs. They all show the low luminosity (1041–1043 erg s−1) and no significant variability in the γ-ray band. Their γ-ray average spectra can be well fitted by a power-law function. These properties of γ-rays are clearly different from the γ-ray emitting CSOs CTD 135 and PKS 1413+135, for which the γ-rays are produced by a restarted aligned jet. In the L γ − Γ γ plane, the three CSOs are also located at the region occupied by radio galaxies (RGs) while CTD 135 and PKS 1413+135 display a similar feature to blazars. Together with a similar radio emission property to γ-ray emitting RGs Cen A and Fornax A, we speculate that the γ-rays of the three CSOs stem from their extended mini-lobes. The broadband spectral energy distributions of the three CSOs can be well explained by the two-zone leptonic model, where their γ-rays are produced by the inverse Compton process of the relativistic electrons in extended regions. By extrapolating the observed Fermi/LAT spectra to the very high energy band, we find that TXS 0128+554 among the three CSOs may be detected by the Cherenkov Telescope Array in the future.

Combustion performance and flame front morphology of producer gas from a biomass gasification-based cookstove

The present work investigates the last phase of the biomass gasification-based cookstove, which is the combustion process of biomass producer gas (BGP). A constant volume combustion bomb and kinetic simulation are used to characterize the behavior of the biomass producer gas and its pollutant emissions under a conventional premixed combustion process. The BGP combustion processes of two types of biomasses available in Colombia (Pinus Patula and Cordia Alliodora) are studied under certain conditions of pressure, temperature, and equivalence ratio. To characterize the combustible mixture of gases obtained after biomass gasification, a combustion chamber with cylindrical geometry equipped with a Schlieren optical diagnostic system to visualize the combustion process, and a piezoelectric pressure transducer to register the instantaneous pressure are used. By visualizing the flame front, the flame propagation speed at which the flame propagates through the combustion chamber and the morphology of the flame are studied. Instantaneous pressure is the input of a two-zone diagnostic model through which variables, such as burning velocity, temperature, mass burned fraction, etc., are obtained to characterize the combustion process of the mentioned gasification gases. Experimental results are compared and complemented with kinetic modeling results obtained with the Cantera package using the Gri-Mech 3.0 and Aramko 1.3 kinetic mechanisms, in terms of laminar burning velocity and NO, NO2, CO, and CO2 exhaust emissions. Results show that Cordia Alliodora presents higher burning velocities than Pinus Patula BP. However, lower CO2 emissions are obtained during the Cordia Alliodora combustion, and NOx emissions are not influenced by the type of biomass considered.

Adaptive Control of a Two-zone Model of Electric Ship Power Electronics Power Distribution System

Power system operations must be robust to unexpected non-ideal conditions. In an attempt to create more robust Power Electronic Power Distribution Systems in ships in the event of changes in the ambient characteristics such as temperature or load changes, a Model Reference Adaptive Control was implemented. Our approach was based on distributed control of the converters in a two-zone model and for the two types of converters present in the system. This work used adaptive law based on instantaneous cost to modify the existing control coefficients in an online mode. The result is that the designed adaptive control can readjust the transient and steady state responses of the converters when they deviate from their original forms in case of any of the unexpected conditions.

Two-zone subgrid combustion model for large eddy simulations of buoyant diffusion flames

This work presents the development of a novel two-zone subgrid combustion model (SCM) recently designed to incorporate the finite-rate kinetics in the large eddy simulations of buoyant turbulent diffusion flames in which turbulence might not be fully developed. The reaction rate is evaluated as the mass-weighted average of contributions by the higher-temperature flamelet zone and the lower-temperature surrounding. The volume fraction of the flamelet zone and the time scale responsible for the inter-zone transfer are determined by the expected thickness of the laminar diffusion flamelet existing in the grid cell, and by wrinkling its surface by the unresolved turbulent eddies. This approach is applicable to an arbitrary level of turbulence (quantified by the subgrid Reynolds number) thereby overcoming the limitation of the conventional eddy dissipation concept formulated for the high-Re flames. Advancing the previous study [1], the flamelet status at every time step is evaluated by solving transient equations of heat and mass transfer between the subgrid zones. Effective kinetic parameters for the simplified global reaction of fuel oxidation are derived from experimental data for autoignition delay in methane-air and n-heptane-air mixtures and are shown to capture transition from the low- to high-temperature measurement datasets including the NTC (negative temperature coefficient) interval. With these kinetic parameters, the SCM is validated against the measurements in the unconfined methane-air flame produced by the UMD line burner and then applied to predict the development and extinction of the heptane-air flame in an under-ventilated enclosure. In the first case, flame extinction is predicted at the oxygen concentrations consistent with the experimental values. In the second scenario, global low-frequency oscillations of the heat release rate are predicted prior to flame extinction.