Instability Mechanism Research Articles

Instability processes in short and long laminar separation bubbles

This work studies the link between the bursting process of a flat plate laminar separation bubble and the modification of the stability characteristics of the separated shear layer due to changes in the flow parameters. A vast population of short and long laminar separation bubbles was surveyed by means of Particle Image Velocimetry instrumentation for different values of the Reynolds number, the free-stream turbulence intensity and the streamwise pressure gradient. A fine-step variation of the free-stream velocity allowed us to determine the critical Reynolds number at which bursting occurs. Successively, the most amplified wavelength and frequency were computed for both the short and the long bubble regimes. Once scaled with the boundary layer displacement thickness at separation, the average wavenumber of the vortices shed by the bubble was found to be constant and equal to about 0.9 in the short regime, accordingly to previous studies. Differently, this quantity reduces to about 0.6 in the long bubble regime, and a marked change in the Strouhal number of vortex shedding occurs. Also, the temporal growth of spanwise vortices was seen to occur in the recirculation region of long type bubbles, being linked to an absolute instability of disturbances. The currently acquired data demonstrate the existing link between the bursting process of a laminar separation bubble and a marked change in the instability mechanisms driving the transition process of the boundary layer. A simplified correlation for the prediction of bursting is provided in this work as a function of the free-stream turbulence intensity and the streamwise pressure gradient.

A Review of Stability of Dam Structures in Coal Mine Underground Reservoirs

Coal has remained the primary component of China’s energy structure, and high-intensity extraction has continued in the central and western coal-producing regions of China. In contrast to the abundant coal resources, water resources have become extremely scarce in these regions, creating a conflict between coal resource extraction and water resource conservation. The coal mine underground reservoir (CMUR), as a typical technology for combined coal and water extraction and water-preserving coal mining, has been applied in numerous mines in central and western China. This effectively alleviates water resource shortages and achieves the goal of water resource conservation. The CMURs utilizes the goaf created by longwall mining as the water storage space. The reservoir dam structure comprises coal pillars, which serve as protective coal pillars in the mining area, and artificial dam structures that filled the gaps between these coal pillars. The stability of the dam structure under the complex stress effects of hydraulic coupling has been identified as the key to maintaining the safe operation of the CMUR. The mechanical properties, stress field, fracture field, and seepage field (“three fields”) change mechanisms, as well as the research results on size optimization of coal pillar dams and artificial dams in CMURs, were systematically reviewed. The core content included the instability and failure mechanisms of dam structures under the comprehensive coupling effects of factors such as dry–wet cycles of mine water, long-term immersion, chemical effects of high-salinity water, dynamic and static loads, and cyclic loads. This paper is considered to have certain reference value for the study of the stability of dam structures in CMURs and to provide some guidance for the safe operation of CMURs.

Effect of double bypass conditions on the thermodynamic characteristics and internal flow mechanisms in a variable core cycle compression system

The performance of variable cycle core compression system (VCCCS) is crucial for achieving mode transition in variable cycle engine (VCE). The double bypass matching characteristics and internal flow mechanisms of the VCCCS under various conditions are investigated in this study. The results indicate that, in the low bypass ratio (LBR) mode, Under the near choke or design point conditions of external bypass, internal bypass throttling causes the excessive blockage of low-energy fluid near the suction surface of high-pressure compressor (HPC) second stator, which leading the flow instability in VCCCS. Under the near stall condition of external bypass, internal bypass throttling causes the CDFS to reach the stall boundary firstly. The flow instability is triggered by the interaction between tip leakage flow and shock waves in CDFS rotor tip. Following mode transition, the CDFS exhibits lower flow and load, resulting in changes in the matching state of HPC due to the strong coupling effects of upstream components, thereby altering the matching characteristics and flow instability mechanisms of the VCCCS. The impact of different bypass conditions on VCCCS performance is elucidated, further establishing the association between bypass exit meridian velocity and VCCCS performance, which is helpful to select key design parameters.

Roles of heat and stress transfer in triggering fault instability in conjugate faulted reservoirs

The presence of multiple conjugate but non-intersecting faults in geothermal reservoirs presents issues related to fault interaction in the presence of complex coupled thermo-hydro-mechanical (THM) processes in influencing the triggering of seismicity. We examine alternate strategies in stimulating such a conjugate-faulted geothermal reservoir analogous to that hosting the Mw 5.5 Pohang earthquake (2017). We evaluate the response of the reservoir to both short-term stimulation (1y) and long-term production (10y), both with and without thermal effects – for a large fault (F1) adjacent to a non-intersecting smaller fault (F2) and in a reverse faulting stress regime. Results suggest that the slip on either fault impacts the stress state on the other fault through stress transfer. Reactivation of the minor fault (F2) transfers stress towards to the upper part of primary fault (F1), inducing instability. The slip of the major fault is delayed by positioning the location of injection away from the junction between the two faults – decreasing the injection depth from 4087.5 m to 3712.5 m delays the time to slip by 2.61 y. Furthermore, thermal stress plays a decisive role in prompting late-stage fault reactivation for long-term fluid circulation where pore pressures have already reached steady state. The pattern of thermal unloading follows the path of fluid transport and heat transfer along the faults. Overall, this study not only advances our understanding of mechanisms of injection-induced fault instability in EGS reservoirs with multiple and closely-interacting faults, but also provides insights into how different injection strategies can delay or mitigate induced seismicity.

Mechanisms and influential factors of soil chromium long-term stability by an accelerated aging system after chemical stabilization

Chemical stabilization is one of the most widely used remediation strategies for chromium (Cr)-contaminated soils by reducing Cr(VI) to Cr(III), and its performance is affected by human and natural processes in a prolonged period, challenging long-term Cr stability. In this work, we established a method for evaluating the long-term effectiveness of remediation of Cr-contaminated soils, and developed an accelerated aging system to simultaneously simulate acid rain leaching and freeze-thaw cycles. The mechanisms and influencing factors of long-term (50-year) change in soil Cr speciation were unravelled after stabilization with Metafix®. Chemical stabilization remarkably decreased the contents of Cr(VI)soil, Crtotal-leach and Cr(VI)leach, among which the removal rate of Cr(VI) in soil was up to 89.70 %, but it also aggravated soil Cr instability. During the accelerated aging process, Crtotal-leach change rates in chemically stabilized soil samples were 0.0462–0.0587 mg/(L·a), and soil Cr became instable after 20-year accelerated aging. The proportion of Cr bound to organic matter and residual Cr increased in soil, and exchangeable Cr decreased. Linear combination fitting results of XANES also showed that Cr(VI) and Cr3+ were transformed into OM-Cr(III), Fh-Cr(III) and CrFeO3 after restoration. During the accelerated aging process, acid rain leaching activated Cr(III) and dissolved Cr(VI), whereas freeze-thaw cycle mainly affected OM-Cr. Chemical stabilization, acid rain leaching and aging time were the major factors influencing the stability of soil Cr, and the freeze-thaw cycle promoted the influence of acid rain leaching. This study provided a new way to explore the long-term effectiveness and instability mechanisms at Cr-contaminated site after chemical stabilization.

Numerical research on disastrous mechanism of seepage instability of karst collapse column considering variable mass effect

In order to reveal the disastrous mechanism of seepage instability of karst collapse column considering variable mass effect, a variable mass fluid–solid coupling mechanical model of water inrush is established, by considering the random distribution characteristics of a collapse column. Taking Qianjin coal mine as the research background, based on the Weibull distribution theory, the heterogeneous distribution characteristics of rock mass is described, and COMSOL Multiphysics numerical simulation software is employed to simulate the seepage characteristics and inrush water changes in collapse columns under different conditions of homogeneity, water pressure, and initial porosity. The research results show that the greater the homogeneity is, the more water conduction channels are formed, and the porosity increases accordingly, when considering the influence of different homogeneity on the seepage characteristics of broken rock mass, which eventually leads to water inrush accidents and a sharp increase in water inflow. Besides, when studying the seepage evolution law of different water pressures on a broken rock mass, an elevation of water pressure dramatically increases the porosity and seepage rate of the water. Over time, the broken rock particles gradually migrate and the fine particles are transported and eroded by the water flow, resulting in changes in the seepage characteristics and the formation of potential water diversion channels. Finally, when taking into account the effect of different initial porosity on the fractured rock mass seepage characteristics, the greater the original porosity is, the higher the seepage velocity is, and the particle migration increases the permeability. This leads to a more pronounced conductive water passage formation, which reveals the disastrous mechanism of seepage instability of karst collapse column considering variable mass effect.

Dynamic failure behaviour of sandstone with filling joints using the Brazilian disc method

Rock masses contain joints often filled with low-strength materials such as mud and gum. The instability mechanism of a filled jointed rock mass under dynamic loads is complex, posing challenges for disaster control. Splitting tests were conducted using a split Hopkinson pressure bar device and a high-speed camera to investigate the effects of weak-filling joints at various angles on the failure mechanism and mechanical properties of sandstone. The typical failure processes, splitting mechanical properties and fracture morphologies were analysed. The results indicated that as the joint angle increased from 0 to 90°, the crack coalescence pattern of the jointed sample under static and dynamic splitting loads evolved from complete separation along the bonding surfaces to shear–slippage failure along the direction of the joint angle and tensile–shear failure with a dominant zigzag track, ultimately leading to typical tensile failure. Under dynamic loads, filled joints reduced the brittleness of the samples, contrary to the behaviour observed under static loads. The peak load and displacement were proportional to the joint angle and impact gas pressure, respectively. The fracture surface morphology precisely reflected the macroscopic fracture mechanism at the microscopic scale.

Numerical Investigation of the Impact of the Rectangular Nozzle Aspect Ratio On Liquid Jet in Crossflow

Abstract High fidelity simulation is conducted to investigate liquid jet in crossflow, specifically regarding the rectangular nozzle. The influence of aspect ratio (AR) of nozzles on the atomization characteristics of liquid column in the process of primary breakup is explored by the analysis of the flow structure of crossflow and liquid column. The aspect ratio is ranging from 1 to 8. The results indicate that as the increase of aspect ratio, the disturbance of crossflow to the liquid on the sides is weakened. While the thickness of liquid column also gradually decreases, which enables smaller disturbances to promote droplet shedding. Therefore, surface breakup first weakens and then strengthens. In the column breakup process, the increase of aspect ratio causes crossflow to become the main factor affecting column breakup, and the influence of air pressure gradually weakens. This indicates a shift in the mechanism of surface instability from “Rayleigh-Taylor” (R-T) instability to “Kelvin-Helmholtz” (K-H) instability.

Editorial: Instability mechanism and disaster prevention of the jointed rockmass

Editorial: Instability mechanism and disaster prevention of the jointed rockmass

Suppression of lift-up effect in the 3D Boussinesq equations around a stably stratified Couette flow

In this paper, we establish linear enhanced dissipation results for the three-dimensional Boussinesq equations around a stably stratified Couette flow, in the viscous and thermally diffusive setting. The dissipation rates are faster compared to those observed in the homogeneous Navier-Stokes equations, in light of the interplay between velocity and temperature, driven by buoyant forces. Our approach involves introducing a change of variables grounded in a Fourier space symmetrization framework. This change elucidates the energy structure inherent in the system. Specifically, we handle non-streaks modes through an augmented energy functional, while streaks modes are amenable to explicit solutions. This explicit treatment reveals the oscillatory nature of shear modes, providing the elimination of the well-known three-dimensional instability mechanism known as the “lift-up effect”.

Macroscopic and mesoscopic mechanism of hydration instability of the rock-grout coupled structure

In order to investigate the macroscopic and mesoscopic mechanism of hydration instability of rock-grout structure under the influence of moisture content, a direct shear test combined with particle flow code (PFC) simulation was conducted subject to various moisture content levels and normal stresses. The results show that a higher moisture content would compromise the load bearing capacity of soft rock anchorage structures by deteriorating the structural integrity of the surrounding rock and the bonding effect between the anchorage interfaces. The load bearing capacity of the surrounding rock is also rapidly reduced. The rock-grout structure has four main shear damage modes, which are influenced by both moisture content and normal stress. When the saturated moisture content is reached, the anchorage structure has lost its bearing capacity, and the rock is muddied and subsequently debonded from the bolt. The energy required to break the internal adhesion of the rock-grout structure under the effect of hydration is greatly reduced, resulting in easy decoupling and dispersion between the rock skeleton particles. In turn, the rock surface particles bonded by the anchor agent are separated from the deeper particles, resulting in the failure of the bonding surface and weakening the coupling effect between the anchor and the surrounding rock. According to the test results, the control measures for surrounding rock of muddy roadway are put forward.

Deep learning-based image segmentation for instantaneous flame front extraction

This paper delves into the methodology employed in examining lean premixed turbulent flame fronts extracted from Planar Laser Induced Fluorescence (PLIF) images at elevated pressures. In such flow regimes, the PLIF signal suffers from significant collisional quenching, typically resulting in image data with low signal-to-noise ratio (SNR). This poses severe difficulties for conventional flame front extraction algorithms based on intensity gradients and requires intense user intervention to yield acceptable results. In this work, we propose Convolutional Neural Network (CNN)-based Deep Learning (DL) models as an alternative to problem specific conventional methods. The pretrained DL models were fine-tuned, employing data augmentation, on a small annotated dataset including a variety of conditions between SNR ≈\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\approx$$\\end{document} 1.6 to 2.6 and subsequently evaluated. All DL models significantly outperformed the best-scoring conventional implementation both quantitatively and visually, while having similar inference times. IoU-scores and Recall values were found to be up to a factor ≈\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\approx$$\\end{document} 1.2 and ≈\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\approx$$\\end{document} 2.5 higher, respectively, with ≈\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\approx$$\\end{document} 1.15 times improved Precision. Small-scale structures were captured much better with fewer erroneous predictions, becoming particularly pronounced for the lower SNR data investigated. Moreover, by applying artificially modeled noise, it was shown that the range of image conditions in terms of SNR that can be reliably processed extends well beyond the images included in the training data, and satisfactory segmentation performances were found for SNR as low as ≈\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\approx$$\\end{document} 1.1. The presented DL-based flame front detection algorithm marks a methodology with significantly increased detection performance, while a similar computational effort for inference is achieved and the need for user-based parameter tuning is eliminated. It enables a very accurate extraction of instantaneous flame fronts in large image datasets where supervised processing is infeasible, unlocking unprecedented possibilities for the study of flame dynamics and instability mechanisms at industry-relevant conditions.

Research updates on filter units for secondary stabilisation of offshore wind farm power cables

This paper presents an experimental and numerical campaign that was carried out to improve the understanding of the performances of filter units, or rock bags, when used as mean of secondary stabilisation for subsea power cables in offshore wind farms. The main parameters of influence are discussed. The capabilities of the experimental facility are presented as well as the experimental approach with a description of the cases tested. The numerical models are also described. Finally some of the findings about the main mechanisms of instability are presented.

Analysis of self-oscillation stability of a pump-turbine in the S-shaped regions of characteristic curves

Abstract The hydraulic oscillation of pumped-storage power station (PSPS) affects the safe and stable operation of the power station. The S-shaped characteristic curves of pump-turbine may sometimes result in instability in speed-no-load operation. To investigate the instability mechanisms and identify the main factors that affect stability, this paper used the overall matrix method to establish a self-excited oscillation calculation model of a PSPS and analyzed the stability characteristics and influencing factors of the S-shaped regions of the speed-no-load opening. The results show that when the slope of the characteristic curve is negative, the natural frequency attenuation factors of the system are negative, demonstrating stability. When the slope of the characteristic curve is positive, the system remains stable if the slope is large. However, if the slope of the characteristic curve is below a certain value, which may lead to instability. Increasing the inlet valve resistance and appropriately changing the valve layout position can enhance the stability of the system. The damping surge chamber fluctuations have no impact on system stability. The study also found that the stability regions of the S-shaped regions are related to the slope of the characteristic curve, the unit operating discharge, and the value resistance coefficient.

Diagnosis and Treatment of Varus Posteromedial Rotational Instability of the Elbow Joint in Children: Re-Understanding of the Injury Mechanism Associated With Coronoid Process Fractures.

To investigate the injury mechanism, diagnosis, and treatment of varus-posteromedial rotational instability of the elbow joint in children. According to the diagnostic criteria of varus posteromedial rotational instability of elbow joint, 16 children with coronoid process fractures treated in our department from July 2013 to July 2017 were re-evaluated. There were 14 males and 2 females, aged 7 to 14 years, with an average age of 11.6 years. Eight cases on left and 8 cases on right side. An associated elbow dislocation occurred in 8 of 16 cases. Nine patients were treated with a lateral soft tissue repair only. In 7 other patients in addition to the lateral soft tissue repair, the coronoid process fractures were treated with open reduction and fixation. At the last clinical follow-up, each elbow joint range of motion was recorded, radiographs were obtained, and functional performance was evaluated by the Mayo elbow performance score (MEPS). The average follow-up time was 81.9 months for the 9 patients treated with lateral elbow soft tissue repair. At the last follow-up, 2 of the patients had MEPS scores as excellent, 1 was good, and 6 were rated as moderate or poor. Four patients had a cubitus varus deformity. The average follow-up time was 30.3 months for the 7 patients treated with both soft tissue repair and coronoid fracture stabilization. The elbow joint MEPS scores for each of these 7 patients was excellent at the last follow-up, and no complications such as cubitus varus occurred. The results of the study suggest that children could also develop elbow varus-posterior medial rotational instability injuries under the same mechanism. Although the morbidity rate is low, due to insufficient understanding of the injury mechanism, it is prone to missed diagnosis, misdiagnosis, and delayed treatment, resulting in severe complications such as elbow instability, dislocation, traumatic arthritis, and elbow stiffness. On the contrary, according to the treatment principle of the posterior medial rotational instability of the elbow joint in adult, while the lateral repair is carried out, strong and effective reduction and fixation of the coronoid process fractures are adopted, it is expected that such children with rare elbow injuries can obtain excellent treatment outcomes.

Thermal-hydraulic phenomena and heat removal performance of a passive containment cooling system according to exit loss coefficient

The natural circulation system has been widely studied for use in various applications because of its inherent advantage. However, it has a key weakness called flow instability that makes the system unstable. Through massive previous research, the mechanisms of flow instability were analyzed, but there was an ambiguous aspect related to the effect of experimental parameters on the phenomenon. Particularly, there has been no report on the heat transfer performance of the system when flow instability phenomena were present. In this study, thermal-hydraulic phenomena of a two-phase natural circulation system that functions as a passive containment cooling system (PCCS) was investigated according to experimental parameters, namely, the temperature boundary (120–158 °C) and exit loss coefficient (0–34.5) under atmospheric pressure conditions. The experimental results showed five different flow types in the loop. The flow modes that occurred by the interaction between flashing and boiling were classified by referring to the mass flow rate, void fraction, and visualization data. The system was more unstable when the temperature boundary conditions increased, but it was more stable when the exit loss coefficient increased. These results have only been confirmed in our research. The reason for the results is that the flow conditions are located on the boundary between Density Wave Oscillation I and the stable flow region, and that boundary does not have clear criteria. In addition, comparing the heat transfer performance of a system by heat rate can confirm the effect of flow instability on the thermal performance of the passive cooling system. As a result, the high exit loss coefficient stabilizes the system better than the low case and has similar heat removal performance.

Aggregation Equivalence Method for Direct-Drive Wind Farms Based on the Excitation–Response Relationship

The grid interconnections for direct-drive wind farms have triggered multiple new sub-synchronous oscillation events, which can prevent the power system from operating safely and stably. However, the excessively high order of the detailed model for large-scale wind farms with multiple direct-drive permanent magnet synchronous generators (PMSGs) connected to power systems poses a challenge when investigating small disturbance stability and instability mechanisms. This study establishes a model of the grid-connected PMSG system based on the voltage/power excitation–response relationship to describe the dynamic characteristics of the port of the PMSG, and the analysis of active and reactive response characteristics of PMSG lays the foundation for model simplification. Based on the unit model, a direct-drive wind farm aggregation equivalence method based on the excitation–response relationship is proposed. The equivalent model obtained by this method is suitable for the small disturbance stability analysis of direct-drive wind farms grid connected system, with good accuracy. The simulation verified the effectiveness of the aggregation model.

Surrounding rock instability mechanism for fault-crossing tunnels in water-rich soft rock

Surrounding rock instability mechanism for fault-crossing tunnels in water-rich soft rock

Numerical Simulation of Rainfall-Induced Erosion on Infiltration and Slope Stability

In slopes where a mixture of coarse and fine particles is present, the infiltration of rainfall can cause the migration of fine particles. This migration alters the hydraulic properties of the soil and has implications for slope stability. In this study, the slope under investigation is a tailings dam composed of loosely consolidated soil with a wide particle size distribution. Due to rainfall infiltration, fine particles tend to migrate within the voids of the coarse particle framework, leading to changes in hydraulic properties and inducing slope instability. The classical internal erosion constitutive model, known as the Cividini and Gioda erosion criterion, is commonly used to predict the behavior and effects of fine particle erosion in geotechnical engineering. However, certain parameters in this erosion criterion equation, such as long-term density, are challenging to obtain through experiments. To investigate the coupled evolution of seepage and erosion within landfill slopes under the influence of rainfall infiltration and to understand the mechanisms of slope instability, this research assumes the erosion of fine particle suspension and adopts the Worman and Olafsdottir erosion criterion to establish a coupled model of unsaturated seepage and internal erosion. The developed model simulates the coupled response of seepage and erosion in unsaturated landfill slopes under three different rainfall intensities. It is then combined with the infinite slope model to quantitatively analyze the impact of fine particle migration on soil permeability and slope stability. The numerical simulations provide the following findings: The Worman and Olafsdottir erosion criterion, unlike the Cividini and Gioda erosion criterion, only requires the determination of the soil’s gradation curve to estimate the erosion rate. Internal erosion primarily occurs within the leading edge of moisture penetration, accelerating the advancement of the wetting front and reducing slope stability. When the rainfall intensity is lower than the saturated permeability coefficient, the influence of internal erosion can be disregarded. However, under rainfall intensities equal to or greater than the saturated permeability coefficient, considering internal erosion results in a difference in the depth of the wetting front of up to 34.2 cm after 6 h in the R2 scenario. The safety factor without considering internal erosion is 1.12, whereas considering internal erosion yields safety factors between 1.08 and 1.09. In the R3 scenario, the difference in the depth of the wetting front reaches 53.8 cm after 6 h, with a safety factor of 1.12 without considering internal erosion and safety factors between 1.06 and 1.07 when considering internal erosion.

Flame propagation characteristics and inherent instability in premixed carbon-free hydrogen/ammonia/oxygen combustibles

Hydrogen-ammonia fuel is an ideal clean energy, and its use in the combustion field is important for achieving energy transformation. This study carried out a series of flame propagation experiments of hydrogen/ammonia/oxygen mixtures and analyzed the instability phenomenon and the competition mechanism of inherent instability during flame propagation at different fuel ratios (0.5–2.0), equivalence ratios (0.5–1.5), and initial pressures (0.1 atm − 1.0 atm). The results show that the hydrogen/ammonia combustion in oxygen is extremely susceptible to instability. Only when the P0 = 0.1 atm, does the flame surface remain smooth in the observable region. Crack initiation and development are observed on the flame surface at P0 = 0.5 atm, and the flame propagated to a late stage with a sudden increase in velocity, which is due to the destabilization of the flame. The increase in fuel ratio gradually shifts the Lewis value away from 1, but does not change the state of thermal-diffusional factor on flame stability. The flame thickness reaches the minimum at φ = 0.8. As the initial pressure increases from 0.1 atm to 1.0 atm, the Lewis number remains almost constant, while the flame thickness, Markstein length, and critical instability radius all decrease rapidly. At the theoretical equivalence ratio and fuel ratio of 1.0, the flame thickness decreased rapidly from 0.143 mm to 0.015 mm, with a decrease of 89.5 %. The flame thickness for hydrogen/ammonia combustion in air is approximately five times that in oxygen for the same operating conditions, implying that the oxygen atmosphere greatly increases the hydrodynamic instability of the hydrogen/ammonia premixed flame. The critical Peclet number (Pecr) of the hydrogen/ammonia/oxygen flames rises exponentially with increasing Markstein number (Ma), and the empirical correlation can be expressed as Pecr = 166.11 × exp (Ma/1.04) + 47.86 (P0 = 0.5 atm) and Pecr = 538.29 × exp (Ma/0.53) + 142.78 (P0 = 1.0 atm).