Deceleration Stage Research Articles

Numerical investigation of the penetration of torpedo anchors in sand-overlying-nonhomogeneous clay deposit

This study presents a comprehensive investigation of torpedo anchors penetrating into sand-overlying-nonhomogeneous clay deposit, with the main objective of predicting the anchor embeddment depth after it penetrates through the top sand layer. The behavior of the torpedo anchor and soil failure mechanisms during the installation process are analyzed using large deformation finite element (LDFE) methods. To validate the numerical models, comparisons are made with data from existing centrifuge and field tests. An extensive parametric study is then conducted, exploring a wide range of soil properties, soil layer thickness, anchor velocities, and parameters related to torpedo anchor geometry. Two intriguing findings emerge from the analysis. Firstly, there is an evolving soil failure, with the strain-softening region expanding in the clay near the boundary between the two soil layers. Secondly, the velocity profile during both the acceleration and deceleration stages varies with different sand layer thicknesses, displaying a nonlinear relationship with the anchor's final embedment depth. Based on the findings, a simple design procedure is developed to account for the influencing factors on the anchor installation depth. This procedure is expected to be valuable for engineers seeking to estimate the behavior of torpedo anchor installations in sand-overlying-nonhomogeneous clay soil deposits.

Physical-chemical coupling excitation of low activity coal gasification slag solid waste and its application as a backfill cementitious material

To realize the green mining and clean utilization of coal resources, environmental issues such as coal mine subsidence and coal gasification slag (CGS) solid waste storage require urgent resolution. High residual carbon and low activity serve as bottlenecks for CGS to achieve safe, efficient and resourceful consumption. Therefore, in this study, we proposed the concept of graded utilization of CGS and conducted a study on physical–chemical coupling to stimulate the hydration activity of CGS. We prepared a modified coal gasification slag-based cement paste backfill material (MCGS-CPB) to meet the technical requirements of coal mine backfilling. Through various experimental tests and microscopic characterization methods (uniaxial compressive strength, hydration heat and microscopic tests, etc.), we elucidated the modification mechanism of CGS, and the synergistic reaction law of MCGS-CPB was clarified. The following results were obtained. (1) The best excitation approach for CGS hydration activity was salt modification, and the early strength of MCGS-CPB prepared with CGS excited by sodium sulfate (SS) developed rapidly, while the later strengths showed expansion and deterioration. The strength development of MCGS-CPB prepared by CGS activated with calcium chloride (CC) was relatively stable, and the excitation effect was ideal. (2) The salt modification of CGS had a significant effect on the early hydration process of MCGS-CPB. The durations of the initial dissolution, acceleration, deceleration and slow reaction stages were prolonged, and the duration of the induction stage was shortened, while the hydration heat release rate increased. (3) The salt modification of CGS had a significant effect on the microstructure of MCGS-CPB, primarily due to the difference in excitation effect of salt modification on CGS on pozzolanic activity. This study provided theoretical guidance for the safe, efficient and resourceful utilization of CGS.

Reappraising Crystallization Kinetics with Overgrowth Chronometry: an in Situ Study of Olivine Growth Velocities

Abstract We investigated the early stages of olivine crystal growth via in situ seeded experiments in a single plagioclase-hosted melt inclusion, using a heating stage microscope. Each experiment was subjected to a cooling ramp of 7800°C/h followed by an isothermal dwell at 19°C, 38°C, 57°C, 77°C, 96°C or 129°C of undercooling. The seeds (6–16 μm in diameter Ø) grew into large crystals (Ø 80–169 μm) in 3 to 30 min through the symmetrical development of tabular, skeletal, and dendritic overgrowths as the undercooling of the system increased. Time-resolved image processing and incremental measurements of the overgrowth thicknesses indicate up to three stages of crystal growth: an acceleration stage, a linear (constant growth rate) stage, and a deceleration stage. At the isotherm, the growth velocities reach a stable maximum that in all experiments corresponds to the period of linear growth. The highest linear values are measured at the $\left\{101\right\}$ interfaces, from 2.1 × 10−8 m/s at 19°C of undercooling to 4.8 × 10−7 m/s at 129°C of undercooling. Crystal growth is slower at other interfaces, in the ranges 1.9–7.6 × 10−8 m/s and 4.5 × 10−9 – 7.6 × 10−8 m/s for the $\left\{100\right\}$ and $\left\{001\right\}$ forms, respectively. Growth in the $<010>$ dimension appears limited to less than 2.4 × 10−8 m/s at 129°C of undercooling. We constrain the uncertainty on these growth velocities, which includes the environmental conditions (± 8.6°C on the nominal undercooling) and the measurements of crystal lengths (underestimated by <16% at most fast interfaces). A systematic and comprehensive review of 19 pre-existing datasets indicates that our linear growth velocities are faster than most growth rates determined at comparable undercoolings. Growth rates determined as half crystal lengths divided by total time are intrinsically low estimates of the true maximum, linear growth velocities, because the total time includes periods of slower or non-growth, and measured crystal dimensions are subject to projection foreshortening or truncation. These errors can lead to values that are several times to several orders of magnitude lower than the true maximum growth rates. This study completes and refines previously published data on the crystallization kinetics of olivine, highlighting the sensitivity of growth rates to specific environmental conditions and measurement methods. We emphasize the importance of symmetrical growth and true maximum growth velocities for interpreting olivine growth histories.

Development and field application of a modified magnesium slag-based mine filling cementitious material

How to make high value use of solid waste is a key issue in achieving sustainable development. In this study, a modified magnesium slag-based cementitious material (M·C) for mine filling was developed using industrial solid waste modified magnesium slag (MMS), fly ash (FA), and desulfurization gypsum (DG); the mechanical properties, stability, hydration heat and synergistic reaction mechanism of M·C were investigated; and the M·C with optimized ratios was used to prepare cemented aeolian sand/coal gangue backfill material for industrial mine backfill tests. The results revealed that (1) the uniaxial compressive strength (UCS) of M·C at 28 d was 5.60–19.30 MPa, and the FA and DG contributed significantly to the development of the UCS of M·C. A combination of 65–90% MMS, 2.5–7.5% DG and 5–35% FA was determined to be optimum for the preparation of M·C. (2) The expansion rate (Ex) of M·C increased rapidly in the early stage, increased slowly after 28 d, and stabilized after 90 d. E28 of M·C was 0.107%–0.188%, which satisfied the requirement of Ex ≤ 0.6% in GB/T 2938. (3) The hydrated exothermic process of M·C was divided into initial dissolution, induction, acceleration, deceleration and slow reaction stages. DG can effectively promote the early hydration process of M·C, while the contribution of FA is not noticeable, and even reduces the hydration heat of M·C. (4) The hydration of MMS generates C-S-H gels and CH to increase the strength and form an alkaline environment. DG promotes the dissolution and polymerization of MMS and FA into C-S(A)-H gels and ettringite, a process that consumes Ca2+ and OH− and promotes the hydration of MMS. (5) The UCS of the 28 d downhole cores from the Changxing and Mahuangliang coal mines were 7.73 and 12.62 MPa, respectively, and the leaching indexes met the requirements stated in GB/T 14848 (Ⅲ), both of which reach the requisites for safe underground backfill. The results of this study demonstrate the feasibility and advancements of the new all-solid waste M·C replacement cement and its backfill application, and also standardizes the production and filling application route of M·C and accelerates the industrialization process of this technology.

Analysis of the aerodynamic characteristics of the entire operation process of an ultra-high-speed elevator under the influence of high blockage ratio, hoistway, and car height parameters

Ultra-high-speed elevators are affected by various factors during operation, resulting in complex airflow disturbances and significant piston effects. To explore the aerodynamic characteristics and ventilation effects of the car at different stages of operation under multiple parameters, this paper first establishes a multi-parameter numerical model for the elevator system and verifies the numerical model through elevator experiments, and then analyzed the general variation characteristics of the pneumatic load during the car's acceleration, constant speed, and deceleration phases. On this basis, the influence of blockage ratio, hoistway and car height, and operating acceleration on multiple types of aerodynamic loads and hoistway ventilation was studied, with a special analysis of the viscous drag and pressure drag of the car. The results show that increasing the blockage ratio significantly increases the ventilation of the hoistway, with an increase in 1 and 0.7 factors in the acceleration and constant velocity stages, respectively. In the deceleration stage, the drag changes the most, with an increase in 3.1 factors, which hinders the ventilation of the hoistway. Meanwhile, the average lateral lift increased by 0.6 factors and the fluctuation increased, exacerbating the lateral vibration of the car. Viscous drag and pressure drag exhibit similar trends at each stage, but pressure drag is much higher than viscous drag. The height of the hoistway has a significant impact on the total drag of the car by increasing the pressure drag. In contrast, car height has the most significant effect on lateral lift and viscous drag.

Quasi-in-situ characterization of microstructure evolution in friction stir welding of aluminum alloy

The in-situ investigation of microstructure evolution during friction stir welding (FSW) can provide very important theoretical guidance to control the microstructure and mechanical properties of the FSW joint. In this study, a quasi-in-situ investigation method of the microstructure evolution in friction stir welding of AA1050 aluminum alloy was developed. The microstructure along the material flow path was characterized and analyzed by taking the strain, strain rate, and temperature into account simultaneously. The results show that the microstructure evolution during the FSW of aluminum alloy can be divided into four stages according to the material flow state and thermal history. Firstly, grain coarsening by static recovery and static recrystallization occurs at the static preparatory stage (or called the preheating stage). Secondly, grain refining by continuous dynamic recrystallization occurs at the acceleration stage, where the temperature gradually increases to nearly a constant. The grain size is mainly controlled by the accumulative total strain. The strain rate only affects the rate of dislocation generation and the type of β-fiber simple shear texture components. Thirdly, at the high-velocity flow stage, a dynamic equilibrium between plastic deformation and dynamic recrystallization is maintained, and the grain size only fluctuates within a small range. Finally, a second grain-coarsening process occurs through normal grain growth due to thermal activation at the deceleration stage. This process is inevitable because it occurs under the cover of the tool shoulder. The grain coarsening during the normal cooling stage was avoided in this study due to the rapid cooling used, which has been widely reported by published documents.

Creep properties and energy evolution characteristics of weakly cemented rock under step loading

The contact mode of particles in weakly cemented mudstone (WCM) and the persistence of cemented materials, as well as the cemented particulate matter composed of them, lead to different mechanical response characteristics from general soft rock during loading, that is, rheological failure occurs under low stress. In order to study the creep characteristics and energy evolution characteristics of weakly cemented rock (WCR) in the western mining area of China, based on the triaxial graded loading creep test of WCR, the creep deformation and failure law of WCM under graded loading was studied, and the energy evolution characteristics of WCR in each creep stage were revealed by energy dissipation theory. The results show that with the increase of confining pressure, the fracture angle decreases, but the failure strength and volume expansion point show the opposite trend. In addition, due to the strong heterogeneity of WCR samples, the contact force between particles is different after loading, which makes the slip between particles and cements and then produces a completely different fracture form from other rocks. The energy growth in the deceleration and steady creep stages is mainly affected by the confining pressure. With the increase of confining pressure, the growth rate of dissipation energy Ud and total energy U gradually tends to be constant, but the growth rate of strain energy Ue remains at a high level. In the accelerated creep stage, the growth rate of strain energy Ue decreases rapidly, and the residual energy storage capacity does not change significantly with the increase of confining pressure. These results revealed the unique creep properties and energy evolution characteristics of the WCR, providing a theoretical basis for long-term stability control of roadways that are built with WCR.

Experimental study on heat transfer and propagation speed of spill fire on porous media substrate with micro-inclination

During the transportation and usage of liquid fuel, leakage from container rupture can cause spill fires. When a spill fire occurs on porous media, the heat transfer mode will change. An experiment with the spill was carried out using a 15 × 150 cm porous media substrate to analyze the spill fire. The results showed that the propagation speed is inversely proportional to the square of the length of the transition zone in the uniform stage. Under horizontal conditions, the model calculation results had an error within 15% of the experimental values. When the substrate has an inclination angle, the speed model has a large error compared to the experimental results due to the influence of gravity. Additionally, the burning rate of spill fire was studied based on heat transfer analysis and experimental measurement. The burning rate of spill fire on porous media substrates was found to be significantly lower than that of a pool fire. In the acceleration and uniform stage, the burning rate increases with the increase in combustion equivalent diameter. In the deceleration stage, the burning rate decreases with the increase in equivalent diameter. The theoretical value of burning rate was consistent with the experimental value in the uniform stage.

Numerical Investigation of the Evolution Process of an Open-Pit Mine Landslide Using Discrete-Element Method

Because of the complexity of geological conditions and disturbance of external factors, the process of instability of open-pit landslides is very complicated. To illustrate this complex process, taking an open-pit coal mine landslide as an example, an accurate landslide geological section was established via detailed field investigation, remote sensing image interpretation, and field borehole data, in this study. A two-dimensional discrete-element model was established by the Fast CPU Matrix Computing of Discrete-Element Method (MatDEM, Version 1.60). The failure evolution of the landslide was investigated; then, the deformation behavior and dynamic characteristics of the landslide were systematically studied. The analyses of the failure process, velocity, displacement, and heat showed that the evolution of the landslide could be divided into the stages of initial deformation, accelerated deformation, and deceleration and stabilization. During the movement of the landslide, the gravitational potential energy of the landslide gradually converted into kinetic energy, heat, and elastic potential energy. The kinetic energy in the initial stage was small but rapidly increased and occupied the dominant position in the second stage. The kinetic energy gradually decreased and tended to be stable in the third stage. The final geometry and accumulation characteristics of the landslide obtained by numerical simulation were consistent with the corresponding field investigation and remote sensing interpretation. This work can provide a reference for the study of the evolution and prevention of open-pit mine landslides.

Cutting Energy Consumption Modelling of End Milling Cutter Coated with AlTiCrN

As an indispensable piece of equipment in the manufacturing industry, the machine tool is low-energy-efficiency and high-energy-consumption in operation. Therefore, it is urgent to establish a cutting energy consumption model to guide production and reduce the energy consumption of the machining process. In this paper, the AlTiCrN-coated cutting tool is taken as the object of study, and the cutting energy consumption model is established. The cutting energy consumption model is composed of a machining time model and a cutting power model. The cutting power model can be divided into the shear deformation power model of the workpiece, the friction power model of the flank surface and the friction power model of the rake surface. The influence of the edge shape is taken into account in the establishment of the friction power model of the flank surface. The machining time model considering the S-type acceleration and deceleration stage is established. The accuracy of the model was verified by experiments. Experimental results show that the model has high accuracy. The Taguchi method was used to carry out the numerical experiment with the cutting energy consumption of the machine tool as the response. The influences of cutting parameters on energy consumption are analyzed. Cutting width is the most important factor, followed by cutting depth, then feed rate and spindle speed. The physical principle of the influence of cutting parameters on cutting energy consumption is revealed.

Dynamics of flames in tubes with no-slip walls and the mechanism of tulip flame formation1

ABSTRACT A hydrogen/air flame propagation and the development of tulip-shaped flame in 2D tubes of different aspect ratios with both closed ends and in a half-open rectangular channel were studied using high resolution direct numerical simulations of the fully compressible Navier – Stokes equations coupled with a detailed chemistry. Flame propagation in a 3D rectangular channel was studied using large eddy simulations and compared with the results of direct numerical simulations of flame propagation in a 2D rectangular channel with the same aspect ratio. It is shown that the interaction of the rarefaction wave generated by the flame at the deceleration stage with the “positive” flow of unburned gas generated by the flame at the previous accelerating stage leads to a significant decrease of the velocity of the unburned gas flow in the near field zone ahead of the flame front. As a result, the thickness of the boundary layer in the near-field zone ahead of the flame increases significantly, and the profile of the axial velocity of the unburned gas in the near-field zone ahead of the flame front takes the form of a tulip or an inverted tulip, which leads to corresponding changes in the velocities of different parts of the flame front, the flame front inversion, and the formation of a tulip-shaped flame.

Transitions and suggestions for China's coastal port reclamation policies

China has the largest extent of reclamation in the world. Since the year 2000, the reclaimed areas have been utilized for ports, industries, and urban expansion. Ports are key to land-sea coordination and regional development. The study of port reclamation patterns and environmental strategies for port clusters has become the focus of coastal zone sustainable development. Ports are the only land usage type which are dependent on reclamation activites, and play important roles in driving economic development and world trade. Since 2002, port reclamation in China has undergone several phases, such as an acceleration stage, peak stage, deceleration stage, and stagnation stage. However, new situations have been demonstrated for port reclamation projects which include the following: Stronger demands for port reclamation and stricter reclamation policies; ecological demands of port reclamation; area in different stages of reclamation; unbalanced spatial-temporal distributions of reclamation activities; coordination of reclamation related policies; and imbalances between filling and dredging practices. Generally speaking, research findings have shown that land use efficiency, reclamation management and ecological restoration require improvement. Management model of port reclamation, adaptation strategies to rising sea level rise, and ecological compensation systems are also needed to be improved. Therefore, further research in the field of land reclamation has important implications for scientifically regulating the spatial patterns of port reclamation activities, as well as promoting the sustainable development of port-dense coastal zones.

The constant/variable kinematics adjustment of the crosshead and the mold’s stability management in injection molding

During the clamping process in an injection-molding machine, the mold’s movement is directly driven by the velocity of the crosshead. The form of the adjustment on the crosshead’s kinematics can influence the final dynamics of the mold, which can contribute to the stability of the clamping duration further. This article is aimed at investigating the kinematics analysis and the stability management strategy of the mold in the early design stage for the clamping mechanism. The two different velocity-controlled forms of the crosshead, constant and variable kinematics adjustments, are applied and compared in the clamping analysis. Three factors of the crosshead, the maximum velocity, the acceleration/deceleration stage adjustment, and the multi acceleration/deceleration process, are validated for the stability control of the mold’s motion in the injection molding process. The results show an extra “fast” process is detected in the constant condition when compared to the variable crosshead’s kinematics adjustment. Furthermore, by the reasonable adjustment of the maximum velocity and two special positions of the crosshead during the acceleration and deceleration stages, the maximum acceleration fluctuation of the mold is decreased by more than 50%, allowing the mold to move more steadily.

Role of H3PO4 concentration on the dissolution and reaction kinetics of phosphate acid based metakaolin geopolymer

Reaction kinetics of phosphate acid based metakaolin geopolymer (PMG) is studied by taking H3PO4 concentration and temperature as variables. Results indicate that the increase in H3PO4 concentration results in the increase in dissolution activation energy (Ed) of metakaolin. The maximum reaction rate of PMG is achieved at 353.15 K with the H3PO4 concentration of 8 mol/L. A relatively higher reaction temperature will lower the maximum heat release (Qmax) of PMG. The initial reaction stage of PMG is controlled by chemical reaction, which turns to diffusion-control mode in later reaction stages. The reaction activation energy (Ea) of acceleration stage is larger than that of deceleration and stabilization stages. In all stages, a higher H3PO4 concentration leads to a higher Ea.

Quality Change in Camellia Oil during Intermittent Frying.

Camellia oil with a high oleic acid content is widely used for frying. To comprehensively describe the quality change in camellia oil during frying, the changes in composition, deterioration indicators, and volatile profiles were investigated. The results showed that tocopherols mainly degraded in the early stage of frying, followed by unsaturated fatty acids (UFA). This caused the carbonyl value and total polar compounds level to significantly increase. Moreover, frying promoted the accumulation of volatile compounds in terms of type and abundance, especially aldehydes, which are related to the degradation of UFA. Principal component analysis showed that the frying of camellia oil was divided into three stages. First, the camellia oil with a heating time of 2.5-7.5 h showed excellent quality, where tocopherol played a major role in preventing the loss of UFA and was in the degradation acceleration stage. Subsequently, as tocopherol entered the degradation deceleration stage, the quality of camellia oil heated for 10.0-15.0 h presented a transition from good to deteriorated. Finally, tocopherol entered the degradation stagnation stage, and the quality of camellia oil heated for 17.5-25.0 h gradually deteriorated, accompanied by a high level of volatile compounds and deterioration indicators. Overall, this work comprehensively determined the deterioration of camellia oil during intermittent frying and offered valuable insights for its quality evaluation.

Cooperative Predictive Position Control of Dual-Motor System

A cooperative predictive position control (CPPC) method for dual-motor system is proposed in this article to improve the synchronization performance, tracking performance, and rapidity. The prediction model for dual-motor system is established to predict future behavior of motor, and the position synchronization error is introduced into cost function to establish algorithm-level connection between motors to improve the synchronization performance. The motor operation process is divided into acceleration stage, constant speed stage, deceleration stage, and position approaching stage, and the adaptive adjustment method of weight coefficient is proposed for different motor operation stages to realize multivariable collaborative optimization control. The experimental results of the proposed CPPC method are presented to verify its effectiveness and superiority.

Modelling the effect of aggressive driver behavior on longitudinal performance measures during car-following

Driving aggression is a major concern during car-following situations as it is closely associated with collision risk and crash severity. Despite the tendency of reckless driving actions and increased crash risk with the vehicle ahead, aggressive driver behavior remains unexplored. The present study aimed to investigate the effects of aggressive driver behavior (three driver categories: aggressive, moderately aggressive, and non-aggressive drivers) and drivers’ tendency to aggressive stimuli (due to lead vehicle (vehicle moving ahead) reckless driving) on car-following behavior. A sample of fifty-eight Indian drivers participated in this study. All the experiments were conducted using a driving simulator. The simulator design includes a lead vehicle dynamic maneuver with rapid accelerations and decelerations (at 1 m/s2, 1.5 m/s2, and 2 m/s2). Three longitudinal performance measures including speed variability (SPV) from LV, speed recovery time (SRCT), and time spent tailgating, were analyzed during LV acceleration stage using generalized linear models (GLM). Further, to assess collision avoidance behavior and crash risk, deceleration adjusting time (DAT) was analyzed during the LV sudden deceleration stage using Weibull Accelerated Failure Time (AFT) analysis. The findings indicate that due to their tendency to risk-raking behavior, aggressive drivers decreased their SPV and SRCT by 25 % and 18 %, respectively. The time spent tailgating increased by 107 % for aggressive drivers. The survival probabilities were decreased by 23.77 % for aggressive drivers at 3 sec DAT. The findings of the study can be used in car-following models to enhance the model performance, improve the reliability of simulation tools, and design interventions to improve safety (in automated driving).

Influence evaluation of producer services agglomeration on economic resilience: Evidence from China’s cities

As a new engine of economic development after urbanization’s structural deceleration stage in China, producer services agglomeration on urban economic resilience (ER) requires analysis. Using data from 264 prefecture-level cities and above in China after the global financial crisis, we examine producer services’ impact in terms of diversification (PSD) and specialization (PSS) on urban ER, and also the mediating effect of industrial structure upgrading (ISU). The results show that Cities with more diversified producer services are more resilient to crises, while the effect direction of PSS is the opposite. ISU is partly how PSD and PSS affect ER. Finally, producer services agglomeration’s impact on ER has significant regional heterogeneity. PSD is very conducive to urban ER in the economically developed eastern region and the less developed western region. The more specialized the development of producer services in eastern and north-eastern regions, the less conducive it is to enhancing ER. Our findings can help stabilize China’s economy and achieve high-quality economic development.

Comprehensive investigation of rock fracture behaviour in clay-rich rocks under the effect of temperature: Experimental study under three loading modes (I, I/II, II)

Rock fracture behaviour is a crucial aspect in many geological and engineering projects and can be affected by different environmental factors such as temperature, especially in projects where the rocks are exposed to a change in temperature such as geothermal energy, oil and gas exploitation, nuclear waste areas, etc. Therefore, this research aims to investigate the effect of temperature on rock fracture behaviour under four loading modes; pure mode I, pure mode II, and two mixed-mode loadings I/II in SCB specimens of clay-rich rocks (mudstone). Thirty-six SCB specimens were prepared and tested under three temperatures (Ambient temperature (AT), 250 and 500 °C), and some important parameters such as fracture toughness, fracture envelope, crack propagation speed, and crack acceleration were measured and discussed as a function of temperature. The main results concluded that due to the gradual loss of water from the structure of high clay content in the mudstone during thermal treatment, the behavior of mudstone gradually changed from ductile to brittle behaviour, and as a result, fracture toughness and crack propagation speed increased significantly. In addition, our results showed that the area under the fracture envelope which represents fracture resistance to growth under mixed-mode loading increased by 1.86 and 2.68 orders in magnitude after thermal treatment at temperatures of 250 and 500 °C, respectively. Under the four loading mode mixities, fracture grows through three main stages, high acceleration (0–5 mm), high deceleration (5–10 mm), and low deceleration (10–15 mm) stages. Crack propagation speed and crack acceleration depend strongly on the mode mixity ratio (KII/KI) and it increased by changing the loading mode from pure mode I to mixed mode loading I/II and pure mode II.

Flame front dynamics, shape and structure on acceleration and deflagration-to-detonation transition

Developing a new generation of rocket engines based on detonation principles needs methods which ensure stability of operation including the onset of detonation wave after mild ignition. This aspect is crucial for ensuring safety of such engines. In the present paper the transition processes associated with flame acceleration and the onset of a detonation mode are under investigation. High-speed flame front photography has been performed during the deflagration of a premixed gas mixture in a long smooth tube. The evolution of flame front structure and shape has been determined, starting from the moment of deflagration initiation and ending with the occurrence of a local explosion leading to the formation of a detonation wave. Four characteristic flame propagation stages have been identified: the initial flame acceleration stage, followed by the apparent velocity deceleration stage, followed by the nearly constant propagation velocity stage, and finally the second acceleration stage, during which detonation is formed. The changes in process dynamics with a change in the initial pressure have been demonstrated. The behavior of flame front at the early stage of acceleration has been identified. The experimental findings are compared with the theoretical models available in the literature. At the initial stage of the accelerated propagation of flame extended along the channel walls — until deceleration — the velocity of the leading tip of the front is effectively described by the exponential relationship proposed by C. Clanet and G. Searby (1996). The most interesting and poorly studied, in the authors' opinion, stage of the deflagration-to-detonation transition process, the intensive repeated acceleration stage, during which flame shape changes abruptly, has been described in detail. The laminar burning velocity of the stoichiometric acetylene–oxygen mixture diluted with either nitrogen or argon at a reduced initial pressure (8–21 kPa) has been identified.