Compression Expansion Research Articles

Study on aero engine thermal performance analysis using a new mathematical model of isentropic expansion (compression) process

At present, for the research of aero engine performance, it is necessary to calculate the overall thermal performance of the aero engine at the beginning of the research. For the mathematical model of the isentropic expansion (compression) process, most researchers use the ideal state value to calculate in the mathematical model, so that the calculated results have a certain error compared with the actual value. Therefore, the new isentropic expansion (compression) process mathematical model based on the Φ function is applied in this paper to the technical aero engine overall thermal performance calculation. After calculation and analysis, it is concluded that the calculation accuracy of the mathematical model provided in this paper is higher than that of the literature program for the overall thermodynamic performance parameters of the aero engine, whether it is the thrust of the aero engine, the unit fuel efficiency, or the thermal efficiency of the aero engine. It shows that it is feasible to revise and improve the mathematical model of isentropic compression and isentropic expansion process in this paper, more accurate calculation results can be obtained by using the new model in the performance analysis program, and more accurate numerical analysis can be carried out.

Comparison of Tabulated and Complex Chemistry Approaches for Ammonia–Diesel Dual-Fuel Combustion Simulation

<div>Using ammonia as a carbon-free fuel is a promising way to reduce greenhouse gas emissions in the maritime sector. Due to the challenging fuel properties, like high autoignition temperature, high latent heat of vaporization, and low laminar flame speeds, a dual-fuel combustion process is the most promising way to use ammonia as a fuel in medium-speed engines.</div> <div>Currently, many experimental investigations regarding premixed and diffusive combustion are carried out. A numerical approach has been employed to simulate the complex dual-fuel combustion process to better understand the influences on the diffusive combustion of ammonia ignited by a diesel pilot. The simulation results are validated based on optical investigations conducted in a rapid compression–expansion machine (RCEM). The present work compares a tabulated chemistry simulation approach to complex chemistry-based simulations. The investigations evaluate the accuracy of both modeling approaches and point out the limitations and weaknesses of the tabulated chemistry approach. When using two fuels, the tabulated chemistry approach cannot reproduce misfiring events due to inherent model limitations. By adjusting the model parameters of the tabulated chemistry model, it is possible to reproduce experimental results accurately for a specific case. However, using the adjusted parameters for simulations with changed injection timing or interaction angle between the sprays shows that no predictive calculations are possible. The parameter set is only valid for a single operation point.</div> <div>Further simulations show that the complex chemistry approach can capture the complex interaction between both directly injected fuels for different operation points. It correctly predicts the ignition as well as heat release. Therefore, the approach allows predictive combustion simulations. Furthermore, it reproduces the occurrence of misfiring in cases of unsuitable interaction of both sprays and injection timing.</div>

Unconventional mechanical and thermal behaviours of MOF CALF-20.

CALF-20 was recently identified as a benchmark sorbent for CO2 capture at the industrial scale, however comprehensive atomistic insight into its mechanical/thermal properties under working conditions is still lacking. In this study, we developed a general-purpose machine-learned potential (MLP) for the CALF-20 MOF framework that predicts the thermodynamic and mechanical properties of the structure at finite temperatures within first-principles accuracy. Interestingly, CALF-20 was demonstrated to exhibit both negative area compression and negative thermal expansion. Most strikingly, upon application of the tensile strain along the [001] direction, CALF-20 was shown to display a distinct two-step elastic deformation behaviour, unlike typical MOFs that undergo plastic deformation after elasticity. Furthermore, this MOF was shown to exhibit a fracture strain of up to 27% along the [001] direction at room temperature comparable to that of MOF glasses. These abnormal thermal and mechanical properties make CALF-20 as attractive material for flexible and stretchable electronics and sensors.

Cyclic behavior of existing RC bridge piers strengthened by exterior ECC and BFRP/ECC composite jacketing at pier base

Engineered cementitious composites (ECC) are a class of high-performance fiber-reinforced cementitious composites. This study proposes using ECC and BFRP grid-reinforced ECC (BFRP/ECC) as external jackets at the base of existing RC bridge piers to enhance seismic performance. Seven 1/5-scale RC bridge piers were tested under reversed cyclic loading, including one unmodified pier as reference and six reinforced piers with varying jacket thickness, height, interface, and BFRP grid mesh size. The test results demonstrate that both ECC and BFRP/ECC composite jacketing effectively enhance ductility. The hysteretic response of the reinforced piers with different interfacial properties, however, varied significantly. The piers with interfacial shear keys exhibited enhanced flexural strength, ductility, and energy dissipation capacity but sustained severe damage as jacketing thickness and height increased. Conversely, the piers with smooth interfaces showed insignificant improvements in flexural strength but significant enhancement in ductility with marginal damage. This behavior can be attributed to the fact that the exterior jacketing in specimens with interfacial shear keys was under bi-axial tension-compression stress state due to flexural compression and lateral expansion of the plastic hinge, while the jacket in specimens with smooth interfaces acted solely as lateral confinement.

Insight into blade tip vibration and deformation characteristics of boiler circulation pumps induced by tip leakage vortex under different tip clearances

For circulating pumps in large power plant boilers, tip leakage flow is the main cause of blade fatigue. To investigate the correlation between tip leakage vortex and blade fatigue, in this paper, the bidirectional fluid structure coupling method is used to simulate the full flow field of the boiler circulating pump under different tip clearance sizes. The accuracy of the delayed detached vortex simulation method is verified by combining the external characteristics and vibration characteristics of the pump. It is obtained that tip leakage vortex is the main cause of blade tip vibration and deformation. Under deep stall conditions, the increase in tip clearance size suppresses the vibration displacement of the blade leading edge, while the opposite is true under optimal conditions. After decomposing tip leakage vortex, it is found that the compression–expansion term played a major role in the deformation of the blade tip, while the viscous dissipation term and the stretching term mainly affected the vibration frequency. At optimal working conditions, the main frequency of blade vibration is basically consistent with the main frequency of vortex generation. In deep stall condition, as the tip clearance size increases, the amplitude of the vibration main frequency decreases and the number of harmonic frequencies decreases, while the optimal condition is the opposite.

Semi-dry and aqueous carbonation of steel slag: Characteristics and properties of steel slag as supplementary cementitious materials

Accelerated carbonation of steel slag presents a viable method for CO2 sequestration and the valorized utilization of steel slag. This study aimed to investigate the impact of semi-dry and aqueous carbonation, as well as the carbonation degree, on the microstructure and properties of steel slag. Additionally, it evaluated the feasibility of utilizing carbonated steel slag as supplementary cementitious materials (SCMs). Steel slag was subjected to CO2 (20% volume percentage) at 2 bar pressure for both semi-dry and aqueous carbonation process at room temperature with water/solid ratio of 0.08 and 5, respectively. The carbonation duration was varied (10 min, 30 min, 60 min, and 120 min) to adjust the degree of carbonation. Mortars and pastes with 30 wt% carbonated steel slag were formulated and their compressive strength and volume expansion values were quantified. The findings revealed that the CO2 uptake values for steel slag after 2 hours of semi-dry and aqueous carbonation were only 3.75% and 1.50%, respectively. Compounds such as Ca(OH)2, Mg(OH)2, and Ca2SiO5 demonstrated high carbonation reactivity during the semi-dry carbonation process, whereas Ca(OH)2 and Mg(OH)2 exhibited only a marginal increase in aqueous carbonated steel slag. The specific surface area of steel slag increased with an increase in the carbonation degree, attributed to the formation of silica gel or amorphous C-S-H during carbonation. However, the hydration activity of carbonated steel slag diminished with an increased degree of carbonation due to the consumption of alkaline minerals. Conversely, pozzolanic activity was enhanced with a higher degree of carbonation, owing to the formation of silica gel. The cumulative heat release of cement containing SDC30 (semi-dry carbonation for 30 min) within 24 h and 72 hours showed increases of 16.0% and 3.5%, respectively, aligning with the compressive strength results. Semi-dry carbonation treatment significantly enhanced the early-stage cementitious properties of steel slag/cement composites. Specifically, the 3-day compressive strength of mortar containing 30 wt% SDC30 increased by 22.7% relative to the reference mortar, composed of 30 wt% uncarbonated steel slag and 70 wt% Portland cement. Carbonation markedly improved the volumetric stability of steel slag with the extent of improvement was positively correlated with the carbonation degree for both semi-dry and aqueous carbonated steel slag. Aqueous carbonation was more effective in promoting the hydration process of free CaO and MgO more effectively than semi-dry carbonation.

Valorization of gypsum waste for the cleaner production of a novel ettringite-based binder

The utilization of gypsum waste (GyW) as an additive for alkali-activated materials has already been addressed. However, using GyW as a partial replacement for aluminosilicate during the alkali-activation process is not sufficient for the full disposal of such waste. Accordingly, this communication focused on full transforming GyW from fertilizers industry into a novel AFt-binder using an eco-friendly method. Various amounts of NaAlO2 were added to gypsum waste (GyW) to yield a hardened binder with a considerable mechanical property. The findings demonstrated that NaAlO2 readily reacted with GyW to produce an AFt-based binder. The highest values for compressive strength, bulk density, and volume expansion were observed at a NaAlO2/GyW ratio of 0.2. This corresponded to the complete conversion of GyW into needle-shaped AFt crystals with a larger diameter compared to those formed in other mixtures. Notably, all hardened mixtures attained their optimum performance after 24 h, and further extension of the curing time did not yield any additional benefits. Consequently, this innovative binder has the potential to be employed as a fast-setting, energy-efficient cement for repair applications.

Development and performance optimization of a new composite sealing material prepared by drilling cuttings

A highly efficient composite sealing material was prepared using drilling cuttings as the base material and a binder, a coagulant, and other additives as auxiliaries. A four-factor, three-level orthogonal test was designed based on the response surface method (RSM), and a response surface regression model was constructed using compressive strength, fluidity, expansion rate, and setting time as performance indexes to analyze the effects of each factor on material performance and optimize the material proportion. The samples were prepared by simulating the grouting process, the permeability of the samples was measured, and the sealability of the material was verified by analyzing the material microscopic morphology. Results showed that the regression model had a high level of confidence and accuracy and could predict the test results accurately within the range of the test. The effects of the interaction between factors on material performance were also examined. The low permeability of the sealing material samples verified the material’s feasibility. Gradual optimization of material performance revealed that the optimal proportion was 52.6% drill cuttings, 44.3% binder, 0.6% coagulant promoter, and 2.5% expansive agent. Under these conditions, the error between the predicted and test values of each material property was less than 5%, and the comprehensive performance was superior. These findings verify the accuracy of RSM and its applicability to the optimization of material performance. This work provides reasonable theoretical guidance for the preparation of drilling cuttings composite (DC) materials in practical engineering.

The structural effect of SiC form ceramic on the compressive strength and thermal expansion properties of Cu–SiC composites

With the development of transportation vehicles towards to the direction of heavy load and high speed, the requirement of Cu matrix brake materials was proposed for improving performance in both mechanical and thermal properties. The Cu-SiCf composites is a new kind of friction brake material. To investigate the structural effect of 3D network structured SiC form ceramic on the compressive strength and thermal expansion properties of Cu-SiCf composites, SiCf was synthesized using a template method in conjunction with reaction sintering, while Cu-SiCf composites were prepared using squeeze casting, and exhibiting a co-continuous phase structure. The results reveal that SiC form ceramic reinforcement and constraint mechanisms enhance the overall strength of the Cu alloy through the 3D network structure. SiC form ceramic plays a role in load transfer and overall strengthening of the Cu alloy. With the synergistic effects of these mechanisms, increasing the SiC form ceramic volume fraction improves the compressive strength and reduces the coefficient of thermal expansion of the Cu-SiCf composites.

Optimizing Cement Asphalt Mortar Mixtures for Bridge Expansion Joints in Tropical Climates: Performance and Durability Assessment

In this study, the suitability of various Cement Asphalt Mortar (CAM) mixtures for bridge expansion joint applications in tropical climates was quantitatively assessed. A comprehensive analysis encompassed key properties, including mixing stability, flowability, unconfined compressive strength, expansion characteristics, and resistance to acidic and alkali environments. The influence of high-temperature exposure on unconfined compressive strength and the microstructural features were also examined. The results revealed a discernible trend: lower cement content, in conjunction with anionic Asphalt Emulsion (AE) or epoxy resin, significantly enhanced mixing stability and flowability while contributing to improved unconfined compressive strength and chemical degradation resistance. Notably, epoxy resin emerged as a valuable component in mitigating high-temperature-induced strength reduction, indicating potential promise for CAM mixture design. SEM analysis visually supported these findings by highlighting the microstructural distinctions among CAM mixtures. Quantitatively, the findings indicated that CAM mixtures with a 25% cement content and 75% anionic AE exhibited an 11% improvement in mixing stability, along with a 13% enhancement in flowability, relative to the control mixture with 100% cement. Additionally, CAM mixtures incorporating epoxy resin (at various percentages) with anionic AE exhibited a significant 15% resistance to high-temperature-induced UCS reduction, surpassing other mixtures. The SEM micrographs visually confirmed the superior microstructural connectivity achieved with epoxy resin, further validating the observed enhancements. These quantitative results offer a robust foundation for tailoring CAM mixture compositions to optimize their suitability for rigorous infrastructure projects in tropical climates.

Spectroscopic investigation of diesel-piloted ammonia spray combustion

This work investigates the flame emissions of diesel-piloted ammonia spray combustion under conditions relevant to internal combustion engines. Due to ammonia’s carbon-free structure, its flame emissions differ significantly from carbon-containing fuels. This work provides new insights into the combustion process by distinguishing pilot and main fuel combustion by analyzing spectrally resolved flame emission footprints between 275nm and 750nm in a rapid compression expansion machine (RCEM). Additional results based on Shadowgraphy (SG), OH*-chemiluminescence (CL), visible natural flame luminosity (NL) imaging, and heat release rate (HRR) analysis describe the combustion process in detail. OH*, NH*, NH2*, as well as broadband CL from NH2*, NO2*, and H2O* contribute to ammonia flame luminosity. The results show that the combustion process encompasses a pilot ignition phase, a flame transition phase, and an ammonia combustion phase, during which only ammonia-related flame emissions occur and the main share of heat is released. Background corrected OH*, NH*, and NH2* intensities reveal the evolution of individual contributions to the flame emissions. NH2*-CL is the main contributor to visible ammonia flame emissions under the conditions investigated. Therefore, visible flame emissions are a suitable indicator for the flame front during the ammonia combustion phase.

Factors affecting compressive strength and expansion due to alkali-silica reaction of fly ash-based alkaline activated mortar

The development of environmentally friendly alkaline-activated materials (AAMs) holds promise, as AAMs can be derived from waste materials. This study aims to investigate the factors influencing (i) compressive strength and (ii) expansion due to alkali-silica reaction (ASR) in AAMs. These factors include alkaline concentration, heat curing conditions, fineness of fly ash, and the liquid alkaline-to-binder (L/B) ratio. The findings indicate that the higher concentrations of NaOH solution led to an increase in AAM compressive strength due to the enhanced dissolution and polymerization rates in a more alkaline environment. Heat curing stimulated chemical reactions and structure formation, while the reduced water content resulted in lower porosity and higher compressive strength in the hardened cement. Finer fly ash yielded greater compressive strength than coarser ash, as its smaller spherical particles contributed to denser and firmer structures. The presence of calcium minerals, from both Ordinary Portland Cement (OPC) and high-calcium fly ash, bolstered the strength of hardened products. Moreover, calcium minerals like CaO, Ca(OH)2, and CaSO4 were found to induce ASR expansion by promoting gel formation, leading to later-stage expansion in the hardened cement or concrete. However, finely milled fly ash as a precursor significantly reduced ASR expansion in AAMs, by approximately 40% compared to ordinary Portland cement. This study provides valuable insights for civil engineers for better understanding of AAM behavior and makes contributions to the safety and sustainability of cement and concrete systems.

Helical model of compression and thermal expansion

A negative linear temperature expansion and a negative linear compressibility were observed for imidazolium benzoate salt. Its strongly anisotropic strain induced by the temperature and pressure changes has been explained by the mechanism of H-bonded helices deformed in the structure. X-ray diffraction and vibrational spectroscopy were used to analyze interactions in the crystal. The Quantum Theory of Atoms in Molecules (QTAiM) approach was applied to analyze the hydrogen bonds and other interactions. In the salt under study, the interactions within the helix are substantially higher in energy than between helices. With decreasing temperature and increasing pressure, the value of the helix pitch increases while the value of the semi-major axis decreases, which results in the negative linear expansion and negative linear compression, respectively.

Evaluation of properties of energy-efficient geopolymer cellular concrete containing basic oxygen furnace slag aggregate

In north-western Kazakhstan, there is a higher demand for energy-efficient construction materials such as cellular concrete in the construction industry due to severe weather conditions. Moreover, construction industries in Kazakhstan are looking into developing new construction materials utilizing industrial by-products. Unlike blast furnace slag (BFS), utilization of basic oxygen furnace slag (BOFS) is limited due to its chemical composition and volumetric instability mainly caused by f-CaO and f-MgO. This study developed a total of nine energy-efficient geopolymer mixtures (3-normal geopolymer mixture and 6-geopolymer cellular concrete). Mix design variables included a partial substitution of river sand (RS) with BOFS aggregate (0%, 50%, and 100%) and three different percentages of a foaming agent (0%, 25%, and 50%). The properties of geopolymer mixtures were evaluated in terms of compressive strength, hardened density, expansion behavior, and thermal conductivity. Test results presented that replacing RS with 50% BOFS aggregates decreased compressive strength, while replacing sand with 100% BOFS aggregate led to increasing the strength regardless of the geopolymer mixture type. Geopolymer cellular concrete had lower compressive strength and thermal conductivity compared to the normal geopolymer mixture due to a higher porous cellularity structure and satisfied the threshold value of expansion due to converting f-CaO and f-MgO to CaSiO3 and MgSiO3 and absorbing the expansion stress by cellular structure.

Conical-cylindrical mixer ejector design model for predicting optimal nozzle exit position

Precisely predicting the optimal nozzle exit position (ONXP) of a vapor ejector is highly desirable for optimizing the performance of an ejector refrigeration system. Based on the theory of the shear mixing-layer and the adiabatic frictional flow equations in a conical-cylindrical mixer, a new design model to predict the ONXP for a conical-cylindrical mixer ejector (CCME) is proposed and verified. Furthermore, the evolution laws of the dimensionless optimal nozzle exit position (DONXP) and critical entrainment ratio (Ercri) for different design conditions (i.e., the expansion ratio E and critical compression ratio Ccri) are investigated. Based on a comparison of the experimental values and the predicted data, the relative deviations of the DONXP, Ercri, and Ccri are within ±14.81%, ±16.99%, and ±10.76%, respectively. The critical entrainment ratio decreases as the critical compression ratio increases but increases as the expansion ratio increases, and both with an increasingly slower rate of change. The variations in the DONXP with the varying critical compression ratio and expansion ratio are similar to that in Ercri. Results further indicate that a well-constructed CCME allows the critical entrainment ratio to attain the design values by adjusting its nozzle exit position to the ONXP when the expansion ratio is changed.

Revaluating coal permeability-gas pressure relation under various gas pressure differential conditions

Identifying changes in coal permeability with gas pressure and accurately codifying mean effective stresses in laboratory samples are crucial in predicting gas-flow behavior in coal reservoirs. Traditionally, coal permeability to gas is assessed using the steady-state method, where the equivalent gas pressure in the coal is indexed to the average of upstream and downstream pressures of the coal, while ignoring the nonlinear gas pressure gradient along the gas flow path. For the flow of a compressible gas, the traditional method consistently underestimates the length/volume-averaged pressure and overestimates mean effective stress. The higher the pressure differential within the sample, the greater the error between the true mean pressure for a compressible fluid and that assumed as the average between upstream and downstream pressures under typical reservoir conditions. A correction coefficient for the compressible fluid pressure asymptotes to approximately 1.3%, representing that the error in mean pressure and effective stress can be on the order of approximately 30%, particularly for highly pressure-sensitive permeabilities and compressibilities, further amplifying errors in evaluated reservoir properties. We utilized this volume-averaged pressure and effective stress to correct permeability and compressibility data reported in the literature. Both the corrected initial permeability and the corrected pore compressibility were found to be smaller than the uncorrected values, due to the underestimation of the true mean fluid pressure, resulting in an overestimation of reservoir permeability if not corrected. The correction coefficient for the initial permeability ranges from 0.6 to 0.1 (reservoir values are only approximately 40% to 90% of laboratory values), while the correction coefficient for pore compressibility remains at approximately 0.75 (reservoir values are only approximately 25% of laboratory value). Errors between the uncorrected and corrected parameters are quantified under various factors, such as confining pressure, gas sorption, and temperature. By analyzing the evolutions of the initial permeability and pore compressibility, the coupling mechanisms of mechanical compression, adsorption swelling, and thermal expansion on the pore structure of the coal can be interpreted. These findings can provide insights that are useful for assessing the sensitivity of coal permeability to gas pressure as truly representative of reservoir conditions.

Adiabatic energy change in the inner heliosheath: how does it affect the distribution of pickup protons and energetic neutral atom fluxes?

ABSTRACT The hydrogen atoms penetrate the heliosphere from the local interstellar medium, and while being ionized, they form the population of pickup protons. The distribution of pickup protons is modified by the adiabatic heating (cooling) induced by the solar wind plasma compression (expansion). In this study, we emphasize the importance of the adiabatic energy change in the inner heliosheath that is usually either neglected or considered improperly. The effect of this process on the energy and spatial distributions of pickup protons and energetic neutral atoms (ENAs), which originate in the charge exchange of pickup protons, has been investigated and quantified using a kinetic model. The model employs the global distributions of plasma and hydrogen atoms in the heliosphere from the simulations of a kinetic-magnetohydrodynamic model of solar wind interaction with the local interstellar medium. The findings indicate that the adiabatic energy change is responsible for the broadening of the pickup proton velocity distribution and the significant enhancement of ENA fluxes (up to ∼5 and ∼20 times in the upwind and downwind directions at energies ∼1–2 keV for an observer at 1 au). It sheds light on the role of adiabatic energy change in explaining the discrepancies between the ENA flux observations and the results of numerical simulations.

A Micromechanical-Based Semi-Empirical Model for Predicting the Compressive Strength Degradation of Concrete under External Sulfate Attack.

As one of the most harmful ions in the environment, sulfate could cause the deformation and material deterioration of concrete structures. Models that accurately describe the whole chemo-transport-mechanical process of an external sulfate attack (ESA) require substantial computational work and contain complex parameters. This paper proposes a semi-empirical model based on micromechanical theory for predicting the compressive strength degradation of concrete under an ESA with basic properties of the undamaged material and limited computational effort. A simplified exponential function is developed for the total amount of the invading sulfate, and a second-order equation governs the chemical reaction. A micromechanical model is implemented to solve the mechanical response caused by an ESA. The model is able to describe the compressive stress-strain behavior of concrete subject to uniaxial loading in good agreement with the experimental results. For the case of a sulfate-attacked material, the relationship between compressive strength and expansion is calculated and validated by the test results. Finally, the deterioration process of compressive strength is predicted with the test results of deformation.

Equation of State for Tantalum at High Pressures in Waves of Shock Compression and Isentropic Expansion

Equation of State for Tantalum at High Pressures in Waves of Shock Compression and Isentropic Expansion

Ignition detection with the breakdown voltage measurement during nanosecond repetitively pulsed discharges

Nanosecond Repetitively Pulsed Discharge (NRPD) is a promising ignition concept for introducing diesel-like process parameters for hard-to-ignite renewable fuels in Spark Ignition (SI) engines. Knowing whether an ignition event initiated by a series of nanosecond electrical discharges was successful or not gives the possibility of using this information for closed-loop ignition control. This paper presents a methodology for detecting successful ignition under NRPD ignition.After a nanosecond discharge, the heat loss from the particles (plasma-gas) between the electrodes and the surrounding gas is different if a robust flame kernel is established. If a flame kernel is present, the heat losses are lower, resulting in a lower local density of the gas between the electrodes. The breakdown voltage value of a nanosecond pulse is proportional to the local density. A control pulse is applied after the main ignition sequence to detect successful ignition. Lower breakdown voltages of the control pulse are present if a robust early flame kernel is present. The control pulse is applied before the pressure rises due to the presence of fast combustion, allowing ignition to be detected during the inflammation phase, thus allowing the possibility to place additional ignition events, if necessary.This technique was experimentally analyzed in a Constant Volume ignition Cell (CVC) and in a Rapid Compression Expansion Machine (RCEM). In the CVC at the ignitability limit, lower breakdown voltages of the control pulse are mostly measured when no pressure rise is measured. In the RCEM, the heat release rate is analyzed with a two-zone thermodynamic model, and the early flame kernel formation is monitored with Schlieren imaging. Some overlap exists in the control pulses' breakdown voltages for the ignition and quenching experiments; nevertheless, the Schlieren videos outline that the overlapping cases have a similar flame kernel formation, and the difference arises thereafter.