Average Propagation Velocity Research Articles

Modelling the average velocity of propagation of the flame front in a gasoline engine with hydrogen additives

The paper presents models for calculating the average velocity of propagation of the flame front, obtained from the results of experimental studies. Experimental studies were carried out on a single-cylinder gasoline engine UIT-85 with hydrogen additives up to 6% of the mass of fuel. The article shows the influence of hydrogen addition on the average velocity propagation of the flame front in the main combustion phase. The dependences of the turbulent propagation velocity of the flame front in the second combustion phase on the composition of the mixture and operating modes. The article shows the influence of the normal combustion rate on the average flame propagation velocity in the third combustion phase.

Flame propagation behaviours in nano-metal dust explosions

Flame propagation behaviours in 40nm titanium, aluminium and iron dust clouds were investigated by high-speed photography. 40nm titanium, aluminium and iron dust flames were all characterized by discrete single glowing burning particles with smoothly spherical front. “Micro explosions” caused by the fragment ejection occurred in both 40nm titanium and aluminium particles burning processes. At nano scales, the heat and mass transfer regimes were absolutely different with micro particles, which resulted in different propagation velocities. The average pulsating flame propagation velocities of 40nm titanium, aluminium and iron dust clouds were 0.565m/s, 0.189m/s and 0.035m/s, respectively. From SEM analysis, it was inferred that 40nm titanium, aluminium and iron particles were burnt in liquid, gas, and solid-phase, respectively. From XPS analysis, it was found that N2 played a significant role in titanium particles combustion. 40nm titanium combustion products contained 43% TiO2 (Ti4+), 27% Ti2O3 (Ti3+), 21% TiO (Ti2+) and 9% TiN (Ti3+). While 40nm aluminium combustion products just contained 100% Al2O3 (Al3+), and 40nm iron combustion products contained 49% Fe2O3 (Fe3+), 26% Fe3O4 (Fe3+/Fe2+), 15% FeO (Fe2+) and 10% iron nonoxides. Inerted by nitrogen for preventing nano‑titanium dust explosions was inadvisable.

Analysis of GaN based high‐power diode lasers after singular degradation events

Damage patterns caused by the Catastrophic Optical Damage (COD) are analyzed in GaN‐based high‐power diode lasers. In contrast to standard failure analysis, the devices are intentionally degraded under well‐defined conditions. We find that defect growth during COD is fed by the optical mode, that is, laser energy. This process involves melting and even vaporization of quantum‐well and waveguide materials. Average defect propagation velocities along the laser axis of 110 m s−1 are observed. COD results in material loss including the formation of an empty channel. This is well consistent with material loss due to ejections of hot material out of the front facet of the device. The laser structure in the immediate vicinity of the empty channel seems to be absolutely undisturbed and no transition regions or remaining material are observed. These results are compared with earlier findings from comparable experiments obtained with GaAs‐based devices.

Effects of particle size distributions on PMMA dust flame propagation behaviors

Experiments of polymethyl methacrylate (PMMA) dust clouds with same Sauter diameters were conducted to reveal the effects of particles size distributions on PMMA dust flame propagation behaviors. High-speed photography was used to capture the flame propagation behaviors and microstructures. The results showed that the combustion behaviors of PMMA dust clouds with different mass fractions of 100nm, 5μm and 30μm PMMA dust particles were complicated. The macroscopical developments of the flames were determined by the major mass proportion dust particles. The flame front became smoother and the average pulsating flame propagation velocity was faster with more proportion of smaller PMMA dust particles. The flame temperatures were detected by a fine thermocouple comprising 25μm-diameter Pt-Pt/Rh13% wires. It was found that the faster the flame propagated, the higher maximum flame temperature was. The maximum temperatures of mixture B and E dust clouds could maintain longer time due to the larger mass fractions of smaller particles. The thermal conversion processes of mixture A–E dust clouds were dominated by the external heat transfer sources, including radiation from burned region and heat convection between particles and gases. And pyrolysis/devolatilization controlled the overall mixture B–E dust clouds combustion processes. Flames of mixtures were coupled with the premixed gas flame of the smaller particles, the diffusion flame of the agglomerates accompanying local premixed flame around split agglomerates, and the diffusion flame accompanying local premixed flame in high pyrolyzates concentration areas of larger particles. The combustion mechanism was determined by the dominant mass fraction dust particles. Smaller particles not only accelerated flame propagation but influenced the flame structure and combustion mechanism.

Modelling of the combustion velocity in UIT-85 on sustainable alternative gas fuel

The flame propagation velocity is one of the determining parameters characterizing the intensity of combustion process in the cylinder of an engine with spark ignition. Strengthening of requirements for toxicity and efficiency of the ICE contributes to gradual transition to sustainable alternative fuels, which include the mixture of natural gas with hydrogen. Currently, studies of conditions and regularities of combustion of this fuel to improve efficiency of its application are carried out in many countries.Therefore, the work is devoted to modeling the average propagation velocities of natural gas flame front laced with hydrogen to 15% by weight of the fuel, and determining the possibility of assessing the heat release characteristics on the average velocities of the flame front propagation in the primary and secondary phases of combustion.Experimental studies, conducted the on single cylinder universal installation UIT-85, showed the presence of relationship of the heat release characteristics with the parameters of the flame front propagation. Based on the analysis of experimental data, the empirical dependences for determination of average velocities of flame front propagation in the first and main phases of combustion, taking into account the change in various parameters of engine operation with spark ignition, were obtained.The obtained results allow to determine the characteristics of heat dissipation and to assess the impact of addition of hydrogen to the natural gas combustion process, that is needed to identify ways of improvement of the combustion process efficiency, including when you change the throttling parameters.

Automatic ELM detection using gSPRT on the COMPASS tokamak

This paper contains the description of the generalized Sequential Probability Ratio Test (gSPRT) method used for automatic ELM detection in different diagnostic signals collected on the COMPASS tokamak. After determination of H-mode region based on Dα signal in a given shot, ELMs are automatically detected in different diagnostic time traces (e.g. magnetic signals and also in Dα signals). The onset time, the maximum location and the peak value, and the duration of each detected ELM is determined. Analyzed diagnostic signals came from different radial positions of the studied plasma volume, thus from arrival times of given ELM into different detectors, the average radial propagation velocity of the ELM event can be also estimated. Comparison of results from gSPRT method with two different, commonly used automatic ELM detection method (threshold technique and correlation based technique) is also reported.

A Study of Polycrystalline Silicon Damage Features Based on Nanosecond Pulse Laser Irradiation with Different Wavelength Effects.

Based on PVDF (piezoelectric sensing techniques), this paper attempts to study the propagation law of shock waves in brittle materials during the process of three-wavelength laser irradiation of polysilicon, and discusses the formation mechanism of thermal shock failure. The experimental results show that the vapor pressure effect and the plasma pressure effect in the process of pulsed laser irradiation lead to the splashing of high temperature and high density melt. With the decrease of the laser wavelength, the laser breakdown threshold decreases and the shock wave is weakened. Because of the pressure effect of the laser shock, the brittle fracture zone is at the edge of the irradiated area. The surface tension gradient and surface shear wave caused by the surface wave are the result of coherent coupling between optical and thermodynamics. The average propagation velocity of laser shock wave in polysilicon is 8.47 × 103 m/s, and the experiment has reached the conclusion that the laser shock wave pressure peak exponentially distributes attenuation in the polysilicon.

The role of global curvature on the structure and propagation of weakly unstable cylindrical detonations

The role of the global curvature on the structure and propagation of cylindrical detonations is studied allowing and without allowing the development of cellular structures through two-dimensional (2-D) and 1-D simulations, respectively. It is shown that as the detonation transitions from being overdriven to the Chapman–Jouguet (CJ) state, its structure evolves from no cell, to growing cells and then to diverging cells. Furthermore, the increased dimension of the average structure of the cellular cylindrical detonation, coupled with the curved transverse wave, leads to bulk un-reacted pockets as the cells grow, and consequently lower average propagation velocities as compared to those associated with smooth fronts. As the global detonation front expands and its curvature decreases, the extent of the un-reacted pockets diminishes and the average velocity of the cellular cylindrical detonation eventually degenerates to that of the smooth fronts. Consequently, the presence of cellular instability renders detonation more difficult to initiate for weakly unstable detonations.

The effect of the level of total sodium deposited in the myocardium on its stiffness

To determine a relationship between the level of total sodium in the myocardium to its stiffness. The investigation enrolled 18 hypertensive patients who had suddenly died; their mean age was 40±10 years; mean waist circumference, 102±12.5 cm; height, 170±7.7 cm; myocardial mass, 319±53 g. The variation in the myocardial level of total sodium averaged 211.7±37.5 (min, 71.5; max, 226.17) mmol/l. The sodium level was ascertained to be affected to the greatest extent by myocardial mass (SS=3615.56; p=0.00029) and age at death (SS=1965.568; p=0.0029), whereas gender and smoking had a considerably lower impact (SS=778.584; p=0.03). A univariate regression analysis showed that there was a relationship between myocardial sodium levels and the thickness of the anterior wall of the left ventricle (β=0.94; p=0.000001; r2=0.88), that of the anterior wall of the right ventricle (β=0.82; p=0.000021; r2=0.66), and that of the interventricular septum (β=0.94; p=0.000001; r2=0.89). The wall thickness of the myocardium was established to depend on its sodium level (SS=21813.89; p=0.000001; r2=0.88): the higher sodium amount in the myocardium, the thicker its walls. The average velocity of acoustic wave propagation was 6.24±0.51 m/sec. A significant correlation was observed between sodium concentrations in the myocardium and its stiffness (β=0.72; p=0.00062; r2=0.49). The level of sodium deposited in the myocardium, which is directly related to dietary sodium intake, is significantly correlated with myocardial stiffness. It can be assumed that the elevated level of sodium deposited in the myocardium is an independent factor that changes the stiffness of the myocardium and appears to influence the development of its diastolic dysfunction.

Electric and Spectroscopic Studies of Pulsed Corona Discharges in Nitrogen at Atmospheric Pressure

This paper is mainly dedicated to the experimental electric and spectroscopic analysis of positive corona discharges in the case of point to plane configuration, generated in nitrogen at atmospheric pressure. The maximum corona current (a few hundreds of mA), the average current (a few μA) and the average propagation velocity (a few 107 cm/s) are analyzed with the variation of the applied voltage (a few kV) and the gap distance (not exceeding 16 mm). By using an ICCD camera, the dynamics of the discharge during the propagation of primary and secondary streamers across the gap distance was analyzed. Spectroscopic study is emphasized in a spectral range from 200 nm up to 500 nm, to determine the important excited species present in the gaseous environment such as the second positive and the first negative systems (SPS and FNS respectively). The identification of the quenching NOγ emission bands is also emphasized.

Gas dynamics and combustion wave expanding velocity of laser induced plasma

Fused silica is an indispensable basic element in a laser system and the weakest link in all components. When the laser interacts with fused silica, the target absorbs the laser energy so that its own temperature rises, and then it melts and vaporizes. The vaporization of the target gasification further absorbs the laser energy and produces a low density ionization reaction, resulting in the laser supported combustion wave (LSCW) phenomenon. In this paper, taking into account the effects of temperature residual, change in target morphology, distribution of splash material, and distribution of target surface airflow condition, we model and simulate the process of LSCW in stages. The laser energy transfer process, including the inverse bremsstrahlung radiation, thermal radiation, heat conduction and convection processes, is simulated by establishing a two-dimensional axisymmetric gas dynamic model. In addition, the LSCW in the visible light band has a strong radiation characteristic, which is significantly different from the laser induced target melting and vaporization phenomenon. The LSCW is easily received and displayed by high-speed camera. Therefore, a shadow system is established to measure the expanding velocity of the combustion wave in the process of fused silica damaged by laser, and the evolution process image of the combustion wave is obtained. The results show that under the action of parallel laser beam, the propagation of the combustion wave is in a steady-state and the gas dynamic behavior is stable. For the pulse widths of 1 ms and 3 ms, the average propagation velocity of the LSCW is calculated to be about 24 m/s, which is consistent with the experimental result in the literature available. This verifies the correctness of our theoretical model. For the pulse width of 3 ms, the average velocity of the flow field near the wavefront is calculated to be about 200 m/s. The numerical relationship between the velocity of the flow field and the propagation velocity of the LSCW is also basically consistent with the theoretical derivation result. Under the action of focused laser beam, the propagation of the combustion wave is unsteady. For the pulse widths of 1 ms, the laser intensity at the front of the plasma decreases gradually and the beam radius becomes larger. For the pulse width of 1.8 ms, both a similar pattern of mushroom cloud in the combustion wave and turbulence are observed, which is basically consistent with the evolution process of the combustion wave appearing in our experiment. The simulation results are in good accordance with the experimental results, and also provide a theoretical and experimental basis for studying the LSCW of fused silica.

Real-Time Hyperbola Recognition and Fitting in GPR Data

The problem of automatically recognizing and fitting hyperbolae from ground-penetrating radar (GPR) images is addressed, and a novel technique computationally suitable for real-time on-site application is proposed. After preprocessing of the input GPR images, a novel thresholding method is applied to separate the regions of interest from background. A novel column-connection clustering (C3) algorithm is then applied to separate the regions of interest from each other. Subsequently, a machine learnt model is applied to identify hyperbolic signatures from outputs of the C3 algorithm, and a hyperbola is fitted to each such signature with an orthogonal-distance hyperbola fitting algorithm. The novel clustering algorithm C3 is a central component of the proposed system, which enables the identification of hyperbolic signatures and hyperbola fitting. Only two features are used in the machine learning algorithm, which is easy to train using a small set of training data. An orthogonal-distance hyperbola fitting algorithm for “south-opening” hyperbolae is introduced in this work, which is more robust and accurate than algebraic hyperbola fitting algorithms. The proposed method can successfully recognize and fit hyperbolic signatures with intersections with others, hyperbolic signatures with distortions, and incomplete hyperbolic signatures with one leg fully or largely missed. As an additional novel contribution, formulas to compute an initial “south-opening” hyperbola directly from a set of given points are derived, which make the system more efficient. The parameters obtained by fitting hyperbolae to hyperbolic signatures are very important features; they can be used to estimate the location and size of the related target objects and the average propagation velocity of the electromagnetic wave in the medium. The effectiveness of the proposed system is tested on both synthetic and real GPR data.

Effects of turbulent intensity on nano-PMMA flame propagation behaviors

Experiments with different ignition delay times (td) corresponding to different residual turbulent intensities after dispersion were conducted to reveal the effects of turbulent intensity on nano-PMMA flame propagation behaviors. The residual vertical root-mean-square (RMS) turbulent velocities and RMS vorticities measured by a particle image velocimetry system (PIV) at td = 0.9 s, 1.0 s and 1.1 s were 0.22 m/s, 0.16 m/s, and 0.15 m/s and 50.40/s, 38.23/s, and 36.68/s, respectively. It was indicated that the residual turbulent intensity in the combustion space decayed with increasing ignition delay time. One-hundred nanometer PMMA dust flames with a nominal concentration of 450 g/m3 were approximately spherical in shape and propagated continuously after ignition at different ignition delay times. As time proceeded, the flame front of td = 0.9 s was still smooth, whereas the flame fronts of td = 1.0 s and 1.1 s were irregular due to the dominant effects of flame self-instability against the residual turbulence. The average pulsating flame propagation velocities of td = 0.9 s, 1.0 s and 1.1 s were 0.67 m/s, 0.54 m/s and 0.44 m/s, respectively. The pulsating level was enhanced by extending the ignition delay time due to the weaker residual turbulent intensity compared with the turbulence induced by the dust flame self-instability. In addition, the suspended particle size distribution was measured by a Phase Doppler Particle Analyzer (PDPA). It was found that the effective suspended particles existed as agglomerates of fine particles rather than as the primary particles themselves.

Combustion behaviors and flame microstructures of micro- and nano-titanium dust explosions

Particle size has significant effect on flame propagation behaviors in dust explosions. In this study, the flame propagation behaviors and microstructures in micro- and nano-titanium dust explosions were observed and compared. Results showed that flame propagation mechanisms in 50nm and 35μm titanium dust clouds were quite different. 50nm titanium dust flame was characterized by discrete single glowing burning particles with smooth spherical flame front. While 35μm titanium dust flame was marked by clusters of glowing burning particles with irregular flame front. 50nm titanium flame velocity was fluctuated more violent and the average flame propagation velocity was faster than that of 35μm titanium flame. In addition, micro explosion phenomenon occurred significantly in the burning process of 50nm titanium particles. SEM photos showed that 50nm titanium particles were approximately spherical shape with observably agglomerations before ignition. However, the combustion products exhibited complicated structures combined the spherical titanium oxides with considerable larger diameters and irregularly spliced smaller titanium oxides. 35μm titanium particles were in irregular shape before ignition, but in spherical shape after combustion. These results indicated that oxidation reaction occurred on the liquid surface of 35μm and 50nm titanium particles. From X-ray photoelectron spectroscopy, it was revealed that the dominant oxidation states of 35μm titanium combustion products was TiO2 (Ti4+), and to a much lesser extent of Ti2O3 (Ti3+). However, 50nm titanium combustion products contained 61% TiO2 (Ti4+), 18% Ti2O3 (Ti3+), 8% TiO (Ti2+) and 13% TiN (Ti3+).

Experimental and numerical investigation on premixed H2/air combustion

Experimental investigation and numerical simulation are performed to obtain the combustion characteristics of premixed hydrogen/air mixtures in a closed container, such as flame propagation velocity, peak pressure and maximum temperature. This paper mainly focused on the influence of initial temperature, hydrogen concentration and steam concentration on hydrogen/air flame propagation. An updated kinetics of hydrogen combustion based on one-step global reaction mechanism was validated by the present experimental data. The numerical results with updated kinetics show a satisfactory agreement with experimental results in term of average flame propagation velocity with the deviation less than 20%. The experimental and simulation results show that the average flame propagation velocity increases with the increase of initial hydrogen concentration and initial temperature, and reduces with the increase of initial steam concentration. The effect of hydrogen concentration on average flame propagation velocity is more significant than that of initial temperature and steam concentration.

A numerical study of species and electric field distributions in pulsed DBD in oxygen for ozone generation

A numerical model including 29 reactions and 11 species is developed to investigate the spatial-temporal distributions of species and electric field in oxygen parallel-plate positive pulsed dielectric barrier discharge for ozone generation. Electron energy conservation equation is coupled to electron continuity equation, heavy species continuity equation and Poisson's equation. The spatial-temporal distributions of species and electric field are obtained at electron avalanche and during streamer propagation. The results show the dominant positive ion, negative ion and excited species are O2+, O3− and O2(∑g+1), respectively. The average streamer propagation velocity is about 5.56 × 104 m/s. Overall electron density almost increases exponentially but with a violent oscillation in the vicinity of anode at electron avalanche. Electron density increases at first and then declines in the vicinity of anode and streamer channel during streamer propagation, and increases gradually in streamer head with the development of streamer, as well as reduces sharply to almost zero near cathode. Electron density almost remains constant in whole streamer channel at any one moment. Other species have a similar spatial-temporal distribution.

Flame propagation behaviors of nano- and micro-scale PMMA dust explosions

Flame propagation behaviors of nano- and micro-polymethyl methacrylate (PMMA) dust explosions were experimentally studied in the open-space dust explosion apparatus. High-speed photography with normal and microscopic lenses were used to record the particle combustion behaviors and flame microstructures. Simple physical models were developed to explore the flame propagation mechanisms. High-speed photographs showed two distinct flame propagation behaviors of nano- and micro-PMMA dust explosions. For nano-particles, flame was characterized by a regular spherical shape and spatially continuous combustion structure combined with a number of luminous spot flames. The flame propagation mechanism was similar to that of a premixed gas flame coupled with solid surface combustion of the agglomerates. In comparison, for micro-particles, flame was characterized by clusters of flames and the irregular flame front, which was inferred to be composed of the diffusion flame accompanying the local premixed flame. It was indicated that smaller particles maintained the leading part of the propagating flame and governed the combustion process of PMMA dust clouds. Increasing the mass densities from 105 g/m3 to 217 g/m3 for 100 nm PMMA particles, and from 72 g/m3 to 170 g/m3 for 30 μm PMMA particles, the flame luminous intensity, scale and the average propagation velocity were enhanced. Besides, the flame front became more irregular for 30 μm PMMA dust clouds.

Characterization of Electrophysiological Propagation by Multichannel Sensors.

The propagation of electrophysiological activity measured by multichannel devices could have significant clinical implications. Gastric slow waves normally propagate along longitudinal paths that are evident in recordings of serosal potentials and transcutaneous magnetic fields. We employed a realistic model of gastric slow wave activity to simulate the transabdominal magnetogastrogram (MGG) recorded in a multichannel biomagnetometer and to determine characteristics of electrophysiological propagation from MGG measurements. Using MGG simulations of slow wave sources in a realistic abdomen (both superficial and deep sources) and in a horizontally-layered volume conductor, we compared two analytic methods (second-order blind identification, SOBI and surface current density, SCD) that allow quantitative characterization of slow wave propagation. We also evaluated the performance of the methods with simulated experimental noise. The methods were also validated in an experimental animal model. Mean square errors in position estimates were within 2cm of the correct position, and average propagation velocities within 2mm/s of the actual velocities. SOBI propagation analysis outperformed the SCD method for dipoles in the superficial and horizontal layer models with and without additive noise. The SCD method gave better estimates for deep sources, but did not handle additive noise as well as SOBI. SOBI-MGG and SCD-MGG were used to quantify slow wave propagation in a realistic abdomen model of gastric electrical activity. These methods could be generalized to any propagating electrophysiological activity detected by multichannel sensor arrays.

Response of the auroral lower ionosphere to solar flares in March 2012 according to ELF observations

The response of the lower ionosphere to the solar flares that occurred in March 2012 is considered. Measurements of the propagation velocity and wave impedance of ELF electromagnetic pulses (atmospherics) performed at Lovozero and Barentsburg high-latitude observatories were used to estimate this response. It was shown that the daily average propagation velocity of atmospherics decreased by 20–30 thousand km/s under disturbed heliogeophysical conditions as compared to the velocity measured under quiet conditions. This is related to a decrease in the effective waveguide height that results from the change in the ionospheric conductivity profile during a solar flare. It was detected that pronounced bursts of wave impedance, the maximums of which exceed the impedance average value by a factor of more than 2, are observed during strong heliogeophysical disturbances. This fact cannot be explained in the scope of a spherically layered model; consequently, such deviations indicate an increase in the D-layer conductivity inhomogeneities.

A computational modeling study on the helium atmospheric pressure plasma needle discharge**Project supported partly by the National Natural Science Foundation of China (Grant No. 11465013), the Natural Science Foundation of Jiangxi Province, China (Grant No. 20151BAB212012), and in part by the International Science and Technology Cooperation Program of China (Grant No. 2015DFA61800).

A two-dimensional coupled model of neutral gas flow and plasma dynamics is employed to investigate the streamer dynamics in a helium plasma needle at atmospheric pressure. A parametric study of the streamer propagation as a function of needle tip curvature radius and helium gas flow rate is presented. The key chemical reactions at the He/air mixing layer which drive the streamer propagation are the direct ionization via collision with electrons, the Penning effect being not so crucial. With increasing the gas flow rate from 0.2 standard liter per minute (SLM) to 0.8 SLM, however, the emissions resulting from reactive oxygen and nitrogen species change from a solid circle to a hollow profile and the average streamer propagation velocity decreases. Air impurities (backdiffusion from ambient air) in the helium jet result in a significant increase in the streamer propagation velocity. Besides, with decreasing the tip curvature radiusfrom 200 μm to 100 μm, the electron avalanche process around the near-tip region is more pronounced. However, the spatially resolved plasma parameters distributions (electron, helium metastables, ground state atomic oxygen, etc.) remain almost the same, except that around the near-tip region where their peak values are more than doubled.