Effect Of Ambient Pressure Research Articles

Numerical investigation on the effect of ambient pressure on mechanical smoke extraction efficiency in tunnel fires

This paper has analyzed the ambient pressure on the effect of the efficiency of the mechanical smoke extraction system in tunnel fires. These were simulated by Large Eddy Simulation (LES) with Fire Dynamics Simulator (FDS). The simulation results indicate that it is not the instance that the higher the exhaust rate, the better the smoke exhaust efficiency of the exhaust vent. There is a critical exhaust rate at which excessive fresh air entrainment, such as plug-holing of the smoke layer, can be avoided. The mechanical smoke exhausting process is affected by two competitive forces: horizontal inertial force and vertical suction force. The thickness and horizontal velocity of the thermal smoke layer in the tunnel increase as the ambient pressure decreases. At this time, the vertical suction force required for the occurrence of the plug-holing phenomenon increases, and thus the critical exhaust rate increases. A linear relationship between the dimensionless critical exhaust rate Vcri∗ and the modified heat release rate Q˙∗ was proposed to characterize the influence of ambient pressure on the mechanical smoke exhaust efficiency. When the exhaust rate is 0.5 times the critical exhaust rate, the smoke mass fraction exhausted from the exhaust vent (E) reaches the maximum value.

Effect of ambient pressure on the extinction limit for opposed flame spread over an electrical wire in microgravity

Parabolic flight experiments were carried out to investigate the effect of ambient pressure on the extinction limit for opposed flame spread over an electric wire insulation in microgravity. Low-density polyethylene insulated Nickel-Chrome wires with inner core diameter of 0.50 mm and insulation thickness of 0.30 mm were examined for ambient pressures ranging from 50 kPa to 140 kPa for an external opposed flow of 10 cm/s. The experiments showed that the limiting volumetric oxygen concentration (LOC) increased as the total ambient pressure decreased. This LOC trend can be explained by radiation loss from wire surface. The radiation loss increased as the ambient pressure decreased – a result that can be explained by the increase in preheat length with decreasing ambient pressure. Moreover, when the data was plotted in a partial pressure vs. total pressure space, it became evident that the limiting oxygen partial pressure (LOPP) decreased with decreasing total ambient pressure. This LOPP trend can be explained by the fact that the flame temperature increased as the ambient pressure decreased under constant oxygen partial pressure. In current fire safety design for spacecraft, tentative oxygen concentration criteria in spacecraft suggested by NASA is assumed as 30% of oxygen concentration, and this value is assumed independent of ambient pressure. However, the present result implies that consideration of the effect of ambient pressure on the flammability limit is necessary, especially with respect to the possibility of an extension of the allowable atmosphere condition for spacecraft cabin in the low pressure region.

Effect of ambient pressure on air side heat transfer and flow characteristics of plain finned tube heat exchanger

Ambient pressure influences on air side heat transfer and flow characteristics of plain finned tube heat exchangers are systematically estimated. Theoretical analysis showed that ambient pressure had impact on the development and separation of boundary layer while air flowed around tubes. Under the experimental conditions, air side heat transfer coefficient dropped by 38.46%–44.12% and jh factor rose by 35.66%–51.89% when the ambient pressure reduced from 100 kPa to 40 kPa. Both row number and fin pitch effect existed at lower ambient pressure, only being less pronounced. Air side heat transfer coefficient corresponding to the same Re decreased as ambient air pressure reduced. The ambient pressure ratio correction index is between 0.27 and 0.49 when the Re is the same.

Optimum pressure range evaluation toward aerodynamic measurements using PSP in low-pressure conditions

In the present study, new performance evaluation parameters for pressure-sensitive paint (PSP) were proposed, and the effects of ambient pressure on the characteristics of the PSP were evaluated. The proposed parameters allow us to determine the optimal pressure range on the basis of the following quantities: (1) the Stern–Volmer coefficient, (2) the normalized pressure sensitivity to intensity changes due to flow-induced pressure fluctuations, (3) the change in the intensity of PSP emissions in response to the given change in pressure, and (4) the signal-to-noise ratio of the change in the PSP emission intensity due to flow-induced pressure fluctuations. The characteristics of several types of PSP were evaluated using the proposed parameters. It was demonstrated that the proposed parameters enable a comparison of the effects of ambient pressure on the characteristics of PSP, and the optimal pressure range for aerodynamic measurements could be successfully identified for the different PSP.

Effects of ambient pressure on smoke movement patterns in vertical shafts in tunnel fires with natural ventilation systems

Smoke exhaust is of vital importance for life safety and structural safety in a tunnel fire. The smoke movement pattern could affect smoke exhaust efficiency, so it is necessary to determine the patterns in shafts in tunnel fires with natural ventilation systems. However, previous studies have focused on this problem at standard atmospheric conditions, but the ambient pressure, which could have an effect on smoke movement characteristics and temperature distribution, decreases in high-altitude areas. First, theoretical analysis is carried out to find that the smoke velocity is higher under reduced pressure due to lower heat loss. In addition, a set of numerical simulations based on Fire Dynamics Simulator (FDS) is conducted to investigate the effects of ambient pressures on smoke movement patterns in vertical shafts in tunnel fires with natural ventilation systems. The results show that the critical Richard number deceases under reduced ambient pressure, and the higher smoke temperature and velocity caused by lower ambient pressure are the reasons for the decrease in critical Ri. We hope that our work can provide a design reference for tunnel natural ventilation system design in high-altitude areas.

Experimental study of the effect of ambient pressure on oscillating behavior of pool fires

Pool fires are known to exhibit self-sustained oscillatory behavior, which plays an important role in the understanding of flame and fire spreading. To investigate the effect of ambient pressure on oscillating behavior of pool fires, the ethanol pool fires with diameters from 2 to 10 cm were conducted under a wide range of pressure from 0.6 to 2.5 atm, where different pressure environments were achieved and maintained using a closed high pressure chamber. Based on the momentum conservation equation for the gas movement driven by buoyant force, the Gr number was derived to illustrate the change of flame structure and oscillation frequency. The results show that the Gr number could explain the flame structure changing from laminar to turbulent. The oscillation intensity, which was redefined using the probability contour of the flame, could be related with Grashof. Furthermore, it was noticed that the influence of ambient pressure and gravity on oscillation frequency depends on the different oscillation mechanisms. Based on the one-dimensional movement of vortex along the axis, a theoretical model was proposed to explain the effect of ambient pressure and gravity on frequency, and then the obtained relationship between Strouhal and Grashof was verified by the measured value.

Macroscopic Spray Behavior of a Single-Hole Common Rail Diesel Injector Using Gasoline-Blended 5% Biodiesel

This research studies the macroscopic spray structure from a single-hole common rail diesel injector using gasoline-blended 5% biodiesel for use in compression ignition engines. To reduce the NOX/PM trade-off emissions, researchers are investigating gasoline used in compression ignition engines, called gasoline compression ignition. As a result that gasoline is injected directly into the combustion chamber, its spray field has a significant effect on combustion and emissions. Due to its low lubricity, gasoline is blended with biodiesel 5%, as a lubricity enhancer, to prevent the failure of the high-pressure injection system. The macroscopic spray structures of this gasoline blend were investigated Schlieren photography and planar laser-induced fluorescence-particle image velocimetry. Injection pressure was handled by a conventional common rail system, while ambient pressure was controlled by supplying nitrogen into the constant-volume combustion chamber. The effects of injection pressure and ambient pressure on the gasoline spray elucidated by Planar laser-induced fluorescence coupled with particle-image velocimetry (PLIF-PIV) imagery and comparisons with variations in neat diesel spray. In addition, the flow field of gasoline spray that formed vortexes and vorticity was characterized. The results show that the injection pressure and back pressure had the same effects on the gasoline spray structure, in terms of the penetration tip and cone angle, as on the diesel spray. However, the injection pressure had a greater effect on the diesel spray than the gasoline at low ambient pressure due to the occurrence of cavitation. Moreover, the images show the remarkable turbulent structure of gasoline spray and indicate air entrainment at the spray tip region.

Heat transfer mechanism of concurrent flame spread over rigid polyurethane foam: Effect of ambient pressure and inclined angle

As a common building façade insulation material with numerous applications, the heat transfer mechanism and flame spread characteristics of rigid polyurethane (RPU) foam should have been concerned. This work aims to give a comprehensive understanding of the coupling effect of sample inclination and ambient pressure on the heat transfer process and flame spread behaviors over RPU foam. A series of contrastive bench-scale tests were performed, in which inclination (denoted θ, 10°–90°) and ambient pressure (P, 65.5 and 100.8 kPa) were varied, and the variation of flame spread rate, flame length and flame stand-off distance were thoroughly investigated. Under the competitive effect of gas-phase heat transfer and surface charring, the extinction phenomena of flame spread were found, and the critical extinction angle were: θcrit,max=85∘, P = 100.8 kPa; θcrit,max=60∘, θcrit,min=25∘, P = 65.5 kPa. Based on the laminar and turbulent flow states, a piece-wise correlation to evaluate flame spread rate was established, which matched the experimental results well. Moreover, spreading flames were classified as pool-like flame and wall-like flame, and a new modified dimensionless correlation and semi-empirical model were proposed to describe the variation of flame length. Average flame stand-off distance was found to linearly decrease with inclination, y‾~θ, and the slopes in the plain and on the plateau were −0.455 and −0.776, respectively. The results of this study can provide a better insight for the fire safety design of energy conservation system of high-rise buildings.

Effect of ambient pressure on fluorescence intensity of inexpensive inorganic fluorescent tracer for particle image velocimetry of gaseous flows near solid objects

Flow dynamics near the spark plug gap have become more important to promote more stable lean/dilute combustion in internal combustion engines. One experimental technique for the measurement of instantaneous gaseous flow in an engine cylinder is particle image velocimetry (PIV). However, PIV measurement of the flow near solid objects is difficult because of the flare due to strong scattered light from the solid objects. Therefore, methods that use fluorescent particles (f-particles) have been proposed to reduce this undesirable effect. The mass production of inexpensive inorganic-based f-particles was previously proposed by one of the authors of this paper, assuming flow measurement in a firing engine. Furthermore, flow measurement near the spark plug gap was attempted using PIV with these f-particles, whereby the unsteady flow motion near the spark plug was successfully detected. However, the measurements were conducted under atmospheric conditions at room temperature. Therefore, as a next step, investigation of the effect of ambient pressure and temperature on the fluorescence intensity of the f-particles, is required to clarify the degree of oxygen quenching and temperature quenching of the f-particles. The effect of ambient pressure on fluorescent intensity was investigated fundamentally using a constant volume vessel. As a result, the intensity of fluorescence emitted from the f-particles was almost equal at pressures between 0.1 MPa and 2.0 MPa.

Technology Focus: High Pressure/High Temperature (March 2020)

Technology Focus One would think that, in these challenging times, high-pressure/high-temperature (HP/HT) technology and projects would be put on the back burner, with everybody chasing zipper-fractured barrels from shale. Well, it is not so. The rewards in new reserves and profits of being able to develop HP/HT resources efficiently are just too attractive to ignore. So, we can enjoy some major developments on the project and technical side in the past year, and the highlights are offshore. Let us look at the Gulf of Mexico, which has moved from being just a pioneering area for deepwater developments to an area where deep water and HP/HT combine to provide the ultimate challenge for the best engineers. In 2019, we saw first oil from Appomattox, the first producing high-temperature development in the deepwater Gulf of Mexico; and we can congratulate Shell (operator), Nexen, and the China National Offshore Oil Corporation for a job well done. This project also served as the developing ground for the recently issued Bureau of Safety and Environmental Enforcement HP/HT guidance documents, which certainly will make the next projects more streamlined. And lest we forget, it is also a “green” project, with the first combined-cycle power generation facility deployed offshore in the Gulf of Mexico. The next challenge is now the “HP” part of HP/HT, and Chevron took up the gauntlet and is putting the final touches on a new generation of drillship together with Transocean as the drilling contractor, with NOV providing dual 20,000-psi subsea blowout preventers. And, with the sanctioning of the Chevron-operated Anchor 20,000-psi development in the Green Canyon area at the end of 2019, it is anybody’s guess where this drillship will be deployed first. Total is the partner on this development. These two major HP/HT breakthroughs have been made in deep water, in the toughest industry environment seen at least in my lifetime. Who says our industry is not resilient and innovative? In this year’s paper selection, we are focusing on three technical areas where HP/HT and deep water come together: very-high-pressure and -temperature (supercritical) fluid properties, the latest expandable technology used for liner hangers, and the effect of ambient pressure on the pressure rating of well-construction equipment. Who would have thought that, in these challenging days, we would be able to celebrate the marriage of deep water and HP/HT? And this is happening not just in the Gulf of Mexico; we also hear some very interesting developments in Asia/Pacific (the South China Sea) and in the eastern Mediterranean. It is not aerospace or Silicon Valley, but our industry that is still the most innovative and pioneering in the word. We can only hope that the many highly qualified graduates, and especially post-graduates, in petroleum engineering looking for their first jobs are invited on this journey and don’t wander into other industries where their skills would be quickly appreciated. They are the ones we need to use all the new digital tools, such as simulator-based predrilling exercises. From the days of Shearwater to Appomattox and Anchor, we have almost no time left to tap the experience of the deepwater and HP/HT pioneers who are at the tail end of their careers and have so much knowledge to transfer. Recommended additional reading at OnePetro: www.onepetro.org. SPE 195353 Active Cooling Method for Downhole Systems in High-Temperature Environments by Wenkai Gao, China National Petroleum Corporation, et al. SPE 195725 Improving Performance in Elgin HP/HT Infill Drilling Despite Increasing Challenges by Mohammad Alahmad, Total, et al. SPE 197207 Improved Well Design 15,000-psi Cemented Completion To Deliver Faster and Cheaper Complex Deep Gas HP/HT Wells by Hashil Nasser Said Al Naabi, Petroleum Development Oman, et al.

Design of Unmanned Helicopter Equipped with Turboshaft Engine for Agriculture Spraying Mission Based on Thermodynamic Analysis

In this study, the thermodynamic cycle of a turboshaft engine whose design parameters were determined in accordance with an agricultural spraying unmanned helicopter was performed. After the solid model of the unmanned helicopter was created, a tank was designed to carry additives and water. The design requirements of the turboshaft engine were determined by deciding the maximum power requirement of the unmanned helicopter in accordance with the design parameters of the tank. The performance parameters of each component of the turboshaft engine are presented through diagrams. In addition, entropy generation (exergy destruction) and loop diagrams are presented to the literature depending on performance parameters. This study provides evidence that the design parameters of a manned and/or unmanned helicopter should be considered depending on the environmental conditions. The effect of ambient temperature and pressure varying with altitude on all components of the engine appears as the rate of exergy destruction. Thereby, it is necessary to analyze the ambient temperature and pressure in determining the required design parameters. The relationship of temperature-pressure-entropy production according to the station numbers of the engine is presented through diagrams. It is clearly seen that entropy production increases with the increase in temperature in the components of the turboshaft. Since the aero-vehicle needs maximum power requirements during the take-off phase and its power is associated with temperature, pressure and many other parameters, the take-off phase is an important process in all flight stages.

Air-Assisted Atomization at Constant Mass and Momentum Flow Rate: Investigation into the Ambient Pressure Influence With the Smoothed Particle Hydrodynamics Method

Abstract A twin-fluid atomizer configuration is simulated by means of the two-dimensional (2D) weakly compressible smoothed particle hydrodynamics (SPH) method and compared to experiments. The gas-to-liquid ratio (GLR), the momentum flux ratio, and the velocity ratio are set constant for different ambient pressures, which lead to different gaseous flow sections. The objectives of this study are (i) to investigate the effect of ambient pressure at constant global parameters and (ii) to verify the capability of 2D SPH to qualitatively predict the proper disintegration mechanism and to recover the correct evolution of the spray characteristics. The setup consists of an axial liquid jet of water fragmented by a coflowing high-speed air stream (Ug = 80 m/s) in a pressurized atmosphere up to 16 bar. The results are compared to the experiment and presented in terms of (i) mean velocity profiles, (ii) drop size distributions, and (iii) Sauter mean diameter (SMD) of the spray. It is found that there exists an optimal pressure to minimize the mean size of the spray droplets. Finally, two new quantities related to atomization are presented: (i) the breakup activity that quantifies the number of breakup events per time and volume unit and (ii) the fragmentation spectrum of the whole breakup chain, which characterize the cascade phenomenon in terms of probability. The breakup activity confirms the presence of the optimal pressure, and the fragmentation spectrum gives information on the type of breakup, depending on the ambient pressure.

An Experimental Study on the Effects of Atomized Rain of a High Velocity Waterjet to Downstream Area in Low Ambient Pressure Environment

A better understanding of the atomized rain characteristics in low ambient pressure areas is beneficial in reducing the jeopardizing effect of flood discharge atomization on high-altitude hydropower stations. A random splash experiment is designed with two measurement planes to investigate the effects of low ambient pressure on downstream atomized rain under the complicated conditions of low ambient pressure (within 0.60P0~1.00P0) and high waterjet velocity (at a magnitude of 10 m/s). The results demonstrate that the atomized rain (rain intensity ≥ 2 mm/h) downstream, characterized by two-dimensional distribution, can be enhanced by decreasing the ambient pressure and by increasing the inflow discharge. When the ambient pressure decreases at the same inflow discharge, both the distance of the rain intensity lines (40 mm/h, 10 mm/h, 2 mm/h) in the horizontal plane from the constricted nozzle outlet and the average rain amount in the inclined plane within the atomized source ratio of ((0~30) × 10−3)% appear as “linear” growth. With the ambient pressure decreasing by 0.10P0, the range of those characteristic rain intensity lines is expanded by 0.68%~1.37%, and the average rain amount is enlarged by 11.06%~20.48%. When keeping the low ambient pressure unchanged, both the point average rain intensity reduction along the releasing centerline and the surface average rain amount growth with increased inflow discharge all follow an exponential function. The aeration reduction in the waterjet boundary and the resistance reduction in atomized water-droplets are contributing factors for the enhancement effect of low ambient pressure. This study can enable the establishment of a foundation to further predict flood discharge atomization in a high-altitude environment.

Effect of processing parameters on forming defects during selective laser melting of AlSi10Mg powder

PurposeTo fabricate a selective laser melting (SLM)-processed AlSi10Mg part with almost full density and free of any apparent pores, this study aims to investigate the effect of ambient argon pressure and laser scanning speed on the particles splash during the AlSi10Mg powder bed laser melting.Design/methodology/approachBased on the discrete element method (DEM), a 3D model of random distribution of powder particles was established, and the 3D free surface of SLM forming process was dynamically tracked by the volume of fluid, where a Gaussian laser beam acts as the energy source melting the powder bed. Through the numerical simulation and process experimental research, the effect of the applied laser power and scanning speed on the operating laser melting temperature was studied.FindingsThe process stability has a fundamental role in the porosity formation, which is process-dependent. The effect of the processing conditions on the process stability and the resultant forming defects were clarified.Research limitations/implicationsThe results shows that the pores were the main defects present in the SLM-processed AlSi10Mg sample, which decreases the densification level of the sample.Practical implicationsThe optimal processing parameters (argon pressure of 1,000 Pa, laser power of 180 W, scan speed of 1,000 mm/s, powder layer thickness of 35 µm and hatch spacing of 50 µm ) applied during laser melting can improve the quality of selective laser melting of AlSi10Mg,Social implicationsIt can provide a technological support for 3D printing.Originality/valueBased on the analysis of the pore and balling formation mechanisms, the optimal processing parameters have been obtained, which were argon pressure of 1,000 Pa, laser power of 180 W, scan speed of 1,000 mm/s, powder layer thickness of 35 µm and hatch spacing of 50 µm. Then, a near-fully dense sample free of any apparent pores on the cross-sectional microstructure was produced by SLM, wherein the relative density of the as-built samples is larger than 97.5%.

Experimental study on flash-boiling spray structure of multi-hole gasoline direct injection injector in a constant volume chamber

The macroscopic and microscopic characteristics of flash-boiling spray were experimentally investigated with various optical measurement techniques. The effects of ambient pressure and fuel temperature on flash-boiling characteristics in multi-hole gasoline direct injection injector were analyzed. The analysis was focused on the spray structure and atomization droplet size distributions. In order to increase the understanding of the flash-boiling spray targeting, three injectors with different spray patterns were investigated under strong flash-boiling condition. The results show that ambient pressure and fuel temperature have significant influence on flash boiling. Both lower ambient pressure and higher fuel temperature could accelerate the flash-boiling process. For the macroscopic characteristics, similar influences could be found with the ambient pressure decreased by 0.4 bar and the fuel temperature increased by 10°C. Further, significant difference could be found within cold-jet spray and strong flash-boiling spray, such as the spatial structure. The spray structure always turns from hollow cone into solid when flash boiling occurs. With a higher fuel superheat degree, the spray droplet distribution moves toward smaller sizes and let the larger droplets reduce due to the promotion of atomization. For the strong flash-boiling spray, the Sauter mean diameter has decreased by 50% compared with cold-jet spray. There is a corresponding relationship between collapsed flash-boiling spray target and weighted geometric center of the injector. Spray collapse could be avoided by increasing the plume distance.

Effect of ambient pressure on spatial structures of electrical exploding aluminum wires in argon gas

The experiments demonstrated a strong dependence of the spatial structures of exploding aluminum wires on ambient pressures. Depending on whether having a current pause, electrical explosion of wires processes was classified into two types and analyzed by stage division. Through shadow, schlieren, and interference images, the spatial structures of exploding wires in two breakdown modes (internal and shunting) were observed. It was indicated that the basic difference is the relative positions of the wire core and the plasma channel. The breakdown mode makes a transition from the shunting mode to the internal mode with increasing pressures. Radial positions of the wire core boundary and the shockwave front were determined by comparing shadowgraphs and multi-frame photographs at different moments. The results demonstrated a gradual separation of the wire core and the shockwave, as well as their parabolic expansion trajectories. Furthermore, the average expansion velocity of wire cores decreases from ∼1.41 km/s to ∼0.78 km/s with the pressure varying from 50 kPa to 400 kPa.

Effects of ambient pressure on diffusion kinetics in coal during methane desorption

To elucidate the influence of ambient pressure on diffusion kinetics during gas desorption, which is of great significance to the efficient development of coalbed methane, a methane desorption experiment was carried out on coal particles under different ambient pressures, and diffusion coefficients for each test time were solved by the unipore diffusion model. The experiment was carried out using our independently developed 'coal particle methane desorption meter' to overcome the difficulty that the existing equipment could not achieve a constant ambient pressure higher than atmospheric pressure. The results show that during methane desorption in coal particles, the diffusion coefficient was not constant but gradually decreased with time, showing a power function relationship. In addition, the diffusion coefficient decreased linearly with increasing ambient pressure, and the degree of influence was related to time. This research will play an important role in improving the unipore diffusion model and elucidating the gas production mechanisms. [Received: February 2, 2018; Accepted: May 9, 2018]

Effects of ambient pressure on diffusion kinetics in coal during methane desorption

To elucidate the influence of ambient pressure on diffusion kinetics during gas desorption, which is of great significance to the efficient development of coalbed methane, a methane desorption experiment was carried out on coal particles under different ambient pressures, and diffusion coefficients for each test time were solved by the unipore diffusion model. The experiment was carried out using our independently developed 'coal particle methane desorption meter' to overcome the difficulty that the existing equipment could not achieve a constant ambient pressure higher than atmospheric pressure. The results show that during methane desorption in coal particles, the diffusion coefficient was not constant but gradually decreased with time, showing a power function relationship. In addition, the diffusion coefficient decreased linearly with increasing ambient pressure, and the degree of influence was related to time. This research will play an important role in improving the unipore diffusion model and elucidating the gas production mechanisms. [Received: February 2, 2018; Accepted: May 9, 2018]

Effect of ambient gas pressure on characteristics of air plasma induced by nanosecond laser

The ambient gas pressure has an important influence on the laser induced plasma characteristics. The effects of gas pressure on the characteristics of air plasma induced by nanosecond laser are studied by using the optical emission spectroscopy, and the relationship between the gas pressure and the spectral intensity, and between electron temperature and electron density of air plasma are discussed. The air gas pressure in chamber is continuously changed in a range from 10 to 100 kPa by using a mechanical pump and measured by using a barometer. The ns laser energy in experiment is fixed at 100 mJ in the whole experiment. The digital delay trigger (Stanford DG535/645) is used to trigger the laser and ICCD synchronously, and the delay and gate time of ICCD are set to be 0 and 5 μs, respectively. The experimental results show that air plasma emission spectrum consists of the line and continuous spectrum, and the spectral intensity of air plasma emission spectrum is dependent on gas pressure in a range from 10 to 100 kPa, and the evolution of atomic spectrum intensity with gas pressure is different from that of ion spectrum intensity. The air density in the region of laser breakdown increases with air pressure increasing, which leads the breakdown probability of air gas to increase, thus resulting in the air plasma spectral intensity increasing. Under the confinement action of the ambient air gas in the expanding region of air plasma, the collision probability and energy exchange probability among particles in the air plasma are both increased, and the trisomic recombination probability of ion-electron-atom is also increased. As a result, the atomic spectral intensity of O Ι 777.2 nm and N Ι 821.6 nm both increase with the air gas pressure increasing, and the spectral intensity is highest at 80 kPa, and then slowly decreases. But the spectral intensity of N II 500.5 nm reaches its maximum value at 40 kPa, and decreases as the pressure becomes greater than 40 kPa. The electron density of the air plasma increases with the air pressure increasing, and the growth rate becomes slow after 80 kPa. The electron temperature of the air plasma reaches a maximum value at 30 kPa. The plasma electron temperature gradually decreases as the pressure becomes greater than 30 kPa. The research results can provide an important experimental basis for studying the laser-induced air plasma characteristics at different altitudes, and also give important technical support for laser atmospheric transmission and atmospheric composition analysis in the future.

Influence of cycling aging and ambient pressure on the thermal safety features of lithium-ion battery

The fire and explosion risks of lithium-ion batteries (LIBs) pose serious threat to the transport and application of LIBs on the aircraft. Therefore, the study of corresponding thermal safety problems in flight condition is of great importance. In this work, a dynamic pressure chamber is used to study the effect of cycling aging and ambient pressure on the thermal safety features of LIBs. The results show that both the thermal runaway onset time and temperature decrease with the increase of cycle number or decrease of external pressure. The interval time between gas release and ignition decreases with the increase of cycle number, while the burning time increases with the decrease of pressure. Moreover, the thermal runaway temperature and heat release rate decrease with the increase of cycle number or decrease of pressure. The dV/dQ and X-ray computed tomography results show that the loss and structural damage of cathode materials, and the side reactions are the main factors for the decrease of battery safety. The easier occurrence of thermal runaway in low pressure is attributed to the larger differential pressure which facilitates the opening of burst disk and the entry of air.