Ambient Gas Pressure Research Articles

Collision of liquid drops: bounce or merge?

Whether colliding drops will merge with or bounce off each other is critical to numerous processes, and the physics involved is notoriously complex. In particular, experiments show that both sufficiently slow and fast head-on drop collisions lead to merging, but that there is often an intermediate regime in which bouncing is observed; these transitions in behaviour were recently discovered to be surprisingly sensitive to the radius of the drops and the ambient gas pressure. We show here that these transitions between bouncing and merging are governed by nanoscale phenomena; namely, gas-kinetic and disjoining pressure effects. To capture these crucial effects, a novel, open-source computational model is developed for the simulation of colliding drops. The model uses a hybrid approach, based on solving the Navier–Stokes equations in the drop with a lubrication approach for the unconventional physics of the gas film. Our simulations show remarkably good agreement with experiments of head-on collisions and also provide new experimentally verifiable predictions.

Zero-g wind tunnel experiments on planetesimal erosion

ABSTRACT Wind erosion thresholds for planetesimals in protoplanetary discs depend on particle size, cohesion, ambient gas pressure, and gravity. In the experiments reported here, we used a low pressure shear wind tunnel for the first time under real zero gravity. That is, we carried out the experiments in the drop tower in Bremen at $10^{-5} \text{g}$. In concert with earlier work, we confirm the applicability of a single semi-empirical equation to describe the threshold shear stress for wind erosion. It can be applied to calculations of unstable orbits of planetesimals and the dissolution of planetesimals passing through evolving planetary atmospheres.

Synthesis and thermal characterization of pulsed laser deposited molybdenum on aluminium nitride substrates for heat sink applications

Thin films of molybdenum (Mo) were grown on aluminium nitride (AlN) substrates by pulsed laser deposition for heat sink applications. The effect of experimental growth parameters on the films’ structural properties were investigated by Scanning Electron Microscopy, X-Ray Diffraction and Atomic Force Microscopy. Thermal characterization was achieved by measuring the in-plane thermal diffusivity of the grown layers by means of a photothermal beam deflection technique using an IR heating laser. Within the experimental parameters studied in this work, a substrate temperature of 600 ˚C and an ambient argon gas pressure of 10 mTorr were identified as the optimal growth conditions for the synthesis of smooth and well-crystallized Mo layers. Concurrently, the thermal diffusivity of the films is significantly affected by film growth parameters. Under the optimal growth conditions, a thermal diffusivity value as high as 5.42x10-5 m2/s was measured, a value that is very close to that of bulk Mo, and which would be a result of the synthesis of polycrystalline Mo films whose grains’ size is greater than the heat carriers mean free path, namely the free electrons. The temperature dependence of the thermal diffusivity was also investigated, and the films were found to be stable up to an operating temperature of 200 °C. Beyond this temperature, photothermal beam deflection imaging shows the onset of film delamination from the underlying substrate.

Enhancing analytical merits of laser-induced breakdown spectroscopy of hydrogen isotopes using an orthogonal double-pulsing scheme

Accurate detection and quantification of hydrogen isotopes in solid materials are vital for diverse applications, including fusion energy, hydrogen storage, and tritium production. Laser-induced breakdown spectroscopy (LIBS) is a well-established, rapid, standoff method for this purpose, but it faces challenges related to the analytical merits required for isotopic analyses. In this study, we enhance the analytical and detection capabilities of traditional single-pulse LIBS by implementing an orthogonal double pulsing approach, focusing on the analysis of a range of 2H concentrations in Zircaloy-4 substrates (acting as a proxy for 3H). The double-pulse experiments employed an orthogonal re-heating configuration with two nanosecond Nd:YAG lasers. We systematically evaluated critical parameters affecting the signal intensity in double-pulse LIBS, including interpulse delay, ambient gas pressure, and heating laser energy. Our results demonstrate that employing an orthogonal double-pulse scheme significantly enhances 2Hα emission while minimizing line broadening and self-absorption, ultimately improving the technique's analytical capabilities.

A Phenomenological Thermal Spray Wall Interaction Modeling Framework Applied to a High-Temperature Ignition Assistant Device

Abstract Airborne compression ignition engines must operate with reliable ignition systems to achieve proper ignition at every cycle, particularly at high altitudes. Glow-plug-based ignition-assistant (IA) devices can provide the necessary energy to preheat the fuel and ensure ignitability of the fuel-air mixture. Ignitability of liquid sprays can be facilitated via direct impingement onto the hot IA surface, however this comes with adverse effects on the IA durability. Therefore, optimizing an IA's design requires detailed understanding of the physics of fuel spray impingement of superheated surfaces. While spray impingement on relatively low wall temperatures has been extensively studied and appropriate numerical models have been proposed through the years, fundamental understanding of high-speed liquid spray impingement on superheated walls is still elusive. This work aims to formulate a phenomenological thermal spray-wall interaction framework for modeling the film-boiling-induced heat transfer, atomization, and dispersion of fuel spray droplets impinging onto a superheated IA device. A qualitative comparison of the new phenomenological model is performed against optical experiments from the literature of an F-24 fuel spray injected onto an IA device located 12 mm away from the injector tip. The temperature of the IA was set at 1400 K. The fuel injection pressure was 400 bar, while the ambient gas pressure and temperature were 30 bar and 800 K, respectively. The performance of the phenomenological model is evaluated in comparison with two other state-of-art models from the literature. A qualitative analysis of the different spray and fuel-air mixture characteristics is performed to outline the differences in the predictions offered by the new phenomenological model and the two state-of-art spray-wall interaction models.

Topological signatures of Mo2TiC2O2

Mo2TiC2 is the Ordered Double Transitional Metal Layered Carbides (ODTMLC) derived from its parent MAX phases Mo2TiAlC2 by a wet chemical etching. Its oxidation was done by a new ablated plasma thrust method in which the MXenes were at 750 °C under an oxygen background in the pulsed laser deposition chamber. The reflective high electron energy diffraction technique assures the oxidation at the ambient gas pressure p = 0.1 mbar, which was described in the previous paper. The obtained Mo2TiC2O2 was transferred for their topological test under angle-resolved photoemission spectroscopy, circular dichroism test, and Chemical Potential (CP) analysis. An indirect energy bandgap of 125 meV was obtained. The sine function of α along with period π and β with period 2π shows that there is a possibility of helical spin textures in both α (electron-like pocket around Γ̄) and β (elliptical electron-like pocket around M̄). The CP analysis shows the possibility of at least 100 meV bandgap creation on a single surface so that the surface charges will flow without any effect of bulk. The Mo2TiC2O2 can be used as topological insulating material.

Synthesis of Ti2AlC MAX phase and Ti2C MXene by activated combustion

The preparation of the Ti2AlC MAX phase by activated self-propagating high-temperature synthesis from a mixture of elemental powders using the thermokinetic coupling approach is reported. The underlying mechanism in the Ti–Al–C system has been elucidated. Ti2C MXene was produced by exfoliation of its MAX counterpart. The combustion wave sensitive parameters were determined, the influence of promoter and ambient gas pressure on the phase composition and evolution of the microstructure was revealed. The Ti2AlC MAX phase was successfully prepared in an energy-efficient pathway with some controlled amount of TiC. The interaction mechanism, simulated at comparatively higher heating rates (β = 600–4800 °C·min−1), demonstrated the evolution of the following phases: Ti3Al, TiC, Ti3AlC, Ti2AlC, T3AlC2, Al3Ti. Etching for only 2h in combination with double ultrasonic treatment made it possible to delaminate the nanolayered structure of the parent MAX phase and obtain Ti2C MXene.

Gigahertz semiconductor laser at a center wavelength of 2 µm in single and dual-comb operation.

Dual-comb lasers are a new class of ultrafast lasers that enable fast, accurate and sensitive measurements without any mechanical delay lines. Here, we demonstrate a 2-µm laser called MIXSEL (Modelocked Integrated eXternal-cavity Surface Emitting Laser), based on an optically pumped passively modelocked semiconductor thin disk laser. Using III-V semiconductor molecular beam epitaxy, we achieve a center wavelength in the shortwave infrared (SWIR) range by integrating InGaSb quantum well gain and saturable absorber layers onto a highly reflective mirror. The cavity setup consists of a linear straight configuration with the semiconductor MIXSEL chip at one end and an output coupler a few centimeters away, resulting in an optical comb spacing between 1 and 10 GHz. This gigahertz pulse repetition rate is ideal for ambient pressure gas spectroscopy and dual-comb measurements without requiring additional stabilization. In single-comb operation, we generate 1.5-ps pulses with an average output power of 28 mW, a pulse repetition rate of 4 GHz at a center wavelength of 2.035 µm. For dual-comb operation, we spatially multiplex the cavity using an inverted bisprism operated in transmission, achieving an adjustable pulse repetition rate difference estimated up to 4.4 MHz. The resulting heterodyne beat reveals a low-noise down-converted microwave frequency comb, facilitating coherent averaging.

Influence of fuel characteristics on the alternative jet fuel atomization at non-reacting conditions

Synthetic paraffinic kerosene jet fuel derived from natural gas (NG-SPK) is gaining importance as a low-emission alternative fuel for aviation combustors. In this context, the influence of fuel properties on the atomization of NG-SPK jet fuel at non-reacting, evaporating conditions is investigated. The spray results of NG-SPK jet fuel are compared with the reference, Jet A-1 fuel. This study considers a simplex atomizer, commonly deployed as a pilot injector in aviation gas turbine combustors for ignition at sea level and in high-altitude relight conditions. The local atomization characteristics like droplet concentration, mean diameters, and mean axial droplet velocity is obtained experimentally using phase Doppler anemometry technique at different ambient gas pressures of 100, 500, and 900 kPa while maintaining the ambient temperature at 400 K. The fuel dispersion parameters are measured at two nozzle pressure differentials of 300 and 900 kPa. The NG-SPK jet fuel exhibited higher droplet concentrations for the conditions studied than conventional fuel. Furthermore, the droplet size distribution of NG-SPK jet fuel revealed a higher percentage of smaller droplet diameters than Jet A-1 fuel. This difference in droplet characteristics can positively influence the heat release patterns under reacting conditions.

Influence of sample temperature and ambient argon pressure on LIBS spectra of extracted animal fat

This work explores the influence of sample temperature, ambient gas pressure, laser energy, and detector gate delay (DGD) on the LIBS emissions of extracted animal fat. The emissions from laser-induced plasma of extracted animal fat were acquired with varying sample temperatures, ambient argon pressure, laser energy, and detector gate delay. A thermoelectric system fitted with a temperature-controlling sensor was used to control the sample temperature. For maximum optical emission intensity, the detector gate delay (DGD) shifted toward a lower value with the increase in sample temperature for all ambient argon pressures. The nine-fold signal enhancement, ten times improvement in signal-to-noise ratio (SNR), and significantly reduced self-absorption were observed in all detected emission lines recorded at 200 mJ laser energy –2 ºC sample temperature, 375 Torr ambient argon pressure, and 2 µs DGD. The repeatability of emission signals was evaluated by calculating the relative standard deviation (RSD) of the emission line (Zn I (636.23 nm)) at different sample temperatures, which improved from 35 % to 6 % when the sample changed from liquid to solid (frozen). During the semi-liquid form of fat, the decreasing ambient gas pressure did not significantly affect signal repeatability. In addition to diagnosing the fat plasma, the plasma temperature and electron number density have been evaluated as a function of sample temperature and ambient argon pressure.

Combustion Synthesis of MAX Phases: Microstructure and Properties Inherited from the Processing Pathway

The MAX phases exhibit outstanding combination of strength and ductility which are unique features of both metals and ceramics. The preparation of pure MAX phases has been challenging due to the thermodynamic auspiciousness of intermetallic formation in the ternary systems. This review demonstrates the power of the self-propagating, high-temperature synthesis method, delivers the main findings of the combustion synthesis optimization of the MAX phases, and reveals the influence of the combustion wave on the microstructure features thereof. The possibility of using elements and binary compounds as precursors, oxidizers, and diluents to control the exothermicity was comparatively analyzed from the point of view of the final composition and microstructure in the following systems: Ti-Al-C, Ti-V-Al-C, Cr-V-Al-C, Ti-Cr-Al-C, Ti-Nb-Al-C, Ti-Al-Si-C, Ti-Al-Sn-C, Ti-Al-N, Ti-Al-C-N, Ti-Al-B, Ti-Si-B, Ti-Si-C, Nb-Al-C, Cr-Al-C, Cr-Mn-Al-C, V-Al-C, Cr-V-Al-C, Ta-Al-C, Zr-S-C, Cr-Ga-C, Zr-Al-C, and Mo-Al-C, respectively. The influence of sample preparation (including the processes of preheating, mechanical activation, and microwave heating, sample geometry, porosity, and cold pressing) accompanied with the heating and cooling rates and the ambient gas pressure on the combustion parameters was deduced. The combustion preparation of the MAX phases was then summarized in chronological order. Further improvements of the synthesis conditions, along with recommendations for the products quality and microstructure control were given. The comparison of the mechanical properties of the MAX phases prepared by different approaches was illustrated wherever relevant.

Optical emission features of laterally colliding laser-produced plasmas at different ambient pressures

Optical emission characteristics of seed plasma and stagnation region of laterally colliding laser produced aluminium plasmas are investigated under various background gas pressures of air and argon ambient. The intensity of ionic and neutral emissions from seed plasma region increases with the increase of ambient pressure. Neutral emission from the stagnation region increases whereas the intensity of ionic emission decreases with the increase of ambient pressure. Decrease of collision rate between the seed plasmas results in the decrease of ionic emission from the interaction region. For higher inter seed plume separation, maximum spectral emission from stagnation region is observed at an intermediate pressure range of 10−2 mbar. Emission intensity and the life time of both ionic and neutral emission are greater in heavier Ar ambient. In the case of seed plasmas, the electron density decreases whereas the electron temperature increases with the increase of ambient gas pressure and is greater in Ar ambient. The variations of plasma parameters are quite less in stagnation region.

Line intensity calculation of laser-induced breakdown spectroscopy during plasma expansion in nonlocal thermodynamic equilibrium.

We present a new, to the best of our knowledge, simulation method for laser-induced breakdown spectroscopy during the plasma expansion phase in nonlocal thermodynamic equilibrium. Our method uses the particle-in-cell/Monte Carlo collision model to calculate dynamic processes and line intensity of nonequilibrium laser-induced plasma (LIP) in the afterglow phase. The effects of ambient gas pressure and type on LIP evolution are investigated. This simulation provides an added way to understand the nonequilibrium processes in more detail than the current fluid and collision radiation models. Our simulation results are compared with experimental and SimulatedLIBS package results and show good agreement.

Spray characteristics of natural gas-based alternative jet fuel at high pressure ambient conditions

Alternative fuel like Gas-To-Liquid (GTL) jet fuel derived from natural gas is gaining importance in aviation industry due to their cleaner-burning nature. In this context, this study focuses on the role of ambient conditions on the atomization characteristics of GTL jet fuel at inert conditions, and the outcomes are compared with those of the reference, Jet A-1 fuel. For this purpose, a simplex atomizer, commonly utilized as a pilot-injector in aviation combustors for ignition at sea-level and in high-altitude relight conditions, is considered. The local atomization characteristics like droplet concentration, mean diameter and droplet mean axial velocity are obtained experimentally by employing the phase Doppler anemometry technique at different conditions (ambient gas pressures of 100, 500, and 900 kPa, and ambient temperature at 400 K). The fuel dispersion parameters are measured at different locations far from the atomizer while maintaining the pressure differentials across the atomizer at 300 and 900 kPa. For the conditions studied, the dispersion features are greatly influenced by the ambient conditions. At higher ambient gas pressures, GTL jet fuel exhibited higher droplet concentrations than conventional fuel, and smaller droplet and Sauter mean diameters. This difference in droplet characteristics can positively influence the energy release patterns of GTL jet fuel in a combustion environment.

Direct synthesis of oxalic acid via oxidative CO coupling mediated by a dinuclear hydroxycarbonylcobalt(III) complex

Oxidative coupling of CO is a straightforward and economic benign synthetic route for value-added α-diketone moiety containing C2 or higher carbon compounds in both laboratory and industry, but is still undeveloped to date. In this work, a rare coplanar dinuclear hydroxycarbonylcobalt(III) complex, bearing a Schiff-base macrocyclic equatorial ligand and a μ-κ1(O):κ1(O’)-acetate bridging axial ligand, is synthesized and characterized. The Co(III)-COOH bonds in this complex can be feasibly photocleaved, leading to the formation of oxalic acid. Moreover, the light-promoted catalytic direct production of oxalic acid from CO and H2O using O2 as the oxidant with good selectivity (> 95%) and atom economy at ambient temperature and gas pressure based on this dicobalt(III) complex have been achieved, with a turnover number of 38.5. The 13C-labelling and 18O-labelling experiments confirm that CO and H2O act as the sources of the -COOH groups in the dinuclear hydroxycarbonylcobalt(III) complex and the oxalic acid product.

Microphone signal specialities in laser powder bed fusion: single-track scan and multi-track scan

Acoustic emission is a promising candidate among online process monitoring methods in laser powder bed fusion. Although previous studies have demonstrated the feasibility for acoustic-based monitoring, an underlying understanding of the acoustic specialities related to laser–powder interactions in the enclosed chamber is scarce. This research employed a microphone to collect the acoustic emission signal during laser printing processes. Two contents are contained in the raw data: static pressure variation and acoustic signal. The features of each content are investigated and it follows several significant findings. First, more violent fluctuations of ambient gas static pressure in the chamber are due to the high temperature gradient of ambient gas and recoil pressure over the laser-material interaction zone, which typically occurs in a very low frequency band (<22.4 Hz). Second, spectral analysis for the acoustic signals has indicated that standing wave patterns are commonly occurrences in the chamber. The fundamental frequency of the relevant acoustic emission strongly depends on the number of melt tracks scanned per unit time. Additionally, with investigations of single-track scan and multi-track scan, it is certain that more powder particles participated in laser melting can result in larger acoustic emission energy in a relative high frequency range (>10 kHz). For powder bed additive manufacturing, these results offer a guidance for signal processing and machine learning to dig out the signal signatures of a specific defect in terms of acoustic monitoring.

Understanding the effect of ambient gas pressure on the nanoparticle formation in electrically exploding wires

In this paper, a computational model characterizing the preparation of metallic nanoparticles by electrically exploding wires from the onset of current flowing through the wire to the final moment of nanoparticle formation in a gaseous environment is constructed. The computational model consists of a 1D magnetohydrodynamic model, a simplified magnetohydrodynamic model with two-temperature approximation, and a set of general dynamic equations based on the nodal approach, corresponding to the phase transition stage, plasma evolution stage, and nanoparticle growth stage, respectively. The numerical investigation on the formation of nanoparticles is performed with “cold-start” conditions. The computational predictions for the dependence of nanoparticle size on proportion under argon gas pressure of 10 kPa demonstrate that the nanoparticles of 21 nm in diameter account for the maximum proportion of 4.3%. It coincides with the experimental measurements for nanoparticles of 19 nm in diameter with the maximum proportion of 3.5%. The computational model is employed to reveal the influence of ambient gas pressures on the process of nanoparticle formation. The variation trends for parameters of exploding products, cooling rate, and nanoparticle diameter with the largest proportion on ambient gas pressures are discussed. The size distribution of nanoparticles under different argon gas pressures matches relatively well with relevant experimental data. This computational model bridges the gap between the electrically exploding wires and the growth of nanoparticles, providing theoretical support for the regulation and control technology in nanoparticle synthesization by electrically exploding wires.

Assessment of an E10 gasoline surrogate: Qualitative and quantitative comparisons of in-cylinder spray morphology

A minimum-component gasoline fuel surrogate that captures both chemical and physical behaviors of a full-distillate fuel is needed for high-fidelity CFD simulations. This study evaluates gasoline spray characteristics in a direct-injection spark-ignition engine under motored operation. Two fuels are compared; PACE-20, which is a 9-component surrogate formulation of RD5-87, is compared with its target fuel RD5-87, which is a full-boiling range research grade E10 gasoline. The spray morphologies of both fuels are recorded for a centrally-located direct-injection 8-hole spray subject to intake air cross-flow during the early part of the intake stroke. High-speed imaging recorded scattered light of the side and axial projections of the liquid spray. Quantitative metrics were developed and employed to facilitate comparison of spray morphologies as well as to identify the transition in spray morphology due to flash boiling. This paper builds on a previous study of RD5-87 where coolant temperature (20°C–100°C), in-cylinder pressure (40–110 kPa), engine speed (650–1950 rpm), and injection pressure (60–180 bar) were systematically changed to span operating conditions with and without flash boiling. Images of the PACE-20 morphology are selected for a sub-set of operating conditions from the previous study where distinctive morphology changes occurred. Visual inspection of the images and quantitative metrics demonstrate that the PACE-20 spray morphology is equivalent to that of the RD5-87 in most cases. The exception was for changes in the ambient-gas pressure where the flash-boiling transition occurred at ∼5 kPa higher in-cylinder pressure for PACE-20. Three empirical metrics, Merging Index, Asymmetry, and Flash Index are proposed here and they were found to be useful both as quantitative comparisons of the fuel morphologies, and for identifying the transition in spray morphology due to flash boiling.

Comparative Study on Structural and Optical Properties of Se85Te6Bi9 Nano-Thin Films Synthesized at Disparate Ambient Argon Pressures

In this research work, we have synthesized amorphous Se85Te6Bi9 Chalcogenide Glasses (ChGs) using Melt Quenching Technique. The glassy nature of the synthesized specimen was confirmed by the DSC thermogram plotted at the heating rate of 20 K/min. The nano-thin films of such synthesized samples at two disparate working pressures (1 torr and 3 torr) of ambient argon gas were made using the Physical Vapour Condensation Technique on the ultrasonically cleaned glass substrates. During the synthesis process of nano-thin films substrate temperature (77 K) was kept constant using liquid nitrogen and the thickness of prepared thin films was 40 nm. The morphological analysis of the prepared nanochalcogenide thin films using FESEM confirmed the nanochalcogenide particle size ranges from 20–80 nm. More aggregation and reduction in particle size on the substrates were observed with an increase in working pressure for Se85Te6Bi9 nano-thin films. HRXRD pattern confirmed the amorphous nature of the synthesized nano-thin films. Based on UV-Visible spectroscopy, the optical parameters such as optical absorption coefficients, optical direct band gaps and extinction coefficients were measured for synthesized Se85Te6Bi9 nanochalcogenide thin films. The value of absorption coefficients and extinction coefficients increases with the increase of ambient argon pressure, whereas the value of the optical direct band gap increases with increasing working pressure due to the quantum size effect and dominance of coulombic interactions. The estimated value of optical direct band gaps are 1.37 eV and 1.47 eV at 1 Torr and 3 Torr respectively making our synthesized nano-thin films a preeminent candidate for solar cell applications and other photonic devices.

Gas Storage Model of Residual Coal Adsorption in Abandoned Mine

Due to the insufficient accurate evaluation model of gas resources in abandoned mines, the development and utilization of abandoned mine gas resources in China are still in the preliminary exploration stage. To this end, in response to the problem of imprecise assessment of gas resource reserves in the adsorbed state of residual coal, the No. 3 coal seam in Jincheng mine was used as the research object, by analyzing the evolution of the ambient gas pressure and temperature of the residual coal in the abandoned mine, and simulating the pressure and temperature environment in which the residual coal is located, the gas adsorption experiment of the residual coal was carried out in this environment, and a gas storage model of the residual coal was constructed with the combination of the adsorption theory. The results show that pressure has a positive effect on gas adsorption by residual coal, while temperature has a negative effect on it. The adsorption potential gradually decreases with increasing pressure and increases with increasing temperature, while the adsorption space increases with increasing pressure and decreases with increasing temperature. The adsorption characteristic curves obtained by the two methods are temperature independent and polynomial in shape, and the fitted correlation coefficient of method 1 is generally higher than that of method 2. The adsorption characteristic curves of two methods are best fitted at [Formula: see text], so the value of [Formula: see text] is most appropriately taken as 2.5 when calculating the virtual saturation vapour pressure. The average relative errors between the predicted and measured values of method 1 and 2 were 2.98% and 7.13%, respectively. The prediction effect of method 1 was significantly better than that of method 2 and met the requirements of engineering applications; therefore, method 1 was used to establish a model for the adsorbed gas storage capacity of residual coal in abandoned mines.