High Power Density Conditions Research Articles

Experimental and numerical studies of the evaporation and combustion characteristics of large-angle impinging sprays

To achieve a large fuel injection mass per cycle, using one injector will need a large nozzle, which may lead to combustion deterioration in high power density (HPD) engines. Therefore, two-injector method was adopted and then impinging sprays by two injectors were developed to promote the air/fuel mixing and combustion performance. This study investigated the spray, evaporation, ignition and soot emission characteristics of twin injectors at different impinging angles (90°, 120°, 150°, and 180°) using various high-speed imaging techniques in a constant volume chamber. Further, CFD simulations were performed to analyze the impinging sprays under HPD condition. Innovatively, a kinetic energy loss model for impinging sprays was proposed to quantify the effect of spray impingement. The results showed that the turbulent kinetic energy increased with the impinging angle, and the kinetic energy loss at any angle could be predicted by the kinetic energy loss of 180° impinging spray. The kinetic energy loss model better characterized the spray impingement process, and then more accurately predicted the atomization quality and mixing capacity compared with spray volume. Spray impingement changed fuel and soot distributions, the peak soot mass decreased with the impinging angle, which was reduced by 36.8 %, 39.8 %, 44.4 % and 45.9 % at 90 ∼ 180° impinging sprays, respectively, comparing to the single spray. These findings recommended that using a large impinging spray angle in real HPD engines can promote spray atomization and reduce soot formation.

Designing hollow mesoporous carbon sphere for high-rate supercapacitor in water-in-salt electrolyte

The expansion of the operational voltage in supercapacitors (SCs) through the utilization of water-in-salt electrolytes (WISEs) has garnered considerable attention for achieving elevated energy density. However, the poor rate performance of assembled SCs persists due to the heightened viscosity of WISEs (e.g., 17.0 mol kg−1 NaClO4), so designing the porous structure of electrode materials to improve the rate performance of SC in WISEs has become a research focus. In this study, we present a straightforward approach to fabricate hierarchical porous carbon spheres, designing to expedite ion infilling and transport within WISE-based SCs. The hollow porous carbon is synthesized via one-step pyrolysis process, wherein the decomposition of inner colloid particles yields gases, while the outer resorcinol–formaldehyde resin carbonizes to form a spherical shell. In the WISE of 17.0 mol kg−1 NaClO4, the resulting symmetric SCs exhibit an operational voltage of 2.6 V and significantly improved rate capability with a capacitance retention of 47.7 % at 100 A g−1. Notably, the energy density can reach 31.0 Wh kg−1 at a power density of 2.4 kW kg−1. Even under high power density conditions of 120.0 kW kg−1, the energy density of 14.7 Wh kg−1 can remain. Additionally, the SC device manifests an impressive capacitance retention of 99.1 % after 20,000 cycles, showcasing exceptional cyclic stability. This research introduces a novel synthesis strategy for hierarchical porous carbon materials, enabling the development of high-rate SCs with both elevated energy density and power density.

Reactive Force-Field Molecular Dynamics Study of SiGe Thin Film Growth in Plasma Enhanced Chemical Vapor Deposition Processes

SiGe thin films, such as hydrogenated amorphous silicon germanium (a-SiGe:H) and hydrogenated microcrystalline silicon germanium (μc-SiGe:H), are formed by plasma enhanced chemical vapor deposition (PECVD) methods from SiH4, GeH4, and H2. Thin film properties such as surface morphology, hydrogen content, and crystal structure are greatly affected by process parameters such as a substrate temperature, gas pressure, and gas composition. For example, SiH3 and GeH3 are dominant species when the SiGe thin film with low defect is deposited under low power density conditions. However, the structural defects are formed by SiH and SiH2 that are highly reactive species under high power density conditions. Although the density and flux of these highly reactive species are much lower than those of the dominant species, the electrical properties of the device are greatly affected by a few structural defects. Therefore, to understand the influence of the composition of each precursor on the thin film properties is important. The purpose of the present study is to clarify the influence of the composition of multiple precursors such as SiH3, GeH3, and H under different substrate temperatures on the surface morphology, hydrogen content, and crystal structure.We performed reactive force-field molecular dynamics (ReaxFF MD) simulations. An existing parameter set was modified to match the dissociation energies in gaseous species because bond formation and breaking are particularly important in this study. The substrate was Si (100)– (2×1), and the size of the simulation box was 30.72 Å× 26.88 Å× 45.00 Å. A periodic boundary condition was applied in the x and y directions, and a fixed boundary condition was applied in the z direction. The first bottom layer of the substrate was fixed to prevent vibrations of the substrate up and down. The second and third layers were controlled constantly at desired temperature by the Berendsen thermostat. The temperature was not controlled on the fourth and subsequent layers to prevent direct influences of changing the velocity of atom due to the temperature control on the surface reaction. The substrate temperature was set at 800 K, 1000 K, 1300 K, and 1600 K. 2000 SiH3 molecules with the velocity corresponding to 1300 K in the direction towards the substrate were deposited one by one onto the substrate surface every 7.5 ps. The initial position of the gaseous species was set at 10 Å above the top of substrate Si atoms, and the in-plane (x-y) position were set randomly. The Velocity-Verlet algorithm was used for time integrations corresponding to 0.25 fs time step. A pre-thermal annealing for 7.5 ps and a post-thermal annealing for 60 ps were performed before and after the deposition to fully equilibrate the substrate and deposited thin films.Figure shows front views of the final configurations and results of analysis when the substrate temperature is 800 K, 1000 K, 1300 K, and 1600 K. The yellow, blue, and red represent substrate Si atoms, added Si atoms, and added H atoms, respectively. Gray shown in the figure are gas-solid surfaces. The surface construction algorithm was used to calculate a surface area that is closely related to the surface morphology. The hydrogen content was calculated using the select type algorithm. The structure identification algorithm for the crystal structure was used to characterize the structure of simulated films. These analyses were carried out by the Open visualization tool (OVITO). The surface area decreases as the substrate temperature increases, indicating that the formation of smoother surface at high temperature. The diffusivity of a SiH3 molecule that reached to the surface increases as the substrate temperature increases, and the atomic scale valleys are filled. This tendency is in good agreement with relevant studies. The hydrogen content was calculated by dividing the number of added H atoms by the total number of atoms existing in the system. The hydrogen content decreases as the substrate temperature increases because a hydrogen desorption is promoted as the substrate temperature increases. Figure 1

Study of microwave discharge at high power density conditions in diamond chemical vapor deposition reactor by optical emission spectroscopy

The paper presents results of measurements of spatial distributions of emission intensity of various lines of argon, atomic hydrogen, C2 and CH molecules in a wide pressure range from 40 to 500 Torr in diamond chemical vapor deposition (CVD) reactor. Microwave discharge plasma was maintained in reactor in continuous mode both in hydrogen and in a mixture of hydrogen-methane with a small addition of argon for diagnostic purposes. Intensity distribution of argon emission was characterized by a pronounced maximum near the substrate in reactor, which indicates that microwave power density (MWPD) absorbed in the plasma is also concentrated mainly near the substrate. Maximum emission intensity of hydrogen atoms is located at some distance from the substrate, and maxima of spatial distributions of emission intensity of carbon-containing species C2 and CH are shifted farther away from the substrate. This leads to the fact that the shape and size of discharge, which are observed visually or with help of a camera, significantly differ from the distribution of MWPD. There is a difference in the shape of spatial distribution of argon lines emission (750.4 nm and 811.5 nm), that can be explained by a change in the mechanism for quenching the excited levels of argon in different regions of discharge. This imposes some restrictions on the use of argon for measuring characteristics of microwave discharge.

Understanding strong knocking mechanism through high-strength optical rapid compression machines

Strong knocking combustion has become the greatest challenge for advanced internal combustion engines to pursue thermal efficiency limits at high power density conditions. Arising from enclosed space and extreme combustion situations, the fundamental mechanism for strong knocking combustion has still not been fully understood. In this study, synchronization measurement was performed through simultaneous pressure acquisition and high-speed direct photography, and knocking experiments were comparatively conducted under spark-ignition (SI) and compression-ignition (CI) conditions in a high-strength optical rapid compression machine (RCM) with flat piston design. Strong knocking phenomena were reproduced through varying initial thermodynamic conditions, and localized autoignition (AI) initiation and reaction wave evolutions were visualized, companied by synchronous pressure and temperature trajectories. The results show that compared with initial temperature, initial pressure and equivalence ratio exhibit greater influence on the variations of knocking severity. The weighting of different contributors can be further quantified by an effective energy density that shows positive but nonlinear correlations with knocking severity. However, the distinctions between CI and SI knocking characteristics at identical effective energy density also reflect the essential role of the interplay between primary flame propagation and end-gas AI progress. Visualized combustion images show that through improving end-gas thermodynamic state and reactivity sensitivity, the primary flame propagation can enhance localized AI initiation and secondary intensive AI evolutions, facilitating combustion mode transitions into developing detonation. The significant influence of primary flame propagation is diminished until ignition delay time becomes sufficiently short. Finally, with estimated thermal heterogeneities in flat-piston RCM configurations, the ignition modes of strong knocking cycles are quantified by a non-dimensional ignition regime diagram, and favorable scaling agreements with strong and mixed ignition regimes are observed.

Influence of power density on high purity 63 mm diameter polycrystalline diamond deposition inside a 2.45 GHz MPCVD reactor

63 mm diameter polycrystalline diamond (PCD) films were synthesized via a microwave plasma chemical vapor deposition (MPCVD) reactor in 99% H2–1% CH4 atmosphere. Two different conditions, i.e. the typical condition (input power of 5 kW and gas pressure of 13 kPa) and the high power density condition (input power of 10 kW and gas pressure of 18 kPa), were employed for diamond depositions. The color changes of the plasma under the two proposed conditions with and without methane were observed by photographs. Likewise, the concentrations of hydrogen atoms and carbon active chemical species in plasma were analyzed by optical emission spectroscopy (OES). The morphologies and purity of the PCD films were investigated by scanning electron microscopy (SEM) and Raman spectroscopy, respectively. Finally, the transmission spectrum of the polished PCD plates was characterized by a UV–Vis–NIR spectrometer. Experimental results showed that both the concentrations of hydrogen atoms and carbon radicals increased obviously, with the boost input power and higher pressure. The films synthesized under the high power density condition displayed higher purity and more uniform thickness. The growth rates in 10 kW and 18 kPa reached ~7.7 µm h−1, approximately 6.5 times as much as that occurred in the typical process. Moreover, the polished plates synthesized under the high power density condition possessed a relatively high optical transmittance (~69%), approaching the theoretical values of approximately 71.4% in IR. These results indicate that the purity and growth rate of big-area PCD films could be simultaneously increased with power density.

Chemical analysis of H2–B2H6 and H2–CH4–B2H6 microwave CVD plasmas used for diamond deposition

This study provides a detailed analysis of the H2/B2H6 chemistry and a first evaluation of H2/CH4/B2H6 plasmas for conditions close to boron doped diamond microwave plasma assisted chemical vapor deposition. A H2/B2H6 chemical scheme composed of 38 reactions has been formulated from the research literature and finalized by an equilibrium approach of the gas composition. 1D thermochemical modeling of the gas phase provides a spatial description of the temperatures and of the mole fractions of the species. A comparison with the mole fractions of atomic boron and diborane measured by optical emission spectroscopy and tunable diode laser absorption spectroscopy in the mid-infrared region, respectively, have been performed on a wide range of operating conditions, i.e. for pressure values of p = 60–300 mbar, power values of P = 0.6–4 kW, diborane admixtures of 30 and 66 ppm and methane admixtures of up to 5% diluted in H2. For the H2/B2H6 plasmas, the agreement between the experiments and the 1D model is satisfactory, especially for high power density conditions. At low power density, the electron processes would appear to be necessary to describe the diborane dissociation. Finally, the effect of methane addition to the H2/B2H6 mixture on the mole fraction of the atomic boron was investigated. At high pressure/power conditions, no significant influence was observed, whereas at low power densities the measured boron atom mole fractions decreased when methane was added.

Determining electron temperature and electron density in moderate pressure H2/CH4 microwave plasma

The electron temperature and electron density are measured in a microwave (MW) plasma-assisted chemical vapour diamond deposition reactor for different experimental conditions by varying the substrate temperature, methane content and MW power density. Optical emission spectroscopy (OES) and MW interferometry are used to probe the discharge generated in a stainless steel resonant cavity excited at a frequency of 2.45 GHz. Changing the substrate temperature from 630 to 900 °C does not show any significant influence on the electron temperature or on the electron density. Increasing the methane content from 0 to 10% does not lead to any modification of the electron temperature or density. However between 10% and 20% CH, a decrease of the electron density is observed which may be attributed to soot particle formation.The electron density increases in the range of (1.2–10) × 1011 cm−3 from moderate power density conditions (50 hPa/1000 W) to high power density conditions (250 hPa/3500 W). OES measurements show that the electron temperature exhibits a flat axial profile in the plasma bulk and ranges from 14 000 K at (25 hPa/600 W) to 10 500 K at (400 hPa/3000 W).

선박 주 추진기관으로 사용가능한 대형 디젤엔진의 배기가스 특성 분석

본 논문은 선박의 주 추진기관으로 사용가능한 육상 운송수단에 탑재되어있는 대형 디젤엔진에서 배출되는 입자상 물질과 질소산화물의 배출특성을 ESC mode에 적용시켜 분석하는데 초점을 두었다. 입자상 물질은 엔진의 고부하 영역에서 다량으로 배출되는 특성을 확인 할 수 있었으며 질소산화물의 경우 배기가스 온도 측정형태와 동일한 경향으로 배출되었다. 매연여과장치를 부착하여 입자상 물질의 배출특성을 분석한 결과 mode별 1/100∼1/1,000배까지 저감되었다. 그러나 급격한 엔진 부하 변동 시 저감효과는 떨어졌으며 고속-고부하 영역에서는 매연여과장치내부의 온도상승으로 일부 재생현상이 발생하여 입자상 물질의 배출 증가 결과를 초래하는 현상이 발생하였다. 시험엔진에서 배출되는 입자상 물질의 입경분포를 분석한 결과 대부분 100㎚ 이하의 크기를 가지는 나노 입자가 배출되었으며 매연여과장치를 부착 및 탈착하여 측정한 결과 배출수준에는 차이가 있지만 입자상 물질의 입경분포는 비슷한 경향으로 분석되었다.

The Aspects of Practical Application of Ultrafine-Grained Titanium Alloys Produced by Severe Plastic Deformation

The mechanical and physical properties of ultrafine-grained titanium alloys produced by severe plastic deformation are considered. It is found that the formation of ultrafine-grained structure in these materials causes a significant enhancement in their mechanical properties at room temperature and in their resistance to hydrogen embrittlement as well as a change in their acoustic properties. Moreover, superplasticity is realized in these materials at less elevated temperatures relative to the respective coarse grained counterparts. It is shown that the above changes in material properties permit optimization of conditions by the production of items from the titanium alloys, e.g. medical implants having the requisite strength and stepped waveguides having long life even in the high power density conditions of an ultrasound system.

Chemical vapor deposition of homoepitaxial diamond films

AbstractThis paper presents a review of state‐of‐the‐art reports of homoepitaxial diamond growth. Crystal growth and characterization methods that are commonly used for homoepitaxial diamond studies are first described briefly. Then, effects of each process parameter on the growth mode of homoepitaxial diamond are discussed. Methane concentration and substrate temperature were two dominant parameters that determine the growth rate and crystalline quality of diamond films, while conditions of diamond substrates including crystal orientation and pretreatments should be optimized for high‐quality diamond deposition. Attempts to increase growth rate of homoepitaxial diamond films while keeping their crystalline quality are discussed in the later section. Increase of microwave power density for plasma‐assisted chemical vapor deposition was found to be effective for the high‐rate growth of diamond. Free‐exciton recombination emission was intensively observed from homoepitaxial films grown at higher growth rate of 3.5 μm/h under the high power density condition. In addition, boron‐doped samples grown under the high‐power conditions showed high Hall mobility of 1620 cm2 V s−1 at room temperature. The plasma power density is thus one of the important parameters of diamond deposition from the viewpoint of electronic device application. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

LASER MICROPROBE MASS SPECTROMETRY: PRINCIPLE AND APPLICATIONS IN BIOLOGY AND MEDICINE

Laser microprobe mass spectrometry (LMMS) is an interesting technique for micro- and surface analysis. It employs local ionization by a focused laser under high power density conditions and subsequent mass analysis of the generated ions. This paper surveys the main LMMS instruments and their operational principles. Sample preparation is discussed in the context of biological materials. The problem of quantification is addressed. Selected examples show the way that precise information on the molecular composition can be deduced from the detected signals. Both inorganic and organic substances can be identified, even without reference spectra, from in-situ analysis with a lateral resolution in the order of 1 to 5μm.