NATURAL CONVECTION FROM DISCRETE HEAT SOURCES IN A VERTICALLY VENTED RECTANGULAR ENCLOSURE

Natural convection in a discretely heated, vertically vented enclosure has been investigated experimentally. A vertically vented enclosure is one whose top and bottom boundaries are partially open, allowing ambient fluid to be drawn in by buoyancy. Mach-Zehnder interferometry was used to visualize the temperature field within the enclosure and to determine the local and average heat transfer characteristics of the discrete heat sources. A smoke-generation technique was used to visualize the flow structure. The experimental parameters investigated include Grashof number, vent gap width, and heater locations for a dual heater configuration and a single-enclosure aspect ratio. The vent gap width was varied between a fully open condition (discretely healed parallel plates) and nearly closed. The Mach-Zehnder interferograms and local heat transfer results suggest a complex buoyancy-driven flow field that depends intimately on the rent gap width. For some geometric configurations, the heat transfer coefficient was found to be nearly uniform across the length of the heater, while for others traditional boundary-layer-type heat transfer characteristics were found. In general, the heat transfer coefficient increased with increased vent gap spacing. However, a maximum in average Nusselt number was observed for a dimensionless gap spacing of G/W – 0.67 for one of the two dual-heater placement configurations studied. The heat transfer characteristics were compared to those of a single isolated plate and unobstructed parallel-plate channels.     

Non-contact acoustic tests based on nanosecond laser ablation: Generation of a pulse sound source with a small amplitude

A method to generate a pulse sound source for acoustic tests based on nanosecond laser ablation with a plasma plume is discussed. Irradiating a solid surface with a laser beam expands a high-temperature plasma plume composed of free electrons, ionized atoms, etc. at a high velocity throughout ambient air. The shockwave generated by the plasma plume becomes the pulse sound source. A laser ablation sound source has two features. Because laser ablation is induced when the laser fluence reaches 10¹²–10¹⁴ W/m², which is less than that for laser-induced breakdown (10¹⁵ W/m²), laser ablation can generate a lower sound pressure, and the sound source has a hemispherical radiation pattern on the surface where laser ablation is generated. Additionally, another feature is that laser-induced breakdown sound sources can fluctuate, whereas laser ablation sound sources do not because laser ablation is produced at a laser beam–irradiation point. We validate this laser ablation method for acoustic tests by comparing the measured and theoretical resonant frequencies of an impedance tube.     

The study of geometric effects on the explosion front propagation in a horizontal channel with a layer of spherical beads

Experiments were performed in a horizontal channel partially filled with a layer of 12.7 mm ceramic-oxide beads filled with a nitrogen-diluted stoichiometric methane–oxygen mixture, i.e., CH₄ + 2(O₂ + 2/3N₂). Ionization probes and pressure transducers were used to track the explosion front velocity in the 1.22 m long, 76 mm wide and 152 mm high horizontal channel. Schlieren photography and smoked foil techniques are used to gain insight into the explosion front structure. The explosion propagation phenomenon was characterized by the combustion in the bead layer and the unobstructed gap above. It was determined that for a fixed gap height the bead layer thickness had very little effect on the explosion propagation phenomenon. However, for a fixed bead layer height the explosion propagation was strongly influenced by the gap height. The combustion products vented from the bead layer behind the flame propagating in the gap affects the structure of the shock-flame front in the gap and the maximum flame velocity achieved. The coupling between the vented products and the flame velocity in the gap was strongly influenced by the gap height. The gap height also affects the structure of the detonation wave propagating in the gap following DDT that always occurred in the gap. The DDT run-up distance was found to increase with increasing gap height and inversely with initial pressure.     

Effect of ambient air pressure on synthesis of copper and copper oxide nanoparticles by wire explosion process

Nanoparticles of copper and copper oxide were synthesized by wire explosion technique in the environment of different ambient air pressure. Copper wire with a diameter of 125 μm and 6.1 cm in length has been exploded in air at 1 bar, 500 mbar, 100 mbar and 50 mbar. Current density in the order of 10⁶ A cm⁻² has been applied to disintegrate the wire by discharging a 1.85 μF capacitor at 10 kV. It is found that the particle size distribution for nano-sized particles follows the log-normal distribution and the median diameter is found to decrease from 31.3 nm to 23.6 nm as the pressure is decreased from 1 bar to 50 mbar.     

Natural Convection from a Corrugated Heated Surface at the Bottom of Vented Rectangular Enclosure

Natural convection heat transfer characteristics enclosures from a tilted rectangular enclosure heated at the corrugated bottom wall and vented by uniform slots opening at the top wall are experimentally investigated. The experiments were carried out to study the effects of the angle of opening of the corrugated surface, opening ratio, enclosure's tilt angle, and Rayleigh number on the passive cooling of the enclosure. The experiments were carried out at Rayleigh numbers ranging from 2 × 10⁸ to 1.52 × 10⁹ for enclosure tilt angles ranging from 0° to 90° and angle of opening of corrugated surface ranging from 0° to 25°. The top venting arrangement was studied at different opening ratios of 1, 0.75, 0.5, and 0.25. The results gave an optimum angle of opening of the corrugated surface at which Nusselt number is maximum.     

Energetic ion generation by Coulomb explosion in cluster gas and a low-density plastic foam with an intense femtosecond laser

The energy distributions of protons emitted from the Coulomb explosion of hydrogen clusters by an intense femtosecond laser have been experimentally obtained. Ten thousand hydrogen clusters were exploded, emitting 8.1-keV protons under laser irradiation of intensity 6 × 10¹⁶W/cm². The energy distributions are interpreted well by a spherical uniform cluster analytical model. The maximum energy of the emitted protons can be characterized by cluster size and laser intensity. The laser intensity scale for the maximum proton energy, given by a spherical cluster Coulomb explosion model, is in fairly good agreement with the experimental results obtained at a laser intensity of 10¹⁶–10¹⁷ W/cm² and also when extrapolated with the results of three-dimensional particle simulations at 10²⁰–10²¹ W/cm². Energetic proton generation in low-density plastic (C₅H₁₀) foam by intense femtosecond laser pulse irradiation has been studied experimentally and numerically. Plastic foam was successfully produced by a sol-gel method, achieving an average density of 10 mg/cm³. The foam target was irradiated by 100-fs pulses of a laser with intensity 1 × 10¹⁸ W/cm². A plateau structure extending up to 200 keV was observed in the energy distribution of protons generated from the foam target, with the plateau shape explained well by Coulomb explosion of lamella in the foam. The laser-foam interaction and ion generation were studied qualitatively by two-dimensional particle-in-cell simulations, which indicated that energetic protons are mainly generated by the Coulomb explosion. From the results, the efficiency of energetic ion generation in a low-density foam target by Coulomb explosion is expected to be higher than in a gas-cluster target.     

超长建筑物爆炸泄压特性的试验研究

本文报导了在长径比等于7的7m3长方体试验装置内用液化石油气作的大量试验．定量地给出了液化石油气浓度、点火位置、点火源能量、泄压口封盖物和可燃气局部分布等因素对泄放爆炸压力的影响规律。     

Air Flow in Aluminum Foam: Heat Transfer and Pressure Drops Measurements

Abstract

Metal foams are cellular structure materials that present open cells, randomly oriented and mostly homogeneous in size and shape. Cellular structure materials, and particularly open-cell metal foams, have been proposed as possible substitutes for traditional finned surfaces in electronics cooling applications. This article presents the heat transfer and pressure drops measurements obtained during air flow through an aluminum foam, which has 40 pores per inch with 0.63 mm pore diameter. The specimen has been inserted in a new open-circuit type wind channel with a rectangular cross-section that has recently been built at the Department of Fisica Tecnica of the University of Padova. The experimental heat transfer coefficients and pressure drops have been collected by varying the air flow rate supplied by the screw compressor that provides a variable volumetric air flow ranging between 0–90 m³h^?1 at a constant gauge pressure of 7 bar. The specific heat flux has been simulated by powering with a 25-kWm^?2 copper heater attached at the bottom of the aluminum foam base plate. The experimental results are reported in terms of heat transfer coefficients, mean normalized wall temperatures, and pressure drops.     

Neural network prediction of cycle-to-cycle power variability in a spark-ignited internal combustion engine

Cycle-to-cycle variation (CCV) limits how lean a spark-ignited (SI) internal combustion engine (ICE) can stably operate at, restricts efficiency, and increases emissions through incomplete combustion. Therefore, a way to cleaner, more efficient SI ICEs is to minimize the CCV. Current methods to study CCV include experimental investigations and CFD-based numerical simulations. This study, in contrast, investigates the ability of neural networks to accurately model the indicated mean effective pressure (IMEP) and its coefficient of variation (COV of IMEP). Experimental data from a previous study of spark-ignited propane/air combustion in the TCC-III engine was used to train and evaluate a neural network. An optimized network was generated that utilizes 109 experimental inputs and is operated with 15 neurons in one hidden layer to determine IMEP for 18 engine operating conditions, with 625 individual consecutive engine cycles for each condition. The impact of training set size and the number of input parameters was also investigated. The average deviation for IMEP from the experimental measurements is 0.7–2.2% for the training data set and less than 12% for the entire predicted range of operating conditions. Data sets consisted of tests under rich, lean, and stoichiometric conditions without and with 9% nitrogen dilution. Predicted COV of IMEP strongly correlates with experimental data (R²?=?0.8453). However, a systematic over prediction of COV of IMEP for low COVs was observed while higher COVs were under-predicted by the neural network. The cause for this systematic behavior has not yet been identified but histograms of the predicted IMEP data indicate that this could be related to missing physical parameters that have a significant impact on combustion variability.     

Conduction transients in single-crystal octadecane

Charge transients were measured in high-quality single-crystal octadecane after electron beam irradiation. The signals produced were of significantly greater duration than the ionization pulse but were not interpretable by conventional uniform field time-of-flight analysis. By assuming that the specimens contained a strong inbuilt polarization field, it was possible to predict the properties of the observed charge transients and obtain values for the positive and negative drift mobilities. The results indicated a positive species with a mobility of 0.7 × 10^?4 m² V^?1 s^?1 and a negative carrier value of 1.6 × 10^?4 m² V^?1 s^?1. The standard deviations observed in the data were large, being 0.3 × 10^?4 m² V^?1 s^?1 and 0.8 × 10^?4 m² V^?1 s^?1, respectively. This scatter was significantly in excess of the estimated experimental error.     

强点火作用下C3HF7对甲烷-空气爆炸的抑制

为解决瓦斯输送过程中的爆炸安全问题,探索寻找绿色环保且阻火性能优越的新型抑爆剂,开展了当量比下甲烷-空气预混气体爆炸传播过程中的七氟丙烷抑爆效果研究。实验采用长径比L/D=108的水平管道爆炸特性测试系统,研究了在强点火作用下不同体积分数的七氟丙烷对9.5%甲烷-空气预混气体最大爆炸压力、最大压力上升速率和火焰传播速度的影响。实验结果显示:将2.5 m长的管段作为七氟丙烷抑爆区时,七氟丙烷阻断9.5%甲烷-空气预混气体爆炸火焰传播的最小体积分数为5%;当七氟丙烷的体积分数为1%~4%时,不仅无法阻断爆炸火焰的传播,而且与对照组相比,会使火焰传播速度加快;当七氟丙烷的体积分数为1%~6%时,爆炸源及管道末端处的爆炸压力峰值随着七氟丙烷体积分数的增加而逐渐减小;当七氟丙烷的体积分数为3%时,抑爆区处的爆炸压力峰值与对照组相比增幅为10.9%。     

Stochastic effects on the isothermal explosion arising in the oxidation of iron (II) by nitric acid

The oxidation of ferrous ion by nitric acid provides us with an example of an isothermal chemical explosion. We propose a chemical scheme with only two independent variables capable of explaining the explosive behavior, and examine the behavior of the fluctuations during the explosion period. The analysis is based on the numerical integration of the time evolution equations corresponding to the first and the second moments of the probability distribution of the number of particles of Fe²⁺ and HNO₂. During the time interval of explosion the fluctuations present a peak whose maximum value increases as the volume of the system decreases. Comparison with the results of deterministic analysis is performed.     

Characteristics of the underwater explosion of a nonideally detonating aluminum-rich energetic material

An experimental study of the characteristics of the explosion of mixtures of ammonium perchlorate, aluminum, and nitromethane with a large excess of aluminum (1.45 to 1.66 g/cm³ in density) confined in plastic enclosures and immersed in small elastic-wall reservoirs with water is conducted. It is shown that composite charges, 20 mm in diameter, surrounded by a water layer of thickness 20–30 cm and detonate in a nonideal detonation mode. High-speed cinematography records show the possibility of the intense mixing of the detonation products with the surrounding water and of the burning of excess aluminum particles in a heterogeneous cloud. The time scales of the development of secondary energy release by burning of aluminum particles in water are estimated. The possibility of controlling the characteristics of the pressure waves generated by the explosion, for example, by means of a preliminary bubbling of the water with air near the charge, is demonstrated.     

N2Broadening in theCO 2–0 Band around 4167 cm

N₂broadening coefficients have been measured for 65 lines of the¹³C¹⁶O 2–0 band using a Fourier transform spectrometer. These lines are located in the spectral range 4011–4252 cm⁻¹. The spectra were recorded with 99% isotopically pure¹³CO in a White-type cell at a resolution of 0.005 cm⁻¹. Voigt profiles convolved with the FTS apparatus function were fitted to the experimental lineshapes using a nonlinear least-squares fit technique. From the fits the Lorentzian HWHM was determined as function of N₂pressure. Pressure broadening coefficients formbetween −33 and +34 were obtained with uncertainties of 5.8%. The results are compared to earlier published N₂broadening coefficients and our measurements in the 2–0 band of¹²C¹⁶O. To our knowledge this is the first investigation of¹³CO pressure broadening.     

In-water gas combustion for thrust production

The paper presents the results of experimental study for hydrodynamic processes occurring during combustion of a stoichiometric mixture propane-oxygen in combustion chambers with different configurations and submerged into water. The pulses of force acting upon a thrust wall were measured for different geometries: cylindrical, conic, hemispherical, including the case of gas combustion near a flat thrust wall. After a single charge of stoichiometric mixture propane-oxygen is burnt near the thrust wall, the process of cyclic generation of force pulses develops. The first pulse is generated due to pressure growth during gas combustion, and the following pulses are the result of hydrodynamic pulsations of the gaseous cavity. Experiments demonstrated that efficient generation of thrust occurs if all bubble pulsations are used during combustion of a single gas combustion. In the series of experiments, the specific impulse on the thrust wall was in the range 10⁴–10⁵ s (10⁵–10⁶ m/s) with account for positive and negative components of impulse.     

Wettability of ZnO: A comparison of reactively sputtered; thermally oxidized and vacuum annealed coatings

We have studied the wettability of sputter deposited ZnO, thermally oxidized Zn-ZnO and vacuum annealed ZnO coatings. The X-ray diffraction patterns showed the formation of hexagonal-wurtzite structure of ZnO, which was further confirmed by micro-Raman spectroscopy data. The X-ray photoelectron spectroscopy data indicated that the sputter deposited ZnO coatings were more stoichiometric than thermally oxidized Zn-ZnO and vacuum annealed ZnO coatings. The wettability measurements indicated that water contact angles of 158.5° and 155.2° with sliding angles of 2° and 4° were achieved for thermally oxidized Zn-ZnO and vacuum annealed ZnO coatings, respectively. The superhydrophobicity observed in thermally oxidized Zn-ZnO and vacuum annealed coatings is attributed to the nanorod cluster like morphology along with the presence of high fraction of micron scale air pockets. The water droplet on such surfaces is mostly in contact with air pockets rather than solid surface, leading to high contact angle. Whereas, the sputter deposited ZnO coatings exhibited a maximum water contact angle of 110.3°. This is because the sputter deposited ZnO coatings exhibited a densely packed nanograin-like microstructure without any air pockets. The work of adhesion of water was very low for thermally oxidized Zn-ZnO (5.06 mJ/m²) and vacuum annealed ZnO coatings (6.71 mJ/m²) when compared to reactively sputtered ZnO coatings (90.41 mJ/m²). The apparent surface free energy (SFE) for these coatings was calculated using Neumann method and the SFE values for sputter deposited ZnO, thermally oxidized Zn-ZnO and vacuum annealed ZnO coatings were 32.95, 23.21 and 18.78 mJ/m², respectively.     

Optical probing of laser-induced shock waves in air

Shock Waves (SW) were produced in air by focusing the (0.25 J, 6 ns) second-harmonic ( = 532 nm) Nd : YAG laser light into a stainless-steel cylindrical cell at a pressure from 200 to 760 Torr. The laser fluence at the focal point is > 5 GW/cm². The spatial variation and consequently the time evolution of the radial propagation velocityU of the generated shock waves were measured via a simple optical system utilizing a HeNe laser beam triply intersecting the propagating shock wave at three successive positions. Using a reflector, we were able to probe the traveling SW in six consecutive positions during its round trip. Good agreement was obtained between the experimental results and the predictions of the point strong explosion theory. It is shown that this method is simple with a fairly good precision. It therefore appears to be useful for the determination of the SW dynamic parameters, namely its Mach number, the pressure at the SW front, the thickness of the compressed air layer and the energy consumed in producing this layer.     

Effects of phase explosion in pulsed laser deposition of nickel thin film and sub-micron droplets

Nickel (Ni) thin films were deposited on glass substrates in high vacuum and at room temperature with third-harmonic or 355-nm output from a nanosecond Nd:YAG laser. At low laser fluence of 1 J/cm², the deposition rate was about 0.0016 nm/shot which increased linearly until 4 J/cm². Above 4 J/cm², the onset of phase explosion in the ablation abruptly increased the optical emission intensity from laser-produced Ni plume as well as thin-film deposition rate by about 6×. The phase explosion also shifted the size distribution and number density of Ni droplets on its thin-film surface. On the other hand, the surface structures of the ablated Ni targets were compared between the scan-mode and the fixed-mode ablations, which may suggest that droplets observed on the thin-film surface were caused by direct laser-induced splashing of molten Ni rather than vapour-to-cluster condensation during the plume propagation.     

Development of gas-dynamic perturbations propagating from a distributed sliding surface discharge

Results are presented from experimental studies of the plasma layer structure of a distributed sliding surface discharge excited in quiescent air and in a uniform gas flow behind a plane shock wave at gas densities of 0.03–0.30 kg/m³. The dynamics of weak shock waves generated after discharge initiation was studied. According to the experimental and simulation results, 40% of the discharge energy transforms into heat within a surface gas layer in the energy input stage, which lasts up to 200 ns.     

Dense-phase pneumatically conveyed coal particle velocity measurement using electrostatic probes

The fundamental measurement theory of an electrostatic probe and cross-correlation velocity measurement method are introduced in this paper. The effects of the probe's geometric parameters, including the length of the probe; the thickness, length and relative permittivity of the dielectric pipe; the radius of the screen on the dimensionless calibration coefficient (k), and the statistical error of the transit time (τ_m) of the correlation velocity measurement system using electrostatic probes were investigated theoretically. Finally the measurement system was applied in a 10 mm bore horizontal section of a dense-phase pneumatic conveying system under high pressure circulating pulverized coal over a superficial air velocity range of 0.5–7 m/s for a particle concentration 0.052–0.141 m³/m³. The experimental results that were obtained demonstrate that the system is capable of providing solid particle velocity measurements with repeatability better than 10% under the given experimental conditions.