Effect Of Ambient Atmosphere Research Articles

Effects of Ambient Atmosphere on Flame Spread over Solid Fuel. Effects of Ambient Temperature and Limiting Oxygen Concentration for Downward Flame Spread.

The downward flame spread over solid fuel has been studied. Paper sheets are used as samples, and experiments are conduced in a combustion chamber with a vertical duct to obtain uniform ambient atmosphere, keeping the inflow velocity constant. The gas composition and temperature can be changed to examine the effects of ambient atmosphere. The downward flame spread rate has been measured at lower oxygen concentration and preheated temperature (∼150°C). Results show that, flame spread rate is increased with an increase of ambient temperature. As the oxygen concentration is decreased, the flame spread rate is decreased monotonically. Then, the flame can not propagate and the flame extinction occurs at some oxygen concentration. We define this condition as the limiting oxygen concentration for downward flame spread. It can be reduced as the ambient temperate is increased. To discuss these experimental facts, we estimate the net heat flow into the preheat region, which is an important factor to sustain the flame spread. The net heat flow into the preheat region is decreased as the oxygen concentration is smaller. It is interesting to note that this heat flow near the flame extinction is smaller with an increase of the ambient temperature. These experimental data are useful to make clear the controlling mechanism of flame spread.

Experiments on the instability modes of buoyant diffusion flames and effects of ambient atmosphere on the instabilities

Large scale dynamic behavior of buoyant diffusion flames were studied experimentally. It was found that buoyant diffusion flames originating from circular nozzles exhibit two different modes of flame instabilities. The first mode results in a sinuous meandering of the diffusion flame, characteristic of flames originating from small diameter nozzles. This instability originates at some distance downstream of the nozzle exit in the contraction region of the buoyant flame envelope and develops into a sinuous motion of the flame. The second mode is the varicose mode which develops very close to the nozzle exit as axisymmetric perturbations of a contracting flame surface. In this mode, flame oscillations result in the formation of toroidal vortical structures that convect through the flame and cause periodic burn out at the flame top resulting in the observed flame height fluctuations. The average flame heights are found to be typically shorter for these flames. The oscillation frequencies and their scaling for the two modes are also different with the sinuous mode having higher frequencies than the varicose mode. It was also observed that the instability can switch from one mode to the other and the probability of observing the varicose mode appears to increase with increasing Richardson number. Additionally, the feasibility of altering the behavior of buoyant diffusion flames was explored through variation of the oxidizer medium density. It was found that the flame oscillations can be completely suppressed for flames burning in helium rich helium–oxygen mixtures. At lower helium concentrations, the oscillation frequency can be significantly reduced. In order to enhance the buoyancy effect, CO2–O2 mixtures were also studied. However, the density increase and its effects on flame oscillation frequency were found to be small compared to those flames burning in air. These experiments point towards the feasibility of altering buoyant flame behavior under earth gravity and studying the large scale dynamical aspects of buoyant flames without the need of variable gravity environment.

Effect of Ambient Atmosphere on Flame Spread at Microgravity

The effects of atmosphere composition on the rate of opposed-flow flame spread (Sf ) over thin solid fuel beds at microgravity (μg) were measured and compared to earth gravity (lg) results and theoretical predictions. Two modifications to standard atmospheres were considered. First, the effects of sub-flammability-limit concentrations of a gaseous fuel (CO or CH4) were measured and compared to an existing theoretical model that was extended to ug conditions. The agreement between the model and experiment is reasonable considering the simplicity of the model. Notably, both model and experiment show that the effect of added gaseous fuel is greater at μg than lg. Secondly, the effect of diluent type on Sf -was studied by comparing results using He, N2, Ar, CO2 and SF6 diluents. It was found that, in agreement with prior studies in N2 diluent, for He, N2 or Ar diluents, Sf was larger at 1g than μg. In contrast, for CO2 diluent, Sf was slightly lower at 1g than at μg and for SF6 diluent, Sf was much lower at 1g than μg. Moreover, unlike He, N2 and Ar, for CO2 and especially SF6 diluents the minimum O2 concentration required to support flame spread was lower at μg than 1g. For SF6, the minimum O2 concentration at μg was even lower than the upward (concurrent-flow) limit. This unusual behavior is proposed to be a result of reabsorption of radiation emitted from the gases, which is significant only for gases with small mean absorption lengths.

Effects of Atmosphere on Sintering of Alumina

The effect of ambient atmosphere on the sintering of oxides was studied ever since the first experiments on sintering of aluminium oxide spheres were married out Several effects of atmosphere on sintering have been found to be important. These depend on the nature of interaction, whether physical or chemical, of the active species present in the atmosphere with the solid undergoing sintering. Atmosphere control of point defect concentration is touched upon. Atmospheric effects on technical ceramics are briefly reviewed. Sintering behaviour of alumina in gaseous atmospheres is analysed on the basis of such reactions. Microstructure development during atmosphere resintering is discussed.

Atmospheric effects on cratering efficiency

Laboratory experiments permit quantifying the effects of an atmosphere on cratering efficiency by hypervelocity impacts. Three separable processes have been identified: ambient atmospheric pressure, aerodynamic drag, and projectile‐atmosphere interactions. The effect of ambient atmosphere can be described by a dimensionless pressure ratio, P/δv2 for a pressure P, target density δ, and impact velocity. The use of a helium atmosphere minimized potential effects of aerodynamic drag (low density) and projectile‐atmosphere interactions (low mach numbers) and revealed a power‐law exponent of −0.23 for the dimensionless pressure term for impacts into pumice. Similar exponents but different degrees of reduced cratering efficiency are observed for targets composed of loose fine‐grained sand, low‐density microspheres, and mixtures of coarse sand with small amounts (<10%) of fine‐grained particulates. Consequently, cratering efficiency in an atmosphere does not appear to depend on material properties of the target (internal angle of friction) but on atmospheric pressure and constituent grain size. Correcting the data for atmospheric pressure allowed testing for the possible role of aerodynamic drag through the use of higher‐density atmospheres (same pressure) and various targets with contrasting ejecta sizes but the same bulk density. The results reveal that the effect of aerodynamic drag deceleration d can be viewed as a correction to gravity g in the dimensionless gravity‐scaling π2 parameter. The derived power law exponent for drag‐controlled crater growth in an atmosphere closely matched the exponent for gravity‐controlled crater growth in a vacuum. If it is further assumed that a combination of pressure and drag jointly limit crater growth, then the empirical power law exponent reasonably matches values derived from dimensional analysis and a coupling parameter theory. After correcting for pressure and drag, systematic offsets of the data remained and appeared to depend on Mach number. The possible role of the supersonic wake trailing the projectile was tested by performing projectile‐less impacts where only the wake was allowed to interact with the target. The colliding projectile wake was found to affect crater scaling in two ways. At low mach numbers, the trailing wake gases augmented cratering efficiency by creating backpressure in the transient cavity; that is, the effect of the projectile wake was decoupled from the impact and could be viewed as a negative ambient pressure. At high mach numbers, the disturbed wake gas became coupled with the impactor, thereby changing the effective size of the energy source. Both effects could be documented in the data and in observed phenomena during impact. These results not only reconcile previous studies where atmospheric pressure effects were found to be minimal but could have potential implications for interpreting the cratering record on planets with atmospheres.

Effect of oxidising atmosphere on the conductivity and photoconductivity of cofacial metallophthalocyanine polymers

The conducting and photoconducting properties of the polymeric cofacial stacked metallophthalocyanines, poly (fluoroaluminium phthalocyanine)[Al(pc)F] n, poly ((phthalocyaninato) siloxane)[Si(pc)0] n and poly ((phthalocyaninato) stannic oxide) [Sn(pc)0] n have been investigated in high vacuum and atmospheres of NO 2 and O 2. The materials were fabricated as compressed pellets or thin evaporated films. Standard 2 and 4 probe conductivity measurements have been used to study the systems. It has been found that NO 2 and O 2 increase the conductivity of the pellet form according to a diffusion process. This is also true for the effect of O 2 on the thin films. However, NO 2 is shown to follow Elovich adsorption kinetics when doping the films. The effect of ambient atmosphere, light intensity and temperature on the magnitude and kinetics of photoconduction of these polymers is also reported.

Photocurrent kinetics in metal phthalocyanine crystals, films and pellets

The rise and decay kinetics of photoconductivity are reported for copper phthalocyanine [Cu(pc)], lead phthalocyanine [Pb(pc)] and chloro-(phthalocyaninato)-aluminium [Al(pc)Cl] single crystals. These show the rise to the first order (τ < 0.004, 0.68 and 0.36 (platelet), 0.071 (needle) s, respectively). The decay generally fits two consecutive first-order processes, both slower than the rise. The differences in lifetimes are interpreted in terms of trapping at defect lattice sites. The effects of ambient atmosphere and physical form are also reported. Exposure to NO2 or O2 decreases the rise time and increases the decay time. ‘Photocurrent kinetics’ of Pb(pc) pellets are shown to be due to thermal effects, as are those of poly-(phthalocyaninato siloxane). Thin films of these phthalocyanines show faster kinetics than single crystals. These results would not be expected for a polycrystalline structure and are discussed with reference to X-ray diffraction data obtained using synchrotron radiation.

Effect of Ambient Atmosphere on YBa2Cu3O7-y

The reactivity of sintered samples of YBa2Cu3O7-y with moisture at elevated temperatures has recently been reported [1]. The reaction sequence proposed was:Such instability of the 1–2–3 phase has serious implications regarding not only the use of the sintered material, but powder processing as well. While the need for fine particle 1–2–3 powder exhibiting good sinterability is recognized, it is also anticipated that the degradation of this material will accelerate with decreasing particle size because of the larger surface area. Thus, we have conducted a series of experiments to monitor the effect of ambient atmosphere on fine particle 1–2–3 powder. The powder was monitored as a function of time by x-ray diffraction, scanning electron microscopy, sintering characteristics and magnetic flux exclusion.

Effect of Ambient Atmosphere on Solid State Reaction of Kaolin-Salt Mixtures

AbstractThe reaction of kaolin with NaCl was followed by dynamic thermal analysis and mass spectrometry under N2, CO2, and air atmospheres and in a 10−5-torr vacuum. The weight loss was a function of the atmosphere used and, according to mass spectrometry, was due to the evolution of H2O, HCl, and very small amounts of H2. HCl was formed only after the release of 85% of the hydroxyl content of the kaolin. When the clay was pretreated with saturated salt solution, H2O and HCl evolved in more or less the same temperature range, indicating that only some of the OH groups reacted with the chloride ion. High-temperature X-ray powder diffraction patterns showed that the sodium ion reacted with the noncrystalline metakaolin to give NaAlSiO4. Chemical analysis showed that the reaction of kaolinite and sodium chloride started below 400°C. The rate of the reaction increased at higher water vapor concentration. From mass spectrometric data, the NaCl-treated kaolin appeared to adsorb CO2. Desorption at several distinct temperatures suggests that CO2 was adsorbed by different parts of the structure, i.e., holes and channels. X-ray powder diffraction and infrared absorption data indicate that the kaolinite structure persisted even after it had been heated with NaCl in a CO2 atmosphere to as high as 800°C.

Effect of Ambient Atmosphere on Defect Generation in MgO during Mechanical Activation

AbstractAs previous studies have shown (Spitsyn et al.; Henderson, Wertz; Steinike et al. 1981, 1984; Hennig et al.), mechanical activation of MgO in air results in the generation of anion and cation vacancies in the lattice, thus causing heterogeneous lattice distortions. These anion and cation vacancies are identified as F+‐ and V−‐centres, respectively, by EPR spectroscopy. F+‐centres are anion vacancies filled with electrons, V−‐centres are cation vacancies filled with holes. The electrons and defect electrons filling the vacancies are generated essentially during γ‐irradiation. Since the effect of the ambient atmosphere on the generation of electronic and structural defects upon mechanical activation is known (Bystrikov et al.), the studies of mechanical MgO activation in air were extended to the generation of above defects in an argon atmosphere. The results are compared with those obtained in air.

Catalytic effects of transition metals on oil shale pyrolysis

The decomposition of kerogen of Green River formation oil shale, impregnated with transition metals, was investigated following nonisothermal thermogravimetric techniques, in the presence of hydrogen at atmospheric pressure. Experimental data were best fit by a first-order kinetic model with respect to kerogen. The effects of ambient atmosphere and heating rate were also investigated. In the absence of metals, inert and reducing atmospheres were found not to affect kinetic parameters. The frequency factor was found to increase with heating rate at a constant activation energy. The presence of metals in the oil shale matrix and hydrogen in the retorting atmosphere did not alter the kinetic order of kerogen decomposition, but it lowered apparent activation energies and increased the rate constants. These results are discussed in terms of dissociative hydrogen adsorption on the metal crystallites and hydrogen spill over to the organic matter, resulting in hydrogenation and/or cracking reactions.

Effects of Ambient Atmosphere on Aluminum - Copper wirebond Reliability

The effect of ambient atmosphere on the stability of Al-Cu bonds aged at 150 to 300°C was studied. Data show that atmosphere plays a major role in determining the failure mode of wires bonded and aged under otherwise identical conditions. Bonds aged in air, nitrogen, and vacuum all experienced an initial decrease in pull strength due to annealing of the wire. The mechanical strength of vacuum and nitrogen aged samples continued to degrade with time. A shift in the primary failure mode from wire breaks to bond lifts was noted. The pull strengths of air-aged samples remained stable through 1600 h. The dominant failure mode was wire breaks. Al-Cu intermetallic phases were formed under all conditions. Surface diffusion was suggested by the presence of a groove at the bond perimeter. With time, the groove propagated along the metallic Cu/Cu intermetallic interface, undercutting the bond. The formation of copper oxides in air-aged samples retarded this process. An effective activation energy of 0.45 eV was calculated for the Al-Cu failure mechanism in vacuum.

The effect of ambient atmosphere in the annealing of indium-tin oxide films

Isochronal annealing experiments on dc‐sputtered indium tin oxide (ITO) films in inert (N2), reducing (N2/H2) and oxidizing atmospheres were cumulatively performed over the 50→500→50 °C range. Three anneal regimes have been identified. In region I, TA=50→200 °C, crystallization occurs, resulting in a sharp drop in sheet resistance (Rs) due to increasing mobility. TA?200 °C results in a minimum Rs. In region II, 200→500→200 °C, Rs is proportional to TA, increasing (decreasing) during the forward (reverse) anneal cycle. This behavior is apparently due to a temperature‐dependent active oxygen concentration and its effect on the carrier concentration. In region III, 200→50 °C, Rs is constant with TA. Optical transmission and x‐ray diffraction experiments were performed at 100 °C intervals. Successive anneals tended to increase the transmission in the visible and near‐UV regions and to decrease it in the near‐ and far‐IR region. Strong evidence of the Burstein‐Moss shift was observed and an extrapolated intrinsic band gap of 3.85 eV was determined. Free‐carrier absorption over the 2–5‐μm regions was evident after the 200 °C anneal for all ambients. From the x‐ray data, no evidence of crystallinity was observed in the as‐deposited case and for anneals up to 100 °C. For anneals in the 300–500 °C range, a grain size of the order of 600 A with an orientation normal to the (222) plane was observed for all ambients.

The effect of ambient atmosphere in the annealing of indium tin oxide films

Isochronal annealing experiments on dc-sputtered indium tin oxide (ITO) films in inert (N2), reducing (N2/H2) and oxidizing atmospheres were cumulatively performed over the 50→500→50 °C range. Three anneal regimes have been identified. In region I, TA=50→200 °C, crystallization occurs, resulting in a sharp drop in sheet resistance (Rs) due to increasing mobility. TA?200 °C results in a minimum Rs. In region II, 200→500→200 °C, Rs is proportional to TA, increasing (decreasing) during the forward (reverse) anneal cycle. This behavior is apparently due to a temperature-dependent active oxygen concentration and its effect on the carrier concentration. In region III, 200→50 °C, Rs is constant with TA. Optical transmission and x-ray diffraction experiments were performed at 100 °C intervals. Successive anneals tended to increase the transmission in the visible and near-UV regions and to decrease it in the near- and far-IR region. Strong evidence of the Burstein-Moss shift was observed and an extrapolated intrinsic band gap of 3.85 eV was determined. Free-carrier absorption over the 2–5-μm regions was evident after the 200 °C anneal for all ambients. From the x-ray data, no evidence of crystallinity was observed in the as-deposited case and for anneals up to 100 °C. For anneals in the 300–500 °C range, a grain size of the order of 600 Å with an orientation normal to the (222) plane was observed for all ambients.

The effect of ambient atmosphere on the gold-to-alumina solid state reaction bond

The effect of ambient atmosphere on the gold-to-alumina solid state reaction bond

Effects of Ambient Atmosphere on the Stability of Barium Titanate

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