Ignition of a two-fuel hydrogen–silane mixture in air

Based on the previously developed model of detailed kinetics, the ignition delay time of two-fuel hydrogen–silane–air mixtures is calculated. The effect of the silane concentration and the temperature of the mixture on the ignition delay time is determined. It is shown that addition of a small (within 20%) amount of silane to the hydrogen–air mixture in the temperature range from 1200 to 2500 K leads to significant reduction of the ignition delay time of the mixture, whereas there is only a minor decrease in mixtures with silane concentrations higher than 20%.     

Continuous spin detonation of a heterogeneous kerosene–air mixture with addition of hydrogen

Regimes of continuous spin detonation of two-phase kerosene–air mixtures with small addition of hydrogen are obtained for the first time in a flow-type annular cylindrical combustor 503 mm in diameter.     

Modelling the coupled transfer of mass and thermal energy in the vapour–liquid region of a nitrogen–oxygen mixture

Current non-equilibrium distillation models do not explicitly include the coupling between thermal and mass fluxes. We present a calculation model for the coupled transfer of mass and thermal energy in the vapour–liquid region of a binary mixture. The region is modelled as a vapour–liquid interface in between two homogeneous films. The entropy production in the vapour–liquid region can be calculated using both irreversible thermodynamics and the entropy balance. The film thickness ratio is found by requiring the entropy production calculated with the two methods to be equal, while keeping the vapour film thickness fixed. Using a nitrogen–oxygen mixture as example, we show that neglecting the coupling between thermal and mass fluxes can have a large impact on the magnitude and direction of the theoretical (net) fluxes. The size of the impact depends on the vapour film thickness, but it is significant for all thicknesses. By increasing the number of control volumes that is used to represent the liquid and vapour films, we also show that the fluxes depend highly on the resistivity profiles in the films. They depend slightly on the interface resistance. A sensitivity analysis of the transport properties shows that accurate values of the Maxwell–Stefan diffusion coefficients in both homogeneous phases and of the liquid phase heat of transfer are most important. Especially the measurable heat flux at the liquid boundary of the system is sensitive to neglect of coupling, to neglect of the interface resistance and to uncertainties in the transfer properties.     

Numerical analysis of combustion of a hydrogen–air mixture in an advanced ramjet combustor model during activation of O<Subscript>2</Subscript> molecules by resonant laser radiation

This paper presents a numerical study of the combustion of a hydrogen–air mixture in a model ramjet combustor with separate hydrogen and air supply during activation of O₂ molecules by resonant laser radiation at a wavelength of 762.3 nm and 193.3 nm. The calculation is made using the parabolized Navier–Stokes equations taking into account chemical reactions, laser irradiation, and the nonuniformity of air parameters at the combustor inlet due to the complex gas-dynamic structure of the flow in the air intake. It is shown that the combustion completeness at the combustor outlet can be increased by a factor of 2.8 by redistributing the hydrogen supply through the system of fuel tank pylons. Further increase in the combustion completeness can be obtained by exposure of a narrow flow region to resonant laser radiation, more effectively at a wavelength of 193.3 nm. The combination of laser exposure with hydrogen supply redistribution increases the combustion efficiency by a factor of more than 4.7 compared to the base case. In this case, this provides a 95% increase the longitudinal force component in the portion of the internal engine duct that provides a positive contribution to the thrust. Estimation of the energy efficiency of using laser radiation shows that the laser energy input required to achieve this effect is 40–80 times (depending on the fuel supply method) less than the increase in the chemical energy (compared to the case of no laser exposure) released due to fuel combustion.     

Initiation of homogeneous combustion in a high-velocity jet of a fuel–air mixture by an optical discharge

The effect of a focused pulsed-periodic beam of a CO₂ laser on initiation and evolution of combustion in subsonic and supersonic flows of homogeneous fuel–air mixtures (H₂ + air and CH₄ + air) is experimentally studied. The beam generated by the CO₂ laser propagates across the flow and is focused by a lens at the jet axis. The flow structure is determined by a schlieren system with a slot and a plane knife aligned in the streamwise direction. The image is recorded by a high-speed camera with an exposure time of 1.5 μs and a frame frequency of 1000 s^?1. The structure of the combustion region is studied by an example of inherent luminescence of the flame at the wavelengths of OH and CH radicals. The distribution of the emission intensity of the mixture components in the optical discharge region is investigated in the present experiments by methods of emission spectroscopy.     

Simulation of the separation of a helium–methane mixture in a flat membrane module

The problem of the membrane separation of a helium–methane mixture containing 0.1 wt % He, which imitates the composition of a helium-rich natural gas field, has been numerically analyzed. The helium and methane flux distribution along the length of a flat-sheet membrane module have been calculated. The distribution of the average mass concentration of helium in the channels is presented. The gas pumping expenditures have been estimated. The optimal working parameters of the gas mixture have been determined. Heating the mixture above 400 K does not enhance the helium–methane separation efficiency.     

Promoting effect of halogen- and phosphorus-containing flame retardants on the autoignition of a methane–oxygen mixture

This paper presents a numerical and experimental study of the effect of flame-retardant additives on the autoignition of methane behind shock waves. It is shown that at a temperature of 1300–1900 K, the compounds CCl₄, CF₃H, and (CH₃O)₃PO not only do not suppress ignition but significantly reduce the induction time of methane–oxygen mixtures. A kinetic mechanism is proposed which relates the promoting effect to the reactivity of the pyrolysis products of the additives.     

An evaluation of the effectiveness of applying a gelatin–copper complex in the low-temperature bleaching of cotton with hydrogen peroxide

A gelatin–copper complex was prepared and then added as catalyst to a hydrogen peroxide bleaching bath. Cotton fabric was bleached with the new system at low temperature (70 °C). The effects of the gelatin–copper complex on the whiteness, capillary effect, damage, and bleaching rate of the bleached fabric and on the decomposition ratio of hydrogen peroxide were evaluated. These effects were compared with the effectiveness of traditional high-temperature bleaching and low-temperature bleaching without the gelatin–copper complex. The results showed that the gelatin–copper complex enhances the bleaching effectiveness of hydrogen peroxide. The whiteness of cotton fabric bleached with the catalytic complex is comparable with the whiteness achieved with a conventional peroxide system. The catalytic bleaching technology not only realises low-temperature and low-alkali bleaching of cotton with hydrogen peroxide but also reduces fabric strength loss, which meets the requirements of industry for continued development of the wet processing of textiles.     

Characteristics of non-platinum cathode catalysts for a hydrogen–oxygen fuel cell with proton- and anion-conducting electrolytes

Cathode catalysts for a hydrogen–oxygen fuel cell (FC) with proton-conducting (acidic) and anion-conducting (alkaline) electrolytes are synthesized via the pyrolysis of nitrogen-containing iron and cobalt complexes on the surfaces of highly disperse carbon materials. The catalysts are characterized by X-ray photoelectron spectroscopy (XPS) and tested under model conditions on a thin-layer disk electrode and as a part of a membrane electrode assembly of hydrogen–oxygen FCs. The properties of the CoFe/C system formed via the pyrolysis of macroheterocyclic cobalt and iron compounds on carbon materials (XC-72 soot and multiwall nanotubes (MNTs)) are described for the first time. According to XPS data, the surface of the CoFe/C catalytic systems is enriched with carbon (95.5 at %) and contains nitrogen (2 at %), oxygen (2 at %), and metals (0.5 at %). According to the results from electrochemical measurements under model conditions, the CoFe/MNT catalytic systems approaches 60% Pt/C (HiSPEC9100) commercial platinum catalyst according to their activity in the oxygen reduction reaction in an alkaline medium (0.5 M KOH). The half-wave potentials are 0.85 and 0.88 V for CoFe/MNT and 60% Pt/C (HiSPEC9100) catalysts, respectively. The maximum specific powers of hydrogen–oxygen FCs with anion-conducting electrolytes are 210 mW/cm² (60% Pt/C (HiSPEC9100) based cathode) and 180 mW/cm² (CoFe/MNT based cathode). The characteristics of a membrane electrode assembly with a non-platinum cathode correspond to the best analogs described in the literature. The results of this work show the prospects for further studies on scaling this technology for the synthesis of the proposed non-platinum cathode catalysts and optimizing the architecture of the membrane electrode assembly of FCs based on them.     

Determination of the optical constants of carbon black in the combustion products of a hydrogen fuel at λ=10.6 μm

Literature data on the optical constants n and of carbon black were analyzed over a wide spectral range. The values of n and at =10.6 m were determined by measuring the sizes and concentrations of carbon black particles in the combustion products of kerosene from the attenuation of the transmitted radiation in the short-wavelength range of the spectrum and from the absorption coefficient of the particles, which is proportional to their volume concentration and determined by the optical properties of carbon black.B. I. Stepanov Institute of Physics, Belarus' Academy of Sciences, 220602, Minsk. Translated form Fizika Goreniya Vzryva, No. 1, pp 70–73, January–February, 1995.     

Method of power density determination in microwave discharge,sustained in hydrogen–methane gas mixture

We present the results of studying a continuous microwave discharge maintained at a frequency of 2.45 GHz in a CVD reactor based on a cylindrical resonator excited at the TM₀₁₃ mode. The discharge was ignited in hydrogen and a gaseous mixture of hydrogen and methane and was studied by the method of optical emission spectroscopy. Density of atomic hydrogen and gas temperature were measured, as well as the spatial distribution of both optical emission intensity of the plasma and intensity of the Hα line of atomic hydrogen. The main parameters of the discharge were calculated numerically using the two-dimensional self-consistent model of the discharge. Basing on the obtained results, we proposed a method for high-precision experimental determination of the plasma volume and calculation of the specific energy contribution to the plasma, i.e., microwave power density in plasma (MWPD), with minimal errors. According to the calculations, in the experiment performed, the microwave power density in the plasma varied from 50 to 550 W/cm³ as the gas pressure increased from 80 to 350 Torr. The method allows one to perform unified MWPD calculations in different CVD reactors and to compare diamond film deposition regimes.     

Experimental measurements of effective diffusion coefficient of oxygen–nitrogen mixture in PEM fuel cell diffusion media

PEM fuel cells are increasingly designed to operate at high current densities. At these densities, mass transport limitations become very significant, but they are not well understood, with many modeling studies but few experimental observations. The use of accurate transport coefficients to simulate the mass transport at high current densities is crucial. In this study, experimental measurements have been carried out to determine the effective diffusion coefficient in the carbon paper gas diffusion layer that is commonly used in PEM fuel cells. It was found that almost all the existing theoretical models significantly overpredict the effective diffusion coefficient by as much as 4–5 times; thus, underestimating the transport limitations considerably. Further, the effects of temperature, Teflon treatment for hydrophobicity and porosity on the effective diffusion coefficient were investigated. It was found that temperature does not affect the overall diffusibility of the gas. The diffusibility is decreased with the increase of Teflon treatment and decrease in porosity. Further work on better understanding the diffusion process in the gas diffusion layer is under way.     

Laser diffraction characterization of droplet size distributions produced by vibrating mesh nebulization in air and a helium–oxygen mixture

Medical aerosols may be delivered in combination with gases other than air, thus it is of interest to assess the effects of gas properties on the characteristics of the administered aerosol separately from effects on ventilation distribution and particle deposition mechanisms. This work investigated the influence of the supplied gas, either air or a mixture containing 78% helium and 22% oxygen, on droplet sizes produced by a vibrating mesh nebulizer incorporated in a combined gas–aerosol delivery system. Droplet size distributions were measured by laser diffraction. Nebulization was performed using three different meshes, producing droplets with nominal volume median diameters (VMDs) of 3, 4, or 5 μm. Measured VMDs were stable, in that they were in all cases within ±10% of their nominal values, and unaffected by humidity or dilution of the aerosol stream. While VMDs were consistently 5–10% smaller in helium–oxygen than in air, this variation was small compared to the variation between meshes. Accordingly, unlike jet nebulizers, vibrating mesh nebulizers having high output rates can be operated in helium–oxygen with only minor impact on emitted droplet sizes. This will be attractive in the design of controlled clinical studies investigating aerosol delivery in helium–oxygen. In assessing such therapies, it is important to distinguish effects of gas properties on the characteristics of the administered aerosol from effects on particle and fluid mechanics influencing the regional distribution of aerosol in the lung. Use of an aerosol delivery device that is virtually unaffected by changing gas properties, such as that tested in the present study, is a straightforward way to make such a distinction.     

Co-combustion of rice husk with coal in a cyclonic fluidized-bed combustor (ψ-FBC)

Thailand is well-endowed with renewable energy resources. In Thailand, rice husk, a by-product of the rice-milling process and one of the most potentially sustainable cultivated biomasses, has an annual energy equivalent of 6.6 × 10⁷ GJ. Using rice husk alone, however, can be problematic, particularly if there is a deficit during the off-season. Coal, the most abundant fossil fuel, has thus been considered an appropriate supplementary fuel. This paper describes the combustion characteristics of co-firing rice husk with bituminous coal in a 120 kW_th-capacity cyclonic fluidized-bed combustor (ψ-FBC), and how excess air ratios and fuel blends impacted emissions and combustion efficiency (E_c). Overall, excess air and blending ratios did not have tremendous effects on E_c, easily achieving >97%. Radial temperature profiles revealed that vortex combustion prevailed along the combustor walls. Concurring with axial temperature profiles, axial O₂ profiles suggested that the combustion was confined chiefly to regions under the vortex ring. Despite massive CO production in the lower section, CO emissions were satisfactory (range 60-260 ppm, at 6% O₂). Due to the high bed temperatures, NO_x appeared rather high (260-416 ppm, at 6% O₂). Not only were NO_x emissions affected by coal ratio, it were also highly reliable on the operating conditions. SO₂ emissions varied directly, but not proportionally, with the sulfur content of the fuel mixture.     

Modeling of pressure losses for the slug flow of a gas–liquid mixture in mini- and microchannels

In addition to the previously constructed model of the hydrodynamics of a gas-liquid slug flow, a mathematical model is developed that describes pressure losses taking into account the rearrangement of a velocity profile in liquid slugs and energy losses on the formation and renewal of interfacial area during the motion of bubbles. The contribution of different forms of pressure losses in capillaries is analyzed. It is shown that in microchannels tangential stresses on the surface of a bubble substantially affect the total pressure losses. It is found that the length of bubbles does not affect the rate of surface formation and respective pressure losses if bubbles have the same velocity. The results of modeling are in satisfactory agreement with experimental data of other researchers.     

Interruption of Detonation Wave Propagation in Monofuel–Air Mixtures by a Layer of Inhomogeneous Inert Particles

Combustion, Explosion, and Shock Waves - This paper describes the results of a numerical study pertaining to the interruption of detonation wave propagation in monofuel–air mixtures by a...     

Emission control in the combustion of coal–water and organic coal–water fuels

Promising methods for decreasing anthropogenic emissions due to the combustion of coals of different ranks and coal–water fuel (CWF) and organic coal–water fuel (OCWF) slurries on their basis are considered. The maximum concentrations of the main anthropogenic emissions of sulfur, nitrogen, and carbon oxides (SO _x , NO _x , and CO _x ) formed upon the combustion of solid fuels in a powdered state and as the components of CWF and OCWF slurries were determined. The concentrations of the most hazardous oxides formed upon the combustion of coals of different ranks (brown and black coals) and CWF and OCWF slurries were compared. The experimental results substantiated the use of CWF and OCWF slurries for emission control in coal-burning power engineering. The addition of a combustible liquid component to a CWF slurry (the production of an OCWF slurry) makes it possible to ensure acceptable environmental and energy characteristics.     

Quantification of powder holdups in the presence of a cavity with lateral gas–powder injection in a packed bed

The lateral flow of gas–powder through a packed bed in a cold model is studied to understand the flow and holdup behaviour of powder in the presence of a cavity, nozzle (tuyere) protrusion, and decreasing gas condition, a system used in the ironmaking blast furnace. Experiments conducted in the current study included a two-dimensional (2D) slot-type packed bed. A previously published mass balance and elutriation velocity concept formed the basis for accurately quantifying the static and dynamic powder holdups. Experiments conducted under different conditions such as powder size and flux, gas flow rate, and packed particle density and size resulted in quantifying the powder holdups. The pressure drop in both horizontal and vertical directions is studied in all two-phase flow experiments. The formation of the static holdup with time in the packed bed is studied. The reproducibility of the experiments was confirmed. The static holdup inside the packed bed at various locations along the vertical direction (i.e., height) is also quantified. The static holdup correlation developed based on experimental data resulted in a 95% confidence interval. Static powder holdup increases with a decrease in the superficial gas velocity, an increase in the size of the powder particle, and powder flux. Dynamic holdup also showed a similar trend.     

End combustion products of mixtures of ultrafine aluminum with a zirconium—Aluminum alloy in air

The chemical composition of the products formed during combustion in air of powder mixtures of a commercial zirconium-aluminum alloy (mass concentrations of zirconium and aluminum 84 and 16%, respectively) and ultrafine Al powder produced by electric detonation of conductors in argon is studied. The capacity of the mixtures for chemical fixation of air nitrogen is analyzed. It is established that under certain conditions, the end products can contain up to 60% AlN + ZrN mixture. Translated fromFizika Goreniya i Vzryva, Vol. 36, No. 2, pp. 56–59, March–April, 2000. This work was supported by the Russian Foundation for Fundamental Research (Grant No. 98-02-16321).     

Correct use of the Langmuir–Hinshelwood equation for proving the absence of a synergy effect in the photocatalytic degradation of phenol on a suspended mixture of titania and activated carbon

This study is a critical approach to the widespread use of the first order form of the Langmuir–Hinshelwood (LH) equation for analyzing kinetics in heterogeneous photocatalytic processes. The different kinetic protocols analyzed have been applied to the results, published in the literature, of the photocatalytic degradation of phenol in an aqueous solution by a physical mixture of TiO₂ particles and activated carbon (AC), the impact of which has been enormous over the last decade. It is commonly accepted that there is a strong synergy in this mixture due to the transfer of phenol from the activated carbon particles to TiO₂. However, we found in this study that the apparent synergy between activated carbon and TiO₂ particles arises from the erroneous use of the first order form of the LH equation. When applying the extended form of the LH equation, that includes the inhibitory effect of the phenol concentration, AC/TiO₂ synergy should be disregarded. In this physical mixture the activated carbon merely alleviates the inhibitory effect of the phenol concentration by decreasing its initial value.