Effects Of Water Vapor Addition Research Articles

Sooting properties of laminar coflow non-premixed ethylene/hydrogen flames influenced by water vapor addition to the oxidizer

A numerical study was conducted to quantify the effects of water vapor addition to oxidizer on soot formation in ethylene/hydrogen flames in a benchmark coflow configuration. The objective was to quantify the dilution, thermal and chemical effects in terms of their contributions to the change in sooting properties and flame radiation. The conditions are selected to keep constant the heat release rate of the flame while adding hydrogen and to maintain the mass flow of air of the oxidizer while adding water vapor. On the one hand, a consistent decrease of flame height, soot volume fraction, carbon-to-soot conversion efficiency and total radiation emitted by the flame was observed when increasing the mole fraction of hydrogen on the fuel side. On the other hand, water vapor addition to the coflow slightly increases the flame height, and reduces soot volume fraction and temperature. A consistent decrease is observed on the local radiative power per unit volume by soot, CO and CO2. However, the total radiative losses are enhanced by the water vapor presence at high temperatures. In general, the dilution effects are dominant for the change of soot loading and radiation, whereas the thermal effects are dominant for carbon-to-soot conversion efficiency. Finally, a chemical pathway analysis was conducted to quantify the changes in the main reactions for soot precursor formation affected by addition of hydrogen and water vapor. These results provide fundamental insights to researchers and practitioners dealing with fires caused by hydrogen-enriched mixtures, which are attempted to be extinguished by traditional means by adding water.

Effects of water vapor addition on downstream interaction in CO/O2 counterflow premixed flames

The effects of H2O addition on downstream interaction in counterflow premixed CO/O2 flames are investigated by varying the global strain rate (ag) and CO mole fractions (XCO,L, XCO,U) in the lower and upper nozzles, respectively. For interacting premixed CO/O2 flames, the flammable region is very narrow such that the flames cannot be sustained for ag > 11.75 s−1. When 1.0% vol H2O is added to O2/CO2 mixtures, the flammable region is appreciably extended. At low strain rate, the lean-lean and rich-rich extinction boundaries show strong and weak interactions similar to those observed previously in hydrocarbon fuels. The flammable lean-lean and rich-rich regions gradually shrink with the increase of ag. When XCO,U is small for asymmetric lean double flames at low strain rate, the extinction boundary exhibits weak interaction behavior, where the weaker flame is parasitic to the stronger flame by XCO,L. The stronger flame experiences heat loss to the weaker flame. As the strain increases, the reaction cannot be completed due to the reduction in the flow time. The thermal energy loss by incomplete reaction leads to the flame extinction. This effect changes the qualitative nature of extinction boundary as strain rate increases, resulting in the extinction boundary having only the strong interaction mode having near constant (XCO,L + XCO,U) and bending toward larger XCO,L, and eventually forming an island shape at higher strain rate. The local equilibrium temperature (LET) concept is introduced to explain these flame extinction mechanisms. Local temperature behaviors are well explained by investigating major reaction contributions to heat release rate. In all cases, LET decreases by the effect of preferential diffusion because of the Lewis number of deficient reactant being larger than unity. For asymmetric double flames, conductive heat transfer (CHT) from the stronger to weaker flame reduces the LET of the stronger flame. Flame extinction mechanism can be explained by introducing a loss ratio. For CO/O2 flames, the effects of incomplete reaction as well as preferential diffusion and CHT lead to flame extinction. For (CO/O2 + 1.0% H2O) flames with the increase of strain rate, thermal energy loss by incomplete reaction becomes appreciable, as compared with the effects of preferential diffusion and CHT.

Effects of Water Vapor Addition on Downstream Interaction in Co/O2 Counterflow Premixed Flames

Effects of Water Vapor Addition on Downstream Interaction in Co/O2 Counterflow Premixed Flames

Effects of Water Vapor Addition on the Flame Structure and Soot Formation in a Laminar Ethanol/air Coflow Flame

ABSTRACT Ethanol is a promising alternative fuel to gasoline engines; however, the problem of particulate matter (PM) emissions still exists when ethanol is applied to gasoline direct injection (GDI) engines, especially under high load. Water injection is proved to be an effective way to reduce the PM emissions and engine knocking, simultaneously. In this work, a numerical calculation was carried out in an ethanol/air coflow diffusion flame to investigate the effects of water vapor addition on the flame structure and soot formation. A complete step-by-step decoupling method was proposed to numerically isolate the dilution, density, transport, thermal, radiative and chemical effects of water vapor by introducing several virtual species. Results show that the temperature distribution of the flame is mainly affected by the dilution effect and thermal effect of water vapor, followed by radiative effect and chemical effect. The mole fraction of OH radical increases because of the enhancement of the reaction H2O + H <=> OH + H2 and O + H2O <=> OH + OH, while the conversion of CO to CO2 is promoted through the reaction CO + OH <=> CO2 + H, due to the chemical effect. Soot volume fraction is also reduced, where the dilution and chemical effects play the most important roles. Hydrogen-abstraction-carbon-addition (HACA) surface growth and OH oxidation are the processes controlling the soot mass addition and consumption, respectively. HACA rate is mainly affected by the dilution and chemical effects. However, chemical effect has little influence on the OH oxidation rate, since the collision probability of OH with soot particles remains almost unchanged as a result of the increase of OH concentration and the decrease of soot volume fraction under the chemical effect.

Effects of water vapor addition on NO reduction of n-decane/air flames

ABSTRACTNO producing mechanism and effects of additional water vapor on NO reduction of n-decane/air flames were examined numerically in the present study. A new phenomenon was found that the variation trend of contribution of chemical effect of water vapor on NO reduction with pressure changes with the equivalence ratio. To illustrate this, NO emissions without and with water vapor addition were examined. Results showed that the new phenomenon is because that NO-producing pathways vary with both equivalence ratio and pressure, and hence the reactions through which the added water vapor influences NO production also vary with equivalence ratio and pressure.

The Effect of Water Vapor Addition on the Ignition Delay of Iso-octane/Air Mixture

The Effect of Water Vapor Addition on the Ignition Delay of Iso-octane/Air Mixture

Premixed combustion of low-concentration coal mine methane with water vapor addition in a two-section porous media burner

During the transport of low-concentration coal mine methane (CMM), water mist spraying into the pipeline is used to eliminate the risk of explosion, but it leaves abundant vapor in the methane. This study was aimed to explore the effects of water vapor addition on the low-concentration CMM combustion in porous media. Thereby, a 2D numerical model based on a two-section ceramic foam burner setup with high flame stability was established and multi-step kinetics mechanisms were imported to the model. In this paper, the effects of vapor concentrations on the temperature distribution, flame stability limit, and chemical reaction during low-concentration CMM combustion in ceramic foam were investigated. Results indicate that with the increase of vapor mole fraction in the inlet methane, the overall temperatures in the downstream section of the burner gradually decreased, while the vapor mole fractions were linearly and negatively correlated with the peak temperatures in the burner. A small amount of vapor was involved in the chemical reactions of combustion, and with the increase of vapor mole fraction, more vapor took part in the reactions when the vapor addition into the inlet methane was unchanged. As the vapor mole fraction in the low-concentration CMM increased, the velocity range of flame stability limit was gradually narrowed down. In addition, the lower limit of velocity changed very slightly and maintained at 0.13–0.20 m/s, while the upper limit dropped obviously. The key elementary reactions underlying the effect of vapor on combustion reactions were determined by defining the changing rate of peak reaction rate. Addition of vapor into methane affected the peak rate of each elementary reaction, and altered the area of axial region where elementary reactions occurred.

Thermal and Chemical Effects of Water Addition on Laminar Burning Velocity of Syngas

An experimental and numerical study on thermal and chemical effects of water vapor addition on the laminar burning velocities of syngas was conducted using a constant-volume chamber and CHEMKIN package. The experimental conditions in the present study for CO/H2/air/H2O mixtures with hydrogen fraction in syngas were from 5% to 50%, initial temperature of 373 K, pressures of 0.1 and 0.5 MPa and water dilution ratios from 0% to 30%. The measured laminar burning velocity data were compared with simulations with three mechanisms, the San Diego mechanism and those of Davis et al. and Li et al. The experimental data showed a reasonable agreement with the calculated values at low and high pressures when the content of H2 in the fuel was low. However, when H2/CO ratio in the fuel was higher (75/25 and 95/05), all three mechanisms overpredicated the laminar burning velocity for fuel-rich mixtures. Sensitivity analysis was performed to identify the possible sources of discrepancy between the experimental data and ca...

Effects of water vapor addition to the air stream on soot formation and flame properties in a laminar coflow ethylene/air diffusion flame

The effects of adding water vapor to the air stream on flame properties and soot volume fraction were investigated numerically in a laminar coflow ethylene/air diffusion flame at atmospheric pressure by solving the fully elliptic conservation equations and using a detailed C2 reaction mechanism including PAH up to pyrene and detailed thermal and transport properties. Thermal radiation was calculated using the discrete-ordinates method and a statistical narrow-band correlated-k based wide band model for the absorption coefficients of CO2 and H2O. Soot formation was modeled using a PAH based inception model and the HACA mechanism for surface growth and oxidation. Addition of water vapor significantly reduces radiation heat loss from the flame primarily through reduced soot loading and flame temperature. The added water vapor affects soot formation and flame properties through not only dilution and thermal effects, but also through chemical effect. The chemical effect is as significant as the dilution and thermal effects. The primary pathway for the chemical effect of water vapor is the reverse reaction of OH+H2↔H+H2O. Our numerical results confirm that the reduced H radical concentration leads to lower PAH concentrations and consequently lower soot inception rates. In contrast, the radiation effect due to the added water vapor was found to have a minor influence on both flame structure and soot formation in the laminar diffusion flame investigated.

Influence of Water Vapor Addition on the Surface Modification of Polyethylene in an Argon Dielectric Barrier Discharge

In this paper, water vapor up to 41% is added to an argon dielectric barrier discharge at medium pressure to profoundly investigate the effect of water vapor addition on the surface modification of polyethylene (PE). Contact angle measurements show that the wettability of PE significantly increases after a pure argon plasma treatment. However, the addition of water vapor to the argon feeding gas can give an extra 30% decrease in water contact angle. This extra decrease is due to the incorporation of additional oxygen containing groups such as CO, CO, OCO, and OCO. AFM results reveal that the surface roughness of PE only increases for plasma treatments with no or low water vapor concentrations. All this indicates that a water vapor based plasma can be an excellent tool for the surface activation of PE.

The effects of water dilution on hydrogen, syngas, and ethylene flames at elevated pressure

This work investigates experimentally and numerically the kinetic effects of water vapor addition on the burning rates of H2, H2/CO mixtures, and C2H4 from 1atm to 10atm at flame temperatures between 1600K and 1800K. Burning rates were measured using outwardly propagating spherical flames in a nearly constant pressure chamber. Results show good agreement with newly updated kinetic models for H2 flames. However, there is considerable disagreement between simulations and measurements for H2/CO and C2H4 flames at high pressure and high water vapor dilution. Both experiments and simulations show that water vapor addition causes a monotonic decrease in mass burning rate and the inhibitory effect increases with pressure. For hydrogen flames, water vapor addition reduces the critical pressure above which a negative pressure dependence of the burning rate is observed. However, for C2H4 flames, the burning rate always increases with pressure. The results also show that water vapor addition has the same effect as a pressure increase for H2 and H2/CO flames, shifting the reaction zone into a narrower window at higher temperatures. For all fuels, water vapor addition increases OH formation via H2O+O while reducing the overall active radical pool for hydrogen flames. For C2H4, the additional HO2 production pathway through HCO results in a dramatic difference in pressure dependence of the burning rate from that observed for hydrogen. The present work provides important additions to the experimental database for syngas and C0–C2 high pressure kinetic model validations.

Effects of water vapor addition on the laminar burning velocity of oxygen-enriched methane flames

The effects of steam addition on the laminar burning velocity of premixed oxygen-enriched methane flames are investigated at atmospheric pressure. Experiments are carried out with an axisymmetric burner on which laminar conical flames are stabilized. A newly devised steam production system is used to dilute the reactants with water vapor. The oxygen-enrichment ratio in the oxidizer, defined as O 2/(O 2 + N 2) (mol.), is varied from 0.21 (air) to 1.0 (pure oxygen). The equivalence ratio ranges from 0.5 to 1.5 and the steam molar fraction in the reactive mixture is varied from 0 to 0.50. For all compositions examined, the reactive mixture is preheated to a temperature T u = 373 K. Laminar flame speeds are determined with the flame area method using a Schlieren apparatus. The deviations induced by stretch effects due to aerodynamic strain and flame curvature are assessed using Particle Imaging Velocimetry measurements and flame images, and these data are used to estimate the uncertainty of the flame speed measurements. The experiments are completed by numerical simulations conducted with the PREMIX code using different detailed kinetic mechanisms. It is shown that the laminar flame speed of CH 4/O 2/N 2/H 2O ( v) mixtures features a quasi-linear decrease with increasing steam molar fraction, even at high steam dilution rates. Numerical predictions are in good agreement with experimental data for all compositions explored, except for low dilution rates X H 2 O < 0.10 in methane–oxygen mixtures, where the flame speed is slightly underestimated by the calculations. It is also shown that steam addition has a non-negligible chemical impact on the flame speed for methane–air flames, mainly due to water vapor high chaperon efficiency in third-body reactions. This effect is however strongly attenuated when the oxygen concentration is increased in the reactive mixture. For highly oxygen-enriched flames, steam can be considered as an inert diluent.

In situ FTIR studies of CO oxidation over Fe-free and Fe-promoted PtY catalysts: Effect of water vapor addition

In the quest of better CO elimination catalysts, Fe oxide was employed to promote platinum supported on HY zeolite. Fe-free and Fe-promoted PtY catalysts were prepared by ion exchange method and calcined at 500 °C. The prepared samples were characterized using XRD, FTIR, TEM, CO adsorption and CO-TPD. The particles size calculated from XRD patterns and TEM images was ≈18–20 nm in PtY and ≈8–15 nm in PtFeY. The CO removal performance over the prepared solids has been investigated by means of in situ technique using FTIR spectroscopy. Platinum surface modified with Fe oxide has a pronounced catalytic activity towards the CO oxidation more than Fe-free Pt containing catalyst. Reduction of the prepared catalysts with H 2 gas prior to admission of gas mixtures increased the catalytic activity more than that of as prepared catalysts. A strong positive water-effect was established when a trace amount of water (more realistic conditions) was added to the gas mixture (CO + O 2) during the CO oxidation particularly over PtFeY. In presence of water, the amount of CO 2 adsorbed over PtFeY is ≈5 and ≈600 times more than that of water free reaction over PtFeY and PtY catalysts, respectively.

Nanostructure Fabrication by Electron-Beam-Induced Deposition with Metal Carbonyl Precursor and Water Vapor

Nanorod fabrication is performed by electron beam induced deposition (EBID) with iron carbonyl [Fe(CO)5] and tungsten carbonyl [W(CO)6] precursors. The effects of water vapor addition to each metal carbonyl on the microstructure and composition of the obtained nanorods are studied. Normally, EBID-fabricated metal nanorods consist of an amorphous phase containing a considerable amount of carbon. However, it is found that water vapor addition to iron carbonyl can effectively reduce the carbon content of the nanorods and induce the formation of carbon-free crystalline Fe3O4 nanorods with increasing partial pressure ratio of water vapor to iron carbonyl. In contrast, for tungsten carbonyl, water vapor addition has no obvious effect on carbon content reduction. The obtained nanorods consist of a carbon-rich amorphous matrix containing tungsten oxide nanocrystals inside.

CO oxidation over CeO 2-promoted Cu/γ-Al 2O 3 catalyst: Effect of preparation method

The effect of preparation method on the properties of CeO 2-promoted Cu/γ-Al 2O 3 catalysts for CO oxidation was investigated. The effect of water vapor addition on the catalyst activities was also studied. Experimental results show that copper catalyst activity highly depends on the preparation method. The superior activity of the catalyst can be attributed to the existence of highly dispersed copper oxide species, which strongly interact with the ceria. Addition of water vapor into the feed stream decreases the catalyst activity. The resistance of the catalysts to this deactivation is also highly dependent on the preparation method as well as on the calcination temperature.

Effect of water on electrochemical oxygen reduction at the interface between fluorite-type oxide-ion conductors and various types of electrodes

The effect of water vapor addition on the electrochemical oxygen reduction was investigated for the interfaces between fluorite-type oxide-ion conductors (YSZ, ceria) and various types of electrodes at the temperature range 750–1100 K in air. The interface conductivity of YSZ electrolyte/gold electrode at open circuit condition increased when a slight amount of water vapor was added to air at T=865 K. However, the interface conductivity of YSZ/platinum decreased during a long-term exposure in humid atmospheres. The improvement was correlated to the enhancement of oxygen exchange through water molecules, and the degradation is probably due to the blocking of oxygen molecule migration by the adsorbed water molecules.

Chemical and physical effects of water vapor addition on diffusion flames

The use of water mist to extinguish fires is a problem of particular interest since the banning of halogen-based agents for environmental reasons. This interest is reflected in the large number of studies performed on the main mechanisms of extinguishment: heat extraction, oxygen displacement and attenuation of radiant heat fluxes. In contrast, little is still known about the chemical and some other aspects of water vapor addition in hydrocarbon diffusion flames. In this paper, a phenomenological study is conducted of the effect of water vapor addition, through the base of a small-scale heptane pool fire. Heptane, supported on a pool of liquid water, is burned as a pool flame while the water underneath is heated to boiling. This is a unique and original method in that boiling the water under the fuel is used for adding water vapor at the base of a diffusion flame where chemical reactions and air entrainment take place. This region is the most interesting regarding flame reactivity and soot formation and, in spite of the perturbing influence of the injected water vapor, the major characteristics of the flame are conserved. The procedure permits measurements of exhaust gas composition by means of a flow through system. Measurements of temperature, CO and CO 2 species concentrations, and monochromatic absorption coefficient which can be associated with soot concentration, are performed. It is seen that adding water vapor in such a way affects both physical phenomena and chemical reactions. That water vapor inhibits the soot formation and shifts CO to CO 2 is further confirmed by injection of an inert gas instead of water vapor. However, the resulting heat release is not efficient enough to counteract the cooling effect of water vapor and the temperature is significantly decreased.