Effect Of Gas-phase Reactions Research Articles

Computational Analysis and Regression Laws for Nozzle Erosion Prediction in Hybrid Rockets

The erosion of the nozzle throat can represent one of the major limitations against the future widespread use of hybrid rocket engines (HREs) in the space industry. In fact, nozzle erosion in HREs can be more severe and harder to predict than in solid rockets due to the higher concentration of oxidizing species in the combustion products and to mixture ratio shifts and/or throttling. Therefore, an accurate understanding of the erosion phenomenon is of fundamental importance for the technological advancement of HREs. This work is focused on the investigation of graphite nozzle erosion in HREs burning high-density polyethylene with two different oxidizers, oxygen and nitrous oxide. First, the results of a computational fluid dynamics parametric analysis are used to derive closed-form regression laws for the rapid estimation of nozzle throat erosion and wall temperature depending on chamber pressure and mixture ratio. Then, a one-dimensional transient heat conduction solver is loosely coupled with the aforementioned regression laws, allowing to reconstruct the transient heating process within the solid. The obtained numerical results are validated against experimental data. Finally, the effect of gas-phase reactions on the heterogeneous reactions occurring at the nozzle surface is highlighted when moving from fuel-rich to oxidizer-rich conditions.

Effect of Gas-Phase Reaction on the CVD Growth of Graphene

Effect of Gas-Phase Reaction on the CVD Growth of Graphene

Combustion and heat transfer characteristics of co-firing biomass and coal under oxy-fuel condition

A numerical investigation was conducted on co-firing biomass and coal under oxy-fuel condition with the refined combustion model. The effects of gas-phase reactions caused by various volatiles of coal and biomass were considered. The influences of biomass fraction and combustion atmosphere on the reaction rate of volatile, the distribution of wall heat flux, and the burnout behavior were analyzed. The results show that the reaction is more active for the volatile of Paulownia wood chips than that of coal. The average wall heat flux suffers a decline when the combustion atmosphere is switched from air to oxy-fuel condition, while the O2 fraction is 24%. The wall heat flux of hopper rises by 29.97% to 162.7% as the biomass fraction increases from 10% to 30% under various atmospheres. Due to the variation in trajectories of fuel particles and the change of temperature within the furnace, the combustion atmosphere and the biomass share fraction affect the burnout performance to some extent. The working performances of all cases were evaluated, and the optimal operation conditions for co-firing different co-firing scenarios were recommended.

Gas phase combustion in the vicinity of a biomass particle during devolatilization – Model development and experimental verification

A numerical and experimental study on the devolatilization of a large biomass particle is carried out to quantify the effect of homogeneous volatile combustion on the conversion of the particle and on the temperature and species distribution at the particle vicinity. A global chemical kinetic mechanism and a detailed reaction mechanism are considered in a one dimensional numerical model that takes into account preferential diffusivity and a detailed composition of tar species. An adaptive moving mesh is employed to capture the changes in the domain due to particle shrinkage. The effect of gas phase reactions on pyrolysis time, temperature and species distribution close to the particle is studied and compared to experiments. Online in situ measurements of average H2O mole fraction and gas temperature above a softwood pellet are conducted in a reactor using tunable diode laser absorption spectroscopy (TDLAS) while recording the particle mass loss. The results show that the volatile combustion plays an important role in the prediction of biomass conversion during the devolatilization stage. It is shown that the global reaction mechanism predicts a thin flame front in the vicinity of the particle deviating from the measured temperature and H2O distribution over different heights above the particle. A better agreement between numerical and experimental results is obtained using the detailed reaction mechanism, which predicts a wider reaction zone.

Investigation of wall chemical effect using PLIF measurement of OH radical generated by pulsed electric discharge

Near-wall distribution of OH radical concentration provides a measure of the wall chemical effect near a solid surface. However, it is not a straightforward process to isolate the wall chemical effect from the effect of gas-phase reactions because the OH radical distribution can be influenced by both the wall chemical effect and the effect of heat and radicals released by gas-phase reactions. In the present study, instead of the flame, OH radicals generated by a pulsed electric discharge is used in order to eliminate the interference from the gas-phase reaction. OH field with comparable concentration to a methane/air premixed flame has been achieved by tuning the input electric power of pulsed electric discharge. The wall chemical effect is investigated by comparing OH distributions near the quartz wall and the quartz wall with 100-nm-thick alumina coating, while the wall thermal boundary condition is kept identical. High-resolution near-wall measurement of OH distribution was carried out by microscopic planar laser-induced fluorescence (PLIF), and the result was analyzed with the aid of numerical simulations with a surface reaction mechanism. It is found that the initial sticking coefficients estimated on the quartz/alumina surfaces are almost the same with the results in our previous methane/air flame experiment (Saiki and Suzuki, 2013). In the present OH field with electric discharge, it is easier to investigate the radical quenching effect, as the wall chemical effect on OH is decoupled from the gas-phase reaction. The chemical action defined as the wall-normal OH concentration gradient divided by the local OH concentration, which is an index of the wall chemical effect, increases with increasing wall temperature in the OH field generated by pulsed electric discharge.

An effect of gas-phase reactions on the vertically aligned CNT growth by temperature gradient chemical vapor deposition

Gas-phase reactions among carbon precursors could have a significant effect on the catalytic synthesis of vertically aligned (VA-) carbon nanotubes (CNTs). State-of-the-art adjustment of the gas-phase reaction conditions is done ad hoc with low reproducibility, ascribed partly to an incomplete understanding of the mechanism how the gas-phase reaction influences the growth kinetics of VA-CNTs. Here, we investigate this mechanism and propose that in the presence of prominent gas-phase reaction, the growth kinetics characterization shifts from a single rate-limiting reaction to a multi-route reaction with each pathway having its own rate-limiting step. The rate of the gas-phase reaction could be tailored by the partial pressure and thermal history of a carbon feedstock (e.g., acetylene). According to in-situ mass spectrometry and numerical simulation, a rich set of thermally rearranged secondary species (e.g., C4H4 and C6H6) are found above the catalyst surface. It appears that these secondary species, along with a chief precursor (C2H2), can give variation to a yield of CNTs through various reaction pathways with altering the overall growth rate of VA-CNTs. Our findings would further the current understanding of the VA-CNT growth from thermally rearranged precursors, instrumental to applications such as diameter-controlled growth and CMOS-compatible low-temperature growth.

Effect of gas-phase reaction on catalytic reaction for H2/O2 mixture in micro combustor

The catalytic reaction characteristics of the hydrogen and oxygen mixture in the catalytic micro-combustor were detected by thermocouple, infrared thermal imager and OH-PLIF. The equivalence ratio of transition stage from coupling reaction (coupled homogeneous-heterogeneous combustion) to pure catalytic reaction (heterogeneous combustion) was obtained. The three-dimensional pure catalytic reaction model with detailed catalytic reaction mechanism was established in a rectangular catalytic micro-combustor. The simulation model was validated with experimental data. The effects of the intermediate and final products from gas-phase reactions on the pure catalytic reaction were discussed as well as the effects of gas-phase reactions on the catalytic reaction in the process of coupling reactions were studied. The intermediate product OH radical can improve the hydrogen conversion of the surface reaction, and the effect of O radical is not obvious. The final product H2O has an inhibitory effect on the surface reaction. Since the mass fraction of H2O is much higher than other gas-phase reaction products, the dominant effect of the gas-phase reaction on the catalytic reaction is suppression. In the coupling reactions, the fuel consumed by the gas-phase reaction weakened the catalytic reaction.

Effect of Volatile Reactions on the Thermochemical Conversion of Biomass Particles

A numerical and experimental study on the conversion of a biomass particle is carried out to quantify the effect of homogeneous volatile combustion on the biomass pyrolysis. The numerical domain consists of a particle and its surrounding and the model considers detailed chemical kinetic mechanism for reaction of pyrolysis products. A detailed pyrolysis model is employed which provides the composition of pyrolysis products. The effect of gas phase reaction on the conversion time and temperature of the particle is analyzed and it was shown that the gas phase reactions results in shorter pyrolysis time. H2O mole fraction and temperature above a biomass pellet from wheat straw (WS) and stem wood (SW) were experimentally measured using tunable diode laser absorption spectroscopy (TDLAS) while recording the particle mass loss. The TDLAS data were used to validate the numerical model developed for biomass conversion. The results showed that by considering the gas phase reactions a good agreement between the measurement and the model prediction for mass loss and temperature can be achieved. For H2O mole fraction on top of the particle, on the other hand, some discrepancy between the model prediction and the experimental data was observed. Nevertheless, the difference in H2O mole fraction would be much larger by neglecting the gas phase reaction at the particle boundary.

Computational Results Show Gas Phase Reactions Have Great Impact on the Deposition Rate of Silicon in Siemens CVD Reactors

Over 80% of solar grade silicon is produced by using Siemens process. The chemical and physical phenomenon involved in the Siemens CVD reactors is very complex. For the purpose of finding the effect of gas phase reactions on the deposition rate of silicon in Siemens CVD reactors, four different gas phase reaction routes were applied in computational simulations using a commercial software Ansys Fluent. The simulation results show that the gas phase reaction mechanisms have great impact on the predicted Si growth rates. Specifically, the silicon growth rates decrease with an increase in HCl concentration on the rods’ surfaces with a fixed surface temperature. The formation of SiCl4, however, shows insignificant impacts on the growth rate of Si, and the surface concentration of SiCl4 is not directly associated with that of HCl.

Laser deposition of iron in oxygen atmosphere

Abstract Iron oxide films were produced by pulsed laser deposition (PLD) of 57Fe metal in an oxygen atmosphere and their compositions were studied by Mössbauer spectroscopy. The effects of gas-phase reactions were investigated by varying the pressure of O2 gas or an O2/Ar gas mixture. When PLD was performed in a high-pressure O2 atmosphere, the main product in the film was trivalent iron oxide particles. When the O2 pressure was reduced, hematite Fe2O3 became dominant in the film, while wüstite FeO was produced at very low O2 pressures. PLD in an O2/Ar gas mixture produced films of trivalent iron oxide particles and hematite solid, but wüstite was not produced. Increasing the substrate temperature during deposition induced annealing of the films, reducing the lattice defect density. X-ray diffraction patterns were obtained to confirm the assignments, and the surface morphologies of the films were investigated by scanning electron microscopy.

Influence of Gas-Phase Reactions on the Growth of Carbon Nanotubes

The effect of gas-phase reactions on the growth of carbon nanotubes is investigated by a new synthesis method of infrared light local heating chemical vapor deposition where only the substrate can be heated. The simulation of temperature distributions surrounding the heated substrate shows that the region with high temperatures over 600 °C is enlarged with the increase of the substrate temperature. The expansion of the high-temperature region is speculated to result in the growth of carbon nanotubes at a substrate temperature of 770 °C and the successful growth of carbon nanotubes with a higher density at a higher temperature of 900 °C. It is analyzed that polymerization reactions of acetylene gas that occurred in the high-temperature region generate unsaturated carbon chains mainly comprising C4H4. These unsaturated carbon chains, in addition to the acetylene gas, with a high temperature should be important carbon sources for the growth of carbon nanotubes.

The effect of gas-phase reactions on the quantitation of cyclic hydrazone libraries by electrospray ionization (ESI) mass spectrometry

Using mass spectrometry coupled with LC analysis we report evidence of diastereomer dependent fragmentation and oligomerization reactions in the ionization of acyl-hydrazone-based libraries of cyclic oligomers. These effects can significantly affect the accuracy of MS-based quantitations, but also provide a venue for examining ionization effects in dynamic combinatorial libraries (DCLs).

Interactions Between Surface Reactions and Gas-phase Reactions in Catalytic Combustion and Their Influence on Ignition of HCCI Engine

The catalytic combustion of methane in a microchannel whose surface was coated with platinum (Pt) catalyst was studied by numerical-simulation. The effects of gas-phase reactions on the whole catalytic combustion process were analyzed at a high inlet pressure. A sensitivity analysis of the detailed mechanisms of the surface reaction of methane on Pt revealed that the most sensitive reactions affecting the heterogeneous ignition are oxygen adsorption/desorption and methane adsorption, and the most sensitive reactions affecting the homogeneous ignition are OH and H 2O adsorption/desorption. The combustion process of the homogeneous charge compression ignition (HCCI) engine whose piston face was coated with Pt catalyst was simulated. The effects of catalysis and the most sensitive reactions on the ignition timing and the concentration of the main intermediate species during the HCCI engine combustion are discussed. The results show that the ignition timing of the HCCI engine can be increased by, catalysis, and the most sensitive reactions affecting the ignition timing of the HCCI engine are OH and H 2O adsorption/desorption.

Effect of physico-chemical degradation of epoxy resin on partial discharge behavior

Epoxy resin coated electrode specimens were subjected to partial discharges (PD) in air at twice the discharge inception voltage for durations ranging to 5000 h. The PD behavior was characterized by a transition from initially large pulse type discharges (- 200 to 300 pC) to either small pulse (- 1 pC), pseudoglow, glow discharge, or a combination thereof. Although these different forms of discharges were capable of occurring simultaneously, each type tended to prevail over certain periods of the exposure time. The physical and chemical nature of the degradation products formed on the surfaces of the epoxy resin also varied accordingly, indicating that each discrete form of discharge exerts its particular effect on the overall degradation process. Droplet formation on the central portion of the epoxy surfaces typified the large pulse discharge regime, whilst crystals formed within the transition region over which small discharge pulses (- 1 pC) were usually superimposed upon a continuous glow (pseudoglow and true glow regime). The droplets were identified as being partially comprised of a mixture of acids, mainly formic, glycolic, glyoxalic and nitric acids, whilst the crystals consisted of hydrated oxalic acid. Since similar results were obtained with discharges in both air and nitrogen, the effects of gas phase reactions on the PD activity would appear to be only of secondary importance compared to the reactions taking place on the surface of the epoxy resin. Since the test cell design did not allow any pressure variation within the discharge gap, the observed discharge transition cannot be attributed to pressure changes such as may take place within occluded cavities. Nevertheless, the transition observed here is similar to what is found with actual stator bar type insulation, suggesting that pronounced chemical and physical modifications on the surface of the physical cavity inclusions in the bar insulation may account for the characteristic PD behavior observed as a function of time.

Gas phase reactions of trimethylamine alane in low pressure organometallic vapor phase epitaxy of AlGaAs

We have investigated the effects of gas phase reactions between trimethylamine alane (TMAA), triethylgallium (TEG), and arsine on AlxGa1−xAs films grown by low pressure organometallic vapor phase epitaxy. The reactor used in this study provides for independent observation of the effects of TEG-TMAA and TMAA-arsine gas phase reactions. Gas phase reactions involving TMAA and TEG result in the formation of nonvolatile compounds upstream, which condense on the reactor wall, resulting in a reduction of growth rate and a degradation of the deposition uniformity. The TMAA-arsine reaction produces a compositional dependence on the gas phase stoichiometry (V/III ratio). Both of these effects are more severe for higher TMAA fluxes. High quality AlGaAs with excellent thickness and compositional uniformity was produced by spatially separating the TMAA and TEG in the gas phase which minimizes the parasitic reactions.

Contributions of Gas Phase Reactions to CVD Processes

ABSTRACTAn analytical model to account for the effects of gas phase reactions in CVD processes is presented. In this model a system with only two reactions, gas phase production of an intermediate and reaction of the intermediate on the surface of the substrate to form the film is considered. The reaction kinetics, convective and diffusive transport mechanisms are coupled to analyse the thickness distribution over the length of the reactor.Theoretical deposition rate profiles in the direction of gas flow are shown with varying deposition temperatures and gas flow velocities for both surface and gas phase reaction controlled mechanisms. It has been shown that by analyzing the nature of the deposition rate profiles, the rate limiting step in a CVD process can be identified.