Detailed Numerical Comparison of Laminar Burning-Speed of Stratified Hydrogen-Air and Methane-Air Mixture with Corresponding Homogeneous Mixture Using Open-Source Code

2020 ◽  
pp. 1-17
Author(s):  
Mohammad Sadik Rizvi
Keyword(s):  
Equivalence Ratio ◽  
Molecular Diffusion ◽  
Transport Model ◽  
Lewis Number ◽  
Homogeneous Mixture ◽  
Chemical Effect ◽  
Diffusion Effect ◽  
Flammability Limit ◽  
Significant Difference ◽  
Laminar Burning Speed

Abstract Detailed numerical study of laminar burning speed for fuel-air mixture is conducted using laminarReactingLMFoam solver which is a modified version of reactingfoam solver based on OpenFoam code. It accounts for detailed mixture averaged transport property calculation for reacting flow using low-Mach number governing equations. Effect of various equivalence ratio gradients is studied on stratified hydrogen-air and methane-air mixture with mixture-averaged transport model and unity Lewis number for all species and corresponding laminar burning speed is compared with homogeneous mixture. For both the fuel-air mixture, rich to lean stratified mixture resulted in a higher laminar burning speed and no significant difference was noticed for lean to rich stratified mixture when compared to homogeneous mixture at same local equivalence ratio. Increased burning speed is explained based on higher burnt gas temperature and molecular diffusion of lighter species from burnt gas referred to “Chemical Effect” in this study. Effect of thermal and molecular diffusion from the burnt gas on laminar burning speed is studied for stratified and homogeneous mixture using mixture-averaged transport model and unity Lewis number for all species. It is shown that molecular diffusion effect from burnt gas (“Chemical Effect”) are more prominent as compared to thermal diffusion effect. Extension in lean flammability limit for stratified mixture of both the fuel is shown based on higher heat release rate as compared to homogeneous mixture and extension in flammability limit for stratified mixture is explained based on higher Chemical Effect from burnt gas.

On the extinction limit and flammability limit of non-adiabatic stretched methane–air premixed flames

1997 ◽  
Vol 342 ◽  
pp. 315-334 ◽  
Author(s):  
YIGUANG JU ◽  
HONGSHENG GUO ◽  
KAORU MARUTA ◽  
FENGSHAN LIU
Keyword(s):  
Equivalence Ratio ◽  
Flame Temperature ◽  
Lewis Number ◽  
Flammability Limit ◽  
Radiative Heat Loss ◽  
Stretch Rate ◽  
Stable Branch ◽  
Extinction Limit ◽  
Radiation Extinction ◽  
Normal Flame

Extinction limits and the lean flammability limit of non-adiabatic stretched premixed methane–air flames are investigated numerically with detailed chemistry and two different Planck mean absorption coefficient models. Attention is paid to the combined effect of radiative heat loss and stretch at low stretch rate. It is found that for a mixture at an equivalence ratio lower than the standard lean flammability limit, a moderate stretch can strengthen the combustion and allow burning. The flame is extinguished at a high stretch rate due to stretch and is quenched at a low stretch rate due to radiation loss. A O-shaped curve of flame temperature versus stretch rate with two distinct extinction limits, a radiation extinction limit and a stretch extinction limit respectively on the left- and right-hand sides, is obtained. A C-shaped curve showing the flammability limit of the stretched methane–air flame is obtained by plotting these two extinction limits in the mixture strength coordinate. A good agreement is shown on comparing the predicted results with the experimental data. For equivalence ratio larger than a critical value, it is found that the O-shaped temperature curve opens up in the middle of the stable branch, so that the stable branch divides into two stable flame branches; a weak flame branch and a normal flame branch. The weak flame can survive between the radiation extinction limit and the opening point (jump limit) while the normal flame branch can survive from its stretch extinction limit to zero stretch rate. Finally, a G-shaped curve showing both extinction limits and jump limits of stretched methane–air flames is presented. It is found that the critical equivalence ratio for opening up corresponds to the standard flammability limit measured in microgravity. Furthermore, the results show that the flammability limit (inferior limit) of the stretched methane–air flame is lower than the standard flammability limit because flames are strengthened by a moderate stretch at Lewis number less than unity.

scholarly journals Interaction of equivalence ratio fluctuations and flow fluctuations in acoustically forced swirl flames

2021 ◽  
pp. 175682772110155
Author(s):  
Vincent Kather ◽  
Finn Lückoff ◽  
Christian O. Paschereit ◽  
Kilian Oberleithner
Keyword(s):  
Equivalence Ratio ◽  
Transfer Functions ◽  
Transport Model ◽  
Mode Conversion ◽  
Fuel Injector ◽  
One Dimensional ◽  
Flow Fluctuations ◽  
Non Linear ◽  
Non Local ◽  
Acoustic Forcing

The generation and turbulent transport of temporal equivalence ratio fluctuations in a swirl combustor are experimentally investigated and compared to a one-dimensional transport model. These fluctuations are generated by acoustic perturbations at the fuel injector and play a crucial role in the feedback loop leading to thermoacoustic instabilities. The focus of this investigation lies on the interplay between fuel fluctuations and coherent vortical structures that are both affected by the acoustic forcing. To this end, optical diagnostics are applied inside the mixing duct and in the combustion chamber, housing a turbulent swirl flame. The flame was acoustically perturbed to obtain phase-averaged spatially resolved flow and equivalence ratio fluctuations, which allow the determination of flux-based local and global mixing transfer functions. Measurements show that the mode-conversion model that predicts the generation of equivalence ratio fluctuations at the injector holds for linear acoustic forcing amplitudes, but it fails for non-linear amplitudes. The global (radially integrated) transport of fuel fluctuations from the injector to the flame is reasonably well approximated by a one-dimensional transport model with an effective diffusivity that accounts for turbulent diffusion and dispersion. This approach however, fails to recover critical details of the mixing transfer function, which is caused by non-local interaction of flow and fuel fluctuations. This effect becomes even more pronounced for non-linear forcing amplitudes where strong coherent fluctuations induce a non-trivial frequency dependence of the mixing process. The mechanisms resolved in this study suggest that non-local interference of fuel fluctuations and coherent flow fluctuations is significant for the transport of global equivalence ratio fluctuations at linear acoustic amplitudes and crucial for non-linear amplitudes. To improve future predictions and facilitate a satisfactory modelling, a non-local, two-dimensional approach is necessary.

scholarly journals Solar response in tropical stratospheric ozone: a 3-D chemical transport model study using ERA reanalyses

2011 ◽  
Vol 11 (24) ◽  
pp. 12773-12786 ◽  
Author(s):  
S. Dhomse ◽  
M. P. Chipperfield ◽  
W. Feng ◽  
J. D. Haigh
Keyword(s):  
Standard Model ◽  
Transport Model ◽  
Stratospheric Ozone ◽  
Chemical Transport ◽  
Stratospheric Aerosol ◽  
Data Sets ◽  
Chemical Transport Model ◽  
Model Simulations ◽  
Model Study ◽  
Significant Difference

Abstract. We have used an off-line 3-D chemical transport model (CTM) to investigate the 11-yr solar cycle response in tropical stratospheric ozone. The model is forced with European Centre for Medium-Range Weather Forecasts (ECMWF) (re)analysis (ERA-40/operational and ERA-Interim) data for the 1979–2005 time period. We have compared the modelled solar response in ozone to observation-based data sets that are constructed using satellite instruments such as Total Ozone Mapping Spectrometer (TOMS), Solar Backscatter UltraViolet instrument (SBUV), Stratospheric Aerosol and Gas Experiment (SAGE) and Halogen Occultation Experiment (HALOE). A significant difference is seen between simulated and observed ozone during the 1980s, which is probably due to inhomogeneities in the ERA-40 reanalyses. In general, the model with ERA-Interim dynamics shows better agreement with the observations from 1990 onwards than with ERA-40. Overall both standard model simulations are partially able to simulate a "double peak"-structured ozone solar response with a minimum around 30 km, and these are in better agreement with HALOE than SAGE-corrected SBUV (SBUV/SAGE) or SAGE-based data sets. In the tropical lower stratosphere (TLS), the modelled solar response with time-varying aerosols is amplified through aliasing with a volcanic signal, as the model overestimates ozone loss during high aerosol loading years. However, the modelled solar response with fixed dynamics and constant aerosols shows a positive signal which is in better agreement with SBUV/SAGE and SAGE-based data sets in the TLS. Our model simulations suggests that photochemistry contributes to the ozone solar response in this region. The largest model-observation differences occur in the upper stratosphere where SBUV/SAGE and SAGE-based data show a significant (up to 4%) solar response whereas the standard model and HALOE do not. This is partly due to a positive solar response in the ECMWF upper stratospheric temperatures which reduces the modelled ozone signal. The large positive upper stratospheric solar response seen in SBUV/SAGE and SAGE-based data can be reproduced in model runs with fixed dynamical fields (i.e. no inter-annual meteorological changes). As these runs effectively assume no long-term temperature changes (solar-induced or otherwise), it should provide an upper limit of the ozone solar response. Overall, full quantification of the solar response in stratospheric ozone is limited by differences in the observed data sets and by uncertainties in the solar response in stratospheric temperatures.

Laboratory Studies of the Effects of Dissolved Organic Material on the Adsorption of Organochlorine Pesticides by Sediments and Transport in Rivers

1993 ◽  
Vol 28 (8-9) ◽  
pp. 199-208 ◽  
Author(s):  
J.-Y. Ding ◽  
S.-C. Wu
Keyword(s):  
Organic Matter ◽  
Molecular Diffusion ◽  
Transport Model ◽  
Water System ◽  
High Rate ◽  
Sensitivity Analyses ◽  
Overlying Water ◽  
Sediment Bed ◽  
Three Phase ◽  
Phase Transport

In this study experiments simulating sediment/water system were carried on with sediments spiked with aldrin, heptachlor epoxide and p,p'-DDE. It was expected that these hydrophobic contaminants would be released to the overlying water column from sediment bed with molecular diffusion and co-diffusion with dissolved organic matter (DOM) as well. A three-phase-transport model including aqueous, solid and mobile adsorptive phases was developed and used to describe the behavior of these contaminants and to explain the results of the experiments. Sensitivity analyses show that observable effects of DOM occur only under conditions of high partition coefficient (Koc) of the contaminant and high rate of transfer from sediment organic matter to DOM. In this study, owing to the low concentration of DOM and relatively low hydrophobicity of the compounds, the DOM-associated pollutant flux does not significantly contribute to the total flux. Also, the simulated results of the model can reasonably explain the variations of the concentrations of the spiked compounds observed in the microcosms.

scholarly journals Evaluation of the effect of two commercially available non-alcoholic mouth rinses on the microhardness of composite material - An invitro study

2019 ◽  
Vol 1 (Issue 1) ◽  
pp. 14-21
Author(s):  
Reshma Rajasekhar ◽  
Baby James ◽  
Minimol K Johny ◽  
Jose Jacob
Keyword(s):  
Artificial Saliva ◽  
Composite Resins ◽  
Chemical Effect ◽  
Mouth Rinses ◽  
Significant Difference ◽  
Nanohybrid Composite ◽  
Group 5 ◽  
Vicker’S Microhardness ◽  
Posterior Restorations ◽  
Group 1

Composites have been widely used for anterior as well as posterior restorations due to increased aesthetic demands. Composite restorations placed in patients oral cavity is subjected to masticatory load, pH changes and chemical effect due to mouthwashes, beverages, food etc and the detrimental effects associated with these challenges on the microhardness of composite has to be determined. The study was conducted to evaluate the effect of two commercially available non-alcoholic mouth rinses on the microhardness of micro-hybrid and nanohybrid composite resins. Forty disc-shaped composite specimens were made from each type of composite materials and were divided into 5 groups with 8 samples each, Group 1= Microhybrid+Chlorhex, Group2=Microhybrid + Hexidine, Group3= Nanohybrid + Chlorhex, Group4= Nanohybrid+ Hexidine, Group 5= Nanohybrid and Micro hybrid + artificial saliva(Control).The samples were immersed in mouthrinses for 24 hrs and microhardness was subsequently measured using a Vicker’s microhardness tester. There was no significant difference in microhardness values of tested composite resins in either mouth rinses when compared to composites immersed in artificial saliva. Non-alcoholic mouth rinses do not affect the microhardness of micro-hybrid and nanohybrid composites.

The Explosion Severity of Biogas(CH4-CO2)/Air Mixtures in a Closed Vessel

2019 ◽  
Vol 964 ◽  
pp. 33-39
Author(s):  
Nur Aqidah Muhammad Harinder Khan ◽  
Siti Zubaidah Sulaiman ◽  
Izirwan Izhab ◽  
Siti Kholijah Abdul Mudalip ◽  
Rohaida Che Man ◽  
...  
Keyword(s):  
Maximum Rate ◽  
Equivalence Ratio ◽  
Pressure Rise ◽  
Flammability Limit ◽  
Flammability Limits ◽  
Spherical Vessel ◽  
Suppression Effect ◽  
Explosion Overpressure ◽  
Deflagration Index ◽  
Lower Flammability Limit

Biogas which consists of methane (CH4) and carbon dioxide (CO2) could explode when diluted to a certain degree with air in the presence of ignition source. The maximum explosion overpressure (Pmax), the maximum rate of pressure rise (dP/dt)max, flammability limits, and deflagration index are the most important explosion severities parameters to characterize the risk of explosion. In this research paper, the effect of equivalence ratio (ER) of biogas/air mixtures and the effect of CO2 concentrations presence in biogas were studied in a 20 L spherical vessel. The values of Pmax and (dP/dt)max of biogas/air mixtures were more severe at ER 1.2. At various CO2 content, Pmax and (dP/dt)max of biogas/air mixtures were the least affected at 45% vol/vol of CO2. On the other hand, deflagration index (KG) of biogas/air mixtures trend was the most severe at 35% vol/vol of CO2 content despite the lowest Pmax and (dP/dt)max at 45% vol/vol of CO2 content. The lowest values in Pmax and (dP/dt)max were due to the diffusivity properties of CH4 that had surpassed the CO2 suppression effect. Furthermore, the presence of CO2 in biogas/air mixtures had increased the upper flammability limit and lower flammability limit of biogas.

Significance of biofilm structure on transport of inert particulates into biofilms

1998 ◽  
Vol 38 (8-9) ◽  
pp. 163-170 ◽  
Author(s):  
S. Okabe ◽  
H. Kuroda ◽  
Y. Watanabe
Keyword(s):  
Molecular Diffusion ◽  
Domestic Wastewater ◽  
Honeycomb Structure ◽  
Convective Transport ◽  
Transport Efficiency ◽  
Particulate Transport ◽  
Fluorescent Microbeads ◽  
Significant Difference ◽  
Rapid Transport ◽  
Biofilm Structure

Evolutional changes in interior structures of mixed population biofilms grown on domestic wastewater were quantitatively analyzed using a cryosectioning technique and an image analysis. Meanwhile, transport of particulates into the biofilms was also experimentally investigated using fluorescent microbeads as tracers to relate the biofilm structure and particulate transport into the biofilm. Microscopic observation of the cryomicrotomy biofilm sections indicated the biofilms were very porous and consisted of interwinded filamentous biomass acting as a framework of the biofilm. A honeycomb structure was often found, which would make the biofilm more resistant to water flow. There were micropores with the diameter of about 10 μm microcolony aggregates attached to filamentous biomass and macropores with the diameter of 20–200 μm in the biomass matrix. These pores did not clog during two months of cultivation. Areal porosity was about 30% in the bottom biofilm and more than 90% in the surface. Significant difference in transport efficiency was not observed for various sizes of microbeads due to the presence of macropores. Therefore, even 10 μm tracer beads could quickly traverse throughout a biofilm 640 μm thick via water channels or macropores and then penetrated into the micropores. Convective transport from the bulk to the bottom biofilm, rather than molecular diffusion, was responsible for this rapid transport. Based on experimental results, it can be concluded that the biofilm structure seems to be well designed to maximize the transport efficiency of substrates and products and the strength of biofilm structure.

Evaluation of the Accuracy of Selected Syngas Chemical Mechanisms

2015 ◽  
Vol 137 (4) ◽  
Author(s):  
Fahad M. Alzahrani ◽  
Yinka S. Sanusi ◽  
Konstantina Vogiatzaki ◽  
Ahmed F. Ghoniem ◽  
Mohamed A. Habib ◽  
...  
Keyword(s):  
Equivalence Ratio ◽  
Laminar Flame ◽  
Computational Cost ◽  
Operating Conditions ◽  
Average Error ◽  
Operating Pressure ◽  
Chemical Mechanisms ◽  
Flame Thickness ◽  
Reduced Mechanism ◽  
Significant Difference

The implementation of reduced syngas combustion mechanisms in numerical combustion studies has become inevitable in order to reduce the computational cost without compromising the predictions' accuracy. In this regard, the present study evaluates the predictive capabilities of selected detailed, reduced, and global syngas chemical mechanisms by comparing the numerical results with experimental laminar flame speed (LFS) values of lean premixed (LPM) syngas flames. The comparisons are carried out at varying equivalence ratios, syngas compositions, operating pressures, and preheat temperatures to represent a range of operating conditions of modern fuel flexible combustion systems. NOx emissions predicted by the detailed mechanism, GRI-Mech. 3.0, are also used to study the accuracy of the selected mechanisms under these operating conditions. Moreover, the selected mechanisms' accuracy in predicting the laminar flame thickness (LFT), species concentrations of the reactants, and OH profiles at different equivalence ratios and syngas compositions are investigated as well. The LFS is generally observed to increase with increasing equivalence ratio, hydrogen content in the syngas, and preheat temperature, while it is decreased with increasing operating pressure. This trend is followed by all mechanisms understudy. The global mechanisms of Watanabe–Otaka and Jones–Lindstedt for syngas are consistently observed to over-predict and under-predict the LFS up to an average of 60% and 80%, respectively. The reduced mechanism of Slavinskaya has an average error of less than 20%, which is comparable to the average error of the GRI-Mech. 3.0. It however over-predicts the flame thickness by up to 30% when compared to GRI-Mech. 3.0. The NO prediction by Li mechanism and the reduced mechanisms are observed to be within 10% prediction range of the GRI-Mech. 3.0 at intermediate equivalence ratio (φ=0.74) up to stoichiometry. Moving toward more lean conditions, there is significant difference between the GRI-Mech. 3.0 NO prediction and those of the reduced mechanisms due to relative importance of the prompt NOx at lower temperature compared to thermal NOx that is only accounted for by the GRI-Mech. 3.0.

Laminar premixed flame extinction limits. II Combined effects of stretch and radiative loss in the single flame unburnt-to-burnt and the twin-flame unburnt-to-unburnt opposed flow configurations

2005 ◽  
Vol 462 (2066) ◽  
pp. 349-370 ◽  
Author(s):  
Graham Dixon-Lewis
Keyword(s):  
Lewis Number ◽  
Combustion Products ◽  
Maximum Temperature ◽  
Radiative Loss ◽  
Conductive Heat ◽  
Flammability Limit ◽  
Opposed Flow ◽  
Flame Stretch ◽  
Flow Configurations ◽  
Stagnation Plane

Numerical methods have been used to examine the effects of (a) stretch alone, and (b) a combination of stretch and radiative loss, on the properties and extinction limits of methane–air flames near the lean flammability limit. Two axisymmetric opposed flow configurations were examined: (i) a single flame, unburnt-to-burnt (UTB) system in which fresh reactant is opposed by a stream of its own combustion products at the unburnt temperature, and (ii) a symmetric unburnt-to-unburnt (UTU) configuration where twin flames are supported back to back, one on each side of the stagnation plane. The maximum temperatures achieved in the UTB system are always away from the stagnation plane. For a fixed sufficiently sub-adiabatic product stream temperature, increasing flame stretch or gaseous radiative emissivity, or a combination of both, will augment downstream conductive heat loss, leading to a reduction in T max and eventually to an abrupt extinction if the loss rate is sufficiently large. The UTU system is more complex, and offers the additional possibility of purely stretch-induced extinctions where the flames are forced together back-to-back so that radiative loss is restricted to upstream of the maximum temperature. Extinction in these cases occurs by straightforward truncation of the hot sides of the reaction zones. At sufficiently low stretch, near and at the standard flammability limit, radiative loss makes a major contribution to the overall extinction mechanism in both configurations. The detailed effects of flame stretch on extinction behaviour depend on the diffusion characteristics within the near-limit mixtures, in particular the Lewis number, Le, of the deficient component. The effect of high stretch is always to attenuate the composition range of flammability. However, for Le<1 this range is extended at low to moderate stretch, particularly in the UTU situations where downstream radiative loss is not present at extinction. Lewis number effects for a global methane–air chemistry, and with assumed Le≥1, are discussed in the light of numerical results previously presented by Ju et al . ( Ju et al . 1998 Combust. Flame 113 , 603–614).

Determination of Granites' Mineral Specific Porosities by PMMA Method and FESEM/EDAX

2006 ◽  
Vol 985 ◽  
Author(s):  
Marja Siitari-Kauppi ◽  
L. Penttinen ◽  
M. Siitari-Kauppi ◽  
U. Alanso ◽  
M. Garcia-Gutierrez ◽  
...  
Keyword(s):  
Diffusion Coefficients ◽  
Molecular Diffusion ◽  
Total Porosity ◽  
Mineral Surfaces ◽  
Quartz Crystals ◽  
Apparent Diffusion Coefficients ◽  
Significant Difference ◽  
Apparent Diffusion ◽  
The Matrix ◽  
Granite Samples

AbstractOver extended periods, long-lived radionuclides (RN) or activation products within geologic disposal sites may be released from the fuel and migrate to the geo/biosphere. In the bedrock, contaminants will be transported along fractures by advection and retarded by sorption on mineral surfaces and by molecular diffusion into stagnant pore water in the matrix along a connected system of pores and micro-fissures.The objective of this paper was to determine the connective porosity and mineral-specific porosities for three granite samples by 14C methylmethacrylate (14C-PMMA) autoradiography. Scanning electron microscopy and energy-dispersive X-ray analyses (FESEM/EDAX) were performed in order to study the pore apertures of porous regions in greater detail and to identify the corresponding minerals. Finally, the porosity results were used to evaluate the diffusion coefficients of RNs from previous experiments which determined apparent diffusion coefficients for the main minerals in three granite samples by the Rutherford Backscattering technique.The total porosity of the Grimsel granite (0.75%) was significantly higher than the porosities of the El Berrocal and Los Ratones granites (0.3%). The porosities of the Grimsel granite feldspars were two to three times higher than the porosities of the El Berrocal and Los Ratones granites' feldspars. However, there was no significant difference between the porosities of the dark minerals. A clear difference was found between the various quartz grains. Quartz crystals were nonporous in the El Berrocal and Los Ratones granites when measured by the PMMA method, but the quartz crystals in the Grimsel granite showed 0.5% intra granular porosity. The apparent diffusion coefficients calculated for uranium diffusion within Grimsel granite on different minerals were very similar (210-13 ± 0.5 m2/s), but differences within both Spanish granites were found from one mineral to another (9 ± 110-14 m2/s in feldspars and 4.5 ± 0.510-14 m2/s in quartz) - always presenting lower diffusion values than in the Grimsel granite.

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