Low Burning Rate Research Articles

Subgraph Sampling Methods for Social Networks: The Good, the Bad, and the Ugly

The trajectories of social processes (e.g., peer pressure, imitation, and assimilation) that take place on social networks depend on the structure of those networks. Thus, to understand a social process or to predict the associated outcomes accurately, marketers would need good knowledge of the social network structure. However, many social networks of relevance to marketers are large, complex, or hidden, making it prohibitively expensive to map out an entire social network. Instead, marketers often need to work with a sample (i.e., a subgraph) of a social network. In this paper we evaluate the efficacy of nine different sampling methods for generating subgraphs that recover four structural characteristics of importance to marketers, namely, the distributions of degree, clustering coefficient, betweenness centrality, and closeness centrality, which are important for understanding how social network structure influences outcomes of processes that take place on the network. Via extensive simulations, we find that sampling methods differ substantially in their ability to recover network characteristics. Traditional sampling procedures, such as random node sampling, result in poor subgraphs. When the focus is on understanding local network effects (e.g., peer influence) then forest fire sampling with a medium burn rate performs the best, i.e., it is most effective for recovering the distributions of degree and clustering coefficient. When the focus is on global network effects (e.g., speed of diffusion, identifying influential nodes, or the “multiplier” effects of network seeding), then random-walk sampling (i.e., forest-fire sampling with a low burn rate) performs the best, and it is most effective for recovering the distributions of betweenness and closeness centrality. Further, we show that accurate recovery of social network structure in a sample is important for inferring the properties of a network process, when one observes only the process in the sampled network. We validate our findings on four different real-world networks, including a Facebook network and a co-authorship network, and conclude with recommendations for practice.

Heat transfer in the rotary ash cooler with residual char combustion considered

In circulating fluidized-bed (CFB) boilers, some residual char is left in the ash when the ash is drained into the rotary ash cooler (RAC). The residue char will combust in the RAC given oxygen is present. It is important to understand and predict the influence of residual char combustion on the heat transfer process in the RAC in order to avoid slagging and fouling inside the RAC. In this paper, an improved heat transfer model of the RAC was developed in which the residual char combustion process was considered. Based on the analysis, the shrinking sphere model was selected to describe the residual char combustion. The improved model was validated by the final ash temperature and cooling water temperature data measured from a few RACs equipped with the coal fired CFB boilers with different steam output. The predictions of the improved model agree well with the field measurement data, within an error range of ± 10%. The simulation results also show that the heat released from residual char combustion counts for about 60–80% of the total heat released by ash in the inlet section of the RAC, and the influence of residual char combustion decreases rapidly along the ash flow. The final ash temperature increases accordingly with the increase of inlet carbon content, and decreases with the increases of residual char size. As particle size is larger than 3 mm, the final ash temperature changes little at a given carbon content due to a low mass burning rate. It is suggested that combustion of residual char should be considered when the inlet carbon content is over 2.5%; otherwise, the combustion of residual char in the RAC could be neglected.

Dark zones of solid propellant flames: Critically assessed datasets, quantitative model comparison, and detailed chemical analysis

The formation of propellant dark zone (DZ) structures in the gaseous flames above many solid propellants has been a subject of recurrent interest to us for about 20 years. The DZ structure is controlled by small molecule chemistry. The DZ chemistry is very important in controlling both the flame structure at low pressure (10–100 atm) and burning rates at high pressure (above ∼500 atm), even though DZs collapse at higher pressures. We developed a detailed, frequently updated mechanism to model it. This report reviews prior work, introduces our most recent modeling results, and documents the first quantitative tests of several key assumptions. All relevant experimental literature is critically assessed to identify datasets for testing our model. Comparison of predictions and experimental results shows reasonable agreement, and thus we advocate use of our mechanism. But the precision of both experiments and predictions is not tight, nor are there many test datasets. Further experimentation is needed, and types of study that would be of greatest benefit are suggested. A detailed discussion of the chemistry that controls DZ structure is presented.

Combustion and emission characteristics of HCNG in a constant volume chamber

Finding an alternative fuel and reducing environmental pollution are the main goals for future internal combustion engines. Hydrogenmethane (HCNG) is now considered an alternative fuel due to its low emission and high burning rate. An experimental study was carried out to obtain fundamental data for the combustion and emission characteristics of pre-mixed hydrogen and methane in a constant volume chamber (CVC) with various fractions of hydrogen-methane blends. A pre-mixed chamber was used to obtain a good mixture of these gases. Exhaust emissions were measured using a Horiba exhaust gas analyzer for various fractions of hydrogen-methane blends. The results showed that the rapid combustion duration was shortened, and the rate of heat release elevated as the hydrogen fraction in the fuel blend was increased. Moreover, the maximum mean gas temperature and the maximum rate of pressure rise also increased. These phenomena were attributed to the burning velocity, which increased exponentially with the increased hydrogen fraction in the fuel blend. Exhaust HC and CO2 concentrations decreased, while NOX emission increased with an increase in the hydrogen fraction in the fuel blend. Our results could facilitate the application of hydrogen and methane as a fuel in the current fossil hydrocarbon-based economy and the strict emission regulations in internal combustion engines.

Emission factors from residential combustion appliances burning Portuguese biomass fuels

Smoke from residential wood burning has been identified as a major contributor to air pollution, motivating detailed emission measurements under controlled conditions. A series of experiments were performed to compare the emission levels from two types of wood-stoves to those of fireplaces. Eight types of biomass were burned in the laboratory: wood from seven species of trees grown in the Portuguese forest (Pinus pinaster, Eucalyptus globulus, Quercus suber, Acacia longifolia, Quercus faginea, Olea europaea and Quercus ilex rotundifolia) and briquettes produced from forest biomass waste. Average emission factors were in the ranges 27.5-99.2 g CO kg(-1), 552-1660 g CO(2) kg(-1), 0.66-1.34 g NO kg(-1), and 0.82-4.94 g hydrocarbons kg(-1) of biomass burned (dry basis). Average particle emission factors varied between 1.12 and 20.06 g kg(-1) biomass burned (dry basis), with higher burn rates producing significantly less particle mass per kg wood burned than the low burn rates. Particle mass emission factors from wood-stoves were lower than those from the fireplace. The average emission factors for organic and elemental carbon were in the intervals 0.24-10.1 and 0.18-0.68 g kg(-1) biomass burned (dry basis), respectively. The elemental carbon content of particles emitted from the energy-efficient "chimney type" logwood stove was substantially higher than in the conventional cast iron stove and fireplace, whereas the opposite was observed for the organic carbon fraction. Pinus pinaster, the only softwood species among all, was the biofuel with the lowest emissions of particles, CO, NO and hydrocarbons.

Thermal decomposition behaviors and burning characteristics of ammonium nitrate/polytetrahydrofuran/glycerin composite propellant

Ammonium nitrate (AN)-based propellant is plagued by several drawbacks: low burning rate, poor ignitability, and low energy. The application of an energetic binder is an effective approach to the practical use of AN-based propellants but has not been used to date because the synthesis processes are complicated and expensive. Polytetrahydrofuran (PTHF) is a raw material for rubber products and is not an energetic material. The repeating unit of PTHF has one oxygen atom, and although PTHF could not become solid with use of a curing agent alone, PTHF became rubbery with the use of glycerin as a crosslinking modifier. Thus the PTHF/glycerin mixture would be a useful binder to improve the burning characteristics of AN-based propellants, due to the oxygen atoms in the mixture. The use of PTHF/glycerin binder is theoretically effective for enhancing the performance of AN-based propellant, but the combined AN/PTHF/glycerin propellants failed to ignite; they only generated a flashing flame at the ignition event. The ignition failure of the AN/PTHF/glycerin propellant occurred because the melted binder covered the burning surface and interfered with the evolution of the AN decomposition gases and the heat flux feedback from the flame to AN. The AN/PTHF/glycerin propellant was then burned with the addition of a catalyst, and the effect of PTHF/glycerin binder on the improvement of burning characteristics was apparent. The PTHF/glycerin binder was an effective binder material for improving a performance of the AN-based composite propellant.

Tailored flame retardancy via nanofiller dispersion state: Synergistic action between a conventional flame-retardant and nanoclay in high-impact polystyrene

Two preparation techniques attempting to disperse nanoclays in high-impact polystyrene matrix yielded different clay dispersion states either as intercalated or phase-separated morphologies. By this means, the influence of micro- and nanocomposite formation on the synergistic flame retardancy between nanoclays and a conventional mineral-type flame-retardant additive, namely aluminium tri-hydroxide, was investigated in terms of limiting oxygen index, horizontal burning rates and cone calorimetric fire properties. Reductions in peak heat release rates in the cone calorimeter were doubled with nanocomposites relative to microcomposites, attributed to char enhancement and lower mass loss rates. This was accompanied by higher limiting oxygen index, lower burning rates and better mechanical properties. In particular, the formation of nanocomposites allowed for the recovery of tensile strength reductions caused by high loadings of aluminium tri-hydroxide in the polymer.

Aluminum Burn Rate Modifiers Based on Reactive Nanocomposite Powders

AbstractAluminum powders have long been used as additives in propellants, pyrotechnics, and explosives. Aluminum has a high enthalpy of combustion but relatively low burn rate. Addition of reactive nanocomposite powders can increase the burn rate of aluminum and thus the overall reaction rate of the energetic formulation. Replacing only a small fraction of the fuel by a nanocomposite material can enhance the reaction rate with little change to the thermodynamic performance of the formulation. This research showed the feasibility of the above concept using nanocomposite powders prepared by arrested reactive milling (ARM), a scalable “top‐down” technique for manufacturing reactive nanomaterials. The nanocomposite materials used in this study were 2B+Ti, and Al‐rich thermites: 8Al+3CuO, and 8Al+MoO3. The reactive nanocomposite powders were added to micrometer‐sized aluminum powder and the mixture was aerosolized and burned in a constant volume chamber. The combustion atmosphere was varied using oxygen, nitrogen, and methane. The resulting pressure traces were recorded and processed to compare different types and amounts of modifiers. Additives of nanocomposite powders of 8Al+MoO3 and 2B+Ti to micrometer‐sized aluminum were found to be effective in increasing both the rate of pressure rise and maximum pressure in the respective constant volume explosion experiments. It was observed that 20 wt.‐% of additive resulted in the best combination of the achieved burn rate and pressure.

Burning Characteristics of Ammonium Nitrate‐based Composite Propellants Supplemented with Ammonium Dichromate

AbstractAmmonium nitrate (AN)‐based composite propellants have attracted much attention, primarily because of the clean burning nature of AN as an oxidizer. However, such propellants have some disadvantages such as poor ignition and low burning rate. Ammonium dichromate (ADC) is used as a burning catalyst for AN‐based propellants; however, the effect of ADC on the burning characteristics has yet to be sufficiently delineated. The burning characteristics of AN/ADC propellants prepared with various contents of AN and ADC have been investigated in this study. The theoretical performance of an AN‐based propellant is improved by the addition of ADC. The increase in the burning rate is enhanced and the pressure deflagration limit (PDL) becomes lower with increasing amount of ADC added. The increasing ratio of the burning rate with respect to the amount of ADC is independent of the AN content and the combustion pressure. The optimal amount of ADC for improving the burning characteristics has been determined.

Chemical, Microphysical and Optical Properties of Primary Particles from the Combustion of Biomass Fuels

Biomass fuel combustion for residential energy significantly influences both emissions and the atmospheric burden of aerosols in world regions, i.e., east and south Asia. This study reports measurements of climate-relevant properties of particles emitted from biomass fuels widely used for cooking in south Asia, in laboratory experiments simulating actual cooking in the region. Fuel burn rates of 1-2 kg h(-1) for wood species, and 1.5-2 kg h(-1) for crop residues and dried cattle dung, influenced PM2.5 emission factors which were 1.7-2 g kg(-1) at low burn rates but 5-9 gkg(-1) at higher burn rates. Total carbon accounted for 45-55% and ions and trace elements for 2-12% of PM2.5 mass. The elemental carbon (EC) content was variable and highest (22-35%) in particles emitted from low burn rate combustion (wood and jute stalks) but significantly lower (2-4%) from high burn rate combustion (dried cattle dung and rice straw). The mass absorption cross-section (MAC, m2 g(-1)) correlated with EC content for strongly absorbing particles. Weakly absorbing particles, from straw and dung combustion, showed absorption that could not be explained by EC content alone. On average, the MAC of biofuel emission particles was significantly higher than reported measurements from forest fires but somewhat lower than those from diesel engines, indicating potential to significantly influence atmospheric absorption. Both for a given fuel and across different fuels, increased burn rates result in higher emission rates of PM2.5, larger organic carbon (OC) content, larger average particle sizes, and lower MAC. Larger mean particle size (0.42-1.31 microm MMAD) and organic carbon content, than in emissions from combustion sources like diesels, have potential implications for hygroscopic growth and cloud nucleation behavior of these aerosols. These measurements can be used to refine regional emission inventories and derive optical parametrizations, for climate modeling, representative of regions dominated by primary particles from biomass fuel combustion.

On the effect of filters for the design of solid propellant gas generators

Solid propellant gas generator (SPGG) plays a role as a turbo pump starter in liquid propellant propulsion systems by supplying pressurized gas to power turbines for engine start. For such a purpose, the propellant should burn with a low flame temperature and the combustion gas should not contain corrosive constituents. Stabilized AN-based propellants have been used as the most appropriate propellant compositions that meet these requirements. However, because of its low burning rate, the burning area of the propellant intends to increase to satisfy the required mass flux and consequently the burning area increment brings on the SPGG size augmentation. A flow restriction such as filters, which are usually employed to filter off impurities in the combustion gas, is applied to decrease the SPGG size by rising up the combustion pressure resulting in increasing the burning rate. Pressure loss coefficient effects on the combustion pressure and on the SPGG/turbines system are investigated with the experimental results. Then the feasibility of the SPGG size reduction by the employment of filters has been studied. The preliminary results of this study show that the considerable reduction of SPGG size would be achievable just by installing a filter with a relatively high pressure loss coefficient.

Co-combustion of textile residues with cardboard and waste wood in a packed bed

The combustible fraction of the municipal waste is mostly bio-derived. Energy recovery of the wastes that cannot be economically recycled is a key part of sustainable energy policy and waste management. Textile residues have high energy content. When burned alone in a packed bed system, however, their combustion efficiency is low due to the irregular propagation of the ignition front and the low burning rates. In order to achieve more efficient combustion of textile residues, a series of co-combustion tests were carried out for various mixture compositions and air flow rates in a packed bed combustor. The combustion performance of these materials was evaluated by using quantitative measures such as ignition rate, burning rate and equivalence ratio. Co-combustion of textile residues with cardboard for a textile fraction of up to 30% achieved satisfactorily high burning rate and low unburned carbon content in the bottom ash. The mixture was more resistant to convective cooling by air, which significantly expanded the range of air flow rate for combustion at high burning rates. In co-combustion with a material that has a very low ignition front speed such as waste wood, the propagation of the ignition front was governed by textile residues. Therefore, the co-combustion of textile residues can be better performed with a material having similar ignition front speeds, in which the two materials simultaneously burn at the ignition front.

Measuring the violence of dust explosions with the “20 l sphere” and with the standard “ISO 1 m 3 vessel” : Systematic comparison and analysis of the discrepancies

On the basis of a systematic testwork with a number of different dusts, the explosion indices as determined within the 20 l sphere and with the ISO-VDI 1 m 3 vessel have been compared. The repeatability has been assessed and since some systematic deviations appear a refined physical analysis of the explosion processes is developed. It appears in particular that the cube root law supposed to link both vessels is not verified. A striking illustration of this appears when a dust with a significant explosion severity inside the 20 l sphere is not even explosible in the larger vessel. It is strongly suggested that the ignition energy is forcing very significantly the explosion in the smaller vessel inducing several tens of Celsius degrees of preheating. It is shown also that the inner level of turbulence is decreasing very fast in the 20 l sphere during the flame development so that difficult-to-ignite mixtures would tend to burn at a lower combustion rate. It is further demonstrated that the major bias between the chambers can be explained and quantified with these elements. A correlation with the standard 1 m 3 vessel and a grid of interpretation of the data is proposed.

Low-temperature combustion of energetic materials in filled polymer systems

In systems filled by inert additives, combustion of cellulose nitrate (CN) proceeds in a flameless low-temperature regime with a low linear burning rate. At a standard temperature, the exponent in the low of CN combustion in ballasted mixtures with inert additives in the pressure range of 0.1 to 10 MPa is several times lower than that of pure cellulose nitrate and amounts to 0.23. The qualitative and quantitative composition of gaseous products of flameless CN conversion is found. It is noted that this composition approximately corresponds to data available in the literature for the products of thermal decomposition of cellulose nitrate at comparatively low temperatures. Based on this fact and on a weak dependence of the CN burning rate on pressure in ballasted systems, the process under these conditions is assumed to be controlled by conversion of the energetic component predominantly in the condensed phase. In the case of a composite consisting of cellulose nitrate, silicon carbide, and polymer binder, for samples 10–25 mm in diameter, armoring exerts practically no effect on combustion parameters. Combustion of the same mixture with smaller diameters of non-armored samples is unstable. The presence of a liner establishes a clearly expressed critical combustion diameter in the examined systems.

DECOMPOSITION AND PERFORMANCEOFNEW HIGH NITROGEN PROPELLANTS AND EXPLOSIVES

As of late, molecules with high nitrogen content have received increased attention, due in large part to their novel energetic materials properties. At the Los Alamos National Laboratory, we continue to pursue the development and characterization of new high-nitrogen materials for applications in a wide variety of fields. In this work three molecules, triaminoguanidinium azotetrazolate (TAGzT), 3,6-bis-nitroguanyl-1,2,4,5-tetrazine and its corresponding bis-triaminoguanidinium salt, are studied all of which are high-nitrogen compounds with little or no oxygen, however, retain energetic material properties as a result of their high heats of formation. Because of this, the decomposition of this class of compounds have limited or no secondary oxidation reactions of carbon and hydrogen. Other materials discussed for comparison include 3,3'-azobis(6-amino-1,2,4,5-tetrazine)-mixed N-oxides (DAATO{sub 3.5}) and 3,6-bis(1H-1,2,3,4-tetrazol-5-ylamino)-s-tetrazine (BTATz) and the nitramine octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX). The fact that many of these molecules approach 80% nitrogen content makes them potentially useful as gas generants or energetic materials with low flame temperatures, while simultaneously increasing the impulse of gun or rocket propellants. The burning rate, flash pyrolysis (T-jump/FTIR spectroscopy), explosive sensitivity and performance properties were determined. Some examples of interesting behaviors include that TAGzT exhibits one of the fastest low pressure burning rates yet measured for an organic compound,more » and 3,6-bis-nitroguanyl-1,2,4,5-tetrazine has one of the lowest pressure exponents yet measured for a pure organic compound.« less

Historical Fire Cycles in the Canadian Rocky Mountain Parks

Abstract The subject of this article is the fire history of seven contiguous national and provincial parks in the Canadian Rocky Mountains, four to the east of the continental divide and three to the west. Seven centuries of time-since-fire data were combined and arrayed in three sets: all seven parks together, then the east-side and west-side groups separately. Fire cycles were estimated by four different methods: two of them statistical and two deterministic. Statistical evidence of distinct changes in fire regime, always toward a lower burning rate, was uncovered, with transitions in 1760 and 1940 on the east side, and in 1840 on the west side. The principal conclusions were: The seven parks as a unit supported a fire cycle of 60–70 yr for nearly five centuries before 1760. The burning rate then dropped sharply and a longer cycle of about 175 yr prevailed until 1940. Fire history in the combined large parks on the east side was similar to the above. By contrast, the smaller west-side parks group sustained a fire cycle of about 90 yr for at least five centuries before 1840; the burning rate then decreased irregularly to 2000. On the east side, the near absence of burned area after 1940 was attributed to the onset of effective fire protection. Possible prior causes of the earlier transitions on east and west sides include changes in climate or aboriginal fire practices, but this issue is left unresolved.

Kinetics of heterogeneous reactions of carbon and oxygen during combustion of porous carbon particles in oxygen

A model of combustion of a high-porosity carbon particle in oxygen is considered, which takes into account heterogeneous and homogeneous chemical reactions inside the particles and radiative heat transfer. The boundaries of the domain where the burning rate depends on the particle temperature are determined. The possibility of two combustion regimes is demonstrated: regime with a high burning rate, where the carbon-oxygen reaction proceeds in a layer adjacent to the particle surface, and regime with a low burning rate, where the reaction proceeds in the entire particle volume. In the regime with a high burning rate, the main product of the reaction between carbon and oxygen is carbon monoxide, whereas both carbon monoxide and carbon dioxide can be formed in the regime with a low burning rate. The kinetic equations of heterogeneous reactions C + O2 = CO2 and 2C + O2 = 2CO are determined, which reveal the retarding effect of carbon monoxide and dioxide on the rates of these reactions.

Modelling Turbulent Deflagrations in Type Ia Supernovae

We present an overview of the current state of multidimensional modelling of type Ia supernovae and an example for surprising consequences of impoving the physics of the model. In this case, an improved handling of subgrid scale turbulence gives rise to lower burning rates and a decreased global energy release. While this result is too preliminary to be interpreted quantitatively, it shows that much work remains to be done before the turbulent deflagration model can be declared to be understood and/or insufficient to explain normal type Ia supernovae.

Effect of Airflow Setting on the Organic Composition of Woodheater Emissions

Fine particle emissions from woodheaters are large contributors to ambient atmospheric pollution in a number of regional centers in Australia. The health impact of woodsmoke is not limited to the particle loading alone, and a wide range of toxic inorganic and organic compounds are also emitted as gases or adsorbed onto particles. The organic composition of woodsmoke was determined from two heater models operated using different airflow settings. Particle emission factors varied between 3 and 36 g per kg dry wood burned, with higher burn rates (open airflow) producing significantly less particle mass per kg wood burned than the low burn rates (closed airflow). Over 90 organic compounds were quantified from the vapor- and particle-phases, including furans, methoxyphenols, and other substituted aromatics, PAHs, maltols, and the sugar levoglucosan. Emission factors for the majority of the particle-phase compounds increased as the airflow was progressively closed, but decreased for some PAHs and other compounds found predominantly in the vapor-phase. Levoglucosan was the single most abundant compound, contributing 5-16% of the total particle mass. Although there was some variation in levoglucosan emissions between heater models, the fact that levoglucosan emissions vary relatively little between airflow conditions for a given heater provides the potential to use it as a general tracer for woodsmoke. In contrast, the mass fractions of many other particle-phase compounds were considerably higher when operated with a closed airflow.

이중추력형 로켓 모터의 개발

본 논문은 국내 최초로 그레인의 형상을 조절하여 설계한 이중추력형 로켓 모터의 개발에 관하여 기술하였다. 저연소속도 저연 추친제의 조성 및 finocyl형 그레인을 이용한 로켓 모터의 설계와 연소시험 데이터의 분석/평가 결과를 상술하였다. 개발된 이중추력형 로켓 모터는 비행시험에 성공적으로 적용되어 유도탄의 성능향상에 크게 기여하였고, 개발 결과는 향후 유사한 로켓 모터의 개발에 유용하게 사용될 것이다. This paper describes the development of the dual-thrust rocket motor, which gets a significant change in thrust level by varying the burning area of the propellant grain. Reduced smoke propellant of low burning rate was formulated and the finocyl type grain was designed to get the boosting- and sustaining-phase of the thrust level. And the motor firing data were analyzed in detail. Developed motor was applied to the missile system to implement the successful flight test and this development helped to upgrade the performance of the missile system. The results will be usefully applied to the development of the similar rocket motors.