Equilibrium States of Two Stochastic Models in Mathematical Ecology

The aim of this study is to investigate about the complex phenomenology associated with the interaction of a particle-laden turbulent flow with the slag-covered wall of an entrained-flow gasifier. Recent observations, indeed, highlighted that this phenomenology can have an impact on the global gasifier performance greater than that expected from previous analyses. The design of new generation of entrained-flow coal gasifiers aims at favoring ash migration/deposition onto the reactor walls, whence the molten ash (slag) flows and is eventually drained separately at the bottom of the gasifier. In terms of efficiency, the oxidation of the volatile compounds released around the particles depends upon its mixing with the fresh oxidant mixture. Therefore combustion efficiency is influenced by the spatial distribution of the particle phase, with an homogeneous distributions favoring a better mixing. From the observation that a significant number of coal particles can spent most of the time in the gasifier close to the slag layer, where usually their concentration largely increase, leads to the need to understand the effective conditions experienced before complete conversion. An experimental evidence of a picture for the fate of coal particles has been recently assessed by analyzing the chemical composition of samples of coarse slag and slag fines generated in the ELCOGAS entrained-flow gasifier located in Puertollano, Ciudad Real (Spain). Quantitative SEM-EDX analysis of the coarse slag revealed the presence of small marks with a significant carbon content as high as 48.8%-54.2%. This fact can be explained by assuming the entrapment of not fully burned coal particles into the slag. The results of the SEM analysis performed on whole slag fines particles showed that the carbon content was larger than the value obtained from the inspection of coarse slag particles. This is particularly evident for porous particles where C-content ranged between 82.3% and 86.5%. A considerable amount of unreacted coal is therefore entrapped into the slag matrix. From this observations emerges that a certain level of spatial non homogeneity of the solid phase distribution exists. In a recently published study by Montagnaro and Salatino (2010), these data have been interpreted by assuming that different regimes of particles-slag interaction can occur: either char entrapment inside the melt or carbon-coverage of the slag may occur, depending on properties like char density, particle diameter and impact velocity, slag viscosity, interfacial particle-slag tension. Occurrence of char entrapment prevents further progress of combustion/gasification. On the contrary, if char particles reaching the wall adhere to the slag layer's surface without being fully engulfed, the progress of combustion/gasification is still permitted. The observed high rate of coal conversion can actually be explained only if this second regime establishes on the slag surface. The addressed considerations highlights the technological need to build up methods for the prediction of the mechanism particles clustering and segregation in condition representative of coal particle flying and converting into a gasifier. Actually a comprehensive numerical simulation of the whole range of spatial and temporal chemical and turbulent time scales involved in a full scale gasifier, is still unfeasible due to the high computational cost: the scales of turbulence involved in the gasification processes range from sub-micron scale up to the integral scale of a gasifier reactor chamber (of the order of tens of meters). To overcome this difficulty, the approach proposed in this study is based on the development of a multilevel approach.. In a first level, the motion of particles representing classes of partially converted coal in a 3-dimensional representation of the gasifier is modeled with a Computational Fluid Dynamic (CFD) approach.. Turbulence of the flow field is described adopting the Reynolds Averaged Navier Stokes (RANS) approach, while particle motion is resolved with a Lagrangian Particle Tracking (LPT) approach. The use of the RANS method for the gas phase coupled with the LPT for the solid phase in this analysis is twofold. Firstly it has been used to address the behavior of coarse and fine coal particles trajectories when subjected to a swirl motion which induced a turbulent field. This model, while avoiding the great complexity and computational effort required by comprehensive numerical CFD models of gasifiers already proposed in the literature, is sufficient to characterize the range of conditions, in terms of momentum possessed and direction, that the different particles show when approaching the gasifier walls. The second aspect concerns the identification of regions where different mechanisms for the coal clustering becomes foreseeable: distinct regions close to the wall have been identified: finer particles could be mainly responsible of particle layering near the solid walls as they, after their first impinging on the wall, assumes a pathway parallel to the wall; in contrast, larger particles continue to bounce over the walls along the whole length of the gasifier. The identification of these two different regions and the characterization of particle classes representative of partially burned coal particles, was the basis for the proper setup of numerical simulations based on a Large Eddy Simulation (LES) approach in two completely different configurations. This level aims at a detailed investigation of the mechanisms of slag-particle interaction. The first is a plane particle-laden channel flow, that well represents the main features of the gasifier regions where particles move parallel to the wall. The second is a periodic particle laden curved channel flow, that best represent regions close to the wall but dominated by the external swirling flow. For these two configurations the particle interaction with the slag has been treated as a rebound on a not perfectly elastic wall. A parametric study has been conducted obtaining results for different particle sizes (representing different particle inertia) and different momentum restitution in the particle-wall impact. Numerical multiphase simulations are based on the Eulerian-Lagrangian approach implemented in the OpenFOAM CFD framework. Both RANS and LES turbulence models are implemented for the gas phase. The equations of particles motion were solved via a Lagrangian particle tracking algorithm with the TrackToFace method. Simulations were performed involving a number of particles from 10^5 to 10^6, a level of detail that allowed to obtain a clear picture of the multiphase flow behavior responsible for char deposition phenomena. Numerical simulation results with the LES approach do confirm the establishment of a region near the wall slag layer (the dense-dispersed phase leading to the formation of the slag fines), in which particles impacting the slag accumulate to an extent depending on the system fluid-dynamics and on parameters such as particles Stokes number and restitution coefficient. However, particle concentration near the wall in all the simulated cases does not appear perfectly steady not evenly spatially distributed. Interestingly, the segregation of char particles near the wall is more evident for the curved channel flow geometry and is enhanced for coarser particles, making evident the role played by the effective impact with the slag not recovered by the simpler models adopted in the RANS simulations.

Host races of phytophagous insects are sympatric populations that have different host preferences and between which gene flow is restricted because of the difference in host preference. Sympatric, host-associated sibling species are sympatric populations that use different host plants and that do not interbreed because of the presence of isolating mechanisms not related to host preference (Jaenike, 1981; see Mayr, 1970, and Bush, 1969, for slightly different definitions of these terms). Although the existence of host races and host-associated sibling species of phytophagous insects has been suspected for over 50 years (e.g., Thorpe, 1930), the evolutionary mechanism of host race formation remains controversial. In particular, there is disagreement about the importance of sympatric divergence in generating host races and species. White (1978) has argued that sympatric speciation must be invoked to explain, in part, the great diversity of specialized herbivorous insects, while Bush (1974, 1975) has forcefully advocated the operation of sympatric divergence in the creation of sympatric host races of tephritid flies. By contrast, Futuyma and Mayer (1980; see also Futuyma, 1983a) have argued that there is no reliable experimental evidence to support these claims and that models of sympatric speciation are based on assumptions that are probably not met by most phytophagous insects. Several authors have developed formal models of sympatric divergence and speciation (Maynard Smith, 1966; Dickinson and Antonovics, 1973; Caisse and Antonovics, 1978; Pimm, 1979; Felsenstein, 1981). Although these models do not account for the evolution of differences in host plant preference associated with purported sympatric speciation in phytophagous insects, Bush and Diehl (1983) and Rausher (1984) have suggested how modification of these models could remedy this problem. A common assumption of all these models is that fitness on one host is negatively correlated with fitness on a second. This type of negative correlation is manifested in these models in the assumption that genetic variation exists at loci that exhibit a genotype x host plant interaction, i.e., at loci at which some genotypes have high fitness on one host but low fitness on a second host while other genotypes have low fitness on the first but high fitness on the second host. This negative correlation seems to be necessary for sympatric speciation because it permits linkage disequilibrium to be established between loci influencing, say, viability on different hosts and loci influencing host preference. The resulting coadapted preference-viability gene complexes represent incipient host races or species. Moreover, the breakdown of these coadapted gene complexes by recombination provides the selection pressure that improves reproductive isolation, whether isolation is achieved via mating on the host (e.g., Bush, 1974, 1975) or by a separate assortative mating locus in linkage disequilibrium with loci affecting preference and fitness (e.g., Felsenstein, 1981). Without a crossing interaction, one homozygote genotype would have maximal fitness on both host species. Consequently, unless reproductive isolation were achieved instantaneously by a mutation

Physical and chemical characterization of gas-borne particles in the nanometer to sub-micrometer size range is important in many applications, such as nanoparticle related health studies, the monitoring of powder or dust distribution processes, assessing the release of engineered nanoparticles (ENP) from ENP reinforced materials due to abrasion or combustion, or exposure studies related to ENP containing spray products. Such investigations are also highly valuable in the development of technical processes, e.g. to determine distinct size fractions containing specific elements or isotopes in terms of dealing with nuclear waste, or to evaluate particulate matter in process gases and emissions from thermal waste treatment, gas turbines, or other combustion engines. Such particle speciation includes knowledge about particle size distribution and chemical composition, and if possible obtained with a high time resolution. However, most of the available techniques are offline methods and/or not able to provide such physical and chemical information simultaneously. The Scanning Mobility Particle Sizer (SMPS) is a well-established and widely used equipment for determining size distribution and number concentration of such particles, within scan durations of a few minutes. Inductively Coupled Plasma Mass Spectrometry (ICPMS) is a highly sensitive multi-element technique, which allows determining the elemental composition of normally liquid samples, with excellent detection limits and a wide dynamic concentration range. In this work, SMPS and ICPMS are coupled to one hyphenated system. A Rotating Disk Diluter (RDD) allows directing a well-defined flow of a diluted aerosol into subsequent measuring equipment, which besides is mainly argon based, due to the dilution effect. Therefore an RDD is implemented as sample introduction interface, and the SMPS flow concept is re-designed to fulfil the requirements of both different analytical methods. This coupling strategy allows achieving simultaneous information on particle size and elemental composition, with SMPS-typical time resolutions of a few minutes, and offers high flexibility in terms of dealing with different aerosols, loaded with particles in the nanometer to sub-micrometer size range. Proper SMPS operation under argon atmosphere instead of air is validated. The developed setup is tested with a model aerosol containing air-borne silver nanoparticles. The capabilities are then demonstrated on uniformly composited metal aerosols, as well as an aerosol mixture containing smaller gold and larger silver nanoparticles. Sensitivity and limit of detection, related to particle number and mass concentration, are determined for several elements and particle diameters. After the successful characterization, the instrumentation is applied in two research applications. The first is a study on aerosol particles, released by commercial consumer spray products. In the second application, particulate metal emissions from the thermal treatment of differently impregnated wood samples are investigated, with the focus on alkali metals.

Entrained flow pyrolysis and gasification of selected biomass – an experimental and modeling study

Modeling of residential outdoor exposure to traffic air pollution and assessment of associated health effects

Purpose / Context - Epidemiological studies have consistently shown that fine and course airborne particles (PM2.5 and PM10), as well as ultrafine (UF) particles measured in terms of particle number (PN) concentrations, are toxic to human health. A number of studies on particle concentrations in households were conducted worldwide; however, no such studies have so far been conducted in Vietnam. Methodology / Approach - Using two Nano-Tracers, the authors have simultaneously and continuously measured both indoor and outdoor number concentrations of UF particles at one low rise residential house and one apartment at a high rise building in Hanoi in order to quantify the concentrations and develop an understanding of factors driving them. Results - Daily average indoor and outdoor PN concentrations ranged from 14.5 to 19.8 x 103 p/cm3 and from 33.4 to 35.5 x 103 p/cm3, respectively. However, mean concentrations of indoor and outdoor PN during rush-hours were higher and increased up to the maximum of 23.1 and to 57.8 x 103 p/cm3, respectively. Key findings / Implications - Inspection of time series of particle concentration and subsequent statistic analysis showed that outdoor PN concentrations were strongly influenced by the outdoor vehicle emissions, while indoor PN concentrations were contributed by both indoor and outdoor sources. Originality – It is the first time, UF particle number concentrations outside and inside the residential houses in Hanoi were quantified. Outdoor particle concentrations were found strongly influenced by vehicle emisisions, while indoor particle levels affected by both indoor and outdoor sources.

Aiming at the weight degeneracy phenomena in particle filter algorithm, a resampling method improving the diversity based on genetic particle filter was presented. Taking the advantage of genetic algorithm (GA) in selection, crossover and inheritance to make up for the shortcoming of resampling. Genetic operation on particles in real number domain is adapted to reduce the complex of the genetic algorithm. And the evolutionary idea of genetic algorithm was combined with particle filter, by using selection, and mutation to improve the weight degeneracy and diversity of particle filter. This genetic particle filter was applied in the established GPS system nonlinear dynamic state space model. The experimental results based on the collected real GPS data is compared with the tradition particle filter, and compared with the effective number of particles and particle distribution. The experimental results indicated that the genetic particle filter can increase the number of particle, and effectively solve the particle degradation phenomena, the estimation accuracy of genetic particle filter is better than that of particle filter (PF).

Particles existing in a hard disk drive are known as a major source of TA(thermal asperity). Researchers have investigated how particles induce the TA phenomena, but have not verified yet the reason why and how particles are generated in a HDD. The objective of this study is to investigate why and how particles are generated, and in what condition, the largest number of particles is generated. The number of particles generated in a HDD was measured over the landing zone after various rest times of slider and during various motions and positions of slider. It is found that the large number of particles was generated when the HDD was turned on after a long rest time of slider and that a few of particles were continuously generated when the slider flied over the disk surface. It is thought that the number of particles generated in a HDD was related to the rest time of slider because the rest time of slider increased stiction, and that there were intermittent contacts between the slider and the disk surface when the slider flied over the disk surface.

Ultrafine particle emission and its potential health risk from residential solid fuel combustion

Cloud chamber observations of cosmic rays have been made in a B-29 aircraft, flying at altitudes of 30,000 ft. A 17-cm cloud chamber in a magnetic field of 7500 gauss was employed. From curvature measurements on 245 cloud tracks, the energy spectrum of cosmic rays at 30,000 ft has been determined. In contrast to the roughly equal numbers of positive and negative particles which are fom1d at sea level, the high altitude data show that positive particles dominate the negative particles by a ratio of 2:1. The different forms of the positive and negative spectra show that there exists among the positives a type of particle which is not represented among the negatives. The data are consistent with the hypothesis that these positives are protons, measured energies of which extend to 2.5 Bev. At least three-fourths of the protons probably are secondary particles. The remainder may well come directly from a primary proton component.

Experimental measurements of the particle pressure were obtained for a liquid fluidized bed and for a vertical gravity driven liquid-solid flow. The particle, or granular, pressure is defined as the extra pressure generated by the action of particles in a particulate multi-phase flow. Using a high-frequency-response pressure transducer, individual collisions of particles were collected and measured to obtain a time-averaged particle pressure. Results were obtained for a number of different particles and for two different test section diameters. Results show that the particle pressure experiences a maximum at intermediate concentrations, and that its magnitude is scaled with the particle density and the square of the terminal velocity of the particles. The particle pressure was found to be composed of two main contributions: one from pressure pulses generated by direct collisions of particles against the containing walls (direct component), and a second one from pressure pulses due to collisions between individual particles that are transmitted through the liquid (radiated component). The direct component of the particle pressure was studied by an analysis of particle collisions submerged in a liquid. A simple pendulum experiment provides controlled impacts in which measurements are made of the particle trajectories for different particles immersed in water. The velocity of the approaching particle is measured using a high speed digital camera; the magnitude of the collision is quantified using a high frequency response pressure transducer at the colliding surface. The measurements show that most of the particle deceleration occurs at less than half a particle diameter away from the wall. The measured collision pressure appears to increase with the impact velocity. Comparisons are drawn between the measured pressures and the predictions by Hertzian theory. A simple control-volume model is proposed to account for the effects of fluid inertia and viscosity. The pressure profile is estimated, and then integrated over the surface of the particle to obtain a force. The model predicts a critical Reynolds number at which the particle reaches the wall with zero velocity. Comparisons between the proposed model and the experimental measurements show qualitative agreement. Experiments involving binary collisions of particles were performed to investigate the radiated component of the particle pressure. This component results from the pressure front generated by the impulsive motion of a fluid resulting from a collision of particles in a liquid. When the two particles come into contact, the impulsive acceleration due to the elastic rebound produces a pressure pulse, which is transmitted through the fluid. A simple dual pendulum experiment was set up to generate controlled collisions. Measurements were obtained for a range of impact velocities, angles of incidence, and distances away from the wall for different pairs of particles. The magnitude of the impulse pressure appears to scale with the particle impact velocity and the density of the fluid. Based on the impulse pressure theory, a prediction for pressure generated due to the collision can be obtained. The model appears to agree well with the experimental measurements. The fluctuating component of the solid fraction was studied, as one of the sources of the particle pressure. The instantaneous cross-sectional averaged solid fraction was measured using an impedance meter. The root-mean square fluctuation of the solid fraction signal was measured in a liquid fluidized bed and a vertical gravity-driven flow, for different particle sizes and densities. Two types of fluctuations were identified: low-frequency large-scale fluctuations which dominate at high concentrations, and high-frequency small-scale fluctuations which are dominant at intermediate solid fractions. The effect of each type was isolated by filtering. When the large-scale fluctuations were present, the magnitude of the rms fluctuation was found to scale with particle diameter, but when eliminated the mean fluctuation appeared to scale with the particle mass instead.

Waste discharge by the Los Angeles County Sanitation District (LACSD) affects sunlight in the ocean. Increased sunlight attenuance affects productivity and changes in ocean surface color could be useful for monitoring dispersal. Temperature, sunlight irradiance in five colors, light beam attenuance, and particle size distributions were simultaneously measured as a function of water depth. A limited number of samples were analyzed by a co-worker for particulate percent organic carbon and particulate carbon isotope ratio. Background field stations included Catalina, Dana Point, and Rocky Point. Stations in the vicinity of the LACSD outfall at Whites Point represented a dispersing sewage field. The most significant result was that pollutants decreased the optical albedo of ocean water, increasing the extinction of sunlight irradiance as a function of optical depth. Additional findings were that 1) pollutants decreased euphotic zone depths by as much as 60 percent (depending upon particle concentrations), 2) water surface color spectra indicated that pollutants increased light absorption at short wavelengths (violet, blue and green), 3) the average particle concentration in polluted waters was twice that of background waters, 4) the LACSD discharge caused a bimodal increase in particle numbers as a function of particle size; there was a dramatic increase for particles less than 1.5 microns and a secondary increase at 8 microns, 5) assuming real particle refractive indices (no light absorption by particles), scattering by natural and sewage particles was maximum between particle diameters 3 and 8 microns, 6) calculations assuming complex refractive indices (absorbing particles) indicated that light absorption by sewage particles was maximum for particles less than 1.5 microns in diameter, and 7) heavy (high specific gravity) particles had lower percent organic carbon and a lower carbon isotope ratio.

Annihilating branching processes

As an alternative to conventional Eulerian methods in the field of computational fluid dynamics (CFD), smoothed particle hydrodynamics (SPH) has been developed. Its mesh-free method is useful in problems, especially, where a free surface is present. Although researchers are currently able to simulate up to hundreds of millions of particles in a volume, the analysis and visualization of the particle datasets are still limited especially, due to the lack of connectivity between particles and the number of particles. In this paper, we present a volumetric evaluation technique of SPH particle data using hierarchical particle data structures. Our approach does not resample or triangulate the meshless particle data on grid structures, instead, the evaluations are performed in a fragment program with the original SPH kernels used in the CFD simulations. The SPH particle information and the hierarchical data structures are stored in 2D and 3D textures respectively and the 3D texture stores the access pointers, such as texture coordinates, to the 2D textures. To achieve interactive frame rates during interaction we suggest to control the kernel radii while generating the hierarchical data structures. This approach allows us to visualize over a million of SPH particles interactively.

Objective To assess the occupational exposure doses received by the physicians in clinical practice at Shanghai Proton and Heavy Ion Center (SPHIC). Methods A total of 40 patients treated from September to November in 2016, including 20 proton cases and 20 carbon cases at SPHIC, were selected using simple random sampling method. Particle type, total particle number and prescribed doses were recorded for all the cases. The dose rates in the control room were measured by using a photon and neutron personal radiation detector during patient treatment. The dose rates around the surface of the patient′s tumor 1 min after completion of beam delivery and the dose rates about 30 cm to the tumor surface (where a physician stands) were also measured during unfixing and assisting the patients. Finally, the dose rates surrounding the fixtures, couch, robotic arm and window of BAMS were measured. The factors affecting the occupational exposure of physician were analyzed and the annual dose equivalent was assessed for physicians in SPHIC. Results Proton and heavy ion released nearly all energy in the tumor for Bragg peak advantage, so there was no induced radioactivity in the treatment room. However, the tumor became the main induced radioactivity source to the occupational exposure dose to physicians in clinical practices. The dose rate around the surface of the patient′s tumor 1 min after completion of beam delivery was (20.68±21.91) μSv/h, which was the highest in the working places of physicians, thus regarded as the main source. A significant positive correlation (r=0.828, P<0.05) was shown between dose rates and total number of particles delivered for the treatment. The dose rate measured in the control room was (0.08±0.01)μSv/h, and the dose rate measured surrounding the fixtures, couch, robotic arm and BAMS window was (0.09±0.01)μSv/h. No neutron was detected. The dose rate about 30 cm to the tumor surface (where physicians stand) was (2.03±2.84) μSv/h during unfixing and assisting the patients. The average annual dose to physicians was about 0.508 mSv. Conclusions The average annual dose to physicians was at a low level in SPHIC Key words: Proton and heavy ion therapy; Occupational exposure; Radiation dose; Physician