Babak Shotorban

COSMIC DUST AGGREGATION WITH STOCHASTIC CHARGING

The coagulation of cosmic dust grains is a fundamental process which takes place in astrophysical environments, such as presolar nebulae and circumstellar and protoplanetary disks. Cosmic dust grains can become charged through interaction with their plasma environment or other processes, and the resultant electrostatic force between dust grains can strongly affect their coagulation rate. Since ions and electrons are collected on the surface of the dust grain at random time intervals, the electrical charge of a dust grain experiences stochastic fluctuations. In this study, a set of stochastic differential equations is developed to model these fluctuations over the surface of an irregularly-shaped aggregate. Then, employing the data produced, the influence of the charge fluctuations on the coagulation process and the physical characteristics of the aggregates formed is examined. It is shown that dust with small charges (due to the small size of the dust grains or a tenuous plasma environment) are affected most strongly.

The Role of Moisture on Combustion of Pyrolysis Gases in Wildland Fires

The role of water vapor, originated from the moisture content in vegetation, on the combustion process was investigated via simulating an opposed diffusion flame and a laminar premixed flame with pyrolysis gases as the fuel and air as the oxidizer. The fuel was mixed with water vapor, and the simulation was repeated for various water mole fractions. In both of the diffusion and premixed flames, the smaller the water mole fraction, the higher the maximum temperature. No reactions occurred when the water mole fraction was 0.65 or larger in the diffusion flame, and 0.70 or larger in the premixed flame. The maximum energy release rate and the flame speed decreased with the increase of the water mole fraction in the premixed flame. In both flames, O2 and H were the components that showed dramatic changes with the change of the water mole fraction.

Effects of distribution of bulk density and moisture content on shrub fires

Formulation of a physics-based model, capable of predicting fire spread through a single elevated crown-like shrub, is described in detail. Predictions from the model, obtained by numerical solutions to governing equations of fluid dynamics, combustion, heat transfer and thermal degradation of solid fuel, are found to be in fairly good agreement with experimental results. In this study we utilise the physics-based model to explore the importance of two parameters – the spatial variation of solid fuel bulk density and the solid fuel moisture content – on the burning of an isolated shrub in quiescent atmosphere. The results suggest that vertical fire spread rate within an isolated shrub and the time to initiate ignition within the crown are two global parameters significantly affected when the spatial variation of the bulk density or the variation of fuel moisture content is taken into account. The amount of fuel burnt is another parameter affected by varying fuel moisture content, especially in the cases of fire propagating through solid fuel with moisture content exceeding 40%. The specific mechanisms responsible for the reduction in propagation speed in the presence of higher bulk densities and moisture content are identified.

Stochastic fluctuations of dust particle charge in RF discharges

In addition to RF oscillations, intrinsic stochastic fluctuations due to the discreteness of electrons and ions could be important to the charging of a dust particle in RF discharges. These fluctuations are studied in the present work for three cases [M. Bacharis et al., Plasma Sources Sci. Technol. 19, 025002 (2010)] relevant to RF discharges employing a recently proposed model [B. Shotorban, Phys. Rev. E 83, 066403 (2011)] valid for stochastic charging at nonstationary states. The cases are concerned with a time varying electron number density relevant to sheaths, a time varying electric field relevant to the bulk plasma, and a time-dependent bi-Maxwellian distribution of electrons in a low pressure discharge. Two dust particles with different sizes are individually studied in each case. The radius of one is ten times larger than the radius of the other. In all of the cases, for the larger dust particle, the root-mean-squre of charge stochastic fluctuations is about an order of magnitude smaller than th...

Physics-Based Modeling of Live Wildland Fuel Ignition Experiments in the Forced Ignition and Flame Spread Test Apparatus

ABSTRACT A computational study was performed to improve our understanding of the ignition of live fuel in the forced ignition and flame spread test apparatus, a setup where the impact of the heating mode is investigated by subjecting the fuel to forced convection and radiation. An improvement was first made in the physics-based model WFDS where the fuel is treated as fixed thermally thin elements and then it was utilized in this study. This improvement included bound water in addition to free water in fuel moisture content. The fuel was assumed to undergo evaporation of free and bound water, pyrolysis and char oxidation. Fuels with different moisture contents ranging from 0% to 130% were simulated under an identical heating condition. The simulated and the experimental ignition times compared reasonably well with each other. The time evolutions of simulated and experimental mass loss rates also compared well with each other. For all fuel moisture contents, it was observed that the release of bound moisture starts at temperatures greater than 200°C long after ignition time. This observation was consistent with the release of moisture observed at high temperatures in the experiments of live fuels.

An Investigation of the Influence of Heating Modes on Ignition and Pyrolysis of Woody Wildland Fuel

The ignition of woody wildland fuel modeled as a one-dimensional slab subject to various modes of heating was investigated using a general pyrolysis code, Gpyro. The heating mode was varied by applying different convective and/or radiative, time-dependent heat flux boundary conditions on one end of the slab while keeping the other end insulated. Dry wood properties were used for the slab. Initially, wood was treated as chemically inactive and following this it is presumed to decompose via a single-stage kinetic model involving two solid phase species coupled with one gas phase species. This single-step model approximation for wood degradation was validated with experimental results. Critical time was defined as the time when the temperature of the heated side reached a critical value at which the ignition was assumed to take place. The chemically inactive assumption led to a significant underprediction of the critical time for a broad range of convective heat source temperatures at a fixed Biot number. When thermal decomposition was included, the critical time was quite sensitive to radiative and convective source temperatures and the Biot number during combined mode of heating. Time evolution of the mass loss and charring rates was weakly influenced by convective heating during the combined mode of heating. The variation of Biot number had little influence on this evolution when a combined mode of heating was applied.

Interactions of fires of neighbouring shrubs in two- and three-shrub arrangements

A physics-based computational model was utilised to better understand the interactions of fires generated by burning of neighbouring shrubs. The model included large-eddy simulation for flow field turbulence and a two-phase approach for the coupling of solid fuel and gas phases. Two different arrangements consisting of two and three identical shrubs placed adjacent to each other were considered. All shrubs were simultaneously ignited from their base with the aid of separate ground fuels. Both crown and ground fuels were modelled as porous media with thermophysical properties of chamise and excelsior respectively. Modelling results indicated that the peak mass-loss rate and the vertical fire spread rate within a shrub decrease when the shrub separation distance increases. At zero separation, heat release rate normalised by the number of shrubs is enhanced by 5 and 15% for the two-shrub and the three-shrub arrangements, respectively. Generation of strong vorticity by higher gravitational torque appeared to be the cause for enhanced burning in the three-shrub arrangement. This effect was seen to be much weaker for the two-shrub arrangement. Interactions between the individual fires cease for a centre-to-centre distance of 1.5 and 2 times the shrub diameter for the two-shrub and the three-shrub arrangement respectively.

Intrinsic fluctuations of dust grain charge in multi-component plasmas

A master equation is formulated to model the states of the grain charge in a general multi-component plasma, where there are electrons and various kinds of positive or negative ions that are singly or multiply charged. A Fokker-Planck equation is developed from the master equation through the system-size expansion method. The Fokker-Planck equation has a Gaussian solution with a mean and variance governed by two initial-value differential equations involving the rates of the attachment of ions and electrons to the dust grain. Also, a Langevin equation and a discrete stochastic method are developed to model the time variation of the grain charge. Grain charging in a plasma containing electrons, protons, and alpha particles with Maxwellian distributions is considered as an example problem. The Gaussian solution is in very good agreement with the master equation solution numerically obtained for this problem.

Waste Heat Utilization in Natural Gas Pipeline Compression Stations by an Organic Rankine Cycle

Utilizing compressors, natural gas compression stations (NGCS) supply the required pressure to transport the natural gas in pipelines. This study proposes to utilize the waste heat of gas turbines (GT), employed in these stations, to produce shaft power for compressors. A NGCS coupled to an Organic Rankine Cycle (ORC) is simulated by THERMOFLEX in this study. Ten different working fluids are compared against each other in the ORC to select a working fluid with the highest possible efficiency and obtainable power, and lowest possible environmental impacts. According to the results, n-Pentane is selected to be the working fluid, as compared to others, it better meets these criteria. The effects of the variation of the ambient temperature and the condenser pressure on the net power of the cycle working with n-Pentane are investigated in details. It was found that the utilization of waste heat in NGCS by an ORC can improve the energy efficiency and generates electricity for near rural areas.

Bistable Intrinsic Charge Fluctuations of a Dust Grain Subject to Secondary Electron Emission in a Plasma

A master equation was formulated to study intrinsic charge fluctuations of a grain in a plasma as ions and primary electrons are attached to the grain through collisional collection, and secondary electrons are emitted from the grain. Two different plasmas with Maxwellian and non-Maxwellian distributions were considered. The fluctuations could be bistable in either plasma when the secondary electron emission is present, as two stable macrostates, associated with two stable roots of the charge net current, may exist. Metastablity of fluctuations, manifested by the passage of the grain charge between two macrostates, was shown to be possible.