Dinesh Gera

Computer simulation of bubbles in large-particle fluidized beds

A distinct element model is used to study the hydrodynamics of large-particle fluidized beds. The computed bubble rise velocity, voidage variations, averaged particle/particulate and fluid velocities are compared with the other continuum theory based on two fluid model. Based on the averaged particle/particulate velocities in a grid cell, deformation of the particle layers predicted by the two fluid model and the distinct element method are also compared. The predicted characteristics of bubble formation, motion, and eruption at the bed surface are in good qualitative agreement with the experimental observations. The quantitative differences in predicting the above parameters along with the advantages and limitations of two approaches for the case of a single isolated bubble rising in a two-dimensional fluidized bed are discussed.

Parametric studies on the formation of diesel particulate matter via nucleation and coagulation modes

Abstract The objective of this study is to develop a physical model that accurately accounts for the nucleation, coagulation, and condensation processes in the formation of particulate matter (PM) inside the exhaust plume of the diesel-fueled vehicles. The PM concentration has been predicted based on the fuel sulfur content, fuel-to-air ratio, exhaust flow rate, and the ambient conditions. It was predicted that the critical nucleus diameter of the particles decreased by approximately 30% and the number concentration increased by a factor of 6 with the increase in relative humidity from 10% to 90% for a fuel with 50 ppm sulfur content. The parametric studies suggested that the condensation effects are very important near the stack. Ignoring the contribution from condensation term decreased PM count median diameter from 52 to 10 nm . A fair agreement is observed between the numerically predicted PM size distribution and concentration and the experimentally measured values.

Bubble rise velocity in two-dimensional fluidized beds

Abstract Using a method analogous to that of Davies and Taylor [7], a simple model incorporating wake effects is employed to derive the velocity of a two-dimensional gas bubble rising in a fluidized bed. Predicted values of bubble rise velocity agree well with the experimental data. Effects of wake angle and bubble aspect ratio on rise velocity are also discussed.

Effect of voidage variation and bubble aspect ratio on throughflow in 2-D elliptical bubbles

Abstract The first principles hydrodynamic model of gas-fluidized beds has been employed to study the effects of voidage variation and bubble aspect ratio on the magnitude of throughflow velocity in an elliptical bubble. The equations for the throughflow are derived without the restrictive assumption of constant voidage along the surface of the bubble. Numerical calculations of voidage variations and throughflow in elliptical cap bubbles have been compared with the corresponding values in circular cap bubbles. It was observed that the throughflow velocity component at the nose of an elongated bubble is higher than that in a flattened bubble. Comparison with the sparse experimental data on throughflow in single isolated bubbles shows fairly good agreement.

Energy variation trends in the fatigue growth of surface cracks under tension and bending

Abstract There is a dramatic change in the shape of the semi-elliptical surface cracks, growing due to combined cyclic bending and tensile loading of plates. It is possible to predict the change of aspect ratio ( a c ) during the propagation of the crack. The growing cracks change their shapes such that they follow preferred propagation patterns (PPPs). Some of the important characteristics during crack propagation are determined. In particular, strain energy release rate (G) variation during crack growth is presented and discussed. It is shown that the shape of any growing crack changes such that G for the crack tries to converge to the preferred propagation trend. Also, some of the geometrical variations during the propagation of the cracks are discussed.

Emissions from RDF/coal blended fuel combustion

Abstract The significant volume and weight reduction along with the energy potential of MSW, in the form of refuse derived fuel (RDF), has made its incineration an attractive alternative. However, the gaseous emissions such as, CO 2 , CO, NO x and SO 2 , which are the byproduct of the combustion process pose serious environmental problems. These problems are compounded by the presence of certain highly regulated hazardous wastes such as, dioxins and furans in the exhaust stream. In the present investigation, different compositions of RDF/Coal blends were examined and the gaseous emissions from the combustion of the briquetted fuel were measured. Also, a direct comparison of emissions from RDF/coal blend incineration with coal combustion is presented in this paper. The potential for recovery of ferrous and non- ferrous metals provides an additional economic motivation for the use of RDF/Coal blended briquettes.

Measurement of fecal coliform bacteria from domestic waste

The objective of this research is to estimate the concentrations of fecal coliform bacteria from the septic tank effluent and the sand columns discharge. Collected samples of effluent from the septic tank were analyzed for number of pathogens using sand filter. Estimates on raw waste strength were made by measuring settleable solids and recording the volume feed and time feed for the septic tank each day. Biochemical Oxygen Demand (BOD), Total Suspended Solids (TSS), and pH were also measured for the septic tank effluent to ensure that the waste is within the normal (allowable safe) range. Average FC, BOD, and TSS values were measured to be in the range of 435,000–52,530,00 colonies per 100 ml, 130–230 mg/L, and 80–307 mg/L respectively for septic tank influent. A removal of the order of 99% for FC, TSS, and BOD in column effluent was achieved using the sand filters.

Effect of bubble coalescence on throughflow velocity in a 2-D fluidized bed

Abstract A theoretical analysis is presented of the effect of bubble coalescence on throughflow gas velocity. In order to predict the motion of interacting two-dimensional circular bubbles, bubble velocity is approximated by adding to its rise velocity in isolation the velocity that the particulate phase would have at the position of the nose if the other bubble were absent. It is inferred that throughflow gas velocity is significantly increased during bubble interaction as compared with the case of single isolated bubbles. Also, it was found that a lateral motion of the rear bubble is induced if the interaction bubbles are not vertically aligned.

On the computation of total strain energy release rate in surface cracks under cyclic loading

Abstract Computation of strain energy release rate in surface cracks plays an important role in determining the preferred propagation pattern (PPP). Semi-elliptical cracks, growing due to cyclic loading of plates, undergo significant shape change during the propagation process. These growing cracks change their shapes such that they follow preferred propagation patterns. It is shown that for a given cyclic-loading field, the PPP represents an upper limit on the aspect ratio of any surface crack growing due to this cyclic loading. Numerical integration techniques have been employed to compute total strain energy (TSE) release rate. The variation of TSE with crack depth ( a / t ) for different bending ratios ( R b ) is presented and discussed. Some initial analyses have already been presented in a prior paper [D. Gera and M. A. Mahmoud, Engng Fracture Mech . 41 , 515–528 (1992)], which discussed the energy variation trends in the fatigue growth of surface cracks under tension and bending.

Estimation of fecal Coliform removal from domestic sewage in two sand filter column systems

The objective of the present research is to compare fecal coliform removal from two different sand column systems treating septic tank effluent over a range of hydraulic loading rates. Drain time measurements were performed to compare the hydraulic characteristic of the column systems at different hydraulic loading rates. Biochemical oxygen demand (BOD), total suspended solids (TSS), and pH were also measured for the septic tank effluent. A removal of the order of 99% for fecal coliform, TSS, and BOD in column effluent was achieved using the sand filter columns. The ‘break‐in’ period for determining the effectiveness of the sand filter columns is also discussed.