DSpace at IIT Bombay
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The DSpace digital repository system captures, stores, indexes, preserves, and distributes digital research material.2015-10-07T02:46:36ZOptimal Paired Choice Block Designs
http://dspace.library.iitb.ac.in/jspui/handle/100/17352
Optimal Paired Choice Block Designs
Singh, Rakhi; Das, Ashish; Chai, Feng-Shun
Choice experiments mirror real world situations closely and helps manufacturers, policy-makers and other researchers in taking business decisions on their product characteristics based on its perceived utility. In a paired choice experiment, several pairs of options are shown to respondents. The respondents are asked to give their preference among the two options for each of the choice pairs shown to them. In order to conduct an experiment, a choice design is customarily used to efficiently estimate the parameters of interest which essentially consists of either the main effects only or the main plus two-factor interaction effects of the attributes. Traditionally,
every respondent is shown the same collection of choice pairs under an
untenable assumption that the respondents are alike in every respect. Also, as the
attributes or the number of levels under each attribute increases, the number of
choice pairs in an optimal paired choice design increases rapidly. To address these
concerns, under the multinomial logit model or the linear paired comparison model, we first incorporate the respondent effects and then present optimal designs for the parameters of interest. We provide optimal paired choice designs for estimating the main effects for symmetric and asymmetric multi-level attributes with smaller number of choice pairs shown to each respondent. We also provide optimal paired choice designs for estimating the main effects only and the main plus two-factor interaction effects under the main plus two-factor interaction effects model.
2015-09-16T00:00:00ZA Process Model for Underground Coal Gasification: Part-II Growth of Outflow Channel
http://dspace.library.iitb.ac.in/jspui/handle/100/17351
A Process Model for Underground Coal Gasification: Part-II Growth of Outflow Channel
Samdani, Ganesh; Aghalayam, Preeti; Ganesh, Anuradda; Sapru, R.K.; Lohar, B.L.; Mahajani, Sanjay
Underground Coal Gasification is a process of gasifying coal in-situ to produce syn-gas. The gas thus produced, passes through the outflow channel that leads to the production well. As explained in part-I of this paper (Samdani et al., 2014), cavity growth between injection and production wells happens in two distinct phases. This paper presents an unsteady state model for cavity growth and gas production in phase-II wherein, the growth occurs mostly in horizontal direction towards the production well through the outflow channel. This phase of UCG lasts much longer than phase-I, in which growth takes place in vertical direction till the cavity hits the overburden. In the model for phase-II, the outflow channel is divided in small sections along its length and each section includes three subzones i.e. rubble zone, void zone and roof at the top. A compartment model is developed to reduce the complexity caused by non-ideal flow patterns and changing sizes of different subzones inside the outflow channel. The subzones and the sections are linked appropriately, for mass and energy flow, to give overall performance during Phase-II of UCG. The proposed approach combines chemical reactions, heat and mass transfer effects, spalling characteristic and complex flow patterns to achieve meaningful results. In all, seven gas species, three solid species and eleven reactions are included. The simulation results such as variation in solid density, dynamics of different zones, exit gas quality are presented. The model is validated by comparing the predicted exit gas quality and that observed during similar laboratory scale experiments. Finally the results are also compared with field scale experiments. This model along with the Phase-I model provides a complete modeling solution for UCG process.
2015-03-20T00:00:00ZA Process Model for Underground Coal Gasification- Part-I: Cavity Growth
http://dspace.library.iitb.ac.in/jspui/handle/100/17350
A Process Model for Underground Coal Gasification- Part-I: Cavity Growth
Samdani, Ganesh; Aghalayam, Preeti; Ganesh, Anuradda; Sapru, R.K.; Lohar, B.L.; Mahajani, Sanjay
In underground coal gasification (UCG), a cavity is formed in the coal seam due to consumption of coal. The irregular-shaped cavity consists of three distinct zones: a spalled-rubble zone on the cavity floor, a cavity roof at the top and a void zone between the two. During UCG, the coal seam between the injection and production wells undergoes two distinct growth phases. In phase-I, coal/char near injection well gets consumed and cavity grows in a vertical (radial) direction and hits the overburden. Phase-II starts thereafter, in which the cavity grows in the horizontal direction towards the production well. The geometry and flow patterns are distinctly different in these two phases and should be considered as two separate events while modeling UCG process. This part of the paper presents an unsteady-state model for gas production during the initial vertical growth of the cavity in phase-I. A computationally less expensive compartment modeling approach, based on computational flow dynamics (CFD), is used to establish non-ideal flow patterns in the cavity. Furthermore, the model also incorporates reaction kinetics, heat transfer, mass transfer, intra-particle diffusional limitations and thermo-mechanical failure (spalling) of coal by using required parameters for coal of interest. The simulations are performed for a typical Indian lignite and the results are interpreted to demonstrate potential of the developed model. Simulations results such as dynamics in rubble, void and roof zone are explained using different parameters including reaction fronts, gas composition and exit gas calorific value. The average calorific value of exit gas was observed to be relatively steady in spite of changes occurring in each zone. Finally the simulation results are analyzed by comparing with the results of the reported laboratory-scale experiments performed on the same coal under UCG-like conditions.
2015-03-20T00:00:00ZOn optimal two-level supersaturated designs
http://dspace.library.iitb.ac.in/jspui/handle/100/17349
On optimal two-level supersaturated designs
Singh, Rakhi; Das, Ashish
A popular measure to assess two-level supersaturated designs is the $E(s^2)$ criteria. Recently, Jones and Majumdar (2014) introduced the $\mbox{{\it UE}}(s^2)$ criteria and obtained optimal designs under the criteria. Effect-sparsity principle states that only a very small proportion of the factors have effects that are large. These factors with large effects are called {\it active} factors. Therefore, the basis of using a supersaturated design is the inherent assumption that there are very few active factors which one has to identify. Though there are only a few active factors, it is not known a priori what these active factors are. The identification of the active factors, say $k$ in number, is based on model building regression diagnostics (e.g. forward selection method) wherein one has to desirably use a supersaturated design which on an average estimates the model parameters optimally during the sequential introduction of factors in the model building process. Accordingly, to overcome possible lacuna on existing criteria of measuring the goodness of a supersaturated design, we meaningfully define the $ave(s^2_k)$ and $ave(s^2)_\rho$ criteria, where $\rho$ is the maximum number of active factors. We obtain superior $\mbox{{\it UE}}(s^2)$-optimal designs in ${\cal D}_U(m,n)$ and compare them against $E(s^2)$-optimal designs under the more meaningful criteria of $ave(s^2_k)$ and $ave(s^2)_\rho$. It is seen that $E(s^2)$-optimal designs perform fairly well or better even against superior $\mbox{{\it UE}}(s^2)$-optimal designs with respect to $ave(s^2_k)$ and $ave_d(s^2)_\rho$ criteria.
2015-02-04T00:00:00Z