Phase appearance or disappearance in two-phase flows

This paper is devoted to the treatment of specific numerical problems which appear when phase appearance or disappearance occurs in models of two-phase flows. Such models have crucial importance in many industrial areas such as nuclear power plant safety studies. In this paper, two outstanding problems are identified: first, the loss of hyperbolicity of the system when a phase appears or disappears and second, the lack of positivity of standard shock capturing schemes such as the Roe scheme. After an asymptotic study of the model, this paper proposes accurate and robust numerical methods adapted to the simulation of phase appearance or disappearance. Polynomial solvers are developed to avoid the use of eigenvectors which are needed in usual shock capturing schemes, and a method based on an adaptive numerical diffusion is designed to treat the positivity problems. An alternate method, based on the use of the hyperbolic tangent function instead of a polynomial, is also considered. Numerical results are presented which demonstrate the efficiency of the proposed solutions

Analytical methods for lignocellulosic biomass structural polysaccharides

The use of lignocellulosic biomass has been postulated as a potential pathway toward diminishing global dependence on nonrenewable sources of chemicals and fuels. Before a specific feedstock can be selected for biochemical conversion into biofuels and bio-based chemicals, it must first be characterized to evaluate the chemical composition of the cell walls. Polysaccharides, specifically cellulose and hemicellulose, are often the focal point of these appraisals, since these constituents are the dominant substrates converted into monomeric sugars like glucose and xylose. These monosaccharides can be transformed, using microorganisms like yeast, into substances such as ethanol. Plant species containing abundant polysaccharides are highly desirable, as higher quantities of sugars should translate into larger end-product yields. Given the vast pool of potential feedstocks, qualitative and quantitative analytical methods are needed to assess cell wall polysaccharides. Many of these tools, such as wet chemical and chromatographic techniques, have been ubiquitously used for some time. Shortcomings in these analyses, however, prevent their usage in screening large sample sets for quintessential, high-yield, fuel-producing traits. This chapter briefly summarizes how analytical spectroscopy can lessen some of these limitations and how it has been utilized for polysaccharide analysis