Showing papers by "Stefan Stefanov published in 2006"

Biotransformation of Phytosterols to Androstenedione in Two Phase Water-oil Systems

Abstract: The microbial transformation of phytosterols to androstenedione (androst-4-ene--3,17-dione, AD) and androstadienedione (androsta-1,4-diene-3,17-dione, ADD) in two-phase water-oil systems by means of the strain Mycobacterium sp. MB 3683 has been studied. The effect of some process conditions, like the agitation speed, the age and amount of inoculum, the temperature and some additional carbon sources have been investigated. The highest conversion rates were attained with 10-15 % of inolucum of age t = 16-20 h at T= 34-35 °C and n = 400 min -1 . Media containing high concentrations of carbohydrates have a negative impact on the process, while the natural nitrogen sources influence beneficially the bioconversion. The use of silicon oil and polypropylene-glycol as solubilizing agents was found suitable for the above-mentioned biotransformation and permitted to increase considerably the amount of the substrate in the system. On the other hand, the biotransformation rate depended on the amount of the oils, because they inhibited the microbial cells growth.

The critical accommodation coefficient for velocity inversion in rarefied cylindrical Couette flow in the slip and near free-molecular regimes

Abstract: This paper investigates velocity inversion in rarefied cylindrical Couette flow at the two extremes of the Knudsen number regime. For low Knudsen numbers, velocity inversion is independent of the accommodation coefficient of the inner cylinder and will only occur when the accommodation coefficient of the outer cylinder lies below a critical value. It is shown that this critical value depends upon the Knudsen number and the ratio of the outer and inner cylinders. In addition, velocity inversion is also investigated in the free-molecular and near free-molecular regimes. For the specific case of specular reflection at the outer cylinder, the present study has identified two distinct categories of molecules within the flow. The first category consists of particles that collide alternately with the inner and outer cylinders whilst the second consists of particles that never collide with the inner cylinder. It is shown that recognizing these different categories of particles is the key to understanding the velocity inversion process in the near free-molecular flow regime. Finally, for large but finite Knudsen numbers, our DSMC simulations demonstrate that velocity inversion does occur provided the accommodation coefficient of the outer cylinder is sufficiently small.

Micro-scale cavities in the slip - and transition - flow regimes

Abstract: highlight the fact that many micro-flows that have been considered are simple planar flows and typical classification systems are defined on such flows. We show that for complex flows, such as the driven cavity, non-equilibrium effects are more appreciable and their onset occurs at lower Knudsen numbers than expected.