Simón Bolívar University

About: Simón Bolívar University is a education organization based out in Caracas, Venezuela. It is known for research contribution in the topics: Population & Crystallization. The organization has 5912 authors who have published 8294 publications receiving 126152 citations.

Slip Factor for Centrifugal Impellers Under Single and Two-Phase Flow Conditions

Abstract: Throughout the history of turbomachines investigators have tried to develop reliable methods for prediction of centrifugal pump behavior. Among the parameters available to estimate the performance of this kind of machine is the slip factor. In spite of being regarded as a variable of great significance in the analysis of turbomachinery, there seem to be a misconception regarding its concept and application. Indeed, empirical correlations have been widely used to estimate the slip factor, even in the case of two-phase flow applications, where it has not been investigated. Moreover, these correlations provide a constant value of the slip factor for a given impeller only at the best efficiency point, which is an important restriction to the pump performance prediction, considering that slip factor varies with the pump flow rate. In this study, three-dimensional computational fluid dynamics simulations were carried out on an impeller of known geometry (Ns = 1960) from which values of slip factor were calculated for both single- and two-phase flow (water and water-air). These results include curves of the slip factor as a function of the specific capacity and the gas-void fraction. Additionally, results for the slip factor in the case of single-phase flow (water) are given for various centrifugal impellers (N S = 1157, 1447, 1612, and 3513) in order to illustrate the influence of the flow rate on this parameter.

Nucleation and crystallization of PLDA-b-PE and PLLA-b-PE diblock copolymers

Abstract: The nucleation and crystallization of two types of strongly segregated poly(lactide)-block-polyethylene diblock copolymers with an approximate 50/50 composition has been investigated. One material contains an amorphous PLDA block (PLDA-b-PE) and the other contains a semicrystalline PLLA block (PLLA-b-PE). The overall isothermal crystallization rate of the PLLA block was slowed down as compared to homo-PLLA by the covalently bonded PE chains that were molten at the PLLA crystallization temperatures. This crystallization rate depression of the PLLA block produces a coincident crystallization process when PLLA-b-PE is cooled down from the melt at rates larger than 2°C/min. The overall crystallization rate of the PE block is faster when it is covalently bonded to previously crystallized PLLA than when it is attached to a rubbery PDLA block, this results from a nucleation effect of PLLA on the PE block. Polarized Light Optical Microscopy (PLOM) confirmed the confined nature of the crystallization process within lamellar microdomains for both diblock copolymers, since neither PLLA nor PE are capable of breaking out and spherulites can not be formed.