Finite element analysis of concrete cracking at early age

The study of the early age concrete properties is becoming more important, as the thermal effects and the shrinkage, even in the first hours, could generate cracks, increasing the permeability of the structure and being able to induce problems of durability and functionality in the same ones. The detailed study of the stresses development during the construction process can be decisive to keep low the cracking levels. In this work a computational model, based on the finite element method, was implemented to simulate the early age concrete behavior and, specially, the evaluation of the cracking risk. The finite element analysis encloses the computational modeling of the following phenomena: chemical, thermal, moisture diffusion and mechanical which occur at the first days after the concrete cast. The developed software results were compared with experimental values found in the literature, demonstrating an excellent approach for all the implemented analysis.

Finite element and dimensional analysis algorithm for the prediction of mechanical properties of bulk materials and thin films

In this work, the applicability of a new algorithm for the estimation of mechanical properties from instrumented indentation data was studied for thin films. The applicability was analyzed with the aid of both three-dimensional finite element simulations and experimental indentation tests. The numerical approach allowed studying the effect of the substrate on the estimation of mechanical properties of the film, which was conducted based on the ratio h(max)/l between maximum indentation depth and film thickness. For the experimental analysis, indentation tests were conducted on AISI H13 tool steel specimens, plasma nitrated and coated with TiN thin films. Results have indicated that, for the conditions analyzed in this work, the elastic deformation of the substrate limited the extraction of mechanical properties of the film/substrate system. This limitation occurred even at low h(max)/l ratios and especially for the estimation of the values of yield strength and strain hardening exponent. At indentation depths lower than 4% of the film thickness, the proposed algorithm estimated the mechanical properties of the film with accuracy. Particularly for hardness, precise values were estimated at h(max)/l lower than 0.1, i.e. 10% of film thickness. (C) 2010 Published by Elsevier B.V.

Ethanol production process from banana fruit and its lignocellulosic residues: Energy analysis

Tropical countries, such as Brazil and Colombia, have the possibility of using agricultural lands for growing biomass to produce bio-fuels such as biodiesel and ethanol. This study applies an energy analysis to the production process of anhydrous ethanol obtained from the hydrolysis of starch and cellulosic and hemicellulosic material present in the banana fruit and its residual biomass. Four different production routes were analyzed: acid hydrolysis of amylaceous material (banana pulp and banana fruit) and enzymatic hydrolysis of lignocellulosic material (flower stalk and banana skin). The analysis considered banana plant cultivation, feedstock transport, hydrolysis, fermentation, distillation, dehydration, residue treatment and utility plant. The best indexes were obtained for amylaceous material for which mass performance varied from 346.5 L/t to 388.7 L/t, Net Energy Value (NEV) ranged from 9.86 MJ/L to 9.94 MJ/L and the energy ratio was 1.9 MJ/MJ. For lignocellulosic materials, the figures were less favorable: mass performance varied from 86.1 to 123.5 L/t, NEV from 5.24 10 8.79 MJ/L and energy ratio from 1.3 to 1.6 MJ/MJ. The analysis showed, however, that both processes can be considered energetically feasible. (C) 2010 Elsevier Ltd. All rights reserved.

Exergy and thermoeconomic analysis of a turbofan engine during a typical commercial flight

In an energy perspective of cost-reduction and configuration-optimization, it becomes necessary to develop and use advanced tools for the analysis, design and improvement of energy conversion systems. In the aeronautical industry, such trend is fundamental since this industry has evolved to design extremely complex aircrafts, with highly integrated systems, requiring more information in order to evaluate the whole system. The aim of this paper is to present an exergy-based analysis as to evaluate the global performance of a typical turbofan engine and its components. The study presents values for exergy efficiency over the whole flight cycle, critical equipment and flight phases considering exergy destruction and estimating internal and exhaust flow costs. (C) 2009 Elsevier Ltd. All rights reserved.

Energy and exergy analysis of ethanol production process from banana fruit

An alternative for ethanol production, is the use of vegetable waste, such as excess of banana production, that are evaluated in 2,400,000 t/year, which includes: residual banana fruit and lignocellulosic material. This paper analyzes the energetic and exergetic behavior to carry the process developed at laboratory scale to a plant processing of banana for the ethanol production, involving: growing and transport of the vegetable material, hydrolysis of banana fruit, sugar fermentation, ethanol distillation and utility plant. Finally, energy and exergy indicators are obtained. The results show a positive energy balance when banana fruit is used for ethanol production, but some process modification must be done looking for improving the exergetic efficiency in ethanol production.

Floquet stability analysis of the flow around an oscillating cylinder

This investigative work is concerned with the flow around a circular cylinder submitted to forced transverse oscillations. The goal is to investigate how the transition to turbulence is initiated in the wake for cases with different Reynolds numbers (Re) and displacement amplitudes (A). For each Re the motion frequency is kept constant, close to the Strouhal number of the flow around a fixed cylinder at the same Re. Stability analysis of two-dimensional periodic flows around a forced-oscillating cylinder is carried out with respect to three-dimensional infinitesimal perturbations. The procedure consists of performing a Floquet type analysis of time-periodic base flows, computed using the spectral/hp element method. With the results of the Floquet calculations, considerations regarding the stability of the system are drawn, and the form of the instability at its onset is obtained. The critical Reynolds number is observed to change with the amplitude of oscillation. With respect to instabilities, unstable modes with the same symmetry as mode A of a fixed cylinder are observed; however, they present different wavelengths. Also, the instabilities observed for the oscillating cylinder are distinctively stronger in the braid shear layers. Other unstable modes similar to mode B are found. Quasi-periodic modes are observed in the 2S wake, and subharmonic mode occurrences are reported in P + S wakes. (C) 2009 Elsevier Ltd. All rights reserved.

Reliability Analysis in Reciprocating Compressors for Refrigeration Systems

The most-used refrigeration system is the vapor-compression system. In this cycle, the compressor is the most complex and expensive component, especially the reciprocating semihermetic type, which is often used in food product conservation. This component is very sensitive to variations in its operating conditions. If these conditions reach unacceptable levels, failures are practically inevitable. Therefore, maintenance actions should be taken in order to maintain good performance of such compressors and to avoid undesirable stops of the system. To achieve such a goal, one has to evaluate the reliability of the system and/or the components. In this case, reliability means the probability that some equipment cannot perform their requested functions for an established time period, under defined operating conditions. One of the tools used to improve component reliability is the failure mode and effect analysis (FMEA). This paper proposes that the methodology of FMEA be used as a tool to evaluate the main failures found in semihermetic reciprocating compressors used in refrigeration systems. Based on the results, some suggestions for maintenance are addressed.

Combined production of sugar, ethanol and electricity: Thermoeconomic and environmental analysis and optimization

Many works have shown the potential of the Brazilian sugarcane industry as an electricity supplier. However, few studies have studied how this potential could be achieved without jeopardizing the production of sugar and ethanol. Also, the impact of modifications in the cogeneration plant on the costs of production of sugar and ethanol has not been evaluated. This paper presents an approach to the problem of exergy optimization of cogeneration systems in sugarcane mills. A general model to the sugar and ethanol production processes is developed based on data supplied by a real plant, and an exergy analysis is performed. A discussion is made about the variables that most affect the performance of the processes. Then, a procedure is presented to evaluate modifications in the cogeneration system and in the process, and their impact on the production costs of sugar, ethanol and electricity. Furthermore, a discussion on the renewability of processes is made based on an exergy index of renewability. As a general conclusion, besides adding a new revenue to the mill, the generation of excess electricity improves the exergo-environmental performance of the mill as a whole. (C) 2010 Elsevier Ltd. All rights reserved.