Lavinia Grosu

An engineer–oriented optimisation of Stirling engine cycle with finite–size finite–speed of revolution thermodynamics

Finite–time thermodynamics takes into account exo–irreversibilities which are not managed by reversible thermodynamics, especially reservoir–to–gas temperature gaps. In todays works, thermodynamicists use the working gas mass as the main imposed parameter. In this paper, the study of the Stirling engine cycle is carried out with a motorist–oriented viewpoint: instead of the working gas mass, the reference parameters are those linked to engineering constraints; they are the maximum pressure, the maximum volume, the extreme temperatures and the overall thermal conductance; the adjustable parameters are the volumetric compression ratio, the hot–to–cold conductance ratio and the regenerator efficiency. Analytical normalised expressions of the operating characteristics of the engine, such as power, work, efficiency, speed and their optima are set up. Classical results of analytical optimisation are found back and new ones are established, leading to new conclusions. Some dimensionless and dimensional numbers are put into evidence.

Entropic-Skins Geometry to Describe Wall Turbulence Intermittency

In order to describe the phenomenon of intermittency in wall turbulence and, more particularly, the behaviour of moments and and intermittency exponents ζP with the order p and distance to the wall, we developed a new geometrical framework called “entropic-skins geometry” based on the notion of scale-entropy which is here applied to an experimental database of boundary layer flows. Each moment has its own spatial multi-scale support Ωp (“skin”). The model assumes the existence of a hierarchy of multi-scale sets Ωp ranged from the “bulk” to the “crest”. The crest noted characterizes the geometrical support where the most intermittent (the highest) fluctuations in energy dissipation occur; the bulk is the geometrical support for the whole range of fluctuations. The model assumes then the existence of a dynamical flux through the hierarchy of skins. The specific case where skins display a fractal structure is investigated. Bulk fractal dimension and crest dimension are linked by a scale-entropy flux defining a reversibility efficiency (d is the embedding dimension). The model, initially developed for homogeneous and isotropic turbulent flows, is applied here to wall bounded turbulence where intermittency exponents are measured by extended self-similarity. We obtained for intermittency exponents the analytical expression with γ ≈ 0.36 in agreement with experimental results.

Energy and exergy analyses of a solar-driven absorption cooling system

In this paper, energy and exergy analyses of an absorption cooling system using H2O/LiBr and solar energy as heat source are conducted. A simple schematic of a chiller and an improved one, are compared using Thermoptim and EES software. An increase in the coefficient of performance (COP) from 0.69 to 0.8 was noticed. Numerical simulations are performed for six values of the condensing/absorption temperatures, which vary in the interval 31–36°C. The increase in these temperatures implies a decrease in the COP from 0.8 to 0.56. The exergy analysis of the overall system and of each component shows a decrease in the exergetic efficiency of the system from 0.27 to 0.18. A detailed exergy analysis of the absorber reveals the real causes of exergy destruction. A new exergy destruction indicator was introduced, which shows the importance of the irreversibility of each component, relative to the starting potential.

An exergetic approach for materials fatigue

An entropy approach has been recently developed, based on the irreversible thermodynamics and entropy production. It has been observed that, for low cycle fatigue, metals undergoing cyclic load reach fracture, at a certain level of entropy production called fracture fatigue entropy (FFE) (Naderi and Khonsari, 2010), solely dependent on the material. Until now, we only used the two principles of thermodynamics separately to follow the behaviour of a solid continuum problem as low cycle fatigue. We propose here to use the two laws of thermodynamics together via the Gouy-Stodola equation and the concept of exergy to provide a new description for metals undergoing low cycle fatigue.

Fractal Representation of Exergy

We developed a geometrical model to represent the thermodynamic concepts of exergy and anergy. The model leads to multi-scale energy lines (correlons) that we characterised by fractal dimension and entropy analyses. A specific attention will be paid to overlapping points, rising interesting remarks about trans-scale dynamics of heat flows.