Abstract
A second law analysis in combustion systems is performed along with an exergy loss study by quantifying the entropy generation sources using, for the first time, three different approaches: a classical-thermodynamics-based approach, a novel turbulence-based method and a look-up-table-based approach, respectively. The numerical computation is based on a hybrid filtered Eulerian stochastic field (ESF) method coupled with tabulated detailed chemistry according to a Famelet-Generated Manifold (FGM)-based combustion model. In this work, the capability of the three approaches to capture the effect of the Re number on local exergy losses is especially appraised. For this purpose, Sandia flames D and E are selected as application cases. First, the validation of the computed flow and scalar fields is achieved by comparison to available experimental data. For both flames, the flow field results for eight stochastic fields and the associated scalar fields show an excellent agreement. The ESF method reproduces all major features of the flames at a lower numerical cost. Next, the second law analysis carried out with the different approaches for the entropy generation computation provides comparable quantitative results. Using flame D as a reference, for which some results with the thermodynamic-based approach exist in the literature, it turns out that, among the sources of exergy loss, the heat transfer and the chemical reaction emerge notably as the main culprits for entropy production, causing 50% and 35% of it, respectively. This fact-finding increases in Sandia flame E, which features a high Re number compared to Sandia flame D. The computational cost is less once the entropy generation analysis is carried out by using the Large Eddy Simulation (LES) hybrid ESF/FGM approach together with the look-up-table-based or turbulence-based approach.
Highlights
Most energy conversion systems feature, typically, 20–30% of exergy destruction in fuel combustion [1]
Sources of irreversibilites may include heat transfer, mechanical dissipation, mass transfer and diffusion, chemical reactions, phase change, inelastic material deformation and breakup, etc. [1,2,3,4,5,6]. Such irreversibilities lead to a destruction of available energy into internal energy in the system, which causes a raise in the system entropy [1,2,3,4]
The present paper focuses on the second aspect by assuming that models that describe the evolving transport processes in combustion systems are available
Summary
Most energy conversion systems feature, typically, 20–30% of exergy destruction in fuel combustion [1]. The objective of the present paper is threefold: (a) to carry out EGA by applying the hybrid filtered Eulerian stochastic field (ESF) method coupled with the FGM chemistry tabulation strategy; (b) to suggest two novel methods for quantifying entropy generation sources in addition to the classical-thermodynamics-based one; (c) to assess the capability of the novel approaches to capture the effect of the Re number on local exergy losses in terms of accuracy and computational costs. Knowledge of the sub-grid evolution of the controlling variables is needed All this information is delivered in the present study by means of the transported filtered density function, T-FDF, following the ESF approach in which the chemical source term is provided via the look-up table. The filtered mean and sub-grid variance of the variable φα, which are the first and second moment, respectively, can be derived as:
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