Entropy parameters for falling film absorber optimization

Niccolo Giannetti, Andrea Rocchetti, Arnas Arnas, Kiyoshi Saito, Seiichi Yamaguchi

Research output: Contribution to journalArticle

11 Citations (Scopus)

Abstract

A local entropy generation analysis, for water vapor absorption in LiBr-H2O solution, is performed referring to velocity, temperature and concentration fields obtained from the numerical solution of mass and energy transport equations. The hydrodynamic description is based on Nusselt boundary layer assumption and the actual amount of irreversibility introduced is determined for an absorptive falling film over a cooled horizontal tube inside the absorber. Results are explored in different operative conditions in order to examine the impact of the various irreversibility sources in a wide operative range. A least irreversible solution mass flow-rate can always be identified. Furthermore, a simple and general thermodynamic analysis, carried out regarding a refrigerating and a heat boosting applications, makes evidence of a dimensionless group "Q/σT" that separates the weight of the irreversibilities and gives the way to an optimization criterion applied to the absorber in order to improve the whole system efficiency. Both thermodynamic equilibrium and sub-cooling conditions of the solution at the inlet are considered for typical temperature and concentration of refrigerators' absorbers and heat transformers' absorbers. Results suggest the importance to work at reduced mass flow-rates with a thin uniform film. In practice, tension-active additives are required to realize this condition. Also, it is highlighted that the two parameters defined with reference to the dimensionless group "Q/σT" can be maximized by specific values of the tube radius, operative Reynolds number, solution sub-cooling and temperature difference between the wall and the inlet solution.

Original languageEnglish
Pages (from-to)750-762
Number of pages13
JournalApplied Thermal Engineering
Volume93
DOIs
Publication statusPublished - 2016 Jan 25

Keywords

  • Absorber
  • Entropy generation
  • Falling film
  • Horizontal tube
  • Irreversibility
  • Optimization

ASJC Scopus subject areas

  • Energy Engineering and Power Technology
  • Industrial and Manufacturing Engineering

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