Antun Galović

Analytical Entropy Analysis of Recuperative Heat Exchangers

The analytical solutions for the temperature variation of two streams in parallel flow, counter flow and cross-flow heat exchangers and related entropy generation due to heat exchange between the streams are presented. The analysis of limiting cases for the relative entropy generation is performed, and corresponding analytical expressions are given. The obtained results may be included in a more general procedure concerning optimal heat exchanger design.

Sand as a compressible fluid

The macroscopic volume changes of granular bodies during compression and shearing are investigated in terms of compressible fluid. The proposed analogy between sand body and van der Waals fluid, and appropriate form of van der Waals state equation, is demonstrated interpreting common soil mechanics tests in which the deviator stress is proportional to the mean pressure. The equation fits well the large range of applied pressures, and the proposed regular behavior is observed despite the severe grain crushing during loading. The common position of sands in reduced coordinates is found. The simple expression for pressure in sand in isothermal conditions is also proposed.

Another view on the optimization of the Brayton cycle with recuperator

Nondimensional expressions for the work output, thermal efficiency, heat input, and entropy generation number in the Brayton cycle with a recuperator are given as functions of seven input parameters: pressure ratio, temperature ratio, recuperator effectiveness, turbine and compressor isentropic efficiencies, nondimensional pressure drop in the combustor and recuperator and the ratio of specific heat capacities. The expressions for pressure ratios which maximize or minimize the considered quantities are derived. Since it is not possible to maximize all the quantities at a unique value of pressure ratio, three optimization criteria: (i) maximum work output, (ii) maximum thermal efficiency, and (iii) maximum of their weighted sum are defined. For each criterion two diagrams from which one can select optimal pressure ratio and read out all basic quantities defining the Brayton cycle are provided. In the case of criterion of maximum thermal efficiency, when the recuperator effectiveness is lower than the certain limit, the Brayton cycle without a recuperator is better than that with it. In the case of other two criteria, the Brayton cycle with a recuperator is always better than that without it, regardless of the value of the recuperator effectiveness.

Exergy analysis of a Brayton cycle with variable physical properties and variable composition of working substance

The exergy analysis of a Brayton cycle is performed in the paper. The four input variables: the ratio of the compressor exit and inlet pressures, the ratio of inlet temperature of gases in turbine and inlet temperature of air in compressor and the isentropic efficiencies of the compressor and turbine are analysed. The temperature ratio is varied in a way that the compressor inlet temperature of air is maintained constant, while the turbine inlet temperature of flue gases is varied from 900 to 1200°C. In the combustion chamber methane is completely combusted with excess air ratio which is determined by the temperature of flue gases at the turbine inlet. The analysis further includes variability of the molar heat capacities of air and flue gases with temperature and variability of their heat capacity ratios. The exergy destruction in turbine, compressor and combustion chamber and also total exergy efficiency of the cycle are considered in the analysis.

Entropy analysis of adiabatic mixing of three streams of different ideal gases at equal pressures and temperatures

A non-dimensional analytical solution of entropy generation during adiabatic mixing of three streams of different ideal gases at equal pressures and temperatures is presented. The molar fractions of the three streams are the only relevant variables. The functional dependences of the entropy generation on two variables, i.e., the molar fractions of two gases, are given and the maximum entropy generation is expressed. The results are presented in three- and two-dimensional diagrams. The maximum entropy generation characteristic is shown in a two-dimensional chart. The adiabatic mixing of two different gases is also analysed, as the special case of the above-mentioned general model.