Entropijska produkcija pri stacionarnom provodenju topline kroz stijenku cilindra s toplinskim izvorom

U radu je detaljno analiziran problem minimizacije entropijske produkcije za model stacionarnog provođenja topline u stijenci cilindra s toplinskim izvorom i nametnutim izotermnim rubnim uvjetima. Problem je riješen uvođenjem relevantnih bezdimenzijskih varijabli: bezdimenzijske izdašnosti toplinskog izvora, omjera vanjskog i unutrašnjeg polumjera cilindra, kao i omjera rubnih temperatura. Pomoću tih veličina i bezdimenzijskog polumjera izvedeni su analitički izrazi za temperaturno polje, te lokalnu i ukupnu entropijsku produkciju te je postavljen kriterij za postojanje minimuma entropijske produkcije. Rezultati provedene analize su obrazloženi te je prikazano područje vrijednosti omjera polumjera cilindra, te omjera rubnih temperatura, za određene vrijednosti izdašnosti toplinskog izvora, u kojem postoji minimum entropijske produkcije

Detailed analysis of the effect of the turbine and compressor isentropic efficiency on the thermal and exergy efficiency of a Brayton cycle

Energy and exergy analysis of a Brayton cycle with an ideal gas is given. The irreversibility of the adiabatic processes in turbine and compressor is taken into account through their isentropic efficiencies. The net work per cycle, the thermal efficiency and the two exergy efficiencies are expressed as functions of the four dimensionless variables: the isentropic efficiencies of turbine and compressor, the pressure ratio, and the temperature ratio. It is shown that the maximal values of the net work per cycle, the thermal and the exergy efficiency are achieved when the isentropic efficiencies and temperature ratio are as high as possible, while the different values of pressure ratio that maximize the net work per cycle, the thermal and the exergy efficiencies exist. These pressure ratios increase with the increase of the temperature ratio and the isentropic efficiency of compressor and turbine. The increase of the turbine isentropic efficiency has a greater impact on the increase of the net work per cycle and the thermal efficiency of a Brayton cycle than the same increase of compressor isentropic efficiency. Finally, two goal functions are proposed for thermodynamic optimization of a Brayton cycle for given values of the temperature ratio and the compressor and turbine isentropic efficiencies. The first maximizes the sum of the net work per cycle and thermal efficiency while the second the net work per cycle and exergy efficiency. In both cases the optimal pressure ratio is closer to the pressure ratio that maximizes the net work per cycle