Efficient use of energy is one of the major objectives in designing modern energy conversion systems. Entropy generation and exergy loss analysis are useful means of investigating the sources of irreversibility and improving the efficiency of combustion systems. The amount of entropy generated in a system is directly connected with the amount of available work and hence, the energy efficiency of the system.
A new methodology is being developed for LES of turbulent reacting flows, incorporating the second law of thermodynamics. The entropy transport equation is introduced in LES. The filtered form of this equation includes the effect of unclosed subgrid scale entropy generation. The filtered density function (FDF) methodology provides an effective means to close these terms. The FDF includes the complete statistical information about the joint entropy, turbulent frequency, velocity and scalar within the SGS. An exact transport equation is developed for the FDF, which includes the effects of chemical reaction in closed forms. The unclosed terms in this equation are modeled by considering a system of stochastic differential equations. The modeled FDF transport equation is solved by a Lagrangian Monte Carlo method. LES/FDF is employed to simulate a 3D turbulent temporally developing mixing layer, involving transport of scalars and entropy as shown in the figure above.
Figures below show some of the comparisons with the DNS data. The one on the right shows the variation of filtered entropy across the layer. The one on the left, shows the average filtered entropy generation rate which includes the contributions due to heat and mass transfer and turbulent dissipation.