Wall heat flux influence on the thermodynamic optimisation of irreversibilities of a circulating fluidised bed combustor

Abstract

In the study the comparison of irreversibilities was done when the wall condition of the combustor was changed from adiabatic to negative heat flux, for incoming air temperature of 400 K. The reactant mixture of solid pitch pine wood fuel and air was varied from a rich to a lean mixture. A non-adiabatic non-premixed combustion model of a numerical code (ANSYS FLUENT 16.2) was used to simulate the combustion process where the solid fuel was modelled by using the ultimate analysis data. The irreversibilities generated were arrived at by computing the entropy generation rates due to the combustion and frictional pressure drop processes. For the combustor where the wall condition was changed from adiabatic to negative heat flux (that is heat leaving the system) the minimum irreversibilities generated changed from occurring at an equivalence ratio of 1.67 for an adiabatic wall condition to 1.34 for a negative heat flux wall condition. It was also found that the penalty paid when deviating from the equivalence ratio of 1 at which minimum irreversibilities are generated is drastically increased due to less heat in the combustor resulting more unburnt fuel exiting the combustor.