A thermodynamic model of a compartment fire with one state variable (corresponding to the dimensionlesstemperature of the smoke layer) based on energy conservation is studied from the point of view of the Second Law (exergyanalysis). To define the exergy rate of the process, a novel concept of mean thermodynamic temperature is introducedfor the smoke layer. This parameter is the equivalent of the Carnot factor in the energy transfer process for an opensystem under steady-state conditions. The exergy analysis shows that in order to satisfy the Second Law, not all the solutionsof the thermodynamic model are physically accessible in the phase space. The physically accessible part of the phasespace is delimited by a boundary representing the non-negative entropy generation condition.
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