Theory of Effective Heat-Absorbing and Heat-Emitting Temperatures in Entropy and Exergy Analysis with Applications to Flow Systems and Combustion Processes

Abstract

The heat integral in the entropy equation is separated into heat-absorbing and heat-emitting parts. Defining the effective heat-absorbing and heat-emitting temperatures for the boundary of the system over which the heat flows, we derive a general formula for the real working power in terms of effective temperatures with corresponding entropy generation rates. The method gives an explicit solution to the classical problem of how to determine the correct temperature T_ in each case for the power loss = (temperature T_) × (entropy generation rate σ). For different types of heat exchangers, we derive mathematical presentations for the effective heat-absorbing and heat-emitting temperatures; we thus distinguish the entropy generation rates, or exergy losses, for each fluid flow separately. The mathematical formulae for combined-flow systems are illustrated by a district heating system. As an example of chemical processes, we analyze the entropy generation rate in a combustion chamber, and we solve the problem of the real power loss of isobaric combustion. We also show that an adiabatic reversible combustion of carbon in air chambers could be self-pressurized theoretically, even up to several hundred bar, without compressors – and that the efficiency of the gas turbine process could be the same as when fuel cells are used.