Second-Law Analysis of Irreversible Losses in Gas Turbines
Mar 18, 2019 / Author by Jin Yan, Du Juan
Text SizeDBS
Several fundamental concepts with respect to the second-law analysis (SLA) of the turbulent flows in gas turbines are discussed in this study. Entropy and exergy equations for compressible/incompressible flows in a rotating/non-rotating frame have been derived. The exergy transformation efficiency of a gas turbine as well as the exergy transformation number for a single process step has been proposed. The exergy transformation number will indicate the overall performance of a single process in a gas turbine, including the local irreversible losses in it and its contribution to the exergy obtained the combustion chamber. A more general formula for calculating local entropy generation rate densities is suggested. A test case of a compressor cascade has been employed to demonstrate the application of the developed concepts.
Several fundamental aspects with respect to the SLA of the turbulent flows in gas turbines were discussed in this study. Entropy and exergy equations for compressible/incompressible flows in a rotating/non-rotating frame were derived. The derivation shows that the Navier–Stokes equations and the energy equation are sufficient to satisfy the second law of thermodynamics, thus it is not necessary to solve the entropy and exergy equations to evaluate their quantities. The entropy and exergy can be determined by the post processing of CFD simulations. However, the entropy and exergy equations and their budgets are helpful tools for analyzing the irreversible processes in gas turbines.
The exergy transformation efficiency ？E of a gas turbine as well as the exergy transformation number ？iE of a single process step i was proposed in this study. ？iE in a turbine cascade, a compressor cascade, or the combustion chamber are suggested to be calculated by Equations. The value of ？iE indicates the overall effects of an irreversible process, including its destruction of exergy and its contribution to the potential exergy obtained in the combustion chamber. ？iE can be used to assess the performance of an isolated process in a gas turbine, since only local flow and temperature fields are required to calculate its value.
The methods for calculating the local entropy generation rate densities were discussed. It was suggested to use turbulence production rates instead of the turbulence dissipation rates to calculate the local entropy generation rate densities. The assumption behind this approximation is that the turbulence production rate is in balance with the turbulence dissipation rate when the domain is sufficiently large. An advantage of Equations is that they are independent from the choices of turbulence models. However, more systematic studies, e.g., LESs of cascade flows at high Mach numbers, are still required to further validate these equations.
A test case with respect to a compressor cascade has been employed for applying the concepts developed in the study. The numerical results show that the entropy generation rates calculated by Equations are almost identical. The exergy transformation number suggests an optimal incidence angle at which the compressor cascade works with the best overall performance.
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Second-Law Analysis of Irreversible Losses in Gas Turbines
Mar 18, 2019 / Author by Jin Yan, Du Juan Text SizeDBSSeveral fundamental concepts with respect to the second-law analysis (SLA) of the turbulent flows in gas turbines are discussed in this study. Entropy and exergy equations for compressible/incompressible flows in a rotating/non-rotating frame have been derived. The exergy transformation efficiency of a gas turbine as well as the exergy transformation number for a single process step has been proposed. The exergy transformation number will indicate the overall performance of a single process in a gas turbine, including the local irreversible losses in it and its contribution to the exergy obtained the combustion chamber. A more general formula for calculating local entropy generation rate densities is suggested. A test case of a compressor cascade has been employed to demonstrate the application of the developed concepts.
Several fundamental aspects with respect to the SLA of the turbulent flows in gas turbines were discussed in this study. Entropy and exergy equations for compressible/incompressible flows in a rotating/non-rotating frame were derived. The derivation shows that the Navier–Stokes equations and the energy equation are sufficient to satisfy the second law of thermodynamics, thus it is not necessary to solve the entropy and exergy equations to evaluate their quantities. The entropy and exergy can be determined by the post processing of CFD simulations. However, the entropy and exergy equations and their budgets are helpful tools for analyzing the irreversible processes in gas turbines.
The exergy transformation efficiency ？E of a gas turbine as well as the exergy transformation number ？iE of a single process step i was proposed in this study. ？iE in a turbine cascade, a compressor cascade, or the combustion chamber are suggested to be calculated by Equations. The value of ？iE indicates the overall effects of an irreversible process, including its destruction of exergy and its contribution to the potential exergy obtained in the combustion chamber. ？iE can be used to assess the performance of an isolated process in a gas turbine, since only local flow and temperature fields are required to calculate its value.
The methods for calculating the local entropy generation rate densities were discussed. It was suggested to use turbulence production rates instead of the turbulence dissipation rates to calculate the local entropy generation rate densities. The assumption behind this approximation is that the turbulence production rate is in balance with the turbulence dissipation rate when the domain is sufficiently large. An advantage of Equations is that they are independent from the choices of turbulence models. However, more systematic studies, e.g., LESs of cascade flows at high Mach numbers, are still required to further validate these equations.
A test case with respect to a compressor cascade has been employed for applying the concepts developed in the study. The numerical results show that the entropy generation rates calculated by Equations are almost identical. The exergy transformation number suggests an optimal incidence angle at which the compressor cascade works with the best overall performance.