Linear harmonic analysis of Stirling engine thermodynamics
The analysis involves linearization of the pressure waveform and represents each term in the conservation equations by a truncated Fourier series, including enthalpy flux discontinuity. Second-Law analysis is presented of four important loss mechanisms that result from adiabatic cylinders, transient heat transfer in semiadiabatic cylinders, pressure drop through the heat exchangers, and gas leakage from the compression space. The four loss mechanisms, all leading to efficiency reduction below the Carnot level, are characterized by irreversible thermodynamic processes that occur when heat is transferred across a finite temperature difference; when gases at two different temperatures are mixed; or when there is a mass flow through a pressure difference. The allocation of each individual loss mechanism is derived precisely in terms of entropy production but evaluated by use of pressure, temperature, and mass oscillations calculated from the linear harmonic approximation. When the theory is applied to an engine of Sunpower's RE-1000 dimensions, it reveals clearly that the adiabatic loss (due to temperature fluctuations in the cylinders) consists of two components: gas mixing and heat transfer across a temperature difference. The theory further shows that the adiabatic effect is more important than the transient heat transfer loss if the gas-to-cylinder heat transfer rate is small (i.e., nearly adiabatic conditions); the reverse is true for intermediate heat transfer rates; and both losses vanish at very high heat transfer rates. In addition, entropy analyses of pressure drop and mass leakage for isothermal cylinders shed some light on coupling between the different individual loss mechanisms.
- Research Organization:
- Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)
- DOE Contract Number:
- AC05-84OR21400
- OSTI ID:
- 6618631
- Report Number(s):
- ORNL/CON-155; ON: DE84016646
- Country of Publication:
- United States
- Language:
- English
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Linear harmonic analysis for Stirling machines and second law analysis of four important losses
Linear harmonic analysis for Stirling machines and second law analysis of four important losses