A Framework for the Analysis of Thermal Losses in Reciprocating Compressors and Expanders
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چکیده
This article presents a framework that describes formally the underlying unsteady and conjugate heat transfer processes that are undergone in thermodynamic systems, along with results from its application to the characterisation of thermodynamic losses due to irreversible heat transfer during reciprocating compression and expansion processes in a gas spring. Specifically, a heat transfer model is proposed that solves the onedimensional unsteady heat conduction equation in the solid simultaneously with the first law in the gas phase, with an imposed heat transfer coefficient taken from suitable experiments in gas springs. Even at low compression ratios (of 2.5), notable effects of unsteady heat transfer to the solid walls are revealed, with thermodynamic cycle losses of up to 14% (relative to equivalent adiabatic and reversible processes) at intermediate Péclet numbers (i.e., normalised frequencies) when unfavourable solid and gas materials are selected, and closer to 10-12% for more common material choices. The contribution of the solid towards these values, through the conjugate variations attributed to the thickness of the cylinder wall, is about 8% and 2% points, respectively; showing a maximum at intermediate thicknesses. At higher compression ratios (of 6) a 19% worst case loss is reported for common materials. These results suggest strongly that, in designing highefficiency reciprocating machines, the full conjugate and unsteady problem must be considered and that the role of the solid in determining performance cannot, in general, be neglected. Address correspondence to Dr C. N. Markides, Department of Chemical Engineering, Imperial College London, London SW7 2AZ, United Kingdom. E-mail: [email protected] Tel: +44 20 759 41601
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تاریخ انتشار 2013