Paper presented at the 9th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Malta, 16-18 July, 2012.
The establishment of a reliable methodology for the design of a host of practical thermal and thermodynamic systems demands the development of a framework that can accurately predict their performance by accounting formally for the underlying unsteady and conjugate heat transfer processes that are undergone as part of their operation. The purpose of this paper is to present a framework that includes such a description and to show results from its application to the characterisation of a reciprocating compression and expansion process. Specifically, an unsteady and conjugate heat transfer model is proposed that solves the one-dimensional unsteady heat conduction equation in the solid simultaneously with the first law in the gas phase, with an imposed heat transfer coefficient taken from relevant experimental studies in the literature. This model is applied to the study of thermal losses in gas springs. Beyond the explicit inclusion of conjugate heat transfer, the present model goes beyond previous efforts by considering the case of imposed volumetric compression and by allowing the resulting gas pressure to vary accordingly. Notable effects of the solid walls of the gas spring are revealed, with worst case thermodynamic cycle losses of up to 14% (relative to equivalent adiabatic and reversible processes) for cases in which unfavourable solid and gas materials are selected, and closer to 11% for more common material choices. The contribution of the solid towards these values, through the dimensionless thickness of the gas spring cylinder wall, is about 8% and 2%, respectively, showing a non-monotonic trend with the thermodynamic losses; increasing with increased solid thickness, reaching a maximum and then decreasing again. 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 the performance of the cycle undergone by the gas cannot, in general, be neglected.