Unsteady Heat Transfer During the Rapid Compression and Expansion of Air

The instantaneous unsteady heat transfer during rapid compression and expansion of air within a pneumatically driven piston in a cylinder arrangement which offers simple, well-controlled and known boundary conditions was examined. Values of the instantaneous apparent overall heat flux from the cylinder gas to the wall surfaces were calculated using a thermodynamic analysis of the experimentally measured pressure and volume temporal development. Corresponding heat flux values were also calculated through the application of a zero-dimensional model that incorporates the use of the k–ε turbulence model. Comparison of the results of the model with corresponding experimental data showed fair to good agreement for the wide range of compression ratio values used (8.4∼24.3). Also, correlation of the derived data using an effective velocity which is based on the distribution of mean kinetic energy, turbulence energy and piston motion and a characteristic length that is a function of the instantaneous height between the piston top and the cylinder head and bore diameter as the parameters to use when calculating the Reynolds, Prandtl and Nusselt numbers resulted in the following workable relationship: Nu = 0.01Re^0.205 Pr^0.033.