Evaluation of the instantaneous unsteady heat transfer in a rapid compression-expansion machine

Investigation is made of the instantaneous unsteady heat transfer within a pneumatically driven rapid compression-expansion machine that offers simple, well-controlled and known boundary conditions. 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 estimated through the application of a three-dimensional computational fluid dynamics code, KIVA3. Correlation of the derived data using the mean piston speed and cylinder bore diameter as the characteristic parameters for the Reynolds and the Nusselt numbers resulted in Nu = 0.010 x Re-1.188 for all compression ratios (8.4 similar to 24.3). Correlation of the derived data when using the instantaneous height between the piston top and the cylinder head, the maximum gas velocity and the kinetic mean gas velocity obtained when using the KIVA3 code as the characteristic parameters for the Reynolds and the Nusselt numbers was found to produce values of the Nusselt number that were almost independent of the corresponding calculated values of the Reynolds number during the major part of the compression and expansion strokes. A power relationship between the cylinder pressure and gas temperature and the apparent heat transfer coefficient was demonstrated.