The application of energy and entropy balances for treatment of flow and temperature data at a real wall in an unsteady state

The results of an analytical and experimental treatment of the entropy and energy balances undergone in the wall region of a flat surface exposed to fluctuating heating or cooling flow are reported. The wall region examined was a shell exposed on one surface to ambient laboratory conditions and on the other by a flow of heated or cooled air. Sensors affixed to the surface measured the flow and temperature fluxes over and through the wall. The increases in entropy caused by heat transfer across the wall were calculated as a function of the stresses present. Numerical models are defined for the heat transfer and storage process in a homogeneous wall immersed in an unsteady-state medium. The formulation is used to describe the changing conditions within the wall, taking into account the presence of thermal resistance which slowly varies as a function of the flow and temperature variations. It is found that, for disturbances varying with a period greater than the wall time constant, thermal conductive losses are inversely proportional to the thermal resistance measured at steady state.