The effusivity model for multi-layer walls

The effusivity model for describing the time response of walls to changes in heat flux is extended to two-layer walls using an integral method. Based on a simple integral method, shown to obtain results close to the ones of the effusivity method for the one-layer case, a simple correction of the expressions for temperature variation due to a step change in the heat flux is obtained. The results are shown to be in good agreement with the exact solution. It is shown that, depending on the ratio of the effusivities of the two layers, significant deviations from the results obtained using the effusivity of the outer layer can be expected.