Performance analysis of coupled quasi-steady state air flow calculation and dynamic simulation of hygrothermal transport inside porous materials

Abstract

The hygrothermal simulation model DELPHIN performs a transient simulation of energy and moisture transport in porous building materials and air gaps. The governing partial differential equations are solved with an advanced error control time integration algorithm. In comparison to the time scales of heat and moisture transport, air flow can be treated sufficiently well as steady-state process. The pressure and flow field needs to be updated whenever ambient air pressures change significantly or free convection is induced due to temperature or humidity variations. Within DELPHIN the air flow pattern is computed with a Darcy-type steady-state flow calculation.The calculation of the flow field is a time-consuming task and should only be done when necessary. A central question that impacts both physical results and performance is the update criteria for the quasi-steady flow field. Indeed, if the flow field is updated in with every change of temperature or moisture, we observe in some cases significant performance losses. For all simulation cases we use an error controlled time integration method.In the article we identify free convection inside an air gap as a performance critical simulation scenario. Dominating convective transport is responsible for divergence of the fully coupled simulation. Reduction of update frequency stabilizes simulation run without loss of calculation efficiency. These results may be generalized for less critical simulation cases.