Comparison of zonal and CFD modelling of natural ventilation in a thermally stratified building.

This paper compares two well-known modelling approaches for natural ventilation in a multi-zone building with thermal stratification and large openings. The zonal approach in this paper assumes a fully mixed condition in each zone, and considers the bi-directional flows through all large openings. The zonal model is integrated into a thermal analysis code to provide simultaneous prediction of both ventilation flow rates and air temperatures in each zone. The CFD approach uses a finite-volume method for turbulent flows. A simple pressure boundary condition is used at all external openings in CFD. There is reasonable agreement between the overall ventilation flow rates and average zonal air temperatures predicted by the two modelling approaches. It is found that the multi-zone approach predicts a lower neutral level for the building than predicted by CFD. This might be mainly due to the fact that the thermal stratification is neglected in the present multi-zone model. It is explained by a new emptying air-filling box model for natural ventilation of single-zone buildings.