Evaluation of the effectiveness of the ventilation tower use in comparison with the wall face-to-face route: Study on draft and ventilation promotion using a ventilation tower in dense urban areas part 1

1. Introduction: Even if it is difficult to take advantage of natural ventilation due to ambient conditions, it is beneficial to carry out designing which improves ventilation performances. Several design techniques aimed at promoting ventilation have been available, but have not gained in popularity because they often cause degradation and leak in the roof. In addition, in densely urban areas, it is difficult to predict the prevailing wind direction, and there are possibilities that a wind direction is changed due to rebuilding of the surrounding buildings. Furthermore, due to very narrow building intervals in the residential area of the urban areas, ensuring ventilation amount through the existing wall face-to-face is not easy. This paper focuses on the residential ventilation tower as a measure to address these problems. It aims to realize securing ventilation, which is often difficult through the existing wall window path, as a construction building frame without any use of electricity or moving parts. 2. Method: In order to quantitatively evaluate the performance of the ventilation tower, a calculation of ventilation tower size which demonstrates an equivalent airflow resistance value as through the existing wall face openings was carried out using a CFD. In the procedure, a virtual chamber to avoid effects of external wind and inlet conditions into the room was provided on all of the openings, then the flow rate count, α, in the ventilation tower inlet and each opening was calculated. The area of the ventilation tower entrance and ventilation tower opening area, which shows an equivalent ventilation resistance when using α, were calculated and set as Ventilation tower 1 (VT1) and Ventilation tower 2 (VT2) for evaluation in this study. 3. Result: The ventilation tower parts of VT1 and VT2 are attached to the roof of the house model, and wind tunnel experiments were carried out. The chamber pressure, the amount of ventilation and a rooftop velocity were measured. The results of the experiment confirmed that the route using the ventilation tower holds about 70% of the ventilation capacity of the existing wall face route without any dependency on the wall opening position and wind direction. 4. Conclusions: By creating a mesh for analysis which reproduced the housing model and comparing it with the obtained wind pressure coefficients and amount of ventilation in the experiment, the accuracy of CFD calculation was confirmed. Since the actual phenomena reproducibility of CFD with accuracy was confirmed, we went on to a case study with conditions with buildings in the surrounding area. No significant differences were noted in the case of assumed suburbs with 30% gross building coverage, whereas it was confirmed that the ventilation tower to function effectively in the densely built.