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Using natural ventilation is effective to save energy, and it is essential for energy conservation and decreasing running cost [1]. However, in office buildings located in where mid- to -high-rise buildings are densely distributed, the way of ensuring stable ventilation is very important matter of natural ventilation system. In this research, we focus on the ventilation performance of an office building where the natural ventilation system is introduced by utilizing the buoyancy force through a ventilation shaft. First, the air change rate (ACH) of target rooms were estimated using CO2 generated from occupants as tracer gas in the measurement that was conducted in the autumn in 2017. In addition, influence of the outdoor condition on airflow rate was investigated by flow network calculation based on the wind pressure coefficient of the target building that was obtained from CFD analysis.
Since the target building is located in a densely built-up block area, wind-induced natural ventilation seems to be unstable and difficult to apply. Therefore, a natural ventilation shaft is introduced and the room air is ventilated by buoyancy. A 10-story office building was analyzed, and the natural ventilation system is introduced to 4F to 9F floors. There exist two natural ventilation shafts, one is for the lower floors (4-7 F), and the other is for the higher floors (8-9 F), to prevent the backflow from the shaft at the upper floors.
In this study, in order to understand the indoor environment and the air change rate (ACH) when natural ventilation openings are open, several kinds of measurements were conducted in the office rooms of lower floors (4F - 7F) from 5th October 2017 to 8th November 2017. In the measurement, the following measurements were conducted, (a) CO2 concentration, temperature, and humidity in the indoor occupied area, (b) pressure difference at the natural ventilation shaft, (c) number of people existing in the room by the observation camera. The amount of CO2 generated per person in the room was first estimated when the mechanical ventilation was operated. Then, the air change rate (ACH) was obtained during natural ventilation as well.
By estimating the airfow rate, the air change rate (ACH) of the target room for each floor was found to be 1.0 to 2.5[1/h]. In addition, based on the pressure difference measurement at the natural ventilation shaft, it was observed that the outdoor air occasionally flows backward from the shaft to the office room despite that it was assumed to be an exhaust path. This is because the temperature inside the shaft has not sufficiently increased.
Then correlation between airflow rate and the following outdoor condition v, Δtr, Δts (external wind speed, temperature difference between indoor and outdoor air, temperature difference between outside and the inside the shaft) was calculated, and Δtr showed the greatest correlation. In the target building, it was found that Δtr (the difference between the indoor and outdoor temperature) was the main driving force.
In order to calculate the airflow rate by flow network model, CFD analysis assuming a model experiment in the wind tunnel was conducted to obtain the wind pressure coefficient in the 8 wind direction. As a result of calculation of airflow rate by the flow network model changing the outside air temperature and the external wind, the backflow from the shaft to the office room was seen in some wind direction. However, no backflow occurred when temperature difference between indoor and outdoor air was large. It is a challenge to ensure a sufficient temperature difference within the shaft to achieve stable ventilation.