On-site assessment of the discharge coefficient of open windows

• In situ discharge coefficients of windows were measured with the flow driven by a fan and by wind.
• A good agreement was obtained with the two methods.
• The discharge coefficient of a side-hung casement window ranged from CdCd = 0.41 to CdCd = 0.81.
• The discharge coefficient of a bottom hung casement window was CdCd = 0.84.
• The impact of CdCd of a roller shutter on a side-hung casement window was analysed.

For assessing the discharge coefficient of an opening it is necessary to measure, as accurately as possible, the flow rate and the corresponding pressure difference that occurs when the fluid passes through it. When the openings are as big as windows applied in common building spaces the open section of the window is not negligible when compared with the cross-section of the room. In these conditions, the flow velocity in the room is not negligible and the obstructions (e.g., furniture) are prone to have a higher influence on measurements. In addition, the external windows are subject to the wind action, which may also impair the measurement. Even though these are not the ideal conditions, they are the common conditions in which natural ventilation occurs. In this paper, on site measurements of the discharge coefficient of open windows were carried out and the feasibility of these measurements was evaluated. Two methods were used (flow driven by a fan and flow driven by wind action) and compared. It is shown that the discharge coefficient of a side-hung casement window with a roller shutter may range from CdCd = 0.41 to CdCd = 0.81 and that the discharge coefficient of a bottom hung casement window is CdCd = 0.84.