火災時の機械排煙口への空気混入限界の定式化と実験による検討 : 二層ゾーンモデルにおける機械排煙口への空気混入の定式化(その1)

Abstract

Formulation of critical condition for onset of air entrainment into mechanical smoke vent was carried for two-layer environment where upper part of room is occupied by hot smoke layer while lower part for room is occupied by normal temperature air. Assuming that induced air flow is pushed back downward by negative buoyant force by hot smoke layer, critical condition for onset of air entrainment was described by Froude number whose characteristic dimension is short edge length of vent opening. Flow to vent were classified into 3 flow patterns, plane sink, line sink and point sink. Relationships between critical smoke layer thickness and Froude number were derived by dimensional analysis. Critical smoke layer thickness of each flow pattern is proportional to 2,2/3 or 2/5-th power of Froude number, respectively. Model scale experiments were carried out in order to determine proportional coefficients of this formula. Varying aspect ratio between unity and infinite, critical condition was determined by eye-observation for multiple combinations of venting velocity and smoke layer temperature. As a result, line and point sink patterns were demonstrated. It was found that flow to vent could be regarded as point sink when smoke layer is thick enough. As smoke layer thickness is decreased, flow pattern changes to line sink. Branching Froude number of flow pattern depends on aspect ratio of vent opening. The results were summarized in a simple formula to predict critical condition for the onset of air entrainment into mechanical smoke vent on ceiling or on wall for various aspect ratio.