The effectiveness of an air curtain in the doorway of a ventilated building

Abstract

AbstractAir curtains are used to reduce the heat and mass exchange across open doorways. Their sealing ability is assessed in terms of the effectiveness $\def \xmlpi #1{}\def \mathsfbi #1{\boldsymbol {\mathsf {#1}}}\let \le =\leqslant \let \leq =\leqslant \let \ge =\geqslant \let \geq =\geqslant \def \Pr {\mathit {Pr}}\def \Fr {\mathit {Fr}}\def \Rey {\mathit {Re}}E$, the fraction of the exchange flow prevented by the air curtain compared to an unobstructed open door. Previous work has studied air-curtain effectiveness when the doorway is the only means of ventilating a space. In this paper, we examine the effects of additional displacement ventilation on the dynamics of the air curtain and the resulting changes in its effectiveness. The main controlling parameter is the deflection modulus $D_{m}$, which is the ratio between the momentum flux of the air curtain and the transverse forces due to the displacement ventilation. For a relatively warm interior, we find that, for small values of $D_{m}$, the air curtain is drawn inside the space by the ventilation flow. For large values of $D_{m}$, the flow through the doorway is controlled by the air curtain. A smooth transition occurs between these two regimes, and we estimate the $D_{m}$ value for the onset of this transition. Our model provides a quantitative prediction of $E(D_{m})$ in the ventilation-driven regime, and gives a qualitative description of the other two regimes. Laboratory experiments were conducted to test the proposed model. The experimental data were compared to theoretical predictions, and good agreement was found.