The effect of tunnel height on critical ventilation velocity in tunnel fires

[[abstract]]隧道火災發生時，因隧道之獨特空間，會助長煙的危害，故煙控系統為隧道火災防制之最重要工作。煙控設計需將濃煙抑制使其不往上游繼續蔓延移動，而其所需之最小風速稱為「臨界風速」（ucr critical ventilation velocity）。有很多學者以理論、實驗或模擬探討不同隧道斷面對臨界風速之影響，但大多探討隧道寬度而非高度。本研究之隧道總長8 m、寬0 6 m而高度分別為0 3 m、0 4 m、0 5 m、0 6 m、0 8 m進行實驗，共有5種火源分別為2 7kW、5 4kW、8 1kW、10 8kW、13 5kW，有兩種方式判別臨界風速，而後進行電腦數值模擬及理論公式分析。結果顯示，當隧道寬高比 > 1時，隧道高度降低時臨界風速會隨著增加，本研究將所有實驗規格帶入先前研究所發展之臨界風速預測公式u"cr"=0 68399〖(Q^*)〗^(1/3) 〖("g" H ¯)〗^(1/2)，發現實驗之臨界風速與預測公式之風速趨勢具有非常好的相關性，但於本實驗規格隧道寬高比小於1時將無法符合上述風速趨勢及預測公式之風速趨勢。[[abstract]]Ventilation is an effective method for controlling smoke during a tunnel fire The “critical ventilation velocity” ucr is defined as the minimum velocity at which smoke is prevented from spreading under longitudinal ventilation flow in tunnel fire situations Many theoretical experimental and numerical researches have investigated the effect of different geometries of cross section of tunnel on the ucr Most of the researches discussed the tunnel width and tunnel height was not discussed This study used an 1:20 small-scale tunnel (8 m in length 0 3 m、0 4 m、0 5 m、0 6 m、0 8 m in height and 0 6 m in width) to understand the influence of tunnel height on critical ventilation velocity Gasoline in square pans (2 7 kW、5 4 kW、8 1 kW、10 8 kW、13 5 kW for free burning) was used as fire source There are two ways to determine the ucr Results show that the tunnel height significantly influences the smoke flow in tunnel fire and ucr When the tunnel aspect ratio is greater than 1 the flow velocity was higher in tunnels with lower heights However when the tunnel aspect ratio is less than 1 the flow velocity was lower in tunnels with lower height