Experimental and Theoretical Study on Flow Behavior from a Fire Source under the downward Flow

在洁净室中，火灾引起的向上气流和向下通风气流之间的相互作用是火灾安全和烟雾探测中值得关注的问题。一般来说，火灾早期产生的浮力羽流非常微弱，在火灾产品到达火灾探测器之前，很容易被通风的环境空气流稀释。为了保证洁净室足够的消防安全，根据向上的火流和向下的通风流复合产生的流动行为来设计早期火灾探测系统是很重要的。全尺寸模型洁净室尺寸为6.4m(W) × 9m(L) × 3.2m(H)和3.8m(W) × 3.8m(L) × 3.0m(H)，采用6kW的线或圆形模型火源。甲醇被用作燃料。温度、速度和动压力每10秒测量一次，同时通过烟丝和/或烟蜡烛进行流动可视化。通风速度为20 ~ 45 cm/sec。以特征形式v/Q^<1/5>的ter线上升速度和火源上方羽流的温升（DELTAT）与热释放率z/Q^<2/5>的特征高度相对应。这些实验数据是在向下通风的情况下得到的。v / Q ^ < 1/5 > = (z / Q ^ < > 2/5) ^ n, DELTAT = B (z / Q ^ < > 2/5) ^ 2 n - 1 > <。在24 ~ 25 cm/sec下，羽流区域的A和B分别为A = 0.2 ~ 0.3和B = 3 ~ 5, n = -1/3。结果表明，除火焰间歇区外，沿火焰羽流中心线基于温升和速度的流动行为与没有向下通风流的火焰羽流相似。A和B的下降速率大于无通风流的流量。向上的火羽流和向下的通风流诱导的复杂流动使得内部的火产物（烟雾和气体）迅速扩散；几分钟后打扫干净。除非出现非常快速的火灾探测系统，否则没有足够的时间来保护洁净室的洁净度不受火灾烟雾扩散的影响。少

An interaction between fire induced upward flow and downward ventilation flow in a clean room is a matter of some concerns in the fire safety and smoke detection. Generally, the buoyant plume from an early stage of fire is very weak and is easily diluted with ambient air flow of the ventilation before the fire products reaching to a fire detector. In order to ensure adequate fire safety for clean rooms, it is important to design the early fire detecting system depending on the flow behavior produced by the complex of upward fire flow and downward ventilation flow.Full scale model clean rooms, 6.4m(W) x 9m(L) x 3.2m(H) and 3.8m (W) x 3.8m(L) x 3.0m(H), were used with a line or a circular model fire source of 6kW. Methylalcohol was used as a fuel. Temperature, velocity, and dynamic pressure were measured every 10 sec and flow visualization by a smoke wire and/or smoke candles was also carried out simultaneously. Downward ventilation velocity ranging 20 cm/sec - 45 cm/sec was employed.Cen … More ter-line upward velocity in a characteristic form of v/Q^<1/5> and temperature rise (DELTAT) in a plume above a fire source were plotted against characteristic height with heat release rate of z/Q^<2/5>. These experimental data were obtained with downward ventilation flow. v/Q^<1/5> = A (z/Q^<2/5>)^n, DELTAT = B (z/Q^<2/5>)^<2n-1>. The values of A and B for the plume region with downward ventilation flow of 24 - 25 cm/sec are estimated as A = 0.2 - 0.3 and B = 3 - 5, respectively with n = -1/3. It was clearly found that the flow behavior based on temperature rise and velocity along the center-line of the fire plume are similar to those in the fire plume without downward ventilation flow except the flame intermittent region. The decreasing rates of A and B are greater than those in the flow without ventilation flow. Complex flow induced by upward flow of fire plume and downward ventilation flow made rapid diffusion of fire products (smoke and gas) within ;the clean room in a few minutes. There is no enough time to protect the cleanliness of the clean room from smoke diffusion from a fire unless very rapid fire detecting system will presents. Less