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

洁净室内火灾引起的上行气流和下行气流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，在火灾产物到达火灾探测器之前，很容易被通风系统的环境气流稀释。为了保证洁净室足够的火灾安全，根据向上火灾流动和向下通风流动复合产生的流动特性，设计早期火灾探测系统是很重要的。全比例模型洁净室，6.4m(W)×9m(L)×3.2m(H)和3.8m(W)×3.8m(L)x 3.0m(H)，采用6kW的直线或圆形火源。甲醇被用作燃料。每10秒测量一次温度、速度和动态压力，同时还通过烟丝和/或烟蜡烛进行流动显示。下行通风速度范围为20厘米/秒-45厘米/秒。Cen…以v/q^&lt；1/5&gt；1/5&gt；的特征形式绘制了较多的三线上升速度和火源上方羽流的温升(DELTAT)与特征高度的关系图，放热率为z/q^&lt；2/5&gt；这些实验数据是在向下通风的情况下获得的。V/Q^&lt；1/5&gt；=A(z/Q^&lt；2/5&gt；)^n，DELTAT=B(z/Q^&lt；2/5&gt；)^&lt；2n-1&gt；。对于下行气流为24-25 cm/s的烟羽区域，A和B的值分别为A=0.2-0.3和B=3-5，n=-1/3。结果表明，除了火焰间歇区外，基于温度升高和沿烟羽中心线速度的流动特性与没有下行气流的羽流相似。A、B的下降率均大于无通风时的下降率。火羽向上流动和向下通风产生的复杂流动使火灾产物(烟雾和气体)在几分钟内在洁净室内迅速扩散。除非有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清洁度，使其不受火灾烟雾扩散的影响。较少

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