【作者】 陈兵；

【导师】 陆守香；

【作者基本信息】 中国科学技术大学 ， 安全技术及工程， 2011， 博士

【摘要】 作为典型受限舱室火灾的一种,发生在机舱等船舶顶部开口舱室内的火灾具有易发、频发和危害性大的特点,且由于其开口位置的特殊性,导致顶部开口舱室中的火灾发展过程与具有门窗等竖直开口的普通建筑空间火灾有很大的不同。因此,研究船舶顶部开口舱室中火灾的发展过程,对认识顶部开口舱室火灾的发展规律以及火灾的控制和扑救具有重要的意义。本文在100cm(长)×100cm(宽)×75cm(高)的实验舱内,开展了10cm、14cm和20cm等三种直径的庚烷油池火的实验研究,水平的方形开口位于实验舱的顶部拐角,其大小由25cm2(5cm×5cm)增大到900cm2(30cm×30cm)。实验研究了不同开口尺寸对池火燃烧过程的影响;随后对热烟气在舱室内的填充过程以及热烟气和冷空气在顶部水平开口处的交换流动过程开展了研究;最后分析了顶部开口舱室中火灾环境下的传热模式,探讨了热量在舱室内的分配规律,并建立了顶部开口舱室火灾动力学模型。论文的具体工作包括:分析了不同开口尺寸对池火发展过程的影响。根据火焰熄灭的原因,把池火熄灭分为“缺氧熄灭”模式和“燃料耗尽熄灭”模式。在“缺氧熄灭”模式下,随着燃烧的进行,火焰形态由稳定的规则形状变成在油池面上不断游走的不稳定形状,火焰卷吸含氧量持续减少的成分不断变化的烟气混合物,且开口尺寸对质量损失速率的影响甚微;在“燃料耗尽熄灭”模式下,火焰可以维持较为稳定的形态进行燃烧,其卷吸的氧气含量在经历一个下降阶段后,保持在一个相对恒定的值直至火焰熄灭,且由于达到沸腾燃烧状态的原因,燃烧速率有较为明显的增大。根据氧耗原理,计算发现给定舱室条件下,油池火的燃烧效率随火源直径的增大而减小。研究了顶部开口舱室中火灾烟气的填充过程。通过对比分析发现基于温度分层的烟气层高度判断方法在顶部开口舱室中具有很好的适用性。在顶部开口舱室火灾中,烟气迅速沉降至舱室底部,在两种熄灭模式下,整个舱室均可用“单区模型”来描述。根据“单区模型”假设,建立了顶部开口舱室中烟气温度预测模型,并发现在应用模型时,随着燃烧的进行,不断增大热损系数的取值,可以较好的预测烟气温度,这也说明越来越多的热量用于加热舱体和通过舱室壁面向外散失。探讨了热烟气和冷空气在顶部水平开口处的流动过程。利用激光散射法清晰的显示出火灾条件下冷热气体在顶部水平开口处的流动过程。当开口较小时,水平开口处形成“瞬时单向流”,且烟气向外流出的时间占主导地位。随着开口的增大,开口处转变为“双向交换流”,热烟气流出的主导地位不断减弱。通过计算不同流动方式下水平开口两侧的压力差和密度差,验证了盐水模拟实验中获得的从“单向流”向“双向流”转变的临界压力差,在火灾条件下具有较好的适用性。当开口处膨胀压力差大于临界压力差后,流体呈单向流方式通过开口,且体积流率主要由膨胀压力差决定;而当膨胀压力差小于临界值时,流体的流动方式为双向流,流率取决于压力差和密度差共同作用;而当压力差为零时,开口处仍呈现双向流特征,流体的流率由开口上下方的密度差控制,且此时密度差引起的体积流率达到最大。最后,给出了热烟气和冷空气通过顶部水平开口的质量流率计算方法。随着开口的增大,冷热气体通过开口的质量流率均增大,且在较大开口尺寸下,膨胀压力差的作用可以忽略,空气流入舱室的质量大于烟气流出的质量。研究了顶部开口舱室中火灾环境下的传热模式和热量分配规律,并建立了火灾参数的动力学预测模型。分析了顶部开口舱室中,火焰、气体以及舱体之间的对流、辐射和传导传热,并在热量守恒方程的基础上,计算发现火灾释放出来的绝大部分热流用于加热舱室壁面并通过壁面向外散失,而用于加热舱室内气体和通过顶部开口散失的热量仅占总热量的10%左右。随着燃烧的进行,热损系数不断变大,在燃烧的后期,其值保持在0.9～1的范围内。依据“单区模型”假设以及质量、能量、组分守恒方程,建立了顶部开口舱室中气体温度、氧气浓度和压强的预测模型。预测模型的计算结果和实验测量值具有相似的变化趋势和较好的符合度。更多还原

【Abstract】 As a typical compartment fire, the fire occurring in ship engine room with ceiling vent has resulted in the greatest number of fire fatalities due to the abundance of ignition sources in close proximity with flammable liquids. Compartment fire with vertical wall vents or horizontal ceiling vents might have different behaviors, because the flow exchange at the openings and vents is of considerable importance in the compartment fire growth and spread. It is therefore very important that carrying on the research on the fire development in the compartment with ceiling vent, to gain the knowledge of how the fire grows in such compartment at different ventilation conditions and how to control the fire spread and rescue.Experimental studies were carried out in a compartment with dimensions of 100cm (length)×100cm (width)×75cm (height). Rectangular openings with various sizes from 25 cm2 (5cm×5cm) to 900 cm2 (30cm×30cm) were located at one corner of the ceiling. Heptane pool fires with diameter of 10cm, 14cm and 20cm were adapted as fire source. In present study, the effects of opening size on the pool fire behavior were studied. Furthermore, the filling process of smoke in the enclosure together with the exchange flow at the ceiling opening is investigated. Then, detailed analysis of the heat transfer mode in compartment fires with ceiling openings is conducted to understand the heat distribution in the fire and finally establish the fire dynamic model for enclosure fires with ceiling openings. The detailed work is as follows:The effect of ceiling vent size on the burning behavior of pool fire. Two kinds of burning are decided according to the reason for fire extinction as“oxygen-lack”mode and“fuel exhaust”mode. When under the“oxygen-lack”mode, flame would change from steady burning state to an unsteady state in which flame wonders around the fuel surface. The fire plume entrains contaminated gas with decreasing oxygen concentration and the effects of the opening sizes on fuel mass loss rate are rare. When under the“fuel exhaust”mode, flame keeps in steady burning state, the oxygen concentration in the entrained gas stays at a relatively unchanged value after the initial decreasing stage, and the fuel mass loss rate increases dramatically due to the boiling of the fuel. According to the oxygen consumption, the combustion efficiency decreases with the increasing of pool diameter for a given compartment.The filling process of smoke in compartment fire with ceiling vent. By comparison, it can be observed that the determination method of layer interface based on the temperature stratification can be adapted in enclosure fires with ceiling openings. Smoke layer quickly descends to the enclosure floor, so“single zone”model is proper for both“oxygen-lack”and“fuel exhaust”conditions. Based on the main assumption of“single zone”model, a prediction model for gas temperature in enclosure fires with ceiling openings is established. As the burning continues, the heat loss fraction should increase accordingly to get a good prediction. This indicates that more heat from the fire is converted from the walls to the outside environment and used to heat the walls.The basic nature of the gas flow through the horizontal ceiling vent in the compartment fire. The flow patterns across the ceiling vent are observed to study the basic characteristics of the flow resulting from the imposed pressure and temperature across the vent by use of the laser sheet arrangement. Under the small veiling vent condition, the downward flow decreases to zero, a unidirectional upward flow is found at the vent. As the vent size increases, the flow is bidirectional, the ambient air descends in the compartment and the smoke rises up across the vent. The calculating results show that when the thermal expansion pressure is larger than the critical pressure, the flow is unidirectional and the flow rate is determinated by the thermal expansion; when the pressure is smaller than the critical pressure, a bidirectional symmetric flow across the vent is observed, with the mass flow is taken as the pressure-and buoyancy-driven; for a zero pressure difference, the pressure-driven flow drop to zero and the two-way flow rate are only determinated by the buoyance-driven. The mass flow rates across the ceiling vent are given. The inflowing and out flowing mass rates increase as ceiling vent increases, and under much larger vent size, the effect of thermal expansion pressure can be ignored, the fresh air flow rate is larger than that of smoke.The heat transfer mode in ceiling vent compartment fire and predicting model of fire dynamic parameters. We analyse the heat transfer including convection, radiation and conduction among the flame, gases and the compartment. Based upon the heat conservation equation, we found that most of the heat release from the fire was lost to the ceiling and walls, only approximately 10% of the total heat generated by combustion heated the gases and leaked from the ceiling vent. With the burning of the fuel, the heat loss coefficient became larger and larger and maintained within 0.9~1 at the late stage of the fire. In addition, a model for predicting the temperature, oxygen concentration and pressure in a compartment with a ceiling vent was proposed based upon the one zone assumption and the conservation equations of mass, energy and components. The change tendency and the value of the predicted and the experimental results show a good agreement.更多还原