流量分布系数对低压热释放速率测量的影响

热释放速率是反映火灾规模的重要参数,但高海拔低压地区可燃物燃烧的热释放速率测量一直未得到有效解决。为此,引入烟气管道流量分布系数,并对其进行低压修正,基于耗氧原理分析了低压条件下可燃物燃烧的热释放速率测量原理,提出了低压燃烧热释放速率的计算方法。在此基础上,设计并搭建了低压燃烧舱模拟高海拔低压环境,开展了不同静态低压条件下的正庚烷池火试验,并对烟气管道流量分布系数与不同的静态环境低压进行线性关系、指数关系等拟合,发现试验工况压力下的流量分布系数与响应的环境压力呈指数函数关系,且拟合度高达97.2%。采用指数函数关系并根据能量守恒对试验测量出的低压热释放速率进行修正,误差不超过5%,表明该方法可行。

Influence of the flowing rate distribution coefficient on the HRR measurement under low pressures

In this paper,we would like to propose the principle for measuring the HRR under low pressures from the theoretical point of view and the method for computing and modifying the HRR under such pressures based on the oxygen consumption calorimetry.As is known,heat releasing rate (HRR) is one of the principal indexes for characterizing the fire size,though,there still exists a great deal of difficulties for measuring the HRR at high altitude under low-pressure conditions.This may involve the necessity to introduce and modify a new variable named the flow rate distribution factor in the smoke duct of the HRR measuring system at low pressures.What is more,it is also necessary to disclose the HRR dependence on the pressure extent comprehensively by designing a controlled oxygen and pressure environment through the experiments to trace the fire behaviors at the different pressure levels,especially under low pressures.For the above mentioned purpose,we have developed and built up an altitude combustion chamber with the oxygen control mode of 2 × 3 × 4.65m3 in volume to create a needed high-altitude environment.And,then,we have done a series of n-heptane pool fire tests both under the standard pressures and the low different fixed pressures in the higher altitude combustion chamber on the basis of the proposed method for measuring HRR under the aforementioned conditions.Furthermore,the relationship between the flow rate distribution factor and the different fixed low pressures has been gained with the linear and exponential fitting method.Thus,it has made us believe that the exponential relationship for the nheptane pool fire tests under the configured low pressures with their correlation degree being 97.2％ is far more suitable to be recommended.What is more,the modified HRR measuring method based on the exponential relationship between the flowing rate distribution parameters and the low pressures can thus be founded based on the conservation of the total energy released mode,by which the error of the calibrated HRR can be reduced by less than 5％.Therefore,the above research results demonstrate that the method for accounting the HRR under low pressures is rational and practicable.