Study Of The Thermal Stratification And Movement Mechanism Of Fire-induced Smoke In High Aspect Ratio Channel

High Aspect Ratio Channels are required in the model society. The channels, such as large building corridors, road or railway tunnels, mining laneways, and urban underground traffics and so on, are important for the social and economic development. However, the fire in the channels once takes place the fire-induced ruined situation generally is very serious, especially for busy road or railway tunnels, where more casualties will easy happen and its bad social effect is great. So, it is necessary to study how to control this type fire in enclosure with almost closed space. According to the channel fires in the world, the main factor which can result in fatal and injured is the fire-induced poison smoke. So how to control fire smoke is very important for the channel fire controlling. As the channel fire smoke, there are many affecting factors to it, such as combustible types, fire power, ventilation and exhaust fume, fire location and so on, whose effects are on the flow and thermal stratification stability of the fire smoke, and these factors increase the difficulty of the channel fire smoke controlling. The thermal stratification stability of the hot smoke is essential for the channel fire smoke controlling and personal evacuation. Lots of researches on fire smoke thermal stratification had been carried out, but these researches mainly focus on far flow field from fire source and for near fire flow field is absent. It is especially noteworthy, when the channel fire develops from the initial stage to the flashover the air compress wave from the jump of flashover will circularly move because of its reflection on the channel end. This movement will result in the pulsation or oscillation of the channel fire smoke. So this situation generally requires more serious to control the fire smoke. Thus, the fire smoke movement in High Aspect Ratio Channel was selected as the research object. The experimental study was carried out by use of a small-scale model apparatus in order to ascertain the effect of fire power and longitudinal ventilation on the stability of the fire smoke thermal stratification in the channel. Then, with theoretical study and numeral simulation, characteristic mechanism of the fire smoke pulsation in the channel was investigated here.The small scale set-up was introduced first in the present study. Its configuration, measure methods and how to obtain experimental data and to analyze it were explained.The experimental research of the fire smoke thermal stratification in the set-up was carried out. The effect of the different fire powers and longitudinal ventilation on the fire smoke thermal stratification during the flow field quasi-steady stage was investigated in detail in the downstream of fire source. The main conclusions include as follow:(1) By the stratification intensity and curve investigations to the fire smoke thermal stratification for near fire flow field, it shows that the stratification intensity and curve are able to characterize well the fire hot flow field. So, they are powerful tools for the fire hot flow field. Such as, under the condition of different longitudinal ventilation switch, some rules about the stratification curve were obtained. When the switch changes from low air speed (namely low shift) to high one (namely high shift) the stratification intensity firstly decreases and approaches to stability (that is to say, the no strong thermal stratification between the hot layer and cold layer comes into being); when the switch changes from high air speed (namely high shift) to low one (namely low shift) the stratification intensity once increases and approaches to stability again.(2) The present study also shows that different fire powers have minor effect on the stratification intensity of hot fire smoke vertical thermal stratification in the quasi-steady zone of the channel fire flow field. However, different longitudinal ventilation has apparently effect on the stratification intensity, especially on the pole value of the stratification intensity.(3) The fire power curve was discussed theoretically. The two key factors, which were the max heat release rate (viz. HRR) and the total energy of the channel fire, were obtained. They both can affect the shape of the fire power curve and their ratio (viz. the ratio of the max HRR and the total energy) has important effect on whether the plateau stage of the fire power curve can appear. The larger the ratio is, the less probability the fire power curve with a plateau stage is; vice versa. Under the condition of nature ventilation, the fire power has minor effect on the fire smoke thermal stratification, and there is an optimal fire power, which makes the thermal stratification become the best stability and most stratification intensity.For the thermal oscillation character of the channel fire smoke, the theoretical analysis and numerical experiment were carried out. As the fire with big power, the air compress wave from the jump of the flashover has apparently effect on the flow field in the channel. By dealing proximately with one dimension for a channel fire flow field, its primary oscillation value was induced, and the oscillation results of the famous Runehamar tunnel fire experiment was analyzed theoretically. It was certificated that the theoretical value of the oscillation period accords well with the experimental one (viz. the period of4s and18s during the twice experiment, respectively) of the two full-scale fire investigations in the Runehamar tunnel, and the probable error analysis was given. By the numerical experiment of3dimensions channel with300m length, the movement mechanism of the flashover compress wave was ascertained. It was found that the wave asymmetry period is about1.7s, which is proximate to the channel theoretical value of1.76s. For the effect of the wave on the thermal stratification in the channel, the thickness of the hot layer near the fire changes with the asymmetry wave. Another vehicle-induced air compress wave, which has effect on the thermal stratification near the fire in the channel, was investigated. It is found that the magnitude of the vehicle-induced disturbing wave is stronger than that of the flashover-induced disturbing wave, which reflects and decomposes many times in the channel. So, the effect of the vehicle-induced disturbing wave on the thermal stratification is obvious, and the effect produces an "extrusion" phenomenon to the thermal stratification in the channel.Through the numerical experiment, the study of the channel with one closed-end was carried out. For the fire during the tunnel being constructed (viz. the channel with one closed-end), the critical criterions on safety evacuation were firstly provided in order to protect presently the constructor in the tunnel. Then, according to the factional engineering conditions, some relative characters and boundary conditions were setup and the tunnel physical model was built. Lastly, by use of the FDS, the investigation on the fire smoke evolution in the channel with one closed-end was carried out. The main conclusions include as follow:(1) For the horizontal tunnel, the related analysis shows that, only considering the temperature along tunnel longitudinal direction, the temperature distribution complies with certain rules, that is, there are a certain lag among the temperature rising curves and their fluctuation, which is relative to the detailed tunnel parameters and ambient condition, e.g. the mixing and cooling effects of air flow from the duct open on the hot plume. At the same time, the temperature rising rate is different at various inspecting points before reaching the critical temperature.(2) The analysis also shows that the cost time required for the critical temperature and the critical visibility to arrive at these points conforms to certain rule, that is, the critical temperature appearance is prior to the critical visibility before60m point in the tunnel and reverses after60m point.(3) For the influence of the tunnel inclination on the critical temperature and the critical visibility, two key inclinations are obtained by a series of numerical simulations. For the C point there is a key slope scope, namely from-5to0degree or so and the other key slope point is approximately5degree for the E point.(4) On the whole, the CO concentrations at all inspecting points are not over400ppm, namely they are always less than the critical value of500ppm, this show that the influence of the CO concentration on the occupant safety evacuation is small for the tunnel fire. In the case of forced ventilation, the CO concentration distribution tends to steady at250s in the typical one closed-end tunnel. That is to say, at that moment, before90m the CO concentration vertical distribution presents minor steady two layers and after90m the CO concentration become relative uniformity and the constant of130ppm or so in the end. By the simulation, we can see that the regularity of the CO concentration changing is very accordance with the regularity of the smoke flow. It is possible to discover the smoke flow regularity by inspecting low CO concentration changing.In summary, by the combine method of theoretical analysis, small-scale model and numerical experiment, the fire flow field characters and the fire-induced smoke evolution in High Aspect Ratio Channel with longitudinal ventilation were investigated here. The definition of thermal stratification intensity in the channel was given and some rules of the fire smoke movement during its dynamical state were obtained under different flow field characters. The simple disturbing theory in the channel fire was built and the law of intersection between the critical temperature and critical visibility was given. These results will be help to understand further the fire smoke movement mechanism for us and offer the science base on how to control the channel fire.