The Fire Problem - Research Paper Example

Table of Contents Table of Contents 1 Abstract 2 Introduction 2 The Fire Problem 2 Methodology 5 Results and discussion 6 Processes of flame spread 7 Diffusion flame spread 8 Thermal Model 9 Transition to Turbulence 11 References 14 Abstract The fires problem is one of the hazard pose a threat to life and property. Flames behave differently under various conditions which include: the oxygen available, combustible material, orientation of surfaces, etc. this a complex phenomena which is influenced by multiple factors that includes ignition, heat release rate, flame spread and the generation of different products of combustion like carbon IV oxide and carbon II oxide. The research was performed by studying various sources in the library and in the internet and also performing experiments to simulate a real life situation. Introduction The Fire Problem The flammability of a material is a complex event which is influenced by multiple factors that includes ignition, heat release rate, flame spread and the generation of different products of combustion like carbon IV oxide and carbon II oxide. In order to better protect the people and property from risk posed by the unwanted fires, it become necessary to understands all these factors under different conditions. Babrauskas and Vytenis 1992 suggested that heat release rate is the main variable in fire hazard; but Kashiwagi and Ito argued effectively that, the flame spread over the surface of combustible material such a wall and a floor is the main variable responsible for the growth of fire at the initial stages of fire. In addition, the angle of orientation of the combustible material has been found to contribute to the flame spread; specifically, upward flame spread on a vertical surface has been recognized as important since it is often present during the development of fires (Pizzo et al., 2009). Fires on materials burning in the vertical position spread very rapidly and therefore the flame spread is the most hazardous. Although many studies have been done on the flammability of the materials, many of them give their results of some measurement or observations used to derive a relative ranking level. There have been fewer attempts to relate the measured test results to the theories of ignition, combustion or spread. Therefore, the results can be used in a limited way. Although thermal properties of a material influence the flame spread rate, numerous studies have shown that the controlling mechanism of upward flame spread mechanism is heat transfer flame the flame to the unburnt material. This makes knowledge of the flame heat flux to a given oriented surface very essential as well as the length of flame extension, or preheats distance. These two factors are also emphasized by Quintiere et al., (1988). As a result, the knowledge of flame to surface heat flux and flame length as studied here would provide the required information for flame spread on the surface. The main purpose of this work is to research on flame spread of the surface of the material and investigate the effect of orientation and inclination angle on the flame spread on the surface under different conditions. Due to the fact that the synthetic polymers in buildings and other structures have increased significantly, it becomes necessary to understand and evaluate these materials and the hazards they pose, especially on inclined surfaces. Polymers do not char or drip heavily and therefore the most well known polymer used in the most flame spread research and thermophysical properties is PMMA. In compliance with the aim, the objectives of this work have been put together into a set of issues, which is to help us illuminate the major one. They are the building blocks of this research. They include: a) Provide data and analysis of research on flame spread. b) Carrying out risk assessments and maintain safety records as appropriate. c) Synthesise results and conclusions of the study with reference to the limitations and generalisations; In accordance with these objectives, the literature sources have been chosen. There is a great number of works dedicated to the analysis of flame spread in a working environment in general and in the synthetic material in particular. From a number of sources, we have sorted out the ones which can form the best basis for our research in accordance with the issue under consideration. They, in their turn, provide a foundation for development of the theory and to provide better understanding on the subject matter. This is reliability of the prediction of fire processes depends on the amount of knowledge and quality on the processes for example in fires are of combustion. Thus this study explores combustion phenomena, like ignition, flame spread, fire induced gas flow, flame radiation and fire behavior. However, it is seems that the knowledge obtained from the studies on combustion phenomena is rarely used in protection against fires. Sometimes it can be argued out that if the people dealing with building design, fire modeling, fire suppression system, or fire fighting would have sufficient knowledge of combustion phenomena they can be able to obtain better results. On the other hand, the data and models on basic combustion phenomenon can be used develop the procedure for fire protection. It is therefore necessary to obtain basic knowledge on combustion in fires for fire protection, and as there are many changes in scientific, engineering, financial and social environment, there must be continual effort in this field. The issues presented in this paper are limited to the physical aspects of fire combustion, but does not mean that chemical aspects is less important. Methodology To define the problem in more detail and to give a specific aim and goal of the project, the work began with two studies. These included the study of literature flame spread on material and conducting practical test using PMMA material in the lab. This gave a first impression of the situation in literature and in practice. This has been summarized in the figure below. Figure 1.1: Summary of research approach Data were taken to show the flame spread characteristics of PMMA burning in inclination angles of 00 (Horizontal), 450 and 900 (Vertical). A flame was applied at the bottom of the PMMA inclined at an angle. The experiment would end when the flame reached the other end of the burning material. Flame spread was observed in upward facing orientation which was also gravity assistance direction. Time was estimated for the flame to cover the length of the burning material which measured 25 cm and 20 cm. To compare the effect of the outside conditions and the inside conditions, the experiments were done indoors and outdoors. In literature review, the information is taken from different type of sources such as libraries, internet and journals. Although there was some data which were collected earlier, there was lack of enough evidence. Therefore, data from experiments and advice from professionals increased the value of research in this work. The information gathered has been analysed and compared with information which was gathered before for literature review. As a theoretical analysis of one dimension was performed to predict the flame spread as a function of material properties, sample orientation, and flame spread direction. Results and discussion 15cm 0 degrees 15 cm 450 15 cm 900 15 cm Time (sec) Length (cm) 0 degree 45 degree 90 degree 0 0 0 0 3 6 9 12 15 20 cm Time (sec) Length (cm) 0 degree 45 degree 90 degree 0 0 0 0 3 6 9 12 Processes of flame spread Ignition This is the initial process of fire incidences. From the studies which has been done in the past by various scholars on this topic, which provide different mechanisms of various types of ignitions. Quantities which characterize the ignition includes the temperature, ignition delay time and minimum ignition energy have been analysed. The results of the experimental and theoretical studies on the subject are obtained from an idealized condition, such that the data are based ideal conditions, and therefore the process of predicting the ignition on a practical environment pose problems (Babrauskas and Vytenis, 1992). Some effects of the boundary conditions cannot easily be predicted. Flame Spread Flame spread is the second stage of fire development. Various studies have been done on flame spread under different conditions and various models have been proposed. The mechanisms for different flame types have been explored and large amount of data have been recorded and analysed. Compared to other fire development stages, this has been the most studied. The main issue is the accurate prediction of complicated phenomena observed in real fire situations. This research attempts to solve this problem by re-examining the data obtained through experiments, observations, analysis and numerical simulations models. The result of this study is expected to provide appropriate application to real life case. Diffusion flame spread The flame spreading over a polymethyl methacrylate (PMMA) is mainly through diffusion. Therefore, the basic understanding the flame spread mechanisms and the characteristics of the leading flame edge are essential. Although the characteristics of the flame the leading flame edge is blue followed by a narrow dark zone, it not a prefixed combustion. A blue leading flame is not one of the characteristics of a prefixed flame, but under certain circumstances, a diffusion flame is blue and the color does not necessarily indicate the prefix flame. i) ii) From the observations, the flame spread types were separated into three regions mainly characterised by air stream velocities in or outside the room. In (ii) the velocity of air-stream above the flame is higher compared to (i). The flame spread increases with the increase in air-stream velocity. In Region II, representing the range of air-stream velocities Thermal Model Thermal model is used to describe flame spread system. As seen in the figure below, a non-premixed, primarily laminar flame is fixed at the base, where x = 0, and the burning material is supported between an ideal thermal boundary layer (TBL) and the polymer, which facilitate fairly constant heat flux from to . The pyrolysis region, has a constant surface temperature that is equivalent to the pyrolysis temperature of PMMA, which is approximately 630K. The temperature is maintained as heat transfer from the flame to the PMMA surface is balanced by conduction, radiation and the process of pyrolysis endothermic. The figure shows a thermal model for surface flame spread, adapted from Fundamentals of Fire Phenomena by Quintiere (2006). As seen from the figure above, the vaporized fuel moves up through natural convection, and react with oxygen to burn above the pyrolysis region until it is consumed entirely. As the fire burns from, the unburnt solid surface is heated until it reaches its pyrolysis temperature and it begins to vaporize. Above the flame height, there exists a thermal column which preheats the unburned region, but at a lower rate as compared to the burning region. In this model, the energy required for pyrolysis of the unburnt region basically comes from gas phase heat transfer from the flame to the surface. Kashiwagi and Ito mention that the conduction through the PMMA solid is negligible, which account for 6% of total heat transfer. Thus, only the measurements obtained here can be used determine the distribution of the primary source of heat on the unburnt region. Although this can be sufficiently be used to predict the spread of the flame, other methods such as the solid phase combustion solver can be used to predict and account for conduction through the sample, in the direction the spread of the flame. Due to the fact that the measured flame to surface heat flux is the most laminar and fairly transparent, the heat transfer is mainly convective in nature and has a small radiative element. As a result, when determining the heat transfer to the material sample, it is considered to be arising from the thermal boundary layer hot gases which flow naturally across the surface but not just from the flame itself. Thus, even as the flame is the energy source for this gas streaming, at the higher heights where the gas flow transform from laminar to turbulent, this heat flux will remain although the flame flickers. If the radiative heat transfer is measured and the remaining component of net heat flux is treated as pure convection, more details of the gas flow field can be estimated, for example the temperature and an effective convective heat transfer coefficient of the evolving boundary layer. This temperature profile can be seen in the figure above as a solid black curve and in ideal conditions it goes beyond the flame as a constant value shown by the dotted line. As such, heat transfer from the flame to surface, , can be described as: Where is the coefficient of the convective heat transfer; is the effective flame temperature of the boundary layer flow and is the temperature of the surface. Transition to Turbulence As the angle of orientation increases, the rate of heat release per unit length, at which the flame transform from laminar to turbulent form within a shorter time. This transition is significant since it coincide with the change in the heat transfer mechanism above the surface of the combustible material. In the laminar region, which is closer to the surface of the material, the flame is thinnest, and the heat transfer from the flame to the surface occurs primarily by convection; however, as the height of the flame increases and hence the flame thickness and turbulence, radiation becomes dominant. The figure below shows the flame spread on the material surface. The flame shows the simple buoyancy induced flow along an inclined surface. From the observation it can be seen that the flame height is related to the pyrolysis height, which in turn is related to the angle of inclination of the combustible material. Considering a given angle of inclination, say 450, the transition to turbulence might be predicted better through a geometric parameter rather than by a measure of heat release rate. This analogy help us visualize the flow field of the flame induced by a vertically spreading flame in a simple way that is well known: a hot surface in a cold environment will induced buoyancy induced flow. Although, the presence of a flame in a reacting environment makes the nature of the flame spread more complex, it is useful and valid to make approximation in an effort to predict the behavior of the flame spread. The relationship of the laminar to the turbulence transition has highlighted here because the flame spread rate per unit length can be used with pyrolysis length to estimate the flame height. The relationship that has been suggested is: Where is the length of the flame. is the pyrolysis length While a and n are constants. (Consalvi et al., 2008) And Where is Froude number and the constants, as suggested by Consalvi et al., would take one of the depending whether the critical value of heat release rate of a sample, Q = 20 kW/m, is exceeded. References Numerical analysis of the heating process in upward flame spread over thick PMMA slabs. Consalvi, J.L., Pizzo, Y. and Porterie, B. 2008, Fire Safety Journal, pp. 351-362. Spread of a Laminar Diffusion Flame. DeRis, J. N. s.l. : National Bureau of Standards, 1969, Proceedings of the Combustion Institute, pp. 241-252. Quintiere, James G. Fundamentals of Fire Phenomena. s.l. : Wiley & Sons Ltd., 2006. Heat Release Rate: The Single Most Important Variable in Fire Hazard. Babrauskas, Vytenis.1992, Fire Safety Journal, Volume 18, pp. 255-272. Characterization of Flame Spread over PMMA Using Holographic Interferometry Sample Orientation Effects. Kashiwagi, Takashi and Ito, Akihiko. 1988, Combustion and Flame, pp. 189-204. Width effects on the early stage of upward flame spread over PMMA slabs: Experimental observations. Pizzo, Y., et al. 2009, Fires Safety Journal. Flame Spread Over Combustible Surfaces for Laminar Flow Systems, Part I: Excess Fuel and Heat Flux. Annamalai, A. and Sibulkin, M. 1979, Combustion Science and Technology, pp. 167-183. The Application of Flame Spread Theory to Predict Material Performance. Quintiere, James G. 1988, Journal of the Research of the National Bureau of Standards, pp. 61-70. Controlling Mechanisms of Flame Spread. Fernandez-Pello, A.C and Hirano, T. 1983, Comustion Science and Technology, pp. 1-31. Read More