Experimental Investigations on Fires in Inclined Trenches - Research Paper Example

A Report on Experimental Investigations on Fires in Inclined Trenches – Trench Effect Name Institutional Affiliation Date Table of Contents Abstract 2 Introduction 3 Aims and Objectives 6 Hypothesis 6 Background 6 Material and Equipment 7 Results , Discussion and Analysis 9 Conclusion and Recommendations 10 References 11 Abstract This experiment was objectively carried out to investigate the various behaviors of fire burning in an inclined and enclosed trench. Trench effect, which is one of the common occurrences in trench fires, is the main factor on investigation in this experiment. The experiment involves setting up of similar factors subjected to a model of a trench to aid in giving more sufficient information on fire behaviors. The results presented by this experiment are derived from study of the fire, inclusive of height of fire measurements, the heat produced, the intensity of the fire and duration of burning. Effect of wind on fires burning in a trench is also taken to consideration where fans are used to blow wind into the trench model. Introduction This experiment examine a phenomenon which results from combination of different mechanisms which force a fire to move up on a relative inclination surface. Two of the main mechanisms involved are the Coanda effect and flashover effect. The Coanda effect refers to the tendency of fluids to follow a certain angle of elevation or inclination created by barriers. If a fire happens to burn in an open space, it entrains air required for its combustion from all sides of the flame. Creation of barriers on any of the fire sides prevents the fire from getting air from those sides. For example, if the fire is set near a building and its flames are next to a wall, there is an obvious air entrainment limitation from the side which brings an imbalanced flow of air to the flame. This tends to have a deflection effect on the fire, deflecting it towards the wall (Atkinson, Drysdale & Wu 1999). Following the entrainment of air to the flame restriction, products of incomplete combustion, such as, pyrolysing products move up the flame plume before completely burning up and releasing heat contained in them. This scenario leads to creation of a relatively elongated flame which has higher temperatures at the top and is known as the Coanda effect. Thus, Coanda effect is illustrated as shown in Figure 1. Figure 1: Diagrammatic Representation of the Coanda Effect Under the Coanda effect, the behaviour of deflecting towards close surface of an air stream moving at a high speed is evident. Pressure imbalance caused by the speed of the moving air track is the main reason bringing about the deflections of the flame. The air track creates a zone of low pressure while the areas farther from the wall maintain a constant pressure which bends the high speed air track attaching it to the wall. The air track continues to move at a high speed but remains completely attached to the wall. The other mechanism is Flashover. It refers to a rapid and sudden spread of fire which is brought about by overheating of most of the surfaces surrounding a fire (Woodburn & Drysdale 1998). These surfaces, when overheated, produce hot gases which, sometimes, auto ignite themselves leading to a fire outburst. Initially some of the gases present can be flammable but the heating effects are what bring about auto ignition. Holding the assumption that all other conditions affecting fire spread are constant, the spread rate of a fire increases by an increase in slope angle. Further, the rate of spread of a fire increases as slope increases (Wu & Drysdale 1996). The trench effect can only be noticed in a case where fire burns in close proximity to a surface of steep inclination. Fire flames spread in inclination to the surface as explained by the Coanda effect. As fire continues burning the material and surfaces around it are superheated and produce hot gases which are heated past their ignition point. This triggers auto ignition and the fire burns as per the principles of the flashover theory. The flames produced are subjected to the Coanda effect and they spread over the inclined surface to its end and continue burning until the source fuel of the fire is completely exhausted (Wu & Drysdale 1996). These experiments are meant to give informed knowledge on fire behavour and its mechanisms. A full understanding of the experiment on fire behaviour with respect to an inclined surface is achieved at the end of the experiments. Aims and Objectives (i) To find out the effect of a top barrier on an inclined trench fire which is tested by having a roof on the inclined trench. (ii) To determine an angle of inclination of the trench. (iii) To determine the effects of increase or decrease of the inclination angle on the fire combustion rate, burning intensity and flame length. (iv) To determine the total time taken by a flame to spread to a given distance in the model of inclined trench fire. (v) To find the relation between the flame angle at the bottom and the height of the flame. (vi) To determine how the angle of inclination of a trench fire affects the rate of spread of the fire and temperature in the inclined trench. Hypothesis This report describes experiments carried out to test entrenchment of fire. The results obtained will be recorded or tabulated, analysized and the results obtained will be used in realization of the experiment objectives. Background In recent past, there has been occurrence of fires in courier passage entrances which have been quite unpredictable in how they burn, thus making it hard for the fire fighters to counter the fire. For example, in the case of King’s Cross Fire in Central Landon a fire station caught fire, starting from the elevator and it led to occurrence of a flashover which filled the underground rail station with hot gases and smoke. There later occurred a fire breakout which spread rapidly in the station. This triggered investigations on the mechanisms involved in inclined trench fires which led to a number of experimentations on the same. It was later on realized that the occurrences of the Kings Cross Fire scene were due to the trench effect on the fire (Woodburn & Drysdale 1998). Material and Equipment The trench fire experiment requires a number of equipments for it to give efficient and effective findings on the areas of interest. Below is a description of the material and equipment required: The trench model is about 2.5 meters long, 25 centimeters height and 20 centimeters width. The model test trench should have a single side and the base made from a monolux board of 2.5 centimetres thickness. The remaining side of the model test trench is made up of glass which is 0.7centimeters thick. A strong steel or metallic frame is required to support the modelled test trench in the varied angles of inclination or elevation. The steel frame should be flexible enough to support the test trench in angles varying from 0-45 degrees. The trench roof is designed to have a frame that is flexible and adjustable to allow adjusting of the height between the floor and the roof of the test trench at the same time maintaining the same angle of inclination as the floor. The type of fuel used in the experiment should not have any type of contaminations on it. A calibrated plane cardboard would be most appropriate, with the calibrations done at pre-determined intervals to allow timing of the rate of fire growth. For the trench effect to take place, the fire flame on the test experiment should be fully fledged in that it should fill the trench with respect to its width. This is quite a hard task as, in most cases, the flame takes the shape which is not favorable for this experiment. The cardboard is the main cause of the shape of the fire as it tends to layer out as it burns thus causing the unsteady shaped flame. However, this can be countered by use of calibrations on the cardboard. Experiment Procedure Angle of inclination of the test trench is measured by use of an inclinometer and the distance between the floors to the roof is put into record. The cardboard is placed into the trench directly lying on the steel frame which supports it and is in line with the lower part of the trench. The cardboard should be placed in such a way that the markings are on the top side where they can be easily read and recorded in the process of the experiment. Determination and recording of the exact locations of the thermocouples The thermocouples should be placed strategically to be able to show any heat changes that take place in the process of the experiment no matter how small the change may be. A table should be constructed where the numerical findings from the experiments should be recorded. Further, the cardboard should be ignited ensuring that it is all burning in flames. When the front of the flame reaches 10cm mark on the cardboard, timing of fire spread rate begins. At the start of the test experiment, the inclination of the trench should be at 30 degrees and it should be gradually decreased at intervals of 10 degrees till the trenching effect is no more observed. At 30 degrees tests should be conducted first with a fan blowing air directly to the test trench and secondly with the fan blowing along the trench on ts angle of inclination. Results , Discussion and Analysis The trench angle is 150C. When the trench inclinations are steeper than the critical angle, the flame is deflected towards the trench till it gets attached to the floor. The geometry of the trench is characterized by its aspect ratio which is, the height to width ratio. The critical angle for any fire in an inclined trench relies on the geometry of both the trench and the burner. For example, thinner and longer burners with trench walls that are high will tend to produce plumes in the trench that will seem 2-dimensional at the middle of the trench. Ventilation determines the ability to get rid of heat, fire gases and remove smoke form the structure. Poor ventilations can increase the risk of fire spreading or even contribute to injury on the firefighters. The gases, heat and smoke out of an incipient fire can rise and after meeting the vertical restraint, they start to flow laterally or mushroom down on walls. In other words, the combustion products will start flowing along the paths of least resistance. In such circumstances, forced ventilations can allow or force the removal of gases, smoke and heat from the structure and can exchange it with fresh air. Forced ventilation, if well designed, will minimize the rate of fire spread but increase the temperatures. The forced air supplies more oxygen, thus supporting combustions and more heat release in the process. Conclusion and Recommendations In this experiment, just like other experiments, a number of errors are involved and which would affect the final results. The errors cut across human and instrument errors. Human errors can include the rounding off of the numbers, recording errors, poorly taking the readings especially reading of numbers bellow the meniscus. Further, experiment errors entail those errors attributed to the poor condition of the measuring instruments. When instruments have overstayed, some numbers on their calibrated scales would be faded thus providing misleading readings. In general, the combination of all errors leads to distortion of the results and offers explanations as to why the results obtained from the experiments differ from the theoretical values. References Top of Form Bottom of Form Top of Form Bottom of Form Top of Form Bottom of Form Top of Form Atkinson, G., Drysdale, D., & Wu, Y. (1999). Fire driven flow in an inclined trench. Fire Safety Journal. 25, 141,147-145,158. Woodburn, P., & Drysdale, D. (1998). Fires in inclined trenches: time-varying features of the attached plume. Fire Safety Journal. 31, 165-172. Woodburn, P., & Drysdale, D. (1998). Fires in inclined trenches: the dependence of the critical angle on the trench and burner geometry. Fire Safety Journal. 31, 143-164. Wu, Y., & Drysdale, D. D. (1996). Upward flame spread on inclined surfaces. Edinburgh, University of Edinburgh. http://www.aspresolver.com/aspresolver.asp?ENGV;2208331. Bottom of Form Read More