Wind Effect in Combustion Process - Literature review Example

Introduction In this section there is presentation of a review of relevant literature on theories, past research papers and discourses on the manner in which compartment fire will behave under the influence of wind. With a thematic approach being taken, the literature review presentation is put into sections whose design is to give a framework on the level of knowledge at different areas of compartment fires and the behavior exhibited on subjection to wind. First the literature review starts by provision of an overview of compartment fires and the key components then proceeds to discuss induction of wind and how it impact on the fire dynamics. There is also presentation of a number of key components of combustion so as to give perspective relating to influence that wind has on behavior of compartment fires. The important part of this section involves analyzing of discourses regarding fire behavior on the basis of configuring of compartment and scenarios that involves external factors changes, where there is presentation the theories, methods and findings in order to allow the outputs of the study to be compared. The source of documentation in this literature are majorly peer-reviewed journal articles in which there is presentation of experimental findings, in addition to other references including advisory documents, academic texts, government reports and company publications which were a good source of insights with regards to how important this topic is and how the findings could be of importance in fire safety sector. Overview of Compartment fire and their development stages Like any fire compartment fire has all characteristics of growth of any fire. The description of the compartment fire is very similar to any fire that occurs in confined space, where there is ultimate air supply as well as the thermal surroundings of the fire [1]. All these factors take a centre stage when it comes to controlling the spreading and growth of compartment fires, duration and the maximum burning rate. HM Fire Service Inspectorate [2], observation is that compartment fires have same principles of operation just in the case of a burning candle. In the candle first the wick is lit causing the immediate wax to melt and the molten wax rises up the wick and this then turns to flammable vapour which in turn burns forming the flame. A similar thing happens in a compartment fire as can be seen from figure 2.1. When the compartment fire starts, the initial process of combustion will be small in magnitude, with low heat level being generated. But with production of flammable gases by the consumable materials, the intensity of fires increases accompanied by an increase in temperature. It happens that all the gases produced in the compartment may not be consumed and due to reduction in the oxygen levels there will be reduction in the intensity of fire [3]. The flammable gases will burst into flames when oxygen is introduced to the compartment as a result or ventilation or the windows being broken and this is referred to as flashover [2]. The temperature of the fuel is raised to high levels resulting to more generation of flammable gases that will eventually ignite [4]. With sufficient oxygen being supplied and combustible material being available, there will be growth of fire to a maximum limit, up the point where there will be reduction flammable gases thus causing the fire to die down. Heat and smoke are the major products that come out of the combustion process and the same are the most dangerous too [5] [4]. Figure 1: Stages of compartment fire development In some criteria which have been applied in the demarcation of flashover point a temperature of 500-6000C has been used, or the radiation on the floor of the compartment having a range of 15 to 20kW/m (Walton, W.D. & Thomas, P.H., 1995). The appearance of from the openings in the compartment is also used as a criterion of locating flashover. Many things that are linked to flashover come as a result of a number of mechanisms that are linked to orientation, properties and position of the fuel, and the prevailing conditions in the upper layer as well as the geometry of the compartment. Flashover is not a mechanism but rather it should be seen as a phenomenon which is associated with a thermal instability [4]. The fully developed stage is reached after the growth stage through flashover. This stage where there is maximum release of energy in the compartment and the amount of oxygen available puts a limit to the level of energy released. This is when conspiring ventilation-controlled burning where the oxygen that is required to support the combustion process is believed to enter the compartment through the openings that are not sufficient to supply oxygen that can support fuel controlled combustion. In the ventilation controlled fires the un-burnt gasses gather in the upper level of the compartment and with the gases trying to make their way through openings, the gases catches fire and as a result we have flames sticking out through openings. This stage is associated with very high temperatures in the range of 700 to 12000C [2]. The decay stage is the last stage which comes after the fully developed stage at which point most of the fuel has been consumed and there is limited energy release resulting to lowered temperature in the compartment. At this stage there the fire may go from ventilation controlled to fuel controlled. Wind induced internal flows Wind velocity plays an important role in the behaviour of fires in a compartment [6] [7]. Particularly in high-rise buildings, literature shows that one of the floor in a building being on fire, may result to the ambient wind at this level changing velocity as a result of building blockage [8]. In the case of strong ambient winds, the general principle is that the wind may drive large volumes of air into a compartment resulting in the expansion of the fire [8]. This makes the time it takes for flashover to happen in a windy environment to be shortened as compared to the normal environment with no wind [9]. The windy environment also results to a shorter fully developed fire stage. This is critical in firefighting, as with the case of horizontal wind flow across the compartment, the reliability of predicting the flashover will be minimal, decreasing the level of survival of occupants and even exposing rescue personnel to increased risks [10]. According to management theory there is indication that openings in the walls may allow heat to escape and this would reduce the level of heat that can be build up in a compartment [11]. Wind gaining entry into through ventilation will affect the manner in which the fire behaves with an increased likelihood of ejection of flames through windows where winds are present [12]. The rate of spread of fire in a compartment is also influenced by wind. Larger flame length will be produced when winds blow flames in the direction of a building [9] and this is likely to facilitate fire accessing additional fuel sources resulting to a faster rate of fire spread [13]. The fire plume behaviour as well as the un-burnt gases bahaviour is bound to be affected when wind flows across a compartment [14]. Wind characteristics Nearer to the earth’s surface there will be high variation of wind velocity with height and the region is referred to as terrestrial boundary layer. Figure 3 shows graphical presentation of the terrestrial boundary layer with the curves giving the various surface roughness. Category 1 curve represents plain area such as dessert or seas while category 4 represents areas that exhibit high roughness such as areas with many buildings. Wind is to play a major role of dispersion of fire and it has impact on fire containment, with the spread of the fire being dependant of the speed at which the wind in moving, the direction, turbulence level and persistence [15]. Fire service authorities have noted, without doubt, the difficulties and danger that firefighters are exposed to in managing wind-driven fire [16]. A number of studies have been commissioned to understand how winds can results to smoke hazard in a refuge floor, that is meant to be protected by cross-ventilation that occurs naturally by wind induction [9] [17]. However, little attention has been given to the influence of wind on the dispersion of smoke in or from the fire compartment [18]. Evidence from research have indicated that when wind is a critical factor in a compartment fire, this will have a severe effect on behavior of the fire. From experiment performed by Chen et al. [19] it revealed that ambient wind pressure could have effect on the way flame/plume ejected from the compartment via windows behaves. Figure 3 Fire attributes affected by wind Wind is likely to have a substantial effect movement and spread of gases and smoke in the a compartment. A positive pressure will be caused by wind when the wind originates from the wind ward direction with the leeward side will have a negative pressure. In case the origin of fire is the windward side, then this side will have a considerable contribution on the development of fire and on the spreading of smoke. But in the case where the fire origin is on the leeward side then the spread of fire will be restricted by the wind. When fire is in its full development stage, there will be flame ejections from the windows and any other openings. According to the findings of Himoto et al. [12] involving a reduced-scale experiment, his conclusion was that; window flame trajectory, temperature rise in the compartment and the speed of wind outside the compartment are the major forces behind fire spread resulting to the spread of fire into other compartment. Effect of Wind on Compartment fires Studies which have been undertaken on effect of wind on tunnel fires have shown that Heat Release Rate (HRR) is increased in presence of wind. A study undertaken by Ingason [20] as an attempt of understanding the impact of longitudinal ventilation of fire growth and HRR revealed that when wood cribs are used in a model scale test, the maximum HRR would be increased by 1.4 to 1.55 in comparison to measurements taken at ambient conditions while there was an increase in fire growth rate by up to a factor of 3 as a result of ventilation. The study also indicated that there was a maximum velocity beyond which further increase in velocity did not result in increase HRR. Fire growth and HRR show significant variation when there is change in wind velocity. When Lönnermark and Ingason [21], performed a study with the aim of establishing how cross-sectional area and wind velocity affected tunnel fire, a positive relationship was found to exist between HRR of crib and the porosity, with a range of 1.3 to 1.7 times above values that were taken on the outer side of the tunnel at ambient conditions at various wind velocities. Heat release rate (HRR) of fires HRR is defined as being the rate at which energy is released when there is combustion process, while burning rate is a measure of chemical reactions in the process of combustion. HRR is given by Both HRR and burning rate are all relevant in characterization of fire behaviours with a comparison HRR vs time being very important in the assessment of fire hazards [4] [33]. In certain scenario the relationship between HRR and time gives what is referred to as design fire curve [14]. The temperature level recorded in the compartment depends on heat gain and mass loss rate. From a study conducted by Lönnermark and Ingason [21] it was revealed that Mass Loss Rate (MLR) for fuel and the HRR had a relationship with dimensions of the compartment. Other facts about HRR is that it varies with duration of time and it is low at the inception stage of fire with a significant at the pint when the fire is fully developed. The rate of development of fire, peak HRR and duration of burning all depends on both the characteristics and ventilation level as can be seen in figure There can increased fire growth rate up by a factor of 3, due to effect of the ventilation, clearly showing the influence wind has on the HRR as shown in Figure 2.4. Burning Rate Having accurate prediction of the rate at which burning takes place is very important owing to the fact that it plays a major role when it comes to estimating specific fire effects inside the compartment. For instance when looking at the effect of post-flashover fires on structures, it is important to put into consideration the severity of the burning and the period of time the burning happened. The fuel loading can be utilized in the description of the expected duration of burning on terms that there is enough air. Some authors have used burning rates in describing mass loss rate of fuel which in meaning are different even though the trend exhibited by the two are close. Mass loss rates involve the condensed fuel decomposing into hot gases as a result of re-radiation from the surrounding boundaries that are at high temperatures. On assumption that no inverts are in the fuel, the mass loss rate and burning rate relationship can given by is given by Fuel mass loss rate = Burning rate + Rate of unburned fuel gases and soot [23]: If it is assumed that there are no inverts present in the fuel Alternatively this can be written as  The other parameter used in describing the relationship between burning rate and rate of loss of mass is the global equivalent ratio. Usually in this case no provision is given for the unburned gases and the relation is given by [11];[22].  Here r gives stoichiometric mass of the fuel to air ratio. In a situation where the global equivalent ratio is greater than zero (> 1) – it will mean that there is a burning regime that is referred to as under-ventilated or fuel rich compartment fires, which means that no enough oxygen is supplied that can result to complete combustion of fuel. But in the case where < 1 it would be an indication that there is excess supply of oxygen (over-ventilated) in which case there will be no unburned fuel. From the equation above, the induced mass inflow of air getting into the compartment fire depends on ventilation factor and can be given as  ; Hence, the compartment equivalency ratio is  In this evaluation, it can be seen hat equivalence ratio may be defined as the ratio of mass loss rate of fuel and the induced mass inflow of air in the room vent normalized by stoichiometric ratio. Therefore , it will follow that the mass loss rate of a fuel should accurately be known to ensure correct prediction rate of burning as observed in under-ventilated regimes where burning is dependant the air amount supplied and the rate at which fuel is consumed (stoichiometric mass fuel to air ratio). Research by Past research work by Parkes and Fleischmann [24] revealed that global equivalent ratio for a range of opening factors, for vent with  was approximately 1.7 and that for  was closer to a global equivalence ratio of 3. 2.8 Smoke Spread by Wind In compartment fire there can be generation of a lot of smoke [25] with the spread of the smoke being dependant on factors such as buoyancy and restriction from doors [14].wind effect on the compartment affects smoke spread [11] where the wind brings about positive pressures that is exerted on the windward side while negative pressure is created on the leeward side. All this can have a substantial effect on compartment fire smoke bahaviour. There is a lot of influence of wind on the behavior exhibited by compartment fire with smoke movement being affected substantially [6]. An experiment by Chen et al. [26], there was investigation on the effect of wind on movement of smoke and temperature level in ventilation controlled fire in a two vent compartment where the findings were for the case of opposing wind, the wind force and thermal heat in the compartment will be in a state of competition. In this study it was demonstrated that the direction which the smoke will take will be dependent on stronger component between the thermal heat and wind force [9]. 2.9 Smoke Temperature The presence of ambient wind substantially affect the steady temperature of the smoke [23]. When the ambient wind speed goes beyond a certain critical speed, it will result to steady smoke temperature being achieved[23]. Smoke temperature can be in any of the three states, any of the two stable states or it can be in the unstable state. When there is a change in the speed of wind, the steady smoke temperature jumps from one state to another when the critical speed point is reached [25]. This critical temperature wind speed is affected by heat loss experienced in compartment walls. In a situation where there is opposition forces affecting the mass movement, the wind force and thermal buoyancy will compete in the fire compartment in which case one will be dominant in the determination of smoke motion. A critical wind speed that coincides with a certain critical Froude number can then be determined[4]. Where the ambient wind speed is beyond the critical value, the smoke is driven by wind to move in the downward direction, otherwise smoke should be moving in the upward direction with thermal buoyancy being the driving force. Compartment Fires with one Opening (Ventilations) Works on one compartment fire spill plumes is attributed to Yokoi [58] in which a scale model of 0.4x0.4x0.2m was used in studying the distribution of temperature in the spill plume with the opening being of various sizes. Yokoi [27] proposed a dimensionless temperature distribution on the trajectory axes of the plume ejected from the window. There was development of a model that can be used in the prediction of the trajectory of the flame ejected from the window from a compartment using reducing scale experiments that is attributed to Himoto et al.[12]. Another experiment was performed by Yamaguchi and Tanaka [28] which involved 0.5m cube with the aim of establishing the window had on the behavior of the spill plume. Oleszkiewicz [29] study involved qualitative observation of external flame height of the flame tip where a number of full-scale experiments were involved. In a more recent study by Lee[30] a number of reduced scale experiments involving 0.5m cubic compartment were conducted with the aim of establishing the flame height and the heat flux in under ventilated fire involving one window in a compartment. From Lee’s study it was found that time-variation of gas temperatures in the compartment has a correlation with both the dimensions of the window and compartment [31]. It was found from the study that the mean of the spill flame height was dependant on the combustion of the unburned fuel outside the compartment meaning that where there was more unburned fuel moving outside the compartment the spill flame height was bigger. Summary of the Literature Review In this literature review the key features of compartment fires that are of great importance in this research have been identified assessed. In the literature a green light has been given on behavior of compartment fires in addition looking at the influence of wind on heat release rate and mass flow rate with an insight being provided into studies that have been undertaken in the past years. With the understanding wind effect on compartment fire may have two effects which are in opposition to each other means that the two options were all to be put into consideration. The works that have been done in previous research though quite different with the present study gives the needed plat form of carrying out this study. In literature it is seen that increasing wing speed is likely to bring about increased intensity of fire inside the compartment is there is increased supply of oxygen an important component in combustion and this leads to increased temperature levels in the compartment. There is also the possibility that the wind may blow heat away resulting to diluted combustible gases in the compartment which in turn will decrease temperature and this will hasten fire extinction. References [1]B. Merci and P. Vandevelde (2007), “Experimental study of natural roof ventilation in full-scale enclosure fire tests in a small compartment,” Fire Safety Journal , vol. 42, no. 8, pp. 523-535, 2007. [2]H. F. S. Inspectorater (1997), “Compartment Fires and Tactical Ventilation,” Home Office , UK. [3]D. Rasbash,(2004), Evaluation of fire safety, Chichester: England: John Wiley. [4]CFBT-US(2010), “Compartment Fire Development & Flashover,” CFBT-US, Chile. [5]R. J. Whelan(1995), The Ecology of Fire, Cambridge: Cambridge University Press. [6]G. Hadjisophocleous and D. Yung(1992), “A Model for Calculating the Probabilities of Smoke Hazard From Fires in Multi-Storey Buildings,” Journal of Fire Protection Engineering , vol. 4, no. 2, pp. 67-79, 1992. [7]K. Richardson (2000), “Fire Safety in High-Rise Apartment Buildings,” Ken Richardson Fire Technologies Inc, Canada. [8]H. Chen, N. Liu and W. Chow(2011), “Wind tunnel tests on compartment fires with crossflow ventilation,” Journal of Wind Engineering and Industrial Aerodynamics, vol. 99, no. 1, pp. 1025-1035. [9]D. Weise and G. Biging(1994), “Effects Of Wind Velocity And Slope On Fire Behavior,” Fire Safety Science - Proceedings of the Fourth International Symposium 4 , pp. 1041-1051. [10]O. Sugawa, D. Momita and W. Takahashi(1997), “Flow Behavior Of Ejected Fire Flamelplume From An Opening Effected By External Side Wind,” Fire Safety Science - Proceedings of the 5th International Symposium 5 , pp. 249-260. [11]J. G. Quintiere(1998), Principles of fire behavior, Albany N.Y: Delmar Publishers, 1998. [12]K. Himoto, T. Tsuchihashi, Y. Tanaka and T. Tanaka(2009), “Modeling thermal behaviors of window flame ejected from a fire compartment,” Fire Safety Journal , vol. 44, no. 2, pp. 230-240. [13]D. e. a. Yang(2010), “Comparison of FDS predictions by different combustion models with measured data for enclosure fires,” Fire Safety Journal, vol. 45, no. 5, pp. 298-313. [14]L.-G. Bengtsson(2001), Enclosure fires, Karlstad: Swedish Rescue Services Agency, 2001. [15]BRE(2010), “Fire Risk Assessment Protecting People, Property and the Planet,” BRE Global, Watford. [16]G. Rein, X. Zhang, P. Williams, B. Hume, A. Heise and A. Jowsey,(2007) “Multi-story Fire Analysis for High-Rise Buildings,” Edinburgh Research Archive, London , 2007. [17]H. Ingason and A. Lönnermark,(2010) “Effects of longitudinal ventilation on fire growth and maximum heat release rate,” Fourth International Symposium on Tunnel Safety and Security, pp. 395-406. [18]NIST.GOV,(2015). “Wind Driven Fires,” [Online]. Available: http://www.nist.gov/fire/wdf.cfm. [Accessed 12 July 2015]. [19]H. Chen, N. Liu, L. Zhang, Z. Deng and H. Huang(2009), “Experimental Study on Cross-ventilation Compartment Fire in the Wind Environment,” Fire Safety Science, vol. 9 , pp. 907-918. [20]H. Ingason,(2005). “Model scale tunnel fire tests-Longitudinal ventilation,” SP Technical Research Institute of Sweden, Borås . [21]A. Lönnermark and H. I. Ingason,(2007). “The Effect of Cross-sectional Area and Air Velocity on the Conditions in a Tunnel during a Fire,” SP Technical Research Institute of Sweden, BORÅS. [22]D. Drysdale,(2011). An introduction to fire dynamics, Chichester: Wiley. [23]Utiskul, Y., et al. (2005). Compartment fire phenomena under limited ventilation. Fire safety journal, 40(4), 367-390. [24]Parkes AR. Fleischmann CM (1997) Effects of ventilation on the compartment enhanced mass loss rate. Fire Safety Science-Proceedings of the Fifth International Symposium. p. 415}26. [25]D. T. L. Yung,(2008). Principles of fire risk assessment in buildings, Chichester, U.K: Wiley. [26] U. N. R. Commission(2007). “Verification & Validation of Selected Fire Models for Nuclear Power Plant Applications, Volume 7:Fire Dynamics Simulator,” NUREG-1824 and EPRI 1011999, Washington. [27]Yokoi S.(1960), Study on the Prevention of Fire Spread Caused by Hot Upward Current, Japan, Report 34, Report of the Building Research Institute. [28]Yamaguchi, J.I. and Tanaka T.(2005). Temperature profiles of window jet plume, Fire Sci.Technol. 24 (1) (2005) 17–38. [29]Oleszkiewicz, I. (1989) Heat Transfer from a Window Fire Plume to a Building Façade,HTD – Collected papers in Heat Transfer, vol. 123, Book No. H00526. [30]Lee Y.P.(2006). Heat Fluxes and Flame Heights in External Facade Fires, FireSERT,University of Ulster. [31]Delichatsios, M.A. Lee, Y.P. Tofilo, P. (2006). A new correlation for gas temperature inside a burning enclosure, Fire Safety J. 44 (2009) 1003–1009. Read More