Engineering Design Project: Smoke Hazards - Assignment Example

Smoke hazards The term smoke is used in the description of the airborne products of combustion generated by the fire with substantially large volume of air that will be entrained in them as a result of their motion. The smoke particles may be a combination of both liquid and solid particles but make up the gaseous mass. Almost all fires are accompanied by smoke production and where these fire happen to be enclosed in a building; these bring about a highly hazardous environment to the building occupants and could also be a great damage to property. Rarely in fire incidences do victim succumb to fire burns but rather the causes of death are usually the inhalation of smoke. The bulk of combustions products compost of carbon dioxide and water usually will have highly toxic gases mostly carbon monoxide and to some extent they may include hydrogen cyanide and some other rare gas species. Irritant gases such as acrolein may have be of great harm to the people trying to make their escape from the building. The products of combustion which are in solid or liquid form will bring about reduced visibility in the smoke which is an additional problem that is brought about by the smoke. Suffocation of the people that could be trapped in the building as a result of there being reduced level of oxygen brought about by the combustion process. The heat generated by the combustion products may be another hazard to those in the building as they are likely to be immersed in the smoke or the heat may reach them through thermal radiation with the hot smoke layers being the source. Smoke will result to damaged property as the fires are likely result into production of soot and also there will be generation of gases such as hydrogen chloride which are highly corrosive. This is likely to bring about huge financial losses because of the equipment being damaged, the need for the system to be cleaned up and losses as a result of the disruptions. Tenability and willingness to enter or ability to move through smoke In a building, if the situation is such that smoke is mixed down almost to the level of the floor, the this could mean that while some of the may be willing to pass through the dense smoke , but some of the occupants may not be willing to make their entry into escape routes that appear to be full of smoke and instead this occupants may turn back or they may not in a position to locate the exit. For scenario where heat has not risen considerably to a level of raising any form of concern, then the initial effects of smoke will depend of visibility distance and the irritating effects that come as a result of the occupants’ direct exposure. Research findings has shown that close 30% of people will make a decision of turning back instead of making their way through areas that are full of smoke(Bryan, J.L.,1995; Wood, P.G.,1972) and the visibility level that will result to people turning back is of three metres. The optical distance is 0.33 (D.m-1) and extinction coefficient of 0.76 with women being reported to turn back more than men. The behavior exhibition will also be depend on whether layering give the people a chance of crouching down so as to reach a level where smoke intensity is relatively low or whether there is low placed lighting that could enhance visibility. Putting all this into consideration with respect to parameters that includes size and how complex the building is, design limit with regards to optical density of smoke will be used. BS 7899-1 and BS 7899-2 is a good guide on how this is to be accomplished. For approximation sake, the assumption could be that occupants will not enter an escape route that has a visibility of less than 3 metre (D·m–1 = 0.33), extinction coefficient 0.76). But where this happens and the ability of them progressing will depend on optical density and irritancy of smoke. Zonal model One zone model may not be sufficient in addressing the range problems at hand due fact that they have basis on some assumptions which are made according experience and the observations made with regards to the way fire develops for any particular environment. Heat content of the plume is given by where  is the fraction of total heat release lost through convection ranging from 0.4 to 0.9; is heat release rate convected from the plume and  gives the overall heat release from the fire source Other zone model expressions are where  and the smoke mass flow rate at a distance of z from fuel surface is given by  (PD 7974-2:2002 p 21) Where  represents the source linear dimensions; z represents the plume height above fuel surface, gives the virtual source above the fuel surface while  represents the heat output of fire through convection Air mass flow rate of the air that is inside the fire is given by  (PD 7974-2:2002 p 21) where p represents the perimeter of the source CFD Modeling In CFD modeling there is use of latest techniques that are used in simulation of flow in fluids involving the full breadth of the engineering application. By use of CFD modeling it is possible to have direct solution to the three dimensional time-dependant equations that gives description of the laws of conservation factoring in the boundary conditions of a particular situation by use numerical methods. The CFD viability with its high level of sophistication depends critically on the speed and memory of the current computer. In comparison to the zonal models , the CFD models are complex and much more expensive to apply but their application is much more universal and does not depend on priori assumption as in the case of zonal models. In CFD the basic equation that is set in simulation of fires in enclosures is given by  (PD 7974-2:2002 p 36) Where  is a generic variable which may represent 3 Cartesian velocity components , enthalpy (h) or the fraction mass of a particular species ( ). Here, the mass continuity representation is given by   is a source term that should be appropriate to  that may incorporate for instance the effect of chemical production and radiative heat losses. While Cartesian grid may not be essential its assumption serves simplicity. With all dependant variables in the previous equation being time averaged quantities and neglecting fluctuations in density, thus we have There is incorporation of the turbulent and molecular diffusion in the diffusion term by the exchange of . Usually in majority of CFD models the assumption is that Reynolds stresses and scalar fluxes that involves correlations of the fluctuating properties, may be modeled through engagement of gradient transport hypothesis expressed as Where  gives the fluctuating component of velocity while  is the generic variable. In order for the local value to be determined for  there will be need to have solution for transport equations for  which is the turbulent kinetic energy and also a solution for  which is the rate of dissipation (Mitler , 1991 p711).There is also need to give special attention to effect of buoyancy with regard to extra turbulence production for the case of rising plume and inhibition for the case of stratified layers. Then the conservation models are discretized and the solution obtained through iteration on a numerical grid many control volumes in the range of hundreds of thousands to millions that will fill the computational domains a process that employ the ‘guess and correct operations’. By having solutions of the equations with the relevant boundary conditions being put in place, the major features of a smoke motion problem for a fire of known size can sufficiently be captured (Miles, 1997; p 237).. However, in CFD modeling incorporating the combustion model is of great importance when it comes to modeling extended release heat over a volume whose determination is through the local mixing conditions. Caution should be taken before one makes their mind of representing a fire by a heat source if one is in doubt of the resulting consequences. ASET calculations The equation to be used is Available Safe Escape Time (ASET)= In the equation Z = is the height of the door H= is ceiling height above the fire = growth time (=600sec for slow reaction) A = Area of the room = 0.937 References Miles, S. , Kumar, S. and Cox, G. (1997). The Balcony Spill Plume-some CFD Simulations, pp 237-247, Proc 5th International Symposium on Fire Safety Science, IAFSS. Mitler , H E. (1991).Mathematical Modelling of Enclosure Fires-Numerical Approaches to Combustion Modelling, p711 Progress in Astronautics and Aeronautics, 135,1991. PD 7974-6:2004. The application of fire safety engineering principles to fire safety design of buildings- Part 6: Human factors: Life safety strategies-Occupant evacuation, behavior and condition (Sub-system 6) PD 7974-6:2004. The application of fire safety engineering principles to fire safety design of buildings- Part 6:Spread of smoke and toxic gases within and beyond the enclosure of origin (Sub-system 2) Purser, D.A. (2002).Toxicity Assessment of Combustion Products. The SFPE Handbook of Fire Protection Engineering 3rd ed). DiNENNO, P.J (ed.), National Fire Protection Association, Quincy, MA 02269, pp. 2/83 – 2/171. Wood, P.G.(1972).( The behaviour of people in fires. Fire Research Station, UK, Fire Research Note 953, . Read More