Enclosure Fire Dynamics - Assignment Example

Enclosure Fire Dynamics Author’s name School name T1. Analyze thermal explosion in a vessel with cold walls, including the mechanism of self-accelerating reaction and nature of induction period? Thermal explosion refers to a spontaneous ignition in a homogeneous mixture caused by an increase in temperature that induces the acceleration of the reaction (Terao, 1987). The thermal explosion in a vessel with cold walls first involves the following, reduction in the temperature of the gaseous mixture in the vessel due to thermal conduction to the cold walls of the vessel. This happens so that we obtain a temperature that is the same for both the vessel walls and the gaseous mixture. Terao (1987 pp. 4-7) says that; for thermal explosion to take place, the temperature of the walls should be kept at a constant, T0. We assume that the state of the mixture is homogeneous. The reaction heat is used to heat the mixture while part of it is transferred out of the vessel through the walls. The transferred heat (Q2) is proportional to the temperature difference between the mixture, T, and that of the wall, T0. (i.e. dQ2/dt=C (T-T0) where C is the heat conductivity of the gaseous mixture which is dependent on the vessel size. If the amount of heat that is transferred to the outside of the vessel is less than the amount of reaction heat, an explosion occurs. On the other hand, if the transferred heat is more than the reaction heat, then no ignition will occur. In thermal explosion, the flame is usually unstable against small perturbations due to gas expansion for reactions that are exothermic. The instability is further amplified by gravity or RT instability hence propagating the flame. Secondary instability leads to formation of fractal flame structure. After this formation, the flame propagation regime approaches a regime of self-similar acceleration regime. During the induction period, the mixture is adiabatically compressed keeping the pressure and temperature almost constant while all the other phenomena proceeds steadily (Terao, 1987 p. 14). T2. Using Semenov diagrams, explain the effect of heat transfer on thermal heat balance in a vessel with cold walls, critical conditions of thermal explosion, and pre-explosion heating. Semenov, 1927 cited in Liberman 2008, p.81 uses q+ to represent the amount of heat that is released during a combustible reaction of gaseous mixture and q- to represent the total heat flux from a vessel with a given volume and surface area. It is assumed that the temperature of the vessel, T0, is equal to the temperature of the gaseous mixture, T, at the beginning in order for the reaction progress. The gaseous mixture conducts some of its heat energy to the vessel with cold walls in order for T equaling with T0. This can be represented using Semenov diagram shown below. G 1 q+, q- 2 F H 3 T0 TF TH TG T Fig. 1. A Semenov graph for heat release and heat loss as a function of temperature. If the heat transfer is large enough, then the heat loss line crosses the heat influx curve as shown by line 1. If the heat transfer is less, then there heat loss line graph does not cross the heat influx curve (Liberman 2008, p.82). Thermal heat balance in a vessel with cold wall arises when there is a balance between the heat produced and heat lost (i.e. q+= q-). We assume the burn up of the initial reactants. Critical thermal conditions are the conditions at which a critical temperature TH is reached and the heat loss graph is tangential to the heat influx curve. It is also referred to as a thermal explosion limit. It determines the point at which the heat loss is in equilibrium with heat production and any slight increase or decrease in temperature will not result in an explosion since there will be no heat removed from the gaseous mixture to the vessel walls. Liberman (2008, p.84) defines pre-explosion as the heating that occurs when the gaseous mixture can not be able to ignite itself since the heat released is less than that lost as shown in line 3. This pre-heating is necessary since it raises the amount of heat produced such that the line can be able to cut the curve, reaching at a point like G, then it will be able to ignite. At this point there is instability triggering ignition which only occurs if T>TG. T3. Define explosion, deflagration and detonation. Explain the development and main features of detonation. According to Liberman (2008), an explosion is a violent exothermal reaction process where the pressure and temperature are increased rapidly. It can either develop spontaneously or be triggered by a source of ignition. He defines deflagration as a burning regime which occurs as a result of heat diffusion whereby the burning gas transfers heat to a fresh fuel directly. It is also referred to as a slow combustion flame. In the case of detonation, the combustion propagation regime involves shock waves. These shock waves travel through the combustible mixture compressing and heating the gas that lies behind the shock front. The shock waves must be sufficiently strong in order to raise the temperature to a considerably sufficient point which will ignite combustion at the back of the shock front. Thus, this phenomenon involves an explosive decomposition that is propagated by a shockwave that traverses high explosive material at high speed. The heating and compression of the shock initiates a chemical reaction which gives energy that sustains the shock waves. The detonation initiation is regarded as a sudden rise in pressure when an explosive suspension in the gaseous products forms and explodes. The detonation features include: detonation pressure, detonation speed and temperature distributions. T4. Review the main features of diffusion combustion. Explain the types of diffusion flames (momentum and buoyancy dominated fires). Compare the Froude number for these types of fire. According to Poinsot and Veynante, 2005, the diffusion flame combustion occurs when the oxidizer and the fuel are not mixed prior to entering at the chamber of combustion. For combustion to proceed, the mixing of the fuel and oxidizer must be brought into the reaction zone. The reaction zone where they meet has the maximum temperature and the flame diffuses towards the oxidizer and fuel. Turbulent flows characterize the combustion process and the mixing process manages the movement of the turbulent eddies randomly across the combustion layer. This influences the flame front thickness. Another characteristic feature of diffusion combustion, according to Friedman (2008), is that they do not propagate. Usually, the diffusion combustion flames are located at the point where the oxidizer and fuel meet making it a useful property for safety functions. However, this can be regarded as a consequence when it interacts with turbulence. Without propagation, the flame can not enforce its dynamics in the flow field. Besides, they are more sensitive to turbulence. The diffusion combustion flames are sensitive to stretch as compared to the pre-mixed flames. They have a smaller magnitude critical stretch value than the pre-mixed flames. The turbulent fluctuations can therefore quench it thus no justification for flamelet assumptions as for the case of the pre-mixed combustion. Momentum dominated fires, according to Toden 1988, have flames that occur when “the burning rate of a flame exceeds a certain level and the flow field ceases to be lamina.” Increase in the mass results into change of flow field, from buoyancy dominated to momentum dominated. On the other hand, the buoyant dominated fires are those flames that separate the fuel from the oxidizer with different densities and widen perpendicularly from a solid horizontal boundary. Buoyant flames are enhanced by the differential effects of oxidizer, fuel and combustion product streams. The Froude number is the inertial to buoyant forces ratio. The momentum dominated fires have Froude number greater than 1 (i.e. Fr>>1) whereas the buoyant dominated fires have a Froude number less than 1 (i.e. Fr Read More