Enclosure Fire Dynamics - Coursework Example

Name : xxxxxxxxxxx Institution : xxxxxxxxxxx Course : xxxxxxxxxxx Title : Enclosure fire dynamics Tutor : xxxxxxxxxxx @2010 Enclosure fire dynamics 1 There is exhaustive development of density, velocity, temperature, as well as fuel mass fraction areas over the induction period and the whole period is divided into three phases. During the initial phase, which takes place on the acoustic time range of the vessel, conduction subjugated boundary layers produce an acoustic field within a nondissipative interior centre section. The second phase takes place on the conduction time range of the vessel. There is poitwise rivalry between reaction-generated heat releases, conduction, in addition to compression. The third phase, of exceptionally restricted interval, and it is dominated by the development of a tiny self-focusing hot spot entrenched in an almost invariant conduction-dominated section filling most of the vessel. The speedy gas expansion within the hot spot is the source more spectacular gas dynamical processes. 2 Semenov diagram is used in illustrating the energy balance within the system; this means that the ration that is found between heat gain which is coincidentally the heat release within reaction and heat loss which is the external cooling. On the other hand, equilibrium point is depends on the heat transfer coefficient, h, wall surface, S and volume, V. As a result, incase the heat rate is not above the critical value, there is no occurrence of equilibrium. The run away temperature increases and for this reason the substance explodes (Karlsson & Quintiere 2000). Pre-explosion heating refers to the maximum temperature increase that can be established without any explosion. Here, the equilibrium state is still present. Pre-explosion heating: T pre exp1- T0 = RT20/E Mathematically, if the numerical value of the parameter within brackets happens to be larger than right hand side number, the alternative is to calculate the explosion. If this is not the case, then the expectation is that there will be thermal equilibrium at temperature below pre-explosion heating. For cylindrical vessel S = πd ^ H, V= πd2/4H For spherical vessel S =πd2, V = πd3/6 With this, it is possible to calculate the critical value, dcr, of a vessel diameter. If d< dcr subsequently the loss rate sets equilibrium between heat release rate and thermal equilibrium devoid of any explosion. It is not advisable to store of combustible materials when d> dcr since there is a probability of thermal explosion occurring. There are practical factors that have the probability of improving the ability of unprompted ignition and accordingly fire consists of the body material size, ambient temperature as well as the thermal insulation which are the key factors (Karlsson & Quintiere 2000). 3 Explosion refers to the very high rate of heat production. However, explosion does not require combustion wave to pass through exploding medium. Deflagration refers to the combustion wave that which has the ability of spreading at a subsonic speed. Detonation entails an exothermic front increasing the speed through a medium that finally forces a shock front propagating directly in front of it. (Zel'dovics 2004). Development In detonation, there is normally a propagation of a shock wave resulting from combustion. It propagates through a reactive mixture or exothermic compound. Detonation takes place in a similar manner within solids, liquid and gaseous substances as well. In this regard, the rate at which the combustion spreads is in general supersonic in comparison to the media that burning has not taken place (Babrauskas 2003). Features Characteristic of detonation at T0 = 25 °C, P0 = 100 kPa include: Mixture Velocity (m/s) Pressure (MPa) Temperature (K) 80%H2 + 20%O2 3390-3408 1.80 3439 67%H2 + 33%O2 2825-2841 1.88 3679 25%H2 + 75%O2 1163-1737 1.42 2667 CH4 + O2 2728-2639 3.16 3332 CH4 +1.5 O2 2470-2535 3.16 3725 4. The key features of diffusion combustion include: There is separation of the oxidant and the fuel. The region where the gases normally blend is where the combustion occurs. There is diffusion of both the oxidant and the fuel into the region within space. In this region, swift chemical reactions allied to burning occur. Component diffusion is the main factor that determines the burning rate and not the speed of the chemical reaction. Types of diffusion flames Basically, the behavior of the flame is largely determined by the speed of the fuel. Momentum jet fires Momentum jet fires are characterized by elevated flow rate as well as turbulent fuel flow. Momentum jet fire can be in actual fact illustrated by an emergency flare which normally reduces the pressure in a compent of chemical plant. Buoyancy dominated fires In buoyancy dominated fires, the rate of fuel flow is low. A characteristic example is the burning of a condensed fuel, either a solid or liquid when the momentum of volatiles rising from the surfaces is not important. The relative importance of momentum as well as buoyancy within the flame is determined through using Froude number: Fr = v2/gD = Kinetic jet energy / Potential energy of gravity Where V= (Q/p ∆ Hc) 1/π D2/4 which is the fuel velocity at a surface of condensed fuel or at the gas burner outlet. Momentum jet flames Fr Read More