FIRE ASSESSMENT

Fire Assessment

Fire Assessment

Question 1:

An exothermic chemical reaction proceeding in a gas contained in, say, a spherical vessel can end in an explosion; this is because the release of enthalpy results in the temperature of the reacting mixture increasing, with the consequence that the rates of both reaction and the production of heat continue to grow. However, an explosion is inhibited by heat being conducted to the walls of the containing vessel. Sometimes this loss of heat is augmented considerably by natural convection occurring in the reacting gas. The original works of Semenov and Frank-Kamenetskii considered, respectively, the two cases, in which: (a) convection is so vigorous that the temperature in the vessel is uniform; and (b) the transport of heat occurs by conduction only.

For well-mixed systems, the reacting gas explodes when the Semenov number exceeds the critical value of 1/e. When heat is transferred by thermal conduction alone, explosion occurs when the parameter (sometimes called the Frank-Kamenetskii number) is greater than a critical value dcr. This critical value depends on the geometry of the system and has been calculated numerically for reactions without consumption of reactant as 3.32 for a sphere, 2.0 for a cylinder and 0.88 for parallel plates; we note that these results are independent of the order of the reaction provided there is no consumption of reactant. However, in most reacting systems heat loss occurs due to the combined effects of natural convection and heat conduction. Recently, Liu et al.

proposed that in such systems the occurrence or not of an explosion depends on the relative magnitudes of three timescales: that for chemical reaction to heat up the fluid to ignition, the timescale for thermal conduction and that for natural convection. They summarised their results in a new two-dimensional regime diagram, in which Frank-Kamenetskii's purely conductive system and Semenov's well-mixed system appear as two limiting cases, represented by the two axes (Archer 1997).

The plane in between the two axes contains all the systems with different relative magnitudes of heat loss by conduction and by natural convection. This approach has the advantage of quantifying separately the stabilising effects of conduction and natural convection on an explosion (Quintiere 2006).

Systems neglecting the consumption of reactant have been widely studied. However, in addition to heat loss by conduction and convection, an explosion is inhibited by a drop in the rate of reaction, because one or more of the reactants inevitably disappears.

Question 2:

Thermal therapies can be hyperthermic (energy source) or cryothermic (energy sink) technologies. The hyperthermic studies included both studies where hyperthermia was the treatment as well as where hyperthermia a side effect. Studies which reported hyperthermia as a side effect of another treatment included modeling of heat generation and temperature distribution in drilled bone and temperature modeling in a total knee joint replacement using patient-specific kinematics . Hyperthermic treatment studies included an analysis of the thermal action of a short light pulse on biological tissues ; a vascular model for heat transfer in an isolated pig kidney during ...