Question

4a. What is the thermal explosion model of flashover in a compartment fire? Critically explain using a Semenov diagram and a temperature-time history. (250 words) [10 marks] 4b. Give and explain three alternative definitions of "flashover fire in compartment". (125 words) [5 marks] 15 marks]

Answer 1

to flames 2 or an area in - * This radiation of energy stos 40) Flashover : Flashover by definition in the sudden involvement et a room or an area in floor from flame celling caused by thismal radiation feed back. & Thermal radiation feedback in the energy of the fire being radiated back to the contents of the roong from the walls, floor, and ceiling, radiation of mergy to the content of the room 311 raise all the contents to their ignition temperature. * when the content of the room suddenly and simultaneously ignite this af blabolar * This simply means that Hashover ir a temperature driven event. * It reruined that the fire's energy be radiated back to the content to produce a a rapid rise in temperature and somu tomeoun nghitim. * Flashover indicates that the fire has grown to the Runy developed stage figure 1 Temp Flashover Bacle dratt Crouth ral Fully developed dtage Declining du duge Time De Another important cmcept do undmatand flashover. to understand in the physics of the de A Thermal explosion occurs when a chemical syatem uno an exothermic reaction during which inculticront heat m ohe dydtem so that the reach proces become per miary

forms as intermidion, no to the temperation & Semenor explained the long induction period by by unstable, but long lived Species that forms oo and their undago ditteront reading according to the * This concept can be represented in the form of the to Competing fuel (A) reaction routes after the formato umetable intermediate mot formation of in branching step I Chon- * T (non-chain branching Fig (2) A - M . Cchain branching step) & Route a ne controlled by an acorvation enorgy proceso larger them that at n & Figure 3 shows the variation at the reaction rate of each step at a tunction of temperature * The nombru in figa correspond to the temperature position denignatim Fig] Arrhenius plot of the semmor Stept in hydro Carbon midation sanching system Jince the * At point I in figi one ha chain branching system tempered the SA low and alorit in large thua akalent and the system is nm explorive . do As the temperature i inneased point the rate constants a - the chain steps in the dysteing increase and of our crope do > acrit and syutom exploder At spill higher temperature point 3 The non chain branching route I go becomes of astor A Although this steps to faster, a is always less than xerir ithus the system carly explode. * Ratsing temperature along route I still further leads to near do tout that it become felt heating and hence explosive again point ya

6 A flashover in the near simultaneous Ignition at most of the directly exposed comburfble material in an enclosed area when contain organic materials are heated, they undergo thermal de composition and velase flammable gaver. Tyment * A lean Harhover (some time is called rollolar qr the Ignition at the gar layer onder the ceiling, leading to total involvement of the compartment. The tual fair ratio que at the bottom region of the flammability range. to A Ach flashover occurs when the flammable gavef alte Egnited while do the upper region at Hammable rouge Cie rich). This can happen in rooms where the fire subsided because of lace of mygen, The Ignition Source can be a smouldering object or the stirring up of embers by the air trade such an event do backdratt. * A delayed Hauhonor occurs when the colder gray smoke cloud Ignites after congregating outside ett anta room of origin. This results in a volaple situakon, and if the ignition occinar at the ideal mixture, the result can be a violent some gas explodion A bot rich Hauhonor occurs when the hot sonke with Hamma ble gao rako above the upper limit of Hammablity range stemperature higher than the ognition temperces the leaves the compartment