Influence of the place of the expiration of gases from the placement at the fire for critical time of formation of dangerous factors of the fire

In normative documents for definition of time of approach of critical values of dangerous factors of the fire (DFF) use medium-volume characteristics of temperature, concentration of O2 oxygen, toxiferous gases and smoke. The purpose of this work – to show that for calculating time of approach of dangerous factors of the fire it is necessary to consider the place of the expiration of gases and their initial parameters. The expressions received in work can serve for determination of values of the dangerous factors of the fire parameters indoors depending on time, taking into account the place of the expiration of gases and their initial parameters. From the dependences given in work it is visible that differences in time taking into account the place of the expiration for the room of 5000 m are made by not less than 30 c with on reaching critical temperature and critical concentration of O2. Especially it should be noted that at change of height of a working zone the time difference increases to 40 c with in both parameters.


Introduction
Death of people at the fires results from action on an organism of dangerous factors of the fire. Treat these factors: temperature of gases indoors, the heat flux falling on people, the under concentration of oxygen, the increased concentration of toxiferous gases, the poor visibility. The last factor works indirectly, increasing a response time of people in a dangerous zone. In recent years in world practice much attention was paid to improvement of criteria of impact on people of dangerous factors of the fire. One of more modern normative documents establishing the specified criteria is the international standard [1]. According to this standard, the critical duration of the fire of t kr T on a heat flux and high temperature is determined by a first passage time on the ways of evacuation by an efficient thermal dose of Q ef . the efficient thermal dose is determined by a formula [1] Where t Radmissible time of influence of a caloradiance during a time term Δt, mines, t kadmissible time of influence of high temperature during a time term Δt, mines, q -intensity of a heat flux of kW/m 2 , Δta time term of influence, mines, t 1 and t 2 -border of a time frame during which evacuation of people is possible, mines, Q ef Tit is necessary to equate to 1. According to [2] t kr f on toxiferous t kr T.g gases. is determined by the least of values of a first passage time on the ways of evacuation of an efficient dose of X ef D or efficiency concentration of X ef K of size equal 1 taking into account a combined effect of all gases. For determining t kr Vvisibility loss time, according to [3] it is necessary to consider influence of an optical density of smoke, feature of an arrangement of an inventory of emergency exits. In [4] critical distances of loss of visibility depending on room space and probability of operation of the warning system and management of evacuation and its type are specified. There are not carried to number of initial factors which need to be considered such as: the place of the expiration of gases from the room on an incipient state of the fire, dependence of initial partial density of oxygen on the reference temperature of the environment. For determining fire risks when the probability of various script of development of the fire [5][6][7][8], accounted for of the expiration of gases not with the instantaneous medium-volume parameters and with the particular reference temperature of 20 0 C indoors is considered, should not be excluded, at least a priori. For example, in a situation when indoors pressuretight doors, there are air vents or lamps, it is necessary to consider that gases with parameters of an upper of an affluent zone expire. On the contrary, in case of lack of exhaust lamps and open or leaky doors, on an incipient state of the fire from the room air will expire. The offered work considers these unaccounted situations within integral model [9]. Initial values of dangerous factors of the fire are accepted to [10] corresponding clean air.

Materials and Methods
Parameters of the expiring gases at the fire for integral model of an incipient state of development of the fire indoors are regulated in [9] and are determined by a formula: and F у , provides the regulated DFF critical value at its limiting admissible value at height of a working zone. However this value distribution of DFF on height does not correspond to the maintenance of DFF in volume of the room as the integral on height accounted for offered distribution to 33% exceeds the common maintenance of the considered DFF indoors, and for the partial density of oxygen its negative value at height 3h/4 is possible.
To eliminate the existing defects of value distribution of DFF on height, it is necessary to carry out its correction [9]. Correction is that the DFF parameters depending on height are defined by expressions: These expressions give for y < 0,65h the F kr values coinciding with calculated on (1) A visual idea of distribution of the DFF parameters on height of the room is given to tab. 1 and fig. 1. The expiration of cold air from the lower levels From the equation of balance of weight we have: where  mthe medium-volume density of gases indoors, kg/m 3 ; V -volume of the room, m 3 ; mass rate of a burnup, kg/s; G ga consumption of the pushed-out gases, kg/s.
Where   For a case of the circular fire: where U lthe peripheral speed of flame spread on the square on which fire loading, m/s is placed.
Similarly we receive expressions for critical first passage times of critical values of partial density of oxygen, partial density of toxiferous gases and an optical density of smoke: -предельно допустимая парциальная плотность кислорода; = 0,226 L O2oxygen consumption at combustion of combustible material, kg/kg; t.g p.d marginal partial density of toxiferous gas, kg/m 3 ; L t.gselection of toxiferous gas at combustion of combustible material, kg/kg; where Ti sttemperature of the expiring gases, K. Follows from balance of weight: Let's define time of approach of bottlenecks of dangerous factors of the fire at height no more 0,65h because with such heights the balance of masses will remain. Having integrated (11), we will receive: From balance of mass of oxygen we have:  1istdensity of the expiring gases, kg/m 3 .

Conclusion
The expiration of gases from more low levels of rooms, that is with the under maintenance of dangerous factors of the fire, also leads to decrease of critical heating-up periods of dangerous factors of the fire in comparison with the accepted offer on the expiration of gases with medium-volume indexes of dangerous factors of the fire, which in turn affects the calculation of the time of evacuation of people from the room or building.