Kazuo Akita

Critical phenomenon in compartment fires with liquid fuels

The objective of this paper was to analyze in detail the behaviour of liquid-fuel compartment fires and to present precise information on the whole aspect of the fire phenomenon for use in modelling compartment fires. Two hundred experiments were conducted to research the behaviour of liquid-fuel compartment fires as a function of the ventilation parameter A H and compartment size, where A is the area of the ventilation opening and H is its height. Especially the influence of the compartment on the burning behaviour was investigated, using various size compartments, and the total aspect of this influence was presented. A critical phenomenon was observed for a pertinent condition of the ratio of compartment size to fuel tray size. This phenomenon, which is closely associated with the enhancement of burning rate and oscillatory combustion, may be substantially controlled by the dynamic equilibrium between the rate of fuel gas supply and the rate of air supplied by ventilation. A maximum burning rate may be obtained at an extreme value on this equilibrium. For methanol pool fires, the maximum burning rate was up to 7.2 times the open burning rate. The temperature within the compartment near the maximum burning condition was unexpectedly uniform and isothermal, due to fluctuation of turbulent flames and recirculation of gas. These experimental results were compared with previous research in detailed discussion.

Burning characteristics of methanolwaterair mixtures in a constant volume combustion vessel

Abstract Explosion pressures, unburned methanol, and formaldehyde emissions from combustion of methanolwaterair gaseous mixtures in a constant volume vessel are studied as functions of equivalence ratio and water content. These quantities take their extreme values at the equivalence ratio = 1.0–1.1, irrespective of presence or absence of the primary water vapor. The emissions increase with increasing water content. These emission characteristics are discussed on the basis of wall quenching. The larger amount of formaldehyde emission with increasing water content may be due primarily to the increase in quench volume. In addition, the laminar burning velocity is found to be predicted satisfactorily in terms of a semiempirical formula based on Semenovs thermal theory.

An analysis of bottom stagnation region combustion of polymeric material pieces under natural convection

Abstract Bottom surface combustion of polymeric material pieces in free convective flows is studied theoretically. The radiative heat exchange at the gas-condensed material interface and the difference of the diffusion coefficient for the pyrolysis gas from that for other species are considered in the analysis. Numerical calculations are performed for cases of cylindrical and spherical PMMA (polymethylmethacrylate) pieces. For each case, the gas temperature, gas velocity, and species profiles across a flame established at the bottom stagnation region and the surface regression rate are examined. Several novel aspects are elucidated. As the diameter d of the cylindrical or spherical PMMA piece increases, the flame temperature T f decreases and the distance y f from the gas-condensed material interface to the flame front increases. At a small but finite value of d − 1 4 , T f decreases rapidly as d − 1 4 decreases. When the Schmidt number Sc F for the pyrolysis gas is small, i.e., when the diffusion coefficient D F for the pyrolysis gas is large, a maximum of y f appears at a near-extinction condition. As Sc F increases, both T f and y f decrease. Since the effects of T f and y f on the regression rate ν s counteract each other, the dependence of ν s on Sc F is very slight. The experimentally obtained values of y f and ν s for cylindrical PMMA pieces are compared with their numerically obtained values. The numerical results are shown to coincide with the experimental results when a large value of Sc F is assumed.

An approximate solution of the equation forself-ignition

Abstract A method is describe for obtaining approximate solution of the equation for self-ignition for the case when temperature increases with time. The method has been used in investigating the self-ignition of spherical severdust heated in a uniform temperature field fror several ambient temperatures and reactions radii, ligand agreement is reported between theory and observation fo the qualitative behaviour of temperature rise with time but results fell short of expectations with respect to the magnitude of the ignition time, which may be due to consumption of material by chemical reaction for which no allowance was made.

Reaction kinetics of nitrogen dioxide with methanol in the gas phase.

The reaction between diluted nitrogen dioxide and methanol in Nz has been investigated in the gas phase at room temperature. The observations that 1 mol of methyl nitrite is produced from 2 mol of NOz and that almost no methyl nitrate is produced support that the reaction proceeds as CH30H + 2NOZ CH30N0 + HN03. The rate depends on the second power of the NOz concentration. It depends on the first power of the methanol concentration at low concentration in accord with previous researchers but gradually becomes independent with the increase of the concentration. From the above kinetics as well as the rate constant, the reaction is considered to proceed via the formation of a certain kind of asymmetric NOz dimer which subsequently reacts with methanol bimolecularly.

Experimental study on radiative ignition of polymethylmethacrylate

An experimental study has been conducted to explore the behavior of the radiative ignition of polymethylmethacrylate (PMMA) by examining in detail the ignition delay and the temperature and fuel concentration distributions in the gas phase during ignition. The ignition delay of the transition ignition is found to change discontinuously at a certain radiant flux, which is about 60 J/(cm2 sec) in the present case. For a radiant flux above this critical value, ignition is observed to occur at the plume axis away from the PMMA surface, and for a radiant flux below the critical value, it is observed to occur near the PMMA surface close to the plume boundary. It is shown that ignition occurs at a point where both the temperature and fuel concentration are in the proper range. Based on the present experimental results, the criterion of ignition is discussed.

Experimental study of bottom surface combustion of polymethylmethacrylate

An experimental study was conducted to explore the characteristics of bottom surface combustion of polymethylmethacrylate (PMMA). In the experiments, in order to confine the regression caused combustion to the bottom surface, the side surfaces were protected by asbestos boards. Several minutes after ignition, a steady flat flame with a distinct blue zone was established beneath the bottom surface. The regression rate v of the bottom surface was found to be almost constant during the combustion period. It was shown that v decreased as the piece dimensional L increased and that the relation between them could be expressed as v = a L −0.25 + b , where a and b are constants. The characteristics of bottom surface combustion were compared with those of top surface combustion. The regression rate of the former was found to be one-third that of the latter, From the temperature-time diagram for bottom surface combustion, a distinct layer, which could not be observed in top surface combustion, was observed between the solid and gas phases. The properties of this sublayer could be considered different from those of the solid or gas phase. Through the discussion of heat balances at phase boundaries, the ratio of the regression rate in bottom surface combustion to that in top surface combustion was shown to be almost equal to the ratio of the heat fluxes into the solid phase. The sublayer was attributed to a smaller heat flux.

New modeling of liquid or thermoplastic pool fires in compartment

A new model of compartment fires with liquid or thermoplastic fuels was proposed, in which a new concept of combustion efficiency based on the mixing process of fuel gas and fresh air was considered. This new concept was formulated by the parameter “μ” related to the residence time and mixing time. A simple one zone model was used in order to demonstrate the effect of the mixing process. Theoretical results were in good agreement with the experimental results of methanol and PMMA compartment fires, and especially the scale effect of compartment was predicted successfully. Further the similarity law for this scale effect was investigated, and the upper and lower limits of flashver were decided by use of a new number “F.” This F number was found to be the key parameter for the prediction of compartment fire behavior.

Emission characteristics of methanol combustion in constant volume spherical vessels

Abstract Emission characteristics of methanol combustion were studied by means of constant volume combustion in spherical vessels, the crevice volume of which was minimized as much as possible. CO, unburnt methanol (UBM), formaldehyde and NO x were important primary emission species from the environmental viewpoint. The yields of these emissions were measured as functions of equivalence ratio ∅. CO and NO x emissions corresponded to calculated equilibrium compositions at the adiabatic flame temperatures. On the other hand, UBM and formaldehyde were not predicted. UBM had a minimum value at ∅ ≈ 1.1. UBM and formaldehyde emissions were pressure-dependent, indicating that these emissions were related to quenching phenomena. By employing a larger vessel with a smaller surface/ volume ratio, these emissions could be reduced considerably. Methyl nitrite was produced as an important emission species via post-combustion reactions between UBM and NO x . Water addition was found not to affect the emission characteristics seriously.