Time scales ignition

It has already been shown that the Cone calorimeter smoke parameter correlates well with the obscuration in full-scale fires (Equation 1). At least four other correlations have also been found for Cone data (a) peak specific extinction area results parallel those of furniture calorimeter work [12] (b) specific extinction area of simple fuels burnt in the cone calorimeter correlates well with the value at a much larger scale, at similar fuel burning rates [15] (c)maximum rate of heat release values predicted from Cone data tie in well with corresponding full scale room furniture fire results [16] and (d) a function based on total heat release and time to ignition accurately predicts the relative rankings of wall lining materials in terms of times to flashover in a full room [22]. 

The ignition process for FRC materials can be described in terms of the relationship between time to ignition and heat flux. A technique developed by FMRC using its Small-Scale Flammability Apparatus (2-6) was used for the quantification. 

Experimentally, time to ignition is measured at various heat flux values and critical heat flux for ignition and TRP are quantified using techniques such as the one used in the FMRC Small-Scale Flammability Apparatus ( 4,6). 

The examples that appeared in Section C were with regard to linseed and lung oils, damp leaves, and pulverized coal. In each case a surface reaction occurred. To be noted is the fact that the analyses that set the parameters for determining the ignition condition do not contain a time scale. In essence then, the concept... 

Results of an experimental program in which aluminum particles were burned with steam and mixtures of oxygen and argon in small-scale atmospheric dump combustor are presented. Measurements of combustion temperature, radiation intensity in the wavelength interval from 400 to 800 nm, and combustion products particle size distribution and composition were made. A combustion temperature of about 2900 K was measured for combustion of aluminum particles with a mixture of 20%(wt.) O2 and 80%(wt.) Ar, while a combustion temperature of about 2500 K was measured for combustion of aluminum particles with steam. Combustion efficiency for aluminum particles with a mean size of 17 yum burned in steam with O/F) / 0/F)st 1-10 and with residence time after ignition estimated at 22 ms was about 95%. A Monte Carlo numerical method was used to estimate the radiant heat loss rates from the combustion products, based on the measured radiation intensities and combustion temperatures. A peak heat loss rate of 9.5 W/cm was calculated for the 02/Ar oxidizer case, while a peak heat loss rate of 4.8 W/cm was calculated for the H2O oxidizer case. 

The flame lift-off height, which is related to the ignition distance, was inversely affected by the excitation frequency. Since the flow time scale decreased with increasing frequency, the data were plotted as a function of the Damkohler number in Fig. 29.14, where the characteristic flow time scale was estimated by large-eddy turnover time as 1/17 and the characteristic chemical reaction time was computed using an ignition delay model [21] for ethylene jet. While the results did not show any evidence of critical Damkohler number, the range... 

Fig. 17.15 The top panel shows the transient surface-state composition during catalytic ignition on a long time scale. The lower panel shows the transient response of the Stefan velocity and the pressure-curvature eigenvalue on a very short time scale during the ignition transient. The zero point for the abscissa scales is arbitrary.

The time to ignition as a function of incident radiant heat flux can also be measured in the ISO ignit-ability test apparatus. This apparatus and its use are described in ISO 5657. Bench-scale heat release calorimeters such as the Cone Calorimeter (Section 14.3.3.2.1) and the Fire Propagation Apparatus (Section 14.3.3.2.3) can also be used to obtain this kind of data. 

In the past, combustion modeling was directed towards ffuid mechanics that included global heat release by chemical reaction. The latter was often described simply with the help of thermodynamics, assuming that the chemical reactions are much faster than the other processes like diffusion, heat conduction, and flow. However, in most cases chemistry occurs on time scales which are comparable with those of flow and molecular transport. As a consequence, detailed information about the individual elementary reactions is required if transient processes like ignition and flame quenching or pollutant formation shall be successfully modeled. The fundamental concept of using elementary reactions to describe a macroscopic... 

Given ideal, well-stirred conditions, the heat release rate could be interpreted from (6.13) under non-stationary conditions, but accurate measurements of (dT/dt) would also be required. The rate of temperature change is always more important than the heat loss rate during the late stages of the development of ignition, because the chemical time-scale is much shorter than the Newtonian cooling time-scale. 

Spontaneous ignition and associated features of organic gases and vapours are a consequence of the exothermic oxidation chemistry discussed in Chapter 1, but the way in which events unfold is determined by the physical environment within which reaction takes place. The heat transfer characteristics are probably most important, as may be illustrated with respect to the different consequences of adiabatic and non-adiabatic operation in a CSTR (Section 5) [117]. The notion of adiabatic operation may seem remote from any practical application, but this idealized condition may be approached if the chemical time-scale is considerably shorter than the time-scale for heat losses. 

The combustion process is initiated by an ignition source converting some number of methane molecules into free radicals. Free radicals are in turn converted to OH free radical. Possible oxygenated compounds include aldehyde, alcohol, carboxylic acid, and oxide. The hydroxyl free radical then reacts with methane and is regenerated. The successive (chain type) combustion reaction is impeded by destruction of the OH radicals. Solid surfaces often destroy the OH radicals before they can react with hydrocarbons. The same effect is exploited in a porous-bed flame arrestor. In general, the combustion rates are very fast and nearly measurable with a few exceptional situations where time scales can be expanded to microseconds (KT6 s). The... 

The ignition/extinction results and responses to changes in load provide information about the time scales for the response of the fuel cell. The time constant for transitioning to steady state during startup is 100 s. Five of the key time constants associated with PEM fuel cells are listed in Table 3.1.They include the characteristic reaction time of the PEM fuel cell (ti), the time for gas phase transport across the diffusion layer to the membrane electrode interface (T2), the characteristic time for water to diffuse across the membrane from the cathode to the anode (ts), the characteristic time for water produced to be absorbed by the membrane (T4), and the characteristic time for water vapor to be convected out of the fuel cell (T5). Approximate values for the physical parameters have been used to obtain order of magnitude estimates of these time constants. 

Spray combustion is initiated by an ignition process, which occurs at relatively low temperatures and, therefore, its chemical time scales are comparable with the ones of the flow. Consequently, fluid dynamics effects play an important role, and species transport equations are essential for monitoring the ignition progress. Also, the chemical reactions are dominated by various reaction paths, which lead to a large number of intermediate species that can exhibit tmexpected behavior. The ignition delay characteristics are illustrated for different gas pressures of a u-heptane fuel in Fig. 13.4. The data in this figure has been obtained from computations based on a detailed reaction mechanism as reported by Inhelder et al. [25]. 

We emphasize that only brittle disruption was capable to ignite the reaction. A fast interaction of the needle with the san )le, involving time scales of order 0.01-0.1 s is necessary to induce the effect. This is reminiscent of what happens in shock waves. If the m hanical loading of the needle is much slower (times scale of the order of 1 ms), the reaction is not ignited. It means that a plastic deformation is not capable to excite the reactions, and, as suggested by theory, only elastic deformations and following brittle disruption is a necessary condition for cold ignition of cryochemical reactions in solids. 

---