Chemical heat release rate

The heat generated in a fire is due to various chemical reactions, the major contributors being those reactions where CO and COg are generated, and O2 is consumed, and is defined as the chemical heat release rate (3). Techniques are available to quantify chemical heat release rate using FMRC s Flammability Apparatus (2-6), Ohio State University (OSU) Heat Release Rate Apparatus (J 3) and the NIST Cone Calorimeter (J jO. Techniques are also available to quantify the convective heat release rate using the FMRC Flammability Apparatus (2, 3) and the OSU Heat Release Rate Apparatus (J 3) The radiative heat release rate is the difference between the chemical and convective heat release rates (2,3). In the study, FMRC techniques were used. 

Heat Release Rate. It has been shown that the heat generated in chemical reactions leading to the generation of CO and C02 and depletion of 02 can be used to calculate the chemical heat release rate using the following relationships (2,3) ... 

Since we are not concerned presently with ignition and extinction phenomena (23,24, 25,26,27) caused by slow chemical heat release rates, the gas-phase chemical reactions can be assumed to occur at rates much faster than the gas-phase heat and mass transfer rates. This implies that the gas-phase consists of two (one in the case of pure vaporization) convective-diffusive regions separated by a fiame of infinitesimal thickness, at which the outwardly diffusing fuel vapor reacts stoichiometrically and completely with the inwardly diffusing oxidizer gas. 

Figure 11.1. Chemical heat release rate for a 100-mm diameter and 25-mm thick horizontal slab of polypropylene exposed to 50 kW/m of external heat flux in normal air, under well-ventilated condition, in the ASTM E2058 Apparatus [31]. Airflow rate 2.9 X 10 m /s. Data up to about 900 s are for the combustion with very small bubbles formed at the surface of the solid slab of PP. Beyond about 1150 s, the data are for the combustion with deep liquid pool over the solid slab of PP. Data were measured in our laboratory.

Chemical heat release rate is the actual heat that is released in the combustion of a polymer [21, 22]. It has a convective and a radiative component. Chemical heat release rate is always less than the heat release rate for complete combustion as the polymers do not burn completely. Chemical heat release rate is determined from the mass generation rate of CO2 corrected for the mass generation rate of CO (defined as the caibon dioxide generation calorimetry) and from the mass depletion rate of O2 (defined as the oxygen consumption calorimetry). 

Chemical heat release rate per unit surface area of the polymer (kW/m )... 

The chemical heat release rate is determined from the carbon dioxide generation (CDG) and oxygen consumption (OC) calorimetries [2,3]. In the CDG calorimetry, the chemical heat release rate is determined from the mass generation rate of CO2 corrected for CO [2,3]. In the OC calorimetry, the chemical heat release rate is determined from the mass consumption rate of O2 [2,3,24]. The convective heat release rate is determined from the gas temperature rise... 

The chemical heat release rate is determined from the following relationships ... 

For the determination of the chemical heat release rate, mass generation rates of CO2 and CO are measured and actual values of Mi 2 Mi are used for accuracy or the average values for approximate results. The CO2 and CO measurement details are described in Tewarson [2]. 

The chemical heat release rates determined from the CDG and OC calorimetries are very similar. 

The fire property associated with smoke damage is the ratio of the yield of smoke-to-the yield of CO2. The ratio increases with increase in the equivalence ratio or ventilation restriction. The yield of smoke is proportional to the smoke generation rate and the yield of CO2 is proportional to the chemical heat release rate. The higher the ratio of the yield of smoke to the yield of CO2, higher the damage due to smoke relative to the damage due to heat. 

When the amotmt of an agent applied to a burning polymer is close to the amount required for flame extinction, first flame instability sets in, followed by flame liftoff from the surface and finally the flame is extinguished, as indicated in Fig. 53.7 for the flame extinction of PMMA by Halon 1301. Initially there is a rapid decrease in the chemical heat release rate as Halon is added to the flame. There is a gradual increase in the chemical heat release rate between 5.40% and 6.25% of Halon unto flame extinction. The increase in the chemical heat release rate appears to be due to increase in the flame luminosity (increase in the flame radiative heat flux transferred back to the fuel surface). 

FIGURE 53.7. Chemical heat release rate versus time for the combustion of 100 mm x 100 mm x 25 mm thick horizontal slab of polymethylmethacrylate exposed to 40 kw/m in coair flow with varying Halon 1301 concentration at a velocity of 90 mm/s in the ASTM E 2058 FPA. Numbers and their locations represent Halon 1301 concentrations in volume percents and application times. Times for flame instability, liftoff, and extinction are also indicated. 

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