Mass loss rates heat fluxes

Mass Loss Rate as a Function of External Heat Flux. The technique for the measurement of mass loss rate as a function of heat flux was developed in 1976 at FMRC using the Small-Scale Flammability Apparatus (8 ). Several other flammability apparatuses are now available for such measurements, such as OSU Heat Release Rate Apparatus (13) and NIST Cone Calorimeter (1 4). 

For the assessment of flame heat flux, expected in large-scale fires, 0.10 x 0.10 m samples with edges covered tightly with heavy duty aluminum foil, were burned in 40 oxygen concentration without the external heat flux. Mass loss rate was measured and Equation (1) was used to calculate flame heat flux. 

A vertical slab of PMMA bums in air at an average mass loss rate of 15 g/m2 s. The air temperature is 20 °C. Assume that the heat transfer coefficient of the slab is 15 W/m2 K. Determine the net radiative heat flux to the surface. What amount comes from the flame ... 

Some materials exhibit nearly steady mass loss rates when exposed to a fixed radiant heat flux. The surface temperature for these materials reaches a steady value after a short initial transient period, and all terms in Equation 14.7 are approximately constant at a specified heat flux level. L can then be obtained by measuring steady mass loss rates at different radiant heat flux levels, and... 

FIGURE 19.14 Comparison of the mass loss rate history of PA6 and PA6/NC (UBE) at different heat fluxes (sample thickness is 6mm). 

FIGURE 19.34 Calculated heat flux ratio, ratioflux, against pyrolyzed depth, S[)yro, for (a) EVA and (b) PBT nanocomposites at 50kW/m2. Two lines represent the best fits of the calculation results, which are also used to predict the mass loss rate. 

Delichatsios, M. A. (2005) Piloted ignition times, critical heat fluxes and mass loss rates at reduced oxygen atmospheres, Fire Safety Journal 40(3) 197-212. 

With empirical pyrolysis models, a material s burning rate is zero until its surface is heated to its ignition temperature (Tig), at which time ignition occurs. After ignition, the mass loss rate of a fuel element is estimated from the net heat flux to the fuel s surface (q"a) divided by the effective heat of gasification (A7/g) ... 

Table 2 Cone calorimeter data for modified bisphenol A vinyl ester (Mod-Bis-A Vinyl Ester), bisphenol A novolac vinyl ester (Bis-/Novolac Vinyl Ester) and methylenedianiline and benzyldimaine (BDMA) cured epoxy resins and their intercalated nanocomposites ( ) containing 6% dimethyl dioctadecylammonium-exchanged montmorillonite. Heat flux = 35 kW/m, HRR = heat release rate, MLR = mass loss rate. He = heat of combustion, SEA = specific extinction area [121]...

Results are shown in Table 2.5. All additives were effective in reducing peak HRR to some extent. Of the additives studied ATH (sample 1196) shows 20% and 25% decreases in peak HRR at radiant heat fluxes at 50 and 75 kW/m, respectively, and 24% and 13% decreases in average HRR at 50 and 75 kW/m respectively. Also, average mass loss rates were decreased by 27% and 24% at 50 and 75 kW/m. ... 

In the mass pyrolysis technique, the mass loss rate is measured as a function of external heat flux in the presence of co-flowing nitrogen or air with an oxygen concentration of 10% by volume, and the data are used in Eq. (53.1). The heat of gasification is determined from the linear regression... 

The steady state relationship for polymer gasification rate or mass loss rate is similar to the relationship for the pyrolysis condition [Eq. (53.1)], except for an additional term for the flame heat flux [2,3] ... 

In the flame radiation scaling technique, mass loss rate is measured with co-flowing air having various oxygen mass fractions. Flame heat flux is calculated by using the mass loss rate data in Eq. (53.5), along with the values of the heat of gasification and surface radiation loss measured in pyrolysis [13], The convective component of the flame heat flux is determined from the combustion of methanol dominated by convective heat transfer [13]. The flammability... 

The asymptotic values for the mass loss rate in combustion and flame heat flux determined from the radiation... 

TABLE 53.5. Asymptotic mass loss rate and flame heat flux. 

Mass loss rate (kg/m -s) X 10 Flame heat flux (kW/m ) ... 

Apart from this information, the graphs of mass loss rates (dm/dt) vs. time for neat PP and PP-MAPP-Cloisite 20A indicate the depression of the degradation (fuel) products under the isothermal pyrolysis conditions at 600°C (Fig. 10). It is well known that the temperature of 600°C corresponds to an incident heat flux of 35 kW/m this is referred to the real scale fire scenario [21]. 

In the present study the combustibility of polypropylene nanocomposite was evaluated by a cone calorimeter. The tests were performed at an incident heat flux of 35 kW/m using the cone heater [21]. Peak heat release rate (RHR), mass loss rate (MLR), specific extinction area (SEA) data, earbon monoxide and heat of eombustion data measured at 35 kW/m2, are presented in Figs. 11 and 12. 

The RHR plots for PP-MAPP-Cloisite 20A nanocomposite and PP at 35 kW/m heat flux shown in Figure indicate a 60% - decrease of peak of RHR (Fig. 11). Comparison of the Cone calorimeter data PP and PP-MAPP- 7% Cloisite 20A reveals that the specific heat of combustion (He), specific extinction area (SEA), a measure of smoke yield, and carbon monoxide yields are practically unchanged this suggests that the source of the improved flammability properties of these materials is due to differences in condensed-phase decomposition processes and not to a gas-phase effect. The primary parameter responsible for the lower RHR of the nanocomposites is the mass loss rate (MLR) during combustion, which is significantly reduced from the value observed for the pure PP (Fig. 12). It is supposed, that this effect is caused by ability to initiate the formation of char barrier on a surface of burning polymeric nanocomposites that drastically limits the heat and mass transfer in a burning zone. 

The Cone Calorimeter was based on a design by Vytenis Babrauskas (Fire Research, National Bureau of Standards) in the early 1980s [4]. This apparatus allows simultaneous and continuous determinations to be made of heat release rate, smoke production rate, mass loss rate and concentration of the various combustion gases formed. It is capable, at the same time, of being used to obtain ignition, heat of combustion and soot production data for the materials tested. As an example, PP has a rate of heat release maximum of 1800 kW/m (heat flux = 50kW/m thickness of the sheet = 3 mm). 

The cone calorimeter. This instrument measures ignition characteristics, heat release rate, and sample mass loss rate. For the experiments reported below, an external radiant heat flux of 50kW/m was applied. 

A radiant gasification apparatus, somewhat similar to the cone calorimeter above, measures the mass loss rate and temperatures of the sample exposed to a fire-like heat flux in a nitrogen atmosphere. Thus, there is no burning. 

Figure 8.11 compares the predicted mass loss rates (MLRs) with the measured ones under several heat fluxes. Not only does the model predict correctly the trends of the experimental data, but the predicted MLRs are also in quantitative agreement with the measurements. Figure 8.11 implies, as noted in [11], that the correlation between ratiOflux and Spyro (cf. eq. (8.6.1)) is independent of the external heat flux. It depends only on the thickness of the material that has pyrolyzed or, equivalently, on the amount of nanoclay accumulated on the surface after polymer pyrolysis. The result in Figure 8.11 demonstrates that the methodology is applicable not only to pure polymer nanocomposites but also to polymer blend nanocomposites. 

Figure 9.7 (a) Comparison of the HRR plot for nylon 6 and nylon-6 silicate nanocomposites (mass fraction 5%) at 35 KW/m heat flux (b) the mass loss rate data for nylon 6 and nylon-6 clay nanocomposites [78]. Reproduced from [78] by permission of Elsevier Science Ltd., UK. 

Heat flux, 35 kW/m. He, specific heat of combustion SEA, specific extinction area g , ignition time. Peak heat release rale, mass loss rate, and SEA data are reproducible to within 10%. The heat of combustion and the time to ignition data are reproducible to within 15%. The cone data reported are the average of three repheated samples. The samples are square plates 100 mm large and 8 mm thick. 

---