And external heat flux

FIGURE 15.20 Temperature at the back of coated (system A) steel plates plotted against time, for different intumescent coating thicknesses (d) and external heat fluxes. 

The following is the most commonly used expression for the relationship between the ignition time and external heat flux based purely on the thermal arguments [14, 15, 21, 22, 34, 35] ... 

Figure 11.4. Reiationship between the time to ignition measured in the ASTM E2058 Apparatus and external heat flux for 100-mm square and 25-mm thick siab of PMMA. The surface was coated biack in the experiments. Data are taken from Ref. [39].

Three factors that have the most dominant effects on the fire intensity and thermal and nonthermal hazards are (1) availability of air (oxygen), (2) fire properties, generic nature, and mode of vaporization (decomposition) of the polymers, and (3) flame and external heat flux and surface re-radiation loss. 

In the ignition tests, 155-xl55-mm samples are exposed to various external heat flux values and times to flame attachment are measured [15,16]. The values of k, p, Cp, ATig are determined from the relationship between the time to flame attachment and external heat flux [15,16]. These values can be used to calculate the TRP value [Eq. (53.4)]. 

For a thermally thin sample of thickness b and at low external heat fluxes near the critical value, the ignition time and external heat flux are related... 

Hicks showed that the time required to achieve ignition depends on the manner in which the external heat flux is applied. If the propellant surface is heated continuously, the surface temperature will continue to rise until runaway reaction conditions are reached [curve (a) of Fig. 3]. If the heat flux is terminated just before runaway reaction conditions pre achieved, then a sudden drop in surface temperature can occur, followed by a long time-delay before the surface temperature again begins to mse [as shown in curve (b)]. If the flux is removed too soon, the temperature will drop continuously and ignition will not be achieved [as shown in curve (c)]. 

The relation between the emf of the thermoelectric pile and the heat flux from the calorimeter cell will be first established. Let us suppose (Fig. 8) that the process under investigation takes place in a calorimeter vessel (A), which is completely surrounded by n identical thermoelectric junctions, each separated from one another by equal intervals. The thermocouples are attached to the external surface of the calorimeter cell (A), which constitutes the internal boundary (Eint) of the pile and to the inside wall of the heat sink (B), constituting the external boundary (Eext) of the thermoelectric pile. The heat sink (B) is maintained at a constant temperature (6e). 

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). 

Experiments can be performed where chemical, convective and radiative heat release rates can be measured at various external heat flux values. Linear relationships should be found for the experimental data, where the slope is equal to xj (AH /L). 

For the quantification of fire propagation behavior of the FRC materials, 0.10 m wide and 0.61 m long vertical sheets with thickness varying from 3 mm to 5 mm were used. The bottom 0.15 m of the sheet was exposed to 50 kW/m2 of external heat flux in the presence of a 0.01 m long pilot flame to initiate fire propagation. For the simulation of large-scale flame radiation, experiments were performed in k0% oxygen concentration. 

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. 

Note that if this net flux is for a heat flux meter cooled at 7 ,XJ, the ambient temperature, the gage directly measures the external incident radiative and flame heat fluxes. 

After ignition of the edge, the external heat flux is removed and it is no longer felt by the wood, in any way. A researcher measures the heat flux of the flame, ahead of the burning wood, as it spreads steadily and horizontally along the edge. That measurement is depicted in the plot below. Compute the flame speed on the wood, based on this measurement. 

By substituting for the flame convective heat flux from Equation (9.108) for steady conditions and no external heat flux (and no char and water), the analogue to Equation (9.116) follows for the diffusion flame ... 

The Primary Reformer is a steam-hydrocarbon reforming tubular furnace that is typically externally fired at 25 to 35 bar and 780°C to 820°C on the process side. The reformer tubes function under an external heat flux of 75,000 W/m2 and are subject to carburization, oxidation, over-heating, stress-corrosion cracking (SCC), sulfidation and thermal cycling. Previously SS 304, SS 310 and SS 347 were used as tube materials. However these materials developed cracks that very frequently led to premature tube failures (see Table 5.10)88. 

FIGURE 6.12 (See color insert following page 530.) HRR as a function of time of pure TPU and TPU/ FQ-POSS composite (external heat flux = 35kW/m2) (a) and intumescent char residue at the end of the cone experiment (b). 

FIGURE 6.17 HRR curves as a function of time of PET, PET/OP950, and PET/OP950-OMPOSS (total loading = 20 wt % substituting of 2 wt % OP950 by OMPOSS external heat flux = 35kW/m2). 

Silicone materials exhibit relatively low rates of heat release, a uniquely low dependence of rate of heat release on external heat flux. One of the causes of the lower burning rate is attributed to the accumulation of the silica ash layer at the silicone fuel surface. This accumulation of amorphous silica ash at the surface results from the deposition of silica particles, one of the major combustion products of silicone oligomers (cyclic and/or linear structures) in the gas phase. 

Higher irradiation levels give better reproducibility, more clearly defined ignition, and shorter measurement times, but correspond to more developed fires. Thus particularly for flame-retarded polymers, a smaller irradiation level often corresponds better to the fire protection goals addressed. Cone calorimeter results for the HRR at small irradiances correspond to flammability tests such as LOI and UL 94, if a reasonable set of materials are compared and the behavior is not dominated by dripping effects. Thus different considerations govern the choice of external heat flux.76 77... 

The external heat flux (q"x) from the cone heater does not exclusively determine the heat flux important for samples pyrolysis in the cone calorimeter, since the reradiation from the hot sample surface (q"eTad), the loss by thermal conductivity into the specimen and the surroundings ( I SS), and the heat flux from the flame (q Lmt) are also of the same order of magnitude.82 85 Thus, the heat flux effective with respect to pyrolysis during a cone calorimeter run (qeii) is the result of the external heat flux and the material s response (qeB = q L + < L - gCad - qLs). 

Figures 15.8 and 15.9 illustrate examples of how cone calorimeter data can be used in the development of flame-retarded materials. PA 66-GF without Pred showed typical fire behavior for noncharring polymers containing inorganic glass fiber as inert filler,69 when high external heat flux is applied. The shape of the HRR curve is divided in two different parts. In the beginning, the surface layer pyrolysis shows a sharp peak, followed by a reduced pyrolysis rate when the pyrolysis zone is covered by the glass fiber network residue layer. When Pred was added, the PA 66-GF samples were transformed into carbonaceous char-forming materials, which led to a...

FIGURE 15.10 HRR and THR for PP-g-MA/LS nanocomposite plotted against the amount of clay added (0, 2.5, 5, 7.5, 10wt.% clay external heat flux = 30kW m-2). 

It is striking that this system shows such a clear differentiation between the behavior of the two probably most important fire risks PHRR and THE, when the external heat flux is varied. The flame retardancy effect with respect to the THE decreases with increasing irradiation, whereas the relative flame retardancy effect with respect to the PHRR increases. The latter clearly indicates the predominant influence of the barrier effect on the HRR. 

Hence, a linear dependency of the HRRst from the external heat flux is expected (Figure 15.17) based on Schartel and Braun.55 The slope (referred to as the heat release parameter, HRP) is interpreted as a fire response parameter,60-108 whereas the intercept (HRR,) is considered a flammability parameter,62 107 at least on this length scale. 

Figure 15.21 (right) uses the Petrella approach, to assess the fire risks of both PA 66-GF and PA 66-GF/Pred at different external heat fluxes.54 55 81 Thus, the assessment is valuable for different applications, fire scenarios, and Are tests, as these correspond to different external heat fluxes and define different demands on Are retardancy in terms of the long duration and growth of a Are. The data for both materials show that with increasing heat flux the Are hazards increase in terms of Are growth, as expected, since a higher irradiance results in an increase in fuel production rate. The THE of... 

FIGURE 15.21 Using the Petrella plot for comprehensive and reasonable scientiflc assessment of flame retardancy by comparing different approaches, or by comparing the effects for different irradiations. THE stands for the Are load and PHRR/tig for the fire growth rate hence, the two most important fire risks are monitored at the same time. An ideal flame retardancy would decrease both hazards significantly as is the case for the combination of both flame retardants on the left (comparison of HIPS), HIPS/Pmd, HIPS/Mg(OH)2, and HIPS/Pmd/Mg(OH)2) and for low external heat flux on the right (comparison of PA 66-GF and PA 66-GF/Pred for different irradiations). 

PA 66-GF is fairly constant, since the polymer is nearly completely combusted for all irradiances used, whereas the THE decreases most for the flame-retarded material at low irradiance, because the char formation is at the highest level. This effect diminishes with increasing irradiance. In the case of Pred in PA 66-GF, combustion is complete at the highest external heat flux, with or without flame retardant, so the THE is almost the same, but the fire growth index is almost halved. Conversely, at low irradiance, not only is the fire growth index reduced, but the THE is almost halved as well. The change in the fire scenario changes the effectiveness of Pred added to PA 66-GF in two of the most important fire properties. Pred in PA 66-GF works best for low external heat flux. Flammability tests like LOI and, much more important, UL 94, are fire scenarios with low external heat flux. 

Bartholmai M, Schriever R, Schartel B. Influence of external heat flux and coating thickness on thermal insulation properties of two different intumescent coatings using cone calorimeter and numerical analysis. Fire Mater. 2003 27 151-162. 

Prior to the tests, all the samples were dried in a vacuum oven at 80°C for at least 72 h to minimize the moisture effect and then transferred to a desiccator. Measurements were carried out on a cone calorimeter provided by the Dark Star Research Ltd., United Kingdom. To minimize the conduction heat losses to insulation and to provide well-defined boundary conditions for numerical analysis of these tests, a sample holder was constructed as reported in [14] with four layers (each layer is 3 mm thick) of Cotronic ceramic paper at the back of the sample and four layers at the sides. A schematic view of the sample holder is shown in Figure 19.12. Three external heat fluxes (40, 50, and 60kW/m2) were used with duplicated tests at each heat flux. 

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