Surface Regression Rate Research Articles

Radiative ignition mechanism of solid fuels

Transmittance of external radiation from a CO2 laser through a boundary layer of decomposition products over a vertical sample surface is measured during the ignition period. The results indicate that there is significant absorption of the external radiation for PMMA, and a lesser but still not negligible amount, for red oak. An increase in gas phase temperature over surface temperature is observed over much of the ignition interval. Using the experimentally measured incident flux at the sample surface, surface temperature history was calculated from a model that included re-radiation and convection losses from the surface, endothermic decomposition and conduction into the material. The results confirm the significant effect of gas phase absorption on surface temperature. Steady-state-derived surface regression rate expression was used for PMMA in this model. The results raise questions about the validity of such data for the dynamic heating conditions during the ignition period. Further studies needed to understand the radiative ignition mechanism are identified.

Combustion of irradiated dry and wet oak

Combustion of dry oak and wet oak with 50 wt% water content at a fire level surface heat flux of 2 cal/cm 2 sec was experimentally investigated. Experimental measurements included thermal-physical properties, mass loss rate, surface regression rate, solid density, temperature, pressure, and gas composition. Effects of absorbed water on wood pyrolysis and combustion are examined from these measurements. It is found that the absorbed water mainly delays the temperature rise in the wet wood and the vaporized water convectively cools the pyrolyzing wood. The pyrolysis process is thus delayed in the wet wood and the volatile combustible gases are diluted by the water vapor. Decomposition of water and reaction between water and wood char (carbon) do not appear to occur significantly. At steady-state burning, surface regression rates are the same for both dry and wet wood, apparently because the regression rate is primarily governed by the oxygen supply to the char surface.

Deflagration and extinction of fuel droplet in a weakly-reactive atmosphere

The combustion of a fuel droplet in an oxidizing environment consisting of a small quantity of its own vapor is analyzed, in the limit of large activation energy, using matched asymptotic technique. The zeroth-order solution yields explicit expresssions for the steady deflagration characteristics indicating that with increasing ambient fuel vapor concentration the flame size and temperature both increase whereas the droplet surface regression rate remains unchanged. The first order expansion yields the flame structure and a set of numerically-obtained universal curves representing the extinction limits of the system.

Propagation and extinction of downward burning fires

Measurements are reported on downward burning of PMMA fuel rods in an apparatus similar to that used in the Limiting Oxygen Index flammability test. As the ambient oxygen concentration is reduced the flame propagation rate decreases. The propagation rate V does not go to zero as the oxygen concentration Y ox is reduced, rather extinction occurs at a finite value of V. Although the propagation rate decreases as Y ox is reduced, the surface regression rate is not reduced. In agreement with this finding, the calculated heat flux per unit area to the pyrolysis region does not decrease as Y ox decreases. Thus, extinction is not caused by reduction of heat transfer from the flame to the burning region. To investigate the cause of extinction a previously developed analysis for the effect of surface heat flux on flame propagation is applied to the present problem. It is found that flame propagation is sensitive to the amount of heat transferred to the fuel surface ahead of the flame, and that extinction occurs (at a finite propagation rate) for sufficiently low values of forward heat flux. Comparisons of predicted and measured values of V are made using heat flux distributions measured at values of Y ox for atmospheric and extinction conditions and reasonable agreement is found. A transient measurement near the extinction limit confirms that the Limiting Oxygen Index is a measure of the critical condition for downward flame propagation rather than a more general limit for burning.

Flame Propagation Measurements and Energy Feedback Analysis for Burning Cylinders

Abstract Flame spreading velocity measurements were made using PMMA cylinders as fuel rods. The diameter of the rods varied from 1½10; to 1½2 inch, and burning was observed at angles of orientation from θ = −90° (vertically downward burning) θ = +40°. The length of the pyrolysis zone was measured and used to calculate an average surface regression rate. For the axisymmetric case of vertically downward burning, surface temperatures were measured in the pyrolysis zone and in front of the flame; internal temperatures were measured in front or the flame. The measured temperatures were used to evaluate a number of energy balances. As a result, values were obtained for the energy feedback from the flame to the fuel by the various modes of heat transfer.

Laminar combustion of polymethylmethacrylate in O 2/N 2 mixtures

The results of experiments on the combustion of polymethylmethacrylate (PMMA, Plexiglas) slabs in laminar flows of O2/N2 mixtures are reported. The Reynolds Number, based on streamwise distance from the leading edge of the slab, ranged from 1.5×103 to 6×104, the pressure from atmospheric to 1/15 atm, and the mole fraction of oxygen from 0.2 to unity. Measurements were made of surface regression rates, of flame standoff distances, of temperatures of the regressing surface, and of conditions for flame extinction. Regression rates agree reasonably well with theoretical results obtained from the flame-surface approximation for the boundary layer on a semi-infinite flat plate. Standoff distances agree with the same type of model, if account is taken of the axial pressure gradient in the test section under hot-flow conditions, and of the reduction in flame temperature produced by dissociation and by radiative heat loss from the flame. The observed relationship between regression rate and surface temperature differs from that obtained earlier in hot-plate linear-pyrolysis experiments, and agrees with theoretical results that employ measured bulk degradation kinetics to calculate surface pyrolysis rates. Since these earlier calculations, based on the assumption of end initiation and unzipping, were believed to be accurate only for a degree of polymerization (DP) less than 2×103, and since the DP of our PMMA samples was 1 to 6×104, it appears that the earlier model may remain a useful approximation for a higher DP than expected. Observed extinction conditions are discussed in the light of some recent developments in extinction theory. Under the assumption that the gas-phase combustion process which occurs in the vicinity of extinction has an over-all order of 2, an extinction criterion is defined in terms of a critical value for the ratio of the square of the test-section pressure to the free-stream oxidizer mass flux. This critical value is influenced by the mole fraction of oxygen primarily through its dependence on flame temperature. The experimental results are consistent with an activation energy of roughly 20 kcal/mole, and a preexponential factor of the order of 1014 cm3/mole sec for the over-all combustion of gaseous PMMA degradation products with O2 in a diffusion flame near the extinction limir.

Linear pyrolysis of polymethylmethacrylate during combustion.

Polymethylmethacrylate (PMM) surface linear pyrolysis produced by intense surface heating has been studied experimentally. Surface heating was produced by means of a nearby flame established between emerging pyrolysis-product vapors and an impinging jet of oxygen and nitrogen. Surface regression rates between 0.08 and 0.32 mm/sec were achieved by varying the oxygen impingement rate. Very small thermocouples (15/x) embedded in the specimens yielded subsurface distributions. Interpretation of these indicated that: 1) a thin liquidlike transition region existed on the PMM surface; 2) the correlation of regression rate with at the interface between the solidlike and liquidlike regions is generally consistent with regression rate-surface temperature data from hot-plate pyrolysis data of others; 3) the correlation of regression rate with at the interface between the liquidlike region and vapor phase was less consistent with that of others, but was consistent with a correlation of regression rate with our optically measured surface temperature. The implications of these findings are discussed, particularly with regard to analysis of linear pyrolysis data obtained by other techniques. It is concluded that the linear pyrolysis of PMM is sufficiently complex to preclude unambiguous, detailed interpretation on the basis of currently available information.

Kinetics of Polymer Pyrolysis from Surface Regression Rates

A method of calculating polymer pyrolysis kinetic parameters from surface-regression-rate data and temperature-profile measurements has been presented. This method is based on the assumption that bond rupture, rather than evaporation and/or diffusion of the monomer, is the rate-limiting step. To apply this method it is first necessary to determine the temperature profile, then the rate is calculated as a sum of the rates of a series of thin isothermal slabs, each at the appropriate temperature along the profile. Because the slope of the temperature profile increases as the regression rate increases, the reaction-zone thickness decreases correspondingly. Thus, the size of the reaction zone is inversely proportional to the rate. This characteristic leads to activation energies about double those obtained by conventional calculations. The results of several sample calculations are tabulated. In addition, certain weaknesses in the desorption-rate-limited mechanism previously proposed are pointed out.