Akifumi Takahashi

A study on flammability limits of fuel mixtures

Flammability limit measurements were made for various binary and ternary mixtures prepared from nine different compounds. The compounds treated are methane, propane, ethylene, propylene, methyl ether, methyl formate, 1,1-difluoroethane, ammonia, and carbon monoxide. The observed values of lower flammability limits of mixtures were found to be in good agreement to the calculated values by Le Chateliers formula. As for the upper limits, however, some are close to the calculated values but some are not. It has been found that the deviations of the observed values of upper flammability limits from the calculated ones are mostly to lower concentrations. Modification of Le Chateliers formula was made to better fit to the observed values of upper flammability limits. This procedure reduced the average difference between the observed and calculated values of upper flammability limits to one-third of the initial value.

Rate constants for the reactions of OH radicals with CF3OCF=CF2 and CF3CF=CF2

Abstract The rate constants for the reactions of OH radicals with trifluoromethyl trifluorovinyl ether (CF 3 OCF=CF 2 ) and hexafluoropropene (CF 3 CF=CF 2 ) have been measured over the temperature range 250–430 K. Kinetic measurements have been carried out using the flash photolysis and laser photolysis methods combined, respectively, with the laser-induced fluorescence technique. The Arrhenius rate constants have been determined as k (CF 3 OCF=CF 2 )=1.01 −0.04 +0.04 ×10 −12 exp [(320±10)/T] , and k (CF 3 CF=CF 2 )=8.74 −0.33 +0.34 ×10 −13 exp [(260±10)/T] cm 3 molecule −1 s −1 .

Burning velocity measurements of nitrogen-containing compounds

Burning velocity measurements of nitrogen-containing compounds, i.e., ammonia (NH3), methylamine (CH3NH2), ethylamine (C2H5NH2), and propylamine (C3H7NH2), were carried out to assess the flammability of potential natural refrigerants. The spherical-vessel (SV) method was used to measure the burning velocity over a wide range of sample and air concentrations. In addition, flame propagation was directly observed by the schlieren photography method, which showed that the spherical flame model was applicable to flames with a burning velocity higher than approximately 5 cm s(-1). For CH3NH2, the nozzle burner method was also used to confirm the validity of the results obtained by closed vessel methods. We obtained maximum burning velocities (Su0,max) of 7.2, 24.7, 26.9, and 28.3 cm s(-1) for NH3, CH3NH2, C2H5NH2, and C3H7NH2, respectively. It was noted that the burning velocities of NH3 and CH3NH2 were as high as those of the typical hydrofluorocarbon refrigerants difluoromethane (HFC-32, Su0,max=6.7 cm s(-1)) and 1,1-difluoroethane (HFC-152a, Su0,max=23.6 cm s(-1)), respectively. The burning velocities were compared with those of the parent alkanes, and it was found that introducing an NH2 group into hydrocarbon molecules decreases their burning velocity.

Prediction of flammability of gases by using F-number analysis.

A novel method of predicting flammability limits has been proposed. This method utilizes a new flammability index called F-number. For this purpose, an empirical expression of F-number has been derived to account for the flammability characteristics of various organic substances. The analysis has been done by fitting to the observed values of F-number for a wide variety of organic gases and vapors. As a result, it has been found that F-number is an excellent tool to analyze the flammability characteristics of various substances. It has also been shown that the values of upper and lower flammability limits can be derived from F-number together with the stoichiometric concentration corrected for the effect of selective diffusion.

A Numerical Study of Low Temperature Silane Combustion

Abstract A mechanism of low temperature silane combustion has been proposed in the present work based on the assumption that a trace amount of water vapor helps the occurrence of spontaneous ignition at room temperature. This assumption has been made based upon the fact that the combustion product of silane influences positively the occurrence of spontaneous ignition. Energetic calculation of the reaction path way for low temperature silane combustion also supports this assumption [Kondo et al., 1999]. A numerical model has been constructed which can interpret the spontaneous ignition limit at room temperature, the ignition delay times, and the second explosion limit of silane mixtures simultaneously.

Calculation of minimum ignition energy of premixed gases.

The minimum ignition energy of premixed gases has been calculated by using two theoretical expressions and compared with the experimental data. One expression considers the amount of energy that the minimal flame should have, and the other the heat loss from the surface of the minimal flamelet. The former is a cubic function of the quenching distance while the latter is a quadratic function of quenching distance. It has been found that the latter expression gives a better fit to the experimental data than the former, though the discrepancy is considerable even for the latter expression. The calculated widths of the fronts of the minimal flame for various fuels were about one-order of magnitude smaller than the corresponding experimentally determined quenching distances, although no clear correlation relationship between the two quantities was found.

Theoretical calculation of heat of formation for a number of moderate sized fluorinated compounds

Abstract Hydrofluorocarbons (HFCs) and hydrofluoroethers (HFEs) are considered to be potential replacements for chlorofluorocarbons (CFCs). The values of heat of formation have been calculated by using both the bond additivity corrected MP2 method (BAC-MP2/6-31G ∗∗ ) and atom additivity corrected MP2 method (AAC-MP2/6-31G ∗∗ ) for moderate sized HFCs and HFEs for which the literature values are not available. The reliability of the calculated values by these methods has been examined by comparing them with the values obtained with G2 and G2MP2 methods as well as with the literature values for a set of 42 simpler molecules.

Flammability limits of olefinic and saturated fluoro-compounds.

Flammability limits were measured for a number of olefinic and saturated fluoro-compounds in a 12l spherical glass vessel. The obtained data together with the ones of previous studies have been analyzed based on the F-number scheme of flammability limits. The flammability limits of these compounds have been found to be explained very well by the present scheme of interpretation. The flammability limits are dependent upon distribution of F atoms in a molecule as well as upon F-substitution rate itself. It has been found that -O-CF(3) group in a molecule conspicuously decreases the flammability of the compound, while -C-CF(3) group does not much. For olefinic compounds, distribution of F atoms around double bonds markedly diminishes the flammability of the molecule.

RF number as a new index for assessing combustion hazard of flammable gases.

A new index called RF number has been proposed for assessing the combustion hazard of all sorts of flammable gases and their mixtures. RF number represents the total expectancy of combustion hazard in terms of flammability limits and heat of combustion for each known and unknown compounds. The advantage of RF number over others such as R-index and F-number for classification of combustion hazard has been highlighted.

Reinvestigation of Flammability Limits Measurement of Methane by the Conventional Vessel Method with AC Discharge Ignition

The flammability limits of methane have been reinvestigated by using the AC discharge ignition method. This work has partly been motivated by the necessity of evaluating the flammability characteristics of CFC alternatives. In the study, the effects of spark duration time and spark gap on the flammability limits of methane have extensively been explored. The resulting flammable range tends to become wide if the spark duration time is too long and/or the spark gap is too large. The effects of inadequate spark duration time and spark gap are exaggerated if the height of the experimental vessel is too small. As a result, the spark duration time of 0·1–0·2 sec combined with the spark gap of 6–8  mm has been found to make an optimum condition for the correct measurement of the flammability limits by the AC discharge ignition method.