Kenji Takizawa

Flammability assessment of CH2CFCF3: Comparison with fluoroalkenes and fluoroalkanes

The burning velocity, flammability limits, and heat of combustion of CH(2)CF=CF(3) (1234yf) have been studied to elucidate the fundamental flammability properties of this new alternative refrigerant with low global-warming potential. The burning velocity of 1234yf was measured independently by schlieren photography and the spherical vessel method. In the spherical vessel method, the burning velocities of 1234yf and its analogues CH(2)=CFCHF(2) (1243yf) and CH(2)=CHCF(3) (1243zf) as well as those of typical fluoroalkanes CH(2)F(2) (HFC-32) and CH(3)=CHF(2) (HFC-152a) were measured in mixtures of air at various O(2)/(N(2)+O(2)) ratios. The maximum burning velocity of 1234yf was found to be 1.2+/-0.3 cm s(-1), which was approximately one-fifth that of HFC-32 (6.7 cm s(-1)) and one order of magnitude less than those of 1243yf (19.8 cm s(-1)) and 1243zf (14.1 cm s(-1)). The flame propagation of 1234yf was highly sensitive to flame temperature compared to that of the other compounds. The measured flammability limits and calculated heat of combustion of 1234yf were also determined.

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.

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.

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.

Effects of temperature and pressure on quenching distances of difluoromethane (R32) and ammonia (R717)

Temperature and pressure effects on quenching distance have been studied for difluoromethane (R32) and ammonia (R717), two mildly flammable refrigerant gases. By studying ignition and quenching behavior in conditions beyond standard test conditions, the authors aimed at understanding whether combustion takes place in practical situations. First, the quenching distance of R32 and R717 in the temperature range of 298 to 333 K and pressure range of 0.79 to 1.12 atm was measured. The quenching distance decreased slowly with increasing temperature and decreased steeply with increasing pressure. Second, the authors examined the relationship between quenching distance and burning velocity at various temperature and pressure. It was found that quenching distance is essentially related to (unburned gas density × burning velocity)−1, and changes in quenching distance observed in the present experiment can be explained by the temperature and pressure dependences of burning velocity and unburned gas density. It was also found that the quenching distance and burning velocity for R32 were not affected much by the humidity of air in the practical condition.

Rate Constants for the Reactions of OH Radical with the (E)/(Z) Isomers of CF3CF═CHCl and CHF2CF═CHCl

The rate constants for the reactions of OH radical with ( E)- and ( Z)-isomers of CF3CF═CHCl and CHF2CF═CHCl have been measured over the temperature range of 250-430 K. Kinetic measurements have been performed using flash and laser photolysis methods combined with laser-induced fluorescence. Arrhenius rate constants have been determined as k(( E)-CF3CF═CHCl) = (1.09 ± 0.03) × 10-12 · exp[(50 ± 10)K/ T], k(( Z)-CF3CF═CHCl) = (8.02 ± 0.19) × 10-13 · exp[-(100 ± 10)K/ T], k(( E)-CHF2CF═CHCl) = (1.50 ± 0.03) × 10-12 · exp[(160 ± 10)K/ T], and k(( Z)-CHF2CF═CHCl) = (1.36 ± 0.03) × 10-12 · exp[(360 ± 10)K/ T] cm3 molecule-1 s-1. Infrared absorption spectra have also been measured at room temperature. The atmospheric lifetimes of ( E)-CF3CF═CHCl, ( Z)-CF3CF═CHCl, ( E)-CHF2CF═CHCl, and ( Z)-CHF2CF═CHCl have been estimated as 8.9, 20, 4.6, and 2.6 days, respectively, and their global warming potentials and ozone depletion potentials were determined as 0.23, 0.88, 0.060, and 0.016 and 0.00010, 0.00023, 0.000057, and 0.000030, respectively. Additionally, the rate constants for OH radical addition and IR spectra of these compounds were determined computationally. Consistent with experiment, our calculations indicate that the reactivity toward OH radical addition is reduced as ( Z)-CHF2CF═CHCl > ( E)-CHF2CF═CHCl > ( E)-CF3CF═CHCl > ( Z)-CF3CF═CHCl, where the ( E)/( Z) reactivity is reversed for CF3CF═CHCl and CHF2CF═CHCl. The calculations reproduced the observed temperature dependencies of the rate constants for the OH radical reactions, which is slightly positive for ( Z)-CF3CF═CHCl but negative for the other compounds.

Rate Constants for the Reactions of OH Radicals with Fluorinated Ethenes: Kinetic Measurements and Correlation between Structure and Reactivity

The rate constants for the reaction of OH radicals with four fluorinated ethenes (CF2═CHF, ( E)-CHF═CHF, CF2═CH2, and CHF═CH2) have been measured over the temperature range of 250-430 K. Kinetic measurements have been carried out using flash photolysis and laser photolysis methods combined with a laser-induced fluorescence technique. The Arrhenius expressions for the rate constant have been determined as k(CF2═CHF) = (3.12 ± 0.11) × 10-12 exp[(270 ± 10)/ T], k(( E)-CHF═CHF) = (3.75 ± 0.08) × 10-12 exp[(230 ± 10)/ T], k(CF2═CH2) = (1.15 ± 0.07) × 10-12 exp[(230 ± 20)/ T], and k(CHF═CH2) = (1.16 ± 0.09) × 10-12 exp[(390 ± 20)/ T] cm3 molecule-1 s-1. Infrared absorption spectra of the fluorinated ethenes have been measured at room temperature. The atmospheric lifetimes and global warming potentials of the fluorinated ethenes have been estimated. The correlation between the reactivity and the structure of the halogenated ethenes has been investigated by considering the structure containing the atoms attached to the carbons on both sides of the double bond. The calculated rate constants of 14 halogenated ethenes showed agreement with the measured rate constants within a factor of 2, except for that of one compound.