INFLUENCE OF PRESSURE ON THE FLAMMABILITY LIMITS OF HYDROGEN - EFFECTS OF VESSEL SIZE AND DIRECTION OF FLAME PROPAGATION

The flammability limits of hydrogen are known to follow characteristic pressure dependence. The relationship between the limit flame temperature and pressure is determined according to the mechanism of flame quenching. In the present study, by using rather small sized vessels (20 mm and 35 mm in diameter), the flammability limits of hydrogen hae been measured at pressures of 1-50 kgf.cm-2. The measurements have also been made to examine the effect of direction of flame propagation ; upward, downward and horizontal propagations (Table 1). Pressure dependences of the limit flame temperature were analyzed, using equations which were derived from competitive reaction model and conductive heat loss model, for upward propagation, and convection model for downward and horizontal propagations, respectively (Tables 2 and 3). It has been found that the vessel size does not change much the pressure dependence of the lower flammability limits. On the other hand, the direction of flame propagation does change the lower limits, though the way is very similar for the two vessels (Figs. 2 and 3). It seems that the mechanism which determines the lower limits of hydrogen is the competition between the chain branching and chain breaking reactions of hydrogen combustion. As for the upper limits (Figs. 4 and 5), the pressure dependence has been found to vary drastically according to the experimental conditions. The mechanism of the propagation limit seems to change for different propagation directions and for different pressure regions. For the lower pressure region, the pressure dependence of the limit flame temperature is well explained by the competitive reaction model. And for the higher pressure region, the heat loss mechanism seems to be effective for upward propagation, and the convective mechanism seems responsible for both downward and horizontal propagations.