OCR Text |
Show the experimental data were available in the literature. Results of the lean-flammability limit calculations are presented in Table 1 together with the corresponding experimentally determined values for upward flame propagation. Generally, these deviations are well within the range of the associated experimental uncertainties for all the fuel/diluent mixtures examined. The larger deviations occurred when greater amounts of a diluent were present in the fuel mixture. Calculated values of the rich flammability limits of fuel-diluent mixtures, in accordance with the method described for methane, ethane, propane, butane, hydrogen, and carbon monoxide are presented in Table 2 together with the experimental data for upward flame propagation. As in the case of the lean limits, higher accuracy in prediction is obtained with fuel mixtures that contain smaller concentrations of a diluent . The accuracy is also higher when the diluent is nitrogen. Accuracy in prediction of the rich limits was higher with methane, hydrogen and carbon monoxide than with the other fuels considered. The proposed approach considers changes in the thermodynamic properties of the components of the limit mixtures but not their transport properties, which also influence the flame propagation process. The transport properties of carbon dioxide differ substantially from those of air and nitrogen. This would affect the accuracy of the calculated values of especially the rich flammability limits of fuel mixtures containing this diluent. However, the deviation appears still to be within the range of experimental uncertainties for low and moderate concentrations of the diluent (up to about 60%) in the mixtures. However, the method described does not appear to predict satisfactorily the rich flammability limits of ethylene/diluent mixtures, especially when carbon dioxide is the diluent. A peculiarity in the behaviour of ethylene/carbon dioxide mixtures in air at their rich limits had earlier been noted experimentally [10]. Therefore, great care should be taken in consideration of the rich flammability limits of fuel mixtures containing ethylene as one of the combustible components and carbon dioxide as the diluent. It has been shown [10, 13] that the experimentally established values of the lean and rich flammability limits of fuel-diluent mixtures L ^ and L^R can be correlated in terms of the corresponding lean and rich limits LF R and LF L of the pure fuel in air according to the following simple relationships: j- - Y~ + H 0 " I» (1) and 7- = -r~ + «* d - rF) (2> ^m,R ^F,R where aL and aR are constants that depend on the type of fuel and diluent and YF is the fractional concentration of the fuel in the fuel-diluent mixture. The corresponding values of these constants were established from experimental data for a range of c o m m o n gaseous, fuels and diluents, at mainly ambient conditions [13]. The method for calculating the limits of fiiel/diluent mixtures thus described can be used to calculate the value of these constants aL and aR over a wide range of operating conditions in accordance with Equations 1 and 2. Once the values of these constants are obtained for a given fuel and diluent, then the lean and/or rich flammability limits can be easily and |