OCR Text |
Show - 14 - EFFECTS OF FUEL TYPE Methane is well known to exhibit ignition delay times substantially longer than other n-alkane fuels [10] or mixtures of methane and other fuels such as ethane or propane [6, 7, 12, 13, 14] which are commonly present in natural gas. Model computations were carried out for a series of propane-air mixtures and the results compared with those for methane-air mixtures. At each value of * and f, values of x were found to be significantly smaller than those computed with methane as the fuel (Figures 2 and 3). Retaining a limiting value of T ^ 1 msec for stable operation, the model predicts a lean limit for propane of $ ^0.35. At this time it is not clear whether or not this difference in the lean limit could actually be exploited experimentally, since as pointed out by Keller [15], the intake process will change as the fuel is changed from methane to propane, even though all of the physical equipment in unchanged. That is, because propane is so much larger a molecule than methane, a much smaller mass and volume of propane is needed to provide the necessary fuel for the oxygen. An inlet jet of propane at $ = 0.35 will not possess the same momentum, energy, and mixing rate as an inlet jet for methane at $ = 0.5. Although the use of propane as a fuel may not be as directly relevant to current pulse combustion applications as methane or natural gas, some experimental analysis of propane in existing pulse combustors would help efforts to understand the relative roles of mixing and kinetics. An alternative means of changing the methane ignition delay time without significantly changing the intake properties is to add small amounts of ethane to the methane. This also provides a fuel mixture which is nearer to natural gas than is pure methane. As noted earlier, both experimental studies and modeling analyses have shown that the presence of |