It is well known that diesel fuel consist a large number of hydrocarbons with one or more carbon double bond in their molecular structure. Various studies have been conducted to conform that the presence of the double bond affects the fuel reactivity and its flame structure. To perform the computational fluid dynamics (CFD) analysis of the fuel combustion property with the presence of the unsaturated carbon to carbon double bond, various detailed and reduced kinetics models have developed. Along with the computational studies, numerous experimental studies have also been conducted using shock tube and RCM to study the effect of double bond on the fuel ignitability. The major outcomes from these studies are that at lower temperature the ignition delay is quite higher for alkenes (hydrocarbon containing double bond) in comparison to the alkanes. At intermediate temperature (1000 K-1200 K), the ignition delay is dependent on the radicals formed by the decomposition of the stabilized radicals present in the combustion chamber. On the other hand, at higher temperature (> 1300K), the ignition phenomenon is determined by the initiation reactions. Presence of the double bond helps in the formation of allyl radicals which are more energetic in comparison to the methyl radicals formed by the decomposition of the alkanes. Therefore, the alkenes have shorter ignition delay than alkanes at higher temperature.
Numerous engine experiments using different biodiesel fuels have also been performed to study the effect of unsaturated bond on the NOx and soot emissions. But it is not feasible to study effect of presence of double bond on the flame structure through an engine experiment. Therefore a recent numerical study has been conducted using the Sandia constant volume reactor to simulate more realistic spray flames and to study the effect of the unsaturated bond on the flame structure and combustion phenomenon in spray flames under diesel engine conditions.
The above mentioned study was only conducted for higher initial temperature (at 1300K) in order to study the n-heptane and 1-heptene flame characteristics on the basis of same ignition delay. But there is no numerical study has been reported yet for the lower and intermediate initial temperature. The present study focuses on the effect of presence of double bond on the ignition and flame structure of the n-heptane and 1-heptene flame at the initial temperature ranging from 1000K to 1200K in the Sandia constant volume reactor at the diesel engine condition. This reactor is being extensively used to produce the experimental results, which can be downloaded from the Engine Combustion Network (ECN) website, for the reacting and non-reacting spray simulations.
N-heptane and 1-heptene flames are simulated by using the reduced CRECK mechanism. The simulations have been performed at the initial temperature of 1000K, 1100K and 1200K temperature. Since at lower temperature (<1000K) the ignition delay is too high for 1-heptene flame and therefore it is not possible to perform the simulation at that temperature because of the lack of the computational power. Therefore these three intermediate temperatures has been chosen to study the effect of initial temperature and the presence of unsaturated bond on the ignition and flame structure of the spray flames. The flame structure of n-heptane and 1-heptene flame is shown by the contour plot of temperature and the heat release rate and by the scatter plot of heat release rate in a ɸ/T plane. Finally, the formation of ignition kernels has been discussed in order to study the flame structure of the n-heptane and 1-heptene spray flame.
The second part of the thesis focuses on the effect of the presence of double bond on the ignition and combustion of biodiesel/alcohol fuel blends. Partially premixed flames are simulated in a constant-volume reactors using different n-heptane/iso-pentanol and 1-heptene/iso-pentanol blends. A kinetic model for the blends is developed by combining a reduced mechanism for iso-pentanol with a reduced CRECK mechanism for n-heptane and 1-heptene. Four different blends considered include 90%n heptane/10%iso-pentanol (by volume), 80%n-heptane/20%iso-pentanol, 70%n-heptane/30%iso-pentanol, and 50%n-heptane/50%iso-pentanol. To examine the effect of the presence of double bond, similar blends are considered with 1-heptene and pentanol. The main conclusion drawn from this study is that when iso-pentanol was added with n-heptane, the ignition delay increases whereas when the blend was formed with 1-heptene, the ignition delay decreases and there was a significant variation in the location of the formation of ignition kernel. As the concentration of iso-pentanol was increased in the 1-heptene/iso-pentanol blend, the zone in which ignition occur shifts from lean premixed to non-premixed reaction zone. Fuel vapor penetration of both the fuels shows that the ignition is controlled by the presence of iso-pentanol which explains the shift in the ignition kernel formation location
