Study On Laminar-premixed Combustion Characteristics Of Butanol Blended Fuels

In recent years, since the depletion of fossil fuel and many environmental issuesdue to its heavy use, considerable efforts have been spent on finding clean renewablealternative fuels, and n-butanol is the right one of them. Compared with ethanol, awidely used biofuel in the market, n-butanol has a higher heating value, and itsfugacity and corrosivity are weaker. The physical and chemical properties ofn-butanol are similar to those of gasoline, and n-butanol and gasoline can be solublewith each other at any proportion. Therefore n-butanol is a promising alternative fuel.At present, the research about n-butanol is mainly on engine test benches, and there isless study on fundamental combustion characteristics of n-butanol. Thus in thisdissertation, the premixed laminar combustion characteristics of n-butanol areinvestigated experimentally and numerically.Premixed laminar flames of n-butanol–ethanol/n-heptane/iso-octane–airmixtures at different equivalence ratios and initial pressures are experimentallystudied by employing outwardly expanding spherical flames. The effects of initialconditions and additives on laminar burning velocity and flame instability ofn-butanol–air flames are detailedly analyzed. The results show that both unstretchedflame speeds and laminar burning velocities of the mixtures studied first increase andthen decrease with the increasing equivalence ratio, and they reach their peaks atequivalence ratio of about1.1. When elevating initial pressure, the laminar burningvelocity decreases, and the cracks on the flame surfaces appear earlier, indicating thatthe flames tend to be more instable. Among these four fuels, the flames of ethanolpropagate fastest, followed by flames of n-butanol and n-heptane, and the iso-octaneflames propagate slowest. When adding n-heptane and iso-octane into n-butanol–airmixtures, the laminar burning velocity decreases, but the Markstein length increases,implying the flames become more stable. The opposite phenomena are observedwhen ethanol is added into n-butanol–air mixtures.One-dimensional plane flames of n-butanol–air mixtures at different equivalenceratios and initial pressures are simulated with CHEMKIN PRO software. Structuresof one-dimensional flames and sensitivity analysis of laminar burning velocity arepresented. The results show that at lean-fuel side, the experimental values of laminar burning velocity agree well with the similated values, while at equivalence ratioabove1.2, the experimental values are larger than the modeled values. In theone-dimensional flame structures, when increasing the initial pressure, the final flametemperature increases, but the molar fractions of main species remain unchanged. Thelaminar burning velocity of n-butanol–air flames are insensitive to the elementaryreactions related to n-butanol itself, but sensitive to those involving OH, O and H.