Modelin combustion of multicomponent fuel droplets: formulation and application to transportation fuels

摘要

The quasi-steady, spherically symmetric combustion of multicomponent isolated fueldroplets has been modeled using modified Shvab-Zeldovich variable mechanism. Newlydeveloped modified Shvab-Zeldovich equations have been used to describe the gas phasereactions. Vapor-liquid equilibrium model has been applied to describe the phase change at thedroplet surface. Constant gas phase specific heats are assumed. The liquid phase is assumed tobe of uniform composition and temperature. Radiative heat transfer between the droplet andsurroundings is neglected.The results of evaporation of gasoline with discrete composition of hydrocarbons have beenpresented. The evaporation rates seem to follow the pattern of volatility differentials. Theevaporation rate constant was obtained as 0.344mm2/sec which compared well with the unsteadyresults of Reitz et al. The total evaporation time of the droplet at an ambience of 1000K wasestimated to be around 0.63 seconds. Next, the results of evaporation of representative dieselfuels have been compared with previously reported experimental data. The previous experimentsshowed sufficient liquid phase diffusional resistance in the droplet. Numerical results areconsistent with the qualitative behavior of the experiments. The quantitative deviation during thevaporization process can be attributed to the diffusion time inside the droplet which isunaccounted for in the model. Transient evaporation results have also been presented for therepresentative diesel droplets. The droplet temperature profile indicates that the droplettemperature does not reach an instantaneous steady state as in the case of single-componentevaporation.To perform similar combustion calculations for multicomponent fuel droplets, no simplemodel existed prior to this work. Accordingly, a new simplified approximate mechanism formulticomponent combustion of fuel droplets has been developed and validated against severalindependent data sets. The new mechanism is simple enough to be used for computationalstudies of multicomponent droplets.The new modified Shvab-Zeldovich mechanism for multicomponent droplet combustion hasbeen used to model the combustion characteristics of a binary alcohol-alkane droplet andvalidated against experimental data. Burn rate for the binary droplet of octanol-undecane wasestimated to be 1.17mm2/sec in good concurrence with the experimental value of 0.952mm2/secobtained by Law and Law. The model has then been used to evaluate the combustioncharacteristics of diesel fuels assuming only gas phase reactions. Flame sheet approximation hasbeen invoked in the formulation of the model.