Date of Award
Doctor of Philosophy
Drop vaporization modeling is an important stage in spray combustion simulation. To improve the overall accuracy of multidimensional engine simulation, drop vaporization models based on multicomponent strategies was developed in this study. Petroleum fuels such as diesel fuel and gasoline are the major engine fuels composed of hundreds of hydrocarbon species. Therefore, a multicomponent model based on continuous thermodynamics (CT) was used for the vaporization modeling of petroleum fuels. Using the model, the molecular weights of petroleum fuel components is described by a gamma distribution, which is a probability distribution function. The model was applied in the vaporization simulation of single petroleum fuel drops. Results show a continuous shift in the molecular weight distribution and an increase in the mean molecular weight of the drop during the vaporization process, indicating that lighter components in the liquid drop vaporize earlier. The model was validated by simulating the diesel spray combustion in a constant volume combustion chamber, and good levels of agreement with experimental results were obtained in both flame structures and soot distributions.
The application of biofuels such as biodiesel and ethanol in engines has become increasingly important for reasons of environmental sustainability. Therefore, another emphasis of the present study is on the vaporization modeling of biofuels under engine operating conditions. Since biodiesel compositions are not universal due to the uncertainty in feedstock, biodiesel vaporization (BV) model and a discrete component (DC) approach based on its five dominant fatty acid components were developed for the fuel. Methods of predicting the critical and physical properties of biodiesel components were presented. Experimentally measured vaporization results of biodiesel were used to demonstrate the validity of the vaporization models. Moreover, biofuels are often used in engines by blending with petroleum fuels, and the detailed understanding of the vaporization process is essential to designing a clean and efficient combustion system. In the present study, a hybrid drop vaporization model was developed, i.e., continuous thermodynamics is used to describe the distribution of petroleum fuel components, and a discrete component approach was used to model biofuels. Practical fuel blends including those of diesel-biodiesel and gasoline-ethanol were studied.
Bio-oil is a bio-renewable fuel produced by fast pyrolysis of biomass and has complex composition due to the uncertainties in feedstock. The present study is further focused on the multicomponent vaporization modeling of bio-oil based on its major components selected from previous studies. A discrete component approach was developed and the physical properties of bio-oil major components were predicted. Results show a continuous variation of drop composition and indicate that levoglucosan survives other fuel components and dominates the late stage of the vaporization process. Vaporization behaviors of the mixtures of bio-oil and other practical fuels were also studied, and results show the much longer lifetime of bio-oil components than other fuels.
In addition to the consideration of fuel components, drop vaporization modeling under high-pressure conditions is also emphasized in this study. Traditional vaporization models are based on the Raoult's law, which is a phase equilibrium only true at low-pressure conditions. However, in engine operating conditions, liquid fuel drops vaporize in high-pressure environments. To extend the multicomponent vaporization models to high-pressure conditions, continuous thermodynamics was coupled general phase equilibrium which is characterized by the equality of the fugacity of each component in both phases. Correlations were developed to relate the parameters of Peng-Robinson equation of state and fugacity coefficients with molecular weight. Equations to calculate the molecular weight distribution in the vapor phase were also derived. The high-pressure vaporization model was validated using the experimental data of n-heptane drops under different ambient temperatures and pressures. Good levels of agreement between the predicted and measured drop vaporization histories were obtained. The high-pressure model was used to predict the diesel-biodiesel drop vaporization in diesel engine conditions. Results show that the drop lifetime increases with the biodiesel volume fraction in the fuel blend. Only the lighter components of diesel fuel vaporize at the beginning, and biodiesel components do not vaporize until sometime in the vaporization process. This phenomenon was further confirmed by the predicted distributions of both fuel vapors resulting from a biodiesel-diesel spray.
Zhang, Lei, "Multicomponent drop vaporization modeling of petroleum and biofuel mixtures" (2011). Graduate Theses and Dissertations. 10268.