Guang-Sheng Zhu

A model for high-pressure vaporization of droplets of complex liquid mixtures using continuous thermodynamics

Abstract This paper presents a comprehensive model for the transient high-pressure vaporization process of droplets of complex liquid mixtures with large number of components in which the mixture composition, the mixture properties, and the vapor–liquid equilibrium (VLE) are described by using the theory of continuous thermodynamics. Transport equations, which are general for the moments and independent of the distribution functions, are derived for the semi-continuous systems of both gas and liquid phases. A general treatment of the VLE is conducted which can be applied with any cubic equation of state (EOS). Relations for the properties of the continuous species are formulated. The model was further applied to calculate the sub- and super-critical vaporization processes of droplets of a representative petroleum fuel mixture – diesel fuel. The results show that the liquid mixture droplet exhibits an intrinsic transient vaporization behavior regardless of whether the pressure is sub- or super-critical. The regression rate of the liquid mixture droplet is reduced significantly during the late vaporization period. The comparison with the results of a single-component substitute fuel case emphasizes the importance of considering the multi-component nature of practical mixture fuel and the critical vaporization effects in practical applications. This paper provides a practical means for more realistically describing the high-pressure vaporization processes of practical fuels.

Gas-phase unsteadiness and its influence on droplet vaporization in sub- and super-critical environments

Abstract This paper aims to investigate quantitatively the influence of gas-phase unsteadiness on the droplet vaporization process in sub- and super-critical environments. Two comprehensive models of high-pressure droplet vaporization, including a transient model and another assuming gas-phase quasi-steadiness, are presented. Both models are first compared with experimental data and then used to calculate vaporization processes of single droplets of different initial sizes for environmental conditions in which the ambient pressure and temperature range from 1–150 atm and 500–2000 K, respectively. The unsteady effects are quantified by introducing characteristic time scale ratios. It is shown that strong gas-phase unsteadiness exists during the early period of the vaporization process. The unsteadiness attains a maximum value in the gas near the droplet surface and decreases quickly to a nearly steady value within a short distance from the surface. With increasing ambient pressure, the unsteadiness increases nearly linearly at low ambient temperatures and rapidly at high ambient temperature. Gas-phase unsteadiness also increases with increasing ambient temperature and is affected even more strongly by temperature. Compared to the transient model, the quasi-steady model predicts a smaller regression rate initially and a larger regression rate during the later period. The differences between the predicted regression rates, and thus between the predicted vaporization processes, are magnified with increasing ambient temperatures and/or pressures. The vaporization process predicted using the quasi-steady model reaches the critical mixing state earlier than that predicted using the transient model. These conclusions also apply for the vaporization processes of single droplets of different initial sizes.

Engine Fuel Droplet High-Pressure Vaporization Modeling

The objective of this investigation was to characterize the high-pressure vaporization processes of engine fuels, which are too complex in composition to be described with conventional methods. To do so a comprehensive model was developed for the transient vaporization process of droplets of practical engine fuels using continuous thermodynamics in which high-pressure effects are fully considered. Transport equations are derived in a spherical coordinate system for the semi-continuous systems of both gas and liquid phases. A general treatment of vapor-liquid equilibrium is presented, which can be applied with any type of cubic equation of state. Relations for the properties of the continuous species are formulated. The model is further applied to calculate the transient high-pressure vaporization processes of droplets of representative engine fuels-diesel and gasoline. The high-pressure vaporization processes of droplets of two single-component fuels are also predicted for comparison. The results clarify the characteristics of the vaporization processes of engine fuel droplets and indicate the significant effects of fuel type on the vaporization behavior. The comparison with the results of the single-component cases also emphasizes the importance of considering the influence of multicomponent fuels in practical applications.

On Non-Equilibrium Turbulence Corrections in Multidimensional HSDI Diesel Engine Computations

The introduction of high-pressure injection systems in D.I. diesel engines has highlighted already known drawbacks of in-cylinder turbulence modeling. In particular, the well known equilibrium hypothesis is far from being valid even during the compression stroke and moreover during the spray injection and combustion processes when turbulence energy transfer between scales occurs under non-equilibrium conditions. The present paper focuses on modeling in-cylinder engine turbulent flows. Turbulence is accounted for by using the RNG k-e model which is based on equilibrium turbulence assumptions. By using a modified version of the Kiva-3 code, different mathematically based corrections to the computed macro length scale are proposed in order to account for non-equilibrium effects. These new approaches are applied to a simulation of a recent generation HSDI Diesel engine at both full load and partial load conditions representative of the emission EUDC cycle. The numerical results show that the proposed corrections improve the physical behavior of the combustion model by a self-scaling of the eddy-turnover time depending on the engine operating conditions. The overall achievement is the extension of modeling reliability over a wider range of operating conditions and in particular over those that are of interest in the European emission test cycle. INTRODUCTION Complying with automotive emission standards is a very complicated task since a reduction of NOx and soot engine-out levels is continuously required to limit air pollution. Engine performance and emissions are controlled by a great number of parameters related to the injection system, combustion chamber geometry, boost pressure and EGR percentage. Due to the expense of experimental investigations on engines, the development process must be supported by CFD simulations in order to reduce the cost and time required to bring a new engine into the market. In order to be useful in engine design, CFD has to be reliable. Models must describe correctly the in-cylinder processes and they must require very limited tuning of the empirical model constants. Despite the fact the many efforts have been spent in order to provide reliable predictions over a wide range of engine operating conditions, spray combustion simulations have often followed experimental development of engine combustion chambers and injection systems due to shortcomings in the models used and the limited accuracy of experimental data used as input parameters [1]. As it is well known, good NOx and soot engineout level predictions are strictly linked not only to the models themselves but also to the accuracy in determining local equivalence ratio and turbulence distribution. The available models for NOx and soot have proved to be able to capture at least the trade-off if local conditions are correctly predicted [2,3]. Unfortunately, as pointed out earlier, in almost all practical cases a proper 2001-01-0997 On Non-Equilibrium Turbulence Corrections in Multidimensional HSDI Diesel Engine Computations G. M. Bianchi and P. Pelloni DIEM University of Bologna G.-S. Zhu and R. Reitz University of Wisconsin Madison Copyright

A Nonequilibrium Turbulence Dissipation Correction and Its Influence on Pollution Predictions for DI Diesel Engines

A correction for the turbulence dissipation rate, based on nonequilibrium turbulence considerations from rapid distortion theory, has been derived and implemented in combination with the RNG k-e model in a KIVA-based code. This correction reflects the time delay between changes in the turbulent kinetic energy due to changes in the mean flow and its turbulence dissipation rate, and it is shown that this time delay is controlled by the turbulence Reynolds number. The model correction has been validated with experimental data in the compression and expansion phase of a small diesel engine operated in motored mode. Combustion simulations of two heavy-duty DI diesel engines have been performed with the RNG k-e model and the dissipation rate correction. The focus of these computations has been on the nitric oxide formation and the net soot production. These simulations have been compared with experimental data and their preditions are explained in terms of the turbulence dissipation effect on the transport coefficients for mass and heat diffusion. It has been found, that the dissipation correction yields consistent results with observations reported in previous studies.