Single-component Surrogate Research Articles

Large eddy simulation of spray and combustion characteristics of biodiesel and biodiesel/butanol blend fuels in internal combustion engines

Biofuel is a crucial renewable and environmentally friendly energy source for addressing greenhouse gas emissions and other energy-related issues. Biodiesel and butanol, among alternative biofuels, possess complementary physical and chemical properties, offering multiple possibilities for their use in existing internal combustion engines. However, biodiesel’s distinctly different physical and combustion properties from conventional diesel fuels make its combustion process substantially different. The complex composition of biodiesel presents significant challenges in accurately simulating its spray combustion characteristics. This paper presents a systematic evaluation of six single-component surrogate fuel models and a five-component model for the prediction of biodiesel spray characteristics under various conditions using large-eddy simulation (LES). The results show that single-component surrogate fuel models can only predict the gaseous penetration of biodiesel but not the liquid-phase penetration. A five-component fatty acid methyl ester surrogate fuel model is proposed, demonstrating an accurate simulation of biodiesel spray evaporation characteristics under different conditions. Based on the five-component evaporation model, LES is utilized to examine three strategies of biodiesel/butanol-fueled internal combustion engines: direct injection of pure biodiesel in conventional diffusion-controlled combustion (CDC) engines, direct injection of biodiesel–butanol blend in CDC engines, and biodiesel/butanol reactivity-controlled compression ignition (RCCI) engines. The simulation results are validated against engine experiment results, showing that the five-component model can successfully predict spray and combustion characteristics in internal combustion engines. The RCCI concept can significantly reduce NOx emissions; however, CO and UHC emissions are higher than in the CDC engines due to incomplete combustion in the fuel-lean butanol/air mixture.

Development of a practical reaction model of polycyclic aromatic hydrocarbon (PAH) formation and oxidation for diesel surrogate fuel

A practical reaction model for the formation and oxidation of polycyclic aromatic hydrocarbons (PAHs) up to coronene (A7) was developed focusing on two diesel surrogate fuels, i.e., a single-component surrogate including n-heptane and a multi-component surrogate including iso-octane, n-decane, methylcyclohexane, and toluene, for engine simulations. The important PAH relevant reactions are identified by using the global sensitivity analysis of the up-to-date detailed mechanisms. In the reduced PAH reaction model, the up-to-date reaction channels involving PAH formation were considered, and the variations and characteristics of the formation pathways of benzene and large PAHs for large alkanes, aromatics, and cycloalkanes with the fuel molecular structure were captured with the present under wide operating conditions. By integrating the PAH reaction model with a reduced oxidation mechanism of the two diesel surrogates, the final chemical mechanism of diesel oxidation consists of 137 species and 447 reactions. The chemical mechanism was extensively validated against the fundamental experimental data including ignition delay times in shock tubes and species profiles in premixed laminar flames, diffusion flames, and jet-stirred reactors (JSR). The performance of the mechanism for the predictions of PAHs from the oxidation of individual component, their mixtures, and diesel fuel was comprehensively evaluated. The computational results indicate that the effect of fuel structure on the PAH formation is satisfactorily reproduced by the present mechanism. Based on pathway analysis in the premixed flame, the contribution of the components in the diesel surrogate for the PAH formation behavior is understood.

An adaptive region segmentation combining surrogate model applied to correlate design variables and performance parameters in a transonic axial compressor

Surrogate models have been widely applied to correlate design variables and performance parameters in turbomachinery optimization applications. With more design variables and uncertain factors taken into account in an optimization design problem, the mathematical relations between the design variables and the performance parameters might present linear, low-order nonlinear or even high-order nonlinear characteristics, and are usually analytically unknown. Therefore, it is required that surrogate models have high adaptability and prediction accuracy for both the linear and nonlinear characteristics. The paper mainly investigates the effectiveness of an adaptive region segmentation combining surrogate model based on support vector regression and kriging model applied to a transonic axial compressor to approximate the complicated relationships between geometrical variables and objective performance outputs with different sampling methods and sizes. The purpose is to explore the prediction accuracy and computational efficiency of this adaptive surrogate model in real turbomachinery applications. Three different sampling techniques are studied: (1) uniform design; (2) Latin hypercube sampling method; (3) Sobol quasi-random design. For the low dimensional case with five variables, the adaptive region segmentation combining surrogate model performs better (not worse) than the single component surrogate in terms of prediction accuracy and computational efficiency. In the meanwhile, it is also noted that the uniform design applied to the adaptive surrogate model has more advantages over the Latin hypercube sampling method especially for the small sample size cases, both performing better than the Sobol quasi-random design. Moreover, a high dimensional case with 12 variables is also utilized to further validate the prediction advantage of the adaptive region segmentation combining surrogate model over the single component surrogate, and the computational results favor it. Overall, the adaptive region segmentation combining surrogate model has produced acceptable to high prediction accuracy in presenting complex relationships between the geometrical variables and the objective performance outputs and performed robustly for a transonic axial compressor problem.

Insights into the Effects of Mechanism Reduction on the Performance of n-Decane and Its Ability to Act as a Single-Component Surrogate for Jet Fuels

In this study, a detailed chemical reaction mechanism of n-decane containing 1034 species and 4268 reactions has been reduced at three different reduction levels to study the effects of subsequent mechanism reductions on the performance of n-decane. The detailed and reduced mechanisms were then used to validate ignition delays, laminar flame speeds, flame species, and species in a jet-stirred reactor. The one-half reduced mechanism performed nearly the same as the detailed mechanism in most of the cases. The one-fourth and one-eighth reduced mechanisms performed fairly well as compared to the detailed mechanism in some cases. The differences were further elaborated by sensitivity analyses of ignition delays and laminar flame speeds at different conditions followed by reaction pathway analysis of the detailed and one-eighth reduced mechanisms. These analyses indicated the absence or presence of certain reaction classes in reduced mechanisms that shaped the particular behavior of the mechanisms. To evaluate the capability of n-decane as a single-component surrogate for jet fuels, the experimental data of real-life jet fuels were used to validate the ignition delays and laminar flame speeds using the reaction mechanisms. Among all of the tested fuels, the ignition delay of jet A was reproduced fairly well by the detailed mechanism followed by jet S8 and jet RP-3 fuels with a noticeable discrepancy in the negative temperature coefficient (NTC) region. The one-eight reduced mechanism performed well in the NTC region. The laminar flame speeds of jet A and jet S8 fuels were predicted quite well by the detailed mechanism with the one-fourth reduced mechanism performing well at fuel-lean conditions. This strengthened the capability of n-decane as a single-component surrogate for jet fuels.

Multicomponent Evaporation Characteristics of Biodiesel Sprays

The vapor mass fraction distribution of the multicomponent surrogate of diesel and biodiesel fuel is studied using multicomponent evaporation model at engine-relevant conditions. The numerical model is validated with experimental liquid tip penetration and vapor penetration for diesel and biodiesel. The vapor distribution for a multicomponent surrogate is significantly different than that of single-component fuel. The tetradecane, single-component surrogate for diesel has different vapor mass distribution unlike in multicomponent diesel fuel. The spatial mass fraction distribution profile of biodiesel is found to be the function of volatility differential of the pure components and their proportion in the composition. The vapor mass fraction distribution for biodiesel of palm and soybean is found to be similar to that of their single-component surrogate due to comparable volatility of components of these biodiesels. However, methyl laurate in coconut biodiesel has volatility differential with other components and also has a significant share of 40% to 50% in the coconut biodiesel composition. The change in percentage of methyl laurate in coconut composition is found to have strong impact on vapor distribution. The vapor mass fraction distribution is observed to be affected by the change in the biodiesel composition specifically volatile component.

Effects of Fuel Composition on Charge Preparation, Combustion and Knock Tendency in a High Performance GDI Engine. Part II: LES Analysis

As discussed in the Part I of this paper, a numerical activity is carried out in order to analyse the effects of fuel composition modelling in a turbocharged GDI engine for sport car applications. While Part I analyses the “ensemble averaged” macroscopic effects on spray evolution, mixture stratification, combustion and knock tendency, in Part II of this paper cycle-to-cycle variations are analysed and discussed using a multi-cycle LES numerical framework, again comparing results from a more traditional single-component fuel surrogate model to those of a multi-component one. A purposely developed numerical approach is applied to properly account for the effects of the Discrete-Continuous-Multi-Component fuel formulation on the charge preparation: just before the spark timing, each vaporized fuel fraction is lumped back into a single-component surrogate fuel to allow the combustion model (ECFM-3Z, in its LES formulation) to take place. At the beginning of a new injection process, the numerical framework for the injected spray is switched back to Multi-Component, thus allowing each fuel fraction to independently spread, vaporize and diffuse in the combustion chamber according to the cycle-specific characteristics. A detailed comparison between the two fuel formulations is carried out on both average and rms values of the most influencing fields just before the spark discharge.

Low temperature oxidation of n-hexane in a flow reactor

The risk of igniting a flammable mixture in fuel tank vapor space is a major concern in aviation safety. In order to analyze the hazards and develop mitigation strategies, it is necessary to characterize the explosive properties of kerosene vapor–air mixtures over wide ranges of initial conditions. n-Hexane has been extensively used in our laboratory as a single component surrogate of kerosene. In the present study, hexane oxidation by oxygen was studied in a flow reactor at equivalence ratios of 0.7, 1 and 1.5 for mixtures diluted at 90% with nitrogen. Residence time was set at 2s and the pressure at 100kPa. The evolution of the gas phase composition at the reactor exit was studied over the range 450–1000K. Laser-based diagnostics and gas chromatography analysis were used to characterize the exit mixture composition. The chemical species measurements revealed three distinct regimes of oxidation, namely (i) the cool flame region from 600 to 650K, (ii) the NTC region between 675 and 775K, and (iii) the high temperature oxidation regime from 800K. The modeling study demonstrated the capability of reproducing most of the trends observed experimentally.

Ignition behavior and surrogate modeling of JP-8 and of camelina and tallow hydrotreated renewable jet fuels at low temperatures

The autoignition characteristics of the conventional jet fuel, JP-8, and the alternative jet fuels, camelina and tallow hydrotreated renewable jet (HRJ) fuels, are investigated using a rapid compression machine and the direct test chamber charge preparation approach. Ignition delay measurements are made at low compressed temperatures (625K⩽Tc⩽730K), compressed pressures of pc=5, 10, and 20bar, and equivalence ratios of ϕ=0.25, 0.5 and 1.0 in air. The HRJ fuels ignite more readily than JP-8 for all tested conditions, consistent with derived cetane number data in the literature. The camelina and tallow HRJ fuels exhibit similar autoignition characteristics, but the two fuels can be distinguished under stoichiometric conditions. Kinetic modeling is conducted with a 2-component surrogate (10% n-dodecane/90% 2-methylundecane) and a single component surrogate (2-methylnonane) to evaluate the potential to predict ignition behavior of the HRJ fuels. Modeling results indicate that the surrogate fuels can only provide useful predictions at a limited set of conditions (pc=5bar and ϕ=1.0), and that the agreement of the model and experimental data improves with decreasing compressed pressure. Under most conditions, the 2-component surrogate provides better prediction of ignition behavior, but the single component surrogate is superior at low pressures near the negative temperature coefficient region.

Experimental characterization of aviation-fuel cavitation

The results of an experimental investigation of the gaseous cavitation of JP-8 aviation fuel in a converging-diverging nozzle are presented. Fuel cavitation is experimentally characterized by high-speed digital imaging, static pressure distributions, and nonintrusive void fraction and bubble velocity measurements. For comparative purposes, experiments were performed using distilled water and dodecane for the same nozzle and nozzle pressure ratios. Dodecane, the largest component of JP-8 by weight, served as its single-component surrogate. For each working fluid, the experiments examined two different flow regimes: an initially single-phase liquid flow in which no cavitation occurred and another that evolved into two-phase cavitating flow. Additional experiments were performed to study the effect of air bubbles injected into either water or JP-8 at the nozzle inlet. For a sufficiently low range of imposed back pressures, gaseous cavitation led to choked flow for each working fluid. The character of the cavitation in the three fluids was different. These differences are highlighted and plausible mechanisms responsible for the observed behavior are discussed.

EVAPORATIVE DIESEL SPRAY MODELING

A one-dimensional nonevaporative spray model has been extended to take into account the effects of evaporation under direct-injection diesel engine conditions. The approach is based on the assumption that evaporation is limited by the mixing process between fuel and ambient gas, and makes use of nonideal gas state relationships for the description of the whole spray. Experimental results have been obtained by means of backlight illumination from a hot spray test rig. Comparisons between experimental and modeling results confirm the initial hypotheses. The model accurately predicts the influence of fuel type and both ambient and injection conditions on liquid spray penetration. An adequate prediction of the liquid penetration of a reference diesel fuel is achieved by considering the state relationships of a single-component surrogate fuel.