Multicomponent Fuels Research Articles

A physics-based approach to modeling real-fuel combustion chemistry - I. Evidence from experiments, and thermodynamic, chemical kinetic and statistical considerations

Real distillate fuels usually contain thousands of hydrocarbon components. Over a wide range of combustion conditions, large hydrocarbon molecules undergo thermal decomposition to form a small set of low molecular weight fragments. In the case of conventional petroleum-derived fuels, the composition variation of the decomposition products is washed out due to the principle of large component number in real, multicomponent fuels. From a joint consideration of elemental conservation, thermodynamics and chemical kinetics, it is shown that the composition of the thermal decomposition products is a weak function of the thermodynamic condition, the fuel-oxidizer ratio and the fuel composition within the range of temperatures of relevance to flames and high temperature ignition. Based on these findings, we explore a hybrid chemistry (HyChem) approach to modeling the high-temperature oxidation of real, distillate fuels. In this approach, the kinetics of thermal and oxidative pyrolysis of the fuel is modeled using lumped kinetic parameters derived from experiments, while the oxidation of the pyrolysis fragments is described by a detailed reaction model. Sample model results are provided to support the HyChem approach.

Multicomponent fuel droplet combustion investigation using magnified high speed backlighting and shadowgraph imaging

The liquid-phase processes occurring during fuel droplet combustion are important in deciding the behaviour of the overall combustion process, especially, for the multicomponent fuel droplets. Hence, understanding these processes is essential for explaining the combustion of the multicomponent fuel droplet. However, the very fast combustion of the too small fuel droplet makes experimental investigation of these processes uneasily affordable. In the present work, a high speed backlighting and shadowgraph imaging of the multicomponent fuel droplet combustion including liquid-phase dynamics are performed. Two categories of multicomponent fuels – in which diesel is the base fuel – are prepared and utilized. The first category is biodiesel/diesel and bioethanol/diesel blends, while the second category is the water-in-diesel and diesel-in-water emulsions. Specific optical setups are developed and used for tracking droplet combustion. The first setup is associated with the backlighting imaging with the resulting magnification of the droplet images being 30 times the real size. The second optical setup is used for shadowgraph imaging, with the resulting magnification being 10 times the real size. Using these setups, spatial and temporal tracking of nucleation, bubble generation, internal circulation, puffing, microexplosion, and secondary atomization during the combustion of isolated multicomponent fuel droplets are performed. Spatial and temporal tracking of the sub-droplets generated by secondary atomization, and their subsequent combustion, in addition to their overall lifetimes have also been performed. Accordingly, a comparison of the burning rate constant between the parent droplet and the resulting sub-droplets is carried out. The rate of droplet secondary atomization is higher than those obtained by relatively low imaging rate. Additionally, it is shown that during a large portion of its entire lifetime, the droplet geometry has been affected by combustion significantly.

Fuel Property Effects on Liquid and Vapor Penetrations of Evaporating Sprays

The purpose of this study is to investigate the effect of fuel properties on liquid and vapor penetrations in evaporating spray systems. A recently developed model, which can simultaneously account for the finite thermal conductivity, finite mass diffusivity and turbulence effects within atomizing multi-component liquid fuel sprays, is utilized for the numerical predictions. Two different multi-component fuels with different boiling temperatures, densities and other thermal properties are implemented in the KIVA-3V computational fluid dynamics (CFD) code to study the evaporation behaviors. A six-component surrogate fuel is used to emulate the relevant volatility property of the real diesel fuel, and a second bi-component fuel is chosen to represent a low boiling-temperature fuel. The numerical results are compared with the experimental data, and the representative results are obtained. For a lower density and lower boiling temperature fuel, the liquid penetration length is shorter. However, the vapor penetration lengths are not affected by the fuel type in terms of fuel volatility. Available experimental data are used for validation and appraisal of the multi-component evaporation model.

多成分燃料に対応できる簡略化反応スキームを用いた HCCI 燃焼制御の検討

多成分燃料に対応できる簡略化反応スキームを用いた HCCI 燃焼制御の検討

Laser-Driven Calorimetry of Single-Component Liquid Hydrocarbons.

A novel laser-heating technique, referred to as the laser-driven thermal reactor (LDTR), was used to determine sample thermal (exothermic/endothermic) behavior, specific heat release rate, and total specific heat release of three volatile single-component liquid hydrocarbons, i.e., n-decane, n-butylcyclohexane and n-butylbenzene. The objective was to demonstrate measurement repeatability and extend the LDTR model from earlier investigations. The experimental apparatus consists of a copper sphere-shaped reactor mounted within a vacuum chamber, with sample and substrate supported on a thermocouple near the center of the reactor. The reactor is heated from opposing sides by a near-infrared laser to achieve nearly uniform sample temperature. The change in temperature with time (i.e., thermogram) is compared to a previously recorded baseline (no liquid sample) thermogram. A model, based on thermal energy conservation, is used to evaluate the thermograms for the thermochemical characteristics of interest. This study represents a step toward applying this technique to more complex volatile multi-component fuels of unspecified composition. Results for the LDTR were compared with a differential scanning calorimetry/thermal gravimetric analysis (DSC/TGA) instrument. The model analysis was extended to include an estimation of the hydrocarbon mass change with increasing temperature, based on the temporal change in the specific heat release rate. An estimate of the total specific heat release was obtained for these three liquid hydrocarbons and found to be consistent with the literature values when the measurements were carried out under suitable operating conditions.

Effects of the direct-injected fuel’s physical and chemical properties on dual-fuel combustion

An experimental and computational study was conducted to explore the effects of the physical properties of the high reactivity fuel in a Reactivity Controlled Compression Ignition (RCCI) combustion dual-fuel strategy. The objective is to systematically isolate the effects of the boiling characteristics of the direct injected fuel in dual-fuel combustion strategies with different levels of fuel stratification. In all studies, iso-octane was used as the low reactivity fuel. The effect of high reactivity fuel physical properties was investigated by comparing the results of engine experiments using two fuels with equal cetane numbers (CN), but different boiling characteristics. The two fuels are 1) a certification grade Ultra Low Sulfur Diesel (ULSD) fuel with a cetane number of 45 and 2) a blend of 21% iso-octane and 89% n-heptane with a cetane number of 45. Computational fluid dynamics (CFD) modeling using the KIVA-3v code with a discrete multi-component evaporation model capable of capturing important physical property influences and a multi-fuel chemistry model capable of describing the chemical kinetics of single and multi-component fuels was used to explain the observed differences in the experiments. It was found that the different boiling curves of the two fuels have minimal effect on the combustion phasing at early and late injection timings. Differences in the combustion phasing were explained by differences in the chemical characteristics of each fuel that could not be matched solely by fixing the CN.

Experimental investigation on the effect of n-pentanol blending on spray, ignition and combustion characteristics of waste cooking oil biodiesel

Due to their excellent physicochemical properties, biodiesel and n-pentanol are regarded as two promising alternative biofuels for automobile. However, the fundamental data of spray and combustion characteristics of n-pentanol/biodiesel blends are still scarce. The objective of this work is to investigate the effects of n-pentanol addition to waste cooking oil (WCO) biodiesel in different ratios (0%, 20%, and 40% in volume) on spray, ignition and combustion characteristics in a constant volume combustion bomb (CVCB). Ignition and combustion tests were performed at five ambient temperatures (800, 900, 1000, 1100 and 1200K) with three oxygen concentration (10%, 15% and 21%), and spray tests were conducted at the same temperature ranges but with no oxygen involved to prevent combustion happening. The result shows liquid length decreases with increase of ambient temperature, and the falling slope increases significantly with the blended n-pentanol concentration. The liquid length of B60P40 is about 6% longer than that of B100 at 800K condition, while it becomes about 10% shorter than B100 when ambient temperature rises to 1200K. Although the cetane number of n-pentanol is much less than biodiesel, an increase of n-pentanol ratio has a promoting effect on the ignition event under most conditions. Under these conditions, addition of 40% n-pentanol has cut the ignition delay of biodiesel, on average, by 30.2%. The natural flame luminosity is found to be reduced significantly with an increasing n-pentanol ratio, meaning that addition of n-pentanol could reduce the soot level under all conditions. For example, the time integrated natural luminosity (TINL) of B60P40 is only 41.2% of that of B100 under 1000K temperature conditions. The result also suggests that, for multi-component fuels, flame lift-off length (FL) is the most reliable factor that influence the soot concentration level under spray combustion processes, rather than ignition delay or soot formation time. Overall, blending n-pentanol into biodiesel has a great soot oxidation potential and could maintain suitable ignition phase at the same time.

Reconstruction of chemical structure of real fuel by surrogate formulation based upon combustion property targets

The global chemical character of complex chemical fuel mixtures is explicitly determined by evaluating the abundances of chemical functional groups present within them rather than by applying a traditional interpretation based upon molecular species composition. Statistical analyses of the relationships among each chemical functional group and specific combustion property targets (CPTs) of the fuel are rigorously developed. The results demonstrate that the four CPTs currently used in aviation kerosene surrogate formulation - H/C molar ratio, derived cetane number (DCN), average molecular weight (MW), and threshold sooting index (TSI) - effectively constrain the chemical functional group distribution of the fuel, and, hence, the global combustion behaviors of pre-vaporized fuel/air mixtures. Successful emulation of the CPTs for a target real fuel involves developing a surrogate mixture that defines an “equivalent” chemical functional group distribution to that of the target fuel. Among the CPTs used for real fuel surrogate development, DCN does not abide by a linear blending rule, which generally frustrates development of surrogates. However, a quantitative structure–property relation (QSPR) regression for DCN is demonstrated here using the chemical functional group approach. Results of the regression reveal that the (CH2)n group plays the most significant role in determining the fuel autoignition propensity, followed by the influences of CH3 and benzyl-type groups. The QSPR functional group approach extends to provide a powerful tool to address potential preferential vaporization effects dictated by fuel distillation characteristics. Further analysis of fuel chemical property variation (DCN and H/C ratio) over the distillation curve (and other physical properties) provides a foundation for understanding the complex combustion behaviors of multi-phase and multi-component fuels relevant to real gas turbine engine applications.

5-2-4 多成分燃料における液滴燃焼時の二次微粒化発生に関する研究

5-2-4 多成分燃料における液滴燃焼時の二次微粒化発生に関する研究

Vaporization order and burning efficiency of crude oils during in-situ burning on water

In order to improve the understanding of the burning efficiency and its observed size dependency of in-situ burning of crude oil on water, the vaporization order of the components in crude oils was studied. The vaporization order of such multicomponent fuels was assessed by studying the surface temperature, flame height, burning rate and burn residues of three alkanes (n-octane, dodecane and hexadecane), a mixture of these alkanes (1:1:1 volumetric ratio) and two crude oils (light and medium-light crudes). The experimental results were compared to four models for the vaporization order of multicomponent fuels. The alkanes were tested as benchmark fuels with a uniform vaporization order, for which all components evaporate simultaneously. As expected, these pure fuels showed a steady state burning with a near-constant surface temperature, flame height and burning rate. The alkane mixture showed similar steady state results but became dominated by the heaviest component towards the end of the burning. These results indicate that the lightest components had been depleted from the mixture. A near-uniform vaporization order in which the lighter components evaporate preferably best matched these results. The crude oils did not show any steady state behavior, but instead had an increasing surface temperature and decreasing burning rate and flame height, indicating a volatility controlled vaporization order. An increasing concentration gradient from the medium to heavy fraction in the burn residues furthermore showed that the vaporization was diffusion-limited. Analysis of the heat transfer balance for the crude oils indicated that the energy available for evaporation decreased over time due to increasing heat losses, which were caused by the volatility controlled vaporization order. Presumably, larger scale fires can overcome these heat losses, as they typically have higher burning rates, which increase the heat feedback to the fuel surface and therefore can result in the higher burning efficiencies.

Effects of iso-octane/ethanol blend ratios on the observance of negative temperature coefficient behavior within the Ignition Quality Tester

An ignition delay study investigating the reduction in low temperature heat release (LTHR) and negative temperature coefficient (NTC) region with increasing ethanol concentration in binary blends of ethanol/isooctane was conducted in the Ignition Quality Tester (IQT). The IQT is advantageous for studying multi-component fuels such as iso-octane/ethanol which are difficult to study at lower temperatures covering the NTC region in traditional systems (e.g., shock tubes, rapid compression machines, etc.). The high octane numbers and concomitant long ignition delay times of ethanol and iso-octane are ideal for study in the IQT allowing the system to reach a quasi-homogeneous mixture; allowing the effect of fuel chemistry on ignition delay to be investigated with minimal impact from the fuel spray due to the relatively long ignition times. NTC behavior from iso-octane/ethanol blends was observed for the first time using an IQT. Temperature sweeps of iso-octane/ethanol volumetric blends (100/0, 90/10, 80/20, 50/50, and 0/100) were conducted from 623 to 993K at 0.5, 1.0 and 1.5MPa and global equivalence ratios ranging from 0.7 to 1.0. Ignition of the iso-octane/ethanol blends in the IQT was also modeled using a 0-D homogeneous batch reactor model. Significant observations include: (1) NTC behavior was observed for ethanol/iso-octane fuel blends up to 20% ethanol. (2) Ethanol produced shorter ignition delay times than iso-octane in the high temperature region. (3) The initial increase in ethanol from 0% to 10% had a lesser impact on ignition delay than increasing ethanol from 10% to 20%. (4) The 0-D model predicts that at 0.5 and 1.0MPa ethanol produces the shortest ignition time in the high-temperature regime, as seen experimentally.

Coupling multicomponent droplet evaporation and tabulated chemistry combustion models for large-eddy simulations

An evaporation model for multicomponent fuels and emulsions is implemented in OpenFOAM within the Euler–Lagrangian formulation. The model is suitable for pressures typical of heavy duty gas turbines, up to 30bar, and considers an increased evaporation rate of the most volatile component when the droplet temperature reaches its boiling temperature. Evaporation of emulsion droplets is modeled following a shell approach, assuming that the water in the droplet core does not evaporate. A diffusivity model is introduced to account for the transport of the dispersed phase from the core to the outer shell. The combustion model is based on the Flamelet Generated Manifold approach. The mixture fraction is calculated from the species mass fractions to couple it with the evaporation model. Flamelet tables are created for different enthalpies and mixture fractions so that they can be used in non-premixed simulations. The thermo-chemistry model considers up to two fuel components by using 4-D tables. Experimental data for n-decane droplets are used to validate the evaporation model. Results from the combustion model are compared with the detailed chemistry solutions. Following the separate evaluation of each model, 1-D spray simulations for 0% and 10% initial water contents are performed to verify the coupling between the models, so that they can be applied to partially pre-vaporized spray flames.

Impingement characteristics of an early injection gasoline direct injection engine: A numerical study

This article describes the use of a Lagrangian discrete droplet model to evaluate the liquid fuel impingement characteristics on the internal surfaces of an early injection gasoline direct injection engine. This study focuses on fuel impingement on the intake valve and cylinder liner between start of injection and 20° after start of injection using both a single- and a multi-component fuels. The single-component fuel used was iso-octane and the multi-component fuel contained fractions of iso-pentane, iso-octane and n-decane to represent the light, medium and heavy fuel fractions of gasoline, respectively. A detailed description of the impingement and liquid film modelling approach is also provided. Fuel properties, wall surface temperature and droplet Weber number and Laplace number were used to quantify the impingement regime for different fuel fractions and correlated well with the predicted onset of liquid film formation. Evidence of film stripping was seen from the liquid film formed on the side of the intake valve head with subsequent ejected droplets being a likely source of unburned hydrocarbons and particulate matter emissions. Differences in impingement location and subsequent location of liquid film formation were also observed between single- and multi-component fuels. A qualitative comparison with experimental cylinder liner impingement data showed the model to well predict the timing and positioning of the liner fuel impingement.

Group contribution method for multicomponent evaporation with application to transportation fuels

Transportation fuels consist of a large number of species that belong to different families of compounds. Predictions of physical and chemical properties of such multicomponent fuels require the accurate description of evaporation rates of all species initially contained in the liquid phase. Existing computational approaches are yet to do this without difficulty due to the lack of experimental data for physical properties of fuel components. In addition, a vast number of assumptions, which include idealized compositions and interpolation equations calibrated using experiments, are being introduced to make the problem tractable. By addressing this issue, an approach based solely on functional groups that are present in the components of the fuel is developed. This formulation is coupled to a multicomponent evaporation model to account for non-ideal behavior of mixtures and the consideration of pressure and temperature variations. This is done by using the fuel composition to evaluate the acentric factor and critical properties of each fuel component followed by the evaluation of certain physical properties using a group contribution method. This procedure thus allows the evaluation of evaporation rates of practical fuels based only on the intrinsic functional groups and their relative abundance. Studies of common fuel blends including those belonging to the Jet-A and JP-8 classes related to evaporation rates, distillation characteristics and speciation are presented. A discussion of the effect of polar species on evaporation properties utilizing the UNIFAC approach for non-ideal mixtures is presented. Due to the generalized nature of this approach, the proposed method provides an effective alternative to existing computational techniques dedicated to multicomponent droplet evaporation.

The role of preferential evaporation on the ignition of multicomponent fuels in a homogeneous spray/air mixture

This work examines the importance of preferential evaporation and liquid species diffusion on the ignition of multicomponent surrogate fuels in homogeneous mixtures. To this end, a model is developed that considers the evaporation of a monodisperse droplet cloud in a homogeneous and isobaric gas-phase environment. The mathematical model accounts for physico-chemical relevant processes, involving (i) species diffusion in the liquid phase, (ii) two-phase thermodynamic equilibrium and (iii) gas-phase chemistry. After evaluating the accuracy of individual model components against available experimental data, the model is applied to investigate the ignition of a surrogate mixture for a Jet-A fuel (POSF 4658) at conditions of interest to gas turbine engines. A parametric study is performed to examine effects on the interaction between multicomponent evaporation and ignition. It is found that the maximum reactivity occurs for conditions at which the ignition time in the gas phase is comparable to the evaporation time. At these conditions, the mixture composition is significantly affected by preferential evaporation, which in turn affects the reactivity of the gas phase. It is shown that intradroplet diffusion plays a major role on the radial droplet composition, thereby conditioning the evaporation rates of single species. A comparison with a zero-diffusivity model for the treatment of droplet internal diffusion emphasizes the importance of liquid diffusion on the autoignition time, and confirms the need for properly accounting for a detailed description of multicomponent evaporation effects.

TECHNOLOGY FOR EFFICIENT USAGE OF HYDROCARBON-CONTAINING WASTE IN PRODUCTION OF MULTI-COMPONENT SOLID FUEL

The paper considers modern approaches to usage of hydrocarbon-containing waste as energy resources and presents description of investigations, statistic materials, analysis results on formation of hydrocarbon-containing waste in the Republic of Belarus. Main problems pertaining to usage of waste as a fuel and technologies for their application have been given in the paper. The paper describes main results of the investigations and a method for efficient application of viscous hydrocarbon-containing waste as an energy-packed component and a binding material while producing a solid fuel. A technological scheme, a prototype industrial unit which are necessary to realize a method for obtaining multi-component solid fuel are represented in the paper. A paper also provides a model of technological process with efficient sequence of technological operations and parameters of optimum component composition. Main factors exerting significant structure-formation influence in creation of structural composition of multi-component solid fuel have been presented in the paper. The paper gives a graphical representation of the principle for selection of mixture particles of various coarseness to form a solid fuel while using a briquetting method and comprising viscous hydrocarbon-containing waste. A dependence of dimensionless concentration g of emissions into atmosphere during burning of two-component solid fuel has been described in the paper. The paper analyzes an influence of the developed methodology for emission calculation of multi-component solid fuels and reveals a possibility to optimize the component composition in accordance with ecological function and individual peculiar features of fuel-burning equipment. Special features concerning storage and transportation, advantages and disadvantages, comparative characteristics, practical applicability of the developed multi-component solid fuel have been considered and presented in the paper. The paper cites practical results pertaining to usage of hydrocarboncontaining waste for the equipment applied for production of multi-component solid fuel. Data on economic expediency for usage of the multi-component solid fuel with high thermo-technical characteristics in the boiler-houses operating on local solid fuel have been analyzed in the paper. The paper shows a perspective evaluation, applicability and practical significance of the solution of the problem on efficient usage of hydrocarbon-containing waste while producing the multi-component solid fuel.

Development of a quasi-dimensional vaporization model for multi-component fuels focusing on forced convection and high temperature conditions

A new quasi-dimensional multi-component vaporization model considering the finite thermal conductivity and mass diffusivity within the droplet was constructed. First, the heat flux of conduction, enthalpy diffusion, and radiation absorption in the gas phase were calculated based on the Fourier’s law, a multi-diffusion sub-model, and a simplified analytical solution, respectively. The phase equilibrium at the gas–liquid interface was calculated by the ideal and real gas approaches according to the ambient pressure. For the liquid phase, the assumption of the quadratic polynomial distributions of the temperature and component concentration within the droplet was proposed in the quasi-dimensional model. Then, the proposed vaporization model was extensively validated by the experimental measurements, and good agreements were observed. Based on the computational results, the vaporization and movement behaviors of fuel droplets under forced convection conditions were further understood. Finally, by comparing with the zero-dimensional vaporization model with uniform temperature and component concentration distributions within the droplet and the one-dimensional vaporization model with finite thermal conductivity and mass diffusivity in the radical direction of the droplet, it is found that the quasi-dimensional model agrees better with the one-dimensional model than the zero-dimensional model, especially for the conditions with high ambient temperature and velocity. Sensitivity analysis indicates that the temperature gradient within the droplet plays a significantly important role in the droplet vaporization process.

Flammability and volatility attributes of binary mixtures of some practical multi-component fuels

Gasoline compression ignition (GCI) engines could be more efficient than most advanced SI engines while running on lower octane fuel. GCI engines may utilize a mixture of different fuels and fuel components such as gasoline and diesel or diesel and naphtha. The risks and hazards associated with such mixtures must be studied to ensure safe fuel storage, shipping and dispensing. In this work, flash point and vapor pressure measurements of different binary multi-component hydrocarbon mixtures are presented along with calculated lower and upper flammability limits. An equation has been developed to correlate flash point with other fuel properties. The flash point of a mixture approaches the flash point of the more volatile component, falling rapidly in some cases, as the more volatile component concentration increases. Vapor pressure is inversely related to flash point for a given mixture. Diesel/light straight run naphtha mixtures and diesel/gasoline mixtures exhibit similar flash point versus vapor pressure trends. Flammability limits were calculated using Le Chatelier’s Mixing Rule and modified Burgess–Wheeler Law. Hydrocarbon mixtures have similar lower and upper flammability limits over a range of temperatures. The vapor pressure of fuels and fuel blends has been used to determine the safe operating region as a function of blending formula and temperature. This work demonstrates that normal butane can be used to formulate blends of gasoline and naphtha with diesel, which are safe to handle and meet seasonal vapor pressure requirements.

1601 直噴ガソリン機関に適用可能な多成分燃料噴霧モデル(OS7-1 複雑流体の流動現象・流れの制御・混相流の応用技術)

1601 直噴ガソリン機関に適用可能な多成分燃料噴霧モデル(OS7-1 複雑流体の流動現象・流れの制御・混相流の応用技術)

Different spray droplet evaporation models for non-ideal multi-component fuels with experimental validation

Herein three heat and mass transfer models were considered: the well-mixed (WM) model, the frozen evaporation (FZ) model and the diffusion controlled (DC) model for multi-component droplet evaporation were compared and validated against experimental data. The test fuels were mixtures of n-decane (a surrogate fuel for kerosene) and alcohol (ethanol or butanol) with differing volumetric ratios. Since the test fuels are non-ideal solutions, the activity coefficient of each fuel component was calculated by using the UNIFAC method and was incorporated with the heat and mass transfer models to account for the non-linearity effect on fuel evaporation process. Single-camera and PIV (Particle image velocimetry) measurements yielded the velocity distributions of fuel droplets at two cross-sections of the spray cone with different axial distances. The Sauter Mean Diameter (SMD) of spray droplets were measured by a LSA-III Malvern particle size analyzer. For butanol–decane blends, all the three different models can predict the evaporation accurately with little variation. Whilst, prediction of evaporation for ethanol–decane blends suffers much more noticeable variation among the models because of the relatively higher saturation vapor pressure of ethanol. A great deviation between the predicted droplet size and the measured SMD was observed for E10 (10% ethanol by volume) because the activity coefficient of ethanol for E10, and thus the evaporation rate, is more susceptible to the variation of the ethanol mole fraction. The well mixed model can be considered suitable for predicting slow evaporation processes with low injection pressures.