Multicomponent-liquid–fuel vaporization with complex configuration

An analysis is presented for multicomponent-liquid–fuel vaporization in a general geometrical situation, e.g., a dense spray. Variable transport properties and only Stefan flow are considered. The problem is separated, using a mass-flux potential function, into a one-dimensional problem for the quasi-steady, gas-phase scalar properties and a three-dimensional problem for the mass-flux potential. The theory predicts scalar gas-phase profiles and vaporization rates for any value of the Lewis number. Transient heat- and mass-diffusion in the liquid interiors is considered with special attention given to the fast- and slow-diffusion limits. Eight droplets in a cubic array are considered in the calculations with a blended liquid mixture of heptane, octane, and decane. Comparisons are made amongst the results for the various liquid-diffusion models: transient behavior, the fast-vaporization limit, and the slow-vaporization limit.     

Advances in droplet array combustion theory and modeling

A review of research on the subject of the vaporization and burning of fuel droplets configured in a prescribed array is presented, including both classical works and research over the past decade or two. Droplet arrays and groups and the relation to sprays are discussed. The classical works are reviewed. Recent research on transient burning and vaporization of finite arrays with Stefan convection but without forced convection is presented, including extensions to non-unitary Lewis number and multi-component, liquid fuels. Recent results on transient, convective burning of droplets in arrays are also examined. In particular, transient convective burning of infinite (single-layer periodic and double-layer periodic) and finite droplet arrays are discussed; attention is given to the effects of droplet deceleration due to aerodynamic drag, diameter decrease due to vaporization, internal liquid circulation, and arrays with moving droplets in tandem and staggered configurations. Flame structure is examined as a function of spacing between neighboring droplets and Damköhler number: individual droplet flames versus group flames and wake flames versus envelope flames. Based on existing knowledge of laminar droplet array and spray combustion theory, experimental evidence, and turbulent studies for non-vaporizing and non-reacting two-phase flows, comments are made on the needs and implications for the study of turbulent spray and array combustion.     

The effect of Lewis and Damk?hler numbers on the flame propagation through micro-organic dust particles

In this study, the role of Lewis and Damköhler numbers on the premixed flame propagation through micro-organic dust particles is investigated. It is presumed that the fuel particles vaporize first to yield a gaseous fuel, which is oxidized in the gas phase. In order to simulate the combustion process, the flame structure is composed of four zones; a preheat zone, a vaporization zone, a reaction zone and finally a post flame zone, respectively. Then the governing equations, required boundary conditions and matching conditions are applied for each zone and the standard asymptotic method is used in order to solve these differential equations. Consequently the important parameters on the combustion phenomenon of organic dust particles such as gaseous fuel mass fraction, organic dust mass fraction and burning velocity with the various numbers of Lewis, Damköhler and the onset of vaporization are plotted in figures. This prediction has a reasonable agreement with experimental data of micro-organic dust particle combustion.     

Transient vaporization and burning in dense droplet arrays

Three-dimensional droplet-array combustion with an unsteady liquid-phase and a quasi-steady gas-phase is studied computationally by a generalized approach using a mass-flux potential function. Symmetric and asymmetric droplet arrays with non-uniform droplet size and non-uniform spacing are considered. Burning rates are computed and correlated with the number of droplets, an average droplet size, and an average spacing for the array through one similarity parameter for arrays as large as 1000 droplets. Total array vaporization rates are found to be maximized at a specific droplet number density that depends on liquid volume within the array. An unsteady liquid-phase model with either a uniform or a radially varying temperature distribution is coupled with the quasi-steady gas-phase solution for decane, heptane, and methanol fuels. Droplet interactions and liquid-phase heating have been shown to almost double the lifetime when compared to an isolated droplet. Depending on fuel type, initial temperature, and array geometry, droplets may initially burn with individual flames, transition to a single group flame, and transition back to individual flames as vaporization progresses. In most cases, group combustion occurs upon ignition and is the dominant mode of combustion.     

Correlations of vaporization performance of conventional and biofuel sprays in a crossflow heated chamber

Pre-vaporization and pre-mixing are the two main features of LPP type of combustor that operate on liquid fuels. The pre-vaporization length scale is one of its most important design parameters. In this study, the goal is to put forward a simulation based correlation for fuel vaporization performance as a function of dimensionless parameters for crossflow type of injections. Two types of fuels are studied here: jet-A and one of its potential biofuel substitutes, RME. Different sets of spray simulations are considered for crossflow type of injections. Correlations are provided for both jet-A and RME's vaporization performance as a function of non-dimensional inlet air temperature, fuel/air momentum flux ratio and normalized spray traverse distance.     

Utilizing preferential vaporization to enhance the stability of spray combustion for high water content fuels

In a previous study on the stability of spray combustion for mixtures of alcohols (ethanol or 1-propanol) and water, the feasibility of burning fuels heavily diluted with water was demonstrated. In that study it was found that the preferential vaporization of alcohols in water can significantly enhance flame stability. Due to their high volatility and high activity coefficient in aqueous solution, the alcohols quickly evaporate from the droplets and generate a concentrated fuel vapor at the base of the jet. Therefore, a flame can be ignited and stabilize even though the water content of the fuel is quite high (up to 90 wt%) (Yi and Axelbaum, 2013). In this study, we develop a procedure for selecting chemical fuels showing strong preferential vaporization in water. t-Butanol was identified as an excellent candidate based on its physical and chemical properties, including activity coefficient, vapor pressure, heat of vaporization and heat of combustion. Flame stability was evaluated for aqueous solutions of both ethanol and t-butanol using a spray burner where the extent of swirl was adjustable. Under both high and low swirl intensity, the flame stability of t-butanol aqueous solutions was better than that of ethanol. The characteristic time for fuel release from a droplet was modeled for both ethanol and t-butanol. The time to release 99% of the fuel from the droplet for t-butanol is over 70% shorter compared to that for ethanol, which supports the improved flame stability observed for t-butanol in the experiments.     

The prediction of mass transfer rates when equilibrium does not prevail at the phase interface

It is shown that departures from thermodynamic equilibrium at the interface call for only small modifications in standard procedures for calculating mass transfer rates. If enthalpy-composition diagrams are used for representing thermodynamic properties and evaluating mass transfer driving forces, the modifications comprise the replacement of the single curve, valid for equilibrium mixtures in contact with the neighbouring phase, by a family of curves; the parameter of this family is the ratio of the mass transfer conductance to a molecular flux.

These and other aspects of mass transfer theory are illustrated by reference to: the vaporization of water into air; the combustion of carbon in air; the pyrolysis of a solid or liquid without chemical reaction; and the combustion of a pellet of ammonium perchlorate in a stream of fuel gas. The examples discussed numerically relate to axi-symmetrical stagnation point flows with laminar boundary layers.     

Modeling of a conceptual self-sustained liquid fuel vaporization – combustion system with radiative output using inert porous media

The present model is based on a combined self-sustained liquid fuel vaporization – combustion system, where the liquid fuel vaporization occurs on a wetted wall plate with energy transferred through the plate from the combustion of vaporized oil. The vaporization energy has been derived through the radiative interaction of the vaporizing plate and an upstream end surface of the porous medium. The inert porous medium, used in the flow passage of combustion gas, is allowed to emit and absorb radiant energy. The radiative heat flux equations for the porous medium have been derived using the two-flux gray approximation. The work analyzes the effect of emissivities of vaporizing plate and porous medium, the optical thickness of medium and equivalence ratio on the kerosene vaporization rate, combustion temperature and radiative output of the system. Combination of low and high emissivities of vaporizing plate and porous medium respectively with low optical thickness of medium makes the system operable over a wide range of power. The study covers the data concerning the design and operating characteristics of a practical system.     

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

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.     

Liquid-fuel burning with nonunitary Lewis number

An analysis is presented for liquid-fuel vaporization and burning with nonunitary Lewis number (i.e., nonsimilar heat and mass diffusion) in a general geometrical situation, e.g., a dense spray. Variable transport properties are considered and only Stefan flow is allowed. The analysis builds on the approach of Imaoka and Sirignano for unitary Lewis number. Fickian diffusion with differing diffusivities for each species is considered. It is shown that the problem can conveniently be separated, using a mass-flux potential function, into a one-dimensional problem for the quasi-steady, gas-phase scalar properties and a three-dimensional problem for the mass-flux potential, which satisfies Laplace's equation. This allows some previous calculations of the potential function for unitary Lewis number to be used for the potential-function solution. The scalar properties are shown to be functions of the mass-flux potential only. It is demonstrated that a mass-flux-weighted sensible specific enthalpy is more natural and convenient than the traditional mass-weighted value. This modification results in a new definition of the Lewis number. A generalization of the classical Spalding heat transfer number is presented. The theory predicts scalar gas-phase profiles, flame position, and vaporization rates. Quantitative results are presented for special cases where the Lewis number is piecewise constant. The thin-flame temperature and the effective latent heat of vaporization can be determined as functions of the liquid-surface temperature via solution of nonlinear algebraic equations; these values do not depend on the specific configuration and therefore have some universality.     

Modeling of trickle flow liquid fuel combustion in inert porous medium

Present work is a numerical analysis of fuel oil combustion inside an inert porous medium where fuel oil flows through the porous medium under gravity wetting its solid wall with concurrent movement of liquid fuel and air under steady state conditions. A one-dimensional heat transfer model has been developed under steady state conditions using a single step global reaction mechanism. The effects of optical thickness, emissivity of medium, flame position and reaction enthalpy flux on radiation energy output efficiency as well as the temperature, position and thickness of vaporization zone have been presented using kerosene as fuel. Low values of optical thickness and emissivity of porous medium will ensure efficient combustion, maximize downstream radiative output with minimum upstream radiative loss.     

Flame propagation through an air-fuel spray mixture with transient droplet vaporization

Flame initiation and propagation through an air/fuel vapor/fuel drop system is numerically modeled in a cylindrical one-dimensional closed combustor. An unsteady formulation of the flow problem eliminates the cold-boundary difficulty and gas-phase ignition problem. A velocity lag between the gas and the liquid phase is allowed and unsteady heat transfer to the droplets is taken into account. The surface temperature of the droplet is evaluated by using an unsteady spherically symmetric formulation of the droplet heat conduction problem with no internal motion and with a time-varying heat flux specified at the surface as a boundary condition. Results have been obtained for two commercially important fuels, namely, n-octane and n-decane. The activation energy and the preexponential factor in the Arrhenius-type expression for chemical rate, along with initial temperature, initial droplet size, stoichiometric ratio, and diffusivity are parametrically varied and flame speed and flame temperatures are observed. Flame speed is seen to increase with increasing preexponential factor, decreasing activation energy, increasing ambient temperature, decreasing initial droplet radii, and increasing diffusivities. It is also observed that unlike premixed combustion, heterogeneous combustion gives rise to local variations of equivalence in the axial direction. This phenomenon could give rise to a secondary diffusion flame in the wake of a propagating flame and produce local variations in the flame temperature.     

Vaporization of polydisperse fuel sprays in a laminar boundary layer flow: A sectional approach

The effect of the presence of a cold wall on the downstream changes in size distribution of a spray of fuel droplets undergoing vaporization and combustion is theoretically analyzed. The fuel is considered to be in the form of discrete liquid droplets which have an arbitrary range of sizes and differ in their rates of vaporization. In fact, the total number of discrete droplet sizes needed to simulate actual fuel sprays can be immense. To avoid the dimensionality problem associated with the discrete form of population balance equations of an ensemble of individual burning or evaporating particles, “sectional conservation equations” are used. The method, based on dividing the droplet size domain into sections and dealing only with one integral quantity in each section (e.g., number, surface area of droplets, or volume), has the advantage that the integral quantity is conserved within the computational domain and the number of conservation equations required is simply equal to the number of sections. Employing known solutions for the boundary layer flow field, the “sectional size conservation equations” are solved assuming that droplets follow streamlines. New solutions for the changes in size distributon of droplets as a function of temperature and distance from the wall are presented. Since the present analysis uses an arbitrary droplet size distribution as an initial condition, it may be used to evaluate the performance of various atomizers, as demonstrated in the present study.     

High-pressure vaporization modeling of multi-component petroleum–biofuel mixtures under engine conditions

Numerical simulation of the vaporization of multi-component liquid fuels under high-pressure conditions is conducted in this study. A high-pressure drop vaporization model is developed by considering the high-pressure phase equilibrium which equates the fugacity of each component in both liquid and vapor phases. Peng–Robinson equation of state is used for the calculation of fugacity. To model the vaporization of diesel fuel under high-pressure conditions, continuous thermodynamics based on a gamma distribution is coupled with phase equilibrium by correlating the parameters of the equation of state with the molecular weights of the continuous components. The high-pressure vaporization model is validated using the experimental data of n-heptane drops under different ambient pressures and temperatures. Good levels of agreement are obtained in drop size history. Predicted results of the vaporization of diesel fuel drops show that increasing ambient pressure leads to a shorter drop lifetime under high temperature conditions (e.g., 900 K). On the other hand, at a slightly lower temperature of 700 K, the drop lifetime increases as the ambient pressure increases. It is found that the net affects of high ambient pressure on drop vaporization are determined by two competing factors, namely, reduced mass transfer number and reduced enthalpy of vaporization. The model was further applied to biodiesel and its blends with diesel fuel. The fuel blend is modeled based on a method that continuous thermodynamics is used to model diesel fuel and biodiesel is modeled as a mixture of its five representative components. Results of single drop vaporization history show that drop lifetime increases as the volume fraction of biodiesel in the fuel blend increases. This phenomenon reveals the low vaporization rate of biodiesel that has a higher critical temperature than diesel fuel. It is also observed that the volume fraction of biodiesel in the fuel blend increases during vaporization and its vapor concentrates near the tip of the liquid spray while diesel fuel vapor is around the entire liquid spray.     

Dispersion and vaporization of biofuels and conventional fuels in a crossflow pre-mixer

In lean premixed pre-vaporized (LPP) combustion, controlled atomization, dispersion and vaporization of different types of liquid fuel in the premixer are the key factors required to stabilize the combustion process and improve the efficiency. The dispersion and vaporization process for biofuels and conventional fuels sprayed into a crossflow pre-mixer have been simulated and analyzed with respect to vaporization rate, degree of mixedness and homogeneity. Two major biofuels under investigation are Ethanol and Rapeseed Methyl Esters (RME), while conventional fuels are gasoline and jet-A. First, the numerical code is validated by comparing with the experimental data of single n-heptane and decane droplet evaporating under both moderate and high temperature convective air flow. Next, the spray simulations were conducted with monodispersed droplets with an initial diameter of 80 μm injected into a turbulent crossflow of air with a typical velocity of 10 m/s and temperature of around 800 K. Vaporization time scales of different fuels are found to be very different. The droplet diameter reduction and surface temperature rise were found to be strongly dependent on the fuel properties. Gasoline droplet exhibited a much faster vaporization due a combination of higher vapor pressure and smaller latent heat of vaporization compared to other fuels. Mono-dispersed spray was adopted with the expectation of achieving more homogeneous fuel droplet size than poly-dispersed spray. However, the diameter histogram in the zone near the pre-mixer exit shows a large range of droplet diameter distributions for all the fuels. In order to improve the vaporization performance, fuels were pre-heated before injection. Results show that the Sauter mean diameter of ethanol improved from 52.8% of the initial injection size to 48.2%, while jet-A improved from 48.4% to 18.6% and RME improved from 63.5% to 31.3%. The diameter histogram showed improved vaporization performance of jet-A.     

AN ADVANCED SPRAY MODEL FOR APPLICATION TO THE PREDICTION OF GAS TURBINE COMBUSTOR FLOW FIELDS

It is well known that fuel preparation, its method of injection into a combustor, and its atomization characteristics have a significant impact on emissions. A simple dilute spray model, which assumes that droplet heating and vaporization occur in sequence, has been implemented in the past within computational fluid dynamics (CFD) codes at General Electric (GE) and has been used extensively for combustion applications. This spray model coupled with an appropriate combustion model makes reasonable predictions of the combustor pattern factor and emissions. To improve upon this predictive ability, a more advanced quasi-steady droplet vaporization model has been considered. This article describes the evaluation of this advanced model. In this new approach, droplet heating and vaporization take place simultaneously (which is more realistic). In addition, the transport properties of both the liquid and vapor phases are allowed to vary as a function of pressure, gas phase temperature, and droplet temperature. These transport properties, which are most up to date, have been compiled from various sources and appropriately curve-fit in the form of polynomials. Validation of this new approach for a single droplet was initially performed. Subsequently calculations of the flow and temperature field were conducted and emissions (NOx, CO, and UHC) were predicted for a modern single annular turbofan engine combustor using both the standard spray model and the advanced spray model. The effect of the number of droplet size ranges as well as the effect of stochastic treatment of the droplets were both investigated.     

Second-order conditional moment closure modeling of turbulent piloted Jet diffusion flames

A second-order CMC model for a detailed chemical mechanism is presented and applied to turbulent piloted jet diffusion flames, Sandia Flames D, E, and F. Second-order corrections are made for three rate-limiting steps of methane-air combustion, while first-order closure is employed for all the other reaction steps. The effect of a pilot flame is well predicted by the conditional scalar dissipation rate derived from a trimodal PDF for fuel, air, and pilot streams. Results show significantly improved predictions of conditional mean temperature and mass fractions of major and stable intermediate species on the fuel-rich side. Conditional mean OH and NO mass fractions are also in good agreement, although underestimated extinction seems to be responsible for overpredicted NO downstream for Flame E. Results for Flame D are in better agreement than those for Flame E which involves more significant local extinction. Second-order correction of only the chain-branching step, H + O₂ → OH + O, is shown to capture most of the improvements by correction of the three rate-limiting steps. The predictions for Flame F do not seem to be satisfactory due to underpredicted conditional variances and covariances. Further study may be needed to clarify major causes of the deviation and to suggest possible remedies for more accurate prediction of Flame F.     

Infrared thermography and numerical study of vaporization characteristics of pure and blended bio-fuel droplets

The combustion dynamics and stability are dependent on the quality of mixing and vaporization of the liquid fuel in the pre-mixer. The vaporization characteristics of different blends of bio-fuel droplets injected into the air stream in the pre-mixer have been modeled. Two major alternate fuels analyzed are ethanol and Rapeseed Methyl Esters (RME). Ethanol is being used as a substitute of gasoline, while RME has been considered as an alternative for diesel. In the current work, the vaporization characteristics of a single droplet in a simple pre-mixer has been studied for pure ethanol and RME in a hot air jet at a temperature of 800 K. In addition, the behavior of the fuels when they are mixed with conventional fuels like gasoline and diesel is also studied. Temperature gradients and vaporization efficiency for different blends of bio-conventional fuel mixture are compared with one another. Smaller droplets vaporize faster than larger droplets ensuring homogenous mixture. The model was validated using an experiment involving convection heating of acoustically levitated fuel droplets and IR-thermography to visualize and quantify the vaporization characteristics of different bio-fuel blends downstream of the pre-mixer.     

Burning syngas in a high swirl burner: Effects of fuel composition

Flame characteristics of swirling non-premixed H₂/CO syngas fuel mixtures have been simulated using large eddy simulation and detailed chemistry. The selected combustor configuration is the TECFLAM burner which has been used for extensive experimental investigations for natural gas combustion. The large eddy simulation (LES) solves the governing equations on a structured Cartesian grid using a finite volume method, with turbulence and combustion modelling based on the localised dynamic Smagorinsky model and the steady laminar flamelet model respectively. The predictions for H₂-rich and CO-rich flames show considerable differences between them for velocity and scalar fields and this demonstrates the effects of fuel variability on the flame characteristics in swirling environment. In general, the higher diffusivity of hydrogen in H₂-rich fuel is largely responsible for forming a much thicker flame with a larger vortex breakdown bubble (VBB) in a swirling flame compare to the H₂-lean but CO-rich syngas flames.     

Effects of nozzle geometry on direct injection diesel engine combustion process

The aim of the current article is to link nozzle geometry, and its influence on spray characteristics, with combustion characteristics in the chamber. For this purpose, three 6-hole sac nozzles, with different orifices degree of conicity, have been used. These nozzles had been geometrically and hydraulically characterized in a previous publication, where also a study of liquid phase penetration and stabilized liquid length in real engine conditions has been done. In the present work, CH and OH chemiluminescence techniques are used to thoroughly examine combustion process. CH-radicals are directly related to pre-reactions, which take place once the fuel has mixed with air and it has evaporated. On the other hand, OH-radicals data provide information about the location of the flame front once the combustion has begun. The analysis of all the results allows linking nozzle geometry, spray behaviour and combustion development. In particular, CH-radicals have shown to appear together with vapor spray, both temporally and in their location, being directly related to nozzle characteristics. Additionally, analysis of ignition delay is done form OH measurements, including some correlations in terms of chamber properties, injection pressure and nozzle diameter.