Transient burning of a convective fuel droplet

The transient burning of an n-octane fuel droplet in a hot gas stream at 20 atmosphere pressure is numerically studied, with considerations of droplet regression, deceleration due to the drag of the droplet, internal circulation inside the droplet, variable properties, non-uniform surface temperature, and the effect of surface tension. An initial envelope flame is found to remain envelope in time, and an initial wake flame is always transitioned into an envelope flame at a later time, with the normalized transition delay controlled by the initial Reynolds number and the initial Damkohler number. The initial flame shape is primarily determined by the initial Damkohler number, which has a critical value of Da₀=1.02. The burning rates are modified by the transition, and are influenced by the intensity of forced convection which is determined by initial Reynolds number. The influence of surface tension is also studied as the surface temperature is non-uniform. Surface tension affects the liquid motion at the droplet surface significantly and affects the change of surface temperature and burning rate modestly. The influence of surface tension generally increases with increasing initial Reynolds number within the range without droplet breakup. We also studied cases with constant relative velocity between the air stream and the droplet. The results show that in these cases the initial envelope flame still remains envelope, but the evolution from an initial wake flame to an envelope flame is inhibited. Validation of our analysis is made by comparing with a published porous-sphere experiment Raghavan et al. (2005) [6] which used methanol fuel.     

Evaporation of an oil‐in‐water type emulsion droplet on a hot surface

Evaporation characteristics of an Oil‐in‐Water (O/W) emulsion droplet were examined experimentally. The evaporation time per unit of initial surface area of a droplet τ^* was used to estimate the evaporation characteristics of droplets with different diameters and to compare a water droplet and an emulsion droplet. Results show that τ^* of an O/W emulsion droplet is shorter than a water droplet in the Leidenfrost film boiling regime. The four evaporation modes of O/W type emulsion droplets were observed. These depended on the mixing ratio of water and oil, G_S, and hot surface temperature, T_W. Increasing G_S increases the emulsion droplet's Leidenfrost temperature when the droplet is used as a die‐cast releasing agent. Microexplosions were observed during Leidenfrost film boiling when T_W was greater than 250^°C. © 2005 Wiley Periodicals, Inc. Heat Trans Asian Res, 34(7): 527–537, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20081     

A numerical study of the effect of turbulence on mass transfer from a single fuel droplet evaporating in a hot convective flow

A three-dimensional numerical model is developed to investigate the effect of turbulence on mass transfer from a single droplet exposed to a freestream of air. The freestream temperature, turbulence intensity and Reynolds number are varied to provide a wide range of test conditions, whereas the ambient pressure is kept atmospheric. The turbulence terms in the conservation equations of the gas-phase are modelled by using the shear-stress transport (SST) model. A Cartesian grid based blocked-off technique is used in conjunction with the finite-volume method to solve numerically the governing equations of the gas and liquid-phases. This study showed that the vaporization Damköhler number proposed in the literature to correlate the effect of turbulence on the droplet's vaporization rate is invalid at air temperatures higher than room temperature. Additionally, an attempt is made to correlate the effect of the freestream turbulence on the droplet's mass transfer rate by using Sherwood number over a wide range of freestream temperatures.     

Numerical modelling of a three-zone combustion for heavy fuel oil in inert porous media reactor

Numerical model for heavy fuel oil and air mixtures combustion is presented to simulate the behavior of the fuel in an inert porous medium reactor for hydrogen production. Three-zone combustion of oil and petroleum cokes separated by temperature ranges starting from ambient temperature to 560 K, from 560 K to 673 K, and above 673 K, is presented. Hydrogen production is achieved using water gas shift equilibrium reaction on the combustion products at different temperatures. Results show a high enthalpy contribution due to coke combustion formed in the low temperature oxidation reaction, being the most important reaction in relation to its zone size. Simulations increasing filtration velocity (from 0.05 to 0.9 m/s) has a favorable effect on the maximum temperature and the combustion front velocity. The effect of the simplified combustion model lowers computational time, with acceptable results for temperature as well as hydrogen production in contrast to laboratory tests and other software simulation such as COMSOL Multiphysics.     

Modeling hydrogen production in a catalytic-inert packed bed reactor by rich combustion of heavy fuel oil

This work presents simulation results for the production of hydrogen by the rich combustion of heavy fuel oil in a dual zone packed bed reactor. The first zone provides catalytic-thermal cracking of the fuel and is followed by a second zone for partial oxidation reforming of the cracked products. The kinetic model for the heavy fuel oil reactions in the catalytic zone uses decalin as a model compound. The partial oxidation reforming zone uses model compounds for the product groups formed from decalin cracking, and uncracked decalin. The hybrid reactor model is compared to results from a model of an inert (non-catalytic) porous media reactor. The work considers equivalence ratios from 1 to 2, filtration velocities between 15.0 and 65.5 cm/s, heat loss from 10 to 108% and particle diameter between 3 and 7 mm, and evaluates their effect on conversion. The simulations with the hybrid reactor model, in slightly rich conditions (equivalence ratio = 1.3) and constant filtration velocity of 19.3 cm/s deliver maximum hydrogen production for an optimal length of the intermediate zone. Considering this optimization: the total energy conversion efficiencies improve with the increase of the equivalence ratio due to the presence of hydrocarbon species generated by the cracking process. It is observed that the hybrid reactor model makes a better use of vaporized fuel, compared to a model for an inert packed bed reactor, when the deposits of carbonaceous material in the latter exceed 7.4%.     

液滴在倾斜表面上的固化特征研究

采用平滑铝表面为基底,以去离子水为介质,对低温条件下倾斜表面上运动液滴的固化特征进行了研究,分析了液滴的固化时间特征,并探讨了基底倾斜角、液滴体积以及基底表面浸润性对液滴固化时间的影响。结果表明：当基底倾斜角小于液滴在铝表面上的临界滑动角时,液滴固化时间保持不变;当基底倾斜角大于临界滑动角时,随着倾斜角增大,液滴固化时间变短。随着液滴体积增大,虽然湿接触面积增大,但是液滴高度也增大,液滴固化时间随之延长。基底表面的疏水性越好,液滴与固体表面的湿接触面积越少,液滴高度越高,因此液滴固化时间越长。基于传热学理论建立了双圆法模型,利用其预测运动液滴的固化时间,并将计算值与实验值进行了比较,发现两者吻合良好。     

Thermodynamic analysis of in-situ hydrogen from hot compressed water for heavy oil upgrading

Due to many benefits of heavy oil upgrading in the green medium of hot compressed water (HCW), the present study considers the thermodynamic analysis of in-situ hydrogen created by partial oxidation of light hydrocarbons (HC) in HCW. The aim is seeking the upgrading condition where light hydrocarbons create hydrogen (H₂) and carbon monoxide (CO) assisted by partial oxidation of light hydrocarbons. The formed CO collaborates in in-situ active hydrogen through water gas shift reaction (CO+H₂O↔H₂+CO₂) which is more effective than external hydrogen for hydrogenation of heavy oil in HCW. Applying the powerful capability of Aspen Plus®, i.e., sensitivity analysis, the effect of significant parameters, such as temperature, pressure (water density), water to oil ratio, and oxygen (O₂) to oil ratio are studied comprehensively in order to maximize the amount of active hydrogen. The results indicate that higher temperatures and the amount of water (H₂O/heavy oil) are two favorable factors to increase the contribution of active hydrogen, while the pressure is not a determinant factor at supercritical condition (P ≥ 25 MPa). The formation of methane is also decreased at high temperature which is desired for upgrading system. The higher amount of water implies more quantity of O₂ since partial oxidation affords the enthalpy of auto-thermal reforming of HO. Hence there should be a compromise in the selected ratios of H₂O/HC and O₂/HC in HCW upgrading system. A set of experiments are conducted in order to compare the simulation and experimental results. Although the experimental results are established on kinetic data which also reflect the physical effect of HCW during HO upgrading, however, the thermodynamic study provides valued information, in agreement with experiments, that improves our understanding of HO upgrading in HCW with less coke.     

Experimental validation of a lumped model of single droplet deformation,oscillation and detachment on the GDL surface of a PEM fuel cell

Proton Exchange Membrane Fuel Cell (PEMFC) performance significantly depends on electrodes water content. Liquid water emerging from the Gas Diffusion Layer (GDL) micro-channels can form droplets, films or slugs in the Gas Flow Channel (GFC). In the regime of droplets formation, the interaction with the gas flow leads to an oscillating mechanisms that is fundamental to study the detachment from the GDL surface. In this work, a numerical model of a droplet growing on the GDL surface is developed to describe the interaction between droplet and gas flow. Therefore, a lumped force balance is enforced to determine the center of mass motion law. Oscillation frequencies during growth and at detachment are found as a function of droplet size. The model is also exploited to find the relationship between droplet critical detachment size and gas velocity. The numerical results are compared with the experimental data previously published by the authors as well as with other experimental results available in the literature. The matching between the numerical and experimental data is very good. The low computational burden and the conciseness of the proposed approach make the model suitable for applications such as control and optimization strategies development to enhance PEMFC performance. Additionally, the model can be exploited to implement monitoring and diagnostic algorithm as well.     

Mono-component fuel droplet evaporation in the presence of background fuel vapor

A simplified set of equations is examined for the problem of droplet evaporation. The equations employ the Clausius–Clapeyron (CC) boundary condition for the surface fuel-vapor, which is responsible for mathematical behaviors that include an initial condensation stage of droplet swelling followed by an evaporation stage. Numerical methods of analysis are used in conjunction with an asymptotic analysis of each of the three stages: (I) condensation; (II) transition; and (III) evaporation. Droplet evaporation in partial condensation environments is discussed.     

Behaviour of a prestressed concrete pressure vessel with hot liner

At Seibersdorf Research Centre a prototype vessel has been built to verify the concept of a Prestressed Concrete Pressure Vessel (PCPV) with elastic hot liner and elevated wall temperature which was developed there. The vessel is designed for a liner temperature of 300°C and 95 bar inner pressure.To investigate the behaviour of the vessel, it is extensively instrumented with different types of gauges, measuring temperatures, strains, stresses, deformations and prestress load. After erection and pressure test the vessel was submitted to an intensive test programme. The first period consisted of a series of test cycles with increasing temperature and pressure load. The second period was a long term test at full load conditions. The results of the intensive measurement will be interpreted and the behaviour of the liner, the concrete and the prestressing system will be discussed.     

Numerical simulation of droplet impact and evaporation on a porous surface

Numerical simulation is performed for the evaporation of a droplet impacted on a porous surface. A level-set formulation for tracking the droplet deformation is extended to include the effects of evaporation coupled to heat and mass transfer, porosity and porous drag and capillary forces. The local volume averaged conservation equations of mass, momentum, energy and vapor fraction for the porous region are simultaneously solved with the conservation equations for the external fluid region. The computations demonstrate not only the evolution of the liquid-gas interface during the whole period of droplet penetration and evaporation in a porous medium, but also the associated flow, temperature and vapor fraction fields. The effects of impact velocity, porosity and particle size on the droplet deformation and evaporation are quantified.     

Effect of heavy fuel oil/natural gas co-combustion on pollutant generation in retrofitted power plant

The purpose of this paper is to examine effect of co-combustion of heavy fuel oil and natural gas on the pollutant formation: CO₂, SO₂, SO₃, NO_X and soot. The analysis was carried out by means of numerical simulation on the case of retrofitted steam generator furnace of Thermal Power Plant Sisak (Croatia). Comprehensive mathematical model of the furnace with detailed 3D mesh was set up to include all relevant aerodynamic and thermo-chemical processes in the furnace. Dedicated model for SO₃ formation was developed earlier and used in this work. By increasing the natural gas contribution in overall fuel fired in the furnace, emissions of CO₂, SO₂, soot, NO_X and SO₃ decreased. Heat transferred to the furnace walls and temperature field in the furnace were also examined in order to establish regions of safe and efficient boiler operation for different operational regimes.     

Advanced models of fuel droplet heating and evaporation

Recent developments in modelling the heating and evaporation of fuel droplets are reviewed, and unsolved problems are identified. It is noted that modelling transient droplet heating using steady-state correlations for the convective heat transfer coefficient can be misleading. At the initial stage of heating stationary droplets, the well known steady-state result Nu=2 leads to under prediction of the rate of heating, while at the final stage the same result leads to over prediction. The numerical analysis of droplet heating using the effective thermal conductivity model can be based on the analytical solution of the heat conduction equation inside the droplet. This approach was shown to have clear advantages compared with the approach based on the numerical solution of the same equation both from the point of view of accuracy and computer efficiency. When highly accurate calculations are not required, but CPU time economy is essential then the effect of finite thermal conductivity and re-circulation in droplets can be taken into account using the so called parabolic model. For practical applications in computation fluid dynamics (CFD) codes the simplified model for radiative heating, describing the average droplet absorption efficiency factor, appears to be the most useful both from the point of view of accuracy and CPU efficiency. Models describing the effects of multi-component droplets need to be considered when modelling realistic fuel droplet heating and evaporation. However, most of these models are still rather complicated, which limits their wide application in CFD codes. The Distillation Curve Model for multi-component droplets seems to be a reasonable compromise between accuracy and CPU efficiency. The systems of equations describing droplet heating and evaporation and autoignition of fuel vapour/air mixture in individual computational cells are stiff. Establishing hierarchy between these equations, and separate analysis of the equations for fast and slow variables may be a constructive way forward in analysing these systems.     

Flame spread behaviour of blended fuel droplet array

Experimental investigation of the effect of blended fuel on flame spread along droplet array has been conducted. Flame spread rate is measured using high‐speed chemiluminescence images of an OH radical. The flame spread is observed with the initial droplet diameter, droplet spacing, and the mixing ratio of n‐heptane and n‐hexadecane. The mode of flame spread is categorized into two types: a continuous mode and an intermittent one. It is seen that flame spread rate is sensitively dependent on the relative flame position to droplet spacing. For a large droplet, the flame spread time is governed by a volatile fuel (heptane), but for a small droplet, it is controlled by a less volatile fuel (hexadecane). Copyright © 1999 John Wiley & Sons, Ltd.     

A numerical investigation of evaporation characteristics of a fuel droplet suspended from a thermocouple

Numerical simulations of combined natural convection–conduction in a droplet of n-dodecane suspended from a thermocouple were carried out, taking into consideration evaporation, and the effect of thermocouple diameter on the evaporation characteristics was investigated. The calculated temperature history of the droplet is in good agreement with experimental results; both show that the rate of heating decreases with increasing thermocouple diameter. The maximum error in temperature due to the thermocouple increases linearly with increasing thermocouple diameter. Thus, in investigations involving a droplet suspended from a thermocouple, it is preferable to use a thermocouple with the smallest possible diameter.     

Effects of gravity and ambient pressure on liquid fuel droplet evaporation

An axisymmetric numerical model has been developed to conduct a study of single droplet evaporation over a wide range of ambient pressures both under normal and microgravity conditions. Results for droplet lifetime as a function of ambient pressure and initial droplet diameter are presented. The enhancement in the droplet evaporation rate due to natural convection is predicted. This enhancement becomes more dominant with increasing ambient pressure due to the increase in the Grashof number. The higher the ambient pressure, the closer the Grashof number remains to its initial value throughout most of the droplet lifetime because of the droplet swelling and the heat-up of the droplet interior. Results should be particularly of interest to researchers conducting experiments on droplet evaporation at elevated pressures within a normal gravity environment. The model developed is in good agreement with experimental data at low pressures. Explanations have been provided for its deviation at high pressures.     

Numerical investigation of the cooling effectiveness of a droplet impinging on a heated surface

Computational fluid dynamics numerical simulations for 2.0 mm water droplets impinging normal onto a flat heated surface under atmospheric conditions are presented and validated against experimental data. The coupled problem of liquid and air flow, heat transfer with the solid wall together with the liquid vaporization process from the droplet’s free surface is predicted using a VOF-based methodology accounting for phase-change. The cooling of the solid wall surface, initially at 120 °C, is predicted by solving simultaneously with the fluid flow and evaporation processes, the heat conduction equation within the solid wall. The range of impact velocities examined was between 1.3 and 3.0 m/s while focus is given to the process during the transitional period of the initial stages of impact prior to liquid deposition. The droplet’s evaporation rate is predicted using a model based on Fick’s law and considers variable physical properties which are a function of the local temperature and composition. Additionally, a kinetic theory model was used to evaluate the importance of thermal non-equilibrium conditions at the liquid–gas interface and which have been found to be negligible for the test cases investigated. The numerical results are compared against experimental data, showing satisfactory agreement. Model predictions for the droplet shape, temperature, flow distribution and vaporised liquid distribution reveal the detailed flow mechanisms that cannot be easily obtained from the experimental observations.     

Resident time of a compound drop impinging on a hot surface

The resident time of a water-in-diesel compound drop impinging on a hot surface at a temperature higher than the Leidenfrost temperature was investigated experimentally. Past experimental evidence suggested that the resident time of a pure liquid drop was independent of the impact velocity. And this independency could also be seen for compound drops. For both pure drops and compound drops, the resident time became longer with increasing outer diameter of the drop. For water-in-diesel compound drops of a given outer diameter, the resident time decreased as the volume of the core water drop increased. By using a modified Weber number which took into account of the two interfaces of the compound drop, a correlation of the non-dimensional resident time was obtained and was in good agreement with the experimental data.