Numerical simulation of emulsified fuel spray combustion with puffing and micro-explosion

The purpose of this study was to develop numerical simulation of spray combustion of emulsified fuel with considering puffing and micro-explosion. First, a mathematical model for puffing was proposed. In the proposed puffing model, the rate of mass change of a droplet during puffing was expressed by the evaporation rate of dispersed water and the mass change rate due to fine droplets spouted from the droplet surface. The mass change rate due to fine droplets was related to the evaporation rate of the dispersed water and each liquid content. This model had only one experimental parameter. The essential feature of this model was that it was simple to apply to numerical simulation of spray combustion. First, the validity of the proposed puffing model was investigated with the experimental results for a single droplet. The calculated results for a single droplet with the experimental parameter varying from 5.0 to 10 were in good agreement with the experimental results. Moreover, numerical simulation of spray combustion of emulsified fuel was carried out. The occurrence of puffing and micro-explosion was determined by the inner droplet temperature. When micro-explosion occurred, a droplet changed to vapor rapidly. When the proposed puffing model was used in numerical simulation of spray combustion, the experimental parameter in the puffing model was determined for each droplet by random numbers within the range 5.0-10. The calculated results of spray combustion of emulsified fuel without considering puffing or micro-explosions were different from the experimental results even where combustion reactions were almost terminated. Meanwhile, the calculated results when considering puffing and micro-explosions were in good agreement with experimental results at the same location.     

Comparison of ignition,injection and micro - Explosion characteristics of RP-3/ ethanol and biodiesel / ethanol mixed drops

The ignition, injection, and micro-explosion characteristics of aviation fuel (RP-3)/ethanol mixed droplets and biodiesel/ethanol mixed droplets at different proportions under high temperature conditions (420 °C) were compared using an experimental setup. A device for measuring small droplet volumes was designed using an infusion set and different types of needles, and a corresponding equation was established. Mixed droplets suspended on high-temperature resistance nichrome wire with a diameter of 0.2 mm were heated by sending them to a position approximately 2 mm from the forklift preheating plug using a moving rail. SLR and high-speed cameras were used to observe the flame structure as well as the injection and micro-explosion of the mixed droplets during combustion, respectively. Expansion, injection, and micro-explosion were observed in the biodiesel/ethanol mixed droplet experiments when the biodiesel content was 60%. Although the micro-explosion of mixed droplets of aviation fuel/ethanol was not observed, expansion and ejection of the droplets were observed. Image Pro-plus software was used to calculate the diameters at different times in the combustion cycle of the droplets. Through this analysis, the occurrence of micro-explosion was described, and a model for the calculation of micro-explosion strength was established.     

Microexplosion of aluminum slurry droplets

The microexplosion of a slurry droplet is experimentally and theoretically investigated. The microexplosion was considered to be caused by the shell formation and the following pressure build-up in the shell which would be promoted by the suppression of evaporation, subsequent superheating and heterogeneous nucleation of a liquid carrier. Experimentally, the microexplosion phenomena was examined for various surfactant concentrations and particle loading under different ambient temperature ranges (500–900 K). To closely investigate the pressure build-up and the heterogeneous nucleation, a numerical model was introduced by considering the three stages such as the shell formation, suppression of evaporation and pressure build-up inside. The microexplosion time was estimated by postulating the limit of superheat for heterogeneous nucleation. The simulation yielded a reasonably good agreement with experimental results for Al/n-heptane slurry droplets under various solid loadings (10–40 wt.%).     

Lower limit of homogeneous nucleation boiling explosion for water

In this paper, the lower limit for the occurrence of homogeneous nucleation boiling explosion during water heating at atmospheric pressure has been determined by applying a new theoretical model proposed by the authors. Two different cases of water heating have been considered for the study of homogeneous nucleation boiling explosion. In one case, the liquid on the surface is linearly heated at a rate of 10 K/s to 10⁹ K/s. In another case, the liquid suddenly contacts with a high temperature surface such as in case of quenching with impinging jet or droplet. With the assumption of liquid boiling without any cavity or surface effect, the liquid temperature limit at which homogeneous boiling explosion occurs essentially corresponds to a value of 302 °C even though the surface is heated very slowly. On the other hand, during water contact with hot surfaces, the occurrence of the homogeneous boiling explosion within a characteristic time period of 1 ms is obtained at a maximum liquid temperature of 303 °C for a limiting steady state boundary temperature of about 304 °C. From the definition of the steady-state interface boundary temperature of two 1-D semi-infinite body contact problem, the lower limiting surface temperatures for the occurrence of the homogeneous nucleation boiling explosion have been determined for water contact with various solid surfaces at different initial liquid temperatures ranging from 0 °C to 100 °C. The effects of the parametric variation in the boundary heating conditions on various characteristics of the homogeneous boiling explosion such as liquid temperature and time of boiling explosion, heat-flux across the liquid–vapor interface at the boiling explosion, etc. are determined and compared with other results reported in the literature.     

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     

Development of a mathematical model for predicting water vapor mass generated in micro-explosion

This paper describes a mathematical model for predicting the mass of water vapor generated in micro-explosion. First, a single droplet experiment was carried out. A W/O (water/oil) emulsified fuel droplet suspended by a thermocouple was heated by a halogen spot heater, and micro-explosion was observed using a high-speed video camera. The progress of the coalescence of the dispersed water droplet was observed while droplet was heated, and an aggregated water droplet was formed in the oil layer. Based on the measured micro-explosion characteristics, a mathematical model for predicting water vapor mass generated in micro-explosion was proposed. The size of the aggregated water droplet just before micro-explosion was measured to verify the proposed mathematical model. Under certain assumptions, mass and energy conservation equations were applied to micro-explosion process, and an equation to calculate water vapor mass generated in micro-explosion was derived. The derived equation and some measurement results provide enough information to calculate water vapor mass generated in micro-explosion. The calculated diameter of the water droplet, which changed to vapor in micro-explosion, was compared to that of the aggregated water droplet just before micro-explosion. The calculated results roughly agreed with experimental ones, and the validity of the proposed model was verified.     

An experimental investigation on spreading of droplets with evaporation and nucleation

An experimental investigation was conducted to visually observe the dynamic characteristics of water droplets with evaporation and nucleation on stainless steel and polished silicon surfaces. The water droplet diameter, contact area, and spreading speed were measured using a high‐speed CCD camera at surface temperatures ranging from 110^°C to 190^°C, and a model was proposed to describe the dynamic behavior of droplet spreading. The spreading of water droplets under evaporation and nucleate boiling is highly dependent on the dynamic bubble behavior in the droplets, particularly bubble volume, bubble interaction, as well as the surface properties and temperature. Water droplets were easiest to spread at the surface temperature of 130 ^°C, and the spreading tendency increased with increasing surface coarseness. © 2008 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20231     

Interaction of the transfer processes in semitransparent liquid droplets

The study presents the mathematical model of unsteady heat transfer in evaporating semitransparent droplets of non-isothermal initial state and the numerical research method, evaluating selective radiation absorption and its influence on the interaction of transfer processes. The relation of the transfer processes inside droplets and in their surroundings and the necessity of thorough research of these processes are substantiated. When modeling the combined energy transfer in water droplets, the evaluation of thermoconvective stability in evaporating semitransparent liquid droplets is presented; the influence of the droplet initial state on its heating and evaporation process is investigated. The influence of heat transfer peculiarities on the change of the evaporating droplet state is indicated. Main parameters, which decide the peculiarities of the interaction of unsteady transfer processes in droplets and their surroundings, are discussed. The results of the numerical research are compared to the known results of the experimental studies of water droplet temperature and evaporation rate.     

Monodisperse droplet heating and evaporation: Experimental study and modelling

Results of experimental studies and the modelling of heating and evaporation of monodisperse ethanol and acetone droplets in two regimes are presented. Firstly, pure heating and evaporation of droplets in a flow of air of prescribed temperature are considered. Secondly, droplet heating and evaporation in a flame produced by previously injected combusting droplets are studied. The phase Doppler anemometry technique is used for droplet velocity and size measurements. Two-colour laser induced fluorescence thermometry is used to estimate droplet temperatures. The experiments have been performed for various distances between droplets and various initial droplet radii and velocities. The experimental data have been compared with the results of modelling, based on given gas temperatures, measured by coherent anti-stokes Raman spectroscopy, and Nusselt and Sherwood numbers calculated using measured values of droplet relative velocities. When estimating the latter numbers the finite distance between droplets was taken into account. The model is based on the assumption that droplets are spherically symmetrical, but takes into account the radial distribution of temperature inside droplets. It is pointed out that for relatively small droplets (initial radii about 65 μm) the experimentally measured droplet temperatures are close to the predicted average droplet temperatures, while for larger droplets (initial radii about 120 μm) the experimentally measured droplet temperatures are close to the temperatures predicted at the centre of the droplets.     

New approaches to numerical modelling of droplet transient heating and evaporation

New approaches to numerical modelling of droplet heating and evaporation by convection and radiation from the surrounding hot gas are suggested. The finite thermal conductivity of droplets and recirculation in them are taken into account. These approaches are based on the incorporation of new analytical solutions of the heat conduction equation inside the droplets (constant or almost constant h) or replacement of the numerical solution of this equation by the numerical solution of the integral equation (arbitrary h). It is shown that the solution based on the assumption of constant convective heat transfer coefficient is the most computer efficient for implementation into numerical codes. This solution is applied to the first time step, using the initial distribution of temperature inside the droplet. The results of the analytical solution over this time step are used as the initial condition for the second time step etc. This approach is applied to the numerical modelling of fuel droplet heating and evaporation in conditions relevant to diesel engines, but without taking into account the effects of droplet break-up. It is shown to be more effective than the approach based on the numerical solution of the discretised heat conduction equation inside the droplet, and more accurate than the solution based on the parabolic temperature profile model. The relatively small contribution of thermal radiation to droplet heating and evaporation allows us to take it into account using a simplified model, which does not consider the variation of radiation absorption inside droplets.     

An overview of utilizing water-in-diesel emulsion fuel in diesel engine and its potential research study

The need for more efficient energy usage and a less polluted environment are the prominent research areas that are currently being investigated by many researchers worldwide. Water-in-diesel emulsion fuel (W/D) is a promising alternative fuel that could fulfills such requests in that it can improve the combustion efficiency of a diesel engine and reduce harmful exhaust emission, especially nitrogen oxides (NO_x) and particulate matter (PM). To date, there have been many W/D emulsion fuel studies, especially regarding performance, emissions and micro-explosion phenomena. This review paper gathers and discusses the recent advances in emulsion fuel studies in respect of the impact of W/D emulsion fuel on the performance and emission of diesel engines, micro-explosion phenomena especially the factors that affecting the onset and strength of micro-explosion process, and proposed potential research area in W/D emulsion fuel study. There is an inconsistency in the results reported from previous studies especially for the thermal efficiency, brake power, torque and specific fuel consumption. However, it is agreed by most of the studies that W/D does result in an improvement in these measurements when the total amount of diesel fuel in the emulsion is compared with that of the neat diesel fuel. NO_x and PM exhaust gas emissions are greatly reduced by using the W/D emulsion fuel. Unburnt hydrocarbon (UHC) and carbon monoxide (CO) exhaust emissions are found to be increased by using the W/D emulsion fuel. The inconsistency of the experimental result can be related to the effects of the onset and the strength of the micro-explosion process. The factors that affect these measurements consist of the size of the dispersed water particle, droplet size of the emulsion, water-content in the emulsion, ambient temperature, ambient pressure, type and percentage of surfactant, type of diesel engine and engine operating conditions. Durability testing and developing the fuel production device that requires no/less surfactant are the potential research area that can be explored in future.     

An experimental investigation of the breakup characteristics of secondary atomization of emulsified fuel droplet

In this study, the breakup characteristics of secondary atomization of an emulsified fuel droplet were investigated with a single droplet experiment. In the single droplet experiment, the emulsified fuel droplet suspended from a fine wire was inserted into an electric furnace, and then secondary atomization behavior was observed using a high-speed video camera. Moreover, a mathematical model to calculate the generated water vapor at micro-explosion was proposed by using the mass and energy conservation equations under some assumptions. In the proposed model, that can be calculated by using the inner droplet temperature history at micro-explosion. As a result, puffing and micro-explosion occurred even when the fine ceramics fiber was used. The proposed model showed that about 50–70 wt% of water in the emulsified fuel changed to water vapor instantaneously at the occurrence of micro-explosion. The mass of water necessary for micro-explosion was shown. The breakup time was closely related to the superheat temperature just before the occurrence of micro-explosion.     

Numerical study on flash‐boiling spray of multicomponent fuel

Flash‐boiling occurs when a fuel is injected into a combustion chamber where the ambient pressure is lower than the saturation pressure of the fuel. It has been known that flashing is a favorable mechanism for atomizing liquid fuels. On the other hand, alternative fuels, such as gaseous fuels and oxygenated fuels, are used to achieve low exhaust emissions in recent years. In general, most of these alternative fuels have high volatility and flash‐boiling takes place easily in the fuel spray when injected into the combustion chamber of an internal combustion engine under high pressure. In addition the multicomponent mixture of high‐ and low‐volatility fuels has been proposed in the previous study in order to control the spray and combustion processes in an internal combustion engine. It was found that the multicomponent fuel produces flash‐boiling with an increase in the initial fuel temperature. Therefore, it is important to investigate these flash‐boiling processes in fuel spray. In the present study, the submodels of a flash‐boiling spray are constructed. These submodels consider the bubble nucleation, growth, and disruption in the nozzle orifice and injected fuel droplets. The model is implemented in KIVA3V and the spray characteristics of multicomponent fuel with and without flashing are numerically investigated. In addition, these numerical results are compared with experimental data obtained in the previous study using a constant volume vessel. The flashing spray characteristics from numerical simulation qualitatively show good agreement with the experimental results. In particular, it is confirmed from both the numerical and experimental data that flash‐boiling effectively accelerates the atomization and vaporization of fuel droplets. This means that a lean homogeneous mixture can be quickly formed using flash‐boiling in the combustion chamber. © 2006 Wiley Periodicals, Inc. Heat Trans Asian Res, 35(5): 369–385, 2006; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20117     

Modeling of axially and transversely injected precursor droplets into a plasma environment

Transport model of droplets injected axially or transversely into a plasma jet is presented. The droplets contain a ceramic precursor that precipitates in the vaporizing droplets. Droplet temperature and solute concentration distributions were predicted to estimate precipitation zones based on a homogeneous nucleation hypothesis. Effects of droplet size, injection velocity and injection angle were parametrically studied and the results were compared for axial and transverse injection. Axial injection is found to result in more rapid heat-up and precipitation than transverse injection. Computations suggest that small droplets form solid spherical particulates whereas larger droplets may form ceno-spheres and thin shell structures.     

On the suppression of negative temperature coefficient (NTC) in autoignition of n-heptane droplets

Autoignition of n-heptane droplets under microgravity is investigated numerically. The comprehensive model, considering the transience in both the gas and liquid phases and non-ideal thermophysical properties, includes the 116-step reaction mechanism of Griffiths. Two-stage ignition manifests for ambient temperature less than 900 K at elevated pressures of 0.5 and 1.0 MPa. The predicted first delays and total delays agree well with the experimental data in the literature. The second delay decreases greatly with increasing pressure because a stronger Stefan flow supplies more fuel vapor for reaction as the cool flame shifts closer to the droplet to enhance evaporation. The Stefan flow effect, in combination with the inhomogeneous temperature and fuel vapor distributions, explains why the NTC (negative temperature coefficient) present in homogeneous mixtures is not observed in droplet ignition experiments. Near the minimum ignition diameter, the ignition delay increases for smaller droplets at T_∞ = 700 K, P_∞ = 1.0 MPa. For a droplet smaller than the minimum ignition diameter, only first ignition with cool flame is reached. The absence of ZTC (zero temperature coefficient) in our simulations may be attributed to the weaker inverse temperature dependence of the reaction mechanism adopted.     

The effect of internal diffusion on an evaporating bio-oil droplet – The chemistry free case

The chemical diversity of the components in bio-oil has a significant effect on its evaporation. The low boiling point compounds, such as simple acids and water, evaporate away from the surface at a faster rate than the internal diffusion. As a result a significant proportion still remains in the droplet core late in the evaporation process. As the droplet temperature increases, these trapped chemicals, in the centre of the droplet, can reach a sufficiently high temperature to cause them to vapourise resulting in rapid expansions, which can fracture the droplet. In this study a numerical model of a stationary, spherically symmetric evaporating bio-oil droplet, in a hot ambient atmosphere, is used to investigate the effect of the rate of diffusion on the evaporation process.     

Hysteresis Phenomena at the Onset of Subcooled Nucleate Flow Boiling in Microchannels

In this paper, attempts were made to experimentally investigate the boiling incipience in a narrow rectangular vertical channel of 1 mm depth with an external 40 mm wide wall heated uniformly and others assumed quasiadiabatic. The “boiling front” location was determined from the temperature distribution of the heated wall obtained from liquid crystal thermography. Boiling incipience occurs when a considerable rise in the wall temperature above the saturation temperature takes place. Thus, boiling incipience is accompanied by “nucleation hysteresis.” The impact of various factors on the boiling incipience in microchannels, such as pressure, the inlet liquid subcooling, and flow velocity, were investigated.     

Investigation of heat and mass transfer between the two phases of an evaporating droplet stream using laser-induced fluorescence techniques: Comparison with modeling

Heat and mass exchanges between the two phases of a spray is a key point for the understanding of physical phenomena occurring during spray evaporation in a combustion chamber. Development and validation of physical models and computational tools dealing with spray evaporation requires experimental databases on both liquid and gas phases. This paper reports an experimental study of evaporating acetone droplets streaming linearly at moderate ambient temperatures up to 75 °C. Two-color laser-induced fluorescence is used to characterize the temporal evolution of droplet mean temperature. Simultaneously, fuel vapor distribution in the gas phase surrounding the droplet stream is investigated using acetone planar laser-induced fluorescence.Temperature measurements are compared to simplified heat and mass transfer model taking into account variable physical properties, droplet-to-droplet interactions and internal fluid circulation within the droplets. The droplet surface temperature, calculated with the model, is used to initiate the numerical simulation of fuel vapor diffusion and transport in the gas phase, assuming thermodynamic equilibrium at the droplet surface. Influence of droplet diameter and droplet spacing on the fuel vapor concentration field is investigated and numerical results are compared with experiments.     

Evaporation and condensing augmentation of water droplets in flue gas

The unsteady heat and mass transfer of sprayed water in the flue gas is modelled according to the iterative method of numerical research. The complex “droplet problem” covers the analysis of combined energy transfer in a semitransparent droplet, also combined heating and evaporation of the droplet. The surface temperature of the evaporating droplet is determined, at which the balance of energy fluxes taken to the surface and taken from the surface is reached. The thermal state mode of an evaporating droplet depends on the way of droplet heating as well. The change of thermal state and phase transformations parameters of water droplets warming in flue gas is analysed in the universal time scale. The initial evaluation of heat energy accumulated in exhaust flue gas utilization by water injection is presented.     

Microexplosion and ignition of biodiesel/ethanol blends droplets in oxygenated hot co-flow

Microexplosion and ignition characteristics of biodiesel/ethanol blends were studied by suspending single droplets on fibers in a tubular oven at different ambient conditions. The range of ethanol content in initial fuel, as an important parameter, was changed from 0% to 100%.Heating temperature and gas flow were also reference variables, which were respectively changed from 300 °C to 500 °C and from 0 L/min to 10 L/min. Behaviors of droplet microexplosion and ignition were recorded by high speed camera. Temporal variation of droplet size, time to microexplosion and ignition delay time were all analyzed for every cases. The results showed that microexplosion could be found in most cases due to high volatility distinction of biodiesel and ethanol and be expedited and intensified at fuel for nearly equivalent volume mixing, or at enough high temperature and in a certain gas flow. However, ignition was achieved only in some situations which biodiesel content in initial fuel and temperature was relatively high and gas flow rate was moderate. Further, under the condition that the temperature and flow rate remain unchanged, when the ethanol content reached 50%, the micro-explosion intensity which was named the normalized diameter reaches the maximum (1.3). At the same time, the delay time of micro-explosion is also reduced to the minimum (0.23 s). Under the condition that the ratio and flow rate remained the same, when the ambient temperature rose to 400 °C, the micro-explosion intensity reached the maximum. At this time, the normalized diameter of the mixed droplets approached 1.5. Under the condition that the temperature and ratio remained the same, when the flow rate was 5 L/min, the ignition delay time was the lowest (about 2.8 s), when the flow rate was 10 L/min, the micro-explosion intensity was the largest, and the normalized diameter reached 1.8. Beyond this, it was found that raising the ratio of ethanol in the blended fuel could increase the burning rate but lower the ignitability, and the ignition delay time could be shortened when the droplet exhibited microexplosion and fuel of near equi-volume blends, experiencing the most violent microexplosion, optimized the improvement, which could be found more obviously at high temperature and high speed airflow.