Liquid Length Research Articles

Numerical modeling and analysis of coaxial electrohydrodynamic jet printing

Coaxial electrohydrodynamic jet (CE-Jet) printing is an encouraging method for fabrication of high-resolution micro and nanostructures in MEMS systems. This paper presents a novel simulation work based on phase field method which is considered as a precise technique in fluid dynamics. The study explores influence of various parameters such as applied voltage, needle-substrate distance, dynamic viscosity, relative permittivity, needle size and flow rate on stability and resolution of CE-Jet morphologies. The morphology of CE-Jet exhibits that width of cone-jet profile and printed structures on substrate were directly proportional to relative permittivity and flow rate. In addition, it was inversely proportional to dynamic viscosity and applied voltage. The study examine that CE-Jet length of inner liquid is inversely proportional to needle-substrate distance in same time. It was later verified in experimental study by producing stable CE-Jet morphology with 300 μm diameter using optimized parameters (i.e., DC voltage 7.0 kV and inner liquid flow rate 400 nl/min) as compared to other validation studies such as 400 μm and 500 μm. The CE-Jet printing technique investigates significant changes in consistency and stability of CE-Jet morphologies and makes Jet unique and comparable when adjustment accuracy reaches 0.01 mm. PZT sol line structures with a diameter of 1 µm were printed directly on substrate using inner needle (diameter of 120 µm). Therefore, it is considered as a powerful tool for nano constructs production in M/NEMS devices.

Dynamic behavior and heat transfer characteristics of a droplet impacting on the micro corrugated tube

Liquid droplets impacting on rough or structured surfaces occur ubiquitously in nature and are applicable in many industrial activities. In this study, a 3-D numerical method has been selected to explore the dynamic behavior and heat transfer characteristics of an individual droplet impacting on the micro corrugated tube. The differences in dynamic behavior and heat transfer characteristics between single droplet impacting on a micro corrugated tube and a smooth circular tube were compared. Besides, how impact velocity and wettability affect hydrodynamics and heat transfer was studied. It turns out that the liquid film morphology, spreading length and liquid film contact area on the micro corrugated tube differ from those on the smooth circular tube due to the corrugations’ inhibition and anisotropy of the cylindrical surface. Furthermore, a considerably larger heat transfer coefficient is recognized within the former, owing to the greatly smaller liquid film contact area and larger liquid film average temperature on the micro corrugated surface than those on the smooth circular surface. As impact velocity rises, the global heat flow increases and finally remains unchanged on the micro corrugated tube and the smooth circular tube. With a growing surface hydrophobicity, structures (rough or smooth surfaces) play a less important role in affecting the droplet's morphology variation, and the heat transfer coefficient is barely affected, but the tendency to decline occurs in the global heat flow. This study can enrich and develop the dynamics and heat transfer theory of droplets impacting on rough surfaces and provide guidance for engineering applications such as spray cooling and anti-icing etc.

Influence of alkyl chain length of imidazolium-based ionic liquid on the crude oil-aqueous solution IFT under different ionic strengths

Due to the unique potentials of ionic liquid (ILs) for interfacial tension (IFT) reduction, a large number of studies concentrated on the applications of these chemicals. In this way, the current study compares the IFT of 1-dodecyl-3-methylimidazolium chloride ([C12mim]Cl) and 1-octadecyl-3-methylimidazolium chloride ([C18mim]Cl) in the presence and absence of NaCl, to investigate the effect of alkyl chain length on the ILs performance. The measured IFTs reveal that, [C18mim]Cl has a higher influence on the IFT reduction in comparison to [C12mim]Cl, in the absence of ions. However, in the presence of NaCl, longer alkyl chain has a destructive effect on the IFT reduction which means the better efficiency of [C12mim]Cl for IFT reduction compared to [C18mim]Cl. According to the findings, the critical micelle concentrations (CMC), the functionality of IFT and the alkyl chain length of imidazolium based ILs depend not only on the number of methyl presented in the cationic part of the IL, but also on the ionic strength of the aqueous solution. The common effect of alkyl chain length (i.e. reduction of surface activity as a function of cation chain length) can be reversed at the presence of the salt, as the surface activity of shorter cation chain length is higher in the presence of NaCl.

Experimental analysis and transient numerical simulation of a large diameter pulsating heat pipe in microgravity conditions

A multi-parametric transient numerical simulation of the start-up of a large diameter Pulsating Heat Pipe (PHP) specially designed for future experiments on the International Space Station (ISS) are compared to the results obtained during a parabolic flight campaign supported by the European Space Agency. Since the channel diameter is larger than the capillary limit in normal gravity, such a device behaves as a loop thermosyphon on ground and as a PHP in weightless conditions; therefore, the microgravity environment is mandatory for pulsating mode. Because of a short duration of microgravity during a parabolic flight, the data concerns only the transient start-up behavior of the device. One of the most comprehensive models in the literature, namely the in-house 1-D transient code CASCO (French acronym for Code Avancé de Simulation du Caloduc Oscillant: Advanced PHP Simulation Code in English), has been configured in terms of geometry, topology, material properties and thermal boundary conditions to model the experimental device. The comparison between numerical and experimental results is performed simultaneously on the temporal evolution of multiple parameters: tube wall temperature, pressure and, wherever possible, velocity of liquid plugs, their length and temperature distribution within them. The simulation results agree with the experiment for different input powers. Temperatures are predicted with a maximum deviation of 7%. Pressure variation trend is qualitatively captured as well as the liquid plug velocity, length and temperature distribution. The model also shows the ability of capturing the instant when the fluid pressure begins to oscillate after the heat load is supplied, which is a fundamental information for the correct design of the engineering model that will be tested on the ISS. We also reveal the existence of strong liquid temperature gradients near the ends of liquid plugs both experimentally and by simulation. Finally, a theoretical prediction of the stable functioning of a large diameter PHP in microgravity is given. Results show that the system provided with an input power of 185W should be able to reach the steady state after 1min and maintain a stable operation from then on.

Modelling and Validating the Spray Characteristics of a Co-axial Twin-Fluid Atomizer Using OpenFOAM

Today, the applications of sprays cover a wide range of fields. Their role in internal combustion engines is instrumental in maintaining higher engine efficiency. A deeper understanding of the liquid-gas phase interaction in sprays is crucial to the atomization process. The methods and models used in the simulations have their challenges due to the various discretization schemes and solutions used. To develop and validate the computational models, well defined experimental data is required. In the present work, spray characteristics were studied numerically through OpenFOAM. As the spray characteristics are closely linked with the liquid breakup length, this study focuses on the primary breakup phenomena and the breakup length of the liquid jet emanating from the twin-fluid co-axial flow atomizer. Numerical simulations were performed for a wide range of initial conditions and the breakup length of the spray was validated against the experimental observed by Sivadas et al., [26]. These simulations were carried out using a Eulerian based VOF solver that models the fluid as a continuum. K-Epsilon model was used to predict the turbulent nature of the spray. The air and water velocities were varied between 19.0 to 31.3 m/s and 0.7 to 1.8 m/s respectively. The proposed model was able to predict the computed breakup length within 20% of the experimental values. The present model can be further extended to test for a co-axial swirl injector to predict finer spray formation.

A transient analysis of slug flow in a horizontal pipe using slug tracking model: Void and pressure wave

In the oil and gas industry, the transient flow arises in daily situations, such as the start-up and shutdown of lines, pigging, artificial lifting, and drilling operations. These disturbances affect the temporal behavior of pressure and void fraction, which could cause separator flooding, system vibration, and production line facilities damage. Nevertheless, there are few numerical studies to describe this phenomenon. In this article, the transient slug flow is studied using the slug tracking model, which predicts the transient behavior and the evolution of slug flow properties. This study demonstrates the slug tracking model's capability to reproduce the transient flow as well as the pressure and void fraction wave's behavior and velocity. Experimental data presented by Maria and Rosa, 2016 were used to assess the model performance. The experimental data show a 6.0 s time delay between the two steady states. This time delay does not influence the void fraction wave, and its numerical maximum deviation is 9.0%. On the other hand, the numerical performance for the pressure wave velocity depends on the time delay. Using the experimental time delay, the maximum deviation for the pressure wave velocity is 6.8%. The slug tracking model is combined with a mass–dashpot–spring analogy to avoid the experimental time delay dependency. Using this methodology, the maximum deviation for pressure wave velocity is 8.2%. Some slug flow properties, such as the bubble nose translational velocity, the bubble and liquid slug lengths, are also compared to experimental data. The comparison is for averaged values and statistical distribution. The bubble nose velocities and pressure deviations are less than 1.5%, while the bubble and liquid slug lengths deviations are 4.1% and 5.3%, respectively. Most slug flow parameters have normal statistical distribution, but slug length follows a close to log-normal distribution. The model reproduces this behavior.

Experimental study of the effect of various collision angles and critical conditions on marine engine’s twin-spray collision process

To enhance the fuel-gas mixing and phase transition process, the fuel is injected by twin injectors in a large-bore low-speed two-stroke marine engine, while the cylinder condition has reached the transcritical and supercritical conditions. The twin-injector configuration has a great potential for further optimization, but the exploration on the outcome of collision and phase transition was still limited. Therefore, this work aims to study the effect of various collision angles (60°, 90°, 120°, 150°) and critical conditions (sub/trans/supercritical) on the twin-spray collision process using optical techniques. A wide range of experimental cases are conducted to provide an analysis and database for future modeling validation. The post-collisional spray structures, spatial distribution, and periphery features are analyzed to characterize the droplet’s collision. The results show that with the collision angle increasing, the higher collision velocity enhances the mass transfer while the minor vertical component results in a smaller axial dispersion. Because of the trade-off relationship between the vertical velocity component and pre-collision penetration, a higher reduction in droplet momentum results in a slighter collision behavior. At the collision angle of 150°, the subcritical condition tends to result in an off-axis collision. Under the transcritical (P) condition, the probability of head-on collision increases and presents a wider spatial distribution. But under the supercritical condition, because of the existence of the liquid collision, the thermal conversion among phases is accelerated, while the ambient resistance is reduced. Moreover, an exponential correlation of collision liquid length is formulated to predict the axial dispersion based on various critical conditions.

Statistical analysis of Taylor bubble formation in a capillary pipe

An experimental study of the Taylor regime in a pipe with an inner diameter of 2 mm has been carried out. A method for studying the Taylor regime in a capillary pipe using automatic image analysis to measure the main characteristics of bubbles has been developed and applied for the first time. The dependences of the gas bubble and liquid slug lengths on the gas and liquid velocities are studied. It was found that in the region of the stable Taylor regime, the standard deviation of the bubble sizes is close to the accuracy of the research method. The dispersion of the bubble size distribution increases near the regime boundary. It is shown that, based on the statistical analysis of a large amount of data, it can be concluded that there is a coalescence and fragmentation of bubbles. Keywords: capillary pipe, two-phase flow, Taylor regime, statistical analysis.

Статистический анализ формирования пузыря Тейлора в капиллярной трубке

An experimental study of the Taylor regime in a pipe with an inner diameter of 2 mm has been carried out. A method for studying the Taylor regime in a capillary pipe using automatic image analysis to measure the main characteristics of bubbles has been developed and applied for the first time. The dependences of the gas bubble and liquid slug lengths on the gas and liquid velocities are studied. It was found that in the region of the stable Taylor regime, the standard deviation of the bubble sizes is close to the accuracy of the research method. The dispersion of the bubble size distribution increases near the regime boundary. It is shown that based on the statistical analysis of a large amount of data, it can be concluded that there is a coalescence and fragmentation of bubbles.

The Corrected Distortion model for Lagrangian spray simulation of transcritical fuel injection

In this work, we present a detailed implementation and validation of the droplet modeling framework proposed by Dahms and Oefelein (2016) into the engine commercial CFD software CONVERGE using the User Defined Function (UDF) interface. The model accounts for the nonlinear deformation and oscillation experienced by liquid spray droplet injected into high pressure and temperature. Lagrangian spray simulations of Engine Combustion Network (ECN) Spray A are performed. Model validation against standard experimental measurements of liquid velocity, vapor mixture fraction is conducted. To perform more rigorous model validation, new experimental measurements based on Diffused Back Illumination (DBI) are introduced. The new measurements are processed for Projected Liquid Volume (PLV), which offers as close as possible one-to-one model validation for liquid penetration while offering new insights into the spray physics. Comparison with a One-D model based on adiabatic mixing theory by Siebers (1999) and Desantes et al. (2007) are also conducted. Through these model validation exercises, it is shown that the new framework improves liquid-phase penetration predictions, following a tendency for enhanced evaporation, compared to the standard approach for both Reynolds Average Navier Stokes (RANS) and Large Eddy Simulation (LES). At the liquid length, maximum mixture fraction values predicted by the new approach are in good agreement those of an adiabatic mixing model. Qualitative analysis of the spray behaviors during the early stage of the injection process reveals that the proposed framework predicts significant increase in droplet evaporation rate with lower drop drag compared to the current standard approach.

Large-eddy simulation of the injection timing effects on the dual-fuel spray flame

Large-eddy simulations (LES) coupled with a partially-stirred reactor model and a finite-rate chemistry are carried out to investigate the effects of n-heptane injection timing on the methanol fueled dual-fuel (DF) combustion. Methanol is premixed with air in a constant volume chamber (T=1000 K, ρ=14.8 kg/m3) to form a homogeneous mixture (equivalence ratio ϕm of 0.3). Liquid fuel n-heptane is provided from a high pressure injector to mimic the pilot fuel injection in DF engines. First, mesh sensitivity analysis and LES model validation are conducted. The experimental data of Spray-H (n-heptane fueled) from the Engine Combustion Network is used for model validation. It is shown that the present mesh and LES model are capable of replicating the liquid and vapor penetration length, mixture fraction, temperature distribution, pressure rise profile and ignition delay time (IDT). Second, the effects of n-heptane injection timing are investigated, by varying the start of injection (SOI) time. The LES results reveal that there are three stage heat releases in the DF combustion. With the delay of SOI, the mass fraction of hydrogen peroxide in the ambient mixture increases, leading to an early formation of hydroxyl. Therefore, the two-stage IDTs of n-heptane decrease, while the ambient methanol IDT increases. Results also show the cool flame and high-temperature flame evolution after methanol auto-ignition. The cool flame disappears while the high-temperature flame is found near the injector nozzle, which leads to a relatively high heat release rate.

Large-Eddy Simulation of Laser-Ignited Direct Injection Gasoline Spray for Emission Control

Large-Eddy Simulations (LES) of a gasoline spray, where the mixture was ignited rapidly during or after injection, were performed in comparison to a previous experimental study with quantitative flame motion and soot formation data [SAE 2020-01-0291] and an accompanying Reynolds-Averaged Navier–Stokes (RANS) simulation at the same conditions. The present study reveals major shortcomings in common RANS combustion modeling practices that are significantly improved using LES at the conditions of the study, specifically for the phenomenon of rapid ignition in the highly turbulent, stratified mixture. At different ignition timings, benchmarks for the study include spray mixing and evaporation, flame propagation after ignition, and soot formation in rich mixtures. A comparison of the simulations and the experiments showed that the LES with Dynamic Structure turbulence were able to capture correctly the liquid penetration length, and to some extent, spray collapse demonstrated in the experiments. For early and intermediate ignition timings, the LES showed excellent agreement to the measurements in terms of flame structure, extent of flame penetration, and heat-release rate. However, RANS simulations (employing the common G-equation or well-stirred reactor) showed much too rapid flame spread and heat release, with connections to the predicted turbulent kinetic energy. With confidence in the LES for predicted mixture and flame motion, the predicted soot formation/oxidation was also compared to the experiments. The soot location was well captured in the LES, but the soot mass was largely underestimated using the empirical Hiroyasu model. An analysis of the predicted fuel–air mixture was used to explain different flame propagation speeds and soot production tendencies when varying ignition timing.

Numerical study of spray combustion and soot emission of gasoline–biodiesel fuel under gasoline compression ignition-relevant conditions

A blend of biodiesel and gasoline fuel has been proposed for powering gasoline compression ignition (GCI) engines to improve overall reactivity, which is essential for the auto-ignition quality of GCI engines. Previous studies emphasized the differences in the physicochemical properties of hydrogenated catalytic biodiesel (HCB) and gasoline, which significantly altered the combustion characteristics. This study is a further step in this direction to gain a deeper understanding on the spray combustion and emission behavior of this promising fuel based on computational fluid dynamic simulation in a constant volume chamber. N-Hexadecane and toluene reference fuel were selected as surrogates for HCB and gasoline fuel, respectively. A reduced kinetic mechanism for HCB-gasoline surrogate was developed and combined with a reduced PAH mechanism to simulate HCB-gasoline spray combustion and soot formation. Liquid penetration length, ignition delay, lift-off length, and soot formation for HCB and HCB/gasoline surrogates were validated against available experimental data in the literature. The proposed mechanism along with a multi-step phenomenological soot model could capture the spray characteristics and soot formation of the tested fuels with acceptable accuracy under GCI engine combustion conditions. The numerical results showed larger spray/flame development for HCB fuel as compared with that of HCB/gasoline blend prior to ignition and at ignition stage, which was mainly due to the less fuel spray mixing with the surrounding fresh air. Furthermore, the outer diffusion flames for HCB and HCB/gasoline blend were identical and existed on the stoichiometric region (at ϕ ∼ 1), while the rich pre-mixed core of HCB was located at higher equivalence ratio, which was more favorable for precursors formation. HCB/gasoline fuel had a lower soot mass yield compared with HCB because of its lower C2H2 concentration, which slowed down the surface growth step. In addition, the effects of ambient oxygen level and ambient temperature on flame structure and soot formation were also investigated. The current study provides a useful information about spray combustion and soot formation of HCB–gasoline blend, which has practical applications in GCI engines.

Interaction between multiple bubbles in microchannel flow boiling and the effects on heat transfer

Flow boiling in microchannels have widely been studied in order to design more efficient cooling systems with numerical simulations forming a crucial part to deal with the areas that cannot be investigated experimentally. Previously published research largely focussed on the behaviour of a single bubble. Here, we focus on the behaviour of multiple bubbles. In this study, the influence of the distance between the bubbles (liquid slug length) is investigated in both an axisymmetric and a planar domain for two and three bubbles present. In this regard, an interface-tracking adaptive mesh refinement model was implemented to improve simulation time. Results show that the heat transfer was improved with sequential bubbles, and a 50% increase in heat transfer coefficient was observed for the cases investigated with three bubbles present. The heat transfer also improved the closer the bubbles were together.

Experimental investigation of flow and pressure drop characteristics of air-oil slug flow in a horizontal tube

This study examines two-phase flow and pressure drop characteristics associated with slug flow in a tube with an inner diameter of 4 cm. Experiments are conducted using air and low viscosity mineral oil as working fluids at atmospheric pressure and two different oil temperatures, 25 and 45°C. The superficial gas and liquid velocities are in the range of 0.33–2.43 m/s and 0.20–1.20 m/s, respectively. Flow visualization images and temporal records of pressure drop oscillation are presented for different gas and liquid superficial velocities. The relevant flow parameters associated with slug flow, such as the elongated bubble velocity, elongated bubble length, liquid slug length, and liquid film thickness, are measured, and the relationship between the slug flow parameters and the superficial gas and liquid velocities is investigated. The measured pressure gradient data are also compared to predictions of previous homogeneous equilibrium models and semi-empirical correlations for two-phase frictional pressure gradient. A theoretical slug flow model for predicting the total pressure gradient of slug unit is presented using empirical correlations for the slug flow parameters. The new model accurately predicts the experimental pressure gradient data, evidenced by an overall mean absolute error of 19.3%.

Impact of coolant temperature on the combustion characteristics and emissions of a stratified-charge direct-injection spark-ignition engine fueled with E30

The direct injection spark ignition (DISI) engine has received considerable attention due to its potential to increase the power density of traditional spark ignition engines while significantly improving fuel economy through lean, unthrottled combustion. However, the market introduction of DISI engines operated in a lean combustion mode is inhibited by their unsatisfactory emissions, especially during cold start conditions that make proper mixture formation more challenging. Ethanol-blended gasoline, now a widely used fuel, makes the cold start of a DISI engine more difficult, leading to higher HC and soot emissions because of the high latent heat of vaporization of ethanol relative to gasoline. This work investigated the impact of coolant temperature on the characteristics of combustion and emissions in a stratified-charge DISI engine fueled with an E30 fuel (i.e. 30% ethanol in gasoline), while the coolant temperature was alternated between four levels (45, 60, 75, and 90 °C) to simulate different conditions throughout the warm-up process. The experiments showed that the coolant temperature affected the post-spark inflammation time, as well as the speed, intensity, and stability of the combustion process in the engine. When the coolant temperature rose, the engine produced more NOX and less CO, PM and HC. In addition, high-speed direct photography was used to obtain crank-angle resolved images of fuel sprays and flames in the cylinder. As the coolant temperature rose, the liquid spray lengths became shorter, reducing the possibility of wall wetting, and reduced irradiance from soot particles also indicated less non-premixed combustion. The in-cylinder imaging results are consistent with the observed combustion and emission characteristics and shed light on the underlying processes. Some potential solutions to the emissions challenges faced here could be either raising in-cylinder temperatures by using trapped residuals or modifying the injection schedule, for example by increasing the number of injections or to inject later in the cycle into a higher-density environment.

The Effect of Fermentation Length and Dosage of Liquid of Organic Fertilizer Banana Peel on the Growth of Red Spinach (Amaranthus Tricolor L.)

Fertilizer substitutes elements that have been used by plants. There are two available fertilizers, inorganic and organic fertilizers. Long-term use of inorganic fertilizers on a land negatively affects the crop and the soil fertility, hence, the organic one is a better choice. This study aims to determine the effect of liquid organic fertilizer of banana peels on the growth of red spinach and determine the best treatment for red spinach growth. This study used a Completely Randomized Design consisted of 2 factors (dosage of POC banana peel and length of fermentation time). Each has 4 levels, 3 replicates. The results of data analysis using ANOVA test show that the treatment of fermentation length and dosage of POC banana peel increase growth parameters of red spinach include plant height, leaf number, leaf area, fresh weight, and dry weight, while the interaction treatment of fermentation length and dosage of POC banana peel affects almost all growth parameters, except for the leaf area parameter that shows no interaction. DMRT (α 5%) test shows that the best combination treatment for plant height and fresh weight is a dosage of 8 ml POC of banana peel that have been fermented for 15 days, and the best treatment for leaf number and dry weight is a dosage of 8 ml POC of banana peel that have been fermented for 10 days.

Investigating the in-flight droplets' atomization in suspension plasma-sprayed coating

A three-dimensional unsteady numerical model is employed to study the in-flight droplets/particles' atomization in the suspension plasma spraying (SPS) process. A user-defined function (UDF) written in C programming adds the electromagnetic fields to the fluid flow field. A two-way coupled Eulerian-Lagrangian technique simulates the sprayed droplets' interaction with the plasma flow. The developed model is applied to investigate the droplets' atomization behavior in the SPS process. Droplets' atomization is simulated by applying the Kelvin-Helmholtz Rayleigh-Taylor (KHRT) breakup model. This model considers the effects of both aerodynamic forces (Wave model) and Rayleigh-Taylor instabilities on the atomization process combing a liquid core region (Levich model). Previous works to estimate the KHRT breakup model's coefficients were performed on a liquid jet (e.g., fuel jet) into the air. Because the density ratio of suspension in the plasma is higher than that of the liquid jet into the air (suspension density is higher than a liquid droplet and plasma density is lower than air), the model's coefficients should be modified for the suspension atomization in the SPS process. For this reason, nine case studies are investigated to see the effect of KH and RT instabilities and the liquid core length on the droplets/particles' atomization. The results obtained from the suggested values show a good agreement compared to the existing data.

A novel model of liquid film flow and evaporation for thermal protection to a chamber with high temperature and high shear force

Thin liquid film evaporation is a prevailing cooling technique for thermal protection to the inner wall of a chamber with high-intensity combustion. A novel model is developed to investigate the flow and evaporation of liquid film in a rocket combustion chamber with high temperature and high shear force, in which shear stress at the interface between liquid film and high temperature combustion gas with high velocity, capillary pressure of liquid and disjoining pressure associated with liquid film evaporation are taken into account. A numerical algorithm of the backward solution is deliberately adopted to solve this model so as to eliminate the entrance effect caused by different inlet conditions of liquid film. For given temperature and hot gas rate in a chamber, a stably evolution regime of liquid film with the thickness from thick to thin is depicted corresponding to the total process of being sheared into film and being evaporated to dryness. Results indicate that the three forces mentioned above play critical parts in flowing and evaporating of liquid film during its different evolution regimes. It was shown that shear stress and capillary pressure have a positive effect on the flow and evaporation of film, as well as a negative effect from the disjoining pressure. The effects of these forces are different dependent on different initial average thickness of film, and the distribution profile of these forces are totally illustrated in detailed. The prediction of liquid film length, from being injected to being evaporated, by the proposed model is in a good agreement with available experimental data. All findings are expected to provide a fundamental guideline for the design and optimization of efficient thermal protect to the combustion chamber of space engine.

Numerical investigation of soot emission sources in a direct-injection spark-ignition engine based on comprehensive breakup model validation

Direct injection system is widely adopted in spark-ignition engines to achieve higher thermal efficiency, but it accompanies a penalty in particulate emission, especially when engine is not fully warmed-up. Split injection strategy is known to be an effective measure to reduce engine-out particulate emissions. To better understand the role of split injections, this study aims to analyze the effect of split injection strategy on the sources of soot formation using computational fluid dynamics simulation. To accurately predict changes in particulate mass and number associated with split injection strategy, it is vital that spray models be carefully validated against the experimental data since spray dynamics govern the formation of soot emission sources, such as local fuel-rich mixtures and wall-deposited fuel-films. To this end, a set of spray experiments for free sprays is performed to measure liquid penetration length and droplet size distribution, and hence a comprehensive validation is conducted for spray breakup models. Then, engine simulations are carried out to predict the change in soot sources according to split injection, and the trend of simulation results is compared against the measured engine-out particulate mass and number. Simulation results indicate that breakup model validation using both penetration length and droplet size data is critical for predicting fuel spray dynamics and formation of sources of soot emission. It is also revealed that the piston wetting decreases as the number of injections increases because less amount of fuel is injected when piston is closer to the injector. Lastly, the late evaporation of heavy gasoline components from fuel-film appears to be a significant contributor to soot precursors formation.