Spray Wall Research Articles

An Analysis of the Combustion Behavior of Ethanol, Butanol, Iso-Octane, Gasoline, and Methane in a Direct-Injection Spark-Ignition Research Engine

Future automotive fuels are expected to contain significant quantities of bio-components. This poses a great challenge to the designers of novel low-CO2 internal combustion engines because biofuels have very different properties to those of most typical hydrocarbons. The current article presents results of firing a direct-injection spark-ignition optical research engine on ethanol and butanol and comparing those to data obtained with gasoline and iso-octane. A multihole injector, located centrally in the combustion chamber, was used with all fuels. Methane was also employed by injecting it into the inlet plenum to provide a benchmark case for well-mixed “homogeneous” charge preparation. The study covered stoichiometric and lean mixtures (λ = 1.0 and λ = 1.2), various spark advances (30–50° CA), a range of engine temperatures (20–90°C), and diverse injection strategies (single and “split” triple). In-cylinder gas sampling at the spark-plug location and at a location on the pent-roof wall was also carried out using a fast flame ionization detector to measure the equivalence ratio of the in-cylinder charge and identify the degree of stratification. Combustion imaging was performed through a full-bore optical piston to study the effect of injection strategy on late burning associated with fuel spray wall impingement. Combustion with single injection was fastest for ethanol throughout 20–90°C, but butanol and methane were just as fast at 90°C; iso-octane was the slowest and gasoline was between iso-octane and the alcohols. At 20°C, λ at the spark plug location was 0.96–1.09, with gasoline exhibiting the largest and iso-octane the lowest value. Ethanol showed the lowest degree of stratification and butanol the largest. At 90°C, stratification was lower for most fuels, with butanol showing the largest effect. The work output with triple injection was marginally higher for the alcohols and lower for iso-octane and gasoline (than with single injection), but combustion stability was worse for all fuels. Triple injection produced a lower degree of stratification, with leaner λ at the spark plug than single injection. Combustion imaging showed much less luminous late burning with tripe injection. In terms of combustion stability, the alcohols were more robust to changes in fueling (λ = 1.2) than the liquid hydrocarbons.

Soot Evolution With Cyclic Crank-Angle-Resolved Two-Color Thermometry in an Optical Diesel Engine Fueled With Biodiesel Blend and ULSD

Biodiesel is a desirable alternative fuel for the diesel engine due to its low engine-out soot emission tendency. When blended with petroleum-based diesel fuels, soot emissions generally decrease in proportion to the volume fraction of biodiesel in the mixture. While comparisons of engine-out soot measurements between biodiesel blends and petroleum-based diesel have been widely reported, in-cylinder soot evolution has not been experimentally explored to the same extent. To elucidate the soot emission reduction mechanism of biodiesel, a single-cylinder optically-accessible diesel engine was used to compare the in-cylinder soot evolution when fueled with ultra-low sulfur diesel (ULSD) to that using a B20 biodiesel blend (20% vol./vol. biodiesel ASTM D6751-03A). Soot temperature and KL factors are simultaneously determined using a novel two-color optical thermometry technique implemented with a high-speed CMOS color camera having wide-band Bayer filters. The crank-angle resolved data allows quantitative comparison of the rate of in-cylinder soot formation. High-speed spray images show that B20 has more splashing during spray wall impingement than ULSD, distributing rebounding fuel droplets over a thicker annular ring interior to the piston bowl periphery. The subsequent soot luminescence is observed by high-speed combustion imaging and soot temperature and KL factor measurements. B20 forms soot both at low KL magnitudes over large areas between fuel jets, and at high values among remnants of the fuel spray, along its axis and away from the bowl edge. In contrast, ULSD soot luminescence is observed exclusively as pool burning on the piston bowl surfaces resulting from spray wall impingement. The soot KL factor evolution during B20 combustion indicates earlier and significantly greater soot formation than with ULSD. B20 combustion is also observed to have a greater soot oxidation rate, which results in lower late-cycle soot emissions. For both fuels, higher fuel injection pressure led to lower late-cycle soot KL levels. The apparent rate of heat release (ARHR) analysis under steady skip-fire conditions indicates that B20 combustion is less sensitive to wall temperature than that observed with ULSD due to a lesser degree of pool burning. B20 was found to have both a shorter ignition delay and shorter combustion duration than ULSD.

On the Behavior of Water Droplets When Moving Onto Blade Surface in a Wet Compression Transonic Compressor

The process of wet compression in an axial compressor is an intricate two-phase flow involving not only heat and mass transfer processes but also droplet breakup and even formation of discontinuous water film on the blade surface and then breaking into droplets. In this paper, the droplet-wall interactions are analyzed using the theory of spray wall impingement through two computational models for an isolated transonic compressor rotor (NASA rotor 37). Model 1, representing spread phenomenon, assumes that all droplets impacting on the blade are trapped in the water film and subsequently released from its trailing edge and enter the wake region with an equivalent mass flow but bigger in diameter and smaller in number. Whereas, the model 2, representing splashing phenomenon, assumes that upon impacting on the blade, the droplets will breakup into many smaller ones. The three-dimensional flow simulation results of these two models are analyzed and compared in this paper.

Combustion Simulation of a Diesel Engine in the pHCCI Mode with Split Injections by the Spatially Integrated CMC Model

Simulation is performed for a diesel engine in the partial homogeneous charge compression ignition (pHCCI) mode with split injections. The spatially integrated CMC model involves an additional heat loss term for the conditional enthalpy and the evaporation source terms for the mean mixture fraction variance. Two flame structures are defined to consider different evaporation and flame histories of the two split injection fuel groups. An independent transport equation is solved for each fuel vapor group with no mutual interaction between flame structures. The local mean mixture fraction and its variance determine the probability density functions (PDF) and the scalar dissipation rates (SDR). Calculated pressure traces show reasonable agreement with measurements for different loads and ratios between the two split injections. The overpredicted rate of pressure rise at ignition is attributed to simultaneous burning of all combustible premixture in the CMC model. The flammable limits are applied to consider extinction of lean mixture, wall film, or other possible factors for incomplete combustion of the pilot injection fuel, which burns completely after mixing with the second main injection fuel. Further research may be required for improved spray atomization and wall film models, which play a significant role in heat release and emissions of pHCCI engines.

Investigation of charge preparation strategies for controlled premixed charge compression ignition combustion using a variable pressure injection system

This paper uses a multi-dimensional computational fluid dynamics (CFD) code coupled with detailed chemistry, the KIVA-CHEMKIN code, to provide guidelines for solving problems with premixed combustion strategies, namely, lack of combustion phasing control, excessive pressure rise rate, and spray wall impingement due to early injections. A multiple injection concept is used to control combustion phasing and reduce the rate of peak pressure rise. To address spray—wall impingement, an adaptive injection strategy (AIS) using two-injection pulses at different injection pressures is employed. The combustion process considered is at a mid-load operating condition for the light-duty engine of the present study (nominal indicated mean effective pressure (IMEP) of 5.5 bar and high speed, 2000 r/min) and the effects of first and second pulse injection pressure and timing, swirl ratio, and spray targeting are explored. The investigation showed that an optimized low-pressure early cycle injection combined with a high-pressure near top dead centre (TDC) injection allows combustion phasing to be well controlled while achieving premixed compression ignition (PCI)-like emissions levels. An improved solution was found with near-zero nitric oxides (NO x) and soot, a net indicated specific fuel consumption (ISFC) of only 175 g/kW h, and a peak pressure rise rate of ∼8 bar/deg.

A laser induced fluorescence technique for quantifying transient liquid fuel films utilising total internal reflection

This paper describes the development of a laser induced fluorescence (LIF) technique to quantify the thickness and spatial distribution of transient liquid fuel films formed as a result of spray–wall interaction. The LIF technique relies on the principle that upon excitation by laser radiation the intensity of the fluorescent signal from a tracer like 3-pentanone is proportional to the film thickness. A binary solution of 10% (v/v) of 3-pentanone in iso-octane is used as a test fuel with a Nd:YAG laser as the excitation light source (utilising the fourth harmonic at wavelength 266 nm) and an intensified CCD camera is used to record the results as fluorescent images. The propagation of the excitation laser beam through the optical piston is carefully controlled by total internal reflection so that only the fuel film is excited and not the airborne droplets above the film, which had been previously shown to induce significant error. Other known sources of error are also carefully minimised. Calibrated temporally resolved benchmark results of a transient spray from a gasoline direct injector impinging on a flat quartz crown under atmospheric conditions are presented, with observations and discussion of the transient development of the fuel film. The calibrated measurements are consistent with previous studies of this event and demonstrate the applicability of the technique particularly for appraisal of CFD predictions. The potential utilisation of the technique under typical elevated ambient conditions is commented upon.

Experimental study of pollutant emission reduction for near-stoichiometric diesel combustion in a three-way catalyst

An experimental study was performed to examine the possibility of simultaneous reductions of NO x, hydrocarbons (HC), CO, and soot emissions for near-stoichiometric diesel combustion using a three-way catalyst and a diesel particulate filter (DPF). The DPF was deactivated in order to ensure the non-regeneration state and was just used as a soot filter trap upstream of the three-way catalyst. The goal of the research was to investigate the efficiency of emission conversion for an automotive diesel engine with a general three-way catalyst normally used for gasoline-powered vehicles under near-stoichiometric combustion operation. A modified single-cylinder engine was used for the experiments that considered throttled diesel combustion at 65–90 kPa intake pressure to achieve near-stoichiometric conditions, and a common rail injection system with 90 MPa injection pressure was used for mixture preparation and to avoid spray wall impingement. The results showed that the three-way catalyst was able to reduce NO x and CO emissions by up to nearly 98 per cent, and the DPF trapped soot emissions up to 99.6 per cent for near-stoichiometric operation. However, the particular palladium/rhodium catalyst formulation used in the study was less effective for HC emissions, which were only reduced by up to 57 per cent. It is concluded that the use of the present proposed three-way catalyst/DPF aftertreatment system provides significant promise for future efficient, low emission diesel engines.

Numerical and experimental study on effect of wall geometry on wall impingement process of hollow-cone fuel spray under various ambient conditions

The spray–wall impingement process in gasoline direct injection (GDI) engines, which is caused by the interaction among spray, wall and air to move the air–fuel mixture near the spark plug, directly influences the engine performance and emissions. Therefore, a detailed understanding of this process is very important in designing an injection system and controlling a strategy of GDI engines. The purpose of this study is to understand the spray–wall impingement characteristics for more efficient designing of the injection system in GDI engines and to supply the fundamental data under engine operation conditions. The wall impingement processes of hollow-cone fuel spray according to ambient gas conditions and wall geometry are calculated by validated spray models. The calculated results were compared with the experimental results obtained by the laser-induced exciplex fluorescence (LIEF) technique. It was found that the spray and vortex cloud at the high ambient pressure were distributed at inner area of cavity and the more fuel film mass observed at this condition. The fuel film mass decreased with the increase of ambient temperature, while the fuel film mass increased at high cavity angles.

Computational Analysis of the Oxidizer Preflow in an Upper-Stage Rocket Engine

A detailed computational analysis of the oxidizer preflow during startup of a storable propellant upper-stage rocket engine is presented. This low-pressure flow regime is controlled by various two-phase phenomena, particularly flash atomization and flash evaporation of superheated liquid oxidizer, leading to a significant temperature drop in the combustion chamber. To account for these phenomena, physical models are extended to low-pressure conditions and implemented into the framework of an iterative Euler―Lagrange method. A new semi-implicit concept of the spray source term linearization is employed, improving the robustness of the numerical solution procedure by accounting for intense coupling of vapor flow and spray. Following the analysis of the three-dimensional two-phase flowfield and spray deposit distribution in the combustion chamber and nozzle extension, the influence of liquid injection temperature and initial droplet size distribution is investigated by detailed parametric studies. The results indicate a particular importance of the spray―wall interaction and secondary-droplet breakup for the coarser sprays and a bimodal deposit distribution on the chamber walls. Computational results agree well with experimental data and are used to derive transfer functions describing global preflow dynamics on a system level.

FORMATION AND DEVELOPMENT OF WALL LIQUID FILMS DURING IMPACTION OF GASOLINE FUEL SPRAYS

Laser-induced fluorescence (LIF) measurements and model predictions of the liquid film thickness formed on a flat surface during impingement of fuel sprays formed from low-pressure gasoline injectors are reported. The obtained measurements guide the development of a mathematical model employed to numerically predict the formation and transport of the wall liquid film. The governing film flow equations are based on the continuous Eulerian approach and are formulated according to the thin-film boundary layer framework. Extended parametric studies investigate the sensitivity of the model to different type and order of spatial and temporal discretization schemes. Governing parameters for the spray representation and wall impingement process are addressed, and their influence upon the behavior of the fuel film development is investigated. Droplet impact pressure is identified as the main driving force of the film transport process, as revealed through an order-of-magnitude analysis on the terms considered in momentum conservation equations. Model parameters defining the characteristics of the incident fuel droplets are additionally proved influential. Predictions under different injection conditions are compared against the temporal evolution of liquid film thickness at various locations over the impact wall. The numerical results capture the wavy variation of film thickness during the fuel film formation and transport along the solid surface from the time of impingement until equilibrium conditions have been reached. The latter is achieved at the final stages of mass exchange with the impinging jet, and it is well reproduced, although a tendency to under predict the peak thickness values during the initial and more transient stages of the formation process is revealed.

Study of Adhesion Fuel Behavior Formed by Spray Wall Impingement in PFI Engine

Study of Adhesion Fuel Behavior Formed by Spray Wall Impingement in PFI Engine

Spray impact onto flat and rigid walls: Empirical characterization and modelling

An experimental study of spray impact onto horizontal flat and rigid surfaces is presented and used as input data for a new empirical model. A phase Doppler instrument has been used to measure drop size and two components of velocity directly above the target. The average film thickness formed due to spray impact has been measured using a high-speed CCD camera. The spray–wall interaction has been characterized in terms of correlations for the velocity and trajectory of secondary droplets and the mass and number ratio of the secondary spray. The novel aspect of the model is that the correlations are based on mean statistics over many events and not on the outcome of single drop impact experiments. Furthermore a rather large range of oblique impact angles have been studied and incorporated into the empirical models as an influencing factor.

Experimental and numerical analysis of high pressure diesel spray–wall interaction

The interaction between impacting and splashed droplets and air motion plays a fundamental role on the mixture formation process, which is a crucial aspect for the correct operation of modern DI Diesel engines as it greatly influences the combustion process and the exhaust emissions. A complete understanding of spray impingement is quite complex. A mixed numerical–experimental approach is proposed in this paper.The experimental tests are carried out with a high pressure (up to 120MPa) diesel spray emerging from an axial disposed single-hole nozzle in an optically accessible vessel, pressurized up to 5.0MPa at ambient temperature. The jet impacts on a flat stainless steel wall heated up to 500°C by a 200W temperature regulated electrical resistance wire. The experimental analysis is performed using a Bosh tube as the injection mass flow meter, a pulsed laser sheet generated on the second harmonic of a Nd-YAG laser and a synchronized CCD camera. Digital image post-processing allows extraction of the radial penetration and thickness growth of the impacted fuel versus injection pressure, vessel back-pressure and wall temperature. Moreover, a procedure to relate light intensity to average fuel density is proposed.The numerical analysis is carried out by means of a multi-dimensional numerical tool, based on the KIVA-3V code. The spray–wall interaction is simulated through a phenomenological splash model available in literature and validated for low injection pressures (up to 300bar) and ambient back-pressure. The comparison between experimental and numerical results demonstrates the inability of the model in predicting high pressure spray–wall interaction, especially under increasing back-pressures. Based on the experimental evidences, a modified version of the model is proposed and the new model is proven to be an adequate representation for different injection pressures and back-pressures.

1834 噴霧壁面衝突時の燃料付着挙動に関する検討(J15-1 噴霧による混合気形成および燃焼(1),J15 噴霧による混合気形成および燃焼)

1834 噴霧壁面衝突時の燃料付着挙動に関する検討(J15-1 噴霧による混合気形成および燃焼(1),J15 噴霧による混合気形成および燃焼)

Dynamics of electrically charged transient evaporating sprays

AbstractThe evolution of an evaporating spray plume typical of those under consideration for use in direct injection spark ignition (DISI) engines, for early and late fuel injection strategies is investigated. Here the effect of electric charge, present on individual drops, upon the spray dispersal and evaporation rate is investigated with the aim of optimizing these parameters with respect to typical engine timescales and injection strategy. The predictions suggest that applying electric charge to drops in sprays injected early into the intake stroke does not have a beneficial effect. The spray evaporation rate is not significantly enhanced, and the long time interval between fuel injection and ignition actually promotes spray wall deposition. Conversely, applying electric charge to sprays injected late encourages secondary atomization and the increase in surface area greatly improves the evaporation rate. This is also true at higher engine speeds, corresponding to a much reduced time between fuel injection and ignition. Therefore it is suggested that the selective use of electric charge is viable way of tuning the spray character without effecting fuel metering when moving from an early to a late injection strategy in DISI engines when variable loads are required. Copyright © 2006 John Wiley & Sons, Ltd.

Flow characteristics of spray impingement in PFI injection systems

The present paper addresses an experimental study of the dynamic exchanges between the impact of an intermittent spray and the liquid film formed over the target, based on detailed phase Doppler anemometry (PDA) measurements of droplet size, velocity and volume flux in the vicinity of the impact. The flow configuration is that of a pulsed injector spraying gasoline onto a flat disc to simulate the port fuel injection (PFI) of an internal combustion engine operating at cold-start conditions. The measurements evidence that the outcome of impact cannot be accurately predicted based on the characteristics of the free spray, but requires precise knowledge of the flow structure, induced by the target. The implications for spray–wall interaction modelling are then discussed based on the application of conservation equations to the mass, momentum and energy exchanged between the impinging droplets and the liquid film. The results show that the liquid film starts to form in the vicinity of the stagnation region at early stages of injection and a non-negligible proportion of droplets impinging at outer regions splash after interaction with the film. Film disruption is mainly driven by the intermittent axial momentum of impinging droplets, which enhances the vertical oscillations. The radial momentum imparted to the liquid film at the stagnation region is fed back onto secondary droplets emerging later during the injection cycle at outwards locations, where momentum of impacting droplets is much smaller. As a consequence, although the number of splashed droplets is enhanced by normal momentum, their size and ejection velocity depends more on the radial spread induced onto the liquid film and, hence, on the radial momentum at impact. The analysis further shows that existing spray–wall interaction models can be improved if the dynamic exchanges between the impacting spray and the liquid film are accounted.

Laser Optical Flow Measurements and Computational Fluid Dynamic Calculation of Spray Tower Hydrodynamics

Spray towers for gas scrubbing and similar separation technology applications often show complicated three-dimensional gas flow patterns. Due to non-optimal inlet duct configuration and nozzle arrangement inhomogeneous gas distributions in the tower cross-section may result with a fraction of the gas stream bypassing the dense spray region. Thereby, the axial gas flux locally may reach a multiple of the towers superficial velocity. To maintain the efficiency of the spray tower the maldistribution effects are counterbalanced by the amount of the liquid throughput implying increased operating costs. In the current investigation the mechanisms of the non-ideal gas and spray motion are analysed with the help of Laser Doppler and Phase Doppler measurements in single spray cone experiments as well as in a pilot-sized spray tower. In addition to the gas velocity profiles and the spray's size distribution, the gas phase pressure drop and the spray's wall deposition flux was also measured for the countercurrent spray/gas operation. The experimental pilot-plant data were used to evaluate the results of computational fluid-dynamics simulations, based on the particle-in-cell (PIC) methodology for the polydispersed spray/gas-flow. The spray-nozzle flow is demonstrated to influence the gas motion to a considerable extend on the scale of the near nozzle region as well as in the overall spray tower flow field. A substantial amount of the total liquid injected is lost in form of spray/wall deposition.

Multidimensional modeling of Dimethyl Ether (DME) spray combustion in DI diesel engine

In the present study a modified CFD code KIVA3V was used to simulate the spray combustion in a small DI diesel engine fueled with DME. The improved spray models consider more spray phenomena such as cavitation flow in nozzle hole, jet atomization, droplet second breakup and spray wall interaction. Otherwise, a reduced DME reaction mechanism is implemented in the combustion model, and a new turbulent combustion model-Partial Stirred Reactor (PaSR) model is selected to simulate the spray combustion process, the effects of turbulent mixing on the reaction rate are considered. The results of engine modeling based on those models agreed well with the experimental measurements. Study of temperature fields variation and particle traces in the combustion chamber revealed that the engine combustion system originally used for diesel fuel must be optimized for DME.

Spray cooling characteristics of water and R-134a. Part I: nucleate boiling

An experimental study was made in the first of two papers to determine the effect of liquid sprays used to cool a hot surface. Both pure water and R-134a were served as a working medium sprayed from a single circular nozzle onto a Cu (oxygen free) metal of an electrically heated surface which was heated to an initial temperature with a range of wall superheat for steady-state nucleate boiling experiments using thermocouples for heat transfer measurements. Cooling characteristics (boiling curves) were obtained over a range of spray mass flux, Weber number, wall superheat and degree of subcooling. Boiling visualization was also conducted with varied heat flux levels at a specified We for R-134a and water.

A direct injection diesel combustion model for use in transient condition analysis

A phenomenological multizone combustion model is developed for use in the transient condition analysis of a turbocharged direct injection (DI) diesel engine. The main processes in the cylinder, such as spray penetration, droplet evaporation, air-fuel mixing, spray wall interaction, ignition and fuel combustion, are taken into consideration. In order to predict cylinder pressure and NOx emissions in a wide range of engine speeds, loads and air-fuel ratios under transient conditions, the model has been improved in many respects, especially in the velocity distribution of fuel spray, zonal entrainment and the calculation of gas properties and chemical equilibrium compositions of combustion products including solid carbon. With respect to the turbocharged engine with a high pressure in the cylinder, the compression factor is added to the ideal gas equation of state in order to simulate cylinder pressure accurately. Cylinder pressure, heat release and NOx emission histories for different operating conditions are calculated for two types of engines. The agreement between calculated and measured results is good for a wide range of operating conditions. This indicates that the model has the ability to predict cylinder pressures and NOx emissions in both steady state and transient operating conditions.