Air Mixture Formation Research Articles

Impact of flash boiling spray on soot generation of a rich fuel–air mixture under various ambient pressures

Particulate emission is one of the critical challenges for the modern gasoine direct injection (GDI) engines. Especially under cold start conditions, the global equivalence ratio inside the cylinder is often set quite rich, leading to the generation of a large amount of soot. The fuel distribution state, such as homogeneous mixing and heterogeneous mixing with local rich pockets prior to the combustion is the key factor affecting soot generation under cold start conditions in GDI engines. Flash boiling injection, as a method to promote fuel air mixing, could be a potential solution to sooting problems of rich mixture combustion under cold start conditions. However, the unclear influences of flash boiling spray to fuel–air mixing state and subsequent soot generation pose a challenge to soot reduction in GDI engines. This work aims to study the effect of various flash boiling degrees on fuel–air mixture formation and subsequent soot generation utilizing a customized constant volume combustion chamber (CVCC) incorporated with the gasoline direct injection scheme under moderately cold start ambient temperature of 30 °C. A global high equivalence ratio of 1.7 representing extremely rich combustions under cold start conditions is selected for strong sooting propensity. By setting different fuel temperatures (Tf= 30 °C and 120 °C) and various ambient pressures (Pa= 40 kPa, 100 kPa and 160 kPa), various spray flash boiling states (subcooled, transitional flash boiling and flare flash boiling sprays) are achieved, and finally various fuel distribution states of rich mixtures prior to combustion are obtained. High speed color camera and soot sampling TEM grids were used to capture and evaluate the characteristics of spray atomization, combustion and soot aggregate structures. Based on this investigation, the effect of flash boiling degree on fuel–air mixing and subsequent soot generation is analyzed. The results show that flash boiling spray enhances fuel atomization and produces much more homogeneous rich mixture with smaller local rich pockets and richer global premixed gaseous portion. Over-rich global premixed mixture combustion is prone to nuclei inception and agglomeration, producing soot aggregates with small primary particles. While the local rich pocket combustion is prone to surface growth, producing larger primary particles. Flash boiling spray could reduce the size of soot particles under cold start conditions by generating smaller local rich pockets.

Effects of fuel injection pressure and quantity on low octane gasoline sprays in a simulated high-pressure ambient environment of a gasoline compression ignition engine

Gasoline Compression Ignition technology using low-octane gasoline-like fuel can lead to very low NOx and Soot emissions and enhanced thermal efficiency than conventional diesel engines. The difference in test fuel properties is a significant reason for the difference in the fuel–air mixture formation in Gasoline Compression Ignition engines offering these advantages. In this study, microscopic and macroscopic spray characterisation of baseline diesel, 50 % v/v diesel blended with 50 % v/v gasoline (G50), and 20 % v/v diesel blended with 80 % v/v gasoline (G80) are conducted in a high-pressure ambient environment relevant to Gasoline Compression Ignition engines. The study was performed in a constant volume spray chamber in non-reacting conditions using Phase Doppler Interferometry and Diffused Backlit Illumination techniques. The test fuels were injected into a 30 bar ambient pressure environment at 400, 500, and 700-bar fuel injection pressures. The liquid spray penetration length of gasoline-diesel blends was slightly shorter than baseline diesel, but their spray cone angles and entrained air mass were significantly higher. The average axial droplet velocity was higher, and the Sauter Mean Diameter was lower for gasoline-diesel blends than baseline diesel 40 mm away from the injector nozzle exit, indicating superior mixture formation for the gasoline-diesel blends. The effect of droplet collision and injected fuel quantity on droplet size distribution was assessed in all test fuels. Droplet collisions increased the Sauter mean diameter of the spray at farther distances. Smaller fuel injection quantities decreased the Sauter mean diameter of the spray, increasing the probability of unburnt hydrocarbon emission producing ultra-lean regions in the engine combustion chamber.

Macroscopic and microscopic spray characterisation of Low-Octane blends for gasoline compression ignition engines

Gasoline compression ignition is an advanced combustion technology that controls harmful emissions from low-octane fuels used in compression ignition engines. Fuel’s physical properties and injection strategy influence the fuel–air mixture formation and combustion. High volatile fuel and fuel injection pressure improve the spray atomisation and fuel–air mixture formation. Gasoline-diesel blends are widely investigated for gasoline compression ignition engine applications. This study investigated microscopic and macroscopic spray characteristics of 70 % v/v gasoline and 30 % v/v diesel blend (G70), 80 % v/v gasoline and 20 % v/v diesel blend (G80), and baseline diesel injected at different fuel injection pressures. Parameters such as liquid fuel spray penetration length, spray cone angle, and droplet size and velocity distributions were assessed. The results of these parameters help determine the effect of the fuel injection parameters on combustion. It was found that though the liquid spray penetration length was not significantly different for the three test fuels, G70 exhibited a slightly lower liquid spray penetration length than the other two test fuels. Increasing the diesel fraction in the test blend reduced the spray cone angle and improved the axial dispersion of the fuel spray. The spray droplet diameter reduced with increasing gasoline fraction in the test blend and the fuel injection pressure. Sauter mean diameter decreased by up to 20 % for G80 than baseline diesel and up to 36 % with an increased fuel injection pressure from 300 to 700 bar. This study addressed a significant research gap in the spray characteristics of gasoline-diesel blends and provided a benchmark for simulation based model validation.

Direct Numerical Simulation of Atomization Characteristics of ECN Spray-G Injector: In-Nozzle Fluid Flow and Breakup Processes

AbstractIn order to sustain applications dealing with various liquid fuels in internal combustion engine (ICE), it is essential to make available prediction methodologies that allow an early evaluation of the potential usefulness of such fuels in terms of favourable mixture preparation process already in realistic configurations. Since the air-mixture formation and subsequent processes are predominantly governed by the fuel injection, a DNS based numerical investigation coupled with VOF as an interface tracking method is carried out in this paper to gain better insight on the fuel injection from an industrial injector "Spray G" configuration. Chosen from Engine Combustion Network (ECN), this is a gasoline direct injector (GDI) featuring 8-holes orifices and operating with high injection pressure (200 bar). Under consideration of the required computational cost associated with DNS, only the 1/8 of the nozzle geometry including one orifice is used. The numerical simulation is accomplished for the quasi-steady injection condition with nozzle needle fully opened. The obtained results are first validated with available experimental data for nozzle mass flow rate and spray spread angle showing a good agreement. Then, a detailed numerical analysis is provided for the in/near nozzle flow evolution especially for flow turbulence, primary and secondary atomization. Furthermore, droplet statistics in terms of droplet shape, and droplet size-velocity distribution together with a breakup regime map are reported. Finally, a 2-D data curation technique is proposed to extract the droplet statistics along selected planes and evaluated by direct comparison with three-dimensional droplet data, which may allow handling of the DNS data in more feasible and economical way especially for time series data with higher frequency. The comprehensive DNS data generated by this DNS-VOF approach enable not only to carry out detailed numerical analysis of in- and near-nozzle physical phenomena for which experimental data are still scarce, but also to provide a hint of more reliable injector boundary conditions useful for lower order spray injection method based on Lagrangian particle tracking.

Determination of the Effect of the Addition of Second-Generation Biodiesel BBuE to Diesel Fuel on Selected Parameters of “B” Fuels

Progressive and increasingly noticeable climate change is forcing the search for new energy sources to reduce greenhouse gas emissions, especially carbon dioxide. One way to reduce greenhouse gas emissions is by gradually replacing fossil fuels with biofuels. The authors of this work addressed the production of second-generation biofuel. The purpose of this study was to produce second-generation biodiesel from babassu palm oil (BBuE) and first-generation biodiesel from rapeseed oil (RME), to study their properties, and to determine the effect of the addition of these biodiesels to diesel fuel on selected properties of “B” fuels that affect the fuel–air mixture formation process and the combustion process. Biodiesel from babassu oil was produced because it is non-edible and has a different composition than canola oil. Then, fuels were prepared that were mixtures of diesel oil and biodiesel containing from 10 to 40% (v/v) BBuE or RME (B10, B20, B30, and B40).Tests were conducted on selected physicochemical properties of the obtained fuels. “B” fuels prepared with BBuE and DF were shown to have more favorable fuel properties than those obtained from RME and DF. Fuels that are blends of BBuE and DF have slightly higher heating values, higher cetane number values, a more favorable distillation curve, lower dynamic viscosity values as a function of temperature, and marginally lower flash point values compared to the corresponding blends of RME and DF. Esters with shorter molecules have been shown to have more favorable fuel properties.

Characterizing the role of fuel injection strategies on performance, combustion, and emissions in intelligent charge compression ignition (ICCI) mode

In a novel combustion mode, intelligent charge compression ignition (ICCI), the effects of fuel injection strategies were studied on a modified single-cylinder engine with the given engine load and speed. Commercial gasoline and diesel were used to represent low-reactivity fuel and high-reactivity fuel, respectively. The in-cylinder stratification, fuel–air mixture formation, heat release rate profile and combustion phasing were investigated in detail. Engine thermal efficiency, fuel consumption, gaseous and solid emissions were further studied. In the single-injection strategy and the multiple-injection strategy, different diesel injection timings were configured to find the influence of their changing trends on engine performance, combustion and emissions. In the single injection strategy, the diesel injection of 60 °CA BTDC can lead to the highest indicated thermal efficiency. In contrast, only a specific injection strategy influences the heat release in small-scale split injection. Using multiple injections of high-reactivity fuel and adjusting the combustion phasing decrease the excessive in-cylinder pressure and maximum pressure rise rate in the single injection. Meanwhile, alternate fuel injection to build a stratified environment in the cylinder can maintain the indicated thermal efficiency at a high level. The indicated thermal efficiency is close to 50% when the diesel first injection timing is 60 °CA BTDC, the second injection timing is 38 °CA BTDC or 100 °CA BTDC. The single diesel injection strategy of 60 °CA BTDC reaches the best emissions, in which the NOx emissions are lower than 0.5 g/kWh. The multiple split injection strategies can further reduce carbon monoxide and total hydrocarbon emissions.

ECN Spray G injector: Numerical modelling of flash-boiling breakup and spray collapse

Flash boiling is the sudden change from liquid to vapour phase and in IC engines it typically occurs during the injection process with a gasoline direct injection (GDI) setup and when high fuel temperatures result in saturation pressures higher than the in-cylinder ones. The flash boiling, due to the complex evolution of a multi-phase flow and a rapid droplet vaporisation, promotes spray atomisation and affects significantly the spray structure and fuel–air mixture formation, with consequences for the engine performance and pollutant emissions. Hence, a deep knowledge of such phenomenon is required to improve the combustion process and reduce the pollutant production. The breakup mechanism of a superheated spray is quite different from a regular one, because it features the nucleation and growth of bubbles within liquid droplets. In this paper, an innovative flash-boiling breakup model was embedded in a Eulerian–Lagrangian two-phase solver using OpenFOAM library. The model assigns to the droplets a radial velocity component due to the bubble explosion, making it capable of reconstructing the spray expansion. The code was tested using experimental data of the ECN Spray G injector covering various thermodynamic conditions. The numerical model achieves a good level of agreement with the experimental data and, in particular, it is able to reconstruct the spray collapse. Besides, it provides useful information regarding parameters that are intrinsically difficult to measure experimentally.

Experimental and CFD analysis of Re-entrant Combustion Chamber on Diesel Enriched with Hydrogen

In view of global warming and diminution of sources for traditional fuels, investigators paid their attention towards finding the substitutes in all sectors such as agriculture, transportation and industries. One of the best alternative and unique option for overcoming these limitations is usage of hydrogen gas as fuel for petroleum-based engines. Further to have better quality of fuel and air mixture formation, a significant part is played by the geometry of the combustion chamber. To have quality burning of the fuel, an effort is targeted to investigate the impact of re-entrant chambers over the conventional type without and with hydrogen enrichment at distinct flows of diesel. Initially, experiments were conducted on toroidal chamber geometry of the piston bowl with diesel and with hydrogen enrichment. With a reduction of 30.65%, 44.11% and 18.78% in emission particulates of UHC, CO and smoke density correspondingly with addition of hydrogen as that of with diesel at peak load, further there is growth in NOx emissions about 10.16%. The variations of emission particulates with replacement of modified geometry, i.e. re-entrant type are detected as further reduced. To validate the reduction in emission trends, CFD simulations are carried out on fuel mass fraction and the findings presented.

CFD Analysis of the Fuel–Air Mixture Formation Process in Passive Prechambers for Use in a High-Pressure Direct Injection (HPDI) Two-Stroke Engine

The research on two-stroke engines has been focused lately on the development of direct injection systems for reducing the emissions of hydrocarbons by minimizing the fuel short-circuiting. Low temperature combustion (LTC) may be the next step to further improve emissions and fuel consumption; however, LTC requires unconventional ignition systems. Jet ignition, i.e., the use of prechambers to accelerate the combustion process, turned out to be an effective way to perform LTC. The present work aims at proving the feasibility of adopting passive prechambers in a high-pressure, direct injection, two-stroke engine through non-reactive computational fluid dynamics analyses. The goal of the analysis is the evaluation of the prechamber performance in terms of both scavenging efficiency of burnt gases and fuel/air mixture formation inside the prechamber volume itself, in order to guarantee the mixture ignitability. Two prechamber geometries, featuring different aspect ratios and orifice numbers, were investigated. The analyses were replicated for two different locations of the injection and for three operating conditions of the engine in terms of revolution speed and load. Upon examination of the results, the effectiveness of both prechambers was found to be strongly dependent on the injection setup.

Experimental study on combustion characteristics of a CI engine runs on a renewable fuel

ABSTRACT Currently, the world is focusing on utilising biofuels as much as possible for internal combustion (IC) engine applications. Several strategies are being employed to improve fuel efficiency and decrease exhaust emissions of compression ignition (CI) engines which run on different biofuels. Fuel air mixture formation is one of the key aspects that influence engine behaviour in terms of combustion, emission, and performance of CI engines. In this research work, an attempt was made to induce air motion in a CI engine which was operated with an orange oil diesel (O-D) blend. For this purpose, a single cylinder, 4-stroke, air-cooled, direct injection (DI) diesel engine producing power of 4.4 kW at a constant speed of 1500 rpm was used. The piston of the engine was modified by two means to create turbulence; (i) piston with micro holes, and (ii) piston with micro holes and spiral grooves. The combustion characteristics of the test engine run on the O-D blend, without and with the change in piston geometries were evaluated, compared with the baseline diesel fuel operation, and presented in this paper.

Numerical investigation of twin swirl application in diesel engine combustion

Unlike conventional Diesel engine Combustion Chambers (CC) which have single swirl, MR-Process CC has twin swirl that promotes fuel air mixture formation by enabling ideal vaporization of fuel spray directed towards tangentially to piston walls. For the initial studies 2-valve (one intake manifold) engine CC geometry was modified aiming to create twin swirls from the initial single swirl and this limited version of MR-Process named as Quasi MR-Process. It is concluded in these studies that a 4-valve engine is needed for an ideal twin swirl formation. However, design and application of 4-valve engine head with two intake manifolds that satisfies ideal twin swirl conditions inside the CC is a challenging task. In addition, proposed unique MR-Process CC is not known before and there is no available experimental data for the optimum values of injection characteristics, air flow field and swirl conditions. Optimum intake manifold and MR-Process CC shape design and optimum spray injection angle determination will be an expensive and time-consuming task if only utilized from experimental studies. In this study numerical analysis of MR-Process CC were performed to investigate the feasibility of twin swirl initiation and the results are presented. This study aims to reveal effectiveness/potential of twin swirl application on Diesel engines utilizing closed cycle simulations. For this purpose, the existing swirl model in open source KIVA3V-R2 code was modified to create perfect initial twin swirls at the start of the compression stroke. Then different angular velocities of the initial swirls and injection directions of fuel sprays were applied to obtain optimum fuel air mixture that ensues to increase efficiency and decrease harmful exhaust emissions. The analyzed results showed that MR-Process has potential to obtain better fuel air mixture, hence reduce emission levels while increasing efficiency of Diesel engines. This study also presents a basis for further full-cycle investigations of MR-Process CC.

The effect of hydrogen injection parameters on the quality of hydrogen–air mixture formation for a PFI hydrogen internal combustion engine

To research the quality of the hydrogen–air mixture formation and the combustion characteristics of the hydrogen fueled engine under different hydrogen injection timings, nozzle hole positions and nozzle hole diameter, a three-dimensional simulation model for a PFI hydrogen internal combustion engine with the inlet, outlet, valves and cylinder was established using AVL Fire software. In the maximum torque condition, research focused on the variation law of the total hydrogen mass in the cylinder and inlet and the space distribution characteristics and variation law of velocity field, concentration field and turbulent kinetic energy under different hydrogen injection parameters (injection timings, nozzle hole positions and nozzle hole area) in order to reveal the influence of these parameters on hydrogen–air mixture formation process. Then the formation quality of hydrogen–air mixture was comprehensively evaluated according to the mixture uniformity coefficient, the remnant hydrogen percentage in the inlet and restraining abnormal combustion (such as preignition and backfire). The results showed that the three hydrogen injection parameters have important influence on the forming quality of hydrogen–air mixture and combustion state. The reasonable choice of the nozzle hole position of hydrogen, nozzle hole diameter and the hydrogen injection time can improve the uniformity of the hydrogen–air mixing in the cylinder of the hydrogen internal combustion engine, and the combustion heat release reaction is more reasonable. At the end of the compression stroke, the equivalence ratio uniform coefficient increased at first and then decreased with the beginning of the hydrogen injection. When hydrogen injection starting point was with 410–430°CA, equivalence ratio uniform coefficient was larger, and ignition delay period was shorter so that the combustion performance index was also good. And remnant hydrogen percentage in the inlet was less, high concentration of mixed gas in the vicinity of the inlet valve also gathered less, thus suppressing the preignition and backfire. With the increase of the distance between the nozzle and the inlet valve, the selection of the hydrogen injection period is narrowed, and the optimum hydrogen injection time was also ahead of time. The results also showed that it was favorable for the formation of uniform mixing gas when the nozzle hole diameter was 4 mm.

The influence of the enrichment injection angle on the performance of a pre-chamber spark ignition natural-gas engine

Using an enriched pre-chamber is an effective way to extend the lean limit, to reduce the nitrogen oxide emissions and to avoid abnormal combustion in spark ignition natural-gas engines. Enrichment injection in the pre-chamber of a spark ignition natural-gas engine determines the flow field and the fuel–air mixture formation quality in the pre-chamber and has a profound influence on the combustion performance of the engine. In order to study the characteristics of enrichment injection in the pre-chamber of a natural-gas engine, two-dimensional particle image velocimetry measurements and three-dimensional computational fluid dynamics calculations were carried out. The influence of the enrichment injection angle on the engine performance was investigated with the aid of a computational fluid dynamics simulation tool. The results indicate that a change in the enrichment injection angle directly affects the gas motion, the fuel–air mixture formation, the flame propagation and the formation of nitrogen oxides in the pre-chamber and further influences the penetration of the flame jets, the combustion temperature distribution and the formation of nitrogen oxides in the main chamber. There is an optimal injection angle for this research engine. Of the four injection angles that were investigated, an injection angle of 14° results in the lowest nitrogen oxide emissions.

Investigation of Swirl Ratio Impact on In-Cylinder Flow in an SIDI Optical Engine

Advanced powertrain technologies have improved engine performance with higher power output, lower exhaust emission, and better controllability. Chief among them is the development of spark-ignition direct-injection (SIDI) engines in which the in-cylinder processes control the air flow motion, fuel–air mixture formation, combustion, and soot formation. Specifically, intake air with strong swirl motion is usually introduced to form a directional in-cylinder flowfield. This approach improves the mixing process of air and fuel as well as the propagation of flame. In this study, the effect of intake air swirl on in-cylinder flow characteristics was experimentally investigated. High-speed particle image velocimetry (PIV) was conducted in an optical SIDI engine to record the flowfield on a swirl plane. The intake air swirl motion was achieved by adjusting the opening of a swirl ratio (SR) control valve which was installed in one of the two intake ports in the optical engine. Ten opening angles of the SR control valve were adjusted to produce an intake SR from 0.55 to 5.68. The flow structures at the same crank angle degree (CAD), but under different SR, were compared and analyzed using proper orthogonal decomposition (POD). The flow dominant structures and variation structures were interpreted by different POD modes. The first POD mode captured the most dominant flowfield structure characteristics; the corresponding mode coefficients showed good linearity with the measured SR at the compression stroke when the flow was swirling and steady. During the intake stroke, strong intake air motion took place, and the structures and coefficients of the first modes varied along different SR. These modes captured the flow properties affected by the intake swirl motion. Meanwhile, the second and higher modes captured the variation feature of the flow at various CADs. In summary, this paper demonstrated a promising approach of using POD to interpret the effectiveness of swirl control valve on in-cylinder swirl flow characteristics, providing better understanding for engine intake system design and optimization.

Alternative method for bulk modulus estimation of Diesel fuels

The knowledge of Diesel fuel properties has great relevance for the analysis and comprehension of phenomena related to fuel injection, fuel–air mixture formation and diesel combustion processes. This work proposes an alternative method for estimating the bulk modulus of diesel fuels by means of an experimental installation commonly used for determining fuel injection rates from common rail injection systems. Three fuels were tested in the mentioned installation: commercial Diesel fuel (blended with 5.8% of biodiesel), hydro-treated vegetal oil (HVO) and gas to liquid (GTL) fuel from natural gas by means of low temperature Fischer Tropsch process. Results were obtained by testing two different injectors (solenoid operated and piezoelectric injectors) under different injection pressures and energizing times. Fuel temperature at inlet of the high pressure injection pump was controlled, keeping constant the pressure inside the fuel injection indicator. From the experimental work, data analysis and post-processing, bulk modulus of fuels tested has been estimated and compared to results obtained by diverse authors with different experimental installations and methods. Results obtained in this work show small differences compared to published data. Additionally, the initial time lag between the signal of the electric pulse when the injector is energized and the beginning of the injection rate profile depends only on the type of injector without influence of type of fuel and operating conditions. However, the rear time lag between both mentioned profiles depends only on injector type (when the solenoid operated injector was tested) while, with piezoelectric injector, it also depends on both energizing time of the injector and injection pressure.

Effects of combustion chamber geometry on combustion characteristics of a DI diesel engine fueled with calophyllum inophyllum methyl ester

This paper describes the results of an experimental investigation carried out in a single cylinder, variable compression ratio, CI engine fueled with Calophyllum Inophyllum Methyl Ester (CIME) blended with diesel. An earlier investigation made by the investigators using CIME blends in conventional Hemispherical Combustion Chamber (HCC), showed that the blend B20 provides the optimum results and hence B20 blend was used as a test fuel for further investigations. Further, the emissions like unburned hydrocarbon (UBHC), carbon monoxide (CO) and smoke have been noticed higher with diesel. In this investigation, attempts have been made to reduce the emissions and improve the combustion characteristics by enhancing the fuel–air mixture preparation and its turbulence by changing the design of piston bowl geometry. For this, a Modified Hemispherical Combustion Chamber (MHCC) has employed and the results were compared with conventional Hemispherical Combustion Chamber (HCC). The fuel–air mixture formation in the cylinder was simulated at different positions of the piston (at TDC, mid of stroke and at BDC) using Ansys Fluent software. Conclusively, from the investigations, MHCC was recognized as an ideal choice of combustion chamber design for the entire range of operations of the engine using the blend (B20) than that of HCC.

Numerical study of emergency cold helium relief into tunnel using a simplified 3D model

Helium–air mixture formation and its propagation along tunnels is an important issue for the safe operation of cryogenic machines, including superconducting accelerators or free electron lasers. This paper proposes the use of a simplified 3D numerical model for the simulation of mixture parameters, such as temperature and helium content. This model has been validated through experimental results obtained in the laboratory, with a circular tunnel representing the LHC tunnel, scaled at 1:13. The exemplary analysis of helium flow to a rectangular tunnel is presented in this paper. The mixture flow patterns, as well as chosen parameters, are also presented and discussed.

Numerical investigation of the spray–mesh–turbulence interactions for high-pressure, evaporating sprays at engine conditions

This work presents a numerical methodology to simulate evaporating, high pressure Diesel sprays using the Eulerian–Lagrangian approach. Specific sub-models were developed to describe the liquid spray injection and breakup, and the influence of the liquid jet on the turbulence viscosity in the vicinity of the nozzle. To reduce the computational time and easily solve the problem of the grid dependency, the possibility to dynamically refine the grid where the fuel–air mixing process takes place was also included. The validity of the proposed approach was firstly verified simulating an evaporating spray in a constant-volume vessel at non-reacting conditions. The availability of a large quantity of experimental data allowed us to investigate in detail the effects of grid size, ambient diffusivity and used spray sub-models. In this way, different guidelines were derived for a successful simulation of the fuel–air mixture formation process. Finally, fuel injection and evaporation were simulated in an optical engine geometry and computed mixture fraction distributions were compared with experimental data.

Safety oriented analysis of cold helium–air mixture formation and stratification

Cryogenic installations of superconducting accelerators contain significant amount of high density helium, mostly filling the cryostats and headers located in underground tunnels and caverns. In the case of emergency relief of the helium, oxygen deficiency hazard arises. The paper proposes a description of helium–air mixture formation based on turbulent jet physics. A condition of stratified flow onset is given resulting from dimensional analysis. A performed experiment of cold helium discharge to an instrumented test tunnel enabled identification of different stratified flow types together with corresponding measurements of oxygen concentration and temperature drop. The results can be used for safety analysis of oxygen deficiency hazard in confined areas, especially in the tunnels.

A222 フラクタル次元解析による非定常噴流の形状特性

Explanation of injected fuel and air mixture formation process in combustion chamber is necessary to reduce exhaust gas of a car. The shape of fuel jet is shown quantitatively by fractal dimension. And the unsteady jet shape characteristic is clarified. Till now, helium jet has been visualized by the Schilieren method. By this method, the three dimensional data on a jet is photography in two dimensions, and it cannot extract only the outline of a jet. So water jet with tracer is injected into a water, and the two dimensional section of a jet was visualized by the laser light sheet method. The photograph of jet is taken by high speed camera. The outline of jet was extracted by image processing. The jet shape characteristic was found by searching for a fractal dimension. And it becomes clear that the jet shape characteristic changes with differences in nozzle shape (diameter and length).