Effect of injection conditions on penetration and drop size of HCCI Diesel sprays

Abstract. The development of direct injection strategies in modern Diesel engines needs increasingly high injection pressures and better fuel distribution in the combustion chamber. The ongoing study of the HCCI concept will probably require early fuel injection in air at low pressure and density, and also fuel composition will probably require some modification to reach a perfect mixing and to match evaporation and ignition requirements. In this work different common rail nozzles, fed with fuel supplied at constant pressure in the range from 30 to 100 MPa, are used to produce sprays in air at ambient temperature and pressure ranging from 1 to 7 bar, to investigate the spray penetration as a function of air and fuel pressure. From experimental results a scale low is then deduced, which is able to account for different penetration curves in the various tests by a unique common behaviour: a linear penetration part, whose length is function of the air density and of the nozzle diameter, followed by a decrease of the tip velocity. For a reduced set of experimental conditions drop size and velocity are measured by phase Doppler anemometry; time averaged mean diameter is then computed and analysed as a function of the fuel injection pressure, and shows a clear reduction of the drop diameter with increasing injection pressure

Misure di Flusso

Capitolo 10: Misure di flusso tramite sonde e tecniche ottiche: anemometria laser Doppler, anemometria da immagini di particelle, velocimetro fase-Doppler; visualizzazioni del flusso, misure di portate volumetriche e di portata massica

REDUCTION OF NOx EMISSION BY AIR STAGING IN A SWIRLED NATURAL GAS BURNER

The present work describes the experimental investigation on NOx emissions of partially premixed flames characterised by air staging through radial injection of premixed jets into a annular cross-flow of swirling air. The influence of the level of premix has been studied by monitoring the NOx and CO emissions and by measuring the isothermal flow pattern by particle image velocimetry (PIV). Three main parameters were varied independently: the fuel flow rate (thermal power), the overall equivalence ratio Φg and the swirl strength S. NOx emissions increase with increasing partial premixing and reach a maximum value at a premix equivalence ratio Φ p ≈ 5, followed by a dramatic decrease as Φ p approaches values ≈ 3. The benefits of air staging are more significant at medium swirl intensities (S ≈ 1) and moderately lean global conditions (Φ g ≥ 0.8) since otherwise flame blow off occurs. In the most favourable conditions, single digit NO x levels were reached without penalizing combustion performances or increasing CO emissions. Such conditions may be conveniently imposed by adjusting the swirl strength, the equivalence ratio of the premixed jets and the global equivalence ratio. A preliminary analysis, based on a characteristic mixing length deduced for isothermal jets discharging into uniform cross-flow, has been used to better understand the results and their implications. PIV measurements of the isothermal flow field were used to confirm such analysis and to correlate emissions with the turbulent mixing of the premixed first stage with the secondary swirling air. PIV maps have been compared with the corresponding flame photographs and the structure and dynamics of the different combustion regimes have been qualitatively interpreted and correlated with the emission levels

EFFECT OF AIR STAGING ON A COAXIAL SWIRLED NATURAL GAS FLAME

In the framework of a research program to investigate air staging applied to swirling flames, an experimental investigation aimed at studying pollutant emissions is reported. Staged combustion is accepted as an effective way to reduce nitrogen oxides in gas turbine combustors and, in the present study, is applied to a swirled flame fuelled by natural gas, to analyse the potential for further reducing nitrogen oxides emissions in a coaxial, non-premixed combustor under overall lean conditions. The results indicate that the full benefits of air staged combustion in swirling flames depend mainly on the method of fuel injection. The most common method of axial injection of a premixed jet into a co-axial swirling airflow does not realize the required fast mixing. By radial injection of the premixed first stage in the secondary co-axial airflow, more efficient and faster centrifugal mixing is achieved, and hence shorter residence time and higher combustion intensity. This fuel injection strategy results in a more stable flame and dramatic reduction of NOx emissions when the first stage equivalence ratio is approaching the stoichiometric value. Still photographs of the flame and isothermal flow patterns are also reported to help correlate flame morphology and mixing features with nitrogen oxides emission

Effect of Injection Strategy and Fuel Pressure on Diesel Spray Penetration: Application to HCCI Engines

ABSTRACT Direct injection strategies play an important role in modern Diesel engines and high-pressure common rail injection systems made easier to achieve better engine performances and lower fuel consumption. It is generally recognized that the design of the fuel injection strategy and in particular the performance of the injection system represent the key factor in meeting the increasing demand for improvement of engine performance and reduced exhaust emissions. The recent introduction of the homogeneous charge compression ignition (HCCI) combustion concept, as an attractive alternative for future IC engines, poses an increased demand for better fuel injection strategies and improvement of the mixture formation process. The HCCI concept requires early fuel injection in a gas environment at lower pressure and density, compared to the conventional Diesel engines, and thus the injection strategy needs modification to assure sufficient mixing and to match evaporation and ignition requirements, while avoiding fuel impingement on the combustion chamber surfaces. This paper describes spray penetration characteristics of a common-rail piezo-type injector operated at medium-high pressures. Optical diagnostics and CCD camera photography are currently applied to generate an extensive data base which is intended for validation of CFD simulations of HCCI engines. The preliminary results show that a proper selection of the injection strategy together with high injection pressures can fulfil the main requirements of HCCI engine conditions, still avoiding impingement of the spray on the wall. However, poor mixture formation is achieved and intense gas motion may be necessary inside the engine cylinder

Analysis of local entrainment rate in the initial region of a isothermal free swirling jets by Stereo PIV

Entrainment is of crucial importance in many engineering applications, while the relevance of swirling flows is related to their ability to improve mixing in the initial region of a jet, as a consequence of a larger entrainment of ambient air. This is particularly effective at swirl levels above the critical value associated with the onset of vortex breakdown and the generation of the instability referred to as the precessing vortex core (PVC). Despite the importance of the entrainment process in swirling flows, there have been very few quantitative data in the literature. In view of the above observation, the initial region of a swirling gas jet was investigated experimentally using Stereo Particle Image Velocimetry (SPIV) in order to evaluate the local entrainment rate as a function of swirl number and axial distance from the nozzle. The experiments were carried out on a model burner used to generate isothermal swirling air jets at Reynolds number, Re = 21800. This configuration was chosen due to its relevance to industrial burners and the ability to reproduce a round, turbulent gas jet issuing into ambient air, since this is the most investigated case and allows comparison with previously published results with and without swirl. Measurements were made for the region extending from the nozzle exit to 2.5 nozzle diameters downstream. Several swirl conditions were investigated, characterized by swirl numbers ranging from S = 0 to S = 2.2. Time-averaged SPIV allowed simultaneous evaluation of the swirl number and characterization of the structure of swirling flows in the near-field region before and after the development of the inner recirculation zone. The entrainment rate was evaluated by using the integral method applied to the measured SPIV velocity maps. The results provided quantitative evidence that the entrainment rate in the near field is enhanced considerably by the swirl, especially after the onset of vortex breakdown and PVC, reaching values significantly larger than those measured in non-swirling jets. The analysis of the average local entrainment rate revealed a large variability with both the swirl number and the axial distance from the nozzle, leading to the conclusion that the swirl affects the entrainment process in a complex way. The significance of the present results consists in a better understanding of the mixing in the initial region of a typical burner and the possible contribution to validation of numerical modelling of the combustion process

Investigation of fine scale structure of turbulent and molecular diffusion in coaxial jets of He/CO2 in air by LDA and Rayleigh scattering

none4F. Cozzi; A. Olivani; L. Caratti; A. CogheCozzi, Fabio; A., Olivani; L., Caratti; Coghe, ALDO SEBASTIAN