Investigations Of Spray Characteristics And Optical Measuring Methods For Multi-swirl Air-blast Atomizer

Fuel is injected into combustor in the form of spray by fuel nozzles, the new concepts of combustion are proposed and developed for high combustion efficiency, low pollution emission and wide range of operating conditions for combustion stability, which are dependent on the new technique of atomization forming the spray by multi-swirl airblast atomizer. The low emission and high temperature rise combustors propose new requirements and challenges for atomization, inspect of the role of atomization, the nozzles play the roles of fuel-air mixing and evaporation of droplet, the key in determining the performance of combustor are the performance of atomization, organization of air, and degree of matching between them. The complex of atomization leads to lack of mechanism and model of atomization, and now the atomization technique is bottleneck in the development of combustors. In order to make deeper understand of phenomena and mechanism in atomization, and providing support in designing the multi-swirl airblast atomizer, experimental investigation of characteristics of multi-swirl airblast atomizer is done in this work.Understanding of the physics and mechanisms associated with the formation of sprays and subsequent behavior of the two-phase flow that are normally generated in the process depends on the diagnostic methods for sprays, significant progress has been made with the benefit of developments in optical methods. Well-devoloped Phase Dopple Particle Analyzer (PDPA) method basing on point interferometry and Fraunhofer diffraction technique basing on line-of-sight measurement give the statistical results, limiting its usage in transient spray characteristics. The planar laser sheet imaging method offers several advantages over PDPA and Fraunhofer diffraction technique, having the ability of characterizing the two-dimension transient flow structure in spray. However, further effort is required to improve the accuracy of existing techniques, such as errors induced by attenuation of laser in spray, the non-linearities of Mie scattering for smaller droplets, and overlapping of the droplet interferograms in dense spray etc. Hence, this dissertation is organized as follows:the development of planar laser sheet diagnostic methods and experimental setup for visualizing the two-phase flow in atomization, experimental investigation of the characteristics of swirl cup airblast atomizer and scaling down method for atomization experiments, the optimization for performance of fuel staged and multi-point injection airblast atomizer. The main contents and conclusions of this dissertation are as follows,(1) Planar laser diagnostic methods are used to characterize the two-phase flow in spray, and the experimental setup is constructed. Planar laser imaging methods provide temporally and spatially resolved spray measurements, allow one to characterize he spray more quickly and efficiently than single point measurements of PDPA. The spray characteristics evaluated include droplet velocity, spray cone angle, patternation of spray, diameter of droplet and its distribution. The experimental facility is controlled and monitored by MCView data acquisition system and allows taking measurements of sprays generated by pressure-swirl atomizer and airblast atomizer repectively. Droplet velocity is obtained by Particle Image Velocity (PIV) technique, spray cone angle and patternation are obtained by fuel planar laser induced fluorescence (fuel-PLIF) technique, and the method for correction of fuel distribution measurement error induced by laser attenuation in spray is proposed, fuel distribution is studied in the form of axis center section plane and across-sectional plane perpendicular to the axis respectively. The droplet size are measured by PDPA, Fraunhofer diffraction method and laser sheet imaging method combining LIF and Mie scattering respectively, the geometry correction, background correction, sheet correction for detection systems are done for the spray image of LIF and Mie scattering. Furthermore, the results of droplet size are calibrated by PDPA and Fraunhofer diffraction method respectively. The comparison between results of them indicates the validation of LIF/Mie method for droplet diameter measurements. In addition, the instability of waves and breakup process of the liquid sheet are studied by combining high-speed camera and illumination techniques.(2) The performance of pressure-swirl atomizers used in the swirl cup was experimentally studied by the methods developed in the first part, including effects of pressure differential of fuel on the spray cone angle, droplet velocity, droplets size and spatial distribution of fuel in spray. The results indicated that there is a maximum value for spray cone angle with increasing of pressure differential, and then decreases gradually with increase of pressure differential, the penetration of droplets increases with increase of pressure differential firstly and then decreases, Investigations of Spray Characteristics and Optical Measuring Methods for Multi-swirl Air-blast Atomizer droplet size decreases quickly with increase of pressure differential, the droplet size will not vary when the pressure differential larger than20bar. The peak of radial fuel distribution at different axial position will disappear with the development of the spray when the pressure differential of fuel is less than10bar, while there still is a peak at periphery of the spray at axial position of50mm with pressure differential of fuel larger than10bar. The correction method for fuel distribution measurement error is used to evaluate the performance of circumferential fuel distribution of eighteen nozzles used in an aero engine, the results indicate that there are many factors leads to the ununiformity of fuel distribution, such as coaxialness of various parts in nozzle, roundness of orifice, and roughness of interior wall etc. The phenomena of liquid sheet breakup in spray was studied by high speed camera, the development of instable waves on the liquid sheet and process of liquid sheet disintegrates into ligaments and small droplet is obtained, the spectrum analysis of waves on liquid sheet indicates that the main frequency of instable waves is100Hz.(3) The internal flow field was studied by numerical simulation, including motive for formation of air core, velocity distribution in air and liquid phases, the relationship between thickness of liquid sheet at nozzle orifice and diameters of droplet in spray, the relationship between the velocity distributions in nozzle orifice and spray cone angle. The results illustrate that the axial and tangential velocity in swirl chamber reaches a maximum at the air-liquid interfaces, the size of air core increases with increase of mass flow rate, the contractions and expansions of air core in axial direction are induced by vortex in air core. The air core is a strong vortex with swirling flow, leading to instable wave on liquid sheet. The Gortler vortex in liquid phase and asymmetric vortex in gas phase are other motives for instable waves on liquid sheet. The spray cone angle based on the velocity distribution is larger than the experimental results of downstream spray, as the downstream spray interacts with the air. The results of thickness of liquid sheet explain experimental results of droplet diameter.(4) The characteristics of spray generated by an air blast atomizer with Venturi prefilmer was studied by laser diagnostic methods, furthermore, the strategies of scaling down the flow condition for spray experiments of air blast atomizer was proposed. The contents include the influence of pressure loss in swirler and performance of primary atomization on the characteristics of air blast atomizer for both ignition and idle condition of an engine. Results show that droplet velocity is determined by both swirling air and degree of droplets following the swirling air, while the degree of droplets following the swirling air is related to the primary atomization. Droplet diameter is also determined by both velocity of swirling air and primary atomization, the liqid sheet on prefilmer and strength of shearing on liquid sheet affects the performance of secondary atomization. The primary swirling air has the role of reopening the primary spray cone, while the secondary swirling air affects the fuel distribution by forming the recirculation zone. Scaling strategies of conserving velocity of air in swirler, fuel/air momentum flux ratio and AFR (air/fuel mass flow rate ratio) are in current use for full size atomizers, based on the results of sensitivity analysis of parameters on the characteristics of spray, the velocity of air in swirler is a key factor in determining the performance of atomizer. Hence, when the spray characteristics of an airblast fuel injector are to be measured in a rig at pressure lower than that of the engine, the velocity of air in swirler must be set such that the physical behaviours of interest are similar in the rig and engine.(5)The pilot nozzle of a fuel-staged and multi-point injection airblast atomizer is optimized for widening the operation range for lean combustion and enhancing fuel/air mixing by optimizing the performance of atomization. Based on the analysis of combustion performace of previous experimental results, such as ignition, lean blowout, combustion efficiency and emissions, the pilot nozzle influencing the stability of lean combustion is redesigned. The performances of the new designed pilot nozzle both working single and working with swirling air are evaluated respectively, spray cone angle of the new designed pilot nozzle varies between75°and85°, and mean droplet diameter varies between20μm and30μm in full range of working condition. The results of pilot nozzle working together woth swirling air show that most of the droplets in primary spray will impinge on the venturi prefilmer to form liquid sheet, in order to ensure the performance of seconday atomization, the relative pressure loss of swirling air should be larger than2.0%, the uniform distribution of droplet diameters in radial direction provides favorable condition for fuel/air mixing. The experimental results of ignition indicate that the new pilot nozzle can match with the multi-swirl, and the performance of ignition is greatly improved.