Investigations Of Sprav Characteristics Of Air-blast Fuel Nozzle For Small Aircraft Engine

With the development of aircraft engine, heavier thrust, higher efficiency and lower pollution emission are required. To meet these requirements, higher temperature rise and lower pollution emissions are required for engine combustion chamber. Due to the combustion chamber temperature increases, the overall equivalent ratio the combustion chamber needs to be increased. With fuel flow rate increasing, the quantity of air which participates in the combustion should be increased. It is important that fuel and air are able to blend more effectively before combustion. Higher turndown ratio is required with fuel flow rate increases. In addition, good performance at high altitude is required for light power aircraft engine. At high altitude, fuel flow rate is low while the mass flow rate of air is high. The combustion chamber can perform well only if the fuel is well atomized. Airblast nozzle is widely used in advanced aircraft engines because it can provide well blending between fuel and air. The swirler-cup is widely used on small aircraft engines. Due to the pressure nozzle used on the swirler-cup, the spary characteristics of swirler-cup is like the pressure nozzle when the fuel mass flow rate is low. The quality of spray is bad. Installation accuracy has significant influence on the spray characteristics, such as diameter distribution, fuel concentration distribution. The OTDF on the outlet of the combustion chamber will be influenced. The application of pure airblast nozzle can avoid some inherent drawbacks of swirler-cup. But the influencing mechanism of nozzle structure and aerodynamic parameters on spray characteristics is complex. The understanding of atomization mechanism cannot fully guide the design of airblast nozzle. Hence, this dissertation is organized as follows:the design of a pure airblast nozzle, experimental investigation of spray characteristics of the pure airblast nozzle and method of scaling down experiments, comparison between pure airblast nozzle and swirler-cup on spray characteristics and performance of ignition and lean blow out. The main concepts and conclusions of this dissertation are as follows,1) According to literature investigation and operation conditions of a small aircraft engine, this dissertation designs a pure airblast nozzle. CFD results indicates that the relative total pressure loss across the nozzle is blow 2.5% at max thrust conditions. The stream line shows a good center recirculation zone is formed which can increase stability of flame in combustion chamber.2) Experimental investigation on the spray characteristics of the pure airblast nozzle is carried out by LSA and kerosene-PLIF. The results indicate that the SMD of the spray is influenced mainly by the velocity of the air across the nozzle. Fuel/air mass flow ratio AFR has light influence on SMD. The R-R distribution drop size parameter n is small over all the experimental conditions, n decreases with the increasing of Ua and AFR. The spray cone angle decreases lightly while Ua or AFR increases. The droplet diameter distribution conforms to the SMD correlation which used in prompt atomization mechanism. Hence, the scaling down parameter is Ua and AFR. The spray cone angle remains 60°in scaling down experiments.3) A comparison between pure airblast nozzle and swirler-cup on spray characteristics is made. The results indicate that the SMD of pure airblast nozzle is more sensitive to Ua than that of swirler-cup. The SMD of pure airblast nozzle is less sensitive to AFR than that of swirler-cup. The fuel mass flow has no significant influence on fuel spatial distribution of pure airblast nozzle. While the fuel mass flow has significant influence on fuel spatial distribution of swirler-cup. Within the spray cone formed by pure airblast nozzle, the radial distribution of fuel concentration 1 changes less than swirler-cup.4) A comparison between pure airblast nozzle and swirler-cup on ignition and lean blow out perfomence is made. The results indicates that swirler-cup has better ignition perfomence when Ua is low. The fuel/air mass flow ratio f/a at lean blow out condition(LBO) decrease quickly with the increasing of Ua. LBO of both nozzle increases with increasing of Ua. The LBO of pure airblast increases faster than that of swirler-cup with increasing of Ua. The LBO of pure airblast nozzle is lower when Ua is low. With Ua increasing, LBO of pure airblast nozzle will exceed the value of swirler-cup.5) The ignition performance of pure airblast nozzle is mainly restricted by SMD performance while the lean blow out performance is mainly restricted by fuel spatial distribution.