Pin fins are normally used for cooling the trailing edge region of a turbine, where their aspect ratio (height H/diameter D) is characteristically low. In small turbine vanes and blades, however, pin fins may also be located in the middle region of the airfoil. In this case, the aspect ratio can be quite large, usually obtaining values greater than 4. Heat transfer tests, which are conducted under atmospheric conditions for the cooling design of turbine vanes and blades, may overestimate the heat transfer coefficient of the pin-finned flow channel for such long pin fins. The fin efficiency of a long pin fin is almost unity in a low heat transfer situation as it would be encountered under atmospheric conditions, but can be considerably lower under high heat transfer conditions and for pin fins made of low thermal conductivity material. A series of tests with corresponding heat transfer models has been conducted in order to clarify the heat transfer characteristics of the long pin-finned flow channel. It is assumed that heat transfer coefficients can be predicted by the linear combination of two heat transfer equations, which were separately developed for the pin fin surface and for tubes in crossflow. To confirm the suggested combined equations, experiments have been carried out, in which the aspect ratio and the thermal conductivity of the pin were the test parameters. To maintain a high heat transfer coefficient for a long pin fin under high-pressure conditions, the heat transfer was augmented by adding a turbulence promoter on the pin-finned endwall surface. A corresponding equation that describes this situation has been developed. The predicted and measured values showed good agreement. In this paper, a comprehensive study on the heat transfer of a long pin-fin array will be presented.
An LPP (Lean Pre-mixed Pre-vaporized) combustor is one of the most promising systems to make it possible to reduce NOx emission drastically. To realize low NOx combustors using liquid fuel, uniformity and fine atomization of fuel droplets are essential requirements. Droplet diameters of a fuel nozzle designed for LPP combustor as determined by PDPA measurement system are presented in this paper. An annulus pre-mixing duct was employed for the LPP fuel nozzle test model. Spray tests were conducted at pressures from 0.18MPa to 0.53MPa. Pre-mixing air velocity was also varied. Data show that the test nozzle produces a fine spray. In this paper, fuel droplet size distribution and velocity are presented and effects of air pressure and velocity on atomization are discussed. SMD of fuel droplets increases with the increases of ambient pressure. This is inconsistent with the trend determined by other works. But when the effect of fuel flow rate (or fuel film thickness) is considered, these inconsistencies can be resolved.
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