Studio sperimentale degli aspetti aero-termici associati al raffreddamento dei primi stadi delle turbine a gas

Experimental study of aero-thermal characteristics of gas turbine first-stages cooling The aim of the present work is to experimentally investigate aero-thermic characteristics related to hot spot propagation in gas turbines and the gas turbine internal blade cooling. The experiments were carried out in the Aerodynamics and Turbomachinery Laboratory of DIME/MASET Department of the University of Genoa. In the Introduction, the issues related to the machine performance improvement and the consequent gas turbine temperature and thermal stress increase are introduced. Finally, the main internal and external gas turbine blade cooling technologies are presented. The Ph.D. Thesis is divided into two sections: in the first part, “Experimental study of heat transfer in forced convection in ribbed channels”, the heat transfer of gas turbine blades is experimentally studied. The channels inside the blades are corrugated by so-called ribs, which have the task of increasing the heat transfer between the coolant and the inner surface of the blade, by means of flow turbulence and heat exchange surface increase. Part 1 - Chapter 1 describes the phenomenology and state of the art regarding heat transfer in ribbed channels, with a review of the main experimental studies and the description of most important parameters reported in the literature. Part 1 - Chapter 2 presents the experimental facility and the thermal and aerodynamic measurement techniques. In the Part 1 - Chapter 3 the test methodology used in the experiments is described as well as the procedure of data processing. Temperature measurements required for the analysis were carried on by means of the liquid crystal thermography. The use of liquid crystals in experiments on turbomachinery represents an exciting new approach with significant margins of development: it is an optical technique that guarantees very limited intrusiveness. Finally, in Part 1 - Chapter 4, the experimental results are presented: smooth channel and four ribbed channel configurations were investigated, varying the disposition of the ribs, the Reynolds number and the type of electric heater used. First, the channel turbulence measures are analyzed, then the pressure drop and the heat transfer of the channel are presented, varying the tested configurations. The map of the Nusselt number permits to identify the most thermally stressed areas and the heat transfer enhancement, by means of the ribbed disposition optimization. The channel performance of the different configurations has also been studied by analyzing local and global quantities. In the second part, “Experimental study of the effects induced by hot-streaks in an axial turbine model”, the hot-streaks influence, propagation and mixing with gas turbine mainstream are experimentally investigated. Furthermore, the heat transfer with the stator blades is been analyzed. Part 2 - Chapter 1 describes the phenomenology and the state of the art regarding the hot spot propagation in the gas turbine and a review of the main studies is presented, most of which are numerical studies. In Part 2 - Chapter 2 the single stage axial gas turbine model, the experimental facility and the measurement techniques used to investigate the hot-streak propagation are presented. The test methodology and the data processing are also described. Finally, in Part 2 - Chapter 3, the experimental results of the study are presented: several hot-streak radial and circumferential positions were investigated and the pressure and thermal field through the gas turbine stage were analyzed to study the propagation and mixing experienced by the hot jet. Finally, the Conclusions summarizes the main experimental results obtained during the whole research activity.