Gas turbines

Ofodike A. Ezekoye

J.T. MacGuire Professor
Dept. of Mechanical Engineering, The University of Texas at Austin Austin, TX 78712

Gas turbines are used in a variety of power generation and propulsion system applications. These range from the generation of electricity in electrical power grids to the propulsion of planes, ships, trains, and tanks. Gas turbines can be modeled as operating in various modifications of the thermodynamic Brayton cycle. The Brayton cycle is an air-standard idealization of the gas -turbine process. The major components of gas turbines are the compressor which compresses air or the primary working fluid, combustor where a fuel is burned and heat is added to the working flow, a turbine, through which the heated flow expands, and an exhaust flow, which can be used to generate thrust in some aero-propulsion applications. These key elements are modeled in the ideal (reversible) Brayton cycle as an isentropic compression process, isobaric heat addition process, isentropic expansion process, and isobaric cooling (heat rejection) process. The reversible or idealized Brayton cycle T-s and P-h diagrams are shown below.

In reality, the processes require more detailed modeling considerations for design and analysis considerations. One useful insight gained from the ideal Brayton cycle analysis relates to the thermal efficiency of the Brayton cycle. It is easily shown that the thermal efficiency is:

where rp is the pressure ratio. The pressure ratio is directly related to key temperature ratios across the gas turbine exhaust. The trend of improving gas turbine efficiency has resulted in increased thermal stress on turbine exhaust vane materials. Improvements in metallurgical properties of critical elements has been important in achieving these required efficiency improvements. Equally important has been improvements in gas turbine vane heat transfer strategies.

Heat transfer characterization of gas turbines has focused on the conjugate heat transfer analysis of conduction and convection in the post-combustor flow. Temperatures in excess of 1400 K are exhausted from the combustor region. The flow over the turbine vanes and blades is a complex three-dimensional flow. Gas turbine heat transfer researchers have worked to develop cooling strategies that incorporate cooling channels within the blades and transpirational cooling flows outside the vanes. Work has focused on optimizing the cooling effectiveness of the hole placement and geometries while enforcing constraints on pressure drop, manufacturability, and other issues.

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