A conjugate heat transfer analysis methodology has been defined and applied to an Air Force film cooled turbine vane consisting of 648 cooling holes. An unstructured computational mesh was used to model both the fluid and metal sides of the turbine vane. A summary of the numerical methods employed by Code Leo is provided along with a description of the coupling procedure employed between the fluid and heat conduction computations. Numerical simulations were conducted at multiple mesh resolutions to assess accuracy and repeatability. A detailed review is presented for the numerical solution obtained from a fine mesh consisting of 24 million elements (8 million solid, 16 million fluid) covering all 648 film holes. Results showed that cooled air from the film holes formed a protective layer around the airfoil surfaces and endwalls as intended. Low metal temperatures were present not only on the external surfaces exposed to hot gas, but also around the entrances to the film cooling holes. Cooled air was also observed to pile up along the pressure surface at mid-span. Solution convergence was achieved in approximately 15,000 iterations and 100 hours elapsed time on a dual-socket Intel E5504 workstation. The combination of fast turnaround time with accurate metal temperature prediction will enable conjugate heat transfer analysis to be easily incorporated into routine design processes to better address durability goals.
Conjugate heat transfer analysis was conducted on a 648 hole film cooled turbine vane using Code Leo and compared to experimental results obtained at the Air Force Research Laboratory Turbine Research Facility. An unstructured mesh with fully resolved film holes for both fluid and solid domains was used to conduct the conjugate heat transfer simulation on a desktop PC with eight cores. Initial heat flux and surface metal temperature predictions showed reasonable agreement with heat flux measurements but under prediction of surface metal temperature values. Root cause analysis was performed, leading to two refinements. First, a thermal barrier coating layer was introduced into the analysis to account for the insulating properties of the Kapton layer used for the heat flux gauges. Second, inlet boundary conditions were updated to more accurately reflect rig measurement conditions. The resulting surface metal temperature predictions showed excellent agreement relative to measured results (+/− 5 degrees K).
This paper describes a procedure for assessing the efficiency of a turbine operating under unsteady periodic flow at the inlet. Time-resolved flow simulations of the modified Garrett turbine case A3K7 are conducted with time-varying inlet conditions upstream of the turbine. A mass-weighted moving average is applied to the instantaneous data over a period corresponding to the pulsing frequency of the time-varying inlet conditions. The resulting time averaged data is then used to assess the convergence of the numerical solution and evaluate the turbine performance. Three methods of computing the time-averaged turbine efficiency are presented. The first method, referred to as the TP method, is based upon the use of mass-weighted average of the total temperature (Tt) and total pressure (Pt). The second method, the TS method, uses the mass-weighted average of total temperature (Tt) and entropy (S). The third method, the WS method, employs the moving average of specific work output (Ws) and work lost due to entropy (S) increase. A comparison of time-averaged specific work output, stage efficiency, and rotor efficiency is made for three turbine operating configurations: partial admission, synchronized pulsation, and sequential pulsing flow conditions of the sector inlets. Results show that for the tested rotor speed, the efficiency increases as the inlet pulsing frequency is reduced, and that when inlet sectors are not opened simultaneously, a large drop in efficiency occurs due to spillage of high energy fluid between the open and closed sectors. Additionally, it is shown that due to the non-convective nature of total pressure, the TP method for efficiency is not reliable while the TS and WS methods are able to produce consistent results.
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