Modern gas turbine blades are designed to operate at high temperatures well above allowable metal temperatures, since increased turbine inlet temperature leads to better thermal efficiency.  As a result of our pursuit for better thermal efficiency, the turbine blade cooling has received growing and unremitting attention. In most of the practical gas turbines, the turbine blades of high pressure stage are usually too small to employ blade cooling techniques effectively. Many approaches, including novel material or alloy design, improved cooling techniques, and better manufacturing methods have been used to increase the operating temperature limit of the turbine blades and vanes to their current levels. In the cooling technique, internal cooling channels are located in the body of blade and turbine component. Bleed air from compressor is forced through these cooling passages (internal cooling) and openings at the blade external surface (external film cooling). Many cooling strategies including impingement cooling, film cooling and ribbed serpentine passages are employed to maximize the heat transferred from the blade to the coolant. In ribbed serpentine passages, repeated ribs are used on the channel walls as turbulence promoters to achieve heat transfer augmentation.



This study presents turbulent flow and heat transfer predictions in a two pass cooling ribbed duct with 180 deg turn, using a zonal or two layer wall model. The boundary layer type equation are solved in the inner layer on a virtual grid, imbedded in outer LES grid and refined only in the wall normal direction.



 











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Fig. 1 (a) Normalized Nusselt number countours on ribbed wall (b) Pitch averaged heat transfer augmentation; Normalized Nusselt number variation in (c) representative fully developed pitch length in the second pass (d) upstream region of bend (e) downstream region of bend



​Predicting complex flow physics and heat transfer in internal cooling passages of turbine blades presents significant challenges. Due to their ease of use and fast turnaround time for calculations, RANS simulations with various turbulence models, are current industry standard. These turbulence models involve lot of approximations and hence are seldom able to accurately reproduce the range of physics encountered in the serpentine internal cooling ducts. Though LES studies have been quite successful in predicting turbulent flow and heat transfer in these geometries, the grid requirements for wall resolved LES still limit their applications to relatively simpler geometries. The wall model approach results in significant saving in computational resources by virtue of the coarser grids that can be used in wall bounded flows. The present study validates the use of WMLES for predicting flow and heat transfer with experiments and elucidates on the detailed flow physics and heat transfer in two pass duct.







All the details are not presented on the website due to copyright and proprietary issues.

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