Airfilm cooling through laser drilled holes

Abstract

One of the major problems in enhancing the specific work output and efficiency in gas turbines is the maximum possible value of the turbine inlet temperature due to blade material properties. To increase this maximum, turbine blades need to be cooled (internal or external), which is usually done by compressor air. Based on its high cooling efficiency, film cooling is one of the major cooling techniques used, especially for the hottest blades. In film cooling cold air is injected into the boundary layer through small nozzles in the blade surface. Impingement of the jets into the (laminar) boundary layer flow is essentially three-dimensional. The collision of the laminar jet with the boundary layer flow produces a local turbulent shear layer and changes the local heat transfer to the blade (when poorly constructed it may even increase the local heat transfer). In this project we have studied local grid refinement methods and their application to flow problems in general and to air film cooling in particular. Local defect correction (LDC) is an iterative method for solving pure boundary value or initial-boundary value problems on composite grids. It is based on using simple data structures and simple discretization stencils on uniform or tensor-product grids. Fast solution techniques exist for solving the system of equations resulting from discretization on a structured grid. We have combined the standard LDC method with high order finite differences by using a new strategy of defect calculation. Numerical results prove high accuracy and fast convergence of the proposed method. We made a review of boundary conditions for compressible flows. Since we would like to use local grid refinement for such flow problems, we studied the spreading of an acoustic pulse. For this model problem we introduced local grid refinement and made a series of tests in order to see if the artificial boundary conditions introduced for the local fine grid cause any reflections of the acoustic waves. The numerical techniques developed have been used to study film cooling. Because this problem concerns the interaction between a main flow and a jet, we also propose a domain decomposition algorithm in order to supply proper boundary conditions for the cooling jet. This domain decomposition combines a structured DNS flow solver for the problem of interest with an unstructured solver for the flow in the cooling nozzle. Additionally we implemented local grid refinement for the flow problem to save computational costs.