Local Heat Transfer Distributions of Small Impingement Configurations: A Detailed Experimental Procedure Using Transient Heat Transfer Methods

Impingement cooling is widely used in the cooling of turbine airfoils at critical regions where high heat transfer rates are required due to the large local convection coefficients that can be achieved. An accurate determination of local heat transfer distributions, is therefore, essential for reliable calculations of blade metal temperatures. In modern cast-in turbine airfoils, e.g. Lutum et al. [1], narrow impingement cooling channels can be formed, where the coolant is practically injected within the wall rather than the hollow of the airfoil, as shown in Figure 1. Narrow impingement channels consist of single or double rows of several cooling jets generating a relatively low amount of crossflow while the small wall thickness of a typical airfoil (~2mm) dictates significant heat transfer in all the interior surfaces of the cavity. A turbine airfoil can include a plurality of such small impingement configurations in the spanwise direction ensuring homogenous distribution of the material temperature. Contrary to the large amount of literature sources for multi-array impingement cooling systems, e.g. a review can be found in Weigand and Spring [2], little amount of research focused on narrow impingement channels with limited information for the sidewalls and impingement plate. A few studies can be found in Gillespie et al. [3], Chambers et al. [4], Ricklick et al. [5], Lamont et al. [6] and Terzis et al. [7]. This paper describes the overall experimental procedure and the heat transfer distributions for all the interior walls of a narrow impingement channel consisting of a single row of five inline jets. Heat transfer coefficients are evaluated with the transient liquid crystal technique considering also thermocouple thermal inertia effects.