Narrow Impingement Channels: Recent Advancements and Future Directions

This paper summarizes the state-of-the-art related to the heat transfer characteristics of narrow impingement cooling channels, which can nowadays be integrated within the external wall of turbine airfoils. Particular emphasis is given to the experimental outcomes of the research activities performed at the Group of Thermal Turbomachinery (GTT) of EPFL over the last 10 years. The influence of various geometrical factors on the heat transfer distributions is demonstrated, aiming to provide a summarized physical knowledge for the thermal design of such configurations. More specifically, effects of channel height and width, streamwise jet-to-jet spacing, impingement hole staggering arrangements, varying jet diameters, convergent and divergent channel geometries, as well as effects of number of holes, are addressed for all channel interior surfaces. Existing correlations are also presented and compared to empirical models derived from traditional multi-array impingement jet systems. Furthermore, future research directions in the form of sequential impingement cascades are also discussed.

Application of the Transient Heater Foil Technique for Heat Transfer and Film Cooling Effectiveness Measurements on a Turbine Vane Endwall

Dominique Charbonnier, Magnus Jonsson, Peter Ott

The paper presents an application of the transient heater foil measurement technique using thermochromic liquid crystals (TLC) to endwall heat transfer and film cooling investigations in a transonic turbine vane cascade. The film cooling configuration consists of an upstream slot, representing the leakage flow area between the interface of the combustor and the turbine, and several rows of cylindrical and fan-shaped holes within the passage. With the transient method chosen, the heat transfer and adiabatic film cooling effectiveness distributions can be obtained simultaneously together with the local heat release within the heater foil. Therefore, the heat release in the foil is not required to be uniform, and the foil can contain discrete holes in the film cooling configuration. Some new developments are presented, which are directed towards improved application of the transient heater foil method for such a complex configuration. This includes tailoring the foil heat-release distribution towards the expected heat transfer patterns, supported by numerical Finite-Element computations and the use of a double-TLC mixture for improved time-wise TLC indications. Additionally, CFDsimulations were used to evaluate the recovery temperature distribution through the vane cascade without film cooling. The experiments were performed in the linear cascade facility at the EPFL-Lausanne. A compressor provides a continuous air flow at near-ambient temperature regulated with heat exchangers. Carbon dioxide is used as coolant in order to achieve engine-representative density ratio between coolant and main flow. Multiple experiments with the same main and coolant flow settings but varying heat flux levels and coolant injection temperatures have been performed and simultaneously analysed using nonlinear regression analysis. The time required between successive experiments to return to homogenous initial conditions, as required by the transient method, has been analysed using an analytical solution for heat-on-heat-off conditions. This permits the model assumption of onedimensional conduction within a semi-infinite wall with a heat releasing layer on the top. Example results for cases without cooling, with film cooling from rows of discrete holes and the addition of slot film cooling are used to illustrate the benefit of the new approaches for the investigated vane cascade.

American Soc Mechanical Engineers, Three Park Avenue, New York, Ny 10016-5990 Usa2008

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

Peter Ott, Alexandros Terzis, Bernhard Weigand

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.

2014

Application of the Transient Heater Foil Technique for Heat Transfer and Film Cooling Effectiveness Measurements on a Turbine Vane Endwall

Dominique Charbonnier, Magnus Jonsson, Peter Ott

American Soc Mechanical Engineers, Three Park Avenue, New York, Ny 10016-5990 Usa2008