Local Heat Transfer in Internally Cooled Turbine Airfoil Leading Edge Regions: Part I—Impingement Cooling Without Film Coolant Extraction

Abstract

An experimental study has been designed and performed to measure very localized internal heat transfer characteristics in large-scale models of turbine blade impingement-cooled leading edge regions. Cooling is provided by a single line of equally spaced multiple jets, aimed at the leading edge apex and exiting the leading edge region in the opposite or chordwise direction. Detailed two-dimensional local surface Nusselt number distributions have been obtained through the use of aerodynamically steady but thermally transient tests employing temperature-indicating coatings. The thin coatings are sprayed directly on the test surface and are observed during the transient with automated computer vision and data acquisition systems. A wide range of parameter combinations of interest in cooled airfoil practice are covered in the test matrix, including combinations of variations in jet Reynolds number, airfoil leading edge sharpness, jet pitch-to-diameter ratio, and jet nozzle-to-apex travel distance. Measured local Nusselt numbers at each chordwise location back from the stagnation line have been used to calculate both the spanwise average Nusselt number and spanwise Nusselt number gradient as functions of chordwise position. Results indicate general increases in heat transfer with approximately the 0.6 power of jet Reynolds number, increases in heat transfer with decreasing leading edge sharpness as well as with decreasing nozzle-to-apex distance, and increases in spanwise average heat transfer with decreasing jet pitch-to-diameter ratio. The latter increases are accompanied by increases in the spanwise gradient of the heat transfer coefficient. Comparison with available prior results of much coarser spatial resolution shows good agreement and establishes confidence in the use of the results for design purposes and as baseline results for comparison with subsequent experiments involving film cooling bleed.

IR thermography in heat transfer measurements on a wing leading edge surface internally impinged by a row of air jets

“…Several papers concerned with impinging jets on concave surfaces are available [6][7]. Bunker and Metzger [6] find out a general increase in heat transfer with decreasing the leading edge sharpness as well as with decreasing the nozzle to apex distance, and increase in span-wise average heat transfer with decreasing the jet pitch to diameter ratio.…”

“…Bunker and Metzger [6] find out a general increase in heat transfer with decreasing the leading edge sharpness as well as with decreasing the nozzle to apex distance, and increase in span-wise average heat transfer with decreasing the jet pitch to diameter ratio. Fenot et al [7] study the heat transfer due to a row of hot jets impinging on a concave surface and present correlations of effectiveness and Nusselt number at stagnation points.…”

“…Both Reynolds and Nusselt numbers are modified to take into account the hole to hole spacing p. Then, they are based on the equivalent slot length s proposed by Jusionis [5]: (6) In this region the modified Nusselt number is averaged: (7) then a correlation with the modified Reynolds number is carried out. It is worth noting that Jusionis [5] found a data correlation of the type: (8) where , and .…”

Rotation Effect on Jet Impingement Heat Transfer in Smooth Rectangular Channels with Film Coolant Extraction

“…The heat transfer coefficients for the jet wall may be up to 50% of those for the target wall. Bunker and Metzger (1990), (1996) presented local mass transfer data by using a naphthalene sublimation technique for a single row of circular jets with radially outward cross flow in the leading edge region of a rotating blade. They showed that rotation reduces the heat/mass transfer coefficients between 10% and 40% depending on the rotating angle.…”

Computational Investigations of Impingement Heat Transfer on an Effused Concave Surface

“…By conducting experiments when a single row of jet impinges on a concave surface with extraction of film coolant, Bunker and Metzger [10,11] noted that the flow extraction showed only a small influence on heat transfer. They also pointed out that the jets inline with film holes would yield higher heat transfer coefficient than when they are in staggered configuration.…”