Experiments are performed to study 'surface curvature effects on the impingement cooling flow and the heat transfer processes over a concave and a convex surface. A single air jet issuing from different size slots continuously impinges normally on the concave side or the convexside of a heated semicylindrical surface. An electrical resistance wire is used to generate smoke, which allows us to visualize the impinging flow structure. The local heat transfer Nusselt number along the surfaces is measured. For impingement on a convex surface, three-dimensional counterrotating vortices on the stagnation point are initiated, which result in the enhancement of the heat transfer process. For impingement on a concave surface, the heat transfer Nusselt number increases with increasing surface curvature, which suggests the initiation of Taylor-Gortler vortices along the surface. In the experiment, the Reynolds number ranges from 6000 to 350,000, the slot-to-plate spacing from 2 to 16, and the diameter-to-slot-width ratio D/b from 8 to 45.7. Correlations of both the stagnation point and the average Nusselt number over the curved surface, which account for the surface curvature effect, are presented. 1 Introduction Impingement cooling has been widely used to cool a heat transfer component exposed to a high temperature or a high heat flux environment. The impingement cooling jet has the advantage that it is readily moved to the location of interest and removes a large amount of heat. It has been widely used in such industrial systems as high-temperature gas turbines, paper drying, glass manufacturing, and high-density electronic equipment. The impinging jet used in these systems is air. Over the past 30 years, impingement cooling heat transfer has been extensively studied. Good review articles are available (Martin, 1977; Becko, 1976). The impinging flow structure (Donaldson and Snedeker, 1971a, 1971b), the local heat transfer, and the correlations of average Nusselt number in terms of relevant parameters have been well studied (Gardon and Cobonpue, 1963; Gardon and Akfirat, 1966; Korger and Krizek, 1965; Zumbrunnen et al., 1989). However, the impingement cooling studied in the past was on a flat plate. The situation of impingement cooling over a curved surface may frequently be encountered. However, the studies of impingement cooling on a curved surface are rela­tively few. Chupp et al. (1969) studied the impingement cooling heat transfer for an array of round jets impinging on a concave surface. The geometric configuration studied is very similar to the case for cooling of the leading edge of a gas turbine airfoil. They measure the local Nusselt number and correlate the av­erage Nusselt number in terms of the Reynolds number, the nozzle-to-plate spacing, and some nondimensional parameters of geometry. However, the local heat transfer obtained is ac­tually an average over a relatively large space. A similar ge­ometry is also studied by Metzger et al. (1969,1972) and Hrycak (1978, 1981). Tabakoff and Clevenger (1972) study three dif­ferent configurations of impinging jets on a concave surface: the single slot jet, the one-dimensional row of round jets, and the two-dimensional array of jets. Both the local and the av­erage Nusselt number are determined. However, the local heat transfer Nusselt number obtained is again an average over a relatively large space. A few correlations of average Nusselt numbers for slot jet impingement cooling over a concave or a

Surface Curvature Effect on Slot-Air-Jet Impingement Cooling Flow and Heat Transfer Process

Fundamental Gas Turbine Heat Transfer

Flow visualization of a round jet impinging on cylindrical surfaces

Detailed heat transfer distributions are presented for an array of in-line jets impinging orthogonally on a target plate with different crossflow orientations. A transient liquid crystal technique was used to measure the detailed heat transfer coefficients on the target surface. Different crossflow directions are created by changing the test section open ends. For each exit orientation, measurements are made at four flow Reynolds numbers between 4.8 x 10 3 and 1.83 X 10 4 . Results show that the now exit crossflow direction significantly affects the flow and heat transfer coefficient distributions on the target plate. Local heat transfer coefficient increases with an Increase in average jet Reynolds number over the entire impingement target surface. Highest heat transfer coefficients are obtained for a crossflow orientation where flow exits in both directions. Nusselt number results are correlated for the various exit flow orientations

Detailed heat transfer distributions under an array of orthogonal impinging jets

Turbulent flow and heat transfer measurements on a curved surface with a fully developed round impinging jet

Although internal impingement cooling of the leading edge of gas-turbine airfoils has been shown to be effective, previously available heat-transfer data are not generally applicable to present-day turbine designs because of the unique geometry requirements. An experimental system which allows the ready acquisition of heat-transfer data necessary for thermal design of turbine airfoils is described. A cold-flow model is developed, and the measurement of local heat-transfer coefficients on a full-size model is accomplished by considering Joulean dissipation in very thin platinum strips bonded to the model. Heattransfer results are given which show the dependence of Nusselt number on Reynolds number, geometry, and chordwise location on the inside leading-edge region of the airfoil. Dimensionless correlations are presented which allow the designer to predict heat transfer for impingement cooling in these geometries for the range of parameters tested.

Tony R. Brown

Papers

Evaluation of internal heat-transfer coefficients for impingement-cooled turbine airfoils.