Abstract
Jet-impingement heat transfer is commonly used for vane leading edges and end-walls of turbine components for cooling the surfaces. One of the factors that limit high heat transfer rates is the effect of the crossflow which builds up downstream and adversely impacts the jet penetration and the impingement heat transfer rates. The present paper investigates the concept of introducing return holes (RH) for the crossflow to prevent its build-up and therefore reduce its deleterious effects. In the present experimental study, a 3 by 9 jet-array impinging on a target surface is considered with and without return holes. The return holes are located in an in-line pattern between the impingement holes. Experiments are conducted in an impingement channel with closed side walls and for jet-to-target distances (H/D) of 1D to 9D and a jet-Reynolds number of 20,000. Two different crossflow schemes combined with three return hole (RH) configurations are studied. The two crossflow arrangements are: (1) one radial exit and RH’s open for the spent air to exit and (2) all radial exits blocked with the spent air exiting through the RH’s only. Three different area-openings for the RH’s are considered and correspond to 33.3%, 66.7%, and 100% of the total return hole area open. In addition, a baseline case with no RH’s and one radial exit is studied. A transient liquid-crystal based study is conducted using a thin sheet of narrowband Thermochromic Liquid Crystal glued on an acrylic plate serving as the target surface. Local heat transfer coefficients are obtained based on the measured surface temperature and the solution of 1D transient heat conduction in the target acrylic plate. Return holes have significant influence on the crossflow-induced degradation effects at small jet-to-target spacing. The all-blocked crossflow scheme demonstrates good uniformity and axisymmetric Nusselt number distributions.
Jet Impingement Heat Transfer on Pinned Surfaces Using a Transient Liquid Crystal Technique