Passage flow structure and its influence on endwall heat transfer in a 90° turning duct: Mean flow and high resolution endwall heat transfer experiments

Three dimensional measurements of the mean velocity field have been made in a square cross section, strongly curved, 90° turbulent duct flow. The mean radius to duct width ratio was 2.3. The study was performed as part of an overall investigation of the physics of endwall convective heat transfer. All three components of the velocity vector and the static and total pressure fields were measured using a five hole probe at four duct cross sections: inlet, 0°, 45° and 90°. Preliminary turbulence measurements using a single sensor hotwire at the inlet cross section were also obtained to provide streamwise fluctuation levels through the boundary layer. The endwall heat transfer coefficient distribution was determined using a steady state measurement technique and liquid crystal thermography. The specific technique was recently developed for arbitrarily specified heater foil geometries used in steady state heat transfer surface construction. The liquid crystal images were processed in the hue domain and extensive image processing routines were developed for local temperature determination on the heat transfer surface. A high resolution heat transfer map of the endwall surface from far upstream of the curve through the 90° cross section is presented. The 3-dimensional velocity field measurements indicate that a highly symmetric, strong secondary flow develops in the duct with a significant transfer of streamwise momentum to the transverse directions. The streamwise component of the vorticity vector was resolved using the cross stream mean velocities as measured by the five hole probe. The cross stream vorticity components within the measurement plane were estimated using the five hole probe data and an inviscid form of the incompressible momentum equation. The development of the total and static pressure fields, the 3-D mean velocity field, and all three components of the vorticity field are discussed. The endwall heat transfer distribution is interpreted with respect to the measured mean flow quantities.