Local heat transfer distribution in a two-pass trapezoidal channel with a 180 [degree] turn via transient liquid crystal technique

Abstract

This experimental research investigates the heat transfer characteristics of air flows in serpentine cooling channels in stationary stator blades of gas turbines. The internal cooling channels are modeled as a smooth two-pass channel of trapezoidal cross section. Attention is focused on the effect of the 180' turn on the local heat transfer distributions on the interior surfaces of the various walls at the turn, under turbulent flow conditions. Transient heat transfer experiments, using encapsulated thermochromic liquid crystals, are conducted to obtain the local heat transfer coefficient distributions for various rates of air flow through the channel, corresponding to Reynolds numbers ranging between 12000 and 88000. The turn induces secondary flows that impinge on the end wall and the outlet outer wall causing high heat transfer. The flow separates at the tip of the middle wall and reattaches on the outlet inner wall. The heat transfer is the lowest on the inlet outer wall. The heat transfer is, in general, much higher on the walls in the turn and downstream of the turn than on the walls upstream of the turn. Heat transfer enhancement due to the turn is lower at higher Reynolds numbers. The trends of the local heat transfer distributions on the various walls are affected by the geometry of the turn, but are relatively independent of the Reynolds number. The results of this research help engineers design effective cooling channels in turbine blades, enabling turbine engines to operate at higher temperatures for improved efficiencies.