| Cooling channels in modern turbine airfoils often include surface features for enhanced heat convection and near 180-degree turns. A sharp turn further induces turbulence level and increases heat transfer rate. However, it also causes significant pressure loss. While significant level of studies have been focused on either surface enhancement, such as vortex generators/turbulators, or turn effects, virtually no study in the open literature has been directed to the combined effects of sharp turn and surface-feature induced heat transfer enhancement. In this study, series of experiments were performed to investigate the heat transfer and pressure characteristics in a high aspect ratio, (4.5:1 width-to-height), two-pass channel, with delta-wing-shaped, cube-shaped and diamond-shaped element arrays placed in both channel passes before and after a 180-degree sharp turn. Transient liquid crystal technique is applied to acquire detailed local heat transfer data both on the channel surface and the turbulator elements, for Reynolds number between 13000 and 32000. To further explore potential design alternatives for enhancement cooling, the density effects of the delta wing turbulators are investigated and the effects of block height of the cube-shaped and diamond-shaped, ranging from ¼, ½, ¾ and full span of the channel height are also evaluated. Present results suggest that a staggered delta wing array can enhance heat transfer rate up to 3.1 fold in the first pass and up to 1.6 fold in the second pass, relative to the fully-developed smooth channel counterpart. When coupled with the 90-degree bend inlet, heat transfer rate can be enhanced up to another 1.9 and 1.3 fold for the first and second pass respectively. The cube-array can enhance heat transfer rate up to 3.5 folds in the first pass and approximately 1.9 fold in the second pass. For the corresponding diamond-shaped block array, the enhancement is up to 3.4 and 1.9 respectively. It is interesting to note that even though the post-turn turbulence transport in the second pass is generally higher than the first turn, the effects of surface-element induced heat transfer enhancement are, in fact, less prominent, in the post-turn region in the second pass. Pressure loss for a diamond block arrays is generally higher than that of the corresponding cube-block array. Pressure losses for sparse and dense delta arrays are approximately the same as the pressure losses of the half height cube and diamond arrays respectively. |
