Flow and Heat Transfer Control in a Rotating Flow Channel with Turbulence Promoters

Abstract

The objective of this research is to study the local heat transfer distribution inside the cooling channel of the turbine blade with rib turbulators in order to give a uniform heat transfer distribution under the rotations. In this research, there are two main parts of the experiment, viz. the experiment in a stationary straight channel and a rotating serpentine channel. For the stationary straight channel, it has a height (H) and width (W) of 75 mm. The rib height-to-hydraulic diameter ratio (e/Dh), the channel length-to-hydraulic diameter ratio (L/D) and rib pitch-to-height ratio (p/e) were selected at 0.133, 8 and 10, respectively. Fifteen different types of rib geometries used in this research, viz. 90°, 30°, 45° and 60° inclined ribs, 30°, 45° and 60° V-shaped ribs, 45° and 60° A-shaped ribs, 60° inclined ribs with different gap positions, and 60° V-shaped ribs with a gap, which these rib types were installed on two side walls. All experimental tests were carried out in the range of Reynolds number (Re) of 10,000 to 30,000. The local heat transfer coefficient distributions were conducted by using steady thermochromic liquid crystals (TLC) method. The friction factor and thermal performance factor were also performed to evaluate for each case. Furthermore, the commercial software, ANSYS ver. 15.0 (Fluent) was utilized to reveal the flow structure and heat transfer characteristic on the surface inside the stationary straight channel.For the rotating serpentine channel, there are two methods to measure the local heat transfer coefficient distribution on the surface, viz. the first method is steady thermochromic liquid crystals (TLC) method. In this method, the rotating serpentine channel has a height (H) and width (W) of 50 mm. The ratio of rib height-to-hydraulic diameter (e/Dh), the channel length-to-hydraulic diameter ratio (L/Dĥ), radius ratio (r/D) and rib pitch-to-height ratio (p/e) was selected at 0.1, 8, 5 and 10, respectively. Seven different types of rib geometries conducted in this method, viz. 90°, 45°, 60° inclined ribs, 60° V-shaped ribs (Case I), 60° V- shaped ribs by having 60° A-shaped ribs in the second-pass (Case II), and 60° V-shaped ribs by having 60° inclined ribs in the second-pass (Case III), and 60° V-shaped ribs with gap (Case IV), which all rib geometries were compared with the smooth serpentine channel. Also, the friction factor and thermal performance factor were also carried out to evaluate for all rib cases. For the second method is the naphthalene sublimation method. The rotating serpentine channel has a height (H) and width (W) of 15 mm. The ratio of rib height-to-hydraulic diameter (e/D1), the channel length-to-hydraulic diameter ratio (L/D), radius ratio (r/D) and rib pitch-to-height ratio (p/e) was selected at 0.133, 11.3, 22 and 10, respectively. Two different types of rib geometries studied in this method, viz. 90° and 60° inclined ribs. Both two methods were defined at the Reynolds number (Re) of 10,000 and rotation number (R,) were varied in ranges from 0.0 to 0.3. Moreover, the 3D flow field structures inside the rotating serpentine channel were revealed by using the commercial software, ANSYS ver. 15.0 (Fluent). For the case of the stationary straight channel, it is found that the heat transfer coefficient distributions on surface for the cases of 60° inclined ribs, 45°, 60° V-shaped ribs and 60° V-shaped ribs with gap were significantly high about 19.3%, 23.5%, 32.6% and 38.7% respectively when compared with the case of 90° ribs because effect of the secondary flow induced by the inclination of rib, attaches on surface. This flow can help to reduce the boundary layer thickness. As a result, the heat transfer rate increases. The highest average heat transfer coefficient on the surface was achieved for the angle of 60° V-shaped ribs with a gap when compared with other rib cases. Because the 60° V-shaped ribs generated the complex secondary flow field that induced by the inclination of rib and effect of flow through a gap. Besides, it is indicated that the case of 60° V-shaped ribs with a gap gives the best thermal performance when compared to the other rib cases. For the case of the rotating serpentine channel, the results show that the trend of the average heat transfer ratios (Nu/Nu。) for all rib cases increases with increasing the rotation number (R.). The average heat transfer ratios in the first-pass were highest on the TS wall (Pressure side wall) followed by the LS wall (Suction side wall). This trend was similar in the turn region as well. Whereas, the average heat transfer ratios in the second-pass was highest for the LS wall followed by the TS wall. Furthermore, the average heat transfer ratios in the cases of V-60° rib having 60° inclined ribs in second pass (case III) and V-60° rib with a gap (case IV) are high about 21.8% and 25.4% when compared with a smooth wall case, and high in range of 10-15% when compared with other rib cases. The friction factor (f/fo) in the case of 60° inclined rib gave the highest f/f, while the 60° V-shaped ribs by having 60° inclined ribs in the second-pass (Case III), and 60° V-shaped ribs with a gap (Case IV) provide the best thermal performance. Also, the flow field structure inside the serpentine channel due to rotation