An experimental investigation of heat transfer characteristics for steam cooling and air cooling in a rectangular channel roughened with parallel ribs

• 2D Nu for steam and air cooling in ribbed channel were obtained by experiments.
• Influences of the P/e on Nu for both coolants were respectively investigated.
• Influences of the α on Nu for both coolants were respectively investigated.
• Area-averaged Nu of steam cooling and air cooling in ribbed channel were compared.

A comparative experimental study on the heat transfer characteristics of steam and air flow in rectangular channels roughened with parallel ribs was conducted with the use of an infrared camera. The effects of Reynolds numbers (Re), rib spacing ratio (P/e) and rib angles (α) on steam and air convective heat transfer were obtained and compared. The blockage ratio (e/Dh) of the tested channel is 0.078 and the aspect ratio (W/H) is fixed at 3.0. In the paper, the Reynolds number for both coolants ranges from 3000 to 15,000, the rib spacing ratios were 8, 10 and 12, and rib angles were 90°, 75°, 60°, and 45°, respectively. For a square channel with 90° ribs with wider rib spacing ratio (P/e = 10 and 12) respectively give a area-averaged heat transfer about 8.4% and 11.4% more than P/e = 8 model. The heat transfer enhancement of both steam and air increased with decreasing the rib angle from 90° to 45°. Although the heat transfer distributions of steam and air flow in the rib roughness channels were similar to each other in all the investigated channels, the steam flow can obtain a high convective heat transfer enhancement capability. The area-averaged Nusselt numbers of steam flow at a Reynolds number of 12,000 were approximately 13.9%, 14.2%, 19.9%, and 23.9% higher than those of air flow for rib angles of 90°, 75°, 60°, and 45°, respectively.