On the thermal and structural characteristics of an artificially generated young turbulent spot

• Turbulent spot effectiveness was proposed to evaluate heat transfer through the spot.
• Contours of Nusselt number and effectiveness of turbulent spot were visualized.
• Non linear behavior of young turbulent spot was explained.
• Turning point from young to mature state of turbulent spot was presented.

The unsteady heat transfer from an isothermal wall to a water flow via a turbulent spot at an early stage was investigated experimentally using thermochromic liquid crystals. With a Reynolds number of approximately 75,000 at the test area, an artificially generated turbulent spot was induced by a water injection in a low-turbulence water tunnel with a turbulent intensity of 0.93%. The ratio of the spot heat transfer rate to the heat transfer rate existing without the spot was determined to evaluate the turbulent spot effectiveness. Selected images of both the Nusselt number and the spot effectiveness were illustrated to visualize the spot development. Interesting results were obtained soon after the spot initiation, as the spot provided an averaged heat transfer up to 10% above the laminar state. The results also indicated a significant influence of the water injection consequent to the non-linear behavior during the young state of the turbulent spot. The turning point from young to mature state was reported. Moreover, the spot footprint appeared as a hand-like shape and propagated downstream with 38% of the free stream velocity.