A comparative study of single-/two-jet crossflow heat transfer on a circular cylinder

This study presents thermo-fluidic characteristics on a circular cylinder subject to the impingement of single-/two-counter jets in crossflow. For a fixed circular jet diameter (Dj), the diameter of a target cylinder (D) varies, D/Dj = 2.5, 5.0, and 10.0. Two separate scenarios are considered and compared; at a fixed jet Reynolds number, Rej = 20,000 and at a fixed total mass flow rate. Results demonstrate that laminar to turbulent transition occurs on a fore cylinder surface which contributes significantly to overall heat transfer. However, it occurs only if the target cylinder is positioned inside the potential core of each jet and only if enough spacing (T) between the jets which is determined by the diameter ratio (D/Dj) as T = πD/2, is ensured. With small spacing, a reverse flow region formed between the jets (=πD/4) suppresses the occurrence of the transition. For a fixed jet Reynolds number, the added second jet improves local heat transfer only on the rear cylinder surface whereas the fore cylinder surface is essentially unaffected by the second jet. For a fixed total flow rate, the single impinging jet removes substantially more heat than that achievable by the two-counter jets in the present D/Dj ranges.