In low Reynolds number flows, viscous foces dominate resulting in a flow characterized by smooth, well ordered streamlines. Such a flow is called laminar flow. In high Reynolds number flows, inertia dominates resulting in turbulent flow. Turbulent flows are characterized by rapidly changing and chaotic velocity profiles. As a result, heat transfer rates are usually much higher in turbulent flows than in laminar flows.
Just how low or high a Reynolds number needs to be in order to determine whether laminar or turbulent flow exists depends on geometry. Here are some generally accepted numbers for the laminar to turbulent transition:
- Internal flow: Recr = 2,300
- External forced flow (Flat Plate): Recr = 5 X 105
- External forced flow (Cylinders): Recr = 2 X 105
- External natural flow (Flat Plate): Grcr = 109