A ratio that defines the rate of fluid flow in pipes, which is used to characterize the particular flow regimes that can occur in the circuit.
The onset of turbulence in water is between 2300-4000. A Reynolds number of 4000 or higher represents turbulent flow, which is preferred for cooling applications; however, the higher the Reynolds number in the circuit, the more energy is required to pump it through the circuit. Hence, the ideal Reynolds number for cooling circuits is 10,000. The pumping losses associated with Reynolds higher than 10,000 outweigh the heat transfer gains that can be achieved with higher Reynolds numbers.